Distinct Fermi Resonance Patterns of Weak Coupling in 2D-IR Spectra of 5-Cyanoindole Revealed by Isotope Labeling.

Fermi resonance is a common phenomenon, and a hidden caveat exists in the applications of infrared probes, causing spectral complication and shorter vibrational lifetime. In this work, using the cyanotryptophan (CNTrp) side chain model compound 5-cyanoindole (CN-5CNI), we performed Fourier transform infrared spectroscopy (FTIR) and two-dimensional infrared (2D-IR) spectroscopy on unlabeled ¹²C¹⁴N-5CNI and its isotopically labeled substituents (¹²C¹⁵N-5CNI, ¹³C¹⁴N-5CNI, ¹³C¹⁵N-5CNI) and demonstrated the existence of Fermi resonance in 5CNI. By constructing the Hamiltonian and simulating 2D-IR spectra, we show that the distinct Fermi resonance 2D-IR patterns in various isotope substituents are determined by the quantum mixing consequences at the v = 1 state, as well as the v = 2 state, where the Fermi coupling and anharmonicity play a crucial role. Our work provides important insights into the elusive type of Fermi resonance, where the coupling is much smaller than the anharmonicity, which is termed the weak coupling case.