Structural changes upon electronic excitation in 1,3-dimethoxybenzene from Franck-Condon/rotational constants fits of the fluorescence emission spectra

•An analysis of the excited state structure of two conformers of 1,3-dimethoxybenzene has been given on the basis of a combined Franck-Condon/rotational constant changes analysis.•The geometry changes are compared to the results of ab initio calculations at the approximate coupled cluster singles and doubles (CC2) level of theory.•The influence of the lone pair orientation of the methoxy oxygen atoms on the electron density differences between both states is surprisingly large. The geometry changes of two rotamers of 1,3-dimethoxybenzene were determined from a combined fit of line intensities in fluorescence emission spectra, obtained via pumping different vibronic transition and the changes of rotational constants upon electronic excitation. The so determined changes are compared to the results of ab initio approximate coupled cluster singles and doubles (CC2) calculations. The bond length variations in the aromatic ring follow the calculated electron density differences from the coupled cluster wavefunctions and are in agreement with excitation to an 1Lb-state for the C2v symmetric B-conformer.