High kinetic stability of HXeBr upon interaction with carbon dioxide: HXeBr···CO2 complex in a xenon matrix and HXeBr in a carbon dioxide matrix

We investigate the conditions when noble-gas hydrides can be found in real environments and report on the preparation and identification of the HXeBr···CO(2) complex in a xenon matrix and HXeBr in a carbon dioxide matrix. The H-Xe stretching mode of the HXeBr···CO(2) complex in a xenon matrix is observed at 1557 cm(-1), showing a spectral shift of +53 cm(-1) from the HXeBr monomer. The calculations at the CCSD(T)/aug-cc-pVTZ-PP(Xe,Br) level of theory give two stable structures for the HXeBr···CO(2) complex with frequency shifts of +55 and +103 cm(-1), respectively. On the basis of the calculations, the experimentally observed band is assigned to the more stable structure with a "parallel" geometry. The HXeBr molecule was prepared in a carbon dioxide matrix and has the H-Xe stretching frequency of 1646 cm(-1), meaning a strong matrix shift and stabilization of the H-Xe bond. The deuterated species DXeBr in a carbon dioxide matrix absorbs at 1200 cm(-1). This is the first case where a noble-gas hydride is prepared in a molecular solid. The thermal stabilities of HXeBr and HXeBr···CO(2) complex in a xenon matrix and HXeBr in a carbon dioxide matrix were examined. We have found a high thermal stability of HXeBr in carbon dioxide ice (at least up to 100 K), i.e., under conditions that may occur in nature.     

Experimental and computational study of HXeY...HX complexes (X, Y = Cl and Br): an example of exceptionally large complexation effect

The complexes of xenon hydrides HXeY (Y = Cl and Br) with hydrogen halides HX (X = Cl and Br) have been studied both computationally and experimentally in a xenon matrix. The experiments revealed three new complexes: HXeBr...HBr, HXeBr...HCl, and HXeCl...HCl. The experimental assignments were done on the basis of the strong H-Xe stretching absorption of HXeY (Y = Cl and Br) molecules and supported by theoretical results. We experimentally obtained monomer-to-complex blue-shifts of this vibrational mode for all the studied systems (up to approximately 150 cm (-1)). The electronic structure calculations revealed three local structures for each HNgY...HX complexes and their computed interaction energies varied between -460 and -2800 cm (-1). The computational estimates of the vibrational shifts were in agreement with the experimental values. We also found possible experimental absorption belonging to HXeBr...(HBr) 2 trimer and its vibrational shift (+245 cm (-1)) is similar to the computational estimate of a cyclic ternary complex (+252 cm (-1)).     

HXeCCH in Ar and Kr matrices

HXeCCH molecule is prepared in Ar and Kr matrices and characterized by IR absorption spectroscopy. The experiments show that HXeCCH can be made in another host than the polarizable Xe environment. The H-Xe stretching absorption of HXeCCH in Ar and Kr is blueshifted from the value measured in solid Xe. The maximum blueshifts are +44.9 and +32.3 cm(-1) in Ar and Kr, respectively, indicating stabilization of the H-Xe bond. HXeCCH has a doublet H-Xe stretching absorption measured in Xe, Kr, and Ar matrices with a splitting of 5.7, 13, and 14 cm(-1), respectively. Ab initio calculations for the 1:1 HXeCCHcdots, three dots, centeredNg complexes (Ng = Ar, Kr, or Xe) are used to analyze the interaction of the hosts with the embedded molecule. These calculations support the matrix-site model where the band splitting observed experimentally is caused by specific interactions of the HXeCCH molecule with noble-gas atoms in certain local morphologies. However, the 1:1 complexation is unable to explain the observed blueshifts of the H-Xe stretching band in Ar and Kr matrices compared to a Xe matrix. More sophisticated computational approach is needed to account in detail the effects of solid environment.     

HXeOBr in a xenon matrix

We report on a new noble-gas molecule HXeOBr prepared in a low-temperature xenon matrix from the HBr and N(2)O precursors by UV photolysis and thermal annealing. This molecule is assigned with the help of deuteration experiments and ab initio calculations including anharmonic methods. The H-Xe stretching frequency of HXeOBr is observed at 1634 cm(-1), which is larger by 56 cm(-1) than the frequency of HXeOH identified previously. The experiments show a higher thermal stability of HXeOBr molecules in a xenon matrix compared to HXeOH.     

