Generation of Naphthoquinone Radical Anions by Electrospray

ARTICLE pubs.acs.org/JPCA

Generation of Naphthoquinone Radical Anions by Electrospray Ionization: Solution, Gas-Phase, and Computational Chemistry Studies Ricardo Vessecchi,*,†,‡ Zeki Naal,‡ Jos
e N. C. Lopes,‡ S
ergio E. Galembeck,† and Norberto P. Lopes*,‡ †

Departamento de Química, Faculdade de Filosofia Ci^encias e Letras and ‡Departamento de Física e Química, Faculdade de Ci^encias Farmac^euticas, Universidade de S~ao Paulo, Ribeir~ao Preto, SP 14040-901, Brasil

bS Supporting Information ABSTRACT: Radical anions are present in several chemical processes, and understanding the reactivity of these species may be described by their thermodynamic properties. Over the last years, the formation of radical ions in the gas phase has been an important issue concerning electrospray ionization mass spectrometry studies. In this work, we report on the generation of radical anions of quinonoid compounds (Q) by electrospray ionization mass spectrometry. The balance between radical anion formation and the deprotonated molecule is also analyzed by influence of the experimental parameters (gas-phase acidity, electron affinity, and reduction potential) and solvent system employed. The gas-phase parameters for formation of radical species and deprotonated species were achieved on the basis of computational thermochemistry. The solution effects on the formation of radical anion (Q•
) and dianion (Q2
) were evaluated on the basis of cyclic voltammetry analysis and the reduction potentials compared with calculated electron affinities. The occurrence of unexpected ions [Q þ 15]
was described as being a reaction between the solvent system and the radical anion, Q•
. The gas-phase chemistry of the electrosprayed radical anions was obtained by collisional-induced dissociation and compared to the relative energy calculations. These results are important for understanding the formation and reactivity of radical anions and to establish their correlation with the reducing properties by electrospray ionization analyses.

’ INTRODUCTION Radical anions are intermediates of several bioorganic and chemical processes.1 These species can be formed by transfer of a single electron to a neutral molecule2 or by homolytic cleavage of a bond from an anion (through radical elimination).3 It has been accepted that the reactivity of radical anions, in the fragmentation and rearrangement processes, is similar to that of a neutral radical. On the other hand, some researchers have shown that both charge and spin may be determining factors for the reactivity of radical anions.4,5 Some studies involving anions in which bond cleavage produces distonic anions have been reported.3,6,7 However, investigations on the fragmentation of a spatially separated radical anion, which produces a localized radical anion and a neutral molecule, are rather scarce.6,8 The recently published work of Magri and Workentin has demonstrated the formation and fragmentation of the distonic radical of endoperoxide by electron transfer in solution and formation of a localized radical anion.7 However, the formation and characterization of radical anions in condensed phase have been extensively studied4
8 and yet the gas-phase radical anion is still little investigated, which indicates an important field of anion radical research. Gaseous radicals have been formed mostly by homolytic cleavage of anions during the collision-induced dissociation (CID).9 More recently, their r 2011 American Chemical Society

formation in electrospray ionization have been demonstrated.10,11 Over the last years, the formation of radical ions in the gas phase has been an important issue concerning electrospray ionization mass spectrometry (ESI-MS) studies.10
13 The ESI ion source is inherently a controlled-current electrolytic flow source.11 Electrolytic reactions in the ESI emitter capillary are continually ongoing, to sustain the production of charged droplets and ultimately of gas-phase ions from this device.10,11,14,15 Radical cation formation during ESI occurs preferentially in the case of organic molecules with low oxidation potential and extensive π systems.10,16 Van Berkel and coworkers11
13 have demonstrated that the relation between oxidation potential and radical cation formation (M•þ) is better established for policyclic aromatic hydrocarbons and metalloporphyrins, but other researchers have more recently shown that these cation radicals also occur for polyenes and carotenoids.15
17 Values of ionization potentials lower than 1.0 V versus ECS and ionization energy lower than 10 eV have been assumed as indicators of the occurrence of radical cation formation during electrospray ionization.11 Received: March 11, 2011 Revised: April 27, 2011 Published: May 11, 2011 5453

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The Journal of Physical Chemistry A Chart 1. 1,4-naphthoquinone Derivatives: Q1, 2-acetylamino-1,4-naphthoquinone; Q2, 2-propyonilamino-1,4naphthoquinone; Q3, 2-butyrilamino-1,4-naphthoquinone; Q4, 2-benzoylamino-1,4-naphthoquinone

