Superoxide Radical Formed on the TiO₂ Surface Produced from Ti(OiPr)₄ Exposed to H₂O₂/KOH

Abstract

In this study, the superoxide radical O₂^•− formed by treating Ti(OR)₄ (R = iPr, nBu) with H₂O₂ in the presence of KOH was detected in the EPR spectra. The g-tensor of this radical differs from the typical values reported for a superoxide on various TiO₂ surfaces. On the other hand, similar g-tensor components g_||(zz = 2.10 ± 0.01, g_⊥ = 2.005 ± 0.003 assigned to the O₂^•− were previously observed for radicals in aqueous solutions in the presence of K₂O, alkaline solutions of DMSO, and water/DMSO mixtures. A common factor in all these systems is the presence of alkali ions. However, there was no structural support for the possible interaction of alkali ions with a superoxide in these systems. The use of multifrequency pulsed EPR techniques in this work revealed the stabilization of the O₂^•− near the K⁺ ion and its involvement in a strong hydrogen bond with the surface. These findings are consistent with the features previously reported for superoxides on a Na pre-covered MgO surface. Interactions with a closely located ²³Na and a strongly coupled ¹H proton were also seen in the HYSCORE spectra but assigned to two different superoxides with various g_zz values presented in the sample.

1. Introduction

Superoxide anion (O₂^•−) is a radical formed after the one-electron reduction of dioxygen O₂, in different chemical processes [1]. In reactions with organic compounds, it can behave as a base, a nucleophile, and an oxidizing or reducing agent [2,3]. O₂^•− is paramagnetic, which has allowed for broad applications of multifrequency, continuous wave (CW) EPR spectroscopy for its studies [4,5,6,7,8].

The treatment of an oxide with a solution of hydrogen peroxide (H₂O₂), followed by drying the obtained solid under a vacuum has been also employed for generations of the superoxide radicals [9]. A product with matrix-bound O₂^•−, produced by treating Ti(OR)₄ (R = iPr, nBu) with H₂O₂, was described and used as a selective heterogeneous catalyst for the oxidation of organic compounds [10]. It is effective at room temperatures and with various solvents including water.

Detailed studies of the catalyst using various experimental methods have shown that O₂^•− is responsible for the reaction, and its exceptional stability results from a stabilization near Ti⁴⁺ on the TiO₂ surface with a contribution of H₂O molecules and/or OH groups [10,11,12]. However, the nature and strength of O₂^•− interactions with the surrounding molecules was not characterized.

In our previous work, paramagnetic O₂^•− intermediates formed during the decomposition of H₂O₂ on the TiO₂ surface have been studied employing X- and Q-band CW and pulsed EPR spectroscopy. Exploiting high-resolution pulsed EPR techniques, i.e., 1D and 2D ESEEM (Electron Spin Echo Envelope Modulation) and ENDOR (Electron-Nuclear Double Resonance), weak interactions between the superoxide unpaired electron and the surrounding protons were quantitatively characterized. This enabled us to modify the model of the O₂^•− with its surrounding environment on the TiO₂ surface [13]. In this work, we found that the superoxide radical with different EPR spectroscopic characteristics is formed in reaction with Ti(OR)₄ (R = iPr, nBu) and H₂O₂ in the presence of a KOH solution. The application of pulsed EPR techniques has led to our finding that the stabilization of this O₂^•− is due to proximity to K⁺ ion and its involvement in a strong hydrogen-bonding interaction with the surface.

2. Materials and Methods

2.1. Preparation of the Catalysts

The TiO₂/O₂^•− catalyst studied in our previous work [13] was prepared from Ti(OiPr)₄ exposed to H₂O₂ following the method described in [10]. The dried powder obtained at the end of this procedure was transferred in quartz X- and Q-band EPR tubes, degassed, sealed, and used in the EPR experiments described in [13] (further called “sample I” in this article). It is known that radiolytically or photochemically generated superoxide reacts with tyrosine, forming phenoxyl radicals of tyrosine [14,15]. We aimed to test the appearance of this species in a similar reaction using tyrosine with superoxide on the surface without prior irradiation. The solubility of tyrosine in alcohol is highly pH-dependent [16]. Initially, we tried to initiate a reaction of tyrosine with superoxide by adding tyrosine to the TiO₂ dispersion powder from an alcohol solution or from its mixture with low concentration KOH. EPR spectra of these samples show a rhombic signal consistent with the spectrum of the O₂^•− radical found in sample I. On the contrary, a signal with an axial g-tensor and increased g_zz was observed in samples with a higher KOH concentration (pH > 10). Similar results were obtained upon joint addition of peroxide and tyrosine in an alkaline alcohol solution to Ti(OCH(CH₃)₂)₄. The presence of an axial EPR signal was also confirmed in the control experiment by adding KOH in methanol without tyrosine to the TiO₂/O₂^•− catalyst (sample II).

