Ab initio study of the KrH+ photodissociation

The multireference spin-orbit configuration interaction method is employed to calculate potential energy curves for the ground and low-lying excited states of the KrH(+) cation. For the first time, the spin-orbit interaction is taken into account and electric dipole moments are computed for transitions to the states responsible for the first absorption continuum (A band) of KrH(+). On this basis, the partial and total absorption spectra in this energy range are obtained. It is shown that the A-band absorption is dominated by the parallel A (1)Sigma(+)<--X (1)Sigma(+) transition. In the low-energy part of the band (<83x10(3) cm(-1)) the absorption is mainly caused by the spin-forbidden b (3)Pi(0(+) )<--X (1)Sigma(+) excitation, while perpendicular transitions to the B (1)Pi and b (3)Pi(1) states are significantly weaker. The branching ratio Gamma for the photodissociation products is calculated and it is shown to increase smoothly from 0 in the red tail of the band to 1 at E>or=90x10(3) cm(-1). The latter value corresponds to the exclusive formation of the spin-excited Kr(+)((2)P(12)) ions, which may be used to obtain laser generation on the Kr(+)((2)P(12)-(2)P(32)) transition.     

乙烯酮光解途径的从头算研究

本文分别考察了C~2~α方式以及C~3-面内弯曲和C~3 一面外弯曲方式下乙烯酮光解脱氢反应(CH~2CO→H~2+C~2O)的状态相关图. 对由此给出的对称性允许的光解脱氢途径进行了从头算研究.发现最具可能的途径是在C~3-面内弯曲方式中^1A″态势能面上进行的绝热光反应,其能垒高度为263.83KJ.mol^-^1.     

Ab initio and RRKM study of photodissociation of azulene cation

The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of the azulene cation at different values of the photon energy. Reaction pathways leading to various decomposition products have been mapped out at the G3(MP2,CC)//B3LYP level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for the dissociation products have been calculated using the steady-state approach. The results show that a photoexcited azulene cation can readily isomerize to a naphthalene cation. The major dissociation channels are elimination of atomic hydrogen, an H2 molecule, and acetylene. The branching ratio of the H elimination channel decreases with an increase of the photon energy. The branching ratio of the acetylene elimination as well as that of the H2 elimination rise as the photon energy increases. The main C8H6+ fragment at all photon energies considered is a pentalene cation, and its yield decreases slightly with increasing excitation energy, whereas the branching ratios of the other C8H6+ fragments, phenylacetylene and benzocyclobutadiene cations, grow.     

Ab initio configuration interaction study of the low-lying 1Sigma+ electronic states of LiCl

Ab initio configuration interaction calculations have been performed for the X 1Sigma+ and B 1Sigma+ electronic states of LiCl. Potential energy curves, dipole moment functions, and dipole transition moments have been computed for internuclear distances between R = 2.5a0 and 50a0. Single- and double-excitation configuration interaction wave functions were constructed using molecular orbitals obtained from a two-state averaged multiconfiguration self-consistent-field calculation. This procedure yielded an accurate energy splitting between the covalent and ionic separated-atom limits. The calculated avoided crossing of the X and B state curves occurs at R = 16.2a0, in close agreement with previous calculations using a semiempirical covalent-ionic resonance model. X 1Sigma+ state spectroscopic constants are in excellent agreement with experimental values.     

Ab initio quantum dynamical study of the multi-state nonadiabatic photodissociation of pyrrole

There has been a substantial amount of theoretical investigations on the photodynamics of pyrrole, often relying on surface hopping techniques or, if fully quantal, confining the study to the lowest two or three singlet states. In this study we extend ab initio based quantum dynamical investigations to cover simultaneously the lowest five singlet states, two π-σ? and two π-π? excited states. The underlying potential energy surfaces are obtained from large-scale MRCI ab initio computations. These are used to extract linear and quadratic vibronic coupling constants employing the corresponding coupling models. For the N-H stretching mode Q(24) an anharmonic treatment is necessary and also adopted. The results reveal a sub-picosecond internal conversion from the S(4) (π-π?) state, corresponding to the strongly dipole-allowed transition, to the S(1) and S(2) (π-σ?) states and, hence, to the ground state of pyrrole. The significance of the various vibrational modes and coupling terms is assessed. Results are also presented for the dissociation probabilities on the three lowest electronic states.     

