Introduction

Organic peroxyl radicals (ROO●) are an important class of reaction intermediate in the mechanisms of low-temperature combustion as well as in the oxidative processing of volatile organic compounds in the Earth’s lower atmosphere [1, 2]. Despite the importance of organic peroxyl radicals in these chemistries, the direct observation of peroxyl formation and decomposition reactions has remained a significant challenge for conventional experimental techniques. This is because, in the gas phase, peroxyl radicals are typically generated in very low concentrations and have a high propensity for subsequent reaction—including decomposition and self-reactions—resulting in relatively short lifetimes. Building on the pioneering work by Kenttämaa and co-workers [3,4,5,6], a number of groups—including our own—have exploited the distonic radical ion approach for studying the chemistry of organic peroxyl radicals in the gas phase using ion cyclotron resonance and ion trap mass spectrometries [7,8,9,10,11,12,13,14,15,16].

Distonic ions have radical centers that are spatially separated from the charge site [17, 18]. This class of radical ions has been much-less studied than conventional radical ions (where charge and radical are co-localized) since the latter are more commonly produced by traditional ionization techniques, namely, electron- and photo-ionization [19]. Despite requiring greater regiochemical control in their synthesis, distonic ions can represent the global minimum on the radical ion surface and, moreover, provide an attractive model for studying radical reactivity by mass spectrometry. The benefits in deploying this strategy lie in the ability to generate high concentrations of reactive species without complicating self- or cross-reactions (owing to Coulombic repulsion of like charges), combined with the power to cleanly isolate and identify reaction products through changes in mass-to-charge ratio (m/z) [6, 20]. The role of the charge itself in modulating the reactivity of the remote radicals, however, has been the subject of recent interest, with computational studies indicating that polarizable peroxyl and nitroxyl radicals may be stabilized significantly by the presence of a negative charge in a distonic radical anion [21, 22]. Stabilization of the radical anion by up to 5 kcal mol−1 relative to neutral radical analogues has been reported, with smaller, but still significant effects, computed over intra-molecular distances > 10 Å. These findings suggest that radical reactivity may be modulated by the presence of a remotely charged moiety with both the charge polarity and charge-radical separation being key determinants of reaction outcomes.

We have previously reported the application of the distonic ion approach to the examination of the reactivity of phenyl radicals with dioxygen [15]. These investigations uncovered a unique route to the generation and isolation of elusive phenylperoxyl radicals that were then subjected to interrogation by photodissociation action spectroscopy [23]. A key observation by Kirk et al. was that reaction efficiencies and product distributions of the aryl radicals were affected by the polarity of the charged moiety [15]. For example, reaction of the p-(N,N,N-trimethylammonium)phenyl radical cation with dioxygen afforded the corresponding [M+O2]•+ phenylperoxyl radical cation as the major product with an overall efficiency of 4.9% (of the collision rate). In contrast, the p-carboxylatophenyl radical anion was observed to react with an efficiency of 9.2% producing an array of products, including [M+O2-O]•− and [M+O2-CHO]•−, arising from decomposition of a phenylperoxyl radical intermediate [15]. To rationalize these observations, we describe here a systematic experimental and theoretical investigation of the effect of (i) charge-tag polarity and (ii) charge-radical separation on the phenylperoxyl-forming reactions of distonic radical ions. Seven charge-tagged phenyl radicals (1–7; Chart 1) were selected for this study in order to provide: good overlap with previously described distonic ion systems (1, 2, 5, and 6) [6, 15], non-delocalizable negative charge carriers through the use of hypervalent trifluoroborate anion moieties (3 and 4), and increased separation between charge and phenyl radical sites in the scaffold of the 4-(1-carboxylatoadamantyl)phenyl radical anion (7). The reaction efficiencies and product distributions of these radical ions are presented along with electronic structure calculations that extend the exploration of charge-radical effects beyond the experimental test set.

Chart 1
figure 1

Structures of the seven distonic radical ions explored in this study

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Methods

Materials

3-Iodo-N,N,N-trimethylanilinium iodide and 4-iodo-N,N,N-trimethylanilinium iodide were previously synthesized according to literature methods [15, 24, 25]. 3-(4-Iodophenyl) adamantane-1-carboxylic acid was obtained from VBR Molecules (Hyderabad, India). Potassium 3-iodophenyltrifluoroborate (≥ 97%) and potassium 4-iodophenyltrifluoroborate (≥ 97%) were purchased from Advanced Molecular Technologies (Scoresby, Australia). 3-Iodobenzoic acid (98%), 4-iodobenzoic acid (98%), and dimethyl disulfide (98%) were purchased from Sigma-Aldrich (St. Louis, MO). Methanol (Optima LC/MS grade) was purchased from Thermo Fisher Scientific (Scoresby, Australia). Compressed helium (ultrahigh purity, 99.999%) was obtained from Coregas (Sydney, Australia). All commercial compounds were used as received without further purification.

