TY - JOUR
T1 - Kinetics of direct and water-mediated tautomerization reactions of nucleobases at low temperatures ⩽200 K
AU - Würmel, Judith
AU - Simmie, John M.
N1 - Publisher Copyright:
© 2023 The Authors. International Journal of Chemical Kinetics published by Wiley Periodicals LLC.
PY - 2024/2
Y1 - 2024/2
N2 - 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- (Formula presented.) 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.
AB - 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- (Formula presented.) 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.
KW - cytosine
KW - kinetics
KW - quantum mechanical tunneling
KW - tautomerization reaction
KW - thermochemistry
KW - thymine
KW - triazine
KW - uracil
UR - http://www.scopus.com/inward/record.url?scp=85174234699&partnerID=8YFLogxK
U2 - 10.1002/kin.21696
DO - 10.1002/kin.21696
M3 - Article
AN - SCOPUS:85174234699
SN - 0538-8066
VL - 56
SP - 105
EP - 116
JO - International Journal of Chemical Kinetics
JF - International Journal of Chemical Kinetics
IS - 2
ER -