Spectroscopic Evidence for Through-Space Arene–Sulfur–Arene

Article pubs.acs.org/JPCA

Spectroscopic Evidence for Through-Space Arene−Sulfur−Arene Bonding Interaction in m‑Terphenyl Thioether Radical Cations Nicolas P.-A. Monney,† Thomas Bally,*,† Takuhei Yamamoto,‡ and Richard S. Glass*,‡ †

Department of Chemistry, University of Fribourg, CH-1700 Fribourg, Switzerland Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States

‡

S Supporting Information *

ABSTRACT: Electronic absorption spectra and quantum chemical calculations of the radical cations of m-terphenyl tert-butyl thioethers, where the S−t-Bu bond is forced to be perpendicular to the central phenyl ring, show the occurrence of through-space [π···S···π]+ bonding interactions which lead to a stabilization of the thioether radical cations. In the corresponding methyl derivatives there is a competition between delocalization of the hole that is centered on a pAO of the S atom into the π-system of the central phenyl ring or through space into the flanking phenyl groups, which leads to a mixture of planar and perpendicular conformations in the radical cation. Adding a second m-terphenyl tert-butyl thioether moiety does not lead to further delocalization; the spin and charge remain in one of the two halves of the radical cation. These findings have interesting implications with regard to the role of methionines as hopping stations in electron transfer through proteins.

I. INTRODUCTION While the role of thiols, e.g., the side-chain functional group in cysteine, in biological and chemical redox reactions has been extensively studied,1−4 this is not the case for thioethers, e.g., the side chain in methionine,5,6 despite their well-known propensity for oxidation by reactive oxygen species.7 Oxidation of this thioether side chain to a sulfoxide may be relevant to a number of diseases,8−14 aging, protection of protein active sites,15 and redox signaling. Oxidation of a methionine side chain to form a sulfilimine reinforces the collagen IV network.16−18 Of particular interest to us is the possible relevance of one-electron oxidation of methionine side chains with regard to long-range electron transfer,19−21 redox signaling, and redox pharmaceuticals. In this context electronrich groups commonly encountered in proteins dramatically affect the oxidation potential of thioethers. In model systems, neighboring S22 or O atoms,23 amides,24,25 and arenes26 have all been found to significantly lower the oxidation potential of thioethers. This is mainly due to the formation, in the radical cations, of two-center−three electron (2c, 3e) bonds27,28 between the sulfur and neighboring atoms carrying lone pairs, or [S ∴ π]+ bonds with neighboring aromatic moieties.29 Interestingly, it has been reported30 that in approximately one-third of all proteins of known structure have the methionine side chains proximate to an aromatic residue (Phe, Tyr, Trp). A structural role for this methionine−aromatic motif has been identified and a stabilization of approximately 1.5 kcal/mol beyond a hydrophobic effect has been estimated for neutral proteins. The much stronger stabilization of sulfur radical cations by neighboring arenes may be relevant to the active site of galactose oxidase. Here the oxidized sulfursubstituted tyrosine radical stacks with a nearby tryptophan © 2015 American Chemical Society

residue, and this interaction is suggested to have profound effects on the radical behavior and enzyme catalysis.31−35 We have previously reported that the oxidation potential of 2-endo-methylthio-6-endo-naphthylnorbornane is lower than that of the corresponding exo compound26 and that computational and spectroscopic analysis of the radical cation of 2-endomethylthio-6-endo-phenylnorbornane under stable ion conditions shows the formation of a novel [S ∴ π]+ bond.29 We have also observed that the electrochemical oxidation potentials of m-terphenyl thioethers 2 are lower than that of thioanisole,36 in spite of the fact that in neutral derivatives of 2 the flanking aryl substituents prevent the thioether group from being coplanar with the central phenyl ring,37 which shuts off the resonance stabilization of the thioether radical cation that prevails in parent thioanisole. Finally, the oxidation potentials of the extended S···π systems 3 were found to be nearly the same as those of compounds 1,38 which seems to indicate that the addition of another m-terphenyl moiety does not lead to further stabilization of the thioether radical cation. Although, due to steric repulsion, neutral m-terphenyl thioethers 2 prefer to adopt conformations where the S−Me bond is perpendicular to the central phenyl ring,17 the ca. 18 kcal/mol resonance stabilization gained upon oxidation39 may overcome this steric repulsion, with the result that the S−Me bond is again parallel to the central phenyl ring. To avoid these structural ambiguities, we synthesized compounds 1 where the bulky tert-butyl group forces the lone pair of the S atom to face the flanking phenyl rings and thus to form upon oxidation an [S ∴ π]+ bond with one or both of these phenyl rings, similar to Received: October 2, 2015 Revised: November 25, 2015 Published: December 4, 2015 12990

