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Spectroscopy and Photochemistry; General Theory

Spectral Characterization of Three-Electron Two-Center (3e-2c) Bonds of Gaseous CHS#S(H)CH and (CHSH) and Enhancement of the 3e-2c Bond on Protonation 3

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Min Xie, Zhitao Shen, Dandan Wang, Asuka Fujii, and Yuan-Pern Lee J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01491 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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The Journal of Physical Chemistry Letters

Spectral Characterization of Three-Electron Two-Center (3e-2c) Bonds of Gaseous CH3S∴S(H)CH3 and (CH3SH)2+ and Enhancement of the 3e-2c Bond on Protonation

Min Xie,*,† Zhitao Shen,† Dandan Wang,‡ Asuka Fujii,‡ and Yuan-Pern Lee*,†,§,¶

†

Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung

University, 1001, Ta-Hsueh Road, Hsinchu 30010, Taiwan, ‡Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan, §Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu 30010, Taiwan, ¶Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan. *e-mail: [email protected] (XM); [email protected] (YPL)

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ABSTRACT

The three-electron two-center (3e-2c) bond plays an important role in structures and electron communication in biological systems involving cationic sulfur compounds. Although the nature of 3e-2c bonds and their theoretical formalism have attracted great interest, direct spectral identifications of 3e-2c-bound molecules are scarce. We observed the infrared spectra of the weakly 3e-2c-bound CH3S∴S(H)CH3 and the strongly 3e-2c-bound (CH3SH)2+ in a supersonic jet using infrared (IR) dissociation with vacuum-ultraviolet photoionization and time-of-flight detection. Protonation of CH3S∴S(H)CH3 to form [CH3(H)S∴S(H)CH3]+ significantly enhances the 3e-2c bond, characterized by a large red shift of the SH-stretching band with enhanced IR intensity, a shortening of the calculated S−S distance from 3.00 to 2.86 Å, and a dissociation energy increased from ~23 to 162 kJ mol−1.

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Cationic methionine and its derivatives play important roles in protein signaling and electron-hopping paths in biological systems.1−4 These systems are thought to be stabilized directly with a three-electron two-center (3e-2c) bond and indirectly with long-range multi-center stabilization.5 These stabilizations of methionine in proteins, such as β-amyloid peptide, human prion protein, and α-synuclein, are strongly linked to the pathogenesis of some neurodegeneration (such as Alzheimer, Jacob-Creutzfeld and Parkingson) diseases.6−9 The concept of the 3e-2c bond had its origin in Pauling's valence-bond theory with reference to simple systems such as He2+ and NO.10,11 This concept was commonly invoked to describe the reactivity of some free radicals or radical cations using molecular-orbital theory.12−14 The history and qualitative theory of such an odd-electron bond were reviewed recently by Clark.15 The 3e-2c bond results from an interaction between a doubly occupied molecular orbital on one atomic center and a singly occupied molecular orbital (SOMO) on another atomic center. This interaction results in stabilization of the radical over two motifs. Asmus and coworkers have reported a 3e-2c S−S bond16−18 and 3e-2c bonds of sulfur with halogen or other atoms of various molecules in solution.19 In the gaseous phase, the 3e-2c-bound dimeric radical cation of bis(isopropyl)sulfide was characterized with ion cyclotron resonance and tandem mass spectrometry.20 Most reports on the 3e-2c S−S bond are associated with dimeric radical cations because of their stability attributed to resonance. The 3e-2c-bound species of a radical and a neutral molecule have been little investigated. The 3e-2c bonding between Cl and NH3 in a helium droplet serves as a rare example of a 3e-2c-bound atom and molecule.21 The 3e-2c-bound structure (H2S∴SH2)+ is predicted to be more stable, by 50−100 kJ mol−1, than the hydrogen-bonded structure (H2S)H+−SH with proton transfer.13,22−26 Although the 3e-2c S−S bond has been characterized with electron paramagnetic resonance, ultraviolet 3



