Ionization Energies of Three Resonance-Stabilized Radicals

Article pubs.acs.org/JPCA

Ionization Energies of Three Resonance-Stabilized Radicals: Cyclohexadienyl (dn, n = 0, 1, 6, 7), 1‑Phenylpropargyl, and Methylcyclohexadienyl Olha Krechkivska,† Callan Wilcox,† Gerard D. O’Connor,‡ Klaas Nauta,† Scott H. Kable,† and Timothy W. Schmidt*,† †

School of Chemistry, UNSW Sydney, Sydney, New South Wales 2052, Australia School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia

‡

ABSTRACT: The ionization energies for three resonance-stabilized radicals are determined: cyclohexadienyl, 1-phenylpropargyl, and methylcyclohexadienyl. The recommended ionization energies are, respectively, 6.820(1), 6.585(1), and 7.232(1) eV. That of cyclohexadienyl is found to be just 0.02 eV above a high level ab initio calculation [Bargholz, A.; Oswald, R.; Botschwina, P. J. Chem. Phys. 2013, 138, 014307], and that of 1-phenylpropargyl is found within the stated error of a recent experimental determination [Holzmeier, F.; Lang, M.; Hemberger, P.; Fischer, I. ChemPhysChem 2014, DOI: 10.1002/cphc.201402446]. The ionization energy of the methylcyclohexadienyl radical is consistent with the ortho isomer. Ionization energies of a range of isotopologues of cyclohexadienyl radical are given, along with their D1 ← D0 origin band positions, which indicate a blue shift of 18 cm−1 per deuterium atom substituted. The ionization energy of cyclohexadienyl, along with the calculated bond dissociation energy of Bargholz et al., affords a new estimate of the 0 K proton affinity of benzene: 739.7 ± 2.0 kJ/mol. The ionization energies are discussed in terms of the interplay between radical and cation stabilization energies.

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reports. The value initially reported by Hemberger et al.22 was at odds with the two-photon detection reported by our group, whereby the combination of 476 and 266 nm photons was found to be sufficient for ionization, implying IE < 7.27 eV. An improved number was reported recently by the Fischer group, 7.24 ± 0.02 eV.23 As will be shown, this number is correct, though our result is reported with improved precision. Cyclohexadienyl radical is formed when a hydrogen atom reacts with benzene in the gas phase, breaking the aromaticity of the ring to yield an sp3 hybridized carbon atom and a 5membered 5-electron radical π-system (Figure 1). This radical has been the subject of numerous studies by laser spectroscopy,24,25 including a rotationally resolved study which revealed accurate structural parameters.26 The rotational constants were recently reproduced to high accuracy by ab initio calculations.27 That study, by Botschwina and co-workers, also investigated the bond dissociation energy and ionization energy, the latter calculated to be 6.803 ± 0.005 eV. The IE has not been reported experimentally. It will be shown that this number is very close to our measurement. Like 1-phenylpropargyl, its selfreaction has also been studied.28 In addition to reporting the IE of cyclohexadienyl radical, we also report that of its isotopologues, C6H6D, C6D6H, and C6D7. The third radical presented here is a methylcyclohexadienyl radical. There are four isomers of this hitherto unobserved

INTRODUCTION The ionization energy of a molecule is an important quantity. It may be used in Hess’s Law calculations of parameters such as bond-dissociation energies and proton affinities and may also be used for molecular identification in photoionization experiments.1−3 Accurate determination of ionization energies is a challenging task. The state-of-the-art methods include vacuum ultraviolet laser velocity-map imaging4−6 and zero electron kinetic energy spectroscopy with pulsed field ionization.7,8 Both of these techniques are able to resolve ionization energies to less than 1 meV, but with sequential 2photon resonant threshold ionization, the quantity may be determined to a few meV accuracy for specific isomers. In this contribution, we present ionization energies of three small resonance-stabilized radicals: cyclohexadienyl, 1-phenylpropargyl, and methylcyclohexadienyl. Resonance-stabilized radicals (RSRs) are important reaction intermediates in combustion and may also play a role in interstellar chemistry.9−19 They are readily formed in electrical discharges of a suitable precursor, and the optical spectroscopy of many have been reported in the past few years by several groups, including our own. We discovered the 1-phenylpropargyl radical to be a ubiquitous product of electrical discharges containing either aromatic or nonaromatic hydrocarbons.15−17,20 Its relationship to propargyl and aromatic molecules has led to suggestions that it may play a role in soot formation, especially through its dimerization.15,21 The ionization energy (IE) of 1-phenylpropargyl has aroused some interest, there being two previous © 2014 American Chemical Society

