2348
J. Am. Chem. SOC. 1993, 115, 2348-2351
Nitro =aci-Nitro Tautomerism Koop Lammertsma* and Bharatam V. Prasad C o n t r i b u t i o n from t h e D e p a r t m e n t of C h e m i s t r y , University of Alabama at B i r m i n g h a m , UAB S t a t i o n , B i r m i n g h a m , A l a b a m a 35294. Received June 18, 1992
Abstract: T h e nitromethane F= aci-nitromethane tautomerism is studied by high-level ab initio methods. The MP2/6-3 1G* geometry of nitromethane compares well with that determined experimentally. T h e GI energy difference between t h e t w o tautomers amounts t o 14.1 kcal/mol in favor of nitromethane. T h e calculated heat of atomization of -570.7 kcal/mol for nitromethane differs by only 2.4 kcal/mol from t h e experimental value. T h e nitromethide anion has C, symmetry with t h e 6-311+G* basis set. Its GI proton affinity is 355.2 kcal/mol, which differs from t h e gas-phase value by 1.4 kcal/mol. T h e anion does not display Y-aromaticity.
Nitronic acids play an important role in thermal and redox reactions, in photochemical processes, in pyrolysis, as well as in the toxicity of nitro compounds.’ They are intermediates, for example, in the Nef reaction,Ib which converts nitroalkanes to ketones. Kinetic studies show similar pKa values for nitronic acids and their carbon counterparts, the carboxylic acids.’ The nitronic acids have received by far the lesser attention, undoubtedly in part due to their tautomeric equilibria with nitroalkanes. H o w e v e r , processes like the keto enol tautomerism are receiving renewed interest largely because of their biochemical importance. Also in the case of nitro compounds, experimental2 and theoretical studied suggest a more prominent place for nitronic acids. In light of the recent gas-phase observation of aci-nitromethane,“ it is surprising that the parent tautomeric process, nitromethane aci-nitromethane, has not been studied in greater detail. Instead, most studies concern the C-N and C-O bond strengths in the related nitromethane methyl nitrite equilibri~m.~In a comprehensive study on the potential energy hypersurface of nitromethane, McKee6 included the nitro e aci-nitro equilibrium ( I ) (a) Nielson, A. T. In The Chemistry of the Nitro and Nitroso Groups; Feuer, H., Ed.; Interscience: New York, 1969; Part 1, p 349. (b) Noland, W. E. Chem. Rev. 1955, 55, 137. (c) The Chemistry of Amine, Nitroso, and Nitro Compounds and Their Derivatives; Patai, S., Ed.; Wiley: New York, 1982: Morrison, H. A., p 165; Chow, Y. L., p 181; Batt, L., p 417. (d) International Agency for Research on Cancer in Monographs on the Evaluation of