Photochemistry in the charge transfer and neutral excited states of HCl in Xe and Kr matrices

HCl-doped Xe and Kr films are irradiated with wavelength dispersed synchrotron radiation in the wavelength range from 200 to 130 nm. The growth of H, Cl, Xe2H+, XeH2, HXeCl, Kr2H+, and HKrCl as well as the decomposition of HCl are recorded by a combination of UV, VIS, and IR spectroscopy. A turnover in the formation of Xe2H+ and Kr2H+ by a predominant two-step reaction on neutral surfaces at low energies to a one-step formation on ionic surfaces is determined at 172 and 155 nm in Xe and Kr, respectively. A potential energy diagram for neutral and ionic states is derived that is consistent with a DIIS calculation, with new UV fluorescence bands from Xe+HCl- centers, with the turnover energies and with a deconvolution of the absorption spectra in neutral and ionic contributions. The cage exit of charged as well as of neutral H, the latter via a harpoon reaction, is discussed for the ionic surfaces. The self-limitation of HCl decomposition on the neutral surfaces due to absorption by H and Cl fragments is treated quantatively. Dissociation efficiencies phi(e), together with absolute absorption cross sections sigma(H) and sigma(Cl) of the fragments, are derived. sigma(H) and sigma(Cl) are of the order of 10(-16) cm(2) compared to 10(-18) cm(2) for sigma(HCl). Dissociation is accompanied by many excitation cycles of the fragments, which leads to light-induced migration of H and recombination. phi(e) therefore represents a product of the cage exit probability phi that was treated theoretically and the survival probability concerning geminate and nongeminate recombination.     

Photofragment slice imaging studies of pyrrole and the Xe...pyrrole cluster

The photolysis of pyrrole has been studied in a molecular beam at wavelengths of 250, 240, and 193.3 nm, using two different carrier gases, He and Xe. A broad bimodal distribution of H-atom fragment velocities has been observed at all wavelengths. Near threshold at both 240 and 250 nm, sharp features have been observed in the fast part of the H-atom distribution. Under appropriate molecular beam conditions, the entire H-atom loss signal from the photolysis of pyrrole at both 240 and 250 nm (including the sharp features) disappear when using Xe as opposed to He as the carrier gas. We attribute this phenomenon to cluster formation between Xe and pyrrole, and this assumption is supported by the observation of resonance enhanced multiphoton ionization spectra for the (Xe...pyrrole) cluster followed by photofragmentation of the nascent cation cluster. Ab initio calculations are presented for the ground states of the neutral and cationic (Xe...pyrrole) clusters as a means of understanding their structural and energetic properties.     

Study of ab initio molecular data for inelastic and reactive collisions involving the H3+ quasimolecule

The lowest two ab initio potential energy surfaces (PES), and the corresponding nonadiabatic couplings between them, have been obtained for the H3+ system; the molecular data are compared to those calculated with the diatomic in molecules (DIM) method. The form of the couplings is discussed in terms of the topology of the molecular structure of the triatomic. The method of Baer is employed to generate "diabatic" states and the residual nonadiabatic couplings are calculated. The ab initio results for these are markedly different from the corresponding DIM data, and show the need to consider the third PES.     

The Xe shielding surfaces for Xe interacting with linear molecules and spherical tops

The 129Xe nuclear magnetic resonance spectrum of xenon in gas mixtures of Xe with other molecules provides a test of the ab initio surfaces for the intermolecular shielding of Xe in the presence of the other molecule. We examine the electron correlation contributions to the Xe-CO2, Xe-N2, Xe-CO, Xe-CH4, and Xe-CF4 shielding surfaces and test the calculations against the experimental temperature dependence of the density coefficients of the Xe chemical shift in the gas mixtures at infinite dilution in Xe. Comparisons with the gas phase data permit the refinement of site-site potential functions for Xe-N2, Xe-CO, and Xe-CF4 especially for atom-Xe distances in the range 3.5-6 A. With the atom-atom shielding surfaces and potential parameters obtained in the present work, construction of shielding surfaces and potentials for applications such as molecular dynamics averaging of Xe chemical shifts in liquid solvents containing CH3, CH2, CF3, and CF2 groups is possible.     

Theoretical studies on the potential energy surfaces and vibrational energy levels of HXeF and HXeCl

The potential energy surfaces for the electronic ground state of the HXeCl and HXeF molecules areconstructed by using the internally contracted multi-reference configuration interaction with theDavidson correction(icMRCI Q)method and large basis sets.The stabilities and dissociation barriersare identified from the potential energy surfaces.The three-body dissociation channel is found to bethe dominate dissociation channel for HXeCl,while two dissociation channels are possible and com-petitive for HXeF.Based on the obtained potentials,vibrational energy levels of HXeCl and HXeF arecalculated using the Lanczos algorithm.Our theoretical results are in good agreement with the avail-able observed values.Particularly,the calculated fundamental frequency of the H—Xe stretching vi-bration including the Xe matrix effect of HXeCl is found to be 1666.6 cm-1,which is only 17.6 cm-1higher than the recently observed value of 1649 cm-1.     

