Inelastic scattering matrix elements for the nonadiabatic collision B(2P1/2)+H2(1Sigmag+,j)<-->B(2P3/2)+H2(1Sigmag+,j')

Inelastic scattering matrix elements for the nonadiabatic collision B(2P1/2)+H2(1Sigmag+,j)<-->B(2P3/2)+H2(1Sigmag+,j') are calculated using the time dependent channel packet method (CPM). The calculation employs 1 2A', 2 2A', and 1 2A" adiabatic electronic potential energy surfaces determined by numerical computation at the multireference configuration-interaction level [M. H. Alexander, J. Chem. Phys. 99, 6041 (1993)]. The 1 2A' and 2 2A', adiabatic electronic potential energy surfaces are transformed to yield diabatic electronic potential energy surfaces that, when combined with the total B+H2 rotational kinetic energy, yield a set of effective potential energy surfaces [M. H. Alexander et al., J. Chem. Phys. 103, 7956 (1995)]. Within the framework of the CPM, the number of effective potential energy surfaces used for the scattering matrix calculation is then determined by the size of the angular momentum basis used as a representation. Twenty basis vectors are employed for these calculations, and the corresponding effective potential energy surfaces are identified in the asymptotic limit by the H2 rotor quantum numbers j=0, 2, 4, 6 and B electronic states 2Pja, ja=1/2, 3/2. Scattering matrix elements are obtained from the Fourier transform of the correlation function between channel packets evolving in time on these effective potential energy surfaces. For these calculations the H2 bond length is constrained to a constant value of req=1.402 a.u. and state to state scattering matrix elements corresponding to a total angular momentum of J=1/2 are discussed for j=0<-->j'=0,2,4 and 2P1/2<-->2P1/2, 2P3/2 over a range of total energy between 0.0 and 0.01 a.u.     

Repulsive double many-body expansion potential energy surface for the reactions N(4S)+H2<-->NH(X3Sigma-)+H from accurate ab initio calculations

A single-sheeted DMBE potential energy surface is reported for the reactions N(4S)+H2<-->NH(X3Sigma-)+H based on a fit to accurate multireference configuration interaction energies. These have been calculated using the aug-cc-pVQZ basis set of Dunning and the full valence complete active space wave function as reference, being semi-empirically corrected by scaling the two-body and three-body dynamical correlation energies. The topographical features of the novel global potential energy surface are examined in detail, including a conical intersection involving the two first 4A' potential energy surfaces which has been transformed into an avoided crossing in the present single-sheeted representation.     

Adiabatic and non-adiabatic quantum dynamics calculation of O(1D) + D2 → OD + D reaction

Adiabatic (1A' or 1A' state) and non-adiabatic (2A'/1A' states) quantum dynamics calculations have been carried out for the title reaction (O((1)D) + D(2) → OD + D) to obtain the initial state-specified (v(i) = 0, j(i) = 0) integral cross section and rate constant using the potential energy surfaces of Dobbyn and Knowles. A total of 50 partial wave contributions have been calculated using the Chebyshev wave packet method with full Coriolis coupling to achieve convergence up to the collision energy of 0.28 eV. The total integral cross section and rate constant are in excellent agreement with experimental as well as quasi-classical trajectory results. Contributions from the adiabatic pathway of the 1A' state and the non-adiabatic pathway of the 2A'/1A' states, increase significantly with the collision energy. Compared to the O((1)D) + H(2) system, the kinetic isotope effect (k(D)/k(H)) is found to be nearly temperature independent above 100 K and its value of 0.77 ± 0.01 shows excellent agreement with the experimental result of 0.81.     

Ab initio prediction of the infrared-absorption spectrum of the C2Cl radical

The three lowest (1(2)A', 2(2)A', and 1(2)A") potential-energy surfaces of the C2Cl radical, correlating at linear geometries with 2Sigma+ and 2Pi states, have been studied ab initio using a large basis set and multireference configuration-interaction techniques. The electronic ground state is confirmed to be bent with a very low barrier to linearity, due to the strong nonadiabatic electronic interactions taking place in this system. The rovibronic energy levels of the 12C12C35Cl isotopomer and the absolute absorption intensities at a temperature of 5 K have been calculated, to an upper limit of 2000 cm(-1), using diabatic potential-energy and dipole moment surfaces and a recently developed variational method. The resulting vibronic states arise from a strong mixture of all the three electronic components and their assignments are intrinsically ambiguous.     

