J. Am. Chem. SOC.1986, 108, 5784-5792
5784
Ab Initio Study of Rearrangements on the CH3N02Potential Energy Surface Michael L. McKee Contribution from the Department of Chemistry, Auburn University, Auburn, Alabama 36849. Received December 27, 1985
Abstract: A theoretical study has been performed for the thermal rearrangements connecting nitromethane, methyl nitrite, nitrosomethanol, and aci-nitromethane by using the 6-31G* basis set to optimize geometries and introducing correlation at = 73.5 kcal/mol) the MP2 level. The lowest calculated rearrangement pathways from nitromethane are-to methyl nitrite (AH* and aci-nitromethane ( A H * = 75.0 kcal/mol). Nitrosomethanol, although predicted to be only 1.6 kcal/mol less stable, is separated by two high barriers from nitromethane. An elimination transition structure connecting methyl nitrite and formaldehyde plus nitroxyl is found to have an enthalpic barrier of 44.1 kcal/mol. Fragmentation reactions have enthalpies of reaction less than calculated barrier heights, suggesting that concerted rearrangements on the CH3N02surface in general will not be observed.
The photodecompositions of nitr~methanel-~ and methyl nitrite'.'+"* have received considerable attention as model systems for studies in propellant ignition, combustion, and atmosphere pollution. Characterization of the above compounds and their decomposition products has included techniques such as micro~ a v e , ' ~ electron . ' ~ , ~ ~impact,,' and NMR.22 The threshold for photolysis of nitromethane is determined to be 86.6 kcal/mol (330 nm),' slightly higher than the threshold for methyl nitrite which is 77.3 kcal/mol (370 nm).' Identification of initial reaction products is difficult since any radicals produced may rapidly undergo secondary reactions. Photolysis of nitromethane can produce cis- and t r a n s - C H 3 0 N 0 as shown in eq 1-3 where reaction 2 is predicted to very rapid.23 CHSNO,
CH3 + N O 2 CH,O
-
+ h~
+ NO
+ NO2
(1)
C H 3 0 + NO
(2)
CH,ONO
(3)
CH3
( I ) Batt. L.; Robinson, G. N. The Chemisrry of Amino, Nitroso, and Nitro Compounds and their Derioatioes; Patai, S . , Ed.; Wiley: New York, 1982; Parts 1 and 2. (2) Jacox, M. E. J . Phys. Chem. 1984, 88, 3373-3379. (3) Jacox, M. E. J . Phys. Chem. 1983,87, 3126-3135. (4) Flicker, W. M.; Mosher, 0. A.; Kuppermann, A. J . Chem. Phys. 1980, 72, 2788-2794. (5) Blais, N. C. J . Chem. Phys. 1983, 79, 1723-1731. (6) Schoen, P. E.; Marrone, M. J.; Schnur, J. M.; Goldbert, L. S. Chem. Phys. Lett. 1982, 90, 272-276. (7) Kwok, H. S.; He, G. Z.; Sparks, R. K.; Lee, Y. T. Znt. J. Chem. Kinet. 1981. 13. 1125-1131. (8) Butler, L. J.; Krajnovich, D.: Lee, Y. T.; Ondrey, G.; Bersohn, R. J . Chem. Phys. 1983, 79, 1708-1722. (9). Taylor, W.D.; Allston, T.D.; Moscato, M. J.; Fazekas, G. B.; Kozlowski, R.; Takacs, G. A. Inr. J . Chem. Kinet. 1980, 12, 231-240. (10) Jacox, M. E.; Rook, F. J . Phvs. Chem. 1982, 86, 2899-2904. ( 1 1 ) Muller, R. P.; Huber, J . R. j . Phys. Chem. 1983, 87, 2460-2462. ( 1 2) Sanders, N.; Butler, J. E.; Pasternack, L. R.; McDonald, J. R. Chem. Phys. 1980, 48, 203-208. (13) Muller, R. P.;Russegger, P.; Huber. J. R. Chem. Phys. 1982, 70, 28 1-290. (14) Muller, R. P.; Huber, J. R. J . Mol. Specrrosc. 