Infrared photodissociation spectra and solvation structures of Mg+(CH3OH)n (n=1–4)

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Abstract

The infrared photodissociation spectra of mass-selected Mg+(CH3OH)n (n=1–4) are measured and analyzed with the aid of density functional theory calculations. Hydrogen bonding between methanol molecules is found to be absent in Mg+(CH3OH)3, but detected in Mg+(CH3OH)4 through characteristic frequency shifts of the OH stretch of methanol. The maximum number of the methanol molecules that can be directly bonded to the Mg+ ion is limited to three and the fourth molecule starts to fill the second solvation shell. The vibrational spectroscopy provides clear evidence for the closure of the first shell at n=3.

Introduction

The process of ion solvation has been studied extensively for a wide variety of ions and solvents [1]. High-pressure mass spectrometry has been used to obtain enthalpies and free energies of association as a function of solvent number [2], [3], providing insight into the presence of solvent shells. Time-of-flight mass spectrometry [4] and flow-tube studies [3] have also found great utility in identifying solvated ions of unusual stability through the observation of magic numbers. More detailed information on the ion solvation has become available through spectroscopic studies of the solvated ions. Vibrational spectroscopy of singly-charged alkali metal ions solvated by H2O and CH3OH has provided evidence for the formation of the second solvation shell before the complete filling of the first shell [5]. Electronic spectroscopy has been applied to singly-charged alkaline earth metals solvated by polar molecules, since the electronic transitions of the single valence electron offer a convenient probe of the local solvation environment around the metal ion [6], [7].

The Mg+(CH3OH)n system has been the subject of previous investigations. Bauschlicher and co-workers [8], [9], [10] carried out extensive theoretical calculations to predict geometrical structures and binding energies for n=1 and 2 as well as vertical transition energies for n=1. In flow-tube reactions studied by Castleman and co-workers [11], successive solvation products were observed without H-atom elimination up to n=6. Stace and co-workers [12] examined the gas-phase chemistry of Mg+ and Mg2+ in association with methanol clusters and found that the Mg+(CH3OH)n ions show `product switching' via H-atom elimination when n⩾3. Lu and Yang [13] performed mass spectrometric experiments with supporting ab initio calculations to facilitate the interpretation of the product switching behavior. Stepwise binding energies were determined in collision-induced dissociation experiments with supporting ab initio calculations by Armentrout and co-workers [14]. Photodissociation spectrum of Mg+(CH3OH)1 in the ultraviolet (UV) region was reported by Duncan and co-workers [15]. More recently, Farrar and co-workers [16] carried out UV/visible photodissociation spectroscopy of Mg+(CH3OD)n (n=1–5). The spectra exhibit substantial red shifts as the second and third methanol molecules are added, but the addition of the fourth or fifth molecule results in small red shift. The observation was interpreted in terms of the closure of the first solvent shell with three methanol molecules.

In this work, we apply vibrational spectroscopy to Mg+(CH3OH)n (n=1–4). The method has capability of probing the solvation structures more directly, since the OH stretch of methanol is extremely sensitive to changes in its bonding environment. Density functional theory (DFT) calculations are also performed to obtain minimum-energy structures and corresponding infrared (IR) spectra of Mg+(CH3OH)n. The comparison of the experimental and theoretical results provides information on the size of the first solvent shell.

Section snippets

Experimental and computational

The IR photodissociation spectra of Mg+(CH3OH)n (n=1–4) are measured by using a triple quadrupole mass spectrometer [17]. The solvated metal ions are produced in a laser vaporization source. The parent ions of interest are isolated by the first quadrupole mass filter. After deflection by an ion bender, the ions are introduced into the second quadrupole ion guide and irradiated by an infrared laser (Continuum, Mirage 3000). The vibrational excitation induces fragmentation of the parent ions.

Minimum-energy structures

Geometries of Mg+(CH3OH)n have been optimized for n=1 and 2 at the SCF/TZ2P level by Bauschlicher and co-workers [8], [9], [10], for n=1–5 at the UHF/6-31G* level by Lu and Yang [13], and for n=1–3 at the MP2/6-31G* level by Armentrout and co-workers [14]. The stable structures obtained from our calculations are consistent with those from the previous calculations, except as noted. We only briefly describe the qualitative aspects of our results as well as the previous ones [8], [9], [10], [11],

Acknowledgments

We thank Dr. K. Ohshimo for his help in equipping our photodissociation spectrometer with the laser vaporization source. This work was supported in part by the Joint Studies Program (2003) of the Institute for Molecular Science.

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