J. Phys. Chem. 1991,95, 1609-1612
1609
Conformational Energies of 2-Fluoroethanol and 2-Fluoroacetaldehyde Enol: Strength of the Internal Hydrogen Bond David A. Dixon* and Bruce E. Smart Central Research and Development Department,t Experimental Station, E. I . du Pont de Nemours & Company, Inc., Wilmington, Delaware 19880-0328 (Received: July 10, 1990)
The structures and energies of several conformers of 2-fluoroethanol and 2-fluoroacetaldehydeenol (I-hydroxy-2-fluoroethylene) have been calculated by using ab initio molecular orbital theory with a triple-{ basis set augmented by polarization functions on all atoms. Correlation and zero-point energy corrections were included. The gauche hydrogen-bonded conformer (GG) is the ground-state structure for CH2FCH20H,in agreement with experiment. At the MP-2 level, the gauche rotamer with no hydrogen bonding (GT) is 1.9 kcal/mol higher in energy, and the all-trans conformer TT is 2.0 kcal/mol higher than GG. The greater stability of gauche-CH2FCH20Htherefore is almost entirely due to hydrogen bonding, and the "gauche effect" contributes only about 0.1 kcal/mol. For CHF=CH(OH) at the MP-2 level, the cis-syn hydrogen-bonded isomer is 3.2 kcal/mol more stable than the cis-anti isomer, and 4.1 kcal/mol more stable than the trans-syn isomer, which is the lowest energy trans conformer. In contrast to the results for CH2FCH20H,the "cis effect" in CHF==CH(OH) (0.9 kcal/mol) is nearly identical with that found in CHF=CHF (1.0 kcal/mol), even though the internal hydrogen bond in the enol is considerably stronger than that in CH2FCH20H.
Introduction Among the 1,2-dihaloethanes, 1,Zdifluoroethane uniquely adopts a gauche conformation.' Both experiment" and theoryZbvc agree that the gauche conformer is preferred by 0.8 kcal/mol in the gas phase. This is the archetypical example of the so-called "gauche effect", whereby vicinal, highly polar bonds unexpectedly tend to favor conformations that maximize the number of gauche interaction^.^ A related but more general phenomenon that similarly runs counter to expectations based on electrostatic or steric considerations is the greater thermodynamic stability of the cis over the trans isomers of ethylenes with electronegative vicinal substituents. This "cis effect" holds for all 1,2-dihaloethylenes (fluoro, chloro, bromo), and the trans-cis energy difference in CHX=CHX progressively increases with the electronegativity of the halogen X to a maximum value of 1.08 & 0.12 kcal/mol in CHF=CHF.4 High-level theoretical calculations also can accurately reproduce this experimental energy differen~e.~ Since the electronegativity of the hydroxyl group is only slightly less than that of the fluorine atom,6 the gauche and cis effects predict 2-fluoroethanol will favor a gauche conformation and cis-2-fluoroacetaldehydeenol, cis-CHF==CH(OH), will be more stable than the trans isomer. There are no data for the enol, but 2-fluoroethanol is estimated experimentally to favor a gauche over a trans conformation by at least 2 kcal/mol in the gas phase.7 Compared to 1,2-difluoroethane, however, 2-fluoroethanol has an additional complicating degree of freedom, namely, its hydroxyl group can intramolecularly hydrogen-bond to the fluorine atom. A hydrogen-bonded gauche conformer indeed is found to be the most stable structure, but it was not possible to determine experimentally the energy of the internal hydrogen bond.7a The relative contribution of the gauche effect versus hydrogen-bonding to the greater stability of the gauche form of 2-fluoroethanol therefore remains unknown. To address this issue, we have calculated the relative energies of various conformers of 2-fluoroethanol by using ab initio molecular orbital theory that has proved to reliably describe structures, energies, and dynamic properties of other fluorocarbons. Following our theoretical work on the energy difference in 1,2difluoroethylenes, we also have calculated the relative energies of several conformers of cis- and trans-2-fluoroacetaldehydeenol to assess the importance of hydrogen-bonding versus the electronic cis effect in this system. Calculations The calculations were done with the programs HONDO* (on an IBM 3081 computer) and GRADSCF~(on a Cray X-MP/28 com-
