Infrared Stark and Zeeman spectroscopy of OH-CO: the entrance channel complex along the OH + CO → trans -HOCO reaction pathway

Sequential capture of OH and CO by superfluid helium droplets leads exclusively to the formation of the linear, entrance-channel complex, OH–CO. This species is characterized by infrared laser Stark and Zeeman spectroscopy via measurements of the fundamental OH stretching vibration. Experimental dipole moments are in disagreement with ab initio calculations at the equilibrium geometry, indicating large-amplitude motion on the ground state potential energy surface. Vibrational averaging along the hydroxyl bending coordinate recovers 80% of the observed deviation from the equilibrium dipole moment. Inhomogeneous line broadening in the zero-field spectrum is modeled with an effective Hamiltonian approach that aims to account for the anisotropic molecule-helium interaction potential that arises as the OH–CO complex is displaced from the center of the droplet.Joseph T. Brice, Tao Liang, Paul L. Raston, Anne B. McCoy and Gary E. Douberl

The ethyl radical in superfluid helium nanodroplets: rovibrational spectroscopy and ab initio computations

Abstract not availablePaul L. Raston, Jay Agarwal, Justin M. Turney, Henry F. Schaefer III, and Gary E. Douberl

Infrared Spectroscopy and Structures of Cobalt Carbonyl Cations, Co(CO)(n)(+) (n=1-9)

Cobalt carbonyl cations of the form Co(CO)(n)(+) (n = 1-9) are produced in a molecular beam by laser vaporization in a pulsed nozzle source. These ions, and their corresponding "argon-tagged" analogues, Co(CO)(n)(Ar)(m)(+), are studied with mass-selected infrared photodissociation spectroscopy in the carbonyl stretching region. The number of infrared-active bands, their frequency positions, and their relative intensities provide distinctive patterns allowing determination of the geometries and electronic structures of these complexes. CO(CO)(5)(+) has a completed coordination sphere, consistent with its expected 18-electron stability, and it has the same structure (D-3h trigonal bipyramid) as its neutral isoelectronic analog Fe(CO)(5). The carbonyl stretches in CO(CO)(5)(+) are less red-shifted than those in Fe(CO)(5) because of charge-induced reduction in the pi back-bonding. Co(CO)(1-4)(+) complexes have triplet ground states, but the spin changes to a singlet for the Co(CO)(5)(+) complex

Liquid hot NAGMA cooled to 0.4 K: Benchmark thermochemistry of a gas-phase peptide

Vibrational spectroscopy and helium nanodroplet isolation are used to determine the gas-phase thermochemistry for isomerization between conformations of the model dipeptide, N-acetylglycine methylamide (NAGMA). A two-stage oven source is implemented to produce a gas-phase equilibrium distribution of NAGMA conformers, which is preserved when individual molecules are captured and cooled to 0.4 K by He nanodroplets. With polarization spectroscopy, the IR spectrum in the NH stretch region is assigned to a mixture of two conformers having intramolecular hydrogen bonds composed of either five- or seven-membered rings, C5 and C7, respectively. The C5 to C7 interconversion enthalpy and entropy, obtained from a van't Hoff analysis, are -4.52 ± 0.12 kJ/mol and -12.4 ± 0.2 J/(mol · K), respectively. The experimental thermochemistry is compared to high-level electronic structure theory computations.Christopher M. Leavitt, Kevin B. Moore III, Paul L. Raston, Jay Agarwal, Grant H. Moody, Caitlyne C. Shirley, Henry F. Schaefer III, and Gary E. Douberl

Helium nanodroplet isolation and infrared spectroscopy of the isolated ion-pair 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

Publication Date (Web): August 21, 2013The ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide was vaporized at 420 K, and the ion-pair constituents were entrained in a beam of liquid He nanodroplets and cooled to 0.4 K. The vapor pressure was optimized such that each He droplet picked up a single ion-pair from the gas phase. Infrared spectroscopy in the CH stretch region reveals bands that are assigned to intact ion-pairs on the basis of comparisons to ab initio harmonic frequency computations of 23 low energy isomers. The He droplet spectrum is consistent with a weighted sum of the computed harmonic spectra, in which the weights are determined from ab initio computations of the relative free energies at 420 K. Anharmonic resonance polyads in the CH stretch region are treated explicitly, which improves the agreement between the experiment and computed spectra for ion-pairs. For isomers having a strong cation···anion hydrogen bonding interaction, the imidazolium C(2)-H stretch fundamental is shifted to lower energy and into resonance with the overtones and combination bands of the imidazolium ring stretching modes, resulting in a spectral complexity in the CH stretch region that is fully resolved in the He droplet spectrum. The assignment of the infrared spectrum to ion-pairs is confirmed through polarization spectroscopy measurements that reveal the permanent electric dipole moment of the He-solvated species to be 11 ± 2 D. The computed permanent electric dipole moments for the low energy isomers of the [emim+][Tf2N–] ion-pairs fall in the range 9–13 D, whereas the computed dipole moments of decomposition products of the ionic liquid are less than 4.3 D.Emmanuel I. Obi, Christopher M. Leavitt, Paul L. Raston, Christopher P. Moradi, Steven D. Flynn, Ghanshyam L. Vaghjiani, Jerry A. Boatz, Steven D. Chambreau, and Gary E. Douberl