Limitations of the diatomics-in-molecules (DIM) method with neglect of overlap

The approximations made in the diatomics-in-molecules (DIM) method are examined by comparing the matrix elements of the DIM calculation with those of the ab initio calculation. It was found that if the overlap between atomic orbitals is small, the ab initio matrix elements can be expressed as the sums of DIM matrix elements and three-center terms ignored in the DIM method. The zero overlap of the atomic orbital approximation used in the DIM method makes the three-center terms simpler and apparently smaller which justifies the applicability of this approximation to the DIM method. Results of calculations show that deviations of the DIM values from accurate values are particularly large in the linear HXH configuration where X is a many-electron atom. It is suggested that a supplementary term be added to the DIM energy (DIM-SC) to take into account the neglected three-center terms.     

Real-time study of the adiabatic energy loss in an atomic collision with a metal cluster

Gas-phase hydrogen atoms are accelerated towards metallic surfaces in their vicinity. As it approaches the surface, the velocity of an atom increases and this motion excites the metallic electrons, causing energy loss to the atom. This dissipative dynamics is frequently described as atomic motion under friction, where the friction coefficient is obtained from ab initio calculations assuming a weak interaction and slow atom. This paper tests the aforementioned approach by comparing to a real-time Ehrenfest molecular dynamics simulation of such a process. The electrons are treated realistically using standard approximations to time-dependent density functional theory. We find indeed that the electronic excitations produce a friction-like force on the atom. However, the friction coefficient strongly depends on the direction of the motion of the atom: it is large when the atom is moving towards the cluster and much smaller when the atom is moving away. It is concluded that a revision of the model for energy dissipation at metallic surfaces, at least for clusters, may be necessary.     

CCl2自由基与H2O分子反应动力学研究

用213 nm激光光解CCl4产生CCl2自由基，用LP LIF技术测定了室温下基态CCl2自由基与H2O分子的反应速率常数为(5.45±0.95)×10－14 cm3•molecule－1•s－1.在G2MP2理论水平上计算了CCl2＋H2O反应的最低单重态势能面，揭示了插入与加成 消除两种反应机理，得到了三个可能的产物通道:HCl＋HClCO、HCl＋trans ClCOH以及HCl＋cis ClCOH.并用RRKM TST和传统过渡态理论计算了这三个通道的分支比及其温度效应.结果说明在低温下（273 K），插入机理的产物通道的分支比远大于加成 消除机理的产物通道， HCl＋HClCO是主要产物，分支比为77.4％，其次是HCl＋cis ClCOH，分支比为22.6％.而在高温下（3000 K），加成 消除机理的反应通道大于插入机理， HCl＋trans ClCOH分支比为82.3％.     

A DIM model for homogeneous noble gas ionic clusters

The method of diatomics-in-molecules (DIM) is applied to the calculation of the energy of the homogeneous noble-gas ionic clusters Ar _n ⁺ and Xe _n ⁺ forn=3, 4, ..., 22. The trimers are stable symmetric linear molecules exhibiting chemical binding, a result in agreement both with ab initio calculations and with previous DIM work. The clusters up ton=13 are best described as a trimer ion surrounded by neutrals, whereby the charge distribution changes slightly with increasingn. Both noble gases exhibit a special stability associated with the completion of the first shell of neutral atoms atn=13. Asn increases from 13 to 22, there is a greater delocalization of the positive charge, the central ion tending to become a linear tetramer, symmetric for Xe and unsymmetric for Ar. Energies of the excited electronic states are reported and the possibility of developing simpler DIM models for the clusters and for mixed noble gases is discussed.     

Potential energy surfaces for HenNe+ ions: ab initio and diatomics-in-molecule results

The potential energy surface of He2Ne+ has been reinvestigated using a combination of ab initio and diatomics-in-molecule (DIM) calculations. In contrast to the reports of two recent studies the ion is found to have an asymmetric linear He-Ne-He structure, with no barrier to formation from the separated atoms on the ground-state surface. The He-Ne+ bond lengths at the potential minimum are 1.51 and 1.81 A, and the total bonding energy is 0.717 eV. Comparing the He2Ne+ energy to that of HeNe+, the bonding energy for the second helium atom is 0.06 eV, about 10% of that of the first He atom. The saddle point between the two equivalent minima is a symmetric structure, 0.0074 eV above the potential minimum. A symmetric geometry becomes the overall potential minimum if the 2s hole on the Ne is excluded from the reference states of a multireference configuration interaction calculation. A DIM potential was created for the HenNe+ family of ions. The DIM potential is consistent with the asymmetric He2Ne+ ion serving as a core; it predicts a slightly more asymmetric geometry than the ab initio results. Additional helium atoms form five-membered rings around the bonds of the core ion to fill the first shell and then add to the ends of the cluster. The asymmetric core ion and the highly compact structure help to account for the lack of apparent shell structure in the mass spectrometry of HenNe+ clusters. Finally, we recommend that the value De=0.63+/-0.04 eV be adopted for the ground state of HeNe+.     