puter). Geometries were gradient-optimized1° and force fields were calculated analytically" at the S C F level. Correlation corrections were made at the MP-2 levelI2for the valence electrons. The geometries for the all-trans and the hydrogen-bonded gauche structures were initially optimized with a double-{ basis set augmented with polarization functions on C and O.I3 On the basis of our experience with 1,2-difl~oroethane~~ and the 1,2diflu~roethylenes,~ a larger basis set is required to calculate properly the relative energies of the two conformers of the ethane and of the cis and trans isomers of the ethylene. Final optimizations and subsequent calculations therefore were done with a triple-[ basis setI4 augmented by two sets of polarization functions on C, F, and 0 and one set of polarization functions on the (1) Summaries of experimental data on 1,2-dihaloethanes: (a) Huang, J.; Hedberg, K. J . Am. Chem. SOC.1990, 112, 2070. (b) Smart, B. E. In Molecular Structure and Energetics; Liebman, J. F., Greenberg, A., Eds.; VCH Publishers: Deerfield Beach, FL, 1986; Vol. 3, Chapter 4. (c) Meyer, A. Y. In The Chemistry of Functional Groups, Supplement D Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1983; Part 1, Chapter 1. (2) (a) Hirano, T.; Nonoyama, S.;Miyajima, T.; Kurita, Y.; Kawamura, T.; Sato, H. J . Chem. SOC.,Chem. Commun. 1986, 606. (b) Dixon, D. A.; Smart, B. E. J . Phys. Chem. 1988,92,2729. (c) Wiberg, K. B.; Murcko, M. A. J. Phys. Chem. 1987, 91, 3616. (3) Wolfe, S.Acc. Chem. Res. 1972, 5, 102. (4) (a) Craig, N. C.; Piper, L. G.; Wheeler, V. L. J . Phys. Chem. 1971, 75, 1453. (b) Craig, N. C.; Overend, J. J . Chem. Phys. 1969, 51, 1127. (c) Craig, N. C.; Entemann, E. A. J . Am. Chem. SOC.1961, 83, 3047. (5) Dixon, D. A,; Smart, B. E.; Fukunaga, T. Chem. Phys. Lett. 1986,125, 447. (6) Representative values from different electronegativity scales: (a) 4.00 (F), 3.64 (OH) (Boyd, R. J.; Edgecombe, K. E. J. Am. Chem. SOC.1988,110, 4182); (b) 3.10 (F), 2.79 (OH) (Mullay, J. J . Am. Chem. SOC.1985, 107, 7271); (c) 3.95 (F), 3.7 (OH) (Wells, P. R. Prog. Phys. Org. Chem. 1968, 6, 111). (7) (a) Huang, J.; Hedberg, K. J. Am. Chem. SOC.1989, 111,6909. (b) Hagen, K.; Hedberg, K. J . Am. Chem. SOC.1973, 95,8263. (c) Buckley, P.; Giguere, P. A.; Yamamoto, D. Can. J. Chem. 1972, 50, 152. (d) Buckton, K. S.;Azrak, R. G. J . Chem. Phys. 1970, 52, 5652. (8) Dupuis, M.; Rys, J.; King, H. F. J . Chem. Phys. 1976, 65, 11 1. ( 9 ) GRADSCF is an ab initio program system designed and written by A. Komornicki at Polyatomics Research. (10) (a) Komornicki, A.; Ishida, K.; Morokuma, K.; Ditchfield, R.; Conrad, M. Chem. Phys. Lett. 1977,45, 595. (b) McIver, J. W., Jr.; Komornicki, A. Chem. Phys. Lett. 1971, 10, 202. (c) Pulay, P. In Applications of Electronic Structure Theory; Schaefer, H. F., 111, Ed.; Plenum Prcss: New York, 1977; p 153. (1 1) (a) King, H. F.;Komornicki, A. J . Chem. Phys. 1986,84, 5465. (b) King, H. F.; Komornicki, A. In Geometrical Deriuatiues of Energy Surfaces and Molecular Properties; Jorgenson, P., Simons, J., Eds.; NATO AS1 Series C, Vol. 166; D. Reidel: Dordrecht, The Netherlands, 1986; p 207. (12) (a) Moller, C.; Plesset, M. S.Phys. Reu. 1934, 46, 618. (b) Pople, J. A,; Binkley, J. S.; Seeger, R. Int. J. Quantum Chem., Symp. 1976, 10, 1. (13) Dunning, T. H., Jr.; Hay, P. J. In Methods ofElectronic Structure Theory; Schaefer, H. F., 111, Ed.; Plenum Press: New York, 1977; Chapter 1
(14) Dunning, T. H. J. Chem. Phys. 1971, 55, 716.