(UV) transient absorption spectroscopy, and X-Ray crystallography16−18,23,27−29 a definitive spectral characterization of the 3e-2c bond is lacking. The presence of a 3e-2c-bound core (H2S∴SH2)+ in gaseous (H2S)n+ (n = 3−6) was reported by Wang and Fujii,22 who demonstrated that the spectral features of the free SH-stretching band of the ion core disappeared only for n = 6, indicating that the 3e-2c bond motif of the ion core is maintained up to the completion of the ﬁrst H-bonded solvation shell. However, these authors reported no spectrum of (H2S∴SH2)+. To provide a direct spectral signature of the 3e-2c S−S bond and to compare the 3e-2c bond of a radical-molecule pair and a dimeric cation, we recorded the infrared-dissociation spectra of jet-cooled CH3S∴S(H)CH3 and (CH3SH)2+. The SH-stretching band with an enhanced intensity and a significant red shift serves as a characteristic spectral signature of a 3e-2c-bound structure; the protonation of this radical-molecule pair CH3S∴S(H)CH3 to become (CH3SH)2+ enhances the 3e-2c bond significantly and is expected to play a critical role in the structural change involving the decreased S−S distance upon protonation of sulfur-containing proteins in a biological system. We produced CH3S-CH3SH on irradiation of a supersonic jet of CH3SH clusters with light at 248 nm from an excimer laser; some CH3SH was dissociated to form CH3S at 248 nm;30 experimental details are described in the Supporting Information. The VUV laser light at 118 nm, generated on tripling the output from a Nd:YAG laser at 355 nm, was employed to ionize the species in the jet. The cations thus produced passed a flight tube before being detected with a MCP detector. As compared in the time-of-flight mass spectra shown in Supporting Information, Figure S1, signals of CH3S+ (m/z = 47) and (CH3S-CH3SH)+ (m/z = 95) appeared when the excimer laser for photolysis was activated. We overlapped the counter-propagating infrared beam with the VUV beam in the ionization region and 4


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monitored the ion signal of (CH3S-CH3SH)+ while tuning the wavelength of the IR light to obtain the (depletion) spectrum of CH3S-CH3SH, as shown in Figure 1(a). The spectrum consists of three prominent bands near 2912, 2941, and 3007 cm−1 in the CH-stretching region and one weak band near 2609 cm−1 in the SH-stretching region. Two extremely weak bands near 2965 and 2983 cm−1, indicated with blue arrows, might also be present. The spectrum is compared with those of CH3S and CH3SH in Figure S2. According to this comparison, the intense bands at 2941 and 3007 cm−1 and the weak band near 2609 cm−1 (red traces) are associated with the CH3SH moiety, whereas the medium band at 2912 cm−1 and perhaps two weaker ones near 2965 and 2983 cm−1 (blue traces) are associated with the CH3S moiety. The SH-stretching band at 2609 cm−1, which is weak and blue-shifted by only 5 cm−1 from that of CH3SH,31,32 indicates that the SH bond is not hydrogen bonded and the interaction between CH3S and CH3SH is small. Possible structures of CH3S-CH3SH were explored with the GRRM program33,34 before we employed the B3LYP-D3(BJ) /aug-cc-pVTZ35 and PW6B95-D3/aug-cc-pVTZ36 methods to optimize the structures and to predict the vibrational wavenumbers and IR intensities. Two 3e-2c-bound structures (1a) and (1b) and one hydrogen-bonded structure (1c) were identified, as shown in Figure 1; critical parameters are shown in Figure S3. The two 3e-2c-bound structures (1a) and (1b) have similar energies, which is ~ 8 kJ mol−1 lower than that of the hydrogen-bonded structure (1c). With the CCSD(T) calculations at the B3LYP geometries, the relative energies of structures (1a), (1b), and (1c) are 0.0, −0.2, and −1.7 kJ mol−1, respectively, indicating that these three structures are similar in energy. The harmonic vibrational wavenumbers and IR intensities of CH3S, CH3SH, and these three structures, calculated with the B3LYP-D3(BJ) and PW6B95-D3/aug-cc-pVTZ methods are compared in Table S1. Stick spectra of three structures of CH3S-CH3SH according to IR 5



