Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 274–281
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Structural, vibrational and thermal studies of a new nonlinear optical crystal tetrapropylammonium dihydrogenmonoarsenate bis arsenic acid Ikram Dhouib a,⇑, Habib Feki b, Philippe Guionneau c, Tahar Mhiri a, Zakaria Elaoud a a
Laboratoire de Physico-chimie de l’Etat Solide, Faculté des Sciences de Sfax, BP. N° 1171, Sfax 3000, Tunisia Laboratoire de Physique Appliquée, Faculté des Sciences de Sfax, BP. N° 1171, Sfax 3000, Tunisia c Institut de Chimie de la Matière Condensée de Bordeaux-ICMCB, 87 av Dr A. Schweitzer, 33608 Pessac Cedex, France b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
New hybrid crystal was synthesized
at room temperature by slow evaporation. This compound crystallizes in monoclinic Ia space group. Two peaks endothermic were shown at almost 370 and 396 K. The experimental vibrational bands have been discussed and assigned to normal mode.
Molecular arrangement for this [(CH3CH2CH2)4N](H2ASO4)(H3ASO4)2
The isolated chains of
The trimmers of anions
cations [N(C3H7)4]
a r t i c l e
i n f o
Article history: Received 4 March 2014 Received in revised form 15 April 2014 Accepted 22 April 2014 Available online 30 April 2014 Keywords: Organic arsenate NLO Vibrational spectroscopy DFT calculations X-Ray diffraction
a b s t r a c t Single crystals of tetrapropylammonium dihydrogenmonoarsenate bis arsenic acid [CH3CH2CH2]4N (H2AsO4) (H3AsO4)2, a potential new nonlinear optical (NLO) material of interest were prepared by the slow evaporation technique and characterized by means of single-crystal X-ray diffraction, thermal analysis, FT-IR and Raman spectroscopy. The title compound belongs to 0the monoclinic 0 space group Ia with 0 the following unit cell dimensions: a = 8.116(2) A Å, b = 33.673(4) A Å, c = 8.689(2) A Å, b = 95.34(2)°. The structure consists of infinite parallel two-dimensional planes built of mutually [H2AsO 4 ] and [H3AsO4] tetrahedra connected by strong O–H O hydrogen bonding giving birth to trimmers. The planes of inorganic groups are alternated with those of the organic cations. The geometry, first hyperpolarizability and harmonic vibrational wavenumbers were calculated by means of density functional theory DFT with the B3LYP/6-31G(d) level of theory. Good consistency was found between the calculated results and the experimental structure, IR, and Raman spectra. The detailed interpretation of the vibrational modes was carried out building on the proposed DFT calculations as primary source of assignment and by comparison with the spectroscopic studies of similar compounds. The first hyperpolarizability btot of the title compound is about 14.6 times more than that of the reference crystal KDP, which may explain the importance of the compound under study. Ó 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding author. Tel.: +216 97019206. E-mail address: [email protected] (I. Dhouib). http://dx.doi.org/10.1016/j.saa.2014.04.120 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
I. Dhouib et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 274–281
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Introduction
X-ray collection
Nonlinear optical (NLO) materials have attracted the attention of researchers owing to their great potential use in fields like laser technology, optical communication, optical data storage and optical signal processing [1–3]. Recently, considerable effort has been made to combine organic compounds with inorganic acids to produce materials having a non-centrosymmetric cell, a good stability and strong nonlinear optical coefficients [4–8]. Extensive literature is available on the organic, inorganic and organic–inorganic NLO materials with deep theoretical and experimental studies. Thanks to its ability to form multidirectional hydrogen bonds and to enhance the macroscopic nonlinearity property, arsenic acid is one of the most important inorganic acids chosen for the synthesis nonlinear materials. A number of arsenate complexes have been studied as promising materials for second harmonic generation [9–11]. On the other hand, there is still growing interest in the study of crystals containing tetra-alkylammonim cations of the general formula (CnH2n+1)4N+ such as (CH3)4N+, (C2H5)4N+ and (C3H7)4N+. The specific geometry of these cations can generate supramolecular networks in one, two or three dimensions. In recent times, the structural and vibrational studies of numerous tetrapropylammonium derivatives have been carried out [12–14]. However, in conjunction with some current research works on these hybrids compounds, the crystal structure and vibrational studies of many organic–inorganic crystals have been investigated in our laboratory [14–17]. The present research work deals with the single X-ray diffraction (XRD) study, the detailed vibrational spectral analysis and the NLO of a new organic–inorganic crystal: tetrapropylammonium dihydrogenmonoarsenate bis arsenic acid. Vibrational spectroscopy is an efficient tool for the characterization of crystalline materials. It is effectively used to identify functional groups and determine the molecular structure of synthesized crystals. The combination of infrared and Raman spectroscopy with quantum chemical computations has been used as an effective tool in the vibrational analysis of complex organic molecular systems [18– 21]. This paper presents the crystal structure, thermal analysis and the detailed vibrational spectral studies assisted by density functional theory (DFT) calculations, as well as the theoretical study of the nonlinear properties of the title compound.
The X-ray data collection of the title compound was performed on Enraf–Nonius CAD4 four circle diffractometer using Mo Ka radiation. The crystal structure was solved with direct methods, from the SHELXS-97 programs, which permitted the location of the AsO4 groups. The remaining non-hydrogen atoms were located by the successive difference Fourier maps using the SHELXL-97 programs [23]. In the final least-squares refinement of atomic parameters with isotropic thermal factors of the H atoms, the hydrogen atoms at H2AsO 4 and H3AsO4 were0 located from the difference Fourier map, and affined (0.79–0.98 Å A). The other H atoms 0 bound to C atoms were fixed geometrically (0.96–0.97 Å A). Corrections were applied for Lorentz, polarization and absorption effects. The final refinement cycles with 2739 reflections corresponding to the criterion I > 2r(I) yielded R = 0.065 and Rw = 0.1284. Crystal data and refinement details are given in Table 1.
Experimental details Crystal growth The crystal of the title compound was prepared by the slow evaporation of an aqueous solution of arsenic acid H3AsO4 and tetrapropylammonium hydroxide [CH3CH2CH2]4NOH in the stoichiometric ratio at room temperature. The obtained solution was mixed well using a magnetic stirrer to ensure homogenous concentration in the entire solution volume. It was allowed to evaporate at room temperature a few days until the colorless parallelepiped shaped monocrystals of the title compound were formed. Repeated crystallization yielded to good quality crystals with the dimensions of 0.4 0.2 0.1 mm3. The reaction scheme involved in the formation of the complex compound is:
½CH3 CH2 CH2 4 NOH þ 3H3 AsO4 ! ½CH3 CH2 CH2 4 NðH2 AsO4 ÞðH3 AsO4 Þ2 þ H2 O The chemical analysis of arsenate and acidic proton was carried out [22]. Density was measured at room temperature by flotation in toluene. The density average value Dm = 1.701 was found to be in good agreement with the calculated one Dx = 1.717.
Spectroscopic measurements The Fourier transform infrared (FT-IR) spectrum of the title compound was recorded in the range of 4000–400 cm1, with samples in KBr pellets using PERKIN–ELMER FT-IR spectrometer. The resolution of the spectrum was ±2 cm1. Besides, the Fourier transform Raman (FT-Raman) spectrum of the same compound was recorded using Horiba Jobin Yvon LabRAM HR 800 Dual Spectrophotometer. The incident laser excitation was 632 nm. The scattered light was collected at the angle of 180° in the region 3600–50 cm1 and the resolution was set up to 2 cm1.
Differential scanning calorimetry Differential scanning calorimetry (DSC) measurements was performed on heating sample from 305 K to 425 K on a SETARAM apparatus (model DSC 92) at a heating rate of 5 K min1.
