The effect of the nature of the substituent on the structure of a 1,3,4-oxathiazol-2-one ring

A series of 5-aryl(thienyl) substituted 1,3,4-oxathiazol-2-ones has been synthesized. The molecular structures of 5-(4-nitrophenyl)-, 5-(5-ethylthieno[2,3-b]thiophen-2-yl)-, 5-(5-ethyl-2-ethylsulfanylthiophen-3-yl)-, and 5-(5-methylsulfonylthiophen-2-yl)-1,3,4-oxathiazol-2-ones have been investigated by X-ray analysis. The thiophene and oxathiazolone fragments are coplanar. The geometric parameters of the oxathiazolone ring are discussed. Electron acceptor substituents in the para position of the benzene ring and in the 5 position of the thiophene ring have the greatest effect on the system conjugation in the oxathiazolone ring.     

A CNDO/2 study on the additivity and the nature of the non-additivity of the substituent effects on 13C NMR shifts in chlorobenzenes and chlorophenols

The general correlation between the electron densities and the ¹³C NMR chemical shifts is found to be quite poor in the cases discussed. The non-additivities of the substituent effects on the chemical shifts and the CNDO/2 electron densities correlate only weakly. However, when the electron densities are made specific to different types of atomic orbitals, the s electrons have a pronounced effect in all the models tested. This is explained by an indirect effect on the 〈1/r³〉 term of the p electrons. Good correlations are found between the sums of the chemical shifts and the corresponding sums of the substituent charge excesses. The different behaviour of OH and Cl substituents in the additivity of the substituent effects can be explained by their different back-bonding properties.     

Palladium(II)-catalyzed ring enlargement of 2-(arylmethylene)cyclopropylcarbinols: strong effect of substituent electronic nature on the reaction pathway

In the presence of Pd(II) catalyst and copper(II) bromide, 2-(arylmethylene)cyclopropylcarbinols undergo ring enlargement to deliver (arylcyclobutenyl)carbinols or hydrogenated furans in good yields under mild conditions; mechanisms accounting for the distinct products have been proposed on the basis of control and deuterium labeling experiments.     

A CNDO/S study on the electronic structure of 1,1′-binaphthyl

CNDO/S CI calculations demonstrate that the changes in fluorescence and S_n ← S₁ properties of 1,1′-binaphthyl in going from rigid to fluid solutions is consistent with a cis → trans rotation of the chromophores. A lack of significant fluorescence dependence on solvent polarity is discussed within the context of the calculations.     

A CNDO/2 molecular orbital study of the electronic and conformational modes of ibotenic acid

CNDO/2 molecular orbital calculations on the non-ionic molecule, zwitterion and mono-anion of ibotenic acid are reported. Internal energy surfaces have been calculated from which minimum energy conformations are predicted. The relative internal energies, dipole moments and net atomic populations at the minimum-energy conformations are given. The importance of using a free energy rather than internal energy approach in the construction of energy surfaces is also discussed.     

Trends in the CNDO/2 molecular orbital study of electronic structure in some neurotransmitter drugs

An investigation of electronic structure in some neurotransmitter drugs has been made using the CNDO/2 semi-empirical molecular orbital method.The electronic structure has been conveniently characterized by the electronic parameters nett atomic population (NAP) and bond index (BI). A variation of these electronic parameters with respect to conformation has been studied and has been found unlikely to exceed 0.1 e in most. cases. Further, the useful extent to which the electronic parameters of some commonly occurring functional groups may be regarded as conformationally invariant has been demonstrated. Also presented are (i) a discussion on the intramolecular close-approach of functional groups — the interaction between terminal —COO^? and ?NH₃⁺ groups of α-ω anaino acids is explicitly considered; (ii) an enquiry into the extension of ‘standard’ (idealized) geometry models in the elucidation of electronic structure.The implication of the results and observations presented here are briefly discussed with reference to classical and quantum structure-activity studies of drug molecules.     

CNDO/2 study of benzene-chlorine complex

We have carried out CNDO/2 calculations for the benzene-chlorine complex in a number of configurations. In order to obtain reasonable complexation energies, it was necessary to exclude chlorine 3d-orbitals from the basis set. When onlys- andp-orbitals were included, we found maximum stability for the complex for an unsymmetrical geometry. The calculated complexation energy was ?3.5 kcal/mole. Configuration-interaction calculations predicted a wavelength of about 270 nm for the charge-transfer band. Most, but not all, geometries were found to produce stable complexes.     

A CNDO/2 study of the ethylene-chlorine reaction

CNDO/2 calculations have been carried out of the energies of reaction and changes in charge distribution for some reactions involving the ethylene chlorine complex. The addition of chlorine to ethylene may involve the attack of a chloride ion on the ethylene chlorine complex.     

A CNDO/S study of trans-indanylideneindane. Implications to the trans-stilbene electronic structure

CNDO calculations have been performed on chiral derivatives of trans-1-(indan-1-ylidene)-indane, a stiff trans-stilbene analogue. The results are in complete agreement with the recently published UV and CD spectra of these compounds and allow state assignments down to 225 nm. The calculations are relevant to the trans-stilbene electronic structure which is under current dispute.     

A further study on the CNDO/2 structures of water dimer

The structure of water dimer has been further studied by the CNDO/2 method. Some results have been found to be different from those in Thiel's paper.     

The effect of solvent on the internal rotation of formamide: A CNDO/2-solvaton method study

The effect of solvent on the barrier to internal rotation of formamide had been studied using a solvaton method within the CNDO/2 parameterization. The experimental trend of increasing rotational barrier with increasing solvent dielectric constant has been reproduced. A critical examination of the manner in which interactions between solute and solvent were allowed to modify solute energies and the polarization of the solute wavefunction was performed.Work performed while A.J.D. was a visting scientist at the Polymer Institute of the Slovak Academy of Sciences sponsored under the exchange program between the U.S. National Academy of Sciences and the Czechoslovak Academy of Sciences during the summer of 1980.     

Water environmental effect on tautomeric equilibria of 2- and 4-oxopyridines. A CNDO/2 study

The supermolecule approach is proposed to evaluate the shift of the tautomeric equilibrium of a molecule when going from the vapour phase to water solution. According to the model, a comparison of the stabilization (hydration) energies of different tautomers of a molecule surrounded by water molecules predicts changes in tautomeric equilibrium upon solvation. An application of the model (within the CNDO/2 method) to 2- and 4-oxopyridine shows that the lactam tautomers of the molecules are more stabilized by water molecules than the corresponding lactim forms by about 7–8 kcal/mole.