Reed and co-workers have shown the formation of radical anions in the gas phase, where radical formation during the CID process occurs after anion formation by electrospray ionization.18 Nevertheless, because ESI begins in the solution phase, the identity of the íons in the mass spectra can be interpreted by gas-phase parameters. In a recent paper, Aschi et al.19 have described that the formation of species during electrospray ionization takes place during desolvation, and it is wrong to compare any insight from mass spectra with solution-phase conditions. On the other hand, Van Berkel et al. have demonstrated that the radical cations in electrospray are better described on the basis of solution parameters in the case of electrospray.12,13 Although the formation of anion radicals has been demonstrated in the literature, only fullerenes have been extensively studied in the ESI source, where their reduction potential and radical anion formation have been compared.20 Dupont and coworkers have shown that the formation of an anion radical for 1,4-benzoquinone and fullerenes (C60
C70) depend on the redox potential, whose values are
0.57 and
0.60 vs ECS, respectively.20 Thus, the knowledge about electrochemical parameters and anion radical formation has led to selection of a compound class that behaves well from an electrochemical viewpoint and allows for radical formation in the condensed phase. The electrochemistry of naphthoquinones depends on the media; indeed, several intermediate species can occur at different pH or under anaerobic conditions.21,22 In aprotic medium, the formation of anion and dianion radicals by reduction is normally evidenced by two monoelectronic reversible waves.23
25 For this reason, 1,4-naphthoquinones have become an important class of compounds, since their biological activities (e.g., anticancer, moluscicidal, trypanocidal action) can be related to their the reduction potential.21,26 In this work, the 1,4-naphthoquinone derivatives (Chart 1) have been synthesized from 1-naphthol27,28 and the biological activities against Biomphalaria glabrata and Aedes Aegypti larvae have been demonstrated.28 Recently, these compounds have emerged as anticancer agents, which has stimulated our research group to use these compounds as prototypes for new derivatives.29 Herein we report the formation of radical anions of 1,4naphthoquinone derivatives in the condensed and gas phases. The influence of solvent system on the formation of these radicals is also investigated in both phases. Furthermore, the gas-phase unimolecular chemistry of radical anions is described for the first time. Thermochemical parameters for radical anions

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have been evaluated by theoretical calculations. Also, the Mulliken spin density values were obtained, in order to understand the charge and spin sites.

’ EXPERIMENTAL SECTION Stock solutions containing the 1,4-naphthoquinone derivatives (Chart 1) were prepared at a concentration of 0.2 mg 3 mL
1 in acetonitrile and acetonitrile/water (1:1). Mass spectrometry analyses were performed on a triple quadrupole Quattro-LC (Micromass). The solutions were infused into the spectrometer through a syringe pump, at 10 μL 3 min
1. The intensities relative of to both species [Q
H]
and [Q•
] generated by ESI mass spectrometry were compared with the varying source conditions, in order to find a better voltage. The voltage in the capillary was 2.7 kV. Initially, the influence of the applied voltage (10, 20, 30, 40, 50, and 60 eV) was tested on the potential cone and on the solvent system. High-resolution ESI-MS analyses were conducted on an Ultro-TOF-Q Bruker Daltonics device, fitted with an electrospray ion source operating in the negative ion mode. Samples were diluted more than 10 times and were then directly infused into the ionization source at a 10 μL 3 min
1 flow rate. Accurate masses were obtained by using TFA-Naþ (sodiated trifluoroacetic acid) as internal standard. The temperature of the source block and desolvation was 150 °C. The optimum energy applied at the capillary emitter was tested, so that a higher intensity of the radical anion [Q•
] would be obtained. The radical anion (precursor ion) was selected and fragmented by CID, using N2 as the collision gas. The variation at Elab was performed in order to obtain the energy-resolved curves. The optimum energy (Elab) to obtain the main fragment (m/z 200) was determined to be 20
25 eV. Electrochemical Procedure. Cyclic voltammetry measurements were carried out with a BAS CV-27 potentiostat. Data were recorded on an Ominographic XY recorder. A conventional electrochemical cell with three electrodes was employed. A glassy carbon disk (area = 0.031 cm2) electrode was used as the working electrode. A sodium chloride saturated Ag/AgCl and a Pt wire were used as reference and counter electrodes, respectively. Prior to all measurements, the working electrode was washed with H2SO4, repeatedly rinsed with distilled water, and polished with alumina. The 1,4-naphthoquinone derivatives (Chart 1) were solubilized in supporting electrolyte (TBAPF6, 0.1 mol 3 L
1 in acetonitrile) to a concentration of 2 mg 3 mL
1. The cyclic voltammograms were obtained in free-oxygen atmosphere, at scan rates of 20, 40, 60, 80, 100, and 150 mV 3 s
1. Prior to the electrochemical measurements, all the stock solutions had been bubbled with Ar for approximately 30 min. The inert atmosphere was used in this study because dissolved oxygen not only can be reduced at potentials close to the quinone reduction potentials but also is capable of oxidizing the radical anion generated in the first reduction step, as in the case of other quinonoid compounds. For the addition of additives (protic solvent), a stock solution methanol was added using 100 μL aliquots, by means of a micropipet. All the electrochemical experiments were accomplished under the same conditions, at 25 °C, in order to obtain consistent data sets. ’ THEORETICAL CALCULATIONS In order to know the best theoretical model to obtain the geometries and energetic parameters of quinonoid compounds 5454

