TRANSIENT RAMAN SPECTRA, STRUCTURE AND THERMOCHEMISTRY OF THE THIOCYANATE - PowerPoint Presentation

Slide1

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, USASlide2

Motivation for (SCN)2- studies

SCN- is

the strongest water structure breaker in the Hofmeister

series of mono anions

(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

halidesSlide3

R

•

cell

P

ulse

of high-energy electrons

monochromator

Detector:

PMT

Photodiode

Oscilloscope

time

Transient

Absorption

Pulse radiolysis setup with optical detectionSlide4

OH-radical induced oxidation mechanism of thiocyanate, SCN-Slide5

Time resolved resonance Raman studies of (SCN)

2- in waterSlide6

Pioneering studies on resonance Raman of (SCN)2-

R. Rossetti, S. M. Beck, and L. E. Brus, JACS, 106 (1984) 981

R. Wilbrandt, H. Jensen, P.

Pagsberg, H. Sillesen,B. Hansen, E. Hester, Chem Phys Lett 60 (1979) 315Slide7

Two-center three-electron bonds (hemibonds)Slide8

RR of (SCN)2- at lower spectral range

- Harmonic frequency

-

Anharmonicity

 Slide9

RR of (SCN)2- at

lower spectral range

- Harmonic frequency

-

Anharmonicity

222 cm

-1

1 cm

-1

D

e

~1.5

eVSlide10

RR of (SCN)

2

-

at h

igher spectral range

R.

Wilbrandt

, H. Jensen, P.

Pagsberg

, H. Sillesen,B. Hansen, E. Hester, Chem Phys Lett 60 (1979) 315Slide11

Side effect and solvation of (SCN)2-

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

Water ClustersSlide12

Solvent isotopic substitution effect

No apparent

effectSlide13

Comparison of Stokes and anti-Stokes resonance Raman

No apparent effectSlide14

Computational description of hemibonded intermediates

M. Yamaguchi, J. Phys. Chem. A 2011, 115, 14620Evidence of proper performance of range-separated hybrid (RSH)

exchange-correlation functionals in description of hemiboded

dihalide anionsSlide15

Optimized geometries of (SCN)2

-Functionals

LC-w

PBE

LC-PBE

LC-BLYP

LC-OLYP

LC-TPSS

w

B97xr(S-S) (Å)

2.6694 (2.6997)

2.6207

2.68480

2.67104

2.63514

2.7293 (2.7606)

r(S-C) (Å)

1.6619 (1.6627)1.6517

1.65977

1.654651.65371.6653 (1.6661)r(CN) (Å)1.1558 (1.1583)1.1492

1.147501.147581.14841.1613 (1.1646)S-S stretching (cm-1)

239 (235.7)256240241.5253.5225 (216)

S-C stretching (cm-1)747 (746)766.6745754.1

761.9746.5 (745)C-N stretching (cm-1)2265.8 (2270)2317.72309.6

2315.9

2316.4

2240.5 (2241)a(SSC) (deg.)

95.8 (96.3)

94.794.9

95

94.5

93.6 (93.8)Tors. angle (CSSC) (deg.)

83.5 (84.7)

79.8

75.3578475.1

78.7

54.3 (53.3)

Calculated with selected range-separated hybrid functionals in PCM water using aug-cc-

pVTZ basis setSlide16

RSH functional based methods in description of (SCN)2

- nature 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

-

Experimental value : 1.5eVSlide17

Complete resonance Raman spectrum of (SCN)2

-Anharmonicity ~1 cm-1Harmonic frequency ~221 cm-1Dissociation energy De ~1.5 eV

Raman shift [cm

-1

]

Assignment

1

220

n

(SS)

2

438

2

n

(SS)

3

501

n(CS)-

n(SS)46543n(SS)5

721n(CS)68864n(SS)

7940n(CS)+n(SS)81080

5n(SS)91159n(CS)+2n(SS)

1012906n(SS)111378n(CS)+2

n(SS)12

1442

7n(SS)

13

1634n

(CN)-2n(SS)

141853

n(CN)-n(SS)

15

2073

n(CN)

16

2293n(CN)+n(SS)17

2512

n(CN)+2n(SS)

18

2731

n(CN)+3n(SS)192794n(CN)+n(SC)20

2951n(CN)+4n(SS)213014n(CN)+n(SS)+ n(SC)

223233n(CN)+2n(SS)+ n(SC)23

3358Water243480Water

2537162n(CN)-2n(SS)2639262n(CN)-

n(SS)2741462n(CN)Slide18

Thermochemistry of (SCN)2-

Average –D

H~ 0.37 eV

Van’t

Hoff analysis

Investigators

D

H [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) 110.460.3Slide19

Comparison of reaction enthalpies of (SCN)2

-

resonance Raman

DH

1.5eV

1.13eV

0.37eV

Van’t

Hoff analysis

D

H

D

H

hyd

(SCN

-

) +

D

H

hyd(SCN) - DHhyd(SCN)2- = 1.13eVDHhyd(SCN-)/DHhyd(SCN)2-=1.42 Born radius of (SCN)2

- ~40% bigger than SCN- -3.2 eV - 0.18 eV - DHhyd(SCN)2- = 1.13eV DHhyd(SCN)2- = -2.25 eVSlide20

The frequency difference between the thermally relaxed and spontaneously created vibrational states of (SCN)2- in water is too small to be observed

ConclusionsI. Janik, I. Carmichael, GNR Tripathi J Chem Phys 146

, 214305 (2017)

Acknowledgement

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 extrapolationCalculations by range-separated hybrid density functionals (wB97x and LC-w

PBE

) 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 solventsSlide21

Thank you for your attention