Mixture Of Rotamers Research Articles

Alpinigenine from Papaver bracteatum Lindl. Restricted rotation in an unusual oxidation product

Alpinigenine (1) was isolated in relatively large amount from a type of Papaver bracteatum Lindl. from Iran. Oxidation of this alkaloid with Jones reagent produced a lactone function and, unexpectedly, converted the NMe into an NCHO, present as a mixture of rotamers. Crystallization afforded one pure component, which equilibrated in solution within 2 h. 13C N.m.r. spectroscopy was used to confirm these results and enabled an insight into the conformation in solution of the seven-membered rings in alpinigenine and its oxidation product. X-Ray analysis determined unambiguously the absolute configuration of alpinigenine.

Conformational Equilibria in Trans 1,2‐Diarylethylenes Manifested In Their Emission Spectra. Part III. (1a). Pyridyl and quinolyl derivatives

AbstractThe emission spectra of four 1,2‐diarylethylenes, in which at least one aryl is 3‐pyridyl or 7 or 8‐quinolyl, vary with the wavelength of excitation, indicating the existence of two or three species of each compound with slightly‐shifted emission (and absorption) peaks, in analogy to our earlier reports (1). At temperatures below ‐70°C in solutions in aliphatic hydrocarbons the normal fluorescence is gradually replaced by a broad and almost structureless emission band in the range 400‐550 nm. The conversion takes time, it is virtually complete at ‐120°C. No similar phenomena were observed in toluene/methylcyclohexane mixtures. The results at temperatures above ‐70°C are ascribed to the existence of an equilibrium mixture of rotamers, while those below ‐70°C are suggested to reflect association phenomena.

Low Temperature Photolysis of o-Benzoquinones. Spectroscopic Detection of Cyclopentadienones and Bis-Ketenes

Abstract The photodecarbonylation of o-benzoquinone to cyclopentadienone at low temperature was monitored by infrared and ultraviolet spectra which showed that the reaction is highly sensitive to substituents. 3,5-Di-t-butyl-o-benzoquinone undergoes clean decarbonylation to give cyclopentadienone under irradiation at −170 °C. Similar irradiation of 4-t-butyl-, 4-methyl-, 3-methyl-, and unsubstituted o-benzoquinones also leads to decarbonylation to cyclopentadienones. However, competitive α-cleavage to bis-ketene becomes important in these systems. Direct spectroscopic observation of the elusive cyclopentadienone was unsuccessful because of the overlapping absorption of the by-product. The infrared spectrum of bis-ketene from 4-t-butyl-o-benzoquinone shows that it exists as a mixture of rotamers, not undergoing thermal interconversion even at −170 °C.

Electronic energy transfer between tyrosine and tryptophan in the peptides Trp-(Pro)n-Tyr.

AbstractA series of peptides, Trp‐(Pro)n‐Tyr, n = 0,…,5 were synthesized. Details of the synthetic method using the Merrifield method are given. The initial objective was to determine the characteristic distance R0 for energy transfer from tyrosine to tryptophan. This objective became submerged in the question of the structure of the chain of prolyl spacers. The efficiency of the energy transfer diminishes as the number of intervening prolines increases. Quantitatively the fit is much better if we assume that the peptides have a polyproline I structure (cis) than if we assume a trans polyproline II structure. Proton nmr spectra on the other hand suggest that the n = 1,3,5 peptides are mixtures of rotamers. Our conclusions are similar to those of previous workers, i.e., that in water there is a transition from a mixture of structures for small n to an all‐trans structure at some n > 5. This means that in water, at least, proline is a dubious spacer for energy‐transfer studies.

Photoelectron spectra of Group 5 compounds. II. Conformational analysis of diphosphine (P2H4)

The photoelectron spectra of certain hydrazines, disulfides, peroxides, and aminophosphines have been assigned to a unique conformer, being present under normal PE spectroscopic conditions. In contrast, different rotamers could be detected in the PE spectra of hexahydropyridazines and tetrasubstituted diphosphines and diarsines and were assumed for polysilanes. The composition of the rotameric mixture (transgauche) obtained for tetramethyldiphosphine from the relative PE peak areas had to be revised.