Vibrational-rotational dependence of molecular properties. Electric field gradients for HCl, LiCl, NaCl and KCl

The vibrational-rotational dependence of the nuclear quadrupole coupling co nstant (NQCC) for the isotopes H-2, Li-7, Na-23, K-39, and Cl-35 is analyse d in detail for the diatomic Group 1 chlorides HCl, LiCl, NaCl and KCl. The potential energy curves were calculated pointwise by using coupled cluster techniques. The electric field gradients (EFGs) and dipole moments were ob tained analytically from a QCISD procedure using the Z vector method. Gener ally the calculated spectroscopic properties are in very good agreement wit h experimental data. Relativistic effects taken into account by a coupled c luster Douglas-Kroll procedure can safely be neglected for the electric fie ld gradients up to potassium. The Inglis model which explains the trend and magnitudes of EFGs within an ionic model of weakly polarized atoms is anal ysed. According to this model the derivatives of the EFG, partial derivativ e(n)q(R)/partial derivative R-n, with respect to the internuclear distance R should show alternating sign behaviour with increasing power n. Hence, th e mechanical anharmonicity (deviation from Hooke's law potential) and the e lectrical anharmonicity (curvature of cl(R)) are of different sign, and we expect partial cancellation of anharmonicity effects in the vibrational dep endence of the NQCCs. Nevertheless, a perturbative vibrational-rotational a nalysis reveals a strong dependence of the chlorine and Group 1 element NQC Cs on the vibrational level due to dominating mechanical anharmonicity.