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desired temperature (between 77 and 300 K) in a thermostated liquid nitrogen-cooled cryostat (Oxford Instruments). Generation of Radical Cations in Argon Matrix. The method and equipment used for the generation and spectroscopy of radical cations in Ar matrices have been previously described.42 The compound, CH2Cl2 (electron scavenger), and argon in a 1:1:1000 ratio was deposited on a cold CsI window (20 K). The sample was then irradiated using a tungsten X-ray tube powered at 40 kV/40 mA. After ca. 90 min of irradiation, the system reaches an equilibrium where no net increase in the amount of radical cations produced is observed. Spectroscopic Measurements. Infrared spectra were recorded with 1 cm−1 resolution on a Bomem DA3 FT-IR interferometer equipped with a KBr beamsplitter and an MCT detector covering the range between 450 and 5000 cm−1. 256 scans were accumulated before the Fourier transformation. Electronic absorption spectra were recorded on a PerkinElmer Lambda 900 UV/vis/NIR spectrometer which covers a spectral range of 190−3300 nm. Photolysis. Photolyses were carried out using a 1000 W high-pressure mercury−xenon lamp. The proper irradiation regions were selected using different cutoff and interference filters. In addition, a low-pressure mercury lamp was used to perform photolyses at 254 nm. Theoretical Methods. The calculations were performed with the following program packages: Gaussian 09 (geometry optimizations, frequency calculations),43 Orca 2.8 (TDB2PLYP calculations),44 and Spartan’10 (molecular editing).45 The molecular orbitals were visualized with the MOplot program.46 Geometry optimizations were done with the B3LYP/6-31G* and B2-PLYP/cc-pVDZ functionals and basis sets. The IR simulations were also done with B3LYP/6-31G*. The electronic absorption spectra were calculated using the TD-B2-PLYP/cc-pVDZ method, which had proven to be quite reliable in our previous work on radical cations showing [S ∴ π]+ bonds.29 The energy changes in the course of the isodesmic reactions shown in Scheme 1 were calculated at the B3LYP/6-31G* level. The optimized geometry of 1•+ was used as a starting point, and that geometry was left unchanged on replacing a phenyl by a methyl group, except that the geometry of the latter was optimized in both steps. This leads of course to entirely hypothetical species, but the energy differences refer to the geometry of 1•+, which is the goal of the exercise.

the case of the 2,6-diarylphenylsilyl cations (where, however, only one [Si···π]+bond is formed).40 To gain more insight into these issues, the radical cations of compounds 1−3 were prepared under conditions where they are stable enough to be analyzed spectroscopically. The results were interpreted on the basis of quantum chemical calculations.

II. EXPERIMENTAL AND THEORETICAL METHODS Synthesis. The syntheses of the compounds used in this study are reported elsewhere.38 Generation of Radical Cations in Freon. A ca. 10−3 M solution of compounds 1−3 in a 1:1 (v/v) mixture of trichlorofluoromethane (Freon 11), CCl3F, and 1,2-dibromo1,1,2,2-tetrafluoroethane (Freon 114 B2, CF2BrCF2Br)41 is placed into homemade cuvettes with a ca. 1 mm optical path length and then frozen at 77 K in liquid nitrogen. It is then irradiated in a Gammacell 220 60Co unit, producing photons of 1.173 and 1.332 MeV, up to a dose of ca. 7 kGy. Under these conditions, only monomeric radical cations are formed. During the measurement of the electronic absorption spectra (EA) in the UV−vis−NIR region, the samples were kept at the Scheme 1

