13C NMR of a Delicious Christmas Treat

As my Santa Claus-like belly may indicate, I love holiday treats. The solid-state ¹³C NMR spectra below were collected from a special sample of a holiday treat prepared by my wife, Patty from her Great Grandma Jennings lab book. The sample was prepared from only four ingredients as follows:

To 227 g of softened butter, 65 g of fructose was added while stirring with a spatula. Slowly, 199 g of flour and 1.26 g of sodium chloride were stirred into the mixture until it became difficult to mix with a spatula. The mixture was kneaded gently until cracks in the surface began to appear after which it was rolled to a thickness of 38 mm and cut into round samples of approximately 51 mm in size. The samples were heated in an oven at 436 K - 450 K for approximately 600 seconds until gold in color.

The bottom trace is a ¹³C CPMAS spectrum and the top trace is a ¹³C MAS spectrum. Both spectra were acquired with high power ¹H decoupling. This pair of spectra serves to illustrate the different types of information available from each of these techniques. The sample is a mixture of rigid and mobile components. The ¹³C CPMAS technique detects mainly the more rigid components as it relies on the dipolar coupling between protons and ¹³C for the cross polarization. The dipolar coupling is averaged to nearly zero for the mobile constituents and therefore they do not appear in the spectrum. The ¹³C CPMAS spectrum therefore, shows primarily all of the rigid constituents (mainly flour and sugar). The ¹³C MAS spectrum with high power ¹H decoupling shows both rigid and mobile constituents. The resonances from the mobile constituents (mainly butter) have sharp lines while the broader lines from the rigid constituents show up at very low intensity as the sensitivity is not enhanced by cross polarization.

Now, you too have enjoyed Patty's delicious shortbread.