Enumeration of the Conformers of Unbranched Aliphatic Alkanes

The Journal of Physical Chemistry A, 1998

We report on the relative conformer energies and rotational energy barriers for meso and racemic 2,4-diphenylpentane (DPP) from ab initio electronic structure calculations. It is found that dispersion interactions between phenyl rings strongly influence the conformational geometries, necessitating the inclusion of electron correlation in both geometry optimizations and energy calculations. Furthermore, basis set superposition contributions to the phenyl-phenyl interactions, estimated by extracting the phenyl rings from the optimized DPP geometries and computing the basis set superposition error for the resulting benzene dimer configurations, are significant. An atomistic molecular mechanics force field is parametrized to reproduce our best values for the conformational energies and rotational energy barriers in DPP obtained from ab initio calculations. Conformational energy contour maps are presented for the DPP enantiomers, and their salient features are discussed. Gas-phase molecular dynamics simulations of DPP have been performed using the quantum chemistry based force field. Important entropic contributions to the conformer populations, due primarily to restricted phenyl group rotation, are discussed.

The Journal of Chemical Physics, 2011

NMR spectra of ethane, propane, and n-butane as solutes in the nematic liquid crystals 4-n-pentyl-4 -cyanobiphenyl (5CB) and Merck ZLI 1132 (1132 are investigated over a wide temperature range. The ratios of dipolar couplings of ethane to propane are constant over the entire temperature range. Assuming that this constancy applies to the butane conformers facilitates the separation of probability from order parameter. This separation allows the investigation of conformational distribution without the need of invoking any model for the anisotropic intermolecular potential. The results give an order matrix that is consistent with that predicted from model potentials that describe the orientational potential in terms of short-range size and shape effects. The isotropic intermolecular potential contribution to the trans-gauche energy difference E tg is found to be temperature dependent with the values and variation in agreement with that found when the same results are analyzed using the chord model for anisotropic interactions [A. C. J. Weber and E. E. Burnell, Chem. Phys. Lett. 506, 196 (2011)]. The fit obtained for 9 spectra in 5CB (63 dipolar couplings) has an RMS difference between experimental and calculated dipolar couplings of 2.7 Hz, while that for the 16 spectra in 1132 (112 couplings) is 6.2 Hz; this excellent fit with nine adjustable parameters suggests that the assumption of equal temperature dependencies of the order parameters for ethane, propane, and each conformer of butane is correct. Also the fit parameters (E tg and the methyl angle increase) obtained for 1132 and 5CB agree. The results indicate that the chord model, which was designed to treat hydrocarbon chains, is indeed the model of choice for these chains. The temperature variation of E tg provides a challenge for theoreticians. Finally, even better fits to the experimental dipolar couplings are obtained when the energy in the Boltzmann factor is used for scaling ethane to butane results. However, in this case the values obtained for E tg differ between 1132 and 5CB.

Chemical Physics Letters, 2015

Interaction energies (E int) were evaluated for n-alkanes dimers (C1-C10) using DFT-D, different functionals, and several basis sets. In addition, calculations were also carried out with DFTB-LD and MM3 and OPLS-AA force fields. Results show linear correlations of E int with respect to those obtained from literature at high levels of theory (MP2 and CCSD(T)). Relationships between E int and experimental heats of vaporization (H v) and critical temperatures (T c) were obtained with MP2, DFT-D, MM and DFTB-LD. This leads to good extrapolations for hairpin-hexadecane using MM3 and DFTB-LD for H v and T c , respectively. Dispersion in DFT is discussed.