Earlier, in aqueous solutions and mixtures containing alkali metals (M), radicals with similar EPR characteristics attributed to O₂^•– were found, which suggests a special role of M⁺ ions in these samples for the radical stabilization. However, no structural EPR support was provided for this hypothesis. Therefore, this paper describes the pulsed EPR characterization of the superoxide radical and its environment in the type II samples containing K⁺ ions.

2.2. EPR Measurements

The CW EPR, two-dimensional, four-pulse hyperfine sublevel correlation (HYSCORE, π/2 − τ − π/2 − t1 − π − t2 − π/2 − τ − echo) [17], and Davies pulsed ENDOR (π − t − π/2 − τ − π − τ − echo) [18]) experiments were performed as previously described elsewhere [13]. The Bruker WIN-EPR software was used for spectral processing.

3. Results and Discussions

3.1. EPR Spectra of Dried TiO₂ + H₂O₂

X- and Q-band EPR spectra of a dried sample of TiO₂ (solution of Ti(OiPr)₄ treated with H₂O₂, sample I) were reported in [13]. They show a signal with rhombic g-tensor produced by the decomposition of H₂O₂ on the TiO₂ surface (Figure S1). The g-tensor principal components (2.024, 2.009, and 2.003) determined from the Q-band spectrum supports the formation of a stable superoxide radical O₂^•− [10], because they are in line with values usually reported for the superoxide on various TiO₂ surfaces (O₂^•−-Ti⁴⁺) (see Table S1).

3.2. EPR Spectra in the Presence of KOH

Figure 1 and Figure S2 show Q- and X-band EPR spectra of the radical formed in sample II. The spectra were obtained as a field-swept two-pulse Electron Spin Echo (ESE) signal and its calculated derivative. In contrast to sample I, the shapes of these spectra possess a typical axial g-tensor anisotropy with principal values g_||(zz) = 2.10, g_⊥(x,y) = 2.002, and a total width of ~20 mT in the X-band. We used the field-sweep ESE because a broad g_||(zz) feature was not clearly resolved in CW EPR spectra. The EPR signals with similar g-tensor components g_||(zz) = 2.10 ± 0.01 and g_⊥ =2.005 ± 0.003 assigned to the superoxide radical have previously been observed in water solutions with the presence of K₂O, alkaline DMSO solutions, and water/DMSO mixtures (

Table 1. Systems where the EPR signal with axial g-tensor was assigned to a superoxide.

Matrix	g||(gzz)	g⊥(gx,y)	Reference
H2O ice (K2O)	2.110		[20]
D2O ice (0.1 mM K2O)	2.110	2.002	[21]
KO2 in DMSO/H2O in the presence of ubiquinone-10	2.108	2.004	[22]
Alkaline DMSO	2.098	2.005	[23,24]
10 μL of 0.5M NaOH/mL of DMSO	2.089	2.007	[25]
TiO2 + H2O2+ KOH	2.10	2.002	This work

). One can note that the common factor in all these systems is the presence of alkaline ions. Earlier studies have found a good correlation of the superoxide g_zz value with the oxidation state of the nearest metal cation [5,19]. Particularly, a comparison of our data with the reported empirical dependence [5] indicates that g_zz = 2.0227 for the superoxide in sample I is consistent with its suggested location near Ti⁴⁺. In contrast, g_zz = 2.10 is within the region reported for M⁺ and may display the radical location near an alkaline cation in sample II as well as in the compounds shown in Table 1.

