ISSN:0026-8976    

DOI:10.1080/002689797170068

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The performance is examined of the B3P86 hybrid exchange-correlation function for a set of 21 sulphur-containing molecules. Optimized geometries, harmonic frequencies and mean molecular polarizabilities are presented, and compared with calculations to the HartreeFock and MP2 levels of theory, and with appropriate experimental results. The hybrid functional predicts geometries at a level comparable with the MP2 results; harmonic frequencies are in closer agreement with experimental results than either MP2 or Hartree-Fock method; mean molecular polarizabilities predicted by the hybrid functional appear to be substantially closer to experiment than those calculated by either of the other two methods.

ISSN:0026-8976    

DOI:10.1080/002689797170077

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The two most important parameters of density functional theory, namely chemical hardness and chemical potential, have been calculated for more than 35 diatomic molecules at HF/6- 31G* level of theory. Correlations between force constant and binding energy with the chemical hardness have been derived and analysed for the above-mentioned 35 molecules. From this study it has been found that the increase in chemical hardness is accompanied by the increases in force constant and binding energy. The ability of chemical hardness and chemical potential to indicate the stability of molecules has been analysed.

ISSN:0026-8976    

DOI:10.1080/002689797170086

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This computational density functional theory (DFT) study provides information about the ground statetrans →cisisomerization process for a set ofall-translinear conjugated polyenes including the undecaene term which can be considered a model of β-carotene. To obtain information about the accuracy of the isomerization barriers computed with the DFT approach we have compared, for a series of shorterall-transpolyenes, the DFT results with those obtained using the CAS-PT2 approach. This comparison has shown that the DFT barriers agree with the CAS-PT2 ones within 1 kcalmo-1. We found for the undecaene term that the singlet ground state central double bond isomerization process occurs through a flat twisted transition state with small rotational barrier (22·9 kcal mol-1) and no special stabilizing effects. We have also investigated, at the DFT level, thetrans→cisisomerization process in the triplet state of undecaene and the DFT results show again two planar low-lying minima (a more stableall-transminimum and a central bond-cisminimum) connected by a twisted transition state (having the same geometry and energy of the twisted singlet ground state transition structure) with an unfavourable intersystem crossing between the two states. These results suggest that the deactivating ability of β-carotene is associated with the attack of the reactive species upon the two singlet closed shell minima.

ISSN:0026-8976    

DOI:10.1080/002689797170095

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A systematicab initiostudy has been made of the bonding between Zn atoms and all first- and second-row main group atoms. Ground state term symbols, equilibrium bond lengths, vibrational frequencies and dissociation energies have been determined, and the nature of the bonding (van der Waals, dative and chemical) has been examined.

ISSN:0026-8976    

DOI:10.1080/002689797170103

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Large-scale coupled cluster calculations were carried out for linear HC7N, HC7NH+and C7N (2Π and2Σ states). Accurate equilibrium geometries (bond lengths accurate to better than 0·001 å) are established. The ground-state rotational constant of HC7NH+is predicted to beB0= 553·8 ±0·5 MHz. Owing to its large equilibrium dipole moment of 6·35 D it may be a suitable candidate for forthcoming experimental investigation by microwave spectroscopy or radio astronomy. Various predictions are made for HC7N and HC7NH+in order to facilitate their detection by means of infrared spectroscopy. Partially restricted open-shell coupled cluster calculations including connected triple substitutions yield the2Π and2Σ states of C7N to be very close in energy and substantial rovibronic interaction is expected.

ISSN:0026-8976    

DOI:10.1080/002689797170112

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Two parallel unimolecular dissociation channels of the acetone cation radical are known experimentally. Methyl loss is the dominant reaction channel for high internal energies of the reactant ion whereas methane elimination is the major reaction channel for low internal energies. However, there exists a controversy about the mechanisms of dissociation of acetone cation radical. High levelab initioMP3/6-311G(d, p)//MP3/6-311G(d,p) calculations have been carried out to investigate the most relevant parts of the C3H6O+·ground state potential energy surface. Reactant and transition state frequencies also have been obtained at the MP3/ 6-311G(d, p) level to include zero-point energy effects. These energy results reinforce the idea that methane elimination at low internal energies may take place only via a tunnelling mechanism through the hydrogen transfer energy barrier. To analyse the influence of the entropic contribution on the mechanism the Gibbs free energy was calculated. Variational calculations at the hydrogen transfer region indicate that conventional transition state theory correctly describes the main features of the system at that bottleneck.

ISSN:0026-8976    

DOI:10.1080/002689797170121

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Using an internally contracted multireference configuration interaction approach, threedimensional potential energy functions of the X2Σ+(AlOH+) and X2Π (HAlO+) states have been generated. The variational calculations of the vibrational levels of AlOH+showed that the large amplitude bending modes exhibit inverse anharmonicity and are coupled with the AlO stretching modes. The energy of the HAlO+isomer at its equilibrium geometry and its lowest dissociation asymptote were calculated to be almost identical. A large barrier on the dissociation path prevents, however, the low lying rovibronic levels to predissociate. Using a variational Renner-Teller approach such rovibronic states forJ= 1/2 and 3/2 were calculated, and strong anharmonic resonances were detected in the excited rovibronic states. The potential energy function connecting the HAlO+and AlOH+isomers includes a conical intersection between the2Σ+and the2Π states. The barrier for isomerization relative to the AlOH+structure was calculated to be 4·34 eV. The MRCI calculations yielded for AlO a large proton affinity of 9·35 eV. Contrary to previous theoretical studies we found that the electronic ground state of AlO+is the1Σ+state and not the3Π state. Theab initiospectroscopic constants for its three lowest electronic states are given.

ISSN:0026-8976    

DOI:10.1080/002689797170130

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

General expressions for predicting line shapes in resonant Auger electron spectra are derived. These formulas take into account explicitly the interference effects due to the presence of several resonances embedded in the continua of different final states having finite lifetimes and include also the effects due to the finite bandwidth of the incident radiation and the finite resolving power of the electron spectrometer. The characteristic features of the Auger resonant Raman effect are derived analytically. The theory is applied to the analysis of high resolution resonant Auger spectra recently measured at the VUV beam line at Elettra, Sincrotrone Trieste.

ISSN:0026-8976    

DOI:10.1080/002689797170149

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The barrier to rotation about the C–N(H2) bond in creatinine in aqueous environments has been studied experimentally, by means of NMR spectroscopy, and theoretically, employing a number of models based upon explicit solvation and continuum descriptions. The measured barrier to rotation (13 kcal mol-1) is obtained only if an explicit solvent is included in calculations on a supermolecule model; the use of a continuum model alone predicts a barrier (9 kcal mol-1), considerably smaller than the experimental value.

ISSN:0026-8976    

DOI:10.1080/002689797170158

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor