The conformational features of a large number of hydroperoxides ROOH and peroxides ROOR', where R and R' are alkyl groups of different and increasing size and phenyl rings, including ortho substituted derivatives, were obtained from molecular mechanics calculations by employing a standard package. For the molecules of small molecular size, comparison was carried out with the results of ab initio calculations. Heats of formation were also obtained from molecular mechanics for hydroperoxides and peroxides: The values are, in general, overestimated. For the molecules containing the CF, group, the calculated values are subject to large errors and heats of formation were obtained from ab initio total energies in the "atom equivalents" scheme. To estimate the homolyhc dissociation energies of the different bonds in the peroxide molecules, heats of formation of R., .OR, and .OOR radicals were employed and several of them had to be calculated. Different approaches were employed-molecular mechanics calculations, ab initio energies within the atom equivalent and isodesmic reaction schemes, and Benson's group additivity rule; values consistent within the different calculation methods were chosen for estimating dissociation energies. The bond dissociation energies indicate different trends in these molecules as a function of the nature of the R and R' groups and the possible electronic effects operating in these molecules are discussed. 0 1993 by John Wiley & Sons, Inc.

The conformational properties of the diheteroaryl ketones of the furan and thiophene series have been studied both experimentally, in the solid state and in solution, and with a theoretical ab-initio approach. X-Ray analysis of the samples which are solid at room temperature was possible only for the compounds which gave suitable crystals, i,e. di(2-thienyl) ketone (2), di(3-thienyl) ketone (5), 2-fury1 3-fury1 ketone ( 7 ) , 2-thienyl 3-thienyl ketone ( 8 ) and 3-fury1 2-thienyl ketone (10). The crystal data are: (2j space group F dd2, a = 13.267(2), b = 21.853(4), c = 6.067(1) 8, Z = 8, R = 0.0637 for 388 observed reflections; ( 5 ) space group P2,/n, a = 6.1 74(1), b = 11.184(3), c = 13.203(2) 8, Z = 4, R = 0.0776 for 1 202 observed reflections; ( 7 ) space group P2,/c, a = 9.900(4), b = 10.934(6), c = 7.485(2) 8, Z = 4, R = 0.0461 for 984 observed reflections; ( 8 ) space group P2,/c, a = 6.1 83(1 ), b = 11 .I 09(1 ), c = 13.379(2) 8, Z = 4, R = 0.0732 for 1 186 observed reflections; (10) space group P i , a = 13.799(2), b = 10.039( 2), c = 6.084(1) 8, 2 = 4, R = 0.0639 for 1 973 observed reflections. The crystal structures of compounds (5) and ( 8 ) are disordered: for the former the disorder concerns the position of one ring, whereas both rings are involved in the latter. For compound (1 0 ) the structure of the two crystallographically independent molecules is the same. From the n.m.r. LIS (lanthanide-induced shifts) measurements (on 'H and I3C) indicative information is obtained on the type of conformer and its relative population present in the solution equilibrium mixture. M O ab-initio calculations, performed by employing the 3-21 G or 3-21 G" (with d-orbitals on the sulphur atom) basis sets for the ground-state conformers of all the molecules considered, show that the solid-state structures are very similar to those calculated for the conformers with the lowestenergy content and the calculated conformer populations are in qualitative agreement with measurements in solution. As a general trend, the solid-state structures, the preferred conformations found in solution and those calculated for the free molecules show that the preferred orientation of one heterocyclic ring with respect to the carbonyl group is of the type found in the corresponding formyl and acetyl derivatives and is maintained in all the diheteroaryl ketones. For the molecules having conformers of similar energy content and different dipole moment, i.e. di(2-furyl) ketone (1 ), it was demonstrated, on the basis of the classical theory of the solvent effect, that the equilibrium mixture of conformers in the vapour phase is different from that in solution and changes in solvents of different polarity. * For part 10 see ref. 25. Supplementary data: (see Instructions for Authors, J. Chem. SOC., Perkin Trans. 2, 1989, issue 1, pp. xvii-xix). Lists of H-atom co-ordinates, bond lengths and angles, and thermal paramters have been deposited at the Cambridge Crystallographic Data Centre.

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Substituent effects o n the electronic structure of 23 biologically active 4-aminoaryl (4-substituted aryl) sulphones were investigated by means of 'H n.m.r., 13C n.m.r., and i.r. spectroscopy, as well as by semiempirical all-valence C N D 0 / 2 calculations, with and without sulphur d orbital participation. Good linear intercorrelations were found among the spectral data and between these and the computed electronic charges and the Hammett op values. On this basis the substituent effects are interpreted in terms of electronic charge perturbation, which is linearly transmitted from the substituent t o the whole molecule, bridging SO, group included. The agreement between experimental and theoretical data is good and the trends d o not depend on the inclusion or exclusion, in the calculations, of the sulphur d orbitals. Strong and linearly related o-n: electron interactions operate between C-1 and C-1' in the C( 1 )SO,C( 1 ') moiety; thus Koch and Moffitt's qualitative x-electron conjugation model for Case 2 type sulphones seems inadequate.

A number of ring-substituted methyl phenyl sulphoxides have been examined with a multinuclear n.m.r. approach in order to obtain experimental evidence of the conformational properties of the methylsulphinyl group bonded to an aromatic ring. Measurements were performed of 'H, 13C, and 170 chemical shifts and LIS (lanthanide-induced shifts) on 'H and 13C, long-range 13C-'H coupling constants. Each of these approaches by itself is not fully conclusive in showing the orientation of the SO bond in the different compounds examined, but by using them in combination a sound picture of the conformational behaviour of these molecules in solution can be obtained. The SO bond is thus almost coplanar with the aromatic ring in ortho-substituted compounds and oriented in the opposite direction with respect to the substituent. The twist-angle probably differs slightly as a function of the ortho-group. An increasing degree of distortion from coplanarity is found in parasubstituted derivatives and ortho-disubstituted compounds. As regards 170 chemical shifts, these were found to span a smaller range than in the corresponding acetophenones.