Table 1 Crystal data, summary of intensity data collection and structure refinement. Compound
[CH3CH2CH2]4N H2AsO4 (H3AsO4)2
Color/shape Molecular weight (g mol1) Space group Temp. (°C)
Colorless/parallelepiped 611.18 Ia 20(2)
Cell constants a (Å) b (Å) c (Å) b (°) Cell volume (Å3) Formula units/unit cell Dcalc (g cm3) lcalc (cm1) Diffractometer/scan Radiation, graphite monochromater Max. crystal dimensions (mm) Reflections measured Range of h, k, l Reflections with F0 > 4r (F0)b GOF P P R = |Fo| |Fc|/ kF0k Rw Tmin Tmax Computer programs
8.116(2) 33.673(4) 8.689(2) 95.34(2) 2364.3(8) 4 1.717 4.27 CAD4. Enraf–Nonius Mo Ka (k = 0.71073) 0.4 0.2 0.1 2739 3/10, 7/42, 11/11 1786 0.952 0.0655 0.1284 0.2801 0.6748 SHELXS-97(Sheldrick, 1990) SHELXL-97(Sheldrick, 1997)
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Computational methods The molecular geometry optimization and vibrational wavenumber calculations of the title compound were performed by DFT method using the Gaussian 03 package [24]. The functional Becke three-parameter hybrid exchange and the functional Lee– Yang–Parr correlation (B3LYP) were utilized in the calculation with the 6-31G(d) basis set. An empirical, uniform scaling factor of 0.967 was used to offset the systematic errors caused by basis set incompleteness, electron correlation neglect and vibrational anharmonicity [25]. The vibrational modes were assigned on the basis of our DFT calculations as a primary source of assignment and by comparison with spectroscopic studies of similar compounds. It is well known that the nonlinear optical response of an isolated molecule in an electric field Ei(x) can be presented as a Taylor series expansion of the total dipole moment, l, induced by the field:
l ¼ l0 þ aij Ej þ bijk Ej Ek þ where a is the linear polarizability, l0 is the permanent dipole moment and bijk are the first hyperpolarizability tensor components. The isotropic (or average) linear polarizability is defined as [26]:
1 3
a ¼ ðaxx þ ayy þ azz Þ The first hyperpolarizability or the second-order polarizability b was calculated using B3LYP/6-31G(d) basis set. The complete equation for calculating the magnitude of the first hyperpolarizability from Gaussian 03 output is given as follows: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi btot ¼ ðbxxx þ bxyy þ bxzz Þ2 þ ðbyyy þ byzz þ byxx Þ2 þ ðbzzz þ bzxx þ bzyy Þ2 The calculated values of the polarizabilities and the hyperpolarizabilities from Gaussian 03 output have been converted from atomic units (a.u) into electrostatic units (esu). The electronic dipole moment li (i = x, y, z), the polarizability aij and the first hyperpolarizability bijk are shown in Table S1 (Supplementary information). The first calculated hyperpolarizability is 100 1031 esu, which is 14.6 times more than that of KDP. This surprisingly high hyperpolarizability corroborates with the NLO activity of the title compound. Results and discussion Structural analysis The title compound is crystallized in the monoclinic space 0 group Ia with 0 the following unit cell dimensions: a = 8.116(2) Å A, 0 b = 33.673(4) Å A, c = 8.689(2) Å A and b = 95.34(2)°. The asymmetric unit is composed of one [H2AsO4] anion, two neutral arsenic acids [H3AsO4] and one [(CH3CH2CH2)4N]+ cation (Fig. 1). Selected bonds length and angles together with the calculated ones are presented in Table 2, which is in accordance with the atom numbering scheme given in Fig. 1a. The optimized geometry of the title compound model is presented in Fig. 1b. As seen in Tables 2 and 3, most of the computed bonds are slightly longer than the experimental one. This discrepancies can be explained by the fact that the calculations relates to the isolated molecule where the intermolecular Coulombic interaction with the neighboring molecules are absent, whereas the experimental result corresponds to interacting molecules in the crystal lattice. The groups of organic and inorganic ring appear to be distorted as seen from contraction and lengthening of internal angles of 109.7°, 109.0°, 109.2°, 109.8° (C1–C7–C9, C10–C8–C2, C11–C6–C4, C12–C5–C3) and 113.2°, 108.9°, 111.2°, 112.4°, 112.9°, 112.1° (O2–As1–O4, O1–
Fig. 1. The asymmetric unit of [(CH3CH2CH2)]4N+(H2AsO 4 ) (H3AsO4)2 observed (a) and calculated (b).
As1–O3, O8–As2–O7, O7–As2–O5, O11–As3–O12, O12–As3–O9) that leads to elongation and shortening of N–C1, C4–C6, C6–C11, As1–O1, As1–O3, As2–O5, As2–O8, As3–O9, As3–O12 bond lengths of 1.53 Å, 1.50 Å, 1.52 Å, 1.700 Å, 1.670 Å, 1.712 Å, 1.672 Å, 1.686 Å, 1.723 Å respectively associated to the charge transfer interaction. The maximum difference does not exceed 0.042 Å for the bond lengths and 3.5° for the bond angles. This result shows that the cluster approach is sufficient to the analysis of the spectra in the solid state. The structure consists of infinite parallel two-dimensional planes built of mutual [H2AsO 4 ], [H3AsO4] tetrahedra and [NC3H7]+4 cations. The projection along the a axis of the atomic arrangement is depicted in Fig. 2. The coordination of arsenic atoms is tetrahedral with As–O distances in the range of [1.657(12); 1.725(12) Å]; [1.663(14); 1.674(13) Å]; [1.674(12); 1.707(11) Å]; and O–As–O angles in the range of [105.9(7)°; 113.3(5)°]; [104.1(8)°; 114.3(6)°] and [105.1(8)°; 114.8(6)°], for [H3As(1)O4]; [H3As(2)O4]2 and [H2As(3)O4], respectively (Table 3). The calculated average values of the distortion indices [27] corresponding to the different distances and angles in [AsO4] tetrahedral DI(As(1)O) = 0.0133, DI(As(2)O) = 0.0030, DI(As(3)O) = 0.0059, DI(OAs(1)O) = 0.0175, DI(OAs(2)O) = 0.0237, DI(OAs(3)O) = 0.0164, DI(OO) = 0.0142, DI(OO) = 0.0113 and DI(OO) = 0.0124 show a high distortion of the (OO) distances of [H3As(1)O4]; [H2As(2)O4]2 and [H3As(3)O4]. The structure is based on sheets
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Table 2 Comparison between the optimized geometrical parameters and the corresponding experimental data of organic part [CH3CH2CH2]4N. Parameters Bond length (Å)
Observed
Calculated B3LYP/6-31G(d)
N–C1 N–C2 N–C4 N–C3 C1–C7 C2–C8 C3–C5 C4–C6 C5–C12 C6–C11 C7–C9 C8–C10 C1–N–C2 C1–N–C3 C1–N–C4 C2–N–C3 C2–N–C4 C4–N–C3 N–C2–C8 N–C4–C6 C5–C3–N C7–C1–N C1–C7–C9 C10–C8–C2 C11–C6–C4 C12–C5–C3
1.50(2) 1.51(2) 1.54(2) 1.55(2) 1.44(3) 1.53(2) 1.51(2) 1.54(3) 1.47(3) 1.48(3) 1.53(3) 1.50(3) 110.9(13) 107.5(12) 111.5(15) 111.9(13) 109.0(12) 106.0(12) 115.3(14) 115.5(14) 115.4(15) 118.4(17) 110(2) 109.2(17) 109.7(17) 109.4(15)
1.53 1.52 1.52 1.50 1.50 1.53 1.52 1.50 1.51 1.52 1.52 1.52 108.8 106.5 111.4 111.7 110.4 108.0 115.5 118.0 115.5 115.6 109.7 109.0 109.2 109.8
Table 3 Comparison between the optimized geometrical parameters and the corresponding experimental data of inorganic part (H2AsO 4 ) (H3AsO4)2. Parameters Bond length (Å)
Observed
Calculated B3LYP/6-31G(d)
As1–O1 As1–O2 As1–O3 As1–O4 O1–As1–O2 O1–As1–O3 O2–As1–O3 O1–As1–O4 O2–As1–O4 O3–As1–O4 As2–O5 As2–O6 As2–O7 As2–O8 O8–As2–O6 O8–As2–O7 O6–As2–O7 O8–As2–O5 O6–As2–O5 O7–As2–O5 As3–O9 As3–O10 As3–O11 As3–O12 O11–As3–O12 O11–As3–O10 O12–As3–O10 O11–As3–O9 O12–As3–O9 O10–As3–O9
1.658(12) 1.664(13) 1.681(12) 1.728(13) 113.3(5) 110.1(6) 109.3(7) 107.3(7) 110.6(6) 105.9(7) 1.671(13) 1.668(14) 1.663(11) 1.662(15) 111.9(7) 109.2(8) 110.0(7) 107.3(10) 104.1(8) 114.3(6) 1.696(12) 1.686(14) 1.672(12) 1.683(13) 114.8(6) 109.1(7) 105.1(8) 109.6(6) 108.7(6) 109.3(7)
1.700 1.701 1.670 1.750 113.2 108.9 110.7 108.8 113.2 108.7 1.712 1.700 1.701 1.672 111.0 111.2 112.4 108.2 104.1 112.4 1.686 1.728 1.681 1.723 112.9 110.9 108.1 112.1 112.1 110.88
of H2AsO 4 and H3AsO4 tetrahedra bonded together by strong intralayer O–H O hydrogen bonds, giving rise to trimmers made up of [(H3AsO4)2 H2AsO4], (dO–O < 2.73 Å) [28,29] as seen in Fig. 3. These hydrogen bonds contribute to the cohesion of the structure. The various hydrogen bond parameters are summarized in Table 4.
