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Attachment of molecular hydrogen to an isolated boron cation: An infrared and ab initio study
No full textStructural properties of the B(+)-H(2) electrostatic complex are investigated through its rotationally resolved infrared spectrum in the H-H stretch region (3905-3975 cm(-1)). The spectrum, which was obtained by monitoring B(+) photofragments while the IR wavelength was scanned, is consistent with the complex having a T-shaped structure and a vibrationally averaged intermolecular separation of 2.26 angstrom, which decreases by 0.04 angstrom when the H(2) subunit is vibraitionally excited. The H-H stretch transition of B+-H(2) is red-shifted by 220.6 +/- 1.5 cm(-1) from that of the free H(2) molecule, much more than for other dihydrogen complexes with comparable binding energies. Properties of B(+)-H(2) and the related Li(+)-H(2), Na(+)-H(2), and Al(+)-H(2) complexes are explored through ab initio calculations at the MP2/aug-cc-pVTZ level. The unusually large red-shift for B+-H(2) is explained as due to electron donation from the H(2) sigma(g) bonding orbital to the unoccupied 2p(z) orbital on the B(+) ion
Infrared spectra of mass-selected Al+-(CH4)(n) n=1-6 clusters
No full textInfrared spectra are recorded for Al+-(CH4)n n = 1-6 clusters in the CH stretch region (2800-3100 cm(-1)). The spectra, which are obtained by monitoring photofragmentation in a tandem mass spectrometer, are dominated by a single, narrow band corresponding to the totally symmetric C-H stretching mode of the CH4 subunits (rendered infrared active through the interaction with the Al+ cation). This band shifts progressively to higher wavenumber as the clusters becomes larger, concomitant with a weakening of the intermolecular Al+center dot center dot center dot CH4 bonds. Supporting ab initio calculations for the n = 1-6 clusters at the MP2/aug-cc-pVDZ level indicate that the Al+ cation is attached to each CH4 sub-unit in a face-bound eta(3) configuration and that when possible the methane molecules are adjacent to one another. Clusters built around an inserted [H-Al-CH3](+) core are also predicted to be stable but lie higher in energy than clusters built around an Al+ core; the latter species are the only ones observed experimentally. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved
Dissociative Photodetachment of the Ethoxide Anion and Stability of the Ethoxy Radical CH3CH2O degrees
No full textThe ethoxy radical is an important species in combustion chemistry; however, considerable debate regarding the fragmentation pathways exists. In order to examine the stability and dissociation dynamics of the ethoxy radical in the two lowest electronic states, dissociative photodetachment experiments at 3.20 eV were carried out on the ethoxide anion, CH3CH2O-, and its per-deuterated isotopologue. Production of excited radicals by photodetachment of the alkoxide anion was found to lead to only CH3 + H2CO products, with no indication of the energetically allowed H-loss channel, H + CH3CHO. Ab initio calculations for the anionic and neutral surfaces, including relevant isomerization and dissociation barriers, were carried out using the CBS-QB3 method to aid in interpretation of the data. The energetics observed in the photoelectron-photofragment coincidence spectra indicate that the calculated barrier (0.70 eV) for the process CH3CH2O -> CH3 + H2CO and the stability of the CH3CH2O radical relative to those products are upper limits
Attaching molecular hydrogen to metal cations: perspectives from gas-phase infrared spectroscopy
No full textIn this perspective article we describe recent infrared spectroscopic investigations of mass-selected M+-H-2 and M+-D-2 complexes in the gas-phase, with targets that include Li+-H-2, B+-H-2, Na+-H-2, Mg+-H-2, Al+-H-2, Cr+-D-2, Mn+-H-2, Zn+-D-2 and Ag+-H-2. Interactions between molecular hydrogen and metal cations play a key role in several contexts, including in the storage of molecular hydrogen in zeolites, metal-organic frameworks, and doped carbon nanostructures. Arguably, the clearest view of the interaction between dihydrogen and a metal cation can be obtained by probing M+-H-2 complexes in the gas phase, free from the complicating influences of solvents or substrates. Infrared spectra of the complexes in the H-H and D-D stretch regions are obtained by monitoring M+ photofragments as the excitation wavelength is scanned. The spectra, which feature full rotational resolution, confirm that the M+-H-2 complexes share a common T-shaped equilibrium structure, consisting essentially of a perturbed H-2 molecule attached to the metal cation, but that the structural and vibrational parameters vary over a considerable range, depending on the size and electronic structure of the metal cation. Correlations are established between intermolecular bond lengths, dissociation energies, and frequency shifts of the H-H stretch vibrational mode. Ultimately, the M+-H-2 and M+-D-2 infrared spectra provide a comprehensive set of benchmarks for modelling and understanding the M+center dot center dot center dot H-2 interaction
