Comparison of Thiyl, Alkoxyl, and Alkyl Radical Addition to Double

Article pubs.acs.org/JPCA

Comparison of Thiyl, Alkoxyl, and Alkyl Radical Addition to Double Bonds: The Unusual Contrasting Behavior of Sulfur and Oxygen Radical Chemistry Isa Degirmenci*,†,‡ and Michelle L. Coote*,‡ †

Chemical Engineering Department, Ondokuz Mayıs University, Samsun 55139, Turkey ARC Centre of Excellence for Electromaterials Science, Research School of Chemistry, The Australian National University, Canberra ACT 2601, Australia

‡

S Supporting Information *

ABSTRACT: High-level ab initio calculations have been used to compare prototypical thiyl, alkoxyl, and alkyl radical addition reactions. Thiyl radical addition to the sulfur center of thioketones is exothermic and rapid, occurring with negative enthalpic barriers and only weakly positive Gibbs free energy barriers. In stark contrast, alkoxyl radical addition to the oxygen center of ketones is highly endothermic and occurs with very high reaction barriers, though these are also suppressed. On the basis of analysis of the corresponding alkyl radical additions to these substrates and the corresponding reactions of these heteroatom radicals with alkenes, it suggested that addition reactions involving thiyl radicals have low intrinsic barriers because their unpaired electrons are better able to undergo stabilizing resonance interactions with the π* orbitals of the substrate in the transition state.

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reactions. Indeed, Lalevée7 has quantified a ∼40 kJ mol−1 difference in intrinsic barriers for these processes, though reasons for this difference have yet to be fully clarified. In the present work, we aim to explore these reactivity differences further by systematically comparing a series of prototypical thiyl radical addition reactions with their alkoxyl and alkyl radical analogues. As part of this work, we present the first theoretical study of thiyl radical addition to thioketones, a reaction with direct relevance to a new class of self-healing materials based on thiruam disulfides.10−12 These materials feature dynamic covalent bonds that homolytically cleave upon exposure to light to form thiyl radicals that can then undergo reshuffling via either a simple homolysis/recombination mechanism or a degenerative chain transfer process involving radical addition to the sulfur side of the CS bond followed by β-scission to regenerate a dithiuram and thiyl radical. The relative importance of these two pathways has yet to be established, and clarifying the reactivities of thiyl radicals in prototypical addition reactions is an important first step toward establishing the relevance of radical transfer in this process.

INTRODUCTION Thiyl radicals are important intermediates in radical biochemistry and organic and polymer synthesis.1 Among other applications, thiols are used in radical polymerization as chain transfer agents,2 where the reactions are generally rapid and highly exothermic, reflecting the greater strength of a C−H bond versus a S−H bond. As a consequence, the reverse reactions are endothermic and typically involve very high reaction barriers (e.g., as much as 90 kJ/mol in the case of hydrogen abstraction from CH4 by •SCH3).3 Interestingly, the high stability of thiyl radicals to hydrogen atom transfer with organic substrates is in direct contrast to their high (albeit reversible) reactivity in radical addition to alkenes,4 a feature taken advantage of in radical thiol−ene processes (Scheme 1).5 Their reactivity has been generally attributed to polar effects, though the structure−reactivity trends also depend on thermodynamic factors.6−9 Nonetheless, this alone cannot account for the high reactivity of thiyl addition to alkenes compared with the considerably more exothermic alkyl addition

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Scheme 1. Radical Thiol−Ene Coupling

COMPUTATIONAL PROCEDURES Standard ab initio molecular orbital theory was carried out using Gaussian 0913 and Molpro 2012.1.14 All geometries were Received: January 18, 2016 Revised: February 28, 2016

© XXXX American Chemical Society

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DOI: 10.1021/acs.jpca.6b00538 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Table 1. Calculated Barriers and Reaction Energies (298 K, kJ mol−1) for Radical Addition to Double Bonds, Radical Stabilization Energies (RSEstd and RSEZ) of the Reactant and Product Radicals (298 K kJ/mol), S−T Gaps of the Substrates (eV), and Mulliken Charge on the Radical Fragment in the Transition State (Q)a reactant radical 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

b

radical

substrate

CH3 • CH3 • CH2CH3 • CH2CH3 • CH3 • CH3 • CH2CH3 • CH2CH3 • OH • OH • OCH3 • OCH3 • OH • OH • OCH3 • OCH3 • SH • SH • SCH3 • SCH3 • CH3 • CH3 • CH2CH3 • CH2CH3 • SH • SH • SCH3 • SCH3