intensities and scaled harmonic vibrational wavenumbers calculated with the B3LYP-D3(BJ)/aug-cc-pVTZ and PW6B95-D3/aug-cc-pVTZ methods are presented in Figures 1(b)−1(d), Figures S4(b)−S4(d), and Table S2. The red and blue sticks in the calculated spectra represent vibrational modes associated with CH3SH and CH3S moieties, respectively. The assignment of observed bands to structure (1c) is less likely because the predicted much enhanced HS-stretching mode red-shifted by 29 cm−1 to 2580 cm−1 of the SH•••S hydrogen-bonded structure is unobserved. The predicted spectra of two 3e-2c structures (1a) and (1b) are similar and difficult to distinguish from one another. Likely both structures exist because of their similarities in energy. The HS-stretching bands of (1a) and

(1b) were predicted to be blue shifted by 6 and 9 cm−1, respectively, and the observed band is blue shifted by 5 cm−1. We hence assigned the observed spectrum to the 3e-2c-bound species, CH3S∴CH3SH. The bands observed at 2941 and 3007 cm−1 are associated with the CH-stretching modes, the band at 2609 cm−1 with the SH-stretching mode of the CH3SH moiety and the band at 2912 cm−1 with the symmetric CH3-stretching mode of the CH3S moiety. The small shift of observed HS-stretching band of CH3S∴S(H)CH3 from that of CH3SH indicates that bonding between CH3S and CH3SH is small. However, it should be noted that many factors affect the band positions and intensities in a complex system. Considering that our calculations are based on (scaled) harmonic vibrations, we cannot positively rule out the possibility that the hydrogen-bonded structure (1c) also contribute to the observed spectrum. In the same system but with the photolysis laser at 248 nm turned off, we produced (CH3SH)2+ and (CH3SH)2+-Ar. Their infrared (action) spectra, shown in Figures 2(a) and 2(b), were recorded on monitoring the loss of these ion signals while tuning the wavelength of the IR light. The spectrum of (CH3SH)2+-Ar shows spectral widths much smaller than those of 6


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(CH3SH)2+, likely because the ionized (CH3SH)2+ is internally hot relative to the Ar-tagged (CH3SH)2+. Because the perturbation of (CH3SH)2+ from Ar is expected to be small, unless noted, we use the data of (CH3SH)2+-Ar in the discussion of CH3SH dimeric cations. The spectra of (CH3SH)2+ and (CH3SH)2+-Ar are compared with those of CH3SH, CH3SH+, and (CH3SH)2 in Figure S5. The most significant difference of the observed spectra of (CH3SH)2+ and (CH3SH)2+-Ar from spectra of CH3SH, CH3SH+, and (CH3SH)2 is the much enhanced SH-stretching band near 2556 cm−1, indicating a strong interaction of the SH moieties in the cationic dimer. Three structures of (CH3SH)2+ initially explored with the GRRM program33,34 were identified: two 3e-2c-bound structures, (2a) and (2b), and one S−H•••S hydrogen-bonded structure (2c), shown in Figure 2; critical parameters are shown in Figure S3. The energies of the two 3e-2c-bound structures are within 1 kJ mol−1, whereas the energy of the hydrogen-bonded structure (2c) is ~64 kJ mol−1 greater. With the CCSD(T) calculations at the B3LYP geometries, the relative energies of structures (2a), (2b), and (2c) are 0.0, 0.5, and 52.5 kJ mol−1, indicating that the hydrogen-bonded structure is much higher in energy. The harmonic vibrational wavenumbers and IR intensities of CH3SH, CH3SH+, and these three structures, calculated with the B3LYP-D3(BJ) and PW6B95-D3/aug-cc-pVTZ methods are compared in Table S3. Stick spectra of these three structures according to IR intensities and scaled harmonic vibrational wavenumbers calculated with the B3LYP-D3(BJ) and PW6B95-D3/aug-cc-pVTZ methods are presented in Figures 2(c)−2(e), Figures S6(c)−S6(e), and Table S2. An assignment of observed bands to structure (2c) is unlikely because more complicated CH-stretching bands of two non-equivalent CH3 moieties of the S−H•••S hydrogen-bonded structure [CH3SH•••S(H)CH3]+ are expected. The predicted spectra of structures (2a) and (2b) are similar; they agree satisfactorily with experiment. Because of the similarity in energies of structures (2a) and (2b), they likely coexist. We hence assigned the 7



