Infrared Laser Spectroscopy of Salt-Solvent Complexes using Helium Nanodroplets

Infrared (IR) spectra of complexes consisting of an alkali halide salt (MX) molecule and one or more protic solvent molecules have been recorded for the first time in this thesis. NaCl and LiI are the principal salts investigated and have been combined with the solvents water and methanol to form complexes of type MX(solvent)n (n = 1-7) in liquid helium nanodroplets. IR spectra were recorded using a depletion technique. For small complexes (n = 1 – 3) the spectra are consistent with formation of contact ion-pair structures in which each solvent molecule forms a single ionic hydrogen bond (IHB) to an intact M+X- ion-pair. For n ≥ 4, the IR spectra suggest that multiple isomers of MX(solvent)n are present in helium nanodroplets. Ab initio calculations were used to support the results by predicting possible structures and their corresponding IR spectra. A mass spectrometric study of a variety of alkali halide salts with a variety of protic and aprotic solvents in helium nanodroplets was also performed. The aim here was to survey the mass spectrometric behaviour of species within the helium nanodroplets in preparation for future IR spectroscopic studies. The salts chosen for this study were NaCl, NaBr, NaF, NaI, LiI, LiCl, CsI and KF, while the solvents chosen were water, methanol, acetone and acetonitrile, i.e. two protic and two aprotic solvents. The ions observed in the mass spectra have been described and assigned to be derived from the neutral species in helium nanodroplets environment.