The double Renner effect in the X(2)A" and A(2)A' electronic states of HO(2)

A theoretical investigation of the X(2)A" and A(2)A' electronic states of the HO(2) radical is reported. Both electronic states have nonlinear equilibrium geometries and they correlate with a (2)Pi state at linear geometries so that they exhibit the Renner effect. In highly excited bending states, there is tunneling between two equivalent minima (with geometries where the H nucleus is bound to one, or the other, of the two O nuclei), and the two linear geometries H-O-O and O-O-H become accessible to the molecule. Thus, HO(2) affords an example of the so-called double Renner effect. Three-dimensional potential energy surfaces for the X(2)A" and A(2)A' electronic states of HO(2) have been calculated ab initio and the global potential energy surfaces for the states have been constructed. These surfaces have been used, in conjunction with the computer program DR [Odaka et al., J. Mol. Structure 795, 14 (2006); Odaka et al., J. Chem. Phys. 126, 094301 (2007)], for calculating HO(2) rovibronic energies in the "double-Renner"-degenerate electronic states X(2)A" and A(2)A'. The results of the ab initio calculations, the rovibronic energies obtained, and analyses of the wavefunctions for selected states are presented.     

Excited state electronic structures and dynamics of NOCl: a new potential function set, absorption spectrum, and photodissociation mechanism

A set of analytical potential energy surfaces (PESs) for six singlet excited states of NOCl are constructed based on multireference configuration interaction calculations. The total absorption cross section at the energy range of 2-7 eV is calculated by quantum dynamics calculations with the present PESs and transition dipole moments. The calculated absorption spectrum agrees well with the experiment. It is also found that the A band with the absorption maximum at 6.3 eV is attributed to the transition to the 4 1A' state, though the excitations to the 3 1A' and 3 1A" states contribute to the spectrum at the energy range between 4 and 5 eV. The spin-forbidden transitions are concluded to be negligibly weak. The mechanism of photodissociation reaction at the energy region corresponding to the A band is examined. The nonadiabatic transition rates from the 4 1A' state to lower singlet and triplet states are estimated by Fermi's golden rule, and the transitions to the 1 1A' and 3 1A' states induced by vibronic coupling are found to be the predominant dissociation pathways. The experimentally observed energy dependence of the recoil anisotropy of the fragments is discussed based on the calculated nonadiabatic transition rates.     

Collision energy dependence of the O(1D) + HCl --> OH + Cl(2P) reaction studied by crossed beam scattering and quasiclassical trajectory calculations on ab initio potential energy surfaces

The dynamics of the O(1D) + HCl --> OH + Cl(2P) reaction are investigated by a crossed molecular beam ion-imaging method and quasiclassical trajectory calculations on the three ab initio potential energy surfaces, the ground 1(1)A' and two excited (1(1)A' and 2(1)A') states. The scattering experiment was carried out at collision energies of 4.2, 4.5, and 6.4 kcal/mol. The observed doubly differential cross sections (DCSs) for the Cl(2P) product exhibit almost no collision energy dependence over this inspected energy range. The nearly forward-backward symmetric DCS indicates that the reaction proceeds predominantly on the ground-state potential energy surface at these energies. Variation of the forward-backward asymmetry with collision energy is interpreted using an osculating complex model. Although the potential energy surfaces obtained by CASSCF-MRCI ab initio calculations exhibit relatively low potential barriers of 1.6 and 6.5 kcal/mol for 1(1)A' and 2(1)A', respectively, the dynamics calculations indicate that contributions of these excited states are small at the collision energies lower than 15.0 kcal/mol. Theoretical DCSs calculated for the ground-state reaction pathway agree well with the observed ones. These experimental and theoretical results suggest that the titled reaction at collision energies less than 6.5 kcal/mol is predominantly via the ground electronic state.     