3.3. Pulsed EPR Characterization of the Radical

Interactions between the O₂^•− species and its environment in samples I and II were probed using HYSCORE and pulsed ENDOR techniques. Our previous HYSCORE studies of the superoxide in sample I have found only weakly coupled protons, with the anisotropic couplings T not exceeding ~2 MHz (Supplementary Materials, Section S1) [13]. These protons produce cross-features located along an antidiagonal crossing the diagonal of the (++) quadrant at the (ν_1H, ν_1H) point, where ν_1H is the proton Zeeman frequency in the applied magnetic field (Figure S3).

In contrast, the X-band HYSCORE spectrum of sample II (Figure 2) is dominated by a pair of cross-ridges 1_H that significantly deviated from the (ν_1H, ν_1H) antidiagonal. This indicates the presence of proton(s) with a substantially stronger anisotropic hyperfine interaction [22] than the protons contributing to the spectra of sample I (Figure S3). One can also note that the ¹H spectrum in Figure 2 clearly shows additional features 1′_H that also deviated from the ¹H antidiagonal but oriented in the opposite matter relative to this line. In the course of the analysis described below, we provide evidence that the 1_H and 1′_H lines are parts of the same cross-feature located on opposite sides of the diagonal of the (++) quadrant. Lines from weaker coupled protons, elongated along the antidiagonal (ν_1H, ν_1H), are also present in the spectra of sample II but possess a lower intensity.

The Q-band HYSCORE spectrum of sample II (Figure 2, right) shows an extended straight ridge around a diagonal frequency of 2.4 MHz with a total length of ~2 MHz. This line is produced by the interaction with ³⁹K (nuclear spin I = 3/2) possessing a Zeeman frequency of 2.409 MHz in the applied magnetic field 1212.5 mT. The natural abundance of ³⁹K is 93.26%. The second stable isotope ⁴¹K has the same nuclear spin and natural abundance of 6.73%. The Zeeman frequency of ⁴¹K in the specified field is 1.32 MHz. However, the spectrum in Figure 2 does not contain any features near this frequency on the diagonal line.

4. Discussions

4.1. Analysis of the ¹H HYSCORE Spectra

Quantitative analysis of ¹H cross-ridges from the HYSCORE spectra of the O₂^•− in samples I and II, based on linear regression of contour line shapes in ν₁² vs. ν₂² coordinates [26], gives isotropic and anisotropic components of hyperfine tensors in axial approximation for protons interacting with the electron spin of the superoxide. Detailed explanations and results of the analysis are provided in the Supplementary Materials, Section S1.

In particular, a representation of the cross-ridges 1_H from the spectra of sample I prepared with H₂O₂ and D₂O₂ in coordinates ν₁² vs. ν₂² gives anisotropic hyperfine coupling T = 2.0 ± 0.2 MHz for the contributing proton(s) (Figure S4 and Table S2) [13]. The estimated value of T ~ 2 MHz is supported by the negligible deviation of the cross-ridges from the antidiagonal in the experimental spectra. A similar analysis of the cross-ridges 1_H and 1′_H with the visible deviation from the antidiagonal (Figure S5) in sample II provides the value of anisotropic coupling of T ~ 7.0 MHz, which significantly exceeds the hyperfine coupling of T ~ 2 MHz for protons interacting with O₂^•− in sample I.

4.2. Evaluation of the Proton Anisotropic Hyperfine Couplings

The anisotropic couplings of T ~ 2 MHz and ~7 MHz for the superoxide-proton interactions in samples I and II indicate different relative locations of the O–O molecule and protons. The value of ~7 MHz is closer to previously reported ¹H hyperfine couplings of T ~ 10 MHz and 9.8 MHz for a species with similar g-tensor generated on from KO₂ reacting with water in a H₂O/DMSO mixture in the presence of ubiquinone-10 [22] and a Na pre-covered MgO surface [27], respectively. The anisotropic hyperfine tensor for the proton located near the O₂^•− is the result of a magnetic dipole–dipole interaction with an unpaired π spin density distributed approximately equally over two oxygens. The tensor depends on a proton position relative to the O–O bond and is generally rhombic [28]. When the interaction between the electron and the proton spins is described by the point dipole approximation the anisotropic parameter is defined by the expression T = $\frac{79}{r^3}$ (MHz) [13], where r is the distance between spins.