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III. RESULTS AND DISCUSSION Parent Thioanisole Radical Cation. To discuss the orientation of the thioether group relative to flanking phenyl rings of 1 to 3, parent thioanisole is taken as a reference for a planar conformation. Although the equilibrium conformation of neutral thioanisole is still under debate due to divergent experimental results,47−51 there is no doubt about the planarity of its radical cation because it profits from a delocalization of the unpaired electron into the main phenyl π-system. The electronic structure of the radical cation of thioanisole was characterized by UV−vis spectroscopy (Figure 1) and by

Table 1. Excited States of Thioanisole Radical Cation Calculated by TD-B2PLYP/cc-PVDZa energy excited state

eV

nm

oscillator strength

main excitations

coeff

1st 2nd 3rd 4th 6th

1.67 2.79 3.42 4.55 4.70

744 445 362 273 264

0.0015 0.1943 0.0017 0.0127 0.1180

32β → 33β 31β → 33β 30β → 33β highly mixed highly mixed

0.96 0.91 0.92

a

Orbital no. 33 corresponds to the SOMO.

According to TD-B2PLYP the second intense band in the UV, around 300 nm, is due to excitations from different sigma orbitals to the SOMO. The observation of these two bands is in agreement with previous pulse radiolysis experiments performed by Baciocchi et al.52 and Korzeniowska-Sobczuk et al.,53 who measured two peaks for the radical cation of thioanisole with λmax of 310 and 530 nm. Admittedly, the quantitative accord between the TDB2PYLP predictions and the experimental spectra is a bit disappointing, in view of the success of this method in predicting the spectra of radical cations where a sulfur p-AO overlaps with a phenyl ring attached to a norbornane framework.29 We therefore sought whether other excited state calculations (CASPT2, SAC-CI, or EOM-CCSD) or larger basis sets would give better results, but this turned out not to be the case; all methods strongly overestimate the energy of both the visible and the UV transition. So we stayed with the otherwise well-proven TD-B2PLYP method for the assignment of the observed spectra. m-Terphenyl tert-Butyl Sulfide 1. In compounds 1 where the flanking phenyl groups force the S−t-Bu bond to be perpendicular to the central phenyl, through-space interactions between the sulfur p orbital with one or two flanking phenyl rings should manifest themselves in the electronic absorption spectra of the radical cations. Indeed, calculations show that neutral 1a has only a single minimum on its potential energy surface where the S−t-Bu bond is exactly perpendicular to the central phenyl ring, as expected (see Figure 3). In the case of 1a•+ things turned out to be a bit more complicated because, unlike neutral 1a, its radical cation has two conformers of similar energy. One of them corresponds roughly to the neutral equilibrium conformation where the S−t-Bu bond is perpendicular to the central phenyl ring and the two flanking phenyl rings are twisted in the opposite direction, while in the other conformer these two phenyl rings are twisted in the same direction and the S−t-Bu bond forms an angle of ca. 65° with the plane of the central phenyl ring (see Figure 3). The latter conformer is 0.8 kcal/mol higher in energy by B3LYP/6-31G* but 0.1 kcal/mol lower in energy by B2PLYP/ cc-pVDZ, but as there is a small barrier separating the two minima, we may assume that the perpendicular conformer, which is formed vertically from neutral 1a, prevails after ionization. What can be seen from these calculations is that the distances between the S atom and the phenyl C atoms labeled “Co” in Figure 3 shrink on ionization (from 3.31 to 3.06 Å for conformer 1 and to 3.10 Å in conformer 2 where the distance to the C atom labeled “C′o” is 3.24 Å). Also, the alignment of the π-MOs of the phenyl rings with the p-lone pair of the S atom improves in the cations, which allows 1a•+ to profit from more [π···S···π]+ bonding.

