Theoretical study of the barriers to internal rotation in dimethyl ether,methoxysilane and disiloxane

The barriers to internal rotation have been computed by ab initio methods for CH₃OCH₃, CH₃OSiH₃ and SiH₃OSiH₃. It is shown that minimal basis set results agree fairly well with experimental results for CH₃OCH₃ and to a lesser extent for SiH₃OSiH₃. To predict a correct ordering of the CH₃ and SiH₃ rotation barriers in CH₃OSiH₃, a split valence basis set has to be used. Attention is also paid to the influence of geometry optimization on the barriers to internal rotation.     

Electronic states and barriers to internal rotation in silaallenes

The electronic states and barriers to internal rotation in allene ( 1a ), 1-silaallene ( 2a ), and 2-silaallene ( 3a ) are investigated computationally using ab-initio molecular orbital methods. Planar geometries with two-, three-, and four-π-electron configurations have been considered as possible transition states ( 1b–3d ). Structures have been optimized at the Hartree–Fock level with a small split valence basis set (3-21G) and higher level calculations with basis sets of split valence (6-31G ) and split valence plus polarization function (6-31G *) quality include correlation energy estimates from Møller–Plesset second-and third-order perturbation theory. The electronic barrier to internal rotation in allene is estimated near 53 kcal/mol whereas the corresponding barriers in 1-silaallene and 2-silaallene are considerably smaller, ca. 35 and 20 kcal/mol, respectively. The transition states are predicted to possess bent geometries in all three molecules with open-shell singlet, three-π-electron configurations in 1 and 2 ( 1c, 2c ) but a closed-shell singlet, two-π-electron configuration in 3 (3d) .     

Quantum mechanical calculations on barriers to internal rotation

SCF LCAO MO calculations are reported for the borazane molecule BH₃NH₃, for different values of the dihedral angle (0°, 20°, 40°, 60°). The lower energy was found equal to -82.59651 a.u. for the staggered form. The theoretical barrier height, 0.00524 a.u.=3.29 kcal/mole, is very close to the one, 0.00577 a.u., computed by Clementi for the ethane molecule. A study of the electronic density maps seems to indicate that some care should be exercised in the use of the population analysis. According to the criterion of Bader andal., the bond density map seems characteristic of ionic binding.

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Zusammenfassung Die Ergebnisse von SCF LCAO MO Rechnungen für Borazan werden mitgeteilt, und zwar für verschiedene Verdrillungswinkel. Die tiefste Energie zeigt die Konformation mit den auf Lücke stehenden H-Atomen. Die Energieschwelle für die innere Rotation ist mit 3.29 Kcal/mol fast genau so groß wie die (von Clementi berechnete) für Äthan.Das Bindungsdichte-Diagramm deutet bei Anwendung des Baderschen Kriteriums auf eine ionische Bindung hin. Eine Untersuchung der Elektronendichte legt ferner die Vermutung nahe, daß die Resultate einer Populationsanalyse mit Vorsicht betrachtet werden müssen.

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Résumé La molécule de borazane a été etudiée par la méthode SCF LCAO MO pour différentes valeurs de l'angle dièdre (0°, 20°, 40°, 60°). L'énergie la plus basse, égale à -82.59651 u.a., a été obtenue pour la conformation decalée. La barrière de rotation théorique, égale à 0.00524 u.a. = 3.29 kcal/mole, est très proche de la valeur 0.00577 u.a. obtenue par Clementi pour la molécule d'éthane. L'étude simultanée de l'analyse de population et des contours de densité isoélectronique semble indiquer qu'une certaine prudence s'impose dans l'interprétation des populations. La liaison B-N apparait comme ionique si l'on on utilise les critères de Baderet al.

