Hydrogen bonding in metal ion solvation: vibrational spectroscopy of Cs+(CH3OH)1–6 in the 2.8 μm region

Hydrogen bond network structures of protonated short-chain alcohol clusters

Protonated alcohol clusters enable extraction of the physical essence of the nature of hydrogen bond networks.

Modeling competitive interactions in proteins: vibrational spectroscopy of M+(n-methylacetamide)1(H2O)n=0-3, M=Na and K, in the 3 microm region

To properly understand the preferred structures and biological properties of proteins, it is important to understand how they are influenced by their immediate environment. Competitive intrapeptide, peptide...water, ion...water, and ion...peptide interactions, such as hydrogen bonding, play a key role in determining the structures, properties, and functionality of proteins. The primary types of hydrogen bonding involving proteins are intramolecular amide...amide (N-H...O=C) and intermolecular amide...water (O-H...O=C and H-O...H-N). n-Methylacetamide (NMA) is a convenient model for investigating these competitive interactions. An analysis of the IR photodissociation (IRPD) spectra of M+(n-methylacetamide)1(H2O)n=0-3 (M=Na and K) in the O-H and N-H spectral regions is presented. Ab initio calculations (MP2/cc-pVDZ) are used as a guide in identifying both the type and location of hydrogen bonds present. In larger clusters, where several structural isomers may be present in the molecular beam, ab initio calculations are also used to suggest assignments for the observed spectral features. The results presented offer insight to the nature of ion...NMA interactions in an aqueous environment and reveal how different ion...ligand pairwise interactions direct the extent of water...water and water...NMA hydrogen bonding observed.

Detection of OH Stretching Mode of CH3OH Chemisorbed on Ni3+ and Ni4+ by Infrared Photodissociation Spectroscopy

Structures of nickel cluster ions adsorbed with methanol, Ni3+ (CH3OH)m (m = 1-3) and Ni4+ (CH3OH)m (m = 1-4) were investigated by using infrared photodissociation (IR-PD) spectroscopy based on a tandem-type mass spectrometer, where they were produced by passing Ni3,4+ through methanol vapor under a multiple collision condition. The IR-PD spectra were measured in the wavenumber region between 3100 and 3900 cm-1. In each IR-PD spectrum, a single peak was observed at a wavenumber lower by approximately 40 cm-1 than that of the OH stretching vibration of a free methanol molecule and was assigned to the OH stretching vibrations of the methanol molecules in Ni3,4+ (CH3OH)m. The photodissociation was analyzed by assuming that Ni3,4+ (CH3OH)m dissociate unimolecularly after the photon energy absorbed by them is statistically distributed among the accessible modes of Ni3,4+ (CH3OH)m. In comparison with the calculations performed by the density functional theory, it is concluded that (1) the oxygen atom of each methanol molecule is bound to one of the nickel atoms in Ni3,4+ (defined as molecular chemisorption), (2) the methanol molecules in Ni3,4+ (CH3OH)m do not form any hydrogen bonds, and (3) the cross section for demethanation [CH4 detachment from Nin+ (CH3OH)] is related to the electron density distribution inside the methanol molecule.

Infrared photodissociation spectroscopy of Mg(+)(H2O)Ar(n) complexes: isomers in progressive microsolvation

Ion-molecule complexes of the form Mg(H2O)Ar(n)+ (n = 1-8) are produced by laser vaporization in a pulsed-nozzle cluster source. These complexes are mass-selected and studied with infrared photodissociation spectroscopy in the O-H stretch region. The spectra are interpreted with the aid of ab initio calculations on the n = 1-5 complexes, including examination of various isomeric structures. The combined spectroscopic and theoretical studies reveal the presence of multiple isomeric structures at each cluster size, as the argon atoms assemble around the Mg(+)(H2O) unit. Distinct infrared resonances are measured for argon-on-metal, argon-on-OH and argon-on-two-OH isomers.

Structures, energetics, and spectra of aqua-cesium (I) complexes: An ab initio and experimental study

The design of cesium-selective ionophores must include the nature of cesium-water interactions. The authors have carried out extensive ab initio and density functional theory calculations of hydrated cesium cations to obtain reasonably accurate energetics, thermodynamic quantities, and IR spectra. An extensive search was made to find the most stable structures. Since water...water interactions are important in the aqua-Cs+ clusters, the authors investigated the vibrational frequency shifts as a function of the number of water molecules and the frequency characteristics with and without the presence of outer-shell water molecules. The predicted vibrational frequencies were then compared with the infrared photodissociation spectra of argon-tagged hydrated cesium cluster ions. This comparison allowed the identification of specific hydrogen-bonding structures present in the experimental spectra.

Infrared studies of ionic clusters: The influence of Yuan T. Lee

Beginning in the mid-1980s, a number of innovative experimental studies on ionic clusters emerged from the laboratory of Yuan T. Lee combining infrared laser spectroscopy and tandem mass spectrometry. Coupled with modern electronic structure calculations, this research explored many facets of ionic clusters including solvation, structure, and dynamics. These efforts spawned a resurgence in gas-phase cluster spectroscopy. This paper will focus on the major areas of research initiated by the Lee group and how these studies stimulated and influenced others in what is currently a vibrant and growing field.

