Friday, June 25, 2010

Quantifying the effect of "small" chemical changes

Physicists say the details don't matter. Chemists say they do. Biologists say the details are a matter of life and death.

If you make a "small" change in a molecule what effect will it have on its properties?

Long ago Hammett found a fairly robust and empirical way [the Hammett equation] to quantify the answer, at least for organic chemical reactions involving aromatic molecules.

A nice J. Phys. Chem. A paper by Cordes et al. considers the following important problem in photophysics. Consider the molecule below which upon irradiation can undergo a conformational change (photoisomerisation).

If the substituents R1 and R2 are changed what effect does that have on the rate of photoisomerisation (non-radiative decay)?

They find the rate of both the forward and backward photochemical reactions [which varies by two orders of magnitude] can be correlated with Hammett's parameters for the substituents.

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