     

Quantum mechanical calculations on barriers to internal rotation

The barrier to internal rotation in hydrazine has been studied by the non empirical SCF-LCAO method, in the gaussian approximation. Calculations have been performed for values 0°, 60°, 120°, 180° and 94° (equilibrium conformation) of the dihedral angle, with all other bond angles and bond lengths fixed. The gaussian basis set used consisted of 9s + 3p for nitrogen and 3s for hydrogen. The calculated total molecular energy for the equilibrium conformation, –111.030 a.u., is 0.865 a.u. higher than the experimental value. The theoretical dihedral angle 94° is in good agreement with experimental indications of 90–95°. The computed rotation barriers are 11.5 kcal/mole for the cis position and 4.7 kcal/mole for the trans.

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Zusammenfassung Die Rotationsbarriere von Hydrazin wurde mit Gaußfunktionen nach einer nicht-empirischen SCF-LCAO-Methode studiert. Rechnungen wurden für die Werte 0°, 60°, 120°, 180°, 94° (Gleichgewichtslage) des Diederwinkels durchgeführt, wobei alle übrigen Bindungswinkel und -längen festgehalten wurden. Der Basissatz von Gaußfunktionen bestand aus 9s- und 3p Funktionen für Stickstoff und 3s-Funktionen für Wasserstoff. Die berechnete Gesamtenergie der Gleichgewichtskonformation, –111,030 at. E. liegt um 0,865 at. E. höher als der experimentelle Wert. Der theoretische Diederwinkel von 94° stimmt gut mit den experimentellen Daten von 90–95° überein. Die berechneten Rotationsbarrieren sind 11,5 kcal/mol für die cis- und 4,7 kcal/mol für die trans-Lage.

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Résumé La barrière de rotation interne de l'hydrazine a été étudiée par la méthode LCAO-SCF dans l'approximation des orbitales gaussiennes. Les calculs ont été effectués pour des valeurs de l'angle dièdre de 0°, 60°, 120°, 180° et 94° (valeur correspondant à l'équilibre), tout en gardant constants les autres angles et les longueurs des liaisons. On obtient pour la position d'équilibre une énergie moléculaire totale de –111.030 u.a., l'écart avec l'énergie expérimentale étant de 0.865 u.a. La valeur obtenue pour l'angle dièdre, 94°, est en bon accord avec les indications expérimentales de 90–95°. Les barrières de rotation théoriques sont de 11.5 kcal/mole pour la position cis et de 4.7 kcal/mole pour la position trans.

     

Potential barriers of internal rotation in halogen-substituted ethanes

A simple model is proposed for calculating torsional vibrational frequencies and torsion potentials in ethane-type molecules. The model is based on internuclear coulomb electrostatic interactions of the end atoms in molecular groups undergoing rotations. The effective charges on the nuclei are considered to be the model parameters. The results of calculations show that the model can reliably reproduce the frequencies of torsional vibrations for rotational conformers and that the effective charges are transferable among halogen-substituted ethanes, both symmetrical and asymmetrical.Orenburg Pedagogical Institute. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 1, pp. 39–43, January–February, 1993.     

Ab initio study of the internal rotation in peroxyformic acid

The internal rotation in peroxyformic acid was investigated using the ab initio SCF MO LCAO method with the STO-3G and 4-31G basis-sets and with experimental and optimum values of the geometrical parameters. Both basis sets yield a rather flat double-minimum potential for the COOH torsion, the trans planar form being lightly preferred in comparison with the cis one. The effects on the internal rotation of the geometry relaxation, of the intramolecular hydrogen bond and of the methyl group of peroxyacetic acid are also discussed.     

