Active Thermochemical Tables: The Partition Function of Hydroxymethyl (CHOH) Revisited.

The best currently available set of temperature-dependent nonrigid rotor anharmonic oscillator (NRRAO) thermochemical and thermophysical properties of hydroxymethyl radical is presented. The underlying partition function relies on a critically evaluated complement of accurate experimental and theoretical data and is constructed using a two-pronged strategy that combines contributions from large amplitude motions obtained from direct counts, with contributions from the other internal modes of motion obtained from analytic NRRAO expressions. The contributions from the two strongly coupled large-amplitude motions of CHOH, OH torsion and CH wag, are based on energy levels obtained by solving the appropriate two-dimensional projection of a fully dimensional potential energy surface that was recently obtained at the CCSD(T)/cc-pVTZ level of theory.

A rigorous full-dimensional quantum dynamics study of tunneling splitting of rovibrational states of vinyl radical CH.

We report a rigorous quantum mechanical study of the rovibrational energy levels of vinyl radical CH. The calculations are carried out using a real two-component multi-layer Lanczos algorithm in a set of orthogonal polyspherical coordinates based on a recently developed accurate ab initio potential energy surface of CH. All well converged 158 vibrational bands up to 3200 cm are determined, together with a comparison to previous calculations and experimental results.

A coherent discrete variable representation method on a sphere.

The coherent discrete variable representation (ZDVR) has been extended for constructing a multidimensional potential-optimized DVR basis on a sphere. In order to deal with the non-constant Jacobian in spherical angles, two direct product primitive basis methods are proposed so that the original ZDVR technique can be properly implemented. The method has been demonstrated by computing the lowest states of a two-dimensional (2D) vibrational model.

An exact variational method to calculate rovibrational spectra of polyatomic molecules with large amplitude motion.

We report a new full-dimensional variational algorithm to calculate rovibrational spectra of polyatomic molecules using an exact quantum mechanical Hamiltonian. The rovibrational Hamiltonian of system is derived in a set of orthogonal polyspherical coordinates in the body-fixed frame. It is expressed in an explicitly Hermitian form.

Full-Dimensional Quantum Calculations of Vibrational Levels of NH4(+) and Isotopomers on An Accurate Ab Initio Potential Energy Surface.

Vibrational energy levels of the ammonium cation (NH4(+)) and its deuterated isotopomers are calculated using a numerically exact kinetic energy operator on a recently developed nine-dimensional permutation invariant semiglobal potential energy surface fitted to a large number of high-level ab initio points. Like CH4, the vibrational levels of NH4(+) and ND4(+) exhibit a polyad structure, characterized by a collective quantum number P = 2(v1 + v3) + v2 + v4. The low-lying vibrational levels of all isotopomers are assigned and the agreement with available experimental data is better than 1 cm(-1).

Rotational and angular distributions of NO products from NO-Rg (Rg = He, Ne, Ar) complex photodissociation.

We present the results of an investigation into the rotational and angular distributions of the NO à state fragment following photodissociation of the NO-He, NO-Ne, and NO-Ar van der Waals complexes excited via the à ← X̃ transition. For each complex, the dissociation is probed for several values of Ea, the available energy above the dissociation threshold. For NO-He, the Ea values probed were 59, 172, and 273 cm(-1); for NO-Ne they were 50 and 166 cm(-1); and for NO-Ar they were 44, 94, 194, and 423 cm(-1).

Vibrational energy levels of the simplest Criegee intermediate (CH2OO) from full-dimensional Lanczos, MCTDH, and MULTIMODE calculations.

Accurate vibrational energy levels of the simplest Criegee intermediate (CH2OO) were determined on a recently developed ab initio based nine-dimensional potential energy surface using three quantum mechanical methods. The first is the iterative Lanczos method using a conventional basis expansion with an exact Hamiltonian. The second and more efficient method is the multi-configurational time-dependent Hartree (MCTDH) method in which the potential energy surface is refit to conform to the sums-of-products requirement of MCTDH.

Accurate quantum dynamics calculations of vibrational spectrum of dideuteromethane CH2D2.

We report a rigorous variational study of the infrared (IR) vibrational spectra of both CH2D2 and (13)CH2D2 isotopomers using an exact molecular Hamiltonian. Calculations are carried out using a recently developed multi-layer Lanczos algorithm based on the accurate refined Wang and Carrington potential energy surface of methane and the low-order truncated ab initio dipole moment surface of Yurchenko et al. [J.

