Hydrogen-Bonding-Induced Enhancement of Fermi Resonances: A Linear IR and Nonlinear 2D-IR Study of Aniline‑d₅

Hydrogen bonding of the amino group of aniline-d₅ results in a huge enhancement of the NH₂ bending overtone absorption strength, mainly attributed to the Fermi resonance effect. A quantitative analysis is presented, using a hybrid mode representation and encompassing experimental data on aniline with 0, 1, or 2 hydrogen bonds to dimethylsulfoxide (DMSO). Changes in enthalpy, hydrogen-bonding-induced frequency shifts, and the transition dipole moment increase of the local N–H stretching oscillator all demonstrate that the hydrogen bond is strongest in the single hydrogen-bonded complex. Linear IR overtone spectra show that the oscillator strength decreases upon hydrogen bonding for the N–H stretching overtones, which is opposite to the effect on the fundamental N–H stretching transitions. Polarization resolved 2D-IR spectra provide detailed information on the N–H stretching overtone manifold and support the relative orientations of the various IR transitions.