Ab initio studies for the photodissociation mechanism of hydroxyacetone

The reaction pathways for CH(3)COCH(2)OH (hydroxyacetone) photodissociation on the low-lying electronic states have been studied with use of the CASSCF energy gradient techniques. The S(0)/S(1) and S(1)/T(1) intersection points were determined by the state-average CASSCF method. Two main reaction pathways, which are possible to the photodissociation, have been studied. It has been found that the mechanism is stepwise, and belongs to Norrish type-I reaction. The n --> pi* excitation leads to the first excited singlet state, followed by the intersystem crossing from S(1) to T(1). On the T(1) potential energy surface, the system can dissociate adiabatically to CH(3)(x) +COCH(2)OH( x) and CH(3)CO(x)+CH(2)OH(x). The COCH(2)OH(x) and CH(3)CO(x) radicals can further dissociate into CO, OH, and other fragments. Our calculated results are in good agreement with recent experimental results.     

Competition between photodissociation and photoionization in methyl iodide

CH₃I has a number of 5pπ → 6p Rydberg states which can be excited by two-photon absorption from the ground state. These two-photon absorptions have been previously detected by molecular ionization with a third photon. They can also be studied by observation of the VUV fluorescence from neutral photofragments (excited iodine atoms) following absorption of a third photon. Similarly it is found that absorption of one photon at 118 nm (10.5 eV) is followed by dissociation into excited iodine atoms as well as ionization. Iodine atom VUV emission is 2–10 times stronger from CD₃I than from CH₃I.     

Ab initio spin-orbit configuration interaction calculations for high-lying states of the HeNe quasimolecule

Multireference configuration interaction (MRD-CI) calculations are reported for a large series of electronic states of the HeNe quasimolecule up to 170000 cm(-1) excitation energy, including those that dissociate to the 3S1 and 2 1S0 excited states of the He atom. Spin-orbit coupling is included through the use of relativistic effective core potentials (RECPs). Good agreement is obtained with experimental spectroscopic data for the respective atomic levels, although there is a tendency to systematically underestimate the energies of the Ne atom by 1000-1500 cm(-1) because of differences in the correlation effects associated with its ground and Rydberg excited states. Potential curves are calculated for each of these states, and a number of relatively deep minima are found. The CI Omega-state wave functions are sufficiently diabatic until r = 4-5 a0 to allow for a clear identification of the He 1s-2s excited states. Electric dipole transition moments are computed between these states and the HeNe X 0+ ground state up to r = 4.0 a0, and it is found that the 2 (1)S0 - X maximum value is over an order of magnitude larger than that for the corresponding (3)S1 - X excitation process.     

硝酸乙酯分子间相互作用的ab initio研究

在abinitio-HF/6-31G水平上求得硝酸乙酯二聚体势能面上的四种优化构型和电子结构。经MP2电子相关校正和基组叠加误差(BSSE)以及零点能(ZPE)校正，求得二聚体的最大结合能为11.46kJ.mol^-^1，还进行HF/6-311G和HF/6-311++G水平的总能量比较计算，发现6-31G基组对计算结合能比较适合，二子体系间的电荷转移很少，对优化构型进行振动分析，并基于统计热力学求得从单体形成二聚体的热力学性质变化。     

甲醛与甲酸相互作用的从头算研究

在MP2／6—31G(d)和MP2(FC)／6—311 G(d，p)水平上，对H2CO和HCOOH以及设计的4种构型H2CO…HCOOH复合物等进行几何全优化计算，经振动频率分析，确认它们为势能面上的稳定驻点．然后在MP2／6—311 G(2df，2p)水平上进行单点能计算和基组重叠误差(BSSE)校正以获得相互作用能，并利用自然键轨道理论探讨H2CO和H(X)OH相互作用的本质。     

Ab initio SCF study of the interaction between L-serine phosphate and ammonia

The possible conformational changes of L -serine phosphate due to the interaction with ammonia are investigated by means of ab initio MO-LCAO-SCF calculations, using a supermolecule approach and an STO -3G basis set. The most favorable conformation of a four-hydrated L -serine phosphate anion is found to be changed by the binding of an ammonium ion. Cointeraction of ammonia and NH suggests another conformational change through the displacement of the bridging water molecule of the polyhydrated L -serine phosphate anion.     

Assignment of the UV photoelectron spectra of the azoles by Ab initio multi-reference configuration interaction calculations

Ab initio multi-reference CI calculations have been performed on pyrrole, pyrazole, imidazole, each of the triazoles and tetrazole. The tautomerism of these species is discussed, and the UV photoelectron spectra are reinterpreted in the light of the CI data. Many shake-up states are evident above ≈ 14 eV; these can cause difficulties in the positioning of LP_N states by CI methods.     

