Quantum Chemistry Guide to PTRMS Studies of As-Yet Undetected

Article pubs.acs.org/JPCA

Quantum Chemistry Guide to PTRMS Studies of As-Yet Undetected Products of the Bromine-Atom Initiated Oxidation of Gaseous Elemental Mercury Theodore S. Dibble,* Matthew J. Zelie, and Yuge Jiao Department of Chemistry, College of Environmental Science and Forestry, State University of New York, Syracuse, New York 13210, United States S Supporting Information *

ABSTRACT: A series of BrHgY compounds (Y = NO2, ClO, BrO, HOO, etc.) are expected to be formed in the Br-initiated oxidation of Hg(0) to Hg(II) in the atmosphere. These BrHgY compounds have not yet been reported in any experiment. This article investigates the potential to use proton-transfer reaction mass spectrometry (PTRMS) to detect these atmospherically important species. We show that reaction of the standard PTRMS reagent (H3O+) with BrHgY leads to stable parent (M + 1) ions, BrHgYH+, for most of these radicals, Y. Rate constants for the proton transfer reaction H3O+ + BrHgY are computed using average dipole orientation theory. Calculations are also carried out on the commercially available compounds HgCl2, HgBr2, and HgI2 to enable tests of the present work. based on the computational work of Goodsite et al.10 Goodsite et al. determined that reaction of BrHg with Br, I, and OH formed thermally stable compounds and computed the rate constant for reaction 2 (but not reaction 3) as a function of temperature at 1 atm of inert bath gas. The kinetics of reaction 2 has since been studied by Wilcox and Okano12 and Balabanov et al.15 Reactions 2 and 3 have been included in modeling studies,16−19 following the explicit assumption of Holmes et al.8 that the rate constant for reaction 3 equals that of reaction 2. Reactions 2 and, presumably, 3 have high rate constants because they are radical−radical recombination reactions that lack activation barriers. We recently suggested that other reactions of BrHg• with atmospherically relevant radicals, Y, (Y = NO, NO2, HOO, O2, ClO, and BrO) would have similarly high rate constants.14 As these radicals are commonly present in enormously higher concentrations than Br or OH, reactions of Y would be more important for BrHg• than reactions 2 and 3, so long as they formed thermally stable compounds BrHgY. Our quantum chemical calculations14 determined that all these radicals, Y, except NO and O2, did form thermally stable compounds; note that Goodsite et al. had obtained the same result for Y = O2.10 As expected, the introduction of the class of reactions

1. INTRODUCTION Mercury is a neurotoxin whose effects are present globally.1 In the developed world, human exposure to mercury comes mostly from consumption of fish, which bioaccumulate mercury from their environment.2 Mercury is emitted to the atmosphere mostly as gaseous elemental mercury (GEM, Hg(0)), which, because of its relatively high equilibrium vapor pressure and low solubility in water, does not readily enter ecosystems.3 By contrast, oxidized mercury compounds (presumably Hg(II)) possess much higher water solubility than Hg(0). Therefore, the details of Hg(0) oxidation largely define when and where mercury enters ecosystems.4,5 Bromine atom has been identified as initiating oxidation of Hg(0) in atmospheric mercury depletion events6 and in the marine boundary layer.7 Modeling studies suggest that oxidation by atomic bromine, alone, is consistent with the atmospheric lifetime of Hg(0), globally.8 Reaction of atomic bromine with Hg(0) proceeds via the reaction9−12 Br • + Hg + M ⇄ BrHg • + M

(1)

Despite the importance of bromine-initiated oxidation of Hg(0) and the modest lifetime of BrHg• with respect to thermal dissociation (reaction 1),10,12−14 the fate of BrHg• radical is very poorly understood. In fact, there have been no experimental investigations of the kinetics or products of reactions of BrHg•. Until very recently, atmospheric models only included two reactions: BrHg • + • Br → BrHgBr

(2)

BrHg • + • OH → BrHgOH

(3)

