The Peroxymonocarbonate Anion HCO₄⁻ as an Effective Oxidant in the Gas Phase: A Mass Spectrometric and Theoretical Study on the Reaction with SO₂

Abstract

The peroxymonocarbonate anion, HCO₄⁻, the covalent adduct between the carbon dioxide and hydrogen peroxide anion, effectively reacts with SO₂ in the gas phase following three oxidative routes. Mass spectrometric and electronic structure calculations show that sulphur dioxide is oxidised through a common intermediate to the hydrogen sulphate anion, sulphur trioxide, and sulphur trioxide anion as primary products through formal HO₂⁻, oxygen atom, and oxygen ion transfers. The hydrogen sulphite anion is also formed as a secondary product from the oxygen atom transfer path. The uncommon nucleophilic behaviour of HCO₄⁻ is disclosed by the Lewis acidic properties of SO₂, an amphiphilic molecule that forms intermediates with characteristic and diagnostic geometries with peroxymonocarbonate.

1. Introduction

The peroxymonocarbonate anion, HCO₄⁻, was first isolated from alkali metal salts [1,2] and characterised by NMR, Raman spectroscopy [3], and X-ray crystallography [4] as the covalent adduct of the hydroperoxide anion HO₂⁻ and CO₂ (HOOCO₂⁻). This anionic peracid, whose structure resembles that of analogous peroxo-species (e.g., peroxymonosulfate, peroxynitrate, and peroxyacetate) [5], is formed by the in situ reaction between hydrogen peroxide and bicarbonate at a near-neutral pH and in the absence of strong and corrosive acids [6]. Accordingly, bicarbonate-activated hydrogen peroxide (BAP) systems provide a source of HCO₄⁻ ions, which are employed as a selective electrophilic oxidant for electron-rich compounds including alkenes [7], amines [8], organic sulphides [9,10,11], and thiols [12]. HCO₄⁻ is also widely exploited in field-scale applications in environmental remediation processes [13,14] and water treatment [15]. No less important is its role in biological systems, where both hydrogen peroxide and bicarbonate are ubiquitous species in extracellular fluids [16,17]. Indeed, HCO₄⁻ has been indicated as a reactive oxygen species (ROS) in the two-electron oxidation of sulphur-containing aminoacidic residues [11,12], with important implications in neurodegenerative diseases [18] and in the peroxidase activity of copper and zinc superoxide dismutase enzymes (Cu, Zn-SOD) [19]. In most of the above cases, the peroxymonocarbonate ion acts as an electrophilic oxidant [7,8,9,10,11,12,13], while only a few studies report its behaviour as a nucleophile [20,21,22,23], although it is believed that the strong oxidant HOO⁻, present in BAP systems, is the actual nucleophilic oxidant [14].

In contrast to the wealth of studies performed in solution, in the gas phase, to the best of our knowledge, only one study has reported the reactivity of an isobaric electrostatic ionic complex [HO₂⸱⸱⸱CO₂] ⁻ towards atmospheric constituents [24], suggesting the possible occurrence of a bound species, the peroxymonocarbonate ion.

In the gas phase, the peroxymonocarbonate anion has been known since 1977, when it was observed in a flame ionisation detector [25,26] upon clustering reactions involving free oxygen radicals with CO₂ or carbonate derivatives, and more recently in air plasma [27,28], either as a free species or as a negative core ion of large water clusters [29]. The lack of gas-phase reactivity studies concerning HCO₄⁻ ions prompted us to investigate the reactions of this anionic peracid in a joint mass spectrometric and theoretical approach. Over the years, this approach has provided a wealth of knowledge on the intrinsic reactivity of isolated ions towards neutral species in a pristine environment, i.e., in the absence of the masking effects of solvents or counter-ions typically operating in bulk environments [30,31,32]. We therefore undertook an investigation of the oxidative behaviour of HCO₄⁻ towards sulphur dioxide by ion trap mass spectrometry, exploiting an in-house-modified experimental set-up to perform ion–molecule reactions [33]. This experimental approach allows one to precisely ascertain the identity of the reactive (oxidant) species, as well as the relationship between the reagent (oxidant), intermediates and the products involved [30,31]. In the last few years, we have thoroughly investigated the reactivity of sulphur dioxide, one of the most produced commodity chemicals worldwide [34] and, at the same time, a harmful pollutant [35], with singly [36,37] and doubly charged [38,39,40] metal oxide anions and non-metal anionic oxides [41,42,43,44]. The very rich chemistry observed is the result of the amphoteric nature of SO₂, whose sulphur atom can either behave as an electron acceptor or as an electron donor. The nucleophilic ability is due to the lone pair on sulphur in a high-lying σ-based HOMO, whereas the electrophilic properties are due to the low-lying π-symmetry LUMO of sulphur dioxide [45]. Interestingly, the donor–acceptor behaviour of SO₂ is reflected in the diagnostic geometrical features of non-metal [46] and metal–SO₂ complexes [45,47].