'Contribution No. 5520.
0022-3654/91/2095-1609$02.50/0
0 1991 American Chemical Society
1610 The Journal of Physical Chemistry, Vol. 95, No. 4, 1991 TABLE I: Geometries for 2-Fluoroethmol Conformers'
r(C-C)
ric-oj r(C-F) r(C-H,) r(C-Hd 4c-H~) r(C-H4) r(0-H) B(CCF) B(CCH1) B(CCH2) B(CCH3) B(CCH4) B(FCH) B(FCH) e(cco) B(OCHJ) B(OCH4) B(HICH2) O(HjCH4)
B(C0H)
GG
TT calc
calc
exptb
1.511 1.403 1.372 1.081 1.081 1.085 1.085 0.941 108.8 111.0 1 1 1.0 109.5 109.5 108.3 108.3 106.7 111.4 1 11.4 109.5 108.4 110.2
1.506 1.399 1.378 1.082 1.081 1.082 1.086 0.943 109.1 111.1 I 1 1.2 108.8 109.5 107.7 107.4 112.3 107.0 11 1.0 110.1 108.1 109.4
1.518 (61 1.432 (16) 1.398 (24)
parameter
108.5 (8)
112.3 (14) 111.7 (50) 1 11.7 (50) [lO5.8Ic
GT calc 1 SO4
1.401 1.369 1.082 1.081 1.088 1.086 0.941 110.7 110.1 110.7 108.0 109.0 107.6 107.9 109.0 110.9 111.2 109.8 108.5 110.2
Bond distances in angstroms; bond angles in degrees. Reference 7a. Assumed value. hydrogens. The heavy-atom polarization functions were each a two-term fiti5to a Slater type orbital with the diffuse exponents set at 0.7 and the contracted exponents set at 2.0, 2.1, and 2.2 for C , 0, and F , respectively.i6 The conformations for 2fluoroethanol examined were the trans,trans (TT), hydrogenbonded gauche,gauche (GG), and gauche,trans (GT) structures, where the first letter is the conformation about the C-C bond and the second is the conformation about the C - 0 bond.
GG
GT
7T
For the enols, calculations were done on the cis and trans isomers with the 0 - H bond syn or anti to the CC double bond, and these are labeled as cis- and trans-syn or cis- and trans-anti, respectively. U
cis-syn
cis-anti
H'
trans-syn
trans-anti
Results and Discussion Geometries. 2-Fluoroethanol. The molecular geometries for 2-fluoroethanol are given in Table I. We first compare the calculated values to the experimental parameters7a for the GG structure. The structure was determined by electron diffraction in the gas phase, and thus all of the geometric parameters could (15) Stewart, R. F. J. Chem. Phys. 1969, 50, 2485. (16) Komornicki, A.; Dixon, D. A. J . Chem. Phys. 1987, 86, 5625.
