Molecular Anion Hydrogen Bonding Dynamics in Aqueous Solution

Abstract

The dynamic nature of hydrogen bonding between a molecular anion, selenocyanate (SeCN(-)), and water in aqueous solution (D2O) is addressed using FT-IR spectroscopy, two-dimensional infrared (2D IR) vibrational echo spectroscopy, and polarization selective IR pump-probe (PSPP) experiments performed on the CN stretching mode. The CN absorption spectrum is asymmetric with a wing on the low frequency (red) side of the line in contrast to the spectrum in the absence of hydrogen bonding. It is shown that the red wing is the result of an increase in the CN stretch transition dipole moment due to the effect of hydrogen bonding (non-Condon effect). This non-Condon effect is similar in nature to observations on pure water and other nonionic systems where hydrogen bonding enhances the extinction coefficient. The 2D IR measurements of spectral diffusion (solvent structural evolution) yield a time constant of 1.5 ps, which is within error the same as that of the OH stretch of HOD in D2O (1.4 ps). The orientational relaxation of SeCN(-) measured by PSPP experiments is long (4.04 ps) compared to the spectral diffusion time. The population decay at or near the absorption line center is a single-exponential decay of 37.4 ± 0.3 ps, the vibrational lifetime. However, on the red side of the line the decay is biexponential with a low amplitude, fast component; on the blue side of the line there is a low amplitude, fast growth followed by the lifetime decay. Both of the fast components have 1.5 ps time constants, which is the spectral diffusion time. The fast components of the population decays are the results of the non-Condon effect that causes the red side of the line to be over pumped by the pump pulse. Spectral diffusion then produces the fast decay component on the red side of the line and the growth on the blue side of the line as the excess initial population on the red side produces a net population flow from red to blue.

Direct observation of dynamic crossover in fragile molecular glass formers with 2D IR vibrational echo spectroscopy

“…1,12,85 2D IR would be highly sensitive to such solvent shell dynamics. [57][58][59] The fast β-process usually appears on a few picosecond time scale with a weak temperature dependence as is observed for the 2D IR data, although it also generally appears as a power law in the MCT equations. This is again attributable to the differences in the FFCF from other correlators, and it has been previously demonstrated that a schematic MCT correlator is as capable of describing a multiexponential decay as a power law.…”

“…This model has had great success in fitting 2D IR data for a variety of systems including organic liquids, water, proteins, liquid crystals, and room-temperature ionic liquids. [55][56][57][58][59][71][72][73][74][75] The multiexponential model was chosen as our starting point, as above T m , these glass forming liquids are simple organic liquids. At these higher temperatures, the CLS(T w ) decay curves for all three samples fit well to biexponential decays.…”

“…The development of longlived selenocyanate probes has been an important advance, as their nitrile stretches can have lifetimes of many hundreds of picoseconds, permitting the collection of data from <1 ps to ∼1 ns. These probes have previously been used in 2D IR spectroscopy to probe dynamics in liquid crystals, 55,56 in solvation shells of water and ionic liquids, [57][58][59] and in proteins. 60 It has been shown that the spectral diffusion reported by probes of similar shape and size as the molecules that make up the bulk liquid is nearly identical to that felt by natural vibrational probes that are intrinsic to the liquid itself.…”

TGA, DSC and DFT Studies of TKX‐50, ABTOX and Their Key Precursors

“…Similar conformational search has been discussed in the literature. [12] The lowest energy conformer of the literature has been compared with that of our study. Since the literature has not provided the corresponding Cartesian coordinates, we have modelled their stable conformer on the basis of visual orientations.…”

“…TKX-50 in recent time followed by many other studies like thermal, compatibility, performance prediction, mechanistic decomposition and computational studies in respect of TKX-50. [9][10][11][12][13][14] ABTOX is a parallel product in the easy one-pot five step synthetic route of TKX-50 starting from glyoxime. Further, its high thermal stability and insensitivity (impact and friction) than TKX-50 prompted us to consider ABTOX for its comparative study with TKX-50.…”