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Figure 1. Cyclic voltammograms of 2-butyrilamino-1,4-naphthoquinone (Q3) (2.0 mg 3 mL
1 in acetonitrile/TBAPF6 0.1 mol 3 L
1). Potential (V) versus Ag/AgClsat. (A, left) Variation in the scan rate. The arrow indicates the scan direction. (B, right) Addition of of MeOH aliquots. Scan rate in mV 3 s
1.

and their radicalar anions, several theoretical models (DFTs and ab initio) were tested, and the results were compared to the X-ray structures of 1,4-benzoquinone and its radical anion, as well as to their ionization energy and electron affinity. The best results were obtained with the B3LYP/6-31þG(d,p) model,30 see Figure S1 and Tables S1 and S2 in the Supporting Information. All geometries were optimized using the Gaussian 03 suite of programs,31 by calculation of harmonic vibrational frequencies employing the B3LYP/6-31þG(d,p) model.30 The electron affinity was computed as the difference in the Gibbs energy between the radical anion and the neutral molecule in the gas phase. Acidity was computed as the Gibbs energy of the deprotonation reaction (MH f M
þ Hþ), where the proton Gibbs energy is
6.28 kcal 3 mol
1, as obtained by the Sackur Tetrode equation.32 Thus, the most stable radical anions were proposed, as well as the fragmentation pathways suggested by Gibbs energies calculations at 298.15 K. For each potential energy profile, the relative Gibbs energies at 298 K, ΔG298, and the relative enthalpies at 298 K, ΔH298, are reported. It is noteworthy that the Gibbs energy values should be interpreted with caution since the thermodynamic equilibrium might not have been reached under CID conditions.32 Mulliken spin densities33 were obtained, in order to understand the charge and spin localization on the radical species. Single-point calculations with more flexible basis set (B3LYP/ aug-cc-pVTZ//B3LYP/6-31þG(d,p) and B3LYP/aug-cc-pVTZ// B3LYP/6-311þþG(3d,2p))30 was made by Turbomole 6.2 software using standard options,31b and the stability of computed relative energies was observed (Table S2_1, Supporting Information).

’ RESULTS AND DISCUSSION This work is organized as follows. First, a discussion of the voltammetric studies in aprotic and protic solvent systems is shown. Then the electrospray ionization mass spectrometry analysis of the radical species is presented, and the theoretical calculations of gas-phase basicities and electron affinities are followed by a description of the plausible formation of this species during the ionization process by analysis of the solvent system, potential cone energy, and emitter voltage. The fragmentation channel and the product ion structures are suggested

Table 1. Reduction Potential As Measured by Cyclic Voltammetry in the Absence of Oxygen (scan rate 60 mV 3 s
1), Gas-Phase Acidity (ΔGacid), in kcal 3 .mol
1, and Electron Affinities (EA), in eV, As Calculated by the B3LYP/6-31þ G(d,p) Model first wave (Q/Q•
) molecule ΔGacid3 EA

a

EIreda

EIredb

second wave (Q•
/Q2-) EIIreda

EIIredb

Q1

328.13 2.14


0.61


0.60


0.95


0.87

Q2 Q3

329.41 2.11 329.20 2.14


0.69
0.62


0.64
0.60


1.04
0.99


0.92
0.80

Q4

324.50 2.22


0.62


0.59


1.07


0.83

Aprotic medium. b Protic medium.

on the basis of the experimental approach and the theoretical calculations of fragmentation mechanisms. The section closes by studying the influence of solvent system on the formation and reaction of radical species during desolvation processes. Electrochemical Studies. A significant amount of research has been directed to the influence of the acidity level of the medium on the electrochemical reduction mechanisms of quinonoid systems.22
25 It has been demonstrated that in aprotic or in low proton availability solvents, a wide spectrum of mechanistic pathways may occur, depending on the stability of the electrogenerated species and on the acidity level of the solution.22
25 All the compounds studied here exhibit similar cyclic voltammogram profiles, with two waves (see Supporting Information). Figure 1A shows a series of consecutive cyclic voltammograms for an electrode in contact with a 2.0 mg 3 mL
1 solution of 2-butyrilamino-1,4-naphthoquinone in acetonitrile (ACN) containing 0.1 M tetrabuthylammonium hexafluorophosphate (TBAH). There are two reversible peaks with a formal potential of
0.59 V (Q•
/Q) and
1.07 V (Q2
/Q•
), ascribed to a quinone-localized reduction process. The ΔEp values for the peaks are 65 mV in a sweep rate range, and the peak currents are directly proportional to the square root of the sweep rate for values up to 150 mV 3 s
1, suggesting a reversible diffusional controlled processes, where EI and EII represent the reduction potential for the Q/Q•
and Q•
/Q2
couples, respectively 5455

