Insights into the Structures of the Gas-Phase Hydrated Cations M⁺(H₂O)_nAr (M = Li, Na, K, Rb, and Cs;n= 3–5) Using Infrared Photodissociation Spectroscopy and Thermodynamic Analysis

Abstract

The hydration of alkali cations yields a variety of structural conformers with varying numbers of water molecules in the first solvation shell. How these ions move from the aqueous phase into biological systems, such as at the entrance of an ion channel, depends on the interplay between competing intermolecular forces, which first must involve ion-water and water-water interactions. New infrared action spectra, using argon as a messenger or "spy", for Li(+), Na(+), and K(+), with up to five water molecules are reported, and new structural conformers determined from ab initio calculations, combined with previous results on Rb(+) and Cs(+), have identified structural transitions at each hydration level. These transitions are a result of the delicate balance between competing noncovalent interactions and represent a quantitative microscopic view of the macroscopic enthalpy-entropy competition between energy and structural variety. Smaller cations (Li(+) and Na(+)), with higher charge density, yield structural configurations with extended linear networks of hydrogen bonds. Larger cations (Rb(+) and Cs(+)), with lower charge density, generate configurations with cyclic hydrogen-bonded water subunits. It appears that K(+) is somewhat unique, with very simple (and predominantly) single structural conformers. This has led to the suggestion that K(+) can "move" easily in or through biological systems, concealing its identity as an ion, under the "appearance" or disguise of a water molecule.

Structural and electrostatic effects at the surfaces of size- and charge-selected aqueous nanodrops

“…First, the number of distinguishable bands is related to the heterogeneity of H-bonding environments of water molecules at the surfaces of these clusters. The frequencies of free OH stretches are exquisitely sensitive to a water molecule's H-bonding environment, 62 – 68 , 79 , 88 , 89 and the free OH stretch of a water molecule red-shifts as the water molecule's participation in H-bonding increases. Using the nomenclature where “A” denotes a H-bond accepted by a water molecule and “D” denotes a H-bond donated by a water molecule, free OH bands appear in increasing frequency following the order AAD < AD ≈ AA < A.…”

“…The effect of a single ion or electron on water structure can be investigated as a function of hydration extent, ranging from a single water molecule up to hundreds of water molecules, and information about ion solvation can be obtained in the limit of infinite dilution. 55 – 61 Infrared photodissociation (IRPD) spectroscopy can probe the structures of numerous gas-phase hydrates, including metal ions, 62 – 68 anions 26 , 28 , 69 – 72 and protonated/ionized amino acids and peptides. 73 – 76 Heteroatom-hydrogen stretching frequencies are sensitive to their local H-bonding environment, and IRPD measurements have been used to elucidate detailed hydration structures of ions with up to ∼20 water molecules attached.…”

Hydrated Sodium Ion Clusters [Na+(H2O)n (n = 1–6)]: An ab initio Study on Structures and Non-covalent Interaction

“…In this work, some of the structures of the Na + (H 2 O) n ( n = 1–6) clusters are adopted from previous literatures (Bauschlicher et al, 1991; Glendening and Feller, 1995; Ke et al, 2015; Fifen and Agmon, 2016). To obtain more isomers for n = 4–6, a global search with the comprehensive genetic algorithm (CGA, Zhao et al, 2016) combined with DMol 3 program (Delley, 2000) based on DFT was executed.…”

“…The infrared (IR) spectra are available in distinguishing the cluster isomers. For hydrated sodium ion clusters, the feature peaks of O–H stretching mode could accurately provide the structure information of clusters (Huang and Miller, 1989; Patwari and Lisy, 2003; Vaden et al, 2004; Miller and Lisy, 2008a,b; Ke et al, 2015), in which the non-covalent interactions, including ion–water interaction and hydrogen bond, weaken the O–H bonds, causing redshifts for O–H stretching vibration modes and producing different feature peaks for different structures (Muller-Dethlefs and Hobza, 2000; Vaden et al, 2002, 2004, 2006; Kozmutza et al, 2003; Bush et al, 2008; Miller and Lisy, 2008a), Using a custom-built, triple-quadrupole mass spectrometer as well as ab initio calculations, Lisy et al reported the IR spectra of Na + (H 2 O) n ( n = 2–5) and Na + (H 2 O) n Ar ( n = 2–5) (Miller and Lisy, 2008a,b; Ke et al, 2015). For n = 4, 3+1+0 is the stable structure with bent hydrogen bonds (Miller and Lisy, 2008a).…”

“…For n = 4, 3+1+0 is the stable structure with bent hydrogen bonds (Miller and Lisy, 2008a). Recently, through straightforward IR spectra for n = 5, they speculated that 4+1+0 and 3+1+1 could be concomitant at 75 K (Ke et al, 2015).…”