Article pubs.acs.org/IC

Properties of Cerium Hydroxides from Matrix Infrared Spectra and Electronic Structure Calculations Zongtang Fang,† K. Sahan Thanthiriwatte,† David A. Dixon,*,† Lester Andrews,‡ and Xuefeng Wang‡,§ †

Department of Chemistry, The University of Alabama, Box 870223, Tuscaloosa, Alabama 34487-0336, United States Department of Chemistry, Box 400319, University of Virginia, Charlottesville, Virginia 22904-4319, United States § Department of Chemistry, Tongji University, Shanghai 200093, China ‡

S Supporting Information *

ABSTRACT: Reactions of laser ablated cerium atoms with hydrogen peroxide or hydrogen and oxygen mixtures diluted in argon and condensed at 4 K produced the Ce(OH)3 and Ce(OH)2 molecules and Ce(OH)2+ cation as major products. Additional minor products were identified as the Ce(OH)4, HCeO, and OCeOH molecules. These new species were identified from their matrix infrared spectra with D2O2, D2, and 18O2 isotopic substitution and correlating observed frequencies with values calculated by density functional theory. We find that the amounts of Ce(OH)3 and of the Ce(OH)2+ cation increase on UV (λ > 220 nm) photolysis, while Ce(OH)2, Ce(OH)4, and HCeO are photosensitive. The observed major species for Ce are in the +III or +II oxidation state, and the minor product, Ce(OH)4, is in the +IV oxidation state. The calculations for the vibrational frequencies with the B3LYP functional agree well with the experiment. The NBO analysis shows significant backbonding to the metal 4f and 5d orbitals for the closed shell species. Most open shell species have the excess spin in the 4f with paired spin in the 5d due to backbonding. The heats of formation of the observed species were derived from the available data from experiment and the calculated reaction energies. The major products in this study are different from similar reactions for Th where the tetrahydroxide was the major species.

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INTRODUCTION Cerium, the most abundant rare earth element, typically has two oxidation states: the cerous +III and the ceric +IV. Cerium hydroxides, Ce(OH)3 and Ce(OH)4, are generally insoluble in water and are only soluble in strong acid.1 Ce(OH)3 can be oxidized to form Ce(OH)4 at room temperature by air.2 Cerium hydroxide can be prepared from a salt of Ce(III) or a Ce(IV) solution and an alkaline solution.3−5 Ce(OH)4 is the precursor to synthesize CeO2 (ceria), a widely used catalyst, by dehydration.5−7 CeO2 nanoparticles are usually a mixture of the Ce(III) and Ce(IV) oxidation states. The catalytic ability of CeO2 is mainly based on the redox chemistry between Ce3+ and Ce4+.8 Water splitting by the reduction of CeO2 to form Ce2O3, followed by the hydrolysis of Ce2O3 to produce H2, is a promising process for hydrogen production.9 CeO2, Ce-doped, and ceria supported catalysts have shown excellent performance for visible light photocatalytic water splitting.10−12 Cerium plays the role of either stabilizing the active catalyst or being directly involved in conjunction with other active species. Its redox and Lewis acid−base properties also allow CeO2 to activate organic molecules for synthesis and for the transformation of biomass.13 CeO2 is an environmentally benign catalyst and is used to eliminate volatile organic carbon species.14,15 Ceria is wellknown as part of the three-way catalyst used to convert automobile exhaust gases into nontoxic gases.16 As CeO2 is quite insoluble, its powder has been used in the polishing or grinding of other materials.17 © XXXX American Chemical Society

A number of small new lanthanide metal bearing molecules have been prepared for spectroscopic investigation using laser ablation of the lanthanides and matrix isolation infrared spectroscopy for characterization. For example, the reactions of Ln with CH3F,18 CH2F2,19 CHF3,20 CH3OH,21 OF2,22 O2,23,24 H2,25−27 N2,28,29 NO,30 CO,31−33 CO2,34 N2O,35,36 and H2O37,38 have been reported. Very recently, the reactions of Ce with CH3OH have been studied by a combination of matrix isolation infrared spectroscopy and electronic structure calculations.39 Molecular metal dihydroxides have been prepared through the matrix reaction of the laser ablated metals with hydrogen peroxide diluted in argon. These investigations have included Groups 2, 3, 4, 11, 12 and Th.40−46 We have extended this work with H2O2 reactions to include cerium as an interesting multivalent metal with the possibility of several products.

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EXPERIMENTAL AND COMPUTATIONAL METHODS

Laser-ablated cerium metal (Johnson Matthey) atoms were codeposited with H2O2 molecules diluted in argon during condensation onto a 4 K cesium iodide window.23−25,47 A urea−hydrogen peroxide complex (Aldrich) maintained behind a Chemglass PyrexTeflon valve at room temperature introduced H2O2 molecules into the flowing argon reaction medium (4 mmol in 1 h) and host deposition Received: November 12, 2015

A

DOI: 10.1021/acs.inorgchem.5b02619 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 1. Observed in Solid Argon and Calculated Frequencies (cm−1) for Cerium Species Containing Hydrogen and Oxygena molecule

symmetry

obs (OH), (OD)

Ce(OH)4 Ce(OH)4b Ce(OD)4 Ce(OH)4−

Td Td Td C1

3714.8

559.8

n.o.

2740.1 not obs

543.4 425, 427

n.o.

Ce(OH)4−b Ce(OH)3

Td C1

3742.7

510.3

n.o.

Ce(OH)3c Ce(OH)3b Ce(OD)3

C3v C3v C1

Ce(OD)3c Ce(OD)3b Ce(OH)2 Ce(OD)2 Ce(OH)2+

C3v C3v C2v C2v C2v

Ce(OD)2+

C2v

Ce(OH)2− Ce(OD)2− OCe(OH) OCe(OD) OCe(OH)2 OCe(OD)2 HCeO DCeO

C2v C2v Cs Cs Cs Cs Cs Cs

2760.5

3719.9 2743.8 3690.7 3693.6 2722.3 2726.2 3733.7 2754.0 3701.5 2727.1 3699.7 n.o. 1286.5d 925.1e

obs Ce−O asym

obs Ce−O sym

calc O−H asyma

calc O−H syma

518.7 505.8 590.7

553.9 537.9 632.1

578.9

617.5

2797.1 (255)

2803.1 (175)

610.6 (264)

654.1 (67)

3926.0 (69) 2858.8 (57) 3899.6 (62) 2840.3 (54) 3896.1 (186) 2837.5 (150) 1308.2 (425)d 932.2 (186)e

3926.0 (69) 2860.4 (62)

520.7 506.1 780.8 780.5 537.0 524.0 793.1 790.4

536.5 (14) 521.2 (10) 529 (179) 516.6 (192) 568.6 (139) 553.6 (134)

750.0 749.5 780.7f 780.3f 796.1 793.5

n.o.

487.2 477.6

(0) (0) (0) (6)

calc Ce−O syma

3903.8 t (732) 3983.4 t (888) 2843.5 (561) 3911.8 (7) 3912.9 (7) 3916.8 (7) 4017.8 t (54) 3919.3 (87) 3920.5 (104) 3797.4e (165) 3996.3e (265) 2854.5 (89) 2855.8 (101) 2766.1e (155) 2911.0e (229) 3914.1 (154) 2851 (12) 3840.1 (356)

496.5

3907.4 3987.8 2848.8 3918.4

calc Ce−O asyma

588.3(0) 584.0 (0) 570.8(0) 482.0 (0)

2767.1 (48) 2913.3 (54) 3915.1 (179) 2853 (124) 3845.9 (267)

544.7 t (834) 533.7 t (915) 531.5 t (774) 427.7 (193) 429.8 (220) 437.1 (235) 449.3 t (831) 524.5 (237) 530.7 (302) 532.1e (564) 522.4e (609) 510.4 (247) 517.4 (2890 520.0e (537) 510.2e (590) 552.7 (175) 539.1 (186) 625.5 (266)

4016.8 (0) 3921.4 (67) 3797.8 (48) 3997.3 (77) 2857.8 (37)

3897.3 (76) 2839.7 (59)

(129) (160) (271) (272) (283) (279) (287) (303)

491.6 (0) 575.6 (30) 568.8 (37) 578.4 (35) 559.2 (20) 551.9 562.1 597.5 581.0 672.7