1984, 104, 209-225. (15) King, D. S.; Stephenson, J. C. J. Chem. Phys. 1985,82, 2236-2239. (16) COX,A. P.; Waring, S . J . Chem. Soc., Faraday Trans. 1972. 2, 1060-1 070. (17) Rook, F. L.; Jacox, M. E. J . Mol. Spectrosc. 1982, 93, 101-106. (18) Ghosh, P. N.; Bauder, A.; Gunthard, H. H. Chem. Phys. 1980, 53, 39-50. (19) Turner. P. H.; Corkill, M. J.; Cox, A. P. J . Phys. Chem. 1979, 83, 1473-1482. (20) Bondybey, V. E.; English, J. H. J. Mol. Spectrosc. 1982, 92,431-442. (21) Flicker, W. M.; Mosher, 0. A.; Kuppermann, A. J . Chem. Phys. 1980, 72, 2788-2794. (22) (a) Lazaar, K. I.; Bauer, S. H. J . Phys. Chem. 1984,88, 3052-3059. (b) Chauvel, J. P.; Friedman, B. R.; Van, H.; Winegar, E. D.; True, N. S. J . Chem. Phys. 1985,82, 3996-3998. Chauvel, J. P.; Friedman, B. R.; True, N. S.; Winegar, E. D. Chem. Phys. Left. 1985, 122, 175-179. (23) Yamada, F.; Slagle, I. R.; Gutman, D. Chem. Phys. Left. 1981, 53, 409-412.
0002-7863/86/1508-5784$01.S0/0
The pyrolysis of nitromethane and methyl nitrite is generally regarded to occur by an initial bond rupture (eq 4 and 5).
---
+
(4) C H 3 N 0 2 C H 3 NO, C H 3 0 N 0 C H 3 0+ N O (5) Secondary reactions of these radicals can then account for a variety of products (eq 6-9). CH30+ NO H2C=0 HNO (6)
2HN0
H2O
CH3 + CH3N02 CH3NO2
+ CH2NO2
+
-+
+
+ N2O
CH,
(7)
+ CHzNO2 + CHZN(0)OH
CH2N02
(8) (9)
The known decomposition-recombination mechanism of nitromethane places a lower limit on the enthalpy of activation for the umimolecular rearrangement in eq IO which must be higher
-
CH3NO2 CH3ONO (10) than the strength of a C-N bond (58.5 kcal/mol') since this would be the enthalpy of activation for the formation of the radicals CH, and NO2. Similarity the barrier for eq 11 must be higher than
-
CH30NO CH3N02 ( 1 1) the strength of the central N - 0 bond which is found" to be 41.2 kcal/mol. The goal of this article is to provide an accurate description of the concerted thermal rearrangement barriers. While the initial step is generally acknowledged to be radical formation it is possible that the later observed products (or unobserved products) are due to unimolecular rearrangements. Indeed, a unimolecular elimination process has been proposed' involving a cyclic transition structure (eq 12). CH3ONO
-
H2C-0
L r i
H
-
CH2=O
t HNO
(12)
N-0
In a related area, the reactions of the metal ion Cot with C H 3 N 0 2and C H 3 0 N 0 have generated a rich field for chemical i n ~ e s t i g a t i o n . ~Reaction ~ pathways begin with the probable coordination of Co' to an oxygen followed by possible concerted rearrangement to a number of different products. It seems likely that the metal assists in the initial nitro-to-nitrite isomerization. If so, the concerted rearrangement surface in the absence of a metal ion would be valuable in interpreting results. Previous theoretical calculations on nitromethane and methyl nitrite have been performed to study such aspects as the multiconfigurational nature of the ground ~ t a t e , ~the ~ - relative ~' stability (24) Cassady, C. J.; Freiser, B. S.; McElvany, S. W.; Allison, J. J . Am. Chem. SQC.1984, 106, 6125-6134. (25) Kleier, D. A.; Lipton, M . A. THEOCHEM. 1984, 109, 39-49. (26) Marynick, D. S . ; Ray, A. K.; Fry, J. L.; Kleier, D. A. THEOCHEM 1984, 108,45-48. Marynick, D. S.; Ray, A . K.; Fry, J. L. Chem. Phys. Letf. 1985, 116, 429-433.