We found that SO₂ was effectively oxidised by HCO₄⁻ to hydrogen sulphate, sulphur trioxide, and SO₃⁻, together with carbon dioxide, hydrogen carbonate, and the hydrogen carbonate radical. Hydrogen sulphite was also observed as a result of a secondary reaction promoted by HCO₃⁻ [48]. Our study adds a new contribution to the gas-phase chemistry of both HCO₄⁻ and sulphur dioxide, highlighting the nucleophilic oxidative properties of peroxymonocarbonate, which are disclosed thanks to the electrophilic character of sulphur dioxide. A brief description of the generation of the anionic peroxide HCO₄⁻ by electrospray ionisation (ESI) in our experimental set-up is also given, whereas experiments on sulphur-containing molecules of atmospheric, environmental, and biological interest are ongoing in our laboratory.

2. Results

2.1. Formation and Characterisation of Peroxymonocarbonate Ion (HCO₄⁻)

The peroxymonocarbonate ion (HCO₄⁻) is typically obtained in solution through a substitution reaction between bicarbonate (HCO₃⁻) and hydrogen peroxide (H₂O₂) [6,7,9], whereas, in the gas phase, different formation pathways that consist of the oxidation of the CO₂ or CO₃⁻ species have been proposed [25,26,27,28]. Accordingly, we generated the HCO₄⁻ ion by two alternative methods and precisely through the electrospray ionisation (ESI) of (1) a solution of sodium percarbonate (Na₂CO₃·1.5 H₂O₂; avail. H₂O₂ 20–30%) or (2) pure water.

The first approach is advantageous as sodium percarbonate is known to be a stable and easy-to-handle, commercially available compound, providing at once proper amounts of HCO₃⁻ and H₂O₂ in water. The peroxymonocarbonate ion was indeed formed in Na₂CO₃·1.5 H₂O₂ solution and gently transferred to the gas-phase environment for structural and reactivity investigations.

The second option is also convenient since, by injecting pure water into the ESI source, line saturation effects and the need for frequent cleaning operations are avoided. Furthermore, the application of in-source electric fields to assess the ionisation process lead to the formation of different oxidising agents [49] that react with O₂ or CO₂ typically found in the ambient air or dissolved in water in different forms (CO_2(aq), HCO₃⁻). The ESI approach is also a gentler ionisation method than atmospheric pressure chemical ionisation (APCI), which permits the application of a negative corona discharge in room air to obtain the HCO₄⁻ ion [27,28,29,50]. According to these studies, we also succeeded in generating the peroxymonocarbonate ion through the APCI of ambient air, but the electrospray ionisation of water proved to be the best approach to assess the stable ionic signal of the HCO₄⁻ species, which is a prerequisite for kinetic measurements. Interestingly, to the best of our knowledge, the formation of the HCO₄⁻ ion under these conditions has never been emphasised by other authors, since the studies analysing the ESI speciation of pure water mainly investigate the aggregation phenomena of H⁺(H₂O)_n and OH⁻(H₂O)_n cluster ions [51].

Focusing on the gas-phase formation pathways of the HCO₄⁻ ion, two different reactions were proposed, namely the three-body association reactions of (1) CO₃⁻ + OH^• + M and (2) HO₂⁻ + CO₂ + M [25,26,27,28,29]. In our experimental conditions, the ESI-(–) mass spectrum of pure water acquired in the low 20–150 mass range shows (Figure 1), among others, intense ionic signals at m/z 32 (O₂^•−), at m/z 50 (the hydrated cluster ion [O₂·H₂O]^•−), and at m/z 60 corresponding to the CO₃⁻ ion. The HO₂⁻ species at m/z 33, commonly obtained through the discharge ionisation of H₂O/O₂ mixtures [52], is present only in a very low amount, as also already reported in the corona discharge ionisation systems by Ninomiya et al. [28], together with other ionic species also present in the ESI-(-) mass spectrum, but not involved in the formation of the HCO₄⁻ ion.

