Reactivity enhancement of ultracold O(3P)+H2 collisions by van der Waals interactions

The role of van der Waals forces in O((3)P)+H(2)(upsilon=1,j=0) collisions is investigated theoretically at low and ultralow temperatures. Quantum scattering calculations have been performed for zero total angular momentum using the lowest London-Eyring-Polanyi-Sato double-polynomial (3)A(") potential-energy surface reported by [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)] and its recent BMS1 and BMS2 extensions developed by [Brandao et al., J. Chem. Phys. 121, 8861 (2004)] which provide a more accurate treatment of the van der Waals interaction. Our calculations show that van der Waals forces strongly influence chemical reactivity at ultracold translational energies. The presence of a zero-energy resonance for the BMS1 surface is found to enhance reactivity in the ultracold regime and shift the Wigner threshold to lower temperatures.     

Isotope branching and tunneling in O(3P)+HD-->OH+D; OD+H reactions

The O((3)P)+HD and O((3)P)+D(2) reactions are studied using quantum scattering calculations and chemically accurate potential energy surfaces developed for the O((3)P)+H(2) system by Rogers et al. [J. Phys. Chem. A 104, 2308 (2000)]. Cross sections and rate coefficients for OH and OD products are calculated using accurate quantum methods as well as the J-shifting approximation. The J-shifting approach is found to work remarkably well for both O+HD and O+D(2) collisions. The reactions are dominated by tunneling at low temperatures and for the O+HD reaction the hydrogen atom transfer leading to the OH product dominates at low temperatures. Our result for the OH/OD branching ratio is in close agreement with previous calculations over a wide range of temperatures. The computed OH/OD branching ratios are also in close agreement with experimental results of Robie et al. [Chem. Phys. Lett. 134, 579 (1987)] at temperatures above 400 K but the theoretical results do not reproduce the rapid rise in the experimental values of the branching ratio for temperatures lower than 350 K. We believe that new measurements could resolve the long-standing discrepancy between experiment and theory for this benchmark reaction.     

A new reaction path for the C + NO reaction: dynamics on the 4A' potential-energy surface

We present a new reaction path without significant barriers for the C + NO reaction, forming ground state N((4)S) and CO. Electronic structure (CASPT2) calculations have been performed for the two lowest (4)A' states of the CNO system. The lowest of these states shows no significant barriers against reaction in the C + NO or O + CN channels. This surface has been fitted to an analytical function using a many-body expansion. Using this surface, and the previously published (2)A' and (2)A' surfaces [Andersson et al., Phys. Chem. Chem. Phys., 2000, 2, 613; Andersson et al., Chem. Phys., 2000, 259, 99], we have performed quasiclassical trajectory (QCT) calculations, obtaining rate coefficients for the C((3)P) + NO(X(2)Pi) --> CO(X(1)Sigma(+)) + N((4)S,(2)D) and C((3)P) + NO(X(2)Pi) --> O((3)P) + CN(X(2)Sigma(+)) reactions. We have also simulated the crossed molecular beam experiments of Naulin et al. [Chem. Phys., 1991, 153, 519] The inclusion of the (4)A' surface in the QCT calculations gives excellent agreement with experiments. This is the first time an adiabatic pathway from C((3)P) + NO(X(2)Pi) to CO(X(1)Sigma(+))+N((4)S) has been reported.     

Ab initio investigations of the radical-radical reaction of O(3P) + C3H3

We present ab initio calculations of the reaction of ground-state atomic oxygen [O((3)P)] with a propargyl (C(3)H(3)) radical based on the application of the density-functional method and the complete basis-set model. It has been predicted that the barrierless addition of O((3)P) to C(3)H(3) on the lowest doublet potential-energy surface produces several energy-rich intermediates, which undergo subsequent isomerization and decomposition steps to generate various exothermic reaction products: C(2)H(3)+CO, C(3)H(2)O+H, C(3)H(2)+OH, C(2)H(2)+CHO, C(2)H(2)O+CH, C(2)HO+CH(2), and CH(2)O+C(2)H. The respective reaction pathways are examined extensively with the aid of statistical Rice-Ramsperger-Kassel-Marcus calculations, suggesting that the primary reaction channel is the formation of propynal (CHCCHO)+H. For the minor C(3)H(2)+OH channel, which has been reported in recent gas-phase crossed-beam experiments [H. Lee et al., J. Chem. Phys. 119, 9337 (2003); 120, 2215 (2004)], a comparison on the basis of prior statistical calculations is made with the nascent rotational state distributions of the OH products to elucidate the mechanistic and dynamic characteristics at the molecular level.     

