Conformational equilibria in some cyclic imines: NH and CH stretching vibrations and the axial lone pair

Abstract

The fundamental NH and CH stretching vibrations of a number of cyclic imines have been examined in dilute CCI, solution. A trat~s orientation of the N lone pair orbital and one or more hydrogen atoms on adjacent carbons lowers the relevant vo, ("Bohlmann" bands) and raises VNH, reflecting an increase in the s-character of the CH bond(s), consistent with a partial delocalization of the lone pair electrons into the CN bond. Conformations in which this interaction occurs are thermodynamically favored, and the AH values for the lone pair axial-equatorial equilibrium in piperidine, pyrrolidine, and indolene are estimated to be 0.4, 0.2, and 0.1 kcal/mole, respectively, in dilute CCI, solution.The effects of hetero ring size, N-substitution, a-methyl substitution, and the solvent environment are investigated.Canadian Journal o f Chemistry, 48,3236 (1970) Introduction Conformational problems which arise from different orientations of the nitrogen atom lone pair orbital in heterocyclic compounds (la) have received much attention in the case of sixmembered rings like piperidine or morpholine, and a variety of methods like i.r. spectroscopy (2-12), n.m.r. spectroscopy (12-22), microwave spectroscopy (23), and dipole moment measurements (24-30) have been employed. The results of these studies have not been uniformlv consistent.

Vibrational overtone spectroscopy of pyrrole and pyrrolidine

“…We will present the infrared absorption spectrum of pyrroline and pyrrolidine in another paper 37. The N-H first overtone transition wavenumbers presented in Table 1 agree with those reported by Baldock and Katritzky 35 . The pyrrolidine v=5 C-H peak at 13,495 cm-1 is over 350 cm-I wide and probably is a convolution of at least two transitions.…”

Theoretical and experimental vibrational spectroscopic study of 4-(1-Pyrrolidinyl)piperidine

“…The free piperidine has a main NH band at 3343 cm À1 with a shoulder at 3315 cm À1 in the infrared spectrum. The dominant high wavenumber component is assigned to the NH equatorial form and the low wavenumber component is attributed to the axial conformation [29]. NH bands of 4-pypp have been observed at 3265 cm À1 (IR-s,b), 3246 cm À1 (R-m), 3226 cm À1 (IR-vw), 3223 cm À1 (R-s).…”

Influence of calculation level and effect of methylation on axial/equatorial equilibria in piperidines

“…1 ᎐ 12 Experimentally, piperidine has been studied mainly by means of indirect techniques, such as ␦ measurements in ae 1 H NMR, 1 dipole moment determinations, 2 analysis of Bohlman bands in the IR spectrum, 3 dynamic 13 C NMR spectra, 4 or calorimetric measurements. 5 After a long controversy, 9, 10 the equatorial preference of the N-H group was definitively established, taking ⌬GЊ s 0.4 " 0.2 kcalrmol as an average value.…”