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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 obtained ; This 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 facilities

14 pags., 6 figs., 6 tabs. -- This article belongs to the Special Issue Terahertz Spectroscopy and Dynamics of Low-Frequency Modes Involving Large Amplitude Molecular Motions ; 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 C 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 region and represent excitations of the ν (skeletal torsion) and the ν (methyl torsion) modes. The third independent variable, the OH torsion, interacts strongly with ν . The A /E splitting of the ground vibrational state has been evaluated to be 0.024 cm as it was expected given the high of the methyl torsional barrier (338 cm ). A very good agreement with respect to previous experimental data concerning fundamental frequencies (ν − ν ~ 1 cm ), and rotational parameters (B CAL − B < 5 MHz), is obtained. ; This 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 facilities.

17 pags., 6 figs., 10 tabs. -- Unified Astronomy Thesaurus concepts: Molecular physics (2058); Molecule formation (2076); Molecular spectroscopy (2095) ; Gas phase formation processes feasible at low temperatures are determined theoretically for 38 isomers obeying the C3O3H6 empirical formula, one of them, the simplest ketose dihydroxyacetone, has been observed in gas phase sources. A preliminary search for isomeric forms first targets ethoxy formic acid (C2H5-O-COOH) as the most stable isomer followed by lactic acid. Profiles corresponding to the minimum energy pathways reveal that the favored conformers of 14 of these isomers can be formed in the gas phase through 29 barrierless processes involving the OH∗, CH3O∗, HCO∗, CH3∗, CH2OH, HCOO∗, and OHCO∗ radicals, all of them observed in the interstellar medium. Kinetic rates are provided at 200, 298, and 500 K, confirming the suitability of 16 processes at low temperatures. Faster processes involve the OH hydroxyl radical whereas, to a lesser degree, the processes involving the HOCO radical and the methoxy methyl radical CH3O∗, are quite significant. Spectroscopic parameters (rovibrational and torsional) are obtained for methoxy acetic acid (CH3-O-CH2COOH) for which two low-lying isoenergetic conformers can be produced from the CH3OCH2∗ radical predicted to be a precursor of abundant observed molecules. Profiles and spectroscopic properties make methoxy acetic acid a good candidate to be detected in the gas phase of extraterrestrial sources. ; This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 872081. This research was supported by the Ministerio de Ciencia, Innovación 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. V.G. acknowledges CONACyT (México) for the postdoctoral grants 2018-000022-01EXT-00386. The author acknowledges the CTI (CSIC) and CESGA and to the "Red Española de Computación" for the grants AECT-2020-2-0008 and RES-AECT2020-3-0011 for computing facilities.

In the present paper, ab initio calculations at MP2/RHF level are performed with different basis sets 6-31G(d,p), 6-311G(d,p), and 6-311(df,p) to determine the potential energy functions, the kinetic parameters, and the dipole moment components as a function of the double methyl rotation in dimethylamine (DMA) and dimethylphosphine (DMP). From the potential energy and kinetic parameters, the torsional energy levels and torsional functions are determined, and from the dipole moment variations, the far infrared spectra are synthesized by calculating both the frequencies and the intensities. The results are in relatively good agreement with experimental spectra. Calculations confirm the assignments performed with the experimental potentials fitted with only five terms. The calculations, however, allow to reassign the observed band at 239.8 cm-1 in DMA and at 177.2 cm-1 in DMP to the superimposition of two different transitions: the 03→04 third sequence and an 10→11 vibrationally excited fundamental. © 1996 American Institute of Physics. ; This work has been supported by the European Union under the Human Capital and Mobility Scheme (contract CHRZ CT 93-0157). The authors also acknowledge the fi- nancial assistance from the ''Comision Interministerial de Ciencias y Technologia'' of Spain through grant no. PB 93- 0185. ; Peer Reviewed

10 pags., 4 figs., 5 tabs. ; Geometric and spectroscopic parameters are determined using explicitly correlated coupled cluster ab initio calculations (CCSD(T)-F12) for five monosubstituted isotopologues of acetone containing 18O, 13C, and D. The far-infrared region is explored with a variational procedure of reduced dimensionality that takes the interconversion of the nine minima of the potential energy surface into consideration. The methyl torsional barrier of the main isotopologue, computed to be 245.7 cm-1, varies slightly with the isotopic substitution. In the monodeuterated species, the two inequivalent internal rotors are hindered by two inequivalent barriers of 244.0 and 244.7 cm-1. The torsional fundamentals of the main variety are localized at 78.636 cm-1 (ν12) and 128.904 cm-1 (ν17). The differences between splitting components are lower than 0.02 and 0.2 cm-1 in the main isotopologue and in the isotopologues containing 18O and 13C, respectively. In the monodeuterated species the subcomponents are separated by ∼15 cm-1. ; This 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, Innovación 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 authors acknowledge the CTI (CSIC), CESGA, and the "Red Española de Computación" for Grants AECT-2020-2-0008 and RES-AECT2020-3-0011 for the computing facilities.

