Structures and bond energies

A theoretical investigation of the M=X bond lengths and the relative r- and 7r-bond energies of H2M=X (for all group 14 M and group 16 X)313a accompanied efforts to synthesize stable R2M=X compounds313, and these results and those of related studies are presented in Table 2956,224,316. 

Calculated reaction energies and activation barriers for the isomerization of RR M=X to the corresponding metallylenes RMXR are given in Table 30. Except for M = C (where 

H2C=0 is more stable than HCOH by ca 52 kcalmol-1), the divalent metallylenes, HMXH, are more stable than the corresponding H2M=X, and the isomerization becomes more exothermic for heavier M e.g. silanone and hydroxysilylene have nearly the same energy, but germanone is by ca 24-31 kcalmol-1 less stable than hydroxygermylene and HPbOH is more stable than H2Pb=0 by 69.5 kcalmol-1316 (Table 30296-316 318-324). However, very high barriers separate the doubly-bonded compounds from their divalent isomers (this was calculated only for M = Si and Ge), so that both isomers are relatively stable kinetically toward this isomerization reaction. 

TABLE 30. Reaction energies (AE) and activation barriers ( a) for the isomerization of RR M=XH to RMXH R  

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B. T. Luke, J. A. Pople, M.-B. Krogh-Jespersen, Y. Apeloig, J. Chandrasekhar, and P. v. R. Schleyer,/. Am. Chem. Soc., 108,260 (1986). A Theoretical Survey of Singly Bonded Silicon Compounds. Comparison of the Structures and Bond Energies of Silyl and Methyl Derivatives. 

Despite the successes of the above mentioned techniques for structure and bond energy analyses, inherent uncertainties and limitations in each method make it important to develop new and independent tests for comparison. In this regard our laboratory has initiated an intensive effort to study the gas-phase photodlssocia-tion of transition metal containing ions [13-16]. 

That paper and their many associated patents show the depth of understanding which Bell and Findlay reached of the mechanism of action and of the importance of the crystal structure and bond energies. Westinghouse also reported on methods of producing pure material and of making satisfactory films, and showed for the first time the very high load-carrying capacity obtainable, in experiments at contact pressures up to 600,000 psi. 

Several studies have demonstrated the ability to observe a complete catalytic cycle in the gas-phase. Wallace and Whetten, and Woste and coworkers combined gas-phase experiments and theoretical calculations to elucidate the fuU catalytic cycle of CO oxidation including intermediate reaction steps [27-29]. Schwarz et al. have also demonstrated a full gas-phase catalytic cycle for the oxidation of CO in the presence of cationic platinum oxide clusters [30]. Furthermore, Armentrout and co-workers have studied the energetics of the individual steps in the overall catalytic cycles and produced a wealth of information on the thermochemistry, structure, and bond energies of transition metal clusters [31]. Clearly, the ability to probe the active sites and intermediates of complex catalytic reactions through gas-phase ion-molecule studies has yielded significant insight into the mechanisms of condensed-phase catalytic processes. 

In the earliest implementation applied to molecular problems, K. Johnson [39] used scattered-plane waves as a basis and the exchange-correlation energy was represented by (13). This SW-Xa method employed in addition an (muffin-tin) approximation to the Coulomb potential of (17) in which Vc is replaced by a sum of spherical potentials around each atom. This approximation is well suited for solids for which the SW-Xa method originally was developed [40]. However, it is less appropriate in molecules where the potential around each atom might be far from spherical. The SW-Xa method is computationally expedient compared to standard ab initio techniques and has been used with considerable success [41] to elucidate the electronic structure in complexes and clusters of transition metals. However, the use of the muffin-tin approximation precludes accurate calculations of total energies. The method has for this reason not been successful in studies involving molecular structures and bond energies [42]. 

Hydrogen cyanide (and organic nitriles, which contain the cyano group) can be catalytically reduced with hydrogen to form amines. Use Lewis structures and bond energies to determine... 

Details of electronic structure and bond energies in molecules... 

Fig. 20.11. The molecular structures and bond energies in phosphorus trihalides and phosphoryl trihalides.

The properties of radiation vulcanized NR have been studied by a number of authors and compared with those of conventional vulcanizates. It has been reported that radiation vulcanized rubber has inferior physical properties compared to sulfur vulcanizates. The maximum tensile strength achieved by the vulcanizates decreases in the order of sulfur vulcanizate > peroxide vulcanizate > radiation vulcanizate. Such differences in properties are attributed to the vulcanizate structure and bond energy. It has also been shown that radiation... 

A. Vfeldkamp and G. Frenking,/. Comput. Chem., 13, 1184 (1992). Theoretical Studies of Organometallic Compounds. III. Structures and Bond Energies of FeCH and FeCH ( = 1, 2, 3). 

Fig. 16 The molecular structures and bond energies of PF3, PF5, F3PO, P4O5, and P4O10 in the gas phase...

Fig. 17 Above-. The molecular structures of SF2, SF4, and SFg determined by gas electron diffraction and mean bond energies (MBE) calculated from thermochemical data. Below. The molecular structures and bond energies of SO, SO2, and SO3...

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