OBSERVATION OF TUNNELING SPLITTINGS IN THE 10 MU-M INFRARED-SPECTRA OF CYCLOPROPYLAMINE AND PROPARGYLAMINE

Three vibrational bands for each of the primary amines, cyclopropylamine and propargylamine, have been studied in the 900-1100 cm(-1) region using a 2 MHz resolution (fwhm) electric resonance optothermal molecular beam spectrometer with a microwave sideband CO2 laser. All the observed bands exhibit tunneling doublets with 3:1 relative intensities, indicative of amine proton interchange. No evidence for tunneling splittings has been observed in the ground-state microwave spectra of these molecules, although such splittings have been observed in the microwave spectra of other primary amines. For trans-cyclopropylamine the v(10) and v(23) fundamental bands and the v(13) + v(27) torsional combination band have been observed near 1020, 1045, and 1006 cm(-1), respectively. The J = 0 tunneling splittings for v(10) and v(23) are 3.6(4) and 21.3(4) MHz. These splittings are essentially independent of J and K-a. For v(13) + v(27) the tunneling splittings vary from 923(14) MHz for K-a = 0 to 675(14) MHz for K-a = 4. Because the tunneling gaps in the ground state are unknown, these splittings measure only the difference in tunneling splitting between the ground and excited states. The rotational progressions for the tunneling sublevels of v(10) and v(23) are well characterized by an asymmetric-top Hamiltonian, except for a Coriolis perturbation of the 928 level of ylo The rotational levels for the combination vibration are poorly characterized by an asymmetric rotor Hamiltonian. For transpropargylamine, the vs and v(9) fundamental bands and an unidentified combination band, presumably in Fermi resonance with v(9), have been observed near 1076, 931, and 929 cm(-1) respectively. The J, K-a = 0 tunneling splittings, measuring the difference in tunneling splitting between the ground and excited states, are +748(1) MHz for ys and -404(2) and -350(4) MHz for the Fermi diad, with the splittings for all three bands showing a strong K-a dependence. The origins of the tunneling splittings are discussed.