Far-infrared spectra and barriers to internal rotation of ethyl nitrate

The far-infrared spectra of gaseous and solid ethyl nitrate, CH₃CH₂ONO₂, have been recorded from 500 to 50 cm⁻¹. The fundamental asymmetric torsion of the trans conformer which has a heavy atom plane has been observed at 112.50 cm⁻¹ with two excited states failing to lower frequencies, and the corresponding fundamental torsion of the gauche conformer was observed at 109.62 cm⁻¹ with two excited states also falling to lower frequencies. The results of a variable temperature Raman study indicate that the trans conformer is more stable than the gauche conformer by 328 ± 96 cm⁻¹ (938 ± 275 cal mol⁻¹). An asymmetric potential function governing the internal rotation about the CH₂O bond is reported which gives a trans to gauche barrier of 894 ± 15 cm⁻¹ (2.56 ± 0.04 kcal mol⁻¹) and a gauche to gauche barrier of 3063 ± 68 cm⁻¹ (8.76 ± 0.20 kcal mol⁻¹) with the trans conformer more stable by 220 ± 148 cm⁻¹ (0.63 ± 0.42 kcal mol⁻¹). Transitions arising from the symmetric CH₃ and NO₂ torsions are observed for both conformers, from which the threefold and twofold periodic barriers to internal rotation have been calculated. For the trans conformer the values are 1002 cm⁻¹ (2.87 kcal mol⁻¹) and 2355 ± 145 cm⁻¹ (6.73 ± 0.42 kcal mol⁻¹) and for the gauche conformer they are 981 cm⁻¹ (2.81 kcal mol⁻¹) and 2736 ± 632 cm⁻¹ (7.82 ± 1.81 kcal mol⁻¹) for the CH₃ and NO₂ rotors, respectively. These results are compared to the corresponding quantities for some similar molecules.     

Steric and electronic contributions to barriers of internal rotation in 1,8-dibenzoylnaphthalenes

Restricted rotation about the naphthalenylcarbonyl bonds in the title compounds resulted in mixtures of cis and trans rotamers, the equilibrium and the rotational barriers depending on the substituents. For 2,7-dimethyl-1,8-di-(p-toluoyl)-naphthalene (1) ΔH° = 3.66 ± 0.14 kJ mol^?1, ΔS° = 1.67 ± 0.63 J mol^?1 K^?1, ΔH^≠_ct = 55.5 ± 1.3 kJ mol^?1, ΔH^≠_ct = 51.9 ± 1.3 kJ mol^?1, ΔS^≠_ct = ?41.3±4.1 J mol^?1 K^?1 and ΔS^≠_ct = ?42.9±4.1 J mol^?1 K^?1. The rotation about the phenylcarbonyl bond requires ΔH^≠ = ?56.9±4.4 kJ mol^?1 and ΔS^≠ = ?20.5±15.3 J mol^?1 K^?1 for the cis rotamer, and ΔH^≠ = 43.5Δ0.4 kJ mol^?1 and ΔS^≠ =± ?22.4Δ1.3 J mol^?1 K^?1 for the trans rotamer. The role of electronic factors is likely to be virtually the same for both these rotamers but steric interaction between the two phenyl rings occurs in the cis rotamer only. Hence, the difference of the activation enthalpies obtained for the cis and trans rotamers, ΔΔH^?1 = 13.4 kJ mol^?1, provides a basis for the estimation of the role of steric factors in this rotation. For the tetracarboxylic acid 2 and its tetramethyl ester 3 the equilibrium is even more shifted towards the trans form because of enhanced steric and electrostatic interactions between the substituents in the cis form. The barriers for the rotation around the phenylcarbonyl bond and the cis-trans isomerization are lowered; an explanation for this result is presented.     

Investigation of the barriers to retarded internal rotation in N-acyl-1-methyl-1,2,3,4-tetrahydroisoquinolines

The effect of a number of alkyl substituents attached to the carbon atom of the acyl group on the activation parameters of retarded internal rotation about the C-N bond in N-acyl-1-methyl-1,2,3,4-tetrahydroisoquinolines was studied. Correlation of the free energies of activation G ₂₉₈ with the steric factors (Es) and the ^* substituent constants was examined.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 502–505, April, 1984.     

