Communication: Spectroscopic characterization of an alkyl substituted Criegee intermediate syn-CH3CHOO through pure rotational transitions Masakazu Nakajima and Yasuki Endo Citation: The Journal of Chemical Physics 140, 011101 (2014); doi: 10.1063/1.4861494 View online: http://dx.doi.org/10.1063/1.4861494 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/140/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Communication: Determination of the molecular structure of the simplest Criegee intermediate CH2OO J. Chem. Phys. 139, 101103 (2013); 10.1063/1.4821165 The microwave and millimeter spectrum of ZnCCH ( X ̃ 2Σ+): A new zinc-containing free radical J. Chem. Phys. 136, 244310 (2012); 10.1063/1.4729943 Rotational spectroscopy and molecular structure of the 1-chloro-1-fluoroethylene-acetylene complex J. Chem. Phys. 134, 034303 (2011); 10.1063/1.3517494 The nature of the complex formed between pyridine and hydrogen bromide in the gas phase: An experimental approach using rotational spectroscopy J. Chem. Phys. 121, 10467 (2004); 10.1063/1.1809577 Microwave spectroscopic and ab initio studies of the hydrogen-bonded trimethylamine–hydrogen sulfide complex J. Chem. Phys. 107, 2227 (1997); 10.1063/1.474619
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 143.160.36.177 On: Thu, 18 Dec 2014 10:31:23
THE JOURNAL OF CHEMICAL PHYSICS 140, 011101 (2014)
Communication: Spectroscopic characterization of an alkyl substituted Criegee intermediate syn-CH3 CHOO through pure rotational transitions Masakazu Nakajima and Yasuki Endoa) Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan
(Received 26 November 2013; accepted 23 December 2013; published online 6 January 2014) An alkyl-substituted Criegee intermediate syn-CH3 CHOO was detected in the gas phase through Fourier-transform microwave spectroscopy. Observed pure rotational transitions show a small splitting corresponding to the A/E components due to the threefold methyl internal rotation. The rotational constants and the barrier height of the hindered methyl rotation were determined to be A = 17 586.5295(15) MHz, B = 7133.4799(41) MHz, C = 5229.1704(40) MHz, and V3 = 837.1(17) cm−1 . High-level ab initio calculations which reproduce the experimentally determined values well indicate that the in-plane C–H bond in the methyl moiety is trans to the C–O bond, and other two protons are directed to the terminal oxygen atom for the most stable structure of synCH3 CHOO. The torsional barrier of the methyl top is fairly large in syn-CH3 CHOO, implying a significant interaction between the terminal oxygen and the protons of the methyl moiety, which may be responsible for the high production yields of the OH radical from energized alkyl-substituted Criegee intermediates. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4861494] In ozonolysis processes of alkenes, a primary organic ozonide, which is produced by the cycloaddition of O3 to a double bond of alkenes, decomposes to a carbonyl compound and a carbonyl oxide, R1 R2 COO, which is known as a Criegee intermediate (CI).1, 2 Since ozonolysis is considered to be an important removal process of alkenes in the troposphere,3 atmospheric significance of its product, CI, has been discussed for decades. Nascent CIs produced from ozonolysis of alkenes are considered to have a large amount of internal energy, because ozonolysis reactions are highly exothermic. One of the atmospherically relevant fates of such energized CIs is unimolecular decomposition yielding the OH radical that is an important initiator of oxidation for atmospheric trace constituents. For the simplest CI, CH2 OO, such OH production is considered to occur dominantly through the cyclic isomerization to dioxirane H2 C − O − O, which further rearranges to vibrationally excited formic acid decomposing to OH + H + CO. A minor pathway producing the hydroxyl radical is the dissociation to OH + HCO through a direct proton migration to the terminal oxygen atom via a four-membered transition state (TS).4 The situation in vibrationally “hot” alkylsubstituted CIs is different from that of CH2 OO. The most important pathway to produce OH is believed to involve a proton migration via a five-membered TS,4, 5 giving a hydroperoxide ˙ which quickly dissociates to OH and a sub(R˙ 1 R2 CO−OH) stituted vinoxy radical. Since such a five-membered TS usually requires lower activation energy than that for the fourmembered TS and the cyclic isomerization to dioxirane, the direct proton migration should be a more efficient pathway for the OH production, responsible for the high OH yields in ozonolysis reactions of alkyl-substituted alkenes.6 a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0021-9606/2014/140(1)/011101/4/$30.00
