ROTATIONAL SPECTRA AND MOLECULAR-STRUCTURES OF N-NITROSO-DERIVATIVES OF PYRROLIDINE, PIPERIDINE, AND MORPHOLINE AND THEIR ISOELECTRONIC FORMYL ANALOGS

Three isoelectronic pairs of compounds, N-formyl- and N-nitrosopyrroli dine, piperidine, and morpholine, have been characterized by microwave spectroscopy. Only one conformational isomer is observed in each comp ound. Both the inertial constants and the N-14 quadrupole coupling con stants determined here show that the molecular geometries of each isol ectronic pair are extremely similar. Rotational spectra of vibrational ly excited species of N-nitrosopyrrolidine and N-nitrosopiperidine are assigned. The range of structural variation among the vibrationally e xcited states is comparable to the structural variation within the iso electronic pairs of compounds, suggesting that a biological receptor f or one compound is likely to bind its isoelectronic analog as well. Ab initio molecular orbital calculations of N-formylpyrrolidine at the M P2/6-31G*//MP2/31G** level reproduce the observed rotational econstan ts extremely well with a model that includes essentially planar bondin g about the amino N atom. (C) 1998 Elsevier Science B.V.