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Examination of the Mechanism of the Yield of NO from Nitroxyl (HNO) in the Solution Phase by Theoretical Calculations Kaiqiang Zhang, and Stefan T. Thynell J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 25, 2017

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The Journal of Physical Chemistry

Examination of the Mechanism of the Yield of N2 O from Nitroxyl (HNO) in the Solution Phase by Theoretical Calculations Kaiqiang Zhang and Stefan T. Thynell* Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, Pennsylvania, 16802

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Corresponding author. Tel.: +1 814 863 0977. E-mail: [email protected] 1 ACS Paragon Plus Environment


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Abstract The dimerization of HNO and subsequent yield of N2O in aqueous solution are studied based on the theoretical calculations and kinetic simulations. The initial dimerization reactions were computed at various levels of theory and large divergence was observed in the predictions of the gas-phase free energies. The T1 diagnostics at CCSD(T)/aug-cc-pVTZ suggests multireference characteristics of the HNO dimers and the transition states. The solution-phase free energies were obtained using the wB97XD method and the SMD solvation model. The pKas of the (HNO)2 tautomers and their first protonated and deprotonated products were estimated using the cluster-continuum approach. The theoretical results confirmed the original conclusion that the favored cis-pathway is comprised of several rapid proton transfer steps leading to either cisHONNOH or cis-HONNO¯ before decomposition. Several new water-catalyzed and H3O+/water catalyzed reactions are presented to explain the fast kinetics observed in the experiments. To validate the proposed mechanism, kinetic simulations with the consideration of diffusion-limited kinetics were implemented on several related systems, based on which the previously reported global rate constant has been explained as the kinetics of the initial dimerization step, and the global kinetics in very dilute HNO solutions.

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1. Introduction The reactivity of HNO (nitroxyl) in a solution phase has been of great interest in several different communities. In propellant studies, HNO has been proposed to be an important intermediate in the decomposition of several nitrogen-containing energetic materials, such as liquid RDX1, 2 and aqueous hydroxylammonium nitrate (HAN).3,

4

Research in biochemistry has suggested that

HNO probably forms in the denitrification process of certain bacteria, and the formation of the N-N bond in the product can occur through the dimerization of nitroxyl.5,

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Recently, many

investigations have been focused on the physiological effect of HNO7-11 and suggested it as a potential pharmacological agent for vasorelaxation in the treatment of heart failure.9-11 One of the most important and well-discussed chemical processes for nitroxyl’s consumption is its rapid dimerization forming hyponitrous acid and the sequential dehydration reaction: HNO + HNO ⟶ HONNOH ⟶ N2 O + H2 O

(R1)

The gas-phase kinetics of R1 are well-studied both experimentally12,

13

and theoretically.14-16

Using the flash photolysis technique to generate gaseous nitroxyl from hydrogen and NO mixtures, Callear and Carr12 reported a 2nd-order decay rate of nitroxyl with a potential energy barrier of 0.91 kcal/mol. Later using a similar technique with different source gases (NH3 and O2), Bryukov et al.13 reported an Arrhenius energy barrier for the dimerization of HNO of 3.6 kcal/mol with a preexponential factor of 3.7×108 M-1s-1. Both experiments were conducted at very dilute initial HNO concentrations (106 M-1s-1),13, 18 the above pH range should be a reflection of the equilibrium among N2 O2 3 and its protonation species instead of the conversion among the species listed in Table 3. Similar to the thermal decomposition of Piloty’s acid (Nhydroxybenzenesulfonamide), which was reported to have the maximum N2O formation for pH higher than 11 at a rate of 4×10-4 s-1.48 Our computed pKa (n = 4) for cis-HONNO¯ is 16.8, which is close to the previous value of 17.5 by Fehling and Friedrichs.19 Both values can explain the experimental observations that cis-ONNO2- is stable in liquid ammonia (pKa=34),49 but highly reactive in water. In the solution phase, proton transfer reactions between the solute and water molecules are considered to be among the fastest groups of reactions ever observed. The observed rate constant for the recombination of H3O+ and a nitrogen/oxygen anion (H3 O+ + A = HA + H2 O) is about 1010-1011 M-1 s-1.50 Our model of diffusion-barrier combined kinetics predicts the rate constants of a barrierless proton transfer reactions to be of the same orders of magnitude, much faster than the reactions with apparent free energy barriers reported in the following section. The fast nature of these proton transfer reactions would lead to fast equilibria among acid/base species.