(20) (23) (71) (57) (83)

a

Calculated using the B3LYP exchange-correlation functional with intensities in km/mol in parentheses unless noted. bM06. cBP86. dCe−H. eCe− D. fCalc CeO, 816.1(260) and 815.8 (260). OCeOH, OCe(OH)2, Ce(OH)2−, Ce(OH)3+, Ce(OH)3−, Ce(OH)4−, CeO2, and CeO2− in order to assist in the identification of possible new product species. Orbital occupancies were determined using NBO655,56 for natural bond orbital (NBO)57−60 population analysis. The open shell DFT calculations were all done in the spin unrestricted formalism. The value of S2 for all open shell species matched the theoretical values to better than 0.01. The density functional theory calculations were done using Gaussian 09.61 The B3LYP optimized geometries were also used for the single point calculations at the correlated molecular orbital theory coupled cluster CCSD(T)62−65 theory with the aug-cc-pVnZ (n = D, T) basis sets for H, O,66 and Stuttgart ECP and basis set for Ce (aug-cc-pVnZ/ Stutt). The open shell calculations were calculated with the R/ UCCSD(T) approach where a restricted open shell Hartree−Fock (ROHF) calculation was initially performed and the spin constraint was then relaxed in the coupled cluster calculation.67,68 The CCSD(T) calculations for selected molecules were also performed at the thirdorder Douglas−Kroll−Hess Hamiltonian69 level with the new ccpVnZ-DK3 basis sets70 for Ce and the DK-contracted versions71 of the standard aug-cc-pVnZ basis sets H and O. The CCSD(T) calculations were done with the MOLPRO program.72,73 The computations for the thorium species were performed at the same levels with the cc-pVTZPP basis set for Th74 and the aug-cc-pVTZ basis set for H and O (ccpVTZ-PP).

line directed at the cold window. We do not know the absolute concentration of H2O2 in these experiments, but our infrared spectra exhibit mostly monomeric H2O2 based on comparison with spectra from the Helsinki laboratory,48 and for this degree of matrix isolation from argon flow and peroxide complex temperature, the concentration of H2O2 should be less than or on the order of 0.2%. Deuterium substituted urea−D2O2 was prepared employing methods to exchange the urea−H2O2 complex with D2O as described previously.40−45,48 On the basis of the relative areas of HDO2 (981 cm−1) and D2O2 (952 cm−1) bands, and the virtual absence of H2O2 (1275 cm−1), our enriched samples contained approximately 10% HDO2 and 90% D2O2.48 Matrix infrared spectra were recorded on a Nicolet 750 spectrometer after sample deposition, after annealing, and after irradiation using a mercury arc street lamp. Additional experiments employed the ultraviolet irradiation of Ce, H2, O2, and mixtures in argon in order to incorporate 18O into the selected reaction product molecules.23,25,40−45 Neon experiments are more difficult to do because the freezing point is closer to our 4 K cold window temperature than in the case of argon. The additional heat load from laser ablation makes freezing even more difficult to do in order to trap and isolate small molecules in solid neon. The product bands observed in solid argon are not strong enough to give us much chance for successful neon matrix experiments. Structures and vibrational frequencies of the Ce(OH)2, Ce(OH)3, and Ce(OH)4 molecules and the Ce(OH)2+ molecular cation were calculated using the B3LYP hybrid functional49,50 with the DZVP2 basis set51 for H and O and the Stuttgart small core relativistic effective core potential (ECP) with its accompanying segmented basis set for cerium52,53 (DZVP2/Stutt). The thermal corrections at 298 K were obtained using the usual statistical mechanical expressions.54 Additional B3LYP computations were performed for CeOH, HCeO,

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RESULTS AND DISCUSSION Infrared spectra for the Ce and H2O2 and Ce, H2, O2 argon matrix systems, with isotopic modifications, and electronic structure calculations of products are presented, followed by comparisons of these results for two different Ce reaction B

DOI: 10.1021/acs.inorgchem.5b02619 Inorg. Chem. XXXX, XXX, XXX−XXX

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Ce(OH)4, is followed better in Figure S1, which shows nine scans of original spectra without the noise added by Coreldraw processing as in Figure 1. Other peaks at 3701.5 and 3699.7 cm−1 will be assigned below to the OCeOH and OCe(OH)2 molecules. Coreldraw is typically employed to prepare figures from original spectra so that intensity and wavenumber axis values can be adjusted, backgrounds can be removed, and peaks can be labeled to aid in their discussion. These advantages, however, come with increasing the noise level in the spectra. Similar behavior is shown in the O−D region for Ce reactions with D2O2. The highest frequency band at 2760.6 cm−1 and the lowest frequency doublet at 2726.1 and 2722.3 cm−1 gain at the expense of the strongest band at 2744.0 cm−1 on UV irradiation. A weak band assigned to Ce(OD)4 appears at 2740.1 cm−1 on annealing, subsides on UV irradiation, and increases slightly on final annealing to 34 K. The corresponding Ce−O(H)((O(D)) and Ce−O stretching modes are illustrated in Figure 2 where products from the

systems to support product identification. In addition to H2O2 and D2O2, a number of absorptions were common to these experiments including OH, OD, HO2, and DO2 radicals,44,75,76 the H2O-O photolysis product,77 and cerium oxides CeO and CeO2 as well as CeO+ and CeO2−.23 The vibrational data are summarized in Table 1. Infrared Spectra. Laser ablated Ce metal reaction products with H2O2 and with D2O2 are presented first, and reasons are given to support our initial identification of the major products, which will be confirmed in detail by isotopic shifts and DFT frequency calculations in subsequent sections. Infrared spectra are compared in Figure 1 for the O−H and O−D stretching

Figure 1. Infrared spectra in the O−H and O−D stretching regions for laser ablated Ce and H2O2 or D2O2 reaction products condensed in excess argon at 4 K. Spectra recorded after (a, g) sample deposition with H2O2 or D2O2 for 60 min, (b, h) after annealing to 20 K, (c, i) after annealing to 24 K, (d, j) after 240−380 nm irradiation, (e, k) after >220 nm irradiation, and (f, l) after annealing to 30 K. Water absorptions are labeled W, and the decomposition complex H2O−O is marked C. Figure 2. Infrared spectra in the Ce−O stretching region for laser ablated Ce and H2O2 or D2O2 reaction products condensed in excess argon at 4 K. Spectra recorded after (a, g) sample deposition with H2O2 or D2O2 for 60 min, (b, h) after annealing to 20 K, (c, i) after annealing to 24 K, (d, j) after 240−380 nm irradiation, (e, k) after >220 nm irradiation, and (f, l) after annealing to 30 K. The label hp denotes H2O2.

regions. The highest frequency product absorption at 3742.7 cm−1 assigned to Ce(OH)3 increases on annealing to 20 and 24 K, reaching double the initial absorbance. Irradiation with mostly visible > 350 nm light had little effect (Figure S1), but two ultraviolet irradiation sequences (Figure 1(d, e)) increase the 3742.7 cm−1 band another 50% and support its assignment to the stable trivalent Ce(OH)3 product. Photolysis also increases the 3730, 3725 cm−1 H2O−O decomposition product complex labeled C.77 The strongest initial product absorption at 3719.9 cm−1 is assigned to Ce(OH)2 and increases 20% on the first annealing and 20% on pyrex transmitting >290 nm irradiation (Figure S1) before decreasing 25% upon UV irradiations (240−380 nm and λ > 220 nm) (Figure 1(d, e)). Weak OH radical absorption at 3553 and 3548 cm−1 (not shown) decreased on annealing and increased on UV irradiation as before.44,75 The lowest frequency absorption in this region at 3692 cm−1 sharpens into a doublet at 3693.7 and 3691.0 cm−1 on annealing to 20 K, then decreases on annealing to 24 K but doubles on UV irradiation, and decreases on final annealing as a reactive species. These absorptions are attributed to a cation using methods employed in the Ce and O2 matrix system23 and are assigned to Ce(OH)2+ on the basis of DFT calculations presented below. An important weak absorption at 3714.8 cm−1 increases on first annealing and on visible photolysis, but decreases on UV irradiation and increases on final annealing. This weak feature, which will be assigned to

reaction of Ce with H2O2 and with D2O2 are compared. The H to D shifts in these modes are small, and matching the H and D counterparts is straightforward. Also, there is an exchange of the highest to lowest frequencies for the products relative to the O−H and O−D stretching regions. The highest frequency bands at 590.7 (578.9, D counterpart) cm−1 and 632.1 (617.5, D counterpart) cm−1 increase on annealing and visible−UV irradiation, and they decrease on final annealing. These bands are assigned to Ce(OH)2+ and Ce(OD)2+, and they exhibit the same behavior as their O−H and O−D stretching counterparts. The stronger−weaker intensity relationship for the lower− higher frequency band pairs is reminiscent of the typical relationship between the antisymmetric and symmetric stretching modes of a species containing two equivalent OH groups. The next band at 559.8 cm−1 has an obvious deuterium counterpart at 543.4 cm−1, and no other bands in this region are associated. The hydrogen counterpart at 559.8 cm−1 increases intensity on early annealing but decreases to 25% on UV irradiation, and none of the major products in Figure 1 C

