Matrix Isolation Spectroscopy and Nuclear Spin Conversion of Propyne Suspended in Solid Parahydrogen.

Parahydrogen (pH2) quantum solids are excellent matrix isolation hosts for studying the rovibrational dynamics and nuclear spin conversion (NSC) kinetics of molecules containing indistinguishable nuclei with nonzero spin. The relatively slow NSC kinetics of propyne (CH3CCH) isolated in solid pH2 is employed as a tool to assign the rovibrational spectrum of propyne in the 600-7000 cm-1 region. Detailed analyses of a variety of parallel (ΔK = 0) and perpendicular (ΔK=±1) bands of propyne indicate that the end-over-end rotation of propyne is quenched, but K rotation of the methyl group around the C3 symmetry axis still persists. However, this single-axis K rotation is significantly hindered for propyne trapped in solid pH2 such that the energies of the K rotational states do not obey simple energy-level expressions. The NSC kinetics of propyne follows first-order reversible kinetics with a 287(7) min effective time constant at 1.7 K. Intensity-intensity correlation plots are used to determine the relative line strengths of individual ortho- and para-propyne rovibrational transitions, enabling an independent estimation of the ground vibrational state effective A″ constant of propyne.

Far-Infrared Investigation of the Benzene-Water Complex: The Identification of Large-Amplitude Motion and Tunneling Pathways.

The far-infrared spectrum of the weakly OH···π hydrogen-bonded benzene-water complex has been studied in neon and argon matrices, below 30 K. The in-plane water libration has been observed in both neon and argon for H2O and D2O complexed with C6H6 and C6D6 but not for the corresponding complexes involving HDO. Both H2O and D2O can tunnel between the two possible hydrogen bonds. This is not possible for HDO. The reported far-infrared observations have implications for the interpretation of the previously obtained molecular beam microwave spectrum of the benzene-water complex.

Photochemistry of glycolaldehyde in cryogenic matrices.

The photochemistry of glycolaldehyde (GA) upon irradiation at 266 nm is investigated in argon, nitrogen, neon, and para-hydrogen matrices by IR spectroscopy. Isomerization and fragmentation processes are found to compete. The hydrogen-bonded Cis-Cis form of GA is transformed mainly to the open Trans-Trans conformer and to CO and CH3OH fragments and their mixed complexes. Different photo-induced behaviours appear depending on the matrix. In nitrogen, small amounts of Trans-Gauche and Trans-Trans conformers are detected after deposition and grow together upon irradiation. The Trans-Gauche conformer is characterized for the first time. In para-hydrogen due to a weaker cage effect additional H2CO and HCO fragments are seen. Calculations of the potential energy surfaces of S0, S1, and T1 states--to analyse the torsional deformations which are involved in the isomerization process--and a kinetic analysis are presented to investigate the different relaxation pathways of GA. Fragmentation of GA under UV irradiation through the CO+CH3OH molecular channel is a minor process, as in the gas phase.

The coupling between translation and rotation for monomeric water in noble gas matrices.

The rotation of water in noble gas matrices has been studied. It is shown that the rotation-translation coupling model of Friedmann and Kimel predicts rotation line spacings, which are close to the experimental observations for H2O, D2O, and HDO, when gas phase rotation constants are used. The model gives intensity estimates in reasonable agreement with the observations for the local oscillator bands, which accompany the rotation spectrum. It also predicts the intensity variations in the bending region of H2O between neon, argon, and krypton matrices.

Water tetramer, pentamer, and hexamer in inert matrices.

The infrared spectrum of water, isolated in inert matrices, has been studied in the interval from 60 to 4000 cm(-1). Experiments with partially deuterated water combined with DFT (density functional theory) calculations have been used to investigate the structure of matrix-isolated water tetramer. A few, strong intermolecular fundamentals of the water tetramer have been observed. Mid-infrared bands due to deuterated pentamers and hexamers have been observed and are used to discuss the assignments of these water clusters.

Observations of host guest interactions specific to molecular matrices: water monomers and dimers in hydrogen matrices.

Water monomers and dimers have been studied at low temperatures in matrices of solid p-H(2), o-D(2), n-H(2), and n-D(2) using infrared spectroscopy. Our data demonstrate interaction mechanisms between host matrix and guest molecules that are different from the ones observed in atomic noble gas matrices. Notably both guest/host rotational--rotational interaction and matrix induced modifications of the guest libration modes are observed. We also show that different types of interaction influence the relaxation times of some of guest modes. Water rotates freely in p-H(2) and o-D(2) but librates in n-H(2) and n-D(2). Rotational relaxation is faster in o-D(2) than in p-H(2) and faster in p-H(2) than in Ne. This is attributed to interactions between water rotation and matrix molecule rotation in p-H(2) and o-D(2). In n-H(2) and n-D(2), a strong water libration band is observed in the far-infrared, and strong water monomer vibration bands have libration satellites. Water dimer bands, close to matrix rotation bands, are perturbed by the matrix motions. The H-bonded isomer H(2)O--HOD rapidly converts to the D-bonded form H(2)O-DOH in p-H(2) and in o-D(2) but slowly in n-H(2) and n-D(2).

