NMR study of molecular motion in oriented long-chain alkanes. III. Oriented n-C32H66

The NMR second moment of a uniaxially oriented mat of single crystals of n-C₃₂H₆₆ (in the orthorhombic form) was measured at temperatures from ?170°C to 70°C and at various alignment angles γ between the orientation axis (preferential direction of the molecular chains) and the NMR magnetic field. Accurate expressions are given for the NMR second moment of an orthorhombic normal paraffin C_nH_2n+2 of arbitrary molecular chain length n for n ≥ 10, in the following states of molecular motion: no motion (a rigid lattice), rotation of CH₃ groups, and rotation of the chains around their axes with superimposed rotation of CH₃ groups. In addition to these well-known motions, n-C₃₂H₆₆ is found to exhibit an α process. The corresponding decrease of the NMR second moment shows the dependence on γ predicted for “flip-flop” motion, i.e., rotational jumps of the chain molecules around their axes through 180° and a simultaneous translation along these axes by one CH₂ group. The overall decrease in second moment occuring at the transition to the hexagonal rotator phase in n-C₃₂H₆₆ can be quantitatively accounted for. The dependence of this decrease on the alignment angle γ, however, is in disagreement with calculations based on a simple rotation of the chains around their axes. Considerable torsion of the chains superimposed on the rotation would improve agreement between theory and experiment.     

Estimation of electrooptical parameters of EХ<Subscript>3</Subscript> molecules in ground and excited states from empirical correlation equations

Basing on the experimental data, it has been shown that for EХ₃ (E = N, P, As, or Sb; X = F, Cl, Br, I, CH₃, С₆Н₅, or SiH₃) compounds the value of the difference between first ionization potentials of atoms and molecules is a function of the bond angle and dipole moment of the molecule. The calculated values of dipole moments of charged atoms and groups are regularly changed within the considered compounds series. The change of the dipole moment of the molecules during the vibrational excitation has been determined, and intensities of symmetric stretching and deformation bands in IR spectra of EF₃ and ECl₃ compounds have been estimated.     

Molecular mechanics assessment of the configurational statistics of polyoxyethylene

Previous investigators have shown that statistical mechanical averages for configuration-dependent physical properties of long unperturbed polyoxyethylene chains are sensitive to the gauche–trans energy difference for rotation about C? C bonds. Agreement between theory and experiment could be obtained only by significant adjustment of this energy away from values predicted by semiempirical conformational energy computations. The present work examines the success of MM 2 in evaluating conformational properties of long unperturbed polyoxyethylene chains. Calculations are performed which identify the rotational isomers, and their energies, for the indicated bonds in CH₃OCH₂CH₂O? CH₂? CH₂? OCH₂CH₂OCH₃. These energies are used to assign statistical weights utilized in the configuration partition function for a rotational isomeric state chain with symmetric threefold interdependent rotations. The customary generator matrix scheme is employed to evaluate the mean-square unperturbed end-to-end distance, mean-square unperturbed dipole moment, and their temperature coefficients. Contrary to computational schemes employed previously, MM 2 is found to provide an estimate of the gauche–trans energy difference for rotation about C? C which is in harmony with the known dimensions and dipole moments of the unperturbed polymer. MM 2 also provides good estimates for most of the other parameters required in the rotational isomeric state treatment. A notable exception is provided by the gauche–trans energy difference for rotation about the C? O bond. This energy difference is overestimated by MM 2.     

Dipole moment derivatives from MINDO/3 semi-empirical MO calculations

Dipole moment derivatives for CO, NO, CO₂, H₂O, HCN, BF₃, CH₄, C₂H₄, C₂H₆, CH₃F, F₂CO and H₂CO molecules have been evaluated using MINDO/3 MO calculations. The values are compared with those obtained by other semi-empirical MO methods.     