Dixon and Smart not be independently refined. An additional difficulty in the experimental refinement is the similarity of the C-F and C - 0 bond distances. Thus in the refinement, the angle O(C0H) had to be fixed and was set at the value found for C H 3 0 H . i 7 Also, the C-H and 0 - H bond distances could not be determined individually. An average value and the difference in the C-H and 0 - H bond distances were determined. Thus, we can only really compare the parameters of the heavy atoms. As expected, the C-F and C-C bond distances are calculated to be 0.01-0.02 8, shorter than the experimental values. However, the difference in the calculated and observed values of 0.033 8, for the C - O bond is greater than the differences usually expected at this level of calculation. The bond angles involving the heavy atoms are in reasonable agreement with the experimental values, although the angle O(C0H) is calculated to be 109.4', compared with the assumed value of 105.8'. Furthermore, the calculated HCH angle is smaller than the experimental value of 111.7'. The internal hydrogen bond is characterized by an H - - - F bond distance of 2.52 8,and is highly bent: O(&H---F) = 99.8'. The calculated torsion angles T(OCCF)= 63.7' and r ( C C 0 H ) = -60.3' compare to the experimental values of 63.7 (8)' and -54.6 (78)', respectively. Thus, there is overall good agreement between the calculated and observed GG structures, especially when the experimental uncertainties are considered. The geometries show some small variations upon rotation, wherein the largest change is in the r(C-F) bond distance. The largest changes in bond angles occur at C I ,and the hydrogenbonded GG structure has a larger O(CC0)compared to the other two structures. 2-Fluoroacetaldehyde Enols. The geometries of the 2fluoroacetaldehyde enols are given in Table 11. The most stable structure (see below) is the cis-syn isomer. The molecular parameters are as expected. The calculated C=C bond distance is similar to those in the 1,2-difl~oroethylenes.~ The C - 0 bond is significantly shorter than that calculated for C2H50H.'* The C-F bond is elongated relative to the C-F bonds in the other syn and trans conformers as well as the C-F bond in 1,2-difluoroethylene.s This is primarily due to the formation of an internal hydrogen bond in the cis-syn isomer. The hydrogen-bond parameters are r(H-- - F ) = 2.366 8, and O ( 0 - H - - -F) = 106.0'. Thus, the hydrogen-bond distance is significantly shorter in the enol than in the alcohol, and the hydrogen-bond angle is also somewhat larger in the enol. These results suggest that the enol has a stronger hydrogen bond than does the alcohol. The only geometry parameters that change for the different conformers and isomers are those for atoms involved in the hydrogen bond. The C - O distance varies from 1.343 to 1.359 8,;O(0CC) varies from 126.0 to 120.8'. The value for r(C-F) varies from 1.340 to 1.327 A. One of the more surprising results of this study is the finding that the anti structures may not be minima but can be transition states on the potential curve for torsion about the C - O bond. At the DZ+Dc,o level, both cis-anti- and trans-anti-CHF=CF(OH) are transition states for torsion. Improvement of the basis set to the TZ+P level makes the cis-anti structure a minimum, but the trans-anti structure remains a transition state. The potential surface for torsion about the C - 0 bond is very flat in the anti region of the cis isomer. A structure rotated by 13' still has all real frequencies and an energy only 0.04 kcal/mol higher at the SCF level and 0.08 kcal/mol higher at the MP-2 level. For the trans isomer, the results are different, with the planar anti conformer still a transition state at the TZ+P level, and the minimum energy anti conformer has a torsion angle T(CCOH)of 39.1 '. Energies. 2-Fluoroethanol. The relative energies are given in Table 111. The GG structure is the most stable one at the SCF and MP-2 levels. At the SCF level, the TT structure is more stable than the GT structure, but this reverses at the MP-2 level. This (17) Ivash, E. V.; Dennison, D. M. J . Chem. Phys. 1953, 21, 1804. (18) At the 3-21G* level, r(C-0) = 1.444 A, as compared to an experimental value of 1.425 A: Hehre, W. J.; Radom, L.; Schleyer, P.v. R.; Pople, J . A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986; p 170.