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The Journal of Physical Chemistry A (Table 1). The small differences in the EIred values of the studied naphthoquinones can be interpreted on the basis of the lateral chain effects, because all substituent groups contribute with the same resonance effects in the quinonoid moiety, as suggested by Macias-Ruvalcaba et al.23 Addition of methanol aliquots results in a positive shift in both reduction waves Q/Q•
and Q•
/Q2
. Representative examples of the voltammetric behavior after MeOH addition to the quinone solution can be observed in Figure 1B. It is noteworthy that this positive shift is more pronounced in the case of the second wave. Some researchers have demonstrated that this behavior could be ascribed to the formation of a hydrogen bond between methanol and the generated radical anion. Gupta and Linschitz24 have shown that the redox process of quinones depends on the medium and that hydrogen bond formation and protonation can be distinguished by the voltammetric behavior of the analyte. Macias-Ruvalcaba et al. have described how it is possible to control protonation reactions and hydrogen bond formation with R-NH-quinones by controlling the concentration and acidity of the additive.23 These results will be used in order to understand the occurrence of electrosprayed radicalar anions. The values of reduction potential (EIred) for 1,4-naphthoquinone derivatives are very close those obtained for 1,4-benzoquinone.20 Thus, the radical anion generation process should occur in ESI-MS analysis, and the radical formation could be estimated through employing the electrochemical studies and computational thermochemistry. Computational Thermochemistry and Structural Analysis of Radical Anions of the 1,4-Naphthoquinone Derivatives. Recent papers have indicated the importance of the basicity (gasphase or solution) of the analyte in electrospray selectivity.34
36 For cation radicals, the redox potential and ionization energy are the most recommended parameters for estimation of analyte selectivity.10 For this reason, the gas-phase acidity was available for compounds, and the lowest values indicate the most acid compound. Electron affinity (EA) corresponds to the energy released during the electron attachment and, consequently, the stability of the anion radical. The EA can be compared to the EIred obtained during the voltammetric analysis (Table 1). In solution, radical anion formation occurs more easily for Q4, in protic medium, because this compound has the most positive EIred value (
0.59 V vs Ag/AgClsat), which indicates that this compound is the stronger oxidizing agent. It is worth mentioning that for compounds Q1, Q2, and Q3, the values of EA are very close, as in the case of the EIred values (see Table 1). These results suggest that the lateral chain contributes to the EA and EIred values in a similar way. The correlation between the EA and Ered values for quinonoid compounds has been recently reported, and the values obtained in the present study are in agreement with those reported in the literature.37 Regarding the acidity values, compounds Q1, Q2, and Q3 exhibit close values. Q4 is the most acid compound with ΔGacid = 324.50 kcal 3 mol
1 (Table 1). The deprotonation site is the N
H bond, as evidenced in recent ESI-MS/MS studies for these compounds.3 In this work, several models have been tested, in order to obtain the best results for the geometries of the anion radical of quinonoid compounds. We came to the conclusion that the B3LYP/6-31þG(d,p) model is the most accurate (see Supporting Information, Tables S1 and S2). Considering the reduced species, changes in the geometry occur mainly for bond length in

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Figure 2. Mulliken spin density for 2-acetylamino-1,4-naphthoquinone anion, Q2•
.

