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