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Two-dimensional infrared (2D IR) spectroscopy, infrared pump–infrared probe spectroscopy, and density functional theory calculations were used to study vibrational relaxation by ring and carbonyl stretching modes in a series of methylated xanthine derivatives in acetonitrile and deuterium oxide (heavy water). Isotropic signals from the excited symmetric and asymmetric carbonyl stretch modes decay biexponentially in both solvents. Coherent energy transfer between the symmetric and asymmetric carbonyl stretching modes gives rise to a quantum beat in the time-dependent anisotropy signals. The damping time of the coherent oscillation agrees with the fast decay component of the carbonyl bleach recovery signals, indicating that this time constant reflects intramolecular vibrational redistribution (IVR) to other solute modes. Despite their similar frequencies, the excited ring modes decay monoexponentially with a time constant that matches the slow decay component of the carbonyl modes. The slow decay times, which are faster in heavy water than in acetonitrile, approximately match the ones observed in previous UV pump–IR probe measurements on the same compounds. The slow component is assigned to intermolecular energy transfer to solvent bath modes from low-frequency solute modes, which are populated by IVR and are anharmonically coupled to the carbonyl and ring stretch modes. 2D IR measurements indicate that the carbonyl stretching modes are weakly coupled to the delocalized ring modes, resulting in slow exchange that cannot explain the common solvent-dependence. IVR is suggested to occur at different rates for the carbonyl vs ring modes due to differences in mode-specific couplings and not to differences in the density of accessible states.
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Experimental two-dimensional infrared spectra of methyl thiocyanate in water and organic solvents
Thiocyanates, nitriles, and azides represent a versatile set of vibrational probes to measure the structure and dynamics in biological systems. The probes are minimally perturbative, the nitrile stretching mode appears in an otherwise uncongested spectral region, and the spectra report on the local environment around the probe. Nitrile frequencies and lineshapes, however, are difficult to interpret, and theoretical models that connect local environments with vibrational frequencies are often necessary. However, the development of both more accurate and intuitive models remains a challenge for the community. The present work provides an experimentally consistent collection of experimental measurements, including IR absorption and ultrafast two-dimensional infrared (2D IR) spectra, to serve as a benchmark in the development of future models. Specifically, we catalog spectra of the nitrile stretching mode of methyl thiocyanate (MeSCN) in fourteen different solvents, including non-polar, polar, and protic solvents. Absorption spectra indicate that π-interactions may be responsible for the line shape differences observed between aromatic and aliphatic alcohols. We also demonstrate that a recent Kamlet–Taft formulation describes the center frequency MeSCN. Furthermore, we report cryogenic infrared spectra that may lead to insights into the peak asymmetry in aprotic solvents. 2D IR spectra measured in protic solvents serve to connect hydrogen bonding with static inhomogeneity. We expect that these insights, along with the publicly available dataset, will be useful to continue advancing future models capable of quantitatively describing the relation between local environments, line shapes, and dynamics in nitrile probes.
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- Award ID(s):
- 1847199
- PAR ID:
- 10532364
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 160
- Issue:
- 11
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0190343