DOI: 10.1021/acs.inorgchem.5b02619 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry follow suit. However, the weaker 3714.8 cm−1 (2740.1, D counterpart) absorptions do relate (see Figure S1 for the best view); accordingly, these bands are assigned to Ce(OH)4 and Ce(OD)4. The deuterium counterpart at 543.4 cm−1 behaves similarly but is not as susceptible to UV irradiation. The band pairs at 518.7 (505.8, D counterpart) cm−1 and 553.9 (537.9, D counterpart) cm−1 increase and decrease similar to the highest frequency pairs, and they are assigned to Ce(OH)2 and Ce(OD)2. The low frequency band at 510.3 cm−1 exhibits annealing and photolysis behavior that matches the highest frequency product bands at 3742.7 (2760.5, D counterpart) cm−1 assigned to Ce(OH)3 and Ce(OD)3, and its deuterium counterpart grows on annealing near 476 cm−1 underneath sharp unassigned bands at 497.0, 493.9 cm−1. New absorptions were observed in the H2O2 experiments at 849.4 cm−1 for CeO+ and at 796.3, 780.7, and 750.0 between CeO at 808.3 and CeO2 at 736.7 cm−1.23 Important structural information regarding these products is obtained from their very small deuterium shifts to 793.7, 780.3, and 749.5 cm−1 using D2O2, which tells us that H and D participate slightly in this vibrational mode. Additional experiments were performed with the reagents Ce + H2 (or D2) + O2 (or 18O2) using the concentrations hydrogen (6% in argon) and oxygen (0.4% in argon), followed by UV irradiation of the deposited samples. Figure S2 shows the O−H stretching region: sample deposition with laser ablated Ce atoms gave only weak bands at 3721.3, 3715, and 3690.6 cm−1. Irradiation at 240−380 nm decreased the 3515 cm−1 peak; produced a weak band at 3743.3 cm−1, which is slightly higher in frequency than the 3742.7 cm−1 Ce(OH)3 band observed using H2O2 above; increased the 3721.3 cm−1 band 4-fold, which is just above the 3719.9 cm−1 Ce(OH)2 absorption; and increased the 3690.6 cm−1 band and its 3693.1 cm−1 satellite 3-fold, which are 0.4 and 0.6 cm−1 lower than observed with H2O2. Next, irradiation with the full light of the mercury arc (λ > 220 nm) doubled these absorptions and produced a new 3715.5 cm−1 peak, which is 0.7 cm−1 higher and stronger than the peak labeled above as Ce(OH)4. Annealing to 20 K had little effect on these bands, but another full arc irradiation decreased the 3715.5 cm−1 band, produced a new 3700.0, 3700.9 cm−1 peak, and increased the three major bands, more for the highest and less for the lowest of them. The experiment with H2 and 18O2 produced shifts in the product bands: the highest absorption at 3732.2 cm−1, the strongest at 3709.6 cm−1, the weak band at 3704.2 cm−1, and the doublet at 3679.1 and 3681.5 cm−1 for shifts of 11.2, 11.6, 11.3, 11.6, and 11.5 cm−1, respectively. In the O−D stretching region, the highest frequency product absorption was observed at 2760.8 cm−1 (2744.7 cm−1 with 18 O2, Figure S3) near the value of 2760.6 cm−1 obtained using D2O2, the strongest band is observed at 2744.4 cm−1 (2727.5 cm−1 with 18O2), the weak band at 2740.3 cm−1 (2723.6 cm−1 with 18O2), and the lowest product doublet slightly shifted to 2725.9, 2721.9 cm−1 (2708.8, 2705.2 cm−1 with 18O2). These bands shifted 16.1, 16.7, 16.7, 17.1, and 16.7 cm−1, respectively, upon 18O2 substitution, which are appropriate for O−D stretching modes. Notice that the O-16 to O-18 shift is larger for O vibration against the heavier D atom than the lighter H atom. Figure S3 shows original spectra in the O−D stretching region for the 18O2 experiment: sample deposition produces weak bands given above in parentheses, except for the lowest doublet. Ultraviolet photolysis increases these bands except for 2723.6 cm−1, but subsequent annealing to 24 K produces at

2723.6 cm−1 the strongest isotopic counterpart of this photosensitive species. Infrared spectra for the Ce−O stretching region are illustrated in Figure 3 for D2 with three oxygen isotopic

Figure 3. Infrared spectra in the Ce−O stretching region for laser ablated Ce, D2 (0.6%), and isotopic O2 (0.4%) reaction products condensed in excess argon at 4 K. Spectra recorded after (a) sample deposition with 16O2 for 60 min, (b) after 240−380 nm irradiation, (c) after >220 nm irradiation, (d) after annealing to 16 K, (e) after annealing to 22 K, and (f) after deposition with statistical 16,18O2 for 60 min, (g) after 240−380 nm irradiation, (h) after >220 nm irradiation, (i) after annealing to 20 K, and (j) after deposition with 18O2 for 60 min, (k) after 240−380 nm irradiation, (l) after >220 nm irradiation, (m) after annealing to 16 K, and (n) after annealing to 22 K.

samples. With 16O2, a weak band at 575.1 cm−1 increases on UV photolysis together with a matrix site splitting at 578.0 cm−1 in the bottom set of spectra, which are shifted slightly from the 578.9 cm−1 position using D2O2. This band shifts to 547.0 cm−1 using 18O2 in the top group of spectra, and it becomes a 1/2/1 triplet at 575.1/561.9/547.4 cm−1 using scrambled 16O2, 16,18O2, 18O2 in the middle spectra. The strongest peak in this region appears on UV irradiation at 506.1 in the bottom, to 484.6 cm−1 in the top, and a triplet with a 491.7 cm−1 intermediate component in the middle spectra. A 543.5 cm−1 band appears in the bottom family of spectra and shifts to 520.9 cm−1 in the top group, which gives a quintet splitting at 543.5, 534.9, 531.0, 525.9, 520.9 cm−1 with 16,18O2. Notice that the 16O to 18O shifts for these Ce−O(D) vibrations for O vibrating against the heavier Ce atom, 28.1, 22.6, and 21.5 cm−1, respectively, are larger than those given above for O vibrating against D (which is more of a D motion against O). Identification of Ce(OH) 2. The strongest product absorption from the Ce and H2O2 reaction in this work appears at 3719.9 cm−1 in Figure 1. This absorption increases on annealing to 20 and 24 K and on visible photolysis, but decreases with UV irradiation while the highest frequency absorption at 3742.7 cm−1 increases intensity. Two bands at 518.7 and 553.8 cm−1 increase and decrease intensity with the 3719.9 cm−1 absorption on annealing and photolysis, and these three are associated with the 505.8, 537.9, and 2744.0 cm−1 peaks on deuterium substitution using D2O2 by their similar changes in intensity and large shift of the latter higher frequency bands for O−H to O−D stretching modes (H/D D

DOI: 10.1021/acs.inorgchem.5b02619 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Calculated Reaction Energies in kcal/mol at 298 K M = Ce reactiona

rxn

M + H2O2 → 1 or 3M(OH)2 M + O2 → MO2 MO2 + H → 2OMOH 2 OMOH + H → xM(OH)2 x M(OH)2 + OH → M(OH)3 x M(OH)2 + H2O2 → 2M(OH)3 + OH M + H2O2 → 2M(OH)2+ + e− M+ + H2O2 → 2M(OH)2+ x M(OH)2 → 2M(OH)2+ + e− x M(OH)2 + H2O2 → M(OH)4 MO2 + H2O → OM(OH)2 OM(OH)2 + H2O → M(OH)4 2 M(OH)3 + OH → M(OH)4 O + xM(OH)2 → OM(OH)2 1 MO2 + e− → 2MO2− 1 M(OH)4 + e− → 2M(OH)4− x M(OH)2 + e− → 2M(OH)2− 2 M(OH)3 + e− → 3M(OH)3− 2 M(OH)3 → 1M(OH)3+ + e− 2 OCeOH → 2HCeO + O

(1) (2a) (2b) (2c) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17)

M = Th

B3LYP/ DZVP2/Stutt

CCSD(T)/aug-ccpVDZ/Stutt

CCSD(T)/aug-ccpVTZ/Stutt

B3LYP/ccpVTZ-PP

CCSD(T)/ccpVTZ-PP

−205.8(T) −225.9 −64.0 −52.0 −115.7 −69.5 −88.8 −219.2 117.4(5.09) −139.8 −36.8 −34.1 −70.3 −138.4 9.7(0.42) 44.7(1.94) 10.1(0.44) 2.3(0.10) 171.6(7.44) 150.4

−194.7(T) −224.6 −48.7 −49.7 −117.1 −74.6 −78.7 −207.2 116.4(5.05) −159.7 −43.5b −38.6b −85.1 −149.8 12.7(0.55) 30.7(1.33)c 10.4(0.45)