0 1986 American Chemical Society
J . Am. Chem. SOC.,Vol. 108, No. 19, 1986 5 1 8 5
Rearrangements on the C H 3 N 0 2Potential Energy Surface
Table I. Total Energies (-hartrees) and Zero Point Energies (kcal/mol) of Various Species on the C H 3 N 0 2 Potential Energy Surface' molecule 1 2 3 4
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
CH3N02 CHIN02 CHjONO CH30N0 CH,ONO CH,ONO CHjONO CH,N(OjOH CHzN(0)OH CHzN(0)OH CHj(0H)NO CHZ(0H)NO CH,(OH)NO CH2(OH)N0 CHI(0H)NO CHz(0H)NO CH,(OH)NO
.
CH20N0H
mol/elec symmetry C.
c, cs
cx
c* c, Cl c 3
c, CJ c, cs cs
c, c,
TS;
Cl Cl
TS.4
NO, + CH, HNO H,C=O CHlN02 + H N O + OCH, H 2 0 HNCO HCNO H,O O N O H + CH2
+
+ +
6-31G*// 6-31G*
MP2/6-31Ga// 6-31G*
MP2/6-31G*b/l 6-31G*
242.255 86 242.255 85 242.280 34 242.280 77 242.28265 242.286 36 242.27064 242.25541 242.23 1 61 242.13350 242.299 68 242.293 40
243.523 38 243.52336 243.527 87 243.527 93 243.525 93 243.528 76 243.51 3 25 243.506 88 243.48696 243.377 I O 243.552 89 243.549 83 243.553 28 243.546 I7 243.545 70 243.546 30 243.540 29
243.661 99 243.661 98 243.665 71 243.666 3 1 243.664 20 243.668 64 243.650 28 243.629 62 243.61350 243.522 82 243.681 85 243.676 21 243.681 93 243.673 87 243.673 87 243.673 79 243.667 78
243.996 77 243.996 76 243.983 81 243.982 74 243.983 18 243.985 57 243.965 42 243.964 90 243.950 30 243.838 IO 243.994 80 255.991 06 243.995 07 243.987 15 243.986 74 243.987 28 243.981 71
244.33794 244.337 92 244.327 24 244.326 60 244.327 74 244.331 48 244.307 81 244.302 77 244.289 88 244.189 18 244.333 98 244.326 44 244.333 74 244.32428 244.327 46 244.324 21 244.31836
243.481 46 243.367 56 243.385 20 243.401 22 243.385 50 243.408 I O
243.623 72 243.486 83 243.508 18 243.531 94 243.527 35 243.549 19 243.59048 243.652 40 243.528 05 243.668 50 243.772 13 243.641 18 243.512 3 1
243.928 I O 243.823 86 243.86064 243.881 58 243.861 80 243.918 16
244.288 06 244.1 59 71 244.1 99 5 1 244.21 6 24 244.21 1 17 244.253 19 244.23261 244.296 91 244.17582 244.255 48 244.426 84 244.3 18 56 244.141 84
242.288 99 242.288 70
C' Cl
c,
ss'4
6-31G// 6-31G*
Cl
TS I TS3
3-21G// 3-21G
242.253 04 242.1 27 5 5 d 242.14262 242.143 69' 242.138 94 242.162 789
c, c, 2Al. 2A21'h 'A', 'A, 2B,, 2S 211,=A'' 'A,, 'AI 'A,, '2' 'AI, 'AI
ZPE/3-21GC 32.58 ( 0 ) 32.55 ( I )
31.79 ( I )
32.29 (0) 31.70 ( I )
30.00 ( 1 ) 30.34 ( I ) 30.32 ( I ) 27.95 ( I ) 28.10 ( I )