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Recall that in an aqueous solution, the cis- pathways are favored over the trans- pathways. Among all the neutral cis- isomers, the one with the lowest free energy is cis-HONNOH (13) (pKa = 5.4). Between the two cis- anionic acids 4 (cis-ONHNO¯) and 12 (cis-HONNO¯), 12 (pKa = 16.3) is favored. Both 12 and 13 can decompose to form N2O directly with calculated barriers of 9.6 and 18.7 kcal/mol, respectively. As a result, the sequential pathways and kinetics after the formation of cis-ONHNHO (2) should be pH-dependent: At relatively low pH, the most feasible reaction channel is (cis-)ONHNHO (2)[↔ ONHNO¯ (4)↔ HONNHO (7)/HONHNO (8)↔ HONNO¯ (12) ↔ HONNOH (13)]→ N2O+H2O In the square brackets are the fast equilibrium steps. At higher pH (>5.4), the reaction pathway would be (cis-)ONHNHO (2)[↔ ONHNO¯ (4)↔ HONNHO (7)/HONHNO (8)↔ HONNO¯ (12)]→ N2O+H2O For the minor trans- pathways, HONNOH (14) and HONNO¯ (15) are also the ones with the lowest formation free energies. The calculated free energy of activation for the decomposition of 15 is 21.8 kcal/mol, whereas no direct decomposition step for 14 has been found. The transpathway for N2O formation over a wide pH range, except for the strongly acidic conditions, would be (trans-)ONHNHO (3)[↔ ONHNO¯ (5)↔ HONHNO (9)/HONNHO (10)↔ HONNO¯ (15)]→ N2O+H2O


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The above pathway is similar to the decomposition mechanism of the trans-hyponitrite compound, in which the highest decomposition rate occurs at pH=8-11 with a rate constant of 5×10-4 s-1. More details about this sub-mechanism will be discussed in Sec. 3.2.1. 3.1.3 Implicit and explicit solvent effects on the gas-phase mechanism Previous theoretical studies14-16 revealed that the gas-phase pathways after the formation of HNO dimers comprise several intramolecular hydrogen-shift reactions, cis-trans isomerization reactions, and the final decomposition step forming N2O and H2O. As discussed by Fehling and Friedrichs,19 the solvation can have a strong impact on the well-developed gas-phase mechanism in various ways. The implicit solvent effect means that elementary reactions in a continuum solvent field may have a shift from the gas-phase equilibria and kinetics, which can be represented by the non-zero differences in the solvation free energies of reactants, products, and TSs. The explicit solvent effect means that the solvent molecules may directly participate in reactions, altering the reaction rates and possibly creating new reaction pathways. Figure 1 presents the theoretical structures and free energies of the species and TSs from the gasphase reaction scheme, which reveals the impact of the implicit solvation effect. The overall scheme is quite similar to the one reported by Fehling and Friedrichs,17 but with a few additions. A new transition state was located between cis-HONHNO (7) and cis-HONNHO (8), corresponding to the H-transfer between the N atoms instead of the shift of H atom along the OH-O intramolecular hydrogen bond, although the latter pathway is still kinetically favored with a much lower free energy barrier. A new intermediate ground-state structure (trans-H2ONNO) was located between the trans- tautomers and the final decomposition products, followed by a TS with a small activation energy but no apparent free energy barrier. A similar ground-state structure was mentioned by Ashcraft et al. using IEFPCM(UAHF) solvation model.51 No such 17 ACS Paragon Plus Environment


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intermediate was found for the cis pathway. By only considering the implicit solvent effect, all the possible pathways after HNO dimers to form N2O and H2O have to overcome at least one free energy barrier greater than 40 kcal/mol, even though the final product is thermodynamically much favored over nitroxyl and all the intermediates. Several water-catalyzed H-transfer reactions were also reported by previous works.19, 51 In Table 4 we present several additional reactions with participation of either one or two H2O molecules. Similar to the previous conclusions, the barriers are significantly lowered with the participation of solvent molecules by forming five-to-seven centered transition states. As a result, most of the pathways in Figure 1 with significantly high barriers now become potentially feasible at room temperature. However, almost all the H-transfer reactions presented in Figure 1 and Table 4 can be realized via the fast acid-base equilibrium as discussed in the last section. Then the gas-phase mechanism except for the final decomposition steps is expected to have minimum effect on the solution-phase kinetics. 3.1.4 Reactions under strongly acidic conditions Previous experiments on hyponitrite (trans-ONNO2 ) reported increased decomposition rates as pH of the solution further decreased to the strongly acidic pH region (pH≤1), which was attributed to the protonation of 14 and the sequential fast decomposition to form N2O and H3O+ (R2).22,23 According to our results, the decomposition of the protonated acid (H2ONNOH+) has a quite low free energy barrier (~4.4 kcal/mol) and thus it decomposes much faster than the neutral acid (14) under mildly acidic conditions. However, the computed pKa of H2ONNOH+ is too low (

HNO - ACS Publications - American Chemical Society

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