−196.3(T) −228.4 −50.2 −52.4 −119.9 −72.2 −80.2 −208.7 116.4(5.05) −160.7 −41.0b −36.8b −88.5 −155.3 13.1(0.57) 29.3(1.27)c 11.5(0.50)

−228.4(S) −256.4 −59.2 −44.2 −121.8 −76.0 −98.5

−224.0(S) −254.2 −59.7 −45.6 −122.7 −74.4 −93.3

130.1(5.64) −199.6 −57.4 −60.0 −123.6

130.8(5.67) −202.4 −59.3 −63.3 −128.0

d

d

158.2(6.86) 143.5

157.7(6.84) 149.4

a

x = 3 for Ce (triplet) and 1 for Th (singlet). S = singlet. T = triplet. Values in parentheses for electron affinities and ionization potentials are in eV. For reactions 9 and 10: CCSD(T)/cc-pVDZ-DK3 ΔH(298) = −41.9 and −36.7 kcal/mol, CCSD(T)/cc-pVTZ-DK3 ΔH(298) = −41.3 and −37.0 kcal/mol, respectively. cCCSD(T)/cc-pVDZ-DK3 and CCSD(T)/cc-pVTZ-DK3, respectively. dCCSD(T) predicts that the electron is not bound for a triplet anion. The singlet anion is even higher in energy. b

antisymmetric O−H stretching modes for Ce(OH)2. The O−D stretching mode for this mixed isotope is covered by the strong 2744.0 cm−1 absorption of Ce(OD)2. Frequencies computed with the B3LYP functional confirm these assignments. First, the computed O−H antisymmetric and symmetric stretching modes are separated by 1 cm−1 and those for the two O−D stretching modes by 2 cm−1. These bandwidths are 4 and 3 cm−1, respectively; so these modes cannot be resolved experimentally. The computed harmonic H/D frequency ratios are 1.3729 and 1.3722, which are higher than the above observed 1.3556 anharmonic value. The O−H and O−D stretching frequencies are computed to be 5.3% and 3.9% higher than the observed argon matrix values, which is mostly due to anharmonicity. We calculate a gas phase anharmonic value of 3736 cm−1 for the asymmetric stretch at the B3LYP level (see the Supporting Information), in excellent agreement with the experimental value of 3720 cm−1, consistent with a 10−20 cm−1 argon matrix shift. Next, notice that the calculated values for the antisymmetric and symmetric Ce− O(H) stretching modes are 34.0 and 43.7 cm−1 higher (6.6% and 7.9%) than the observed values, and the deuterium shifts for these two modes are computed as 13.6 and 16.5 cm−1, respectively, which are in good agreement with the above observed values with the symmetric mode shift being 2.9 and 3.0 cm−1 higher for the calculated and observed frequencies, respectively. The B3LYP calculated anharmonic values are 530 and 569 cm−1 in comparison to the respective experimental values of 519 and 554 cm−1 for the antisymmetric and symmetric Ce−O(H) stretching modes, which is very good agreement considering the potential for an argon matrix shift of about 10 cm−1. The cerium dihydroxide molecule can be prepared by the direct highly exothermic Ce insertion reaction into the weak

ratio 1.3556) and moderate shifts of the lower two bands (12.9, 15.9 cm−1) for Ce−O(H) to Ce−O(D) stretching modes (Figure 2). The 18O shifts measured for these bands using H2 or D2 and 18O2 are (3743.4−3732.2) = 11.2 cm−1, (519.0− 497.6) = 21.4 cm−1, and (554.3−527.9) = 26.4 cm−1 for the hydrogen counterparts and (2744.4−2727.5) = 16.9 cm−1, (506.1−484.6) = 21.5 cm−1 and not observed for the deuterium counterparts. These modes are primarily H or D motion against O and antisymmetric or symmetric motions of O against Ce, respectively. The observed 16O/18O frequency ratios for the lower frequency bands are 1.0430 and 1.0500 and ratios from computed frequencies are 1.0482 and 1.0516, respectively. The symmetric Ce−O(H) stretching mode also has the higher O-16 to O-18 shift in calculated frequencies, 29.3 cm−1, as compared to 25.4 cm−1 for the antisymmetric mode. The asymmetric triplet at 506.1/491.7/484.6 cm−1 shows that this molecule contains two equivalent oxygen atoms, and this antisymmetric mode for Ce(16OH)(18OH) interacts with the higher frequency symmetric counterpart, causing the former mode to be lower than the median isotopic frequency, like we found for CeO2.23 The 505.8 cm−1 deuterium counterpart band acquires a 511.3 cm−1 satellite with growth on annealing, but the 518.7 cm−1 hydrogen counterpart does not. This weak band is due to reaction with HDO2 in the deuterium substituted sample, which absorbs weakly at 3636.2 and 980.9 cm−1,44,48 and this 511.3 cm−1 antisymmetric Ce−O(H), Ce−O(D) stretching mode appears almost halfway between the Ce(OH)2 and Ce(OD)2 counterparts, as predicted by calculations (see Table S4). A weak band is also observed at 3720.6 cm−1 in the experiments with D2O2 and HDO2, which follows the annealing and photolysis behavior found for the 3719.9 and 2744.0 cm−1 bands. This weak band is due to the O−H stretching mode for Ce(OH)(OD), and it falls between the symmetric and the E

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symmetric and asymmetric O−H stretches are predicted to be essentially degenerate with all three functionals. The asymmetric Ce−O(H) stretch is predicted to be substantially lower than the corresponding symmetric stretch and of higher intensity, consistent with the above assignment. Because of the potential for symmetry breaking, the anharmonic frequency corrections are more difficult to calculate here, so we can only estimate them. At the B3LYP level, the average anharmonic value for the asymmetric stretch is 3736 cm−1, which is close to the experimental value of 3743 cm−1. The B3LYP functional gives too large an anharmonic correction for the asymmetric Ce−O(H) stretch, but the average of the anharmonic values for this stretch of 507 cm−1 are consistent with the experimental value of 510 cm−1. These absorptions for Ce(OH)3 also increase on annealing to 20 K while the OH radical concentration decreases, so reaction 3 is a straightforward addition reaction, which can also contribute in the hydrogen/oxygen reactions. However, H2O2 is more abundant, and we expect that more Ce(OH)3 is produced by reaction 4 than 3.

HO−OH bond (reaction 1). The reaction energies are given in Table 2. This reaction proceeds on annealing the solid argon host to 20 and 24 K, so activation energy is not required. However, visible photolysis does promote further reaction. Another mechanism is required for the Ce + D2 + O2 experiments where the product absorption frequencies vary from 0.3 to 1.3 cm−1 in measurements from H2O2 and H2 + O2 reagent experiments. These small differences can arise because the argon matrix packing arrangement around these different reagents will be different, i.e., H2O2 vs H2 + O2. This hypothesis is supported by the 1−2 cm−1 shift in product absorption peaks on annealing cycles, which certainly alters the argon packing around the guest molecules. Figure 3 shows that the major product is CeO2, so this is most likely to be the Ce bearing link to products. The observation of ArnH+ or ArnD+ shows that some H2 or D2 is dissociated in the laser ablation process, and thus, H or D atoms are available for reactions on sample annealing.78 Ce + H 2O2 → Ce(OH)2 (3B1)

(1)

Ce + O2 → CeO2

(2a)

CeO2 + H → OCeOH

(2b)

H + OCeOH → Ce(OH)2

(2c)

Ce(OH)2 + OH → Ce(OH)3

(3)

Ce(OH)2 + H 2O2 → Ce(OH)3 + OH

(4)

+

Identification of Ce(OH)2 . The lowest frequency product band in the O−H stretching region and the only one where antisymmetric and symmetric stretching modes could be resolved is the band which splits at 3691.0 and 3693.7 cm−1 on annealing (Figure 1). The deuterium counterpart does likewise at 2722.3 and 2726.1 cm−1. The corresponding highest frequency bands at 590.7, 632.1 cm−1 in the Ce−O(H) and at 578.9, 617.5 cm−1 in the Ce−O(D) stretching regions exhibit the same behavior, decreasing on annealing and increasing on UV irradiation, particularly with full arc λ > 220 nm. These band pairs in the O−H, O−D, Ce−O(H), and Ce−O(D) stretching regions are appropriate for antisymmetric (lower and stronger) and symmetric (higher and weaker) stretching modes of a new dihydroxide species. The 1/2/1 triplet at 575.1/561.9/ 547.0 cm−1 using D2 and scrambled 16O2, 16,18O2, 18O2 confirms that two equivalent coupled O atoms are involved in this vibrational motion. In the D2O2 experiments, annealing to 24 K allowed resolution of a sharp weak band at 2723.9 cm−1 almost halfway between the antisymmetric and symmetric modes of Ce(OD)2+ at 2722.3 and 2726.1 cm−1, and a sharp weak band was observed in the O−H stretching region at 3693.0 cm−1 also between the two stretching modes for Ce(OH)2+ at 3691.0 and 3693.7 cm−1. These two weak bands are due to the O−H and O−D stretching modes of Ce(OH)(OD)+, which also substantiate this observation of the cerium dihydroxy cation. The experiments with laser ablated Ce + H2 or D2 + O2 observed these bands on sample deposition along with the solvated H+ species ArnH+ (904.3 cm−1) and D+ species ArnD+ (643.6 cm−1), and the latter was destroyed on UV irradiation,74 while Ce(OH)2+ increased with UV light but not annealing. The observation of solvated proton species indicates that these experiments are capable of trapping cations. The proton/ deuteron species are stronger in the O2 experiments than in the hydrogen peroxide samples, which is probably due to a higher concentration of O2 as an electron trap than H2O2. We observed a 27.3 cm−1 18O2 shift for the antisymmetric Ce− O(H) stretching mode of Ce(OH)2+ and 28.1 cm−1 for Ce(OD)2+, and B3LYP predicted comparable 29.0 and 26.6 cm−1 shifts, respectively. These small differences in the shifts

