J . Phys. Chem. 1990, 94, 4856-4861
4856
Theoretical Studies of Rotational Barriers of Heteroatom Derivatives of Methanol Yun-Dong Wu* and K. N. Houk Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90024 (Received: February 1 , 1990)
The rotational barrier about the C-O bond of methanol is well-known to be 1 kcal/mol. In this paper, the rotational barriers OH, and OH2+,are predicted to have the considerably of heteroatom derivatives of methanol, CH30X, where X = F, CI, 0-, higher values of 3.7,3.5,4.3,3.3, and 3.5 kcal/mol, respectively, at the MP4/6-31G** basis set level, with staggered conformers being favored. These conformational preferencesand rotational barriers are rationalized by a combination of antiperiplanar u ~ ~ - delocalization u * ~ ~ and a-type orbital interactions between T * and~ aux ~ orbitals, ~ both of which are maximized in the staggered conformation.
Introduction The rotational barrier of ethane is 3 kcal/mol.' In the eclipsed conformer, there are three pairs of eclipsed CH bonds. It is generally concluded that each CH, CH eclipsing destabilizes this conformer by about 1 kcal/mol. This seems to be quite general in other molecules. For example, methylamine and methanol have rotational barriers of 1.9 and 1.1 kcal/mol, respectively, corresponding to two and one eclipsed CH, XH pair, respectively, in the eclipsed conformations. The molecular orbital rationalization of this phenomenon is illustrated in Scheme 1, which shows the bonding combination of two occupied rCH3 orbitals of the two methyl groups. The four-electron interaction in the eclipsed conformation is larger than in the staggered, since overlap is larger between these filled orbitals in the eclipsed conformer. The two-electron (a-a*) stabilization is small, and also favors the staggered conformation. When one of the hydrogens is replaced by fluorine or chlorine, the rotational barrier increases to 3.3 and 3.7 kcal/mol, respectively.2 It could be concluded that HCCF and HCCCI eclipsing costs about 1.3 and 1.7 kcal/mol, respectively. However, 1,1,1trifluoro- and 1,l ,I -trichloroethane have rotational barriers of 3.0 and 2.9 kcal/mol, respectively, suggesting only 1 kcal/mol de~tabilization.~ We have undertaken a computational study of the rotational barrier of some derivatives of methanol. Here we report our finding that when the hydroxyl hydrogen of methanol is replaced by an electronegative atom or group with lone pairs, F, Cl, 0-, OH, or OH2+, the rotational barrier is considerably increased.
1, X=F 3, x=o5, X=OH2+
2, X=Cl 4, X=OH
Calculations The calculations were performed with Pople's GAUSSIAN 86 p r ~ g r a m . The ~ geometries were optimized a t various levels, but our discussion focuses upon optimizations at the 6-31G* and MP2/6-3 1G*levels. For each compound, staggered and eclipsed ~
~~
(1) For reviews, see: Lowe, J. P. Prog. Phys. Org. Chem. 1968, 6, 1.
Gordy, W.; Cook, R. L. Microwave Molecular Spectra; Interscience: New York, 1970. Pitzer, R. M. Acc. Chem. Res. 1983, 16, 207. (2) Sage, G.; Klemperer, W. J . Chem. Phys. 1963, 39, 371. Danti, A.; Wood, J. L. Ibid. 1959, 30, 582. (3) Wulff, C. A. J . Chem. Phys. 1963, 39, 1227. (4) GAUSSIAN 86; Frisch, M. J.; Binkley, J. S.; Schlegel, H. B.; Raghavachari, K.; Melius, C. F.; Martin, R. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.; Defrees, D. J.; Seeger, R.; Whiteside, R. A.; Fox, D. J.; Fluder, E. M.; Pople, J. A. Carnegie-Mellon Quantum Chemistry Publishing Unit, Pittsburgh PA, 1986.
SCHEME I
eclipsed
staggered
SCHEME I1 slightly longer
Staggered
Eclipsed
structures with C, symmetry were optimized, and the rotational barrier was estimated from the relative energies of the two structures. For methyl hydroperoxide the geometries were restricted in the anti form ( C 3 0 2 0 1 H= 180') for simplicity, although a nonplanar form would most likely be more table.^ Selected geometrical parameters and Mulliken charges of the structures are given in Table I, and the total energies of the structures are given in Table 11. Table I11 summarizes the calculated rotational barriers.
