Ted Schaefer

Calculations of the C(1)C(α) torsional barrier in styrene. Comparison with experiment

Abstract Computations of the geometry optimized conformational energies of styrene at the STO 3G, 4–21G, and 4–31G levels of molecular orbital theory, bear out vibronic level fluorescence spectra showing a large fourfold component for the internal torsional potential function.

STO-3G MO calculations on structures and internal rotational barriers of phenol, benzoyl X(X = H, F, CH3, CN, OCH3), acetyl fluoride, acetyl cyanide, and carbonyl cyanide

Abstract The side chain geometry and some adjacent bond lengths and angles of the ring are optimized at the STO-3G level of molecular orbital theory for the planar and orthogonal forms of benzoyl X(X = H, F, CN, CH3, OCH3). Similar calculations are reported for acetyl fluoride, acetyl cyanide, and carbonyl cyanide, for which experimental structures and reliable internal barriers are available. The calculated barriers for the benzoyl compounds suggest steric hindrance by X in the ground state as a major cause of the variation in the barrier magnitudes. Good agreement between calculated and experimental geometries for acetyl cyanide and carbonyl cyanide, as well as for the internal rotational barrier in the former, are taken to imply a reliable calculated geometry for benzoyl cyanide. A total geometry optimization for phenol agrees fairly well as for the internal rotational barrier in the ture and also with the direction and magnitude of the dipole moment. Optimization of the ring geometry does not lower the calculated internal rotation barrier.

Molecular orbital computations of the internal rotational potentials in 2-, 3-, and 4-fluorostyrene. Comparison with experiment

Abstract Geometry-optimized STO-3G and 6-31G MO computations are presented for the internal rotational potentials in the monofluorostyrenes. These are compared with experimental values, particularly those from fluorescence experiments with a supersonic jet.

Proton Magnetic Resonance Study of Intramolecular Hydrogen Bonding in Halophenols

The long-range spin–spin coupling constants over five bonds between the hydroxyl proton and the ring protons in a series of trihalophenols imply that the intramolecular hydrogen bond strength (negative enthalpy) to fluorine is greater than that to iodine by 75 ± 20 cal/mol, whereas the strengths to chlorine and bromine are 460 ± 60 cal/mol greater than to iodine. If a distinction can be made between chlorine and bromine, then chlorine–hydrogen bonds more strongly by only a few tens of calories per mol. The measurements were made mainly on dilute solutions in carbon tetrachloride at 32 °C.

Some molecular orbital computations of the inversion barrier in 9,10-dihydroanthracene

Abstract Geometry-optimized STO-3G and 4-31G molecular orbital computations yield 8.6 and 7.8 kJ mol −1 , respectively, for the inversion barrier in 9,10-dihydroanthracene. The folding angle in the boat form of the molecule is computed as 140.7 and 141.°3, respectively, to be compared with a value of 144.°7 in the crystal. The computed internal bond angles and carbon-carbon bond lengths agree rather well with the X-ray structure.

STO 3G MO computations of six-fold barriers in some toluene derivatives

Abstract STO 3G MO computations with geometry optimization of six-fold internal rotational barriers in toluene derivatives, 2,6-diX-C6H4CR3 (X = H, F, R = H, F, Cl, CH3 and X = Cl, R = H, F), are reported. The barrier magnitudes are as large as 9.5 kJ mol−1 for X = F, R = CH3. Inclusions of a twelve-fold potential in a fit of the computed energies yields significant values only for X = F, R = CH3, F. For all but R = F, the conformation of lowest energy has a CR bond lying in the benzene plane.

The conformational preference and barrier to internal rotation of an equatorial 3,5-dichlorophenyl group by the J method. Derivatives of cyclohexane, 1,3-dithiane, 1,3-dioxane, and 1,3-dioxolane

We report the preparation and the analysis of the phenyl ring proton magnetic resonance spectra of 3,5-dichlorophenylcyclohexane and of the 2-(3,5-dichlorophenyl) derivatives of 1,3-dioxane, 1,3-dithiane, and 1,3-dioxolane. With the exception of the dioxolanes these compounds exist predominantly as the equatorial isomers. The J method is used to show that the phenyl moiety prefers the conformation in which the α C—H bond lies in the phenyl plane. The predominantly twofold barriers to rotation about the carbon–carbon bond between the two ring systems are 2.0 ± 0.3, 0.4 ± 0.2, 2.2 ± 0.3, 0.85 ± 0.3 kcal/mol for these compounds, in the order given above. The low value for the barrier in the 1,3-dioxane derivative agrees reasonably well with molecular mechanics calculations and with the results of calorimetric and X-ray studies on equatorial 2-phenyl-1,3-dioxane.

A proton magnetic resonance determination of the barrier to internal rotation in benzenethiol in solution

The analysis of the proton magnetic resonance spectrum of benzenethiol in CCl4 solution in the absence of intermolecular proton exchange yields a value of −0.33 Hz for the coupling over six bonds between the sulfhydryl proton and the ring proton in the para position. On the assumption that this coupling depends on sin2θ, where 0 is the angle by which the S-H bond twists out of the aromatic plane, a hindered-rotor treatment yields a maximum value of 1.3 kcal/mole for the twofold barrier to internal rotation. Arguments based on overlap integrals and on related hyperfine couplings in radicals suggest a reduction of this value to 1.1 ± 0.3 kcal/mole. This estimate is compared to other experimental and theoretical data.

A nuclear magnetic resonance determination of the barrier to internal rotation in phenylethane

Abstract The long-range nuclear spin-spin coupling constant between the methylene protons and the ring protons at the para position in 3,5-dibromo-phenylethane in benzene solution is consistent with a two-fold barrier of 1.2 ± 0.1 kcal/mole to rotation about the sp 2 -sp 3 carbon-carbon bond, in agreement with thermodynamic data on phenylethane and with deductions based on hyperfine interactions in related radical anions. The low-energy conformation has a plane of symmetry, the methyl group being situated out of the aromatic plane, in disagreement with Raman depolarization data interpretations and with deductions based on proton chemical shifts.

Some molecular orbital computations of the internal rotational barrier heights in benzaldehyde and its 4-fluoro, 4-cyano and 4-hydroxy derivatives

Abstract Some molecular orbital calculations on the title molecules imply that correlation-gradient computations will be necessary if the internal barriers are to agree with those deduced from torsional frequencies. For example, the energy difference between the planar and perpendicular conformers of benzaldehyde is 34.7 kJ mol−1 for an MP2/6-311G∗//6-31G∗(5D) computation. A calculation of the difference of the zero point energies of the two conformers, using somewhat less flexible basis sets, suggests that the enthalpy difference of the two conformers will not be substantially less than 32.5 kJ mol−1. The barrier height deduced from torsional frequencies is 19.3 kJ mol−1. Computations of the torsional frequencies, using STO-3G and 6-31G bases, suggests that the ensuing two-fold barriers are substantially lower than the computed energy and enthalpy differences of the two conformers for all the compounds. Comparisons are made with the measured barriers in condensed media. The computed dipole moments of the planar and perpendicular conformers imply that the solvent effects on the internal barriers differ in sign among the compounds. For 4-hydroxybenzaldehyde this sign depends on the orientation of the hydroxyl group relative to the CO bond. The need for certain new measurements and computations is emphasized.