Figure 1. Difference spectrum obtained on γ-irradiation of thioanisole in a Freon mixture (5 mM) at 77 K (black) and on X-irradiation of an argon matrix containing thioanisole (red). The bars indicate the excitation energies and transition moments of thioanisole radical cation calculated by TD-B2PLYP (Table 1).

DFT calculations (Figure 2 and Table 1). Its spectrum shows a band in the visible around 530 nm which, according to TDB2PLYP calculations, is due to the second electronic excitation (SOMO-2 → SOMO, electron promotion from the bonding to the antibonding combination of the sulfur p orbital with the π HOMO of the phenyl ring).

Figure 2. MOs of thioanisole radical cation that are involved in the main electronic excitations that appear in the EA spectrum in Figure 1. The main visible excitation is indicated with a red arrow. 12992

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conformer, which shows both a delocalization into one of the flanking phenyl rings and a small delocalization into the central one (Figure S2), shows absorption bands that are blue-shifted relative to those of the perpendicular conformer (Figure S1, turquoise bars). As the spectrum becomes more complicated on annealing, we assume that the spectrum shown in Figure 4 is due to the perpendicular conformer which is formed vertically from neutral 1a on ionization. For this case, the TD-B2PLYP calculations predict that the two observed bands are each composed of two electronic transitions of comparable intensity (see black bars in Figure 4). Similar observations, both experimentally and computationally, were made on the derivatives 1b and 1c (see Tables S1 and S2 and Figures S3−S6). The spectra of the three species look qualitatively similar and so does their electronic structure. Table 2. Excited States of 1a•+ Calculated by TD-B2PLYP/ cc-PVDZa energy excited state

eV

nm

oscillator strength

1st

0.81

1523

0.0599

2nd 3rd 4th 5th

0.90 1.36 1.69 1.79

1371 909 736 693

0.0138 0.0591 0.0095 0.0979

6th

2.08

595

0.0015

Figure 3. B3LYP/6-31G* structures of 1a and its radical cation (for discussion see text).

The electronic absorption spectrum of 1a•+ (Figure 4) shows two main bands around 1330 and 670 nm which bear indeed

a

main excitations

coeff

→ → → → → → → →

0.81 0.10 0.97 0.93 0.96 0.81 0.11 0.96

83β 80β 84β 82β 81β 80β 83β 79β

85β 85β 85β 85β 85β 85β 85β 85β

For orbitals see Figure 5 (84 = SOMO).

The first transition corresponds to a typical “charge resonance band” because it involves the promotion of an electron from the bonding (MO 83) to the antibonding combination (MO 85) of the in-plane p-AO of the S atom (which formally bears the charge and spin), and the out-ofphase combination of the π2-MOs of the flanking phenyl rings, with a slight contribution from another excited configuration (80 → 85), where the S atom is not involved. The second transition involves MO 84 which is mostly centered on the three phenyl rings. The shape of the first absorption band, which extends asymmetrically into the NIR, suggests that the order of the two above transitions is probably inverted relative to the TD-DFT predictions. The third transition corresponds in good part to a chargetransfer excitation from the flanking phenyl rings (where MO 82 is fully localized) to the S atom, while the stronger fifth one is composed of the same configurations as the first one, except that the coefficients are inverted (80 → 85 dominates, while 83 → 85 makes a minor contribution). The shape of the second absorption band, which is again slightly asymmetric toward the NIR, suggests that the third transition is closer to the fifth one than predicted by the calculations, which nevertheless allow an understanding of the electronic structure of 1•+. We tried to address the question by how much the flanking phenyl rings stabilize the S-centered radical cation, using the isodesmic reactions shown in Scheme 1: at the B3LYP/6-31G* level, replacement of the first phenyl ring by a methyl group increases the energy by 10 kcal/mol, a number that is similar to the one obtained, also by an isodesmic reaction, for the energy of interaction of an ionized sulfide with a phenyl ring juxtaposed to the former on a norbornane framework (11.8

Figure 4. Difference spectrum obtained on γ-irradiation of 1a•+ in a Freon mixture (5 mM) at 77 K (black) and on X-irradiation of an argon matrix containing 1a (red, the wiggles are due to interferences). The black bars indicate the main excitation energies and transition moments of 1a•+ calculated by TD-B2PLYP (Table 2, MOs in Figure 5).