Electronic structure and vibrational spectrum of acetylthiocarbamide

An approximate quantum chemical optimization of the geometric parameters of the acetylthiocarbamide molecule CH₃CONHCSNH₂ was carried out using the MNDO/H approximation. Bond lengths, bond angles, enthalpy of formation, total energy, ionization potential, and dipole moment were estimated, and the effective charges on the atoms and the bond orders were calculated. An analysis of the normal vibrations of the acetylthiocarbamide molecule and its deuteroanalog CH₃CONDCSND₂ was carried out. The force fields have been estimated. The frequencies, potential energy distribution among the vibrational coordinates, and the frequencies for the partially and completely deuterated acetylthiocarbamide molecules have been calculated.A. A. Baikov Institute of Metallurgy, Russian Academy of Sciences. A. A. Sechenov Moscow Academy of Medicine. N. S. Kurnakov Institute of General and Inorganic Chemistry. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 2, pp. 58–65, March–April, 1993.     

Influence of substituent electronegativities on conformational preferences in 1,2-disubstituted ethanes

Ab initio molecular orbital theory has been used to examine internal rotation in the 1,2-disubstituted ethanes, CH₃CH₂—CH₂Li, LiCH₂—CH₂Li and LiCH₂—CH₂F. Interpretation of the rotational potential function is facilitated by decomposition into Fourier components. The rotational potential functions for the three molecules are representative of three distinct classes of conformational behaviour which are characterized by the electronegativities of the substituents. A striking individual result is the preferred cis eclipsed conformation for LiCH₂—CH₂F.     

Calculation and properties of non-orthogonal,strictly local molecular orbitals

A general procedure to calculate non-orthogonal, strictly local molecular orbitals (NOLMOs) expanded using only a subset of the total basis set is presented. The energy of a single determinant wave function is minimised using a Newton-Raphson approach. Total energies and barriers to internal rotation for CH₄, NH₃, H₂O, CH₃CH₃, CH₃NH₂, CH₃OH, NH₂NH₂, NH₂OH and HOOH, and certain properties of the NOLMOs present in these molecules, are investigated using the 4-31G basis set.     

An ab initio evaluation of the role of p,π interaction: I. Ethylene derivatives

The RHF/6-311G(d) and MP2/6-311G(d) calculations with full geometry optimization were performed for XCH=CH₂ molecules (X = F, Cl, Br, CH₃, CH₂CH₃, CH₂F, CHO). The p _y electron density distribution in these molecules and the bonding molecular orbitals formed by the p _y orbitals of atoms of the planar fragment of these molecule (atomic orbitals whose symmetry axes are perpendicular to this plane) are not determined by the p,π conjugation between the lone electron pair of the heteroatom in substituent X and π electrons of the C=C bond. Changes in the population of the p _y orbitals of the halogen and carbon atoms in going from X = F to X = Cl and Br are not associated with changes in the extent of this p,π interaction. Taking into account the electon correlation in the MP2 method does not noticeably alter the features of the electron density distribution in these molecules estimated by restricted Hartree-Fock calculations.     

Determination of ion association constants by means of IR absorption band intensities of the molecules forming the first coordination spheres of the cations

The IR spectra of the binary systems CH₃CNLiI, CH₃CNLiClO₄ and CH₃CNLiNCS in the regions of the stretching vibrations of CC and CN bonds of acetonitrile have been recorded. The association constants have been determined on the basis of the analysis of the IR absorption band intensities of acetonitrile molecules. The values of the absorption coefficients of the molecules forming the first coordination spheres of Li⁺ cations have been estimated.     

Calculation of the magnetic properties of C-H bonds and their contribution to the magnetic susceptibility of saturated hydrocarbons

The magnetic susceptibility tensors of the C-H bonds in the CH₄, C₂H₆, C₃H₈, n-C₅H₁₂, iso-C₅H₁₂, and neo-C₅H₁₂ molecules have been calculated by the method of varying the vector potential with the use of a multiparameteric gradient-transformation function, which was constructed in the form of a polynomial in spherical coordinates and takes into account the electron correlation. The influence of the electron correlation on the magnetic properties of C-H bonds is greater than that in homonuclear molecules (H₂) and bonds (C-C) and increases strongly with enhancement of the electron correlation in the wave function not perturbed by an external magnetic field. In contrast to the previously established identity of the magnetic properties of C-C bonds, the magnetic properties of C-H bonds depend both on the structure and geometry of the molecule as a whole and on the location of the bond itself in the molecule. The values of the mean susceptibility of the molecules considered calculated in the additive approximation are in good agreement with the experimental values.     

