The effect of alkylation on the micro-solvation of ethers revealed by highly localized water librational motion

Abstract

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.

Self-Association and Microhydration of Phenol: Identification of Large-Amplitude Hydrogen Bond Librational Modes

“…Furthermore, the spectroscopic detection of these large-amplitude hydrogen bond librational modes reveals the most dominating contributions to the change in vibrational zero-point energy upon complexation (∆ZPE) [25]. These observations help to translate high-level ab initio quantum chemical predictions of electronic equilibrium dissociation energies (D e ) into accurate semi-empirical ground-state dissociation energies (D 0 ), which is notoriously challenging from first principles quantum chemistry alone due to the highly anharmonic nature of this class of large-amplitude vibrational motion [26][27][28][29].…”

Raising the benchmark potential of a simple alcohol-ketone intermolecular balance

“…Annealing processes like in weakly interacting matrix isolation are not possible. 11 Softer degrees of freedom like rotation, intermolecular vibration or translation may however be further cooled, simplifying the spectra. Excitation in stiff vibrations is depopulated inefficiently, but this is of minor concern if the initial temperature of the molecules is sufficiently low.…”