no relation to those of the radical cation of thioanisole (Figure 1). However, on annealing the samples containing ionized 1a we observed slight changes in the spectrum that point toward a conformational relaxation (Figure S1), which could very well correspond to the population of the second, new minimum of 1a•+. Indeed, TD-B2PLYP calculations predict that this 12993

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Figure 5. MOs of 1a•+ that are involved in the main electronic excitations that appear in the EA spectrum in (cf. Table 2).

Figure 6. Difference IR spectra after 2 h photolysis at 254 nm of a matrix containing 1a (green) or 1b (red) with calculations on the complex formed during photolysis of 1a (black) and 1b (blue) compared to the calculated spectrum of isobutylene (purple).

kcal/mol).29 However, in the present case removing a phenyl ring does not only interrupt the [S ∴ π]+ interaction, but also what is left of the conjugation of the flanking with the central phenyl ring, so the stabilization energy is probably a bit lower than the number that results from the first isodesmic reaction. Interestingly, replacing the second phenyl ring by a methyl group entails a much larger increase in energy, even though the geometry of 1•+ was frozen, so the second [S ∴ π]+ interaction should be similar to the first one. However, inspection of the MOs of the product with one Me group shows that the coefficients on the S and the remaining phenyl ring in the SOMO are much larger than in 1•+, which may explain why the interaction with the second phenyl ring becomes much stronger once the first one is removed. In any event, replacing both phenyl rings by methyl groups (while maintaining the geometry of 1•+) increases the energy by almost 38 kcal/mol, which indicates that the stabilization of the central ionized sulfide unit by the two flanking phenyl rings is substantial, even if one subtracts the contribution from the conjugation between the flanking and the central phenyl rings (which is difficult to estimate). In addition to the presence of a second conformer, a side product is formed upon X-radiolysis, and also upon photolysis at 254 nm, which was identified in the IR spectra of 1a•+ and 1c•+ (Figure 6). This species is associated with peaks at 887, 1055, 1377, 1443, 1461, and 1657 cm−1, which are all very close to the major IR bands of isobutylene previously measured in an argon matrix.54 Thus, it appears that isobutylene is formed on irradiation of compounds 1. It is known55,56 that flash vacuum pyrolysis or irradiation of 3-tert-butylthio-2-propenenitrile affords cyanoethenethiol (and therefore presumably also isobutylene). Consequently, the analogous reaction shown in Scheme 2 is proposed to account for the formation of the alkene, which explains the occurrence of the additional IR bands observed. The exact mechanism of this fragmentation on

Scheme 2. Formation of isobutylene on Photolysis of 1 Giving Complex 4

X-irradiation in Ar is not known, but perhaps it also involves excited states of 1. As the two fragments are trapped in the same matrix cavity, they form complex 4. Interestingly, the accord between the calculated and the observed pattern of IR bands is significantly better when the calculations are done on this complex instead of simply adding up the spectra of the individual isolated components. This concerns, for example, the IR band of isobutylene observed at 1278 cm−1 in Ar, which is shifted and whose intensity diminishes in the complex. Fortunately, these photoproducts do not interfere with the visible and NIR spectrum of 1a•+ to 1c•+. m-Terphenyl Methyl Sulfide (2). Armed with the knowledge that the radical cation of thioanisole adopts a planar conformation to benefit from the delocalization of the sulfur p-orbital with the phenyl π-system, and that the spectrum of 1a•+ obtained just after the radiolysis corresponds to a perpendicular conformation that profits from a through-space [π···S···π]+ interaction, it is interesting to look at the behavior of 2, where the t-Bu group is replaced by a methyl group, upon radiolysis. Unlike 1a, neutral 2 can adopt the two different conformations that are similar to those of the radical cation 12994

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The Journal of Physical Chemistry A 1a•+ (Table 3). Considering that the two conformers are close in energy, they should both be present when the sample is Table 3. Conformational Distribution of 2 and 2•+

a

Figure 7. Difference spectrum obtained on γ-irradiation of 2 in a Freon mixture (5 mM) at 77 K (black) and on X-irradiation of an argon matrix containing 2 (red). The bars indicate the main excitations energies and transition moments of 2A•+ perpendicular (black), 2B•+ twisted (green), and 2C•+ parallel (blue) calculated by TD-B2PLYP (Tables S4−S6; orbitals: Figures S7−S9). The spectra of thioanisole radical cation (blue dots) and 1a•+ (black dots) are displayed for comparison.