Instrumentation

Gas phase ion-molecule experiments were performed on a modified [25, 26] linear quadrupole ion trap mass spectrometer (LTQ XL, Thermo Fisher Scientific, San Jose, CA) operating Xcalibur version 3.0 software and equipped with a heated electrospray ionization (HESI) source. The HESI source was connected to both the instrument syringe pump and an HPLC system (Dionex UltiMate 3000 RSLC, Thermo Fisher Scientific, San Jose, CA) via a tee union. Methanol or acetonitrile solutions containing 5–10 μM of individual radical precursors were tee-infused at a rate of 5–10 μL min−1 into an HPLC flow of 100% methanol or acetonitrile (50–100 μL min−1) that eluted into the source. Typical source conditions for negative ion mode were capillary temperature 250 °C, source heater temperature 250 °C, sheath gas flow rate 25–30 arbitrary units, auxiliary and sweep gas flow rates 0–5 arbitrary units, spray voltage − 3.5–4 kV, capillary voltage − 35–40 V, and tube lens − 45–50 V. Typical source conditions for positive ion mode were capillary temperature 250 °C, source heater temperature 50 °C, sheath gas flow rate 25–30 arbitrary units, auxiliary and sweep gas flow rates 0–5 arbitrary units, spray voltage 4–4.5 kV, capillary voltage 10–20 V, and tube lens 35–45 V. For collision-induced dissociation experiments, ions were mass-selected with an isolation width of 1.5–2.0 Th, using an activation q parameter of 0.250. The normalized collision energy applied was typically 25–30 arbitrary units with an excitation time of 30 ms (unless otherwise specified).

Previous work by ourselves and others has indicated that synthesis of distonic radical ions by photodissociation can give greater regioselectivity than collision-induced dissociation [5, 27]. To facilitate photodissociation of trapped ions, instrument modifications that allow laser irradiation of ions confined within the linear ion trap are similar to those reported by Ly and Julian and have been described in detail elsewhere [12, 28]. Briefly, the posterior plate of the mass spectrometer was modified with a quartz window to transmit the fourth harmonic (266 nm) of a Nd:YAG laser (Continuum, Santa Clara, CA). The window was positioned on the backplate to direct laser access to the ions within the ion trap through the 2 mm aperture centered on the back lens. Laser pulses were aligned through the aperture in the back lens by two right-angle steering prisms, adjusted to optimize beam overlap with the ion cloud (see Figure S1, Supporting Information). Laser pulses were synchronized to the beginning of an MS2 ion activation step with a TTL trigger signal generated by the mass spectrometer to the laser via a digital delay generator [29]. In these photodissociation experiments, the normalized collision energy was set to 0 (arbitrary units), such that all product ions arise from excitation by the laser pulse. Only a single laser pulse irradiates the target ions in each MS cycle.

Instrument modifications that allow for neutral reagent molecules to be seeded into the ion trap region of the instrument are similar to those previously described [15, 25]. Briefly, the native helium splitter was bypassed in order to directly connect an external reagent mixing manifold to the ion trap (see Figure S1, Supporting Information). Helium flow to the ion trap was controlled by a variable leak valve (Granville-Phillips Model 203, Boulder, CO) providing a pressure reading of 0.8 × 10−5 Torr on the instrument ion gauge. Neutral reagents, such as dimethyl disulfide, were introduced by placing a droplet of liquid into the end-cap of a stainless steel Swagelok cap and placing under vacuum before opening the line to the manifold. The flow of reagent was controlled by a PEEKsil restriction (100 mm length, internal diameter 25 μm). For observing reactions with dioxygen, no reagent was added and adventitious background air within the instrument provided a concentration of dioxygen that was sufficient for the experiments.

