Density Functional Study on the Reaction Mechanism of Proton Transfer in 2-Pyridone:  Effect of Hydration and Self-Association

Abstract

The proton-transfer mechanism in the isolated, mono, dehydrated forms and dimers of 2-pyridone and the effect of hydration or self-assistance on the transition state structures corresponding to proton transfer from the keto form to the enol form have been investigated using B3LYP and BH-LYP hybrid density functional methods at the 6-311++G (2d, 2p) basis set level. The barrier heights for both H2O-assisted and self-assisted reactions are significantly lower than that of the bare tautomerization reaction from 2-pyridone to 2-hydroxypyridine, implying the importance of the superior catalytic effect of H2O and (H2O)2 and the important role of 2-pyridone itself for the intramolecular proton transfer. Long-range solvent effects have also been taken into account by using the continuum model (Onsager model and polarizable continuum model (PCM)) of water. The tautomerization energies and the potential energy barriers are increased both for the water-assisted and for the self-assisted reaction because of the bulk solvent, which imply that the tautomerization of PY becomes less favorable in the polar solvent.

Solvent-dependent structural dynamics of 2(1H )-pyridinone in light absorbing S4 (ππ*) state

“…The orbitals 35 (HOMO), 36 (LUMO), and 37 (LUMO+1) are respectively π and π* orbitals, while orbital 34 (n orbital) and orbitals 38, 39, and 40 (designated as Ryd 1 , 2, 3 ) are respectively non‐bonding and diffuse orbitals. Therefore the A‐, and B‐band absorption of NHP–(H 2 O) 2 cluster are respectively assigned to two π → π* transitions on the basis of our time‐dependent (TD)‐DFT computations and previous reported results . Two n → π* and three π → Ryd transitions are located, and they have negligible oscillator strength for the A‐ and B‐band absorptions.…”

“…The binding energies and structures of the mono‐ and double‐hydrated clusters of OHP and NHP were studied by using time‐of‐flight mass spectroscopy (TPFMS) and emission spectroscopy as well as ab initio calculations, and the results showed that the ground state hydrogen‐bonding energy for NHP–H 2 O and NHP–(H 2 O) 2 clusters are considerably larger than the corresponding OHP–H 2 O and OHP–(H 2 O) 2 clusters respectively . Density functional theory computations revealed that NHP–(H 2 O) 2 cluster is much more stable than NHP–H 2 O and OHP–H 2 O cluster . Our present combined experimental FT‐Raman spectroscopic and B3LYP/6‐311++G(d, p) computational results are in good agreement with the previous reported conclusion that NHP–(H 2 O) 2 cluster is the predominate species of NHP in water .…”

“…Moreover, the CASSCF and MR‐CI calculations indicate that the excited state tautomerization between the OHP and NHP monomers has a barrier of 18.4 kcal mol −1 in gas phase, which is reduced to ~6.4 kcal mol −1 for NHP–(H 2 O) 2 cluster, and involves a ring‐type transition state having two water molecules as a bridge . It is commonly believed that the excited state tautomerization of NHP in water may proceed via a ring‐type intermediate in which NHP transfers a proton to the solvent which then transfers a proton back to the solute, or it may proceed via a concerted mechanism in which there is a neutral ring‐type intermediate . This means that under 223.1 and 228.7‐nm excitations, NHP–(H 2 O) 2 cluster should have enough available energy to quickly surpass the barrier in water (5.6–6.4 kcal mol −1 , which would be further reduced when taking into account the cage effect of water) and finally lead to a concerted ESPT reaction of NHP.…”

“…The enol–keto tautomerization equilibrium between NHP and OHP depends inherently on the surrounding environment. In this respect, the quantum mechanical calculations have focused on the hydrogen‐bonding interaction between the two tautomers and the solvent molecules, and on the tautomeric mechanism . Barone and Adamo investigated the enol‐keto tautomerization reaction of NHP by means of the density functional/Hartree–Fock hybrid method and using SVP and TZ2P basis sets .…”

“…Chou et al investigated the conjugated dual hydrogen bonds mediating NHP dimerization using MP2/6‐31+G** level of theory, and the result leads to the reverse stability, or (OHP) 2 dimer was more stable than (NHP) 2 dimer, which contradicted the experimental observations. The keto–enol tautomeric reactions involving NHP, NHP–H 2 O, NHP–(H 2 O) 2 , and (NHP) 2 were investigated by employing B3LYP and BH‐LYP/6‐311++G (2d, 2p) levels of theory to examine the hydration effect on the associated transition state structures …”