High‐Resolution Infrared Synchrotron Investigation of (HCN)(2) and a Semi‐Experimental Determination of the Dissociation Energy D (0)

The high‐resolution infrared absorption spectrum of the donor bending fundamental band ν [Formula: see text] 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 ν [Formula: see text] transition has the typical appearance of a perpendicular type band associated with a Σ–Π 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 ν (0) 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 D (0) of this prototypical homodimer.