FOURIER-TRANSFORM ROTATIONAL SPECTRUM AND MOLECULAR-STRUCTURE OF VINYLCYCLOPROPANE

The sensitive method of pulsed-nozzle Fourier-transform microwave spec troscopy has been employed to record selected weaker features of the n atural abundance rotational spectrum of vinylcyclopropane. The weak mu (c) type rotational transitions of the common species were observed fo r the first time. The rotational spectra of all singly substituted C-1 3 isotopic species were also recorded. The resulting rotational consta nts were used in the first microwave determination of the heavy-atom g eometry of vinylcyclopropane and r(s), r(o) and r structural paramete rs were derived. The effect on the r(s) geometry of corrections for bo nd 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. I n the preferred r structure r(C=C)=1.346 Angstrom, r(C--C-ring)=1.466 (3) Angstrom and the two ring CC bond lengths are 1.518(3) and 1.505(3 ) Angstrom, adjacent and opposite to the substituted vertex.