Xenon Matrices Research Articles

Photodecomposition of formohydroxamic acid. Matrix isolation FTIR and DFT studies

Primary dissociation pathways of formohydroxamic acid have been investigated in argon and xenon matrices. The irradiation of the HCONHOH/Ar(Xe) matrices with full output of an Xe arc lamp leads to the formation of hydrogen bonded HNCO–H2O and NH2OH–CO complexes. Two structures were identified for the isocyanic acid–water complex, in the first one HNCO is hydrogen bonded to an oxygen atom of the water molecule and in the second, water serves as a proton donor toward the nitrogen atom of HNCO. For the hydroxylamine–carbon monoxide complex the spectra prove the existence of the structure with the OH group attached to the carbon atom and also suggest the existence of the structure with the NH2 group interacting with the carbon atom. The identification of the products is confirmed by 15N isotopic substitution and by DFT calculations (at the B3LYP level of the theory with 6-311++G(2d,2p) basis set) of the structure and vibrational frequencies of the identified complexes.

Molecular structure and infrared spectra of dimethyl malonate: A combined quantum chemical and matrix-isolation spectroscopic studyElectronic supplementary information (ESI) available: definition of internal symmetry coordinates used in the normal mode analysis of the conformers of dimethyl malonate. See http://www.rsc.org/suppdata/cp/b2/b206270b/

The infrared spectra of dimethyl malonate isolated in low-temperature argon and xenon matrices were studied. Theoretical calculations, carried out at the MP4/6-31G**, MP2/6-31++G** and DFT(B3LYP)/6-311++G** levels, predict two different conformers of nearly equal internal energy, both exhibiting the methyl ester moieties in the cis (C–O) configuration. One of the conformers has C2 symmetry with the two ester groups crossed symmetrically with respect to the C–C–C plane. This structure is doubly degenerated by symmetry. The other form (gauche) belongs to the C1 point group. Four identical-by-symmetry minima on the PES correspond to this structure. The energy of this form is predicted (at the MP4 level) to be slightly lower than that of the C2 conformer. The six minima on the PES can be divided into two groups of three (one C2 isomer and two C1 forms in a group). Each structure from one group is related to its counterpart from the other group by the operation of reflection in the C–C–C plane. In each group, the conformers are separated by low energy barriers (less than 2 kJ mol−1), while conformational interconversions between the two groups imply a transition state structure with a vis-a-vis orientation of oxygen atoms and thus are associated with considerably higher energy barriers. The infrared spectra of the matrix isolated compound were found to closely match the spectrum predicted for the C1 conformer. Annealing of the matrices up to 55 K does not lead to significant changes in the spectra, suggesting that the low energy barriers separating the two conformers allow practically all molecules of dimethyl malonate to transform to the more stable gauche conformer, when they are cooled down after landing on the matrix surface. Spectra of the low temperature solid form of the compound (8 K < T < 200 K) also reveal only the presence of the C1 conformer.

Host–guest charge transfer states: CN doped Kr and Xe

The host–guest charge transfer absorption of CN doped krypton and xenon matrices are identified through direct analogy with the previously assigned transitions of Cl/Kr and Cl/Xe. These intense, structured absorption bands appear with the onset at 245 nm in Kr and 360 nm in Xe. Excitation of the CN/Kr charge transfer band at 193 nm leads to emission over CN(A(2Π)→X(2Σ)) transition, indicating that an efficient curve crossing precludes the ionic state from radiating. No emissions were seen in CN/Xe when excited at 193 nm. The charge transfer absorption spectrum of CN/Kr is reproduced through an extended diatomics-in-ionic-systems treatment, using accurate ab initio pair potentials and transition dipoles as input, without further adjustment. The delocalized hole states are then analyzed in real-space, using atomic bases distributed over as many as eleven shells surrounding the CN− center. The ionic states are well described as J=1/2, 3/2 valence bands bound to CN−, with a substructure that cannot be exclusively assigned to a single quantum number. The strong absorptions terminate on states in which 70%–95% of the hole density remains on the first nearest neighbor shell, with hole densities of 1%–5% extended out to R=8 Å. In higher ionic states, with weaker transition dipoles, the hole density maximizes on shells removed by 10 Å from the ionic center. Although these delocalized states provide channels for charge separation via self-trapping of holes, save for a weak signal from the impurity trapped hole at H+ centers, the experiments do not provide evidence for significant charge separation.

