ABSTRACT
Infrared (IR) spectra of complexes between NaCl and methanol have been recorded for the first time. These complexes were formed in liquid helium nanodroplets by consecutive pick-up of NaCl and CH3OH molecules. For the smallest NaCl(CH3OH)n, complexes where n = 1-3, the IR data suggest that the lowest-energy isomer is the primary product in each case. The predominant contribution to the binding comes from ionic hydrogen bonds between the OH in each methanol molecule and the chloride ion in the NaCl, as established by the large red shift of the OH stretching bands compared with the isolated CH3OH molecule. For n ≥ 4, there is a dramatic shift from discrete vibrational bands to very broad absorption envelopes, suggesting a profound change in the structural landscape and, in particular, access to multiple low-energy isomers.
ABSTRACT
To explore how the ion-pair in a single salt molecule evolves with the addition of water, infrared (IR) spectra of complexes composed of NaCl and multiple water molecules have been recorded for the first time. The NaCl(H2O)n complexes were formed and probed in liquid helium nanodroplets, and IR spectra were recorded for n = 1 â 4. The spectra for n = 1, 2, and 3 are consistent with formation of the lowest energy contact-ion pair structures in which each water molecule forms a single ionic hydrogen bond to an intact Na(+)Cl(-) ion-pair. Alternative structures with hydrogen bonding between water molecules become energetically competitive for n = 4, and the IR spectrum indicates likely the coexistence of at least two isomers.
