Density functional theory calculations were conducted on the title reactions with water molecules. Malonaldehyde, acetylacetone, and malonic acid were adopted as reactants. A reaction of malonaldehyde and (2 + 2) water molecules was found to proceed by a small (ca. 7 kcal/mol) activation energy. Two are reactant and two are catalyst molecules, respectively. The intramolecular hydrogen bond in the enol form is disrupted by the intermolecular one. Tautomerization transition-state geometries for acetylacetone and malonic acid are similar to that of malonaldehyde. Without the enhanced reactivity of water hydrogen-bond networks, tautomerization reactions undergo large activation energies. Various reaction models of (malonaldehyde) n (n = 2, 3, and 4) have given the energetic result. The keto−enol tautomerization has an analogy with the bimolecular nucleophilic elimination (E2) mechanism.
Isomerization reactions of glycine were studied computationally. Three water molecules were found to cause the reaction of glycine with the anti carboxyl group. For the glycine with the syn carboxyl group, three and one molecules are concerned with the reaction; one acts as a catalyst to prevent the local proton transfer. A reaction model composed of glycine and eleven water molecules was examined. An "out-of-plane" path was found to be a favorable stepwise channel with two small activation energies. In the path, an ion-pair intermediate was obtained. From the intermediate, a proton may be relayed outward from the hydrogen-bond network. The possibility of this proton dispersal was discussed in relation to experimental evidence of isoelectric points of zwitterions.
Tautomerization paths of 2(and 4)-hydroxypyridine (called here HP) to 2(and 4)-pyridone (called here PY) with water molecules were investigated by the use of density functional theory calculations. Potential energies were compared for a number of water molecules. The 2-HP molecule was found to be isomerized most readily and concertedly to the 2-PY one via proton relays with two water molecules. The reaction pattern is invariant even when outer water molecules are added. The 4-HP(H(2)O)(n) --> 4-PY(H(2)O)(n) reaction model did not give small activation energies. However, a reaction of (4-HP)(2)(H(2)O)(2) --> (4-PY)(2)(H(2)O)(2) was found to occur readily through a transient ion-pair intermediate. The conversion processes of (2-PY)(2) to the tautomerization reacting system were discussed. The hydrogen-bond directionality regulates the tautomerization paths.
Internal alkyne-to-vinylidene isomerization in the Ru complexes ([CpRu(η(2)-PhC≡CC(6)H(4)R-p)(dppe)](+) (Cp = η(5)-C(5)H(5); dppe = Ph(2)PCH(2)CH(2)PPh(2); R = OMe, Cl, CO(2)Et)) has been investigated using a combination of quantum mechanics and molecular mechanics methods (QM/MM), such as ONIOM(B3PW91:UFF), and density functional theory (DFT) calculations. Three kinds of model systems (I-III), each having a different QM region for the ONIOM method, revealed that considering both the quantum effect of the substituent of the aryl group in the η(2)-alkyne ligand and that of the phenyl groups in the dppe ligand is essential for a correct understanding of this reaction. Several plausible mechanisms have been analyzed by using DFT calculations with the B3PW91 functional. It was found that the isomerization of three complexes (R = OMe, CO(2)Et, and Cl) proceeds via a direct 1,2-shift in all cases. The most favorable process in energy was path 3, which involves the orientation change of the alkyne ligand in the transition state. The activation energies were calculated to be 13.7, 15.0, and 16.4 kcal/mol, respectively, for the three complexes. Donor-acceptor analysis demonstrated that the aryl 1,2-shift is a nucleophilic reaction. Furthermore, our calculation results indicated that an electron-donating substituent on the aryl group stabilizes the positive charge on the accepting carbon rather than that on the migrating aryl group itself at the transition state. Therefore, unlike the general nucleophilic reaction, the less-electron-donating aryl group has an advantage in the migration.
Density functional theory calculations were conducted on the title reactions with explicit inclusion of a variety of water molecules. Concerted reaction paths were examined first in the reaction model, ester(H2O)n --> MeCOOH(H2O)(n-1)EtOH, with n = 1-4. Their Gibbs activation energies are much larger than the experimental value, and the concerted paths are unfavorable. Various stepwise paths were investigated, and the ester(H2O)4 reactant gives a likely stepwise path. The n = 4 based reaction models, n = 4 + 5 and n = 4 + 12, were found to have similar proton-relay shapes with good hydrogen-bond directionality. The distinction of either the concerted or the stepwise path is described by the position of only one proton in the "junction" water molecule.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.