Results and Discussion The geometrical parameters are somewhat basis set dependent. In general, inclusion of correlation (MP2) increases all the bond lengths.6 The trends are followed at all levels. Our discussion will be based on the MP2/6-31G* geometries. As summarized in Scheme 11, there are several interesting geometrical patterns in the staggered and eclipsed structures. (a) Staggered structures have shorter 02-C3 bond lengths than the corresponding eclipsed structures in each of the cases. The bond shortening seems to correlate with the electronegativity of X. It is 0.025,0.015,0.012,0.012, and 0.01 1 %, when X is OH2+, F, CI, OH, and 0-, respectively. The Xl-O2 bond length is only slightly longer in the staggered structures than in the eclipsed structures. (b) C3-02-X1 angle is smaller than the corresponding CCX angle in CH3CH2Xin every case. In the staggered structures, this angle ranges from 103 to 107' when X is F, 0-, OH, and OH2+, and is 109.5' when X is CI. In the eclipsed structures, ( 5 ) For example, 02H2prefers to be C, symmetrical with H-0-0-H dihedral angle of about 115'. See: Radom, L.; Hehre, W.J.; Pople, J. A. J. Am. Chem. SOC.1972, 94, 2371. (6) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986.
0022-3654/90/2094-4856$02.50/0 0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 12, I990 4857
Rotational Barriers of Methanol Derivatives
TABLE I: Selected Geometrical Parameters and Mulliken Net Atomic Charges of CHjOX (X = F, CI, 0-,OH, and OH2+) compd, config 1, X = F, H staggered 6-31G* MP2/6-3 IG* 1, X = F, H eclipsed 6-31G' MP2/6-3 1G* 2, X = CI, H staggered 6-31G* MP2/6-3 IG* 2, X = CI, H eclipsed 6-31G' MP2/6-3 IG* 3, X = 0-, H staggered 6-31G* MP2/6-3 IG* 3, X = 0-, H eclipsed 6-31G* MP2/6-31G* 4, X = OH, H staggered 6-31G* MP2/6-3 IG' 4, X = OH, H eclipsed 6-31G* MP2/6-3 IG* 5a, X = OH,+, H staggered 6-31G* MP2/6-31G* 5a, X = OH2+,H eclipsed 6-31G* MP2/6-3 1G*
Xi02
OzC3
C3H4
Cd.5
C302Xi
H&Jh
H+W2
QXi
QH4
QH,
1.380 1.451
1.403 1.419
1.08 1 1.092
1.081 1.092
105.0 103.2
104.3 103.2
110.4 110.9
-0.219 -0.229
0.189 0.190
0.182 0.188
1.379 1.450
1.416 1.434
1.077 1.088
1.082 1.092
106.9 105.1
109.4 109.1
108.4 108.4
-0.217 -0.226
0.205 0.212
0.184 0.187
1.668 1.716
1.411 1.432
1.081 1.091
1.083 1.093
112.1 109.5
104.7 103.5
111.0 111.4
0.206 0.186
0.195 0.195
0.175 0.179
I .664 1.713
1.419 1.444
1.080 1.090
1.082 1.092
114.5 112.0
111.9 112.0
107.9 107.7
0.214 0.194
0.183 0.188
0.183 0.185
1.458 1.486
1.356 1.382
1.100 1.107
1.097 1.105
106.1 103.0
109.7 108.5
112.6 112.1
-0.649 -0.638
0.038 0.034
0.058 0.064
1.470 1.497
1.362 1.393
1.087 1.093
1.104 1.112
109.2 106.4
110.8 108.3
112.5 112.4
-0.639 -0.627
0.106 0.1 18
0.032 0.034
1.402 1.477
1.396 1.417
1.082 1.091
1.084 1.094
105.8 102.9
105.5 104.3
110.8 110.9
-0.475
0.177
0.166
1.404 1.479
1.406 1.429
1.079 1.089
1.083 1.093
107.5 104.7
110.0 109.6
109.0 108.9
-0.459
0.190
0.167
1.437 1.492
1.451 1.445
1.080 1.094
1.077 1.089
107.4 106.5
100.9 99.7
109.6 110.8
-0.469 -0.503
0.279 0.279
0.245 0.250
1.428 1.480
1.471 1.470
1.076 I .089
1.077 1.089
108.9 107.7
109.3 109.9
105.6 106.0
-0.460 -0.492
0.252 0.258
0.268 0.268
TABLE 11: Total Energies of Staggered and Eclipsed Structures of CHJOX (X = F, CI, 0-,OH, OH,+)" full optimizations MP2/6-31G* geometries compound 1, X = F, staggered
1, X 2, X 2, X 3, X
= F, eclipsed = CI, staggered = CI, eclipsed
