Electric-charge redistribution in the HCl subunit of hydrogen-bonded dimers B • • • HC1 from Cl nuclear quadrupole coupling: determination of the coefficient F zz of the electric-field-induced field gradient in HCl

Abstract

An interpretation is given of the chlorine nuclear quadrupole coupling constants X (Cl) for the series of dimers B • • • HCl and B • • • DCl where B = CO, C 2 H 4 , C 2 H 2 , PH 3 , H 2 S, HCN, CH 3 CN, H 2 O and NH 3 . The factors that contribute to the change in X (Cl) on dimer formation are considered in turn. First, account is taken of the effect of bond lengthening of the HCl subunit that occurs on dimer formation. Secondly, the contribution X E to the change in the coupling constant that arises from the electrical effect of B on the field gradient at the Cl nucleus in the dimer is treated at equilibrium in terms of two contributions according to the equation X E = X P + X Q = ‒ eQ {F zz F z + G zz F zz }/ h . The first term X P results from the polarization of the HCl subunit by the electric field F z due to B. The second term X Q arises from the field gradient F zz due to B but modified by the factor (l + γ zz ) = G zz , where γ zz is the usual Sternheimer antishielding factor. F zz is the corresponding factor associated with the field gradient at the Cl nucleus resulting from the polarization of the HCl subunit by the field due to B. The term X Q is directly evaluated using an available Sternheimer antishielding factor. Thirdly, allowance is made for the effect of averaging over the zero-point bending motion of the dimer. Finally, the remaining term X P has then been calculated for each member of the series B • • • HC1 and shown to be linearly dependent on F z as required by the above expression. Hence it has been possible for the first time to make an experimental determination of an F zz value of a gas-phase molecule and we report F zz = ‒116(6) x 10 10 m -1 for the HCl molecule.