© 2014 American Chemical Society

BrHg • + • Y → BrHgY

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into atmospheric chemistry models dramatically increases the rate of formation of Hg(II).20,21 These findings necessitate experimental studies of the kinetics and products of the reactions of BrHg• with radicals Received: April 28, 2014 Revised: August 7, 2014 Published: August 12, 2014 7847

dx.doi.org/10.1021/jp5041426 | J. Phys. Chem. A 2014, 118, 7847−7854

The Journal of Physical Chemistry A

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•Y. Unfortunately, there has yet to be any experimental report on any of these BrHgY compounds. This article uses quantum calculations to determine the stability of protonated forms of BrHgY with the goal of enabling the interpretation of protontransfer reaction (PTR) mass spectra of BrHgY. PTRMS is a form of chemical ionization mass spectrometry (MS) widely used for quantification of trace gases in the atmosphere.22 PTRMS uses proton transfer from H3O+ as an ionization source. The rate constant for proton transfer for the trace gases (from experiment or calculation) versus that of a species with a known proton-transfer rate constant enables approximate quantification of the trace gas without use of a standard.22,23 Immediately below we discuss the theoretical methods used to characterize BrHgY and various isomers of protonated BrHgY. We include in our studies three HgX2 compounds (X = Cl, Br, and I), as these are commercially available Hg(II) compounds that could be used to test our computational results. We then present new information about BrHgONO, which is potentially one of the more important Hg(II) compounds formed from BrHg•. Next we present calculations of the rate constants for proton transfer from H3O+ to BrHgY based on the average dipole orientation (ADO) approach.24 We proceed to discuss the stabilities and structures of various isomeric forms of protonated BrHgY and determine the stability of the most stable isomer of each BrHgYH+ upon formation in the exothermic H3O+ + BrHgY reaction.

discovered that the frozen core approximation in this revision of Gaussian09 only freezes 10 rather than 18 electrons of bromine and 28 rather than 36 electrons of iodine in postHartree−Fock calculations with this basis set. This means that absolute energies in this article and our previous work are consistently too negative. Several tests convinced us that relative energies are only affected by about 0.02 eV per bromine atom. As this difference is far less than the expected uncertainty in calculated relative energies (∼0.2 eV),14 the improper treatment of core electrons does not affect the interpretation of results. In comparing relative energies below, we mostly only discuss CCSD(T) results (enthalpy differences at 0 K). Absolute energies are listed in Tables S1 and S2 of the Supporting Information at the B3LYP, CCSD, and CCSD(T) levels of theory. Values of the T1 diagnostic of Lee42 are less than 0.026 for all BrHgY and BrHgYH+ species, implying that singlereference methods are adequate to describe these molecules.

3. RESULTS AND DISCUSSION 3.1. Conformational Change and Isomerization of BrHgONO. In our previous work, we only reported on an anti conformer of BrHgONO (Hg−O−N−O dihedral angle of 180 deg). In this work, we report a syn conformer that is 5.3 kcal/ mol lower in energy than the anti conformer (at 0 K). At 298 K, the syn conformer is favored by 5.1 kcal/mol in Gibbs free energy. Therefore, the anti conformer comprises an entirely negligible fraction of BrHgONO at equilibrium under atmospherically relevant conditions. The computed barrier to anti → syn transformation is 12 kcal/mol. Structures of these species are shown in Figure 1, and relative energies are listed in

2. COMPUTATIONAL METHODS All calculations were carried out on the Gaussian09 system of programs.25 Species with doublet or triplet multiplicity were treated with spin-unrestricted wave functions. Geometries of all species were optimized using the B3LYP functional.26,27 The small-core relativistic effective core potentials ECP60MDF,28 ECP10MDF,29 and ECP28MDF30 were used for mercury, bromine, and iodine, respectively. Electrons of these atoms not treated as part of the pseudopotential were treated with a basis set designed to be consistent with the aug-cc-pVTZ basis set.29−31 All other atoms were treated with the standard aug-ccpVTZ basis set.32−35 We refer to this basis set as aVTZ for convenience. This B3LYP/aVTZ method was also used to compute harmonic vibrational frequencies. As we reported previously, this approach results in overestimating the bond lengths of HgBr2 and HgCl2 by about 2%.14 Some preliminary optimizations were carried out using the LANL2DZ basis set,36−38 as were checks of the stability of wave functions, particularly of the singlet versus triplet character of ions with even numbers of electrons. We explored multiple isomers of the protonated BrHgY and their dissociation products but did not attempt to explore conformational space. Relative energies were refined with single-point energy calculations using coupled cluster methods with single and double excitations (CCSD)39 and a perturbative estimate of the triples excitation (CCSD(T)),40 using the aVTZ basis set defined above. These calculations use the (default) frozen core approximation. For this broad survey, we did not include corrections for spin orbit coupling, which could change relative energies by as much as ∼0.1 eV.41 Additional validation of the reliability of B3LYP/aVTZ and CCSD(T)/aVTZ may be found in the work of Wilcox and Okano.12 Note that all calculations in this article and our previous work14 on XHgY compounds (X = Br, Cl) used revision A of Gaussian09. After most of this work was complete, we

