Large Amplitude Motions of Pyruvic Acid (CH3-CO-COOH)

Torsional and rotational spectroscopic properties of pyruvic acid are determined using highly correlated ab initio methods and combining two different theoretical approaches: Second order perturbation theory and a variational procedure in three-dimensions. Four equilibrium geometries of pyruvic acid, Tc, Tt, Ct, and CC, outcome from a search with CCSD(T)-F12. All of them can be classified in the Cs point group. The variational calculations are performed considering the three internal rotation modes responsible for the non-rigidity as independent coordinates. More than 50 torsional energy levels (including torsional subcomponents) are localized in the 406–986 cm−1 region and represent excitations of the ν24 (skeletal torsion) and the ν23 (methyl torsion) modes. The third independent variable, the OH torsion, interacts strongly with ν23. The A1/E splitting of the ground vibrational state has been evaluated to be 0.024 cm−1 as it was expected given the high of the methyl torsional barrier (338 cm−1). A very good agreement with respect to previous experimental data concerning fundamental frequencies (νCAL − νEXP ~ 1 cm−1), and rotational parameters (B0CAL − B0EXP < 5 MHz), is obtainedThis project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 872081. This research was supported by the Ministerio de Ciencia, Inovación y Universidades of Spain through the grants EIN2019-103072 and FIS2016-76418-P. This work has received funding from the CSIC i-coop+2018 program under the reference number COOPB20364. The author acknowledges the CTI (CSIC) and CESGA and to the “Red Española de Computación” for the grants AECT-2020-2-0008 and RES-AECT-2020-3-0011 for computing facilitie

Theoretical spectroscopic characterization at low temperatures of methyl hydroperoxide and three S-analogs

9 pags.; 3 figs.; 6 tabs.© 2015 AIP Publishing LLC. The low temperature spectra of the detectable species methyl hydroperoxide (CH3OOH) and three sulfur analogs, the two isomers of methanesulfenic acid (CH3SOH and CH3OSH) and the methyl hydrogen disulfide (CH3SSH), are predicted from highly correlated ab initio methods (CCSD(T) and CCSD(T)-F12). Rotational parameters, anharmonic frequencies, torsional energy barriers, torsional energy levels, and their splittings are provided. Our computed parameters should help for the characterization and the identification of these organic compounds in laboratory and in the interstellar medium.This research was supported by the MINECO of Spain Grant No. FIS2013-40626-P and by a Marie Curie International Research Staff Exchange Scheme Fellowship within the 7th European Community Framework Program under Grant No. PIRSES-GA-2012-31754. The authors acknowledge the COST Actions CM1002 CODECS and CM1401 “Our Astrochemical History.” The authors acknowledge the CTI (CSIC) and CESGA for computing facilities.Peer Reviewe

Symmetry analysis of internal rotation

Barriers of infinite (or very large) height prevent certain rearrangements of the atoms in a molecule from occurring and thus the complete nuclear permutation inversion (CNPI) group of the molecule can be reduced. Using the example of acetaldehyde molecule, it was pointed out that some barriers of infinite height, in addition to reducing the CNPI group, constrain torsional dynamics explicitly and influence how the torsional coordinate transforms under the MS group. It was proved that the symmetry properties of the torsional potential do not depend on the geometrical symmetry of either the top or the frame.The authors would like to thank Projects E-10/2001 of the Hungarian–Spanish Intergovernmental Cooperations on Science and Technology, BFM2000-1106 and BFM2001- 2315 of the Ministerio de Ciencia y Technologia (Spain), and Grant Nos. OTKA T034327 and T033074 for support.Peer Reviewe

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

10 pages, 5 figures, 9 tables.Band positions and intensities for the far-infrared bands of ethyl methyl ether are variationally determined from a three-dimensional (3D) potential energy surface calculated with CCSD(T)/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 −d3 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 constants), are also provided.This work has also been supported by the Ministerio de Educación of Spain, Grant No. AYA2005-00702 Plan Nacional I+D+I 2004–2007 , CONACYT of Mexico, Grant No. 58728 CoNaCYT, and Computing resources of CESGA.Peer reviewe

Ethane internal rotation-Vibrational Hamiltonian

One of the main purposes of a theoretical spectroscopist is to propose a model to reproduce the xperimental spectra available in the literature as accurate and with as much physical information as possible. In this case it has been studied the Far infrared an Infrared spectra of non-rigid molecules, introducing in the Vibrational Potential their dependence on the large amplitude variables. The molecule that has been selected to test our proposed Hamiltonian is ethane which is a very well studied molecule. The molecule is a non-rigid molecule and their experimental spectra have been reported in the literature. © 2008, Sociedad Química de MéxicoPeer Reviewe

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

Highly correlated ab initio calculations (CCSD(T)) are used to compute gas phase spectroscopic parameters of three isotopologues of the methyl acetate (CH3COOCH3, CD3COOCH3, and CH3COOCD3), 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–CH3) and 413.1 cm−1 (O–CH3), for the cis-conformer. The three fundamental torsional band centers of CH3COOCH3 are predicted to lie at 63.7 cm−1 (C–CH3), 136.1 cm−1 (O–CH3), and 175.8 cm−1 (C–O torsion) providing torsional state separations. For the 27 vibrational modes, anharmonic fundamentals and rovibrational parameters are provided. Computed parameters are compared with those fitted using experimental data

