Structural features of disubstituted polyacetylenes with bulky substituents at double bonds

The structural features of polyacetylenes carrying two substituents at double bonds with the general formula [-C(CH₃)=CR-] _n , where R = Si(CH₃)₃, -Ge(CH₃)₃, or CH(CH₃)₂, are studied. It is shown that the experimental IR and Raman spectra of the polymers and the theoretically calculated vibrational spectra for the model polymers consisting of three units coincide well with experimental data. All bands in the IR spectra are interpreted. The potential curves of internal rotation are calculated and constructed; high values of rotation barriers indicate a high rigidity of chains for all polymers of interest. The orthogonal arrangement of neighboring monomer units and, as a result, the absence of electron-density conjugation over the polymer chain are revealed. Charges on atoms and electron density on bonds of the monomer unit obtained from theoretical calculations indicate the presence of strong polarization of all bonds, including the -C=C- bond. This effect causes a shift in the frequencies of stretching vibrations due to double bonds in the IR spectra of polyacetylenes with Si- and Ge-containing side substituents toward the longwave region. For polyacetylene with hydrocarbon side substituents -CH(CH₃)₂, such polarization is absent.     

Synthesis,structure, and dynamic behavior in solution of arylamino-1,3,5-triazines

Ten unsymmetrically substituted arylamino-1,3,5-triazines were synthesized and studied by dynamic NMR spectroscopy. The free energies of the hindered rotation ΔG^≠?are in 59–77 kJ mol^{? 1} range. Using difference-mode NOE NMR experiments, the structures of the major and minor rotation isomers were proved. The DFT B3LYP/6-31G* calculations were performed. The difference between the calculated rotation barriers and the experimental values obtained by line shape analysis is less than 7.6 kJ mol^?1. The height of the rotation barrier varies in a 18 kJ mol^?1 range depending on the substituents in the triazine ring.     

The role of intra- and intermolecular interactions in determining the conformation and mode of packing of crystalline polymers—I. Poly(cis-1,4-butadiene)

Conformational energy calculations on an isolated chain of poly(cis-1,4-butadiene) have been performed, allowing for the variation of all bond angles and torsion angles. Various minimum energy and high symmetry conformations are accessible to the chain. Packing energy calculations have been performed for chains having tci symmetry, allowing for the variation of symmetry, unit cell constants and internal rotation angles. Unlike the behaviour observed for other polymers, the chain conformation which minimizes the total (conformational plus packing) energy is sensibly different from that of minimum energy for the isolated chain. The agreement between experimental and calculated data of the crystal structure analysis may be considered as very good.     

Meshed tert-butyl gears on a quasirigid backbone

MM3-based calculations showed that bicycles and polycycles with four- to six-membered rings-components of the bi- and polycyclic backbone are sufficiently rigid to keep a syn-periplanar orientation of vicinal tert-butyl substituents. As a result of the spatial proximity of these groups, their rotation occurs in a concerted manner as demonstrated by conformational schemes that are built using MM3-derived methodology. Only correlated disrotation in saturated systems with four- to five-membered rings-components and in the adamantane system leads to isochronism for Me groups of the tert-Bu substituents (i.e., to dynamic gearing in Mislow's terms). Moreover, correlated rotation of these substituents is coupled with a change of the backbone geometry (pseudorotation) except in the most rigid bicyclo[2.1.1]hexa-2-ene system. Thus, a new type of dynamic gearing, correlated rotation–rotation–pseudorotation, is predicted for quasirigid bi- and polycycles with syn-periplanar oriented tert-Bu substituents. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1786–1794, 1998     

Semi-empirical studies of torsional potentials in halonitrosomethanes

Semi-empirical molecular electronic structure calculations have been performed on a number of species of the general formulae CX₃NO and CX₂YNO, where X, Y = Cl, F and H. Potential energy curves for the internal rotation (torsion) of the nitroso functional group and geometric parameters of the molecules have been obtained. These calculations have employed the public domain AMPAC package, using both the MNDO and AM1 models available within that package. Graphical results are presented and compared with previous ab initio calculations and experimental determinations, where they exist. The results of these calculations should prove valuable as an aid in the analysis of the spectroscopy of these species.     