Organo-noble-gas hydride compounds HKrCCH, HXeCCH, HXeCC, and HXeCCXeH: formation mechanisms and effect of 13C isotope substitution on the vibrational properties

We investigate the formation mechanism of HXeCCXeH in a Xe matrix. Our experimental results show that the HXeCCXeH molecules are formed in the secondary reactions involving HXeCC radicals. The experimental data on the formation of HXeCCXeH is fully explained based on the model involving the HXeCC+Xe+H-->HXeCCXeH reaction. This reaction is the first case when a noble-gas hydride molecule is formed from another noble-gas molecule. In addition, we investigate the (12)C/(13)C isotope effect on the vibrational properties of organo-noble-gas hydrides (HKrCCH, HXeCCH, HXeCC, and HXeCCXeH) in noble-gas matrixes. The present experimental results and ab initio calculations on carbon isotope shifts of the vibrational modes support the previous assignments of these molecules. Upon (12)C to (13)C isotope substitution, we observed a pronounced effect on the H-Kr stretching mode of HKrCCH (downshift of 1.0-3.6 cm(-1), depending on the matrix site) and a small anomalous shift (+0.1 cm(-1)) of the H-Xe stretching mode of HXeCCH and HXeCCXeH.     

Theoretical Studies on the Potential Energy Surface and Vibrational Energy Levels of HXeBr

The potential energy surface for the electronic ground state of the HXeBr molecule is constructed from more than 4200 ab initio points calculated using the internally contracted multi-reference configuration interaction method with the Davidson correction （icMRCI ＋ Q）. The stabilities and dissociation barriers are identified from the potential energy surface. The three-body dissociation channel is found to be the dominant dissociation channel for HXeBr. Low-lying vibrational energy levels of HXeBr calculated using the Lanczos algorithm are found to be in good agreement with the available experimental band origins.     

NH3 trapped in nitrogen and rare gas matrices. II. High-resolution spectroscopy of ν2 and calculation of the inversion-translation doubling and proper rotation quadrupling

The high-resolution IR spectrum of the ν₂ absorption band of NH₃ embedded in solid N₂ at 5.5 K exhibits a quadruplet structure. This structure includes the previously mentioned inversion doublet while the additional splitting shows striking nuclear spin species conversion over a very large timescale. The inversion doubling, ? 1.65 cm^?1, is considerably smaller than in rare gas matrices (? 24 cm^?1) and in the gas phase (37 cm^?1). The temperature dependence of the quadruplet frequencies shows in N₂ a larger blue shift than is usually expected and a typical motional narrowing for the doublet structure in the range 8–17 K. The a priori determination of the motions of NH₃ around the equilibrium configurations of the potential surface described in the first paper of this series, shows that the strong coupling between intrinsic inversion and translational space inversion is responsible for the doubling decrease. Such a feature is due to the large equilibrium eccentricity in a N₂ matrix. As this eccentricity is much smaller in rare gas matrices, the coupling is much weaker and the spacing closer to the gas-phase value. The quadruplet structure is due to the vibrational dependence of the hindered proper rotational (spinning) motion in the three-fold wells, characteristically coupled to the nuclear spin species. All numerical predictions are in agreement with experimental measurement.     

Vibrational relaxation of oxygen in an argon cage

The vibrational relaxation of oxygen embedded in an argon cage through vibrational to local translation, rotation, and argon phonon modes has been studied using semiclassical procedures. The collision model is based on the trapped molecule undergoing the restricted motions (local translation and hindered rotation) in a cage formed by its twelve nearest argon neighbors in a face-centered-cubic structure. At 85 K in the liquid argon temperature range, the deexcitation probability of O(2)(v=1) is 5.8 x 10(-12) and the relaxation rate constant with the collision frequency from local translation is 23 s(-1). The rate constant decreases to 5.1 s(-1) at 50 K and to 0.016 s(-1) at 10 K in the solid argon temperature range. Transfer of the vibrational energy to local translation, rotation (both hindered and free), and argon phonon modes is the relaxation pathway for the trapped oxygen molecule.     

HXeBr分子的振动频率和离解途径的理论研究

采用MP2和CCSD(T)方法对HXeBr分子的振动光谱进行了理论研究. 计算结果表明, 经非谐性和基质效应修正后的H—Xe伸缩振动、弯曲振动以及Xe—Br伸缩振动频率分别为1492, 509和174 cm-1, 与实验结果吻合得较好. 此外分别采用单参考组态的CCSD(T)方法和多参考组态耦合簇(MR-AQCC)方法研究了HXeBr分子的稳定性和离解途径. 研究结果表明, 离解途径HXeBr→Xe＋HBr和HXeBr→H＋Xe＋Br的能垒分别为1.39和0.89 eV, 三体离解途径是HXeBr分子的主要离解途径.     

An ab initio potential energy surface and vibrational energy levels of HXeBr

A three-dimensional global potential energy surface for the electronic ground state of HXeBr molecule is constructed from more than 4200 ab initio points. These points are generated using an internally contracted multi-reference configuration interaction method with the Davidson correction （icMRCI ＋ Q） and large basis sets. The stabilities and dissociation barriers are identified from the potential energy surfaces. The three-body dissociation channel is found to be the dominate dissociation channel for HXeBr. Based on the obtained potentials, low-lying vibrational energy levels of HXeBr calculated using the Lanczos algorithm is found to be in good agreement with the available experimental band origins.     

