IR-induced conformer interconversion processes of glycolaldehyde in low-temperature matrices,and ab initio calculations on the energetics and vibrational frequencies of the conformers

Both IR-induced and thermal conformer interconversion processes of glycolaldehyde (CHOCH₂OH) were studied in low-temperature Ar, Kr and Xe matrices. The observed photochemical reaction induced by broad-band IR irradiation is interpreted to proceed from the lowest-energy conformer Cc which prevails after deposition to conformer Tt. In Xe and Kr the photochemical steady state (ratio [Tt]/[Cc]) depends on the matrix temperature; two conformers, Tg and Tt, seem to be in thermal equilibrium, and at elevated temperatures species Tg is enriched, and then the reverse photoprocess Tg → Cc occurs more rapidly than the competing reverse process, Tt → Cc which prevails at 13 K. Because of the thermal equilibrium the routes of the forward and reverse photoprocesses are considered to be different. A thermal reverse process, Tt → Tg → Cc, is observed at temperatures above 30 K in Xe and Kr. The role of the torsional barrier heights in determining the rates of the photoprocesses is emphasized. To aid in the interpretation of the experimental results, ab initio calculations were carried out. Geometries and energies of all conformers of glycoaldehyde as well as the geometries of the most important saddle points were fully optimized at the HF/6-31G** level, and vibrational spectra were calculated on the HF/4-31G level; the latter conformed to the experimental conformer assignment.     

Thermal and IR-induced conformer interconversion processes for 2-chloroallyl alcohol in Low-temperature matrices: Wavelength-dependent conformer steady states

2-chloroallyl alcohol has been studied in low-temperature Ar, Kr, Xe, N₂ and CH₄ matrices. An IR-induced process was found in all media except CH₄ and resulted in a steady state dependent on the Wavelength of the irradiation. At elevated temperatures thermal conformer interconversions occurred. The energetics of the processes are discussed and their potential barriers determined. No mode selectivity was found.     

Ab initio calculation of the inversion barrier in the germyl radical

Equilibrium geometry, vibrational frequencies and inversion barrier of the GeH₃ radical have been calculated using ab initio molecular orbital methods at both the SCF and correlated levels. At the SCF level, the effect of several small and medium size basis sets of Ge on these properties are studied systematically. Electron correlation corrections as incorporatedvia fourth-order unrestricted Møller-Plesset perturbation theory in conjunction with large Gaussian basis sets were found to reduce the GeH₃ inversion barrier appreciably relative to the UHF values. The final barrier height of 4.5 kcal/mol can be compared with a recent estimate of 4.4 kcal/mol inferred from the REMPI study.     

Ab initio torsional potential and transition frequencies of acetaldehyde

High-level ab initio electronic structure calculations, including extrapolations to the complete basis set limit as well as relativistic and diagonal Born-Oppenheimer corrections, resulted in a torsional potential of acetaldehyde in its electronic ground state. This benchmark-quality potential fully reflects the symmetry and internal rotation dynamics of this molecule in the energy range probed by spectroscopic experiments in the infrared and microwave regions. The torsional transition frequencies calculated from this potential and the ab initio torsional inverse effective mass function are within 2 cm(-1) of the available experimental values. Furthermore, the computed contortional parameter rho of the rho-axis system Hamiltonian is also in excellent agreement with that obtained from spectral analyses of acetaldehyde.     

Ab initio study of the inversion barrier in NF3+

Ab initio Hartree–Fock, Møller–Plesset perturbation theory (MP 2), and quadratic configuration interaction, using single and double substitutions (QCISD ), calculations were carried out for the NF₃⁺ ion. Optimized structures were examined at the various levels of theory. Calculation of the inversion barrier height shows the importance of optimizing the geometry at the post-Hartree–Fock level and the inclusion of polarization functions. The best calculated inversion barrier was 13.3 kcal/mol, compared to an experimental value of 17.3 kcal/mol. The dissociation transition state was computed to determine the well depth of the NF₃⁺ ion and its stability toward dissociation. The computed well depth was 28 and 48 kcal/mol at the SCF and MP 2 levels, respectively. © 1994 John Wiley & Sons, Inc.     

