Pyramidal nitrogens calculations

Both AMI calculations on A-acetoxy-A-methoxybenzamide38 and ab initio 6-31G calculations on A-formyloxy-A-methoxyformamide 405 45 predict a strongly pyramidal nitrogen. Its lowest energy conformation at HF/6-31G is depicted in Fig. 5a while structural data are provided in Table 1 together with that for A-methoxyformamide 41. 

Such a substitution may involve substantial changes in the structure and disturb the cyclic electron delocalization. Thus, 1,4-dihydropyrazine (25), as judged from X-ray data on its N-substituted derivatives [83AG(E)171] as well as from the results of MNDO [84JMS( 109)277] and ab initio (6-31G) (88JOC2127) calculations, has a slightly bent boat structure with pyramidalized nitrogen atoms. 

Incorporation of d-functions on the inverting N site is especially important. Such functions contribute more to the pyramidal GS than to the TS and thus stabilize the GS with respect to the TS. In the absence of -functions on N, the inversion barrier in NH3 is found to be much too small and H2N—CN and H2N—SiHs are found to be more stable in the planar form inclusion of -functions (and p on H) leads to the correct NH3 barrier 158> and to pyramidal nitrogen sites in HaN—CN and H2N— SiH3 (Table 8 144>). Of course, the relative error introduced by the absence of -functions is smaller the higher the barrier. This may explain in part why the barriers calculated for aziridine and oxaziridine without including -functions are in satisfactory agreement with experimental values. In PH3, the phosphorus inversion barrier changes from 30.9 to 37.2 kcal/mole on inclusion of two sets of -functions 159>. 

In all cases, the high degree of pyramidality attributed to the presence of two electronegative oxygen atoms at nitrogen, confirms the predictions of HF/6-31G calculations.5 On the basis of geometries for all acyclic amides in the Cambridge Structural Database in 2002,117 amides 31b and 31f are the most pyramidal amides of this type.5... 

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