N-1, F-19, AND C-13 NUCLEAR-MAGNETIC-RESONANCE APPROACHES TO THE BARRIER TO ROTATION ABOUT THE CSP(2)-CSP(3) BOND IN 1,1,1-TRIFLUORO-2-PHENYLETHANE - PROXIMATE COUPLING-CONSTANTS AND MOLECULAR-ORBITAL COMPUTATIONS

The H-1, 'F-19 and C-13{H-1} nuclear magnetic resonance spectra of 1,1 ,1-trifluoro-2-phenylethane, 1, in CS2-C6D12, acetone-d(6), and benzen e-d(6) solutions, on analysis, yield long-range coupling constants fro m which are derived the apparent twofold barriers to rotation about th e Csp(2)-Csp(3) bonds. The twofold barrier is 9.0(2) kJ/mol, independe nt of solvent, 4.0 kJ/mol larger than that of ethylbenzene, also indep endent of solvent. The theoretical barrier heights for the free molecu les at the post-Hartree-Fock level of molecular orbital theory (frozen -core MP2/6-31G) also differ by 4 kJ/mol, but are about 1 kJ/mol high er than the experimental estimates. The,perpendicular conformer is the most stable for both molecules. Comparisons are made with the benzyl halides, in which the internal barriers are remarkably sensitive to so lvent. A spin-spin coupling constant over five formal bonds, (5)J(H, F ) involving the ortho protons in 1, is +0.74(2) Hz and is discussed in some detail in terms of its dependence on internuclear distances (pos sible through-space interactions). The solvent perturbations of (3)J(H , F) and of (2)J(C, F) are of opposite sign. Other long-range coupling constants or their absence are also pointed out. For example, those b etween F-19 and C-13 nuclei or protons at the meta position are effect ively zero; at the para position they are significant.