The simplest alkyl-substituted CI is acetaldehyde oxide, CH3 CHOO, which has two conformers, syn and anti. Since interconversion between them does not readily occur due to the double bond nature of the CO bond of CH3 CHOO, only a proton in the methyl moiety of the syn-conformer is subject to the migration to the terminal oxygen through the fivemembered TS. The probability of such a proton migration in syn-CH3 CHOO is considered to be largely dependent on the magnitude of the interaction between the protons of the methyl moiety and the terminal oxygen atom. This interaction is considered to make the methyl top more hindered, resulting in a high torsional barrier of the methyl internal rotation. An experimental determination of the barrier height of the methyl internal rotation in syn-CH3 CHOO thus attracts our attention in association with the OH production efficiency. Gas-phase spectroscopic characterization for alkyl substituted CIs will thus give us valuable information not only on their geometries but also on their chemistry. The first gas-phase observation of CH3 CHOO was reported very recently by Taatjes et al. using a mass spectrometric technique combined with isomer/conformer selective ionization by a tunable vacuum ultraviolet (UVV) light source.7 They detected the syn- and anti-conformers selectively and demonstrated its conformer-specific reactivity with atmospherically relevant compounds. Only one study has been reported for spectroscopic characterization of gas-phase CH3 CHOO by Beames et al. through UV absorption spectroscopy. However, the spectrum shows only a broad feature without vibrational structure, reflecting the dissociative potential energy surface of the upper electronic state.8 Therefore, vibrational or pure rotational spectroscopy is required in obtaining more detailed information on CH3 CHOO. Such observations have been already reported for the simplest CI, CH2 OO, in the IR9 and MW10, 11 regions. In this
140, 011101-1
© 2014 AIP Publishing LLC
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 143.160.36.177 On: Thu, 18 Dec 2014 10:31:23
011101-2
M. Nakajima and Y. Endo
J. Chem. Phys. 140, 011101 (2014) TABLE I. Experimental and theoretical molecular parameters of CH3 CHOO. Syn Expt.a A (MHz) B (MHz) C (MHz) J (kHz) JK (kHz) K (kHz) δ J (kHz) δ K (kHz) V3 c (cm−1 ) F0 d (GHz) δ e (rad) σ fit (kHz)
17 586.5295(15) 7133.4799(41) 5229.1704(40) 6.729(42) − 20.44(26) 56.15(37) 2.239(11) 7.0(10) 837.1(17) 158.02 (fixed) 0.9212(61) 3.8
Ab initiob
Anti Ab initiob
17 613 7192 5269
48 855 4452 4185
748 158.02 0.973
402 159.39 0.361
a
FIG. 1. Most stable structures of the syn- and anti-conformers of CH3 CHOO.
Result of a least-squares fit using the XIAM program (Ref. 19). Values in parentheses denote 1σ errors, applied to last digits. Values obtained with CCSD(T)-F12a/aug-cc-pVTZ. c Barrier height of the threefold methyl internal rotation. d Rotational constant of the methyl top. e Angle between the principal axis of the molecule and the internal rotation axis.
Communication, we have successfully extended our previous experimental technique to the study on the syn-conformer of CH3 CHOO. The rotational constants and the barrier height of the threefold methyl internal rotation were experimentally determined for this important CI. Prior to a spectral survey, geometrical optimizations of the syn- and anti-conformers were carried out on the ground 1 A potential energy surfaces assuming a Cs geometry using the Molpro 2012.1 program package,12 in order to predict their rotational transition frequencies. Optimized geometrical parameters at the CCSD(T)-F12a/aug-cc-pVTZ level of theory are shown in Table S-I in the supplementary material.13 As shown in Fig. 1, the in-plane C–H bond is directed trans to the C–O bond for the optimized structure of the synconformer, and cis for the anti-conformer, consistent with the results of previous theoretical calculations.8, 14 The synconformer is calculated to be more stable than the anticonformer by 1300 cm−1 without a zero-point energy correction. The methyl torsional barriers are theoretically obtained to be 748 and 402 cm−1 for the syn- and anti-conformers, respectively. Rotational constants derived from the optimized geometries are listed in Table