the quinonoid ring, compared to the bond lengths of neutral compounds. The carbonyl bond lengths increase, and the obtained values are in agreement with those described by L€u et al. during an experimental analysis.38 In addition, Wise and coworkers have compared experimental and computational results for the equilibrium geometries of trimethyl-benzoquinone and plastoquinone by means of B3LYP/6-31G(d,p).39 Their results are in agreement with experimental data, which indicate that the geometries of the radical anions do not sustain modification from substituent effects. As for the radical anions, the Mulliken spin densities were evaluated using the B3LYP/6-31þG(d,p) model, in order to understand the charge and spin localization on the radical anion (Table S4, Supporting Information). The values for 2-acylamino1,4-naphthoquinones were obtained and compared with those achieved for 1,4-naphthoquinone. For the Q1, Q2, Q3, and Q4 derivatives (Chart 1) the spin density is most concentrated at oxygen atoms, but the C(3) atom shows some concentration of spin density (Figure 2), suggesting that the distonic form of radical anions can occur40 (values of spin density are available in Supporting Information, Table S4). ESI-MS and ESI-MS/MS Studies. Mass spectrometry analysis was performed, in order to verify the occurrence of radical anion (Q•
) during electrospray ionization and to find out how solvent systems could affect radical formation. Analysis in acetonitrile (aprotic medium) (Figure 3) showed that the formation of a radical anion occurs in higher proportion compared with acetonitrile/water solvent mixtures (Figures S5
S16, Supporting Information). The intensity of these ions (radical anions and deprotonated molecules) depends on the cone potential energy variation (see Supporting Information). The dianion (Q2
), was not observed in the MS spectrum of any of the molecules. This species occurs specifically in solution, as evidenced by the voltammetric studies. However, the reduction potential values are very close. The fact that doubly charged species do not occur can be explained by the influence of the solvent system on the stability of the anions, a fact that does not occur in the gas phase. All the compounds display the peak [Q þ 15]
in the ESI-MS spectrum, characterized as being triggered by the reaction between the solvent molecule and the radical anion. It is important to mention that the [Q þ 15]
does not appear in the case of analysis in tetrahydrofuran (THF) (Figure S9, Supporting Information). The artifact (unexpected) ions in the electrospray ionization in negative mode have been the theme of 5456

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Table 2. Ions Observed (m/z), in Mass Units (u), during the ESI-MS/MS Analysis of 1,4-Naphthoquinone Derivatives at Elaba ion

Q1

Q2

Q3

Q

215.0545 (52.17)

229.0688 (37.4)

243.0872 (73.0)

A

200.0289 (100)

200.0278 (100)

200.0366 (100)

B

174.0537 (44.8)

174.0522 (52.0)

174.0522 (22.0)

C

173.0450 (33.9)

173.0450 (61.7)

173.0450 (28.0)

D E

160.1133 (52.2) 124.6169 (20.0)

160.1133 (36.5) 124.6169 (13.9)

160.1153 (67.8) 124.6188 (26.8)

•


F

93.5631 (20.0)

93.5631 (13.9)

93.5645 (26.8)

G

66.9518 (14.8)

66.9518 (8.70)

66.9524 (17.0)

a

Values between parentheses are relative abundance at Elab = 20 eV (N2). All spectra are available in the Supporting Information.

Figure 3. ESI-MS spectra for Q2 compound in acetonitrile: (A) highresolution analysis on Q-TOF, m/z 229.0740 (accuracy mass 1.4 ppm); (B) expanded spectrum.

Scheme 1. Proposed Mechanism for Reaction between Anion Radical and Acetonitrile

a recently published communication, which reported the occurrence of the [M þ 12
H]
ion during analysis.41 The authors suggested that these artifact ions are solvent-system dependent, as evidenced by our analysis in THF. Nevertheless, radical anions may react with alkyl halides, such as CH3X, and the X
elimination occurs. Actually, the generated radical attacks the solvent molecule by the mechanism demonstrated in Scheme 1.

This reaction does not take place in solution (during the voltammetric analysis), which leads us to suggest that probably occurs during the desolvation process. The approach of acetonitrile to the radical anion may produce two covalent adducts (ipso or oxo), and the relative enthalpies for formation of the [Q þ 15]
species indicate that the reaction should take place at the oxo position, as suggested at Scheme 1 and Figure S4 (see Supporting Information). In the light of DFT calculations, these reactions are exothermic, which can suggest that they occur for all the studied compounds (see Supporting Information, Table S5). Recently, Bouchoux and collaborators have shown that the reaction with isocyanide and the phenoxy cation may produce several covalent adducts by a similar mechanism in the gas phase.42 In order to confirm the formation of the radical species, CID experiments were conducted in QqQ and Q-TOF devices. All the ESI-MS/MS spectra display the fragment ions with m/z 200 and m/z 173 as the most intense signals (Table 2 and Figures S14, S15, and S16 in the Supporting Information). Thus, in the fragmentation mechanism only these ions were suggested. Comparison of the MS/MS results for the deprotonated molecule and the radical anion species shows that there are differences between the fragmentation mechanisms.3 A previous work developed by our research group with the deprotonated molecule demonstrated that the m/z 199 ion is the most intense fragment in the MS/MS spectra. This ion occurs by radicalar elimination of the lateral chain, in contrast with the even-electron rule.3 On the other hand, for the odd-electron rule is followed in the case of anion radical, so the radical elimination of the lateral chain furnishes a closed-shell species, Scheme 2 and Figures S15
S18 (Supporting Information). Three different mechanisms have been proposed in order to account for the occurrence of m/z 200 ions (Scheme 2). Two mechanisms involve the reduction at C(1)
O(1), Scheme 2 (1) and (2), where the unpaired electron is localized at the O(1) atom. The other mechanism consists of an R-cleavage of the lateral chain, see Scheme 2 (3). On the basis of the possible structures that the radical anions can assume, several fragmentation mechanisms have been proposed, as follows. Pathway (1) (Scheme 2) begins with abstraction of a hydrogen atom from the N
H bond, which is the most acid hydrogen atom in the molecule (as evidenced a recent study), which culminates in sequential •R loss and double bond formation at N
C. The calculated Gibbs energy for this reaction is 78.63 kcal 3 mol
1 (3.40 eV) for Q1, but its value decreases for Q2 and Q3. 5457