Carcinogenic Risk of Chemicals to Man; WHO: Lyon, 1982; Vol. 29, p 331. (2) The Chemistry of Amine, Nitroso, and Nitro Compounds and Their Deriuariues; Patai, S . , Ed.; Wiley: New York, 1982: Schwarz, H., Levsen, K., p 85. (b) fbid. Fry, A. J., p 319. (c) Bernasconi, C. F.; Zitomer, J. L.; Schuck, D. F. J. Org. Chem. 1992, 57, 1 1 32. (d) Miyashita, M.; Awen, B. 2. E.; Yoshikoshi, A. Tetrahedron 1990,7569. (e) Shaw, A. J.; Gescher, A. J. Chromatog. 1990,505,402. (0 Herman, L. W.; ApSimon, J. W. Tetrahedron Lett. 1984, 25, 1423. (9) Veltwisch, D.; Asmus, K.-D. J. Chem. Soc., Perkin Trans. 2 1982, 1143. (h) Srokovi, 1.; VeteSnik, P.; JurlSek, A.; KoviE, J. Collect. Czech. Chem. Commun. 1981, 46, 3122. (i) McKillop, A.; Kobylechi, R. J. Tetrahedron 1974,30, 1365. 6 ) More O’Ferrall, R. A,; Quirke, A. P. Tetrahedron Letf. 1989, 30, 4885. (k) Chattopadhyay, S. K.; Craig, B. B. J. Phys. Chem. 1987, 91, 323. (I) Meese, C. 0.;Giisten, H. Z . Narurforsch. 1986, 416, 265. (m) Terrier, F.; Ah-Kow, G.; Chatrousse, A.-P. J. Org. Chem. 1985,50,4583. (n) Grant, R. D.; Pinhey, J. T.; Rizzardo, E.; Smith, G. C. Ausf. J. Chem. 1985, 38, 1505. ( 0 ) Pollet, P. L.; Perzanowski, H. P.; Gelin, S. Spectrochim. Acta 1984,40A. 1007. (p) Dopp, D. 0. Top. Curr. Chem. 1978, 55,49. (9) Yip, R. W.; Wen, Y. X.;Gravel, D.; Giasson, R.; Sharma, D. K. J. Phys. Chem. 1991,95,6078. (r) Yip, R. W.; Sharma, D. K.; Giasson, R.; Gravel, D. J. Phys. Chem. 1984, 88, 5770. (3) (a) Politzer. P.; Seminario, J. M.; Bolduc, P. R. Chem. Phys. Lett. 1989,158,463. (b) Cox, J. R.; Hillier, I. H. Chem. Phys. 1988, 124, 39. (c) Turner, A. G. J. Phys. Chem. 1986-90,6000. (d) Ritchie, J. P. J. Org. Chem. 1989, 5 4 , 3553. (4) (a) Egsgaard, H.; Carlsen, L.; Florsncio, H.; Drewello, T.; Schwarz, H. Eer. Eunsenges. Phys. Chem. 1989, 93, 76. (b) Egsgaard, H.; Carlsen. L.; E M , S. Eer. Eunsenges. Phys. Chem. 1986, 90, 369. (5) (a) McKee, M. L. J. Phys. Chem. 1989, 93, 7365. (b) McKee, M. L. Chem. Phys. Lett. 1989, 164.520. (c) McKee, M. L. J. Phys. Chem. 1986, 90,2335. (d) Wodtke, A. M.; Hintsa, E. J.; Lee, Y. T. J. Chem. Phys. 1986, 84, 1044. (e) Wodtke, A. M.; Hintsa, E. J.; Lee, Y. T. J. Phys. Chem. 1986, 90, 3549. (0 Dewar, M. J. S.; Ritchie, J. P.; Alster, J. J. Org. Chem. 1985, 50, 1031. (g) Kaufman, J. J.; Hariharan, P. C.; Chabalowski. C.; Hotokka. M. fnr.J. Quantum Chem., Quantum Chem. Symp. 1985, 19,221. See also: (h) Mijoule, C.; Odiot, S.; Fliszar, S.; Schnur, J . M. THEOCHEM 1987, 149, 31 I . (i) Kleier, D. A.; Lipton, M. A. THEOCHEM 1984, 109, 39. (j) Marynick, D. S.; Ray, A. K.; Fry, J . L.; Kleier, D. A. THEOCHEM 1984, 108, 45.