Fig. 2. Projection along the a axis of the atomic arrangement of [(CH3CH2CH2)]4N (H2AsO4) (H3AsO4)2.
The midplanes of these arsenates groups, are located at z = 0.25 and z = 0.75. The organic cation are located at z = 0.25 and z = 0.75. 0 The lengths of the N–C bonds are between 1.50(2) and 1.55(2) Å A and the C–N–C angles range from 106(12)° to 111.9(13)°. The C–C bonds lengths are in the region between 1.44(3) and 1.54(3). The final coordinates and U(eq)or U(iso) are given in Table S2 (Supplementary information).
Vibrational analysis In order to give more information on the crystal structure, we have studied the vibrational properties of our compound using Raman scattering and infrared absorption. Taking into account the effect of intermolecular interactions on geometrical parameters, we have considered the cluster built up from two neutral arsenic acids [H3AsO4], one [H2AsO4] anion linked by O–H O hydrogen bonds and one tetrapropylammonium cation. All the parameters were allowed to relax and all the calculations converged to an optimized geometry which corresponds to an energy minimum as revealed by the lack of imaginary values in the calculated wave numbers. The vibrational spectral analysis is performed based on the characteristic vibration of the arsenate groups and organic cation. The vibrational modes were conducted by the visual inspection of modes animated by using Molekel program [30] and by comparison with the previous theoretical and experimental results reported in the literature for similar compounds. The computed wavenumbers corresponding to different modes along with detailed assignments are listed in Table 5. For visual comparison, the observed and simulated Raman spectra are presented in Fig. 4. The recorded IR spectrum is presented in Fig. 5.
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by comparison with similar organic arsenate compounds, a distinction has been made between the bands originating from the vibrations of arsenate groups and the organic cation. The medium band at 907 cm1 in Raman spectrum is assigned to the asymmetric stretching m3 mode of the arsenate ion. In IR spectrum, this mode also appears as a single medium band at 919 cm1. The theoretically computed values by the B3LYP/6-31G(d,p) method at 913 cm1 show an excellent agreement with the experimental data. The band due to the symmetric stretching mode m1 vibration is located in Raman spectrum at 830 cm1 as a strong band. In IR spectrum this mode appears as a medium band at 813 cm1. The bands due to the symmetric and asymmetric bending vibration m2 and m4 of the arsenate ion are identified in the 400–300 cm1 and 550–400 cm1 frequency regions, respectively. The very strong band observed in the Raman spectrum at 417 cm1 and the strong band at 433 cm1 in IR spectrum can be easily assigned to m4 mode, whereas the symmetric mode m2 appears in the Raman spectrum as a shoulder band at 372 cm1. As seen in Table 5, all modes associated with the arsenates groups are well reproduced by theoretical method. The tetrapropylammonium cation vibrations Numerous functional and skeletal groups such as CH2, CH3, N–C, C–C, CCC, NC4 are present in tetrapropylammonium cation. These groups are manifested in IR and Raman spectra in different ranges with different intensities.
Fig. 3. Projection along the a axis of the inorganic arrangement of [(C3H7)4N] (H2AsO4) (H3AsO4)2.
Table 4 Bond lengths (Å) and bond angles (°) in the hydrogen bonding scheme of [(CH3CH2CH2)4N](H2AsO4)(H3AsO4)2. D–H A
d(D–H) (Å)
d(H A) (Å)
d(D A) (Å)
\D–H A (°)
O(1)–H(1) O(8) O(2)–H(2) O(6) O(5)–H(5) O(11)i O(6)–H(6) O(2) O(10)–H(10) O(7) O(12)– H(12) O(7)ii
0.79 0.85 0.95 0.82 0.81 0.87
1.83 2.02 2.01 1.79 1.90 2.25
2.49(2) 2.56(2) 2.68 (2) 2.56(2) 2.54(2) 2.57(2)
139.6 120.6 125.8 154.6 135.1 101.6
Symmetry code: (i) 1+x, y, z. Symmetry code: (ii) 1/2+x, y, z.
Arsenate groups vibrations The vibrational analysis of an isolated arsenate anion AsO3 4 with Td point group symmetry led to four Raman active normal modes: m1(A1), m2(E), m3(F2) and m4(F2) with average wavenumbers 837, 349, 887 and 463 cm1, respectively [31]. m1 and m3 involve the symmetric and the asymmetric stretching mode of the As–O bond, whereas m2 and m4 involve mainly O–As–O symmetric and asymmetric bending modes. In the title compound, the symmetry of AsO3 4 ions was reduced from Td to C1. In fact, our structural study shows that the As–O distances and the O–As–O angles are distorted with respect to the hypothetical Td symmetry. This symmetry change partially removes the degeneracy of the vibrational wave functions, which would characterize free AsO3 4 . In the 1000–300 cm1 region, the AsO3 stretching and bending vibra4 tions expected to appear, as well as the modes associated with the internal modes of the organic cations. However, in the light of the present calculations as primary sources of assignment and
Methyl vibrations. The asymmetric and symmetric CH3 stretching vibrations are usually observed in the region 3000–2850 cm1 [32]. The weak band at 2987 cm1 in IR spectrum is attributed to the asymmetric stretching mas(CH3) vibration. The Raman counterpart is located as a very strong band at 2998 cm1. The symmetric stretching mode ms(CH3) is observed as a shoulder band at 2897 cm1 in Raman spectrum. In IR spectrum, this mode appears as a very weak intensity band at the 2877 cm1. The asymmetric and symmetric bending modes of the methyl group generally appear in the region 1550–1410 and 1400–1340 cm1, respectively. In the present case, the medium IR band at 1489 cm1 is assigned to CH3 asymmetric bending mode and the corresponding Raman band appears as a strong band at 1462 cm1. The symmetric bending mode ds(CH3) is located at 1392 cm1 as a weak band in the IR spectrum and as a very weak band at 1387 cm1 in Raman spectrum. The methyl rocking vibration is observed as a weak band at 997 cm1 in IR spectrum and at 1001 cm1 as a medium one in Raman spectrum. As shown in Table 5, all calculated wavenumbers related to methyl groups agree well with the experimental values. On the other hand, it is worthwhile to note that the vibrational modes of the CH3 do not deviate much from their expected values, suggesting that the interaction of the groups with the environment is not strong. Methylene group vibrations. The wavenumber of the CH2 vibrational modes depend on its immediate environment. The stretching modes of the CH2 group usually occur in the region 3100–2800 cm1 [33]. In the title compound, the CH2 asymmetric and symmetric stretching modes are observed in Raman spectrum at 2946 cm1 as a very strong band and at 2824 cm1 as a very weak band. In IR spectrum, these modes are probably masked by the broad band around 2900 cm1. The wagging, twisting and the rocking modes of the CH2 group were observed and assigned. NC4 group vibrations. The primitive unit cell of the title compound contains one tetrapropylammonium (NC4)+ cation that is not coupled with the inorganic parts as revealed by the X-ray diffraction. Afterwards, it is convenient as a first approach to consider the NC4 core of the isolated cation with Td symmetry, which exhibits
I. Dhouib et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 274–281 Table 5 Observed and calculated wavenumbers (cm1) of tetrapropylammonium dihydrogenmonoarsenate bis arsenic acid with the proposed assignments. Observed FT-IR
FT-Raman
3660 s,b 3453 w – 2987 w – 2877 vw – – 2386 b,s 1730 w,b 1620 w,b 1489 m 1392 w 1319 w 1288 w 1183 w 1146 w 1122 w 1032 w 997 w 919 m 813 m 786 b,sh – – 433 s – – – – –
– 3473 b 3217 b 2998 vs 2946 vs 2897 sh 2824vw 2753 vw – – – 1462 s 1387 vw 1319 vw 1295 s 1173 w 1151 w 1117 s 1044 vw 1001 m 907 m 830 s 787 w 745 m 676 s 417 vs 372 sh 325 m 283 m 114 m 55 w
Calculated B3LYP/6-31G⁄
Assignments
– 3595 3322 3037 2980 2845 2833 2779 – – – 1466 1394 1320 1300 1183 1163 1127 1091 1019 913 799 777 714 615 396 361 333 275 120 66
m(OH) m(OH O) m(OH O) mas(CH3) mas(CH2) ms(CH3) ms(CH2) ms(CH3) + mas(CH2) Combination bands – – das(CH3) ds(CH3) + das(CH2) x(CH2) + d(OH) x(CH2) + d(OH) m(CC) + twist(CH2) m(CC) + d(CCC) m(NC) + d(CNC) + twist(CH2) mas(NC4) q(CH3) + d(CCC) + c(OH) mas(AsO4) ms(AsO4) + c(OH) ms(NC4) + c(OH) d(CCC) + d(CNC) + q(CH2) d(CCC) + d(CNC) + q(CH2) das(AsO4) ds(AsO4) ds(NC4) + c(CCC) s(CH3) + c(CCC) Lattice mode Lattice mode
m, stretching; d, in-plane bending; c, out of plane bending; q, rocking; x, wagging; twist, twisting; asym, asymmetric; sym, symmetric; w, weak; vw, very weak; s, strong; vs, very strong; m, medium; sh, shoulder.
