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A dielectric and spectrophotometric study of the tautomerization of 2-hydroxypyridine and 2-mercaptopyridine in water.

ABSTRACT: The basic ionization (pk ₁) and acidic ionization (pk ₂) constants and equilibrium constant (K _T) of 2HPy and 2MPy were determined. The pk ₁(s) of their N- and X-methyl derivatives (X = O, S) were also determined. The equilibrium constant of 2MPy is approximately 60 times greater than its oxygen analog, 2HPy. The micro-ionization constants of the functional groups, -NH (pk _A and pk _C) and -XH (pk _B and pk _D), were determined to provide further insights into the ionization equilibria of these N-heteroaromatic XH compounds (2HPy and 2MPy). The relaxation time of water (τ) in aqueous solutions of 2HPy and 2MPy are collectively used with the K _T values to determine the forward (k _f) and backward (k _b) rate constants of tautomerization. Subsequently, the k _f and k _b are used to provide the rationale for the K _T and τ values.

SUBMITTER: Bomzon B

PROVIDER: S-EPMC9048638 | biostudies-literature | 2020 Jan

REPOSITORIES: biostudies-literature

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A dielectric and spectrophotometric study of the tautomerization of 2-hydroxypyridine and 2-mercaptopyridine in water.

Bomzon Biswadeep B Khunger Yashita Y Subramanian Ranga R

RSC advances 20200113 4

The basic ionization (pk 1) and acidic ionization (pk 2) constants and equilibrium constant (K T) of 2HPy and 2MPy were determined. The pk 1(s) of their N- and X-methyl derivatives (X = O, S) were also determined. The equilibrium constant of 2MPy is approximately 60 times greater than its oxygen analog, 2HPy. The micro-ionization constants of the functional groups, -NH (pk A and pk <sub ...[more]

PMID: 35494609

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Project description:The mechanism of acid-catalyzed keto-enol tautomerization of β-cyclopentanedione (CPD) in solution is studied computationally. Reaction profiles are first calculated for a limited solvation environment using ab initio and density functional methods. Barrier heights for systems including up to one hydration shell of explicit water molecules depend strongly on the number of waters involved in proton transfer and to a lesser but significant extent on the number of waters forming hydrogen bonds with waters in the proton-transfer chain (each such water reduces the barrier by 4.4 kcal/mol on average). Barriers of 8-13 kcal/mol were obtained when a full or nearly full hydration shell was present, consistent with calculations for nonacid-catalyzed keto-enol tautomerization of related molecules. The presence of HOCl reduced the barrier by 4.5 kcal/mol in relation to the gas phase, consistent with the well-known principle that keto-enol tautomerization can be acid- or base-catalyzed. The reaction was also modeled beginning with snapshots of reactant conformations taken from a 300 K molecular dynamics simulation of CPD, HOCl, and 324 explicit waters. Reaction profiles were calculated at a QM/MM level with waters in the first hydration shell either fixed or energy-minimized at each step along the reaction coordinate. A substantial variation in barrier height was observed in both cases, depending primarily on electrostatic interactions (hydrogen bonding) with first-hydration-shell waters and, to a lesser extent, on electrostatic interactions with more distant waters and geometric distortion effects. For the lowest barriers, the extent of barrier reduction by waters involved in proton transfer is consistent with the limited solvation results, but further barrier reduction due to hydrogen bonding to waters involved in proton transfer is not observed. It is postulated that this is because highly flexible structures such as extensive hydrogen bonding networks optimal for reaction are entropically disfavored and so may not contribute significantly to the observed reaction rate.

Dataset Information

A dielectric and spectrophotometric study of the tautomerization of 2-hydroxypyridine and 2-mercaptopyridine in water.

Publications

A dielectric and spectrophotometric study of the tautomerization of 2-hydroxypyridine and 2-mercaptopyridine in water.

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