Very Strong Hydrogen Bonds

for the formic acid dimer, which is significantly larger according to the authors. 

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Color from Vibrations and Rotations. Vibrational excitation states occur in H2O molecules in water. The three fundamental frequencies occur in the infrared at more than 2500 nm, but combinations and overtones of these extend with very weak intensities just into the red end of the visible and cause the blue color of water and of ice when viewed in bulk (any green component present derives from algae, etc). This phenomenon is normally seen only in H2O, where the lightest atom H and very strong hydrogen bonding combine to move the fundamental vibrations closer to the visible than in any other material. 

For a review of very strong hydrogen bonding, see Emsley, J. Chem. Soc. Rev. 1980,... 

Gonzales, L., Mo, O., Yafiez, M., Elguerdo, J., 1998b, Very Strong Hydrogen Bonds in Neutral Molecules The Phosphonic Acid Dimers , J. Chem. Phys., 109, 2685. 

You can rule out choice B, hydrogen peroxide, and choice C, water, because the very strong hydrogen bonds between their molecules lower the vapor pressure (the ease at which the liquid evaporates). Although answer A, carbon tetrachloride, the only nonpolar molecule in the list, has only dispersion forces present between molecules, choice D, dichloromethane, has the lowest molecular weight and consequently the lowest amount of dispersion forces. 

Hydrogen Bonding The Role of (Local) Softness in (Very) Strong Hydrogen Bonding... 

At 25°C, the keto-B-oxazoline (from VIII) and keto-B-thia-zoline (from IX) are nearly exclusively (>90 %) in the chelated enamine form both for model compounds and copolymers,either in bulk or in most solvents over a wide range of polarity (from CHCl to DMSO). The keto form does appear only in very strong hydrogen bonding donor solvents like trifluoroethanol (TFE). 

This situation applies with weak hydrogen bonds at one extreme and very strong hydrogen bonds at the other with H and D confined to the same potential well. However, when the potential energy barrier has fallen sufficiently to allow the proton to escape the confines of its parent well, but leaves the deuteron trapped, then different values of the isotopic ratio can be observed (Fig. 7). The effect of isotopic exchange is now much more than merely one of doubling the reduced mass of the vibrating bond. When the proton is above the barrier, the force constant of the A—H bond, k A.—H),... 

Fig. 7 Ir isotope shift associated with weak, strong and very strong hydrogen bonds.

Although there may be other very strong hydrogen-bond systems, it is still the case that the bifluoride ion represents the upper limit of hydrogen-bond strength. (If any system has both H and D above the internal energy barrier. 

An excellent review of bifluorides up to 1980 is available (Gmelin, 1982). An earlier review of very strong hydrogen bonding (Emsley, 1980) assumed that HFj " represented the upper limit of hydrogen bonding and that the proton was in a single minimum potential well. Both assumptions have since been questioned. 

Low-barrier hydrogen bonds (abbreviated LBHBs) are short, very strong hydrogen bonds formed when the electron-pair donors (most commonly oxygen or nitrogen atoms in proteins) sharing the hydrogen have comparable pXa values. 

The position and thermal stability of the 1670 cm band together with the fact that its intensity increases with increased milling time suggests that it may be attributed to water in a very strongly hydrogen bonded environment. It is hypothesised that this band indicates the presence of a type of adhesive water which holds together the aggregates of milled kaolin particles. 

Strong hydrogen bonds can be divided into two classes, the very strong hydrogen bonds with 0.5>5 h>0.4vu which are necessarily linear, and those... 

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