London Dispersion-Assisted Low-Temperature Gas Phase Synthesis of Hydrogen Bond-Inserted Complexes

Abstract

Supersonic expansions of organic molecules in helium carrier gas mixtures are used to synthesize model (pre)reactive complexes at low temperature. Whether or not barriers for hydrogen bond rearrangements can be overcome in this collisional process is not well understood. Using the example of alcohols inserting into intramolecular hydrogen bonds of α-hydroxy esters, we explore whether dispersion energy donors can assist the process in a systematic way. Bromo, iodo, and tert-butyl substitution of benzyl alcohol in the para-position is used to show that the insertion process into methyl glycolate is controllable, whereas it is largely avoided for the chiral methyl lactate homologue. Methyl lactate appears to steer the transient chirality of benzyl alcohol derivatives in a uniform direction relative to the lactate handedness for the OH∙∙∙O=C insertion product, as well as for the competing attachment to the hydroxy group of the ester. A simple rule based on the total binding energy in relation to the rearrangement barrier is tentatively proposed to estimate whether the insertion is feasible or not in such molecular complexes during expansion.

Dispersion Control over Molecule Cohesion: Exploiting and Dissecting the Tipping Power of Aromatic Rings

“…73 Control of the reaction barrier heights in low-temperature supramolecular reactions by London dispersion interaction is a design strategy which may be transferable to room-temperature reactions if the barriers are made higher. 71 In summary, the higher flexibility of benzyl alcohol enables an earlier manifestation of competing London dispersion interactions. The conformational landscape becomes more corrugated, which poses challenges for the theoretical structure and conversion path search as well as for the experiment to reach the lowest-energy minimum.…”

“…This is also due to the dispersion interaction of the phenyl ring with the hydroxyester, which raises the barrier for interconversion. By adding a tert -butyl group in the para position, the barrier is further increased and now the insertion is already incomplete for methyl glycolate . Replacement of the tert -butyl group by a halogen (Cl, Br) also blocks insertion more or less completely but now insertion is thermoneutral or uphill (Figure ), so this is more expected.…”

Subtle hydrogen bond preference and dual Franck–Condon activity – the interesting pairing of 2-naphthol with anisole

“…The energy freed upon complexation is in principle enough to induce thermal isomerisation even in a supersonic jet expansion. 16,74,75 Such processes are still poorly understood, unlike corresponding laser-induced processes. 76,77 Therefore, partial relaxation of t 2NA O to the further solvent-stabilised c 2NA O should not be strictly ruled out but is unlikely to be quantitative.…”

“…Because the driving force of hydrogen bond isomerisation is weak and the barrier rather broad, it is difficult to say whether a subtle energy penalty will result in substantial depopulation by simple carrier gas collisions or more elaborate mechanisms. 16,74,75…”

“…One is the ground state preference for one over other structural arrangements of the complex in the competition of intermolecular forces. It is addressed by the concept of intermolecular energy balances, 14–16 where systems are picked for being ambivalent in terms of two different arrangements of their constituents. 17–24 The other aspect is the influence of molecular excitation of the components on this structural arrangement of the complex, due to the coupling of intramolecular degrees of freedom with intermolecular ones.…”