Figure 1. Structures of syn and anti conformers of BrHgONO (left), TS for their interconversion (bottom), BrHgNO2 (top right), and transition state for isomerization of BrHgNO2 to anti-BrHgONO (bottom right).

Table 1. The good agreement between the CCSD/aVTZ and CCSD(T)/aVTZ results, shown in Table 1, suggests that these methods are performing fairly well in describing this chemistry. We also found a transition state (TS) for the exothermic isomerization of BrHgNO2 to anti-BrHgONO. The barrier to isomerization lies 22 kcal/mol below the energy of BrHg• + NO2, so chemically activated transformation of nascent BrHgNO2 to BrHgONO appears feasible. In addition, this barrier is only 14 kcal/mol, which is low enough that thermal isomerization will likely compete with other fates of stabilized 7848



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in PTRMS instruments does not occur at thermal energies.46 The B3LYP/aug-cc-pVTZ method was reported to perform well for dipole moments and polarizabilities of a series of small organic and inorganic molecules.47 Langevin rate constants only vary by 15% among these compounds, but the polar nature of some of the BrHgY compounds leads to as much as a factor of 2.1 increase in the capture rate constant over the Langevin rate constant. As will be presented below, the computed PA of the HgCl2 and HgBr2 species are less than that of H2O, so charge transfer will typically not occur upon their capture by H3O+. Therefore, calculation of the Langevin rate constants for their reaction with H3O+ would be misleading, and they are not included in Table 2. We see from Table S5 of the Supporting Information that the present calculations underestimate the polarizabilities of HgX2 species by 9−19%. A 20% underestimation of the polarizability of the BrHgY compounds would decrease their ADO rate constants by only 3−9%. 3.3. Proton Affinities of BrHgY and Structures and Stabilities of Protonated BrHgYH+. To help evaluate the accuracy of our results, we begin with a comparison, shown in Table 3, of computed and experimental proton affinities (PAs)

Table 1. Relative Energies (kcal/mol at 0 K, Including ZeroPoint Energy) of BrHgNO2, Conformers of BrHgONO, and Transition States Connecting Them at CCSD/aVTZ and CCSD(T)/aVTZ, Computed at B3LYP/aVTZ Geometries energies (kcal/mol) species

CCSD

CCSD(T)

BrHg + NO2 BrHgNO2 TS [BrHgNO2 → anti-BrHgONO] anti-BrHgONO TS [anti-BrHgONO → syn-BrHgONO] syn-BrHgONO

0.0 −33.2 −19.7 −37.3 −30.7 −42.3

0.0 −35.6 −21.6 −38.4 −31.2 −43.7

BrHgNO2. The CCSD and CCSD(T) energies of this transition state relative to syn-BrHgONO only differ by only 0.3 kcal/mol. As is found for RONO2 → ROONO wave functions, the lowest energy wave function of this TS is not a closed-shell singlet.43 The spin unrestricted Hartree−Fock wave function of the state (nominally singlet) with the lowest energy (absent electron correlation) has ⟨S(S + 1)⟩ equal to 0.17, which might imply the need for a multireference wave function to properly treat this transition state. It is therefore important to point out that the value of ⟨S(S + 1)⟩ is reduced to 0.005 after annihilation of the first contaminating spin component. This fact44 and the excellent agreement of the CCSD and CCSD(T) relative energies both imply that, despite the spin contamination, CCSD(T) yields quite reasonable barrier heights for this TS. 3.2. Rate Constants for PTRMS. Langevin theory45 was used to calculate the rate constant for capture of each BrHgY compound by H3O+. Langevin theory gives the ion−molecule capture rate constant, kL, as kL = 2π (αe 2 /μ)1/2