Received: September 5, 2014 Revised: October 8, 2014 Published: October 9, 2014 10252

dx.doi.org/10.1021/jp508985s | J. Phys. Chem. A 2014, 118, 10252−10258

The Journal of Physical Chemistry A

Article

Figure 1. Structures of radicals investigated in this paper. Clockwise from top left: cyclohexadienyl, methylcyclohexadienyl, and 1-phenylpropargyl radical.

species that results from the association of a hydrogen atom with toluene. Definitive assignment of the observed species will be reported in a future publication.

Figure 2. Observed IE of methylcyclohexadienyl radical plotted against the square-root of grid voltage. The predicted zero-field IE is 6.585(1) eV. Inset: Cartoon of potential described by eq 1.

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dU 1 = 2 −E=0 dz z

EXPERIMENTAL SECTION The determination of ionization energy was performed using our ionization spectroscopy apparatus.10−13 Briefly, radicals were produced in a pulsed discharge nozzle, employing argon gas containing: indene for 1-phenylpropargyl radical; (deuterated) benzene and (heavy) water vapor for cyclohexadienyl radical; or toluene and water vapor for methylcyclohexadienyl radical. It has been reported that water reacts with metastable argon with a cross section exceeding 100 Å2 to yield H atoms and OH radicals.29 Indeed, experiments performed with benzene-d6 failed to produce C6D7 radicals until we added D2O. The products of the discharge were passed through a skimmer into the ionization chamber where they were interrogated with two tunable dye-laser pulses. The first, visible laser pulse excited the radicals to their D1 state, whereupon they were ionized by a second pulse. This UV pulse was generated by frequency-doubling the output of a dye-laser. The ions were extracted perpendicularly into a time-of-flight mass spectrometer. By fixing the second, UV laser pulse at a wavelength sufficient to ionize D1 state radicals, the excitation spectrum of the target radical was (re)produced by monitoring the appropriate m/z signal as a function of the wavelength of the visible laser pulse. Fixing the visible laser to the maximum of the origin band and scanning the UV laser reveals the photoionization efficiency curve, from which the ionization energy was extracted.

z = E−1/2

and Uthresh = −2 E

IE = IE0 − k V

(4)

where V is the applied grid voltage, IE0 is the field-free ionization energy, and k is an experimentally determined constant. The electric field is E = V/d, where d is the distance between our grids (∼1.2 cm). The IE of methylcyclohexadienyl radical was determined at a range of applied voltages. The results are plotted in Figure 2, with a representative photoionization spectrum given in Figure 3. The error of the intercept of 6.585 eV is just below 1 meV. The slope of the fit in Figure 2 is −5.2(6) × 10−4 eV/V0.5. The measurements below for cyclohexadienyl and 1-phenylpropargyl radicals were performed at an applied grid voltage of ∼200 V. Extrapolation of these measurements to zero field yields a value of (−5.2(6) × 10−4 × (200)1/2) eV higher or 7.3(8) meV. To determine the final error on the extrapolated IE, this error of 0.8 meV is added in quadrature to the error on the IE determined from inspection of the photoionization spectrum. Ionization Energy of a Cyclohexadienyl Radical. A representative photoionization spectrum of the cyclohexadienyl radical is given in Figure 3. The ionization onset observed at 6.813 eV is consistent with a zero field IE of 6.820(1) eV, which is our recommended value. The ionization of cyclohexadienyl radical was performed via the 00 level of the D1 state. In so doing, we confirmed the reported origin band wavelength of Nakajima et al.26 The origin band positions of three other isotopologues of cyclohexadienyl radical are given in Table 1. Substitution of a single aliphatic hydrogen for deuterium, from C6H7 to C6H6D or from C6D6H to C6D7, induces a blue shift of the origin band of about 19 cm−1. From the other band origins, it would appear that substitution at the aromatic positions induces a similar shift

RESULTS Determination of Field-Free IE. Our ionization apparatus operates with a static electric field between the extraction and repeller grids. This field acts to diminish the observed IE by pulling down one side of the Coulomb potential (see Figure 2). For a hydrogen atom, in atomic units, the potential experienced by the electron is 1 − Ez r

(3)

As such, the IE is diminished by a quantity proportional to (E)1/2. Empirically, we employed the equation

■

U=−

(2)

(1)

The maximum on the downfield side, corresponding to the classical ionization threshold in the absence of angular momentum considerations, lies on the z axis 10253



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with that observed here for C6H7. The observed shifts are plotted in Figure 4.