ZPE/631G" 34.25 ( 0 ) 34.21 ( I ) 33.13 ( 1 ) 32.23 (0) 33.29 ( I ) 33.69 (0) 32.78 ( I ) 32.99 (0) 33.58 (0) 31.71 ( I ) 33.40 ( I ) 32.99 ( I ) 33.58 (0) 33.29 ( 0 ) 33.03 ( I ) 33.14 ( I ) 33.12 (2) 34.13 (0) 30.74 (1) 31.80 ( I ) 31.39 ( I ) 29.66 (2) 28.62 ( I ) 25.61 (0) 28.40 (0) 24.41 (0) 27.48 ( I ) 28.72 (0) 28.85 (0) 25.80 (0)
30 a Frozen core approximation. bAll orbitals included in correlation treatment. 'Number of negative vibrational frequencies is given i n parentheses. d A stationary point of C, symmetry 46.3 kcal/mol less stable was located and found to have two imaginary frequencies. P A stationary point of Cs symmetry 3.0 kcal/mol less stable was located and found to have two imaginary frequencies. /Super Saddle point. g A stationary point of C, symmetry 11.5 kcal/mol less stable was located and found to have two imaginary frequencies. *Electronic state symmetries of fragments. 'Calculations a t all levels are at 6-31G* optimized geometries except those reported at the 3-21G level, which are optimized at that level.
of the aci forms,28conformational stabilities in m e t h y l nitrite,,' charge distribution^,^^ and vibrational f r e q ~ e n c i e s . ~ 'Also, a MNDO s t u d y of r e a r r a n g e m e n t s in I-nitropropene has been reported.32 The most thorough study of thermolysis of molecules containing NO2 groups is a recent M I N D 0 / 3 s t u d y by Dewar and cow o r k e r ~ . ~Their ~ use of M I N D 0 / 3 r a t h e r t h a n MNDO is surprising since t h e l a t t e r method is normally better suited for systems containing adjacent lone pairs. If t h e experimental heat
of formation is used for NO,, the C-N bond energy of CH3N02 is well reproduced33(calcd 59.5; obsd 60.0 kcal/mol) as well as the 0-N bond energy in C H 3 0 N 0 (calcd 40.4; obsd 42.1 kcal/mol). However, their reported barriers for the nitromethane t o methyl nitrite rearrangement (47.0 kcal/mol) and methyl nitrite decomposition t o formaldehyde plus nitroxyl (32.4 kcal/mol) are much lower t h a n those found in the present study. Since the rearrangement barriers they calculated are smaller t h a n the C-N bond e n e r g y in t h e CH3N02 C H 3 0 N 0 reaction a n d smaller than the 0-N bond energy in t h e C H 3 0 N 0 O=CH, HNO reaction, they conclude t h a t the concerted rearrangement will be favored over (or competitive w i t h ) simple bond fission. In t h e present study, rearrangements are predicted t o have barriers higher
-
-
+
(27) Chabalowski, C.; Hariharan, P. C.; Kaufman, J. J . Inr. J . Quantum Chem. 1983, 17, 643-644.
