The microwave spectrum of cyclopropane-ammonia. A novel structure for cyclopropane complexes

The microwave spectrum of the cyclopropane-ammonia ( CsHs* 14NHs) complex has been observed using a pulsed nozzle, Fourier-transform microwave spectrometer. The spectrum is characteristic of a symmetric top, IS,,= 2668.7 16 l(4), with free internal rotation of the NHs subunit. The spectra of the CsHs*r5NHs, CsDs*r5NHs, and CSHe*‘4NDs isotopomers were also measured. This gives a structure in which the nitrogen of the ammonia interacts with the top of the cyclopropane ring, resulting in a stacked structure with R ,.,,=3.657( 3). Thequadrupolecouplingconstant ofthe nitrogen nucleus is eQq= -2.509(2) MHz. Cyclopropane (CYC) complexes, such as CYC-HCl [I], CYC-HF [2], CYC-HCN [3], and CYC-Hz0 [ 4 1, have been thoroughly investigated by Fourier-transform microwave (FTMW) spectroscopy in order to determine their structure. In each case the HX lies in the heavy atom plane and is hydrogen bonded to the center of one of the C-C bonds. These studies support the Coulson and Moffitt description of cyclopropane [ 5 ] since the interaction occurs at the area of highest electron density of the ring. We have found that the complex of cyclopropane-ammonia does not conform to this pattern. Using FTMW spectroscopy we have found that it is a symmetric top, with a stacked structure, in which the lone pair of the ammonia interacts with the hydrogens above the ring. The only previous report of the CYC*NH3 complex was a matrix-isolation vibrational spectroscopy study which could find no evidence of complex formation in the spectra [ 6 1. The microwave spectra of the CYC-NH3 isotopomers were observed using a Balle-Flygare-type FTMW spectrometer [ 71. A mixture of approximately 1% of each component in argon was maintained at a total pressure of l-2 atm. Transitions found in the C3H6 -“NH3 sample showed the firstand second-order Stark shifts characteristic of a symmetric top. This led to the assignment and fitting of the transitions to Eq. (1) (below), the expression describing a symmetric top with free internal rotation about its C, axis [ 8,9], -2oJ*(J+ 1)m2-20,,,(J+ 1)Km -2H,,,(J+ l)m2K2. (1) The fit for the CJH615NHs species led to assignment of the C3H6i4NHs transitions, found in Table 1, as well as for the C3D6* “NH3 isotopomer. The quadrupole structure for the 14N species was difficult to decipher since the internal rotor states had splittings on the same order of magnitude as the quadrupole splittings. Only the 1 m I= 0, 1 states were observed; given the cold temperature of the expansion, only these states would expected to be populated. A cou0009-2614/94/$07.00