Hannah Weiler-Feilchenfeld

Electronic structure of purine tautomers

Abstract The electronic properties of the four tautomers of purine have been studied theoretically by the PPP-DPB and by the CNDO/2 methods. The dipole moments and UV absorption spectra of simple derivatives of the different purine forms have been determined experimentally. On the whole, they are in good agreement with the predictions. In particular the N(1)H and N(7)H tautomers have moments higher by several Debyes than the N(3)H and N(9)H tautomers; the electronic spectra show a bathochromic shift from the N(9)H to the N(7)H to the N(1)H to the N(3)H form, in this order.

Dipole moments and electronic absorption spectra of aryltricyanoethylenes

The dipole moments of several aryltricyanoethylenes have been measured and found to be higher than those calculated by vector addition. This deviation is probably due to partial charge separation, facilitated by the pronounced ability of the tricyanovinyl group to spread the negative charge. This view is supported by the linear relation between the dipole moment µ and Hammetts σ constant and by the fact that µmeas–µcalc is proportional to Browns σ+ value and to the energy of the longest wavelength transition in the electronic spectra.

Dipole moments of substituted 1,1-dicyanoethylenes

The dipole moments of a number of 2-substituted 1,1-dicyanoethylenes have been measured. The moments are explained by a combination of two effects, partial charge separation and compression of the [graphic omitted] angle. From the data obtained with ω,ω-dicyanofulvenes it is concluded that in compounds (such as I and II) pseudoaromatic structures (such as Ia and IIa) contribute to the ground state of the molecules.

Magnetic circular dichroism studies XXXII: CNDO calculations of the magnetic circular dichroism spectra of 7- and 9-methyladenine

CNDO calculations of the MCD spectra of 7- and 9-methyladenine have been carried out in two approximations. A minor modification of the CNDO/S method of Jaffé and coworkers shows an improvement over Jaffés parameterization with respect to band separations and the signs of the MCDB terms, but is not applicable ton→ π* transitions.

Molecular Factors and Activity in Psychopharmacologically Active Compounds

The determination of the conformation of a pharmacologically active compound either in the solid state, in solution or in the gaseous phase does not necessarily give information on the conformation of this molecule on the biological receptor and no conclusion can therefore be drawn on the fine structure of the receptor site. In the case of relatively rigid molecules, such as the derivatives of dibenzocycloheptatriene (I), one can assume that the conformation found in solution, which has been shown to be non-planar [1], remains unchanged in the receptor-molecule complex or compound. We have shown previously [2] that in the dihydro derivatives of I, the di- benzocycloheptadienes (II), the non-planar structure persists, although the seven- membered ring is capable of inversion, i.e., of passing through a planar configuration.