Structure of benzene from mass-correlated rotational Raman spectroscopy.

We present high resolution rotational Raman spectra and derived geometry parameters for benzene. Rotational Raman spectra with sub-5 MHz resolution were obtained via high-resolution mass-correlated rotational alignment spectroscopy. Isotopologue spectra for C6H6, 13C-C5H6, C6D6, and 13C-C5D6 were distinguished through their correlated mass information. Spectra for 13C6H6 were obtained with lower resolution. Equilibrium and effective bond lengths were estimated from measured inertial moments, based on explicit assumptions and approximations. We discuss the origin of significant bias in previously published geometry parameters and the possibility to derive H,D isotope-specific bond lengths from purely experimental data.

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.

CRASY: correlated rotational alignment spectroscopy of pyridine. The rotational Raman spectrum of pyridine and asymmetric fragmentation of pyridine dimer cations.

We investigated the rotational Raman spectrum of pyridine monomers and pyridine dimers with mass-correlated rotational alignment spectroscopy (mass-CRASY) and ab initio calculations. The mass spectrum showed a strong signal for the protonated pyridine cation, which we assigned to asymmetric fragmentation of the dimer: ab initio calculations revealed facile proton transfer in the dimer cation and thermodynamically favorable asymmetric fragmentation. In the rotational spectrum correlated to the monomer mass channel, we assigned up to 40 lines for rotational states J ≤ 8. No spectrum could be assigned for the dimer, possibly due to the theoretically predicted presence of multiple dimer structures.

De novo structure determination of butadiene by isotope-resolved rotational Raman spectroscopy.

Isotope-selective rotational spectroscopy allows to calculate molecular structures independent of assumptions or theoretical predictions. Here, we present the first de novo structure determination based on mass-correlated rotational Raman spectroscopy, analyzing the carbon atom positions of butadiene. Mass correlation allowed us to analyze signals of rare 13C isotopologues at natural abundance, without interference from the main isotopologue signals. Fitted rotational constants and structural parameters confirm literature data from rovibrational spectroscopy of synthetic isotopologues and electron diffraction experiments.