Hydrogen Bonding Induced Enhancement of Fermi Resonances: Ultrafast Vibrational Energy Flow Dynamics in Aniline‑d 5

• Published in: The journal of physical chemistry. B 2015-02, Vol.119 (6), p.2711-2725
• Main Authors: Costard, Rene, Greve, Christian, Fidder, Henk, Nibbering, Erik T. J
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp509977r

With hydrogen bonding of the amino group of aniline-d 5 we can identify the roles of Fermi enhanced combination and overtone states in intramolecular vibrational re-distribution (IVR) pathways for N–H stretching excitations. Using linear Fourier transform infrared (FT-IR) spectroscopy, ultrafast one- and two-color IR-pump–IR-probe spectroscopy, and femtosecond two-dimensional IR spectroscopy, we can identify the primary accepting modes for N–H stretching excitations. In particular, a key role is played by the δ(NH2) bending degree of freedom, either via its δ = 2 overtone state or via a combination state with the ν(CC) ring stretching mode. No significant transient population in these Fermi enhanced combination/overtone states can be observed, a consequence of similar decay rates of these Fermi enhanced combination/overtone states and of the N–H stretching states. A similar magnitude of the transient response of the two fingerprint modes regardless of direct excitation of the Fermi enhanced combination/overtone levels or of the N–H stretching states suggests an underlying coupling mechanism facilitating common IVR pathways. This mechanism is expected to be of general importance for other organic compounds with hydrogen-bonded amino groups, including DNA bases.