Alexey L. Kaledin

Accurate ab initio and “hybrid” potential energy surfaces, intramolecular vibrational energies, and classical ir spectrum of the water dimer

We report three modifications to recent ab initio, full-dimensional potential energy surfaces (PESs) for the water dimer [X. Huang et al., J. Chem. Phys. 128, 034312 (2008)]. The first modification is a refit of ab initio electronic energies to produce an accurate dissociation energy D(e). The second modification adds replacing the water monomer component of the PES with a spectroscopically accurate one and the third modification produces a hybrid potential that goes smoothly in the asymptotic region to the flexible, Thole-type model potential, version 3 dimer potential (denoted TTM3-F) [G. S. Fanourgakis and S. S. Xantheas, J. Chem. Phys. 128, 074506 (2008)]. The rigorous D(0) for these PESs, obtained using diffusion Monte Carlo calculations of the dimer zero-point energy, and an accurate zero-point energy of the monomer, range from 12.5 to 13.2 kJ/mol (2.99-3.15 kcal/mol), with the latter being the suggested benchmark value. For TTM3-F D(0) equals 16.1 kJ/mol. Vibrational calculations of monomer fundamental energies using the code MULTIMODE are reported for these PESs and the TTM3-F PES and compared to experiment. A classical molecular dynamics simulation of the infrared spectra of the water dimer and deuterated water dimer at 300 K are also reported using the ab initio dipole moment surface reported previously [X. Huang, B. J. Braams, and J. M. Bowman, J. Phys. Chem. A 110, 445 (2006)].

A practical method to avoid zero-point leak in molecular dynamics calculations: Application to the water dimer

We report the implementation of a previously suggested method to constrain a molecular system to have mode-specific vibrational energy greater than or equal to the zero-point energy in quasiclassical trajectory calculations [J. M. Bowman et al., J. Chem. Phys. 91, 2859 (1989); W. H. Miller et al., J. Chem. Phys. 91, 2863 (1989)]. The implementation is made practical by using a technique described recently [G. Czako and J. M. Bowman, J. Chem. Phys. 131, 244302 (2009)], where a normal-mode analysis is performed during the course of a trajectory and which gives only real-valued frequencies. The method is applied to the water dimer, where its effectiveness is shown by computing mode energies as a function of integration time. Radial distribution functions are also calculated using constrained quasiclassical and standard classical molecular dynamics at low temperature and at 300 K and compared to rigorous quantum path integral calculations.

Time averaging the semiclassical initial value representation for the calculation of vibrational energy levels. II. Application to H2CO, NH3,CH4, CH2D2

A method recently developed for calculating vibrational spectral densities of molecules, previously tested successfully on H2O, is applied here to several larger molecules. The method relies on use of a time averaging procedure in the conventional semiclassical (SC) initial value representation (IVR) expression for the spectral density. The convergence of the SC-IVR average over the phase space of initial conditions (of classical trajectories) is greatly enhanced by time averaging and is generally achieved with as few as 1000 trajectories per degree of freedom. Furthermore, meaningful results can be obtained with only a single trajectory propagated for a long time. (For systems with chaotic dynamics, however, the phase space average converges more slowly.) Results for vibrational energy levels of H2CO, NH3, CH4, and CH2D2 are reported and compared with quantum mechanical calculations available in the literature. The accuracy of the time-averaged SC-IVR is very encouraging, with the vibrational energy leve...

Calculation of the Vibrational Spectra of H5O2 + and Its Deuterium-Substituted Isotopologues by Molecular Dynamics Simulations †

In this work, we present infrared spectra of H(5)O(2)(+) and its D(5)O(2)(+), D(4)HO(2)(+), and DH(4)O(2)(+) isotopologues calculated by classical molecular dynamics simulations on an accurate potential energy surface generated from CCSD(T) calculations, as well as on the BLYP DFT potential energy surface sampled by means of the Car-Parrinello algorithm. The calculated spectra obtained with internal energies corresponding to a temperature of about 30 K are in overall good agreement with those from experimental measurements and from quantum dynamical simulations.

Long lived charge separation in iridium(III)-photosensitized polyoxometalates: synthesis, photophysical and computational studies of organometallic–redox tunable oxide assemblies

Keggin and Dawson-type polyoxometalates (POMs) covalently grafted to heteroleptic cyclometalated iridium(III) complexes (POM–[Ir] dyads) have been prepared by postfunctionalization of organosilyl and organotin POM derivatives. Electronic properties of these 4 photosensitized POM–[Ir] dyads were evaluated by electrochemical measurements and theoretical calculations. These studies reveal that the electron acceptor character of the POMs vary with structural class (Keggin vs. Dawson) and chemical anchorage (organosilyl vs. organotin); they reveal the poor electronic interaction between the POMs and the chromophores. Combined transient absorption and spectroelectrochemical measurements provide evidence for the formation of photoinduced electron transfer from the chromophore to the POM. The lifetimes of the charge-separated states (ranging from ns to hundreds of ns) are the longest values reported for covalently bonded photosensitized POMs. The functionalization of the heteroleptic cyclometalated iridium(III) on the picolinate ligand provides directionality to the photoinduced electron transfer by enhancing charge separation and delaying charge recombination The kinetics of the photoinduced electron transfers are rationalized by Marcus theory. We conclude that the charge separation and charge recombination respectively occur in the Marcus normal and inverted regions.