To evaluate the possible formation pathway occurring under ESI conditions, we carried out two different experiments. In the first, we infused aliquots of water (previously decarbonated) enriched with Na₂¹³CO₃ or with NaH¹³CO₃ as a source of ¹³CO₃⁻ ion. As a result, the HCO₄⁻ ion at m/z 77 always largely predominates over the labelled H¹³CO₄⁻ species at m/z 78, even if the H¹³CO₃⁻ ion represents the base peak, thus excluding the CO₃⁻ + OH^⸱ + M reaction as a possible HCO₄⁻ formation pathway.

In the second experiment, we analysed an aliquot of water previously degassed to eliminate the dissolved CO₂ and subsequently placed in a closed vessel saturated with ¹³CO₂ (P ¹³CO₂ = 700 torr). Although the occurrence of the ¹³CO_2(g) ⇄ ¹³CO_2(aq) ⇄ H¹³CO₃⁻ _(aq) equilibrium in water is unavoidable [53], a strong increase in the H¹³CO₄⁻ ion compared to its ¹²C isotopologue was observed, highlighting the occurrence of the HO₂⁻ + CO₂ + M reaction in the ESI generation of the peroxymonocarbonate ion. According to this hypothesis, the neutral CO₂, rather than CO₃⁻ or HCO₃⁻ ions, is supposed to be the oxidation target, as demonstrated by the absence of an increase in the HCO₄⁻ ionic signal when water is enriched with bicarbonate or carbonate species, as previously described.

Interestingly, our results parallel the evidence of the intermediacy of CO₂ in bicarbonate/hydrogen peroxide solutions, where HCO₄⁻ is formed through CO₂ perhydration or base-catalysed perhydration [6,8,11].

Prior to investigating its gas-phase reactivity (see next paragraph), the HCO₄⁻ species was mass-selected and characterised by collision-induced dissociation (CID) experiments. The HCO₄⁻ ion at m/z 77 fragments by releasing a hydroxyl radical counterpart (OH^•; 17 Da) [28], leading to the formation of a carbonate radical anion (CO₃^•−) at m/z 60 (Figure 1, upper inset), which accounts for the higher EA of CO₃^• with respect to HO^• [54]. Likewise, the fragmentation of the ¹³C-isotopologue H¹³CO₄⁻ at m/z 78 gives rise to the corresponding ¹³CO₃^•− species at m/z 61 (Figure 1, lower inset).

2.2. Reactivity of Peroxymonocarbonate Ion (HCO₄⁻) towards Sulphur Dioxide (SO₂)

2.2.1. Mass Spectrometric Results

Regardless of the preparation method (if formed by electrospray ionisation of a percarbonate solution, of water, or of a saturated solution of ¹³CO₂), the peroxymonocarbonate HCO₄⁻ and its heavy ¹³C-form predictably show superimposable reactivity towards sulphur dioxide. They were mass-selected and exposed to SO₂ in the ion trap cell to monitor the formation of ionic products as a function of the activation time. A typical product ion spectrum is reported in Figure 2, showing the formation of ionic signals at m/z 97, 81, 80, and 61, respectively, attributed to HSO₄⁻, HSO₃⁻, SO₃⁻, and HCO₃⁻. Ions at m/z 97 and 81 clearly contain one sulphur atom, as witnessed by the characteristic signature of the ³⁴S peak at [M + 2] (H³⁴SO₄⁻ m/z = 99; H³⁴SO₃⁻ m/z = 83), though not easily distinguishable for the low-intensity SO₃⁻ peak at m/z = 80.

The kinetic profile of the reaction (Figure 3) shows that products are formed through parallel and consecutive reactions, and that the precursor ion conversion is fast and efficient, with a decay constant k_dec at 298 K of 7.2 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ and efficiency (k/k_coll) of 54.8% (

Table 1. Rate constants (cm³ s⁻¹ molecule⁻¹), efficiencies (%), and branching ratios (% Σ) relative to the reaction of HCO₄⁻ + SO₂. See text for rate constant numbering, k_n.

