Ultrafast vibrational dynamics of SCN− and N3− in polar solvents studied by nonlinear infrared spectroscopy

Abstract

In this paper, we report on our investigation into the vibrational dynamics of the antisymmetric stretching modes of SCN(-) and N(3)(-) in several polar solvents. We used an infrared (IR) pump-probe method to study orientational relaxation processes. In two aprotic solvents (N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO)), the anisotropy decay shows a bimodal feature, whereas in other solvents the anisotropy decay can be fitted well by a single exponential function. We consider that the relative contribution of fast-decaying components is smaller in the other solvents than in DMF and DMSO. We discuss the possible origins of the different anisotropy decay behavior in different solvents. From the three-pulse IR photon echo measurements for SCN(-) and N(3)(-), we found that the time-correlation functions (TCFs) of vibrational frequency fluctuations decay on two different time scales, one of which is less than 100 fs and the other is approximately 3-6 ps. In aprotic solvents, the fast-decaying components of the TCFs on a <100 fs time scale play an important role in the vibrational frequency fluctuation, although the contribution of collective solvent reorganization in aprotic solvents was clearly observed to have small amplitudes. On the other hand, we found that the amplitude of components that decay in a few picoseconds and/or the constant offset of the TCF in protic solvents is relatively large compared with that in aprotic solvents. With the formation and dissociation of hydrogen bonds between ion solute and solvent molecules, the spectra of different solvated species are exchanged with each other and merged into one band. We considered that this exchange may be an origin of slow-decaying components of the TCFs and that the decay of the TCFs corresponds to the time scales of the exchange for protic solvents such as formamide. The mechanism of vibrational frequency fluctuations for the antisymmetric stretching modes of SCN(-) and N(3)(-) is discussed in terms of the difference between protic and aprotic solvents.

Vibrational dynamics of azide-derivatized amino acids studied by nonlinear infrared spectroscopy

“…Several groups including us experimentally and theoretically examined the frequency fluctuations of ions in water and pure water and revealed that the characteristic time constant of approximately 1 ps reflects fluctuations of hydrogen-bonding network in liquid water. [9][10][11][12][13][55][56][57][58] Consistent with the results of the threepulse IR photon echo experiment for N 3 − in D 2 O, 12,13 we also found that FFTCFs decay with a time constant of 1.3 ps for N 3 − in H 2 O. Moreover, we showed that FFTCF of N 3 -Ala and N 3 -Pro are also characterized by time constants of about 1.5 ps and 1.0 ps, respectively.…”

“…9 We have reported the temperature dependence of vibrational dynamics of N 3 − in water and the vibrational dynamics of OCN − and SCN − in methanol. [10][11][12][13] Recently, artificial amino acids which are labeled by azide or thiocyanate groups have been shown to be excellent vibrational probes to study the dynamics in water and biological environments. Bloem et al incorporated an azide probe into peptide and studied the spectral diffusion process of the N 3 anti-symmetric stretching mode in protein.…”

Nature of hydration shells of a polyoxy‐anion with a large cationic centre: The case of iodate ion in water

“…Hydration of ions in aqueous media has been a major area of scientific studies [1][2][3][4] in chemical, biological, and many other branches of science. The structure and dynamics of water around ions have been investigated extensively by means of experimental, [5][6][7][8][9][10][11] theoretical, [12][13][14][15][16][17][18][19][20] and also combination of both the methods. [21][22][23][24][25] The polyoxy-anions form a major class of ions which have great relevance to various atmospheric and biological systems.…”

Effect of ion–molecule interaction on fermi-resonance in acetonitrile studied by ultrafast vibrational spectroscopy

“…Accordingly, such ion-molecule interactions have been extensively studied by FTIR experiments and theoretical methods for a few decades [1][2][3][4][5][6][7]. However, the effect of ions on vibrational dynamics of a given molecule has begun to be investigated recently with an advent of time-resolved vibrational spectroscopic techniques such as ultrafast infrared-Raman, infrared pump-probe (IR PP), and two-dimensional IR (2DIR) experiments [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. A vibrationally excited molecule undergoes different relaxation processes on various timescales including vibrational energy transfer to anharmonically coupled vibrational modes [23,24], vibrational energy dissipation into the solvent [25], and resonance energy transfer to nearby vibrational modes with similar vibrational energies [26][27][28][29][30].…”