four normal modes. According to the literature, the vibrations related to NC4 group shown in tetrapropylammonium cation are described as: symmetric stretching mode m1(A) to be found at 752 cm1, asymmetric stretching mode m3(F2) located at
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955 cm1, asymmetric bending mode m4(F2) observed at 455 cm1 and the symmetric bending mode m2(F2) at 372 cm1 [34,35]. In our case the very weak Raman band appearing at 1044 cm1 is related to the m3 mode. Its counterpart appears in IR spectrum as weak band at 1032 cm1. The week band observed in Raman spectrum at 787 cm1 and the shoulder one at 786 cm1 in IR spectrum arises from the symmetric stretching mode m1(NC4). The band assigned to the symmetric bending modes of NC4 core of the isolated cation is observed in Raman spectrum at 325 cm1 with no counterpart in IR spectrum. The asymmetric band m2(NC4) is not observed in IR spectrum and probably masked by the very intense band at 417 cm1, assigned to das(AsO4) in Raman spectrum. It is worthy to note that the vibrational mode of the NC4 group does not deviate much from its expected values, suggesting that the interaction of this group with the environment is not strong. On the other hand, we note that our assignment of NC4 agrees well with the previous reported vibrational studies of the tetrapropylammonium salts [13]. N–C, C–C, C–C–C and C–C–N group of vibrations. The absorption bands arising from N–C symmetric stretching vibrations are observed in the wavenumber region 850–1150 cm1. In the present crystal, the strong Raman band at 1117 cm1 is assigned to this mode. Its IR counterpart is identified at 1122 cm1. The corresponding theoretical value is at 1127 cm1. The absorption bands arising from the C–N–C stretching mode is observed in IR spectrum at 1122 cm1 and in Raman spectrum at 1117, 745 and 676 cm1. The weak bands appearing at 1183 and 1146 cm1 in IR spectrum and at 1173 and 1151 cm1 in Raman are related to C–C stretching mode. The C–C–C vibrational bands have been identified. Table 5 presents a detailed assignment of all observed band related to the organic cation. The hydrogen bond vibrations. It is well known that hydrogen bonding brings about remarkable wavenumber shifts. Whereas the intermolecular hydrogen bonds give rise to broad bands, intramolecular hydrogen bonds produce sharp and well resolved bands. Knowing the bond length, the strength of the hydrogen bond can be determined as very strong (below 2.5 Å), strong (2.5–2.7 Å),
Fig. 4. Raman spectrum of [(CH3CH2CH2)]4N (H2AsO4) (H3AsO4)2 observed (a) and calculated (b).
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Fig. 5. IR spectrum of [(CH3CH2CH2)4]N (H2AsO4) (H3AsO4)2.
normal (2.7–2.9 Å) and weak (above 2.9 Å). X-ray diffraction analysis of the title compound reveals that the structure is based on sheets of H2AsO 4 and H3AsO4 tetrahedra bonded together by strong O–H O hydrogen bonds intralayers, leading to the formation of trimmers. The O–H O bonds in the unit cell are six in number and their bond lengths are in the range of 2.49(2)– 2.68(2) Å. The inspection of the IR spectrum reveals a broad and strong band at 3660 cm1, which may be assigned to the stretching mode m(OH) which is not implied in hydrogen bonding. The weak IR band at 3453 cm1 and the two broad bands in Raman spectrum located at 3473 and 3217 cm1 are accredited to m(O–H O) mode, while the B3LYP/6-31G(d) calculation gives rise to their positions at 3595 and 3322 cm1. The wavenumbers of this mode is shifted towards lower values. In fact, the XRD study of the title compound reveals that the structure is based on sheets of H2AsO 4 and H3AsO4 tetrahedra bonded together by strong intralayer O–HO hydrogen bonds, giving rise to trimmers made up of [(H3AsO4)2H2AsO4]. The in-plane OH bending mode d(OH) leads to a medium band in infrared spectrum at 1319 and 1278 cm1 with a counterpart in Raman spectrum at 1319 and 1295 cm1. The out-of-plane bending c(OH) mode appears in the region 900–700 cm1. As seen in m(O–H O) stretching, wavenumbers are clearly lower than the calculated values and the bending wavenumbers are not much different from the expected range of 1380–1280 cm1, which indicates that the linear distortion is much greater than the angular distortion.
NLO properties Since the values of the polarizability atot and hyperpolarizability btot given by Gaussian 03 are reported in atomic units (a.u.). The B3LYP/6-31G(d) results of the electric dipole moment li (i = x, y, z), polarizability aij and the first hyperpolarizability bijk of the title compound are listed in Table S1. The calculated dipole moment is equal to 39.892 D. Furthermore, the highest absolute value of the dipole moment is observed for the component lx. In this direction, the value is equal to 38.945. The calculated polarizability atot, is equal to 31.56 1024 esu. As can be seen, the first hyperpolarizability btot of the compound has for value 100.47 1031 eus, which is about 14.6 times more than that of the reference crystal KDP (bKDP = 6.85 1031 eus). The above results show that the title compound can be a good material for NLO applications.
Thermal analysis (DSC and ATG) The DSC–ATG of the title compound was conducted in air at the rate of 5 °C/min. One more characteristic feature of the room temperature is its high hygroscopicity. Figs. S1 and S2 illustrate the calorimetric (DSC) and thermogravimetric (TGA) results. In fact, this compound is stable until 340 K, above which a loss of weight appears at 370 K, which is due to the departure of adsorbed water. The endothermic peak observed at T = 396 K is attributed to the melting of the crystal. Conclusion Crystals of a hybrid material of the formulae N(C3H7)4, (H2AsO4) and (H3AsO4)2, have been prepared by the slow evaporation of aqueous solution (CH3CH2CH2)4NOH and H3AsO4 at room temperature and characterized by means of single-crystal X-ray diffraction, thermal analysis, FT-IR and Raman vibrational study. The crystal formed is non-centrosymmetric. The structure consists of a strong two-dimensional character based on sheets of H2AsO 4 and H3AsO4 tetrahedra fused together by strong intralayer O–H O hydrogen bonds, giving rise to trimmers. The planes of inorganic groups are alternated with the planes of the organic cations. The DSC measurement shows two endothermic peaks at about 370 and at 396 K. The structural and vibrational frequency analyses by DFT calculations agree well with the experimental results. Building on the agreement between the experimental and calculated results, the assignment of all vibrational modes of the title compound were examined and proposed in this investigation. The first hyperpolarizability btot of the title compound is about 14.6 times more than that of the reference crystal KDP. This may explain the importance of this compound as a very important new nonlinear optical (NLO) material. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.04.120. References [1] H.O. Marcy, L.F. Warren, M.S. Webb, C.A. Ebbers, S.P. Velsko, G.C. Kennedy, G.C. Catella, Appl. Opt. 31 (1992) 5051.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Structural, vibrational and thermal studies of a new nonlinear optical crystal tetrapropylammonium dihydrogenmonoarsenate bis arsenic acid Ikram Dhouib a,⇑, Habib Feki b, Philippe Guionneau c, Tahar Mhiri a, Zakaria Elaoud a a
Laboratoire de Physico-chimie de l’Etat Solide, Faculté des Sciences de Sfax, BP. N° 1171, Sfax 3000, Tunisia Laboratoire de Physique Appliquée, Faculté des Sciences de Sfax, BP. N° 1171, Sfax 3000, Tunisia c Institut de Chimie de la Matière Condensée de Bordeaux-ICMCB, 87 av Dr A. Schweitzer, 33608 Pessac Cedex, France b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
New hybrid crystal was synthesized
at room temperature by slow evaporation. This compound crystallizes in monoclinic Ia space group. Two peaks endothermic were shown at almost 370 and 396 K. The experimental vibrational bands have been discussed and assigned to normal mode.