The Journal of Physical Chemistry, Vol. 95, No. 4, 1991 1611
2-Fluoroethanol and 2-Fluoroacetaldehyde Enol TABLE II: Geometries for 2-Fluomacetaldehyde Enols"
parameter
cis-syn
cis-anti
cis-anti-rb
trans-syn
trans-anti
trans-anti-P
r(C-C) r( C-0) r(C-F) r(C1-H) r(C2-H) r(0-H) o(occ) B(0CH) B(FCC) B(FCH) O(HCiC2) B(HC2C I ) O(C0H) r(CC0H)
1.309 1.343 1.340 1.070 1.067 0.944 126.0 113.9 120.4 114.3 120.1 125.2 110.3 0.0
1.309 1.348 1.327 1.072 1.068 0.944 123.4 118.2 122.6 114.2 118.5 123.2 110.7 180.0
1.309 1.349 1.327 1.073 1.068 0.940 123.4 118.1 122.6 114.1 118.5 123.2 110.7 13.4
1.309 1.350 1.335 1.07 1 1.071 0.943 125.8 113.0 120.4 113.3 121.2 126.3 111.4 0.0
1.307 1.355 1.334 1.073 1.069 0.940 121.2 1 18.0 120.2 114.0 120.8 125.8 1 10.6 180.0
1.307 1.359 1.332 1.074 1.069 0.942 120.8 117.9 120.5 1 14.0 121.3 125.5 110.4 39.1
" Bond distances in angstroms; bond angles in degrees. Rotated structure. TABLE 111: Relative Energies of 2-Fluoroethnnol Conformers (kcal/mol) DZ+Dc,o" TZ+P* structure AE(SCF) GG 0.00 GT TT I .48
AE(MP-2) AE(SCF) 0.00 0.00 1.90 2.42 1.50
AE(MP-2) AH(MP-2)c 0.00 0.00 2.12 1.93 2.25 2.05
'Total energy (GG) = -252.981 389 au (SCF),-253.513834 au (MP-2). bTotal energy (GG) = -253.036861 au (SCF), -253.735015 au (MP-2).
cZero-pint energy (GG) = 44.46 kcal/mol (scaled).
follows from the results found for 1 ,Zdifluoroethane, where correlation was found to favor the gauche structure.2b Relative to the TT structure, the GG structure is stabilized by 0.75 kcal/mol, whereas the G T structure is stabilized by 0.22 kcal/mol when correlation is included. By comparison, the stabilization at the MP-2 level for the gauche structure of 1,2-difluoroethane is 0.44 kcal/mol. Thus GG-CH2FCH20Hgains slightly more stabilization due to correlation effects than does gaucheCH2FCH2 F. To directly compare our results with experimental data, we corrected the energy differences for zero-point effects. Although these are small, they lower the energy of the TT and G T structures relative to the GG structure by 0.2 kcal/mol. Thus we find that the hydrogen-bonded GG structure is 2.0 kcal/mol more stable than the TT structure and 1.9 kcal/mol more stable than the GT structure. We now can quantify the internal hydrogen-bond strength and gauche effect in CH2FCH20H. The hydrogen bond is worth 1.9 kcal/mol (AHO(GT-GG)) and the gauche effect amounts to only 0.1 kcal/mol (AHO(TT-GT)). Notably, the gauche effect in CHzFCH20H is much smaller than that in CH2FCH2F for which AHo(trans-gauche) = 0.8 kcal/mol.2 Our computed AHo(TT-GT) or AHo(GT-GG) values agree well with most of the reported experimental energy differences. From electron diffraction data,7a the trans-gauche energy difference was crudely estimated to be 2.7 (+1.8, -1.5) kcal/mol, and our calculated values fall within the lower error limit. The difference in energy between the hydrogen-bonded form and the other forms has been c a l ~ u l a t e dfrom ' ~ band intensities in CCI4 solution to be 2.07 f 0.53 kcal/mol, in excellent agreement with our values of 2.0 and 1.9 kcal/mol. Other experimental values range from >2.0 kcal/mol in the gas to N M R measurements in solution that place the energy difference between 1.05 to 1.95 kcal/mol.20 From our force field calculations, we also have derived the entropies of the various conformers: GG (69.19 eu), GT (69.92 eu), and TT (70.40 eu) at 298 K. (The difference in entropies is dominated by the low-energy torsion modes.) Thus we find AGo(TT-GG) = AGo(GT-GG) = 1.7 kcal/mol. There has been one previously high-quality calculation2' on 2-fluoroethanol at the 6-31 IC** level with correlation at the MP-2 (19) Kruger, P. J.; Mettee, H. D. Can.J . Chem. 1964, 42, 326. (20) Pachler, K.G. R.; Wessels, P. L. J . Mol. Srrucr. 1970, 6, 471. (21) Wiberg, K. B.; Murcko, M. A. THEOCHEM 1988, 40, 1.