I. The reactive transient species, syn-CH3 CHOO, was produced in a supersonic jet by a pulsed electric discharge15 of a gas mixture of 1,1-diiodoethane and O2 diluted in Ar. The premix gas of 2% O2 and 98% Ar with the total pressure of 3 atm was passed through a liquid container of 1,1-diiodoethane in order to introduce the precursor molecules with a sufficient partial pressure. In our previous study on CH2 OO,10 the intermediate was produced using the dibromomethane precursor. Although syn-CH3 CHOO could be similarly produced using the 1,1-dibromoethane precursor, the signal intensity was 20–30 times weaker than that obtained with 1,1-diiodoethane. A Balle-Flygare type Fourier-transform microwave (FTMW) spectrometer16 operated in the frequency region of 4–40 GHz was used for the observation of pure rotational transitions of
syn-CH3 CHOO. An FTMW-millimeter wave (mmW) doubleresonance technique17 was also used for observing pure rotational transitions and confirming the assignments of the observed FTMW spectra. The threefold methyl internal rotation in syn-CH3 CHOO makes its pure rotational transitions split into two components, A and E, as the result of the interaction between molecular overall and internal rotations.18 A total of 50 lines including the A and E components of 25 pure rotational transitions were observed for syn-CH3 CHOO in the present study, as summarized in Fig. 2, and their frequencies are listed in Table S-II in the supplementary material.13 A typical FTMW spectrum is shown in Fig. 3(a). The A/E splitting is small, less than 200 kHz for all the observed transitions. Figure 3(b) shows FTMW-mmW double-resonance spectra of the A and E components of 313 − 202 , which were observed by monitoring the FTMW signals of the A and E components of 202 − 101 , respectively. Using the double-resonance technique, rotational assignments including the A/E-splitting could be unambiguously made for the observed lines. All the observed transition frequencies were included in a least-squares fit using the program XIAM19 coded by Hartwig based on the combined axis method developed by Woods.20 The resulting spectroscopic parameters are shown in Table I. The Watson’s A-reduced Hamiltonian was used for calculating overall rotational energies. The standard deviation of the fit is 3.8 kHz, comparable to the expected experimental error. The F0 value which is associated with the moment of inertia of the methyl top could not be determined in the present fit, probably because the observed A/E-splittings are too small. Therefore, it was fixed to the value derived from the ab initio geometry of syn-CH3 CHOO, 158.02 GHz. Since the determined rotational constants agree well with the ab initio values of the syn-conformer of CH3 CHOO, the observed transitions are confirmed to be arising from syn-CH3 CHOO. The δ value, which is the angle between the molecular
b
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 143.160.36.177 On: Thu, 18 Dec 2014 10:31:23
011101-3
M. Nakajima and Y. Endo
FIG. 2. Transitions observed in the present study. Red lines correspond to the transitions observed by FTMW spectroscopy, and blues are those observed by FTMW-mmW double-resonance spectroscopy.
principal axis and the internal rotation axis of the methyl top, is also consistent with that for ab initio geometry. As expected from the observed small A/E splittings, the determined barrier height of the threefold internal rotation V3 is quite high, 837.2 (17) cm−1 , which reasonably agrees with the ab initio prediction. Barrier heights of the methyl internal rotation in small molecules with only one methyl rotor are roughly 400 cm−1 or less for many molecules, e.g., CH3 CHO21 (V3
FIG. 3. Rotational transitions of syn-CH3 CHOO. (a) FTMW spectrum of the 202 − 101 transition. The observed lines are split into two Doppler components because the direction of the supersonic jet expansion is parallel to the standing wave in the Fabry-Pérot cavity of the spectrometer. The feature marked with an asterisk is an artifact. (b) FTMW-mmW double resonance spectra of 313 − 202 observed by monitoring the FTMW signals of the A or E component of 202 − 101 . The direction of irradiated mmW was perpendicular to that of the supersonic expansion.
J. Chem. Phys. 140, 011101 (2014)