Ab initio calculation of resonance Raman cross sections based on excited state geometry optimization

A method for the calculation of resonance Raman cross sections is presented on the basis of calculation of structural differences between optimized ground and excited state geometries using density functional theory. A vibrational frequency calculation of the molecule is employed to obtain normal coordinate displacements for the modes of vibration. The excited state displacement relative to the ground state can be calculated in the normal coordinate basis by means of a linear transformation from a Cartesian basis to a normal coordinate one. The displacements in normal coordinates are then scaled by root-mean-square displacement of zero point motion to calculate dimensionless displacements for use in the two-time-correlator formalism for the calculation of resonance Raman spectra at an arbitrary temperature. The method is valid for Franck-Condon active modes within the harmonic approximation. The method was validated by calculation of resonance Raman cross sections and absorption spectra for chlorine dioxide, nitrate ion, trans-stilbene, 1,3,5-cycloheptatriene, and the aromatic amino acids. This method permits significant gains in the efficiency of calculating resonance Raman cross sections from first principles and, consequently, permits extension to large systems (>50 atoms).     

Ab initio study of the rotational energy barrier in carbonylylide

The rotational energy barrier in carbonylylide CH₂OCH₂ is studied using RHF CI calculations. Depending on the size of the CI and the basis set (STO-3G and 4–31G), values in the range 13–17 $\text{kcal}\text{mol}$ are found. At this level of calculation, the mid-point of the isomerization process can be mainly described by the diradical rather than the zwitterion.     

Ab initio calculation of core-electron binding energies in small molecules

Vertical ionization potentials for core orbitals of HF, H₂O, CO, HCN, and H₂CO are calculated with an ab initio transition operator method, followed by Rayleigh-Schrödinger perturbation theory. The results are improved by using a transition atomic basis set. Relativistic corrections are estimated. The average absolute deviation of our final results from experiment is 0.4 eV for the eight cases studied.     

Vibrational spectrum, ab initio calculation, conformational equilibria and torsional modes of 1,3-dibromopropane

The infrared and Raman spectrum of 1,3-dibromopropane is reported in the crystalline, liquid and gaseous states. These measurements are compared to the results of ab initio calculations carried out using the 6-31+g* Gaussian basis set for a restricted Hartree-Fock computation. The calculation was repeated using second order Moeller-Ploesset perturbation theory to accommodate electron correlation using the 6-31 g* basis set. The three most stable conformers are GG (C2), AG (C1) and AA (C2v), where A and G stand for anti and gauche orientations of the bromomethyl group relative to the plane of the carbon atoms. The point group symmetry of each structure is given in parentheses. The fourth conformer, G'G (Cs) is of such high energy that it is not observed experimentally in isotropic media in either the infrared or Raman spectrum. In the crystalline state, comparison of the infrared and Raman spectrum with that calculated for the C2 conformer shows that only the GG (C2) conformer survives, and the doublet structure of many of the bands in the spectrum indicates at least two molecules per unit cell. The ab initio calculations predict and the temperature dependence of the Raman spectrum of the liquid confirms that the stability order is C2相似文献   

A molecular mechanics calculation of conformational energies and barrier heights in haloalkanes

A molecular mechanics calculation of conformational energies and barrier heights in substituted fluoro- and chloroethanes and -propanes is given which reproduces reasonably well all the available experimental data on these compounds. This was achieved by the use of non-bonded potential functions for chlorine and fluorine very different from those previously used.     

Ab initio calculation of the electronic structure and the barrier to pyramidal inversion for H3S+

The electronic structure of H₃S⁺ is examined by ab initio MO calculations (STO 3G) and is compared with those of other XH₃ type molecules. The barrier to pyramidal inversion and the proton affinity of H₂S are calculated to be 34.8 and 225.05 kcal/mol respectively. Computations are made with respect to a model A₃S⁺ which is employed to discuss the barrier to pyramidal inversion of H₃S⁺, where A corresponds to an electromagnetive substituent or an electron donating one.     

