Low-temperature Rare-gas Matrices Research Articles

Studies of Neutral Rare-Gas Compounds and their Non-Covalent Interactions with Other Molecules

The main focus of this review is a new class of neutral molecules containing chemically bonded rare gas atoms. These compounds are given by the general formula HRgX, where Rg denotes a rare gas atom and X denotes an atom or group of atoms having high electron affinity. They are usually produced by photolysis of a HX precursor in low temperature rare-gas matrices. These novel compounds are highly polar and form weakly bound metastable complexes with other molecules via non-covalent interactions. In this review, we discuss theoretical studies of the structure, bonding, properties and factors influencing the energetic stability of these rare-gas species, the preparation and vibrational spectroscopy of these molecules and some theoretical approaches to understanding the nature of these molecules, with experimental studies discussed where appropriate. The complexes formed by the rare-gas compounds exhibit unusual structural and spectroscopic features, which are also discussed in the review.

Hydrogen-atom inversion of 1,2-dimethylhydrazine in low-temperature rare-gas matrices

Infrared spectra of 1,2-dimethylhydrazine in low-temperature rare-gas matrices were measured with an FTIR spectrophotometer. The observed bands were assigned to two stable isomers, outer–outer and inner–outer, being aided by density-functional-theory calculations. Spectral changes caused by the isomerization from inner–outer to outer–outer were observed when the matrix sample was kept in the infrared radiation of the spectrophotometer, or even in dark without infrared radiation. It is concluded that the isomerization occurs by inversion due to tunneling of hydrogen-atoms and that outer–outer isomer is slightly more stable than inner–outer.

Hydrogen-atom inversion of 1,2-dimethylhydrazine in low-temperature rare-gas matrices

Hydrogen-atom inversion of 1,2-dimethylhydrazine in low-temperature rare-gas matrices

Infrared-induced conformational interconversion in carboxylic acids isolated in low-temperature rare-gas matrices

An overview of our recent studies dealing with infrared-induced conformational interconversion of carboxylic acids isolated in rare-gas matrices is presented. Extensive rotational photoisomerization studies have been performed on formic acid, which is the simplest organic acid enabling this kind of processes. Formic acid has two conformers and interconversion between them can be induced by vibrational excitation. As such, it is an ideal model system to study the conformational dynamics of the carboxylic group. Formic acid molecules were found to be isolated in different local environments within the rare-gas matrices, as shown by the site splitting of the vibrational bands. Narrowband tunable infrared (IR) radiation was used to induce site-selective isomerization processes. The induced changes in the IR absorption spectra allowed for a detailed analysis of the vibrational properties of both conformers of formic acid isolated in solid argon. In particular, derived from the intermode coupling constants the local environment was shown to affects the intramolecular potential energy surface. Tunneling is involved in the rotamerization of formic acid, with the tunneling rate being affected by the local environment. Additionally, formic acid exhibits isomer-selective photodissociation where narrowband IR excitation can control the conformer-dependent photodissociation channels. Tunable IR radiation was also used to promote rotamerization in a series of matrix-isolated dicarboxylic acids (ethanedioic, propanedioic, and 2-butenedioic acids) by exciting the first overtone of the OH stretching mode or a suitable combination mode at similar energies. Efficient isomerization involving rotation around the CO bond was observed in most cases whereas the internal rotation around the CC bond was found to be constrained for ethanedioic and (Z)-2-butenedioic acids.

Experimental and theoretical study of a new C 10N 8 compound. Reversible photoisomerization between 2,3,6,7-tetracyano-1,4,5,8-tetraazanapthalene and 4,5,9,10-tetracyano-1,3,6,8-tetraazacyclodeca-1,2,4,6,7,9-hexaene in low-temperature rare-gas matrices

Reversible photoisomerization of 2,3,6,7-tetracyano-1,4,5,8-tetraazanaphthalene (TTN) in low-temperature rare-gas matrices was investigated by electronic absorption and Fourier transform infrared (FTIR) spectroscopies aided bydensity-functional-theory (DFT) calculations. The reactant, TTN, was found to change to 4,5,9,10-tetracyano-1,3,6,8-tetraazacyclodeca-1,2,4,6,7,9-hexaene (TTH) upon ultraviolet-light irradiation, while the product, TTH, returns to the original reactant, TTN, upon visible-light irradiation. The time-dependent DFT calculation led to the conclusion that the forward and backward photoisomerizations occur by way of the S 3–S 0 transition of TTN and the S 1–S 0 transition of TTH, respectively.

Cis→trans conversion of formic acid by dissipative tunneling in solid rare gases: Influence of environment on the tunneling rate

The relaxation of the higher-energy cis conformer of formic acid to the lower-energy trans form by a tunneling mechanism has been investigated in low-temperature rare gas matrices. In the temperature range 8–60 K, the tunneling takes place dominantly from the vibrational ground state of the cis form and the temperature dependence of the tunneling rate constant is influenced by the interactions with the environment. The temperature-dependent tunneling rates for HCOOH and DCOOH in solid Ar, Kr, and Xe are measured including data for molecules in different local environments within each host. It was found that the medium and the local environment has a significant influence on the tunneling rate.

