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Rotational Constants (rotational + constant)
Selected AbstractsChiral discrimination in hydrogen-bonded complexes of 2-methylol oxirane with hydrogen peroxideINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2009Guiqiu Zhang Abstract A systematic quantum chemical study reveals the effects of chirality on the intermolecular interactions between two chiral molecules bound by hydrogen bonds. The methods used are second-order Møller,Plesset perturbation theory (MP2) with the 6-311++g(d,p) basis set. Complexes via the OH···O hydrogen bond formed between the chiral 2-methylol oxirane (S) and chiral HOOH (P and M) molecules have been investigated, which lead to four diastereomeric complexes. The nomenclature of the complexes used in this article is enantiomeric configuration sign corresponding to English letters. Such as: sm, sp. The relative positions of the methylol group and the hydrogen peroxide are designated as syn (same side) and anti (opposite side). The largest chirodiastaltic energy was ,Echir = ,1.329 kcal mol,1 [9% of the counterpoise correct average binding energy De(corr)] between the sm-syn and sp-anti in favor of sm-syn. The largest diastereofacial energy was ,1.428 kcal mol,1 between sm-syn and sm-anti in favor of sm-syn. To take into account solvents effect, the polarizable continuum model (PCM) method has been used to evaluate the chirodiastaltic energies, and diastereofacial energies of the 2-methylol oxirane···HOOH complexes. The chiral 2,3-dimethylol oxirane (S, S) is C2 symmetry which offers two identical faces. Hence, the chirodiastaltic energy is identical to the diastereomeric energy, and is ,Echir = 0.563 kcal mol,1 or 5.3% of the De(corr) in favor of s,s-p. The optimized structures, interaction energies, and chirodiastaltic energies for various isomers were estimated. The harmonic frequencies, IR intensities, rotational constants, and dipole moments were also reported. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Ab initio and DFT studies on van der Waals trimers: The OCS · (CO2)2 complexesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2002H. Valdés Abstract Ab initio calculations [MP2, MP4SDTQ, and QCISD(T)] using different basis sets [6-31G(d,p), cc-pVXZ (X = D, T, Q), and aug-cc-pVDZ] and density functional theory [B3LYP/6-31G(d,p)] calculations were carried out to study the OCS · (CO2)2 van der Waals trimer. The DFT has proved inappropriate to the study of this type of systems where the dispersion forces are expected to play a relevant role. Three minima isomers (two noncyclic and one cyclic) were located and characterized. The most stable isomer exhibits a noncyclic barrel-like structure whose bond lengths, angles, rotational constants, and dipole moment agree quite well with the corresponding experimental values of the only structure observed in recent microwave spectroscopic studies. The energetic proximity of the three isomers, with stabilization energies of 1442, 1371, and 1307 cm,1, respectively, at the CBS-MP2/cc-pVXZ (X = D, T, Q) level, strongly suggests that the two unobserved structures should also be detected as in the case of the (CO2)3 trimer where both noncyclic and cyclic isomers have been reported to exist. The many-body symmetry-adapted perturbation theory is employed to analyze the nature of the interactions leading to the formation of the different structures. The three-body contributions are small and stabilizing for the two most stable structures and almost negligible for the cyclic isomer. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 444,455, 2002; DOI 10.1002/jcc.10041 [source] Raman and infrared spectra, conformational stability, ab initio calculations and vibrational assignment of dimethylsilylisocyanateJOURNAL OF RAMAN SPECTROSCOPY, Issue 3 2010Gamil A. Guirgis Abstract The Raman (3200-30 cm,1) and/or infrared spectra (3500 to 400 cm,1) of gaseous, liquid and solid dimethylsilylisocyanate, (CH3)2 Si(H)NCO, have been recorded. The MP2(full) calculations, employing a variety of basis sets with and without diffusion functions, have been used to predict the structural parameters, conformational stability, vibrational fundamental wavenumbers, Raman activities, depolarization values and infrared intensities to support the vibrational assignment. The low wavenumber Raman spectrum of the gas with a significant number of Q-branches for the SiNC(O) bend is consistent with an essentially linear SiNCO moiety. The ab initio calculations supported this conclusion as all possible orientations of the NCO moiety lead to nearly the same energy. This result is at variance with the conclusion from the electron diffraction