Communication: Multi-state analysis of the OCS ultraviolet absorption including vibrational structure

The first absorption band of OCS (carbonyl sulfide) is analyzed using potential energy surfaces and transition dipole moment functions of the lowest four singlet and the lowest four triplet states. Excitation of the 2 (1)A' state is predominant except at very low photon energies. It is shown that the vibrational structures in the center of the band are due to excitation of the 2 (3)A' triplet state, whereas the structures at very low energies are caused by bending excitation in the potential wells of states 2 (1)A' and 1 (1)A'.     

A theoretical study of rate coefficients for the O + NO vibrational relaxation

Quasi-classical trajectories have been integrated to study the vibrational relaxation of the O + NO(v) process as a function of the initial vibrational quantum number for T = 298 K, 1500 K, and 3000 K. Two reliable potential energy surfaces have been employed for the A' and A' doublet states of NO2. The calculated vibrational relaxation rate constants show a nearly v-independent behavior at room temperature and a moderate increase with v for higher temperatures. Although deviating significantly from the recommended values, good agreement with recent experimental results has been obtained. The importance of multi-quantum transitions is also analyzed.     

A quasiclassical trajectory study for the N(4S)+O2(X 3Sigmag-)-->NO(X 2Pi)+O(3P) reaction on the new 2A' and 4A' potential-energy surfaces

A quasiclassical trajectory study with the sixth-order explicit symplectic algorithm of the N(4S)+O2(X 3Sigmag-)-->NO(X 2Pi)+O(3P) atmospheric reaction has been performed by employing the new 2A' and 4A' potential-energy surfaces reported by Sayos et al. [J. Chem. Phys. 117, 670 (2002)]. For the translational temperature considered up to 10,000 K, the larger relative translational energy and the higher rovibrational levels of O2 molecule with respect to the previous works have been taken into account, and a clearer database about the character of the total reaction cross section has been presented in this work. The dependence of microscopic rate constants on the vibrational level of O2 molecule at T=3000, 5000, and 10,000 K has been exhibited, and we can see that the values of log10 k(T,v,J) vary almost linearly with the vibrational level of O2 molecule. The thermal rate constants at the translational temperature between 300 and 10,000 K have been evaluated and compared with the experimental and previous theoretical results. It is found that the thermal rate constants determined in this work have a better agreement with the experimental data and can provide a more valid theoretical reference at the translational temperature considered for the title reaction.     

Significant nonadiabatic effects in the S(1D) + HD reaction

A nonadiabatic quantum dynamics calculation involving four coupled potential energy surfaces (two degenerate 3A' ', one 3A', and one 1A') and the spin-orbit coupling matrix for these states is reported for the title reaction. The results show that the important discrepancy between theoretically calculated and experimentally measured intramolecular isotope effects can at least in part be attributed to significant nonadiabatic effects.     

A 193 nm laser photofragmentation time-of-flight mass spectrometric study of chloroiodomethane

The photodissociation dynamics of chloroiodomethane (CH2ICl) at 193 nm has been investigated by employing the photofragment time-of-flight (TOF) mass spectrometric method. Using tunable vacuum ultraviolet undulator synchrotron radiation for photoionization sampling of nascent photofragments, we have identified four primary dissociation product channels: CH2Cl + I(2P(1/2))/I(2P(3/2)), CH2I + Cl(2P(1/2))/Cl(2P(3/2)), CHI + HCl, and CH2 + ICl. The state-selective detection of I(2P(3/2)) and I(2P(1/2)) has allowed the estimation of the branching ratio for I(2P(1/2)):I(2P(3/2)) to be 0.73:0.27. Theoretical calculations based on the time-dependent density-functional theory have been also made to investigate excited electronic potential-energy surfaces, plausible intermediates, and transition structures involved in these photodissociation reactions. The translation energy distributions derived from the TOF measurements suggest that at least two dissociation mechanisms are operative for these photodissociation processes. One involves the direct dissociation from the 2 1A' state initially formed by 193 nm excitation, leading to significant kinetic-energy releases. For the I-atom and Cl-atom elimination channels, the fragment kinetic-energy releases observed via this direct dissociation mechanism are consistent with those predicted by the impulsive dissociation models. Other mechanisms are likely predissociative or statistical in nature from the lower 1 1A' and 1 1A' states and/or the ground X 1A' state populated by internal conversion from the 2 1A' state. On the basis of the maximum kinetic-energy release for the formation of CH2Cl + I(2P(1/2)), we have obtained a value of 53+/-2 kcal/mol for the 0 K bond dissociation energy of I-CH2Cl. The intermediates and transition structures for the CHI + HCl and CH2 + ICl product channels have been also investigated by ab initio quantum calculations at the MP2(full)/6-311G(d) and B3LYP(full)/6-11G(d) levels of theory. The maximum kinetic-energy releases observed for the CHI + HCl and CH2 + ICl channels are consistent with the interpretation that the formation of CHI and CH2 in their ground triplet states is not favored.     

