Pulsed NMR studies of matrix isolated HCl and (HCl)2

Abstract

A proton free probe suitable for pulsed NMR studies of matrix isolated molecules has been used to obtain the proton spectrum of HCl in solid 40Ar for HCl mole fraction 0.003<x<0.02. The spectrum consists of a central Lorentzian associated with the HCl monomer superimposed on a well-defined Pake doublet due to the dimer (HCl)2. The Pake doublet is a result of the proton–proton dipolar interactions between the dimer HCl units. A structural parameter of the (HCl)2 ground state configuration is calculated from the doublet splitting. For the linear hydrogen bond the angle between the HCl unit containing the non-hydrogen-bonded proton and the axis containing the H bond is approximately 122°. An upper limit of 150 μsec for the chlorine spin–lattice relaxation time is calculated from the observed broadening of the Pake line shape. The linewidth of the HCl monomer is roughly an order of magnitude larger than that calculated from model systems used to account for the infrared absorption of matrix isolated HCl. It is suggested that this discrepancy is the result of a much more anisotropic matrix environment than previously assumed.