MULTIMODE quantum calculations of intramolecular vibrational energies of the water dimer and trimer using ab initio-based potential energy surfaces

We report vibrational configuration interaction calculations of the monomer fundamentals of (H(2)O)(2), (D(2)O)(2), (H(2)O)(3), and (D(2)O)(3) using the code MULTIMODE and full dimensional ab initio-based global potential energies surfaces (PESs). For the dimer the HBB PES [Huang et al., J. Chem. Phys 128, 034312 (2008)] is used and for the trimer a new PES, reported here, is used. The salient properties of the new trimer PES are presented and compared to previous single-point calculations and the vibrational energies are compared with experiments.     

Imaging the nature of the mode-specific chemistry in the reaction of Cl atom with antisymmetric stretch-excited CH4

Following up our preliminary communication [Kawamata et al., Phys. Chem. Chem. Phys. 10, 4378 (2008)], the effects of the antisymmetric-stretching excitation of methane on the Cl((2)P(3/2))+CH(4) reaction are examined here over a wide range of initial collision energy in a crossed molecular beam imaging experiment. The antisymmetric stretch of CH(4) is prepared in a single rovibrational state of (v(3)=1, j=2) by direct infrared absorption, and the major product states of CH(3)(v=0) are probed by a time-sliced velocity-map imaging method. We find that at fixed collision energies, the stretching excitation promotes reaction rate. Compared to the ground-state reaction, this vibrational enhancement factor is, however, no more effective than the translational enhancement. The correlated HCl(v'=1) vibrational branching fraction shows a striking dependence on collision energies, varying from 0.7 at E(c)=2 kcal mol(-1) to about 0.2 at 13 kcal mol(-1). This behavior resembles the previously studied Cl+CH(2)D(2)(v(6)=1), but is in sharp contrast to the Cl+CHD(3)(v(1)=1) and CH(2)D(2)(v(1)=1) reactions. Dependences of experimental results on the probed rotational states of CH(3)(v=0) are also elucidated. We qualitatively interpret those experimental observations based on a conceptual framework proposed recently.     

Quasiclassical trajectory study of the reaction H+CH4(nu3 = 0,1)-->CH3+H2 using a new ab initio potential energy surface

Detailed quasiclassical trajectory calculations of the reaction H+CH4(nu3 = 0,1)-->CH3 + H2 using a slightly updated version of a recent ab initio-based CH5 potential energy surface [X. Zhang et al., J. Chem. Phys. 124, 021104 (2006)] are reported. The reaction cross sections are calculated at initial relative translational energies of 1.52, 1.85, and 2.20 eV in order to make direct comparison with experiment. The relative reaction cross section enhancement ratio due to the excitation of the C-H antisymmetric stretch varies from 2.2 to 3.0 over this energy range, in good agreement with the experimental result of 3.0 +/- 1.5 [J. P. Camden et al., J. Chem. Phys. 123, 134301 (2005)]. The laboratory-frame speed and center-of-mass angular distributions of CH3 are calculated as are the vibrational and rotational distributions of H2 and CH3. We confirm that this reaction occurs with a combination of stripping and rebound mechanisms by presenting the impact parameter dependence of these distributions and also by direct examination of trajectories.     

Study of the H+O2 reaction by means of quantum mechanical and statistical approaches: the dynamics on two different potential energy surfaces

The possible existence of a complex-forming pathway for the H+O(2) reaction has been investigated by means of both quantum mechanical and statistical techniques. Reaction probabilities, integral cross sections, and differential cross sections have been obtained with a statistical quantum method and the mean potential phase space theory. The statistical predictions are compared to exact results calculated by means of time dependent wave packet methods and a previously reported time independent exact quantum mechanical approach using the double many-body expansion (DMBE IV) potential energy surface (PES) [Pastrana et al., J. Phys. Chem. 94, 8073 (1990)] and the recently developed surface (denoted XXZLG) by Xu et al. [J. Chem. Phys. 122, 244305 (2005)]. The statistical approaches are found to reproduce only some of the exact total reaction probabilities for low total angular momenta obtained with the DMBE IV PES and some of the cross sections calculated at energy values close to the reaction threshold for the XXZLG surface. Serious discrepancies with the exact integral cross sections at higher energy put into question the possible statistical nature of the title reaction. However, at a collision energy of 1.6 eV, statistical rotationally resolved cross sections managed to reproduce the experimental cross sections for the H+O(2)(v=0,j=1)-->OH(v(')=1,j('))+O process reasonably well.     

