Minimum energy conformation of ortho-xylene in its ground and first excited electronic states

The geometry of ortho-xylene is studied through supersonic jet cooling and one color mass resolved excitation spectroscopy. By examining the S1←S0 transition origin region of various (d0–d2) methyl deuterated o-xylene species, the conformation of the two methyl groups in S0 and S1 can be determined. A comparison between the predicted and experimentally observed number and intensity of origin features for the various partially deuterated o-xylenes shows that two highly symmetric methyl group structures are possible, each structure having C2V point group symmetry—the antiplanar conformation (A1), in which τ1(C2–C1–Cα–Hα)=180°; τ2(C1–C2–Cα′–Hα′)=180°; and the syn, planar conformation (A2) in which τ1=τ2=0°. The experimentally determined structures are consistent with ab initio calculations and microwave studies which predict the anticonformation to be most stable. Spectroscopic data, in conjunction with further ab initio calculations, are used to investigate the ground and excited state potential energy surfaces. In addition, a repulsive, intramolecular, nonbonded interaction between the two methyl groups is identified through an empirical potential energy calculation to be the most important interaction defining the lowest energy structure.