Nonadiabatic quantum dynamics in O(3P)+H2→OH+H: a revisited study

To investigate the extent of nonadiabatic effects in the title reaction, quasi-classical trajectory and nonadiabatic quantum scattering as well as the nonadiabatic quantum-classical trajectory calculations were performed on the accurate ab initio benchmark potential energy surfaces of the lowest (3)A' and (3)A" electronic states [Rogers et al., J Phys Chem A 2000, 104, 2308], together with the spin-orbit coupling matrix [Maiti and Schatz, J Chem Phys 2003, 119, 12360] and the lowest singlet (1) A' potential energy surface [Dobby and Knowles, Faraday Discuss 1998, 110, 247]. Comparison of the calculated total cross sections from both adiabatic and nonadiabatic calculations has demonstrated that for adiabatic channels including (3)A'→(3)A' and (3)A"→(3)A", difference does exist between the two kinds of adiabatic and nonadiabatic calculations, showing nonadiabatic effects to some extent. Such nonadiabatic effects tend to become more conspicuous at high collision energies and are found to be more pronounced with trajectories/quantum wave packet initiated on (3)A' than on (3)A". Furthermore, the present study also showed that nonadiabatic effects can bring the component of forward-scattering in the product angular distributions.     

Quantum chemical and theoretical kinetics study of the O(3P) + C2H2 reaction: a multistate process

The potential energy surfaces of the two lowest-lying triplet electronic surfaces 3A' and 3A' for the O(3P) + C2H2 reaction were theoretically reinvestigated, using various quantum chemical methods including CCSD(T), QCISD, CBS-QCI/APNO, CBS-QB3, G2M(CC,MP2), DFT-B3LYP and CASSCF. An efficient reaction pathway on the electronically excited 3A' surface resulting in H(2S) + HCCO(A2A') was newly identified and is predicted to play an important role at higher temperatures. The primary product distribution for the multistate multiwell reaction was then determined by RRKM statistical rate theory and weak-collision master equation analysis using the exact stochastic simulation method. Allowing for nonstatistical behavior of the internal rotation mode of the initial 3A' adducts, our computed primary-product distributions agree well with the available experimental results, i.e., ca. 80% H(2S) + HCCO(X2A' + A2A') and 20% CH2(X3B1) + CO(X1sigma+) independent of temperature and pressure over the wide 300-2000 K and 0-10 atm ranges. The thermal rate coefficient k(O + C2H2) at 200-2000 K was computed using multistate transition state theory: k(T) = 6.14 x 10(-15)T (1.28) exp(-1244 K/T) cm3 molecule(-1) s(-1); this expression, obtained after reducing the CBS-QCI/APNO ab initio entrance barriers by 0.5 kcal/mol, quasi-perfectly matches the experimental k(T) data over the entire 200-2000 K range, spanning 3 orders of magnitude.     