Figure 4. Observed shifts upon deuteration for excitation30,31 and ionization34 of benzene and cyclohexadienyl radical (this work). Both species exhibit close to linear shifts for excitation, but the IE of cyclohexadienyl shifts up or down depending on the site of deuteration.

Calculated at the (TD)-B3LYP/cc-pVTZ level,32,33 using the Gaussian 09 package, 20 of 33 vibrational frequencies are seen to decrease in the excited state, as compared to the ground state, with the zero-point energy dropping by about 600 cm−1. In the harmonic approximation, the expected shift in the origin band of two isotopologues, a and b, is ΔT0 =

=

Figure 3. Photoionization efficiency curves of the cyclohexadienyl, methylcyclohexadienyl, and 1-phenylpropargyl radicals. The measurement is depressed by 0.0073(8) eV, due to the presence of an electric field (170 V/cm). The red-shaded band indicates an uncertainty of the ionization threshold of ±0.5 meV for cyclohexadienyl radical and ±1 meV for methylcyclohexadienyl and 1-phenylpropargyl radicals. ∗: structure due to 1-color, 2-photon ionization of indene (m/z 116), which contaminates the signal at m/z 115.

ΔT0 (cm−1) C6H7 C6H6D C6D6H C6D7 a

T0 (cm−1) 18 191.5 18 211a 18 301a 18 319a

exp.a

calc.b

+20 +110 +128

+30 +108 +136

1 2

3N − 6

∑

(ωia′ − ωia″) − (ωib′ − ωib″)

(5)

Δωia − Δωib

(6)

i=1 3N − 6

∑ i=1

Lee and co-workers measured several such isotopic shifts in C6D6H and computed anharmonic frequencies using the B3PW91/6-311++G(2d,2p) method.35 Several frequencies are significantly changed, by factors that vary from 0.72 to 1.00. The calculated isotopic frequency ratios were generally found to be within about 1% of the experiment, and overall, the zeropoint energy was reduced by a factor of 0.83. We calculated the harmonic vibrational frequencies of all measured isotopologues in the ground, excited, and ionic states at the (TD)-B3LYP/ccpVTZ level, finding the ground state frequencies to be on average 1.1% higher than the CCSD(T*)-F12 values of Botschwina and Oswald.36 By evaluating changes in the zeropoint energy, we arrive at the calculated origin band shifts given in Table 1, which are in satisfactory agreement with the experiment. The ionization energies of the isotopologues do not show a simple trend with increasing deuteration. Inspection of the observed shifts, in Table 2 (and plotted in Figure 4), shows that deuteration of the sp3 position brings about an increase in IE, with deuteration at other positions causing a decrease in IE. Calculations at the B3-LYP/cc-pVTZ level, given in Table 2, reproduce this behavior. Inspection of individual modes and their shifts upon deuteration, in order to uncover the root causes of the observed IE shifts, is fraught with difficulty. The loss of symmetry in C6H6D and C6D6H makes it difficult to

Table 1. Origin Band Positions of Four Isotopologues of Cyclohexadienyl Radical isotopologue

1 2

c

This work. b(TD)-B3LYP/cc-pVTZ. cRef 26

per hydrogen of about 18 cm−1. This may be compared to benzene itself, where the origin of the S1 ← S0 transition is shifted 200 cm−1 in benzene-d6, a shift of 33 cm−1 per D.30 Ingold and co-workers obtained fluorescence and absorption spectra of a series of deuterated benzenes.31 These show a remarkably constant shift in origin band wavenumber with the number of substituted deuterium atoms, which is consistent 10254



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Table 2. Ionization Energies of Four Isotopologues of Cyclohexadienyl Radical ΔIE (meV)

a

isotopologue

IE (eV)a

exp.a

calc.b

calc.c

C6H7 C6H6D C6D6H C6 D 7

6.820(1) 6.824(1) 6.815(1) 6.821(1)

+4(1) −5(1) +1(1)

+3.9 −6.5 −2.8

+3.3 −8.4 −5.0

This work. b(TD)-B3LYP/cc-pVTZ. cAtomic-block-diagonal Hessian.

