This work demonstrates how large-amplitude OH librational motion of H2O molecules directly reflects the microsolvation of organic compounds. The highly localized OH librational motion of the first solvating H2O molecule gives rise to a strong band origin νlib in the far-infrared spectral region, which is correlated quantitatively with the intermolecular hydrogen bond energy D0.

The high-resolution terahertz absorption spectrum of the large-amplitude intermolecular donor librational band ν of the homodimer (HCN) has been recorded by means of long-path static gas-phase Fourier transform spectroscopy at 207 K employing a highly brilliant electron storage ring source. The rovibrational structure of the ν band has the typical appearance of a perpendicular type band of a Σ-Π transition for a linear polyatomic molecule. The generated terahertz spectrum is analyzed employing a standard semi-rigid linear molecule Hamiltonian, yielding a band origin ν of 119.11526(60) cm together with values for the excited state rotational constant B', the excited state quartic centrifugal distortion constant D' and the l-type doubling constant q for the degenerate state associated with the ν mode. The until now missing donor librational band origin enables the determination of an accurate experimental value for the vibrational zero-point energy of 2.50 ± 0.05 kJ mol arising from the entire class of large-amplitude intermolecular modes. The spectroscopic findings are complemented by CCSD(T)-F12b/aug-cc-pV5Z (electronic energies) and CCSD(T)-F12b/aug-cc-pVQZ (force fields) electronic structure calculations, providing a (semi)-experimental value of 17.20 ± 0.20 kJ mol for the dissociation energy D of this strictly linear weak intermolecular CHN hydrogen bond.

The specific far-infrared spectral signatures associated with highly localized large-amplitude out-of-plane librational motion of water molecules have recently been demonstrated to provide sensitive spectroscopic probes for the micro-solvation of organic molecules [Mihrin et al., Phys. Chem. Chem. Phys. 21(4), 1717 (2019)]. The present work employs this direct far-infrared spectroscopic approach to investigate the non-covalent intermolecular forces involved in the micro-solvation of a selection of seven ether molecules with systematically varied alkyl substituents: dimethyl ether, diethyl ether, diisopropyl ether, ethyl methyl ether, t-butyl methyl ether, and t-butyl ethyl ether. The ranking of the observed out-of-plane water librational band signatures for this selected series of ether–water complexes embedded in inert neon matrices at 4 K reveals information about the interplay of directional intermolecular hydrogen bond motifs and non-directional and long-range dispersion interactions for the micro-solvated structures. These far-infrared observables differentiate minor subtle effects introduced by specific alkyl substituents and serve as rigorous experimental benchmarks for modern quantum chemical methodologies of various levels of scalability, which often fail to accurately predict the structural variations and corresponding vibrational signatures of the closely related systems. The accurate interaction energies of the series of ether–water complexes have been predicted by the domain based local pair natural orbital coupled cluster theory with single-, double-, and perturbative triple excitations, followed by a local energy decomposition analysis of the energy components. In some cases, the secondary dispersion forces are in direct competition with the primary intermolecular hydrogen bonds as witnessed by the specific out-of-plane librational signatures.

The high‐resolution infrared absorption spectrum of the donor bending fundamental band ν 61 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 61 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.