QUADRUPOLE COUPLING IN PURINES AND PYRIMIDINES BY HARTREE-FOCK LATTICE CALCULATIONS OF ELECTRIC-FIELD GRADIENTS

We present ab initio Hartree-Fock lattice calculations on adenine, gua nine and hypoxanthine, and some pyrimidines, including cytosine and ur acil derivatives. The electric field gradients at the nitrogen centres are related to NQR experimental determinations of nuclear quadrupole coupling constants. The calculations were performed as lattice calcula tions in the unit cell environment, with 6-31G or double zeta basis se ts at the SCF level. The present analysis strongly suggests that chi(z z) at N-3 in cytosine, N-3 in guanine are both positive, and approxima tely tangential to the ring at that centre. In contrast, N-7 in guanin e is like most other azine-type N centres, with a largely radial direc tion for chi(zz) The 3-protonated cytosine ring has chi(zz) as the loc al pi-direction.