match vibrational modes of isotopologues with certainty, especially for those modes most affected by deuteration. In C6D7, which retains C2v symmetry, the contributions, from each normal mode, to the calculated isotopic shift of −2.8 meV range from −7.0 to +10.1 meV. By removing terms in the Hessian matrix linking coordinates of different atoms, the effects of deuteration and ionization can be inspected on an atom-by-atom basis. Eigenvalues of this atomic-block-diagonal Hessian were thus calculated. In the ground state, the three vibrational frequencies for the uncoupled sp3 hydrogen are 2812, 1150, and 1097 cm−1, respectively, stretch, in-plane bend, and out-of-plane bend. These frequencies change in the cation to 2850, 1087, and 939 cm−1. As such, the contribution to the zero-point energy from modes involving the sp3 hydrogen drops by 92 cm−1 in the cation. Deuteration at the sp3 position will reduce this zeropoint energy drop upon ionization by a factor of 1/(2)1/2, yielding a shift in IE of (1/(2)1/2 − 1) × (−92) = +27 cm−1 or 3.3 meV. This value compares favorably with the values obtained using the full Hessian matrix (Table 2). Nevertheless, the increase in IE on going from C6H7 to C6H6D and from C6D6H to C6D7 can be, at least partially, explained by the drop in zero-point energy for the sp3 hydrogen upon ionization. More specifically, despite a small increase in stretching frequency, the large drop in both the in-plane and out-plane bending frequencies upon ionization causes an increase in IE when the radical is deuterated at one or both sp3 positions. Contrastingly, the modes involving hydrogens at the sp2 positions all contribute more zero-point energy in the cation than in the radical. The ortho-, meta-, and para-positions, respectively, contribute 66, 43, and 103 cm−1 more zero-point energy in the cation than in the radical, bringing about a predicted shift in IE upon deuteration of −19, −13, and −30 cm−1, respectively. At the ortho- and para-positions, the out-ofplane bending frequencies increase from 720 to 821 cm−1 and 662 to 841 cm−1, respectively, upon ionization while the stretch and in-plane bend remains similar. At the meta-position, there is a moderate increase in all three frequencies. The predicted IE shifts on deuteration, calculated with the atomic-block-diagonal Hessian, are given in Table 2. Ionization of cyclohexadienyl radical results in a species best described as protonated benzene,37 and as such, there emerges a thermochemical cycle linking many of the energetic parameters for which highly accurate experimental and theoretical values are now becoming available. In Figure 5, the various processes are numbered 1−6 and form closed cycles for 1−2−5−6 and 1−3−4−6. Process 1 is the bond dissociation energy of the cyclohexadienyl radical. There is no experimental measurement of this quantity, but it has been calculated to an estimated accuracy of 2 kJ/mol by CCSD(T)-F12 methods.27 Process 2 is the ionization energy of hydrogen, which is accurately known.

Figure 5. Thermochemical cycle linking cyclohexadienyl radical and protonated benzene. The cycle drawn in blue is used to estimate the 0 K proton affinity of benzene from a combination of experimental and theoretical values.

Process 3 is the ionization energy of benzene, which is also accurately known. Process 4, the bond dissociation energy of the cyclohexadienylium cation, is unknown, and process 5, the proton affinity of benzene, has been reported experimentally at 298 K. Finally, process 6, the ionization energy of the cyclohexadienyl radical, has been determined presently. Taking the calculated BDE of the radical as a starting point (1), adding to this the ionization energy of hydrogen (2), and subtracting the IE of cyclohexadienyl radical (6) yields an estimate of the 0 K proton affinity of benzene (5). This value, of 739.7 ± 2.0 kJ/mol, is some 10 kJ/mol less than the value measured at 298 K.38 However, the tabulated value of Hunter and Lias has since been revised down by some 4 kJ/mol in the light of ab initio entropies39 and is probably closer to the mark. Performing the same calculation, but replacing the IE of hydrogen with that of benzene (3) gives the BDE of the cation, which is calculated to be 319.6 ± 2.0 kJ/mol. Recommended values for the energetics depicted in Figure 5 are given in Table 3. Table 3. Experimental and Theoretically Determined 0 K Thermochemical Parameters (kJ/mol) for Cyclohexadienyl Radical and Cation process 1 2 3 4 5 6

description radical BDE hydrogen IE benzene IE cation BDE benzene PA radical IE

experimental

RCCDS(T)-F12 85.7 ± 2.0

a

1312.05b 891.89b c 658.0 ± 0.1

656.4 ± 0.5a

recommended 85.7 ± 2.0 1312.05 891.89 319.6 ± 2.0 739.7 ± 2.0 658.0 ± 0.1

a Ref 27. bFrom http://www.nist.gov/. c298 K values of 750.4 kJ/mol (ref 38) and 746.4 kJ/mol (ref 39).

