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Caged Nitric Oxide–Thiyl Radical Pairs Zhuang Wu, Changyun Chen, Jie Liu, Yan Lu, Jian Xu, Xiangyang Liu, Ganglong Cui, Tarek Trabelsi, Joseph S. Francisco, Artur Mardyukov, Andre K. Eckhardt, Peter R. Schreiner, and Xiaoqing Zeng J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 13, 2019

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Journal of the American Chemical Society

Caged Nitric Oxide–Thiyl Radical Pairs Zhuang Wu,¶,+ Changyun Chen,¶,+ Jie Liu,¶Yan Lu,¶Jian Xu,¶Xiangyang Liu,‡ Ganglong Cui,‡,* Tarek Trabelsi,§ Joseph S. Francisco§,* Artur Mardyukov,# AndréK. Eckhardt,# Peter R. Schreiner,# and Xiaoqing Zeng¶,* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China. ‡Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China. §Department of Earth and Environmental Science and Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States. #Institute of Organic Chemistry, Justus Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen, Germany. ¶

Supporting Information Placeholder ABSTRACT: S-Nitrosothiols (RSNO) are exogenous and endogenous sources of nitric oxide in biological systems due to facile homolytic cleavage of the S–N bonds. By following the photolytic decomposition of prototypical RSNO (R = Me and Et) in Ne, Ar, and N2 matrices (< 10 K), elusive caged radical pairs (RPs) consisting of nitric oxide (NO•) and thiyl radicals (RS•), bridged by O···S and H···N connections, have been identified with IR and UV/Vis spectroscopy. Upon red-light irradiation, both caged radical pairs (RS•···•ON) vanish and reform RSNO. According to the calculation at the CASPT2(10,8)/cc-pVDZ level (298.15 K), the dissociation energy of MeS•···•ON amounts to 4.7 kcal mol–1.

Radical pairs (RPs) are fleeting intermediates generated in a myriad of photolytic, thermolytic, and radiolytic decomposition reactions, during which either a bond is homolytically cleaved or two spin-correlated radicals encounter one another.[1] Nitric oxide (NO•) is a radical that plays essential roles in biological processes such as transmitting cellular signals in the nervous system and fighting against infections in the immune system.[2] As one class of reactive sulfur species, S-nitrosothiols (RSNO)[3] are important exogenous and endogenous sources of NO• in living systems due to facile homolytic cleavage of the S–N bond (eq 1), in which the nitric oxide–thiyl radical pairs (R–S•···•NO) can be involved. Emerging evidences suggest that NO• signals primarily through the formation of RSNO from S-nitrosylation of proteins (SNOproteins), which entails reactions of thiols with the nitrosonium cation (eq 2).[4] Alternatively, SNO-proteins can form through the recombination of NO• with thiyl radicals (R–S•) via the intermediacy of R–S•···•NO (eq 3),[5] since R–S• can be readily generated from one-electron oxidation of thiols.[6] Dissociation of RSNO and the reverse radical recombination also involve in the atmospheric oxidation of volatile organic sulfur compounds in the presence of nitrogen oxides.[7] 2 RSNO → 2 [R–S•···•NO] → 2 NO• + RSSR (1) RSH + HNO2 → RSNO + H2O (2) RS• + NO• → [R–S•···•NO] → RSNO (3) Although NO• release from RSNO has been intensively explored,[8] the radical pairs R–S•···•NO were not considered in these studies. In contrast, the related radical pair H–O•···•ONO, an

