Ab initio determination of the far infrared spectra of some isotopic varieties of ethanol

The far-infrared absorption spectra have been recorded for hydrogen-bonded complexes of water with ethanol embedded in cryogenic neon matrices at 2.8 K. The partial isotopic H/D-substitution of the ethanol subunit enabled by a dual inlet deposition procedure enables the observation and unambiguous assignment of the intermolecular high-frequency out-of-plane and the low-frequency in-plane donor OH librational modes for two different conformations of the mixed binary ethanol/water complex. The resolved donor OH librational bands confirm directly previous experimental evidence that ethanol acts as the O · · · HO hydrogen bond acceptor in the two most stable conformations. In the most stable conformation, the water subunit forces the ethanol molecule into its less stable gauche configuration upon dimerization owing to a cooperative secondary weak O · · · HC hydrogen bond interaction evidenced by a significantly blue-shift of the low-frequency in-plane donor OH librational band origin. The strong correlation between the low-frequency in-plane donor OH librational motion and the secondary intermolecular O · · · HC hydrogen bond is demonstrated by electronic structure calculations. The experimental findings are further supported by CCSD(T)-F12/aug-cc-pVQZ calculations of the conformational energy differences together with second-order vibrational perturbation theory calculations of the large-amplitude donor OH librational band origins. C 2015 AIP Publishing LLC. [http://dx

Section: Vibrational Assignments For the Mixed Ethanol/water Commentioning

confidence: 99%

Spectroscopic identification of ethanol-water conformers by large-amplitude hydrogen bond librational modes

Andersen

Heimdal

Larsen

2015

“…For both species, we perform a thorough comparison of the spectroscopic parameters with those of the corresponding O-analogs. 9,10 The effect of the O → S substitution is thus detailed, as it was done in previous papers where couples of analogs were treated and compared. [34][35][36] …”

Section: Introductionmentioning

confidence: 99%

“…(i) Levels are labeled using the symmetry representations of the G 6 and G 36 molecular symmetry groups (MSG) of ETSH and DMS (as for ethanol 9 and dimethyl-ether, 10 respectively). (ii) Levels are assigned to the different conformers on the basis of the probability integrals involving the torsional wavefunctions ψ(θ ,α),…”

Section: Torsional Analysismentioning

confidence: 99%

See 1 more Smart Citation

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

Senent

Puzzarini

Domı́nguez-Gómez

et al. 2014

Self Cite

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.

“…15 For nonrigid molecules, the theory can help in understanding many aspects related to the effects of barriers on the splitting of the levels. [16][17][18] To take all these effects into consideration, the torsional spectrum of EME is studied, in this paper, from a CCSD͑T͒ three-dimensional potential energy surface ͑3D-PES͒ and a flexible variational model. The 3D-PES determi-nation ͑12 atoms; 181 optimized geometries + zero vibrational energy correction͒ has carried out a large computational effort.…”

Section: Introductionmentioning

confidence: 99%

CCSD(T) study of the far-infrared spectrum of ethyl methyl ether

Senent¹,

Ruiz²,

Villa³

et al. 2009

Self Cite

Band positions and intensities for the far-infrared bands of ethyl methyl ether are variationally determined from a three-dimensional ͑3D͒ potential energy surface calculated with CCSD͑T͒/ cc-pVTZ theory. For this purpose, the energies of 181 selected geometries computed optimizing 3n − 9 parameters are fitted to a 3D Fourier series depending on three torsional coordinates. The zero point vibrational energy correction and the search of a correct definition of the methyl torsional coordinate are taken into consideration for obtaining very accurate frequencies. In addition, second order perturbation theory is applied on the two molecular conformers, trans and cis-gauche, in order to test the validity of the 3D model. Consequently, a new assignment of previous experimental bands, congruent with the new ab initio results, is proposed. For the most stable trans-conformer, the 30 , 29 , and 28 fundamental transitions, computed at 115.3, 206.5, and 255.2 cm −1 , are correlated with the observed bands at 115.4, 202, and 248 cm −1 . For the cis-gauche the three band positions are computed at 91.0, 192.5, and 243.8 cm −1 . Calculations on the −d 3 isotopomer confirm our assignment. Intensities are determined at room temperature and at 10 K. Structural parameters, potential energy barriers, anharmonic frequencies for the 3n − 9 neglected modes, and rotational parameters ͑rotational and centrifugal distortion constants͒, are also provided.