Molecular orbital studies of conformers and the barrier to internal rotation in 1,3-butadiene

The potential curve for rotation around the central bond in 1,3-butadiene has been estimated by ab initio calculations using Gaussian-type basis functions. The calculations, which also include limited geometry variation during rotation, suggest that in the SCF approximation the second stable form of the molecule is a gauche conformation rather than a cis. The predicted energy difference between the planar trans ground state and the stable gauche form is 2.7 kcal/mole and the barrier to internal rotation is found to be 6.0 kcal/mole using a (9,5) basis for carbon and 4s functions on hydrogen.     

Ab initio SCF and CI calculations on the barrier to internal rotation of 1,3-butadiene

Ab initio SCF and CI calculations employing a set of gaussian lobe functions have been carried out for the ground and excited states of five geometrical C₄H₆-structures occurring in the course of rotation from cis-butadiene to the trans-isomer. The rotational potential curves are discussed for the ground and excited states. Particularly the potential curve of the lowest triplet state is considered in this connection thereby substantiating quantitatively the proposed mechanism for induced dimerisation of C₄H₆. Possible assignments of the lowest singlet excited states in trans-butadiene are discussed.

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Zusammenfassung Mit einem Satz von Gaußfunktionen wurden ab initio SCF und CI Rechnungen für den Grundzustand und angeregte Zustände von fünf C₄H₆-Konformeren, die durch Drehung von cis-Butadien in trans-Butadien entstehen, durchgeführt. Die Potentialkurve für die innere Rotation von 1,3-Butadien wird für den Grundzustand und die Anregungszustände diskutiert. Besonders wird in diesem Zusammenhang die Potentialkurve des niedrigsten Triplettzustandes betrachtet, da hierbei der vorgeschlagene Mechanismus der induzierten Dimerisierung von Butadien quantitativ bestätigt wurde. Weiterhin werden mögliche Zuordnungen für die niedrigsten Singulettzustände von trans-Butadien diskutiert.

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Résumé Calculs SCF et CI ab initio en orbitales gaussiennes pour les états fondamentaux et excités de cinq structures géométriques de C₄H₆ intervenant au cours de la rotation du cis-butadiène à l'isomère trans. Les courbes de potentiel correspondant à la rotation pour l'état fondamental et les états excités sont l'objet d'une discussion. En particulier on examine la courbe correspondant à l'état triplet le plus bas confirmant ainsi quantitativement le mécanisme proposé pour la dimérisation induite de C₄H₆. Discussion des attributions possibles des états excités singulets les plus bas dans le trans-butadiène.

     

The barrier to internal rotation in metallocenes

It has been shown by calculation that the eclipsed forms of ferrocene and ruthenocene are more stable than their staggered forms. The main contribution to the energy difference is the induction energy of the metal in the potential field of the rings. Direct ring-ring electrostatic energy also favours the eclipsed forms by a small amount. The calculated barrier in ferrocene is in good agreement with an experimental estimate from electron diffraction.     

On the barrier to internal rotation in phosphineborane

An ab initio STO-3G calculation is carried out on PH₃BH₃. The electronic structure of the molecule is considered, and special attention is paid to the barrier to internal rotation.     

Normal coordinate analyses and barrier to internal rotation of nitroso- and nitroazides

The conformational and structural stability of nitrosoazide NNN-N=O and nitroazide NNN-NO2 were investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311++G** basis set. From the calculations, nitrosoazide was predicted to exist predominantly in the planar trans (NNN and N=O groups are trans to each other) structure with high trans-cis rotational barrier of about 11 kcal mol-1 as a result of pronounced conjugation between the azide group and the N=O bond. The NO2 rotational barrier in nitroazide was predicted from the symmetric potential function to be of about 7 kcal mol-1. The vibrational frequencies were calculated at the DFT-B3LYP level and the infrared and Raman spectra of the cis-trans mixture were plotted. Complete vibrational assignments were made on the basis of normal coordinate calculations for the stable conformers of both molecules. For nitrosoazide, the calculated wavenumbers were compared to the corresponding experimental values obtained from early reported Raman spectrum of the molecule.     

The barrier to internal rotation in Ge2H6

High-quality SCF-MO calculations yield a barrier to internal rotation of 1.70 kJ mole^?1 for Ge₂H₆. A new contraction scheme is reported for Dunning's larger Ge basis set. Geometry optimization, although carried out, is unimportant in this particular calculation. Wavefunctions and properties are reported for GeH₄ and the staggered and eclipsed conformers of Ge₂H₆. The magnitude of the calculated barrier is more physically reasonable than those deduced from experimental data. Comparisons are made across the two series X₂H₆ and CH₃XH₃.     

