Isomerization and Fragmentation Reactions on the [C2SH4] Potential Energy Surface. The Metastable Thione-S-Methylide Isomer

Abstract

Thione S-methylide (TSM), the parent species of the thiocarbonyl ylide family, is a 1,3-dipolar, planar species on the [C2SH4] potential energy surface (PES), which has not shared the richness of studies dedicated to its isomers, the cyclic thiirane (THI), and the keto-enol pair vinyl thiol (VTH)/thioacetaldehyde (THA). While the conrotatory ring closure reaction toward THI was studied in the ‘90s, no complete analysis of the PES is available in the literature. In the present paper, we report a computational study of the reaction scheme linking all species on that PES. We employ several levels of calculation, ranging from density functional theory (DFT), through CCSD(T) based composite schemes, to CASSCF/CASPT2 multi-reference procedures, to find the best description of TSM, its isomers, and the transition states (TSs) ruling their interconversion. Fragmentation of TSM, THA and THI were investigated and compared to the available experimental information. We found that the B2PLYP-D3 functional, contrary to M06-2XD3 or B97X-D, describes well the geometry of both TSM and the transition state connecting it to THI. The reverse barrier, from THI to TSM, amounts to 52.2 kcal mol-1 (to be compared to 17.6 kcal mol-1 for the direct one), thus explaining why, in general, thiocarbonyl ylides cannot be prepared from thiiranes. Conversion of THI to VTH implies also a large barrier, explaining why the reaction has been observed only at high temperatures. The fragmentation of THI to S(3P) or S(1D) and ethylene was also explored, together with the decomposition to H2S plus acetylene. Open species, both in triplet and singlet states, were identified as intermediates in the fragmentations, and their energies were found to be lower than the transition state for the isomerization of THI to VTH, thus explaining the preference for fragmentation over isomerization at relatively low temperatures.

Accurate Biomolecular Structures by the Nano-LEGO Approach: Pick the Bricks and Build Your Geometry

“…In passing, it should be noted that the previous LRA parameterization for the B3LYP and B2PLYP functionals was performed by using the SNSD and cc-pVTZ basis sets, respectively, whereas recent investigations have pointed out the necessity of augmenting the basis set with an additional set of d functions for the S atom (and in general for second-row elements) in order to improve the predictions of structural, spectroscopic, 22 and thermochemical properties. 8,76,77 The CS distance is systematically overestimated at the rev-DSDBPEP86/jun-cc-pV(D+d)Z level of theory by about 0.004 Å, with a maximum error of 0.009 Å, whereas CCS, CSC, and SCH angles are predicted with a remarkable accuracy, with the MADs being 0.09, 0.08, and 0.16°, respectively; thus, they require only minor adjustments. The LRA parameterization for the CS bond length reduces the MAD to 0.0012 Å, and it narrows the errors from the corresponding SE equilibrium values to the range of −0.003 to 0.003 Å.…”

Correcting the Experimental Enthalpies of Formation of Some Members of the Biologically Significant Sulfenic Acids Family

“…Previous works have shown that both methods are equivalent in a number of situations. 66,67 Required geometry optimizations at the DFT level were performed with very tight criteria. Analytical frequency calculations were performed using the DFT methods, and the thermochemical properties calculated using the rigid− rotor/harmonic oscillator approximation.…”