TABLE I V Relative Energies of 2-Fluoroacetaldebyde Enols (kcal/mol)
DZ+Dc,o" structure cis-sy n cis-anti cis-anti4 trans-syn trans-anti trans-anti-r'
AE(SCF) 0.00 4.23d 4.22 3.82 4.29d 4.04
AE(MP-2) 0.00 4.57 4.54 3.93 4.69 4.50
AE(SCF) 0.00 3.30 3.34 4.13 4.57d 4.35
TZ+pb AE(MP-2) 0.00
3.53 3.61 4.45 5.06 4.96
AH(MP-2)c 0.00 3.21 3.27 4.08 4.30 4.40
"Total energy (cis-syn) = -251.785332 au (SCF), -252.309847 au (MP-2). bTotal energy (cis-syn) = -251.843042 au (SCF), -252.516651 au (MP-2). 'Zero-pint energy (cis-syn) = 30.19 kcal/mol (scaled). dTransition state. 'Rotated structure (see Table 11). and MP-3 levels. The geometries were obtained at the 6-31G* level. The results were similar to ours. The GG structure is the most stable, as expected, with AE(TT-GG) = 2.24 kcal/mol and AE(GT-GG) = 2.30 kcal/mol at the MP-2 level. The results do not change significantly at the MP-3 level where AE(TT-CG) = 2.22 kcal/mol and AE(GT-TT) = 2.03 kcal/moi. Although the magnitude of the energies is comparable to our values, we find the TT structure to be less stable than the G T structure at the correlated level. This difference is due to basis set differences and to the use of geometries calculated with a smaller basis set. We calculate at the DZ+DC,o level an H- - -F hydrogen-bond distance of 2.44 A, in good agreement with the value of 2.42 A found at the 6-31G* level. However, the larger TZ+P basis set yields a longer hydrogen bond to 2.52 A, which compares to an approximate experimental value of 2.49 (4) A.7a 2-Nuoroacetaldehyde Enols. The relative energies are given in Table IV. The most stable structure is the cis-syn isomer. The strength of the hydrogen bond can be estimated by comparing the energies of the cis-syn and cis-anti structures. From this difference alone, the strength of the hydrogen bond is 3.30 kcal/mol at the S C F level and 3.53 kcal/mol at the MP-2 level. The syn conformer is still favored over the anti in the trans isomer, where hydrogen bonding to F is not possible. The trans-syn structure is more stable than the trans-anti rotated structure by 0.22 kcal/mol at the S C F level and 0.51 kcal/mol at the MP-2 level. The trans-anti structure is a transition state that is 0.61 kcal/mol higher in energy than the trans-syn structure at the MP-2 level. The energy difference between the cis-syn and trans-syn structures is 4.45 kcal/mol. If we subtract the strength of the hydrogen bond given above, the cis structure is still 0.92 kcal/mol more stable than the trans structure. This is nearly identical with the calculated cis-trans energy difference of 0.95-1 .OO kcal/mol for 1,2-difluoroethylene. Thus, in marked contrast to effects in CHzFCH2Fvs CH2FCH20H,the electronic effect responsible for the cis-trans difference (cis effect) is the same, independent of whether the second substituent is F or OH in the ethylene. The differences in the cis-anti and trans-anti rotated structures are slightly higher, 1.43 kcal/mol, showing that there is an additional electronic effect favoring the cis isomer for this conformation. The zero-point effects are even larger in the fluoroacetaldehyde enols