Table 3. Comparison of Experimental and Computed Proton Affinities (eV at 0 K, Including Zero-Point Energies) of Potential Fragments of Protonated BrHgY Compounds, Computed at B3LYP/aVTZ Geometries

(5)

a

species HgI2 BrHgNO BrHgONOa BrHgNO2 BrHgOO• BrHgOOH BrHgOBr BrHgOCl

117.8 89.1 79.6 83.9 77.8 71.9 88.3 80.2

0.00 3.26 1.17 1.75 1.15 1.46 0.51 0.86

2.3 2.0 1.9 1.8 2.0 1.9 2.0 1.7

× × × × × × × ×

10−9 10−9 10−9 10−9 10−9 10−9 10−9 10−9

ADO rate constant N/A 4.2 × 2.4 × 2.8 × 2.3 × 2.5 × 2.1 × 2.2 ×

exptl

Br O2 NO NO2 BrO H2O

5.86 4.41 5.48 6.18 7.20 7.12

5.86 4.38 5.48 6.07 7.24 7.08

5.76a 4.37a 5.52a 6.14a 7.07b 7.16a

for H2O and some of the potential fragments of the BrHgY compounds. This comparison is limited by the dearth of experimental data. For atomic bromine and the four species listed in Table 2 not containing Br (none contain Hg), the agreement is within 0.10 eV. For BrO, calculations overestimate the PA by 0.17 eV. On the basis of these comparisons and our previous results,14 we suggest that relative energies are uncertain by about 0.2 eV. Most of the protonated species investigated here possess multiple isomers. Proton affinities are listed in Table 4 for each isomer found. The CCSD and CCSD(T) proton affinities are in good, though not great, agreement, with a standard deviation of their difference equal to 0.09 eV and the largest difference being 0.25 eV. In every case except BrHgNO the lowest energy isomer of BrHgYH+ is more stable than the next one found by at least 0.5 eV; therefore, in almost all cases the identity of the most stable isomer seems secure. We successfully found isomers of all BrHgYH+ with the proton bound to all the non-hydrogen atoms of the Y moiety, and often, to the bromine atom. Geometry optimizations starting with the proton attached to the mercury atom resulted in high energy isomers of BrHgYH+. Given that mercury does not form strong bonds to hydrogen, this result is entirely expected, so we did not attempt to find isomers with a hydrogen atom bound to the

Table 2. Polarizabilities and Dipole Moment of BrHgY Species and HgI2 Computed at B3LYP/aVTZ, and Langevin and ADO Rate Constants (cm3 molecule−1 s−1 at 298 K) for H3O+ Capturing These Species Langevin rate constant

CCSD(T)

NIST Database (ref 48). Corrected to 0 K with B3LYP/aVTZ zeropoint energies. bReference 49. Corrected to 0 K with B3LYP/aVTZ zero-point energies.

where α is the polarizability of the neutral (in cm ), e is the charge of the ion (in esu), and μ is the reduced mass of the reactants. Corrections for polar compounds were computed using the ADO approach; specifically, using the empirical fits presented in eq 5 of ref 24. These corrections depend on the temperature as well as the dipole moment and polarizability of the neutral reactant. These rate constants are listed in Table 2. While Langevin theory and the ADO approach have been extensively tested,23 it has been suggested that proton transfer

dipole moment (Debye)