study that the heavy atom skeleton was bent with an angle of 152(5)° with one stable cis conformer. It is believed that this reported angle difference from 180° is due to the shrinkage effect. The SiH distance of 1.486 Å has been obtained from the isolated SiH stretching wavenumber. From the adjustment of the ab initio MP2(full)/6-311+G(d,p) predicted structural parameters, a proposed structure is reported, which is expected to give rotational constants within a few megahertz of the actual ones. These experimental and theoretical results are compared with the corresponding quantities of similar molecules. Copyright © 2009 John Wiley & Sons, Ltd. [source] Degenerate and two-color resonant four-wave mixing applied to the rotational characterization of high-lying vibrational states of formaldehyde (Ã, 1A2),JOURNAL OF RAMAN SPECTROSCOPY, Issue 1-3 2006M. Tulej Abstract Degenerate and two-color resonant four-wave mixing techniques (DFWM and TC-RFWM) are applied to determine rotational constants of high-lying vibrational levels in the first excited singlet state Ã(1A2) of formaldehyde. It has been demonstrated that the sensitivity of the spectroscopic technique is applicable to the low-density environment of a supersonic molecular beam and to predissociating transitions displaying low fluorescence quantum yield. In addition, we take advantage of the superior selectivity of the double-resonance method, TC-RFWM, to isolate and assign transitions in the congested region of the (one-color) DFWM spectra. The line positions of 25 well-isolated transitions are determined in the band and yield the rotational constants A, B, C and the origin ,e. The accuracy of the constants is determined by performing the same procedure for the band where literature data is available for comparison. Copyright © 2006 John Wiley & Sons, Ltd. [source] Molecular Structure of ProlineCHEMISTRY - A EUROPEAN JOURNAL, Issue 18 2004Wesley D. Allen Dr. Abstract The molecular structures of the two lowest-energy conformers of proline, Pro- I and Pro- II, have been characterized by ab initio electronic structure computations. An extensive MP2/6-31G* quartic force field for Pro- I, containing 62,835 unique elements in the internal coordinate space, was computed to account for anharmonic vibrational effects, including total zero-point contributions to isotopomeric rotational constants. New re and improved r0 least-squares structural refinements were performed to determine the heavy-atom framework of Pro- I, based on experimentally measured (A. Lesarri, S. Mata, E. J. Cocinero, S. Blanco, J. C. Lopez, J. L. Alonso, Angew. Chem.2002, 114, 4867; Angew. Chem. Int. Ed.2002, 41, 4673) rotational constant sets of nine isotopomers and our ab initio data for structural constraints and zero-point vibrational (ZPV) shifts. Without the ab initio constraints, even the extensive set of empirical rotational constants cannot satisfactorily fix the molecular structure of the most stable conformer of proline, a 17-atom molecule with no symmetry. After imposing the ab initio constraints, excellent agreement between theory and experiment is found for the heavy-atom geometric framework, the root-mean-square (rms) residual of the empirical rotational constant fit being cut in half by adding ZPV corrections. The most significant disparity, about 0.07 Å, between the empirical and the best ab initio structures, concerns the r(N,,,H) distance of the intramolecular hydrogen bond. Some of the experimental quartic centrifugal distortion constants assigned to Pro- II have been corrected based on data obtained from a theoretical force field. [source] Structure Determination of Resorcinol Rotamers by High-Resolution UV SpectroscopyCHEMPHYSCHEM, Issue 10 2005Grzegorz Myszkiewicz Abstract The rotationally resolved S1,S0 electronic origins of several deuterated resorcinol rotamers cooled in a molecular beam have been recorded. An automated assignment of the observed spectra has been performed using a genetic algorithm approach with an asymmetric rotor Hamiltonian. The structures of resorcinol A and resorcinol B were derived from the rotational constants of twenty deuterated species for both electronic states. The lifetimes of different resorcinol isotopomers in the S1 state are also reported. As is the case for phenol, these lifetimes mainly depend on the position of deuteration. A nearly perfect additivity of the zero-point energies after successive deuterations in resorcinol rotamers has been discovered and subsequently used in the full assignment of the previously reported low-resolution spectra of deuterated resorcinol A. An analogous spectrum is also predicted for the resorcinol B rotamer. [source] | ||||||||||