1612
J . Phys. Chem. 1991, 95, 1612-1618
than in 2-fluoroethanol. The zero-point effect lowers the energy difference between the cis-syn and cis-anti structures by 0.32 kcal/mol, which gives a hydrogen-bond strength of 3.2 kcal/mol. The cis-trans energy difference for the syn conformers decreases to 4.1 kcal/mol. The strength of the hydrogen bond is significantly higher in the enol than in the alcohol, 3.2 kcal/mol in the former and 1.9 kcal/mol in the latter. This difference is also reflected in the relative hydrogen-bond lengths, 2.37 8, in cis-syn-CHF+H(OH) and 2.52 8, in GG-CH2FCH20H. The former is 0.30 8,shorter than the sum of H and F van der Waals radii (2.67 whereas the latter is only 0.15 8, shorter. The stronger hydrogen bond in the enol can be attributed principally to its more favorable planar, five-membered ring geometry for intramolecular bonding.23 Vibrational Spectra. There are several notable features in the calculated vibrational spectra for 2-fluoroethanol. The 0-H stretches shift from 4158 cm-' in the hydrogen-bonded GG structure to 4176 cm-' in the G T structure to 4183 cm-l in the 'IT structure. Such red shifts in the 0 - H stretches have previously been observed in other hydrogen-bonded systems.24 The hydrogen-bonding also raises the torsion frequency about the C - 0 bond from 244 and 241 cm-' for the G T and TT structures to 330 cm-' for the GG structure. Thus, the hydrogen bond helps to lock in the position of the bridging hydrogen. The torsional frequency about the C-C bond, 166 cm-l for GG or TT and 139 cm-' for TT, is only weakly affected by hydrogen-bonding. The value for the torsion about the C-C bond in the GG form is in good agreement with the assigned experimental valueZSof 152 f I O cm-I, especially since we have not scaled26 the calculated values to account for anharmonicity or correlation effects. The frequencies of the 0 - H stretches show some surprising trends in the 2-fluoroacetaldehyde enols. The 0 - H stretch in the (22) Bondi, A. J. Phys. Chem. 1964, 68, 441. (23) There appear to be no special electronic properties in the enol that might favor hydrogen-bonding, e.g., CHF=CH(OH) -CHFCH=O+H. The Mulliken charges on the F atoms, and on the 0 and H atoms of the OH groups, are nearly identical in CH2FCH20Hand CHF=CHOH and are virtually independent of geometry: qF,qo, qH (e) = -0.37, -0.64,0.38 (GG); -0.36, -0.63,0.38 ('IT);-0.36, -0.63,0.37 (GT); -0.35, -0.60,0.40 (cis-syn); -0.33, -0.58, 0.39 (cis-anti):-0.33, -0.60, 0.38 (trans-syn). (24) Pine, A. S.; Lafferty, W. J. J . Chem. Phys. 1983, 78, 2154. (25) Buckton, K. S.; Azrak. R. G.J . Chem. Phys. 1970, 52, 5652. (26) Dixon, D. A. J . Phys. Chem. 1988, 92, 86.
