2D Experiments

Two-dimensional correlation spectra are nowadays standard in NMR spectroscopy. Usually, the more sensitive nucleus (¹H or ¹⁹F) is observed while information on the X nucleus is obtained indirectly. This requires the presence of a reasonable spin-spin coupling, either a one-bond coupling (in case of HSQC or HMQC) or a coupling across several bonds (HMBC). Signals of protons which are not coupled to a X nucleus are suppressed by appropriate pulse techniques.

Especially in case of inverse probeheads where the inner receiver coil is tuned to the proton frequency, a considerable increase in sensitivity is obtained with respect to a direct observation of the less sensitive X nucleus. For example, the direct observation of a ¹⁵N nucleus is tedious and requires a long time due to the low abundance of the ¹⁵N isotope, its low basic frequency as well as the negative gyromagnetic ratio. However, ¹⁵N spectra are readily accessible via HSQC or HMBC correlations in reasonable times.

In principal, a series (TD2) of ¹H spectra (F2 domain) is recorded, 256 being a typical number. Variation of a time delay of the pulse sequence and subsequent Fourier transformation generates the F1 domain. In case of HSQC or HMBC spectra the F1 domain contains information on the chemical shift of the X nucleus. The number TD2 defines the resolution in the F1 dimension and the total required time.

The standard 2D experiments are usually preceded by 1D spectra in order to optimize the F2 and possibly the F1 sweep widths. These spectra are also used for the projections in the printout.

In most 2D pulse sequences gradient spectroscopy (GS) is applied which allows less time-consuming experiments.