Theory of the Hyperfine Splittings of Pihyphen;Electron Free Radicals. III. Methyl Radical in a Pyramidal Configuration: Temperature Dependence of the Hyperfine Splittings

摘要

Nonempirical calculations for methyl radical (CH3middot;) in a pyramidal configurationlpar;C3ugr;rpar;were performed using two minimum basis sets of Slaterhyphen;type orbitals, one in which orbital exponents were chosen according to Slater's rules (unoptimized) and the other in which they were optimized by minimization of the SCF energy. The spinhyphen;restricted SCF plus configurationhyphen;interaction method, including all spinhyphen;adapted configurations with single and double excitations of space orbitals, was employed. The temperature dependence of the contact hyperfine splittings was computed assuming that the variation with temperature arises from the outhyphen;ofhyphen;plane bending motion. Agreement with experiment at various temperatures is good. The optimizedhyphen;basis values of the temperature coefficient of the proton splitting,daHthinsp;sol;thinsp;dT, are about 20percnt;ndash;50percnt; too large and the unoptimizedhyphen;basis results are about 10percnt;ndash;25percnt; too small. Temperature coefficients of the carbonhyphen;13 splitting calculated using the optimized and unoptimized basis sets also bracket the experimental values, from which they deviate by less than 10percnt;, which is within the experimental uncertainty. The variation of the spin densities at the magnetic nuclei with the outhyphen;ofhyphen;plane angle is analyzed in detail. Other results of the calculation are discussed, namely, the equilibrium molecular geometry, an approximate frequency for the bending motion, and the incomplete orbital following.