-
cis-syn structure with the hydrogen bond is 4142 cm-I. There is a significant blue shift to a frequency of 4198 cm-' in the cis-anti form, consistent with the results for 2-fluoroethanol. The trans isomer, however, shows a similar behavior, even though there is no hydrogen bond. The 0-H stretch is at 4156 cm-' in the trans-syn structure and at 4204 cm-I in the trans-anti structure. The rotated trans-anti structure shows a blue shift of only I O cm-' from the trans-syn frequency. Obviously, the trends in 0 - H stretching frequencies are not necessarily diagnostic of hydrogen-bonding. The lowest lying frequencies for the enols also exhibit informative trends. The three lowest frequencies in the cis-syn structure are at 260,419, and 591 cm-I and can be assigned to an in-plane bend, torsion about the OH bond and an out-of-plane bend, respectively. The cis-anti structure has these frequencies at 15 1, 263, and 580 cm-I, which are assigned to torsion about the 0 - H bond, an in-plane bend, and an out-of-plane bend, respectively. Thus, the 0 - H torsion, as expected, is more restricted in the cis-syn structure where there is a hydrogen bond. The other two frequencies show little change on rotation. In the trans-syn isomer, the 0 - H torsion frequency falls between the corresponding frequencies of the two cis conformers at 288 cm-l. The in-plane and out-of-plane bends are at 348 and 402 cm-I, respectively. The rotated trans-anti structure has the 0 - H torsion at a lower frequency of 209 cm-1 and the two bends at 351 and 371 cm-I. The trans-anti structure is a transition state with an imaginary frequency of 1831 cm-l for the 0-H torsion and the in-plane and out-of-plane bends at 342 and 373 cm-l, respectively. Dipole Moments. The calculated dipole moment of 1.56 D for the hydrogen-bonded GG conformer of CH2FCH20His in excellent agreement with the experimental value of 1.51 f 0.02 D.25 This agreement is consistent with the dominance of the hydrogen-bonded structure. The calculated dipole moment of the TT structure is 1.97 D and of the GT structure is 3.03 D. The calculated dipole moment of cis-syn-CHF=CH(OH) is only 0.89 D, compared with 3.35 D for the cis-anti conformer, which again is indicative of hydrogen-bonding in the cis-syn isomer. Even the dipole moment of the trans-syn isomer (1.84 D) is considerably larger than the cis-syn value. The rotated trans-anti conformer has a dipole moment of 2.07 D, and the trans-anti planar transition state has a dipole moment of 2.25 D. Registry No. (Z)-FCH=CH(OH), 371-62-0.
Theoretical Study of Stable Carbocations and Their Interactions with Anions Hiroshi Fujimoto,* Satoshi Denno, and Yasuhisa Jinbu Division of Molecular Engineering, Kyoto University, Kyoto 606, Japan (Received: August 2, 1990)
The relative stabilities of ion-pair and biradical (or covalent) electron configurations are studied with respect to a simple system, CH3--C3H3+,in a C3, symmetrical structure with a view of clarifying the basic feature of hydrocarbon ion pairs. As a possible source of stabilizing cations, the effect of hyperconjugation is analyzed on cyclopropenium ions by representing the interaction between the ring and the substituent groups in terms of paired interacting orbitals. The ability of carbon atoms in cyclopropeniumand tropylium ions for electron acceptance is also discussed by projecting out the reactive unoccupied orbitals each of which has the maximum amplitude on the 2p, atomic orbital of one of the carbon atoms in the threemembered or seven-membered ring. A tropylium ion with an interesting reactivity trend is suggested. Other factors that should stabilize hydrocarbon ion pairs are discussed.
Introduction The formation and cleavage of chemical bonds in organic reactions have long been discussed in terms of two mechanisms, Le., ionic and radical or heterolytic and homo1ytic.I The changes in
bonds are thought generally to take place along with a change in electron distribution. Recently, the possibility of an electrontransfer mechanism has been suggested.2 In that, transfer of an electron between the reagent and the reactant precedes the for-
( I ) See, for example: Lowly, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry; Harper: New York, I98 1.
(2).See, for example: Eberson, L. Electron Transfer Reactions in Organic Chemistry; Springer: Berlin, 1987.
0022-3654/91/2095-1612$02.50/0
0 1991 American Chemical Society
