Probing the global potential energy minimum of (CH2O)2: THz absorption spectrum of (CH2O)2 in solid neon and para-hydrogen

The true global potential energy minimum configuration of the formaldehyde dimer (CH2O)(2), including the presence of a single or a double weak intermolecular CH center dot center dot center dot center dot O hydrogen bond motif, has been a long-standing subject among both experimentalists and theoreticians as two different energy minima conformations of C-s and C-2h symmetry have almost identical energies. The present work demonstrates how the class of large-amplitude hydrogen bond vibrational motion probed in the THz region provides excellent direct spectroscopic observables for these weak intermolecular CH center dot center dot center dot O hydrogen bond motifs. The combination of concentration dependency measurements, observed isotopic spectral shifts associated with H/D substitutions and dedicated annealing procedures, enables the unambiguous assignment of three large-amplitude infrared active hydrogen bond vibrational modes for the non-planar C-s configuration of (CH2O)(2) embedded in cryogenic neon and enriched parahydrogen matrices. A (semi)-empirical value for the change of vibrational zero-point energy of 5.5 +/- 0.3 kJ mol(-1) is proposed for the dimerization process. These THz spectroscopic observations are complemented by CCSD(T)-F12/aug-cc-pV5Z (electronic energies) and MP2/aug-cc-pVQZ (force fields) electronic structure calculations yielding a (semi)-empirical value of 13.7 +/- 0.3 kJ mol(-1) for the dissociation energy D-0 of this global potential energy minimum. Published by AIP Publishing.