Carbon-13 NMR spectra of [cis-2-(N,N-dimethylamino)cyclopentyl]trimethylammonium iodide (4a) and triflate (4b) in acetone-d(6) and in CD3NO2 show inversion of the amine with rotation about the ring-N bond to be unusually slow with Delta H double dagger and Delta S double dagger (4b in acetone-d(6)) of 17 kcal/mol and 14 eu, similar to the values for cis-2-tertbutyl-1-(N,N-dimethylamino)cyclopentane (3) in Et(2)O-d(10) of 16 kcal/mol and 17 eu, indicating that steric and not electrostatic interactions are responsible for the slow rates. In addition, the N+ methyls of 4a and 4b, which are magnetically nonequivalent by C-13 NMR at low temperatures, undergo signal averaging with increasing temperature, the result of rotation dynamics around the ring-N+ bonds giving for 4b in acetone-d(6) Delta H double dagger and Delta S double dagger, respectively, of 7.2 kcal/mol and -9 eu. Similar values are found for tert-butyl rotation in 3 in Et(2)O-d(10). Around room temperature for 4a and 4b, (1)J(N-14,C-13) is resolved and found to be ca. 3.6 Hz. With decreasing temperature, this coupling is averaged by progressively faster N-14 electric quadrupole induced relaxation. Line shape analysis of the C-13 NMR spectra shows E(a) for the correlation time to be 2-2.7 kcal/mol. X-ray crystallography of 4b and (cis-2-tertbutylcyclopentyl)trimethylammonium triflate (7) shows the rings to adopt the half-chair structure with vicinal substituents located at the puckered carbons. In 4b, the N(+)Me(3) group is pseudoequatorial with the NMe(2) group pseudoaxial; whereas, in 7, the tert-butyl group is pseudoequatorial with the N(+)Me(3) group pseudoaxial. Interesting correlations are noted of C-13 NMR shifts of individual methyls with their crystallographic structural assignments.