Nitriles bonding molecular orbitals

According to molecular orbital calculations, the interaciion is mainly electrostatic in character. Only minor variations of the gross atomic population in the associate, as compared with the isolated molecule, are found Consistent with this view, the experimental stretching frequency of the CN bond of organic nitriles in the vapor phase (where supposedly the monomeric form predominates) and in liquid state (where 70—90% are associated) are very amilar, as shown in Table 2. If the small decrease of 15—20 cm is taken as an indication for dimer formation, it may be conduded that the degree of association is similar in the pure liquid and in CCI4 solution. 

The experimental ionization potential of the former is 13.14 eV, that of the latter is 12.21 eV , hence the bonding rr-oibitals are, evidently, the Merest occupied molecular orbitals of the nitrile. Nevertheless, calculations of Del Bene have shown that heterodimers with the rr-orbitals acting as the electron donor are not equilibrium structures on the intermolecular potential surface, i. e., even if formed initially, such dimers would eventually be converted into heterodimers in which the hydrogen bonding involves the lone pair of electrons at the nitrogen. 

Of the 18 systems, some of which are unstable and must be generated in the reaction has been accomplished for at least 15, but not in all cases with a carbon-carbon double bond (the reaction also can be carried out with other double bonds ). Not all aUcenes undergo 1,3-dipolar addition equally well. The reaction is most successful for those that are good dienophUes in the Diels-Alder reaction (15-60). The addition is stereospecific and syn, and the mechanism is probably a one-step concerted process, as illustrated above, " largely controlled by Frontier Molecular Orbital considerations. " In-plane aromaticity has been invoked for these dipolar cycloadditions. " As expected for this type of mechanism, the rates do not vary much with changes in solvent, " although rate acceleration has been observed in ionic liquids. " Nitrile oxide cycloadditions have also been done in supercritical carbon dioxide. There are no simple rules... 

A route to alkylidynes containing metals with high oxidation states involves a metathesis exchange reaction that we have referred to briefly in earlier sections of Chapter 10. Scheme 10.10 shows the triply-bonded ditungsten complex 79 reacting with either alkynes or nitriles to give the corresponding metal carbyne (path a)90 or metal-carbyne plus nitride complex (path b).91 Scheme 10.10 also shows results of molecular orbital calculations at the DFT level.92 In the case... 

Fig. 5.28 Atomic orbitals (left) and resulting molecular orbitals (right) in the nitrile ion (a) bonding and (b) aruibonding.

Nature of the Activation Effect One of the principal questions that may be interpreted with the help of theoretical methods is the reasons for the activation of nitriles toward DCA upon their coordination to a metal center. Traditionally, the reactivity of dipoles and dipolarophiles in the DCA reactions is explained in terms of the frontier molecular orbital (FMO) theory and depends on the predominant type of the FMO interaction. The coupling of nitrones with nitriles is usually controlled by the interaction of the highest occupied molecular orbital (HOMO) of nitrone and the lowest unoccupied molecular orbital (LUMO) of nitrile centered on the C N bond (so-called normal electron demand reactions). For such processes, the coordination of N CR to a Lewis acid (e.g., to a metal) decreases the LUMOncr energy, providing a smaller HOMOjii -one - LUMOncr and, hence, facilitates the DCA reaction (Fig. 13.1a). 

The five HCP valence orbitals (5a through 2jt) are dominated by localized bonding descriptions similar to those of HCN (Table 28). The only differences are that the 6a bond in HCP has some P—C bond character in addition to the C—H bond contribution, and the lone pair electrons are more diffuse on the phosphorus than on nitrogen. The molecular structures of HCN, HCP and a number of substituted species have been determined by microwave techniques. Although studies of the nitriles are well advanced due to the stability of the species and the ease of obtaining the various isotopomers, data on the phosphaalkynes are less complete. The latter are studied by pyrolysis of organophosphorus precursors in flow systems, making isotopic substitution experiments more difficult and costly. In many cases. 

Although nitriles have an unshared pair of electrons, they are not effective hydrogen bond acceptors because the electrons occupy an sp -hybridized orbital. However, because acetonitrile is very polar, it is miscible in water. Propionitrile is moderately soluble in water. Acetonitrile is an excellent polar, aprotic solvent. It has a relatively high boihng point (81.5 °C) for a low molecular weight compound, and a high dielectric constant (38). 

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