Photoinduced rotational isomerization mechanism of 2-chlorobenzaldehyde in low-temperature rare-gas matrices by vibrational and electronic spectroscopies

Rotational isomerization of 2-chlorobenzaldehyde in low-temperature rare-gas matrices has been investigated by vibrational and electronic spectroscopies with aids of the density functional theory (DFT) and configuration interaction single (CIS) calculations. Infrared spectrum of the less stable O-cis isomer, produced from the more stable O-trans isomer upon UV irradiation, is measured with an FT-IR spectrophotometer. The enthalpy difference between the O-cis and O-trans isomers is estimated to be 9.7±0.2kJmol−1 from the temperature dependence of the infrared band intensities. Analyses of the infrared and electronic absorption spectral changes after UV irradiation and the phosphorescence spectra measured at various excitation wavelengths suggest that the rotational isomerization occurs via the intersystem crossing from S1 to T1.

Vibrational spectroscopy of the hydrogen bond: An ab initio quantum-chemical perspective

The hydrogen bond has long been recognized as an important type of intermolecular interaction. Its infrared (IR) spectroscopic signature is the shift to lower frequency and the increase in intensity of the A-H stretching band upon formation of the A-H…B hydrogen bond. Ab initio calculations carried out with an appropriate wavefunction model and basis set, and using the harmonic approximation, can reasonably reproduce the shift of the A-H stretching band upon hydrogen bonding, if the equilibrium structure exists in a relatively deep potential well on the surface, so that both the V=0 and the V=1 vibrational states of the proton-stretching mode are confined within this well. However, if the equilibrium structure is found in a region of the surface which is broad and relatively flat, or if a second region of the surface can be accessed in either the V=0 or the V=1 vibrational state of the proton-stretching mode, then the harmonic approximation fails to describe the anharmonicity inherent in the surface. For such complexes, experimental gas-phase structures and experimental IR spectra obtained in low-temperature rare-gas matrices may give conflicting descriptions of the hydrogen bond, and discrepancies will exist between experimental and computed harmonic IR spectra. Anharmonic frequencies for both fundamental and combination bands are needed to understand and reproduce qualitatively the most important features of the experimental spectra. In this article, an overview of the calculation of anharmonic frequencies is presented, and results of one- and two-dimensional anharmonic treatments of vibration are reported for a variety of hydrogen-bonded complexes. Computed frequencies are compared with experimental gas-phase frequencies when these are available, and with experimental frequencies obtained in low-temperature rare-gas matrices.

Photochemistry of formic acid in rare gas matrices: Double-doping experiments on the 193 nm induced photodecomposition

Photoinduced decomposition of formic acid has been investigated in low-temperature rare gas matrices. The photoproduct ratios were found to differ in Xe matrix from those in Ar and Kr matrices. In xenon, the H 2 + CO 2 dissociation channel was observed to be prominent, whereas in argon and krypton the formation of H 2O + CO was favored. The perturbation upon the photodecomposition mechanism produced by the matrix environment was studied by trapping the precursor in mixed Ar Xe matrices.

IR studies of van der Waals interactions on perfluoro-t-butyl alcohol OH: Part I. Environment effects on monomeric OH and self-associates

The frequency shift Δν of the perfluoro-t-butyl alcohol (PFTB) OH stretching in low temperature rare gas matrices has been studied. Δν depends on the square root of the critical temperature of the rare gases ( T C2) 1 2 . Other matrix results for CH 3OH, HCl and HF can be plotted on the same diagram taking into account the square root of the pair potential of the different oscillators ( T C1) 1 2 . The common straight line Δν∼ ( T C1· T C2) 1 2 starts at 130 K. The cause of this effect is discussed. This relation establishes the validity of the Badger—Bauer rule, the proportionality between Δν and the van der Waals interaction energy, which is already known for H bonds. The free open end OH of PFTB self associates shows a 72% larger matrix sensitivity. This seems to indicate higher van der Waals interactions by OH groups which are polarized by H bonds. Recent experimental demonstrations of H bonds seem to be able to extend the cooperativity between H bonds and van der Waals interactions. The preference of PFTB dimers for the linear conformation is demonstrated. The effects of PFTB H bonds in self associates on Δν due to the different van der Waals effects of rare gas matrices are smaller than of free OH. The first effect may be more like a solvent effect due to different inner pressures of the matrices, and the second effect a direct consequence of the inter- molecular potentials.

Light induced transformations of small nitroso compounds in low temperature rare gas matrices

Light induced transformations of small nitroso compounds in low temperature rare gas matrices

Structure of the HX-HX′ dimers

With the advent of accurate measurements on simple molecular systems, the usual electrostatic + polarization + exchange potential has been more and more subject to severe criticism. Owing to the universality of the sole analytical form actually at our disposal and to its reliability in interpreting the spectral response of hydracids trapped in low temperature rare gas matrices, we apply it to the determination of the geometry of molecular dimers HX-HX′, to the subsequent study of the low frequency coupled internal motions (the three coupled anharmonic librational modes describing the orientation of the internuclear axes and the intermolecular vibration mode) and to the calculation of the principal moments of inertia of the rotating complex. Although the calculated mean intermolecular distance appears 5 per cent shorter than the measured one, the close agreement between the internal zero point motion, averaged angular geometries and the beam experiment data clearly shows that such an analytical potential c...