(28) Murrell, J. N.; Vidal, B.; Guest, M. F. J . Chem. Sac., Farads,,. Trans. 1975, 2, 1577-1582. (29) Cordell, F. R.; Boggs, J. E.; Skancke, A. J. Mol. Srruct. 1980, 64, 57-65. (30) Ritchie, J. P. J . Am. Chem. Sor. 1985, 107, 1825-1837. (31) McKee, M. L. J . Am. Chem. Sor. 1985, 107, 1900-1904. The calculated MNDO isotope frequency shifts were incorrectly reported in this paper for the b, and b, modes of CD2N02. The correct shifts for the b, modes are -133 and -34 cm-I, while for the b, modes the correct shifts are -878. -2, -235, and -45 cm-'. (32) Turner, A. G.; Davis, L. P. J . Am. Chem. Sor. 1984,106,5447-5451. (33) Dewar, M. J. S.; Ritchie, J. P.; Alster. J. J , Org. Chem. 1985, 50, IO3 1-1036.
t h a n bond energies which leads t o the conclusion that the reactions m u s t be either bimolecular or proceed by initial bond cleavage. In addition t o determining rearrangement barriers, t h e present s t u d y will be u n d e r t a k e n t o analyze several r o t a m e r s of s t a b l e species on t h e CH3N0, potential energy surface. Since a major goal of theoretical c h e m i s t r y is t o provide d a t a t o t h e experimentalist t h a t is otherwise difficult t o obtain, a comparison of t h e relative energies of t h e rotamers will aid in interpreting microwave d a t a . A n analysis of t h e vibrational frequencies of several C H 3 N 0 2species will follow in a s e p a r a t e article.
Methods All calculations have been made by using the FALssiAh B ? program ~ y s t e n i . ' ~Geometries have been optimized by using the 3-21G and the 6-31G* basis sets. By using the 6-31G* geometries, the polarization increment (6-3 IC*) and correlation increment (MP2/6-3 IC) starting from the 6-31G basis set have been made in order to determine relative energies at the additivity level ([MP2/6-31G*]). The additivity approximation (eq 13) has been tested and shown to yield relative energies AE[MP2/6-3lG*] = AE(MP2/6-3IG)
+ AE(HF/6-31G*)
-
AE(HF/6-31G) (13)
normally within 5 kcal/mol of those from the full basis set.3i Despite (34) Referecnes to basis sets used are collected here. The program package was used throughout. Carnegie-Mellon University: Binkley, J . S.; Frisch, M.; Raghavachari, K.; Fluder, E.; Seeger, R.: Pople, J . A . 3-21G basis: Binkley, J. S.; Pople, J. A,; Hehre, W . J. J . Am. Chem. SOC.1980, 102, 939. 6-31G basis: Hehre, W. J.: Ditchfield, R.; Pople, J. A . J . Chem. Phys. 1972, 56, 2257. 6-31G' basis: Hariharan, P. C.; Pople, J. A. Theor. Chim. Acra 1973, 28, 213. Gordon, M. S. Chem. Phys. Lerr. 1980, 76, 163. Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J . S.; Gordon, M. S.; Defrees, D. J.; Pople, J. A. J . Chem. Phys. 1977, 77, 3654. MP2 correlation treatment: MQller, C.; Plesset, M. S. Phys. Rec. 1934, 45, 618. Pople, J. A,; Binkley, J . S.; Seeger, R. Int. J . Quantum Chent., SJ'mp. 1979, 13, 225. (35) McKee, M. L.; Lipscomb, W . K. J . A m C h e m Soc. 1981, 103, 4673-4676. Nobes, R. H.; Bouma, W. J.: Radom, L. Chem. Phys. Lett. 1982, 89, 497-500. McKee, M. L.: Lipscomb, W. N. Inorg. Chem. 1985, 24, 762-764. GACSSIAN 82
5186 J . Am. Chem. SOC..Vol. 108, No. 19, 1986
McKee
i)
ti
P)
H\0951(0972)
"\{0.4 0
H
9
b)
H'
1 1 2 ~ 4 : ~
*-f"
HOCN 74.7 ONCO 355.0 17
i) (1413) 1 37/' ti-..,,, ( 1 0 4 8 ) 1 0 4 2 H ) r c 1504(1722) 1.093(1100)
p 7
(O 980'
9)
\P
(1.449) 1,376
2
H,Yc-)
IO
1155(1180) N=O /
