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Insights into the Mechanism of Ground and Excited State Double Proton Transfer Reaction in Formic Acid Dimer Santanab Giri, Rakesh Parida, Madhurima Jana, Soledad Gutierrez-Oliva, and Alejandro Toro-Labbé J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09819 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017
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Insights into the Mechanism of Ground and Excited State Double Proton Transfer Reaction in Formic Acid Dimer Santanab Giri1,*, Rakesh Parida1, Madhurima Jana1, Soledad Gutiérrez-Oliva2, Alejandro ToroLabbe2,* 1 Department of Chemistry, National Institute of Technology Rourkela, Orissa, 769008, India 2 Laboratorio de Química Teórica Computacional (QTC), Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Correo 22, Santiago, Chile
ABSTRACT: The mechanism of ground and excited state double proton transfer reaction in formic acid dimer has been analyzed with the help of reaction force and the reaction electronic flux. The separation of reaction electronic flux in terms of electronic activity and reactivity, NBO, dual descriptor lends additional support for the mechanism. Interestingly we found that, the ground state double proton transfer mechanism is concerted synchronic, whereas the excited state double proton transfer is concerted asynchronic in nature.
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1. INTRODUCTION Proton transfer reaction belongs to a class of reaction in chemistry where proton is transferred from one species to another species. The simplest example of such proton transfer reaction is the interaction between acid and base. For the last two decades, the proton transfer reaction has drawn considerable attention to both experimentalist and theoreticians due to its important role in many chemical and biological processes. Double proton transfer (DPT) in DNA base pair Adenine-Thymine is one of the important examples of such biological process1. Another important example of proton transfer reaction is the proton transfer during the synthesis of ATP2. Several theoretical3,4and experimental5,6proton transfer studies have been performed on different hydrogen-bonding systems. Methods have been developed in both theoretical and experimental point of view to explain the mechanism of the proton transfer reaction. In nature there are many examples of molecule having multiple hydrogen bonding, e.g double helix structure of DNA. In those systems, multiple proton transfer may take place. In particular, double proton transfer in formic acid dimer (FAD) has been studied extensively as a typical example of multiple proton transfer reaction. Formic acid dimer (FAD) which is an eight membered ring with double hydrogen bonds is often chosen as model complex system for studying the DPT not only due to its double H-bonded structure, but also because of the consequence of the proton transfer for the biological systems7e.g enzymatic catalyst8. In summary, proton transfer reactions play a major role in many phenomena in chemistry, physics and biology 9-12. Nowadays with the help of high level ab-initio and density functional theory (DFT) calculations one can fully characterized the reaction mechanism in terms of energetic and structural parameters. Although, it is known that in FAD ground state transfer of acidic protons between the oxygen atoms of different moieties takes place in a concerted tunneling motion
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which means that the two protons simultaneously move from one moiety to another formic acid moiety13-15 the mechanism in excited state is not fully characterize theoretically. Experimentally it is possible to predict the proton transfer tunneling splittings in the ground and vibrationally excited states separately. In this context, an attempt has been made to study the DPT reaction in FAD for both ground and first excited state and analyze the important features of the reaction along the reaction pathway by recovering the electronic structure information in the gas phase with the help of conceptual density functional theory16-21 based reactivity descriptors. To study this type of reaction, conceptual DFT provides different types of reactivity descriptors, such as the chemical potential22,23,electronic flux24,25etc. to understand the electronic activity along the reaction path. It is also possible to separate the electronic flux in terms of nucleophilic and electrophilic reactivity flux in order to have an idea about the pattern of reactivity along the reaction. In this manuscript, our focus has been to explore the ground and first excited state gas phase DPT reaction mechanism in FAD in the light of reaction force, reaction electronic flux, and it’s different components, NBO analysis and dual descriptor. Further attempts have been made to identify the driving force of such reactions. 2. THEORETICAL BACKGROUND 2.1 Reaction Energy and Reaction Force: Chemical reactions can be modeled by constructing a minimum energy profile diagram(( by employing the intrinsic reaction coordinate (IRC)26-30technique. Once we have the energy profile, it is relatively easy to have the reaction force along the reaction pathway. The reaction force31-33 can be defined as, ( = −
(1
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Figure 1. A general reaction force profile for a single step reaction Therefore, according to Figure 1, it is possible to have three distinct reaction region depending upon the position of minimum and maximum. (I) the reactant region ( (II) the transition state region ( and (III) the product region ( Therefore, it is important to characterize different properties along the IRC within the reaction regions.32-35The activation energy (Δ and the reaction energy (Δ can be obtained by decomposing the reaction force in following way,36-39
∆ = ( − ( = ,
(2)
∆ = ( − ( = ! ,
(3)
where, W1, W2, W3 and W4 are the reaction works involved in the reaction and are defined as, = − "
%&'(
%)
( # 0 ; = − "
%+,
%&'(
( # 0 ,
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%
%
= − .% &/0 ( 0 and ! = − .% 1 ( 0 , +,
&/0
(4)
2.2 The Reaction Electronic Flux: Information of electronic activity can be understood by the calculation of chemical potential22,23(2. It is possible to calculate 2 within the density functional theory framework, for a N electron system having total energy, E as,23 4 2 = 3 6 = − 9 . (5 45 7(8
Here,