Interpretation of vibrational absorption intensities: Effective bond charges from rotation free atomic polar tensors

In the present paper a new theoretical framework for analysis of vibrational intensities is presented. In a mathematically straightforward procedure corrected for various rotational contributions, atomic polar tensors are transformed into quantities termed effective bond charges. All rotational contributions to dipole moment derivatives are appropriately considered and eliminated from the equations. The effective bond charges are expected to reflect in a generalized manner polar properties of the valence bonds in molecules. Aside from the usual harmonic approximation no other constraints are imposed on the dipole moment function. The application of the formulation developed is illustrated with calculations employing atomic polar tensors for H₂O, NH₃, CH₄, C₂H₄, C₂H₂ and CH₂O obtained from RHF/6-31G** ab initio calculations.     

Experimentally determined and calculated values of the energies required to form doubly charged ions of the bromomethanes CH3Br,CH2Br2 and CHBr3

Two experimental techniques were used to determine the double ionization energies of CH₃Br, CH₂Br₂ and CHBr₃. In one, these energies were measured directly by double-charge-transfer spectroscopy. In the other, charge stripping of [CH₃Br]⁺, [CH₂Br₂]⁺ and [CHBr₃]⁺ ions was investigated and the ionization energies of the singly charged ions were measured. The double ionization energies of the molecules obtained by adding known single ionization energies of the molecules to the single ionization energies of the ions were in good agreement with those determined by double-charge-transfer spectroscopy. The relevant mean values from the two techniques were 28.9 ± 0.5, 27.5 ± 0.5 and 29.1 ± 0.5 eV for the double ionization energy of CH₃Br, CH₂Br₂ and CHBr₃, respectively. The results of ab initio calculations using second-order Møller-Plesset perturbation theory were in good agreement with the observed double ionization energies; they were consistently slightly lower than the experimental values.     

Potential Functions for Internal Rotation about the Csp2-O Bond and Intramolecular Interactions in p-RC6H4OCH3 Compounds

Potential functions for internal rotation about the Csp²ÄO bond in p-RC₆H₄OCH₃ compounds (R = NH₂, OCH₃, CH₃, H, F, Cl, CN, NO₂) were determined by nonempirical quantum-chemical calculations in the HF/6-31G* and MP2/6-31G* approximations with account taken of correlation energy for all electrons. The molecular conformation is planar. The height of the rotation barrier changes, depending on the electronic effect of para-substituent. Electron-donor substituents reduce while electron-acceptor substituents enhance the stability of the planar conformation. Using the natural bond orbital (NBO) approach, the nature of lone electron pairs on the methoxy oxygen atom was analyzed, and the energies for their resonance interaction with the antibonding aromatic * orbitals were determined. The effect of para-substituent on the electron density distribution over the methoxy group and on the Koopmans first ionization potentials was estimated. Geometric parameters of the molecules under study are given.     

Structure and internal rotation in molecules Cl<Subscript>3</Subscript>P=NCOCX<Subscript>3</Subscript> (X = F and Cl)

Applying the quantum-chemical calculations by MP2/6-31G* method molecular structures of trichlorophosphazenes Cl₃P=NCOCX₃ with X = F and Cl were determined and barriers to intramolecular reorientation of PCl₃ group relative to P=N bond and of CF₃ and CCl₃ groups relative to C-C bond were estimated. The values of potential barriers to internal rotation calculated for isolated molecules were compared with the values of the barriers obtained from the data of NQR and NMR spectroscopy of the compounds in the crystalline state. The structural and dynamic features of the studied molecules are discussed.     