B3LYP/6-31G*. bB2PLYP/cc-pVDZ.

sublimed to be incorporated into an argon matrix or when 2 is dissolved in a Freon mixture. That means that the spectra of 2•+ should contain the traces of at least two conformers. In fact, DFT calculations predict three conformers for 2•+ (see graphics in Table 3). The first two arise almost vertically from the ionization of the two neutral conformers that we call perpendicular (2A•+) and twisted (2B•+) while the third one is formed upon rotation of the thioether group into near coplanarity with the central phenyl ring (2C•+). The latter two conformers are of very similar energy, which illustrates the competition between S-π conjugation and [π···S···π]+ interaction in the radical cations. As seen in Figure 7, three main absorption bands at 520, 730, and 1350 nm are observed, both in a Freon glass and in an argon matrix. According to TD-B2PLYP calculations, the band at 730 nm is due to the perpendicular conformer (2A•+), but the band at 520 nm, which is indeed comparable to that observed for the planar thioanisole radical cation (blue dotted line in Figure 7), is probably due to conformer 2C•+, possibly with some contribution from 2B•+ (all three conformers absorb at 1350 nm; see Tables S4−S6). This is also explained by the orbital interactions that occur in the different conformers. The orbitals of 2A•+ are similar to those of 1a•+ and 2B•+ and has orbitals that look like those of the twisted conformer of 1a•+ and 2C•+ which show similar orbitals to those of the radical cation of thioanisole (Figures S7−S9). Even though the absorptions of 2B•+ cannot be distinguished from those of 2C•+, the spectrum of 2•+ can then be interpreted as a superposition of the spectra of the radical cation of thioanisole and of 1a•+. Regarding the relative intensities of the main absorption bands, the stabilization of the radical cation by resonance with the central aromatic ring

clearly overcomes the steric repulsion between the methyl group and the flanking phenyl rings. Interestingly, the relative population of the different conformers can be varied by changing the sublimation temperature of the sample during argon matrix experiments (Figure 8). Both IR and UV−vis−NIR spectra obtained during two different experiments show the same peaks with different relative intensities. By subliming the sample at 80 °C instead of 60 °C, the intensity of the main IR peak of 2A•+ (1570 cm−1) is lowered, and so is its corresponding visible absorption band at 730 nm (the assignment of the IR peaks can be found in Figure S10). This means that the population of 2B, the higher energy neutral conformer, is enhanced by increasing the temperature. As a consequence, more radical cations with the 2B•+ geometry are produced in the matrix. Since in the perpendicular conformation of 1a•+ there is delocalization involving through space interaction of the sulfur p-orbital and π MO of both o-phenyl rings, the next question is whether this interaction could be further extended by adding another m-terphenyl moiety. Recently, Sun et al.57 have proposed a related type of system where two thioether moieties are placed on either side of a phenyl ring. DFT calculations showed that the spin delocalizes over all three moieties, yielding what Sun et al.57 call a [S:π∴S]+ bond. Bis-m-terphenyl Compound 3. Thus, we synthesized compound 3, where the above type of bonding, involving the central phenyl ring, could be realized or where alternatively an 12995

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Figure 10. SOMO of 3•+ calculated with B3LYP/6-31G* (left-hand side) and UHF/6-31G* (right-hand side).

