Exchange perturbation theory of the Rayleigh-Schrödinger type in first and second orders is applied to a calculation of barriers of internal rotation about single bonds in molecules. The barrier is assumed to arise as a result of exchange interactions between nonbonded atoms; the electrons of these atoms are replaced by one effective electron per atom, with a Gaussian charge distribution. Spins of effective electrons on different atoms are taken to be parallel. The method is applied to a large variety of molecules; calculated results for the barriers are in substantial agreement with experiment. A decomposition of the interaction energy for different conformers and for the barrier itself is given in terms of two- and many-atom exchange interactions.

AbstractsIn a previous publication [l] we analyzed the stability of rare gas halides on the basis of exchange perturbation theory of the Rayleigh-Schrodinger type, using a three-center, four-electron model. In this paper, the analysis is extended to a (n + 1)-center, (n + 2)-electron model for rare gas halides of composition RX, , where R is the rare gas atom and X the halogen atom, in order to investigate the validity of the three-center, fourelectron model. The compounds analyzed are XeF, , XeCl, , KrF, and KrCl, with n = 3 and 4, in different geometric configurations and for different states of total spin S.As before, we use exchange perturbation theory in first and second orders. The results are in good agreement with those obtained in the previous analysis and with experiments. Specifically, it is found that chlorides of rare gas atoms are not stable, that XeF4 has the square-planar configuration and that trifluorides cannot exist. The possible existence of KrF, is discussed.Dam une publication rtcente [l], nous avons analyst la stabilitt des halogtnures des gaz rares sur la base d'une thtorie de perturbation du type de Rayleigh-Schrodinger gknkraliste pour les forces d'tchange, en utilisant un modtle A quatre tlectrons sur trois centres. Dans cet article, l'analyse est gtntraliste A un systtme de (n + 2)-tlectrons sur (n + 1)-centres, ceci pour ttudier les halogtnures des gaz rares de composition RX, , oh R est l'atome des gaz rares et X l'halogtne, et pour ttablir la validitt du modkle de quatre tlectrons sur trois centres. Les composts analysts sont XeF, , XeCl, , KrF, et KrCl, , avec n = 3 et n = 4, pour difftrentes configurations gtomttriques et pour difftrents ttats de spin total S. Comme auparavant, nous utilisons la thtorie de perturbation du type d'tchange en premier et deuxitme ordres. Les rksultats sont en bon accord avec ceux obtenus dans l'analyse prtctdente et avec l'exptrience. En particulier, on trouve que les chlorures des gaz rares ne sont pas stables, que XeFp a la configuration de forme plan-carrke et que les tri-fluorures ne peuvent pas exister. L'existence possible de KrF4 est discutte.I n einer kiirzlich erschienenen Arbeit [l] wurde das Problem der Stabilitat der Edelgashalogenide auf der Basis eines "drei-Zentren, vier-E1ektronen"-Modells unter Verwendung von Austausch-S torungsrechnung vom Typ Rayleigh-Schr8dinger analysiert. 335

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Exchange perturbation theory of the Rayleigh–Schrödinger type is applied, in first and second order, to a calculation of the conformations of simple molecules containing a double bond and of barriers to internal rotation around a single or a double bond. Indirect exchange (’’superexchange’’) interactions between nonbonded atoms via the double bond are considered explicitly. In the model, the electrons of the nonbonded atoms are replaced by one effective electron per atom, those of the double bond by two spin-paired effective electrons. The charge distribution of the effective electrons are of Gaussian form. The method is applied to acetaldehyde, the acyl halides and related compounds, and to the ethylene molecule; calculated results are in good agreement with experiment. A decomposotion of the interaction energy is given, for different conformers, in terms of two- and many-atom interactions.

Conformational analysis of simple molecules containing lone pair electrons such as H2O2, NH2OH, NH2NH2, is carried out as an extension of a recently proposed model where the barrier to internal rotation is assumed to arise only as a result of exchange interactions, calculated to first order using perturbation theory, among nonbonded atoms. Lone pair electrons associated to atoms located on the axis of rotation are effectively included in the model as additional nonbonded atoms. It is shown that exchange interactions among the nonbonded atoms of the extended cluster can be taken alone as responsible for the observed conformations and barriers. The model is then applied to molecules as propane and its derivatives, dimethylamine, dimethyl ether, with two consecutive axes of internal rotation and to the conformational analysis of polyoxymethylene. The results are in good agreement with experiments. The possibility of application to molecules of biological interest is discussed.