Molecular arrangement for this [(CH3CH2CH2)4N](H2ASO4)(H3ASO4)2
The isolated chains of
The trimmers of anions
cations [N(C3H7)4]
a r t i c l e
i n f o
Article history: Received 4 March 2014 Received in revised form 15 April 2014 Accepted 22 April 2014 Available online 30 April 2014 Keywords: Organic arsenate NLO Vibrational spectroscopy DFT calculations X-Ray diffraction
a b s t r a c t Single crystals of tetrapropylammonium dihydrogenmonoarsenate bis arsenic acid [CH3CH2CH2]4N (H2AsO4) (H3AsO4)2, a potential new nonlinear optical (NLO) material of interest were prepared by the slow evaporation technique and characterized by means of single-crystal X-ray diffraction, thermal analysis, FT-IR and Raman spectroscopy. The title compound belongs to 0the monoclinic 0 space group Ia with 0 the following unit cell dimensions: a = 8.116(2) A Å, b = 33.673(4) A Å, c = 8.689(2) A Å, b = 95.34(2)°. The structure consists of infinite parallel two-dimensional planes built of mutually [H2AsO 4 ] and [H3AsO4] tetrahedra connected by strong O–H O hydrogen bonding giving birth to trimmers. The planes of inorganic groups are alternated with those of the organic cations. The geometry, first hyperpolarizability and harmonic vibrational wavenumbers were calculated by means of density functional theory DFT with the B3LYP/6-31G(d) level of theory. Good consistency was found between the calculated results and the experimental structure, IR, and Raman spectra. The detailed interpretation of the vibrational modes was carried out building on the proposed DFT calculations as primary source of assignment and by comparison with the spectroscopic studies of similar compounds. The first hyperpolarizability btot of the title compound is about 14.6 times more than that of the reference crystal KDP, which may explain the importance of the compound under study. Ó 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding author. Tel.: +216 97019206. E-mail address: [email protected] (I. Dhouib). http://dx.doi.org/10.1016/j.saa.2014.04.120 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
I. Dhouib et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 274–281
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Introduction
X-ray collection
Nonlinear optical (NLO) materials have attracted the attention of researchers owing to their great potential use in fields like laser technology, optical communication, optical data storage and optical signal processing [1–3]. Recently, considerable effort has been made to combine organic compounds with inorganic acids to produce materials having a non-centrosymmetric cell, a good stability and strong nonlinear optical coefficients [4–8]. Extensive literature is available on the organic, inorganic and organic–inorganic NLO materials with deep theoretical and experimental studies. Thanks to its ability to form multidirectional hydrogen bonds and to enhance the macroscopic nonlinearity property, arsenic acid is one of the most important inorganic acids chosen for the synthesis nonlinear materials. A number of arsenate complexes have been studied as promising materials for second harmonic generation [9–11]. On the other hand, there is still growing interest in the study of crystals containing tetra-alkylammonim cations of the general formula (CnH2n+1)4N+ such as (CH3)4N+, (C2H5)4N+ and (C3H7)4N+. The specific geometry of these cations can generate supramolecular networks in one, two or three dimensions. In recent times, the structural and vibrational studies of numerous tetrapropylammonium derivatives have been carried out [12–14]. However, in conjunction with some current research works on these hybrids compounds, the crystal structure and vibrational studies of many organic–inorganic crystals have been investigated in our laboratory [14–17]. The present research work deals with the single X-ray diffraction (XRD) study, the detailed vibrational spectral analysis and the NLO of a new organic–inorganic crystal: tetrapropylammonium dihydrogenmonoarsenate bis arsenic acid. Vibrational spectroscopy is an efficient tool for the characterization of crystalline materials. It is effectively used to identify functional groups and determine the molecular structure of synthesized crystals. The combination of infrared and Raman spectroscopy with quantum chemical computations has been used as an effective tool in the vibrational analysis of complex organic molecular systems [18– 21]. This paper presents the crystal structure, thermal analysis and the detailed vibrational spectral studies assisted by density functional theory (DFT) calculations, as well as the theoretical study of the nonlinear properties of the title compound.
The X-ray data collection of the title compound was performed on Enraf–Nonius CAD4 four circle diffractometer using Mo Ka radiation. The crystal structure was solved with direct methods, from the SHELXS-97 programs, which permitted the location of the AsO4 groups. The remaining non-hydrogen atoms were located by the successive difference Fourier maps using the SHELXL-97 programs [23]. In the final least-squares refinement of atomic parameters with isotropic thermal factors of the H atoms, the hydrogen atoms at H2AsO 4 and H3AsO4 were0 located from the difference Fourier map, and affined (0.79–0.98 Å A). The other H atoms 0 bound to C atoms were fixed geometrically (0.96–0.97 Å A). Corrections were applied for Lorentz, polarization and absorption effects. The final refinement cycles with 2739 reflections corresponding to the criterion I > 2r(I) yielded R = 0.065 and Rw = 0.1284. Crystal data and refinement details are given in Table 1.
Experimental details Crystal growth The crystal of the title compound was prepared by the slow evaporation of an aqueous solution of arsenic acid H3AsO4 and tetrapropylammonium hydroxide [CH3CH2CH2]4NOH in the stoichiometric ratio at room temperature. The obtained solution was mixed well using a magnetic stirrer to ensure homogenous concentration in the entire solution volume. It was allowed to evaporate at room temperature a few days until the colorless parallelepiped shaped monocrystals of the title compound were formed. Repeated crystallization yielded to good quality crystals with the dimensions of 0.4 0.2 0.1 mm3. The reaction scheme involved in the formation of the complex compound is:
½CH3 CH2 CH2 4 NOH þ 3H3 AsO4 ! ½CH3 CH2 CH2 4 NðH2 AsO4 ÞðH3 AsO4 Þ2 þ H2 O The chemical analysis of arsenate and acidic proton was carried out [22]. Density was measured at room temperature by flotation in toluene. The density average value Dm = 1.701 was found to be in good agreement with the calculated one Dx = 1.717.
Spectroscopic measurements The Fourier transform infrared (FT-IR) spectrum of the title compound was recorded in the range of 4000–400 cm1, with samples in KBr pellets using PERKIN–ELMER FT-IR spectrometer. The resolution of the spectrum was ±2 cm1. Besides, the Fourier transform Raman (FT-Raman) spectrum of the same compound was recorded using Horiba Jobin Yvon LabRAM HR 800 Dual Spectrophotometer. The incident laser excitation was 632 nm. The scattered light was collected at the angle of 180° in the region 3600–50 cm1 and the resolution was set up to 2 cm1.
Differential scanning calorimetry Differential scanning calorimetry (DSC) measurements was performed on heating sample from 305 K to 425 K on a SETARAM apparatus (model DSC 92) at a heating rate of 5 K min1.