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The Journal of Physical Chemistry A Scheme 2. Fragmentation Mechanism for Formation of m/z 200 (A) from Q•
a

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Scheme 3. Fragmentation Mechanisms for the Anion Radical of 2-Acylamino-1,4-naphthoquinone Derivativesa

a

Values between parentheses are relative Gibbs energies at 298 K. Italic numbers are enthapy values at 298 K.

a

Values are the relative Gibbs energies for reactions at 298 K. Values between parentheses are the relative enthalpies at 298 K. All values are given in kcal 3 mol
1. Geometries, vibrational frequencies, and absolute energies are available in the Supporting Information.

Pathway (2) is triggered by the five-membered ring, from which •R leaves through homolytic cleavage of the C(O)
R bond, thus forming a planar ion, Scheme 2. This step is an endothermic process with ΔH298 = 46.28 kcal 3 mol
1, but the values decrease for Q2 and Q3. The Gibbs energy for the fragmentation step is 35.85 kcal 3 mol
1 (1.55 eV), which is approximately 8% of Elab. Kanawati and co-workers have shown in their studies that up to 26% Elab can be transferred during a collision,43 but if we consider only simple collisions, the maximum transferred energy would be 2.07 eV. Formation of m/z 173 ions is suggested to occur by one of the following two ways: C(1)
O(1) reduction or C(4)
O(2) reduction, Scheme 3. Ketene loss would occur through formation of a sevenmembered ring between the R-carbonyl hyrogen and O(1) forming a hydroxyl group via hydrogen atom transfer. This mechanism is the same that described by our research group for formation of the m/z 174 ion from protonated 2-acylamino1,4-naphthoquinone during electrospray analysis.44 By considering the reduction at C(4)
O(2), the formation of m/z 173 could occur; however, this step is more energetic than the one discussed above. The detailed analysis of the percentage of ions in the MS/MS spectra and the variation in Elab enable one to conclude that the formations of m/z 200 and m/z 173 occur by two competitive

channels. At applied Elab, the m/z 200 occurs in higher proportion than m/z 173. This is easily understood if one analyzes ΔG298 and ΔH298 for each reaction. The formation of m/z 200 requires lower energy than the formation of m/z 173 (Schemes 2 and 3), which allowed us to confirm that the gas phase is involved in the fragmentation reaction.

’ CONCLUSIONS Our studies enable identification of a radical and a dianion of 2-acylamino-1,4-naphthoquinone derivatives in solution during cyclic voltammetry analyses. All the studied compounds exhibit two voltammetric waves, indicating a quinonoid moiety. Electrochemical analysis allowed for characterization of the reduction process as being a well-behaved redox system. The ESI-MS studies were conducted in order to understand the influence of the solvent system on radical anion formation from 1,4-naphthoquinone in the gas phase. The intensity of the radical anion and deprotonated molecule depends on the employed solvent system, evidenced by ESI-MS analysis. The occurrence of the unexpected ions [Q þ 15]•
can be attributed to reaction between the anion radical and the solvent molecule, and these results are in agreement with those obtained by thermochemical calculations. The MS/MS analysis for the radical anion demonstrated the presence of m/z 200 ion and the m/z 173 base peaks, which were suggested to be due to radical elimination of the lateral chain by the odd-electron rule. The energies calculated for the formation of these ions indicate that the radical elimination occurs via formation of a six-membered ring. The density spin analysis allowed us to propose that the anion radical can assume several resonance forms. The spin and charge sites are concentrated on 5458

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The Journal of Physical Chemistry A the quinonoid carbonyl. Finally, on the basis of our results, the formation of radical anions for quinonoid compounds can be exploited by of electrospray ionization analyses and these results should be interesting for studies on natural and synthetic naphthoquinones.