CCSD

a

3

polarizabilities (Bohr3)

species

10−9 10−9 10−9 10−9 10−9 10−9 10−9

Syn conformer only (which dominates at equilibrium). 7849



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difference is slightly less than the 0.2 eV uncertainty in our calculations. This means that HgI2 may not be a reliable standard for PTRMS quantification of the BrHgY species. We next turn to Y = NO and NO2. While we discuss some of the less stable [BrHgYH+] isomers, we identify the PA of a BrHgY compound as the value computed for the most stable BrHgYH+ isomer. Two of the isomers of protonated BrHgNO shown in Figure 3 are nearly equally stable: BrHgN(H)O+ and an ion−molecule complex HBr−HgNO+. The CCSD and CCSD(T) levels of theory yield contradictory predictions as to which is most stable (see Table 4). At CCSD(T), the proton affinities of the most stable isomer of BrHgNO is fully 1 eV higher than that of HgBr2. A third isomer of BrHgNO, with the proton bound to the oxygen, is about 1.4 eV less stable than the other two. We find three isomers upon protonation of syn-BrHgONO. The most stable appears to be an ion−molecule complex with the structure BrHgOH−NO+. This structure leads to the highest PA (8.27 eV) of any of the BrHgY species discussed herein. Other isomers have structural formulas of BrHgONOH+ and BrHgON(H)O+, but these are significantly less stable than BrHgOH−NO+. Protonation of BrHgNO2 also leads to three isomers, of which the most stable is BrHgN(O)OH+, leading to a PA of 8.07 eV. As can be seen from Figure 4, this species is not as stable as the BrHgOH− NO+ species derived from BrHgONO. The other two isomers of BrHgNO2H+ (see Figure 3) are ion−molecule complexes that are 1.6−2.3 eV less stable than BrHgN(O)OH+. We did not find protonated structures of either BrHgNO2 or BrHgONO with the proton on the Br atom. Just as there is a TS connecting BrHgNO2 to antiBrHgONO, there is a TS connecting BrHgN(O)OH+ to antiBrHgOH−NO+. The lowest energy (spin unrestricted singlet) SCF wave function of this TS has ⟨S(S + 1)⟩ equal to 0.094, which reduces to 0.0008 after annihilation of the first spin contaminant. The CCSD and CCSD(T) relative energies of this transition state agree to within 0.8 kcal/mol. As argued in section 3.1 for the TS connecting BrHgNO2 to BrHgONO, these facts imply that the CCSD(T) energies should be reasonably accurate despite the spin contamination of the wave function. Figure 4 shows the relative energies for the H3O+ + BrHgNO2/BrHgONO system. The BrHgOH + NO+ asymptote lies 0.13 eV above H3O+ + BrHgONO. This energy gap is within the error of our calculations and makes it possible that protonation of BrHgONO by H3O+ will produce some NO+. Consider the addition of a proton to BrHgOO• or BrHgOOH. Although BrHgOO• is kinetically rather unstable with respect to dissociation to BrHg + O2, it presumably exists in equilibrium with BrHg• in air. The most stable isomer of protonated BrHgOO• places the hydrogen on the radical center: BrHgOOH+. The structures of all three isomers are depicted in Figure 5. The computed proton affinity is 8.06 eV (see Table 3). The most stable isomer we found for protonated BrHgOOH possesses the structural formula BrHgO(H)OH+. Protonation of the other oxygen atom or the bromine atom lead to less stable structures (shown in Figure 5). The relative stability of isomers of protonated BrHgOBr and BrHgOCl are very similar to each other: the hydrogen preferentially adds to the oxygen atom, with protonation of either halogen atom being less favored by about 1 eV. Figure 6 depicts the structures of these species. Table 5 provides systematic data on the enthalpy of dissociation of all isomers of protonated BrHgY into fragments

Table 4. CCSD/aVTZ and CCSD(T)/aVTZ Proton Affinities (eV at 0 K, Including Zero-Point Energies) of HgX2 and BrHgY Compounds, Computed at B3LYP/aVTZ Geometries; For Each Protonated BrHgY for Which Multiple Isomers Were Found, the Proton Affinity is Presented in Bold-Face for the Most Stable Isomer proton affinity neutral compd IHgI BrHgBr ClHgCl BrHgNO

BrHgONO (syn)

BrHgNO2

BrHgOO•

BrHgOOH

BrHgOBr

BrHgOCl

structure of BrHgYH+

CCSD

CCSD(T)