= 0-, staggered 3, X = 0-, eclipsed 4, X = OH, staggered 4, X = OH, eclipsed Sa, X = OH2+,staggered Sa, X = OH2+,eclipsed
"he
3-21G 212.621 7 21 2.61 7 08 57 1.063 22 571.05842 188.122 84 188.1 I8 64 188.763 1 1 188.758 99 189.063 61 189.059 89
6-3 IG* 21 3.765 46 213.75985 573.871 77 573.866 80 189.166 73 189.16095 189.796 15 189.790 97 190.079 69 190.074 82
MP2/6-3 1G* 214.261 26 214.255 1 1 574.332 73 574.327 05 189.669 12 189.661 68 190.299 44 190.293 92 190.576 86 190.571 30
MP2/6-31G** 214.27569 2 14.269 72 574.33908 574.333 42 189.684 78 189.677 63 190.32461 190.319 20 190.609 7 1 190.604 18
MP3/6-3 lG** 214.288 35 214.28282 574.363 14 574.357 77 189.698 55 189.692 02 190.34049 190.335 44 190.629 06 190.62404
MP4/6-3 lG** 214.31385 214.307 95 574.383 49 574.377 95 189.723 65 189.716 82 190.364 07 190.358 83 190.653 69 190.648 19
values are negatives of energies, in hartrees.
TABLE 111: Rotational Barriers (kcalhol) of CHIOX (X = F. CI, 0-.OH, OH2+) compd
1,X=F 2, x = CI 3, x = 04, X = OH 5a. X = OH2+
3-21G 2.9 3 .O 2.6 2.6 2.3
6-31G* 3.5 3.1 3.6 3.3 3.1
MP2/6-31G* 3.9 3.6 4.7 3.5 3.5
this angle is slightly larger than the corresponding angle in the staggered structures. It ranges from 105 to 108' when X is F, 0-,OH, and OH2+, and is 112' when X is CI. (c) The H4-C3-02 angle in the staggered structures, where it is antiperiplanar to the 02-X, bond, is considerably smaller than tetrahedral angle, except when X is 0-. It is 103O, 104O, 1 0 9 O , and 104' when X is F, CI, 0-,and OH, respectively. It is only 100' when X is OH2+. On the other hand, the H5-C3-02 angle in these staggered structures is 1-3' larger than tetrahedral angle. In the eclipsed structures, both H4-C3-02 and HS-C3-02 angles are near tetrahedral, with H5-C3-02 slightly smaller. (d) In the staggered structures, the C3-H4 and C3-H5 bond lengths are very similar. In the eclipsed structures, however, C3-H4, which is eclipsing with respect to the 02-X, bond, is longer than the C3-H5 bond, except when X is OH2+. The C3-H4 bond elongation is most significant when X bears negative charge (0-). This bond length difference disappears or reverses when X is OH?+.
MP2/6-31G** 3.7 3.6 4.5 3.5 3.5
MP3/6-31G** 3.5 3.4 4.1 3.2 3.2
MP4/6-31G** 3.7 3.5 4.3 3.3 3.5
In general, the calculated rotational barriers increase as the quality of the basis set improves.' Correlation energy corrections increase the rotational barriers further. At the best level of calculations (MP4/6-3 lG**//MP2/6-3 1G*) the barriers range from 3.3 to 4.3 kcal/mol. These barriers are 3-4 times higher than that of methanol. Our calculated rotational barrier for methyl hypochlorite, 3.5 kcal/mol, is close to the 3.06 f 0.15 kcal/mol rotational barrier measured experimentally by microwave spectroscopy.8 We did not calculate zero-point energy contributions to the rotational barrier, which may have a small e f f e ~ t . Ex~ (7) Usually calculations for anionic species with diffuse function basis sets give better results. However, our 6-31+G* calculations for methyl peroxide anion gave almost identical geometries and rotational barrier as the 6-3 1G* calculations. For relevant references, see: Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J . Compur. Chem. 1983,4, 294. Frisch, M. J.; Pople, J. A.; Binkley, J. S . J . Chem. Phys. 1984, 80,3265. (8) Rigden, J. S.; Butcher, S. S. J . Chem. Phys. 1964, 40, 2109. (9) Wiberg, K . B.; Murcko, M. A. J . Am. Chem. SOC.1988, 110, 8029.