= 407.7 cm−1 ), CH3 OH22 (V3 = 373.6 cm−1 ), and CH3 OO23 (V3 = 327.6 cm−1 ). In the present experiment, there could be a chance for anti-CH3 CHOO to be produced in the discharged jet simultaneously with syn-CH3 CHOO, although the syn-conformer is more stable than the anti-conformer by 1300 cm−1 , based on the present ab initio result. In fact, recent experimental studies on CH3 CHOO, which was produced from the same precursors CH3 CHI2 and O2 , estimated that roughly 3% and 10% of CH3 CHOO existed as the anti-conformer under jetcooled8 and flow cell7 conditions, respectively. We thus surveyed rotational transitions of the anti-conformer by FTMW spectroscopy, and easily observed several transitions arising from anti-CH3 CHOO with moderate intensities. Under our experimental condition, the anti-conformer was estimated to be produced with the amount ∼20% of the syn-conformer, based on the observed spectral intensities. In contrast to synCH3 CHOO, large A/E-splittings were observed in the rotational spectra of the anti-conformer (the largest is more than 1.5 GHz), and it is concluded that a least-squares fitting using the program XIAM19 is unsatisfactory because the residuals in the fit were significantly larger than the expected experimental errors for several observed line positions. Although the measurements of the anti-CH3 CHOO spectra and the rotational analysis are still in progress and the details will be reported elsewhere, our preliminary least-squares fit using the program XIAM gives the rotational constants of the anticonformer, A = 48 494, B = 4435, and C = 4167 MHz, which are very close to the ab initio values listed in Table I. The V3 barrier height obtained in the analysis is roughly 400 cm−1 for the anti-conformer, which is comparable to those of CH3 CHO, CH3 OH, and CH3 OO. The experimentally determined barrier height for syn-CH3 CHOO is more than twice as large as their values, clearly indicating that the methyl top in syn-CH3 CHOO is strongly hindered by the interaction between the protons in the methyl moiety and the terminal oxygen atom. The interaction responsible for the strong hindrance to the methyl torsional motion should be an attractive interaction but not repulsive one, because the ab initio calculations show that the syn-conformer is more stable than the anti-conformer. The ab initio equilibrium geometry of syn-CH3 CHOO shows that two protons in the methyl moiety are directed to the terminal oxygen that has three lone-pairs in a zwitterionic canonical structure, R1 R2 C = O⊕ − O , which is considered to significantly contribute to the electronic structure at least in CH2 OO.10 The three lone-pairs and the σ -bonding electrons on the terminal oxygen atom are spatially distributed as the electron repulsion becomes minimum, that is, occupy the valence sp3 orbitals. Then, two of the three lone-pairs seem to be interacting with two protons of the methyl moiety in the most stable geometry of syn-CH3 CHOO. This attractive interaction is considered to promote the proton migration from the methyl moiety to the terminal oxygen atom through the five-membered TS, resulting in high OH yield from energized syn-CH3 CHOO. Similar interactions can also be expected in syn-conformers of other singly alkyl-substituted CIs and doubly substituted CIs, responsible for the efficient OH productions in ozonolysis reactions of alkyl-substituted alkenes.6
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 143.160.36.177 On: Thu, 18 Dec 2014 10:31:23
011101-4
M. Nakajima and Y. Endo
The present study was supported by JSPS KAKENHI Grant Nos. 25410004 (M.N. and Y.E.) and 19205002 (Y.E.). 1 R.
Criegee and G. Wenner, Justus Liebig Ann. Chem. 564, 9 (1949). Criegee, Angew. Chem., Int. Ed. Engl. 14, 745 (1975). 3 R. Atkinson and J. Arey, Chem. Rev. 103, 4605 (2003). 4 D. Johnson and G. Marston, Chem. Soc. Rev. 37, 699 (2008). 5 H. Niki, P. D. Maker, C. M. Savage, L. P. Breitenbach, and M. D. Hurley, J. Phys. Chem. 91, 941 (1987). 6 J. H. Kroll, T. F. Hanisco, N. M. Donahue, K. L. Demerjian, and J. G. Anderson, Geophys. Res. Lett. 28, 3863, doi:10.1029/2001GL013406 (2001); J. H. Kroll, J. S. Clarke, N. M. Donahue, J. G. Anderson, and K. L. Demerjian, J. Phys. Chem. A 105, 1554 (2001); J. H. Kroll, N. M. Donahue, V. J. Cee, K. L. Demerjian, and J. G. Anderson, J. Am. Chem. Soc. 124, 8518 (2002). 7 C. A. Taatjes, O. Welz, A. J. Eskola, J. D. Savee, A. M. Scheer, D. E. Shallcross, B. Rotavera, E. P. F. Lee, J. M. Dyke, D. W. K. Mok, D. L. Osborn, and C. J. Percival, Science 340, 177 (2013). 8 J. M. Beames, F. Liu, L. Lu, and M. I. Lester, J. Chem. Phys. 138, 244307 (2013). 9 Y.-T. Su, Y.-H. Huang, H. A. Witek, and Y.-P. Lee, Science 340, 174 (2013). 2 R.