IHgIH+ HBrHgBr+ HClHgCl+ BrHgNOH+ BrHgN(H)O+ HBr−HgNO+ BrHgONOH+ BrHgON(H)O+ BrHgOH−NO+ BrHgN(O)OH+ BrHgH−NO2+ BrHg−HNO2+ BrHgOOH+ BrHgO(H)O+ HBr−Hg−OO+ BrHgOOH2+ BrHgO(H)OH+ HBrHgOOH+ HBrHgOBr+ BrHgOHBr+ BrHgOBrH+ HBrHgOCl+ BrHgOHCl+ BrHgOClH+

7.24 6.98 6.77 6.88 8.26 8.15 7.85 7.66 8.34 8.23 6.29 5.74 8.17 7.64 7.51 7.64 8.34 6.90 6.84 8.25 6.95 6.77 8.03 7.03

7.23 6.94 6.73 6.67 8.01 8.08 7.72 7.59 8.27 8.07 6.48 5.80 8.06 7.48 7.31 7.56 8.22 6.93 6.84 8.13 6.91 6.77 7.92 7.01

mercury atom for each and every BrHgY. Such isomers are reported where found. We start by discussing protonation of HgX2 species, the structures of which are shown in Figure 2. The proton affinities

Figure 2. Structures of [HgX2H]+ with bond distances in Å.

decrease from HgI2 to HgBr2 to HgCl2 in steps of 0.3 and 0.2 eV. As seems reasonable, the XHg−XH bond is significantly longer (by ∼0.22 Å) than the X−HgXH bond. Protonation also causes the X−Hg−X angle to deviate slightly from the 180 deg value of the linear triatomics. Cartesian coordinates of the three HgX2H+ and all BrHgYH+ species are provided in the Supporting Information. Unfortunately, the computed PAs for HgBr2 and HgCl2 are smaller than that of H2O (7.16 eV).48 This means that these molecules are probably not suitable as standards to help quantify signals from the BrHgY compounds. The computed PA of HgI2 is only somewhat higher than that of H2O, and the 7850



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Figure 3. Structures of multiple isomers of protonated BrHgNOx species. Left, [BrHgNOH]+; center, [BrHgONOH]+ and the transition state (TS) for isomerization of BrHgN(O)OH+ to BrHgOH−NO+; right, [BrHgNO2H]+.

that could form while keeping the Y moiety intact. In each case the most stable isomers of the BrHgYH+ are enthalpically stable with respect to these decomposition reactions, as are, in fact, nearly all of the other isomers we found. Note that in computing energies of fragments for Table 5, we attempted to use the fragment isomer and spin state of lowest energy. The lowest energy dissociation pathways for BrHgYH+ lead to production of BrHg+ + YH or HBr + HgY+. As both these pathways lead (mostly) to pairs of closed-shell singlets (except in the case of Y = O2 and HO2), these observations are entirely reasonable. Rather than the absolute stability of the protonated BrHgY, what is most relevant for interpretation of mass spectra from PTRMS is the enthalpy change for the net reaction BrHgY + H3O+ → [BrHgYH]+ + H 2O

+

Figure 4. Relative energies for proton transfer from H3O to BrHgONO and BrHgNO2, dissociation of BrHgOH−NO+ to BrHgOH + NO+, and transition state for isomerization of BrHgN(O)OH+ to BrHgOH−NO+.

→ fragments + H 2O

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This information is provided in Table 6 for the most stable isomer of each [BrHgYH]+ for the five most favorable classes of

Figure 5. Structures of multiple isomers of protonated BrHgOO and BrHgOOH. Left, BrHgOOH+; right, BrHgOOH2+. Selected internuclear distances are listed in Å. 7851



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Figure 6. Structures of multiple isomers of protonated BrHgOX species. Left, BrHgOBrH+; right, [BrHgOClH]+. Selected internuclear distances are listed in Å.