Small linear carbon chains: vibrational and electronic states

First General Meeting in Prague, May 25 - 29, 2015The study of carbon chains type Cn and their ions has been attracted a significant effort due to their connection with the astrophysical observations, because pure carbon chains are abundant species in several sources such as the carbon rich circumstellar envelopes. In addition, they can play important roles in the reactivity of large systems containing carbon atoms. Carbon molecules have been considered responsible of the Diffuse Interstellar Bands (DIBs) [1]. Small chains are building blocks of larger species such as the fullerenes and the PAHs. In spite of their astrophysical relevance, few bare chains have been observed in gas phase extraterrestrial sources. The shortest chain C2, was the first detected, followed by C3 and the linear-C5 [2-3]. Detections were performed through the analysis of Infra-Red active vibrational excitations or through their electronic transitions because they present a zero dipole moment. All the Cn chains present a large number of isomers, however, in this work we focus on the linear ones. The large stability of charged linear chains was used as argument to predict the presence of anions in the interstellar medium. We present computed molecular properties calculated using highly correlated ab initio methods (CCSD(T)-F12, MRCI/CASSCF). We determine structures, infrared frequencies and excitations energies to the lowest electronic states and electron affinities of the small chains type Cn (n=3,4,5,6,7). Special attention is given to the anions which relevance for reactivity is evident. Non-adiabatic effects and spin-orbit effects will be predicted.Peer Reviewe

A new ab initio potential energy surface for the collisional excitation of N2H(+) by H2

10 pags.; 14 figs.© 2015 AIP Publishing LLC. We compute a new potential energy surface (PES) for the study of the inelastic collisions between N2H+ and H2 molecules. A preliminary study of the reactivity of N2H+ with H2 shows that neglecting reactive channels in collisional excitation studies is certainly valid at low temperatures. The four dimensional (4D) N2H+–H2 PES is obtained from electronic structure calculations using the coupled cluster with single, double, and perturbative triple excitation level of theory. The atoms are described by the augmented correlation consistent triple zeta basis set. Both molecules were treated as rigid rotors. The potential energy surface exhibits a well depth of ≃2530 cm−1. Considering this very deep well, it appears that converged scattering calculations that take into account the rotational structure of both N2H+ and H2 should be very difficult to carry out. To overcome this difficulty, the “adiabatic-hindered-rotor” treatment, which allows para-H2(j = 0) to be treated as if it were spherical, was used in order to reduce the scattering calculations to a 2D problem. The validity of this approach is checked and we find that cross sections and rate coefficients computed from the adiabatic reduced surface are in very good agreement with the full 4D calculationsThis research was supported by the CNRS national program “Physique et Chimie du Milieu Interstellaire.” F.L. and Y.K. also thank the Agence Nationale de la Recherche (ANR-HYDRIDES), contract No. ANR-12-BS05-0011-01. We acknowledge Laurent Pagani for stimulating this work.Peer Reviewe

Numerical Simulation of the Flow Inside a Scroll Compressor Equipped with Intermediate Discharge Valves

This paper presents the results of CFD simulations of the compression process of a scroll compressor. The compressor geometry accounts for the final scroll gas pockets, two Intermediate Discharge Valves (IDVs), the central discharge zone and the upper shell. The numerical model uses a real gas equation of state to determine gas properties during the compression process and accounts for the motion of the orbiting scroll and IDVs using a mesh smoothing and remeshing algorithm. The IDVs are represented as a spring mass system with their movement controlled via the pressure difference around the valve. Appropriate pressure based boundary conditions are used at entry to the third gas pocket and at the exit of the upper shell. An initial analysis of the results has shown that it is possible to achieve time accurate results of the pressure field throughout the flow domain and also determine the impact of the IDVs on scroll performance. The results will also enable a more thorough analysis of the fluid flow and compression process inside the scroll in order to improve its performance

An analysis of the methyl rotation dynamics in the S0 (X̃ 1A1) and T1 (ã 3A2) states of thioacetone, (CH3)2 CS and (CD 3)2 CS from pyrolysis jet spectra

Jet-cooled, laser-induced phosphorescence excitation spectra (LIP) of thioacetone (CH3)2CS/(CD3)2 CS have been recorded over the region 16 800-18 500 cm-1 using the pyrolysis jet spectroscopic technique. The responsible electronic transition, T 1 ←-S0, ã 3 A ″ ← X̃ 1A1, results from an n → π* electron promotion and gives rise to a pattern of vibronic bands that were attributed to activity of the methyl torsion and the sulphur out-of-plane wagging modes. The intensities of the torsional and wagging progressions in the excitation spectra were interpreted in terms of a C2υ-Cs molecular distortion of the triplet molecule from its singlet ground state equilibrium structure. A complete unrestricted Hartree-Fock (UHF) ab initio molecular orbital (MO) structural optimization of the T1 state predicted that the sulphur was displaced by 27.36° from the molecular plane and the methyl groups were rotated by 10.93° in clockwise-counterclockwise directions. Restricted Hartree-Fock (RHF) calculations were used to generate the F(θ1,θ2) potential surface governing methyl rotation for the S0 state. This was incorporated into a two-dimensional Hamiltonian, symmetrized for the G36 point group and solved variationally for the torsional frequencies. The calculated frequencies of 159.97/118.94 for the ν17 (b1) mode of S0 (CH3)2CS/(CD3)2 CS were found to agree with the experimental values, 153.2/114.7 cm-1. © 1991 American Institute of Physics.The authors are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support of this work. D. J. C. acknowledges the support of the National Science Foundation through Grant No. CHE-8914403.Peer Reviewe