Crosspeaks

Crosspeaks are signals in two-dimensional NMR spectra that arise at coordinates corresponding to the frequencies of two nuclei, but are not on the diagonal. The diagonal contains signals where the two axes share the same nucleus frequency, i.e., the peak at (ω1, ω2) with ω1 = ω2. Crosspeaks off the diagonal indicate correlations between spins and arise from interactions that link the nuclei being observed in the experiment.

In correlation experiments such as COSY (Correlation Spectroscopy), crosspeaks reflect scalar (through-bond) coupling between nuclei, typically protons, indicating which nuclei are coupled within a spin system. In NOESY (Nuclear Overhauser Effect Spectroscopy) and ROESY (Rotating-frame NOE), crosspeaks arise from through-space dipole-dipole interactions and provide information about spatial proximity between nuclei. In heteronuclear experiments like HSQC (Heteronuclear Single Quantum Coherence) or HMBC (Heteronuclear Multiple Bond Correlation), crosspeaks reveal correlations between protons and heteroatoms such as 13C or 15N.

Interpretation: The presence, absence, and intensity of crosspeaks help assign resonances and establish connectivity and spatial relationships. Crosspeak patterns are used to infer molecular structure, sequence, and conformation. The exact pattern depends on the experiment, its mixing time, and relaxation properties.

Appearance: In a 2D spectrum, the crosspeak appears at the intersection of the frequency axes of the two coupled nuclei. Depending on symmetry and labeling, some crosspeaks may be symmetric about the diagonal.

Limitations: Overlap, broad peaks, weak coupling, and decoupling schemes can complicate interpretation. Proper analysis often requires complementary experiments.