Masashi Tsuge

Tunneling isomerization of small carboxylic acids and their complexes in solid matrixes: a computational insight.

We have studied hydrogen-atom tunneling in the cis-to-trans conformational change of some carboxylic acid monomers and formic acid (FA) complexes and dimers at the MP2(full) and CCSD(T) levels of theory within the Wentzel-Kramers-Brillouin approximation. The barrier for the minimum energy path, where the OH bond length and the COH bending angle are optimized, is found to be a good approximation for providing the highest barrier transparency. The matrix effect on the transmission coefficients of cis-FA monomer, trans-cis FA dimer (tc1), and cis-acetic acid monomer are modeled by the polarizable continuum model (PCM) at the MP2(full) level of theory in different environments. For the cis-FA monomer and trans-cis FA dimer (tc1), the calculated transmission coefficients agree with the experimental lifetimes observed in noble-gas solids. However, this method cannot reproduce the experimental results obtained for cis-acetic acid. Moreover, the long lifetime of cis-FA and cis-acetic acid in the N2 environment cannot be reproduced either, which is most probably due to specific interactions that are not included in the PCM. The calculation for cis-HCOOD shows a strong decrease of the barrier transparency compared to that for cis-HCOOH, which is consistent with the experiments. In general, good agreement is observed between the calculated barrier transparency (including PCM) and experimental tunneling rate. However, some exceptions are found, which shows that additional factors influence the tunneling rate.

Infrared Characterization of the HCOOH···CO2 Complexes in Solid Argon: Stabilization of the Higher-Energy Conformer of Formic Acid

The complexes of formic acid (HCOOH, FA) with carbon dioxide are studied by infrared spectroscopy in an argon matrix. Two trans-FA···CO(2) and one cis-FA···CO(2) complexes are experimentally identified while the calculations at the MP2(full)/6-311++G(2d,2p) level of theory predict one more minimum for the cis-FA···CO(2) complex. The complex of the higher-energy conformer cis-FA with CO(2) is prepared by vibrational excitation of the ground-state trans-FA conformer combined with thermal annealing. The lifetime of the cis-FA···CO(2) complex in an argon matrix at 10 K is 2 orders of magnitude longer than that of the cis-FA monomer. This big difference is explained by the computational results which show a higher stabilization barrier for the complex. The solvation effects in solid argon are theoretically estimated and their contribution to the stabilization barriers of the higher-energy species is discussed. The relative barrier transmissions for hydrogen tunneling in the cis-FA···CO(2) complex and cis-FA monomer are in good agreement with the experimental decay rates.

Experimental and computational study of the HXeI⋯HY complexes (Y = Br and I)

The complexes of HXeI with hydrogen halides HY (Y = Br and I) are studied computationally and experimentally in a xenon matrix. The calculations at the CCSD(T)∕def2-TZVPPD level of theory predict several energy minima for the HXeI···HY complexes with interaction energies from -4.69 to -0.23 kcal mol(-1). We have identified three bands of the HXeI···HI complexes in the H-Xe stretching region with the monomer-to-complex blue shifts from +37 to +96 cm(-1), and three bands of the HXeI···HBr complexes with blue shifts from +88 to +157 cm(-1). The structural assignments are done on the basis of the strong H-Xe and HY stretching bands and the decomposition rates upon broadband IR irradiation. The experimental bands with larger shifts are assigned to the most stable structures of the HXeI···HY complexes with the Y-H···I hydrogen bond.

Matrix-isolation and computational study of the HXeY⋯H2O complexes (Y = Cl, Br, and I)

The HXeY⋯H2O complexes (Y = Cl, Br, and I) are studied theoretically and experimentally. The calculations at the CCSD(T)/def2-TZVPPD level of theory predict two stable structures for Y = Cl and Br and one structure for Y = I, with interaction energies up to about -7 kcal mol(-1). In the experiments, we have identified several infrared absorption bands originating from the H-Xe stretching mode of these complexes in a xenon matrix. The monomer-to-complex frequency shifts of this mode are up to +82 cm(-1) (Y = Cl), +101 cm(-1) (Y = Br), and +138 cm(-1) (Y = I), i.e., the shift is smaller for more strongly bound molecules. Based on the agreement of the experimental and theoretical results, the observed bands are assigned to the most stable planar structure with an O-H⋯Y-Xe hydrogen bond.