CH2CH2 CH2C(CH3)2 CH2CH2 CH2C(CH3)2 OCH2 OC(CH3)2 OCH2 OC(CH3)2 CH2CH2 CH2C(CH3)2 CH2CH2 CH2C(CH3)2 OCH2 OC(CH3)2 OCH2 OC(CH3)2 CH2CH2 CH2C(CH3)2 CH2CH2 CH2C(CH3)2 SCH2 SC(CH3)2 SCH2 SC(CH3)2 SCH2 SC(CH3)2 SCH2 SC(CH3)2

•

‡

‡

ΔHrxn

ΔGrxn

ΔH

ΔG

−89.6 −86.6 −89.1 −86.9 −28.5 2.0 −40.9 −10.3 −108.4 −105.8 −75.1 −76.5 145.9 171.0 164.1 188.2 −31.6 −36.5 −27.3 −30.7 −116.9 −87.6 −119.8 −91.5 −69.7 −49.7 −73.4 −50.9

−45.9 −40.0 −42.2 −36.8 19.6 55.1 10.5 46.2 −71.8 −68.4 −30.6 −30.7 184.9 214.2 209.9 239.7 4.3 2.5 15.2 14.4 −72.6 −39.3 −70.7 −38.7 −34.0 −11.3 −29.7 −5.3

36.3 33.7 31.2 29.4 80.0 88.8 62.2 76.4 3.6 −4.4 28.2 16.1 133.2 149.9 151.9 167.9 1.3 −11.9 11.4 −0.1 7.7 10.4 −0.2 1.1 −21.3 −32.9 −10.6 −19.7

77.8 78.3 76.0 77.5 124.6 134.9 110.7 124.3 34.8 33.1 70.7 62.1 172.4 191.0 197.6 216.0 36.4 26.6 52.3 44.8 44.8 51.1 38.8 44.9 13.5 4.4 27.4 22.3

product radical

Q

S−T gap

RSEStd.

RSEZ

RSEStd.

RSEZ

−0.01 −0.02 0.00 −0.01 0.10 0.10 0.12 0.11 −0.09 −0.09 −0.09 −0.10 −0.02 −0.03 −0.01 −0.03 −0.09 −0.12 −0.06 −0.08 0.03 0.03 0.04 0.04 −0.06 −0.08 −0.02 −0.04

4.58 4.60 4.60 4.60 3.98 4.48 3.98 4.48 4.58 4.65 4.58 4.65 3.98 4.48 3.98 4.48 4.58 4.65 4.58 4.65 2.10 2.48 2.10 2.48 2.10 2.48 2.10 2.48

0.0 0.0 12.9 12.9 0.0 0.0 12.9 12.9 −59.0 −59.0 −5.43 −5.43 −59.0 −59.0 −5.43 −5.43 56.2 56.2 73.6 73.6 0.0 0.0 12.9 12.9 56.2 56.2 73.6 73.6

0.0 0.0 1.5 1.5 0.0 0.0 1.5 1.5 87.9 87.9 110.8 110.8 87.9 87.9 110.8 110.8 66.4 66.4 68.3 68.3 0.0 0.0 1.5 1.5 66.4 66.4 68.3 68.3

11.4 27.1 11.6 28.0 30.7 35.4 30.7 35.6 9.6 24.0 4.2 22.7 23.1 28.6 23.8 30.1 13.7 34.1 14.6 32.7 40.9 44.7 42.1 47.7 33.2 41.4 34.4 40.0

0.8c 5.9c 1.0 7.7 6.2 15.0 6.6 15.4 −1.2 1.9 −6.5 −1.5 −1.0 0.6 −0.3 0.9 1.1 10.4 2.3 12.4 27.0 20.8 28.5 24.1 14.5 12.4 16.0 11.7

a

Barriers, enthalpies, and radical stabilization energies were calculated at the G3(MP2)-RAD//MP2/6-31G(d) level of theory. S−T gaps were calculated using MP2/6-31G(d) electronic energies at 0 K. bRadical addition is calculated for attack on the left-hand side of the substrate. cReference 16.