A neutral xenon-containing radical,HXeO

We report an open-shell species containing xenon, HXeO ((2)Sigma), prepared by UV photolysis of H(2)O/Xe or N(2)O/HBr/Xe solid mixtures at 7 K and subsequent thermal mobilization of oxygen atoms at >/=30 K. The H-Xe stretching absorption of HXeO in solid Xe is at 1466.1 cm(-1), and it shifts to 1070.3 cm(-1) upon deuteration. The extensive ab initio calculations indicate that HXeO is intrinsically stable, owing to significant ionic and covalent contributions to its bonding. The formation of HXeO ((2)Sigma) radicals in these experiments suggests extensive stabilization and thermal mobility of singlet ((1)D) oxygen atoms in solid Xe and holds promises for the stability of the HKrO and HArO species.     

Modifying the fullerene surface using endohedral noble gas atoms: density functional theory based molecular dynamics study of C70O3

We have performed a series of ab initio molecular orbital and molecular dynamics calculations to ascertain the influence of an endohedral noble gas atom on the reactivity of the surface of the model system C(70)O(3). Our simulations show that the minimum energy pathways for the ozone ring-opening reaction are influenced by the presence of the endohedral atom. The effect is isomer dependent, with the enthalpy of the reaction increasing for a,b-C(70)O(3) and decreasing for e,e-C(70)O(3) when doped with the heavy noble gas atoms Xe and Rn.     

UV photolysis products of propiolic acid in noble-gas solids

Photolysis (193 nm) of propiolic acid (HCCCOOH) was studied with Fourier transform infrared spectroscopy in noble-gas (Ar, Kr, and Xe) solid matrixes. The photolysis products were assigned using ab initio quantum chemistry calculations. The novel higher-energy conformer of propiolic acid was efficiently formed upon UV irradiation, and it decayed back to the ground-state conformer on a time scale of approximately 10 min by tunneling of the hydrogen atom through the torsional energy barrier. In addition, the photolysis produced a number of matrix-isolated 1:1 molecular complexes such as HCCH...CO2, HCCOH...CO, and H2O...C3O. The HCCH...CO2 complex dominated among the photolysis products, and the computations suggested a parallel geometry of this complex characterized by an interaction energy of -9.6 kJ/mol. The HCCOH...CO complex also formed efficiently, but its concentration was strongly limited by its light-induced decomposition. In this complex, the most probable geometry was found to feature the interaction of carbon monoxide with the OH group via the carbon atom, and the computational interaction energy was determined to be -18.3 kJ/mol. The formation of the strong H2O...C3O complex (interaction energy -21 kJ/mol) was less efficient, which might be due to the inefficiency of the involved radical reaction.     

Generation and orientation of organoxenon molecule H-Xe-CCH in the gas phase

We report on the first observation of the organoxenon HXeCCH molecule in the gas phase. This molecule has been prepared in a molecular beam experiment by 193 nm photolysis of an acetylene molecule on Xe(n) clusters (n approximately 390). Subsequently the molecule has been oriented via the pseudo-first-order Stark effect in a strong electric field of the polarized laser light combined with the weak electrostatic field in the extraction region of a time-of-flight spectrometer. The experimental evidence for the oriented molecule has been provided by measurements of its photodissociation. For comparison, photolysis of C(2)H(2) on Ar(n) clusters (n approximately 280) has been measured. Here the analogous rare gas molecule HArCCH could not be generated. The interpretation of our experimental findings has been supported by ab initio calculations. In addition, the experiment together with the calculations reveals information on the photochemistry of the HXeCCH molecule. The 193 nm radiation excites the molecule predominantly into the 2 (1)Sigma(+) state, which cannot dissociate the Xe-H bond directly, but the system evolves along the Xe-C coordinate to a conical intersection of a slightly nonlinear configuration with the dissociative 1 (1)Pi state, which then dissociates the Xe-H bond.     

How Many Water Molecules Does it Take to Dissociate HCl?

