BARRIERS TO INTERNAL-ROTATION IN CF3CFCL2, CF3CHCL2, CF3CF2BR, CF3CF2CL, CF3CH2BR, AND CF3CH2CL - F-19 AND H-1 DYNAMIC NMR-STUDIES - ABINITIO MOLECULAR-ORBITAL BARRIER CALCULATIONS

Decoalescence of the F-19 and H-1 NMR spectra of the title compounds occurs at low temperatures due to slowing trifluoromethyl rotation. Theoretical simulations of the exchange-broadened spectra allow measurement of the rate of rotation as a function of temperature and calculation of the activation parameters (DELTAG(double-dagger), kcal/mol; DELTAS(double-dagger), cal/(mol.K); DELTAH(double-dagger), kcal/mol) for CF3CFCl2 (6.1 +/- 0.1 at 125 K; -5 +/- 3; 5.6 +/- 0.3), CF3CHCl2 (5.6 +/- 0.1 at 122 K; -3 +/- 3; 5.2 +/- 0.3), CF3CF2Br (4.9 +/- 0.1 at 103 K; -2 +/- 3; 4.7 +/- 0.3), CF3CF2Cl (4.7 +/- 0.1 at 106 K; -4 +/- 3; 4.2 +/- 0.3), CF3CH2Cl (4.2 +/- 0.1 at 93 K; -1 +/- 4; 4.3 +/- 0.4), and CF3CH2Br (4.2 +/- 0.1 at 92 K; -4 +/- 4; 3.9 +/- 0.4). The barriers in CF3CF2Cl, CF3CF2Br, CF3CH2Cl and CF3CH2Br are among the lowest measured by using NMR spectroscopy. For CF3CF2Cl, CF3CF2Br, CF3CH2Cl, and CF3CH2Br, the NMR-measured barriers are 1-2 kcal/mol lower than those measured by using microwave and/or infrared spectroscopy. Ab initio molecular orbital barrier calculations (3-21G basis set) for CF3CFCl2 (5.5 kcal/mol), CF3CHCl2 (5.2 kcal/mol), CF3CF2Br (4.4 kcal/mol), CF3CF2Cl (4.3 kcal/mol), CF3CH2Cl (4.3 kcal/mol), and CF3CH2Br (4.2 kcal/mol) agree very well with the NMR-determined barriers and less well with the microwave/infrared barriers. The ab initio molecular orbital calculations give insight into the geometric adjustments that occur in the eclipsed transition state conformations.