Ξ values for indirect referencing

In the previous post I mentioned that indirect referencing was required to compare spectra collected in methanol-d₄ and methanol-d₃. Indirect referencing relies on precisely determined ratios of the ¹H gyromagnetic ratio and the gyromagnetic ratio of the nucleus being referenced. Unfortunately, many different values of this ratio, known as Xi (Ξ), have been reported. Here I test several of these Ξ values to see how closely they match referencing to an internal standard.

Indirect referencing

Most chemists use the residual protonated solvent peak to reference their spectra, but in two dimensional heteronuclear spectra, like HMBCs, the residual solvent peak is often distorted or not present at all. The ¹H dimension of such experiments can usually be referenced by aligning it to a 1D ¹H spectrum. The heteronuclear dimension, however, is more difficult to reference correctly. The best way to reference such spectra is to use indirect referencing to calculate the chemical shifts based on the frequency ratio of the nuclei involved.

Indirect detection

The previous post examined ¹³C sensitivity using direct detection of ¹³C. Here I look at obtaining ¹³C chemical shifts using indirect detection.