observed bands at 2547, 2940, and 3011 cm−1 (or 2556, 2947, and 3025 cm−1 for the Ar-tagged species) to these 3e-2c-bound structures [CH3(H)S∴S(H)CH3]+. The much enhanced band of the SH-stretching mode characterizes a strong 3e-2c interaction between two CH3SH moieties. On comparison of the experimental spectra of CH3SH, CH3SH+, and (CH3SH)2 in Figure S5, one finds that the SH-stretching band of the 3e-2c-bound (CH3SH)2+ at 2547 cm−1 is nearly the average of the corresponding wavenumbers of the SH-stretching bands of CH3SH+ and CH3SH at 2502 and 2604 cm−1, respectively, but the intensity of the 3e-2c-bound (CH3SH)2+ is much increased relative to those bands of CH3SH+ and CH3SH. The spectral signature of the 3e-2c bond in this case is hence an enhanced IR intensity and a significant shift from those of the individual molecule. The SH-stretching band of the CH3SH shifted from 2604 cm−1 to the red by 57 cm−1 to 2547 cm−1 of (CH3SH)2+; the shift is 48 cm−1 relative to (CH3SH)2+-Ar. This significant shift indicates a strong interaction between these two moieties; the nearly equivalent shifts from the bands of the SH-stretching modes of CH3SH and CH3SH+ indicate that the positive charge is nearly equally shared by these two CH3SH moieties in (CH3SH)2+. The order of the 3e-2c bond of (CH3SH)2+ is calculated to be 0.486; this 3e-2c bond can hence be called a hemi-bond. In contrast, for CH3S∴S(H)CH3, its SH-stretching band at 2609 cm−1 is blue-shifted only 5 cm−1 from that of CH3SH; neither was its intensity significantly enhanced, indicating a weak 3e-2c bond. The order of the 3e-2c bond of CH3S∴S(H)CH3 is calculated to be 0.279. The spin densities of two 3e-2c-bound structures of CH3S∴S(H)CH3, (1a) and (1b), and two 3e-2c-bound structures of [CH3(H)S∴S(H)CH3]+, (2a) and (2b), are shown in Figure S7. The spin density (indicating unpaired electrons) only appears on the two S atoms, indicating that the unpaired electron is shared between the two S atoms, even though the spin density of 8


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the S atom in CH3S is only slightly delocalized over the S atoms of CH3SH in CH3S∴ S(H)CH3, indicating a weak interaction of the 3e-2c bond. In contrast, the spin density is nearly equally delocalized over the two S atoms in [CH3(H)S∴S(H)CH3]+, indicating equal sharing of the charge between two S atoms. This delocalization is further supported by values of the integrated charge distribution of 0.5 on each CH3SH moiety in [CH3(H)S∴S(H)CH3]+. The charge distribution affects the dissociation energy, D0, corrected for basis-set superposition error and zero-point energy. Values of D0 were calculated to be 23.3 and 23.1 kJ mol−1 for structures (1a) and (1b) of CH3S∴S(H)CH3 and 161.7 and 161.2 kJ mol−1 for structures (2a) and (2b) of 3e-2c-bound (CH3SH)2+, respectively. The S−S distance ~2.86 Å of the 3e-2c-bound (CH3SH)2+ is significantly smaller than the corresponding distance, 3.00 Å, for CH3S∴S(H)CH3, indicating a much stronger 3e-2c bond of the former. The protonation of the weakly bound CH3S∴S(H)CH3 by H3O+ to form hemi-bound [CH3(H)S∴S(H)CH3]+ is a barrierless process (Figure S8). Such a protonation results in a significant enhancement of the 3e-2c binding, as is discernible from the significantly shortened S−S bond and increased bond-dissociation energy. Such an enhancement upon protonation can be rationalized with the molecular orbitals shown in Figure 3. As the singly occupied molecular orbital (SOMO) of CH3S lies at a much higher energy than the highest-occupied molecular orbital (HOMO) of CH3SH, their interaction results in little stabilization of the doubly occupied orbital (HOMO−2) in CH3S∴S(H)CH3. In contrast, upon protonation of CH3S to CH3SH+, the SOMO has an energy significantly decreased from −0.148 to −0.342 hartree; the energies of SOMO and the doubly occupied orbital (HOMO−1) in the 3e-2c-bound (CH3SH)2+ are consequently significantly smaller than those of CH3S∴ S(H)CH3, leading to a much enhanced 3e-2c bond.