Mössbauer study of the Sn resonance in rare-gas matrix-isolated Sn(IV)-and Sn(II)halide molecules

Mössbauer absorption spectra of rare-gas matrix-isolated SnX₄ and SnX₂ molecules (X = F, Cl, Br, I) have been measured at matrix temperatures of a bout 5 K. The hyperfine interaction (hfi) parameters of ¹¹⁹Sn in argon matrix-isolated SnX₄ (X = Cl, Br, 1) molecules are identical with those of the corresponding crystalline compounds. This fact reveals that the inter-molecular interactions are negligible in the crystalline compounds as far as concerning the electronic structure of Sn⁴⁺. The ¹¹⁹Sn hfi parameters of rare-gas matrix-isolated SnX₂ molecules differ from those measured in the crystallin compounds. This arise from the totally different coordination of tin in the two situations. The analysis of the hfi parameters using a simple bonding model yields information about the ionicity of the Sn-halogen bonds and the bonding angle in these molecules. The observed isomer shifts and quadrupole interactions can only be explained in this model with a bonding angle θ = 95° ± 2° for all SnX₂ molecules and a slight increase of θ from Sn1₂ to SnF₂.     

Characterization of intensity-dependent optical rotation phenomena in chiral molecules in solution

The rotation of the plane of polarization of linearly polarized light by chiral molecules in solution is due to a forward scattering event. Ordinary optical rotation, a single-photon effect, is independent of intensity. As the light intensity is increased, other effects can appear, such as two-photon scattering or alignment of the molecule by one photon and scattering with a change of polarization by another. Both of these effects result in intensity-dependent (or nonlinear) optical rotation. A polarimeter was used to measure the nonlinear optical rotation of solutions in a heterodyne experiment. No nonlinear optical rotation was found in molecules lacking an absorption band near the laser frequency. In the three pyrimidine nucleosides studied, which do have such an absorption band, a nonlinear optical rotation was identified that was cumulative with each laser pulse. The effect persisted with a time constant that was on the order of seconds and characteristic of the molecule.     

Comparative matrix isolation infrared spectroscopy study of 1,3- and 1,4-diene monoterpenes (α-phellandrene and γ-terpinene)

In the present work, γ-terpinene (a 1,4-diene derivative) and α-phellandrene (1,3-diene derivative) were isolated in cryogenic argon matrices and their structures, vibrational spectra, and photochemistries were characterized with the aid of FTIR spectroscopy and quantum chemical calculations performed at the DFT/B3LYP/6-311++G(d,p) level of approximation. The molecules bear one conformationally relevant internal rotation axis, corresponding to the rotation of the isopropyl group. The calculations provide evidence of three minima on the potential energy surfaces of the studied molecules, where the isopropyl group assumes the trans, gauche+, and gauche- conformations (T, G+, G-). The signatures of all these conformers were identified in the experimental matrix infrared spectra, with the T forms dominating, in agreement with the theoretical predicted abundances in gas phase at room temperature. In situ UV (λ > 200 nm) irradiation of matrix-isolated α-phellandrene led to its isomerization into an open-ring species. The photoproduct was found to exhibit the ZE configuration of its backbone, which to be formed from the reactant molecule does not require extensive structural rearrangements of both the reagent and matrix. γ-Terpinene was photostable when subjected to irradiation under the same experimental conditions. In addition, the liquid compounds at room temperature were also investigated by FTIR-ATR and FT-Raman spectroscopies.     

Halogenated xenon cyanides ClXeCN, ClXeNC, and BrXeCN

We report on the preparation and characterization of three new noble-gas molecules ClXeCN, ClXeNC, and BrXeCN. These molecules are synthesized by 193 nm photolysis and thermal annealing of ClCN and BrCN in a xenon matrix. The absorption spectra are measured in the mid- and far-infrared regions, and the assignment is supported by isotope substitution and quantum chemical calculations at the B3LYP and MP2 levels of theory. The present results demonstrate a way to prepare other noble-gas molecules of this type.     