Neural network iterative diagonalization method to solve eigenvalue problems in quantum mechanics.

We propose a multi-layer feed-forward neural network iterative diagonalization method (NNiDM) to compute some eigenvalues and eigenvectors of large sparse complex symmetric or Hermitian matrices. The NNiDM algorithm is developed by using the complex (or real) guided spectral transform Lanczos (cGSTL) method, thick restart technique, and multi-layered basis contraction scheme. Artificial neurons (or nodes) are defined by a set of formally orthogonal Lanczos polynomials, where the biases and weights are dynamically determined through a series of cGSTL iterations and small matrix diagonalizations.

Doppler-Resolved Kinetics of Saturation Recovery.

Frequency-modulated laser transient absorption has been used to monitor the ground-state rotational energy-transfer rates of CN radicals in a double-resonance, depletion recovery experiment. When a pulsed laser is used to burn a hole in the equilibrium ground-state population of one rotational state without velocity selection, the population recovery rate is found to depend strongly on the Doppler detuning of a narrow-band probe laser. Similar effects should be apparent for any relaxation rate process that competes effectively with velocity randomization.

Multi-layer Lanczos iteration approach to calculations of vibrational energies and dipole transition intensities for polyatomic molecules.

We report a rigorous full dimensional quantum dynamics algorithm, the multi-layer Lanczos method, for computing vibrational energies and dipole transition intensities of polyatomic molecules without any dynamics approximation. The multi-layer Lanczos method is developed by using a few advanced techniques including the guided spectral transform Lanczos method, multi-layer Lanczos iteration approach, recursive residue generation method, and dipole-wavefunction contraction. The quantum molecular Hamiltonian at the total angular momentum J = 0 is represented in a set of orthogonal polyspherical coordinates so that the large amplitude motions of vibrations are naturally described.

A complex guided spectral transform Lanczos method for studying quantum resonance states.

A complex guided spectral transform Lanczos (cGSTL) algorithm is proposed to compute both bound and resonance states including energies, widths, and wavefunctions. The algorithm comprises of two layers of complex-symmetric Lanczos iterations. A short inner layer iteration produces a set of complex formally orthogonal Lanczos polynomials.

Origin of anomalous electronic circular dichroism spectrum of RuPt2(tppz)2Cl2(PF6)4 in acetonitrile.

We report a theoretical study of the structures, energetics, and electronic spectra of the Pt(II)/Ru(II) mixed-metal complex RuPt2(tppz)2Cl2(PF6)4 (tppz = 2,3,5,6-tetra(2-pyridyl)pyrazine) in acetonitrile. The hybrid B3LYP density functional theory and its TDDFT methods were used with a complete basis set (CBS) extrapolation scheme and a conductor polarizable continuum model (C-PCM) for solvation effects. Results showed that the trinuclear complex has four types of stable conformers and/or enantiomers.

Electronic structure and quantum dynamics of photoinitiated dissociation of O2 on rutile TiO2 nanocluster.

The adsorption and photoinitiated dissociation of molecular oxygen on reduced rutile TiO2 nanocluster have been studied using a hybrid density functional theory (DFT)/time-dependent DFT approach and a time-dependent wavepacket dynamics method. Results show that the most favorable state for O2 at the bridging row O-vacancy site of TiO2 is O2(2-) with an orientation parallel to the surface. We find that its dissociation in the electronic ground state involves a spin forbidden intersystem crossing, and therefore has a large barrier along the reaction pathway.

Exploring the ring-opening pathways in the reaction of morpholinyl radicals with oxygen molecule.

Quantum chemistry calculations using hybrid density functional theory and the coupled-cluster method have been performed to investigate the ring-opening pathways in the oxidation of morpholine (1-oxa-4-aza-cyclohexane). Hydrogen abstraction can form two different carbon-centered radicals, morpholin-2-yl or morpholin-3-yl, or the nitrogen-centered radical, morpholin-4-yl, none of which are found to have low-energy pathways to ring-opening. Extensive exploration of multiple reaction pathways following molecular oxygen addition to these three radicals revealed two competitive low energy pathways to ring-opening.

The kinetics study of the S + S2 → S3 reaction by the chaperone mechanism.

