Friday, July 29, 2016

Another example of competing quantum effects in hydrogen bonds

Previously, I have highlighted how one of the organising principles for understanding quantum nuclear effects in hydrogen bonding is that of competing quantum effects.
This idea features in this talk and this recent review about water.

Basically, as the strength of the hydrogen bond in an X-H...Y systems increases, the zero point energy associated with the X-H stretch (bending) vibrational modes increases (decreases).
The effect manifests in a wide range of isotope effects where hydrogen is replaced with deuterium.
The relative magnitude of these competing effects changes with the bond strength, and so the sign of the isotope effects can be positive or negative.

This week I learned of another nice example of competing quantum effects in the paper.

Why Does Argon Bind to Deuterium? Isotope Effects and Structures of Ar·H 5O 2 + Complexes Laura R. McCunn, Joseph R. Roscioli, Ben M. Elliott, Mark A. Johnson, and Anne B. McCoy

The figure below shows the ground state geometry of the system before deuterium substitution.

When one H is replaced by a D it prefers to be one of the end H's not the central one, again due to zero point energy considerations. The paper answers the question as to where the Ar binds: to one of the end H's or the D? It turns out it is due to the D.
The H-bonding (or D-bonding) to the Ar lowers the stretch frequency and increases the bend frequency. It turns the zero point energy is lowered the most by D-bonding.

The conclusion nicely puts the work in a broader context.
in the case of deuterated water dimer, the deuterium-bound conformers of H2O · D2O or (HOD)2 have lower ZPEs than the H-bound conformers. 
Likewise, in the case of I-·DOH, for example, the D atom is preferentially in the bound position, whereas in F- · HOD, the H atom is in the bound position...  Cl- · DOH behaves like the I- complex. The OH-stretch frequency of the halide-bound OH bond in F- · H O is considerably lower than the OH-stretch frequency in water. This large difference in ZPE is the driving force for the H being in the shared position. This is analogous to the situation of the shared H in Zundel. In I-·H2O, the difference between the OH-stretch frequencies is small, and it is the dependence of the lower frequency in-plane and out-of- plane bends upon the location of the D atom that determines the energy ordering of the two isomers.

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