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Irek Janik, G.N.R. Tripathi, Ian Carmichael

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1 Irek Janik, G.N.R. Tripathi, Ian Carmichael
TRANSIENT RAMAN SPECTRA, STRUCTURE AND THERMOCHEMISTRY OF THE THIOCYANATE DIMER RADICAL ANION IN WATER Irek Janik, G.N.R. Tripathi, Ian Carmichael Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46655, USA

2 Motivation for (SCN)2- studies
(SCN)2- serves as a dosimeter in pulse radiolysis (SCN)2- is used in competition kinetics to determine OH reactivity with other solutes (SCN)2- is one of the simplest models of small symmetric hemibonded intermediates SCN- (pseudo-halide) redox transformations model redox transformation in halides SCN- is the strongest water structure breaker in the Hofmeister series of mono anions

3 Pulse radiolysis setup with optical detection
Pulse of high-energy electrons Transient Absorption monochromator R• Detector: PMT Photodiode cell time Oscilloscope

4 OH-radical induced oxidation mechanism of thiocyanate, SCN-

5 Time resolved resonance Raman studies of (SCN)2- in water

6 Pioneering studies on resonance Raman of (SCN)2-
R. Wilbrandt, H. Jensen, P. Pagsberg, H. Sillesen,B. Hansen, E. Hester, Chem Phys Lett 60 (1979) 315 R. Rossetti, S. M. Beck, and L. E. Brus, JACS, 106 (1984) 981

7 Two-center three-electron bonds (hemibonds)

8 RR of (SCN)2- at lower spectral range
When deriving the Raman effect, it is generally easiest to start with the classical interpretation by considering a simple diatomic molecule as a mass on a spring where m represents the atomic mass, x represents the displacement, and K represents the bond strength. When using this approximation, the displacement of the molecule can be expressed by using Hooke’s law as, By replacing the reduced mass (m1m2/[m1+m2]) with μ and the total displacement (x1+x2) with q, the equation can be simplified to,By solving this equation for q we get, where νm is the molecular vibration and is defined as, From equations it is apparent that the molecule vibrates in a cosine pattern with a frequency proportional to the bond strength and inversely proportional to the reduced mass. From this we can see that each molecule will have its own unique vibrational signatures which are determined not only by the atoms in the molecule, but also the characteristics of the individual bonds. 𝐸 𝑞 = 𝐷 𝑒 1− 𝑒 −𝑎 𝑞− 𝑞 𝑒 2 𝜈= 𝜔 𝑒 − 𝜔 𝑒 𝜒 𝑒 𝜐− 𝜔 𝑒 𝜒 𝑒 𝜐 2 𝜔 𝑒 - Harmonic frequency - Anharmonicity 𝐷 𝑒 = 𝜔 𝑒 𝜔 𝑒 𝜒 𝑒 𝜔 𝑒 𝜒 𝑒

9 RR of (SCN)2- at lower spectral range
𝝎 𝒆 𝝌 𝒆 𝝎 𝒆 222 cm-1 1 cm-1 De ~1.5 eV When deriving the Raman effect, it is generally easiest to start with the classical interpretation by considering a simple diatomic molecule as a mass on a spring where m represents the atomic mass, x represents the displacement, and K represents the bond strength. When using this approximation, the displacement of the molecule can be expressed by using Hooke’s law as, By replacing the reduced mass (m1m2/[m1+m2]) with μ and the total displacement (x1+x2) with q, the equation can be simplified to,By solving this equation for q we get, where νm is the molecular vibration and is defined as, From equations it is apparent that the molecule vibrates in a cosine pattern with a frequency proportional to the bond strength and inversely proportional to the reduced mass. From this we can see that each molecule will have its own unique vibrational signatures which are determined not only by the atoms in the molecule, but also the characteristics of the individual bonds. 𝜈= 𝜔 𝑒 − 𝜔 𝑒 𝜒 𝑒 𝜐− 𝜔 𝑒 𝜒 𝑒 𝜐 2 𝜔 𝑒 - Harmonic frequency - Anharmonicity 𝐷 𝑒 = 𝜔 𝑒 𝜔 𝑒 𝜒 𝑒 𝜔 𝑒 𝜒 𝑒

10 RR of (SCN)2- at higher spectral range
R. Wilbrandt, H. Jensen, P. Pagsberg, H. Sillesen,B. Hansen, E. Hester, Chem Phys Lett 60 (1979) 315

11 Side effect and solvation of (SCN)2-
How Anion Chaotrope Changes the Local Structure of Water: Insights from Photoelectron Spectroscopy and Theoretical Modeling of SCN− Water Clusters M.Valiev, SHM. Deng, Xue-Bin Wang, J. Phys. Chem. B 2016, 120, 1518.