Interactions in open-shell clusters: ab initio study of pre-reactive complex O(3P) + HCl

Van der Waals interactions between the ground-state triplet O(3P) atom and the closed-shell HCl molecule are investigated in the pre-reactive region. Three adiabatic (two of A' symmetry and one of A' symmetry) and four non-relativistic diabatic potential energy surfaces are obtained by combining a restricted open-shell coupled cluster approach with the multireference configuration interaction method. The lower A' adiabatic potential surface has a single minimum (D(e) = 589 cm(-1)) for a linear O...HCl configuration. The upper A' potential has a weak (D(e) = 65 cm(-1)) minimum for a linear HCl...O configuration. The A' adiabatic potential has a weak (124 cm(-1)) T-shaped minimum. Adiabatic potentials intersect once in the O...HCl linear configuration and twice in the linear HCl...O geometry. The role of electrostatic interactions in shaping these potentials is discussed. The effects of spin-orbit coupling on this interaction are also investigated assuming a constant value of the SO parameter.     

Computational study on the structures of the [H, Si, N, C, O] isomers: possible species of interstellar interest

Ab initio methods have been used to study the lowest lying [H, Si, N, C, O] isomers, which are of astrochemical interest. Over 20 [H, Si, N, C, O] isomers in the 1A' electronic state have been investigated at the MP2/aug-cc-pVTZ level of theory. Of these, the seven lowest isomers have been further investigated using different levels of theory, including B3LYP and QCISD(T). It has been found that the relative energies of the isomers in their ground electronic state (1A') are very dependent on the level of theory used with either the cis-HOSiCN or cis-HOSiNC isomers being the lowest in energy. Overall, the four lowest isomers are within 6 kcal/mol of each other, and a further three isomers are less than 15 kcal/mol higher in energy than the lowest lying isomer, including HSiNCO, which has recently been detected spectroscopically. Natural bond analysis has been carried out on the ground electronic states of the seven lowest lying isomers to examine their electronic structure. The enthalpies of formation of the seven lowest isomers have also been evaluated using the G3MP2 and G3B3 multilevel methods and show that the isomers are relatively thermodynamically stable. The structures and energies of lowest lying 1A' and 3A' electronic states of these isomers have also been investigated and show that for most of the isomers the optimized structures in these excited electronic states correspond to a transition state structure.     

OUH体系的结构和分析势能函数

采用密度泛涵B3LYP方法优化出了OUH分子的各种结构，确定了最稳定构型和离解能，以及它们的谐性力常数，并导出双原子分子UH,UO的Murrell-Sorbie势能函数及其光谱数据。采用多体项展式方法，导出OUH(X^4A')基态分子的分析势能函数，获得OUH(X^4A')体系的势能面，考察了这个势能函数的基本性质，正确地复现出OUH分子的平衡结构特征，结果表明：U+OH,O+UH,H+UO的反应均为无阈能的放热能反应。为进一步探讨OUH体系的反应动力学过程打下了基础。     

Spectroscopic implications of partially quenched orbital angular momentum in the OH-water complex

The OH monomer orbital angular momentum is predicted to be partially quenched in the OH-water complex because of the significant splitting of the OH monomer orbital degeneracy into (2)A' and (2)A' ' electronic states. This orbital angular momentum quenching and the associated decoupling of the electron spin from the a inertial axis are shown to have dramatic effects on the rotational band structure of the microwave and infrared transitions of the OH-water complex. At the ab initio values for the splitting between the (2)A' and (2)A' ' surfaces, simulated spectra of a- and b-type bands, such as those expected for the OH radical stretch and water asymmetric stretch, are predicted to have a noticeably different appearance than the well-established limiting cases associated with fully quenched or completely unquenched orbital angular momentum. Spectral identification of the OH-water complex in the gas phase will require explicit consideration of this quenching phenomenon.     

3A''and 3A"Energy Surfaces for the VS+ + CO2→VO+ + COS Reaction

The title reaction proceeds via a one- and two-step mechanism along the 3A" and 3A' surfaces, respectively. The 3A' excited state surface has a lower barrier than the 3A" ground state surface (11.6 and 16.0 kcal/mol, respectively). We suggest that, at low energies, the reaction proceeds along the A' surface and then makes an intersystem crossing to the A" surface and yields the ground state product VO (3∑), which may explain the small VO cross-section at low energies observed in the experiment