Infrared Spectroscopy of NaCl(CH₃OH)_n Complexes in Helium Nanodroplets

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 CH 3 OH molecules. For the smallest NaCl(CH 3 OH) 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 CH 3 OH 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. ■ INTRODUCTIONAlkali halides (MX) are archetypal univalent salts. The crystalline structure of the solid is disrupted by water, and these salts readily dissolve to form separated ions in dilute aqueous solutions. If the solid represents one extreme, then the counterpart is an isolated MX molecule. How might this smallest entity of salt, a single MX molecule, behave in the presence of a small water cluster, (H 2 O) n ? This problem can be addressed in experiments by forming isolated MX(H 2 O) n complexes and then using techniques, such as spectroscopy, to determine their properties. Such studies offer insight into the interaction between the constituent ionic particles and the solvent and can be reinforced through quantum chemical calculations. Understanding these interactions and the balance between them is critical in being able to carry out accurate simulations of bulk solutions. An early infrared (IR) spectroscopic study of a NaCl/H 2 O mixture in an argon matrix provided a tentative assignment of vibrational bands of the NaCl(H 2 O) complex.2 The first detailed spectroscopic study of several different NaCl(H 2 O) n complexes (n = 1−3) was obtained by microwave spectroscopy, and this revealed important structural information. 3,4 We have recently shown that complexes between NaCl and water molecules can be formed inside liquid helium nanodroplets, and this made it possible to record the first IR spectra of several small NaCl(H 2 O) n complexes. 5 The spectra for n = 1−3 are consistent with structures in which an ionic hydrogen bond (IHB) is formed between a single OH group in each water molecule and the Cl − in the salt. Evidence for interwater hydrogen bonding only begins to appear for n ≥ 4.An alternative solvent to water is methanol. Most solid alkali halides will dissolve in methanol, but their solubility is far lower than that in water. 6,7 The reduced solubility of salts in methanol when compared with that in water is a consequence of the hydrophobic (CH 3 ) group in the former. It would therefore be interesting to investigate how this amphiphilic character affects the interactions between a single salt molecule and one or more methanol molecules. It is well-known that...

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