Theory and application of photoelectron spectroscopy. 100. Variable-temperature photoelectron spectral study of 1,3-dithiol-2-one and 4,5-disubstituted 1,3-dithiol-2-ones. Thermal generation of 1,2-dithiete, 3,4-disubstituted 1,2-dithietes, and dialkyl tetrathiooxalates

Abstract

The thermal fragmentation of 1,3-dithiol-2-one, its 4,5-dimethyl, dicyano, monocyclic 4,5-dialkylthio (R4 = R5 = SCH3, SCzH5, SCH(CH&), and bicyclic alkylthio derivatives (R4v5 = SCH2S, S(CH&3, S(CH,),S) was investigated by variable-temperature photoelectron spectroscopy and in part by the matrix-isolation technique and infrared spectroscopy. The photoelectron spectra of the reactive intermediates 1,2-dithiete, 3,4dimethyl-1 ,Zdithiete, and diethyl tetrathiooxalate are presented and interpreted. The new reactive species are unambiguously identified by photoelectron spectral and quantum chemical means. In addition, the photoelectron spectra of the stable (i.e., isolable at room temperature) compounds 3,4-bis(trifluoromethyl)-1,2-dithiete and dimethyl tetrathiooxalate are discussed. The influence of substituents on the relative stabilities of the 1,2-dithiete vs. the 1,2-dithione structures is studied using the MNDO method. The theoretical as well as the experimental results show that 1,2-dithiete and 3,4-dimethyl-1,2-dithiete are thermodynamically favored above their 1,2-dithione counterparts.Variable-temperature photoelectron spectroscopy (VTPES) is a valuable tool for generating and observing reactive species in thermal reaction^.^^^ Our present experimental setup allows for the detection of reactive intermediates having a lifetime of ca.

Ab initio MO calculations on 1,2-dithietes and valence isomers

“…The barrier and the heat of isomerization amounts to 7.3 and 4.9 kcal/mol, respectively, at the highest level of theory employed in this article [CASPT2 (8,8)/6-31G(2 df)//MP2/6-31G*l. These values include zero-point energies. There are only small effects by electron correlation and the use of basis sets with more polarization functions; the barriers amount to 7.0 kcal/mol at the RHF/6-31G*//RHF/6-31G* and to 7.7 kcal/mol at the CASPT2(10,10)/6-3lG*//MP2/6-31G* lev- In contrast to RHF/6-31G* and MP2/6-31G* calculations, the gauche structure of lb is no longer energetically favored over the planar cis structure at the CASSCF and CASPT2 levels with an active space of (6,6) or (10,10). For MP2/6-31G*-optimized geometries, there is no energy difference between the cis and gauche structure at the CASSCF and CASPT2 levels.…”

“…The stabilization energies of 1,2-dithiete and 1,2-dithiin according Eqs. (6) and (7) are now almost equal (cf. …”

“…The ring is more stable by 3.8 (3.5 including ZPE) kcal/mol at CASPT2 (8,8)/6-31G(2 df)// MP2/6-31G* level. If the AN0 basis sets of Widmark et al [36] are used, there is only a small change in relative energies: 3.8 kcal/mol at the CASPT2 (6,6) cluding zero-point energies (cf. Table VIII).…”

“…For MP2/6-31G*-optimized geometries, there is no energy difference between the cis and gauche structure at the CASSCF and CASPT2 levels. According to CASPT2 (6,6) calculations with AN0 basis sets, the planar cis structure is lower in energy by 1.1 up to 2.3 kcal/mol than is the gauche structure (cf. Table VIII).…”

“…1,2-Dithietes have also been generated by pyrolysis of 1,3-dithiol-2-ones in the gas phase or were trapped in an argon matrix at 10 K [81. Photoelectron [6] and low-temperature matrix IR spectra [81 supported the cyclic 1,2-dithiete structure. The analysis of the gas-phase microwave spectrum of the parent compound is compatible with the planar ring structure of l a with C,, symmetry [71.…”