	Rate Const. (×10−10) a	Eff.% b	Branching Ratio%
kdec	7.2	54.8
k1	4.6	35.1	64.5
k2	2.5	19.1	34.8
k3	0.050	0.4	0.70
k4	14.0 c	>100

^a ±30%; ^b k/k_coll; ^c see ref. [48].

). Three parallel reaction pathways can be identified in the following:

HCO₄⁻ + SO₂ → HSO₄⁻ + CO₂

(1)

HCO₄⁻ + SO₂ → HCO₃⁻ + SO₃

(2)

HCO₄⁻ + SO₂ → HCO₃ + SO₃⁻

(3)

The main reaction channel 1 consists of a formal SO₂/CO₂ switching, leading to the formation of the bisulphate HSO₄⁻ product ion at m/z 97, accounting for a branching ratio of 64.5% (k₁ = 4.6 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹) and efficiency of 35.1% (Table 1). From another point of view, this reaction can also be considered as a formal hydroperoxide anion HOO⁻ transfer. Pathway 2 gives rise to the HCO₃⁻ ion at m/z 61 with the release of SO₃ as the neutral counterpart through an oxygen atom transfer (OAT) reaction to SO₂ occurring with a rate constant of k₂ = 2.5 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ (eff. = 19.1%) and a branching ratio of 34.8%. Finally, the very minor path 3, accounting for less than 1% of the products, leads to SO₃⁻ at m/z 80, with a k₃ = 5.0 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ (reaction 3, eff. = 0.4%) through oxygen ion transfer (OIT). The preferential formation of products from reaction 2 with respect to those of reaction 3 accounts for the considerably higher electron affinity of the bicarbonate radical (EA HCO₃ = 3.68 eV) [55] with respect to SO₃ (EA SO₃ = 1.90 eV) [54].

The HCO₃⁻ ion formed by reaction 2 is known to be reactive towards SO₂ to form HSO₃⁻ [48], and indeed it is rapidly depleted in a consecutive reaction forming the bisulphite anion at m/z 81 and carbon dioxide (Equation (4)), as illustrated in Figure 2b.

HCO₃⁻ + SO₂ → HSO₃⁻ + CO₂

(4)

This process occurs at a collision rate, with a measured rate constant k₄ of 1.40 × 10⁻⁹ cm³ molecule⁻¹ s⁻¹ (Table 1), in good agreement with previous data [48]. Spectra relative to the reaction of the H¹³CO₄⁻ ion are reported in Figure S1 (Supplementary Materials).

The reaction of HCO₄⁻ + SO₂ → HSO₄⁻ + CO₂ described in Equation (1) is an irreversible process, since no displacement of SO₂ by CO₂ occurred when the HSO₄⁻ ion was reacted with CO₂, not even with the high pressure of carbon dioxide or extension of the activation time up to the maximum value of 10 s. Similar irreversible switching reactions were also observed in the gas phase by reacting carbonate cluster ions with SO₂ and resulting in the complete conversion of carbonate into sulphite ions [43].

2.2.2. Theoretical Calculations

The oxidation routes of SO₂ promoted by HCO₄⁻ were investigated using density functional calculations. Optimised geometries of relevant stationary points and transition states on the B3LYP/aug-cc-pV(T+d)Z potential energy surface for the processes are reported in Figure 4, while charge density distributions of relevant minima are shown in Figure 5. Complete geometrical parameters of minima and saddle points are shown in Figure S2 (Supplementary Materials). Enthalpy changes (kcal mol⁻¹) at 298.15 K computed at the B3LYP/aug-cc-pV(T+d)Z and CCSD(T)/aug-cc-pV(T+d)Z levels of theory are detailed in

Table 2. Enthalpy changes (kcal mol⁻¹, 298.15 K) computed at the B3LYP/aug-cc-pV(T+d)Z and CCSD(T)/aug-cc-pV(T+d)Z levels of theory for selected reaction of HCO₄⁻ + SO₂.