On the structure of the matrix isolated water trimer.

Infrared spectra of partially deuterated water trimers have been investigated. It is found that HDO(H(2)O)(2) has a single, bound OD stretching fundamental, (HDO)(2)H(2)O two bound OD stretches. (HDO)(3) has a single, bound OD stretch and (H(2)O)(3) has a pair of bound OH stretches. Ab initio and discrete Fourier transform (DFT) calculations predict that the water trimer has C(1) symmetry with six different, isoenergetic minima. These calculations consequently give three numerically different OD stretches for HDO(H(2)O)(2), six for (HDO)(2)H(2)O, three for (HDO)(3), and three bound OH stretches for (H(2)O)(3). The connection between the observations and the pseudorotation of the trimer is discussed with the help of Wales' pseudorotation model. It is found that pseudorotation is sufficiently fast to average the effective symmetry of the A(3) trimer to C(3h) and to eliminate the difference between the different ab initio minima for A(2)B. The only exception is (H(2)O)(3) where the splitting between the different bound OH stretches is largest. Here a doublet is observed due to incomplete averaging. DFT calculations indicate that the D-bonded form of HDO(H(2)O)(2) is between 50 and 60 cm(-1) more stable than the H-bonded form. The energy difference is determined by differences in zero point vibration energy of intermolecular librations of the two forms. Attempts to measure the energy difference indicate that the energy difference is larger, of the order of 100 cm(-1).

Rotation of water in solid parahydrogen and orthodeuterium.

The far-infrared spectra of solid orthodeuterium and solid normal deuterium are presented and compared to the corresponding spectra of solid parahydrogen and solid normal hydrogen. Spectra of water in orthodeuterium are compared to spectra of water in parahydrogen. The water rotation constants in orthodeuterium are approximately 80% of the rotation constants of water in parahydrogen. The S(0)(0) band of orthodeuterium gets a strong satellite in the presence of water. The position and width of the satellite depends on the isotopic composition of the water present. If there is a corresponding satellite in parahydrogen it is weak and closer to the S(0)(0) band of the matrix. The conclusion of the paper is that interaction between guest rotation and the rotation of matrix molecules must be taken into account to explain the reduction of the rotation constants in orthodeuterium.

Acceptor switching and axial rotation of the water dimer in matrices, observed by infrared spectroscopy.

Several isotopologues of the water dimer have been studied in different matrices (Ne, Ar, Kr, and p-H(2)) at very low temperatures. A fine structure, which is more or less matrix independent and very similar for different intramolecular fundamentals of the same isotopologic dimer, is present on the high wavenumber side of the main component. The bound OD (OH) stretches of the donor have temperature dependent components. The fine structure and temperature dependency is interpreted as evidence for acceptor switching and rotation of the water dimer around its O-O axis in the matrices studied here. The slow nuclear spin equilibration in H(2)O inhibits the thermal equilibration between the acceptor switching states in H(2)O-DOH and H(2)O-DOD. The condensed environment slows down the acceptor switching rate compared to the gas phase. The antisymmetric stretch of the proton acceptor is assigned by combining information from different matrices with the rotation-acceptor switching model.

Complex formation of small molecules during isolation in low temperature matrices: water dimers in p-H2 and Ne matrices.

The concentrations of water dimer are compared in Ne and p-H(2) matrices at low temperatures, using infrared spectroscopy. Additional data are given for o-D(2) and Ar matrices. For a given monomer concentration, the dimer concentration is significantly higher in solid Ne (or Ar) than in solid p-H(2). In p-H(2), the dimer concentration is only slightly higher than expected for a random distribution of water in the matrix. The dimer concentration in o-D(2) matrices is intermediate between p-H(2) and noble gas matrices. This strongly suggests that most dimers form on the surface of the growing matrix, and not as the result of diffusion in the bulk of the matrix.

Intermolecular vibrations of different isotopologs of the water dimer: Experiments and density functional theory calculations.

Far infrared spectra of seven different isotopologs of the water dimer have been measured in neon matrices at 2.8 K. The experiments are interpreted with the aid of density functional theory calculations, in particular the calculated harmonic isotopic shifts were utilized. All six intermolecular vibrational modes of the water dimer and the fully deuterated water dimer are assigned based the isotopic shifts induced. 31 of a total of 42 intermolecular fundamental modes of the seven different H, D, and (18)O containing water dimers have been experimentally observed and assigned accordingly. The overall agreement between the calculations and the experiments of all isotopologs results in a complete and consistent description of these modes.