Vibrational Characterization Of Hemi-bonded Halide-thiocyanate Dimer Radical Anions (xscn).− In Water

Janik, Ireneusz

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https://hdl.handle.net/2142/116679 Copy

Description

Title

Vibrational Characterization Of Hemi-bonded Halide-thiocyanate Dimer Radical Anions (xscn).− In Water

Author(s)

Janik, Ireneusz

Contributor(s)

Bhattacharya, Susmita

Issue Date

2022-06-23

Keyword(s)

Mini-symposium: Benchmarking in Spectroscopy

Abstract

Time resolved Raman studies of halide-thiocyanate dimer radical anions, (X-SCN)$^{.-}$ (for X=Cl, Br, I), were performed in resonance with their peak of light absorption wavelength at 415 nm. In two of the experiments (for X$^{-}$=Br$^{-}$ or I$^{-}$) the apparent Raman spectrum contains signatures of three hemibonded intermediates present simultaneously in mutual equilibria with their precursor and successor hemibonded radical counterparts: X$_{2}$$^{.-}$ + SCN$^{-}$ = (XSCN)$^{.-}$ + X$^{-}$ = (SCN)$_{2}$$^{.-}$ + X$^{-}$. In order to extract (X-SCN)$^{.-}$ (for X=Br, I) from the composite spectrum additional experiments were performed to generate pre-resonance spectra of X$_{2}$$^{.-}$ and (SCN)$_{2}$$^{.-}$ at 415 nm in order to collect and then subtract their contributions from the composite spectrum. Ten Stokes Raman bands of the halide-thiocyanate radical anions (X-SCN)$^{.-}$ (for X=Br, I) were observed in the 60-2400\wn region. They were assigned in terms of the strongly enhanced 198 and 174\wn, weakly enhanced 719.5 and 729\wn, and moderately enhanced 2069 and 2078\wn fundamentals, their overtones, and combinations in BrSCN$^{.-}$ and ISCN$^{.-}$, respectively. On attempt to record chloride intermediate only characteristic bands coming from the mixed contributions of Cl$_{2}$$^{.-}$ and (SCN)$_{2}$$^{.-}$ have been apparent. Quantum chemical calculations using a range-separated hybrid density functional ($\omega$B97x) with flexible augmented correlation-consistent basis sets support the spectroscopic assignments of the strongest fundamental vibrations to a predominantly S-X (X = Br, I) stretching mode and the features around 720\wn and 2070\wn to CS and CN symmetric stretching modes, respectively. Interestingly, CS and CN bond stretching vibrational frequencies in asymmetrical (X-SCN)$^{.-}$ anion radicals are shifted a few wavenumbers down or up in comparison to the symmetrical (SCN)$_{2}$$^{.-}$ molecule in BrSCN$^{.-}$ or ISCN$^{.-}$, respectively. Considering that ClSCN$^{.-}$ seems to have vibrational frequencies almost identical to (SCN)$_{2}$$^{.-}$ does not grant any systematic correlation between hemi-bond polarization in this array of molecules and vibrational frequencies of CS and CN bonds. A possible explanation of such an observation can relate to a counteracting induction and migration effects in $\sigma$ and $\pi$ bonds, respectively, upon charge migration across the molecule.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

text

Language

eng

Handle URL

https://hdl.handle.net/2142/116679

DOI

https://doi.org/10.15278/isms.2022.RI07

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Copyright 2022 held by the authors

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