Nonempirical Calculations of the Isotropic Hyperfine Splitting Constants of 13C, 14N, and 17O in Simple Radicals

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Abstract
The isotropic hyperfine splitting constants in ESR spectra for simple radicals 13C1H3, 14N1H2, and 17O1H have been calculated nonempirically by using a configuration‐interaction method correct to the second order in the energy matrix elements. Included in the treatment are all the singly excited configurations plus certain types of doubly excited configurations. Each configuration is expressed in terms of the self‐consistent‐field molecular orbitals and virtual orbitals derived for the ground state. The molecular orbitals are expanded about a single center with an extended basis set of Slater‐type orbitals. The calculated hyperfine splitting constants for 13C, 14N, and 17O are found to be 2.7, 0.9, and −0.5 G, respectively, when all the virtual orbitals with symmetry species A1′ or A1 are used; the values are improved to 40.6, 19.0, and −35.4 G, respectively, when certain virtual orbitals with undesirable characteristics are excluded. Though not to be given serious consideration because of the nature of the wavefunctions employed, calculations for 1H are carried out and the values are found to range from −4 to −7 G. Attention is called to the possible danger in using virtual orbitals indiscriminately for calculations on ground‐state properties other than energy. Based on the experience gained from this work we suggest that only the orbitals that are better approximations to the excited‐state molecular orbitals be used.