Article pubs.acs.org/JPCA
Exploring Water Catalysis in the Reaction of Thioformic Acid with Hydroxyl Radical: A Global Reaction Route Mapping Perspective Gurpreet Kaur and Vikas* Quantum Chemistry Group, Department of Chemistry & Centre of Advanced Studies in Chemistry, Panjab University, Chandigarh 160014, India ABSTRACT: Hydrogen abstraction pathways, in the gas-phase reaction of tautomers of thioformic acid (TFA), TFA(thiol), and TFA(thione), with hydroxyl radical in the presence and absence of single water molecule acting as a catalyst, is investigated with high-level quantum mechanical calculations at CCSD(T)/6-311++G(2d,2p)//MP2/ 6-311++G(2d,2p), CCSD(T)/6-311++G(d,p)//DFT/BHandHLYP/6-311++G(d,p), and DFT/B3LYP/6-311++G(2df,2p) levels of the theory. A systematic and automated search of the potential energy surface (PES) for the reaction pathways is performed using the global reaction route mapping (GRRM) method that employs an uphill walking technique to search prereaction complexes and transition states. The computations reveal significant lowering of the PES and substantial reduction in the activation energy for the hydrogen abstraction pathway in the presence of water, thereby proving water as an efficient catalyst in the reaction of both the TFA tautomers with OH radical. The hydrogen-bonding interactions are observed to be responsible for the large catalytic effect of water. Notably, in the case of TFA(thiol), formyl hydrogen abstraction is observed to be kinetically more favorable, while acidic hydrogen abstraction is observed to be thermodynamically more feasible. Interestingly, in the case of TFA(thione), reaction pathways involving only formyl hydrogen abstraction were observed to be feasible. The water-catalyzed hydrogen abstraction reaction of TFA with hydroxyl radical, investigated in this work, can provide significant insights into the corresponding reaction in the biological systems.
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water,13 sulfuric acid−water,14 water−nitrous oxide,15 water− nitric acid,16 and the H2O−OClO complexes.17,18 Computationally, the exploration of water catalysis in a variety of reaction systems has been an active area of research in recent times. In 2009, Luo et al.10 carried out computational investigation using the global reaction route mapping (GRRM) method10,19−30 on water-catalyzed gas-phase reaction of formic acid with hydroxyl radical. GRRM is a fast emerging method for an automated exploration of unknown isomers, synthetic routes, and dissociation channels (DCs) along chemical reaction pathways. The GRRM method utilizes an anharmonic downward distortion following (ADDF)19−22 approach through a scaled hypersphere search (SHS) technique22−28 as an uphill walking method to search reaction pathways on a potential energy surface (PES). It should be noted that the mapping of reaction routes from an equilibrium structure (EQ) to transition state (TS) or DC by uphill walking has remained a major challenge, although the search for the reaction routes by downhill walking along the intrinsic reaction coordinates (IRCs)31−34 usually involves steepest descent approaches. Notably, employing a full-ADD-following method, almost all of the reaction routes including very high-energy pathways can be explored around an EQ to obtain global reaction route map on a PES automatically. However, a full-ADD search is quite
INTRODUCTION The reaction of hydroxyl (OH) radical with polar molecules in water is a topic of considerable attention owing to its importance in variety of chemical, industrial, atmospheric, astrophysical, and biological processes.1−5 Water can easily form complexes with active species such as OH radicals and polar molecules, thereby significantly altering reaction barrier and product selectivity.6−8 Such prereaction complexes can, spatially, influence reaction dynamics through steric hindrance, favoring a reaction site leading to site-selectivity in the reaction.6−10 Moreover, water serves as a very efficient collision partner,11 facilitating stabilization of the intermediates, which, in turn, controls the overall reaction yield. A number of reactions involving molecule−hydroxy radical−water complexes have been studied showing the catalytic influence of water.6−10 Vohringer-Martinez et al.6 provided an example of gas-phase catalysis involving the reaction between hydroxyl radical and acetaldehyde, which is accelerated by the participation of single molecule of water. They contended with conviction that this is because hydrogen-bonded complexes of CH3CHO and H2O form and that these complexes react faster with OH radicals than do individual molecules of CH3CHO.7,12 Besides these, several molecule− water complexes have been determined, exhibiting new chemistry different from the one of the isolated molecules. Furthermore, the impact of water−molecule complexes on the photochemistry of the atmosphere is well-described by ozone− © 2014 American Chemical Society
Received: April 1, 2014 Revised: May 16, 2014 Published: May 19, 2014 4019
dx.doi.org/10.1021/jp503213n | J. Phys. Chem. A 2014, 118, 4019−4029
The Journal of Physical Chemistry A
Article
Figure 1. Reaction pathways for noncatalytic formyl and acidic hydrogen abstraction of TFA(thiol), along paths I(a) and II(a), respectively. Relative energies (in kilocalories per mole) with respect to isolated reactants A1, depicted in bold, are computed at the CCSD(T)/6-311++G(d,p)// BHandHLYP/6-311++G(d,p) level including ZPE correction. The distances (in angstroms), depicted with and without parentheses, were optimized at the MP2/6-311++G(d,p) and BHandHLYP/6-311++G(d,p) levels, respectively.