	ΔH°298.15		Barrier Height
	B3LYP	CCSD (T)	B3LYP	CCSD (T)
HCO4− + SO2 → HCO3− + SO3	−28.2	−28.9
HCO4− + SO2 → HCO3 + SO3−	−1.7	5.9
HCO4− + SO2 → HSO4− + CO2	−77.5	−81.0
HCO4− + SO2 → MIN1	−12.9	−12.7
MIN1 → MIN2	−2.2	−3.0	−0.7	−0.9
MIN2 → MIN3	−65.8	−72.6	5.0	5.7
MIN3 → MIN4	−1.2	−0.7	4.8	5.3
MIN4 → HSO4− + CO2	4.7	8.1
MIN3 → HCO3− + SO3	52.8	59.4

. In the following, only the CCSD(T) values will be considered.

HCO₄⁻ features a five-membered-ring intramolecular hydrogen bond between the carbonyl oxygen and the hydrogen atom bound to the peroxidic oxygen [28]. The initial attack of the peroxymonocarbonate anion involves this oxygen atom interacting with the incoming SO₂ and leads to an encounter complex (MIN1) stabilised by a binding energy of 12.7 kcal mol⁻¹. The two moieties, at a distance of 2.37 Å, almost conserve their original geometric features, with SO₂ binding to the peroxyl oxygen in a bent coordination mode: the angle between the peroxyl O–S bond vector and the SO₂ plane is around 100°, with the dihedral OSOO angle equal to 102.1°. With a negative small activation barrier, MIN1 converts to a slightly more stable MIN2, in which the proton is now bound to the carbonyl oxygen. The saddle point TS12 is highly stabilised by the sharing of the proton between the two oxygen atoms. In MIN2, the peroxidic O–S bond shortens to 2.04 Å, developing a small negative charge on SO₂ (−0.27 e⁻) and still binding with a bent geometry (angle between peroxyl O–S bond vector and the SO₂ plane is 100°, with the OSOO angle equal to 100.1°). In MIN2, an incipient SO₃ moiety can be recognised and the charge density distribution on the two entities, SO₃ and HCO₃, is ₋0.83 e⁻ on the former and −0.17 e⁻ on the latter, therefore defining a sulphur trioxide anion and a bicarbonate radical. Proceeding along the reaction coordinate, MIN2 easily converts into MIN3, overcoming a barrier of only 5.7 kcal mol⁻¹. In TS23, the easy formation of an S–O bond and the breaking of the O–O bond lead to a pyramidal SO₃ and an HCO₃ moiety. From this saddle point, a second oxygen atom is transferred from the carbonate to the sulphur trioxide moiety, forming an incipient sulphate in MIN3. This minimum is highly stabilised by 88.3 kcal mol⁻¹ (Table 2) with respect to the entrance asymptote, thanks to the formation of a covalent S–O bond in the SO₃ moiety and to the strong interaction of the sulphur atom with the oxygen of the bicarbonate, preluding at the formation of a sulphate species. MIN3 is further stabilised by an intramolecular hydrogen bond, forming a six-membered ring. The charge density distribution of this stationary point is almost equally distributed between the SO₃ (−0.47 e⁻) and HCO₃ (−0.53 e⁻) groups; nonetheless, in MIN3, one can also consider a (distorted) tetrahedral SO₄ and a bent HCO₂ moiety. Therefore, two competing paths leading to the final products can be envisaged from this point. The first is the dissociation of MIN3 into HCO₃⁻ + SO₃ or into HCO₃ + SO₃⁻, which is driven by the difference in AE between HCO₃ and SO₃. Accordingly, the exit path into HCO₃⁻ + SO₃ experimentally observed (reaction 2) largely prevails over the product couple HCO₃ + SO₃⁻, accounting for the much higher electron affinity of the bicarbonate radical (EA HCO₃^• = 3.68 eV) [55] with respect to SO₃ (EA SO₃ = 1.90 eV [54], 2.22 eV [56]). The difference in the EA values of SO₃ is due to the uncertainty attached to the heat of formation of SO₃⁻ [54,56]. Accordingly, the enthalpy of reaction 3 is slightly positive (see also Table 2), which accounts for the low branching observed. A quite high dissociation energy of MIN3 (59.4 kcal mol⁻¹, Table 2) is required both to break the hydrogen bond and the S–O bond, and to change the SO₃ geometry from pyramidal to trigonal planar, to eventually release sulphur trioxide and the bicarbonate anion. This dissociation energy is, nevertheless, provided by the exothermicity of the process, calculated to be 28.9 kcal mol⁻¹ (Table 2). Alternatively, MIN3 can isomerise with only 5.3 kcal mol⁻¹ to the isoenergetic MIN4, a loose ion-neutral complex between the hydrogen sulphate anion and CO₂, from which only 8.1 kcal mol⁻¹ is required to release the products. The products HSO₄⁻/CO₂ are formed in a very exothermic process, calculated to be 80.9 kcal mol⁻¹ (Table 2). Regarding the competition between the two product couples, HSO₄⁻/CO₂ and HCO₃⁻/SO₃, the experiments revealed a branching ratio of 3:1. Accordingly, the most stable HSO₄⁻/CO₂ products are also formed through the faster path, occurring through a reaction coordinate developing well under the entrance asymptote, whereas HCO₃⁻/SO₃, although forming from direct dissociation, requires a quite large amount of energy to attain the geometry of the products.