Ion-Molecule Kinetics

Pseudo-first-order rate constants, k1 (s−1), were obtained by recording a series of mass spectra as a function of storage time for the reaction between the radical ion and neutral reagent. Each spectrum recorded was an average of at least 60 individual spectra. The reaction time was defined as the interval between the isolation of the selected radical ion and ejection of all ions from the trap for detection. Reaction times of 50–10,000 ms were set using the activation time parameter within the instrument control software. A plot of the mean radical ion abundance against reaction time yielded a single exponential relationship. Fitting the data to Eq. (1) gave the pseudo-first-order rate constant for the reaction, k1 (s−1). Second-order rate constants, k2 (cm3 molecule−1 s−1), were obtained from k1 and the concentration of dioxygen ([O2]) present in the ion trap region of the mass spectrometer (Eq. (2)) [30], where [O2] was determined from the calibration reaction of 3-carboxylatoadamantyl radical anion with dioxygen under the same instrumental conditions [25]. Reaction efficiencies, Φ (i.e., the percentage of ion-molecule collisions that resulted in product formation), were determined according to Eq. (3), where the collision rate constant, kcoll, is determined by average dipole orientation (ADO) theory at 307 K [31]. The temperature of ions stored within a linear quadrupole ion trap has been estimated at 318 ± 23 K [32], consistent with an earlier estimate of 307 ± 1 K [25], which can be taken as the effective temperature for ion-molecule reactions observed herein [33, 34]. While the accuracy of the rate constant measurements is estimated to be ± 50%, the precision of the measurements is better than ± 10%.

$$ {\left[{R}^{\bullet}\right]}_t={\left[{R}^{\bullet}\right]}_0{e}^{-{k}_1t} $$
(1)
$$ {k}_2=\frac{-{k}_1}{\left[{O}_2\right]} $$
(2)
$$ \varPhi =\frac{k_2}{k_{coll}}\ast 100 $$
(3)

Computational Methods

The Gaussian 09 suite of programs was used for all computations [35]. Electronic energies and thermally corrected (298 K) enthalpies for all ground-state species were computed using the hybrid meta-GAA M06-2X functional [36, 37] and the 6–311++G(d,p) basis set. DFT calculations for doublet states employed an unrestricted formalism. This computational strategy has previously been shown to be effective in modeling the thermochemical and spectroscopic properties of gas phase arylperoxyl radicals [23]. All stationary points on the potential energy surface were verified to be either local minima (no imaginary frequencies) or transition states (one imaginary frequency) by computation of analytic vibrational frequencies. Transition states were connected to minima by calculation of the intrinsic reaction coordinate (IRC). Outputs from all calculations discussed in this study are summarized in the Supporting Information.

Results and Discussion

Formation of Charge-Tagged Phenyl Radicals in the Gas Phase

Aryl iodide precursor ions were generated by electrospray ionization (in either positive or negative ion mode), mass selected, and subsequently irradiated at 266 nm within the ion trap mass spectrometer to liberate the distonic radical ions 1–7 (Chart 1). Photodissociation of aryl iodides has been demonstrated as an effective means of generating aryl radicals with high regiospecificity, and the approach has been deployed to great effect by Julian and co-workers for molecular structure elucidation via radical-directed dissociation [28, 38]. In this study, the putative structure of the radical ions was confirmed for each instance by observing the gas phase reaction with dimethyl disulfide [11, 25, 39,40,41]. For all radical ions 1–7, the reaction resulted in thiomethyl abstraction consistent with the distonic radical ion structures shown in Chart 1 (see example spectrum in Figure S2, Supporting Information).

Charge Polarity Effect on Products of Phenyl Radical Reactions

Both para- and meta-(N,N,N-trimethylammonium)phenyl radical cations (1 and 2) were prepared by 266 nm photodissociation of mass-selected 4- and 3-iodo-N,N,N-trimethylanilinium cations, respectively. Mass selection and isolation of m/z 135 within the ion trap allowed for observation of the reactions of the radical cations 1 and 2 with background oxygen at timescales ranging from 100 to 10,000 ms. These observations revealed that reaction of 1 and 2 with O2 predominantly resulted in the formation of phenylperoxyl radical cations (m/z 167) 1-OO and 2-OO, respectively (Figure 1). Two trace product ions of m/z 151 and m/z 138 were also observed arising from the peroxyl radicals (see ×50 magnification in Figure 1). Decomposition of 1-OO and 2-OO by cleavage of the O–O bond results in the loss of an O (3P) atom, ultimately forming p- and m-(N,N,N-trimethylammonium)phenoxyl radical cations of m/z 151. The product ion of m/z 138 corresponds to the loss of a formyl radical (− HCO, − 29 Da) from the peroxyl radical intermediate to form a charge-labeled cyclopentadienone. These observations are consistent with our previous exploration of the reaction of positively charged arylperoxyl radicals with dioxygen [15].