reaction coordinate calculations (IRCs).21 However, because the aim of the present work was to study reactivity differences rather than quantify actual reaction rates, the reaction precomplexes were not explicitly studied.

optimized at the (U)MP2/6-31G(d) level of theory, and frequencies were also calculated at the same level of theory. Improved single-point energies were calculated using the highlevel composite ab initio G3(MP2)-RAD15 that approximates (UR)CCSD(T) calculations with a large triple-ζ basis from calculations with a double-ζ basis set via basis set corrections carried out at the R(O)MP2 level. Previous benchmarking studies have validated this method for a large test set of bond energies and associated radical stabilities.16 Partition functions and associated zero-point vibrational energies (ZPVEs), thermal corrections, and entropies were calculated using the standard textbook formulas for the statistical thermodynamics of an ideal gas under harmonic oscillator/rigid rotor approximations. In carrying out these calculations, standard frequency scaling factors for the ZPVE (0.9670), for thermal correction (1.0059), and for entropy correction (1.0178) were applied. It is well-known that OH radical addition to olefins proceeds with submerged activation barriers, and this phenomenon is explained with the existence of prereactive complexes.17−20 Similar behavior is also observed for the sulfurcentered radical reactions in the present work. In such cases, the nature of the transition state geometries and the existence of the prereactive complexes were established using intrinsic

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RESULTS AND DISCUSSION

Table 1 shows the reaction barriers and energies for a test set of prototypical radical addition reactions. In all cases, the calculations were performed for radical addition at the unsubstituted carbon (in the case of alkenes) or heteroatom (in the case of ketones and thioketones). This is the kinetic preference for alkenes and thioketones and was selected for the ketones to enable a consistent comparison of results. To assist in the analysis of the results, Table 1 also shows the singlet− triplet (S−T) gaps of the substrates, the charge on the radical fragment in the transition state, and the radical stabilization energies (RSEs) of the reactant and product radicals. The RSEs were calculated via both the standard method (RSEstd, eq 1) and Zavitsas’s inherent RSE method (RSEZ, eq 2; see ref 22 for further details). RSEstd(R•) = D[H−CH3] − D[H−R] B

(1)


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The S−T gaps of the substrates correspond to their π−π* excitation energies and can used as a measure of “inherent” πbond strength, which in turn affects both the reaction exothermicities and intrinsic barriers. In the present examples, the average S−T gap of the four alkenes studied is 4.60 eV, slightly higher than that for the four analogous ketones (4.23 eV) and considerably higher than that for the thioketones (2.29 eV). The differences in alkenes and ketones are not large enough to reverse their reactivities or exothermicities, with the latter presumably governed by differences in the strength of the forming C−O versus C−C σ-bonds. However, the considerably weaker π-bond in the thioketones almost certainly contributes to their very low barriers and large exothermicities. Effect of Radical. Comparing next the effect of the attacking radical in addition to the same CC substrates, we note that the trends are strikingly contrathermodynamic (see Figure 2). In particular, the thiyl additions have considerably

1 (D[H3C−CH3]calc − D[R−R]calc ) (2) 2 The former method provides a direct measure of the propensity of radicals to undergo hydrogen atom transfer with an organic substrate, whereas the latter provides a better means of comparing the inherent stabilities of different classes of radicals. Energies for charge transfer from the radical to substrate (R+A−) and from the substrate to radical (R−A+) are provided in the Supporting Information. Below, we use the data in Table 1 to compare radical addition of thiyl radicals to thioketones with their oxygen- and carbon-based analogues. To assist in interpreting the results, we first study the effect of the substrate in the same alkyl radical addition reactions and then the effect of the radical in addition to the same alkenes, before finally comparing thiyl addition to thioketones with alkoxyl addition to ketones. Effect of the Substrate. The factors affecting the relative activation barriers of alkyl radical addition to CC, CO, and CS double bonds have been previously elaborated.23 In these prototypical reactions, the barriers are largely governed by reaction exothermicity, with addition to CO having the highest barrier and lowest exothermicity, addition to CC bonds being intermediate, and addition to CS bonds having the greatest exothermicity and lowest barriers (see Figure 1). RSE Z(R•) =

Figure 2. Average barriers and enthalpies (298 K) for the addition of alkyl, alkoxyl, and thiyl radicals to alkenes. The average charge on the radical fragment in the transition state (Q) and the standard (RSEstd) and inherent (RSEZ) radical stabilization energies for the reactant radicals (kJ/mol) are also shown.