conformers. Therefore, in the present work, the search for prereaction complexes in the gas-phase reaction of only transTFA with the hydroxyl radical, in the presence and absence of single water molecule, was carried out. The initial exploration for the reaction pathways was performed employing a largeADD-following method in GRRM at DFT/BHandHLYP/6311++G(d,p) level of theory using Becke-halfand-half-Lee− Yang−Parr (BHandHLYP)37 exchange-correlation (XC) functional of the density functional theory (DFT). In the exploration through GRRM, mainly the five largest ADDs were searched for the present study. Furthermore, the binary complexes, HSCOH-H2O, depicted in Figures 1−4 for various reaction pathways of TFA(thiol) and TFA(thione), were obtained by removing OH from ternary prereaction complexes obtained by ADDF search. Besides this, various TS structures were located intuitively and applying saddle-point optimization through the GRRM method. The harmonic vibrational frequency analysis was further performed to check if the optimized structure located is a stationary state or a TS. The frequency analysis also provided the zero-point energy (ZPE) correction. Furthermore, the IRC calculations were performed on all identified TSs to testify the right connectivity between reactants and products. To analyze the single-point energies accurately, we performed computations at the CCSD(T)/6-311++G(d,p) level on the DFT/BHandHLYP/6-311++G(d,p) geometries. For comparison, the geometries obtained at DFT/BHandHLYP/6-311++G(d,p) are further optimized at MP2/6-311+ +G(2d,2p) level, and single-point energies were then determined at the CCSD(T)/6-311++G(2d,2p)//MP2/6311++G(2d,2p) level. Besides these, GRRM computations at the level of DFT/B3LYP/6-311++G(2df,2p) were also carried to analyze the reaction pathways. In the present work, all required computations for the GRRM program are performed along with Gaussian 03 quantum chemistry software.38 The above choice of the theory for the present work is based on the fact that the use of CCSD(T)/BHandHLYP can be
expensive, but a less expensive approach, namely, large-ADDfollowing method, is available in GRRM, which allows a quick exploration of low-energy structures along low barrier pathways, and it has been successfully applied to various hydrogenbonded25−27and catalytic systems.28−30 In this work, through automated search of reaction intermediates with GRRM, we present a detailed investigation on the role of water catalysis in the gas-phase reaction of hydroxyl radical with the tautomeric forms of thioformic acid (TFA), namely, TFA(thiol) and TFA(thione). Because the prereaction complexes, in such type of reaction systems, exhibit strong effect over the kinetics of reaction,6−8,10 GRRM method is highly useful for search of these complexes, even in the lowenergy regions of the PES. Furthermore, the thio-carboxylic acids have been an area of keen research owing to their numerous biochemical and pharmaceutical applications. For example, TFA can be used to estimate relevant structural and dynamical features of the covalently bonded enzyme reaction intermediate such as acyl enzyme formed by cysteine or serine protease-catalyzed reactions.35 Besides this, thiol-ethers and esters of TFA have considerable potential applications in the manufacture of different drugs.36 Moreover, the interactions of TFA with water are known to be quite important for a detailed understanding of its hydrogen-bonding ability because it serves as an appropriate model to study conformational changes and its effect in biological molecules.36 The paper is organized as follows: The next section describes the computational methodology employed for exploring different pathways of hydrogen abstraction reaction for both the tautomers of TFA. This is followed by the Results and Discussion, which presents detailed analysis on the mechanism of hydrogen abstraction reactions. Finally, the last section makes a few concluding remarks.
