The Structures of Cyclopropane−Amine van der Waals Complexes

Abstract

The rotational spectra of the cyclopropane−trimethylamine (CYC·TMA) and cyclopropane−dimethylamine (CYC·DMA) complexes have been observed using a pulsed nozzle, Fourier transform microwave spectrometer. The spectrum of CYC·TMA is characteristic of a symmetric top, Bo = 1172.2800(1) MHz. Unlike the symmetric top complex of cyclopropane−ammonia (Chem. Phys. Lett. 1994, 218, 349−352), the CYC·TMA complex did not exhibit effects from internal rotation about the C3 axis. The CYC·DMA complex has an asymmetric top spectrum with A = 5095.137(9) MHz, B = 1456.477(2) MHz, and C = 1301.365(1) MHz. Both structures have the amine sitting above the ring, with the lone pair pointing toward the cyclopropane. A comprehensive discussion of the three cyclopropane−amine complexes in terms of spectral characteristics and constants, structure, strength, and dynamics is included, as well as an effort to understand the origin of these interactions from an examination of the electrostatic potential of cyclopropane. We also contrast the cyclopropane−base complexes with the cyclopropane−acid complexes found in the literature.