Ab initio studies of ultrafast x-ray scattering of the photodissociation of iodine

We computationally examine various aspects of the reaction dynamics of the photodissociation and recombination of molecular iodine. We use our recently proposed formalism to calculate time-dependent x-ray scattering signal changes from first principles. Different aspects of the dynamics of this prototypical reaction are studied, such as coherent and noncoherent processes, features of structural relaxation that are periodic in time versus nonperiodic dissociative processes, as well as small electron density changes caused by electronic excitation, all with respect to x-ray scattering. We can demonstrate that wide-angle x-ray scattering offers a possibility to study the changes in electron densities in nonperiodic systems, which render it a suitable technique for the investigation of chemical reactions from a structural dynamics point of view.     

The primary step in the ultrafast photodissociation of the methyl iodide dimer

This paper shows the results of combined experimental and theoretical work that have unravelled the mechanism of ultrafast ejection of a methyl group from a cluster, the methyl iodide dimer (CH(3)I)(2). Ab initio calculations have produced optimized geometries for the dimer and energy values and oscillator strengths for the excited states of the A band of (CH(3)I)(2). These calculations have allowed us to describe the blue shift that had been observed in the past in this band. This blue shift has been experimentally determined with higher precision than in all previously reported experiments, since it has been measured through its effect upon the kinetic energy release of the fragments using femtosecond velocity map imaging. Observations of the reaction branching ratio and of the angular nature of the fragment distribution indicate that two main changes occur in A-band absorption in the dimer with respect to the monomer: a substantial change in the relative absorption to different states of the band, and, more importantly, a more efficient non-adiabatic crossing between two of those states. Additionally, time resolved experiments have been performed on the system, obtaining snapshots of the dissociation process. The apparent retardation of more than 100 fs in the dissociation process of the dimer relative to the monomer has been assigned to a delay in the opening of the optical detection window associated with the resonant multiphoton ionization detection of the methyl fragment.     

Ab initio calculation of the dipole moment function of hydrogen iodide

SCEP/CEPA and MC SCF potential energy and dipole moment functions for hydrogen iodide have been calculated. Spectroscopic constants and vibrational dipole matrix elements obtained from the CEPA functions are in good agreement with experimental data. In contrast to previous results for hydrogen fluoride, the MC SCF dipole moment function is less accurate than the CEPA function.     

Ab initio study of water clustering in the presence of a methyl radical

The geometrical structure and binding energy of small clusters of methyl radical and water molecules (up to five water molecules) in gas phase and water media have been investigated at the MP2 level of theory using 6-311++G(2df,2p) basis set. The complexes characterized contain OH···O, CH···O, and OH···C attractive interactions with stabilization energies in the range 6–143 kJ mol^?1. The solvent has an enhancing influence on the stabilities of studied clusters. The atoms in molecules theory were also applied to explain the nature of the complexes. The interaction energies have been partitioned with the natural energy decomposition analysis showing that the most important attractive term corresponds to the charge transfer one.     

Ab initio study on the ground and low-lying excited states of cesium iodide (CsI)

Potential energy curves (PECs) for the ground and low-lying excited states of the cesium iodide (CsI) molecule have been calculated using the internally contracted multireference configuration interaction calculation with single and double excitation method with the relativistic pseudopotentials. PECs for seven Lambda-S states, X 1Sigma+, 2 1Sigma+, 3Sigma+, 1Pi, and 3Pi are first calculated and then those for 13 Omega states are obtained by diagonalizing the matrix of the electronic Hamiltonian H(el) plus the effective one-electron spin-orbit (SO) Hamiltonian H(SO). Spectroscopic constants for the calculated ground X 0+-state PEC with the Davidson correction are found to agree well with the experiment. Transition dipole moments (TDMs) between X 0 and the other Omega states are also obtained and the TDM between X 0+ and A 0+ is predicted to be the largest and that between X 0+ and B 0+ is the second largest around the equilibrium internuclear distance. The TDMs between X 0+ and the Omega=1 states are estimated to be nonzero, but they are notably small as compared with those between the 0+ states. Finally, vibrational levels of the X 0+ PEC for the two isotopic analogs, (133)CsI and (135)CsI, are numerically obtained to investigate the isotope effect on the vibrational-level shift. It has been found that the maximized available isotope shift is approximately 30 cm(-1) around nu=136.