Tuesday, July 20, 2010

The challenge of water

As I have discussed in some previous posts, water is amazing stuff. It is also amazing how it continues to be a struggle to understand and to model its basic properties. Today Francesco Paesani (UC San Diego) gave a stimulating talk about his recent work on this problem. Ultimately, he wants to modeling the chemistry of aqueous aerosol surfaces (this is relevant to climate change). Here are a few things I learnt that I found particularly interesting:
67 anomalous properties of water are documented here.

What is the structure of liquid water?
Two extreme pictures are
1. local orientational order due to strong near-tetrahedral hydrogen-bonds which minimise enthalpy.
2. local orientational disorder characterised by nondirectional H-bonds.
A recent PNAS paper that real water actual fluctuates significantly between the two.
This is controversial.

Is the surface of neat water neutral, acidic, or basic?
There is significant controversy about this although some consider a recent C&E News story: Storm in a teacup amplifies the controversy [in the time honoured tradition of journalism].

One really needs to include quantum nuclear dynamics to realistically model water.
Classical molecular dynamics with the standard force field gives a melting temperature of 215 K.
To do quantum dynamics one needs a high quality potential energy surface extracted from high level quantum chemistry on the water dimer. This pairwise interaction is about 85 per cent of the total interaction. One can then try to get the rest of the interactions from explicit polarisation.

Francesco then calculated a wide range of properties including the lineshapes of infra-red spectra and three-photon echo vibrational spectra.

He then considered the controversial question of how do hydrogen bonds form and break in liquid water.
This aims to test a proposal of Laage and Hynes for a:
water reorientation mechanism involves large-amplitude angular jumps, rather than the commonly accepted sequence of small diffusive steps, and therefore calls for reinterpretation of many experimental data wherein water rotational relaxation is assumed to be diffusive.

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