Journal of Molecular Structure, 2012

The infrared spectra of 3-butyn-1-ol, HC"CCH 2 CH 2 OH, have been recorded as a vapour in the range 3600-50 cm À1 and as a liquid between 3600 and 400 cm À1. Additional spectra of the alcohol isolated in an argon matrix at ca. 5 K were obtained and spectra were recorded after annealing to various temperatures between 10 and 35 K. Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 143 K. Spectra of an amorphous solid were recorded at 78 K. In spite of several attempts and many different annealing temperatures, the sample crystallized neither in the IR nor in the Raman cryostats. In the variable temperature Raman spectra, some bands of the liquid changed in relative intensity and were interpreted in terms of conformational equilibria between two of the five possible conformers. Complete assignments were made for all the bands of the most stable conformer gg, in which the OH group is approaching the triple bond, forming an intramolecular hydrogen bond. From various bands assigned to a second conformer aa, in which OH is oriented anti to the AC"CA bond, or a third conformer ag, the conformational enthalpy difference was found to be D conf H(ag-gg) = 0.9 kJ mol À1 in the liquid. The two highest energy conformers g'g and ag were not detected. Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. The calculations revealed that gg was the low energy conformer and CBS-QB3 calculations suggested the gg conformer was more stable by 5.4 and 4.2 kJ mol À1 relative to ag and aa, respectively, in the vapour. Vibrational wavenumbers and infrared and Raman band intensities for the three low energy conformers are reported from B3LYP/cc-pVTZ calculations.

Quantum Chemistry - Molecules for Innovations, 2012

Quantum Chemistry-Molecules for Innovations 104 tetravalent and can form valence bonds with other carbon atoms yielding to chains. These carbon chains can sometimes have closed arrangements, forming rings. (Drayer, 1993) Van't Hoff and Le Bel proposed that the four valences of the carbon atom were not planar, but directed into three-dimensional space. Van't Hoff specifically proposed that the spatial arrangement was tetrahedral. Later, he used the tetrahedron as a graphic representation of the valence arrangement around the carbon atom and also used this model to explain the physical property of optical activity.(Ramberg & Somsen, 2001) A compound containing a four different substituted carbon-described by Van't Hoff as asymmetric carbon-would be capable of existing in two distinctly different nonsuperimposable forms. Finally, he stated that the asymmetric carbon atom was the cause of molecular asymmetry and optical activity.(Drayer, 1993) Le Bel, in turn, also published his stereochemical ideas in 1874, but with a different approach to the problem from that presented by Van't Hoff. His hypothesis was not based on the tetrahedral model for the carbon atom and the fixed valences between the atoms. His investigation was into the asymmetry as a whole, without evaluating the individual atoms. The full system was considered in his evaluation, and his interpretation could be inserted into the field that is currently understood as molecular asymmetry. He mentions the tetrahedral carbon atom only in special cases, and not as a general principle. Many molecules confirm Le Bel's concepts of molecular asymmetry. Allenes, spiranes, and biphenyls are some examples of asymmetric molecules that do not contain any asymmetric carbons. Van't Hoff's and Le Bel's different approaches can be explained by the origin of their formation. Van't Hoff, based on Kekulé tetrahedron models, suggested the concept of the asymmetric carbon atom. On the other hand, Le Bel based his investigations on Pasteur's considerations of the connections between optical rotation and molecular structure.

Journal of Molecular Structure: THEOCHEM, 1997

Semiempirical molecular orbital calculations in the gas phase and in solution have been carried out to study the conformational equilibria of 2-substituted-1,3-dioxanes with substituents Y =-OCH3,-C6H5,-p-C6H4CI and 2-substituted-1,3dithianes with substituents Y =-COzH,-C02CH3,-SCH3,-OCH3,-SC6H5,-COCsH5. The influence of the solvent has been studied with a self-consistent reaction field (SCRF) continuum model. The effects of steric and electrostatic interactions on the axial/equatorial preference have been investigated. The entropic contributions to the free energy change have been evaluated for two model compounds. The present results are compared with the available experimental data. 0 1997 Elsevier Science B.V.