Table 5. Enthalpy of Dissociation (in eV at 0 K, Including Zero-Point Energy) of Protonated BrHgY and HgX2 (X= I, Br, Cl) Compounds into Various Fragments, Computed at CCSD(T)//B3LYP/aVTZ; For the HgX2 Compounds, Y = X protonated species

BrH+ + Hg +Y

BrH + HgY+

BrHgH+ +Y

BrHgH + Y+

BrHg+ + YH

BrHg + YH+

HgH+ + BrY

HgH + BrY+

BrH + Hg +Y

Br+ + HHgY

Br + HHgY+

IHgIH+ BrHgBrH+ ClHgClH+ [BrHgNOH]+ [BrHgN(H)O]+ HBr−HgNO+ syn-[BrHgONOH]+ syn-[BrHgON(H)O]+ syn-[BrHgOH···NO+] [BrHgN(O)OH]+ BrHgH···NO2 BrHg−NO2+ [BrHgOOH]+ [BrHg···O(H)O]+ HBr···Hg+···OO [BrHgOOH2]+ [BrHgO(H)OH]+ [HBrHgOOH]+ HBrHgOBr+ BrHgOHBr+ BrHgOBrH+ HBrHgOCl+ BrHgOHCl+ BrHgOClH+

4.14 5.08 5.74 2.04 3.38 3.45 4.47 4.34 5.03 4.47 2.88 2.21 3.21 2.63 2.46 4.25 4.92 3.64 4.11 5.40 4.17 3.91 5.07 4.15

1.79 1.75 1.58 −0.92 0.42 0.48 1.95 1.81 2.50 1.95 0.35 −0.32 1.64 1.06 0.89 2.21 2.88 1.60 1.83 3.12 1.89 1.59 2.74 1.82

3.03 3.81 4.24 0.78 2.12 2.19 3.21 3.08 3.76 3.21 1.62 0.95 1.95 1.37 1.20 2.99 3.66 2.38 2.85 4.14 2.91 2.65 3.80 2.89

3.86 5.04 6.11 −0.43 0.91 0.97 2.31 2.18 2.86 2.31 0.72 0.05 3.59 3.01 2.84 3.85 4.52 3.24 2.69 3.98 2.76 3.01 4.16 3.24

1.79 1.75 1.58 0.62 1.96 2.02 1.70 1.56 2.25 1.70 0.10 −0.57 1.75 1.17 1.00 1.16 1.83 0.55 0.53 1.82 0.60 0.43 1.58 0.67

3.64 4.35 4.89 1.70 3.04 3.11 3.55 3.42 4.10 3.55 1.96 1.28 3.97 3.39 3.22 2.69 3.36 2.07 2.01 3.29 2.07 2.42 3.57 2.66

2.78 2.90 2.94 0.75 2.09 2.15 3.21 3.08 3.76 3.21 1.62 0.94 3.20 2.62 2.45 3.01 3.68 2.40 2.55 3.84 2.61 2.46 3.61 2.70

4.20 5.35 6.31 2.76 4.10 4.17 5.22 5.09 5.77 5.22 3.63 2.95 6.01 5.43 5.26 5.13 5.79 4.51 4.65 5.94 4.71 4.87 6.02 5.10

3.98 3.78 3.48 0.74 2.09 2.15 3.18 3.05 3.73 3.18 1.59 0.91 1.91 1.33 1.16 2.96 3.63 2.34 2.81 4.10 2.88 2.62 3.77 2.86

3.86 5.04 6.11 5.20 6.54 6.61 5.76 5.63 6.32 5.77 4.17 3.50 6.06 5.48 5.31 5.61 6.28 4.99 2.32 3.61 2.38 4.77 5.92 5.00

3.03 3.81 4.24 1.04 2.38 2.45 2.45 2.32 3.00 2.45 0.86 0.19 2.70 2.11 1.94 1.42 2.08 0.80 1.59 2.87 1.65 1.64 2.79 1.88