The dipole–dipole interaction is described by an axially symmetric tensor with diagonal principal values

T = [T_xx, T_yy, T_zz] = T [−1, −1, 2]

(1)

in the principal axes coordinate, with the z axis directed along the $\overrightarrow{\mathit{r}}$ direction.

A proton near the superoxide oxygens O₁ and O₂, carrying unpaired spin densities ρ₁ and ρ₂, experiences a local magnetic field, which is a vector sum of two contributions depending on the O₁–H (r₁) and the O₂–H (r₂) distances (Figure S9). The principal values of the rhombic hyperfine tensor in this case are [28]

T = [½(T₁ + T₂ − 3δ), − (T₁ + T₂), ½(T₁ + T₂ + 3δ)]

(2)

where δ = [T₁² + T₂² + 2T₁T₂cos(2α + 2β)]^1/2, T₁ = 79ρ₁/r₁³, T₂ = 79ρ₂/r₂³. Equation (2) transforms to the traditional axial form

T = [−(T₁ + T₂), −(T₁ + T₂), 2(T₁ + T₂)]

(3)

for the proton located on the O₁––O₂ line with β = 180°and δ = T₁ + T₂.

The relations between the sides and angles of the triangle HO₁O₂ (Figure S8)

$$r_2^2={[r_1^2+r_{O-O}^2-2r_1{r}_{O-O}\cos \beta ]}^{1/2} \mathrm{and} \alpha =\arcsin \left[\frac{r_1}{r_2}sin\beta \right]$$

(4)

allow us to define the tensor components based on one distance (r₁, O₁–H distance) and one angle (β, angle between H–O₁ and O₁–O₂).

It has been shown that the unpaired spin density is almost equally (ρ_1,2 ~ 0.5) distributed on the 2p_π^x of each oxygen in superoxide radicals studied in a solution [29], on a MgO surface [30], and generated in TiAlPO-5 [31]. The reported O–O distance in the superoxide varies between 1.32–1.35 Å [29,32,33,34] and increases under the influence of hydrogen bonds [29,32].

For a direct comparison with the HYSCORE determined values of T = 2 [13], 7 (this work) and 10 MHz [22,27], we calculated the term T₁ + T₂ from Equation (2), which lacks the rhombic term 3δ and is equal to T = 39.5 $\left[\frac{1}{r_1^3}\mathrm{+}\frac{1}{r_2^3}\right]$ for ρ₁ ≈ ρ₂ ≈ 0.5. This term is shown in the form of contours, where each point defines r₁ and β with the selected T (2, 7, or 10 MHz) (Figure 3), i.e., as a function of the O₁–H distance and the angle β between the H–O₁ and O₁–O₂ directions. Calculated graphs show that T = 2 MHz corresponds to the H–O₁ distance 2.97–3.4 Å for the angles β < 180°. On the contrary, this distance is ~1.0–1.2 Å less for T = 7 or 10 MHz (

Table 2. Distances of a proton location relative to a superoxide radical at different values of the anisotropic hyperfine coupling.

T, MHz	r1 (H–O1), Å	β, Degree	r1 at β = 180°, Å	r1 at β = 120°, Å	References a
2	2.97–3.4	180–75	2.97	3.08	[13]
7	1.89–2.24	180–73	1.89	1.95	This work
10	1.67–1.99	180–70	1.67	1.71	[22,27]

^a References, where parameters T from first column were experimentally determined. Adapted by permission from Copyright Clearance Center: Springer Nature, Samoilova et al. [13]. Copyright 2022.

).

Available models of a hydration shell for the aqueous O₂^•− include four water molecules, with two waters forming hydrogen bonds with each oxygen atom (Figure S9). The hydrogen bond lengths vary between 1.72–1.94 Å [30,35,36]. The lower limit of the H–O₁ distance 1.89 Å obtained for T = 7 MHz is still within the interval shown above.