J. Chem. Phys. 140, 011101 (2014) 10 M.
Nakajima and Y. Endo, J. Chem. Phys. 139, 101103 (2013). C. McCarthy, L. Cheng, K. N. Crabtree, O. Martinez, T. L. Nguyen, C. C. Womack, and J. F. Stanton, J. Phys. Chem. Lett. 4, 4133 (2013). 12 H.-J. Werner, P. J. Knowles, G. Knizia, F. R. Manby, M. Schütz et al., MOLPRO, version 2012.1, a package of ab initio programs, 2012, see http://www.molpro.net. 13 See supplementary material at http://dx.doi.org/10.1063/1.4861494 for the ab initio geometries and the observed transition frequencies. 14 K. T. Kuwata, M. R. Hermes, J. Carlson, and C. K. Zogg, J. Phys. Chem. A 114, 9192 (2010). 15 Y. Endo, H. Kohguchi, and Y. Ohshima, Faraday Discuss. 97, 341 (1994). 16 T. J. Balle and W. H. Flygare, Rev. Sci. Instrum. 52, 33 (1981). 17 Y. Sumiyoshi, H. Katsunuma, K. Suma, and Y. Endo, J. Chem. Phys. 123, 054324 (2005). 18 W. Gordy and R. L. Cook, Microwave Molecular Spectra (John Wiley Sons, New York, 1984). 19 H. Hartwig and H. Dreizler, Z. Naturforsch. 51a, 923 (1996). 20 R. C. Woods, J. Mol. Spectrosc. 21, 4 (1966). 21 I. Kleiner, J. T. Hougen, J.-U. Grabow, S. P. Belov, M. Yu. Tretyyakov, and J. Cosléou, J. Mol. Spectrosc. 179, 41 (1996). 22 L.-H. Xu and J. T. Hougen, J. Mol. Spectrosc. 173, 540 (1995). 23 K. Katoh, Ph.D. thesis, the University of Tokyo, 2007. 11 M.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 143.160.36.177 On: Thu, 18 Dec 2014 10:31:23
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 143.160.36.177 On: Thu, 18 Dec 2014 10:31:23
THE JOURNAL OF CHEMICAL PHYSICS 140, 011101 (2014)
Communication: Spectroscopic characterization of an alkyl substituted Criegee intermediate syn-CH3 CHOO through pure rotational transitions Masakazu Nakajima and Yasuki Endoa) Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan
(Received 26 November 2013; accepted 23 December 2013; published online 6 January 2014) An alkyl-substituted Criegee intermediate syn-CH3 CHOO was detected in the gas phase through Fourier-transform microwave spectroscopy. Observed pure rotational transitions show a small splitting corresponding to the A/E components due to the threefold methyl internal rotation. The rotational constants and the barrier height of the hindered methyl rotation were determined to be A = 17 586.5295(15) MHz, B = 7133.4799(41) MHz, C = 5229.1704(40) MHz, and V3 = 837.1(17) cm−1 . High-level ab initio calculations which reproduce the experimentally determined values well indicate that the in-plane C–H bond in the methyl moiety is trans to the C–O bond, and other two protons are directed to the terminal oxygen atom for the most stable structure of synCH3 CHOO. The torsional barrier of the methyl top is fairly large in syn-CH3 CHOO, implying a significant interaction between the terminal oxygen and the protons of the methyl moiety, which may be responsible for the high production yields of the OH radical from energized alkyl-substituted Criegee intermediates. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4861494] In ozonolysis processes of alkenes, a primary organic ozonide, which is produced by the cycloaddition of O3 to a double bond of alkenes, decomposes to a carbonyl compound and a carbonyl oxide, R1 R2 COO, which is known as a Criegee intermediate (CI).1, 2 Since ozonolysis is considered to be an important removal process of alkenes in the troposphere,3 atmospheric significance of its product, CI, has been discussed for decades. Nascent CIs produced from ozonolysis of alkenes are considered to have a large amount of internal energy, because ozonolysis reactions are highly exothermic. One of the atmospherically relevant fates of such energized CIs is unimolecular decomposition yielding the OH radical that is an important initiator of oxidation for atmospheric trace constituents. For the simplest CI, CH2 OO, such OH production is considered to occur dominantly through the cyclic isomerization to dioxirane H2 C − O − O, which further rearranges to vibrationally excited formic acid decomposing to OH + H + CO. A minor pathway producing the hydroxyl radical is the dissociation to OH + HCO through a direct proton migration to the terminal oxygen atom via a four-membered transition state (TS).4 The situation in vibrationally “hot” alkylsubstituted CIs is different from that of CH2 OO. The most important pathway to produce OH is believed to involve a proton migration via a five-membered TS,4, 5 giving a hydroperoxide ˙ which quickly dissociates to OH and a sub(R˙ 1 R2 CO−OH) stituted vinoxy radical. Since such a five-membered TS usually requires lower activation energy than that for the fourmembered TS and the cyclic isomerization to dioxirane, the direct proton migration should be a more efficient pathway for the OH production, responsible for the high OH yields in ozonolysis reactions of alkyl-substituted alkenes.6 a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0021-9606/2014/140(1)/011101/4/$30.00
The simplest alkyl-substituted CI is acetaldehyde oxide, CH3 CHOO, which has two conformers, syn and anti. Since interconversion between them does not readily occur due to the double bond nature of the CO bond of CH3 CHOO, only a proton in the methyl moiety of the syn-conformer is subject to the migration to the terminal oxygen through the fivemembered TS. The probability of such a proton migration in syn-CH3 CHOO is considered to be largely dependent on the magnitude of the interaction between the protons of the methyl moiety and the terminal oxygen atom. This interaction is considered to make the methyl top more hindered, resulting in a high torsional barrier of the methyl internal rotation. An experimental determination of the barrier height of the methyl internal rotation in syn-CH3 CHOO thus attracts our attention in association with the OH production efficiency. Gas-phase spectroscopic characterization for alkyl substituted CIs will thus give us valuable information not only on their geometries but also on their chemistry. The first gas-phase observation of CH3 CHOO was reported very recently by Taatjes et al. using a mass spectrometric technique combined with isomer/conformer selective ionization by a tunable vacuum ultraviolet (UVV) light source.7 They detected the syn- and anti-conformers selectively and demonstrated its conformer-specific reactivity with atmospherically relevant compounds. Only one study has been reported for spectroscopic characterization of gas-phase CH3 CHOO by Beames et al. through UV absorption spectroscopy. However, the spectrum shows only a broad feature without vibrational structure, reflecting the dissociative potential energy surface of the upper electronic state.8 Therefore, vibrational or pure rotational spectroscopy is required in obtaining more detailed information on CH3 CHOO. Such observations have been already reported for the simplest CI, CH2 OO, in the IR9 and MW10, 11 regions. In this