intermediate in the decomposition of peroxynitrous oxide HO– ONO in physiological media, has been thoroughly studied.[9] Rebinding R–S• and NO• can also yield isonitrosyl isomer RS–ON, which was computationally located as a true minimum with a long S–O bond length (ca. 2.3 Å).[10] Indeed, formation of X–ON (X = H,[11] Cl and Br,[12] CN,[13] and HO[14]) from the association of NO• with X• has been observed. Bonding analyses suggest that HO–ON, bearing the longest O–O bond (1.9149±0.0005 Å),[14] can be best represented by mixed resonance structures of a radical pair (H–O•···•ON) and a nitrene-like zwitterion (H–O–O+=N–).[15] In contrast, H–ON[11] and Me–ON [16] are triplet oxynitrenes. It is known that weakly interacting complexes can be isolated in cryogenic matrices,[17] herein, we report the observation of caged radicals pairs RS•···•ON (R = Me and Et), linked by O···S and H···N interactions, in the decomposition of RSNO in solid Ne, Ar, and N2 matrices. The IR and UV/Vis spectroscopic data and computational results suggest that both are singlet species and interconvert with RSNO under irradiation conditions. Generally, S-nitrosothiols display two absorptions in the regions of 330–350 (nO → π*) and 550–600 nm (nN → π*).[8e] Visible light irradiation of MeSNO mainly results in conformational changes.[18] In our experiments, a 365 nm UV-lamp was used, whose irradiation was found to effectively break the S–N bond in the gas phase (Figures S1 and S2). A typical IR difference spectrum showing the change MeSNO in Ar-matrix upon irradiation is shown in Figure 1A. Depletion of MeSNO occurs, and a new species with a strong IR band at 1820.9 cm–1 forms. It is redshifted by 50.5 cm–1 compared to free NO• (1871.4 cm–1),[12] which is also produced in trace amount (Figure 1C). The counterpart MeS• was not detected due to low IR intensities.[19] The frequency of 1820.9 cm–1 is close to that for the N–O stretching vibration in Br–ON (1820.0 cm–1).[12] The assignment to an N–O stretching vibration is supported by a 15N-isotopic shift of 34.6 cm–1 (Figure 1I). Moreover, a weak IR band at 3606.6 cm–1 with a 15N-isotopic shift of 60.7 cm–1 for its overtone is observed. Additionally, several weak IR bands (Figure 1C) appear in the ranges of 3100–2900, 1400–1200, and 1000–900 cm–1, which are close to those of MeSNO for the stretching, deformation, and rocking vibrations of the CH3 group. One band at 743.0 cm–1 belongs to the same carrier, and it is close to the C–S stretching vibration in MeSNO (cis: 731.3 cm–1; trans: 736.7 cm–1).

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Figure 1. (A) IR difference spectrum (res. 0.25 cm–1) reflecting the generation of syn MeS•···•ON (b) from the 365 nm irradiation (10 min) of MeSNO (a: syn; a': anti) in Ar-matrix (10.0 K); (B) IR difference spectrum reflecting the reverse conversion upon subsequent 830 nm irradiation (10 min); (C–F) Parts of the expanded IR difference spectra. (G–J) Parts of the expanded IR spectra reflecting the shifts of the selected IR bands upon 15N-labeling. The IR band of NO is labeled with c. Like isonitrosyl halides X–ON (X = Cl and Br),[12] this new species is light-sensitive, even the irradiation from the IR spectrometer can lead to 90% depletion in one hour, and the depletion can be significantly enhanced by using red-light (830 nm). As a result, MeSNO reforms (Figure 1B). Unlike the quantum mechanical tunneling (QMT) in the X–ON → X–NO (X = Cl and Br) transformation (8.5–25 K),[12,20] no noticeable changes occur when the matrix containing the photolysis products of MeSNO is either kept in the dark (10 K) or warmed up 25 K. The reproducible photoisomerization with MeSNO in different matrices (Figures S3-S4) allows unambiguous identification of all the IR bands for this new species (4000–550 cm–1, Table S1). Among these IR bands, a weak one at 2603.8 cm–1 (Figure 1H) distinguishes itself from the other two (2939.4 and 2836.0 cm–1) for the C–H stretching vibrations by showing a shift (Δ(14/15N)) of 0.3 cm–1 upon 15N-isotope labeling. This shift renders an assignment to either a combination with the strongest band at 1820.9 cm–1 (Δ(14/15N) = 34.6 cm–1) or an overtone of the IR fundamental at 1317.6 cm–1 (Δ(14/15N) = 1.0 cm–1, Figure 1J) unlikely. The frequency agrees with the calculation at 2679 cm–1 (CCSD(T)F12/VTZ-F12) for the C–H stretching vibration in MeS•···•ON. Therefore, the involved hydrogen atom should be structurally nonequivalent with the other two in the methyl group since the other two IR bands display negligible 15N-isotopic shifts (Figure 1G). The photosensitivity of MeS•···•ON towards red light is consistent with the calculated vertical transition at 779 nm (Table S2). Due to its low intensity (oscillator strength f = 0.0008), this transition could not be observed in the UV/Vis spectra for the 365 nm photolysis products of MeSNO in solid Ar-matrix (Figure S5). Whereas, the predicted strong absorption at 264 nm (f = 0.4155) for MeS•···•ON appears at 268 nm. By analogy, EtSNO isomerizes to EtS•···•ON upon UV light irradiation (365 nm, Figure 2A), and the reverse conversion occurs under the red-light irradiation (Figure 2B). In Ar-matrix, the N–O stretching vibration appears at 1821.8 cm–1 (N2-matrix: 1828.2 cm–1; Ne-matrix: 1827.3 cm–1, Figures S6-S7), which differs from that of MeS•···•ON by less than 2 cm–1. This strongest band shows 15N-shift of 31.8 cm–1 (Figure 2E), and its overtone at 3163.2 cm–1 exhibits a shift of 62.7 cm–1. In addition to the bands for the ethyl group in the region of 3100–1200 cm–1, a weak band at 720.1 cm–1 for the C–S stretching mode can be assigned (Table S3).