Theoretical analysis of the internal rotation in aminoborane and borylphosphine

Using a recently proposed orbital deletion procedure and the block-localized wavefunction method, the rotational barriers in H₂BNH₂ and H₂BPH₂ are analyzed in terms of conjugation, hyperconjugation, steric effect and pyramidalization. With the zero-point energy corrections, the π-binding strengths in the planar H₂BNH₂ and H₂BPH₂ are both around 20 kcal/mol at the HF level using the 6-311+G** basis set. With the deactivation of the π atomic orbitals on the boron atom and the evolution from a planar structure to a 90°-twisted structure, the steric repulsion between the B‐H and the N‐H or P‐H is relieved and moreover, the negative hyperconjugation from the lone electron pair or pairs on the nitrogen or phosphorus atoms to the antibonding orbital ^χ* _B H ₂ of the BH₂ group stabilizes the twisted structure by 7.4(8.8) or 4.0(5.0) kcal/mol at the HF/6-31G*(6-311+G**) level. However, the repulsive interaction between the lone pair(s) and the two BH σ bonds is so prominent that the overall steric effect contributes 20.3(22.9) and 19.3(19.8) kcal/mol to the rotational barriers in H₂BNH₂ and H₂BPH₂ with the 6-31G*(6-311+G**) basis set. The present techniques and analyses may also give some clues to justify the parameterization in the empirical molecular mechanics methods. Received: 17 April 1998 / Accepted: 17 September 1998 / Published online: 1 February 1999     

Semiempirical calculations of internal barriers to rotation and ring puckering: II. Amindo/3 study

The MINDO/3 technique is examined for its applicability to rotational barriers and ring puckering of strained-ring compounds. Comparisons are made with MINDO/2' and INDO on the compounds ethane, acetone, isobutylene, 3-oxetanone, cyclobutane, cyclobutanone, methylenecyclobutane, and 1,1-difluorocyclobutane. The MINDO/3 method improperly predicts all ring compounds to energetically favor the planar conformation as does its predecessor. Some improvement in bond lengths and bond angles is observed by MINDO/3 but the rotational barriers are still underestimated.     

Conformational analysis,barriers to internal rotation and vibrational assignment for dimethylethylamine

The IR (50–3500 cm^?1) and Raman (20–3500 cm^?1) spectra have been recorded for gaseous and solid dimethylethylamine. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. Due to the fact that three distinct Raman lines disappear on going from the fluid phases to the solid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers in the fluid phases with the gauche conformer being more stable and the only one present in the spectra of the unannealed solid. From the temperature study of the Raman spectrum of the liquid a rough estimate of 3.9 kcal mol^?1 has been obtained for ΔH. Relying mainly on group frequencies and relative intensities of the IR and Raman lines, a complete vibrational assignment is proposed for the gauche conformer. The potential functions for the three methyl rotors have been obtained, and the barriers to internal rotation for the two CH₃ rotors attached to the nitrogen atom have been calculated to be 3.51 and 3.43 kcal mol^?1, whereas the barrier for the CH₃ rotor of the ethyl group has been calculated to be 3.71 kcal mol^?1. The asymmetric torsional mode for the gauche conformer has been observed in both the IR and Raman spectra of the gas at 105 cm^?1 with at least one hot band at a lower frequency. Since the corresponding mode has not been observed for the trans conformer, it is not possible to obtain the potential function for the asymmetric rotation although estimates on the magnitudes of some of the terms have been made. Significant changes occur in the low-frequency IR and Raman spectra of the solid with repeated annealing; several possible reasons for these changes are discussed and one possible explanation is that a conformational change is taking place in the solid where the trans form is stabilized by crystal packing forces. These results are compared to the corresponding quantities for some similar amines.     

Conformations and barriers to internal rotation in trans-diarylethylenes: Theoretical investigation using a new INDO-type method (C-INDO)

C-INDO, a new INDO-based technique specially devised for conformational studies of conjugated systems, is used to investigate conformational equilibria in ground state trans-diarylethylenes, as revealed by a number of emission spectroscopic observations (both steady-state and time resolved). Conformations, ΔE and barriers to internal rotation are provided for all the possible rotamers of 1-StN (1-styrilnaphthalene), 2-StN, 2,2′-DNE (2,2′-dinaphthylethylene) and 1,2′-DNE. Energetic parameters are used to predict the relative abundances of the distinct rotameric species at equilibrium as well as to estimate the rate at which equilibria are established (at different T). The results prove to be a consistent basis on which experimentally observed behaviour can be rationalized.     

