Corey J. Weinheimer

Size selectivity by cation–π interactions: Solvation of K+ and Na+ by benzene and water

Size-specific interaction of alkali metal ions with aromatic side chains has been proposed as a mechanism for selectivity in some K+ channel proteins. Experiments on gas-phase cluster ions of the form M+(C6H6)n(H2O)m, with M=Na or K, have demonstrated that the interaction between benzene and K+ is sufficiently strong to result in partial dehydration of the ion, i.e., benzene will displace some water molecules from direct contact with the ion. In sharp contrast, there is no evidence that benzene can displace water from the first hydration shell of Na+. The resistance of Na+(H2O)4 towards dehydration in an aromatic environment suggests a molecular-level mechanism for the low permeability of Na+ through the pore region of K+ channel proteins: the hydrated Na+ ion is too large to pass, while K+ can shed enough of its hydration shell to fit through the pore. These results also suggest that it may be possible to design a new class of ionophores that take advantage of the cation–π interaction to confer ion selec...

Microscopic hydration of the fluoride anion

A combined experimental and theoretical investigation of the step-wise hydration of the fluoride ion has been performed in order to characterize the details of its solvation at the microscopic level. The comparable anion–water and water–water interactions pose a challenging experimental/theoretical problem due to competing intermolecular forces in these small ionic clusters. Vibrational spectra of size-selected F−(H2O)3−5 in the O–H stretching region, coupled with high level ab initio calculations, have been used to analyze the spectra and assign the structures of these species. The interaction between the fluoride anion and water plays the dominant role, resulting in internally solvated clusters. The microhydration of fluoride ion is thus qualitatively different from the other halide ions.

Competitive solvation of K+ by benzene and water: Cation-π interactions and π-hydrogen bonds

The competition between ion–molecule and hydrogen bond interactions in K+(benzene)1–5(water)1,2 is examined using infrared spectroscopic and mass spectrometric methods. The cation-π interaction and π-hydrogen bond play an important role in the structure of the mixed cluster ions. Important observations include: the preferential binding of benzene vs water to K+; the “dehydration” of the potassium ion by benzene; and the observation of water acting as a double proton donor with π-hydrogen bonds to two benzenes.

Vibrational predissociation spectroscopy of Cs+(H2O)1−5

Infrared spectra of Cs+(H2O)1−5 were obtained from vibrational predissociation of mass‐selected cluster ions in a triple quadrupole mass spectrometer using a pulsed‐tunable infrared laser in the 2.6–3.0 μm region. By comparison to size‐selective infrared spectra of neutral water clusters, the structure of hydrogen‐bonded water clusters complexed to the Cs+ can be observed for cluster ions with three or more water molecules. The onset of hydrogen bonding is also marked by the presence of structural isomers. There is also evidence for an unusual change in the vibrational transition moments for the symmetric and asymmetric O–H stretch, for isolated (non‐hydrogen‐bonded) water molecules, where the symmetric stretch is substantially enhanced.

The solvation of chloride by methanol—surface versus interior cluster ion states

A combined experimental and theoretical structural study of methanolated chloride anions has been conducted, utilizing infrared vibrational spectroscopy and ab initio electronic structure calculations. These results represent the first such study for an anion with a nonaqueous solvent. The principal question addressed is whether the cluster ions assume structures reflecting surface or interior solvated states. The vibrational spectra in the O–H stretching regions, for Cl−(CH3OH)1–8,10,12, and calculated O–H vibrational bands for Cl−(CH3OH)1–4, consistently indicate that the chloride anion undergoes surface solvation. The behavior is remarkably similar to that of hydrated anions (chloride, bromide, and iodide) with large polarizabilities. This suggests that the asymmetric hydration of these anions lies not necessarily in the nature of the solvent, but in the nature of the anion.

Evaporatively cooled M+(H2O)Ar cluster ions: Infrared spectroscopy and internal energy simulations

Rotationally resolved IR spectra of M+ (H2O)Ar cluster ions for M=Na, K, and Cs in the O-H stretch region were measured in a triple-quadrupole mass spectrometer. Analysis of the spectra yields O-H stretch vibrational band origins and relative IR intensities of the symmetric and asymmetric modes. The effect of the alkali-metal ions on these modes results in frequency shifts and intensity changes from the gas phase values of water. The A-rotational constants are also obtained from the rotational structure and are discussed. Experimentally, the temperatures of these species were deduced from the relative populations of the K-rotational states. The internal energies and temperatures of the cluster ions for Na and K were simulated using RRKM calculations and the evaporative ensemble formalism. With binding energies and vibrational frequencies obtained from ab initio calculations, the average predicted temperatures are qualitatively consistent with the experimental values and demonstrate the additional cooling resulting from argon evaporation.

Vibrational and unimolecular dissociation of mixed solvent cluster ions: Na+((CH3)2CO)n(CH3OH)m

Abstract The competitive solvation of the sodium ion by acetone and methanol has been investigated by vibrational spectroscopy of the C–O and O–H stretching modes of methanol and by unimolecular dissociation of mass-selected cluster ions using a tandem mass spectrometer. The onset of hydrogen bonding was detected by substantial shifts in the C–O (+12 to +16 cm −1 ) and O–H (−200 cm −1 ) stretches, as well as by significant increases in the intensity and width of the O–H bands. These onsets were observed when a total of five molecules were present about the ion. The unimolecular dissociation rates of metastable ion clusters of composition Na + ((CH 3 ) 2 CO) 1–9 and Na + ((CH 3 ) 2 CO) 1–8 (CH 3 OH) 1 were also measured using the same experimental apparatus. A significant increase in rate was observed when seven or more acetone molecules were present, suggesting a solvent shell size of six.