James M. Lisy

Infrared vibrational predissociation spectroscopy of water clusters by the crossed laser‐molecular beam technique

Water clusters formed in a molecular beam are predissociated by tunable, pulsed, infrared radiation in the frequency range 2900–3750 cm−1. Absorption spectra of the clusters are obtained by detecting the recoiling fragments off‐axis from the molecular beam as a function of laser frequency using a rotatable mass spectrometer. By carefully adjusting the expansion conditions of the molecular beam and monitoring the largest cluster observable, excessive contamination by clusters larger than the specific one of interest is avoided. It is found that the spectra of clusters containing three or more water molecules absorb over the same frequency range as the liquid. Dynamical information on the predissociation process is obtained from the measured angular and velocity distributions of the fragments. An upper limit to the excited vibrational state lifetime of ∼1 μs is observed for the results reported here. The most probable dissociation process concentrates the available excess energy into the internal motions of...

Vibrational predissociation spectra of (HF)n, n = 2–6

Using molecular beam techniques and a tunable infraredlaser, the vibrational predissociation spectra for (HF)n, n = 2 to 6, in the 3000 to 4000 cm−1 range are presented. The vibrational bands have been assigned to intramolecular HF stretching modes and combinations of intra‐ and intermolecular modes. The structures of (HF)n, n = 3 to 6, were found to be cyclic, i.e., each HF molecule is both a proton donor and acceptor.

Entropic Effects on Hydrated Alkali-Metal Cations : Infrared Spectroscopy and ab Initio Calculations of M+(H2O)x=2-5 Cluster Ions for M = Li, Na, K, and Cs

A delicate balance between competing and cooperating noncovalent interactions determines the three-dimensional structure of hydrated alkali-metal ion clusters. A critical factor influencing the balance reached is the internal energy content (or effective temperature) of the ion cluster. Cold cluster ions (approximately 50-150 K) have little internal energy, and enthalpic contributions have a greater influence on the relative population of low-lying minima. In clusters whose internal energy distributions correspond to temperatures approximately 250-500 K, entropic effects are expected to influence which structural isomers are present, favoring those where free energy has been minimized. Infrared photodissociation spectra of M(+)(H2O)(x=2-5) (approximately 250-500 K) are reported for M = Li, Na, K, and Cs to explore ion dependencies and entropic effects on the observed three-dimensional structure.

Infrared Spectroscopy of Ionophore-Model Systems: Hydrated Alkali Metal Ion 18-Crown-6 Ether Complexes

We report our efforts to study host-guest complexes in the gas phase using a combination of cluster spectroscopy and density functional theory. Mass-selected M(+)(18-crown-6 ether)(H(2)O)(1-4) complexes for the alkali metal ion series were probed using infrared predissociation (IRPD) spectroscopy in the OH stretching region. As the degree of hydration is increased, the IRPD spectra undergo significant changes as the strong 18c6...M(+) interaction weakens and allows H(2)O...H(2)O hydrogen-bonding interactions to compete. The size of the ion is important in determining when this transition occurs. For the smaller ions, Li(+) and Na(+), the 18c6...M(+) interaction proves to be more resilient and is still dominant with two and three waters present. The potassium cation, with its optimum size match with the 18-cown-6 ether cavity, serves as a bridge between the larger and smaller alkali metal ions. In particular, we found a structure for K(+)(18-crown-6 ether)(H(2)O)(2) that appears to be a building block for K(+)(18-crown-6 ether)(H(2)O)(3) complexes and is also believed to be present in Rb(+)(18-crown-6 ether)(H(2)O)(2,3) and Cs(+)(18-crown-6 ether)(H(2)O)(2,3). With four waters present, we were unable to spectrally resolve features associated with individual water molecules due to broad hydrogen bonding. However, results for Cs(+)(18-crown-6 ether)(H(2)O)(4) suggest that H(2)O...H(2)O hydrogen bonding has become the dominant interaction present at this size.

Exploring gas-phase ion-ionophore interactions: infrared spectroscopy of argon-tagged alkali ion-crown ether complexes.

Argon-tagged alkali metal ion-crown ether complexes were generated in the gas phase and investigated using a combination of infrared predissociation (IRPD) spectroscopy, density functional, and symmetry-adapted perturbation theory (SAPT). The IRPD spectra of M(+)(12-crown-4 ether)Ar and M(+)(18-crown-6 ether)Ar, where M = Li, Na, K, Rb, and Cs, were collected in the CH stretching region, using the argon-messenger technique. The gas-phase neutral vibrations for each crown ether shift to higher frequency when complexed to an alkali metal ion. Similarities in the experimental IRPD spectra of Li(+)(12-crown-4 ether)Ar and Li(+)(18-crown-6 ether)Ar indicate that the binding of Li(+) is similar for both crown ethers. In the 12-crown-4 ether systems, there are trackable changes with ion size in the IRPD spectra until the point is reached where the ion is too large to bind to the interior of the macrocycle. Starting with Na(+), the 18-crown-6 spectra vary only slightly as the ion size is increased. The overall profiles of the IRPD spectra indicate that a similar configuration of the complexes is adopted for the ions larger than Na(+). The calculated SAPT interaction energies mirror the trends exhibited by the IRPD spectra and provide interesting insights on the roles of the different interaction energy components in binding the cation to the crown-ether.

