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Time-dependent fifth-order bands in nominally third-order 2D IR vibrational echo spectra
Megan C Thielges1, Michael D Fayer
1Department of Chemistry, Stanford University, Stanford, California 94305, USA.
Fifth-order signals in 2D IR spectra of carbon monoxide (CO) in cytochrome P450(cam) were observed. These novel signals, arising from higher-order vibrational transitions, require careful interpretation to distinguish them from other spectral features.
Area of Science:
- Physical Chemistry
- Spectroscopy
- Biophysics
Background:
- Intense mid-infrared laser pulses have advanced 2D IR vibrational spectroscopy.
- Higher pulse energies increase the probability of fifth-order contributions to spectra.
Purpose of the Study:
- To report and characterize novel fifth-order signals in 2D IR spectra.
- To investigate these signals in carbon monoxide (CO) bound to cytochrome P450(cam) with norcamphor.
- To differentiate these signals from third-order interactions and other spectral phenomena.
Main Methods:
- Utilized 2D IR vibrational spectroscopy with intense mid-IR laser pulses.
- Analyzed spectra of CO bound to the active site of cytochrome P450(cam) with norcamphor.
- Performed model calculations to reproduce observed signal behavior.
Main Results:
- Observed two novel fifth-order bands with unique time-dependent behaviors.
- These bands correspond to a ν = 1-2 vibrational transition and exhibit sign changes.
- Model calculations successfully reproduced the appearance and time dependence of these bands, considering differential vibrational relaxation rates.
Conclusions:
- Fifth-order signals can appear in 2D IR spectra under high-energy pulse conditions.
- Understanding these signals is crucial to avoid misidentification with other spectral processes like chemical exchange or energy transfer.
- The observed phenomena are explained by accounting for faster vibrational relaxation from ν=2 to ν=1 compared to ν=1 to ν=0.
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