Protons solvated in noble-gas matrices: interaction with nitrogen

The interaction of a (NgHNg)(+) cation (Ng = Ar and Kr) with a nitrogen molecule is studied. The structure, energetics, and vibrational properties of these complexed systems are computationally studied at the MP2/6-311++G(2d,2p) level of theory. The computations reveal two stable structures, linear and T-shaped configurations, with BSSE corrected interaction energies of the order of -1000 cm(-1). The (NgHNg)(+)[dot dot dot]N(2) complexes are characterized experimentally by IR absorption spectroscopy in solid Ar and Kr matrices. The spectra show that only one complex structure is present, as evidenced by the single nitrogen-induced nu(3) band. According to the computational results, the linear structure is more probable in the experiments. However, our results show that the one-to-one complex at the present computational level does not accurately agree with the matrix-isolation experiments, the differences originating possibly from the influence of the surrounding matrix. Based on the current data, the mechanism of cation decay in noble-gas matrices is discussed. The observed similar decay of (NgHNg)(+) and its N(2) complex indicates that the solvated proton is unable to tunnel and is therefore immobile in noble-gas matrices. The observations for the cation decay are consistent with the electron neutralization mechanism.     

Implications of comparative spectral doublets observed for neon-isolated and gaseous tropolone(OH) and tropolone(OD)

Spectral doublet separations reported for gas phase and neon matrix-isolated samples of tropolone(OH) and tropolone(OD) are found to support recent work suggesting the possibility that tropolone has a slightly nonplanar geometry in the S1 (A 1B2) (pi*-pi) electronic state. Tautomerizations of gaseous tropolones in the S0 and S1 states are governed by equal double-minimum potential energy functions (PEFs), but interactions in the neon matrix environment transform the tautomerization PEFs of the slightly nonplanar S1 tropolones into unequal double-minimum PEFs. The spectral doublets reported for the zero-point S1-S0 transitions imply energy minima for the nonplanar S1 state in a neon matrix are offset by about 7 cm-1, and tunneling splittings in the symmetric double minimum PEFs of the gaseous molecules are damped about 2 cm-1 by the matrix environment. This means gas phase tunneling splittings smaller than 2 cm-1 are fully quenched in the neon matrix, and gas phase tunneling splittings near 20 cm-1 are damped by only 10%.     

Long-range hydrogen-bond structure in aqueous solutions and the vapor-water interface

There is a considerable disagreement about the extent to which solutes perturb water structure. On the one hand, studies that analyse structure directly only show local structuring in a solute's first and possibly second hydration shells. On the other hand, thermodynamic and kinetic data imply indirectly that structuring occurs much further away. Here, the hydrogen-bond structure of water around halide anions, alkali cations, noble-gas solutes, and at the vapor-water interface is examined using molecular dynamics simulations. In addition to the expected perturbation in the first hydration shell, deviations from bulk behavior are observed at longer range in the rest of the simulation box. In particular, at the longer range, there is an excess of acceptors around halide anions, an excess of donors around alkali cations, weakly enhanced tetrahedrality and an oscillating excess and deficiency of donors and acceptors around noble-gas solutes, and enhanced tetrahedrality at the vapor-water interface. The structuring compensates for the short-range perturbation in water-water hydrogen bonds induced by the solute. Rather than being confined close to the solute, it is spread over as many water molecules as possible, presumably to minimize the perturbation to each water molecule.     

Vibrational isotope effect and molecular structure of M-Xy type molecules

A general theory on the vibrational isotope effect of linear MXY type molecules is presented. Here, XY denotes diatomic species such as CO and CN, while M typically represents a metal atom such as Li and Pt. It is shown that the MXY and MYX structures can be distinguished by comparing the isotope shift of the MXY (or MYX) molecule with that of free XY. Applications of the theory to problems of structure determination and band assignments are made to demonstrate its utility.     

The IR spectrum of formic acid in an argon matrix

A new method of preparation of matrix-isolated molecules of formic acid at low temperatures has been developed. A set of experimental frequencies and intensities of the IR spectrum of monomer molecules of formic acid isolated in a low temperature argon matrix has been obtained. It is shown on the basis of analysis of the IR spectrum that the new method eliminates association and decomposition of molecules of the acid during preparation of the sample.     

A microscopic model for the fluctuations of local field and spontaneous emission of single molecules in disordered media.

We develop a microscopic model to describe the observed temporal fluctuations of the fluorescence lifetime of single molecules embedded in a polymer at room temperature. The model represents the fluorescent probe and the macromolecular matrix on the sites of a cubic lattice and introduces voids in the matrix to account for its mobility. We generalize Lorentz's approach to dielectrics by considering three domains of electrostatic interaction of the probe molecule with its nanoenvironment: (1) the probe molecule with its elongated shape and its specific polarizability, (2) the first few solvent shells with their discrete structure and their inhomogeneity, (3) the remainder of the solvent at larger distances, treated as a continuous dielectric. The model is validated by comparing its outcome for homogeneous systems with those of existing theories. When realistic inhomogeneities are introduced, the model correctly explains the observed fluctuations of the lifetimes of single molecules. Such a comparison is only possible with single-molecule observations, which provide a new access to local field effects.