The ground-state vibrational spectra of triptycene (9,10-dihydro-9,10[ 1',2']benzenoanthracene) were studied by fluorescence emission and IR and Raman spectroscopies, as well as by semiempirical (AMI) and ab initio (HartreeFock) quantum chemical calculations. Comparison of experimental vibrational frequencies and intensities in the range 60-1oOO cm-l with the semiempirical and the ab initio values is made. Excellent agreement is found between experiment and ab initio calculation with respect to vibrational frequencies. Agreement with the frequencies predicted by the semiempirical AM1 calculation is less satisfactory. The interpretation and assignment of the lowest frequency vibrational modes is also discussed in terms of a vibrational excimer model in a basis of symmetry-adapted combinations of local benzene ring coordinates. The two lowest frequency modes at 64 and 21 1 cm-' are identified by shape and symmetry (e' and a i , respectively), which is very important for the understanding of the Jahn-Teller effect in the first excited state 'E' of triptycene.
A new formalism has been developed in order to evaluate intermolecular interaction energies for inorganic and organometallic complexes in the framework of the extended Hückel method. In order to provide the shortest possible response time on an interactive computer graphics facility, this model should require the minimum amount of computer time, which explains why approximate procedures are used to evaluate electrostatic, charge transfer and exchange repulsion components. When applying this model to typical examples of electrophilic addition reactions to organometallic complexes, it is found that it is essential to take account of charge transfer interactions, the electrostatic component alone being not sufficient, even qualitatively, for a proper description of the reaction mechanism. The results, presented as color-coded dot molecular surfaces, show a very good agreement with experiment as to the site of attack, namely (i) on metal for the electrophilic attack on Fe(cp)2, Fe(CO)5 and X(cp)(CO)2, X = Co, Rh; (ii) on the cp ligand for the nucleophilic attack on Co(cp)2+ and Rh(cp)2+; (iii) on bz for the nucleophilic attack on Fe(cp)(bz)+. Finally, modellizations of the nucleophilic attack on a coordinated olefin and of the relation between structure and acidic properties of zeolites are presented and discussed.
A new formalism has been developed to evaluate from extended-Hiickel wavefunctions the intermolecular interaction energy E,,, between an organometallic substrate S and a model electrophile or nucleophile reactant R. Calculated as the sum of electrostatic, charge-transfer and exchange-repulsion components, E,,, is used as a local reactivity index to interpret and predict the regio-and stereoselectivity of electrophilic and nucleophilic addition reactions to organometallics. The methodology developed incorporates molecular graphics as an important ingredient, as the reactivity index is represented using either: (i) a mapping onto the molecular surface by S by means of a color code or (ii) the generation of 3D isoenergy contour surfaces. This model is shown to be able to rationalize the activation of a benzene ring by complexation to the Cr(CO), moiety and also to describe adequately the sequential addition of a carbanion nucleophile and of an electrophile to (q6-benzene)tricarbonylchromium derivatives.Comments Inorg. Chem.
The spin distribution in a stable nitroxide biradical that shows ferromagnetic interactions in the solid phase has been studied at three levels of theory: First, at the UHF level; then, including correlation effects in U M P~ calculations; and finally, the results are compared with the spin density obtained using the local density functional (LDF) approximation. It is shown that LDF spin densities are closer to U M P~ than to UHF predictions; the difference between the UHF and the ( U M P~, LDF) results points to a redistribution of the spin repartition between N and 0 due to electronic correlation. For planar conformations of the NO group, there is symmetric distribution ( D u ) of the spin density on the adamantane skeleton. For nonplanar nitroxides, the molecule is chiral ( C * ) , which results in a breakdown of the spin transmission on part of the adamantane cage. 0 1993 John Wiley & Sons, Inc.
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