C—N bond lengths

Since HF calculations have a tendency to underestimate the N—N and the C—N bond lengths in triazoles [98JPC(A)620, 98JPC(A) 10348], the structural parameters should be computed at least at the DFT or MP2 levels. This is particularly true if electron-donating substituents are attached to the ring. Nitrogen NMR shielding tensors were computed for a set of methylated triazoles and tetrazoles but will be discussed in the context of tetrazoles (cf. Section IV,B). 

As expected, the nitronate fragment is planar in all nitronates. The C=N bond is shortened (in comparison with standard C—N bond), and its length is similar to the C=N bond lengths in reference structures (86) and (87). Two... 

One could expect a similar bond length-barrier relation to hold for the rotations of dimethylamino and acetyl groups, but here the data are much less convincing. In a pair like 93a and b with AG (C—N) = 13.4 and 17.0 kcal/mol, respectively (Table 10), the C—N bond lengths fall in the correct order, but otherwise no correlation is observed. 

The structure of the corresponding [Co(terpy)2]3 + ion in the complex [Co(terpy)2][Cl]3 is also available.146 In this case too, the C-N bond lengths of the central pyridine (1.86 A) are shorter than those of the peripheral pyridines (1.93 A). 

FIGURE 5. Histogram showing distribution of calculated gas phase C=N bond lengths in the aldoximes and ketoximes in Table 2... 

FIGURE 15. Histogram showing distribution of C—N bond lengths in crystal phase hydroxamic acids... 

---