Reaction 2a can readily be benchmarked with experiment as the heats of formation of Ce(g) and CeO2(g) are known from experiment79,80 with respective values of 100.4 ± 0.5 and 128.6 ± 4.8 kcal/mol giving a value of 229 ± 5 kcal/mol, in excellent agreement with the CCSD(T)/aug-cc-pVTZ/Stutt result. Identification of Ce(OH)3. The highest frequency product absorption at 3742.7 cm−1 increases 100% on annealing to 20 and 24 K and half again more on UV irradiation at the expense of Ce(OH)2. Our B3LYP calculations predict this antisymmetric mode for Ce(OH)3 to be ∼5 cm−1 higher than that for Ce(OH)2, and we find it to be 23 cm−1 higher experimentally. The deuterium counterpart at 2760.5 cm−1 (H/D ratio 1.3560) increases a little more on annealing and less on UV irradiation than the hydrogen species. At first, we hypothesized that the 559.8 cm−1 Ce−O(H) stretching mode might be associated with the 3742.7 cm−1 band because it increases on annealing and visible photolysis, but it decreases to 25% on UV irradiation while the absorption at 3742.7 cm−1 increases. However, a broader band at 510.3 cm−1 falls below the antisymmetric mode for Ce(OH)2 as predicted, and it follows the 3742.7 cm−1 band on initial annealing, photolysis, and UV irradiation. Furthermore, the broader band at 510.3 cm−1 probably shifts the 14 cm−1 calculated on D substitution for this mode, which puts it at 495.6 cm−1 underneath the sharp unassigned 493.9, 497.0 cm−1 bands. The H2 or D2, O2 photolysis experiments do not produce these sharper bands, and the broader band is found at 510.4 and 489.1 cm−1 for Ce(OH)3 and Ce(18OH)3 and at 496 cm−1 for Ce(OD)3 with its Ce(18OD)3 counterpart at 476.3 cm−1 (in Figure 3). The observed 21.3 and 20.7 cm−1 O-16 to O-18 shifts compare favorably with calculated 23.4 and 21.8 cm−1 values. Thus, we assign the doubly degenerate O−H and Ce−O(H) stretching modes for Ce(OH)3 at 3742.7 cm−1 and at 510.3 cm−1. At the B3LYP level, there is a slight symmetry breaking for Ce(OH)3, so we repeated the calculations with the BP8681,82 and M0683,84 functionals and the same basis sets. Both additional functionals gave C3v structures with 2A1 states and bracketed the B3LYP O−H stretches. The calculated F

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Inorganic Chemistry Ce(OH)2 → Ce(OH)2+ + e−

could be due to small differences in the actual angle in the matrix and the computed DFT gas phase value, which affects the magnitude of the 18O shift in this vibrational motion. Finally, we observed 11.6 and 11.5 cm−1 O-18 shifts for the two O−H stretching modes and 17.2 and 16.7 cm−1 for the two O− D stretching modes for this cation, which are again appropriate isotopic shifts for these modes. At the B3LYP level, the two O18 shifts are 12.6 and 13.0 cm−1 for the O−H stretching and 18.1 and 18.7 cm−1 for O−D stretching. When 0.1% CCl4 is added to the argon reagent stream to serve as an electron trap, the intensities of the cation absorptions in a similar experiment with H2O2 increase 3-fold relative to the neutral Ce(OH)2 absorption intensities, as illustrated in Figure S4. This arises because electrons produced in the ablation process that might neutralize this cation are trapped instead by CCl4 through dissociative attachment as chloride anions.85−87 Our B3LYP calculations predict the cation O−H stretching absorptions to be 69 and 73 cm−1 lower than for Ce(OH)2, and we observe them to be only 30 cm−1 lower. We suggest that this difference could be due in part to more strongly bound argon atoms in the cation complex with argon than in the neutral argon matrix isolated species. We modeled the interaction of Ar with Ce(OH)2+ as Ar4Ce(OH)2+ with the Ar attached to the Ce. The results are summarized in the Supporting Information. At the B3LYP level, the total binding energy of the 4 Ar to the cation is 8 kcal/mol with predicted blue shifts of ∼15 cm−1, which places the calculated results in better agreement with experiment. We also calculated the structure of Ar4Ce(OH)2+ with the ωB97X-D exchangecorrelation functional to include a better treatment of weak nonbonded interactions. The total binding energy of the 4 Ar atoms increases to 12 kcal/mol, and the blue shifts remain the same, ∼15 cm−1. In contrast, the Ce−O(H) shifts in the cation are predicted to red shift by 16−19 cm−1 on addition of 4 Ar atoms, which improves the agreement with experiment. These results are consistent with previous results for the Sc(OH)2+ ion in an Ar matrix.41 Our calculations also predict the antisymmetric and symmetric O−H stretching modes to be separated by 1 cm−1 for the Ce(OH)2 absorption and 5 cm−1 for the Ce(OH)2+ cation. Experimentally, these modes are not resolved for the former and resolved by 2.7 cm−1 for the Ce(OH)2+ cation and 3.8 cm−1 for the Ce(OD)2+ cation. The Ce(OH)2+ cation can be formed upon sample deposition through the exothermic autoionization process (reaction 5). The gas phase reaction of thermal Ce atoms and O2 has produced CeO+ and CeO2+ through autoionization as well,88 and we also observe CeO+ in argon matrix experiments with H2O2 and O2 reagents.23 In addition, Ce+ cations are also produced on laser ablation, and their reaction with H2O2 can contribute to the product yield (reaction 6). The analogous Group 3 metal M(OH)2+ cations have been observed and characterized in previous matrix isolation investigations.40,41 In addition, reactions 5 and 6 can be driven by full mercury arc irradiation. Our computed ionization potential for Ce(OH)2 (reaction 7) is 5.05 eV (116 kcal/mol), and the full mercury arc lamp UV radiation can produce up to 130 kcal/mol (220 nm). Therefore, direct photoionization reaction 7 can also contribute to the yield of Ce(OH)2+ in these experiments. Ce + H 2O2 → Ce(OH)2+ + e−

(5)

Ce+ + H 2O2 → Ce(OH)2+

(6)

(7)

The relatively low ionization potential for Ce(OH)2 is consistent with the stability of the +III oxidation state in the lanthanides. Identification of Ce(OH)4. The high photosensitivity of the 559.8 cm−1 band in the Ce−O(H) stretching region to UV irradiation was matched only by the weaker 3714.8 cm−1 band in the O−H stretching region for the H2O2 reaction, but subsequent annealing increased the subject band intensities. Perdeuterated counterparts were observed at 2740.1 cm−1 (H/ D ratio 1.3557) and at 543.4 cm−1 for the largest, 16.4 cm−1, deuterium shift in the Ce−O(H) stretching mode found for any of the above-discussed product species. In fact, this deuterium shift depends on the number of OH groups in the molecule, which suggests a spectator mass effect for the heavier D. We found a 14.7 cm−1 shift in the Ce−O(D) mode for Ce(OH)3, 12.9 cm−1 for Ce(OH)2, 11.8 cm−1 for the dihydroxy cation, and 9.6 cm−1 for OCeOH. The oxygen/hydrogen experiments gave more intense slightly shifted bands for this important product species (Figures S2 and S3). With H2 and 16O2 or 18O2, the bands were observed at 3715.5 or 3704.2 cm−1 for an 11.3 cm−1 shift and at 560.2 or 538.3 cm−1 for a 21.9 cm−1 shift. With D2 and 16 O2 or 18O2, the bands were observed at 2740.3 or 2702.3 cm−1 for a 16.7 cm−1 shift and 543.5 or 520.9 cm−1 for a 22.6 cm−1 shift. Our B3LYP frequency calculations predict the antisymmetric O−H stretching mode for Ce(OH)4 to be 15.5 cm−1 below this motion for Ce(OH)3, and we observe it to be 27.9 cm−1 lower. Likewise, the antisymmetric Ce−O(H) stretching mode for Ce(OH)4 is computed to be 20.2 cm−1 higher than this mode for Ce(OH)3, and we observe it to be 49.5 cm−1 higher. The quintet multiplet splitting at 543.5, 534.9, 531.0, 525.9, 520.9 cm−1 with D2 and 16,18O2 is expected for the tetrahydroxide, as observed for Th(OH)4.46 All of the above isotopic spectroscopic data and their relationship to computed frequencies and product structure confirm this identification of Ce(OH)4. It was not possible to calculate anharmonic frequency corrections using Gaussian 09 due to the Td symmetry of the molecule. The most likely route for the production of Ce(OH)4 is a second H2O2 reaction with Ce(OH)2 (reaction 8), which is highly exothermic. The analogous reaction has previously been proposed for the Group 4 and thorium tetrahydroxide molecules.43,46 Ce(OH)2 + H 2O2 → Ce(OH)4