Journal of Computational Chemistry, 2004

The conformational free energies for some 2-substituted butanes where X ϭ F, Cl, CN, and CCH were calculated using G3-B3, CBS-QB3, and CCSD(T)/6-311ϩϩG(2d,p) as well as other theoretical levels. The above methods gave consistent results with free energies relative to the trans conformers as follows: X ϭ CCH, g؉ ϭ 0.77 Ϯ 0.05 kcal/mol. g؊ ϭ 0.88 Ϯ 0.05 kcal/mol; X ϭ CN, g؉ ϭ 0.85 Ϯ 0.05 kcal/mol, g؊ ϭ 0.75 Ϯ 0.05 kcal/mol; X ϭ Cl, g؉ ϭ 0.70 Ϯ 0.05 kcal/ml, g؊ ϭ 0.80 Ϯ 0.05 kcal/mol; and X ϭ F, g؉ ϭ 0.53 Ϯ 0.05 kcal/mol, g؊ ϭ 0.83 Ϯ 0.05 kcal/mol. The conformational free energies also were estimated using the observed liquid phase IR spectra and intensities calculated using B3LYP/6-311ϩϩG** and MP2/6-311ϩϩG**. The rotational free energy profiles for all of the compounds were estimated at the G3-B3 level.

Bulletin of the Chemical Society of Japan, 2003

The dipole moments of (AE)-(1,1 0-biindene)-3,3 0-dione 1, (AE)-2,2 0 ,3,3 0-tetrahydro-(1,1 0-binaphthalene)-4,4 0 (1H,1 0 H)-dione 2, and (AE)-(1,1 0-bibenzosuberene)-5,5 0-dione 3 in carbon tetrachloride and benzene were measured over a range of temperatures. Analyses of the relative permittivity data in carbon tetrachloride and benzene showed that at 25 C, 1 exists predominantly in the gauche conformation, whilst 2 and 3 favor the trans form. The experimentally derived values of the energy difference between the gauche and trans rotamers and the gauche/trans population quotient were compared with values predicted by molecular-orbital calculations. The crystal and molecular structures of 2 and 3 were determined by single-crystal X-ray diffraction methods. Both compounds exist in the trans conformation in the solid-state. Dynamic 1 H NMR was employed to 3 in order to determine the activation parameters for the barriers to rotation in solution.

The Journal of Chemical Physics, 2002

An accurate calculation of the energy differences between stationary points on the torsional potential energy surface of n-pentane is performed using ab initio Hartree-Fock theory, advanced many-body methods such as MP2, MP3, CCSD, and CCSD͑T͒, as well as density functional theory, together with basis sets of increasing size. This study focuses on the four conformers of this compound, namely, the all staggered trans-trans ͑TT͒, trans-gauche ͑TG͒, gauche-gauche (G ϩ G ϩ ), and gauche-gauche (G ϩ G Ϫ ) structures, belonging to the C 2v , C 1 , C 2 , and C 1 symmetry point groups, respectively. A focal point analysis up to 635 basis functions is carried out to determine when the series of relative energies of the four conformers approach convergence. It is found that relative to the minimum energy TT conformer, the energy differences of the TG, G ϩ G ϩ , and G ϩ G Ϫ conformers obtained from ab initio methods are 0.621, 1.065, and 2.917 kcal mol Ϫ1 , respectively. Converged energy differences obtained with three density functionals, B3PW91, B3LYP, and MPW1K, are found to be considerably higher than those computed ab initio. Mole fractions of the various conformers are evaluated at different temperatures from thermostatistical data accounting for vibrational and rotational entropies, as well as zero-point vibrational energies in the rigid rotor-harmonic oscillator approximation.

Tetrahedron, 1982

2-Oxo-1,3,2_dioxathiane and all methyl-and several alkyl-substituted 2-0x0-1,3,2_dioxathianes were prepared for a 'H NMR conformational study. The conformational energy of the axial S=O group in Ccl,,-AG$ = 14.8 ? 0.3 kJ mol-', was determined by chemical equilibration of the epimeric cis_4,6diiethyl derivatives and it was found to decrease with the increasing solvent polarity. The conformational equilibria of alkyl-substituted derivatives were solved and the proportions of the conformers estimated using 'H NMR chemical shifts, vicinal coupling constants and in three cases also dipole moments. The configurational interactions in the C&-C, moiety are close to the corresponding values of I ,3-dioxanes.