Restricted Hartree-Fock Method Research Articles

Heteroatomic molecules: calculation of proton and 14N hyperfine splittings

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.

Rotating barrier end groups in organic cumulenes

An elementary explanation in terms of MO theory is given for the stability of the various conformations of symmetry D 2h and D 2j for the organic cumulenes CnH4. Results are given for the rotation barriers for the end groups as derived by the restricted Hartree-Fock method in the semiempirical approximation for closed and open shells. It is found that the barrier is ~0.2 eV for an infinitely long cumulene with equal bonds. The origin of the effect is discussed, which is absent in the simple and extended forms of Huckel's method. It is shown that the ex-extended form of Huckel's method incorrectly gives barrier alternation.