Kinetics of the tautomerization of thioimidic acids R−C(SH)NH → R−C(S)NH2: For R = H, F, HO, CN, NC, H2N, HC(O), HC(S), HC≡ C, CH3, CF3, H2C=CH, HOCH2, H2NCH2,CH3C(O), C2H5, (CH3)2CH, C6H5

Self Cite

Detailed chemical kinetic mechanisms for the synthesis of complex organic molecules in the interstellar medium are at an early stage of developement. That such synthesis must take place is well‐known from chemical analysis of sampled asteroids. As molecular complexity increases the number of possible structural isomers also increases with the consequence that the nascent species may adopt a different spatial arrangement, to the lowest energy one. As part of a program of investigations of the hydrogen atom transfer reaction or tautomerization of imidic acid–amide species H‐O=C‐N‐ O=C‐N‐H we have studied the kinetics for a number of nucleobases, namely cytosine, thymine and uracil where a cyclic form of tautomerism (lactim–lactam) is encountered. Together with a fourth, 5‐aza‐uracil (1,3,5‐triazine‐2,4(1H,3H)‐dione), we report on the rates of reaction at low temperatures 50–200 K for both the direct unimolecular process and the similar transformation mediated by an additional water molecule. We show that these tautomerization reactions can be categorized into three classes, and highlight the importance of quantum mechanical tunneling on the rate constants at these low temperatures. We further present some thermochemistry data, such as formation enthalpies, entropies, isobaric heat capacities and enthalpy functions.

Kinetics of direct and water‐mediated tautomerization reactions of nucleobases at low temperatures ⩽200 K

Würmel,

Simmie

2023

Self Cite

Kinetics of direct and water‐mediated tautomerization reactions of five‐membered cyclic amides or lactams

Würmel,

Simmie

2024

Self Cite

As part of a series of studies of hydrogen‐atom transfer or tautomerization reactions of imidic acid–amide species, , we report the rate constants for a set of 24 five‐membered cyclic compounds at low, 50–300 K, and high, 500–1500 K, temperatures. These rate constants are for both the high temperature direct reaction and for that mediated by an additional water molecule which facilitates the hydrogen transfer reaction at low temperatures. In the latter case we show that the rate of reaction from a pre‐reaction complex can be rapid at temperatures down to 50 K on a 1 ms timescale and is dominated by quantum mechanical effects as evaluated by small‐curvature and quantised‐reaction‐states tunnelling. In addition, we present thermochemical data such as enthalpies of formation, entropies, isobaric heat capacities and enthalpy functions for these largely unknown species which span a range of compounds from pyrolidinone to oxo‐tetrazoles.

Kinetics of direct and water‐mediated tautomerization reactions of four‐membered cyclic lactims to amides or lactams

Würmel,

Simmie

2024

As part of a series of studies of hydrogen‐atom transfer or tautomerization reactions of imidic acid‐amide species, ──═─ ═──, we report the rate constants for a set of 16 four‐membered cyclic compounds at low, 50–300 K, and high, 500–1500 K, temperatures. The compounds are labeled according to the two ring groups X and Y, which can be , NH, CH, N, O, or C(O) and which are at some remove from the reactive site. These rate constants are for both the direct reaction and for that mediated by an additional water molecule, which facilitates the hydrogen transfer reaction. In the latter case, we show that the rate of reaction from a pre‐reaction complex is rapid at temperatures down to 50 K and dominated by quantum mechanical effects as evaluated by small‐curvature and quantized‐reaction‐states tunneling. In addition, we present thermochemical data such as enthalpies of formation, entropies, isobaric heat capacities, and enthalpy functions for these largely unknown species, which span a range of compounds from ‐propiolactone to 1,3‐diazetidine‐2,4‐dione.