DEPT

Ordinary carbon-13 NMR tells you how many kinds of carbon a molecule has, but it does not tell you how many hydrogens hang off each carbon — a quaternary carbon with no hydrogens looks much like a CH carbon. DEPT is a clever variant of the carbon experiment that fills this gap: it sorts the carbons by how many hydrogens each one carries. It is one of the simplest of the so-called pulse-sequence tricks that make modern NMR so powerful.

Without diving into the radio-pulse choreography, here is what DEPT delivers. The most common version, DEPT-135, produces a carbon spectrum in which the peaks point in informative directions: CH and CH3 carbons point up, CH2 carbons point down, and carbons bearing no hydrogen at all (quaternary carbons) do not appear. So in one glance the spectrum is color-coded by hydrogen count — you instantly see which carbons are CH3 or CH (up), which are CH2 (down), and, by comparing with the plain carbon spectrum, which are bare quaternary carbons (present there but missing in DEPT).

This sorting is a huge help in assembling a structure. Knowing that a carbon at a certain shift is a CH2 rather than a CH3 immediately constrains how it can connect to the rest of the molecule. DEPT is a gateway to the broader world of two-dimensional NMR — experiments like COSY and HSQC that spread the data across two axes to show directly which carbon is bonded to which hydrogen and which atoms are neighbors. For large or complex molecules, these two-dimensional methods turn an impossible puzzle into a solvable one.