lower exothermicities than the other reactions but are essentially barrierless. This fits with previous experimental and theoretical studies that report these reactions as being rapid and often reversible.1,4−9 In contrast, the alkyl and alkoxyl additions have significantly higher barriers despite being considerably more exothermic. The exothermicity differences between the alkoxyl and thiyl additions are also striking, particularly as the substrate is common and the product radicals have very similar radical stabilities. Interestingly, the reactant thiyl radicals have high standard and inherent radical stabilities, whereas the alkoxyl radicals have very high inherent stabilities but negative standard RSEs. This implies that both types of radicals are inherently stable (high RSEZ), but alkoxyl radicals readily undergo hydrogen atom transfer reactions (low RSEstd) due to formation of a strong O−H bond, whereas thiyl radicals do not as the resulting weak S−H bond is too weak. Presumably, the low exothermicity in the thiyl addition reaction is likewise due to formation of a relatively weak S−C σ bond, though this does not explain its low addition barrier compared with the considerably more exothermic alkoxyl and alkyl radical addition reactions. As highlighted previously,4−9 polar effects clearly play a role in reducing the barrier heights in thiyl addition to alkenes. However, the present results indicate that this by itself does not explain their high reactivity as the extent of charge transfer in

Figure 1. Average barriers and enthalpies (298 K) for the addition of carbon-centered radicals to alkenes, ketones, and thioketones in Table 1. Average charges on the radical fragment in the transition states (Q), radical stabilization energies (RSEstd) of the product radicals (kJ/mol), and S−T gaps of the substrates (eV) are also shown.

For addition to the alkenes, the carbon-centered attacking radical is weakly electrophilic, having a lower charge transfer energy for R−A+ versus R+A− (see Supporting Information). However, the extent of polar stabilization of the transition state is negligible (average charge on the radical Q = −0.01), and indeed, it is well-known that the polarity can be reversed with appropriate combinations of electron-donating and -withdrawing substituents on the radical and alkene, respectively.24−26 For addition to the more electrophilic ketones and thioketones, the alkyl radical acts as a nucleophile even in these prototypical reactions, and charge separation is more important (Q = +0.11 and +0.04, respectively), though in the case of the ketones, these polar effects are not significant enough to overcome the thermodynamic factors, and the barriers remain very high. C


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The Journal of Physical Chemistry A the transition state is similar for both thiyl and alkoxyl radical reactions (average Q = −0.09 in both cases), despite their vastly different behavior. Instead, the present results indicate that the lower reactivity of thiyl (versus alkyl and alkoxyl) radicals in addition reactions is due primarily to their lower intrinsic barriers, a feature that, as noted above, has been quantified previously in comparisons of alkyl and thiyl radicals.7 Moreover, although less striking than the thiyl versus alkyl/alkoxyl reactivity differences, the barrier heights for alkoxyl radical addition to alkenes are also considerably lower than those for the corresponding alkyl addition reactions, despite having similar exothermicities. While this in part reflects increased polar effects in the former reactions,27 it also supports the notion that the intrinsic barriers for heteroatom radical addition reactions are generally smaller than those for alkyl radicals. This is most clear in the alkoxyl addition to ketone reactions (see below) where, in the absence both polar effects and a thermodynamic driving force, the barriers are submerged. Under a normal curve-crossing model analysis28−30 of radical addition to double bonds,24 the intrinsic barrier is related to the S−T excitation gap of the substrate, which is equal to the energy difference between the reactant (RA) and product (RA3) electronic configurations in the reactant geometry. In practical terms, it represents the energy required for bond breaking without compensation from geometry relaxation or bond formation. However, in the present examples, the substrates (and hence their S−T gaps) are common. The transition structures themselves are also very similar (see Figure 3). Instead, the reduced intrinsic barrier heights imply that the

Figure 4. Reactant and product configuration for radical addition to double bonds and an orbital interaction diagram showing how the formation of a strong early R−X σ-bonding interaction (and hence stabilization of the product configuration) is favored when the energy of the SOMO is higher. In the series •CH3, •OH, and •SH, •SH has the highest HOMO energy (see Table S2 of the Supporting Information).

