The Structures of Cyclopropane−Amine van der Waals Complexes

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.