A. E. Bruno

Thioketone spectroscopy: An analysis of the lower electronic transitions in thioacetone and thioacetaldehyde

Abstract Visible and ultraviolet spectra of the unstable species, thioacetaldehyde, CH 3 CHS, thioacetone, (CH 3 ) 2 CS, and thioacetone- d 6 have been studied in the gas phase. The valence n → π* excitations, A ← X and a ← X, have been identified. Rydberg n → 4s, 4p y and 4p z and π → π* valence excitations have been found in thioacetone. In thioacetaldehyde a Rydberg-valence interaction mixes the n ,4s and π,π* states which leads to a broad absorption of mixed character between 200 and 220 nm.

Thiocarbonyl spectroscopy: Methyl torsional vibrations and internal rotational barriers of thioacetaldehyde in its ã 3A‘ and X̃ 1A’ states

Thioacetaldehyde was prepared by the vacuum flash pyrolysis of 1, 3, 5‐trimethyl‐s‐trithiane and was observed spectroscopically in a rapidly flowing system. The 614 nm absorption system of CH3CHS/CH3CDS/CD3CHS/CD3CDS was recorded photographically at path lengths of 72 m. Phosophorescence excitation and emission spectra of CH3CHS were observed with a cw dye laser. The assignment of the first absorption system was made to the spin forbidden transition a 3A‘←X 1A’. The bands observed in the region of the origin proved to have a very complex structure, and were assigned to torsional transitions. The observation of several hot band progressions which could be fitted to levels derived from microwave data gave the parameters V‘3 =534.3 cm−1, F‘=6.9950 cm−1, and ν0=16 293.8 cm−1 for CH3CHS. The fit to the excited state torsional levels was less satisfactory and gave V3 =94.18 cm−1, F’=5.3608 cm−1. An analysis of the data for the other isotopic species showed that the torsional motion in the upper state was coup...

Potential energy surfaces for the molecular and free radical dissociations of H2CS, F2CS and Cl2CS: An ab initio SCF MO study

Abstract Energies along the planar symmetric (C 2v ) and planar assymetric (C s paths to molecular dissociation of the ground state thiocarbonyl halides, F 2 CS and Cl 2 CS, together with their transition state geometries, have been calculated by ab initio SCF MO methods using the STO-3G and 4-31G basis sets. For comparison, results on H 2 CS at similar levels of calculation are also included in this report. In addition, the 4-31G ** basis set has been employed to predict the geometries of the ground state species and the endothermicities of their free radical dissociations. The results of experiments in which the lowest excited singlet states of these molecules have been photoexcited are interpreted in light of these calculations. Thermodynamic data for both molecular and free radical dissociations are evaluated and discussed.

Vibronic analyses of the Rydberg and lower intravalence electronic transitions in thioacetone

Abstract The absorption spectrum of thioacetone, (CH3)2CS, has been surveyed over the region 700-190 nm, and five distinct absorptions have been identified. These are the spin-allowed and spin-forbidden n → π ∗ , A ( 1 A 2 ) ← X ( 1 A 1 ) , and a ( 3 A 2 ) ← X ( 1 A 1 ) transitions; the orbitally allowed π → π ∗ , B ( 1 A 1 ) ← X ( 1 A 1 ) transition; and the Rydberg transitions n → 4s, C ( 1 B 2 ) ← X ( 1 A 1 ) , n → 4pz, D ( 1 B 2 ) ← X ( 1 A 1 ) , and n → 4py, E ( 1 A 1 ) ← X ( 1 A 1 ) . In addition, the laser phosphorescence excitation spectrum has been obtained for the a ← X transition. Partial vibrational assignments have been obtained for all of the excited electronic states. The analyses showed that the a , A , and C states are all planar or pseudo-planar. Of interest is the vibrational activity of the methyl torsional modes. The presence of torsional progressions in the visible absorption spectrum led to the conclusion that the CH3 groups are rotated in the a state by 60° from the X state. Observation of torsional sequence transitions rather than progressions in the C ← X spectrum indicates that the methyl groups do not change their configuration on excitation into the C state.

Decay dynamics of the lowest triplet and lowest excited singlet states of thioacetaldehyde and thioacetone

Abstract The lifetimes of the emission observed when thioacetone and thioacetaldehyde are excited in their a-X and A-X absorption systems have been measured as a function of pressure. The lifetimes of the a states under collision-free conditions are in the 10 μs region, vary slightly with vibrational energy content and, in the case of thioacetone, on deuteriation. Collision-induced intersystem crossing is identified as a likely mechanism of radiationless decay of the a states, and is also implicated in the decay of the A states in both molecules.