Structure and rotational isomerism of chloroacetyl chloride molecules

The structure of chloroacetyl chloroide (CH₂ClCOCl) molecule in different conformations arising from rotation of the CH₂Cl group about the C-C bond was determined by the Hartree-Fock RHF/6-31G(d) quantum-chemical calculations. The energy difference between the two stable rotamers was estimated at 5.9 kJ mol^?1, and barriers to intramolecular reorientations of the CH₂Cl group were calculated.     

Investigation of electronic structure in chlorine-substituted methanes

The electronic structure, dipole moments, and nuclear magnetic resonance and nuclear quadrupole resonance constants of the CH₄, CCl₄, CHCl₃, CH₂Cl₂, and CH₃Cl molecules were calculated by the Parr-Pariser-Pople (PPP) and Wolfsberg-Helmholz (WH) methods, with self-consistency of the charges on the atoms. Conclusions were reached on the applicability limits of these methods. The calculated values were compared with experimental data.     

Vibrational spectra of ethyl iodedes

The infrared spectra of CH₃CH₂I, CD₃CH₂I, and CH₃CD₂I of the vapors and the solids at 170°C have been recorded from 4000-200 cm^?1. The Raman spectra of the liquids and vapors have also been recorded and depolarization values have been measured. Assignment of the eighteen fundamental vibrations has been based on depolarization values, band contours, group-frequency correlations, and normal coordinate calculations. A critical discussion of the CH stretching assignments in CH₃CH₂X molecules is presented.     

A PCILO Study of Conformation and Internal Rotation in Mono-substituted Benzenes

The effect of Kékulé representation and hybrid function of O-atoms in the PCILO-CNDO framework of conformation and internal rotation in mono-sub stituted benzenes Ph-X (X?NH₂, OH, OCH₃, CH₃, CHO, NO₂) is studied. Three variational criteria for the choice of the appropriate third-order energy, proposed to symmetrize the PCILO results, are critically examined in relation with the height of rotational barrier in these molecules. The study shows that, in all cases, the most stable conformation is qualitatively correct predicted by the PCILO method. Since the barrier to internal rotation in the studied aromatic systems arises predominantly from delocalization effect, it is proposed to employ the arithmetic mean of the third-order energy of the two Kékulé structures. In molecules, in which the third-order energy between the two Kékulé structures is larger than 2 kcal/mol, however, the lower third-order energy representation alone seems to be appropriate. In phenol and anisole the spa-hybridization type of the O-atoms offers better values of rotational barrier, whereas in the sp³-type the delocalization is overestimated in the planar conformation.     

Molecular orbital calculation of Mössbauer parameters for tin compounds in low temperature matrices

Mössbauer parameters of tin compounds, Sn(CH₃)_nCl_4–n (n=0, 1, 2, 3, 4), isolated in low temperature matrices are correlated with electronic properties at the tin nuclei obtained by molecular orbital calculations. The Mössbauer isomer shift and quadrupole splitting show good correlation with electron density and electric field gradient estimated by molecular orbital calculations, respectively. Structures of novel species (Sn(CH₃)₂CH₂ and [Sn(CH₃)₂CH₂]₂) produced via photodissociation of Sn(CH₃)₄ in low temperature matrices were estimated by means of molecular orbital calculations as compared with Mössbauer parameters.     

Interpretation of infrared intensities of propane and deutero-propanes by electrooptical parameters

The interpretation of IR intensities of CH₃CH₂CH₃, CD₃CH₂CD₃ and CH₃CD₂CH₃ in terms of bond parameters (electrooptical parameters, e.o.p. s) is discussed. Sets of e.o.p. s derived from a previous study on methanes and ethanes are used as starting points to derive more complete sets of e.o.p. s which fit the intensities of propane very well. The intensities from ethanes have been used as a constraint on the e.o.p. s of CH₃ groups. The results are discussed and compared with values from quantum mechanics for the derivatives of the molecular dipole moment. Since the parameters derived in this work predict satisfactorily the IR intensities of polyethylene, they should form a good starting set for the interpretation of the intensities of n-paraffins.