A "one dimensional" NMR-spectrum has two dimensions: the x-axis shows resonance frequency (chemical shifting in ppm) and the y-axis the signal intensity. In contrast to this both axes of 2D-NMR-spectrum describe frequency axes. The third (z-axis) of 2D-NMR-spectrum is the intensity axis. The usual diagram of 2D-NMR-spectrum is displayed as a sectional view (level curve visualisation, comparable with a map). Usually in a 2D-NMR-spectrum the two frequency axis are subdivided in F2 (directly measured dimension) and F1 (indirectly measured dimension). The most important two-dimensional NMR-techniques for structure determination are the [1H,1H]-COSYprocedure, the[1H,1H]-or ROESY procedure, the[1H,13C]-HSQC- or HMQC procedure and the [1H,13C]-HMBC procedure.
The [1H,1H]-COSY procedure
Within the COSY-experiments scalar linkages of proximate protons are detected. Only rarely are there detectable longer range chemical shifts. These linkages are especially useful for allocation of 1H-NMR-signals and represents beside the 1D-1H-NMR-spectrum an important form of analyse.
The [1H,1H]-NOESY- or ROESY procedure
The NOESY- resp. ROESY-experiment can be used for the assignment of neighbouring protons by a so called nuclear Overhauser effect. The NOESY resp. ROESY procedure is especially meaningful for the clarification of stereo chemical structures.
The [1H,13C]-HSQC- or HMQC procedure
With the HSQC (HMQC) procedure a direct allocation of 1H- resp. 13C-NMR-signals can be achieved by investigation of direct coupling of protons to carbon-atoms. This provides information about the carbon scaffold of the molecule. Since in this way an "indirect" 13C-spectrum is generated evidence about the direct carbon-atom bound protons is obtained with relatively small sample amounts.
The [1H,13C]-HMBC procedure
The HMBC procedure is an invaluable tool for structure determination of natural compounds. In contrast to HSQC or HMQCNMR the "long-range"- coupling of protons to more distant carbon-atoms is analysed. If there are sufficient protons in a molecule then the determination of the structure of the carbon-scaffold becomes possible by indirect measurement of 1H-NMR-spectra.