Ab initio SCF calculations on hydrogen bonded cresol isomers

Ab initio GAUSSIAN 80 calculations with two different basis sets (STO-3G and 4–31 G*) were performed on hydrogen bonded cresol isomers for comparison with experimental data from free jet fluorescence excitation spectroscopy. Form-cresol, the calculated barriers for hindered internal rotation of the OH-group and the CH₃-group are in good agreement with experiment. The calculations show the trans-linear configuration ofp-cresol·B-clusters (B = H₂O, CH₃OH) to be more stable than the all-planar configuration. This agrees with CI calculations and microwave spectroscopic investigations of the water dimer. Calculations of both the intermolecular stretch and bend frequencies ofp-cresol·B-clusters show little dependence on the all-planar or trans-linear configuration but a strong dependence on the choice of the basis set. With the minimal basis set STO-3G, the vibrational energies are generally too high. The agreement between the calculated vibrational frequencies from the 4–31 G* basis set and the experimental values is fair.     

Intramolecular Diels-Alder reaction of N-allyl 2-furoyl amides: effect of steric strain and amide rotational isomerism

Intramolecular Diels-Alder reactions of various N-allyl 2-furoyl amides with different substituents on the nitrogen atom were investigated. The reaction of amides having bulky substituents proceeded at a faster rate than the analogs whose substituents were of less bulkiness. From the systematic experimental survey of the substituent effects and the energetic evaluation based on the DFT calculations at the B3LYP/6-31G(d) level, the enhanced reactivity was ascribed to the relief of steric strain upon cyclization rather than the amide rotational isomerism governed by the bulky substituents.     

The effect of side substituents on rotation hindrance in polyheteroarylenes

Hindered rotation was considered in calculations of the conformational parameters of a series of polyheteroarylenes with bulky side substituents by the Monte-Carlo method. Within the range of experimental errors, the results of calculations for several polyarylates coincide with the values of conformational rigidity, determined from hydrodynamic experimental data. The proposed procedure was used to estimate the rigidities of a number of polymers with bulky side substuents for which experimental determination is difficult. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1287–1296, July, 1998.     

Hula-twist within the confinement of molecular crystals

E/Z-photoisomerizations within molecular crystals are varied. Existing cases are summarized. They require crystal lattices that allow for long-range molecular movements in the nontopotactic solid-state mechanism. Reactivity and directionality can be foreseen on the basis of the crystal packing. The reacting crystal changes continuously by phase rebuilding, phase transformation and disintegration. Two possibilities for the chemical mechanism exist: (1) highly space-demanding (cooperative) double-bond rotations; and (2) space-conserving hula-twist (HT) motions while the substituents move within their planes and only one C-H unit undergoes out-of-plane translocation. If internal rotation cannot be reasonably modeled within the crystal lattice, HT remains the only choice, as in the case of trans-1,2-dibenzoylethene. Direct experimental proof is still lacking because the differences in the conformational outcome could not be assessed in the studied examples. Density functional theory calculations of cis-1,2-dibenzoylethene revealed very low differences in energy content of the helical (s-cis, s-cis)- and the almost orthogonal (s-cis, s-trans)-cis-conformers. The almost orthogonal (s-cis, s-trans)-cis-conformer that is found in the pure crystal is very similar to the calculated counterpart. It is suggested that more favorable initial conformers might be obtained by proper vinylic substitution. The stereochemical outcome of highly space-demanding thermal vinylic-bond rotations followed by cyclizations of conjugated bisallenes to give bismethylene cyclobutenes excludes the alternative HT mechanism (double-bond isomerization) in the present cases. But space-conserving HT might be a mechanistic alternative in less-substituted cases under photoexcitation. The stereochemical consequences are discussed.     