140, 011101-1
© 2014 AIP Publishing LLC
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 143.160.36.177 On: Thu, 18 Dec 2014 10:31:23
011101-2
M. Nakajima and Y. Endo
J. Chem. Phys. 140, 011101 (2014) TABLE I. Experimental and theoretical molecular parameters of CH3 CHOO. Syn Expt.a A (MHz) B (MHz) C (MHz) J (kHz) JK (kHz) K (kHz) δ J (kHz) δ K (kHz) V3 c (cm−1 ) F0 d (GHz) δ e (rad) σ fit (kHz)
17 586.5295(15) 7133.4799(41) 5229.1704(40) 6.729(42) − 20.44(26) 56.15(37) 2.239(11) 7.0(10) 837.1(17) 158.02 (fixed) 0.9212(61) 3.8
Ab initiob
Anti Ab initiob
17 613 7192 5269
48 855 4452 4185
748 158.02 0.973
402 159.39 0.361
a
FIG. 1. Most stable structures of the syn- and anti-conformers of CH3 CHOO.
Result of a least-squares fit using the XIAM program (Ref. 19). Values in parentheses denote 1σ errors, applied to last digits. Values obtained with CCSD(T)-F12a/aug-cc-pVTZ. c Barrier height of the threefold methyl internal rotation. d Rotational constant of the methyl top. e Angle between the principal axis of the molecule and the internal rotation axis.
Communication, we have successfully extended our previous experimental technique to the study on the syn-conformer of CH3 CHOO. The rotational constants and the barrier height of the threefold methyl internal rotation were experimentally determined for this important CI. Prior to a spectral survey, geometrical optimizations of the syn- and anti-conformers were carried out on the ground 1 A potential energy surfaces assuming a Cs geometry using the Molpro 2012.1 program package,12 in order to predict their rotational transition frequencies. Optimized geometrical parameters at the CCSD(T)-F12a/aug-cc-pVTZ level of theory are shown in Table S-I in the supplementary material.13 As shown in Fig. 1, the in-plane C–H bond is directed trans to the C–O bond for the optimized structure of the synconformer, and cis for the anti-conformer, consistent with the results of previous theoretical calculations.8, 14 The synconformer is calculated to be more stable than the anticonformer by 1300 cm−1 without a zero-point energy correction. The methyl torsional barriers are theoretically obtained to be 748 and 402 cm−1 for the syn- and anti-conformers, respectively. Rotational constants derived from the optimized geometries are listed in Table I. The reactive transient species, syn-CH3 CHOO, was produced in a supersonic jet by a pulsed electric discharge15 of a gas mixture of 1,1-diiodoethane and O2 diluted in Ar. The premix gas of 2% O2 and 98% Ar with the total pressure of 3 atm was passed through a liquid container of 1,1-diiodoethane in order to introduce the precursor molecules with a sufficient partial pressure. In our previous study on CH2 OO,10 the intermediate was produced using the dibromomethane precursor. Although syn-CH3 CHOO could be similarly produced using the 1,1-dibromoethane precursor, the signal intensity was 20–30 times weaker than that obtained with 1,1-diiodoethane. A Balle-Flygare type Fourier-transform microwave (FTMW) spectrometer16 operated in the frequency region of 4–40 GHz was used for the observation of pure rotational transitions of
syn-CH3 CHOO. An FTMW-millimeter wave (mmW) doubleresonance technique17 was also used for observing pure rotational transitions and confirming the assignments of the observed FTMW spectra. The threefold methyl internal rotation in syn-CH3 CHOO makes its pure rotational transitions split into two components, A and E, as the result of the interaction between molecular overall and internal rotations.18 A total of 50 lines including the A and E components of 25 pure rotational transitions were observed for syn-CH3 CHOO in the present study, as summarized in Fig. 2, and their frequencies are listed in Table S-II in the supplementary material.13 A typical FTMW spectrum is shown in Fig. 3(a). The A/E splitting is small, less than 200 kHz for all the observed transitions. Figure 3(b) shows FTMW-mmW double-resonance spectra of the A and E components of 313 − 202 , which were observed by monitoring the FTMW signals of the A and E components of 202 − 101 , respectively. Using the double-resonance technique, rotational assignments including the A/E-splitting could be unambiguously made for the observed lines. All the observed transition frequencies were included in a least-squares fit using the program XIAM19 coded by Hartwig based on the combined axis method developed by Woods.20 The resulting spectroscopic parameters are shown in Table I. The Watson’s A-reduced Hamiltonian was used for calculating overall rotational energies. The standard deviation of the fit is 3.8 kHz, comparable to the expected experimental error. The F0 value which is associated with the moment of inertia of the methyl top could not be determined in the present fit, probably because the observed A/E-splittings are too small. Therefore, it was fixed to the value derived from the ab initio geometry of syn-CH3 CHOO, 158.02 GHz. Since the determined rotational constants agree well with the ab initio values of the syn-conformer of CH3 CHOO, the observed transitions are confirmed to be arising from syn-CH3 CHOO. The δ value, which is the angle between the molecular
b
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 143.160.36.177 On: Thu, 18 Dec 2014 10:31:23
011101-3
M. Nakajima and Y. Endo
FIG. 2. Transitions observed in the present study. Red lines correspond to the transitions observed by FTMW spectroscopy, and blues are those observed by FTMW-mmW double-resonance spectroscopy.