Octa-1,3,5,7-tetraene: Ab initio geometries and vibrational spectra of some stable structures

Complete geometry optimizations of trans, Trans, trans, Trans, trans-, gauche, Trans, trans, Trans, trans-, trans, Trans, trans, Cis, trans-, and gauche, Trans, trans, Cis, trans-octa-1,3,5,7-te-traenes were carried out at the RHF/6-31G level. Characteristic changes in the geometry are found in going from the planar conformers of octatetraene to the corresponding higher-energy stable forms. The harmonic force constants were computed for the above conformers at the RHF/6-31G//RHF/6-31G level using analytical second derivatives. The computed force fields of these four molecules were then corrected using empirical scale factors transferred fromtrans-buta-1,3-diene. To account for the vibronic coupling effect, which may be a characteristic of oligoenes, a special scale factor was introduced for the two internal coordinates which correspond to stretching the central C=C double bonds. A complete assignment of the experimental spectra oft, T, t, T, t-octatetraene is also given.On leave from Laboratory of Molecular Spectroscopy, Department of Chemistry, Moscow State University, Moscow 119899, U.S.S.R.Preliminary results were reported at the Thirteenth Austin Symposium on Molecular Structure, Austin, TX, USA March 12–14, 1990, S 5, p. 91. and at the Second World Congress of the Theoretical Organic Chemists, Toronto, Canada July 8–14, 1990, BP-35 (Canada).     

Conformational structures, energies, rotational barrier heights and torsional force constants in halogenated methyl-silanes obtained by molecular-mechanics calculations

Conformational structures, energies, barrier heights and torsional force constants in halogenated (F, Cl, Br) methyl-silanes have been obtained by molecular-mechanics calculations. A comparison of the ethan-like molecules with central bonds C---C, Si---C and Si---Si has been made. It is predicted that, for halogenated methyl-silanes, the difference in conformational energy between anti and gauche conformers is small.     

Ab initio calculation of the lowest singlet and triplet states in CH2, CHF,CF2, and CHCH3

The lowest singlet and triplet states of the radicals CH₂, CHF, CF₂, and CHCH₃ have been investigated both in SCF and IEPA approximation (“independent electron pair approach” to account for electron correlation). The SCF calculations yield triplet ground states for CH₂, CHF, and CHCH₃, and a singlet ground state for CF₂. Electron correlation stabilizes the singlet state by about 14 kcal/mole with respect to the triplet for all four radicals leading to a singlet ground state also for CHF. The final triplet-singlet energy separations are 10, 6, ?11, ?47 kcal/mole for CH₂, CHCH₃, CHF, CF₂, respectively. Values for equilibrium bond angles, ionization potentials and bond energies are also given.     

Ab initio interpretation of Hund's rule for the methylene molecule: Variational optimization of its molecular geometries and energy component analysis

Hund's spin‐multiplicity rule for the ground state of the methylene molecule CH₂ is interpreted by Hartree‐Fock (HF) and multi‐reference configuration interaction (MRCI) methods. The stabilization of the triplet ground state ( ³B₁) relative to the second singlet excited state ( ¹B₁) is ascribed to the greater electron‐nucleus attraction energy that is gained at the cost of increasing the electron‐electron repulsion energy and with the aid of a reduction in the nucleus‐nucleus repulsion energy. The highest spin‐multiplicity in the ground state of CH₂ is accompanied by a set of three characteristic features, i.e., elongation of the internuclear distances, reduction in the bond angle, and contraction of the valence electron density distribution around the nuclei involving expansion of the core electron density distribution. The present calculations fulfill the virial theorem to an accuracy of ?V/T = 2.000 for both HF and MRCI. Accordingly, the molecular geometries are optimized for each of the two states. The inclusion of correlation by MRCI method reduces the energy splitting between the two states by about 14%. The energy splitting is analyzed by the correlational virial theorem 2T^c + V^c = 0 to make a clear interpretation of the correlation effect.© 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Ab initio studies of structural features not easily amenable to experiment. 30. Conformational analysis and molecular structures of propanal and butanal