The recombination of S atoms has been found to be stepwise from the smallest unit, the elemental S atom, to the most abundant molecule S(8). The reaction between S + S(2) → S(3) has not been reported either experimentally or by theory, but may be a key intermediate step in the formation of sulfur aerosols in low-O(2) atmospheres. In this work, the kinetics of this reaction is reported with Ar gas used as the chaperone molecule in the production of S(3) via two complex intermediates: SAr + S(2) and S(2)Ar + S.

HOCO radical chemistry.

Free radicals are important species in atmospheric chemistry, combustion, plasma environments, interstellar clouds, and biochemistry. Therefore, researchers would like to understand the formation mechanism, structure, stability, reactivity, spectroscopy, and dynamics of these chemical species. However, due to the presence of one or more unpaired electrons, radicals are often very reactive and have short lifetimes, which makes it difficult to conduct experiments.

Ab initio and RRKM study of the reaction of ClO with HOCO radicals.

The reaction pathways for the ClO + HOCO reaction have been explored using the coupled-cluster method to locate and optimize the critical points on the ground-state potential-energy surface. Results show that the ClO + HOCO reaction can produce Cl + HOC(O)O, HOCl + CO(2), HCl + CO(3), and HClO + CO(2) via an addition or a direct hydrogen abstraction reaction mechanism. The reaction kinetics has been studied using the variational RRKM theory.

Spherical electron cloud hopping molecular dynamics simulation on dissociative recombination of protonated water.

Dissociative recombination (DR) of H(3)O(+) with electrons at zero collision energy has been studied by a direct ab initio molecular dynamics method on four low-lying electronic states of the system. Initial conditions for trajectories are determined by a spherical electron cloud hopping (SECH) model, while nonadiabatic effects are considered through a surface hopping scheme. The energies, forces, and nonadiabatic coupling strengths (NACS) used in trajectory propagations are calculated on-the-fly via state-average complete active self-consistent field (CASSCF) theory with full valence electrons.

Theoretical study of the reaction of CH3 with HOCO radicals.

The reaction of HOCO radicals with CH3 radicals is examined using the coupled cluster method to locate and optimize the critical points on the ground-state potential energy surface. The results show that the CH3 + HOCO reaction can produce both the H2O + CH2CO and the CH4 + CO2 products through acetic acid and enediol intermediates. Direct ab initio dynamics calculations determine the thermal rate coefficients to be k(T/K) = 3.

Energetics and molecular dynamics of the reaction of HOCO with HO(2) radicals.

The energetics of the reaction of HOCO with HO(2) have been studied using the quadratic configuration interaction with single and double excitations (QCISD(T)) method and a large basis set on the singlet and triplet potential energy surfaces of the system. The results show that the ground-state O(2)+HOC(O)H products can be produced by a direct hydrogen abstraction via a transition state with a small barrier (1.66 kcal/mol) on the lowest triplet surface.

Ab initio and direct dynamics study of the reaction of Cl atoms with HOCO.

The reaction of Cl with HOCO has been examined using the coupled-cluster method to locate and optimize the critical points on the ground-state potential energy surface. The results show that the reaction produces the HCl and CO(2) products as experimentally observed. The reaction occurs via a HOC(O)Cl intermediate with an estimated heat of formation of -97.

Potential energy surface and quantum dynamics study of rovibrational states for HO(3) (X (2)A'').

An analytic potential energy surface has been constructed by fitting to about 28 thousand energy points for the electronic ground-state (X (2)A'') of HO(3). The energy points are calculated using a hybrid density functional HCTH and a large basis set aug-cc-pVTZ, i.e.

Energetics and kinetics of the reaction of HOCO with hydrogen atoms.

The potential energy surface for the reaction of HOCO radicals with hydrogen atoms has been explored using the CCSD(T)/aug-cc-pVQZ ab initio method. Results show that the reaction occurs via a formic acid (HOC(O)H) intermediate, and produces two types of products: H(2)O+CO and H(2)+CO(2). Reaction enthalpies (0 K) are obtained as -102.

A spherical electron cloud hopping model for studying product branching ratios of dissociative recombination.

A spherical electron cloud hopping (SECH) model is proposed to study the product branching ratios of dissociative recombination (DR) of polyatomic systems. In this model, the fast electron-captured process is treated as an instantaneous hopping of a cloud of uniform spherical fractional point charges onto a target M+q ion (or molecule). The sum of point charges (-1) simulates the incident electron.