The microwave spectrum and structure of the pyridine–CO complex

Abstract

Rotational spectra of five isotopomers of the pyridine-CO complex with D, 13 C, 15 N, and 18 O have been measured over the 8 -18 GHz frequency range. Rotational constants, centrifugal distortion constants, and 14 N quadrupole coupling constants have been fitted to the measured transition frequencies of each isotopomer. Two different structures have been found to be compatible with the moments of inertia of all isotopomers. Both have CO lying in the plane of pyridine approaching simultaneously N and ortho-H but with two different orientations of CO.

Microwave spectra and structure of the fluorobenzene-carbon monoxide van der Waals complex

“…The orientation of CO, which is given by angle ½, is not always uniquely determined as in the complexes with pyrrole [14] and pyridine [15]. The centre of mass which ® xes the a principal axis of the complex is nearly in the middle of the C± O bond.…”

“…The C± O bond length of 1.1267 A Ê for the complexed CO is slightly shorter than the corresponding value of 1.129 48 A Ê for the free CO [43] from the substitution coordinates. A more pronounced shortening was observed in pyrrole± CO [14], pyridine± CO [15] and isoxazole± CO [17], and was attributed to large amplitude motions. CO was almost parallel to the ring plane, with the carbon slightly closer to it; the same e ect was observed in the benzene± CO complex [13].…”

“…In the complexes of pyridine [15] and isoxazole [17], CO binds in the plane of the aromatic molecule. It forms an angle of about 62± 648 to the line connecting the centres of mass of the monomers.…”

“…Here, a geometry with CO, again above the p electron system in the symmetry plane perpendicular to the ring plane, was determined from spectra containing di erent CO isotopomers. In the pyridine± CO complex, it came as a surprise that CO attaches in the ring plane [15]. Detailed investigations of isotopomers revealed that CO lies in the region between the nitrogen and the ¬ hydrogen of pyridine.…”

Millimeter and submillimeter wave rotational spectrum of pyridine in the ground and excited vibrational states

“…Improved constants for the various isotopomers were obtained by Mata et al [8] between 14 and 27 GHz to yield what is currently the best r s structure for pyridine. As an aside, a further isotopomer, 4-13 C-15 N, was incidentally measured between 8 and 15 GHz while examining the spectrum of the pyridine-CO complex by Bettens and Bauder [9].…”