2D IR spectra of cyanide in water investigated by molecular dynamics simulations

Abstract

Using classical molecular dynamics simulations, the 2D infrared (IR) spectroscopy of CN(-) solvated in D2O is investigated. Depending on the force field parametrizations, most of which are based on multipolar interactions for the CN(-) molecule, the frequency-frequency correlation function and observables computed from it differ. Most notably, models based on multipoles for CN(-) and TIP3P for water yield quantitatively correct results when compared with experiments. Furthermore, the recent finding that T1 times are sensitive to the van der Waals ranges on the CN(-) is confirmed in the present study. For the linear IR spectrum, the best model reproduces the full widths at half maximum almost quantitatively (13.0 cm(-1) vs. 14.9 cm(-1)) if the rotational contribution to the linewidth is included. Without the rotational contribution, the lines are too narrow by about a factor of two, which agrees with Raman and IR experiments. The computed and experimental tilt angles (or nodal slopes) α as a function of the 2D IR waiting time compare favorably with the measured ones and the frequency fluctuation correlation function is invariably found to contain three time scales: a sub-ps, 1 ps, and one on the 10-ps time scale. These time scales are discussed in terms of the structural dynamics of the surrounding solvent and it is found that the longest time scale (≈10 ps) most likely corresponds to solvent exchange between the first and second solvation shell, in agreement with interpretations from nuclear magnetic resonance measurements.

Solvation of fluoro-acetonitrile in water by 2D-IR spectroscopy: A combined experimental-computational study

“…FFCFs of other vibrational chromophores often exhibit a more or less pronounced initial oscillatory contribution, sometimes even leading to anticorrelations at short times. That has been observed, for example, for CN − in D 2 O, 29 HOD in D 2 O, 57 or N − 3 in D 2 O. 31 In contrast, here the FFCF decays monotonically to zero, similar to what was found for NMA in D 2 O.…”

“…29,34 For cyanide in water, a force field that correctly describes solvation free energies, vibrational relaxation and the 1-dimensional spectroscopy was found to closely match the experimentally measured spectral diffusion dynamics. 29 For the case of NMA solvated in water a force field reproducing thermodynamic data from experiment was found to lead to somewhat faster dynamics (around 1 ps compared to between 1 and 1.6 ps, depending on the experiment). 13,14,34 This second approach attempts to treat the dynamics and computation of a physico-chemical observable on the same footing.…”

“…FPCs have proved to be useful to describe the electron density as a molecule undergoes conformational changes 34 and to reproduce up to the quadrupole moment in diatomic molecules. 29,49,50 The nuclear charges are a function of the coordinate of interest, which is the CF bond length in the present case. From the electronic structure calculations for the CF-Morse potential described above, nuclear charges are fitted to the electrostatic field.…”

“…29 Essentially two approaches have been pursued to perform such simulations: (a) running an MD simulation with a standard force field in order to sample the conformational space and then calculating instantaneous frequency shifts of the chromophore of interest, utilizing maps that have typically been pre-calculated on a higher ab-initio level of theory, [30][31][32][33] or (b) developing better, physics-based empirical force fields that are used for both the MD trajectory and the spectral calculation in a consistent manner. 29,34 For cyanide in water, a force field that correctly describes solvation free energies, vibrational relaxation and the 1-dimensional spectroscopy was found to closely match the experimentally measured spectral diffusion dynamics. 29 For the case of NMA solvated in water a force field reproducing thermodynamic data from experiment was found to lead to somewhat faster dynamics (around 1 ps compared to between 1 and 1.6 ps, depending on the experiment).…”

Quantitative Atomistic Simulations of Reactive and Non-Reactive Processes

“…Such an approach was found to quantitatively describe the splitting, red shift and line shape for infrared spectra. [4,9,11] A signature application for such a MTP force field was the investigation of the 1d-and 2d-spectroscopy of CN -in water. Using a force field optimized for the vibrational relaxation of cyanide in water, the spectroscopic properties could be faithfully reproduced.…”