Abinitiostudy of the electronic structure and hyperfine coupling properties in simple hydrocarbon radicals. II. Shorthyphen;range and longhyphen;range interactions in alkyl free radicals

摘要

Nonempirical calculations of the groundminus;state energy, proton and carbonminus;13 coupling constants of methyl, ethyl,nminus;propyl, and cyclopropyl radicals have been performed in the frame of spinminus;restricted LCAOminus;SCF openminus;shell and firstminus;order doubleminus;perturbation theories. The rotation barrier is negligible for the ethyl radical. The two barriers of thenminus;propyl radical are, respectively, 0.3 kcal/mole (0.4 exptl) and 4.7 kcal/mole for the rotations about the Cdot;minus;Cagr;and Cagr;minus;Cbgr;bonds. The cyclopropyl radical is found to be nonplanar with an outminus;ofminus;plane angle of 41deg; and an inversion barrier of 3.80 kcal/mole. For the equilibrium conformations, the computed carbonminus;13 splittings of the radical carbon of methyl (1), ethyl (2),nminus;propyl (3), and cyclopropyl (4) are +31.04, +37.90, +37.72, and +138.83 G, respectively; the theoretical agr; minus;carbonminus;13 splittings are minus;18.14 (2), minus;17.15 (3), and minus;8.35 G (4); the bgr; minus;carbonminus;13 splitting is +11.8 G for the stable conformation of thenminus;propyl radical. The calculated coupling constants are negative for agr; hydrogens, namely minus;31.03 (1), minus;33.82 (2), minus;34.34 (3), and minus;9.20 G (4); they are found positive for bgr; hydrogens, i.e., +16.8 (2), +23.58 (3), and 12.12 G (4); the ggr; couplings of the freely rotating hydrogens of thenminus;propyl radical are negative minus;0.21 G. The variation of these couplings with conformational and torsional effects has also been investigated. The radical carbonminus;13 splitting is quite insensitive to rotations of substituents but increases markedly with outminus;ofminus;plane deviations at the radical site. The coupling constants of agr; nucleiaHagr;,aCagr;are also insensitive to conformational modifications. For bgr; atoms, the contact hyperfine splittings are found to follow the relationabgr;=B0+B2cos2Vthgr; independently of the chemical nature of the nuclei,B0being negative if vibronic effects are neglected. Longminus;range couplings in ggr; position are generally negative, except in the region depicted by the sominus;calledWminus;plan arrangement. The role of delocalization and spin polarization is discussed for each kind of coupling. It is shown that intrabond excitations of the CHrarr;CHast; type dominate the spin polarization at the proton in all positions, while innerminus;shell excitations are found to be essential in the case of the radical carbon.