and reported nitromethane to be 21.8 kcal/mol (MP2/6-31G*) more stable with a barrier of 75.0 kcal/mol for the thermally forbidden [ 1,3]-H transfer. However, recent studies on nitro compounds h a v e illustrated that larger basis sets and extensive electron correlation treatments are required for reliable energy eval~ations.~~-g.~ This has also been demonstrated by Wiberg et aLSaand Radom and co-workerssbin an evaluation of the keto * enol equilibrium in acetaldehyde. For example, the 16.5 kcal/mol energy difference at MP2/6-31G in favor of acetaldehyde reduces to 10.4 kcal/mol at MP3/6-31++G**8~9and to 11.2 kcal/mol at Gaussian (Gl) theory. Hence, we decided to evaluate the nitromethane * aci-nitromethane tautomerism using Pople’s G1 theory.’O Computational Methods All a b initio molecular orbital calculations” were carried out using the GAUSSIAN 90 suite of programs.’* T h e nitromethane (1, 2, C, symmetry), mi-nitromethane (3, 4, C,symmetry), and nitromethane anion (5, C , symmetry) structures were optimized at the SCF level with the split-valence 3-21G and the d-polarized 6-31G* basis sets and with inclusion of the effects of all electron correlation by using Maller-Plesset (MP) p e p r b a t i o n theory a t second order, Le., MP2/6-31G*. The latter geometries, which a r e shown in Figure 1 (see also Table I for nitromethane), were used for energy evaluation a t the G1 level.Ioa In this approach additive energy corrections a r e made to those obtained with frozen core MP full fourth-order (including singles, doubles, triples, and quadruples) calculations using an essentially triply split valence basis set that includes d- and for hydrogen ppolarization functions, Le., MP4/631 1G**. T h e corrections include additional diffuse (+) functions, and extra d- and f-functions (2df) for nonhydrogen atoms, quadratic configuration interaction (QCI or QCISD(T)), an empirical higher level correlation ( H L C ) based on the number of electrons ( H L C = -0.19n,, - 5.95n,), and 0.95 scaled” zero-point vibrational MP2/6-31G* energy corrections (instead of the 0.9 scaled HF energies)’@and is shown in the equation: GI = MP4/6-311GZ* + M ( + ) AE(2df) M(QC1) AE(HCL) scaled Z P E ( I ) The magnitude of these corrections is illustrated by comparison of the energies with those a t MP4/6-31 l++G** in which diffuse and polarization functions were added to both hydrogen and nonhydrogen atoms. Absolute energies are given in Table I1 and those a t G1 theory in Table Ill; all relative energies are summarized in Table IV. Ionization energies
+
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+
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(6) McKee, M. L. J. Am. Chem. Soc. 1986, 108, 5784. (7) Head-Gordon, M.; Pople, J. A. Chem. Phys. Lett. 1990, 173, 585. (8) (a) Wiberg, K. B.; Breneman, C. M.; LePage, T. J. J. Am. Chem. Soc. 1990, 112,61. (b) Smith, B. J.; Nguyen, M. T.; Bouma, W. J.; Radom, L. J. Am. Chem. Soc. 1991, 113, 6452. (9) Apeloig, Y.; Arad, D.; Rappoport, 2.J. Am. Chem. SOC. 1990, f l 2 , 9131. (IO) (a) Pople, J . A.; Head-Gordon, M.; Fox, D. J.; Raghavachari, K.; Curtiss, L. A. J. Chem. Phys. 1989, 90, 5622. (b) Gaussian-2 (G2) was not feasible due to disk space limitations: Curtiss. L. A.; Jones, C.; Trucks, G. W.; Raghavachari, K.; Pople, J . A. J. Chem. Phys. 1990, 93, 2537. ( I I ) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. A6 Initio Molecular Orbital Theory; Wiley: New York, 1986. (12) GAUSSIAN 90, Revision I; Frisch, M. J.; Head-Gordon, M.; Trucks, G . W.; Foresman, J. B.; Schlegel, H. B.; Raghavachari, K.; Robb, M.;Binkley, J . S.; Gonzalez. C.; DeFrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S . ; Pople, J . A . Gaussian, Inc.: Pittsburgh, PA, 1990. ( 1 3 ) DeFrees. D. J.; McLean, A. D. J. Chem. Phys. 1985, 82, 333.
