Determination of the kinetic energy parameters of non-rigid molecules

Highly correlated ab initio calculations (CCSD(T)) are used to compute gas phase spectroscopic parameters of three isotopologues of the methyl acetate (CH 3 COOCH 3 , CD 3 COOCH 3 , and CH 3 COOCD 3 ), searching to help experimental assignments and astrophysical detections. The molecule shows two conformers cis and trans separated by a barrier of 4457 cm −1 . The potential energy surface presents 18 minima that intertransform through three internal rotation motions. To analyze the far infrared spectrum at low temperatures, a three-dimensional Hamiltonian is solved variationally. The two methyl torsion barriers are calculated to be 99.2 cm −1 (C-CH 3 ) and 413.1 cm

Section: Full Dimensional Anharmonic Analysismentioning

confidence: 94%

“…35 The pseudopotential V´as well as the B qiqj depend on the isotopic substitution. For each isotopologue and geometry, they can be calculated using the subroutine MATRIZG 27,28 implemented in the code ENEDIM 26 and fitted to equations formally identical to Eq. (2).…”

Section: M=0mentioning

confidence: 99%

Highly correlated ab initio study of the far infrared spectra of methyl acetate

Senent

Domı́nguez-Gómez

Carvajal

et al. 2013

“…For this purpose, the torsional energy levels were obtained by solving the following two-dimensional Hamiltonian, 67,68 …”

Section: Torsional Analysismentioning

confidence: 99%

“…For their definitions, the readers are referred to Ref. 67. For each NS structure, these parameters were determined with the code ENEDIM.…”

Section: Torsional Analysismentioning

confidence: 99%

Theoretical spectroscopic characterization at low temperatures of detectable sulfur-organic compounds: Ethyl mercaptan and dimethyl sulfide

Senent

Puzzarini

Domı́nguez-Gómez

et al. 2014

Microwave and ab initio studies of rare gas-methane van der Waals complexes J. Chem. Phys. 120, 9047 (2004); 10.1063/1.1691743 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Highly correlated ab initio methods are used for the spectroscopic characterization of ethyl mercaptan (CH 3 CH 2 32 SH, ETSH) and dimethyl sulfide (CH 3 32 SCH 3 , DMS), considering them on the vibrational ground and excited torsional states. Since both molecules show non-rigid properties, torsional energy barriers and splittings are provided. Equilibrium geometries and the corresponding rotational constants are calculated by means of a composite scheme based on CCSD(T) calculations that accounts for the extrapolation to the complete basis set limit and core-correlation effects. The ground and excited states rotational constants are then determined using vibrational corrections obtained from CCSD/cc-pVTZ force-field calculations, which are also employed to determine anharmonic frequencies for all vibrational modes. CCSD(T) and CCSD force fields are employed to predict quartic and sextic centrifugal-distortion constants, respectively. Equilibrium rotational constants are also calculated using CCSD(T)-F12. The full-dimensional anharmonic analysis does not predict displacements of the lowest torsional excited states due to Fermi resonances with the remaining vibrational modes. Thus, very accurate torsional transitions are calculated by solving variationally two-dimensional Hamiltonians depending on the CH 3 and SH torsional coordinates of ethyl mercaptan or on the two methyl groups torsions of dimethyl-sulfide. For this purpose, vibrationally corrected potential energy surfaces are computed at the CCSD(T)/aug-cc-pVTZ level of theory. For ethyl mercaptan, calculations show large differences between the gauche (g) and trans (t) conformer spectral features. Interactions between rotating groups are responsible for the displacements of the g-bands with respect to the t-bands that cannot therefore be described with one-dimensional models. For DMS, the CCSD(T) potential energy surface has been semi-empirically adjusted to reproduce experimental data. New assignments are suggested for the methyl torsion bands of ETSH and a reassignment is proposed for the infrared bands of DMS (0 3 → 0 4 and 1 0 → 1 1). Our accurate spectroscopic data should be useful for the analysis of the microwave and far infrared spectra of ETSH and DMS recorded, at low temperatures, either in laboratory or in the interstellar medium.

“…͑2͒. Another important advantage of using TNUM is that the same formulation of the kinetic energy operator is valid for reduced dimensionality models if the G͑q͒ matrix is replaced by the corresponding reduced dimensionality G red ͑q͒, which is calculated using a procedure 37 equivalent to the one originally proposed by Wilson et al 39 The coordinates not included in the reduced dimensionality model can be frozen or considered as a function of the active coordinates, 36,37,[40][41][42][43] which was done here. The reduced dimensionality kinetic operator calculated in this manner is exact.…”

Section: Effective 1d Potential Energy Function and Kinetic Energymentioning

confidence: 99%

Treatment of nonlinear optical properties due to large amplitude anharmonic vibrational motions: Umbrella motion in NH3

Luis

Reis

Παπαδόπουλος

2009

A general reduced dimensionality finite field nuclear relaxation method for calculating vibrational nonlinear optical properties of molecules with large contributions due to anharmonic motions is introduced. In an initial application to the umbrella ͑inversion͒ motion of NH 3 it is found that difficulties associated with a conventional single well treatment are overcome and that the particular definition of the inversion coordinate is not important. Future applications are described.