Metal-metal interactions in chain compounds of gold(I) : syntheses and crystal structures of chlorotetrahydrothiophenegold(I), bromotetrahydrothiophenegold(I) and iodotetrahydroselenophenegold(I)

An ab initio computational study on a series of sulfoxides is reported. The SO bond dissociation energy (BDE) of sulfoxides in which the sulfur atom is included in a formally aromatic ring (e.g., thiophene sulfoxide) is found to be decreased by as much as 25 kcal/mol, compared to DMSO. A complementary effect is observed for sulfoxides in which the sulfur is included in a formally antiaromatic ring (e.g., thiirene sulfoxide), in which SO BDEs are increased by as much as 15 kcal/mol. Both effects are attenuated by benzannulation. Examination of calculated geometries and isodesmic reactions with pure hydrocarbons leads to the conclusion that the observed effects are due to a severe disruption of the (anti) aromaticity of the sulfur-containing ring on oxidation. The cyclic sulfoxides appear to be neither significantly aromatic nor antiaromatic by energetic considerations. No significant SO bond strength effect is observed for simple conjugation. Inversion barriers for several sulfoxides are also calculated.

Effects of Conjugation and Aromaticity on the Sulfoxide Bond1

The 13C NMR spectra have been determined of: (i) aliphatic compounds having at one end a functionalized sulphur atom (SH, S−, SMe, S(O)Me, SO2Me and S+Me2) and (ii) saturated sulphur heterocycles variously substituted at the S-atom . The results are discussed in terms of the familiar deshielding effects for α- and β-carbons and shielding effects for γ-carbons, exerted by the sulphur atom itself and/or by the atoms or groups of which the sulphur function is made up. The γ-effect of the S-atom appears to be nearly independent of the nature of the S-function and of comparable magnitude to that of an aliphatic carbon (−2·5 + −3·0 ppm). Surprisingly, however, a SCH3 group shields the carbon in γ position with respect to CH3 by an amount (−5·4 ppm) which is more than twice that (−2·5 ppm) exerted by the aliphatic γ-carbon on the S-CH3 carbon itself. As to the cyclic compounds, the shieldings of the α- and β-carbons can be rationalized in terms of the conformational orientation of the substituent at sulphur, and the equilibrium distribution of the conformers. The results confirm the great value of 13C NMR for configurational and conformational assignment of S-heterocycles.

13C NMR of organosulphur compounds: I—the effects of sulphur substituents on the 13C chemical shifts of alkyl chains and of S-heterocycles

We report here the results on N-acetyl-N'-methylamides of oxazolidine and thiazolidine (Ac-Oxa-NHMe and Ac-Thz-NHMe) calculated using the ab initio and density functional computations with the reaction field theory at theHF, MP2, and B3LYP levels of theory with the 6-31+G(d) basis set The displacement of the γ-CH 2 group in proline ring by oxygen and sulfur atoms has affected the structure of proline, cis-trans equilibrium, and rotational barrier The up-puckered structure is found to be prevalent for the trans conformers of pseudoproline amides The higher cis populations of pseudoproline amides can be interpreted due to the longer distance between the acetyl methyl group and the 5-methylene group of the ring for the trans conformer of pseudoproline amides than that of the proline amide The changes in charge of the prolyl nitrogen and the decrease in electron overlap of the C-N bond for transition state (TS) structures seem to play a role in lowering rotational barriers of Oxa and Thz amides compared to that of the proline (Pro) amide The calculated preferences for cis conformers in the order of Oxa > Thz > Pro amides and for trans-to-cis rotational barriers in the order of Pro > Oxa > Thz amides in water are consistent with experimental results on pseudoproline-containing peptides The pertinent distance between the prolyl nitrogen and the N-H amide group to form a hydrogen bond might indicate that this intramolecular hydrogen bond could contribute in stabilizing the TS structures of pseudoproline and proline amides and play a role in prolyl isomerization

Cis−Trans Isomerization and Puckering of Pseudoproline Dipeptides

The MM4 force field has been extended to the title class of compounds. The vibrational spectra, structures, conformational equilibria, and heats of formation have been studied for 47 conformers of 29 compounds. In general, the properties may be calculated with accuracy that is competitive with that for hydrocarbons. The structures are better fit than previously because of the inclusion of a torsion–bend interaction term, which has its origin in the lone pair (Bohlmann) effect. Available experimental data do not suffice to yield detailed torsional potentials, or geometries as a function of torsion angle, and these quantities were determined by ab initio calculations at the MP2/6‐31G* level. The rms error in the calculated frequencies of seven representative structures (with a total of 64 experimental and 96 ab initio frequencies) is 25 cm−1. The heats of formation for 23 compounds have a weighted rms error of 0.36 kcal/mol. © 1997 John Wiley & Sons, Inc. J Comput Chem 18: 1827–1847, 1997

Molecular mechanics studies (MM4) of sulfides and mercaptans

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Studies on Molecules with Five-membered Rings. V. Calculation of Conformational Energies and Electron Diffraction Investigation of Gaseous Tetrahydroselenophene.

H. M. Seip

Papers

Studies on Molecules with Five-membered Rings. V. Calculation of Conformational Energies and Electron Diffraction Investigation of Gaseous Tetrahydroselenophene.