Cis– trans isomerization equilibrium in hydroquinone in low-temperature argon and xenon matrices studied by FTIR spectroscopy

Torsional isomerization between cis and trans hydroquinone in low-temperature matrices has been studied by FTIR spectroscopy with an aid of DFT calculations. When the temperature of Xe matrix was changed between 16 and 75 K, the cis/ trans population ratio changed reversibly in conformance with the Boltzmann distribution law, despite the high torsional barrier, 10.8 kJ mol −1 . The cis– trans enthalpy difference was estimated from the population changes to be 0.19±0.06 kJ mol −1 . The cis– trans isomerization equilibrium at low temperature is accounted for in terms of the matrix-induced tunneling of hydrogen atoms.

Raman spectroscopic studies on matrix-isolated phosphorus molecules P4 and P2

The Raman spectra of P4 and P2 molecules have been studied in nitrogen, argon, krypton, and xenon matrices at 15 K. The vibrational frequencies of the P4 molecule are up to 14 cm−1 higher than the experimental gas phase data. This apparent blue-shift is not caused by matrix effects but rather due to an underestimation of the fundamentals in the gas phase as a course of the elevated temperatures. The observed frequencies confirm a recent theoretical prediction on the basis of high-level calculations. From the observed frequencies of the P4 molecule a general valence force field was calculated.

Radiation Chemistry of Organic Molecules in Solid Rare Gas Matrices: 2. Selective Deprotonation of the Primary Radical Cations upon Irradiation of Oxygen-Containing Molecules in Xenon Matrices

The composition of radical products trapped after irradiation of various ethers (dimethyl ether, tetrahydrofuran, methylal, 1,3-dioxolane) or acetaldehyde in xenon matrices at 15–17 K in the presence of electron scavengers was studied by an ESR technique. It was shown that the primary radical cations give corresponding deprotonation products (carbon-centered radicals), rather than stabilize in xenon, under the given experimental conditions. The deprotonation process is characterized by extremely high selectivity; i.e., the only type of radicals resulting from deprotonation at maximum spin density position was observed in each of the cases. The possible mechanism of the reactions and the nature of their selectivity are discussed.

Spectroscopy and energy relaxation processes of Hg-doped solid neon, argon, and xenon

Emission, absorption, and excitation spectroscopy has been used for a detailed analysis of the optical transitions of Hg2 trapped in cryogenic matrices. Upon excitation of electronic states correlating to the 3P1 or the 1P1 asymptote, fast nonradiative relaxation leads to emission from the lowest excited A0g+ state in all matrices, which decays monoexponentially in 1 ms in Ne, 280 μs in Ar, and 12 μs in Xe. In addition, electronically unrelaxed emission of Hg2 is reported in neon and in xenon matrices and attributed to the B1g state in neon and to the B1g state and the C0u− or A0g− states in xenon. The results are rationalized by assuming: (a) that population of the excited states occurs mainly close to the asymptotic limit, where branching is determined by nonadiabatic coupling and energetics, that are strongly environment dependent, and (b) that in Xe matrices the Hg2 states correlating to the 3P1 and 3P0 asymptotic limits are stabilized in different configurations, as a result of the very different solvation properties of the atomic 3P1 and the 3P0 state. Further emission bands are found in the vicinity of the dimer transitions, which we attribute to Hg3 and to site effects on Hg2. In particular, electronically unrelaxed emission from excited states of Hg3 is reported.

