Effect Of Cis-trans Isomerization Research Articles

Homochiral vs. heterochiral preference in chiral self-recognition of cyclic diols.

The structure and clustering propensity of a chiral derivative of cis-1,2-cyclohexanediol, namely, 1-phenyl-cis-1,2-cyclohexanediol (cis-PCD), has been studied under supersonic expansion conditions by combining laser spectroscopy with quantum chemistry calculations. The presence of the phenyl substituent induces conformational locking relative to cis-1,2-cyclohexanediol (cis-CD), and only one conformer of the bare molecule is observed by both Raman and IR-UV double resonance spectroscopy. The homochiral preference inferred for the dimer formation at low enough temperature is in line with the formation of a conglomerate in the solid state. The change in clustering propensity in cis-PCD relative to trans-1,2-cyclohexanediol (trans-CD), which shows heterochiral preference, is explained by the presence of the phenyl substituent rather than the effect of cis-trans isomerism. Indeed the transiently chiral cis-CD also forms preferentially heterodimers, whose structure is very close to that of the corresponding trans-CD dimer.

The Effect of cis-trans Isomerism on the Electronic-vibrational Structure of the Ground and Excited States and Reactivity of 2-(2-Furyl)- and 2-(2-Thienyl)oxazole

The fluorescent properties, structure, and electronic structure of the ground and excited singlet and triplet electronic states of the cis and trans forms of 4,5-dihydro-2-(2-furyl)oxazole, 4,4-dihydro-2-(2-thienyl)oxazole, 2-(2-furyl)oxazole (FO), and 2-(2-thienyl)oxazole (TO) have been studied. The orbital nature of the lower excited singlet and triplet states has been studied by the semiempirical INDO/S (valence approximation) and PPP/S (π approximation) methods. It was shown that for FO and TO molecules the lower triplet state is of the ππ* type, for which delocalization of the electronic excitation on atoms is characteristic. In the singlet excitation state inversion was observed of the energy levels of the delocalized ππ* states and nπ* states localized over several bonds (for the free TO and FO molecules the lower excited singlet states S1* were assigned to ππ* and nπ* types respectively). Owing to the low position of the Tππ* and Tnπ* levels relative to the singlet level of ππ* type, the rate constant for intercombination conversion is greater than the rate constant for radiative decay. Consequently an efficient population of the triplet states of the molecules occurs under conditions of electronic-vibrational excitation. The direction of reactions during synthesis was compared with the localization indices in the ground state for electrophilic, nucleophilic, and radical substitution, and also with the excitation localization numbers Lμ for a wide selection of electronically excited states. It was concluded that the change in the structure of the azole molecule on replacing an O atom by an S atom, or on changing from a partially hydrogenated to a heteroaromatic system, was the main reason for the change of all the spectral parameters characterizing the electronic-vibrational or the spin-orbital interaction of the most reactive groups of atoms in the molecular structure.

Reduction of aliphatic nitro compounds 3. The effect of cis-trans isomerization on the electroreduction of ?-nitrocinnamic acid esters

1. It has been shown, for the first time, that geometrical isomerization affects the electrochemical reduction of the conjugated nitroalkenes. 2. The reduction potentials are lower for the E isomers of the esters of α-nitrocinnamic acids than for the companion Z isomers. 3. The E isomers of these esters adsorb more readily on the dropping mercury electrode than do the companion Z isomers.

Two pathways for the protolysis of 1-(alkythio)-1-buten-3-ynes and the effect of cis-trans isomerism

We have used a spectrophotometric method to examine the kinetics of the hydrolysis of 1-(alkylthio)-1-buten-3-ynes to (alkylthio)butenones and acetoacetaldehyde; the rate of reaction depends markedly on the reactant configuration.

Dispersion curves of poly-L-proline I chain

An analysis of the normal modes of vibration and their dispersion for the poly-L-proline I is carried out using Wilson's GF matrix method as modified by Higgs for an infinite helical system. The conformational dependence of various modes and the effect of cis-trans isomerism is discussed.

The π-bonding effects of other ligands on co-ordinated nitrite ion in palladium (II) and platinum (II) complexes

The preparation of a series of dinitro complexes of palladium (II) and platinum (II) with a variety of neutral ligands is described. The complexes were characterized with respect to their geometric structure ( cis or trans) and the mode of co-ordination of the nitrite ion (N-bonded in all cases) on the basis of their infra-red spectra. A correlation has been found to exist between the π-bonding ability of the neutral ligands and the NO stretching frequencies of the co-ordinated nitrite ion. This is interpreted in terms of the extent of π-bonding between the filled d π orbitals of the metal and the vacant π-antibonding orbital of the nitrite ion. Further support for this hypothesis is found in the effect of cis-trans isomerism on the NO stretching frequencies.

Effect of cis-trans Isomerism on the Urea Inclusion Compound Forming Ability of a Molecule; Study of the Maleate-Fumarate System

Effect of cis-trans Isomerism on the Urea Inclusion Compound Forming Ability of a Molecule; Study of the Maleate-Fumarate System