(8)

In addition, a small amount of water is present, and the reaction of H2O with CeO2 is also exothermic (reactions 9 and 10). CeO2 + H 2O → OCe(OH)2

OCe(OH)2 + H 2O → Ce(OH)4

(9) (10)

The potential energy surface via reactions 9 and 10 to produce Ce(OH)4 is shown in Figure 4. The reaction begins with the Lewis acid−base addition (physisorption) of water to the Ce metal center, forming a Lewis acid−base donor−acceptor bond, followed by a proton transfer from an OH group to a O. The reaction of the second water addition is similar to the first. At the B3LYP/DZVP2/Stutt level, the Lewis acid−base H2O addition energies on both CeO2 and OCe(OH)2 are −14 kcal/ mol. The dissociative addition (chemisorption) energies for the two reactions to produce OCe(OH)2 and Ce(OH)4 are more G

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Ce−O(H) mode at 529.8 cm−1 with a 13.2 cm−1 deuterium shift. The weaker O−H stretch is computed at 3899.6 cm−1, 170.5 cm−1 (4.6%) higher than the observed value and 14.5 cm−1 below the computed value for Ce(OH)2. Our observed value for OCeOH is 19.3 cm−1 below our observation for Ce(OH)2. The calculated anharmonic O−H stretch of 3727 cm−1 is 26 cm−1 above the experimental value, and that for the Ce−O(H) stretch of 506 cm−1 is 19 cm−1 above experiment. The anharmonic correction to the CeO stretch was predicted to increase the frequency by 6 cm−1, decreasing the agreement with experiment. It is interesting to suggest a mechanism for the formation of OCeOH. The strength of the O−H bond makes decomposition of Ce(OH)2 an unlikely source. There is a small amount of HO2 and DO2 produced in these laser ablation experiments, and their reaction with Ce is a possibility. However, we think it more likely that H(D) atom reaction with CeO2, also produced in these experiments, is the best choice (reaction 2). Tentative Identification for OCe(OH)2. The third new absorption in the CeO stretching region at 780.7 cm−1 shifts to 780.3 cm−1 with D2O2, and these bands increase slightly both on annealing and UV photolysis. Their identical 40.1 cm−1 oxygen-18 shifts confirm that this is a terminal CeO stretching mode on still another new molecule. Our calculation for OCe(OH)2 predicts a terminal mode 35.3 cm−1 higher than that for OCeOH, and the 780.7 cm−1 band is 30.7 cm−1 higher than the above 750.0 cm−1 band for OCeOH. The O−H stretching mode for OCe(OH)2 is predicted to be 3 cm−1 lower than for OCeOH, and there is a 1.8 cm−1 lower splitting in the 3701.5 cm−1 band at 3699.7 cm−1, which exhibits a 12.7 cm−1 O-18 counterpart. We fail to find the Ce−O(H) mode expected in the 500−480 cm−1 region, so we must tentatively assign the aforementioned bands to OCe(OH)2. The most direct synthesis of OCe(OH)2 is O atom addition to the major Ce(OH)2 product, and CeO2 hydration is another possibility (reactions 12 and 9).

Figure 4. Potential energy surface for the reaction of CeO2 + 2H2O → Ce(OH)4. The relative energies at 0 K calculated at the B3LYP/ DZVP2/Stutt (in black) and the CCSD(T)/cc-pVTZ-DK3//B3LYP/ DZVP2/Stutt (in blue) levels are in kcal/mol. Atoms: Ce = light yellow, O = red, and H = white/gray.

exothermic, −37 and −35 kcal/mol, respectively. Both the Lewis acid−base addition and dissociative addition energies are less exothermic than those on MO2 (M = Ti, Zr, Hf) metal oxide clusters.89,90 The barriers for the first and the second proton transfer are only 6 and 10 kcal/mol, less than the exothermicities of Lewis acid−base addition steps. Both transition states are below the react asymptote. Therefore, CeO2 can readily react with H2O to produce cerium tetrahydroxide. We also benchmarked the potential energy surface at the CCSD(T)/cc-pVTZ-DK3 level and found good agreement with the Stuttgart ECP and basis set results at the B3LYP level (see Figure 1 and Table 2). The Ce−O(H) bond dissociation energy (BDE) for R = Ce(OH)4 is given by the negative of reaction 11. Ce(OH)3 + OH → Ce(OH)4

(11)

This value is almost 30 kcal/mol less than the BDE for Ce(OH)3 given as the negative of reaction 3. This may be part of the reason why Ce(OH)4 is destroyed by UV photolysis while Ce(OH)3 is produced. Identification of OCeOH. Absorptions at 750.0 and 487.2 cm−1 are produced in both the H2O2 and O2 + H2 reactions with laser ablated Ce, and they decrease on annealing and increase slightly on UV irradiation. These bands appear in the terminal CeO and Ce−O(H) stretching regions, and they shift to 749.5 and 477.6 cm−1 with D2O2. We observe that the magnitude of the H to D shift for a frequency tells us where the H(D) is located with respect to that vibrational mode in the molecule. The small H to D shift and the proximity to the isolated CeO absorption at 808.3 cm−1 identify the new 750.0 cm−1 band as a terminal CeO stretching mode. The medium H to D shift of 9.6 cm−1 and location for the 487.2 cm−1 band identify a Ce−O(H) stretching mode. We observed these bands in the D2, O2 experiments at 749.6 and 477.5 cm−1 and their 18O2 counterparts at 711.2 and 458.9 cm−1, which define shifts of 38.4 and 18.6 cm−1 that are comparable to the shifts for CeO itself (41.0 cm−1) and Ce(OD)2 (21.5 cm−1). Next, we must find an appropriate O−H stretching mode, and the 3701.5 cm−1 band exhibits a 13.0 cm−1 O-18 shift and a possible D counterpart at 2727.1 cm−1. The 3701.5 cm−1 band decreases on 20 and 24 K annealing and increases on >220 nm irradiation like the 750.0 and 487.2 cm−1 bands. Hence, we assign these three absorptions to the OCeOH molecule. The B3LYP calculated frequencies for OCeOH are listed in Table 1. The strongest mode is the CeO stretch computed at 780.8 cm−1 with an 0.3 cm−1 deuterium shift, and next is the

O + Ce(OH)2 → OCe(OH)2

(12)

Reaction 12 is substantially exothermic shown in Table 2. Identification of HCeO. New sharp bands in Figure 3 at 796.3 cm−1 with H2O2 and at 793.7 cm−1 with D2O2 increase on annealing and disappear on near UV−vis (λ > 350 nm) photolysis. The 18O shift for the former is 40.5 cm−1 and for the latter is 39.7 cm−1, which are appropriate for a terminal CeO motion as we found 41.1 cm−1 for CeO itself in these experiments. Weak bands at 1286.5 cm−1 in the Ce−H stretching region and at 925.1 cm−1 in the Ce−D stretching region exhibit similar photosensitive behavior and the H/D ratio 1.3907, which are near observations for CeH2 and CeD2 of 1281.7 and 916.9 cm−1 with a ratio of 1.3979.25 The 18O shift for the Ce−D stretching mode was measured as 0.8 cm−1, which shows that the molecule contains both hydrogen and oxygen, and calculation predicted 1.3 cm−1. Hence, the characterization of Ce−H and CeO stretching modes defines an HCeO molecule. Our calculations predict an H to D shift of 2.7 cm−1 for the terminal Ce16O stretching mode, and we observe 2.6 cm−1, and a 1.8 cm−1 shift for the terminal Ce18O stretching mode, and we observe 1.8 cm−1. These small oxygen isotopic shifts provide a measure of the coupling between the Ce−H and the Ce−O vibrations in this HCeO molecule. With a bond angle of 112.5°, we expect a small H