The potential energy surfaces of the HCl(H₂O)_n (n is the number of water molecules) clusters are systematically explored using density functional theory and high‐level ab initio computations. On the basis of electronic energies, the number of water molecules needed for HCl dissociation is four as reported by some experimental groups. However, this number is five owing to the inclusion of entropic factors. Wiberg bond indices are calculated and analyzed, and the results provide a quadratic correlation and classification of clusters according to the nondissociated, partially dissociated, and fully dissociated character of the H?Cl bond. Our computations show that if temperature is not controlled during the experiment, the values obtained for the dipole moment (or for any measurable property) are susceptible to change, providing a different picture of the number of water molecules needed for HCl dissociation in a nanoscopic droplet.     

Emergence of charge-transfer-to-solvent band in the absorption spectra of hydrogen halides on ice nanoparticles: spectroscopic evidence for acidic dissociation

Extensive ab initio calculations complemented by a photodissociation experiment at 193 nm elucidate the nature of hydrogen halide molecules bound on free ice nanoparticles. Electronic absorption spectra of small water clusters (up to 5 water molecules) and water clusters doped with hydrogen fluoride, hydrogen chloride and hydrogen bromide were calculated. The spectra were modeled at the time-dependent density functional (TDDFT) level of theory with the BHandHLYP functional using the reflection principle. We observe the emergence of a charge-transfer-to-solvent (CTTS) band in the absorption spectra upon the acidic dissociation of the hydrogen halides. The CTTS band provides a spectroscopically observable feature for the acidic dissociation. The calculated spectra were compared with our new experimental photodissociation data for larger water clusters doped with HCl and HBr. We conclude that HCl and HBr dissociate to a large extent on the surface of ice nanoparticles at temperatures near 120 K and photoactive products are formed. The acidic dissociation of HX leads to an enhancement by about 4 orders of magnitude of the HCl photolysis rate in the 200-300 nm region, which is potentially relevant for the halogen budget in the atmosphere.     

Segmented contracted basis sets for atoms H through Xe: Sapporo-(DK)-<Emphasis Type="Italic">n</Emphasis>ZP sets (<Emphasis Type="Italic">n</Emphasis> = D,T, Q)

For the 54 atoms from H to Xe, compact yet accurate segmented Gaussian-type basis sets have been constructed for all electron calculations. Non-relativistic nZP (Sapporo-nZP) sets for Li–Xe and relativistic nZP (Sapporo-DK-nZP) sets for K–Xe are developed (n = D, T, Q), which efficiently incorporate valence and core electron correlations. Test calculations at the coupled-cluster level of theory are performed for spectroscopic constants of 12 hydrides of s- and d-block atoms and 12 diatomics of p-block atoms in their ground states. For all molecules, the calculated spectroscopic constants approach to the experimental values smoothly as the basis set quality increases.     

Classical and quantum studies of the photodissociation of a HX (X=Cl,F) molecule adsorbed on ice

The photodissociation dynamics of a HX (X = Cl,F) molecule adsorbed on a hexagonal ice surface at T = 0 K is studied using time-dependent quantum wave packets and quasiclassical trajectories. The relevant potential energy surfaces are calculated using high-level ab initio methods. We present here two dimensional calculations for the dynamics of the hydrogen photofragment for both HCl and HF molecules. The purpose of this paper is to compare the photodissociation dynamics of the two molecules which are adsorbed on the ice surface with different equilibrium geometries. The total photodissociation cross section and the angular distribution are calculated. The comparison with classical trajectory calculations provides evidence for typical quantum effects and reveals rainbow structures.     

Theoretical study of the decomposition of BCl3 induced by a H radical

We report on a theoretical study of the gas-phase decomposition of boron trichloride in the presence of hydrogen radicals using ab initio energetic calculations coupled to TST, RRKM, and VTST-VRC kinetic calculations. In particular, we present an addition-elimination mechanism (BCl(3) + H → BHCl(2) + Cl) allowing for a much more rapid consumption of BCl(3) than the direct abstraction reaction (BCl(3) + H → BCl(2) + HCl) considered up to now. At low temperatures, T ≤ 800 K, our results show that a weakly stabilized complex BHCl(3) is formed with a kinetic law compatible with the consumption rate measured in the former experiments. At higher temperatures, this complex is not stable and then easily eliminates a chlorine atom. Our work also shows that a very similar mechanism, involving the same intermediate and sharing the same transition state, allows for the elimination of HCl. A dividing coefficient between these two elimination pathways is obtained from both a potential energy surface based statistical analysis and an ab initio molecular dynamics transition path sampling simulation. It finally allows partitioning of the global consumption rate of BCl(3) in terms of the formation of (i) BHCl(3), (ii) BHCl(2) + Cl through a H addition/Cl elimination mechanism, (iii) BCl(2) + HCl through a H addition/HCl elimination mechanism, and (iv) BCl(2) + HCl through direct abstraction.