Quantum and quasiclassical studies of the O(3P)+HCl-->OH+Cl(2P) reaction using benchmark potential surfaces

We have performed quantum mechanical (QM) dynamics calculations within the independent-state approximation with new benchmark triplet A" and A' surfaces [B. Ramachandran et al., J. Chem. Phys. 119, 9590 (2003)] for the rovibronic state-to-state measurements of the reaction O(3P)+HCl(v=2,j=1,6,9)-->OH(v'j')+Cl(2P) [Zhang et al., J. Chem. Phys. 94, 2704 (1991)]. The QM and experimental rotational distributions peak at similar OH(j') levels, but the QM distributions are significantly narrower than the measurements and previous quasiclassical dynamics studies. The OH(low j) populations observed in the measurements are nearly absent in the QM results. We have also performed quasiclassical trajectory with histogram binning (QCT-HB) calculations on these same benchmark surfaces. The QCT-HB rotational distributions, which are qualitatively consistent with measurements and classical dynamics studies using other surfaces, are much broader than the QM results. Application of a Gaussian binning correction (QCT-GB) dramatically narrows and shifts the QCT-HB rotational distributions to be in very good agreement with the QM results. The large QCT-GB correction stems from the special shape of the joint distribution of the classical rotational/vibrational action of OH products. We have also performed QM and QCT calculations for the transition, O+HCl(v=0,T=300 K)-->OH(v'j')+Cl from threshold to approximately 130 kcal mol(-1) collision energy as a guide for possible future hyperthermal O-atom measurements. We find in general a mixed energy release into translation and rotation consistent with a late barrier to reaction. Angular distributions at high collision energy are forward peaked, consistent with a stripping mechanism. Direct collisional excitation channel cross sections, O+HCl(v=0,T=300 K)-->O+HCl(v'=1), in the same energy range are large, comparable in magnitude to the reactive channel cross sections. Although the (3)A" state dominates most collision processes, above approximately 48 kcal mol(-1), the (3)A' state plays the major role in collisional excitation.     

Experimental measurement of the van der Waals binding energy of X-O2 clusters (X=Xe, CH3I, C3H6, C6H12)

Van der Waals binding energies for the X-O(2) complexes (X=Xe, CH(3)I, C(3)H(6), C(6)H(12)) are determined by analysis of experimental velocity map imaging data for O((3)P(2)) atoms arising from UV-photodissociation of the complex [A. V. Baklanov et al., J. Chem. Phys. 126, 124316 (2007)]. Several dissociation pathways have been observed, we focus on the channel corresponding to prompt dissociation of X-O(2) into X+2O((3)P) fragments, which is present for complexes of O(2) with all partners X. Our method is based on analysis of the kinetic energy of all three photofragments, where the O atom kinetic energy was directly measured in the experiment and the kinetic energy of the X partner was calculated using momentum conservation, along with the measured angular anisotropy for O atom recoil. We exploit the fact that the clusters are all T-shaped or nearly T-shaped, which we also confirm by ab initio calculations, along with knowledge of the transition dipole governing radiative absorption by the complex. The effect of partitioning the kinetic energy between translation along the X-O(2) and O-O coordinates on the angular anisotropy of the O atom recoil direction is discussed. Van der Waals binding energies of 110±20 cm(-1), 280±20 cm(-1), 135±30 cm(-1), and 585±20 cm(-1) are determined for Xe-O(2), CH(3)I-O(2), C(3)H(6)-O(2), and C(6)H(12)-O(2) clusters, respectively.     

Communication: rigorous calculation of dissociation energies (D0) of the water trimer, (H2O)3 and (D2O)3

Using a recent, full-dimensional, ab initio potential energy surface [Y. Wang, X. Huang, B. C. Shepler, B. J. Braams, and J. M. Bowman, J. Chem. Phys. 134, 094509 (2011)] together with rigorous diffusion Monte Carlo calculations of the zero-point energy of the water trimer, we report dissociation energies, D(0), to form one monomer plus the water dimer and three monomers. The calculations make use of essentially exact zero-point energies for the water trimer, dimer, and monomer, and benchmark values of the electronic dissociation energies, D(e), of the water trimer [J. A. Anderson, K. Crager, L. Fedoroff, and G. S. Tschumper, J. Chem. Phys. 121, 11023 (2004)]. The D(0) results are 3855 and 2726 cm(-1) for the 3H(2)O and H(2)O + (H(2)O)(2) dissociation channels, respectively, and 4206 and 2947 cm(-1) for 3D(2)O and D(2)O + (D(2)O)(2) dissociation channels, respectively. The results have estimated uncertainties of 20 and 30 cm(-1) for the monomer plus dimer and three monomer of dissociation channels, respectively.     