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This finding of the formation of a hemi-bound S−S upon protonation is expected to have important implications for our understanding of the structural change of a protein upon protonation. Formation of 3e-2c bonds of S atom with other atoms upon protonation is also possible. Sulfur-containing amino acids cysteine and methionine are prominent targets of redox modification under oxidative stress. Cysteine and methionine are prone to one-electron paths that generate intermediary radicals or radical ions, which might react with additional amino acids in proteins, leading to secondary protein modifications. For example, the one-electron oxidation of thiols plays a role in signaling processes and in enzyme regulation.37 The oxidation of methionine at neutral pH inhibits the fibrillation of human recombinant α-synuclein, but this inhibitory effect is eliminated at small pH because α-synuclein is natively unfolded under neutral pH but partially folded under acidic conditions.38,39 The detailed mechanism of these processes has been little investigated. According to our results, the protonation of oxidized thiols in an acidic environment might lead to the formation of a S−S hemi-bond, which can alter the structure of a protein. Such a reaction mechanism deserves further investigation for the radical paths of a sulfur-containing amino acid. In summary, we have achieved a direct spectral identification of 3e-2c-bound CH3S∴ S(H)CH3 and (CH3SH)2+ in a supersonic jet and characterized the strong 3e-2c-bound species (CH3SH)2+ with a SH-stretching band of enhanced intensity and significant red-shifted from that of the unbound species; quantum-chemical calculations support these assignments. The protonation of CH3S-S(H)CH3 to form (CH3SH)2+ induces the charge-dipole interaction and significantly enhances the 3e-2c bond, as indicated by the much enhanced intensity, red shift (~50 cm−1), much decreased S−S distance, and much increased dissociation energy. This enhancement on protonation might play an important role in understanding the structural change involving a S-containing amino acid upon protonation in a biological system.

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Supporting Information Experimental and calculation methods. Figures: time-of-fight mass spectra of (CH3SH)n+ in a supersonic jet, observed IR action spectra of CH3S-CH3SH, CH3S, and CH3SH, geometries of possible conformers CH3S-CH3SH and (CH3SH)2+, comparison of observed with calculated IR spectra of CH3S-CH3SH, (CH3SH)2+, and (CH3SH)2+-Ar, distribution of spin density of CH3S∴S(H)CH3 and (CH3SH)2+, variation of related energy, dO−H and dS−S as H3O+ approaches CH3S-CH3SH. Tables: calculated harmonic vibrational wavenumbers and IR intensities of CH3S, CH3SH, CH3S-CH3SH, and (CH3SH)2+.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by Ministry of Science and Technology, Taiwan (grant No. MOST106-2745-M009-001-ASP and MOST107-3017-F009-003) and the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. The National Center for High-Performance Computation provided computer time.

