CONFORMATIONAL ENERGIES OF 2-FLUOROETHANOL AND 2-FLUOROACETALDEHYDE ENOL - STRENGTH OF THE INTERNAL HYDROGEN-BOND

The structures and energies of several conformers of 2-fluoroethanol and 2-fluoroacetaldehyde enol (1-hydroxy-2-fluoroethylene) have been calculated by using ab initio molecular orbital theory with a triple-zeta 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 CH2FCH2OH, 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-CH2FCH2OH therefore 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 CH2FCH2OH, 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 CH2FCH2OH.