Rate constant for OH(2 Pi)+O(3P)-->H(2S)+O2(3 Sigma g-) reaction on an improved ab initio potential energy surface and implications for the interstellar oxygen problem

The authors report a global potential energy surface for the ground electronic state of HO(2)(X (2)A(")), which improves upon the XXZLG potential [Xu and et al., J. Chem. Phys. 122, 244305 (2005)] with additional high-level ab initio points for the long-range interaction potential in the O+OH channel. Exact J=0 quantum mechanical reaction probabilities were calculated on the new potential and the rate constant for the title reaction was obtained using a J-shifting method. The calculated rate constant is in good agreement with available experimental values and our results predict a significantly lower rate at temperature range below 30 K, offering a possible explanation for the "interstellar oxygen problem."     

Communication: theoretical exploration of Au(+)+H2, D2, and HD reactive collisions

A quasi-classical study of the endoergic Au(+)((1)S) + H(2)(X(1)Σ(g)(+)) → AuH(+) ((2)Σ(+)) + H((2)S) reaction, and isotopic variants, is performed to compare with recent experimental results [F. Li, C. S. Hinton, M. Citir, F. Liu, and P. B. Armentrout, J. Chem. Phys. 134, 024310 (2011)]. For this purpose, a new global potential energy surface has been developed based on multi-reference configuration interaction ab initio calculations. The quasi-classical trajectory results show a very good agreement with the experiments, showing the same trends for the different isotopic variants of the hydrogen molecule. It is also found that the total dissociation into three fragments, Au(+)+H+H, is the dominant reaction channel for energies above the H(2) dissociation energy. This results from a well in the entrance channel of the potential energy surface, which enhances the probability of H-Au-H insertion.     

Angular distributions and angular momentum alignment of O((3)P(J)) atoms formed in the photolysis of O(2)via the Herzberg continuum

Sliced velocity-map imaging has been used to measure photofragment scattering distributions for the O((3)P(2)) and O((3)P(1)) products of O(2) photolysis following laser excitation into the Herzberg continuum between 205 and 241 nm. The images were analysed to extract the photofragment spatial anisotropy parameter, β, together with the alignment parameters a(∥), a(⊥), a(⊥), and Re[a(∥, ⊥)]. Our alignment measurements bridge the gap between the recent 193 nm measurement of Brouard et al., Phys. Chem. Chem. Phys., 2006, 8, 5549 and those of Alexander et al., J. Chem. Phys., 2003, 118, 10566 at 222 and 237 nm, and extend out to the threshold at 241 nm. Our measured parameters show no strong dependence on photolysis wavelength. Near the threshold we were able to separate the contributions from the O((3)P(2)) + O((3)P(2)) and O((3)P(2)) + O((3)P(1)) channels, and found significantly different photofragment alignments for the two cases.     

A new mechanism for the production of highly vibrationally excited OH in the mesosphere: an ab initio study of the reactions of O2(A 3Sigmau+ and A' 3Deltau)+H

In an attempt to explain the observed nightglow emission from OH(v=10) in the mesosphere that has the energy greater than the exothermicity of the H+O(3) reaction, potential energy surfaces were calculated for reactions of high lying electronic states of O(2)(A (3)Sigma(u) (+) and A' (3)Delta(u)) with atomic hydrogen H((2)S) to produce the ground state products OH((2)Pi)+O((3)P). From collinear two-dimensional scans, several adiabatic and nonadiabatic pathways have been identified. Multiconfigurational single and double excitation configuration interaction calculations show that the adiabatic pathways on a (4)Delta potential surface from O(2)(A' (3)Delta)+H and a (4)Sigma(+) potential surface from O(2)(A (3)Sigma(u) (+))+H are the most favorable, with the zero-point corrected barrier heights of as low as 0.191 and 0.182 eV, respectively, and the reactions are fast. The transition states for these pathways are collinear and early, and the reaction coordinate suggests that the potential energy release of ca. 3.8 eV (larger than the energy required to excite OH to v=10) is likely to favor high vibrational excitation.     

