Structural analysis and probing the conformational space of dansylamide by means of gas-phase electron diffraction and quantum chemistry

Dansylamide is perhaps the most ubiquitous fluorophore due to its donor-acceptor bifunctionality and its ability to form intra- and intermolecular hydrogen bonding. Among the diversity of its applications is the development of new generation of biosensors for the in vivo monitoring of traces of metals. The structure and conformational stability of dansylamide in the gas phase were investigated for the first time by a combined gas-phase electron diffraction-mass spectrometry (GED/MS), complemented by quantum chemical calculations. GED data indicate that different skewed conformers exist at T?=?464 K, which are characterized by the deviation of two S–N bonds from the perpendicular orientation relative to the naphthalene plane. Maybe the most indicative structural parameters for electronic interactions between the donor-acceptor substituents and the aromatic naphthalene and the subsequent stabilization of the favorable skewed eclipsed-syn conformer are the dihedral angles C₉–C₁–S–N and C₁₀–C₅–N–C with the experimentally determined values of 66.8° (32) and 68.1° (72), respectively. The role of –SO₂NH₂ by forming intramolecular hydrogen bonds was scrutinized by employing the natural bond orbital approach (NBO), quantum theory atoms in molecules (QTAIM), and molecular electrostatic potential (MESP). The non-planarity of the naphthalene skeleton due to the electronic interactions with the substituents and its consequence for the fluorescence activity of dansylamide have been discussed.