Spectroscopic Study of the Benchmark Mn+-H-2 Complex
No full textWe have recorded the rotationally resolved infrared spectrum of the weakly bound Mn+-H-2, complex in the H-H stretch region (4022-4078 cm(-1)) by monitoring Mn+ photodissociation products. The band center of Mn+-H-2, the H-H stretch transition, is shifted by -111.8 cm(-1) from the transition of the free H-2 molecule. The spectroscopic data suguest that the Mn+-H-2 complex consists of a slightly perturbed H-2 molecule attached to the Mn+ ion in a T-shaped configuration with a vibrationally averaged intermolecular separation of 2.73 angstrom. Together with the measured Mn+center dot center dot center dot H-2 binding energy of 7.9 kJ/mol (Weis, P.; et al. J. Phys. Chem. A 1997, 101, 2809.), the spectroscopic parameters establish Mn+-H-2 as the most thoroughly characterized transition-metal cation-dihydrogen complex and a benchmark for calibrating quantum chemical calculations on noncovalent systems involving open d-shell configurations. Such systems are of possible importance for hydrogen storage applications
State-resolved predissociation dynamics of the formyloxyl radical
No full textDissociation dynamics of the deuterated formyloxyl radical, DCO2, were studied by photoelectron-photofragment coincidence spectroscopy of DCO2-. Photodetachment at 4.27 eV accesses the three lowest-lying electronic states ((2)A(1), B-2(2), and (2)A(2)) of DCO2, leading to vibrational excitation dominated by the (2)A(1) state O-C-O bending mode. Predissociation leads to bending excitation in the CO2 products as observed in the product translational energy distribution. Gating the coincidence spectra by electron kinetic energy for each predissociated vibrational mode in the radical provides a direct measure of the CO2 bending vibration (v(2)) product state distribution, revealing substantially more excitation than ab initio quantum-dynamics calculations predict. (C) 2013 Elsevier B.V. All rights reserved
Infrared spectra of Cl-(C6H6)(m) m=1,2
No full textThe Cl--(C6H6)Ar-n n = 0, 1, 2 and Cl--(C6H6)(2) Complexes are investigated using photodissociation infrared spectroscopy in the CH stretch region and through ab initio calculations at the MP2/aug-cc-pVDZ level. The results indicate that Cl--C6H6 possesses a planar structure in which the benzene molecule is attached to the Cl- anion by a double hydrogen bond. The calculations predict that Cl--(C6H6)(2) has a C-2 symmetry structure in which the two face-to-face benzene molecules are attached to the Cl- anion by double hydrogen bonds. This structure is compatible with the measured Cl--(C6H6)(2) infrared spectrum. (c) 2006 Elsevier B.V. All rights reserved
Mixing laser spectroscopy and mass spectrometry-infrared spectra of metal cation-hydrogen complexes
No full textWe describe recent experiments in which mass spectrometry and laser spectroscopy are combined to characterize Li(+)-H(2), Na(+)-H(2), B(+)-H(2) and Al(+)-H(2) complexes in the gas-phase. The infrared spectra, which feature full resolution of rotational sub-structures, are recorded by monitoring M(+) photofragments as the infrared wavelength is scanned. The spectra deliver detailed information on the way, in which a hydrogen molecule is attached to a metal cation including the intermolecular separation, the force constant for the intermolecular bond and the H-H stretching frequency. The complexes all possess T-shaped equilibrium geometries and display a clear correlation between the length and force constant of the intermolecular bond and the dissociation energy. In contrast, the data do not support any straightforward correlation between the frequency shift for the H-H stretch mode and the dissociation energy
Photoelectron-photofragment coincidence studies of NO--X clusters (X = H2O, CD4)
No full textThe dissociative photodetachment of NO-(H2O) and NO-(CD4) anion clusters was studied at 775 nm (1.60 eV) using photoelectron-photofragment coincidence spectroscopy. The correlation between the photoelectron and photodetached neutral spectra indicates vibrational excitation in the recoiling NO neutral fragments from NO-(H2O), with a progression consistent with vibrational excitation up to v(NO) = 3 in the products. The correlation remains when D2O is substituted for H2O, implying the NO vibrational mode plays a role in the dissociation coordinate of the complex. In contrast, no correlation was observed between photoelectron kinetic energy and kinetic energy release from NO-(CD4). Consideration of the maximum available kinetic energy allows the binding energies to be detemined as 0.57 and 0.07 eV for NO-(H2O) and NO-(CD4), respectively
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