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COMPUTATIONAL METHODOLOGY It should be noted that the trans conformers of TFA(thiol) and TFA(thione) are known36 to be more stable than the cis 4020
dx.doi.org/10.1021/jp503213n | J. Phys. Chem. A 2014, 118, 4019−4029
The Journal of Physical Chemistry A
Article
Table 1. Energy (kcal/mol) of Stationary Points (depicted in Figures 1−4) Relative to Isolated Reactants at ZPE-Corrected CCSD(T)/6-311++G(d,p)//BHandHLYP/6-311++G(d,p) and CCSD(T)/6-311++G(2d,2p)//MP2/6-311++G(2d,2p) Levelsa stationary points
BHandHLYP/6-311+ +G(d,p) + ZPEa
MP2/6-311+ +G(2d,2p) + ZPEb
C1 TS1(a) D1 C2 TS2(a) E1 B3 C3 TS3(a) B4 C4 TS4(a) D2 B5 C5 TS5(a) B6 C6 TS6(a) E2
−3.95 +1.44 −22.84 −4.64 +1.00 −24.85 −3.77 −11.48 −5.15 −5.46 −9.41 −3.70 −22.84 −5.46 −12.99 −7.47 −2.57 −7.59 −2.20 −24.85
−3.77 +1.26 −32.88 −4.58 −32.69 −3.33 −10.35 −3.64 −4.83 −9.10 −2.07 −32.88 −4.83 −14.31
H1 TS1(b) H2 TS2(b) I1 G3 H3 TS3(b) G4 H4 TS4(b) G5 H5 TS5(b) I2
−2.38 +5.77 −2.13 +5.21 −13.24 −3.01 −10.17 +0.38 −8.47 −11.55 −4.33 −2.51 −8.66 +0.56 −13.24
−3.14 +7.66 −4.02 +7.28 −20.58 −3.33 −9.22 +1.19 −8.09 −13.11 −2.38 −2.51 −8.03 +2.70 −20.58
−2.01 −7.53 −32.69
CCSD(T)/6- 311++G(d,p)// BHandHLYP/6-311++G(d,p) + ZPEa TFA(thiol) −3.51 −1.00 −27.30 −4.02 +6.53 −27.11 −3.77 −10.60 −8.35 −5.33 −9.04 −6.34 −27.30 −5.33 −11.61 −7.47 −2.89 −7.28 +4.46 −27.11 TFA(thione) −2.32 +2.38 −2.51 +1.76 −17.38 −3.20 −9.54 −3.58 −7.91 −11.30 −7.28 −2.82 −8.72 −3.64 −17.38
CCSD(T)/6-311++G(2d,2p)//MP2/ 6-311++G(2d,2p) + ZPEb −5.46 −3.14 −30.25 −6.28
T1 diagnostic 0.013(0.017)
0.013(0.017)
−30.43 −4.96 −12.11 −9.48 −6.90 −10.60 −7.97 −30.25 −6.90 −15.81
0.013(0.017) 0.013(0.016)
−4.27 −9.29
0.013(0.016) 0.012(0.016)
0.013(0.016) 0.012(0.016)
0.013(0.017) 0.013(0.016)
−30.43 +76.31 +4.96 −2.51 +4.33 −30.94 −1.57 −5.84 −0.75 −6.15 −11.11 −4.64 −0.88 −4.27 −0.19 −30.94
0.019(0.042) 0.014(0.018)
0.014(0.018) 0.014(0.021) 0.015(0.018) 0.014(0.017) 0.014(0.016) 0.014(0.017)
a
T1 diagnostics listed are performed for prereaction complexes at CCSD/6-311++G(d,p)//DFT/BHandHLYP/6-311++G(d,p) and CCSD(T)/6311++G(2d,2p)//MP2/6-311++G(2d,2p) levels (indicated in parentheses). The values, left blank, correspond to transition states that remained undetected at the MP2/6-311++G(2d,2p) level of the theory.
set superposition error (BSSE) because it may worsen the result.41,42
reliable because the difference in geometries obtained at BHandHLYP and those obtained at CCSD and QCISD are found to be negligible.39 To further evaluate the reliability of these computations with respect to a possible multireference feature of the wave function at the stationary points, we performed T1 diagnostics40 of the prereaction complexes at the CCSD/6-311++G(d,p)//DFT/B3LYP/6-311++G(d,p) level. It should be noted that the T1 diagnostic values, reported in Table 1, for the prereaction complexes along the most probable reaction pathways, particularly water-catalyzed, are found to be