principal axis and the internal rotation axis of the methyl top, is also consistent with that for ab initio geometry. As expected from the observed small A/E splittings, the determined barrier height of the threefold internal rotation V3 is quite high, 837.2 (17) cm−1 , which reasonably agrees with the ab initio prediction. Barrier heights of the methyl internal rotation in small molecules with only one methyl rotor are roughly 400 cm−1 or less for many molecules, e.g., CH3 CHO21 (V3
FIG. 3. Rotational transitions of syn-CH3 CHOO. (a) FTMW spectrum of the 202 − 101 transition. The observed lines are split into two Doppler components because the direction of the supersonic jet expansion is parallel to the standing wave in the Fabry-Pérot cavity of the spectrometer. The feature marked with an asterisk is an artifact. (b) FTMW-mmW double resonance spectra of 313 − 202 observed by monitoring the FTMW signals of the A or E component of 202 − 101 . The direction of irradiated mmW was perpendicular to that of the supersonic expansion.
J. Chem. Phys. 140, 011101 (2014)
= 407.7 cm−1 ), CH3 OH22 (V3 = 373.6 cm−1 ), and CH3 OO23 (V3 = 327.6 cm−1 ). In the present experiment, there could be a chance for anti-CH3 CHOO to be produced in the discharged jet simultaneously with syn-CH3 CHOO, although the syn-conformer is more stable than the anti-conformer by 1300 cm−1 , based on the present ab initio result. In fact, recent experimental studies on CH3 CHOO, which was produced from the same precursors CH3 CHI2 and O2 , estimated that roughly 3% and 10% of CH3 CHOO existed as the anti-conformer under jetcooled8 and flow cell7 conditions, respectively. We thus surveyed rotational transitions of the anti-conformer by FTMW spectroscopy, and easily observed several transitions arising from anti-CH3 CHOO with moderate intensities. Under our experimental condition, the anti-conformer was estimated to be produced with the amount ∼20% of the syn-conformer, based on the observed spectral intensities. In contrast to synCH3 CHOO, large A/E-splittings were observed in the rotational spectra of the anti-conformer (the largest is more than 1.5 GHz), and it is concluded that a least-squares fitting using the program XIAM19 is unsatisfactory because the residuals in the fit were significantly larger than the expected experimental errors for several observed line positions. Although the measurements of the anti-CH3 CHOO spectra and the rotational analysis are still in progress and the details will be reported elsewhere, our preliminary least-squares fit using the program XIAM gives the rotational constants of the anticonformer, A = 48 494, B = 4435, and C = 4167 MHz, which are very close to the ab initio values listed in Table I. The V3 barrier height obtained in the analysis is roughly 400 cm−1 for the anti-conformer, which is comparable to those of CH3 CHO, CH3 OH, and CH3 OO. The experimentally determined barrier height for syn-CH3 CHOO is more than twice as large as their values, clearly indicating that the methyl top in syn-CH3 CHOO is strongly hindered by the interaction between the protons in the methyl moiety and the terminal oxygen atom. The interaction responsible for the strong hindrance to the methyl torsional motion should be an attractive interaction but not repulsive one, because the ab initio calculations show that the syn-conformer is more stable than the anti-conformer. The ab initio equilibrium geometry of syn-CH3 CHOO shows that two protons in the methyl moiety are directed to the terminal oxygen that has three lone-pairs in a zwitterionic canonical structure, R1 R2 C = O⊕ − O , which is considered to significantly contribute to the electronic structure at least in CH2 OO.10 The three lone-pairs and the σ -bonding electrons on the terminal oxygen atom are spatially distributed as the electron repulsion becomes minimum, that is, occupy the valence sp3 orbitals. Then, two of the three lone-pairs seem to be interacting with two protons of the methyl moiety in the most stable geometry of syn-CH3 CHOO. This attractive interaction is considered to promote the proton migration from the methyl moiety to the terminal oxygen atom through the five-membered TS, resulting in high OH yield from energized syn-CH3 CHOO. Similar interactions can also be expected in syn-conformers of other singly alkyl-substituted CIs and doubly substituted CIs, responsible for the efficient OH productions in ozonolysis reactions of alkyl-substituted alkenes.6
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 143.160.36.177 On: Thu, 18 Dec 2014 10:31:23
011101-4
M. Nakajima and Y. Endo
The present study was supported by JSPS KAKENHI Grant Nos. 25410004 (M.N. and Y.E.) and 19205002 (Y.E.). 1 R.
