## Abstract

An interpretation is given of the chlorine nuclear quadrupole coupling constants $\chi$(Cl) for the series of dimers B$\cdots$HCl and B$\cdots$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 $\chi$(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 $\chi_E$ 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 $\chi_E = \chi_P + \chi_Q = -eQ\{\mathbb{F}_{zz}F_z + \mathbb{G}_{zz}F_{zz}\}/h.$ The first term $\chi_P$ results from the polarization of the HCl subunit by the electric field F$_z$ due to B. The second term $\chi_Q$ arises from the field gradient F$_{zz}$ due to B but modified by the factor $(1 + \gamma_{zz}) = \mathbb{G}_{zz},$ where $\gamma_{zz}$ is the usual Sternheimer antishielding factor. $\mathbb{F}_{zz}$ 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 $\chi_Q$ 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 $\chi_P$ has then been calculated for each member of the series B$\cdots$HCl 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 $\mathbb{F}_{zz}$ value of a gas-phase molecule and we report $\mathbb{F}_{zz} = - 116(6) x 10^{10} m^{-1}$ for the HCl molecule.