0002-7863/93/1515-2348%04.00/0 0 1 9 9 3 American C h e m i c a l Society
Nitro
J. Am. Chem. SOC.,Vol. 115, No. 6, 1993 2349
Aci-Nitro Tautomerism
Table I. Experimental and Calculated Geometrical Parameters of Nitromethane geometrical X-ray, single neutron, single MP2/6-31G*, Darameters' crystal, 228 K b crystal, 15 K' microwaved this work 1.4855(9) 1.489(5) 1.449(6) 1.485 C-N 1.2270(9) 1.231(4) 1.240 1.224(5) N-O 1.2225(9) 1.214(3) 1.088( 1) 1.090 C-H4 1.087 C-H, 125.3 125.7 123.7(1) 123.4(4l LONO .. LNCH, 106.4' 107.2 107.1 'Distances are in A, angles in deg. bReference 17. The C-H distances range from 0.7 to 0.95 A, and the N C H angles from 105 to 111". ' Reference 18. The crystal structure of deuterionitromethane was determined. The other N C D angles are 107.7 and 1 0 8 . 5 O . The C-D distances are 1.0751(13), 1.0736(14), and 1.0739(13). dReference 19. '
Table 11. Absolute Energies (in hartreesl of Nitromethane, oci-Nitromethane, and the Nitromethide Anion" compound HF/3-21G HF/6-31G* MP2/6-31G* MP4/6-31G*" MP4/6-31 l++G**b ZPED' -244.345 33 (0) -244.374 27 -244.528 59 30.63 1 c, -242.255 86 (0) -243.661 99 (0) -244.374 25 -244.528 5 5 30.58 -243.661 98 (1) -244.345 3 I (1) 2 c, -242.255 85 1) -243.62963 (0) -244.308 87 (0) -244.33906 -244.491 82 29.39 -242.255 41 (0) 3 c, -243.613 50 (0) -244.296 32 (1) -244.326 36 -244.487 22 28.15 4 c, -242.231 61 (1) 5 c,. -241.663 32 (01 -243.057 73 (01 -243.737 22 (0) -243.762 96 -243.93706 21.56 "Number of imaginary frequencies is given in parentheses. MP4(SDTQ) energies using MP2/6-31G* geometries. 'Scaled to 0.95 in kcal/mol. ~~~
Table 111. Total Energies (in hartrees) and Corrections (in millihartrees) at the G1 Level for Nitromethane, mi-Nitromethane, and the Nitromethide Anion compound MP4/6-311G1* hE(+) W2df) WQCU AE(HLC) ZPE" GI -132.29 +9.55 -73.68 48.82 -244.675 93 1 -244.5 13 04 -15.28 -132.34 +9.55 -73.68 48.73 -244.676 03 2 -244.5 13 03 -15.26 -135.03 +5.96 -73.68 46.84 -244.653 40 3 -244.481 76 -15.73 -134.87 +7.04 -73.68 4 -244.470 8 5 -16.05 45.82 -244.642 60 -140.94 +7.19 -73.68 34.35 -244.109 88 5 -243.900 08 -36.72 "Scaled to 0.95 mhartrees. Table IV. Relative Energies (in kcal/mol) for Nitromethane, aci-Nitromethane, and the Nitromethide Anion MP2/ MP4/ MP4/ MP4/(+ZPE) H F/ H F/ 6-31G* compound 3-21G 6-31G* 6-31G* 6-31 1++G** 6-311++G** 0.00 0.00 0.00 0.00 0.00 1 0.00 0.01 0.0 I 0.03 2 0.01 0.01 -0.02 22.88 22.09 19.3 1 18.07 3 0.27 20.31 4 15.21 30.43 30.76 30.05 25.96 24.08 Table V. Ionization Energies (in kcal/mol) for Nitromethane and mi-Nitromethane MP2/ MP4/ MP4/ F/ HF/ reaction 3-21G 6-31G' 6-31G' 6-31G* 6-311++G** 2+5 371.8 379.2 38 1.6 383.6 371.2 3+5 371.6 388.9 358.7 361.5 35 I .9 Reference 28.
MP4/(+ZPE) 6-31 1++G** 362.1 344.1
Table VI. Vibrational Freauencies of Nitromethane and aci-Nitromethane 1, calc" a' 522 (3) 640 (29) 898 (7) 11 14 (2) a' 1459 (20) 2994 (6) 3095 (4) a" 33 (>O) 402 (