Criegee and G. Wenner, Justus Liebig Ann. Chem. 564, 9 (1949). Criegee, Angew. Chem., Int. Ed. Engl. 14, 745 (1975). 3 R. Atkinson and J. Arey, Chem. Rev. 103, 4605 (2003). 4 D. Johnson and G. Marston, Chem. Soc. Rev. 37, 699 (2008). 5 H. Niki, P. D. Maker, C. M. Savage, L. P. Breitenbach, and M. D. Hurley, J. Phys. Chem. 91, 941 (1987). 6 J. H. Kroll, T. F. Hanisco, N. M. Donahue, K. L. Demerjian, and J. G. Anderson, Geophys. Res. Lett. 28, 3863, doi:10.1029/2001GL013406 (2001); J. H. Kroll, J. S. Clarke, N. M. Donahue, J. G. Anderson, and K. L. Demerjian, J. Phys. Chem. A 105, 1554 (2001); J. H. Kroll, N. M. Donahue, V. J. Cee, K. L. Demerjian, and J. G. Anderson, J. Am. Chem. Soc. 124, 8518 (2002). 7 C. A. Taatjes, O. Welz, A. J. Eskola, J. D. Savee, A. M. Scheer, D. E. Shallcross, B. Rotavera, E. P. F. Lee, J. M. Dyke, D. W. K. Mok, D. L. Osborn, and C. J. Percival, Science 340, 177 (2013). 8 J. M. Beames, F. Liu, L. Lu, and M. I. Lester, J. Chem. Phys. 138, 244307 (2013). 9 Y.-T. Su, Y.-H. Huang, H. A. Witek, and Y.-P. Lee, Science 340, 174 (2013). 2 R.
J. Chem. Phys. 140, 011101 (2014) 10 M.
Nakajima and Y. Endo, J. Chem. Phys. 139, 101103 (2013). C. McCarthy, L. Cheng, K. N. Crabtree, O. Martinez, T. L. Nguyen, C. C. Womack, and J. F. Stanton, J. Phys. Chem. Lett. 4, 4133 (2013). 12 H.-J. Werner, P. J. Knowles, G. Knizia, F. R. Manby, M. Schütz et al., MOLPRO, version 2012.1, a package of ab initio programs, 2012, see http://www.molpro.net. 13 See supplementary material at http://dx.doi.org/10.1063/1.4861494 for the ab initio geometries and the observed transition frequencies. 14 K. T. Kuwata, M. R. Hermes, J. Carlson, and C. K. Zogg, J. Phys. Chem. A 114, 9192 (2010). 15 Y. Endo, H. Kohguchi, and Y. Ohshima, Faraday Discuss. 97, 341 (1994). 16 T. J. Balle and W. H. Flygare, Rev. Sci. Instrum. 52, 33 (1981). 17 Y. Sumiyoshi, H. Katsunuma, K. Suma, and Y. Endo, J. Chem. Phys. 123, 054324 (2005). 18 W. Gordy and R. L. Cook, Microwave Molecular Spectra (John Wiley Sons, New York, 1984). 19 H. Hartwig and H. Dreizler, Z. Naturforsch. 51a, 923 (1996). 20 R. C. Woods, J. Mol. Spectrosc. 21, 4 (1966). 21 I. Kleiner, J. T. Hougen, J.-U. Grabow, S. P. Belov, M. Yu. Tretyyakov, and J. Cosléou, J. Mol. Spectrosc. 179, 41 (1996). 22 L.-H. Xu and J. T. Hougen, J. Mol. Spectrosc. 173, 540 (1995). 23 K. Katoh, Ph.D. thesis, the University of Tokyo, 2007. 11 M.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 143.160.36.177 On: Thu, 18 Dec 2014 10:31:23