Figure 1
figure 2

(a) Mass spectrum arising following the reaction of 4-(N,N,N-trimethylammonium)phenyl radical cation, m/z 135 (1), with O2 for 6 s. (b) Mass spectrum arising following the reaction of 3-(N,N,N,-trimethylammonium)phenyl radical cation, m/z 135 (2), with O2 for 6 s. Trace amounts of hydrogen atom abstraction by 1 and 2 from a small amount of contaminant present in the mixing manifold were also observed

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The reactivity of the p- and m-trifluoroboratophenyl radical anions, 3 and 4, respectively, toward O2 are reported here for the first time. The spectra shown in Figure 2 reveal that the reaction produces only trace amounts of the peroxyl radical adduct with ions corresponding to 3-OO and 4-OO (m/z 176) observable only at ×50 magnification (Figure 2). Numerous product ions consistent with the facile decomposition of these [M+O2]•− adduct ions are observed, pointing to the fleeting lifetime of the arylperoxyl radicals formed in both cases. Scheme 1 presents the possible rearrangement and decomposition pathways available to the peroxyl radical 4-OO and is based upon a previous computational investigation of the potential energy surface for the analogous 4-carboxylatophenyl peroxyl radical anion, 5-OO [15]. The major product ions in the spectra in Figure 2 are observed at m/z 147 corresponding to [M+O2-CHO]− arising from ejection of a formyl radical from the peroxyl radical intermediate (Scheme 1 for 4-OO; for analogous 3-OO, see Scheme S1, Supporting Information). The spectra also reveal a minor product ion at m/z 175 that could be rationalized as a charge-tagged benzoquinone anion arising from the loss of atomic hydrogen. o-Benzoquinones are known to eject carbon monoxide to yield cyclopentadienones [42, 43]; hence, prompt loss of CO from the m/z 175 anion population may also contribute to the abundant product ion signal observed at m/z 147 (Scheme 1). Other major phenylperoxyl radical decomposition products observed arise from cleavage of the oxygen-oxygen bond in the peroxyl radical to form a phenoxyl radical anion (m/z 160), as well as ejection of the BF3 group to form [M+O2-BF3]•− ions at m/z 108. The abundant loss of BF3 from 3-OO and 4-OO (m/z 108) is rather surprising. Calculations predict that direct ejection of BF3 from 3-OO to form a peroxyl phenide radical anion is 26.2 kcal mol−1 above the entrance channel (data not shown). Hence, the loss of BF3 most likely arises from isomerization followed by migration of an oxygen atom around the aromatic ring of 3-OO and 4-OO to form p- and m-benzoquinone radical anions (Scheme 1). Lastly, minor decomposition product ions of m/z 119 were also observed presumably arising from decarbonylation of cyclopentadienone intermediates (m/z 147) to form charge-tagged cyclobutadiene or vinylacetylene anions (Scheme 1). These observations are consistent with recent temperature-dependent studies examining the pyrolysis of cyclopentadienone that have found that vinylacetylene production dominates at lower temperatures (1000–1400 K) [44]. Reactivity of the p- and m-carboxylatophenyl radical anions (5 and 6, respectively) toward O2 was also examined, and the corresponding spectra are shown in Figure S3, Supporting Information. The observed reaction products are consistent with previous reports and are analogous to the channels observed for the trifluoroborates, 3 and 4.

Figure 2
figure 3

(a) Mass spectrum arising from the reaction of 4-(trifluoroborato)phenyl radical anion, m/z 144 (3), with O2 for 6 s. (b) Mass spectrum arising from the reaction of 3-(trifluoroborato)phenyl radical anion, m/z 144 (4), with O2 for 6 s. Trace amounts of hydrogen atom abstraction by 3 and 4 from a small amount of contaminant present in the mixing manifold was also observed

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Scheme 1
scheme 1

Proposed rearrangement and dissociation pathways for the p-trifluoroboratophenyl peroxyl radical anion (4-OO)

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Charge Polarity Effect on the Reaction Potential Energy Surface

The stark difference between the oxidation product distributions of positive and negative charge-tagged phenyl radicals suggests significant modulation of the potential energy surface between the two systems. To assist in rationalizing these experimental observations and to enable comparison to the neutral phenyl radical archetype, the reaction enthalpies (ΔrxnH298) of selected intermediates and transition states for the neutral and charge-tagged phenyl radical plus dioxygen systems were calculated at the (U)M06-2X/6–311++G(d,p) level of theory (Figure 3). A substantial body of theoretical investigations on the phenylperoxyl radical potential energy surface have concluded that isomerization by addition of the peroxyl radical oxygen back onto the ipso-carbon of the aromatic ring to form a dioxiranyl intermediate is the lowest-energy isomerization pathway (N4; Figure 3) [15, 16, 45,46,47,48,49,50,51]. This dioxiranyl radical, along with the phenoxyl radical arising from oxygen-oxygen bond homolysis, thus represent the key reaction intermediates leading to the formation of all product ions (see above), and hence, the influence of the charged moieties on these reaction steps have been examined computationally.