4.3. Interaction with ³⁹K Nucleus

³⁹K possesses the nuclear spin I = 3/2 with a natural abundance of 93.26%. The orientation disordered HYSCORE spectra of the S = 1/2 and I = 3/2 system are influenced by hyperfine and nuclear quadrupole interactions. Both of them are anisotropic. Available data about the nuclear quadrupole coupling constant in ³⁹K⁺ state indicate that it is quite small [37,38]. Model simulations of the HYSCORE spectra from I = 3/2 nuclei with hyperfine and nuclear quadrupole tensors satisfying the conditions [ν_I > A_ZZ > Q_ZZ] have shown that the hyperfine interaction creates cross-ridges normal to the diagonal line in the (++) quadrant, whereas the nuclear quadrupole interaction produces an additional splitting of these cross-features in the direction parallel to the diagonal [39]. This simple manual is helpful for the qualitative analysis of the observed spectrum from ³⁹K (Figure 2).

The spectrum consists of a straight segment normal to the diagonal of the (++) quadrant and is located symmetrically relative to the (ν_39K, ν_39K) diagonal point. The length of the ridge is about ~2 MHz along each coordinate. A projection of the spectrum on each coordinate and stacked presentation of the spectrum shows a weakly resolved triplet structure of the ridge with the hyperfine splitting A ~ 1 MHz between cross-peaks with permutated coordinates of ~2 and 3 MHz. The spectrum does not show any additional splitting of the ridge along the diagonal, assuming the small value of the quadrupole coupling constant of ³⁹K [39].

The crystal structure of an α-potassium superoxide shows an octahedral environment of the superoxide ion near K⁺ ions with smallest contact distances of 2.71 Å in the direction parallel to the O–O bond and another 2.92 Å away, in the direction approximately normal to the O–O bond [40]. The calculated value of the principal component (T₁ + T₂) of the hyperfine tensor defined by Equation (2) for the two indicated locations of ³⁹K⁺ using formulae T = (3.7/2)[1/r₁³ + 1/r₂³] is equal to 0.12 and 0.148 MHz, respectively. The anisotropic width of the single-quantum transitions in the powder spectrum 3T/2 does not explain the line splitting ~1 MHz from ³⁹K, as detected in the HYSCORE spectrum. Consequently, there remains only one source resulting in the observed line shape—the isotropic hyperfine interaction. However, the simulation of spectra with parameters a ~ 1 MHz and T ~ 0.15 MHz did not reproduce the presence of a spectral intensity around the diagonal between two peaks with a ~1 MHz splitting. Additional ideas about hyperfine interactions between ³⁹K and the O₂^•− can be obtained by taking into account the available data on hyperfine couplings between a superoxide and ²³Na or ¹³³Cs on the MgO surface.

4.4. Comparison with Superoxide on a Na or Cs Pre-Covered MgO Surface

The EPR spectrum of superoxide species formed on a Na pre-covered MgO surface [27] shows a formation of two species with g_zz values equal to 2.091 and 2.14. The g_x,y components of both species are close to g = 2 and produce a single intensive line with a width of 1.5–2.0 mT in the spectrum. The species with g_zz = 2.091 have been assigned to superoxide ions on Mg²⁺ matrix sites. The value g_zz = 2.14 is within the range typical for superoxide anions stabilized on monovalent cations; thus, the corresponding EPR signal was designated as a surface O₂^•− − Na⁺ adduct.

The HYSCORE spectra collected in the g_x,y area of the Na/MgO sample contain cross-features from the ²³Na and ¹H nuclei (Figure S10). Two of them belong to ²³Na with strong and weak hyperfine couplings ~17 MHz and ~<3 MHz, respectively. The ¹H spectrum consists of two extended ridges with a clearly visible deviation from the antidiagonal crossing (ν_1H, ν_1H) point of the diagonal, indicating a strong anisotropic interaction between the superoxide and a proton. Computer simulations of the spectrum have provided hyperfine tensors a = −15 ± 2 MHz, T = (−0.1, 3.1, −3.0,) MHz (±0.5 MHz) for the strongly coupled ²³Na from the O₂^•−–Na⁺ adduct and a = −5.0 ± 0.5 MHz, T = (−9.8, 19.6, −9.8) MHz (±0.1 MHz) for the proton that produced extended cross-ridges. It was suggested that this proton belongs to the superoxide stabilized on the Mg²⁺ matrix site in the proximity of a surface OH⁻. Our analysis (Figure 3) shows that the ¹H coupling of T ~ 10 MHz indicates the formation of an H-bond between the superoxide and proton with an O–H distance of ~1.7–2.0 Å. Furthermore, the authors have proposed that the line around the diagonal (ν_Na, ν_Na) point is produced by remote ²³Na nuclei that are randomly distributed between 3.5 Å and 4.5 Å away (i.e., 0.5 >|T| > 0.2 MHz in the point dipole approximation), and the species of both g_zz contribute to this feature [27].

