R. Domínguez-Gómez

Ab Initio Study of the Rotational-Torsional Spectrum of Methyl Formate

The molecular structure of methyl formate is determined from ab initio calculations. The molecule presents two conformers (cis and trans) with a 5.3 kcal mol-1 difference in energy. In the most stable cis conformer, the carbonyl group eclipses the methyl group. The internal rotation barriers are V3(cis) = 368 cm-1 and V3(trans) = 26 cm-1 for the methyl group and VCO = 4826 cm-1 for the CO group. The dependence of the spectroscopic parameters on the torsional motions is detailed. The rotational-torsional energy levels have been calculated variationally up to J = 6 using a flexible model depending on the two torsional modes. Far-infrared frequencies and intensities are determined at room temperature. The rotational parameters have been computed to be A = 20,040.473 MHz, B = 6974.140 MHz, C = 5350.705 MHz, DJ = -0.510 kHz, DJK = 1.566 kHz, and DK = -0.619 kHz; and A = 20,040.492 MHz, B = 6974.399 MHz, C = 5350.851 MHz, DJ = 2.070 kHz, DJK = 14.712 kHz, and DK = 5.898 kHz at the symmetric and E components of the cis ground state, respectively. The corresponding values for trans-methyl formate are A = 47,380.066 MHz, B = 4738.781 MHz, and C = 4430.339 MHz; and A = 47,389.697 MHz, B = 4737.751 MHz, and C = 4429.607 MHz.

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

The far infrared (FIR) spectra of various isotopic species of ethanol (-h6, -d1, and -d3) are analyzed from MP4((SDQT) ab initio calculations using models in one and two dimensions. From the calculated frequencies and intensities, previous assignments of ethanol-h6 and -d1 bands are reviewed. The position of several combination bands are predicted. Ethanol shows two conformers, trans and gauche, and two interacting torsional modes. The torsional barriers have been calculated to be V3(trans)=1226.7 cm−1, V3(gauche)=1296.3 cm−1, VOH(α=62°)=404.1 cm−1, and VOH(α=180°)=423.3 cm−1. The flexible models in one and two dimensions yields the same OH torsional frequencies, whereas they differ in the methyl group state calculations. The fundamental bands of the ethanol-h6 have been evaluated at 205.5 cm−1 (OH torsion) and 257.0 cm−1 (CH3 torsion) and their corresponding intensities to be 18.650×10−4 and 0.662×10−4.

CCSD"T… study of the far-infrared spectrum of ethyl methyl ether

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 nu(30), nu(29), and nu(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 constants), are also provided.

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 (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(-1) (O-CH(3)), for the cis-conformer. The three fundamental torsional band centers of CH(3)COOCH(3) are predicted to lie at 63.7 cm(-1) (C-CH(3)), 136.1 cm(-1) (O-CH(3)), 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.

Structural and spectroscopic characterization of methyl isocyanate, methyl cyanate, methyl fulminate, and acetonitrile N-oxide using highly correlated ab initio methods

Various astrophysical relevant molecules obeying the empirical formula C2H3NO are characterized using explicitly correlated coupled cluster methods (CCSD(T)-F12). Rotational and rovibrational parameters are provided for four isomers: methyl isocyanate (CH3NCO), methyl cyanate (CH3OCN), methyl fulminate (CH3ONC), and acetonitrile N-oxide (CH3CNO). A CH3CON transition state is inspected. A variational procedure is employed to explore the far infrared region because some species present non-rigidity. Second order perturbation theory is used for the determination of anharmonic frequencies, rovibrational constants, and to predict Fermi resonances. Three species, methyl cyanate, methyl fulminate, and CH3CON, show a unique methyl torsion hindered by energy barriers. In methyl isocyanate, the methyl group barrier is so low that the internal top can be considered a free rotor. On the other hand, acetonitrile N-oxide presents a linear skeleton, C3v symmetry, and free internal rotation. Its equilibrium geometry depends strongly on electron correlation. The remaining isomers present a bend skeleton. Divergences between theoretical rotational constants and previous parameters fitted from observed lines for methyl isocyanate are discussed on the basis of the relevant rovibrational interaction and the quasi-linearity of the molecular skeleton.

Ab-initio Harmonic Analysis of Large-amplitude Motions in Ethanol Dimers

At relatively low pressures, dimerization of ethanol yields three different structures trans-gauche, trans-trans and gauche-gauche, each of them with different minimum energy conformations. The energy differences among the stable structures are relatively low. All of them may be present in the same sample. Dimerization shifts the whole spectrum to higher frequencies. The six new intermolecular modes push up the remaining vibrational modes which become constrained by the presence of the second molecule. The normal modes involving the hydrogen bonded atoms show the largest vibrational shifts. Five of the additional modes lies below 100 cm−1 and confer some non-rigidity to the dimer. The harmonic fundamental of the hydrogen bond stretching is located at 173.6, 155.3 and 189.4 cm−1 for the different conformers of the trans-gauche structure, i.e., quite below the OH torsion thaf located at 305.7 cm−1 in the molecule and moves up to 700 cm−1 in the dimers. For the trans-trans and gauche-gauche, this transition lies at 164.8 and 188.7 cm−1 respectively. The pattern observed between 150 and 190 cm−1 may be assigned to this stretching.