In this paper, the far infrared (FIR) methyl and phosphine torsional frequencies and intensities are determined theoretically in ethylphosphine from ab initio calculations. For this purpose, the potential energy function for the double rotation of the methyl and phosphine groups in the electronic ground state is determined in a standard calculation by using the MP2/RHF and a 6-31G(3df,p) basis set, with full optimization of the geometry. The numerical results are fitted to a symmetry adapted analytical form and introduced together with the kinetic parameters into the Hamiltonian operator. The Schrödinger equation for these two motions is solved by developing the solutions on the basis of products of trigonometric functions. From the energy levels, the torsional functions and the dipole moment variations the FIR spectrum is synthesized. A new assignment is proposed for some trans-trans and the gauche-gauche transitions between the phosphine levels. Additional transitions between the methyl levels are also proposed. It is concluded that a two dimensional calculation is indispensable for reproducing the FIR torsional spectrum in the region from 200 to 140 cm-1. © 1997 American Institute of Physics. ; This work has been supported by the European Union under Contract No. CHRZ CT 93-0157 into the Human Capital and Mobility Scheme. The authors also acknowledge the financial assistance of the ''Comision Interministerial de Ciencias y Tecnologia'' of Spain through Grant No. PB 93- 0185. ; Peer Reviewed

9 págs.; 6 figs.; 3 tabs. ; The ground and some excited electronic states of the methyl radical have been characterized by means of highly correlated ab intio techniques. The specific excited states investigated are those involved in the dissociation of the radical, namely the 3s and 3pz Rydberg states, and the A1 and B1 valence states crossing them, respectively. The C¿H dissociative coordinate and the HCH bending angle were considered in order to generate the first two¿dimensional ab initio representation of the potential surfaces of the above electronic states of CH3, along with the nonadiabatic couplings between them. Spectroscopic constants and frequencies calculated for the ground and bound excited states agree well with most of the available experimental data. Implications of the shape of the excited potential surfaces and couplings for the dissociation pathways of CH3 are discussed in the light of recent experimental results for dissociation from low¿lying vibrational states of CH3. Based on the ab initio data some predictions are made regarding methyl photodissociation from higher initial vibrational states. ; This work has been financed by the Spanish MINECO (grants CTQ2012-37404-C02-01, CTQ2015-65033-P, and FIS2013-40626-P) and EU COST Actions CM1401 and CM1405 as well as the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 610256 (NANOCOSMOS). This research has been carried out within the Unidad Asociada Quımica Fısica Molecular between the Departamento de Quımica Fısica of Universidad Complutense de Madrid (UCM) and Consejo Superior de Investigaciones Cientıficas (CSIC). The Centro de Supercomputacion de Galicia (CESGA, Spain) and CTI (CSIC) are acknowledged for the use of their resources. ; Peer Reviewed

10 pags., 9 figs., 4 tabs. ; The electronic states and the spin–orbit couplings between them involved in the photodissociation process of the radical molecules CH3X, CH3X → CH3 + X (X = O, S), taking place after the Ã(2A1) ← [X with combining tilde](2E) transition, have been investigated using highly correlated ab initio techniques. A two-dimensional representation of both the potential-energy surfaces (PESs) and the couplings is generated. This description includes the C–X dissociative mode and the CH3 umbrella mode. Spin–orbit effects are found to play a relevant role in the shape of the excited state potential-energy surfaces, particularly in the CH3S case where the spin–orbit couplings are more than twice more intense than in CH3O. The potential surfaces and couplings reported here for the present set of electronic states allow for the first complete description of the above photodissociation process. The different photodissociation mechanisms are analyzed and discussed in light of the results obtained. ; This work was supported by MINECO (Spain) (grants No. CTQ2015-65033-P, FIS2013-40626-P, and FIS2016-76418-P) and EU COST Action CM1401 as well as the European Research Council under the European Union's Seventh Framework Programme (FP/ 2007–2013)/ERC Grant Agreement No. 610256 (NANOCOSMOS). This research was carried out within the Unidad Asociada Quımica Fsica Molecular between the Departamento de Quımica Fısica of Universidad Complutense de Madrid (UCM) and Consejo Superior de Investigaciones Cientıficas (CSIC). AB acknowledges the financial support from the Tunisian Ministry of Higher Education, Scientific Research, and Technology, of the Short Term Scientific Mission (STSM) program of the COST Action CM1401, and of LSAMA at the Universite´ de Tunis El Manar that made possible research visits to the Instituto de Fısica Fundamental (CSIC). The Centro de Supercomputación de Galicia (CESGA, Spain) and CTI (CSIC) are acknowledged for the use of their resources. ; Peer Reviewed

