Infrared spectroscopy of mass-selected neutral molecular systems without chromophore: the formamide monomer and dimer

Abstract

The paper presents IR spectroscopic data and theoretical frequency calculations, in the C–H and N–H stretch region, on jet-cooled formamide molecules and dimers. By using dipole-bound or quadrupole-bound anion formation, one is able to monitor IR absorption in the neutral beam on mass-selected species. For isolated formamide, resonant IR excitation leads to autodetachment of the excess electron in the dipole-bound anion. For the formamide dimer, the IR spectrum is characteristic from the cyclic dimer, with two neighbouring bonded N–H stretches and a complex spectrum with several lines and a broad absorption band in the C–H region absorption, as for the cyclic formic acid dimer. In this double H-bond configuration, IR absorption cannot lead to vibrational predissociation but it probably produces hot neutral dimers with large-amplitude intermolecular motions that reinforce autodetachment of the nascent weakly bound anions. Thus, the present original IR spectroscopic technique, which allows for rigorous mass-selection of the neutrals, seems to be relevant for any polar neutral species.