25 November 2010 Cover Art For The Journal Of Physical Chemistry A – "Computation Of Deuterium Isotope Perturbation Of 13C NMR Chemical Shifts of Alkanes: A Local Mode Zero Point Level Approach"

A bit of a sneak preview and a chance to get a second use out of an instructive pair of potentials. This new cover for an upcoming article by Kin Yang and Bruce Hudson expands upon the previous Deuterium-for-Hydrogen substitution study summarized on the blog page (and in a publication of the same name) Vicinal Deuterium Perturbations on Hydrogen NMR Chemical Shifts in Cyclohexanes. In brief, exchanging a hydrogen for a deuterium alters ¹³C NMR spectra because the deuterium, being twice the mass, has a narrower nuclear probability distribution. If the C-H/D stretching potential were harmonic (symmetric on both the elongation and contraction sides), this would produce no effect. As this stretching potential is anharmonic (as shown below and the background of the above image), the actual average positions of the D and H differ slightly (but enough!), with the C-D bond being, on average, slightly shorter than C-H (something that a quantum chemical calculation will not tell you without treating the nuclei as quantum mechanical objects). Different average bond length, different affect on a bound ¹³C.

A bit more explanation can be found at www.somewhereville.com/?p=124.

The studies continue, limited largely by the number of simple hydrocarbons with high-quality NMR spectra (and selective deuteration).