Charge and Temperature Dependence of Biomolecule Conformations: K+Tryptamine(H2O)n=0−1Arm=0−1 Cluster Ions

The impact of temperature and charge on the conformation of tryptamine (Tryp) is examined in the gas phase by infrared laser-vibrational predissociation spectroscopy in the 2800-3800 cm(-1) region. Previous studies of neutral Tryp(H(2)O)(n) clusters showed preferential stabilization of specific tryptamine conformers through hydrogen bonding. When complexed with the biologically significant potassium ion, the only conformers found to form under these experimental conditions are built on hitherto unobserved neutral Tryp conformers. The electrostatic interaction between K(+), the tryptamine NH(2) lone pair, and the indole ring in K(+)(Tryp) favors the formation of these new conformers. The observed K(+)(Tryp)(H(2)O) conformers vary significantly from the previously reported neutral Tryp(H(2)O) structure. Using the argon tagging method, we show how variations in temperature impact the observed infrared spectra, demonstrating that different conformers are populated under the different experimental conditions. In addition, the presence of a high-energy conformer of K(+)(Tryp)(H(2)O), trapped by the argon evaporative cooling process, was identified. Exploring the conformational landscape of hydrated cluster ions bearing flexible biomolecules is now possible.

Infrared Spectroscopy of Multiply Charged Metal Ions: Methanol-Solvated Divalent Manganese 18-Crown-6 Ether Systems

We have developed an electrosonic spray ionization source to successfully generate divalent Mn(2+)(18-crown-6)(CH(3)OH)(1-3) complexes in the gas phase. These complexes have been investigated using infrared predissociation spectroscopy in both the CH and OH stretching regions and with density functional theory calculations. To resolve complications from overlapping bands in the CH stretching region due to CH(3)OH and 18-crown-6 CH stretching modes along with strongly perturbed OH stretching modes, we have used d(1) and d(4) methanol substitution. For n = 1, the Mn(2+)...18-crown-6 geometry is highly distorted from its gas-phase neutral configuration in order to maximize favorable electrostatic interactions between the 18-crown-6 macrocyclic oxygens and Mn(2+). For n = 2 and 3, CH(3)OH...CH(3)OH and CH(3)OH...18-crown-6 interactions compete with the Mn(2+)...18-crown-6 interaction, as evidenced by intense hydrogen-bonded OH stretching modes shifted over 500 cm(-1) to lower frequency.

Cooperatively Enhanced Ionic Hydrogen Bonds in Cl-(CH3OH)1-3Ar Clusters

Infrared predissociation (IRPD) spectra of Cl−(CH3OH)1-3Ar and Cl-(CH3OD)1-3Ar were obtained in the OH and CH stretching regions. The use of methanol-d1 was necessary to distinguish between CH stretches and hydrogen-bonded OH features. The spectra of Cl-(CH3OH)2-3Ar show intense features at frequencies lower than the CH stretches, indicating structures with very strong hydrogen bonds. These strong hydrogen bonds arise from structures in which a Cl-···methanol ionic hydrogen bond is cooperatively enhanced by the presence of a second shell and, in the case of Cl-(CH3OH)3Ar, a third shell methanol. The strongest hydrogen bond is observed in the Cl-(CH3OH)3Ar spectrum at 2733 cm-1, shifted a remarkable -948 cm-1 from the neutral, gas-phase methanol value. Harmonic, ab initio frequency calculations are not adequate in describing these strong hydrogen bonds. Therefore, we describe a simple computational approach to better approximate the hydrogen bond frequencies. Overall, the results of this study indicate that high-energy isomers are very efficiently trapped using our experimental method of introducing Cl- into neutral, cold methanol-argon clusters.

Vibrational predissociation spectra and dynamics of small molecular clusters of H2O and HF

Experimental results are presented for the vibrational predissociation spectra in the frequency range 3000–4000 cm–1 for the species (HF)n and (H2O)n, n= 2–6, using molecular-beam techniques and a tunable infrared laser. The observed spectra show a dramatic change between the dimer and larger clusters which is thought to be a result of the cyclic structure of the trimer and larger clusters. The spectra are compared with calculated harmonic force constants of available intermolecular potentials to understand how these small, gas-phase clusters relate to the liquid and solid phases of HF and H2O. Additionally, the angular distributions of the predissociation products show that little energy appears as translational motion of the fragment molecules. This conclusion is consistent with recent theoretical models of the predissociation process. An upper limit of ca. 2 µs is observed for the lifetime of the vibrationally excited clusters.

Infrared Spectroscopy of Li+(CH4)1Arn, n = 1―6, Clusters

Infrared predissociation (IRPD) spectra of Li(+)(CH(4))(1)Ar(n), n = 1-6, clusters are reported in the C-H stretching region from 2800 to 3100 cm(-1). The Li(+) electric field perturbs CH(4) lifting its tetrahedral symmetry and gives rise to multiple IR active modes. The observed bands arise from the totally symmetric vibrational mode, v(1), and the triple degenerate vibrational mode, v(3). Each band is shifted to lower frequency relative to the unperturbed CH(4) values. As the number of argon atoms is increased, the C-H red shift becomes less pronounced until the bands are essentially unchanged from n = 5 to n = 6. For n = 6, additional vibrational features were observed which suggested the presence of an additional conformer. By monitoring different photodissociation loss channels (loss of three Ar or loss of CH(4)), one conformer was uniquely associated with the CH(4) loss channel, with two bands at 2914 and 3017 cm(-1), values nearly identical to the neutral CH(4) gas-phase v(1) and v(3) frequencies. With supporting ab initio calculations, the two conformers were identified, both with a first solvent shell size of six. The major conformer had CH(4) in the first shell, while the conformer exclusively present in the CH(4) loss channel had six argons in the first shell and CH(4) in the second shell. This conformer is +11.89 kJ/mol higher in energy than the minimum energy conformer at the MP2/aug-cc-pVDZ level. B3LYP/6-31+G* level vibrational frequencies and MP2/aug-cc-pVDZ level single-point binding energies, D(e) (kJ/mol), are reported to support the interpretation of the experimental data.