Size-dependent reactivity in open shell metal-ion polar solvent clusters: spectroscopic probes of electronic-vibration coupling, oxidation and ionization

James M. Farrar†
2003 International reviews in physical chemistry (Print)  
This review considers the spectroscopy and structure of clusters formed by stepwise addition of polar solvent molecules such as NH 3 , H 2 O, and CH 3 OH to effective one-electron chromophores that include the singly-charged alkaline earth cations Mg þ , Ca þ , and Sr þ ; atomic sodium; and the Rydberg molecule NH 4 . The absorption of photons by such species results in initial electronic excitation, followed by energy transfer to vibrational degrees of freedom, ultimately leading to
more » ... ading to dissociation. Experimental data are presented to support this electronic-tovibrational energy transfer mechanism. Wavelength-dependent photodissociation signals for all of these species exhibit a similar size-dependence in which large spectral red shifts are observed as the first solvation shell fills. An examination of ab initio calculations on a number of related systems, as well as theoretical models for charge transfer, suggests that non-covalent interactions of solvent molecules with the single valence electron of the cluster core lead to spontaneous ionization of the core with increasing cluster size. The process is analogous to the formation of solvated electrons in condensed phases. The collective results suggest that within the broad topic of solvation and the formation of solutions, clusters can provide a linkage between the properties of the gas phase and those of condensed phases. Exotic systems: Sr Conclusions 43 Acknowledgements 43 References 43 The fundamental role played by ion solvation in this diverse set of problems has led to intense experimental and theoretical activity to understand the structural and dynamical aspects of the process. In particular, gas phase spectroscopy of massselected metal ion-solvent complexes has played a critical role in developing our understanding of these fundamental processes at the most elementary level. Mass spectrometry and optical spectroscopy of size-selected cluster ions are important methods for understanding solvation phenomena, especially the manner in which each successive solvent molecule contributes to the establishment of the properties of bulk solutions [1] . A significant amount of research employing vibrational spectroscopy on size-selected clusters of closed-shell singly-charged ions, i.e. the alkali ions, solvated by H 2 O and CH 3 OH, has provided evidence for the formation of a second solvation shell before the first shell has coordinated fully [18] . Closed-shell ions, while interesting in their own right, provide only a limited perspective on solvent shell formation based on steric and electrostatic factors. Probes of the interactions between open-shell ions and solvent molecules offer additional opportunities to assess solvation in physically realizable bulk systems. However, the study of such systems introduces additional challenges, many of which arise from the existence of more than one oxidation state of the metal. The alkaline earth systems provide a case in point. The singly-charged alkaline earth ions, e.g. Mg þ , Ca þ , and Sr þ , represent examples of systems that do not occur naturally in condensed phase systems, but nonetheless provide insight into the ion-molecule interactions that ultimately lead to solvation. The electronic transitions of the single valence electron in these species provide a convenient probe of the local solvation environment around the ion. Recently, Stace [15] has pointed out the importance of observing the higher oxidation states of clusters formed from polar solvent molecules with alkaline earth ions, and has also commented on the difficulties associated with producing such species. The well-known reactivities of these ions with the solvent H 2 O to form the metal hydroxides, or with CH 3 OH to form metal methoxides and hydroxides, illustrate the propensity of these species to access the higher 2þ oxidation state even in clusters. The coupling of the electronic energy of the atomic ion core with reactive processes involving solvent degrees of freedom [19] [20] [21] provides critical opportunities for probing structural and dynamical features of the solvation process. The excitation energies of electronic states based on the strongly-allowed valence transitions in the alkaline earth ions exceed their electrostatic binding energies with polar solvents [22, 23] such as NH 3 , H 2 O, CH 3 OH, and CH 3 NH 2 ; consequently, those couplings can lead to dissociation by cleavage of specific bonds within the solvent and by the simpler process of solvent evaporation [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] . Those processes can occur both on excited state surfaces and on the ground state surface following internal conversion. The local electronic environment of the metal ion valence electron provides detailed spectroscopic information about excited state and ground state dynamics, probing the solvation process at the level of the potential energy surfaces that govern motions on the atomic length scale. Singly-charged alkaline earth cations are isoelectronic with alkali atoms, and thus a study of the evolution of electronic structure in the vicinity of the atomic centre may also provide insight into the process of spontaneous ionization and formation of solvated electrons. This topic continues to occupy the efforts of experimentalists and theoreticians today [34, 35] . In this review article, we discuss recent studies of the electronic structure and reaction dynamics of alkaline earth cations bound to variable numbers of polar Open shell metal-ion polar solvent clusters 595 J. M. Farrar 596 Figure 1. Schematic diagram of TOF-reflectron tandem mass spectrometer at the University of Rochester.
doi:10.1080/01442350310001616896 fatcat:skeflxrfandfdh6h4ah6rhanru