’ ASSOCIATED CONTENT

bS

Supporting Information. Critical tests (DFT and ab initio level) for geometries and thermochemical parameters for radical anion formation (Figure S1 and Tables S1 and S2), geometries and Mulliken spin density for each molecule can be consulted at Tables S3 and S4, Figure S4 and Table S5 show the energetics for reaction between acetonitrile and radical anion, all ESI-MS spectra for Q1, Q2, Q3, and Q4 are available (Figures S5
S13), Figures S14
S16 display the ESI-MS/MS for radical anion, all cyclic voltammograms are shown at Figures S17
S24, and all geometries, energies, and ZPE values for neutral and anion species at the B3LYP/6-31þG(d,p) level. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT The authors acknowledge the Brazilian foundations FAPESP, CAPES, and CNPq for financial support. Zeki Naal acknowledges the Instituto Nacional de Ci^encia e Tecnologia de Bioanalitica. Ricardo Vessecchi thanks FAPESP for scholarships (Grants 05/01572-1 and 09/08281-3). The authors thank Jos
e Carlos Tomaz for the electrospray ionization analysis and Ali Faez Taha for technical support in Linux OS. ’ REFERENCES (1) Todres, Z. V. Organic Ion Radical: Chemistry and Applications; Marcel Dekker: New York, 2003. (2) Stevenson, J. P.; Jackson, W. F.; Tanko, J. M. J. Am. Chem. Soc. 2002, 124, 4271–4281. (3) Vessecchi, R.; Carollo, C. A.; Lopes, J. N. C.; Crotti, A. E. M.; Lopes, N. P.; Galembeck, S. E. J. Mass Spectrom. 2009, 44, 1224–1233. (4) Tanko, J. M.; Phillips, J. P. J. Am. Chem. Soc. 1999, 121, 6078–6079. (5) Chahma, M.; Li, X. Z.; Phillips, J. P.; Schwartz, P.; Brammer, L. E.; Wang, Y. H.; Tanko, J. M. J. Phys. Chem. A 2005, 109, 3372–3382. (6) Tanko, J. M.; Li, X. Z.; Chahma, M.; Jackson, W. F.; Spencer, J. N. J. Am. Chem. Soc. 2007, 129, 4181–4192. (7) Magri, D. C.; Workentin, M. S. Chem.—Eur. J. 2008, 14, 1698–1709. Magri, D. C.; Workentin, M. S. Org. Biomol. Chem. 2008, 6, 3354–3361. (8) Donkers, R. L.; Workentin, M. S. Chem.—Eur. J. 2001, 7, 4012–4020. (9) Sablier, M.; Fujii, T. Chem. Rev. 2002, 102, 2855–2924. (10) Van Berkel, G. J.; Kertesz, V. Anal. Chem. 2007, 79, 5510–5520. (11) Vessecchi, R.; Crotti, A. E. M.; Guaratini, T.; Colepicolo, P.; Galembeck, S. E.; Lopes, N. P. Mini-Rev. Org. Chem. 2007, 4, 75–87. (12) (a) Van Berkel, G. J. The Electrolytic Nature of Electrospray. In Electrospray Ionization Mass Spectrometry; Cole, R. B., Ed.; Wiley: New York, 1997; pp 65
105. (b) Vanberkel, G. J.; Zhou, F. M. Anal. Chem. 1995, 67, 2916–2923. (c) Vanberkel, G. J.; Zhou, F. M. Anal. Chem. 1995, 67, 3958–3964. (d) Kertesz, V.; Van Berkel, G. J. J. Am. Soc. Mass

