Heteroatomic molecules: calculation of proton and 14N hyperfine splittings

Abstract

Proton and ¹⁴N hyperfine splittings have been calculated for 13 anions and 6 cations of heteroatomic conjugated molecules containing nitrogen, using a variable β ZDO—PPP—π treatment, involving minimization of the energy with respect to bond lengths. Several open shell methods are used and compared. Agreement with experiment is quite satisfactory when either the projected unrestricted Hartree-Fock or preferably the restricted Hartree-Fock method plus configuration interaction, including all single excitations, is used. Various relations between π spin density and hyperfine splitting have been compared and it has been found that for proton or ¹⁴N splittings a relation of the form a=Qρπ, is sufficient. However, small but significant improvement in calculated ¹⁴ N splittings has been found with the use of an added term dependent on the spin density on the atoms neighbouring the nitrogen.