MICROWAVE-SPECTRA OF THE METHYLCYANOPOLYYNES CH3(C-C)(N)CN, N = 2, 3,4, 5

The ground state rotational spectra of four methylcyanopolyynes CH3(C= C)(2)CN, CH3(C=C)(3)CN, CH3(C=C)(4)CN, and CH3(C=C)(5)CN were measured between 6 and 22 GHz by Fourier transform microwave spectroscopy. For CH3(C=C)(2)CN, previously observed by Alexander et al. [J. Mol. Spect rosc. 70, 84-44 (1978)] eight transitions were observed, each with suf ficient resolution to determine the nitrogen quadrupole hyperfine cons tant for the first time: eqQ = -4.25(3) MHz. The longer three methylcy anopolyynes are new, for these, from 10 to 21 rotational transitions w ere detected and precise values for the rotational and the leading cen trifugal distortion constants, and eqQ for CH3(C=C)(3)CN, were determi ned. (C) 1998 Academic Press.