9 pags., 4 figs., 1 tab. -- Open Access funded by Creative Commons Atribution Licence 4.0 ; The photochemistry of the ethyl radical following excitation to the 3p Rydberg state is investigated in a joint experimental and theoretical study. Velocity map images for hydrogen atoms detected from photoexcited isotopologues CH3CH2, CH3CD2 and CD3CH2 at 201 nm, are discussed along with high-level ab initio electronic structure calculations of potential energy curves and non-adiabatic coupling matrix elements (NACME). A novel mechanism governed by a conical intersection allowing prompt site-specific hydrogen-atom elimination is presented and discussed. For this mechanism to occur, an initial rovibrational excitation is allocated to the radical permitting to access this reaction pathway and thus to control the ethyl photochemistry. While hydrogen-atom elimination from cold ethyl radicals occurs through internal conversion into lower electronic states followed by slow statistical dissociation, prompt site-specific Ca elimination into CH3CH + H, occurring through a fast non-adiabatic crossing to a valence bound state followed by dissociation through a conical intersection, is accessed by means of an initial ro-vibrational energy content into the radical. The role of a particularly effective vibrational promoting mode in this prompt photochemical reaction pathway is discussed. ; D. V. C. and S. M. P. contributed equally to the paper. D. V. C. thanks a contract from Spanish MINECO under the FPI predoctoral program. S. M. P. acknowledges nancial support from a postdoctoral contract of the Regional Government of Madrid (Spain) (Grant PEJD-2016/IND-3217). A. B. acknowledges the financial support of the Tunisian Ministry of Higher Education, Scientific Research, and Technology, of the Short Term Scientific Mission (STSM) program of the COST Action CM1401, and of LSAMA at the Universite de Tunis El Manar, that made possible research visits to the Instituto de Fısica Fundamental (CSIC, Spain). This work has been nanced by the Spanish MINECO (Grants No. CTQ2015-65033-P, FIS2016- 76418-P and FIS-2016-77889-R) and EU COST Action CM1401. This research has been carried out within the Unidad Asociada Quımica Fısica Molecular between the Departamento de Quiımica Fisica of Universidad Complutense de Madrid and CSIC. The facilities provided by the Centro de Laseres Ultrarrapidos at Universidad Complutense de Madrid are gratefully acknowledged. The Centro de Supercomputacion de Galicia (CESGA, Spain) and CTI (CSIC) are acknowledged for the use of their resources

8 págs.; 6 figs.; 1 tab. ; The photodissociation dynamics of the methyl radical from the 3s and 3pz Rydberg states have been studied using the velocity map and slice ion imaging in combination with pump–probe nanosecond laser pulses. The reported translational energy and angular distributions of the H(2S) photofragment detected by (2+1) REMPI highlight different dissociation mechanisms for the 3s and 3pz Rydberg states. A narrow peak in the translational energy distribution and an anisotropic angular distribution characterize the fast 3s photodissociation, while for the 3pz state Boltzmann-type translational energy and isotropic angular distributions are found. High level ab initio calculations have been performed in order to elucidate the photodissociation mechanisms from the two Rydberg states and to rationalize the experimental results. The calculated potential energy curves highlight a typical predissociation mechanism for the 3s state, characterized by the coupling between the 3s Rydberg state and a valence repulsive state. On the other hand, the photodissociation on the 3pz state is initiated by a predissociation process due to the coupling between the 3pz Rydberg state and a valence repulsive state and constrained, later on, by two conical intersections that allow the system to relax to lower electronic states. Such a mechanism opens up different reaction pathways leading to CH2 photofragments in different electronic states and inducing a transfer of energy between translational and internal modes. ; S. M. P. acknowledges financial support from Campus de Excelencia Internacional Moncloa and LASING S. A. D. V. C. acknowledges a contract from MINECO under the Fondo de Garantıía Juvenil. A. Z. thanks the support from the European Research Council under the European Union's 7th Framework Program (FP7/2007–2013)/ERC Grant agreement 610256 (NANOCOSMOS). M. G. G. is grateful to Spanish MINECO for a contract through Programa de Técnicos de Apoyo a Infraestructuras. This work has been financed by the Spanish MINECO (grants FIS2011- 29596-C02-01, CTQ2012-37404-C02-01, FIS2013-40626-P and CTQ2015-65033-P) and by COST Actions CM1401 and CM05. This research has been carried out within the Unidad Asociada Química Física Molecular between Departamento de Química Física of Universidad Complutense de Madrid (UCM) and Consejo Superior de Investigaciones Científicas (CSIC). The facilities provided by the Centro de Láseres Ultrarrápidos at UCM are acknowledged. The Centro de Supercomputación de Galicia (CESGA, Spain) and CTI (CSIC) are acknowledged for the use of their resources. ; Peer Reviewed