FTIR spectra of 1.2 M LiNCS in benzonitrile (BN: solid black line) and nitromethane (MeNO2: dashed red line). The peak at ∼2040 cm−1 corresponds to an ion-pair dimer structure with a quadrupole charge distribution (Q-dimer), the peak at ∼2070 cm−1 corresponds to the LiNCS ion pair, and the peak at ∼2100 cm−1 corresponds to the NCS− in a linear ion-pair dimer (L-dimer) that is coordinating two Li+ cations, one with the N atom and the other with the S atom.
Calculated gas phase structures for the (a) ion pair, (b) linear ion-pair dimer (L-dimer) structure, and (c) ion-pair dimer with a quadrupole charge distribution (Q-dimer). The following atomic color scheme are used: Li, mauve; N, blue; C, black; and S, yellow. The size of the spheres do not reflect the electronic charge density.
(a) FTIR spectra of benzonitrile (BN: solid black line) and 1.2 M LiNCS in benzonitrile in the region of the CN-stretch of BN (dashed red line). Note the appearance of a blueshifted CN stretch due to direct coordination of Li+ cations by the cyano-group on BN. (b) FTIR spectra of nitromethane (MeNO2: solid black line) and 1.2 M LiNCS in nitromethane (dashed red line) in the region of the symmetric and anti-symmetric nitro-group stretching vibrations. We collected the pure solvent spectra with a 2 μm path length cell and the ionic solution spectra with a 6 μm path length cell. LiNCS causes a slight broadening of the symmetric and anti-symmetric nitro-group stretching vibrations, but we see no spectral evidence for Li+ coordination by MeNO2.
Logarithmic plot of the time resolved anisotropy for 1.2 M LiNCS in (a) benzonitrile (BN) and (b) nitromethane (MeNO2) measured with polarization selective vibrational pump-probe measurements of the orientational CN stretch of NCS− in the ion pair, Q-dimer, and L-dimer. Note the different time axes for (a) and (b). The solid black lines show the fit of each data set to a tri-exponential decay. The rotational relaxation time constants and amplitudes extracted from the tri-exponential fits can be found in Table II.
Early time decay of the anisotropy for the Q-dimer in benzonitrile (BN) and dimethyl carbonate (DMC). The time dependent decay of the anisotropy shows a strongly damped oscillation, with a frequency of ∼18 cm−1 for BN and ∼15 cm−1 for DMC.
Experimental and calculated absorption peak frequencies, and van der Waals volumes40 of LiNCS ion pair, quadrupole ion-pair dimer (Q-dimer), and linear ion-pair dimer (L-dimer) in MeNO2.
Parameters extracted from the fit of the anisotropy data to a bi-exponential decay function: . The amplitudes have errors of 10% or less.
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