To compare the experimental and calculated spectra, this feature was calculated using a single hyperfine tensor a = −2.5 MHz, T = (−0.5, −0.5, 1.0) MHz (Figure S10). On the other hand, we estimated T_max = (20.9/2)[1/r₁³ + 1/r₂³] for two models of the ²³Na location relative to the superoxide (O–O length 1.33 Å) using a Na–O distance of 2.38–2.39 Å reported for the orthorhombic structure of sodium superoxide [41,42]. The corresponding values are T_max ~ 1 MHz and 1.5 MHz for the ²³Na locations on the line extending the O–O bond and normal to the middle of the O–O bond that is consistent with the value of T_max/2 used for calculating the spectrum. This estimate shows that the signal assigned to randomly distributed ²³Na nuclei at distances in the range between 3.5 Å and 4.5 Å can be produced just by one nucleus located at the Na–O distance found in the crystal structures. More convincing conclusions about the nature of the signal from weakly coupled Na could be supported by the relative intensities of two sodium signals which are not available from the published spectra.

The increased hyperfine parameters for strongly coupled ²³Na is explained by the formation of the ionic sodium superoxide Na⁺O₂⁻ as symmetrical triangular molecules with an interatomic distance of 1.96 Å that was deduced from vibrational spectra [43]. One can recalculate the characteristic hyperfine parameters of the ²³Na⁺O₂^•− species obtained in this work for a superoxide interacting with a nucleus of ³⁹K⁺. The ratio of ²³Na/³⁹K magnetic moments is 5.56. Then, the parameters |a| =15 MHz and |T|= 3 MHz found for the ²³NaO₂ species will give formal values of a = 2.7 MHz and T = 0.54 MHz for ³⁹K. The ratio of the atomic isotropic hyperfine constants 927.1 MHz (²³Na, 3s) and 228.5 MHz (³⁹K, 4s) [44] calculated for unit spin density is 4.06. It leads to a decreasing a value of 0.67 MHz. Another possible factor that may reduce the hyperfine parameters of ³⁹K compared to ²³Na is the larger value of its ionic radius (1.02 Å for ²³Na and 1.38 Å for ²⁹K). Simulations of the ³⁹K HYSCORE spectra with estimated parameters a = 0.6–0.7 MHz and T = 0.6 MHz confirm the increased total length of the cross-ridge and its line shape without well pronounced maxima (Figure S11). Thus, the recalculation analysis assuming similar structural motifs of the MO₂ species predicts significantly lower hyperfine couplings for ³⁹K. However, the value of the anisotropy parameter is greater than the calculated ~0.15 MHz using the crystallographic structure. That increase provides an extended line shape from ³⁹K, which is consistent with the experimental HYSCORE spectra.

DFT calculations of the hyperfine parameters for O₂^•− in Na/MgO were performed for different elements of the MgO surface [27]. Based on the analysis of the full set of g and A tensors, the NaO₂ species formed on an edge site of the MgO surface, where O₂ is simultaneously bound to Na and to MgO, give the best description of the large g_zz and the ²³Na hyperfine interaction observed in the EPR experiments. On the other hand, the DFT analysis of the ¹H tensor determined from the HYSCORE spectrum of O₂^•− in Na/MgO has not been carried out even for the proposed O₂^•−/HMgO center, although this tensor possesses an a_iso = −5 MHz and T_max = 19.6 MHz, which significantly exceeds the a_iso ~ 0 MHz and T_max ~ 10 MHz reported for O₂^•− at the surface of MgO [45].

One can note that the exchangeable proton with T_max ~ 20 MHz was found in the HYSCORE spectra of the superoxide in the KO₂/DMSO/H₂O mixture in the presence of ubiquinone-10 [24]. However, the X-band HYSCORE spectra obtained in this work were not suitable for the detection of ³⁹K signals due to a low Zeeman frequency in this band.