Figure 3
figure 4

Potential energy surface of the positive (red, X = NMe3+) and negative (blue, X = BF3−) charge-tagged and neutral (black, X = H) Ph•+O2 reactions for comparison of the formation of phenoxyl radical and phenyldioxiranyl radical intermediates leading to further phenylperoxyl radical decomposition. Calculated at the (U)M06-2X/6–311++G(d,p) level. Energies for the meta-substituted analogues are shown in parentheses

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The computational results, summarized in Figure 3, illustrate that addition of O2 to the neutral phenyl radical produces the phenylperoxyl radical (N2) with an exothermicity of 46.6 kcal mol−1. Homolytic cleavage of the O–O bond in N2 yields the phenoxyl radical plus atomic oxygen (3P) (N3) that is 7.9 kcal mol−1 below N1. Alternatively, the oxygen-centered radical of the peroxyl moiety in N2 can undergo addition to the ipso-carbon of the aromatic ring forming the dioxiranyl intermediate (N4) that is 26.3 kcal mol−1 below the entrance channel on the neutral surface. This isomerization occurs through the transition state TS N2 → N4 that represents a barrier of 31.3 kcal mol−1 with respect to the phenylperoxyl radical (N2) but is still some 15.3 kcal mol−1 below N1.

In the presence of the positively charged trimethylammonium moiety at the para-position, the addition of dioxygen to the phenyl radical cation (C2) is exothermic by 42.8 kcal mol−1 (Figure 3, red lines). Interestingly, on the cation surface, the minimum for the phenylperoxyl radical C2 lies in a well that is 3.8 kcal mol−1 shallower than the comparable neutral peroxyl radical (N2). The p-(N,N,N-trimethylammonium)phenoxyl radical (C3) formed upon homolysis of the O–O bond in C2 is 7.1 kcal mol−1 higher in energy than the neutral phenoxyl radical and lies just 0.8 kcal mol−1 below the entrance channel (C1). Similarly, the transition state TS C2 → C4 is predicted to lie 12.0 kcal mol−1 below C1, which is 3.8 kcal mol−1 above the comparable barrier on the neutral surface (TS N2 → N4). Furthermore, the dioxiranyl intermediate C4 is also predicted to be 6.2 kcal mol−1 higher in energy than the neutral archetype N4, lying 20.1 kcal mol−1 below C1. Interestingly, when the fixed positive charge-tag is substituted at the meta-position with respect to the radical site, the destabilizing effect of the charge-tag does not vary significantly from the para-substituted isomer (cf. values in parentheses in Figure 3). This finding indicates that the difference between the positively charged and neutral phenyl radical potential energy surfaces is not strongly influenced by formal charge or radical delocalization around the aromatic ring.

Considering the negatively charged analogue, addition of dioxygen to the p-trifluoroboratophenyl radical anion forms the p-trifluoroboratophenylperoxyl radical anion (A2) which resides in a minimum some 51.3 kcal mol−1 below the entrance channel (A1; Figure 3). When normalized to the energy of the entrance channel, the negatively charged peroxyl radical is 4.7 kcal mol−1 below its neutral counterpart N2. Comparison to the corresponding cation C2 reveals that the stabilization of the peroxyl radical by the proximate negative charge is larger in magnitude than the destabilization in the cation (cf. 3.8 kcal mol−1). The p-trifluoroboratophenoxyl radical anion (A3) formed upon cleavage of the O–O bond in A2 is predicted to be 9.6 kcal mol−1 lower in energy than N3, lying 17.5 kcal mol−1 below A1. The barrier to isomerization of A2 to form the dioxiranyl intermediate A4 is predicted to be 24.7 kcal mol−1 (TS A2 → A4) and is thus 4.7 kcal mol−1 lower in energy than the equivalent neutral transition state TS N2 → N4. A4 is computed to be 8.8 kcal mol−1 lower in energy than N4, lying 35.1 kcal mol−1 below the entrance channel. The net lowering of the potential energy surface relative to the entrance channel is also observed when the fixed negative charge-tag is in the meta-position with respect to the radical site (values in parentheses in Figure 3), indicating that resonance effects are only minor contributors to the overall stabilization afforded by the anion moiety.

