Conformational study of cis-cyclononene by dynamic NMR spectroscopy and computational methods

The C-13 NMR spectrum of the olefinic carbons of cis-cyclononene (1) at -18 9.3 degrees C showed three peaks, still exchange-broadened, which were inte rpreted in terms of two conformations. Relative free energies of conformati ons were estimated from ab initio calculations at the HF/6-311G* level and molecular mechanics (MM3) calculations, and possible assignments were made on the basis of these calculations and the experimental data. The major con formation has C-1 symmetry and a population of 66% at -189.3 degrees C. The structure suggested to be the minor conformation also has C1 symmetry, but the deviation from C, symmetry is small, resulting in a time-averaged plan e of symmetry, even at -189.3 degrees C. Free-energy barriers of 4.18 +/- 0 .15 and 4.28 +/- 0.15 kcal/mol for interconversion of the two conformations at -183.1 degrees C were calculated from the rate constants derived from l ine shape matching of the C-13 spectrum of the olefinic carbons at this tem perature. Major-to-minor free-energy barriers for four temperatures from -1 78.3 to -189.3 degrees C were within the range of 4.28 to 4.35 kcal/mol. In terconversion of sites within the major conformation occurs through convers ion to the minor conformation. The effects of a second process were observe d in the H-1 NMR spectra of the allylic protons of 1. Two multiplets of equ al intensity were found by -124.8 degrees C, and a free-energy barrier of 8 .05 +/- 0.2 kcal/mol was calculated at coalescence (-102.3 degrees C). The changes were suggested to be due to slowing the exchange of geminal hydroge n positions.