Other experimental examples relevant to this work are EPR studies of ²³NaO₂, ³⁹KO₂, ⁸⁷RbO₂, and ¹³³CsO₂ in rare gas matrices [46] and superoxides on a ¹³³Cs/MgO surface [47]. Similar to Table 1, the g_zz component of these species varies between 2.10–2.12, and the alkali metals ²³Na and ¹³³Cs produce a resolved hyperfine splitting of the g_zz and g_xx,yy components presented in

Table 3. EPR parameters of MO₂ species in a rare gas and on a MgO surface.

Sample	gzz	T11	T22	T33	aiso	A0	ρו10−3	Ref. a
23NaO2	2.111	−2.8	1.68	−1.12	9	927.1	9.7	[46]
O2•− on Na/MgO	2.14	−0.1	3.1	−3.0	−15	927.1	16.2	[27]
133CsO2	2.107	−0.56	0.84	−0.28	14	2464	5.7	[46]
O2•− on Cs/MgO	2.120	5.1	2.6	−7.7	29.6	2464	12.0	[47]

^a T_ii components of an anisotropic tensor, isotropic constant a_iso, and atomic isotropic hyperfine constant A₀ are in MHz.

.

A comparison with rare-gas matrix-trapped MO₂ molecules shows that the surface-stabilized complexes are characterized by larger a_iso and g_zz parameters. The differences between isolated MO₂ molecules and surface-adsorbed NaO₂/MgO species have been examined with the help of DFT calculations, which illustrated the role of the matrix in the stabilization of the superoxide with particular magnetic characteristics [27]. A set of surface sites was compatible with the observed experimental results, which are characterized by a mutual interaction between the superoxide anion and the Mg²⁺ matrix ions and adsorbed Na⁺ species. Particularly, in the case of the surface-stabilized NaO₂ complex, the unpaired electron is localized in a π* orbital lying in the O₂–Na plane, whereas the π* orbital hosting the unpaired electron is found to be perpendicular to the M(OO) plane for matrix-trapped NaO₂ in the rare gas. This difference will influence the hyperfine interaction with the alkaline atom nucleus [46].

5. Conclusions

Our experiments with the TO₂…O₂^•− catalyst obtained with the joint addition of peroxide and a KOH solution of alcohol show the formation of the superoxide radical with the atypical g_zz = 2.10. A similar EPR signal was previously observed in various systems containing alkaline ions that allowed us to suggest the special role of K⁺ ions in the O₂^•− stabilization. However, ³⁹K nucleus(i) did not produce any resolved features in the reported EPR spectra. In this work, we applied 2D HYSCORE spectroscopy to characterize hyperfine interactions between the O₂^•− and the ³⁹K and ¹H nuclei in its environment. Q-band HYSCORE spectra have shown the presence of ³⁹K near the superoxide. A comparison of the magnetic characteristics and electronic configuration defining the isotropic coupling of ³⁹K with ²³Na in Na/MgO [27] allowed us to predict significantly smaller ³⁹K hyperfine couplings for the similar structural MO₂ motifs. The estimated values give a reasonable agreement between the calculated and experimental ³⁹K HYSCORE spectra. Another finding of this superoxide is the existence of a strongly coupled ¹H with the anisotropic coupling T ~ 7 MHz, which suggests the formation of an H-bond with an O–H distance of <2Å. So far, an interaction with the closely located alkaline ion and the strongly coupled proton T ~ 10 MHz has been reported for the O₂^•− on Na/MgO only [27]. However, two different O₂^•− species with g_zz = 2.014 and 2.091 were present in this sample, and strongly coupled ²³Na with a = 15 MHz and the ¹H with anisotropic coupling T = 9.8 MHz were assigned to different species, − O₂^•−/NaMgO and O₂^•−/HMgO, respectively. Only one type of O₂^•−/KTiO₂ species with g_zz = 2.10 was found in our work. This means that signals from ³⁹K and a strongly coupled proton with T ~ 7 MHz in the HYSCORE spectra are produced by interactions with this superoxide and should be considered as the elements of its structure. Therefore, independent data to support the hypothesis that the presence of a stable superoxide radical in systems containing alkali metals requires the simultaneous proximity of M⁺ ion(s) and the formation of a hydrogen bond with its environment are still needed.