Table 1 Crystal data, summary of intensity data collection and structure refinement. Compound
[CH3CH2CH2]4N H2AsO4 (H3AsO4)2
Color/shape Molecular weight (g mol1) Space group Temp. (°C)
Colorless/parallelepiped 611.18 Ia 20(2)
Cell constants a (Å) b (Å) c (Å) b (°) Cell volume (Å3) Formula units/unit cell Dcalc (g cm3) lcalc (cm1) Diffractometer/scan Radiation, graphite monochromater Max. crystal dimensions (mm) Reflections measured Range of h, k, l Reflections with F0 > 4r (F0)b GOF P P R = |Fo| |Fc|/ kF0k Rw Tmin Tmax Computer programs
8.116(2) 33.673(4) 8.689(2) 95.34(2) 2364.3(8) 4 1.717 4.27 CAD4. Enraf–Nonius Mo Ka (k = 0.71073) 0.4 0.2 0.1 2739 3/10, 7/42, 11/11 1786 0.952 0.0655 0.1284 0.2801 0.6748 SHELXS-97(Sheldrick, 1990) SHELXL-97(Sheldrick, 1997)
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Computational methods The molecular geometry optimization and vibrational wavenumber calculations of the title compound were performed by DFT method using the Gaussian 03 package [24]. The functional Becke three-parameter hybrid exchange and the functional Lee– Yang–Parr correlation (B3LYP) were utilized in the calculation with the 6-31G(d) basis set. An empirical, uniform scaling factor of 0.967 was used to offset the systematic errors caused by basis set incompleteness, electron correlation neglect and vibrational anharmonicity [25]. The vibrational modes were assigned on the basis of our DFT calculations as a primary source of assignment and by comparison with spectroscopic studies of similar compounds. It is well known that the nonlinear optical response of an isolated molecule in an electric field Ei(x) can be presented as a Taylor series expansion of the total dipole moment, l, induced by the field:
l ¼ l0 þ aij Ej þ bijk Ej Ek þ where a is the linear polarizability, l0 is the permanent dipole moment and bijk are the first hyperpolarizability tensor components. The isotropic (or average) linear polarizability is defined as [26]:
1 3
a ¼ ðaxx þ ayy þ azz Þ The first hyperpolarizability or the second-order polarizability b was calculated using B3LYP/6-31G(d) basis set. The complete equation for calculating the magnitude of the first hyperpolarizability from Gaussian 03 output is given as follows: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi btot ¼ ðbxxx þ bxyy þ bxzz Þ2 þ ðbyyy þ byzz þ byxx Þ2 þ ðbzzz þ bzxx þ bzyy Þ2 The calculated values of the polarizabilities and the hyperpolarizabilities from Gaussian 03 output have been converted from atomic units (a.u) into electrostatic units (esu). The electronic dipole moment li (i = x, y, z), the polarizability aij and the first hyperpolarizability bijk are shown in Table S1 (Supplementary information). The first calculated hyperpolarizability is 100 1031 esu, which is 14.6 times more than that of KDP. This surprisingly high hyperpolarizability corroborates with the NLO activity of the title compound. Results and discussion Structural analysis The title compound is crystallized in the monoclinic space 0 group Ia with 0 the following unit cell dimensions: a = 8.116(2) Å A, 0 b = 33.673(4) Å A, c = 8.689(2) Å A and b = 95.34(2)°. The asymmetric unit is composed of one [H2AsO4] anion, two neutral arsenic acids [H3AsO4] and one [(CH3CH2CH2)4N]+ cation (Fig. 1). Selected bonds length and angles together with the calculated ones are presented in Table 2, which is in accordance with the atom numbering scheme given in Fig. 1a. The optimized geometry of the title compound model is presented in Fig. 1b. As seen in Tables 2 and 3, most of the computed bonds are slightly longer than the experimental one. This discrepancies can be explained by the fact that the calculations relates to the isolated molecule where the intermolecular Coulombic interaction with the neighboring molecules are absent, whereas the experimental result corresponds to interacting molecules in the crystal lattice. The groups of organic and inorganic ring appear to be distorted as seen from contraction and lengthening of internal angles of 109.7°, 109.0°, 109.2°, 109.8° (C1–C7–C9, C10–C8–C2, C11–C6–C4, C12–C5–C3) and 113.2°, 108.9°, 111.2°, 112.4°, 112.9°, 112.1° (O2–As1–O4, O1–
Fig. 1. The asymmetric unit of [(CH3CH2CH2)]4N+(H2AsO 4 ) (H3AsO4)2 observed (a) and calculated (b).
As1–O3, O8–As2–O7, O7–As2–O5, O11–As3–O12, O12–As3–O9) that leads to elongation and shortening of N–C1, C4–C6, C6–C11, As1–O1, As1–O3, As2–O5, As2–O8, As3–O9, As3–O12 bond lengths of 1.53 Å, 1.50 Å, 1.52 Å, 1.700 Å, 1.670 Å, 1.712 Å, 1.672 Å, 1.686 Å, 1.723 Å respectively associated to the charge transfer interaction. The maximum difference does not exceed 0.042 Å for the bond lengths and 3.5° for the bond angles. This result shows that the cluster approach is sufficient to the analysis of the spectra in the solid state. The structure consists of infinite parallel two-dimensional planes built of mutual [H2AsO 4 ], [H3AsO4] tetrahedra and [NC3H7]+4 cations. The projection along the a axis of the atomic arrangement is depicted in Fig. 2. The coordination of arsenic atoms is tetrahedral with As–O distances in the range of [1.657(12); 1.725(12) Å]; [1.663(14); 1.674(13) Å]; [1.674(12); 1.707(11) Å]; and O–As–O angles in the range of [105.9(7)°; 113.3(5)°]; [104.1(8)°; 114.3(6)°] and [105.1(8)°; 114.8(6)°], for [H3As(1)O4]; [H3As(2)O4]2 and [H2As(3)O4], respectively (Table 3). The calculated average values of the distortion indices [27] corresponding to the different distances and angles in [AsO4] tetrahedral DI(As(1)O) = 0.0133, DI(As(2)O) = 0.0030, DI(As(3)O) = 0.0059, DI(OAs(1)O) = 0.0175, DI(OAs(2)O) = 0.0237, DI(OAs(3)O) = 0.0164, DI(OO) = 0.0142, DI(OO) = 0.0113 and DI(OO) = 0.0124 show a high distortion of the (OO) distances of [H3As(1)O4]; [H2As(2)O4]2 and [H3As(3)O4]. The structure is based on sheets
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Table 2 Comparison between the optimized geometrical parameters and the corresponding experimental data of organic part [CH3CH2CH2]4N. Parameters Bond length (Å)
Observed
Calculated B3LYP/6-31G(d)
N–C1 N–C2 N–C4 N–C3 C1–C7 C2–C8 C3–C5 C4–C6 C5–C12 C6–C11 C7–C9 C8–C10 C1–N–C2 C1–N–C3 C1–N–C4 C2–N–C3 C2–N–C4 C4–N–C3 N–C2–C8 N–C4–C6 C5–C3–N C7–C1–N C1–C7–C9 C10–C8–C2 C11–C6–C4 C12–C5–C3
1.50(2) 1.51(2) 1.54(2) 1.55(2) 1.44(3) 1.53(2) 1.51(2) 1.54(3) 1.47(3) 1.48(3) 1.53(3) 1.50(3) 110.9(13) 107.5(12) 111.5(15) 111.9(13) 109.0(12) 106.0(12) 115.3(14) 115.5(14) 115.4(15) 118.4(17) 110(2) 109.2(17) 109.7(17) 109.4(15)
1.53 1.52 1.52 1.50 1.50 1.53 1.52 1.50 1.51 1.52 1.52 1.52 108.8 106.5 111.4 111.7 110.4 108.0 115.5 118.0 115.5 115.6 109.7 109.0 109.2 109.8
Table 3 Comparison between the optimized geometrical parameters and the corresponding experimental data of inorganic part (H2AsO 4 ) (H3AsO4)2. Parameters Bond length (Å)
Observed
Calculated B3LYP/6-31G(d)
As1–O1 As1–O2 As1–O3 As1–O4 O1–As1–O2 O1–As1–O3 O2–As1–O3 O1–As1–O4 O2–As1–O4 O3–As1–O4 As2–O5 As2–O6 As2–O7 As2–O8 O8–As2–O6 O8–As2–O7 O6–As2–O7 O8–As2–O5 O6–As2–O5 O7–As2–O5 As3–O9 As3–O10 As3–O11 As3–O12 O11–As3–O12 O11–As3–O10 O12–As3–O10 O11–As3–O9 O12–As3–O9 O10–As3–O9
1.658(12) 1.664(13) 1.681(12) 1.728(13) 113.3(5) 110.1(6) 109.3(7) 107.3(7) 110.6(6) 105.9(7) 1.671(13) 1.668(14) 1.663(11) 1.662(15) 111.9(7) 109.2(8) 110.0(7) 107.3(10) 104.1(8) 114.3(6) 1.696(12) 1.686(14) 1.672(12) 1.683(13) 114.8(6) 109.1(7) 105.1(8) 109.6(6) 108.7(6) 109.3(7)
1.700 1.701 1.670 1.750 113.2 108.9 110.7 108.8 113.2 108.7 1.712 1.700 1.701 1.672 111.0 111.2 112.4 108.2 104.1 112.4 1.686 1.728 1.681 1.723 112.9 110.9 108.1 112.1 112.1 110.88
of H2AsO 4 and H3AsO4 tetrahedra bonded together by strong intralayer O–H O hydrogen bonds, giving rise to trimmers made up of [(H3AsO4)2 H2AsO4], (dO–O < 2.73 Å) [28,29] as seen in Fig. 3. These hydrogen bonds contribute to the cohesion of the structure. The various hydrogen bond parameters are summarized in Table 4.