Quantum and classical studies of the O(3P) + H2(v = 0-3,j = 0) --> OH + H reaction using benchmark potential surfaces

We present results of time-dependent quantum mechanics (TDQM) and quasiclassical trajectory (QCT) studies of the excitation function for O(3P) + H2(v = 0-3,j = 0) --> OH + H from threshold to 30 kcal/mol collision energy using benchmark potential energy surfaces [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)]. For H2(v = 0) there is excellent agreement between quantum and classical results. The TDQM results show that the reactive threshold drops from 10 kcal/mol for v = 0 to 6 for v = 1, 5 for v = 2 and 4 for v = 3, suggesting a much slower increase in rate constant with vibrational excitation above v = 1 than below. For H2(v > 0), the classical results are larger than the quantum results by a factor approximately 2 near threshold, but the agreement monotonically improves until they are within approximately 10% near 30 kcal/mol collision energy. We believe these differences arise from stronger vibrational adiabaticity in the quantum dynamics, an effect examined before for this system at lower energies. We have also computed QCT OH(v',j') state-resolved cross sections and angular distributions. The QCT state-resolved OH(v') cross sections peak at the same vibrational quantum number as the H2 reagent. The OH rotational distributions are also quite hot and tend to cluster around high rotational quantum numbers. However, the dynamics seem to dictate a cutoff in the energy going into OH rotation indicating an angular momentum constraint. The state-resolved OH distributions were fit to probability functions based on conventional information theory extended to include an energy gap law for product vibrations.     

Global potential energy surfaces for O((3)P) + H2O((1)A1) collisions

Global analytic potential energy surfaces for O((3)P) + H(2)O((1)A(1)) collisions, including the OH + OH hydrogen abstraction and H + OOH hydrogen elimination channels, are presented. Ab initio electronic structure calculations were performed at the CASSCF + MP2 level with an O(4s3p2d1f)/H(3s2p) one electron basis set. Approximately 10(5) geometries were used to fit the three lowest triplet adiabatic states corresponding to the triply degenerate O((3)P) + H(2)O((1)A(1)) reactants. Transition state theory rate constant and total cross section calculations using classical trajectories to collision energies up to 120?kcal mol(-1) (～11?km s(-1) collision velocity) were performed and show good agreement with experimental data. Flux-velocity contour maps are presented at selected energies for H(2)O collisional excitation, OH + OH, and H + OOH channels to further investigate the dynamics, especially the competition and distinct dynamics of the two reactive channels. There are large differences in the contributions of each of the triplet surfaces to the reactive channels, especially at higher energies. The present surfaces should support quantitative modeling of O((3)P) + H(2)O((1)A(1)) collision processes up to ～150?kcal mol(-1).     

(infinity)(2)[Cu2(mu5-btb)(mu-OH)(mu-H2O)]: a two-dimensional coordination polymer built from ferromagnetically coupled Cu2 units (btb = benzene-1,2,3-tricarboxylate)

Hydrothermal reaction of Cu(NO(3))(2).3H(2)O, Cd(OH)(2) or Zn(OH)(2) with benzene-1,2,3-tricarboxylic acid (H(3)btb, hemimellitic acid) produced the 2D coordination polymer (MOF) [Cu(2)(mu(5)-btb)(mu-OH)(mu-H(2)O)] () and the 2D hydrogen-bonded complexes [Cd(H(2)btb)(2)(H(2)O)(4)].2H(2)O () and [Zn(H(2)O)(6)](H(2)btb)(2).4H(2)O () which are characterized by single-crystal X-ray diffraction, X-ray powder diffraction and thermoanalysis. Magnetic susceptibility measurements between 1.9-300 K for revealed three magnetic active exchange pathways that link the copper(ii) ions through a long mu-aqua bridge, an anti-syn carboxylate bridge [j(2) = 0.161(1) cm(-1)], and through a mixed mu-hydroxo + syn-syn carboxylate bridge [J = 83(1) cm(-1)]. At temperatures higher than 30 K the system behaves as isolated Cu(2) units with strong ferromagnetic Cu-Cu coupling through the mu-hydroxo and syn-syn carboxylate bridge. The strong ferromagnetic coupling is explained with Hoffmann's approach by means of the concept of counter-complementarity introduced by Nishida et al.[Chem. Lett., 1983, 1815-1818].