Potential functions of internal rotations in n-mononitroalkanes

Internal rotation is studied in n-mononitroalkanes, CH₃(CH₂) _n NO₂, n ?? 7, with the use of the B3LYP/6-311++G(3df, 3pd) and MP2/6-311++G(3df, 3pd) models. Conformations are investigated, and 36 potential functions (V(??)) for all rotations are found. It is shown that as the hydrocarbon chain becomes longer, the potential functions that describe particular molecular fragments change only negligibly beginning from a certain n. Such dependences of V(??) are presented in the form of generalized functions with averaged coefficients (V(??)_av). The V(??)_av values can be recommended for use in simulations. In molecules with n ?? 2, the phenomenon of ??top interactions?? (??movement interactions??) and the gauche effect (the equality between the total energies of gauche ⁺, gauche ^?, and trans-conformers) are observed.     

Potential functions of internal rotation in symmetrical ortho-unsubstituted biphenyls

K. A. Timiryazev Moscow Agricultural Academy. Translated from Zhurnal Strukturnoi Khimii, Vol. 28, No. 4, pp. 56–61, July–August, 1987.     

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.     

Potential function of the internal rotation of biphenyl

The form of the potential function of the internal rotation of the biphenyl molecule has been found for various states of aggregation and temperatures on the basis of an analysis of experimental data. Some chemical and physicochemical characteristics of biphenyl have been discussed.The new publications [1, 2], in which a similar problem was solved for the biphenyl molecule (in the vapor phase), appeared during the preparation of the present communication for press; the conclusions expressed in these publications are in general agreement with our calculations.Translated from Teoreticheskaya i Eksperimental' naya Khimiya, Vol. 24, No. 2, pp. 197–203, March–April, 1988.     

Potential energy calculations for the double internal rotation in acetone and dimethylamine

In the present work, the problem of the determination of the potential energy surface of double rotor molecules is examined in the case of acetone and dimethylamine. From the symmetry adapted functional form for the potential of acetone that of dimethylamine is deduced and the minimum number of conformations to be calculated is derived in order to have a reliable surface (minimal expansion). The potential energy functions for acetone and dimethylamine are then determined using different standard procedures. Special emphasis is put on the electronic correlation effects in the calculation. It is found that these effects significantly improve the potential energy function.     

Distribution of electronic density in n-mononitroalkanes

In the framework of ??quantum theory of atoms in molecule?? (QTAIM) was distribution of electronic density analyzed and inductive effect studied in trans- and gauche-isomers of n-mononitroalkanes CH₃(CH₂) _n NO₂ with n ?? 8. Influence of NO₂ is demonstrated to spread on the four neighboring NO₂ groups. Association of gauche-effect with the electronic density distribution has been given adequate consideration. Definitions were formulated for the standard NO₂ group and perturbed CH₂ groups. It has been suggested the refinements of additive procedures for properties calculation of n-nitroalkanes as well as of big molecules containing the nitroalkane substituents.     

Potential function of internal rotation for isoprene from ab initio calculations and experimental data

Ab initio computations of the potential energy curve of internal rotation around the central single C---C bond of isoprene have been performed at the Hartree—Fock level with a 3·21G basis set. The similarity of the slope of the curve obtained and the potential energy curve calculated for a more complete basis set (7s3p/4s2p) [Kavana-S2ebø, J. Mol. Struct. 106 (1984) 259] is discussed. The values of the Pitzer function F(φ), its Fourier expansion coefficients, and coefficients of the potential energy expansion were calculated from data given in the above reference. The correction of the potential energy expansion coefficients was carried out from frequencies of torsional “hot” bands of isoprene and torsional overtone of its second rotational isometric form. It was shown that the isoprene second isomer is realized as a gauche-form. The potential energy expansion coefficients were obtained as follows: V₁ = 399.9, V₂ = 1330.22, V₃ = 781.8 and V₄ = −175.8 cm⁻¹.     

Potential functions of internal rotation in <Emphasis Type="Italic">n</Emphasis>-alkanes and its contribution to thermodynamic properties

The potential functions of braked internal rotation V(?) in n-alkanes (ethane, propane, butane, n-pentane, n-hexane, n-heptane) were calculated by ab initio and DFT methods with the 6-311++G(3df,3pd) basis set. The functions were approximated as a series of six cosines. The dependences of V(?) on the length of the hydrocarbon chain in n-alkanes were analyzed. The heights of the trans-cis and trans-gauche barriers and the differences between the energies of the trans and gauche conformers were calculated and compared with the experimental data. From the calculated geometric parameters and V(?), the contributions of the braked internal rotation to the enthalpy, entropy, heat capacity, and Gibbs free energy at 298 K were determined. The contributions of internal rotations are transferable within the framework of additive approaches. The generalized function V _av(?) for n-alkanes and averaged contributions of internal rotation of the C-C bonds and CH₃- and -CH₂- tops to the thermodynamic properties were suggested.     

