HEXAFLUOROCYCLOBUTENE - IS THE C-3-C-4 SINGLE BOND ABNORMALLY LONG

A recent investigation of hexafluorocyclobutene by microwave spectrosc opy (MW) has led to values for the lengths of the single bonds in the carbon ring that are less than those from our earlier electron diffrac tion (ED) study by amounts in excess of the estimated uncertainties of the two determinations. In an attempt to resolve the inconsistencies, we have reanalyzed our ED data making use of the rotational constants , which had not been available for the original study, as structural c onstraints. The results of the combined analysis for the lengths (r(al pha)0) of the C-C single bonds adjacent (opposite) the double bond are 1.500(5) angstrom (1.582(8) angstrom); the corresponding r(g) values arc essentially unchanged from those reported before. The r0 MW values from the fitting of data from three isotopic species are shorter than our r(alpha)0 ones by about 0.02 and 0.03 angstrom, respectively. The explanation for the differences remains unclear.