High‐Resolution Infrared Synchrotron Investigation of (HCN)2 and a Semi‐Experimental Determination of the Dissociation Energy D0

The high-resolution infrared absorption spectrum of the donor bending fundamental band nu 6 1 of the homodimer (HCN)2 has been collected by long-path static gas-phase Fourier transform spectroscopy at 207 K employing the highly brilliant 2.75 GeV electron storage ring source at Synchrotron SOLEIL. The rovibrational structure of the nu 6 1 transition has the typical appearance of a perpendicular type band associated with a Sigma-Pi transition for a linear polyatomic molecule. The total number of 100 assigned transitions are fitted employing a standard semi-rigid linear molecule Hamiltonian, providing the band origin nu0 of 779.05182(50) cm(-1) together with spectroscopic parameters for the degenerate excited state. This band origin, blue-shifted by 67.15 cm(-1) relative to the HCN monomer, provides the final significant contribution to the change of intra-molecular vibrational zero-point energy upon HCN dimerization. The combination with the vibrational zero-point energy contribution determined recently for the class of large-amplitude inter-molecular fundamental transitions then enables a complete determination of the total change of vibrational zero-point energy of 3.35+/-0.30 kJ mol(-1) . The new spectroscopic findings together with previously reported benchmark CCSDT(Q)/CBS electronic energies [Hoobler et al. ChemPhysChem. 19, 3257-3265 (2018)] provide the best semi-experimental estimate of 16.48+/-0.30 kJ mol(-1) for the dissociation energy D0 of this prototypical homodimer.