Ionization Energy of the 1-Phenylpropargyl Radical. The photoionization spectrum of the 1-phenylpropargyl radical is shown in Figure 3. Superimposed on the ionization onset are features due to 1-color, 2-photon ionization of the parent species, indene, due to the UV laser pulse. The signal for indene is so strong that it contaminates nearby mass channels. The 1phenylpropargyl radical was found to have a zero-field ionization energy of 7.232(1) eV. This sits comfortably within the stated error of Fischer and co-workers’ most recent result of 7.24(2) eV. Both values are consistent with the original 10255



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respectively.46 Phenylpropargyl was calculated at the G3X(MP2)-RAD level to have an even higher RSE, of 101 kJ/mol.13 The cation stabilization energy (CSE) may be similarly defined as the 0 K heat of the reaction

observation of ionization with cooperative 476 and 266 nm photons. It also sits well with calculations previously reported.13 In the study by the groups of Radom and Schmidt,13 it was shown that adiabatic IEs calculated at the B3-LYP/6-311+ +G(3df,3pd) and G3X(MP2)-RAD levels of theory tended to bracket the experimental IE. The respective calculated values of 7.13 and 7.28 eV are seen to bracket the presently reported experimental value and also that reported by Fischer and coworkers (7.24(2) eV).23 The calculated IEs for the isomeric 3phenylpropargyl radical, 7.14 and 7.29 eV, also bracket the reported value of 7.25(1) eV. Ionization Energy of a Methylcyclohexadienyl Radical. The field-free IE of the detected isomer of the methylcyclohexadienyl radical was determined to be 6.585(1) eV. As will be detailed in a forthcoming publication, the measured IE is consistent with the ortho isomer depicted in Figure 1. Briefly, at the G4(MP2)-6X level of theory, the adiabatic IEs of the ortho, meta, para, and ipso isomers are found to be, respectively, 6.63, 6.72, 6.50, and 6.81 eV.40 For the cyclohexadienyl radical, this level of theory is found to predict a value of 0.03 eV too high. Discounting the calculated values of the methylcyclohexadienyl isomers by a similar amount clearly points to the ortho isomer.

R+ + CH4 → RH + +CH3

(8)

From the reactions given in eqs 7 and 8, it is easy to show that the IE of R· is the IE of the methyl radical minus the difference between the RSE and the CSE of R·: IE(R·) = IE(CH3) − (CSE(R·) − RSE(R·)) = IE(CH3) − ΔSE(R)

(9) 44

Since the CSEs of RSRs outweigh the RSEs, their IEs are lowered compared to the methyl radical by the quantity ΔSE(R). From the IE of allyl radical, 8.13 eV, a ΔSE(allyl) of 1.71 eV is inferred (IE(methyl) = 9.838 eV47). Assuming additivity of ΔSEs, a methyl radical with two vinyl substituents would be stabilized by double this and exhibit an IE of 6.42 eV. Cyclohexadienyl has an IE of 6.82 eV showing that the ΔSE is less than additive, ΔSE(C6H7) = 3.02 eV < 2ΔSE(allyl). The methylcyclohexadienyl radical presented here can be considered a radical stabilized by a methyl group in addition to the dienyl functionality which stabilizes cyclohexadienyl. The observed increase in ΔSE of 0.235 eV is lower than that observed when going from isopropyl to t-butyl radical, where a methyl group is added to further stabilize a secondary radical. Phenylpropargyl exhibits an IE of 7.232 eV, which is very close to the benzyl radical. This is somewhat surprising, given that the RSEs of benzyl and phenylpropargyl are 0.64 and 1.05 eV, respectively.13 From the observed ΔSEs and calculated RSEs, CSEs for benzyl, 3.24 eV, and 1-phenylpropargyl, 3.66 eV, are obtained. Thus, while 1-phenylpropargyl indeed exhibits a larger CSE than benzyl, it is not large enough to engender an IE consistent with additive ΔSEs of phenyl and ethynyl groups (ΔSE(propargyl) = 1.14 eV). Phenylallyl radical13,14 is a similar species to phenylpropargyl, being a three-carbon conjugated radical moiety attached to an aromatic ring. Its ionization energy is lower than benzyl, at 6.9 eV, but not the 1.71 eV lower which would be expected if ΔSEs were additive. While RSEs may exhibit additivity, or even synergy,48 it stands to reason that CSEs do not exhibit additivity. In the valence bond picture, the cation site is stabilized by electron donation, which distributes the excess charge. A vinyl substituent, such as in allyl, effectively redistributes half of the charge. As such, a further vinyl group cannot stabilize to the same extent. For this reason, the observed reductions in IE compared to methyl for carbon-centered radicals will not be additive in terms of the stabilizing substituents.