Table 6. Enthalpy (in eV at 0 K, Including Zero-Point Energy) for the Net Reaction BrHgY + H3O+ → [BrHgYH]+ + H2O→ Fragments + H2O; Computed at CCSD(T)//B3LYP/aVTZ neutral compd BrHgNO syn-BrHgONO BrHgNO2 BrHgOO• BrHgOOH BrHgOBr BrHgOCl

most stable BrHgYH+ +

HBr···HgNO BrHgO(H)NO+ BrHgN(O)OH+ BrHgOOH+ BrHgO(H)OH+ BrHgO(H)Br+ BrHgO(H)Cl+

BrH + HgY+

BrHgH+ + Y

BrHgH + Y+

BrHg+ + YH

Br + HHgY+

−0.44 1.39 1.04 0.74 1.82 2.15 1.98

1.27 2.65 2.30 1.05 2.60 3.17 3.04

0.05 1.75 1.40 2.69 3.46 3.01 3.40

1.10 1.14 0.79 0.85 0.77 0.85 0.82

1.53 1.89 1.54 1.80 1.02 1.90 2.03

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dissociation pathways from Table 5. Table 6 indicates that reactions represented by reaction 6 are significantly endothermic for all BrHgY except BrHgNO. The analogous reactions for HgI2 are endothermic by at least 1.5 eV. We may conclude that PTRMS studies of HgI2 and all BrHgY species except BrHgNO will yield the parent (M + 1) ion. Note that in the case of BrHgONO, Tables 5 and 6 do not include the most favorable dissociation pathway: the one leading BrHgOH + NO+, which was discussed previously. This pathway requires breaking up the NO2 group, and Tables 5 and 6 only list dissociation reactions that keep the Y moiety intact. The existence of this low energy decomposition reaction for protonated BrHgONO led us to consider decomposition reactions involving fragmentation of Y groups in other protonated BrHgY, namely, BrHgOO• and BrHgOOH. For BrHgOOH+, dissociation to the two pairs of products with stoichiometric formulas BrHgO + OH+ or BrHgO+ + OH are endothermic by 5.7 and 2.7 eV, respectively, which are significantly more endothermic than the most favorable of the dissociation reactions considered in Table 5. For BrHgO(H)OH+, there is the possibility of dissociation to the three pairs of products that can be summarized by the stoichiometric formulas [BrHgOH + OH]+, [BrHgO + H2O]+, and [HBrHgO + OH]+. The thermodynamically most favorable of these pathways leads to BrHgO+ + H2O; the variant of reaction 6 leading to these products is endothermic by 0.39 eV. We can conclude that the variants of reaction 5 in which the Y moeity is broken up are all significantly endothermic and therefore unlikely to occur.

Article

ASSOCIATED CONTENT

S Supporting Information *

Absolute energies, zero-point energies, ⟨S(S + 1)⟩ values, and T1 diagnostic values for all species; Cartesian coordinates and harmonic vibrational frequencies for all BrHgYH+ species. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(T.S.D.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge partial support from the National Science Foundation under grants ATG-0937626 and ATG-1141713. We thank Dr. Huiting Mao for providing partial support of M.J.Z. under the second of these two grants.

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REFERENCES

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4. CONCLUSIONS We have analyzed the protonation of BrHgY species for a broad range of Y that are of atmospheric interest. The results provide sufficient detail to guide PTRMS studies of these species. In particular, we have computed capture rate constants for these BrHgY species by the ADO approach and determined that most BrHgYH+ compounds will produce intact parent ions when formed by proton transfer from H3O+. The production of intact parent ions will greatly simplify the interpretation of mass spectral analysis of BrHgY using proton transfer ionization. The only definite exception is BrHgNO. In this case, the ion formed would be HgNO+, detection of which would provide evidence for reaction of BrHg• with NO. Proton transfer to BrHgNO2 may produce BrHgOH + NO+, which would not provide unambiguous evidence for reaction of BrHg• with NO2. However, the neutral BrHgNO2 likely forms BrHgONO immediately upon being formed in the BrHg + NO2 reaction, and BrHgONO is expected to appear as the parent ion in PTRMS experiments. Our analysis includes HgCl2, HgBr2, and HgI2. These commercially available mercury compounds will provide experimentalists a chance to test the accuracies of the proton affinities computed here. Unfortunately, of these three compounds, only HgI2 has a PA large enough to exothermically capture a proton from H3O+. Therefore, only HgI2 might provide experimentalists with the ability to quantify concentrations of BrHgYH+ species by PTRMS. We hope that this study will enable experimental investigations of bromineinitiated oxidation of Hg(0) by experimentalists using PTRMS. 7853



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