Infrared Identification of Proton-Bound Rare-Gas Dimers (XeHXe)+, (KrHKr)+, and (KrHXe)+ and Their Deuterated Species in Solid Hydrogen

Proton-bound rare-gas dimer (RgHRg)(+), in which Rg represents a rare-gas atom, serves as a prototypical system for proton solvation by inert-gas atoms. Until now, only centrosymmetric species with Rg = Ar, Kr, or Xe have been identified with infrared spectra. We employed electron bombardment during deposition of a mixture of Xe (or Kr) in p-H2 at 3.2 K to prepare (RgHRg)(+). Lines at 847.0 and 972.1 cm(-1) are assigned as the Rg-H-Rg antisymmetric stretching (ν3) mode and its combination with the Rg-H-Rg symmetric stretching (ν1 + ν3) mode of (XeHXe)(+) in solid p-H2, respectively. Lines at 871.1 and 974.0 cm(-1) are assigned as the ν3 and ν1 + ν3 modes of (KrHKr)(+) in solid p-H2, respectively. Slightly shifted and broadened lines were observed for these species in solid n-H2. These results agree satisfactorily with reported experimental values of (XeHXe)(+) and (KrHKr)(+) in solid Xe, Kr, and Ar, and with the quantum-chemically predicted anharmonic vibrational wavenumbers of these species in the gaseous phase; the significant spectral shifts in various matrixes are rationalized with the proton affinities of the hosts. When a mixture of Xe and Kr in p-H2 was used, an additional broad feature at 1284 cm(-1) was observed and assigned as the ν3 mode of (KrHXe)(+) in solid p-H2. This line shifted to 1280 cm(-1) in solid n-H2 and the corresponding line of (KrDXe)(+) was observed at 954 cm(-1) in n-D2. The observations of these lines are new; the wavenumbers significantly blue shifted from those of the centrosymmetric (RgHRg)(+) agree with the quantum-chemically predicted anharmonic vibrational wavenumbers of 1279 cm(-1) for (KrHXe)(+) and 916 cm(-1) for (KrDXe)(+). Analysis of the computational results shows that electronic correlation effects play a much greater role for the asymmetric than for the symmetric species. An interpretation for this is provided.

Environmental effects on noble-gas hydrides: HXeBr, HXeCCH, and HXeH in noble-gas and molecular matrices

Noble-gas hydrides HNgY (Ng is a noble-gas atom and Y is an electronegative group) are sensitive probes of local environment due to their relatively weak bonding and large dipole moments. We experimentally studied HXeBr in Ar, Kr, and N2 matrices, HXeCCH in Ne and N2 matrices, and HXeH in an N2 matrix. These are the first observations of noble-gas hydrides in an N2 matrix. An N2 matrix strongly increases the H-Xe stretching frequency of HXeBr and HXeCCH with respect to a Ne matrix, which is presumably due to a strong interaction between the HNgY dipole moment and quadrupole moments of the surrounding lattice N2 molecules. The spectral shift of HXeBr in an N2 matrix is similar to that in a CO2 matrix, which is a rather unexpected result because the quadrupole moment of CO2 is about three times as large as that of N2. The H-Xe stretching frequencies of HXeBr and HXeCCH in noble-gas matrices show a trend of ν(Ne) < ν(Xe) < ν(Kr) < ν(Ar), which is a non-monotonous function of the dielectric constants of the noble-gas solids. The MP2(full) calculations of HXeBr and HXeCCH with the polarizable continuum model as well as the CCSD(T) calculations of the HXeBr···Ng and HXeCCH···Ng (Ng = Ne, Ar, Kr, and Xe) complexes cannot fully explain the experimental observations. It is concluded that more sophisticated computational models should be used to describe these experimental findings.

HXeI and HXeH in Ar, Kr, and Xe matrices: Experiment and simulation

Experimental and theoretical studies of HXeI and HXeH molecules in Ar, Kr, and Xe matrices are presented. HXeI exhibits the H-Xe stretching bands at 1238.0 and 1239.0 cm(-1) in Ar and Kr matrices, respectively, that are blue-shifted from the HXeI band observed in a Xe matrix (1193 cm(-1)) by 45 and 46 cm(-1). These shifts are larger than those observed previously for HXeCl (27 and 16 cm(-1)) and HXeBr (37 and 23 cm(-1)); thus, the matrix effect is stronger for less stable molecules. The results for HXeI are qualitatively different from all previous results on noble-gas hydrides with respect to the frequency order between Ar and Kr matrices. For previously studied HXeCl, HXeBr, and HXeCCH, the H-Xe stretching frequency is reliably (by >10 cm(-1)) higher in an Ar matrix than in a Kr matrix. In contrast, the H-Xe stretching frequency of HXeI in an Ar matrix is slightly lower than that in a Kr matrix. HXeH absorbs in Ar and Kr matrices at 1203.2 and 1192.1 cm(-1) (the stronger band for a Kr matrix), respectively. These bands are blue-shifted from the stronger band of HXeH in a Xe matrix (1166 cm(-1)) by 37 and 26 cm(-1), and this frequency order is the same as observed for HXeCl, HXeBr, and HXeCCH but different from HXeI. The present hybrid quantum-classical simulations successfully describe the main experimental findings. For HXeI in the 〈110〉 (double substitution) site, the order of the H-Xe stretching frequencies (ν(Xe) < ν(Ar) < ν(Kr)) is in accord with the experimental observations, and also the frequency shifts in Ar and Kr matrices from a Xe matrix are well predicted (30 and 34 cm(-1)). Both in the theory and experiment, the order of the H-Xe stretching frequencies differs from the case of HXeCl, which suggests the adequate theoretical description of the matrix effect. For HXeH in the 〈100〉 (single substitution) site, the order of the frequencies is ν(Xe) < ν(Kr) < ν(Ar), which also agrees with the experiments. The calculated frequency shifts for HXeH in Ar and Kr matrices with respect to a Xe matrix (36 and 23 cm(-1)) are in a good agreement with the experiments. The present calculations predict an increase of the H-Xe stretching frequencies in the noble-gas matrices with respect to vacuum.