Figure 2. (A) IR difference spectrum (res. 0.5 cm–1) reflecting the generation of EtS•···•ON (d) from the 365 nm irradiation of EtSNO (d) in Ar-matrix (10.0 K); (B) IR difference spectrum reflecting the reverse conversion upon subsequent 830 nm; (C–D) Parts of the expanded IR difference spectra. (E) Part of the expanded IR spectra reflecting the shifts of the selected IR band upon 15N-labeling. The band for an unknown species is marked with an asterisk. According to the X–NO → X–ON (X = Cl and Br)[12] and R–SO• → R–OS• (X = CF3 and Ph) photoisomerization,[21] formation of RS•···•ON from RSNO might occur through a mechanism of homolytic dissociation (→ RS• + •NO) followed by radical recombination. To capture the free radicals, chemical trapping with O2 was performed by photolyzing MeSNO in O2-doped Ar-matrix, however, the MeSNO ↔ MeS•···•ON interconversion was not perturbed (Figures S8-S9). In line with the formation of NO•, a weak IR band at 1102.3 cm–1 with 18O-isotopic shift of 60.0 cm–1 for MeSOO•[22] was observed. Therefore, the initially generated MeS• and •NO radicals can hardly escape from the matrix cages but recombine to form MeS•···•ON. The energy profile for the interconversion between MeSNO and MeS•···•ON was computationally explored (Figure 3). The S–N


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Journal of the American Chemical Society bond energy in syn MeSNO is 22.7 kcal mol–1, higher than the barrier for conformational conversion (TS1, 10.8 kcal mol–1) but identical with that for the isomerization to anti MeS•···•ON (TS2). Anti MeS•···•ON is unstable due the low dissociation energy and small barriers for reforming syn MeSNO (TS2) and the transformation to syn MeS•···•ON (TS3). Syn MeS•···•ON is slightly more stable than the anti and, importantly, its kinetic stability can be inferred by the moderate barrier (TS4, 6.4 kcal mol–1) for the isomerization to anti MeSNO. It should be noted that the singlereference B3LYP and M06-2X and coupled cluster CCSD(T) methods successfully reproduced the molecular structures for MeS•···•ON but failed in predicting the relative stability, since MeS•···•ON was calculated to be higher in energy than the two free radicals. This could be caused by the more significant multireference character of MeS•···•ON than MeSNO (Tables S4-S7). In the former, two electronic configurations contribute comparably; whereas, in the latter, the closed-shell electronic configuration plays a major role. Structurally, the S–O bond lengths in anti and syn MeS•···•ON (3.060 and 2.528 Å) are significantly longer than the sum of the single-bond covalent radii (1.66 Å)[23] but shorter than the sum of the van der waals radii (3.32 Å).[24] Stabilizing hydrogen-bonding interaction between MeS• and NO• in syn MeS•···•ON is evidenced by the short C–H···O=N contact (2.199 Å), which allows the formation of a planar five-membered ring structure. However, the C–H···O=N hydrogen bond in anti MeS•···•ON (2.604 Å) is close to the sum of the van der waals radii (2.62 Å).[24] The two forms of MeS•···•ON resemble the biradicaloid HO•···•ONO, for which similar five-membered ring structure bearing O–H···O=N contact (2.446 Å) and a weak O–O bond (2.216 Å) was calculated.[9b] The weak interaction between MeS• and NO• in syn MeS•···•ON is also evident from the characteristic highest occupied molecular orbital (HOMO), which consists of in-plane overlap of the sulfur p orbital (lone-pair) from MeS• with the antibonding π orbital (π*) from NO•.