4858
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990
Wu and Houk
SCHEME 111
perimental rotational barriers for methyl hypofluorite, methyl hydroperoxide, and its derivatives are not available. IR and Raman spectroscopic measurements gave a 5.7 kcal/mol barrier to the CF3 rotation in CF300CF3.10J1 Recently Durig et al. reported that the barriers to internal rotation of methyl group of the inner conformer, 8, and outer conformer, 9, of methyl hy-
vo Figure 1. Orbital interactions between the methyl and 0-X n-orbitals.
4.6 kcaVmol
barrier Me
9 outer
8 inner
staggered
X*C&
10
drazine are 4.6 and 4.2 kcal/mol, respective1y.l2 These barriers, once again, are considerably higher than the 1.9 kcal/mol barrier of methylamine. What causes these barriers to be much larger than that of methanol? The effect is not mimicked with second-row heteroatoms. For example, the observed barrier to the methyl rotation in dimethyl disulfide, which is analogous to dimethyl peroxide, is reported to be 1.5 kcal/mol,13which is only slightly higher than 1.3 kcal/mol barrier for methanethiol. Methyl formate and methoxyacetylene have rotational barriers of 1.2 and 1.4 kcal/mol, r e s p e c t i ~ e l y , ~which ~ * ~ ~are also similar to that of methanol. Therefore, the uniquely high rotational barriers of compounds 1-5 are determined by the special electronic features of these systems. When steric effects are present, barriers also increase. For example, the barrier to methyl rotation in methyl ether is about 3 kcal/mol, about 2 kcal/mol higher than that in methanol, as shown in Scheme HI. Pople et al. have argued that, although steric interaction should increase the rotational barrier, the major reason for the high barrier is a a-type orbital interaction between the two methyl groups, which stabilizes the all-staggered conformation.I6 Besides the factors which influence the rotational barrier of ethane and methanol, there could be several additional factors to influence the rotational barriers in the present cases: 1. Since the 02-Xl bond has an associated low-lying a*-orbital, there could be electron delocalization from the antiperiplanar C-H bond in the staggered structure, a factor proposed to account for the gauche effect in 1,2-difluoroethane.I7 o*ox
(IO) Durig, J. R.; Wertz, D. W. Spectrochim. Acta 1968, 24A, 21. (1 1) Crowder, G. A.; Scott, D. W. J. Mol. Spectrosc. 1965, 16, 122. Hubbard, W. W.; Douslin, D. R.; McCullough, J. P.; Scott, D. W.; Todd, S. S.;Messerly, J. F.; Hossenlop, I. A.; George, A.; Waddington, G. J. Am. Chem. SOC.1958,80, 3547. ( 1 2) Durig, J. R.; Lindsay, N. E.; Groner, P. J. Phys. Chem. 1989,93,593. (13) Crowder, G. A.; Scott, D. W. J. Mol. Spectrosc. 1965, 16, 122. (14) Den Engelsen, D.; Dijkerman, H. A.; Kerssen, J. Recl. Trau. Chim.
1965,84, 1357. ( 1 5) Hirano, T.; Nonoyama, S.;Miyajima, T.; Kinita, Y .; Kawamura, T.; Sato, H. J. Chem. SOC.,Chem. Commun. 1986,606. (16) Cremer, D.; Binkley, J. S.;Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1974,96,6900. Hoffmann, R.; Levin, C. C.; Moss, R. A. J. Am. Chem. SOC.1973, 95, 629. (1 7) For general information about anomeric effects, see: Deslongchamps,
P . Stereoelectronic Effects in Organic Chemistry; Pergamon Press, New York, 1983. Kirby, A. J. The Anomeric Effect and Related Stereoelectronic Effects at Oxygen; Springer-Verlag: New York, 1983.
eclipsed
X*C&
12
W H ~ Xstaggered -
11
~ ~ 3 / 1 Ceclipsed 13
Figure 2. Orbital interactions for staggered and eclipsed conformations. The bonding combinations of methyl n-orbitals and 0-X n--orbitals are shown.