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DISCUSSION Within the paradigm of molecular orbital theory, the ionization energies of the resonance-stabilized radicals (RSRs) are a crude measure, following Koopman’s theorem,41 of the energy of the unpaired electron. Yet, the RSRs exhibit similar ionization energies to nonresonance-stabilized systems. Ref 14 contains a compilation of the IEs of some RSRs, most of which are below 7 eV. These may be compared to the t-butyl radical, which has an IE of about 6.7 eV, and the isopropyl radical which requires 7.36 eV for ionization.42 Both of these radicals have barriers to planarity of some tens of meV, so the IEs can be taken as indicative of the planar structure. Indeed, Hückel theory, coupled with Koopman’s theorem, would predict that the IE of an RSR is similar to that of a similar system with an isolated πorbital, since the energy of the unpaired electron is always α (for odd-alternant systems such as those investigated here). Thus, if the stability is not engendered in the unpaired electron, where is it? It is instructive to compare the Hückel electronic energies of benzene and the hypothetical cyclohexatriene. In cyclohexatriene, all six valence electrons have an energy of α + β, whereas in benzene, the four HOMO-electrons have an energy of α + β while the lowest energy π-electrons have an energy of α + 2β. As such, the two compounds would be predicted to have similar IEs. Benzene is much more stable, but the stability is conferred on the low energy π-electrons, which are retained in the cation. Benzene has an IE of 9.24 eV, which is not much higher than cyclohexene, 8.94 eV.43 Because the low energy, stabilized electrons are retained upon ionization, the cations of resonance-stabilized radicals are also resonancestabilized. Indeed, calculations of allyl and propargyl cations show these to be even more stabilized than their neutral counterparts.44 The stability of a radical R· may been defined as the 0 K heat of reaction of the hydrogen transfer from methane:45 R· + CH4 → RH + ·CH3 (7)

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CONCLUSIONS We have reported ionization energies accurate to 1 meV of the cyclohexadienyl and 1-phenylpropargyl radicals and one isomer of the methylcyclohexadienyl radical. These are summarized in Table 4. The value measured for cyclohexadienyl radical was Table 4. Recommended IEs of the Cyclohexadienyl, 1Phenylpropargyl, and Methylcyclohexadienyl Radicals

On this scale, the methyl radical has a radical stabilization energy (RSE) of zero, and the resonance-stabilized radicals allyl, propargyl, and benzyl possess some of the highest RSEs for monosubstituted radicals 69, 55, and 62 kJ/mol, 10256

radical

IE (eV)

cyclohexadienyl 1-phenylpropargyl methylcyclohexadienyl

6.820(1) 7.232(1) 6.585(1)



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found to be just 0.017 eV higher than the ab initio value reported by Botschwina and co-workers.27 Using the calculated bond dissociation energy of the radical from the same ab initio work, we calculate a 0 K benzene proton affinity. Our measured ionization energy of 1-phenylpropargyl was found to be bracketed by our previous calculations, as expected, and was within the error range reported by Fischer and co-workers.23 From an analysis of the observed ionization energies in terms of radical and cation stabilization energies, we conclude that the depression of IEs compared to the methyl radical due to cation stabilization is not, in general, additive.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported under Australian Council’s Discovery Projects funding (DP120102559). T.W.S. acknowledges receipt of Future Fellowship. G.D.O. acknowledges support Australian Postgraduate Award.

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Research scheme an ARC from an

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Ionization Energies of Three Resonance-Stabilized Radicals

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