Figure 3. Zero-point energy corrected potential energy profile (298.15 K) for MeSNO isomers calculated at the CASPT2(10,8)/cc-pVDZ level. The relative energies (in bold) and selected bond lengths (Å, in italics) are depicted. In conclusion, radical pairs RS•···•ON (R = Me and Et) have been trapped in cryogenic Ne, Ar, and N2 matrix cages during the photolytic decomposition of S-nitrosothiols (RSNO). IR spectroscopic data and high-level CASPT2 calculations conclusively suggest the presence of stabilizing C–H···O=N hydrogen-bonding interaction in RS•···•ON. Radical pairs are photolabile and rearrange to RSNO upon red-light irradiation in matrices. The moderate barriers (ca. 5 kcal mol–1) associated with the dissociation and rearrangement reactions of RS•···•ON imply that these intermediates might be also involved in the biochemistry of RSNO in the solvent cages of physiological media.

ASSOCIATED CONTENT Supporting Information

Experimental details, theoretical methods, observed and calculated IR and UV/Vis spectra, calculated vertical transition energies, atomic coordinates, and total energies, including Figures S1−S15 and Tables S1−S7 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors

*X.Q.Z.: [email protected] *G.L.C.: [email protected] *J.S.F.: [email protected] Authors Contributions +Z.W.

and C.Y.C. contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21673147), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_2493).