The normalized reaction coordinate diagrams of the positively and negatively charge-tagged phenyl radicals in Figure 3 give some insight into the differences in experimentally observed reaction products for the two polarities (vide supra). For example, in the positively charge-tagged system, the barriers to direct decomposition or rearrangement of the intermediate peroxyl radicals 1-OO and 2-OO lie much closer to the entrance channel (cf. red lines in Figure 3). These findings suggest that the rate of decomposition will be significantly slower than for the corresponding anions or neutrals and affording a greater contribution from stabilization of the peroxyl adduct. This is consistent with the observation of phenylperoxyl radical formation (i.e., 1-OO and 2-OO) as the predominant reaction product with only trace amounts of the [M+O2-O]•+ and [M+O2-CHO]•+ decomposition products (see Figure 1). Conversely, for the negative ions, the transition states and intermediates in the decomposition pathways are lowered substantially relative to both the cation and neutral analogues (cf. blue lines in Figure 3). These calculations are thus consistent with higher rates for unimolecular transformation of the intermediate peroxyl radicals 3-OO and 4-OO to decomposition products. Such rapid transformations could outcompete stabilization of the adduct ions and explain why only trace amounts of the 3-OO and 4-OO anions are observed experimentally (Figure 2). The computational results illustrated in Figure 3 suggest that the distonic ion reactivity observed in this study is likely to bracket that of the neutral phenyl radicals. It would be interesting therefore to assess whether increasing the distance between charged and radical moieties might allow convergence (from above and below) on the energetics of the neutral phenyl radical archetype.

Charge-Radical Distance Effects on the Reaction Potential Energy Surface

The effect of the intramolecular separation of charged and radical moieties was investigated computationally for the phenyl radical plus dioxygen reaction. The approach used in this study follows the strategy devised by Grob et al. [52] that has been recently been deployed in probing energetic and spectroscopic shifts associated with charge-radical separation in distonic ions [16, 21, 23]. Here, the reaction enthalpies for selected stationary points on the Ph•+O2 potential energy surface were computed for a series of para-substituted phenyl radicals of the form X-(CH2)n-Ph• (where X = −BF3 or (CH3)3N+ and n = 1–7). Comparing the enthalpy of each stationary point—relative to the entrance channel—with the analogous ΔrxnH values derived from neutrals of the form X-(CH2)n-Ph• (where X = H) gives a stabilization or destabilization enthalpy (ΔΔrxnH) resulting from the charge at a distance defined by the number of methylene groups (n) separating the charge carrier from the phenyl ring. In these calculations, the extended hydrocarbon chain was deliberately constrained to an all gauche conformation so as to extend, as far as possible, the distance between the charge and radical sites for a given number of methylene units. Figure 4 plots the ΔΔrxnH values computed (relative to the corresponding neutral radical, indicated as a dashed horizontal line) for (a) the phenylperoxyl radical intermediate (X-(CH2)n-PhOO•), (b) the phenoxyl radical exit channel (X-(CH2)n-PhO•), and (c) the dioxiranyl intermediate. The enthalpy differences are plotted against the reciprocal distance (1/r) between the boron or nitrogen (for X = −BF3 or (CH3)3N+, respectively) and the radical center which is taken as the terminal oxygen atom of the peroxyl radical (Figure 4a) or the para-carbon of the phenyl ring for the phenoxyl and dioxiranyl structures as illustrated in Figure 4b, c, respectively.

Figure 4
figure 5

Stationary point enthalpies relative to the entrance channel for the X-(CH2)n-Ph•+O2 potential energy surfaces (where X = −BF3 (blue circles) or (CH3)3N+ (red circles) and n = 1–7) are plotted as a difference in enthalpy to the corresponding stationary points on the neutral potential energy surface for the H-(CH2)n-Ph•+O2 reaction. Enthalpy differences (at 298 K) were computed at the (U)M06-2X/6–311++G(d,p) level of theory and are plotted (in kcal mol−1) against reciprocal distance between the charge an radical moieties as indicated by the green lines on the representative structures shown. Enthalpy differences are shown for (a) the phenylperoxyl radical (where y[(CH3)3N+] = 37.777x − 1.6043, R2 = 0.99814 and y[−BF3] = − 59.089x + 3.1586, R2 = 0.97784), (b) the phenoxyl radical (where y[(CH3)3N+] = 20.592x − 0.1393, R2 = 0.98642 and y[−BF3] = − 30.923x + 1.0263, R2 = 0.99886), and (c) the dioxiranyl intermediate (where y[(CH3)3N+] = 16.28x − 0.3357, R2 = 0.96722 and y[−BF3] = − 27.167x + 0.8888, R2 = 0.99902). Representative structures for all three reaction intermediates and products are provided