Structure and bonding in proteins: Part 1. The electron organisation in the amide unit

A consistent set of numerical information on the electron organisation in the amide unit has been obtained for a number of small amides, using the readily available Gaussian-70 computer package and a modest amount of computer time. There is good agreement with experimental facts concerning bond lengths, bond angles, rotation barriers and cis/trans energy differences. The results have been analysed in terms of simple molecular orbital theory and other simple valence ideas to make the results intelligible in a physical sense, so that they may be used in larger molecules. The rotation barrier about the CN bond seems to be a simple π-electron conjugation effect, whose magnitude is influenced in the main by the internal geometry of the CNO fragment rather than by substituents.     

Internal Rotation and Equilibrium Structure of the Bromonitromethane Molecule According to Gas Electron Diffraction Data and Quantum Chemical Calculations

The structure and internal rotation of the bromonitromethane molecule are studied using electron diffraction analysis and quantum chemical calculations. The electron diffraction data are analyzed within the models of a general intramolecular anharmonic force field and quantum chemical pseudoconformers to account for the adiabatic separation of a large amplitude motion associated with the internal rotation of the NO₂ group. The following experimental bond lengths and valence angles are obtained for the equilibrium orthogonal configuration of the molecule with C_s symmetry: r_e(N=O) = 1.217(5) Å, r_e(C–N) = 1.48(2) Å, r_e(C–Br) = 1.919(5) Å, ∠_еBr–C–N = 109.6(9)°, ∠_еO=N=O = 125.9(9)°. The equilibrium geometry parameters are in good agreement with CCSD(T)/cc-pVTZ calculations. Thermally averaged parameters are calculated using the equilibrium geometry and quadratic and cubic quantum chemical force constants. The barrier to internal rotation cannot be determined reliably based on the electron diffraction data used in this work. There is a 82% probability that the equilibrium configuration with orthogonal C–Br and N=O bonds is most preferable, and internal rotation barrier does not exceed 280 cm^-1, which agrees with CCSD(T)/cc-pVTZ calculations.     

Density functional calculation of conformations and potentials of internal rotation in polychlorinated biphenyls

Full geometry optimization for all 209 isomers of polychlorinated biphenyls (PCBs) and calculations of internal rotation potentials for 154 isomers have been performed by density functional method B3LYP/6-31G(d, p). Conformations and internal rotation barriers in PCBs were proved to depend on a number of chlorine atoms in ortho-positions and, less, the presence of chlorine atoms in adjacent meta-positions. Subject to the number of chlorine atoms in ortho-and adjacent meta-positions, 209 PCB isomers were classified into 18 groups, within each of them molecules having very close conformations and potentials of internal rotation. It makes possible to evaluate with high accuracy the potential functions of the last 55 PCB molecules for which potential curve calculations have not been made.     

Resonant two-photon ionization spectra of fluorotoluene–argon complexes

The S₁←S₀ transitions of three jet-cooled fluorotoluene–argon complexes are studied by the resonant two-photon ionization technique coupled with time-of-flight mass spectrometry. Detailed spectral analysis assisted by model calculations has allowed us to determine the stretching vibrational frequencies and two bending vibrational frequencies of the complexes. The results show that, the relative positions of the substituents (i.e., F and CH₃) have very little influence on the stretching vibrations of the complexes but strongly affect their bending vibrations. In addition, the Ar atom is found to influence the internal rotation of the methyl group in the complexes.     

Theoretical conformational analysis of a simple sulphilimine model

The topomerization (bond rotation andS-pyramidal inversion) of a simple sulphilimine model, H₂SNH has been studied with the aid ofab initio SCF MO calculations. The highest rotation barrier occurs when the H₂SN moiety is planar, < HSN = 120 °. The maxima of the inversion crossections occur at the planar conformation for all rotation angles α as expected, however, the minima belong to different values when α is varied. The minimum energy path between the two lowest minima of the conformational energy surface consists of a pure inversion section and a section which is mostly rotation. The optimum values of the < HSN bond angles are significantly smaller than the corresponding < RSN bond angles of sulphilimines of bulkierR substituents.     

