Rudy Sebastian

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.

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.

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.

A theoretical potential for the puckering of dibenzo-p-dioxin

Abstract Geometry-optimized STO-3G MO computations at various folding angles are reported for dibenzo-p-dioxin, for which some ambiguity exists concerning its puckering behaviour. The planar molecule is computed to be most stable, but compared to thermal energies the potential is very flat near the minimum. At 300 K the average folding angle is calculated as 168.3°. The theoretical puckering potential appears to be consistent with the available experimental data for solutions of dibenzo-p-dioxin and possibly also for the crystal. The 4-31G MO geometry is given for the planar molecule.

Comparison of LAME analyses for an eight-spin system calibrated by CW and pulsed methods at different larmor frequencies

Abstract Although the statistical error estimates of a LAOCOON-3-type analysis of a high-resolution NMR spectrum in the liquid phase have been widely discussed and precise analyses of spectra calibrated by a given method are available, the problem of systematic experimental errors remains. The analysis of the 1H NMR spectra of p-fluorobenzylthiol, calibrated at 100 MHz in a standard way by cw methods and at 90 MHz in the FFT mode under similar conditions of spectral quality, demonstrates that systematic errors need not be significant at the 0.01-Hz confidence level. Correlation coefficients are useful indicators, of course, in addition to standard errors.

Molecular orbital estimates of the structures and internal rotational barriers in 2-fluorotoluene and 2-chlorotoluene

Abstract The internal rotational barriers in 2-fluorotoluene and 2-chlrrotoluene are computed with the ab initio basis sets, 6-31G and 6-31G*, as well as by single-point MP2/ / 6-31G* calculations. For the fluoro derivative, good agreement with the barrier from rotational spectra is obtained. The computed barrier for 2-chlorotoluene agrees with that derived from dipolar couplings in solution but is possibly smaller than the value implied by the absence of splittings in its rotational spectrum. The 6-31G* structures are given.

Molecular orbital calculations on anthrone

Abstract STO-3G MO and 4-31G MO computations on anthrone have the molecule as most stable in the planar form but also imply that significant out-of-plane motions or folding occurs at ambient temperatures. These results are consistent with a large thermal expansion coefficient for an anthrone crystal, which displays structural disorder, and with large amplitude motions of the oxygen atom. The computations are also consistent with the observation that some anthrone derivatives are planar while others display substantial folding about the C-9, C-10 direction. A comparison is given between the planar 4-31 G structure of anthrone and that of 10,10′-dianthronyl.

Comparison of ab initio and experimentally determined torsional potentials of ethylene in the ground state

Abstract Agreement is found to be excellent between the ab initio potential for a model of torsion in ethylene which follows the minimum energy rearrangement path and that derived from the experimental far-UV resonance Raman spectrum by means of a corresponding theoretical model.

The negative 7J(CHO, CH3) in 4-methylbenzaldehyde. Ionicity of the carbonyl bond

The spin–spin coupling constant over seven bonds between the formyl and methyl protons in 4-methylbenzaldehyde is −0.030 Hz in CS2/C6D12/TMS, and (−)0.035 Hz in acetone-d6, solutions at 297 K. This unexpected result is rationalized in terms of a spin–spin coupling mechanism attributed to the importance of a valence bond structure with an ionic carbonyl bond. The result again emphasizes the sensitivity to substituent perturbations of the six-bond coupling constant in quasi-planar benzaldehyde derivatives. It can have either sign and presents a challenge to its computation from first principles.