From a 1H-decoupled carbon (¹³C) spectrum useful information about ¹³C chemical shifts is obtained. Alternatively, carbon multiplicity and ¹H-¹³C coupling constants can be determined from the gated decoupled methodology and quantitative measurements can be made from the inverse gated methodology.

Easy implementation on AVANCE spectrometer

The basic pulse sequence to record a standard proton spectrum consists of the following steps:

• Relaxation period (d1) to achieve a pre-equilibrium state.
• Read pulse (p1) to create transverse magnetization.
• Acquisition during the t₂ (acq) period.

An important aspect to consider is the broadband proton decoupling. It can be applied during the relaxation and/or during the acquisition periods:

• Gated decoupled methodology: Proton decoupling is only applied during the relaxation period. From the resulting ¹H-coupled ¹³C spectrum carbon multiplicities, and direct and long range ¹H-¹³C coupling constants can be determined.
• Inverse gated decoupled methodology: Proton decoupling is only applied during the acquisition period. In this case, no polarization transfer from ¹H to ¹³C via NOE takes place and therefore, the resulting ¹H-coupled ¹³C spectrum can be used for quantitative measurements.

The ¹³C spectrum is usually acquired, processed and plotted in a fully automated way. Minor changes from a predefined parameter set are required. For routine applications, a 30º-45º  pulse with a short relaxation delay (1 second) is usually applied.

In demanding applications, good accuracy is advisable to obtain reliable integration values. In this case, the ¹³C spectrum is recorded with a 90º pulse and a long relaxation period (5*T₁(¹³C)) is used to avoid partial signal saturation.

For further details on practical implementation on AVANCE spectrometers see:

Recording a 1H-decoupled 13C spectrum

1H-coupled 13C spectrum

1H-decoupled 13C spectrum(without NOE)

The ¹³C spectrum displays resonance intensities vs frequency. The signals are referenced to the TMS signal (0 ppm) or to the internal deuterated solvent.
See 13C Spectra

Assignment of the ¹³C spectrum is usually performed by using other NMR methods. As a general strategy:

• Carbon multiplicity is determined from 1D DEPT experiments.
• Correlation with ¹H nuclei via ¹J(XH) is achieved from HETCOR-type or HMQC-type experiments.
• Correlation with ¹H nuclei via ⁿJ(CH) is achieved from HMBC-type experiments.