A study of the ethane internal rotation barrier

In an effort to deduce the source of the ethane internal rotation barrier, we have investigated the contributions of exchange energy and orthogonality: two effects that are required by the Pauli principle. Fully antisymmetrized, partially antisymmetrized and non-antisymmetrized optimized orbital product wavefunctions were determined. Results show that the exchange energy contribution to the barrier is negligible only when it is evaluated from energy-localized orbitals; even in this case the small total results from cancellation of large contributions. The barrier is apparently caused by the orthogonality that is required between CH orbitals on opposite ends of the molecule. The CC bond has insignificant participation in this effect.     

Microwave spectrum,dipole moment and barrier to internal rotation of trans-methyl glyoxal

The microwave spectrum of the trans conformer of methyl glyoxal has been investigated in the frequency range from 8 to 40 GHz. The rotational constants have been determined for the A state: A = 9102.4332(31), B = 4439.8832(27) and C = 3038.9404(22) MHz. Quantitative measurements of the Stark effect have yielded the components of the electric dipole moment: μ_a = 0.1597(11), μ_b = 0.9620(7) and μ_total = 0.9751(7) D. From the splittings of rotational transitions the three-fold barrier to internal rotation of the methyl top has been found to be V₃ = 269.1 (3) cm^?1.     

Microwave spectrum,barrier to internal rotation and dipole moment in 5-methyl-pyrimidine

The microwave rotational spectrum of 5-methyl-pyrimidine has been investigated in the region from 8 to 27 GHz, the three types of lines to be expected for a molecule of this symmetry and with a very low sixfold barrier hindering internal rotation of the methyl top have been found: m = 0; |m| ≠ 0,3; |m| = 3n. From the m = 0 (a-type transitions) the rotational constants A′ (less methyl top) = 6108.41, B = 2642.198, C = 1844.196 MHz and the dipole moment μ = 2.881D have been determined. From the wide splitting of the lines |m| = 3, |k| = 1 the potential barrier has been derived as V₆ = 11.73 cal/mole.     

Microwave spectrum,structure, dipole moment and barrier to internal rotation of methyltrifluorosilane

The rotational spectrum of methyltrifluorosilane in the ground and the first three excited states of the torsional mode have been investigated in the region of 12.5–40.0 GHz. The rotational transitions for the ¹³C isotopic species have also been measured. The following structural parameters have been determined: r(CH) = 1.081 ± 0.004 Å, ∠ HCSi = 111°1' ± 30', r(CSi) = 1.812±0.014 Å, r(SiF) = 1.574 ± 0.007 Å, ∠ FSiC = 112°20'± 1°6'. The structural parameters are compared to the corresponding ones for similar molecules. The dipole moment was determined to be 2.33 ± 0.10 D. From relative intensity measurements, the barrier to internal rotation was found to be 0.93 ± 0.09 kcal mol⁻¹; this value is consistent with the values obtained for other methylfluorosilanes.     

A nuclear magnetic resonance determination of the barrier to internal rotation in phenylethane

The long-range nuclear spin-spin coupling constant between the methylene protons and the ring protons at the para position in 3,5-dibromo-phenylethane in benzene solution is consistent with a two-fold barrier of 1.2 ± 0.1 kcal/mole to rotation about the sp²-sp³ carbon-carbon bond, in agreement with thermodynamic data on phenylethane and with deductions based on hyperfine interactions in related radical anions. The low-energy conformation has a plane of symmetry, the methyl group being situated out of the aromatic plane, in disagreement with Raman depolarization data interpretations and with deductions based on proton chemical shifts.     

Influence of molecular top deformations on the internal rotation barrier

The influence of molecular top deformations on the potential barrier is investigated. For some ethane-like molecules, it is shown that the deformation makes a pronounced contribution to the height of the torsion barrier. Orenburg State Pedagogical Institute. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 2, pp. 356–358, March–April, 1995. Translated by L. Smolina     

Probing the barrier for internal rotation of the retinal chromophore

Molecular ion calorimetry: A technique for measuring the heat capacity of an isolated gas-phase chromophore is presented and applied to the retinal protonated Schiff base. The potential use of this technique for studying barriers for internal rotations is discussed.     

