Attached Proton Test, an 'APT' experiment for structural elucidation

A key step towards elucidating structures with NMR spectroscopy is the assignment of signals to specific groups within the molecule being analyzed. Two experiments, DEPT (Distortionless Enhancement by Polarization Transfer) and APT (Attached Proton Test), are typically used to aid this process with 13C NMR spectra.[1] Both experiments are similar in that the number of attached protons (i.e. the multiplicity) is revealed by the phase of the 13C NMR signals. The key difference between the DEPT and APT experiment is that signals for quaternary carbons are observed in the APT experiment. As detailed in our previous blog post, signals for the quaternary carbons are suppressed in the DEPT experiment. The figure below displays an example of an APT spectrum recorded on our NMReady-60PRO.

Figure 1.  APT NMR spectrum of diethyl phthalate.

Figure 1. APT NMR spectrum of diethyl phthalate.

In the APT experiment, quaternary and methylene carbons (CH2) are phased negatively while methine (CH) and methyl (CH3) carbons are phased positively. With one experiment all the 13C NMR signals of diethyl phthalate are observed along with information about their multiplicity, allowing for the straightforward assignment of the signals.

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[1]Jacobsen, N. E. “NMR Spectroscopy Explained: Simplified Theory, Applications and Examples for Organic Chemistry and Structural Biology” 2007, John Wiley & Sons, Inc.: Hoboken, Chapter 7.