Mass-Correlated High-Resolution Spectra and the Structure of Benzene

Mass-correlated rotational alignment spectroscopy resolved the rotational Raman spectra for 5 benzene isotopologues with unprecedented resolution. 13-C isotopologues were characterized at natural abundance. Fitted rotational constants allowed the analysis of effective and equilibrium bond lengths for benzene with sub-mÅ uncertainties. We found that previously reported experimental structures were wrong by multiple mÅ, due to unrecognized H/D isotope effects. Our results also refute recent experimental and theoretical literature claims of identical effective C–H and C–D bond lengths in benzene and reveal an isotope effect similar to that in other small molecules.