Fig. 2. Projection along the a axis of the atomic arrangement of [(CH3CH2CH2)]4N (H2AsO4) (H3AsO4)2.
The midplanes of these arsenates groups, are located at z = 0.25 and z = 0.75. The organic cation are located at z = 0.25 and z = 0.75. 0 The lengths of the N–C bonds are between 1.50(2) and 1.55(2) Å A and the C–N–C angles range from 106(12)° to 111.9(13)°. The C–C bonds lengths are in the region between 1.44(3) and 1.54(3). The final coordinates and U(eq)or U(iso) are given in Table S2 (Supplementary information).
Vibrational analysis In order to give more information on the crystal structure, we have studied the vibrational properties of our compound using Raman scattering and infrared absorption. Taking into account the effect of intermolecular interactions on geometrical parameters, we have considered the cluster built up from two neutral arsenic acids [H3AsO4], one [H2AsO4] anion linked by O–H O hydrogen bonds and one tetrapropylammonium cation. All the parameters were allowed to relax and all the calculations converged to an optimized geometry which corresponds to an energy minimum as revealed by the lack of imaginary values in the calculated wave numbers. The vibrational spectral analysis is performed based on the characteristic vibration of the arsenate groups and organic cation. The vibrational modes were conducted by the visual inspection of modes animated by using Molekel program [30] and by comparison with the previous theoretical and experimental results reported in the literature for similar compounds. The computed wavenumbers corresponding to different modes along with detailed assignments are listed in Table 5. For visual comparison, the observed and simulated Raman spectra are presented in Fig. 4. The recorded IR spectrum is presented in Fig. 5.
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by comparison with similar organic arsenate compounds, a distinction has been made between the bands originating from the vibrations of arsenate groups and the organic cation. The medium band at 907 cm1 in Raman spectrum is assigned to the asymmetric stretching m3 mode of the arsenate ion. In IR spectrum, this mode also appears as a single medium band at 919 cm1. The theoretically computed values by the B3LYP/6-31G(d,p) method at 913 cm1 show an excellent agreement with the experimental data. The band due to the symmetric stretching mode m1 vibration is located in Raman spectrum at 830 cm1 as a strong band. In IR spectrum this mode appears as a medium band at 813 cm1. The bands due to the symmetric and asymmetric bending vibration m2 and m4 of the arsenate ion are identified in the 400–300 cm1 and 550–400 cm1 frequency regions, respectively. The very strong band observed in the Raman spectrum at 417 cm1 and the strong band at 433 cm1 in IR spectrum can be easily assigned to m4 mode, whereas the symmetric mode m2 appears in the Raman spectrum as a shoulder band at 372 cm1. As seen in Table 5, all modes associated with the arsenates groups are well reproduced by theoretical method. The tetrapropylammonium cation vibrations Numerous functional and skeletal groups such as CH2, CH3, N–C, C–C, CCC, NC4 are present in tetrapropylammonium cation. These groups are manifested in IR and Raman spectra in different ranges with different intensities.
Fig. 3. Projection along the a axis of the inorganic arrangement of [(C3H7)4N] (H2AsO4) (H3AsO4)2.
Table 4 Bond lengths (Å) and bond angles (°) in the hydrogen bonding scheme of [(CH3CH2CH2)4N](H2AsO4)(H3AsO4)2. D–H A
d(D–H) (Å)
d(H A) (Å)
d(D A) (Å)
\D–H A (°)
O(1)–H(1) O(8) O(2)–H(2) O(6) O(5)–H(5) O(11)i O(6)–H(6) O(2) O(10)–H(10) O(7) O(12)– H(12) O(7)ii
0.79 0.85 0.95 0.82 0.81 0.87
1.83 2.02 2.01 1.79 1.90 2.25
2.49(2) 2.56(2) 2.68 (2) 2.56(2) 2.54(2) 2.57(2)
139.6 120.6 125.8 154.6 135.1 101.6
Symmetry code: (i) 1+x, y, z. Symmetry code: (ii) 1/2+x, y, z.
Arsenate groups vibrations The vibrational analysis of an isolated arsenate anion AsO3 4 with Td point group symmetry led to four Raman active normal modes: m1(A1), m2(E), m3(F2) and m4(F2) with average wavenumbers 837, 349, 887 and 463 cm1, respectively [31]. m1 and m3 involve the symmetric and the asymmetric stretching mode of the As–O bond, whereas m2 and m4 involve mainly O–As–O symmetric and asymmetric bending modes. In the title compound, the symmetry of AsO3 4 ions was reduced from Td to C1. In fact, our structural study shows that the As–O distances and the O–As–O angles are distorted with respect to the hypothetical Td symmetry. This symmetry change partially removes the degeneracy of the vibrational wave functions, which would characterize free AsO3 4 . In the 1000–300 cm1 region, the AsO3 stretching and bending vibra4 tions expected to appear, as well as the modes associated with the internal modes of the organic cations. However, in the light of the present calculations as primary sources of assignment and
Methyl vibrations. The asymmetric and symmetric CH3 stretching vibrations are usually observed in the region 3000–2850 cm1 [32]. The weak band at 2987 cm1 in IR spectrum is attributed to the asymmetric stretching mas(CH3) vibration. The Raman counterpart is located as a very strong band at 2998 cm1. The symmetric stretching mode ms(CH3) is observed as a shoulder band at 2897 cm1 in Raman spectrum. In IR spectrum, this mode appears as a very weak intensity band at the 2877 cm1. The asymmetric and symmetric bending modes of the methyl group generally appear in the region 1550–1410 and 1400–1340 cm1, respectively. In the present case, the medium IR band at 1489 cm1 is assigned to CH3 asymmetric bending mode and the corresponding Raman band appears as a strong band at 1462 cm1. The symmetric bending mode ds(CH3) is located at 1392 cm1 as a weak band in the IR spectrum and as a very weak band at 1387 cm1 in Raman spectrum. The methyl rocking vibration is observed as a weak band at 997 cm1 in IR spectrum and at 1001 cm1 as a medium one in Raman spectrum. As shown in Table 5, all calculated wavenumbers related to methyl groups agree well with the experimental values. On the other hand, it is worthwhile to note that the vibrational modes of the CH3 do not deviate much from their expected values, suggesting that the interaction of the groups with the environment is not strong. Methylene group vibrations. The wavenumber of the CH2 vibrational modes depend on its immediate environment. The stretching modes of the CH2 group usually occur in the region 3100–2800 cm1 [33]. In the title compound, the CH2 asymmetric and symmetric stretching modes are observed in Raman spectrum at 2946 cm1 as a very strong band and at 2824 cm1 as a very weak band. In IR spectrum, these modes are probably masked by the broad band around 2900 cm1. The wagging, twisting and the rocking modes of the CH2 group were observed and assigned. NC4 group vibrations. The primitive unit cell of the title compound contains one tetrapropylammonium (NC4)+ cation that is not coupled with the inorganic parts as revealed by the X-ray diffraction. Afterwards, it is convenient as a first approach to consider the NC4 core of the isolated cation with Td symmetry, which exhibits
I. Dhouib et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 274–281 Table 5 Observed and calculated wavenumbers (cm1) of tetrapropylammonium dihydrogenmonoarsenate bis arsenic acid with the proposed assignments. Observed FT-IR
FT-Raman
3660 s,b 3453 w – 2987 w – 2877 vw – – 2386 b,s 1730 w,b 1620 w,b 1489 m 1392 w 1319 w 1288 w 1183 w 1146 w 1122 w 1032 w 997 w 919 m 813 m 786 b,sh – – 433 s – – – – –
– 3473 b 3217 b 2998 vs 2946 vs 2897 sh 2824vw 2753 vw – – – 1462 s 1387 vw 1319 vw 1295 s 1173 w 1151 w 1117 s 1044 vw 1001 m 907 m 830 s 787 w 745 m 676 s 417 vs 372 sh 325 m 283 m 114 m 55 w
Calculated B3LYP/6-31G⁄
Assignments
– 3595 3322 3037 2980 2845 2833 2779 – – – 1466 1394 1320 1300 1183 1163 1127 1091 1019 913 799 777 714 615 396 361 333 275 120 66
m(OH) m(OH O) m(OH O) mas(CH3) mas(CH2) ms(CH3) ms(CH2) ms(CH3) + mas(CH2) Combination bands – – das(CH3) ds(CH3) + das(CH2) x(CH2) + d(OH) x(CH2) + d(OH) m(CC) + twist(CH2) m(CC) + d(CCC) m(NC) + d(CNC) + twist(CH2) mas(NC4) q(CH3) + d(CCC) + c(OH) mas(AsO4) ms(AsO4) + c(OH) ms(NC4) + c(OH) d(CCC) + d(CNC) + q(CH2) d(CCC) + d(CNC) + q(CH2) das(AsO4) ds(AsO4) ds(NC4) + c(CCC) s(CH3) + c(CCC) Lattice mode Lattice mode
m, stretching; d, in-plane bending; c, out of plane bending; q, rocking; x, wagging; twist, twisting; asym, asymmetric; sym, symmetric; w, weak; vw, very weak; s, strong; vs, very strong; m, medium; sh, shoulder.