Figure 8. Difference IR spectra (top) and EA spectra (bottom) of 2•+ obtained after 1 h 30 min X-irradiation of an argon matrix containing 2 deposited after sublimation at 80° (black curves) and at 60° (red curves).

and S12), presumably due to the lack of dynamic correlation energy driving this delocalization. Thus, we conclude from Figure 9 that in 3•+ spin and charge are largely localized in one of the two m-terphenyl units, which also explains why the oxidation potentials of 2 and 3 are very similar.

even more extended [π···S···π···S···π]+ bond could form. To resolve these issues, 3•+ was generated in a nonpolar frozen solvent (because 3 cannot be sublimed) and investigated spectroscopically. The electronic absorption spectrum of 3•+ is shown in Figure 9 and compared with that of 1a•+ (dotted line), illustrating that

IV. CONCLUSION After having shown that the stabilization of a thioether radical cation that faces an aryl group expresses itself in a clear signature in the electronic absorption spectrum of this radical cation, we went on to study whether a thioether radical cation can profit from simultaneous through-space interaction with two aryl groups. Toward this end we synthesized m-terphenyl tert-butyl thioethers where the bulky alkyl group forces the S−tBu bond to be perpendicular to the central phenyl ring, so that the sulfur p-SOMO of the radical cation is properly oriented to interact with the two flanking phenyl rings which leads to substantial stabilization of that cation. These radical cations show indeed very different spectra than that of thioanisole, where the S−Me bond is coplanar with the central phenyl ring, which allows the system to benefit from resonance stabilization. The electronic transitions seen in these spectra are analyzed in terms of the orbitals that are involved in the excitations. Interestingly, in Ar matrices the X-irradiation that is used to ionize the substrates or UV photolysis leads to the elimination of isobutylene from the tert-butyl thioethers. When the t-Bu group is replaced by a Me group, the mterphenyl thioether radical cations regain the possibility to profit from resonance stabilization by adopting, despite steric constraints, a conformation where the S−Me group is coplanar with the central phenyl ring. The spectra of the corresponding radical cations can be interpreted as being a superposition of the spectra of the “planar” and the “perpendicular” conformations of the thioethers. These results suggest that methionine side chains in proteins which are properly oriented (geometry and distance) between two aromatic side chains are more susceptible to oxidation than methionines with only one nearby aromatic moiety. This may render proteins with such features more susceptible to oxidative damage, facilitate redox signaling, and promote long-range

Figure 9. Difference EA spectra of the 3•+ measured in Freon after γirradiation (black curve) compared to the spectrum of 1a•+ measured in Freon after γ-irradiation (blue dotted curve).

they are very similar, with an intense band at 700 nm and a broad band at ca. 1500 nm. This seems to indicate that the radical cation of 3 is delocalized over only one of the two π-S-π systems and not over both. However, DFT calculations predict the SOMO to be fully delocalized over both m-terphenyl units (Figure 10, top). TD-DFT calculations (performed on the three lowest energy conformers of 3•+) predict an intense absorption band at ca. 2800 nm that could not be found experimentally (see Table S7). But then, DFT methods are known to overdelocalize the spin in radical ions,58 so we subjected 3•+ to an UHF/6-31G* calculation. Here the SOMO shows localization of the spin in one of the two m-terphenyl moieties, but it also shows very little π-S-π delocalization (Figure 10, bottom; the full MOs, calculated by B3LYP and by UHF, are shown in Figures S11 12996

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The Journal of Physical Chemistry A

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electron transfer in which these delocalized species serve as hopping sites. Adding a second m-terphenyl moiety does not change the spectrum of the radical cation, which indicates that the system prefers not to profit from delocalization of spin and charge over both m-terphenyl moieties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b09665. Complete refs 43 and 45, additional spectra, orbitals and excited states for 1b•+, 1c•+, 1a•+ (twisted conformation), 2•+, and 3•+ (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(T.B.) E-mail: [email protected]. *(R.S.G.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support of this work by the Swiss National Foundation SNF (Grant 200020143410) and the U.S. National Science Foundation NSF (Grant 0956581).

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REFERENCES

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DOI: 10.1021/acs.jpca.5b09665 J. Phys. Chem. A 2015, 119, 12990−12998

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