The conformers of bromomethyl dimethyl chlorosilane studied by vibrational spectroscopy and ab initio methods

Bromomethyl dimethyl chlorosilane (CH2Br(CH3)2SiCl) was synthesised and the infrared spectra of the vapour and of the amorphous and crystalline states at liquid nitrogen temperature were obtained. Additional spectra of the compound, isolated in argon, nitrogen and xenon matrices were recorded at 5 and 15K. Raman spectra of the liquid were obtained at various temperatures between 295 and 173K, and spectra of the amorphous and crystalline solids were recorded.The compound is present as anti and gauche conformers in the vapour and in the liquid states. Various infrared and Raman bands present in these phases vanished upon crystallisation. Raman temperature studies in the liquid gave a gauche–anti value of 1.0±0.3kJmol−1,anti was the low energy conformer and was also present in the crystal. The infrared bands diminishing in the argon, nitrogen and xenon matrix spectra after annealing to 28–60K suggested that the anti conformer also had a slightly lower energy than gauche in all the matrices. The conformational barrier was estimated to be 8–10kJmol−1.Ab initio calculations on different levels of approximation gave optimised geometries, infrared and Raman intensities and vibrational frequencies for the anti and gauche conformers. All calculations predicted anti as the low energy conformer. After scaling, a reasonably good agreement between the experimental and calculated wavenumbers for the two rotamers was obtained.

Isomerization and Fragmentation Products of CH2Cl2 and Other Dihalomethanes in Rare-Gas Matrices: An Electron Bombardment Matrix-Isolation FTIR Spectroscopic Study

Isodihalomethanes have been isolated by electron bombardment of CH2Cl2, CD2Cl2, CH2Br2, or CH2ClBr in argon, krypton, or xenon followed by condensation on a 15 K matrix-isolation window. Numerous neutral and ionized decomposition products of dihalomethane ionization were also observed. Irradiation with visible or UV light, isotopic substitution, and previous literature assignments of the matrix-isolated products allow definitive identification of most of the observed product bands in the infrared spectra recorded after electron bombardment matrix-isolation experiments (EBMI). Experiments involving substitution of argon with krypton or xenon gas, mixtures of CH2Cl2 and CH2Br2, rare-gas resonant emission irradiation, and thermodynamic considerations support the proposed mechanism for isomerization of the dihalomethane radical cation in the gas phase. This mechanism involves charge-exchange ionization of dihalomethane followed by gas-phase isomerization, isolation, and stabilization in the solid matrix and subsequent neutralization through electron capture. An upper limit to the barrier for CH2Cl2•+ to CH2ClCl•+ isomerization of 43 kJ mol-1 is deduced following observation of the isodichloromethane product after EBMI of xenon/dichloromethane mixtures. Two isomers of the molecular cation, one resembling the distonic isomer of CH2Cl2•+ (HClC•+−ClH) and the other a complex between CH2Cl+ and a chlorine atom [(CH2Cl+)Cl•] have been distinguished based on their stability with respect to UV−visible light irradiation, their infrared spectra, and published ab initio calculations. Vibrational wavenumbers for isodichloromethane and various other products of dichloromethane EBMI experiments in krypton and xenon matrices are reported for the first time. We propose reasoning for the general observation that ions that have an electron affinity (EA) greater than ∼10.8 eV (the “5 eV rule”) are not observed in argon matrices, but those with EAs less than 10.8 eV are observed.

Empirical Jahn–Teller potential surfaces for silver doped xenon matrices

Potential energy surfaces for 5s and 5p states of Ag atoms in the vibrationally distorted Oh symmetry of a Xe matrix are constructed by numerical diagonalization of the Hamiltonian and using coupling parameters derived from experimental absorption and emission spectra. The 5p state exhibits both spin-orbit and Jahn–Teller coupling. The large structural relaxation after excitation indicated by the large Stokes shift of the emission necessitates higher-order terms in both the vibrational potentials and the Jahn–Teller coupling. Only the two Eg modes are considered. With this restriction the experimental and calculated spectra agree very well. We discuss implications for dynamical studies of wave packet motions.

Infrared studies of the nitrous acid complexes with Xe and N2 in argon, krypton and xenon matrices

The complexes formed by trans- and cis-HONO isomers with xenon were observed and characterized in argon matrices. The complexes of the two isomers with nitrogen in krypton and xenon matrices were also studied.

