Search results for "ab initio"
showing 10 items of 990 documents
Solvation of triplet Rydberg states of molecular hydrogen in superfluid helium
2004
We report ab initio interaction potentials, transition dipole moments, and radiative lifetimes for the four lowest triplet states of ${\mathrm{H}}_{2}:$ $b$ ${}^{3}{\ensuremath{\Sigma}}_{u}^{+},$ $c$ ${}^{3}{\ensuremath{\Pi}}_{u},$ $a$ ${}^{3}{\ensuremath{\Sigma}}_{g}^{+},$ and $e$ ${}^{3}{\ensuremath{\Sigma}}_{u}^{+},$ and their response to the perturbation due to approaching ground state He atom. Hybrid density functional\char21{}quantum Monte Carlo calculations employing the ab initio interaction potentials are then used for calculating the liquid structure around the molecular excimers in bulk superfluid ${}^{4}\mathrm{He}.$ Calculations demonstrate a wide variety of possible solvation …
Structure and properties of hydroxyl radical modified nucleic acid components II. 8-Oxo-adenine and 8-oxo-2′-deoxy-adenosine
1997
Abstract The tautomerism of the 8-oxo-adenine (8-oxo-A) and 8-oxo-2′-deoxy-adenosine (8-oxo-dA) was analysed on the basis of semiempirical, SCF ab initio and DFT density functional quantum chemistry calculations. The results of full gradient geometry optimisation of all possible 8-oxo-A and 8-oxo-dA structures lead to the conclusion that the most stable form is 8-keto-6-amino-tautomer. The second stable tautomer corresponds to 8-hydroxy-isomer. Such an order was unchanged after solvating process. In all studied solvents: water, methanol, acetone, cyclohexane the keto-tautomer proceeds the enol one. The preferred N-glycoside torsion angle corresponds to syn rotamer of 8-oxo-dA in all studied…
Lattice dynamics of CuAlO2 under high pressure fromab initio calculations
2007
The density functional perturbation theory is employed to study the vibrational properties of CuAlO 2 under pressure. The calculations are preformed using the pseudopotential wave method and the local density approximation for the exchange-correlation (XC) potential. The d electrons of Cu are treated as valence states. We present the phonon dispersion curves. Our results are in good agreement with the available experimental Raman scattering experiments. Ab initio calculations show the presence of a dynamical instability, possibly related with the experimentally observed phase transition.
Chemoselective reactions of N1-methyl-2-hydroxy-3-methylamino-3-phenylpropanamide with electrophiles. Synthesis of chiral hexahydro-4-pyrimidinones a…
2002
Abstract The reactivity of (2S,3S)-N1-methyl-2-hydroxy-3-methylamino-3-phenylpropanamide 1 , containing three nucleophilic centres has been studied against dihaloalkanes and aldehydes. Hexahydro-4-pyrimidinones or oxazolidines were obtained chemoselectively. Experimental results were explained by ‘ab initio’ calculations.
Thermochemical properties of small open-shell systems: experimental and high-levelab initioresults for NH2and
2006
The first adiabatic ionization energy and the first singlet–triplet splitting of the amidogen radical (NH2) have been determined by high-level ab initio quantum chemistry based on the coupled-cluster approach (90 041 and 10 319 cm−1, respectively) and by high-resolution pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy (90 083.8 ± 1.0 and 10 222.0 ± 1.3 cm−1, respectively). A comparison between the theoretical and experimental values demonstrates the predictive powers of high-level ab initio theory in the derivation of the thermochemical properties of small molecular systems. The absolute accuracy of better than 100 cm−1 alleviates the experimental search for…
An ab initio study of the low-lying 1 A′ electronic states of indene
1999
Abstract The electronic absorption spectrum of indene in the region of the lowest-lying valence π → π * singlet excited states was studied using multi-configurational second-order perturbation theory through the CASPT2 formalism. Four 1 A′ states were found to be the most important for describing the excitation spectrum up to 6.8 eV. Their transition energies (oscillator strengths in parentheses) were computed to be: 4.46 eV (0.0004), 5.02 eV (0.175), 6.14 eV (0.221), and 6.36 eV (0.434). These states can be labelled and described as the four Platt states of indene: 1 L a , 1 L b , 1 B b , and 1 B a , respectively. This description of the spectrum explains the experimental observations bett…
Low temperature rate coefficients of the H + CH+ → C+ + H2 reaction: New potential energy surface and time-independent quantum scattering
2015
The observed abundances of the methylidyne cation, CH(+), in diffuse molecular clouds can be two orders of magnitude higher than the prediction of the standard gas-phase models which, in turn, predict rather well the abundances of neutral CH. It is therefore necessary to investigate all the possible formation and destruction processes of CH(+) in the interstellar medium with the most abundant species H, H2, and e(-). In this work, we address the destruction process of CH(+) by hydrogen abstraction. We report a new calculation of the low temperature rate coefficients for the abstraction reaction, using accurate time-independent quantum scattering and a new high-level ab initio global potenti…
Electronic structure of the ground and excited states of beta-carboline.
2008
Coupled-cluster calculations are used to compute the energy of conversion between the neutral and the zwitterionic forms of beta-carboline. The stability of the different species is discussed in terms of charge separation and aromatic character, which is related to magnetic criteria. By means of a linear response formalism the vertical excitation energies and oscillator strengths of the lowest singlet states of both structures as well as of the cationic species are determined. General agreement of the relative position and intensity of the different peaks with experimental data is achieved, but the overall spectra are slightly displaced because of solvent effects.
A joint experimental and ab initio study on the reactivity of several hydroxy selenides. Stereoselective synthesis of cis-disubstituted tetrahydrofur…
2001
Abstract The reactivity of several hydroxy selenides bearing an ethereal chain with catalytic amounts of perchloric acid in dichloromethane was investigated. Results showed that the position of the oxygen atom with respect to the seleniranium ring was crucial in order to get a good yield of the cyclized product. The factors on which yields of the 5- endo cyclization of the seleniranium ions depend were analysed by ab initio (HF/3-21G ∗ ) studies. An explanation of the different coordinating ability, towards the positively charged selenium atom, of the allylic OMe and homoallylic OH-2 groups was given.
3He NMR: from free gas to its encapsulation in fullerene
2013
The 3He nuclear magnetic shieldings were calculated for single helium atom, its dimer, simple models of fullerene cages (He@Cn), and single wall carbon nanotubes. The performances of several levels of theory (HF, MP2, DFT-VSXC, CCSD, CCSD(T), and CCSDT) were tested. Two sets of polarization-consistent basis sets were used (pcS-n and aug-pcS-n), and an estimate of 3He nuclear magnetic shieldings in the complete basis set limit using a two-parameter fit was established. Theoretical 3He results reproduced accurately previously reported theoretical values for helium gas, dimer, and helium probe inside several fullerene cages. Excellent agreement with experimental values was achieved. 3He nuclea…