This is a stabilizing two-electron interaction which increases the rotational barrier, since overlap of these orbitals is smaller in the eclipsed conformation. The magnitude of this interaction is dependent on the electronegativity of X. The better the acceptor, X, the larger the delocalization. This effect is strongly indicated by the geometrical features pointed out before. The delocalization in the staggered conformation makes the C3-02 bond shorter and the C3-H4 and 02-X1 bonds longer. The difference between the C3-02 bond lengths of the staggered and eclipsed sturctures can be used to assess the magnitude of the antiperiplanar effect. For example, F is more electronegative than C1, and it does give a larger difference in C3-02 bond length than C1 (0.01 5 A versus 0.012 A). OH2+ is expected to be most electronegative (electron-withdrawing), and it gives the largest difference between the C3-O2 bond length in the staggered and the eclipsed structures. The small H4 N H > 0.22(b) All charged X species have larger stabilizing interactions with Me than with their corresponding neutral species. It is easy to interpret the large stabilization with positively charged species. For example, OH2+is a better acceptor than OH, and the positive charge can be delocalized to the methyl group, which is electron-donating. The protonation energies for methyl anion, ethyl anion, and 2isopropyl anion are estimated to be 417, 421, and 419 kcal/mol in the gas phase:3 respectively, and this is also correctly predicted by Schleyer's ab initio molecular orbital calculation^.^^ This indicates that the methyl groups destabilize the adjacent anion. This is further confirmed by the fact that the methyl anion is observed in the gas phase, but C2-C4 anions are not observed in the gas phase.2s However, there are examples where alkyl groups stabilize anions. It has recently been reported that neopentyl anion ~ ~ well-known that the is observable in the gas p h a ~ e . ~It ~is .also acidity of simple alcohols, R-OH, increases in the order R = Me < Et C i-Pr < ~ - B u . ~ 'This is also true in the simple amine series.28 These results suggest that the increase in the size of R increases the anion stability. Such phenomena are often attributed to the polarizability of anionic alkyl groups. That is, the anion polarizes the alkyl groups adjacent to it and becomes stabilized by electrostatic or dipole effects.29 This is also interpreted by the negative hyperconjugation through n-type orbital interactionsN Our calculational results, that is, larger Me/planar CH2- interaction over Me/pyramidal CHy, support the concept of negative hyperconjugation.
Conclusion In summary, the barrier to the methyl rotation is significantly increased by a geminal 0-X, where X is an electronegative heteroatom functional group. The geometrical features of these molecules are consistent with the concept of a vCH3/y*eX hyperconjugation effect as well as n-type conjugation, especially when X is in the anion form. Acknowledgment. We are grateful to the National Science Foundation for financial support of this research. (22) Reed, A. E.; Schleyer, P. v. R. J . Am. Chem. SOC.1987, 109, 7362. (23) Depuy, C. H.; Bierbaum, V . M.; Damrauer, R. J . Am. Chem. SOC. 1984, 106, 4051. Tumas, W.; Forster, R. F.; Brauman, J. I. Ibid. 1984, 106, 4053. (24) Schleyer, P. v. R.; Spitznagel, G. W.; Chandrasekhar, J. Tetrahedron Lert. 1986, 27, 441 1. (25) Graul, S. T.; Squires, R. R. J . Am. Chem. SOC.1988, 110, 607. (26) Dillard, J. E. Chem. Reu. 1973, 711, 589. (27) Brauman, J. I.; Blair, L. K. J . Am. Chem. SOC.1970, 92, 5986. (28) Brauman, J. I.; Riveros, J. M.; Blari, L. K . J . Am. Chem. SOC.1971, 93, 3914. (29) Pellerite, M. J.; Brauman, J. I. In Comprehensive Carbanion Chemistry; Buncel, E., Durst, T., Eds.; Elsevier: Amsterdam, 1980; Chapter 2. (30) Janousek, B. K.; Zimmerman, A. H.; Reed, K . J.; Brauman, J. I . J . Am. Chem. SOC.1978, 100, 6142.
