The T-1 <- S-0 vibronic spectrum and the structure of the 2-chloroethanal molecule in the T-1 state

The vibronic absorption spectrum of 2-chloroethanal (CH2ClCHO) vapor was me asured with the use of a multipass optical cell with a long optical path (u p to 120 m). The spectrum in the wave length range 347-394 nm was assigned to the T-1 <-- S-0 electronic transition (from the ground S-0 state to the lowest excited triplet T-1 state). The vibrational structure of the spectru m was studied. The T-1 <-- S-0 excitation of 2-chloroethanal conformers was shown to be accompanied by rotation of tops and pyramidal distortions of c arbonyl fragments, which are planar in the S-0 state. The spectral bands we re assigned to two systems of vibronic transitions, conformer I(T-1) <-- tr ans-conformer (S-0) and conformer II(T-1) <-- trans-conformer (S-0) with th e origins (0(0)(0) transitions between zero vibrational levels) at 26650 (e stimate) and 27451 cm(-1), respectively. Several fundamental frequencies, i ncluding torsional and inversion energy levels, were determined for the con formers specified above. The experimental data closely agree with the resul ts of ab initio calculations of the 2-chloroethanal molecule in the S-0 and T-1 states.