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Figure 5. Optimized Ce−O and Ce−H bond distances in Å, ∠O−Ce−O and ∠O−Ce−H bond angles in degrees, and point groups for cerium species with the B3LYP functional. a M06. b BP86.

assignment of the 425, 427 cm−1 peaks to the Ce(OH)4− anion. Both predicted values are consistent with the experimental assignment. It is interesting to note that there is a predicted substantial decrease in the intensity of the O−H stretching modes in the anion as compared to the neutral but little change in the intensity of the Ce−O(H) stretch. The CeO2− anion can be formed here through electron capture reaction 13, and Ce(OH)4− by the corresponding reaction 14.

amount of mode coupling, which depends on the cosine of the angle between the two bonds in question. B3LYP calculations predict a strong Ce−H stretching mode at 1308.2 cm−1, which is only 1.7% higher than the observed band, and a strong CeO stretching mode at 793.1 cm−1, which is closer to the observed band than usually occurs. Our calculations for HCeO and DCeO predict 40.4 and 39.5 cm−1 O-18 shifts, respectively, for the CeO stretching mode, which are in excellent agreement with the observed values. Calculations at the CCSD(T)/aug-cc-pVTZ/Stutt level predict the unobserved CeOH isomer to be 18.1 kcal/mol higher in energy than the observed HCeO form for the doublet state of CeOH and 27.2 kcal/mol higher for the quartet state. Anions. The CeO2− anion is also observed in experiments with oxygen where the yield of CeO2 is large,23 as is shown here in Figure 3, and it is likely to be formed by electron capture (reaction 11). Among the reaction products identified in this work, Ce(OH)4 has the largest electron affinity calculated as ∼1.3 eV at the CCSD(T)/cc-PVTZ-DK3 level. Our frequency calculations for Ce(OH)4− at the B3LYP level predict a weak antisymmetric O−H stretching mode split in the 3912−3917 cm−1 range (symmetry breaking from the Td structure) uncorrected for anharmonicity. At the DFT level with the M06 functional, there is no symmetry breaking and Ce(OH)4 is predicted to be in the 2A1 state in Td symmetry. The O−H anion stretches are predicted to be blue-shifted from the neutral at both levels. A very intense antisymmetric Ce−O(H) stretching mode is predicted with both functionals with B3LYP predicting the band to be in the 428−437 cm−1 range with Ce(OD)4 counterparts 14.4 cm−1 lower. With the M06 functional, the Ce−O(H) antisymmetric stretch is predicted to be at 449 cm−1. We observed a weak doublet absorption at 425, 427 cm−1 below the spectral range shown in Figure 2 for the H2O2 reaction, but not for D2O2, where a red shift below our limit of detection is likely. The 425, 427 cm−1 peaks increased slightly on annealing to 24 K, decreased slightly on >350 nm irradiation, and decreased more on annealing to 30 K: they were not observed in the CCl4 doped H2O2 experiment. A similar band was observed at 459 cm−1 with Y and H2O2, which shifted to 442 cm−1 with D2O2, and the 459 cm−1 band was assigned to Y(OH)4−.42 This evidence supports a tentative

CeO2 + e− → CeO2−

(13)

Ce(OH)4 + e− → Ce(OH)4 −

(14)

Reactions 15 and 16 show the corresponding electron capture reactions of Ce(OH)2 and Ce(OH)3, and reaction 17 shows the ionization energy of Ce(OH)3. The electron affinity of Ce(OH)2 + e− → 2Ce(OH)2−

(15)

Ce(OH)3 + e− → 3Ce(OH)3−

(16)

Ce(OH)3 → 1Ce(OH)3+ + e−

(17)

3

2

2

Ce(OH)2 is comparable to that of CeO2, and the electron affinity of Ce(OH)3 is approximately zero. The ionization potential of Ce(OH)3 is large enough that it will not be ionized by the UV radiation from our lamp. Our calculations predict that the strong antisymmetric O−H stretching mode for 2Ce(OH)2− is blue-shifted 10 cm−1 from that for 3Ce(OH)2. We observed a weak band at 3733.7 cm−1, 14 cm−1 above Ce(OH)2, which increases on annealing to 20 K and is destroyed by near UV−vis irradiation, and its deuterium counterpart at 2754.0 cm−1 (ratio 1.3557) behaves similarly. Unfortunately, we are not able to locate the Ce−O(H) stretching mode in the congested high 400 cm−1 region of lower signal-to-noise. Therefore, we can only tentatively assign the 3733.7 cm−1 band to the 2Ce(OH)2− anion. On the other hand, Ce(OH)3 does not bind an electron at the CCSD(T) level, and at the DFT level, the binding energy is very low, 0.10 eV. At both levels, a triplet (4f16s1) is of lower energy than the singlet. I

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SOMO of Ce(OH)4− is a Ce 4f orbital, in contrast to CeO2−, where the additional electron goes predominantly into a Cecentered 6s orbital with some polarizing p character. The electron affinity for Ce(OH)4 is thus higher than that for CeO2 as the 4f orbital is closer to the nucleus than the 6s orbital and the 4f orbital feels a higher nuclear charge, so the additional electron is better stabilized. Ce(OH)3 would nominally have the Ce in the +III oxidation state, and in this case, the unpaired electron is in the 4f. There is still about 0.7 e spin-paired in the 5d. The Ce in the closed shell cation, Ce(OH)3+ is very similar to that in Ce(OH)4. The Ce in Ce(OH)2 is formally in the +II oxidation state. The ground state is a triplet with a 4f1.16s0.8 configuration with about 0.7 e mostly spin-paired in the 5d due to backbonding Addition of an electron to form the anion of Ce(OH)2 leads to a 4f1 configuration with almost two spin-paired electrons in the 6s and about 0.6 e spin-paired in the 5d. Thus, the electron configuration is like that of the atom except that the 5d occupancy comes from backbonding and is spin-paired. The fact that the electron affinities of CeO2 and Ce(OH)2 are comparable is accidental as the electron configurations are completely different with different amounts of backbonding to the Ce, and the formal oxidation states are also quite different, +III for CeO2− and +I for Ce(OH)2−. Removal of an electron from Ce(OH)2 leads to a doublet with a single electron in the 4f and about 0.6 e spin-paired in the 5d from backbonding. We note that Ce(OH)2+ is unusually stable in these experiments, and that might arise from solvation by the argon matrix as well as the Ce being in the +III oxidation state. HCeO and OCeOH both have the Ce in the formal +III oxidation state like Ce(OH)3. Both have essentially a 4f1 state with about 1 spinpaired electron in the 5d from backbonding. There is an additional 0.22 e in the 6s for HCeO, consistent with the lower electron affinity of H as compared to OH so that there is less ionicity in the Ce−H bond. Comparisons with Thorium. Although Th lies under Ce in the periodic table, Th has a different valence electron configuration, 7s26d2, which is like a Group IV transition metal, as compared to Ce with a valence configuration of 6s25d14f1. As noted previously for the di- and tetrahalides,70 Th behaves more like the Group IV transition metals than does Ce, so it would not be surprising to find that Th and Ce have differing product distributions in reactions with H2O2. Reactions of laser ablated Th with H2O2 yield Th(OH)4 as the major product, Th(OH)2 as a minor product, and no Th(OH)3 was observed.46 The tetrahydroxide has very intense antisymmetric O−H and Th−OH stretching modes predicted by B3LYP calculations46 at 3974.9 and 549.9 cm−1, which were observed in solid argon at 3750.0 and 528.1 cm−1. In contrast, for the reactions of Ce with H2O2, Ce(OH)4 is a minor product and Ce(OH)2 is the major product. The formation of Th(OH)4 from the equivalent of reaction 8 with Th(OH)2 in a ground state singlet (the triplet state is 15.3 kcal/mol higher in energy at the CCSD(T)/cc-pVTZ-PP level) is substantially more exothermic by ∼40 kcal/mol (Table 2). The formation of Th(OH)2 (1A1) is also more exothermic by 28 kcal/mol than for the corresponding Ce reaction to from the triplet dihydroxide (reaction 1). The O−H stretching mode for Th(OH)2 was observed at 3737.8 cm−1, just 18 cm−1 higher than the cerium counterpart, and the strong antisymmetric Th−OH stretching mode at 552.6 cm−1 with the symmetric mode is predicted to be 38.8 cm−1 higher but is too weak to observe, whereas both Ce−O(H) stretching modes were