3. Discussion

Sulphur dioxide is effectively oxidised in the gas phase by the peroxymonocarbonate ion to hydrogen sulphate, sulphur trioxide, and, in a very minor amount, to SO₃⁻. The reactions proceed through formal HOO⁻, oxygen atom, and oxygen ion transfers, respectively. In the absence of solvent molecules, the reaction is found to proceed along a PES initially characterised by the formation of two encounter complexes (MIN1 and MIN2, Figure 4), in which the two reactants, SO₂ and HCO₄⁻, maintain their geometric features and charge distribution. Nevertheless, the geometry of these adducts can be taken as a diagnostic of the bonding mode of sulphur dioxide, i.e., whether SO₂ behaves as a donor or acceptor molecule. In fact, in analogy with known complexes, metallic [46,47] or non-metallic [47], when SO₂ acts as a Lewis acid, it forms complexes with bent geometries, while linear geometries characterise SO₂, which behaves as a Lewis base.

In agreement, the coordination of HCO₄⁻ to SO₂ in MIN1 and MIN2 results in a bent geometry of the complexes (see Figure S3), the angle between the O-S bond vector and the plane of SO₂ moiety being around 100° in both structures, thereby qualifying SO₂ as a Lewis acid, accepting the lone pair of the oxygen atom in the OH group through its lowest unoccupied molecular orbital, and the interaction as HCO₄⁻ → SO₂. The binding energies of the two minima, 12.7 and 15.7 kcal mol⁻¹ (Figure 4, Table 2), are also in line with those calculated for complexes with pure donors, such as ammonia or methyl-substituted N-heterocyclic carbene, ranging from 10 kcal mol⁻¹ (NH₃∙SO₂) to 18 kcal mol⁻¹ (NHC∙SO₂) [47]. Therefore, the Lewis acidic character of SO₂ discloses the nucleophilic nature of HCO₄⁻, which has rarely been observed in solution studies [20,21,22] when not questioned [14].

In solution, the generally accepted mechanism of the oxidation promoted by peroxymonocarbonate follows that of electrophilic peroxides [5]. The reaction occurs via a solvent-aided oxygen transfer mechanism, which involves a nucleophilic attack by the substrate, Nu, at the electrophilic oxygen of HCO₄⁻ with the formation of the oxidised substrate, NuO, and the bicarbonate ion. An example is given by the electrophilic oxidation of sulphides, substrates with clear nucleophilic properties, which selectively leads to sulfoxides [9]. Secondary oxidation to sulfones is due to O₂^•−, generated upon the decomposition of HCO₄⁻, or to H₂O₂, used to activate NaHCO₃, rather than the peroxymonocarbonate itself [14]. Much uncertainty regarding the nucleophilic behaviour of HCO₄⁻ still exists, also regarding the precise nature of the oxidant. Accordingly, the oxidative degradation of organophosphorus compounds, such as Paraoxon, in the NaHCO₃-activated H₂O₂ solutions appears to be attributable to HO₂⁻ or HO⁻ and not to HCO₄⁻ [14]. This problem is easily circumvented in the gas phase, where it is possible to select a precise oxidant, i.e., HCO₄⁻, and study its reactivity towards a selected substrate, i.e., SO₂.