12 Solvent isotopic substitution effect
No apparent effect

13 Comparison of Stokes and anti-Stokes resonance Raman
No apparent effect

14 Computational description of hemibonded intermediates
Evidence of proper performance of range-separated hybrid (RSH) exchange-correlation functionals in description of hemiboded dihalide anions M. Yamaguchi, J. Phys. Chem. A 2011, 115, 14620

15 Optimized geometries of (SCN)2-
Calculated with selected range-separated hybrid functionals in PCM water using aug-cc-pVTZ basis set Functionals LC-wPBE LC-PBE LC-BLYP LC-OLYP LC-TPSS wB97x r(S-S) (Å) (2.6997) 2.6207 (2.7606) r(S-C) (Å) (1.6627) 1.6517 1.6537 (1.6661) r(CN) (Å) (1.1583) 1.1492 1.1484 (1.1646) S-S stretching (cm-1) 239 (235.7) 256 240 241.5 253.5 225 (216) S-C stretching (cm-1) 747 (746) 766.6 745 754.1 761.9 746.5 (745) C-N stretching (cm-1) (2270) 2317.7 2309.6 2315.9 2316.4 (2241) a(SSC) (deg.) 95.8 (96.3) 94.7 94.9 95 94.5 93.6 (93.8) Tors. angle (CSSC) (deg.) 83.5 (84.7) 79.8 75.1 78.7 54.3 (53.3)

16 RSH functional based methods in description of (SCN)2- nature
Experimental value : 1.5eV Relaxed scan of potential energy of (SCN)2•− in vacuum (red) and PCM water (blue) determined using MP2 (solid) or DFT methods (wB97x/ (dotted) and LC-wPBE (dashed)) Potential energy curves of (SCN)2-

17 Complete resonance Raman spectrum of (SCN)2-
I. Janik Kinetics and thermochemistry of hemibonded reaction intermediates Complete resonance Raman spectrum of (SCN)2- Raman shift [cm-1] Assignment 1 220 n(SS) 2 438 2n(SS) 3 501 n(CS)-n(SS) 4 654 3n(SS) 5 721 n(CS) 6 886 4n(SS) 7 940 n(CS)+n(SS) 8 1080 5n(SS) 9 1159 n(CS)+2n(SS) 10 1290 6n(SS) 11 1378 12 1442 7n(SS) 13 1634 n(CN)-2n(SS) 14 1853 n(CN)-n(SS) 15 2073 n(CN) 16 2293 n(CN)+n(SS) 17 2512 n(CN)+2n(SS) 18 2731 n(CN)+3n(SS) 19 2794 n(CN)+n(SC) 20 2951 n(CN)+4n(SS) 21 3014 n(CN)+n(SS)+ n(SC) 22 3233 n(CN)+2n(SS)+ n(SC) 23 3358 Water 24 3480 25 3716 2n(CN)-2n(SS) 26 3926 2n(CN)-n(SS) 27 4146 2n(CN) Anharmonicity ~1 cm-1 Harmonic frequency ~221 cm-1 Dissociation energy De ~1.5 eV

18 Thermochemistry of (SCN)2-
I. Janik Kinetics and thermochemistry of hemibonded reaction intermediates Thermochemistry of (SCN)2- Van’t Hoff analysis ln𝐾𝑒𝑞=− Δ𝐻 𝑅𝑇 + Δ𝑆 𝑅 Investigators DH [eV] JH Baxendale, PLT Bevan, J. Chem Soc. A, (1969) 2240 0.28 AJ Elliot, FC Sopchyshyn, Int. J. Chem. Kinet., 16, (1984) 1247. 0.33 M Chin and PH Wine, J. Photochem. Photobiol. A, 69 (1992) 11 0.46 0.3 Average –DH~ 0.37 eV

19 Comparison of reaction enthalpies of (SCN)2-
I. Janik Kinetics and thermochemistry of hemibonded reaction intermediates Comparison of reaction enthalpies of (SCN)2- resonance Raman DH 1.5eV 1.13eV 0.37eV Van’t Hoff analysis DH DHhyd(SCN-) + DHhyd(SCN) - DHhyd(SCN)2- = 1.13eV -3.2 eV eV - DHhyd(SCN)2- = 1.13eV DHhyd(SCN)2- = eV DHhyd(SCN-)/DHhyd(SCN)2-=1.42 Born radius of (SCN)2- ~40% bigger than SCN-

20 Conclusions Acknowledgement
I. Janik, I. Carmichael, GNR Tripathi J Chem Phys 146, (2017) Fundamental vibrations associated with SS, CS, and CN stretches of (CNS)2- the radical have been obtained by TRRR BDE of hemibond SS of ~1.5 eV was determined by Birge-Sponer extrapolation Calculations by range-separated hybrid density functionals (wB97x and LC-wPBE) support the spectroscopic assignments and thermochemical findings Motion of solvent molecules in the hydration shell has no perceptible effect on the intramolecular dynamics of the radical as no frequency shift or spectral broadening was observed between light and heavy water solvents The frequency difference between the thermally relaxed and spontaneously created vibrational states of (SCN)2- in water is too small to be observed Acknowledgement

21 Thank you for your attention


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