Geometries. The OH bond distances show little variation for the Ce hydroxides (Figure 5). The Ce−O−H bond angle is linear for Ce(OH)4 and deviates from linearity when an electron is added to Ce(OH)4 by ∼30°. The Ce−O−H bond angle in Ce(OH)3 is about 17° from linearity, and that in Ce(OH)2 is almost 20° from linearity. The Ce−O bond distances in the neutral hydroxides do not show any regular pattern with the dihydroxide and tetrahydroxide having comparable bond distances of 2.09−2.10 Å and the trihydroxide having a longer bond distance of ∼2.14 Å. The Ce−O(H) bond distance in OCeOH is the longest value that we predicted for a neutral cerium bearing compound in this study. Addition of an electron to Ce(OH)4 elongates the Ce− O(H) bond by 0.15 Å, but for Ce(OH)2, the Ce−O(H) elongation is only 0.05 Å. The OCeO bond angle in the dihydroxide is 114° in the triplet. This is quite similar to the value of 117° calculated for 3CeF2.70 Heats of Formation. The calculated heats of formation of the various species are given in Table 3. The reactions used to obtain the heats of formation are given as well. The additional experimental information for the heats of formation is taken from the literature.79,80,91,92 Table 3. Calculated Heats of Formation (ΔHf) from CCSD(T) Calculations at 298 K in kcal/mol for Cerium Species Containing Hydrogen and Oxygen molecule

ΔHfa

reaction for ΔHf

Ce(OH)4 2 Ce(OH)4− 2 Ce(OH)3 3 Ce(OH)2

−322.1 −351.4 −242.7 −127.0 −128.3 −129.0 −128.1 −11.6 −139.6 −126.7 −36.3 −128.6 ± 4.8 −141.7 −227.4

(9) (9) + (14) (10) (2c) (1) (8) average ave + (7) ave + (15) (2b)

1

2

Ce(OH)2+ Ce(OH)2− 2 OCeOH 2 HCeO CeO2 2 CeO2− 1 OCe(OH)2 2

b

expt80 expt + (13) (12)

Experimental heats of formation at 298 K in kcal/mol: Ce = 100.4 ± 0.5,79 H = 52.1,91 OH = 9.03 ± 0.01,92 H2O2 = −32.4 ± 0.02,92 H2O = 57.8 ± 0.01,92 O = 58.98 ± 0.0291. bObtained using CeO2 + H → 2 OCeOH → 2HCeO + O. a

Atomic Populations. The Ce atom has a 6s25d14f1 valence configuration. The closed shell molecules CeO2 and Ce(OH)4 nominally have the Ce in the +IV oxidation state. There is clear evidence for backbonding into the metal 4f and 5d orbitals with 0.8 e in the 4f and 1 e in the 5d in Ce(OH)4 (Table 4). CeO2 has more backbonding with about 1.2 e in each of the 4f and 5d. In both cases, there is a very low 6s population, so the backbonding is into the valence 5d and 4f, as one would expect from the bare atom configuration. Addition of an electron to CeO2 goes into the 6s (unpaired) in the anion with charge transfer from the 4f to the 5d, as would be expected from the atomic configuration and the electron occupancy in the neutral. Addition of an electron to Ce(OH)4 is distributed over the molecule with a single electron unpaired in the 4f; i.e., there is a loss of spin pairing in the 4f when an electron is added. The J

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Table 4. Natural Bond Orbital (NBO) Natural Population Analysis (NPA) for Cerium Species Containing Hydrogen and Oxygena molecule 1

Ce(OH)4 Ce(OH)4− 2 Ce(OH)3 1 Ce(OH)3+ 3 Ce(OH)2 2 Ce(OH)2− 2 Ce(OH)2+ 1 CeO2 2 CeO2− 2 OCeOH 2 HCeO 1 OCe(OH)2 2

a

Ce charge 2.15 2.06 2.12 2.26 1.37 0.46 2.21 1.70 0.92 1.77 1.62 2.02

pop Ce 0.82

Ce “spin state”b

Ce 4f α

Ce 5d α

Ce 6s α > 0.05

1.03

4f 5d 4f1.155d0.72 4f1.195d0.68 4f0.835d0.97 4f1.155d0.666s0.83 4f1.125d0.596s1.82 4f1.205d0.62 4f1.245d1.21 4f1.005d1.306s0.91 4f1.285d0.98 4f1.245d0.986s0.22 4f0.925d1.13

pop O 1.77

4f1 4f1

1.07 1.09

0.37 0.34

4f16s1 4f1 4f1

1.08 1.06 1.10

0.42 0.31 0.32

0.80 0.91

6s1 4f1 4f1

0.50 1.14 1.12

0.68 0.51 0.50

0.88 0.11

5.28

2s 2p 2s1.802p5.43 2s1.802p5.41 2s1.772p5.19 2s1.802p5.39 2s1.812p5.40 2s1.802p5.34 2s1.912p4.93 2s1.912p5.01 2s1.822p5.40 2s1.922p5.08 2s1.902p4.89c 2s1.772p5.32

Pop H 1s0.49 1s0.53 1s0.49 1s0.45 1s0.48 1s0.50 1s0.45

1s1.50 1s1.59 1s0.49

Populations in units of electrons. bNominal spin state on Ce from population analysis. cThe O in CeO.

observed for Ce(OH)2 at 518.7 and 553.9 cm−1. For thorium dihydroxide, the O−Th−O bond angle is larger (152.3°) and the H−O−Th bond more nearly linear (168.8°) as compared with Ce(OH)2 with corresponding bond angles of 113.9° and 160.6°. In addition, the Th(OH)2+ cation was not observed, but Ce(OH)2+ is a major product here. The ionization potential of Th(OH)2 is predicted to be 5.67 eV, which places it just outside of the energy of our UV irradiation source, so we would not expect there to be enough energy to ionize Th(OH)2 in a direct reaction like 7.

in the 6s. One of the two unpaired electrons in ground state Ce(OH)2 is in the 6s as well. For the open shell species, there are 0.7−1.3 e in the 5 d, which are spin-paired, and the population is due to backbonding from the ligands to the Ce. The comparable reactions for Th are more exothermic than those for Ce, as found for the reactions of halogens with Ce and Th.70 The reaction of Th with H2O2 produces Th(OH)4 as the major product, whereas Ce(OH)2 is the major product for the reaction of Ce with H2O2. 3

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ASSOCIATED CONTENT

S Supporting Information *

CONCLUSIONS Reactions of laser ablated cerium atoms with hydrogen peroxide in argon proceed spontaneously on codeposition at 4 K and on annealing from 4 K into the 20−24 K range to form Ce(OH)2, Ce(OH)3, and Ce(OH)4, which were identified from matrix infrared spectra of O−H and Ce−O(H) stretching modes with D and 18O isotopic substitution and comparison with frequencies computed by density functional theory using the B3LYP functional. The Ce(OH)2+ cation, also formed on sample deposition, increases on UV irradiation together with Ce(OH)2 and Ce(OH)3, while Ce(OH)4 decreases. Deuterium and oxygen mixtures diluted in argon and condensed at 4 K produced Ce(OD)3, Ce(OD)2, and Ce(OD)2+ as major products. Additional minor products were identified in these H2O2 experiments as HCeO and OCeOH together with CeO and CeO2. The observation of a tetrahydroxide with lanthanide metals is consistent with the +IV oxidation state for Ce. The major products for Ce are in either the +III or +II oxidation state. Product formation is complex due to the presence of a number of intermediates. Ce(OH)2 is generated most likely by the substantially exothermic reaction 1 and can serve as the precursor to form the other species. CeO2 readily reacts with two water molecules to produce Ce(OH)4. The ionization potential to produce Ce(OH)2+ is within the range of the UV irradiation energy. The adiabatic electron detachment energies for the observed anions are less than 1.3 eV. The calculated frequencies including anharmonic effects are in good agreement with experiment. For the closed shell species, there is ∼1 e in the 4f and 5d orbitals due to backbonding, which is spin-paired. Most of the open shell species have the unpaired electron in the 4f with the exception of 2CeO2−, where the unpaired electron is

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02619. Complete citations for refs 61 and 72. Figures: Original, unprocessed infrared spectra recorded in the O−H stretching region for laser ablated Ce, H2O2, Ce, H2, and 18 O2 reaction products; infrared spectra recorded in the O−H stretching region for laser ablated Ce, D2, and 18O2 reaction products; infrared spectra for two identical experiments in terms of laser ablation conditions and H2O2 concentrations with 0.1% CCl4. Tables: Optimized geometry parameters; calculated frequencies and intensities; calculated anharmonic frequencies; calculated isotopic vibrational frequencies and intensities; and Cartesian coordinates, zero point energies, enthalpy correction, and electronic energies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.A.D acknowledges the Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences, catalysis center program and by the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME), an Energy Frontier Research Center funded by the U.S. Department of Energy, K

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Office of Science, Basic Energy Sciences. D.A.D also thanks the Robert Ramsay Chair Endowment, The University of Alabama, for support. L.A. thanks TIAA for retirement funds. X.-F.W. is grateful for support from NSFC Grant (21173158).

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