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127, 3140–3155. (c) Gunaydin, H.; Houk, K. N. Molecular Dynamics Simulation of the HOONO Decomposition and the HO•/NO2• Caged Radical Pair in Water. J. Am. Chem. Soc. 2008, 130, 10036–10037. (10) Yi, J.; Coppens, P.; Powell, D. R.; Richter-Addo, G. B. Linkage Isomerization in Nitrosothiols (RSNOs): The X-ray Crystal Structure of an S-Nitrosocysteine and DFT Analysis of its Metastable MS1 and MS2 Isomers. Comments Inorg. Chem. 2016, 36, 81–91. (11) Maier, G.; Reisenauer, H. P.; Marco, M. D. Isonitroso Hydrogen (Hydroxy nitrene, HON). Angew. Chem. Int. Ed. 1999, 38, 108–110. (12) Maier, G.; Reisenauer, H. P.; Marco, M. D. Isomerizations between Nitrosyl Halides X−N=O and Isonitrosyl Halides X−O−N: A Matrix-Spectroscopic Study. Chem. Eur. J. 2000, 6, 800–808. (13) Maier, G.; Reisenauer, H. P.; Eckwert, J.; Naumann, M.; Marco, M. D. Isomers of the Elemental Composition CN2O. Angew. Chem. Int. Ed. Engl. 1997, 36, 1707–1709. (14) Crabtree, K. N.; Talipov, M. R.; Martinez; O. Jr.; O'Connor, G. D.; Khursan, S. L.; McCarthy, M. C. Detection and Structure of HOON: Microwave Spectroscopy Reveals an O–O bond Exceeding 1.9 Å. Science, 2013, 342, 1354–1357. (15) (a) Takeshita, T. Y.; Dunning, T. H. Jr. Generalized Valence Bond Description of Chalcogen–Nitrogen Compounds. III. Why the NO–OH and NS–OH Bonds are So Different. J. Phys. Chem. A 2016, 120, 6846– 6850. (b) Talipov, M. R.; Timerghazin, Q. K.; Safiullin, R. L.; Khursan, S. L. No Longer a Complex, Not Yet a Molecule: A Challenging Case of Nitrosyl O-Hydroxide, HOON. J. Phys. Chem. A 2013, 117, 679–685. (16) Wasylenko, W. A.; Kebede, N.; Showalter, B. M.; Matsunaga, N.; Miceli, A. P.; Liu, Y. L.; Ryzhkov, L. R.; Hadad, C. M.; Toscano, J. P. Generation of Oxynitrenes and Confirmation of Their Triplet Ground States. J. Am. Chem. Soc. 2006, 128, 13142–12150. (17) For examples, see: (a) Sander, W.; Roy, S.; Polyak, I.; RamirezAnguita, J. M.; Sanchez-Garcia, E. The Phenoxyl Radical–Water Complex—A Matrix Isolation and Computational Study. J. Am. Chem. Soc. 2012, 134, 8222–8230. (b) Henkel, S.; Costa, P.; Klute, L.; Sokkar, P.; Fernandez-Oliva, M.; Thiel, W.; Sanchez-Garcia, E.; Sander, W. Switching the Spin State of Diphenylcarbene via Halogen Bonding. J. Am. Chem. Soc. 2016, 138, 1689–1697. (18) Müller, R. P.; Hube, J. R. Two Rotational Isomers of Methyl Thionitrite: light-induced, Reversible Isomerization in An Argon Matrix. J. Phys. Chem. 1984, 88, 1605–1608. (19) Bahou, M.; Lee, Y. -P. Diminished Cage Effect in Solid p-H2: Infrared Absorption of CH3S Observed from Photolysis in Situ of CH3SH, CH3SCH3, or CH3SSCH3 Isolated in p-H2 Matrices. J. Chem. Phys. 2010, 133, 164316. (20) (a) Schreiner, P. R. Tunneling Control of Chemical Reactions: The Third Reactivity Paradigm. J. Am. Chem. Soc. 2017, 139, 15276–15283. (b) Ley, D.; Gerbig, D.; Schreiner, P. R. Tunnelling Control of Chemical Reactions–The Organic Chemist's Perspective. Org. Biomol. Chem. 2012, 10, 3781–3790. (21) (a) Wu, Z.; Xu, J.; Deng, G. H.; Chu, X. X.; Sokolenko, L.; Trabelsi, T.; Francisco, J. S.; Eckhardt, A. K.; Schreiner, P. R.; Zeng, X. Q. The Trifluoromethyl Sulfinyl and Oxathiyl Radicals. Chem. Eur. J. 2018, 24, 1505–1508. (b) Xu, J.; Wu, Z.; Wan, H. B.; Deng, G. H.; Lu, B.; Eckhardt, A. K.; Schreiner, P. R.; Trabelsi, T.; Francisco, J. S.; Zeng, X. Q. Phenylsulfinyl Radical: Gas-phase Generation, Photoisomerization, and Oxidation. J. Am. Chem. Soc. 2018, 140, 9972−9978. (22) Chu, L. -K.; Lee, Y. -P. Transient Infrared Spectra of CH3SOO and CH3SO Observed with A Step-Scan Fourier-Transform Spectrometer J. Chem. Phys. 2010, 133, 184303. (23) Pyykkö, P.; Atsumi, M. Molecular Single−Bond Covalent Radii for Elements 1−118. Chem. Eur. J. 2009, 15, 186−197. (24) Mantina, M.; Chamberlin, A. C.; Valero, R.; Cramer, C. J.; Truhlar, D. G. Consistent van der Waals Radii for the Whole Main Group. J. Phys. Chem. A 2009, 113, 5806−5812.


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(A) IR difference spectrum (res. 0.25 cm–1) reflecting the generation of syn MeS•···•ON (b) from the 365 nm irradiation (10 min) of MeSNO (a: syn; a': anti) in Ar-matrix (10.0 K); (B) IR difference spectrum reflecting the reverse conversion upon subsequent 830 nm irradiation (10 min); (C–F) Parts of the expanded IR difference spectra. (G–J) Parts of the expanded IR spectra reflecting the shifts of the selected IR bands upon 15N-labeling. The IR band of NO is labeled with c. 503x242mm (300 x 300 DPI)


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(A) IR difference spectrum (res. 0.5 cm–1) reflecting the generation of EtS•···•ON (d) from the 365 nm irradiation of EtS-NO (d) in Ar-matrix (10.0 K); (B) IR difference spectrum reflect-ing the reverse conversion upon subsequent 830 nm; (C–D) Parts of the expanded IR difference spectra. (E) Part of the expanded IR spectra reflecting the shifts of the selected IR band upon 15N-labeling. The band for an unknown species is marked with an asterisk. 258x197mm (300 x 300 DPI)



Zero-point energy corrected potential energy profile (298.15 K) for MeSNO isomers calculated at the CASPT2(10,8)/cc-pVDZ level. The relative energies (in bold) and selected bond lengths (Å, in italics) are depicted.


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