four normal modes. According to the literature, the vibrations related to NC4 group shown in tetrapropylammonium cation are described as: symmetric stretching mode m1(A) to be found at 752 cm1, asymmetric stretching mode m3(F2) located at
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955 cm1, asymmetric bending mode m4(F2) observed at 455 cm1 and the symmetric bending mode m2(F2) at 372 cm1 [34,35]. In our case the very weak Raman band appearing at 1044 cm1 is related to the m3 mode. Its counterpart appears in IR spectrum as weak band at 1032 cm1. The week band observed in Raman spectrum at 787 cm1 and the shoulder one at 786 cm1 in IR spectrum arises from the symmetric stretching mode m1(NC4). The band assigned to the symmetric bending modes of NC4 core of the isolated cation is observed in Raman spectrum at 325 cm1 with no counterpart in IR spectrum. The asymmetric band m2(NC4) is not observed in IR spectrum and probably masked by the very intense band at 417 cm1, assigned to das(AsO4) in Raman spectrum. It is worthy to note that the vibrational mode of the NC4 group does not deviate much from its expected values, suggesting that the interaction of this group with the environment is not strong. On the other hand, we note that our assignment of NC4 agrees well with the previous reported vibrational studies of the tetrapropylammonium salts [13]. N–C, C–C, C–C–C and C–C–N group of vibrations. The absorption bands arising from N–C symmetric stretching vibrations are observed in the wavenumber region 850–1150 cm1. In the present crystal, the strong Raman band at 1117 cm1 is assigned to this mode. Its IR counterpart is identified at 1122 cm1. The corresponding theoretical value is at 1127 cm1. The absorption bands arising from the C–N–C stretching mode is observed in IR spectrum at 1122 cm1 and in Raman spectrum at 1117, 745 and 676 cm1. The weak bands appearing at 1183 and 1146 cm1 in IR spectrum and at 1173 and 1151 cm1 in Raman are related to C–C stretching mode. The C–C–C vibrational bands have been identified. Table 5 presents a detailed assignment of all observed band related to the organic cation. The hydrogen bond vibrations. It is well known that hydrogen bonding brings about remarkable wavenumber shifts. Whereas the intermolecular hydrogen bonds give rise to broad bands, intramolecular hydrogen bonds produce sharp and well resolved bands. Knowing the bond length, the strength of the hydrogen bond can be determined as very strong (below 2.5 Å), strong (2.5–2.7 Å),
Fig. 4. Raman spectrum of [(CH3CH2CH2)]4N (H2AsO4) (H3AsO4)2 observed (a) and calculated (b).
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Fig. 5. IR spectrum of [(CH3CH2CH2)4]N (H2AsO4) (H3AsO4)2.
normal (2.7–2.9 Å) and weak (above 2.9 Å). X-ray diffraction analysis of the title compound reveals that the structure is based on sheets of H2AsO 4 and H3AsO4 tetrahedra bonded together by strong O–H O hydrogen bonds intralayers, leading to the formation of trimmers. The O–H O bonds in the unit cell are six in number and their bond lengths are in the range of 2.49(2)– 2.68(2) Å. The inspection of the IR spectrum reveals a broad and strong band at 3660 cm1, which may be assigned to the stretching mode m(OH) which is not implied in hydrogen bonding. The weak IR band at 3453 cm1 and the two broad bands in Raman spectrum located at 3473 and 3217 cm1 are accredited to m(O–H O) mode, while the B3LYP/6-31G(d) calculation gives rise to their positions at 3595 and 3322 cm1. The wavenumbers of this mode is shifted towards lower values. In fact, the XRD study of the title compound reveals that the structure is based on sheets of H2AsO 4 and H3AsO4 tetrahedra bonded together by strong intralayer O–HO hydrogen bonds, giving rise to trimmers made up of [(H3AsO4)2H2AsO4]. The in-plane OH bending mode d(OH) leads to a medium band in infrared spectrum at 1319 and 1278 cm1 with a counterpart in Raman spectrum at 1319 and 1295 cm1. The out-of-plane bending c(OH) mode appears in the region 900–700 cm1. As seen in m(O–H O) stretching, wavenumbers are clearly lower than the calculated values and the bending wavenumbers are not much different from the expected range of 1380–1280 cm1, which indicates that the linear distortion is much greater than the angular distortion.
NLO properties Since the values of the polarizability atot and hyperpolarizability btot given by Gaussian 03 are reported in atomic units (a.u.). The B3LYP/6-31G(d) results of the electric dipole moment li (i = x, y, z), polarizability aij and the first hyperpolarizability bijk of the title compound are listed in Table S1. The calculated dipole moment is equal to 39.892 D. Furthermore, the highest absolute value of the dipole moment is observed for the component lx. In this direction, the value is equal to 38.945. The calculated polarizability atot, is equal to 31.56 1024 esu. As can be seen, the first hyperpolarizability btot of the compound has for value 100.47 1031 eus, which is about 14.6 times more than that of the reference crystal KDP (bKDP = 6.85 1031 eus). The above results show that the title compound can be a good material for NLO applications.
Thermal analysis (DSC and ATG) The DSC–ATG of the title compound was conducted in air at the rate of 5 °C/min. One more characteristic feature of the room temperature is its high hygroscopicity. Figs. S1 and S2 illustrate the calorimetric (DSC) and thermogravimetric (TGA) results. In fact, this compound is stable until 340 K, above which a loss of weight appears at 370 K, which is due to the departure of adsorbed water. The endothermic peak observed at T = 396 K is attributed to the melting of the crystal. Conclusion Crystals of a hybrid material of the formulae N(C3H7)4, (H2AsO4) and (H3AsO4)2, have been prepared by the slow evaporation of aqueous solution (CH3CH2CH2)4NOH and H3AsO4 at room temperature and characterized by means of single-crystal X-ray diffraction, thermal analysis, FT-IR and Raman vibrational study. The crystal formed is non-centrosymmetric. The structure consists of a strong two-dimensional character based on sheets of H2AsO 4 and H3AsO4 tetrahedra fused together by strong intralayer O–H O hydrogen bonds, giving rise to trimmers. The planes of inorganic groups are alternated with the planes of the organic cations. The DSC measurement shows two endothermic peaks at about 370 and at 396 K. The structural and vibrational frequency analyses by DFT calculations agree well with the experimental results. Building on the agreement between the experimental and calculated results, the assignment of all vibrational modes of the title compound were examined and proposed in this investigation. The first hyperpolarizability btot of the title compound is about 14.6 times more than that of the reference crystal KDP. This may explain the importance of this compound as a very important new nonlinear optical (NLO) material. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.04.120. References [1] H.O. Marcy, L.F. Warren, M.S. Webb, C.A. Ebbers, S.P. Velsko, G.C. Kennedy, G.C. Catella, Appl. Opt. 31 (1992) 5051.
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