Rotational isomerism and barriers to internal rotation in 3-halopropenes byscf-mo methods

Electronic structures of 3 halopropenes have been investigated through semiempiricalscf-mo calculations using valence basis sets of atomic orbitals (ao) constructed from Slater type orbitals (sto). The electronic structures of stable conformers have been predicted and the corresponding calculated dipole moments show good agreement with experimental data. The considerable differences between the dipole moments of various conformers confirm the hindrance to internal rotation about the C−C bond, i.e., the existence of a definite potential barrier to rotation. The barrier heights hindering the internal rotation in each system are also estimated.     

Vibrational spectra of the dithieno[3,2-b: 2′,3′-d] thiophene crystal

A normal mode analysis of dithieno[3,2-b:2′,3′-d]thiophene crystal was carried out using an intramolecular force field refined for the isolated molecule and an intermolecular potential described in terms of interactions among non-bonded atoms. Infrared and Raman data for internal and lattice vibrations are reported, including those for a deuterated isotopomer, which are satisfactorily accounted for by frequency calculations. Cartesian displacements for each normal mode of the isolated molecule are presented and frequency splittings and shifts analysed through calculations at k = 0 which include all crystal vibrational degrees of freedom.     

Bis(dithiomethyl-tetrathiafulvalene) with two phenyl-phosphino bridges

The single crystal X-ray structure of the precursor dithiomethyl-tetrathiafulvalene (MeS)₂TTF is reported, together with theoretical calculations at DFT level, which afforded two energy minima corresponding to cis and trans orientations of the thiomethyl substituents. The bis(tetrathiafulvalene) reaction of this TTF derivative followed by the reaction with phenyldichlorophosphine provides a new rigid bis(tetrathiafulvalene [TTF]) containing a 1,4-dihydro-1,4-diphosphinine ring between the two redox active units. Its solid-state structure, determined by single crystal X-ray diffraction analysis, shows the coexistence of both cis and trans isomers. Cyclic voltammetry measurements are in accordance with the existence of a communication between the two TTFs, as illustrated by the splitting of the oxidation waves.     

Internal rotation and equilibrium structure of the 2-methyl-2-nitropropane molecule from joint processing of gas phase electron diffraction data,vibrational and microwave spectroscopy data,and quantum chemical calculation results

The structure and internal rotation of the 2-methyl-2-nitropropane molecule is studied by electron diffraction and quantum chemical calculations with the use of microwave and vibrational spectroscopy data. The electron diffraction data are analyzed within the general intramolecular anharmonic force field model and the quantum chemical pseudoconformer model, considering the adiabatic separation of the degree of freedom of large amplitude motion, i.e., the internal rotation of the NO₂ group. The equilibrium eclipsed configuration of the C _s symmetry molecule has the following experimental bond lengths and valence angles: r _e(N=O) = 1.226//1.226(8) Å, r _e(C–N)//r _e(C–C) = 1.520//1.515/1,521(4) Å, ∠_еC–C–N = = 109.1/106,1(8)°, ∠_еO=N=O = 124.2(6)°, ∠_eC–C–H_avg = 110(3)°. The equilibrium geometry parameters are well consistent with MP2/cc-pVTZ quantum chemical calculations and microwave spectroscopy data. The thermally average parameters previously obtained within the small vibration model show a satisfactory agreement with the new results. The electron diffraction data used in this work do not allow a reliable determination of the barrier to internal rotation. However, at a barrier of 203(2) cal/mol, which is derived from the microwave study, it follows from the electron diffraction data that the equilibrium configuration must correspond to an eclipsed arrangement of C–C and N=O bonds, which is also consistent with the results of quantum chemical calculations of various levels.     

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.