Double ionization and Coulomb explosion of the formic acid dimer by intense near-infrared femtosecond laser pulses.

Ionization and fragmentation of formic acid dimers (HCOOH)(2) and (DCOOD)(2) by irradiation of femtosecond laser pulses (100 fs, 800 nm, ~1 × 10(14) W/cm(2)) were investigated by time-of-flight (TOF) mass spectrometry. In the TOF spectra, we observed fragment ions (HCOOH)H(+), (HCOOH)HCOO(+), and H(3)O(+), which were produced via the dissociative ionization of (HCOOH)(2). In addition, we found that the TOF signals of COO(+), HCOO(+), and HCOOH(+) have small but clear side peaks, indicating fragmentation with large kinetic energy release caused by Coulomb explosion. On the basis of the momentum matching among pairs of the side peaks, a Coulomb explosion pathway of the dimer dication, (HCOOH)(2)(2+) → HCOOH(+) + HCOOH(+), was identified with the total kinetic energy release of 3.6 eV. Quantum chemical calculations for energies of (HCOOH)(2)(2+) were also performed, and the kinetic energy release of the metastable dication was estimated to be 3.40 eV, showing good agreement with the observation. COO(+) and HCOO(+) signals with kinetic energies of 1.4 eV were tentatively assigned to be fragment ions through Coulomb explosion occurring after the elimination of a hydrogen atom or molecule from (HCOOH)(2)(2+). The present observation demonstrated that the formic acid dimer could be doubly ionized prior to hydrogen bond breaking by intense femtosecond laser fields.

Infrared Spectrum of Toluene: Comparison of Anharmonic Isolated-Molecule Calculations and Experiments in Liquid Phase and in a Ne Matrix.

First-principles anharmonic calculations are carried out for the CH stretching vibrations of isolated toluene and compared with the experimental infrared spectra of isotopologues of toluene in a Ne matrix at 3 K and of liquid toluene at room temperature. The calculations use the vibrational self-consistent field method and the B3LYP potential surface. In general, good agreement is found between the calculations and experiments. However, the spectrum of toluene in a Ne matrix is more complicated than that predicted theoretically. This distinction is discussed in terms of matrix-site and resonance effects. Interestingly, the strongest peak in the CH stretching spectrum has similar widths in the liquid phase and in a Ne matrix, despite the very different temperatures. Implications of this observation to the broadening mechanism are discussed. Finally, our results show that the B3LYP potential offers a good description of the anharmonic CH stretching band in toluene, but a proper description of matrix-site and resonance effects remains a challenge.

Experimental and theoretical study of the HXeI⋯HCl and HXeI⋯HCCH complexes

The HXeI⋯HCl and HXeI⋯HCCH complexes are studied computationally and experimentally in a Xe matrix. In the experiments, three bands of the HXeI⋯HCl complex and one band of the HXeI⋯HCCH complex in the H-Xe stretching region are observed. The monomer-to-complex shifts are +94, +111, and +155 cm(-1) for the HXeI⋯HCl complex and +49 cm(-1) for the HXeI⋯HCCH complex. The bands of the complexed HCl molecules are also observed with large red shifts from the HCl monomer (-187, -252, and -337 cm(-1)). The ab initio calculations at the CCSD(T)/def2-TZVPPD level of theory predict two stable structures for the HXeI⋯HCl complex with interaction energies of -3.72 and -0.28 kcal mol(-1) and one structure for the HXeI⋯HCCH complex with an interaction energy of -2.67 kcal mol(-1) and the calculated monomer-to-complex shifts are in a good agreement with experiment (in the case of HXeI⋯HCl, for the stronger structure). The HXeI molecules are decomposed by broad-band infrared light; however, the decomposition is much more efficient for the HXeI monomer than for the complexes studied here as well as for the previously studied HXeI⋯HI and HXeI⋯HBr complexes. In fact, the decomposition efficiency decreases as the monomer-to-complex shift of the H-Xe stretching mode increases.