The geometries of several conformations of propanal and butanal have been refined by geometrically unconstrained ab initio gradient relaxation on the 4-21G level. Both compounds possess energy minima at O? C? C? C torsional angles of 0° and in the 120° region, and energy maxima in the 70° region and at 180°. The structure of the aldehyde functional group is found to be relatively invariant both when different systems or when different conformations of the same system are compared. Conformationally dependent geometrical trends in propanal and butanal are discussed and found to be subtle yet noticeable.     

Investigation on the potential barrier to internal rotation in molecules——Ab initio calculation and energy partition of the internal rotation barrier in complex molecule H_3N-BH_3

Based on the ab initio/6-31G~* calculation, the potential barrier to internal rotation in mol-ecule H_3N--BH_3 has been studed by means of PD/LSF atomic charge model and Buckingham(exp--6-1)energy partition method. The results indicate that the order of the contributions of the componentsto the total energy barrier △E is |△V_(es)|(electrostatic) >|△V_(et)| (charge transfer)>|△V_(ex)| (exchangerepulsion)>|△V_(di)| (dispersion). For △V_(es) there are maxima at θ= 30°and 90°, and a saddle atθ= 60°. There are good linear relationships for the total barrier △E, △V_(ex) and △V_(di) with cos3θrespectively, and the same for the dipole moment from PD/LSF model (μ_(PD)) and that from abinitio calculation (μ_(QM)) vs. cos3θ respectively.     

Ab initio determination of molecular geometries and vibrational frequencies of CX3 COOH (X=H, F, Cl, Br)

Using Gaussian 03 Revision C.02 version of the quantum chemical program ab initio and DFT computations have been carried out at the rhf/6-31+g*, b3lyp/6-31+g*, b3lyp/6-31++g** and b3lyp/6-311++g** levels to compute optimized geometries, harmonic vibrational frequencies along with intensities in IR and Raman spectra and atomic charges for the acetic (ethanoic) acid and its 1,1,1-tri-halo (fluoro, chloro and bromo) derivatives. The optimized molecular structures for all the four molecules are found to possess Cs point group symmetry. The symmetric stretching mode is found to have lowest magnitude of the three CX3 stretching modes for all the four molecules, whereas the symmetric deformation mode is found to have the lowest magnitude for EA and TFEA and the highest magnitude for TCEA and TBEA. The parallel rocking mode of the CX3 group is found to have lower magnitude than the perpendicular rocking mode for EA and TFEA where reverse is found for TCEA and TBEA. The modes of the COOH group are substituent sensitive except the OH stretching mode. Moreover, the maximum effect is found for the TFEA molecule. The CF3 group is found to have the characteristic frequencies as 235-505, 787, 1150-1190 and 1400 cm(-1) which are due to the modes delta s(CF3), nu s(CF3), nu as(CF3) and nu(C-CF3), respectively.     

Small basis sets for calculations of barrier heights, energies of reaction, electron affinities, geometries, and dipole moments

Beginning with the MIDI! basis set (also called MIDIX), we introduce the MIDIX+, MIDIY, and MIDIY+ basis sets. Using correlated ab initio and hybrid density functional theory, we compare their performance to that of several existing basis sets for electronic structure calculations. The new basis sets are tested with databases of 358 energies of reactions, 44 barrier heights, 31 electron affinities, 18 geometries, and 29 dipole moments. The MIDI!, MIDIX+, MIDIY, and MIDIY+ basis sets are shown to be cost-efficient methods for calculating relative energies, geometries, and dipole moments. The MIDIX+ basis is shown to be particularly efficient for calculating electron affinities of large molecules.Contribution to the Jacopo Tomasi Honorary Issue