ARTICLE

Spectrom. 2006, 17, 953–961. (e) Peintler-Krivan, E.; Van Berkel, G. J.; Kertesz, V. Rapid Commun. Mass Spectrom. 2010, 24, 1327–1334. (13) Kertesz, V.; Van Berkel, G. J. J. Solid State Electrochem. 2005, 9, 390–397. (14) Rohner, T. C.; Lion, N.; Girault, H. H. Phys. Chem. Chem. Phys. 2004, 6, 3056–3068. (15) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64–71. (16) Guaratini, T.; Vessecchi, R. L.; Lavarda, F. C.; Campos, P. M. B. G. M.; Naal, Z.; Gates, P. J.; Lopes, N. P. Analyst 2004, 129, 1223–1226. (17) Guaratini, T.; Vessecchi, R.; Pinto, E.; Colepicolo, P.; Lopes, N. P. J. Mass Spectrom. 2005, 40, 963–968. (18) Reed, D. R.; Hare, M. C.; Fattahi, A.; Chung, G.; Gordon, M. S.; Kass, S. R. J. Am. Chem. Soc. 2003, 125, 4643–4651. (19) Aschi, M.; Fraschetti, C.; Filippi, A.; Speranza, M. J. Mass Spectrom. 2009, 44, 1038–1046. (20) Dupont, A.; Gisselbrecht, J. P.; Leize, E.; Wagner, L.; Vandorsselaer, A. Tetrahedron Lett. 1994, 35, 6083–6086. (21) Monks, T. J.; Hanzlik, R. P.; Cohen, G. M.; Ross, D.; Graham, D. G. Toxicol. Appl. Pharmacol. 1992, 112, 2–16. (22) (a) de Abreu, F. C.; Ferraz, P. A. D.; Goulart, M. O. F. J. Braz. Chem. Soc. 2002, 13, 19–35. (b) Abreu, F. C.; Goulart, M. O. F.; Brett, A. M. O. Electroanalysis 2002, 14, 29–34. (23) Macias-Ruvalcaba, N. A.; Gonzalez, I.; Aguilar-Martinez, M. J. Electrochem. Soc. 2004, 151, E110–E118. (24) Gupta, N.; Linschitz, H. J. Am. Chem. Soc. 1997, 119, 6384–6391. (25) Uno, B.; Okumura, N.; Goto, M.; Kano, K. J. Org. Chem. 2000, 65, 1448–1455. (26) Wilson, I. Chem.-Biol. Interact. 1987, 61, 229–240. (27) Lopes, J. N. C.; Johnson, A. W.; Grove, J. F.; Bulh~oes, M. S. Ci^enc. Cult. (Campinas, Braz.) 1977, 29, 1145. (28) Fieser, L. F.; Fieser, M. J. Am. Chem. Soc. 1934, 56, 1565–1578. (29) Bezerra, D. P.; Alves, A. P. N. N.; Alencar, N. M. N.; Mesquita, R. O.; Lima, M. W.; Pessoa, C.; Moraes Filho, M. O.; Lopes, J. N. C.; Lopes, N. P.; Costa-Lotufo, L. V. J. Exp. Ther. Oncol. 2008, 7, 113–121. (30) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (c) Woon, D. E.; Dunning, T. H., Jr J. Chem. Phys. 1994, 100, 2975–2988. (31) (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (b) TURBOMOLE V6.2 2010, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989
2007, TURBOMOLE GmbH, since 2007; available from http://www. turbomole.com. (32) Range, K.; Riccardi, D.; Cui, Q.; Elstner, M.; York, D. M. Phys. Chem. Chem. Phys. 2005, 7, 3070–3079. McLennan, M. S.; Sutherland, K. N.; Orlova, G. J. Mol. Struct: THEOCHEM 2007, 822, 21–27. McLuckey, S. A. J. Am. Soc. Mass Spectrom. 1992, 3, 599–614. (33) (a) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833–1840. (b) Mulliken., R. S. J. Chem. Phys. 1955, 23, 1841–1846. (34) Ehrmann, B. M.; Henriksen, T.; Cech, N. B. J. Am. Soc. Mass Spectrom. 2008, 19, 719–728. 5459

dx.doi.org/10.1021/jp202322n |J. Phys. Chem. A 2011, 115, 5453–5460

The Journal of Physical Chemistry A

ARTICLE

(35) Tian, Z.; Kass, S. R. J. Am. Chem. Soc. 2008, 130, 10842–10843. (36) Kebarle, P.; Verkerk, U. H. Mass Spectrom. Rev. 2009, 28, 897–917. (37) Zhu, X.-Q.; Wang, C.-H. J. Org. Chem. 2010, 75, 5037–5047. (38) L€u, J.-M.; Rosokha, S. V.; Neretin, I. S.; Kochi, J. K. J. Am. Chem. Soc. 2006, 128, 16708–16719. (39) Wise, K. E.; Grafton, A. K.; Wheeler, R. A. J. Phys. Chem. A. 1997, 101, 1160–1165. (40) Nachtigall, F. M.; Corilo, Y. E.; Cassol, C. C.; Ebeling, G.; Morgon, N. H.; Dupont, J.; Eberlin, M. N. Ang. Chem. Int. Ed. 2008, 47, 151–154. (41) Maire, F.; Lange, C. M. Rapid Commun. Mass Spectrom. 2010, 24, 995–1000. (42) Bouchoux, G.; De Winter, J.; Flammang, R.; Gerbaux, P. J. Phys. Chem. A 2010, 114, 7408–7416. (43) (a) Kanawati, B.; Joniec, S.; Winterhalter, R.; Moortgat, G. K. Int. J. Mass Spectrom. 2007, 266, 97–113. (b) Kanawati, B.; Harir, M.; Schmitt-Kopplin, P. Int. J. Mass Spectrom. 2009, 288, 6–15. (44) Vessecchi, R.; Nascimento, P. G. B. D.; Lopes, J. N. C.; Lopes, N. P. J. Mass Spectrom. 2006, 41, 1219–1225.

5460

dx.doi.org/10.1021/jp202322n |J. Phys. Chem. A 2011, 115, 5453–5460