The prediction of nuclear quadrupole moments from ab initio quantum chemical studies on small molecules. I. The electric field gradients at the 14N and 2H nuclei in N2, NO, NO+, CN, CN-, HCN, HNC, and NH3

Electric field gradients (efg's) at the nitrogen nuclei in N₂, NO⁺, NO, CN, and CN^- and at the nitrogen and hydrogen nuclei in HCN, HNC, and NH₃, calculated using ab initio quantum chemical methods, are reported. Employing extensive Gaussian basis sets, the efg's were computed at the self-consistent field (SCF), singles and doubles configuration interaction [CI(SD)], and coupled pair functional (CPF) levels of theory as the expectation values of the efg operator and also as the energy derivatives of the appropriate perturbed Hamiltonian using the finite field method. Corrections due to zero point vibrational motions were also calculated. The effect of basis set incompleteness on the calculated efg's, together with the experimental nuclear quadrupole coupling constants, are used to estimate the ¹⁴N and ²H nuclear quadrupole moments, and to test the quality of the correlated wave functions generated by the CI(SD) and CPF methods. The recommended values, on the basis of the present calculations, are 2.05±0.02 and 0.29±0.01 fm², respectively, for the ¹⁴N and ²H quadrupole moments.