A new infrared spectroscopy technique for structural studies of mass-selected neutral polar complexes without chromophore

We report gas-phase experimental and theoretical results on the configurations of weakly-bound neutral polar complexes without chromophores: the water dimer and the formamide–water complex. Experimental data are obtained by combining infrared (IR) absorption spectroscopy, in the 2800–3800 cm−1 domain, with the Rydberg electron transfer (RET) technique leading to dipole-bound anion (DBA) formation. In the absence of IR excitation, RET to neutral complexes with a given total dipole moment, and thus a given molecular structure, leads to DBAs which are observed without any possible fragmentation. In the presence of the IR laser, prior to ionisation, resonant IR absorption of intramolecular vibrations of the parent neutral complexes can either induce the breaking of the weak intermolecular bonds (vibrational predissociation of the neutral) or the fast departure of the excess electron after RET (autodetachment of the DBA). Anion signal depletion, monitored at the parent mass, is then a signature of resonant IR absorption from mass- and structure-selected neutral complexes. The validity of the present experimental method and of different types of quantum chemistry calculations is discussed by comparison between calculated harmonic or anharmonic frequencies, the present experimental gas-phase IR spectra, and previous experimental data on these two test-case hydrogen-bonded complexes.