Infrared spectroscopy of Li(NH3)n clusters for n=4–7

Infrared spectra of Li(NH3)(n) clusters as a function of size are reported for the first time. Spectra have been recorded in the N-H stretching region for n=4-->7 using a mass-selective photodissociation technique. For the n=4 cluster, three distinct IR absorption bands are seen over a relatively narrow region, whereas the larger clusters yield additional features at higher frequencies. Ab initio calculations have been carried out in support of these experiments for the specific cases of n=4 and 5 for various isomers of these clusters. The bands observed in the spectrum for Li(NH3)(4) can all be attributed to N-H stretching vibrations from solvent molecules in the first solvation shell. The appearance of higher frequency N-H stretching bands for n > or =5 is assigned to the presence of ammonia molecules located in a second solvent shell. These data provide strong support for previous suggestions, based on gas phase photoionization measurements, that the first solvation shell for Li(NH3)(n) is complete at n=4. They are also consistent with neutron diffraction studies of concentrated lithium/liquid ammonia solutions, where Li(NH3)(4) is found to be the basic structural motif.

Competition between cation-π interactions and intermolecular hydrogen bonds in alkali metal ion-phenol clusters. II. Phenol trimer

The competition between ion-molecule and molecule-molecule interactions was investigated in M+(phenol)3 cluster ions for M=Li, Na, K, and Cs. Infrared predissociation spectroscopy in the O-H stretch region was used to characterize the structure of the cluster ions. By adjusting the experimental conditions, it was possible to generate species where argon was additionally bound in order to investigate cold cluster ions. From a comparison of the M+(phenol)3 spectra with the M+(phenol)3Ar spectra, it is clear that the relative populations of hydrogen-bonded configurations are significantly higher in the colder (argon-bearing) species. For the cold species, the IR spectra were compared with minimum energy ab initio calculations to elucidate the hydrogen-bonded structures. The experimental spectra are most consistent with a cyclic hydrogen-bonded configuration for Cs+(phenol)3 in which the ion binds to the phenol molecules via cation-pi interactions, and noncyclic configurations for Li+, Na+, and K+.

Infrared spectroscopy of the Li+(H2O)Ar complex: the role of internal energy and its dependence on ion preparation

The internal energy or effective temperature of cluster ions has become an important issue in characterizing the structures observed in these species. This report considers the role played by the method of ion preparation (laser vaporization-supersonic expansion versus ion impact-evaporative cooling) in governing the internal energy of a specific species, Li(+)(H(2)O)Ar. Vibrational predissociation spectroscopy of the O-H stretch modes revealed rotational features, which were used to characterize the structure and effective rotational temperature of the cluster ion. In addition, the impact of the lithium ion on the H(2)O molecule was analyzed in terms of the vibrational frequency shifts, relative IR intensities, and H(2)O geometry.

Infrared spectra of O2- x (CO2)n clusters (n=1-6): asymmetric docking at the pi* orbital

Isolated superoxide ions solvated by CO2 have been studied by infrared photodissociation spectroscopy and density-functional theory, using CO2 evaporation upon infrared excitation of the O2- x (CO2)n (n=1-6) parent ions. We can assign the observed frequencies to the asymmetric stretch vibration and its combination bands with the symmetric stretch and the overtone of the bending vibration of CO2 in various binding situations. We interpret our findings with the help of density-functional theory. Our data suggest that only one CO2 moiety binds strongly to the O2-, whereas the rest of the CO2 molecules are weakly bound, which is consistent with the experimental spectra. The lobes of the pi* orbital of O2- provide a template for the structure of the microsolvation environment.

Competition between cation-π interactions and intermolecular hydrogen bonds in alkali metal ion-phenol clusters. I. Phenol dimer

The competition between ion-molecule and molecule-molecule interactions was investigated in M+(phenol)2 cluster ions for M=Li, Na, K, and Cs. Infrared predissociation spectroscopy in the O-H stretch region was used to characterize the structure of the cluster ions. By adjusting the experimental conditions, it was possible to generate species where argon was additionally bound in order to investigate cold cluster ions. The spectra showed the presence of hydrogen bonding in the colder M+(phenol)2Ar cluster ions but the absence of hydrogen bonding in the warmer M+(phenol)2 species. For the cold species, the IR spectra were compared with minimum-energy ab initio calculations to elucidate the hydrogen-bonded structures. In the dominant hydrogen-bonded configurations observed experimentally, the phenol molecules form hydrogen-bonded dimers and the alkali-metal ions bind to the phenol via a cation-pi interaction with the aromatic ring. Increasing the strength of the cation-pi interaction by decreasing the ion size forces the distance between the phenol O-H groups to increase, thus weakening the intermolecular hydrogen bond. Free-energy differences of different configurations relative to the ground state demonstrate that hydrogen-bonded structures are enthalpically favored, while non-hydrogen-bonded structures are entropically favored and are thus observed in the warm cluster ions.