Quenching the Effects of Proton Tunnelling and Methyl Internal Rotations of Dimethylamine by Water Addition
The rotational spectra of dimethylamine4 and its water complex have been investigated. In dimethylamine, the hydrogen atom of the amino group performs an inversion tunnelling motion, causing splittings of all c-type transitions into doublets with a separation of 2646.0 MHz4. The first microwave spectrum of dimethylamine has been recorded in 1968 using a Stark modulated spectrometer with a resolution of 25 kHz. The 14N quadrupole hyperfine structure was reported to be resolved only for some transitions and the internal rotation splittings in the vibrational ground state could not be resolved.
In the present work, the spectra of dimethylamine were remeasured using two molecular jet Fourier transform microwave spectrometers operating from 2 to 40 GHz. The proton tunnelling was analysed, taking Coriolis interactions into account which cause splittings in the order of about 0.2 to 1.2 MHz for all b-type transitions. Furthermore, hyperfine splittings due to the quadrupole coupling of the 14N nucleus were fully resolved. The quadrupole coupling constants Χaa and Χbb - Χcc were determined with high accuracy and calculated well using Bailey’s method5. The splittings into triplets in the order of 200 kHz, arising from internal rotations of two equivalent methyl groups, could be resolved. The addition of water to dimethylamine has deactivated the proton tunnelling inversion as well as decreased the torsional splittings below the resolution limit of 2 kHz.
4J. E. Wollrab and V. W. Laurie, J. Chem. Phys., 1968,48, 5058.
5W. C. Bailey, Chem. Phys., 2000,252, 57.