Theoretical study of the properties of methyl radical

Abstract

A detailed ab initio study of the structure and properties of methyl radical in its ground electronic state is presented. The primary focus is on the spin density distribution and its dependence on basis set, wave function model, and molecular geometry. By using a natural orbital analysis that provides an unambiguous decomposition into core and valence contributions, interesting theoretical and computational results are found that clarify the relationships among CI, UHF, and PUHF models. In particular, it appears that the better agreement with experiment for the PUHF model as compared to UHF is simply fortuitous. A large and negative core contribution found in the ¹³C isotropic hyperfine coupling constant explains why minimal basis set calculations always greatly overestimate this property, regardless of what wave function model is used. All the models studied give good results for anisotropic hfc and for the vibrational corrections to isotropic hfc, although not for the isotropic hfc themselves. Through comparison to the ab initio results, some conditions are revealed where semiempirical methods such as INDO should fail. The dependence of potential energy and various charge density properties on molecular geometry is also studied.