The actual oxidation step of the reaction occurs only in MIN3, in which one peroxidic oxygen atom has been fully transferred to sulphur dioxide, and the second one is still shared by the sulphur and carbon atoms. Accordingly, MIN3 can directly dissociate into HCO₃⁻ + SO₃ or into HCO₃ + SO₃⁻, with a branching that reflects the thermodynamic stability of the products, or it can evolve, through a small barrier, to a loose ion-neutral complex, eventually leading to the separate products HSO₄⁻ + CO₂. Hence, the driving force of the major, highly exothermic, oxidation process is the liberation of the stable carbon dioxide molecule. This circumstance can explain the different reactivity observed in the reaction of the strong nucleophile hydroperoxide, HOO⁻, which reacts with SO₂ at collision rate, forming SO₃⁻ and only a very small quantity of an adduct of formula HOOSO₂⁻ and of unspecified structure [57].

Finally, the oxidation pathways described here show different characteristics compared to the SO₂ oxidation promoted by other anionic systems, such as oxyhalogenated ions (XO_n⁻ X = Cl, Br, I; n = 1,2) [42] or transition metal oxide anions (MO₂⁻, M = Co, Ni, Cu, Zn; CrO₄⁻) [37], whose common feature is that oxidation already occurs within the first highly exothermic intermediate, leading to free or bound SO₃^0/−. The same oxygen transfer process is not observed in the case of HCO₄⁻, in which an intramolecular hydrogen bond engages the carbonyl oxygen in a stable 5-term cycle directing the reactivity towards the peroxidic OH that binds to SO₂. Oxidation is only observed when the peroxidic oxygen is no longer bound to the hydrogen atom and can be eventually transferred to SO₂.

4. Materials and Methods

4.1. Materials

The chemicals employed in this work were commercialised by Merck and used as received from the vendor. The stated purities are as follows: sodium percarbonate salt (Na₂CO₃·1.5 H₂O₂; avail. H₂O₂ 20–30%), Na₂¹³CO₃ (99 atom% ¹³C), NaH¹³CO₃ (98 atom% ¹³C), ¹³CO₂ (99 atom% ¹³C), and SO₂ (99.9%). All the solvents (H₂O HPLC grade, CH₃CN HPLC grade) were purchased from Carlo Erba Reagents S.r.l. and not further purified.

