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

A new chapter in the never ending story of cycloadditions: The puzzling case of SO2 and acetylene

A comprehensive study of the different classes of cycloaddition reactions ([3+2], [2+2], and [2+1]) of SO2 to acetylene and ethylene has been performed using density functional theory (DFT) and composite wavefunction methods. The [3+2] cycloaddition reaction, that was previously explored in the context of the cycloaddition of thioformaldehyde S-methylide (TSM) to ethylene and acetylene, proceeds in a concerted way to the formation of stable heterocycles. In this paper, we extend our study to the [2+2] and [2+1] cycloadditions of SO2 to acetylene, which would produce 1,1-oxathiete-2-oxide and thiirene-1,1-dioxide, respectively. One of the main conclusions is that cyclic 1,1-oxathiete-2-oxide can open through a relatively easy breaking of the SO single bond and rearrange toward sulfinyl acetaldehyde (SA). The SA molecule can easily undergo several internal rearrangements, which eventually lead to sulfenic acid and sulfoxide derivatives of ethenone, 1,2,3-dioxathiole, and CO plus sulfinylmethane. The most probable path, however, produces 2-thioxoacetic acid, whose derivatives (or those of the corresponding acetate) are usually obtained by Willgerodt-Kindler-type sulfuration of acetates. This product can in turn decompose, leading to the final products CO2 and H2CS. Comparison of this decomposition path with that of 2-amino-2-thioxoacetic acid shows that the process occurs through different H-transfer processes.

Toward an Accurate Black-Box Tool for the Kinetics of Gas-Phase Reactions Involving Barrier-less Elementary Steps

An enhanced computational protocol has been devised for the accurate characterization of gas-phase barrier-less reactions in the framework of the reaction-path (RP) and variable reaction coordinate variational transition-state theory. In particular, the synergistic combination of density functional theory and Monte Carlo sampling to optimize reactive fluxes led to a reliable yet effective computational workflow. A black-box strategy has been developed for selecting the most suited density functional with reference to a high-level one-dimensional reference potential. At the same time, different descriptions of hindered rotations are automatically selected, depending on the corresponding harmonic frequencies along the RP. The performance of the new tool is investigated by means of two prototypical reactions involving different degrees of static and dynamic correlation, namely, H2S + Cl and CH3 + CH3. The remarkable agreement of the computed kinetic parameters with the available experimental data confirms the accuracy and robustness of the proposed approach. Together with their intrinsic interest, these results also pave the way toward systematic investigations of gas-phase reactions involving barrier-less elementary steps by a reliable, user-friendly tool, which can be confidently used also by nonspecialists.

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

Sulfenic acids are important intermediates in the oxidation of cysteine thiol groups in proteins by reactive oxygen species. The mechanism is influenced heavily by the presence of polar groups, other thiol groups, and solvent, all of which determines the need to compute precisely the energies involved in the process. Surprisingly, very scarce experimental information exists about a very basic property of sulfenic acids, the enthalpies of formation. In this Article, we use high level quantum chemical methods to derive the enthalpy of formation at 298.15 K of methane-, ethene-, ethyne-, and benzenesulfenic acids, the only ones for which some experimental information exists. The methods employed were tested against well-known experimental data of related species and extensive CCSD(T) calculations. Our best results consistently point out to a much lower enthalpy of formation of methanesulfenic acid, CH₃SOH (Δ_fH⁰(298.15K) = -35.1 ± 0.4 kcal mol^-1), than the one reported in the NIST thermochemical data tables. The enthalpies of formation derived for ethynesulfenic acid, HC≡CSOH, +32.9 ± 1.0 kcal/mol, and benzenesulfenic acid, C₆H₅SOH, -2.6 ± 0.6 kcal mol^-1, also differ markedly from the experimental values, while the enthalpy of formation of ethenesulfenic acid CH₂CHSOH, not available experimentally, was calculated as -11.2 ± 0.7 kcal mol^-1.

Dipolar 1,3-cycloaddition of thioformaldehyde S-methylide (CH2 SCH2 ) to ethylene and acetylene. A comparison with (valence) isoelectronic O3 , SO2 , CH2 OO and CH2 SO

Methods rooted in the density functional theory and in the coupled cluster ansatz were employed to investigate the cycloaddition reactions to ethylene and acetylene of 1,3-dipolar species including ozone and the derivatives issued from replacement of the central oxygen atom by the valence-isoelectronic sulfur atom, and/or of one or both terminal oxygen atoms by the isoelectronic CH₂ group. This gives rise to five different 1,3-dipolar compounds, namely ozone itself (O₃ ), sulfur dioxide (SO₂ ), the simplest Criegee intermediate (CH₂ OO), sulfine (CH₂ SO), and thioformaldehyde S-methylide (CH₂ SCH₂ , TSM). The experimental and accurate theoretical data available for some of those molecules were employed to assess the accuracy of two last-generation composite methods employing conventional or explicitly correlated post-Hartree-Fock contributions (jun-Cheap and SVECV-f12, respectively), which were then applied to investigate the reactivity of TSM. The energy barriers provided by both composite methods are very close (the average values for the two composite methods are 7.1 and 8.3 kcal mol^-1 for the addition to ethylene and acetylene, respectively) and comparable to those ruling the corresponding additions of ozone (4.0 and 7.7 kcal mol^-1 , respectively). These and other evidences strongly suggest that, at least in the case of cycloadditions, the reactivity of TSM is similar to that of O₃ and very different from that of SO₂ .

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

The determination of accurate equilibrium molecular structures plays a fundamental role for understanding many physical–chemical properties of molecules, ranging from the precise evaluation of the electronic structure to the analysis of the role played by dynamical and environmental effects in tuning their overall behavior. For small semi-rigid systems in the gas phase, state-of-the-art quantum chemical computations rival the most sophisticated experimental (from, for example, high-resolution spectroscopy) results. For larger molecules, more effective computational approaches must be devised. To this end, we have further enlarged the compilation of available semi-experimental (SE) equilibrium structures, now covering the most important fragments containing H, B, C, N, O, F, P, S, and Cl atoms collected in the new SE100 database. Next, comparison with geometries optimized by methods rooted in the density functional theory showed that the already remarkable results delivered by PW6B95 and, especially, rev-DSDPBEP86 functionals can be further improved by a linear regression (LR) approach. Use of template fragments (taken from the SE100 library) together with LR estimates for the missing interfragment parameters paves the route toward accurate structures of large molecules, as witnessed by the very small deviations between computed and experimental rotational constants. The whole approach has been implemented in a user-friendly tool, termed nano-LEGO, and applied to a number of demanding case studies.