Hydrogen-bonding, in gas phase

G.S. Denisov, K.G. Tokhadze, Ultrastrong hydrogen bond in gas phase. Dimer of dimethyl -phosphinic acid. Dokl. Phys. Chem. 337, 117-119 (1994)... 

Legon AG, Millen DJ (1987) Directional character, strength, and nature of the hydrogen bond in gas phase dimers. Acc Chem Res 20 39 -46... 

Strongest intermolecular interaction between neutral molecules Examples N-H - N hydrogen bonds between NH3 molecules inpureNHs 0-H - 0 hydrogen bonds H2O molecules in pure H2O Hydrogen bonding in gas-phase molecules... 

Legon, A. C. "Directional Character, Strength, and Nature of the Hydrogen Bond in Gas-Phase Dimers." Acc. Chem. Res., 20,39-46 (1987). 

As an example of hydrogen-bonded molecules which play the role of a bath in relaxation of the excited i (A-H) vibrational level can be the dimers of phosphinic acids studied in the gas phase at high temperatures [43]. In Fig. 8 the results of investigations of profiles of broad IR bands ascribed to i (OH) vibrations with characteristic Hadzi s ABC trio are shown. These spectra are very similar to those recorded for strong hydrogen bonds in condensed phases [44]. 

The hydration of halide anions is of intrinsic interest to the process of solvation. The most important aspect of water, its hydrogen bond network, is directly perturbed by the anion in a simple and direct way. It competes for the protons with its own ionic hydrogen bond. The gas phase studies of the smallest hydrated ions show extremely large shifts in the 0-H stretch in the 0-H-X bond. This strong interaction must play a role in the bulk solvation process. Interesting implications will be discussed in the final section of this chapter. 

Introduction of a second adsorbed ammonia molecule in the neighboring position generates a weak potential barrier in this rotation (broken line in Fig. 7b), which decreases with increase of the distance between the two adsorbed species. The most influenced intramolecular vibration after the adsorption is the umbrella, connected with molecular inversion of the three N-H bonds in gas-phase ammonia. Stronger attraction of the nitrogen atom to the siuface and repulsion of the hydrogens converts the symmetric double-well inversion potential of the gas molecule into a distorted single well (Fig. 8). This change in the potential ciuve modifies the distances between the vibrational levels for this mode. 

Studies have indicated that the association of HF in the gas phase leads predominantly to dimers or hexamers with small amounts of tetramers. Hydrogen bonding in liquids such as sulfuric and phosphoric acids is responsible for them being viscous liquids that have high boiling points. 

C. J. Cramer and D. G. Truhlar, Quantum chemical conformational analysis of 1,2-ethanediol Correlation and solvation effects on the tendency to form internal hydrogen bonds in the gas phase and aqueous solution, J. Am. Chem. Soc. 116 3892 (1994). 

The major activity in gas-phase studies now depends on the use of modem techniques such as ion cyclotron resonance (ICR). Thus, as already mentioned (Section ELD). Fujio, Mclver and Taft131 measured the gas-phase acidities, relative to phenol, of 38 meta- or para-substituted phenols by the ICR equilibrium constant method, and their results for +R substituents led them to suggest that such substituents in aqueous solution exerted solvation-assisted resonance effects. It was later163 shown by comparison of gas-phase acidities of phenols with acidities of phenols in solution in DMSO that solvation-assisted resonance effects could also occur even when the solvent did not have hydrogen-bond donor properties. Indeed for p-NC>2 and certain other substituents these effects appeared to be larger than in aqueous solution. 

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