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(Hemi-Bonded) Structures of (H2S)n+ Clusters in the Gas Phase. Chem. Sci. 2017, 8, 2667−2670. (23) Chaudri, S. A.; Asmus, K.-D. Experimental Evidence for the [H2SSH2]∔ Radical Cation in Solution. Angew. Chem. Int. Ed. 1981, 20, 672−673. (24) Ghanty, T. K.; Ghosh, S. K. Ab Initio CASSCF and DFT Investigations of (H2O)2+ and (H2S)2+: Hemi-Bonded vs Proton-Transferred Structure. J. Phys. Chem. A 2002, 106, 11815−11821. (25) Do, H.; Besley, N. A. Proton Transfer or Hemibonding? The Structure and Stability of Radical Cation Clusters. Phys. Chem. Chem. Phys. 2013, 15, 16214−16219. (26) Stein, T.; Jiménez-Hoyos, C. A.; Scuseria, G. E. Stability of Hemi-Bonded vs Proton-Transferred Structures of (H2O)2+, (H2S)2+, and (H2Se)2+ Studied with Projected Hartree-Fock Methods. J. Phys. Chem. A 2014, 118, 7261−7266. (27) Gauduel, Y.; Launay, T.; Hallou, A. Femtosecond Probing of a 2c/3e Disulfide Bond Making in Liquid Phase. J. Phys. Chem. A 2002, 106, 1727−1732. (28) Zhang, S.; Wang, X.; Sui, Y.; Wang, X. Odd-Electron-Bonded Sulfur Radical Cations: X-Ray Structural Evidence of a Sulfur–Sulfur Three-Electron σ-Bond. J. Am. Chem. Soc. 2014, 136, 14666−14669. (29) Berry, J. F. Two-Center/Three-Electron Sigma Half-Bonds in Main Group and Transition Metal Chemistry. Acc. Chem. Res. 2016, 49, 27−34. (30) Barone, S. B.; Turnipseed, A. A.; Gierczak, T.; Ravishankara, A. R. Quantum Yields of H (2S) and CH3S (2E) from the Photolysis of Simple Organosulfur Compounds at 193, 222, and 248 nm. J. Phys. Chem. 1994, 98, 11969−11977. (31) Fu, L.; Han, H.-L.; Lee, Y.-P.; Infrared Absorption of Methanethiol Clusters (CH3SH)n, n = 2–5, Recorded with a Time-of-Flight Mass Spectrometer Using IR Depletion and VUV Ionization. J. Chem. Phys. 2012, 137, 234307. (32) Xie, M.; Shen, Z.; Pratt, S. T.; Lee, Y.-P. Vibrational Autoionization of State-Selective Jet-Cooled Methanethiol (CH3 SH) Investigated with Infrared + Vacuum-Ultraviolet Photoionization. Phys. Chem. Chem. Phys. 2017, 19, 29153−29161. 14


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(33) Ohno, K.; Maeda, S. A. Scaled Hypersphere Search Method for the Topography of Reaction Pathways on the Potential Energy Surface. Chem. Phys. Lett. 2004, 384, 277−282. (34) Maeda, S.; Ohno, K. Global Mapping of Equilibrium and Transition Structures on Potential Energy Surfaces by the Scaled Hypersphere Search Method: Applications to ab Initio Surfaces of Formaldehyde and Propyne Molecules. J. Phys. Chem. A 2005, 109, 5742−5753. (35) Goerigk, L.; Hansen, A.; Bauer, C.; Ehrlich, S.; Najibi, A.; Grimme, S. A look at the Density Functional Theory Zoo with the Advanced GMTKN55 Database for General Main Group Thermochemistry, Kinetics and Noncovalent Interactions. Phys. Chem. Chem. Phys. 2017, 19, 32184−32215. (36) Zhao, Y.; Truhlar, D. G. Design of Density Functionals That Are Broadly Accurate for Thermochemistry, Thermochemical Kinetics, and Nonbonded Interactions. J. Phys. Chem. A 2005, 109, 5656−5667. (37) Winterbourn, C. C. Are Free Radicals Involved in Thiol-Based Redox Signaling? Free Radic. Biol. Med. 2015, 80, 164−170. (38) Uversky, V. N.; Yamin, G.; Souillac, P. O; Goers, J; Glaser, C. B; Fink, A. L. Methionine Oxidation Inhibits Fibrillation of Human α-Synuclein in Vitro. FEBS Lett. 2002, 517, 239−244. (39) Uversky, V. N.; Li, J.; Fink, A. L. Trimethylamine-N-Oxide-Induced Folding of α-Synuclein. J. Biol. Chem. 2001, 276, 10737−10744.

