Hydrogen cyanide model system

This section presents a discussion of the treatment outlined above applied to a realistic molecular reaction. For simplicity we have chosen the isomerization of hydrogen cyanide (HCN) to form hydrogen isocyanide (HNC) because it is commonly used as model system in the study of isomerization reactions. This reaction, which has been the subject of innumerable studies, is excellent for our purposes. While the system is small (only three atoms) it does possess sufficient complexity to be an excellent test case. The importance of this system as a test case, both experimentally and theoretically, is demonstrated by the size of the literature [82,83-141]. This list is by no means exhaustive, nor particularly representative. 

Hydrogen cyanide is reduced by six electrons to equivalent amounts of NH3 and CH4 (46, 60), a reaction observed with no other homogeneous catalyst and not reported for the molybdenum-thiol-borohydride system proposed as a model of N2ase (55). Methylamine, a 4-electron product, may also be formed in amounts equivalent to 10% of the major products (46). It is not known whether HCN or CN" is the actual substrate, since both species are present at reaction pH. The affinity for cyanide based on equilibrium concentration of HCN is intermediate between that for N2 or C2H2 and N2O or N3, while the affinity based on CN" concentration is substantially greater (28, 46, 50). The rate of electron consumption is less than that seen in N2 reduction, which may indicate partial inhibition of N2ase by cyanide and/or the generation of undetermined products. 

Since hydrogen cyanide and ammonia are two of the most important parent compounds present in comets and since these bodies are intensely irradiated when they pass througih their closest proximity to the Sun, it is not unreasonable to think that some of the organic compounds described above may also be found in comets. The chemical composition of these bodies is considered to be representative of the composition of the solar nebula from which the primitive earth was formed. Therefore, it offers a solar-system model for studies on prebiological chemistry.< >... 

Double and triple bonds between heteroatoms and the carbon atom (N=C, NsC, 0=C) were treated similar to the carbon-carbon multiple bonds A set of p functions was placed on each jr-bond. The model systems inclnde methanimine [47], hydrogen cyanide [44], and formaldehyde [48] the results of the optimizations are presented in Table 1. As can be seen, if the heteroatom is nitrogen, even one 5-type function is sufficient in addition to the p functions used for the r-bonds to achieve the accuracy of the reoptimized AO basis set. For oxygen, two s functions are desirable since the energies obtained with the Islp BF set are only shghtly better than those compnted with the r6-31G basis, and, as we have seen for hydrocarbons, if the BF basis sets optimized for the small molecules are used for bigger systans, their performance with respect to the original AO bases is relatively worse. 

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