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The stabilization of the phenylperoxyl radical computed for a remote, fixed negative charge and the destabilization of the phenylperoxyl radical afforded by a remote, fixed positive charge were both found to decay linearly with 1/r (Figure 4a). The same was found to be true for the phenoxyl radical and dioxiranyl radical intermediates (Figure 5b, c). Interestingly, the enthalpy perturbations arising from a remote, fixed-charge moiety on the reaction intermediates in the Ph•+O2 reaction manifold are persistent over long intra-molecular distances. For example, a (de)stabilization of the phenylperoxyl radical by ~1 kcal mol−1 is calculated even when the charge-radical separation is 14.5 Å (Figure 4a) and a similar (de)stabilization of the phenoxyl radicals and dioxiranyl radical intermediates by ~2 kcal mol−1 over distances of 10.2 Å (Figure 5b, c). The linear fits of these data predict that convergence of the positively and negatively charged distonic radical ions could be expected at charge-radical distances of up to 19 Å. Testing this prediction experimentally, however, presents a challenge in identifying a suitable molecular scaffold capable of maintaining a rigid separation over such distances between a well-defined charged group and an unpaired electron. Nonetheless, the calculations presented in Figure 4 also suggest that a modest increase in charge-radical separation of just 2.4 Å (n = 3) could reduce the stabilizing effect of the negative charge by a factor of 2, and reduce the destabilizing effect of the positive charge by a factor of 1.5. This might suggest, for example, that a rigidly imposed charge-radical separation of 9–10 Å in a negatively charge-tagged phenyl radical may be enough to slow the decomposition and rearrangement reactions of the peroxyl radical intermediate and thus measurably alter the product distribution of the Ph•+O2 reaction.

Figure 5
figure 6

Mass spectra resulting from the reaction of (a) the 4-carboxylatophenyl radical anion, m/z 120 (5), and (b) the 4-(1-carboxylatoadamantyl)phenyl radical anion, m/z 254 (7), with dioxygen for 5 s

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To experimentally test the effect of charge-radical separation on the products of arylperoxyl forming reactions, the reaction of the 4-(1-carboxylatoadamantyl)phenyl radical anion (7) with dioxygen was investigated in the ion trap mass spectrometer and compared side-by-side with the reactions of the 4-carboxylatophenyl radical (5). Molecular orbital calculations estimate the distance between the radical site and the charged moiety in 7 is 9.1 Å, representing an increase in separation of 5.3 Å compared with 5 (data not shown). Figure 5a and b shows the mass spectra obtained after isolation of the radical anions 5 and 7, respectively, in the presence of dioxygen for 5 s. The spectrum obtained from 4-carboxylatophenyl radical anion (m/z 120) is dominated by the phenoxyl radical product ion at m/z 136 with the corresponding phenylperoxyl radical (5-OO) at m/z 152 only present at very low abundance (see ×40 magnification in Figure 5a). In contrast, analogous reaction of the 4-(1-carboxylatoadamantyl)phenyl radical anion (m/z 254) with O2 yields an abundant 7-OO product ion at m/z 286 (Figure 5b), with the phenoxyl radical plus atomic oxygen product channel (m/z 270) making a significantly diminished contribution (see ×40 magnification in Figure 5b). These observations are consistent with the computational prediction (vide supra) that increasing the charge-radical separation raises the barriers toward decomposition and isomerization of the phenylperoxyl radical, thus providing a greater opportunity for stabilization of the adduct ion.

Charge Polarity and Charge-Radical Distance Effects on the Rate of Phenyl Radical Reactions

