The mechanism of tautomerisation and geometric isomerisation in thioformic acid and its water complexes: exploring chemical pathways for water migration

Abstract

A systematic and automated search for chemical pathways of isomerisation between geometric and tautomeric forms of gas-phase thioformic acid (TFA) and its water complexes is performed using a global reaction route mapping (GRRM) method, and an uncovered pathway for cis-trans isomerisation in the thiol form of TFA has been explored through computations performed at CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311++G(2d,2p) level of the coupled cluster and density functional theories. To explore the routes for water migration, a detailed analysis of complexes of TFA with a single-water molecule is presented. Notably, during the isomerisation process in TFA, a positive catalytic effect of water was observed that can arise either by the stabilization of the reactant and/or of the transition state through extensive hydrogen bonding. Interesting behavior of isomeric forms of TFA along the pathways analysed is revealed through the Gibbs free-energy change and its temperature-dependence. The cis form of TFA(thiol) in the complexes of TFA with a single-water molecule is found to be thermodynamically equally feasible as the trans form which though is known to be the most dominating among the isomeric forms of TFA. Besides these, various complexes of TFA with two-water molecules have also been explored to study the hydrogen-bonding interaction through natural bond-orbital (NBO) analysis. The complexes of TFA with two-water molecules have also been characterized using spectral features including vibrational frequency analysis, and the effect of complexation has been observed by noting frequency shift.

Diastereoselective Formation of trans-HC(O)SH Through Hydrogenation of OCS on Interstellar Dust Grains

“…In the absence of other competitive reactions, the cis−HC(O)SH isomer abundance must be small based on the observed specificity of the OCS + 2 H reaction. cis−HC(O)SH is the less stable isomer (cis-trans gap of 3.1 kJ mol −1 (261 K), as mentioned in the introduction) with the direct cis-trans isomerization barrier of 37.2 kJ mol −1 (Kaur & Vikas 2014). Such obstacles (endothermicity and moderate activation barrier) make it difficult to predict a significant abundance of cis−HC(O)SH in cold molecular clouds.…”

“…(Shingledecker et al 2019a(Shingledecker et al , 2020 It is important to note that has been recently proved that in the gas phase of the ISM, thermodynamic equilibrium can be achieved, even at shallow temperatures (García de la Concepción et al 2021), but such equilibrium cannot be taken for granted on dust grains, experiencing constant H accretion, diffusion, and reaction (Wakelam et al 2017a). For HC(O)SH, Rodríguez-Almeida et al (2021) were only able to detect trans−HC(O)SH the most stable structural isomer of the two possible ones, with the cis one lying 3.1 kJ mol −1 above in energy, (Kaur & Vikas 2014).…”

From nitrogen inversion in amines to stereoinversion in aminium salts: role of a single water molecule

“…Recently, we had utilized GRRM to explore isomerization in radical species such as C 6 H and C 6 N radicals [24,25], in the metastable molecular anions of polydeprotonated benzenes [26], and also for tracing the stereoisomerization pathways in chiral molecules like 2-aminopropionitrile [19], and serine [20]. Besides these, the mechanistic investigations of water catalytic pathways in the reaction of hydroxyl radical with volatile organic compounds have also been successfully studied using the GRRM [27][28][29][30][31].…”

“…In recent years, to perform a systematic and an automated exploration of reaction pathways on the potential energy surface (PES) of a molecular system, global reaction route-mapping (GRRM) [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] method has emerged as a quite successful computational strategy. Using a hypersphere search (SHS) method [23], GRRM can explore, in principle, all the isomers of a given molecular formula and respective reaction paths by locating various equilibrium structures (EQs), transition states (TSs) connecting them, and their dissociation channels (DCs).…”

Computational study of the rotational pathways of the amino group in 2-chloroaniline, azines and formamide: One or two rotational barriers?

“…Also, speculation that the presence of energy discontinuities in the rotational pathways (saw-teeth) could be associated with the existence of unidirectional rotations and molecular motors 11,12 cannot be experimentally established. Conversely, computational methods are more suitable for such studies because they can provide detailed information on the different structures and their energies along the reaction pathway 9, [13][14][15][16][17][18] . Despite these advantages, the number of computational publications including complete rotation energy profiles is rather small [2][3][4] , especially in those cases where energy profiles need to be computed along of more than one torsion angle.…”