Fourier transform rotational spectrum and molecular structure of vinylcyclopropane

Abstract

The sensitive method of pulsed-nozzle Fourier-transform microwave spectroscopy has been employed to record selected weaker features of the natural abundance rotational spectrum of vinylcyclopropane. The weak µ_c type rotational transitions of the common species were observed for the first time. The rotational spectra of all singly substituted ¹³C isotopic species were also recorded. The resulting rotational constants were used in the first microwave determination of the heavy-atom geometry of vinylcyclopropane and r_s, r₀ and r* structural parameters were derived. The effect on the r_s geometry of corrections for bond shrinkage on isotopic substitution was examined. The reliability of the various structures for this type of molecule is discussed; it is anticipated that the most reliable structure will be that in which the experimental data are viewed with assumptions concerning the angular geometry of the CH bonds based on results of ab initio calculations. In the preferred r* structure r(CC)= 1.346 Å, r(C—C_ring)= 1.466(3)Å and the two ring CC bond lengths are 1.518(3) and 1.505(3)Å, adjacent and opposite to the substituted vertex.