Polarized Raman spectra,thermodynamic functions and bidders to internal rotation of 2,3-dimethoxy toluene

Polarized Raman spectra of 2,3-dimethoxy toluene have been recorded in the region 50–4000 cm⁻¹ and IR spectra in the region 200–4000 cm⁻¹. All the 63 (40a′ + 23a″) normal modes of vibration have been assigned assuming a C_s point group. Consistent assignments for the internal modes of vibration of methyl (CH₃) and methoxy (OCH₃) groups have been proposed. In addition thermodynamic functions have been computed over the temperature range 100–1500 K on a MIGHTY II computer and barriers to internal rotations for the three methyl (CH₃) tops and the two methoxy (OCH₃) tops about their respective axes have been determined, using the assigned torsional frequencies and assumed structural parameter for the 2,3-dimethoxy toluene. The barrier heights have been found to be greater than 2.5 kcal mol⁻¹ for all five tops.     

The internal rotation potential functions of the methacryloyl chloride molecule in the ground and excited electronic states

The systems of torsional vibration levels of the trans and cis methacryloyl chloride isomers in the ground (S ₀) and excited (S ₁) electronic states obtained by analyzing the vibrational structure of the gas-phase UV spectrum were used to reproduce the internal rotation potential functions of the molecule in both electronic states. The kinematic F factor in the S ₀ and S ₁ electronic states was calculated taking into account the relaxation of geometric parameters depending on the internal rotation angle. The internal rotation potential function parameters in the S ₀ state are substantially different from the parameters obtained using the torsional levels of the IR Fourier transform spectrum; at the same time, they are substantiated by quantum-mechanical calculations.     

Adiabatic approximations to internal rotation

A number of subtle and confusing issues are addressed concerning large amplitude motion (LAM) coordinates (chi) for internal molecular motions, using the methyl rotation in acetaldehyde (CH(3)CHO) as a model problem. If the LAM coordinate is chosen to be one of the H-C-C-O dihedral angles rho(1), rho(2), or rho(3), it lacks the required 2pi3 periodicity, and its use is thus undesirable. An excellent local internal coordinate for this model problem is tau(3)=13(rho(1)+rho(2)+rho(3)-2pi). A similarly good but nonlocal coordinate for the adiabatic approximation of internal rotation is provided by the intrinsic reaction coordinate s. Comparison of the mass-independent V(0)(tau(3)) and the mass-dependent V(0)(s) internal rotation curves shows that the two are virtually identical for the parent isotopolog of acetaldehyde. A unified internal coordinate projection scheme for determining complementary vibrational frequencies and subsequently V(ZPVE)(chi) along a path for LAM has been formulated, where V(ZPVE)(chi) is the zero-point vibrational energy correction to the internal rotation curve. In addition to its simplicity, the projection scheme developed for a distinguished reaction path generated by constrained optimizations is appealing because the vibrational frequencies along the LAM path are invariant to chemically meaningful choices of the internal coordinates for the complementary modes.     

Molecular design of correlated internal rotation

Disrotatory coupling of the internal rotational degrees of freedom in double rotor molecules has been studied for bis(9-triptycyl)methanes (Tp₂CH₂) and bis-(9-triptycyl) ethers (Tp₂O). These molecules undergo rapid internal rotation in a strictly gear-meshed fashion, giving rise to new stereoisomerism in the 2, 2′- and 3,3′-dichloro derivatives. The racemic and meso isomers which arise due to different phase relationship between the substituted benzene rings were separated by HPLC. The high energy barrier to the gear-slipping process was obtained as 32 - 33 and 42 - 43 kcal mol^?1 for Tp₂CH₂ and Tp₂O, respectively, by the isomerization study. The low energy barrier to the gear-meshing process was determined to be ca. 4.5 kcal mol^?1 by analyzing the exciplex fluorescence dynamics for a Tp₂O carrying the naphthalene chromophore on one Tp unit and the tert-amino group on the other. The molecular structure of Tp₂O has been determined by an X-ray analysis; the molecule adopts a bevel gear-shaped structure with the C_s point group in which the CO bond lengths average 1.412 Å and the ∠COC angle is widened to 135.8°. An attempt at reproducing the observed structural features by means of an empirical force-field calculation is described. A possible extension of the concept and further results on correlated internal rotation and phase isomers are discussed.     

Ab initio calculation of the potential functions for internal rotation around the CS bonds in simple ethene thiols

Ab initio calculations with complete geometry optimisation have been used to study internal rotation in compounds of the type XCH = CHSH, X = CN, H, CH₃ and F, with X located trans to the sulphur atom. Potential functions for the CS torsion have been obtained for each case and it has been established that the dominant framework changes accompanying internal rotation in these molecules involve the CCS angle and CS bond length. Furthermore, it has been shown that the nature of the substituent X significantly affects the molecular conformation of the SH group. The observed trends are discussed in terms of a simple model.     

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.