Figure 3. MP2/6-31G(d) optimized geometries of the transition states for CH3•, HO•, and HS• addition to ethylene, along with the SOMOs.

slopes of the RA3 energy curves are steeper for thiyl radicals than those for alkyl or alkoxyl radicals. This would occur if the unpaired electron of the attacking radical is relatively high in energy as this would enable an earlier stronger bonding interaction with the triplet substrate (see Figure 4). Indeed, the thiyl radicals have higher singly occupied molecular orbitals (SOMOs) than the alkoxyl or alkyl radicals (see Table S2 of the Supporting Information). Although the chemistry is very different, similar curve-crossing model arguments have been used to explain the improvement in nucleophilicity down the periodic table in SN2 reactions.28−30 Comparison of Thiyl and Alkoxyl Reactions. Finally, if we compare thiyl radical addition to the S-side of CS bonds with alkoxy radical addition to the O-side of CO bonds, we see diametrically opposed behavior (Figure 5). The alkoxy radical reactions are strongly endothermic, reflecting the high CO π-bond strength, weak O−O bond formed, and higher inherent radical stability of the attacking alkoxyl radical versus

Figure 5. Average barriers and enthalpies (298 K) for the addition of alkoxyl to ketones and thiyl radicals to thioketones. The average charge on the radical fragment in the transition state (Q), S−T gaps of the substrates (eV), and the radical stabilization energies (kJ/mol) for the reactant (RSEstd and RSEZ) and product radicals (Prod. RSEstd) are also shown.

the product alkyl radical. Interestingly, while this endothermicity naturally leads to high reaction barriers, the barriers are submerged, consistent with previous observations.17−20 This implies that the transition states are unusually stabilized relative to the products. This stabilization cannot be attributed to polar effects as, due to the similar electronegativities of the attacking radical and substrate, polar effects are negligible (average Q = −0.02). In other words, as in the alkene additions above, the D


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Infrastructure and financial support from the Australian Research Council Centre of Excellence for Electromaterials Science.

results imply the intrinsic barriers for alkoxyl radical addition to ketones are actually relatively small. In contrast to the alkoxyl reactions, the analogous sulfurcontaining reactions are highly exothermic, which primarily reflects the weaker CS π-bond broken and greater forming S−S versus O−O bond strength. As in the case of the alkoxyl reactions, the barriers are submerged, but due to the thermodynamic driving force, they are now negative in enthalpic terms, though they remain weakly positive in Gibbs free energy terms due to the entropy loss upon forming the transition state. Nonetheless, the calculated gas-phase reaction rates for reactions 25−28 are on the order of 109−1013 L mol−1 s−1, which is at or above the diffusion-control limit, consistent with experimental observations for related systems.31 From a practical perspective, it implies that the chain transfer process is likely to be kinetically competitive with coupling in dithiuram self-healing materials. The submerged barriers and associated high degree of stabilization of the transition state are not attributable to significant polar effects (average Q = −0.05) but again reflect the low intrinsic barriers of thiyl reactions. As explained above, the high-energy SOMOs of the (2nd row, heteroatom) radical are able to undergo early and strong resonance interactions with the π* orbital of the substrate (i.e., triplet configuration), which stabilizes the transition state to a greater extent than expected on the basis of the initial reactant (RA) to product (RA3) excitation energy.