Density functional calculations of beryllium clusters Ben, n=2–8

Neutral beryllium clusters Ben, na 2‐8, were investigated by density functional techniques. To minimize errors, geometry optimization, frequency and energy calculations were all carried out on the same level of theory, employing a large 6-311++G(3df) basis set. The method reproduces well the experimentally known bond length and vibrational frequency of the dimer, but its binding energy is still significantly overestimated. The computed trends of the vibrational frequencies, bond lengths and binding energies of the clusters as a function of the number of atoms are discussed. The binding energies are found to increase rapidly as a function of size, and approach the binding energy of the bulk metal, 54.1 kJ per bond. ” 2000 Elsevier Science B.V. All rights reserved.

Synthesis, Structures, and Photochemistry of Tricarbonyl Metal Polyoxoanion Complexes, [X2W20O70{M(CO)3}2]12– (X = Sb, Bi and M = Re, Mn)

A new series of complexes containing two electron donating groups, {M(CO)(3)}(+) ions, M = Re or Mn, on one polytungstate electron acceptor group have been prepared and characterized. These complexes containing two electron donating groups, {M(CO)(3)}(+) ions, M = Re or Mn, on one polytungstate electron acceptor group have been prepared and characterized. These two-component polyoxometalate (POM) compounds have been made by reaction of solvated {M(CO)(3)}(+) ions (M = Re or Mn) with [X(2)W(22)O(74)(OH)(2)](12-) (X = Sb or Bi) POM multidentate ligands in aqueous solution. These syntheses reveal that the fac-{WO(OH)(2)}(2+) groups in the terminal positions of these two POM ligands are easily replaced by the topologically equivalent units fac-{M(CO)(3)}(+). Four compounds, [X(2)W(20)O(70){M(CO)(3)}(2)](12-) (1a: X = Sb, M = Re; 1b: X = Bi, M = Re; 2a: X = Sb, M = Mn; 2b: X = Bi, M = Mn) have been isolated and characterized of X-ray crystallography, spectroscopic, and computational methods. The charge transfer dynamics, investigated by femtosecond transient absorption (TA) spectroscopy of 1a and 1b combined with the density functional theory (DFT) calculations indicate that both complexes exhibit metal-to-polyoxometalate charge-transfer (MPCT) from the Re centers to the POM ligands, while MPCT from the Mn centers to the POM ligands in 2a and 2b leads to decomposition of starting compounds. The studies suggest a general synthetic route to a potentially very large class of POM-based hybrid compounds.

ELECTRONIC STRUCTURE OF BE2 : THEORETICAL AND EXPERIMENTAL RESULTS

Abstract Potential energy curves and electronic transition moments for Be 2 were calculated using MRSDCI and EOM coupled cluster levels of theory with a (12s6p3d2f1g)/[5s4p3d2f1g] basis set. The properties of low-lying singlet, triplet, and quintet states were predicted. MRSDCI results were found to be in agreement with experimental observations. Predictions for the B 1 Σ + u − A ′ 1 Π g transition were used to guide an experimental search for the A ′ state, which had not been observed previously. Low-lying electronic states of Be 2 were examined using laser excitation techniques. The dimer was formed by pulsed laser ablating Be vapor into a free-jet expansion. Dispersed fluorescence spectra were recorded following excitation of various vibrational levels of the B 1 Σ + u state, and bands of the B 1 Σ + u − A ′ 1 Π g transition were observed. The term energy ( T e =13 711±30 cm −1 ) and vibrational interval (Δ G 1/2 =726±25 cm −1 ) for the A ′ state were determined.

An ab initio direct-trajectory study of the photodissociation of ClOOCl

The photodissociation of chlorine peroxide, ClOOCl, is studied with classical trajectories where the energy and gradient are computed on the fly by means of the state-averaged (sa) complete active space self-consistent field (CASSCF) with the DZP(+) basis set. We show that six electronically excited states are involved in the process of decomposition, which proceeds via several competing pathways and at least three electronically unique fragment channels. The problem is treated in four-dimensional (4D) (C2 constraint) and five-dimensional (5D) (planar constraint) frameworks in order to model the mechanisms of synchronous and asynchronous or stepwise dissociation, respectively. A single trajectory with the initial conditions of a nonvibrating, nonrotating molecule is propagated on each excited state surface for an average time of 10 fs for the purposes of determining the early stages of bond breaking. We show that even in such a short propagation time the pathway competition can be more or less unambiguous...

Ab initio theoretical studies on photodissociation of HNCO upon S1(1A″)←S0(1A′) excitation: The role of internal conversion and intersystem crossing

Photodissociation of isocyanic acid, HNCO, was studied with high-level ab initio methods. Geometry optimizations of stationary points and surface crossing seams were performed with the complete active space self-consistent-field (CASSCF) method, and the energetics were re-evaluated with single-point second-order multireference perturbation theory (CASPT2). The three product channels that participate in the photodissociation process are [1] HN(X 3Σ−)+CO at 86.0 (calculated 79.6) kcal/mol, [2] H+NCO(X 2Π) at 109.7 (108.7) kcal/mol, and [3] HN(a 1Δ)+CO at 122.2 (120.8) kcal/mol. The four electronic states, S0, S1, T1, and T2, that interconnect these channels were studied in detail. S1 exhibits dissociation barriers to both, channel [2] and [3], whose respective reverse heights are 11.3 and 1.2 kcal/mol, in good agreement with experiment as well as previous theoretical works. The two triplets, T1 and T2, show barriers of similar heights for HN bond fission, while S0 has no barriers to either channel. Various ...