Millimeter-wave spectroscopy of rare isotopomers of HC5N and DC5N:: determination of a mixed experimental-theoretical equilibrium structure for cyanobutadiyne

Cyanobutadiyne has been produced by gas phase copyrolysis of pyridine and phosphorus trichloride in a flow reactor. The yield of the reaction is sufficiently good to allow the detection of rotational transitions of the C-13 and N-15 containing species in natural abundance. Normal pyridine and its fully deuterated variant have been used as precursors, making it possible to study the ground-state rotational spectra of 14 isotopomers in the millimeter wave region. Very accurate values of the rotational and quartic centrifugal distortion constants have been obtained for all the isotopic species investigated, and in addition the sextic distortion constant has been precisely determined for the most abundant variants (HC5N)-C-12-N-14 and (DC5N)-C-12-N-14, for which the measurements have been extended Lip to 460GHz. A mixed experimental-theoretical equilibrium structure has been evaluated for cyanobutadiyne combining experimental ground-state rotational constants with theoretically computed zero-point contributions. The r(e), geometry is compared with operationally defined purely experimental structures, namely r(0), r(s), and r(m)((1)) molecular structures. (C) 2004 Elsevier Inc. All rights reserved.