CNDO calculation of the π-electronic structure and barrier to internal rotation in benzenesulphonic acid

The CNDO calculation of the charge densities and bond orders show a distinction between two extreme geometrical configurations of the benzenesulphonic acid, but the barrier to internal rotation of the sulphonic group about the C-S bond, calculated by the same method, has revealed that they are energetically equivalent.     

Microwave spectrum,conformation, barrier to internal rotation and dipole moment of pyruvic acid

Microwave spectra of CH₃COCOOH and CH₃COCOOD are reported. The preferred conformation of the molecule is demonstrated to possess a planar HCCOCOOH skeleton with two out-of-plane hydrogens. The two carbonyl groups are trans to each other and a weak five-membered hydrogen bond is formed between the carboxyl group hydrogen atom and the carbonyl group oxygen atom. The methyl group conformation is discussed. A computer programme based on “the principal axis method” is described in some detail and the results of a least squares analysis of the observed spectra are outlined. The barrier to internal rotation was determined as V₃ = 965±40 cal mol^?1 for both isotopic species. Stark effect measurements yielded μ_a = 2.27±0.02 D, μ_b = 0.35±0.02 D and μ_tot = 2.30±0.03 D for the dipole moment and its components along the principal axes.     

Gaussian molecular orbital calculations of the barrier to internal rotation in the ethyl cation

Extended Gaussian orbital basis set calculations have been carried out on an assumed staggered and eclipsed classical geometrical configuration of C₂H. The best total energies obtained for these geometries were ?78.170692 a.u. and ?78.170674 a.u. respectively, corresponding to a barrier to internal rotation of 1.8 × 10^?5 a.u. or 11 kcal/mole. An analysis of the charge density matrix indicates that charge is distributed in these molecules in a manner consistent with the concept of hyperconjugation.     

Torsional symmetry dependence of S1 dynamics in molecules that undergo methyl internal rotation

The single‐rovibronic‐level fluorescence of “intermediate‐case” molecules that undergo methyl internal rotation is strongly influenced by the torsional symmetry of the lowest excited singlet state (S₁). The most dramatic example of such symmetry dependence comes from our recent finding that the intensities of the e–e transitions in the high‐resolution S₁←S₀ fluorescence excitation spectra of jet‐cooled acetaldehyde become very weak relative to the a–a transitions at higher beam temperatures. In this study, we rationalize this remarkable torsional symmetry dependence of electronic relaxation in acetaldehyde on the basis of internal‐overall rotation coupling that leads to symmetry‐selective increase in the density of states for singlet‐triplet coupling. Related observations by others on aliphatic carbonyls and diazabenzenes are also discussed within the context of the coupling between the internal and overall rotation. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 167–176, 1999     

On the barrier to internal rotation in trichloromethyl trichlorogermane,Cl3CGeCl3

Consideration of the continuing successes of the theory of condensed phase isotope effects in the harmonic approximation leads to the conclusion that relative changes expected in anharmonic constants on condensation are constrained to be of the same order of magnitude or smaller than the corresponding changes in harmonic force constants. Benzene-deuterobenzene isotope efforts are analyzed in an illustrative example and a brief discussion of other cases, including CS₂ and CHCl₃, is given.     

Gas-phase Raman spectra and the potential energy function for the internal rotation of 1,3-butadiene and its isotopologues

The gas-phase Raman spectra of 1,3-butadiene and its 2,3-d(2), 1,1,4,4-d(4), and -d(6) isotopologues have been recorded with high sensitivity in the region below 350 cm(-1) in order to investigate the internal rotation (torsional) vibration. Based on more accurate structural information, the internal rotor constants F(n) were calculated as a function of rotation angle (?). The data for all the isotopologues were then fit using a one-dimensional potential energy function of the form V = (1)/(2)∑V(n)(1 - cos ?). Initial V(n) values were based on those generated from theoretical calculations. The agreement between observed and calculated frequencies is very good, although bands not taken into account were present in the spectra. The energy difference between the trans and gauche forms was determined to be about 1030 cm(-1) (2.94 kcal/mol), and the barrier between the two equivalent gauche forms was determined to be about 180 cm(-1) (0.51 kcal/mol), which agrees well with high-level ab initio calculations. An alternative set of assignments also fits the data quite well for all of the isotopologues. For this model, the energy difference between the trans and gauche forms is about 1080 cm(-1) (3.09 kcal/mol), and the barrier between gauche forms is about 405 cm(-1) (1.16 kcal/mol).