The second-order rate constants for the reaction of charge-tagged phenyl radicals 1–4 with dioxygen were derived from measured pseudo-first-order rate constants and are reported in Table 1. These rate constants range from k2 = 3.6–6.7 × 10−11 cm3 molecule−1 s−1 and are three to five times greater than the comparable rates measured for the neutral phenyl radical previously reported (Table 1) [53,54,55]. In order to normalize for the effect of charge on collision dynamics, the reaction efficiencies were determined through estimation of the collision rate for the different reactions (Table 1). Using an estimated hard sphere collision rate of 3.2 × 10−10 cm3 molecule−1 s−1, the reaction efficiency for the neutral phenyl radical with dioxygen was estimated to range between 4 and 6%. In comparison, efficiencies for the reactions of 1–4 ranged from 6 to 12%. Contrasting the two polarities, the presence of the negative charge leads to a significantly higher reaction efficiency (Φ = 10 and 12%, for 3 and 4, respectively) compared with the positively charged analogues that are much closer to the neutral archetype (Φ = 6 and 7%, for 1 and 2, respectively). Noting that the relative uncertainty in the efficiency determinations is considerably smaller than the absolute uncertainty listed in Table 1, radicals 2 and 4 react slightly faster than their para-substituted isomers 1 and 3, respectively. As well as the position of the charged moiety on the benzene ring, the effect on reaction efficiency of intramolecular charge-radical separation was also explored. The reaction efficiencies of the 4-carboxylatophenyl radical anion (5) and the 4-(1-carboxylatoadamantyl)phenyl radical anion (7) were measured to be 8 (± 1)% and 6 (± 1)%, respectively. This result suggests that removing the negatively charged moiety further from the phenyl radical center retards the reaction efficiency, bringing it closer to the reactivity of the cations 1 and 2 and current estimates of the neutral phenyl radical.

Table 1 Comparison of Second-Order Rate Constants and Reaction Efficiencies for the Ion-Molecule Reactions of Distonic Radical Ions 1–4 with O2
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The reaction efficiency results summarized in Table 1 are interesting to juxtapose against the potential energy surfaces summarized in Figure 3. The computational results in Figure 3 suggest that all Ph•+O2 reactions should proceed by the barrierless recombination of the phenyl radical with dioxygen. Such a barrierless and highly exothermic reaction (ΔrxnH = − 42.4–51.9 kcal mol−1) might be expected to have an efficiency near unity (i.e., 100%). In contrast, the experimental findings resented here indicate reaction efficiencies of < 12% in all instances and, moreover, that the efficiencies are modulated by the polarity, location, and distance between the radical and charged moieties. These findings perhaps point to a more nuanced potential energy surface with both a shallow pre-reactive complex and reef-type barrier just below the entrance channel that represent the rate-determining step in the recombination reaction. Investigation of this extremely flat portion of the potential energy surface using electronic structure calculations is challenging and beyond the scope of the current study. Computational studies of related gas phase ion-molecule reactions, however, have served to highlight the importance of the potential energy landscape surrounding the entrance channel in modulating the reaction kinetics [12, 56]. As indicated in the hypothetical surface in Figure 6, we can conclude based on the experimental findings that—relative to the entrance channel—the barrier to reaction will be lowered in the presence of an anion while the cations have much less impact on the barrier height. Future experiments and high-level calculations will provide insights into the nature of this barrier and its role in dictating reaction rate in arylperoxyl forming reactions.

Figure 6
figure 7

Hypothetical potential energy surface for the formation of a pre-reactive complex upon radical combination of Ph● and O2 giving rise to a small barrier to phenylperoxyl radical formation

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Conclusions

Through a combined experimental and computational examination, we have demonstrated that the polarity of the charge-tag significantly perturbs the potential energy surface of the Ph●+O2 reaction system, altering both the reaction kinetics as well as the reaction products observed. The presence of a fixed-positive charge was found to destabilize influential stationary points on the Ph●+O2 potential energy surface relative to the entrance channel. In contrast, a fixed-negative charge was found to lower the energy of the same stationary points on the corresponding anionic potential energy surface. The opposite but unequal perturbation arising from these charge-tags can account for the different product distributions of the Ph●+O2 reaction and also the measured differences in reaction efficiency. Computational exploration of these effects with increasing intra-molecular separation of charge and radical sites suggests that, in the gas phase, the charge can influence the energetics associated with the formation and fate of peroxyl radicals over surprisingly long distances (e.g., up to 14.5 Å). These predictions were supported by experimental examination of the reactivity of the 4-(1-carboxylatoadamantyl)phenyl radical anion (7), where the molecular structure imposes a charge-radical separation of 9.1 Å. Increasing the charge-radical separation in this system was shown to retard the reaction efficiency and significantly alter the product distribution in favor of the phenylperoxyl radicals. The demonstrated influence of remote charges in modulating peroxyl-forming reactions may have broader implications [21, 22]. In a biological context for example, the presence of remotely charged moieties may afford switching of peroxyl radical reactivity in enzymatic or free radical-induced oxidation chemistries. While the effective range of influence of charge-radical polarization would be diminished in polar solvents, in low-polarity regimes such as hydrophobic pockets of proteins or membrane bilayers, the effect could be comparable to that observed in the gas phase.