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Figure 1. Comparison of observed with calculated IR spectra of CH3S-CH3SH in region 2500−3100 cm−1. (a) Observed spectrum; two weak bands are indicated with blue arrows. (b)−(d) IR stick spectra of structures (1a)−(1c) according to scaled harmonic vibrational wavenumbers calculated with B3LYP-D3(BJ)/aug-cc-pVTZ. The red and blue sticks are associated with the vibrations of the CH3SH and CH3S moieties, respectively. The relative energies in kJ mol−1 calculated with the B3LYP-D3(BJ) method are listed; CCSD(T) values are listed parenthetically.

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Figure 2. Comparison of observed IR spectra of bare and Ar-tagged (CH3SH)2+ with calculated spectra of various isomers of (CH3SH)2+ in region 2400−3150 cm−1. Observed spectra of (CH3SH)2+ and (CH3SH)2+-Ar are shown in (a) and (b), respectively. (c)−(e) are IR stick spectra of structures (2a)−(2c), respectively, according to scaled harmonic vibrational wavenumbers calculated with B3LYP-D3(BJ)/aug-cc-pVTZ. The relative energies in kJ mol−1 calculated with the B3LYP-D3(BJ) method are listed; CCSD(T) values are listed parenthetically. 17



Figure 3. Molecular orbital diagram of CH3SH+, hemi-bonded (CH3SH)2+, CH3SH, CH3S∴ S(H)CH3, and CH3S. Relative energies in hartree were calculated at the B3LYP-D3(BJ)/aug-cc-pVTZ level.

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TOC. The spectrum of (CH3SH)2+ shows a SH-stretching band of significantly enhanced intensity and redshifted from that of CH3S-S(H)CH3, characterizing a strong three-electron two-center (3e-2c) bond. This enhancement of the 3e-2c bond on protonation might play an important role in the structural change of a Scontaining amino acid in a biological system. 44x44mm (300 x 300 DPI)



Comparison of observed with calculated IR spectra of CH3S-CH3SH in region 2500−3100 cm−1. (a) Observed spectrum; two weak bands are indicated with blue arrows. (b)−(d) IR stick spectra of structures (1a)−(1c) according to scaled harmonic vibrational wavenumbers calculated with B3LYP-D3(BJ)/aug-cc-pVTZ. The red and blue sticks are associated with the vibrations of the CH3SH and CH3S moieties, respectively. The relative energies in kJ mol−1 calculated with the B3LYP-D3(BJ) method are listed; CCSD(T) values are listed parenthetically. 92x104mm (300 x 300 DPI)


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Comparison of observed IR spectra of bare and Ar-tagged (CH3SH)2+ with calculated spectra of various isomers of (CH3SH)2+ in region 2400−3150 cm−1. Observed spectra of (CH3SH)2+ and (CH3SH)2+-Ar are shown in (a) and (b), respectively. (c)−(e) are IR stick spectra of structures (2a)−(2c), respectively, according to scaled harmonic vibrational wavenumbers calculated with B3LYP-D3(BJ)/aug-cc-pVTZ. The relative energy in kJ mol−1 calculated with the B3LYP-D3(BJ) method are listed; CCSD(T) values are listed parenthetically. 113x156mm (300 x 300 DPI)



Figure 3. Molecular orbital diagram of CH3SH+, hemi-bonded (CH3SH)2+, CH3SH, CH3S∴S(H)CH3, and CH3S. Relative energies in hartree were calculated at the B3LYP-D3(BJ)/aug-cc-pVTZ level. 64x50mm (300 x 300 DPI)


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3eâ2c - ACS Publications - American Chemical Society

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3eâ2c - ACS Publications - American Chemical Society