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⋯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⋯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 para-hydrogen matrices. A (semi)-empirical value for the change of vibrational zero-point energy of 5.5 $\pm$ 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 $\pm$ 0.3 kJ mol⁻¹ for the dissociation energy D₀ of this global potential energy minimum.

Rate constants for the charge-transfer reaction between ${\mathrm{N}}_2{\mathrm{H}}^{+}$ and Rb in the millikelvin regime are measured in an ion-atom hybrid trap and are found to be lower than the Langevin capture limit. Multireference ab initio computation of the potential energy surfaces involved in the reaction reveals that the low-temperature charge transfer is hindered by short-range features highly dependent on the collision angle and is promoted by a deformation of the molecular frame. The present study highlights the importance of polyatomic effects and of stereodynamics in cold molecular ion-neutral collisions.

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 terahertz absorption spectrum of the large-amplitude intermolecular donor librational band ν18 of the homodimer (HCN)₂ 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.

In a recent theoretical investigation of DCl–H2O, HCl–D2O, and DCl–D2O [Felker et al., J. Phys. Chem. A, 125(29), 6437 (2021)] employing an accurate 9D permutation invariant polynomial-neural network potential energy surface and a highly efficient bound-state methodology, all the intramolecular vibrational eigenstates and dimerization spectral shifts of the three isotopic binary 1:1 complexes have been predicted. By means of dedicated annealing procedures, relative concentration dependencies, and a specialized dual inlet deposition procedure enabling complexation between specific isotopically substituted subunits, the present work identifies the intramolecular vibrational transitions experimentally for these three isotopologues of the binary complex and the most stable cyclic conformations of selectively deuterated mixed (HCl)m(H2O)n (m + n ≤ 4) cluster molecules embedded in inert neon “quantum matrices” at 4 K. The vibrational assignments up to mixed ternary cluster molecules are supported by harmonic CCSD(T)-F12b/cc-pVTZ-F12 frequency predictions in conjunction with anharmonic corrections employing second-order vibrational perturbation theory (VPT2) at the MP2/aug-cc-pVTZ level of theory. While the assigned O–H and O–D stretching transitions in neon are systematically spectrally redshifted by 0.2%–0.5% relative to previously reported observations in supersonic jets, the assigned H–Cl and D–Cl stretching transitions all reveal anomalous excessive spectral redshifts in neon increasing with the size of the cluster molecules. These cluster-size dependent excessive H–Cl/D–Cl spectral redshifts in neon indicate that the extent of charge transfer is enhanced strongly with the complexation of an increasing number of H2O molecules as predicted by quantum chemical models for more than a decade.

High-resolution Fourier transform infrared (FTIR) spectra of methyl fluoride (CH₃F) were recorded in the mid- and far-infrared regions using the Bruker IFS 125HR spectrometers at GSMA (Reims, France) and at the SOLEIL synchrotron facility (Saint-Aubin, France). The measurements cover both the pure rotational transitions of the ground state (10–100 cm⁻¹) and the vibrational triad region (1950–2450 cm⁻¹), which includes the 2${\nu }_3$, ${\nu }_3+{\nu }_6$, and 2${\nu }_6$ bands. Spectra were recorded under various pressure conditions to optimize line visibility, with a high resolution. Line assignments were performed using predictions from the tensorial effective Hamiltonian implemented in the MIRS package, together with a newly developed automated assignment tool, SpectraMatcher, which facilitates line matching and discrimination of CH₃F transitions from overlapping CO₂ features. More than 5000 transitions (up to $J=52$ in the ground state and up to $J=45$ in the triad and $K=19$) were assigned and included in a global fit. The sixth-order tensorial effective Hamiltonian model yielded excellent agreement with experiment, with root mean square (RMS) deviations better than 7 × 10⁻⁴ cm⁻¹ across all regions. This paper presents the first continuous rovibrational study of CH₃F over both the triad and far-infrared ground state regions. The improved accuracy from previous studies stems from the improved set of effective Hamiltonian parameters which will also form a good basis from future applications in atmospheric modelling and spectroscopic databases.