Theoretical study of ketenimine: Geometry,electronic properties,force constants and barriers to inversion and rotation

Non-empirical calculations of the structure and properties of ketenimine have been performed using nine Gaussian basis sets. Values for the bond lengths and angles, HOMO and LUMO energies, atomic charges, overlap populations, dipole moments, bond energies, force constants and barriers to nitrogen inversion and internal rotation are predicted.     

Conformational stability of peroxynitric acid molecule and the barriers to internal rotation about the O−O and N−O bonds

The structure of conformers and potential curves of the internal rotation (PCR) about the O?O and N?O bonds in peroxynitric acid (PNA) were calculated by the unrestricted Hartree-Fock-Roothaan method. The standard valence-split 6–31G and 6–31G^* basis sets were used. The presence of two maxima on every curve has been shown. To refine the values of barriers to the internal rotation in the regions of minima and maxima of PCR, calculations taking into account the electron correlation energy have been carried out at the second- and fourth-order Møller-Plesset level of perturbation theory (MP2 and MP4, respectively). At the MP4/6-61G^* level of approximation, the barriers to the rotation about the O?O bond are equal to 8.6 kJ mol^?1 and 14.7 kJ mol^?1, and both barriers to the rotation about the N?O bond are equal to 33.5 kJ mol^?1. The results are compared with those published for PCR in hydrogen peroxide and peroxynitric acid.     

Theoretical model of internal rotation in monosubstituted derivatives of furfural

The present work explores the effect of substitution in all free positions of furfural on conformational preferences of formyl group by using ab-initio calculations at the MP2/6-31G(p,d) level of theory. Theoretical modeling was made in vacuo. The selected substituents were -CH(3), NH(2), NO(2) and F groups in 3, 4, 5 and ipso carbonyl positions. Geometries of all derivatives were analyzed and it is ascertained that substitution has not important consequences on furan ring geometry. Differences of energy between OO-cis and trans conformers and energy barriers between them are described and extreme cases are explained. Interesting features appear in the cases of -NH(2) and -NO(2) groups, and particularly when the 3 and ipso carbonyl positions are substituted. Variations in energy barriers are correlated with variations in C2-C6 distances for the transition states and planar forms. Substitution effect on Mülliken charges are analyzed and related with internal rotation energy barriers and differences between conformers.     

Microwave spectra and barriers to internal rotation of z- and e-1-propenyl isocyanide

A synthetic procedure yielding a mixture of Z- and E-1-propenyl isocyanide (CH(3)CH═CHNC) is described. The microwave spectrum of this mixture has been recorded in the 12-100 GHz spectral range, and the spectra of the Z and E isomers have been assigned for the first time. Most transitions of the Z form were split into two components of equal intensity due to tunneling of the methyl group, which allowed the barrier to internal rotation of this group to be determined as 4.0124(12) kJ/mol by fitting 568 transitions with a maximum value of J = 46 using the computer program Xiam. This fit had a root-mean-square deviation as large as 4.325. The same transitions were therefore fitted anew using the more sophisticated program Erham. This fit has a rms deviation marginally better (4.136) than the Xiam fit. No split MW lines were found for E-1-propenyl isocyanide. The absence of splittings is ascribed to a barrier to internal rotation of the methyl group that is significantly higher than the barrier of the Z isomer. It is concluded that the barrier must be larger than 6 kJ/mol for the E form. The experimental work was augmented by quantum chemical calculations at CCSD/cc-pVTZ, B3LYP/cc-pVTZ, and MP2/cc-pVTZ levels of theory. The CCSD method predicts rotational constants of the Z and E forms well. The B3LYP barriers to internal rotation of a series of substituted propenes were calculated and found to be in good agreement with experiments. Calculations of the quartic centrifugal distortion constants of the two 1-propenyl isocyanides by the B3LYP and MP2 methods were less successful.