| Proton transfer is important in the chemical reaction and biological process. Nitrogen and oxygen are necessary elements in the organism.The research on the proton transfer associated with nitrogen and oxygen is significative.From the most simple nitrogen-oxygen compound,the work on the isomerization of HNO/HON and pyrazole was carried out in this paper.Proton transfer in the HNO/HON systemA density function theory study is presented for the HNO(?)HON isomerization assisted by m water molecules(m=1-4)on singlet state potential energy surface.Two modes are considered to model the catalytic effect of m water molecules:(1)water molecule(s)directly participates in forming a proton transfer loop with HNO/HON species;and(2)water molecules are out-of-loop,modeling the outer-sphere water effect from the other water molecules directly H-bonded to the loop(referred to out-of-loop waters).Two mechanisms are proposed for the mono-water assisting isomerizations and one for each of the other cases.The reactant and product of all groups have been characterized for all potential energy surfaces.For the monohydration mechanism,the reactant complex is connected to the product complex via two determined saddle points, and the reaction heat is 35.5kcal·mol-1at B3PW91/6-311++G**level.The corresponding forward/backward barrier lowerings are obtained to be 7.4/1.2(AT2)and 34.1/28.3(AT1)kcal·mol-1,respectively,compared with the no-water assisting isomerization barrier T(72.6/31.3 kcal·mol-1).When one to three out-of-loop water molecules are considered,their effects on the three energies are little,and the deviations are not more than 2 kcal·mol-1compared with the original monohydration assisting case (AT1).For the dihydration assisting mechanism,the reaction heat is 30.5kcal·mol-1,and the forward/backward barrier lowerings become 41.7/30.9 kcal·mol-1compared to T. Further increasing 1-2 out-of-loop waters does not obviously change the energetics.For trihydration one,the forward/backward barriers further decrease as 43.5/30.9kcal·mol-1, and the reaction heat decreases by 12.4kcal·mol-1.The same is true for the influence of the additional out-of-loop waters.However,when four water molecules are involved in the reactant loop,the corresponding energy aspect increases slightly.The forward/backward barrier lowerings for tetrahydration mechanism become 41.8/29.4 kcal·mol-1,respectively,smaller by 1.7 and 1.5 kcal·mol-1than the trihydration situation. Therefore,it can be concluded that the most favorable hydration-assisting mode should be one with three in-loop waters.Such hydration assisting isomerization pathways can exist in water-dominated environments,for example,in the organism,and is significant to the energy transferring.Mechanism of the isomerization of pyrazoleThe proton transfer isomerization of the pyrazole and the water assisting effect by looping 1 to 4 water molecules on singlet state potential energy surface have been investigated by using the hybrid density functional theory method(B3PW91)with a 6-311++G**basis set.Two mechanisms were proposed to explain the mono-and multiwater assisting effects,respectively.The reactants and products of all groups have been characterized on their potential energy surfaces.For the isomerization of monomolecule pyrazole,the isomerization energy barrier is 46.4 kcal·mol-1.For the monohydration assisting mechanism,the reactant complex is connected to the product complex via two saddle points.The corresponding isomerization barriers are 46.7 kcal·mol-1and 23.0 kcal·mol-1,respectively.As to the multihydration assisting mechanism,the isomerization barriers are 12.0 kcal·mol-1,10.9 kcal·mol-1and 13.14 kcal·mol-1 accordingly,when the number of the water molecule is 2,3 and 4,respectively.The mutihydration assisting isomerization can occur in water-dominated environments,for example,in the organism,thereby is crucial to energy transference.The deproton energies and dehydrogen energies of monomolecule pyrazole and variedly hydrated pyrazoles are calculated and then found much bigger than the isomerization barriers of their relative complexes,suggesting the impossibility of the deprotonation or dehydrogenation.The isomerization of the pyrazole is a proton-coupling-electron-migration process,but two different mechanisms are noticed, viz.σ-type mechanism andπ-type mechanism.Theπ-bond of the pyrazole participates in the isomerization in theπ-type mechanism,whereas only theσelectron participates in the isomerization in theσ-type mechanism. The proton transfer isomerization of the pyrazole and the water assisting effect by looping 1 to 4 water molecules on singlet state potential energy surface are investigated using the hybrid density functional theory method(B3PW91)with a 6-311++G**basis set.Two mechanisms are proposed to explain the mono- and multi-water assisting effects,respectively.The reactants and products of all groups have been characterized on their potential energy surfaces.For the isomerization of monomolecule pyrazole,the isomerization energy barrier is 46.4 kcal·mol-1.For the monohydration assisting mechanism,the reactant complex is connected to the product complex via two saddle points.The corresponding isomerization barriers are 46.7 kcal·mol-1and 23.0 kcal·mol-1, respectively.As to the multihydration assisting mechanism,the isomerization barriers are 12.0 kcal·mol-1,10.9 kcal·mol-1and 13.14 kcal·mol-1accordingly,when the number of the water molecule is 2,3 and 4,respectively.The mutihydration assisting isomerization can occur in water-dominated environments,for example,in the organism, thereby is crucial to energy transference.The deproton energies and dehydrogen energies of monomolecule pyrazole and variedly hydrated pyrazoles are calculated and then found much bigger than the isomerization barriers of their relative complexes, suggesting the impossibility of the deprotonation or dehydrogenation.The isomerization of the pyrazole is a proton-coupling-electron-migration process,but two different mechanisms are noticed,viz.σ-type mechanism andπ-type mechanism.Theπ-bond of the pyrazole participates in the isomerization in theπ-type mechanism,whereas only theσelectron participates in the isomerization in theσ-type mechanism.Actually,such a hydration assisting isomerization pathway may exist in the water-dominated environments,for example,in the organism,and thus is of interest to the energy transferring and the NO-related metabolism processes.
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