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The transition-metal complex

The 1,3-diaza-2,4-diborobutane (NBNB) molecule serves as the smallest model complex of an intramolecular "dihydrogen bond," which involves a nominally hydrogen-bonding interaction between amine and borane hydrogen atoms. In the present study, the role of this dihydrogen bond in influencing the inherent molecular dynamics of NBNB is investigated computationally with ab initio molecular dynamics and path integral molecular dynamics techniques, as well as vibrational spectra analysis and static quantum chemistry computations. These simulations indicate that the dihydrogen-bonding interaction impacts both the high- and low-frequency motions of the molecule, with the dominant motions involving low-frequency backbone isomerization and terminal amine rotation. Geometric isotope effects were found to be modest. The analysis also addresses the paradoxical fostering of amine rotation via a relatively strong dihydrogen bond interaction. Electrostatic and geometric factors most directly explain this effect, and although some orbital evidence was found for a small covalent component of this interaction, the dynamics and electronic structure suggest that electrostatic contributions are the controlling factors for molecular motion in NBNB.

Because of both experimental and computational challenges, protonated tryptophan has remained the last aromatic amino acid for which the intrinsic structures of low-energy conformers have not been unambiguously solved. The IR-IR-UV hole-burning spectroscopy technique has been applied to overcome the limitations of the commonly used IR-UV double resonance technique and to measure conformer-specific vibrational spectra of TrpH(+), cooled to T = 10 K. Anharmonic ab initio vibrational spectroscopy simulations unambiguously assign the dominant conformers to the two lowest-energy geometries from benchmark coupled-cluster structure computations. The match between experimental and ab initio spectra provides an unbiased validation of the calculated structures of the two experimentally observed conformers of this benchmark ion. Furthermore, the vibrational spectra provide conformer-specific signatures of the stabilizing interactions, including hydrogen bonding and an intramolecular cation-π interaction.