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(1) Deńeś, F.; Pichowicz, M.; Povie, G.; Renaud, P. Thiyl Radicals in Organic Synthesis. Chem. Rev. 2014, 114, 2587−2693. (2) See, for example: Odian, G. Principles of Polymerization; WileyInterscience: New York, 1991. (3) Beare, K. D.; Coote, M. L. What Influences Barrier Heights in Hydrogen Abstraction from Thiols by Carbon-Centered Radicals? A Curve-Crossing Study. J. Phys. Chem. A 2004, 108, 7211−7221. (4) Walling, C.; Helmreich, W. Reactivity and Reversibility in the Reaction of Thiyl Radicals with Olefins. J. Am. Chem. Soc. 1959, 81, 1144−1148. (5) For a review, see for example: Hoyle, C. E.; Bowman, C. N. Thiole-Ene Click Chemistry. Angew. Chem., Int. Ed. 2010, 49, 1540− 1573. (6) Northrop, B. H.; Coffey, R. N. Thiol−Ene Click Chemistry: Computational and Kinetic Analysis of the Influence of Alkene Functionality. J. Am. Chem. Soc. 2012, 134, 13804−13817. (7) Lalevée, J.; Allonas, X.; Fouassier, J. P. Search for High Reactivity and Low Selectivity of Radicals toward Double Bonds: The Case of a Tetrazole-Derived Thyil Radical. J. Org. Chem. 2006, 71, 9723−9727. (8) Lalevée, J.; Allonas, X.; Morlet-Savary, F.; Fouassier, J. P. Respective Contributions of Polar vs Enthalpy Effects in the Addition/ Fragmentation of Mercaptobenzoxazole-Derived Thiyl Radicals and Analogues to Double Bonds. J. Phys. Chem. A 2006, 110, 11605− 11612. (9) Ito, O.; Matsuda, M. New Dual Parameters For Radical Reactivities of Vinyl Monomers. Prog. Polym. Sci. 1992, 17, 827−874. (10) Amamoto, Y.; Kamada, J.; Otsuka, H.; Takahara, A.; Matyjaszewski, K. Repeatable Photoinduced Self-Healing of Covalently Cross-Linked Polymers Through Reshuffling of Trithiocarbonate Units. Angew. Chem., Int. Ed. 2011, 50, 1660−1663. (11) Amamoto, Y.; Otsuka, H.; Takahara, A.; Matyjaszewski, K. SelfHealing of Covalently Cross-Linked Polymers by Reshuffling Thiuram Disulfide Moieties in Air under Visible Light. Adv. Mater. 2012, 24, 3975−3980. (12) Amamoto, Y.; Otsuka, H.; Takahara, A.; Matyjaszewski, K. Changes in Network Structure of Chemicla Gels Controlled by Solvent Quality through Photoinduced Radical Resuffling Reactions of Trithiocarbonate Units. ACS Macro Lett. 2012, 1, 478−481. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision D.01, Gaussian, Inc.: Wallingford, CT, 2009. (14) Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M.; Celani, P.; Korona, T.; Lindh, R.; Mitrushenkov, A. G.; Rauhut, K. R.; et al. MOLPRO 2012.1. http://www.molpro.net (2015). (15) Henry, D. J.; Sullivan, M. B.; Radom, L. G3-RAD and G3XRAD: Modified Gaussian-3 (G3) and Gaussian-3X (G3X) Procedures for Radical Thermochemistry. J. Chem. Phys. 2003, 118, 4849−4860. (16) Coote, M. L.; Lin, C. Y.; Beckwith, A. L. J.; Zavitsas, A. A. A Comparison of Methods for Measuring Relative Radical Stability of Carbon-Centred Radicals. Phys. Chem. Chem. Phys. 2010, 12, 9597− 9610. (17) Greenwald, E. E.; North, S. W.; Georgievskii, Y.; Klippenstein, S. J. A Two Transition State Model for Radical-Molecule Reactions: A Case Study of the Addition of OH to C2H4. J. Phys. Chem. A 2005, 109, 6031−6044. (18) Senosiain, J. P.; Klippenstein, S. J.; Miller, J. A. Reaction of Ethylene with Hydroxyl Radicals: A Theoretical Study. J. Phys. Chem. A 2006, 110, 6960−6970. (19) Golden, D. M. The Reaction OH + C2H4: An Example of Radical Channel Switching. J. Phys. Chem. A 2012, 116, 4259−4266.

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CONCLUSIONS Thiyl radicals are extremely reactive in radical addition reactions, even in the absence of a significant thermodynamic driving force, a property that is harnessed in thiol−ene reactions and dithiuram-based self-healing polymers. While polar effects play a role in their high reactivity, herein we have shown that these cannot fully account for this behavior. Instead, we suggest that their high reactivity compared with their carbon and oxygen analogues stems from the superior ability of their high-energy unpaired electrons to undergo stabilizing resonance interactions with the π* orbital of the incoming substrate.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b00538. Further computational data including complete optimized geometries and associated total energies, entropies, enthalpies, and Gibbs free energies (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (I.D.). *E-mail: [email protected] (M.L.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS I.D. gratefully acknowledges the Scientific and Technological Research Council of Turkey (TUBITAK) under 2219 grant and Naomi Haworth for her valuable support. M.L.C gratefully acknowledges generous allocations of supercomputing time on the National Facility of the Australian National Computational E


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Comparison of Thiyl, Alkoxyl, and Alkyl Radical Addition to Double

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