Vibrational relaxation by methylated xanthines in solution: Insights from 2D IR spectroscopy and calculations

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 (hea...

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Bibliographic Details
Published in:The Journal of chemical physics 2023-01, Vol.158 (4), p.044302-044302
Main Authors: Hanes, Alex T., Grieco, Christopher, Lalisse, Remy F., Hadad, Christopher M., Kohler, Bern
Format: Article
Language:English
Subjects:
Acetonitrile
Anharmonicity
Anisotropy
Asymmetry
Bleaches
Carbonyls
Coupled modes
Couplings
Damping
Decay rate
Density functional theory
Deuterium
Energy transfer
Heavy water
Infrared instruments
Infrared spectroscopy
Mathematical analysis
Physics
Solvents
Spectrum analysis
Stretching
Time constant
Time dependence
Xanthines
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Summary: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.
ISSN:0021-9606
1089-7690
DOI:10.1063/5.0135412