Theoretical Investigation of the Structure and Vibrational Spectrum of the Electronic Ground State X̃(1A‘) of HSiCl

Equilibrium Values for the Si-H Bond Length and Equilibrium Structures of Silyl Iodide and Halosilylenes

The equilibrium structures of silyl iodide, SiH3I, and silylene halides, SiHX (X = F, Cl, Br, I), were determined by using the mixed regression method, where approximate values of the rotational constants are supplemented by the structural parameters of a different origin. For this goal, it is shown that the r(Si-H) bond length can be determined by using the isolated SiH stretching frequency and that an accurate estimation of the bond angles is obtained by an MP2 calculation with a basis set of triple zeta quality. To check the accuracy of the experimental structures, they were also optimized by means of all electron CCSD(T) calculations using basis sets of quadruple zeta quality.

Ab Initio Study on Electronic Excited States of Monochlorosilylene

We perform a high-level ab initio study on 20 electronic states of monochlorosilylene (HSiCl) using an internally contracted multireference configuration interaction method including Davidson correction (icMRCI+Q). The spin-orbit coupling (SOC) effect is investigated, leading to splitting of the 20 spin-orbit-free states into 50 spin-orbit-coupled states. Vertical transition energies, oscillator strengths, and potential energy curves are presented with and without considering the SOC effect. Analysis indicates that the SOC effect plays an important role, especially for the high-lying excited states of HSiCl. The state interaction and the dynamics of the electronic states of HSiCl in the ultraviolet region are discussed based on our calculation results. Our study paves the way to understanding the behavior of electronic excited states of monochlorosilylene.

Accurate Property Prediction by Second Order Perturbation Theory: The REMP and OO-REMP Hybrids

The prediction of molecular properties such as equilibrium structures or vibrationalwavenumbers is a routine task in computational chemistry. If very high accuracy is required, however, the use of computationally demanding ab initio wavefunction methods is mandatory. We present property calculations utilizing the REMP and OO-REMP hybrid perturbation theories showing that with the latter approach, very accurate results are obtained at second order in perturbation theory. Specifically, equilibrium structures and harmonic vibrational wavenumbers as well as dipole moments of closed and open shell molecules were calculated and compared to the best available experimental results or very accurate calculations.OO-REMP is capable of predicting bond lengths of small closed and open shell molecules with an accuracy of 0.2 pm and 0.5 pm, respectively, often within the range of experimental uncertainty. Equilibrium harmonic vibrational wavenumbers are predicted with an accuracy better than 20 cm−1 . Dipole moments of small closed and open shell molecules are reproduced with a relative error of less than 3 %. Across all investigated properties it turns out that a 20 %:80 % MP:RE mixing ratio consistently provides the best results. This is in line with our previous findings featuring closed and open shell reaction energies.

Single vibronic level emission spectroscopic studies of the ground state energy levels and molecular structures of jet-cooled HGeBr, DGeBr, HGeI, and DGeI

Single vibronic level dispersed fluorescence spectra of jet-cooled HGeBr, DGeBr, HGeI, and DGeI have been obtained by laser excitation of selected bands of the A (1)A(")-X (1)A(') electronic transition. The measured ground state vibrational intervals were assigned and fitted to anharmonicity expressions, which allowed the harmonic frequencies to be determined for both isotopomers. In some cases, lack of a suitable range of emission data necessitated that some of the anharmonicity constants and vibrational frequencies be estimated from those of HGeClDGeCl and the corresponding silylenes (HSiX). Harmonic force fields were obtained for both molecules, although only four of the six force constants could be determined. The ground state effective rotational constants and force field data were combined to calculate average (r(z)) and approximate equilibrium (r(e) (z)) structures. For HGeBr r(e) (z)(GeH)=1.593(9) A, r(e) (z)(GeBr)=2.325(21) A, and the bond angle was fixed at our CCSD(T)/aug-cc-pVTZ ab initio value of 93.6 degrees . For HGeI we obtained r(e) (z)(GeH)=1.589(1) A, r(e) (z)(GeI)=2.525(5) A, and bond angle=93.2 degrees . Franck-Condon simulations of the emission spectra using ab initio Cartesian displacement coordinates reproduce the observed intensity distributions satisfactorily. The trends in structural parameters in the halogermylenes and halosilylenes can be readily understood based on the electronegativity of the halogen substituent.

The nuclear-spin-rotation constants of HCY, HSiY, and SiY2 (Y=F, Cl): An ab initio study

The nuclear-spin-rotation constants of fluoro- (HCF) and chloro- (HCCl) carbene, of the corresponding silylenes (HSiF and HSiCl), and of difluoro- and dichlorosilylene (SiF(2) and SiCl(2)) are quantum-chemically investigated employing the coupled-cluster singles and doubles model augmented by a perturbative treatment of triple excitations together with various sequences of correlation-consistent basis sets. Theoretical best estimates are obtained through consideration of corrections for core correlation and of zero-point vibrational contributions. In addition, nuclear quadrupole coupling constants for the chlorine containing species are determined. A thorough comparison with experiment is made.

The accuracy ofab initiomolecular geometries for systems containing second-row atoms

The performance of the standard hierarchy of ab initio models--that is, Hartree-Fock theory, second-order Moller-Plesset theory, coupled-cluster singles-and-doubles theory, and coupled-cluster singles-doubles-approximate-triples theory--in combination with correlation-consistent basis sets is investigated for equilibrium geometries of molecules containing second-row elements. From an analysis on a collection of 31 molecules (yielding statistical samples of 41 bond distances and 13 bond angles), the statistical errors (mean deviation, mean absolute deviation, standard deviation, and maximum absolute deviation) are established at each level of theory. The importance of core correlation is examined by comparing calculations in the frozen-core approximation with calculations where all electrons are correlated.