Electronic Spectra of Porphycenes in Rare Gas and Nitrogen Matrices

Fine-line electronic absorption and fluorescence spectra of porphycene and 2,7,12,17-tetra-n-propylporphycene have been measured in low-temperature nitrogen, argon, krypton, and xenon matrices. A well-resolved emission spectrum of porphycene was also obtained in an isotropic glassy matrix using the fluorescence line-narrowing technique. Analysis of the observed matrix spectral shifts enabled the assignment of the origin of the S0−S2 electronic transition. Vibrational frequencies have been obtained for the ground and two lowest excited singlet states. Vibronic and site structure in the region of S0−S1 and S0−S2 electronic transitions of porphycenes have been compared with the corresponding data for porphyrin. Both similarities and differences were observed. In particular, contrary to the case of porphyrin and its derivatives, no site structure was observed in the spectra of porphycenes in xenon and krypton matrices. This was explained by a model that takes into account different positions of the transition moment directions in the two chromophores with respect to the matrix cage. No evidence was obtained for the presence of the cis tautomeric forms of porphycene.

Matrix Isolation Fourier Transform Infrared and Ab Initio Studies of the 193-nm-Induced Photodecomposition of Formamide

Ultraviolet irradiation of formamide in solid argon forms hydrogen-bonded, carbon- and oxygen-attached complexes NH3−CO and NH3−OC. Computationally, the carbon-attached complex is 1.2 kJ mol-1 more stable than the oxygen-attached complex. However, a thermal equilibrium of the two structures is found experimentally in the matrices. Moreover, the oxygen-attached complex dominates in the 193-nm-induced photolytic complex formation from formamide. In xenon matrices, the photochemistry of formamide increasingly yield the HNCO+H2 binary system. The change in photochemistry of formamide between the argon and xenon enviroments can be attributed to an external heavy-atom effect, where xenon enhances the rate of intersystem crossing from a singlet to a triplet surface.

Theoretical and electron spin resonance studies of the H⋯H, H⋯D, and D⋯D spin-pair radicals in rare gas matrices: A case of extreme singlet–triplet mixing

The H⋯H, H⋯D, and D⋯D spin-pair radicals have been thoroughly investigated in neon, argon, krypton, and xenon matrices near 4 K by electron spin resonance (ESR). A theoretical model has been developed that treats these spin-pairs as weakly interacting atoms. The model includes the effects of Σ/3Σ1 mixing in the analysis of the observed ESR spectral results and yields a consistent set of magnetic parameters for these three isotopomers in all four rare gas hosts. The consideration of H atoms interacting with other H atoms over a distribution of internuclear distances in the rare gas lattice is included in the theoretical and experimental analyses. Application of the model to earlier ESR results for H⋯CH3 reveals a value for its Heisenberg exchange interaction (J) which is found to be considerably larger than that for the H⋯H spin-pair. The effects of methane and neon on the J value are calculated for these spin-pairs. The H⋯H case is unusual in that the nuclear hyperfine interaction (A) is considerably larger than D (the anisotropic dipole–dipole magnetic interaction between electrons) which is much larger than J. The H⋯H spin-pairs exhibit internuclear distances greater than 7 Å and have the following magnetic parameters (MHz) based upon this model of “weakly interacting atoms;” giso=2.0016, Aiso=1426, D=−200, and J=6. Since a distribution of distances is involved, other spin-pairs would be separated by even greater distances in the matrix and thus have smaller absolute values of D and J.

Vibrational relaxation study of O3 in rare gas and nitrogen matrices by time resolved infrared–infrared double resonance spectroscopy

A time resolved infrared–infrared double resonance technique is used to study the vibrational relaxation of O3 in rare gas and nitrogen matrices. A tunable infrared (IR) pulsed source excites the ν1+ν3 level of O3 in the ground electronic state. A continuous wave (cw) CO2 laser probes the populations of the fundamental and v2=1 levels as a function of time. After minimization of thermal effects, the relaxation signal can be analyzed. At fixed probe frequency, the behavior of the rise time of the signals with the pump frequency shows spectral diffusion to occur inside the inhomogeneous profiles. At high concentration in argon (O3/Ar=1/250), intermolecular energy transfer is observed between the two sites. In xenon matrices, it has time to take place at concentrations 1/2000. The relaxation rates of the v2=1 level to the ground state are measured at different concentrations in rare gas and nitrogen matrices. At high dilution, a maximum relaxation time, called intrinsic relaxation time τi, is determined in the different matrices: it covers three orders of magnitude, from a few hundred nanoseconds in neon to 320 microseconds in xenon. The results are discussed and compared with literature data within the frame of the isolated binary collision model.

Laser Irradiation of Monomeric Acetylene and the T-Shaped Acetylene Dimer in Xenon and Argon Matrices

Irradiation of acetylene (4) in an argon matrix with an ArF laser (λ = 193 nm) yields the ethynyl radical C2H· (5) and C2 (6). If the same wavelength is used to irradiate 4 in a xenon matrix, only the infrared signal of the compound Xe–C2 (7) can be detected. The same band is found on irradiation at λ = 248 nm (KrF laser) in a xenon matrix, despite the fact that acetylene (4) does not absorb light of this wavelength. Irradiation of the T-shaped acetylene dimer 9 in an argon or xenon matrix at λ = 193 nm yields butadiyne (11) and vinylacetylene (12). However, irradiation with a KrF laser (λ = 248 nm) in a xenon matrix additionally yields cyclobutadiene (13). The dependence of the mechanisms of the fragmentation and dimerization of acetylene (4) on the matrix material is discussed.

Laser Irradiation of Monomeric Acetylene and the T-Shaped Acetylene Dimer in Xenon and Argon Matrices

Irradiation of acetylene (4) in an argon matrix with an ArF laser (λ = 193 nm) yields the ethynyl radical C2H· (5) and C2 (6). If the same wavelength is used to irradiate 4 in a xenon matrix, only the infrared signal of the compound Xe–C2 (7) can be detected. The same band is found on irradiation at λ = 248 nm (KrF laser) in a xenon matrix, despite the fact that acetylene (4) does not absorb light of this wavelength. Irradiation of the T-shaped acetylene dimer 9 in an argon or xenon matrix at λ = 193 nm yields butadiyne (11) and vinylacetylene (12). However, irradiation with a KrF laser (λ = 248 nm) in a xenon matrix additionally yields cyclobutadiene (13). The dependence of the mechanisms of the fragmentation and dimerization of acetylene (4) on the matrix material is discussed.

Stimulated emission at 10 μm of O3 trapped in low temperature matrices: Threshold measurements and quantum yield evaluation

The recently reported fluorescence, emitted near 10 μm by laser excited ozone molecules trapped in xenon matrices, is actually a stimulated emission. It is observed, at the 2ν3→ν3 frequency, only if the laser pulse, which excites the ν1+ν3 level, contains a sufficient amount of energy. A similar emission is found in the other rare gas matrices and in nitrogen, above a laser energy threshold which is matrix dependent. Above the threshold, the emitted signal grows linearly with the laser pulse energy. An evaluation of the quantum yield is obtained by comparison with the literature data on CO fluorescence. The characteristics of the observed emission (low directionality, laser energy dependence, shortening, and amplification when compared to spontaneous emission) are discussed.

Electronic spectra and symmetry of metalloporphyrins in low-temperature rare gas and nitrogen matrices

The electronic absorption and luminescence spectra of zinc, magnesium and free base porphine have been measured in low temperature nitrogen, argon, krypton and xenon matrices. The absorption spectra of the three compounds show similar multiplet structures, characteristic of each matrix. Analogies are discussed between the splitting of spectral lines in free base porphine, due to phototautomerization, and the crystal field induced lifting of the degeneracy of the Jahn-Teller unstable S 1 state in metalloporphyrins. The similarity between the absorption spectral pattern observed in free base porphine and metalloporphyrins indicates that the symmetry distortion in the latter occurs along the N-N axis. The magnitude of the crystal field splitting is correlated with the nature of the matrix.

Radiation-induced degradation of alkane molecules in solid rare gas matrices

The radiation-induced degradation of heptane molecules in solid argon and xenon matrices at 15 K was studied using low-temperature IR spectroscopy. The total radiation-chemical yield of the destruction of heptane molecules in argon (mole ratio 500:1) was estimated to be 1.4 molecule per 100 eV. Methane, vinyl- and trans-vinylene-type olefins, and allyl-type radicals were identified among the main radiolysis products in both matrices. The C-C bond rupture is favoured in argon probably due to formation of excited heptane cations in the hole transfer in this matrix. An indication of the radical cation trapping was obtained in a xenon matrix containing an electron scavenger (Freon-113). The mechanism of the radiation-induced degradation of alkane molecules and the fate of the primary cations in rigid inert media are discussed.