4.2. Mass Spectrometry

Mass spectrometric investigations were performed by using an LTQ-XL linear ion trap mass spectrometer (ThermoFisher Scientific, Waltham, MA, USA) equipped with an electrospray ionisation (ESI) source. This instrument was previously adapted to realise ion–molecule reaction (IMR) experiments [33,36,37,38,39,40,41,42,43,44]. Accordingly, the gaseous neutral reactant can be introduced into the ion trap through a deactivated fused silica capillary entering the vacuum chamber from a 6.25 mm hole placed in the backside of the mass spectrometer. A Granville-Phillips Series 370 Stabil Ion Vacuum Gauge was used to measure the pressure of the neutral gas, which was kept constant by a metering valve. Owing to the position of the Pirani gauge compared to the ion trap, the actual gas pressure was obtained after reading calibrations [58]. The peroxymonocarbonate ion (HCO₄⁻) was formed in solution by dissolving sodium percarbonate (Na₂CO₃·1.5 H₂O₂) in H₂O/CH₃CN (1:1, v/v) to a millimolar concentration, injected into the ESI source of the instrument at a flow rate of 5 μL min⁻¹. Alternatively, the HCO₄⁻ ion was obtained in the gas phase by directly subjecting pure water to the electrospray ionisation process. Nitrogen was used as a sheath and auxiliary gas at a flow rate of 11 and 2 arbitrary units (a. u.), respectively, a. u. ~0.37 L min⁻¹. Other source parameters were set as follows: spray voltage 3.2 kV, capillary temperature 275 °C, tube lens 15 V, and capillary voltage 10 V. Once optimised, the HCO₄⁻ reactant ion was isolated into the ion trap and exposed to gaseous sulphur dioxide (SO₂). Typical SO₂ pressures ranged between 1.1 × 10⁻⁷ Torr and 6.5 × 10⁻⁷ Torr, with uncertainty of ±30%. During the kinetic experiments, the normalised collision energy was set to zero, whereas the activation Q value was tuned to optimise the stable trapping fields for each ion. The ionic signals of the HCO₄⁻ reactant and reaction products were monitored over time as a function of the SO₂ concentration, and an average of 10 scans was acquired. Xcalibur 2.0.6 software was used to record and process all the mass spectra. Other selected neutrals, such as C₂H₄, ¹³CO₂, C¹⁸O₂, CO, and O₂, proved unreactive with HCO₄⁻. The reaction of HCO₄⁻ ion with gaseous SO₂ can be assimilated to a pseudo-first-order process due to the excess of neutral reactant compared to the precursor ion into the ion trap. Nonlinear least squares regression was performed by using the DynaFit4 software package [59], which simultaneously fit reactant and product concentrations vs. time. The reaction mechanism was verified by fitting the experimental results of the kinetic analysis with the mathematical model proposed for the postulated reaction, which simulated the time progress of the reaction based on the unimolecular rate constants obtained from the software calculations. The pseudo-first-order constants (s⁻¹) were divided into the concentrations of neutral reagent gas to derive the bimolecular rate constants k (cm³ molecule⁻¹ s⁻¹), whereas the branching ratios of the different reaction channels were obtained from the formation constants of the direct products. According to the average dipole orientation (ADO) theory [60], the efficiency of the process was calculated as the ratio of the bimolecular rate constant k to the collision rate constant (k_coll). Approximately 15 independent measurements were performed on different days over a sixfold neutral pressure range to guarantee the linearity of the k values. The standard deviation in the absolute rate constant was typically <10%, although a conservative error of 30% was given owing to the uncertainties affecting the measurement of the neutral pressure. The ionic reactant and products were also characterised by collision-induced dissociation (CID) experiments performed by furnishing energy to mass-selected ions in the presence of helium background gas (P He ca. 3 × 10⁻³ Torr). Normalised collision energies were tuned in a 20–40% range as a function of the isolated species and applied with a standard activation time of 30 ms. Ions were mass-selected with a window of 1 m/z and a Q value optimised to ensure stable trapping fields for all the ionic species under investigation.

The ¹³C-labelled experiments were performed by preparing and mass-analysing millimolar solutions of Na₂¹³CO₃ or NaH¹³CO₃ salt. Analogous studies were also carried out on 1.5 mL of ultrasound-degassed water placed in a double bottle-neck flask that was filled with ¹³CO₂ until a pressure of 700 torr.

4.3. Computational Details

The potential energy surface of the system [HCO₄⁻···SO₂] was investigated localising the lowest stationary points at the B3LYP [61,62] level of theory in conjunction with the correlation consistent valence polarised set aug-cc-pVTZ [63,64,65], augmented with a tight d function with exponent 2.457 for the sulphur atom [66], to correct for the core polarisation effects [67]. This basis set will be denoted aug-cc-pV(T+d)Z. At the same level of theory, we computed the harmonic vibrational frequencies in order to check the nature of the stationary points, i.e., minimum if all the frequencies are real, saddle point if there is one, and only one for the imaginary frequency. The energy of all the stationary points was computed at the higher level of calculation CCSD(T) [68,69,70] using the same basis set, aug-cc-pV(T+d)Z. Both the B3LYP and the CCSD(T) energies were corrected to 298.15 K by adding the zero point energy and the thermal corrections computed using the scaled harmonic vibrational frequencies evaluated at the B3LYP/aug-cc-pV(T+d)Z level. All calculations were performed using Gaussian 09 [71], while the analysis of the vibrational frequencies was performed using Molekel [72,73].

5. Conclusions

Sulphur dioxide is efficiently oxidised in the gas phase by the peroxymonocarbonate ion, HCO₄⁻, through the formal transfer of HO₂⁻, oxygen atom, and oxygen ion, to the hydrogen sulphate anion, sulphur trioxide, and sulphur trioxide anion, respectively. Secondary reactions due to the formation of the hydrogen carbonate anion lead to the formation of hydrogen sulphite. The three oxidative pathways have been characterised using a joint approach of mass spectrometry and theoretical calculations. Contrary to the common behaviour observed in solution, in the gas phase, the peroxymonocarbonate exhibits nucleophilic properties, which are unravelled by the Lewis acid character of sulphur dioxide.