2D NMR: Heteronuclear Single-Quantum Correlation Spectroscopy (HSQC)

HSQC stands for heteronuclear single-quantum correlation spectroscopy, a sensitive 2D NMR technique that correlates heteronuclear spins coupled across a single bond. In a 1H–13C HSQC spectrum, the proton chemical shift appears on the horizontal F2 axis while the carbon-13 chemical shift appears on the vertical F1 axis. Each cross-peak indicates a proton directly attached to a carbon, revealing C–H bond connectivity.

HSQC is similar to the heteronuclear correlation experiment (HETCOR) but offers greater sensitivity, making it the preferred technique for detecting one-bond correlations between hydrogen and heteronuclei. Both methods reveal C–H connectivity, but HSQC's enhanced sensitivity allows researchers to obtain clearer spectra with better signal-to-noise ratios, particularly valuable for complex organic molecules and protein NMR studies.

Cross-peaks in an HSQC spectrum indicate protons directly attached to each carbon in a molecule. The spectrum lacks diagonal peaks, meaning every cross-peak represents a genuine C–H correlation. By drawing imaginary horizontal and vertical lines from each cross-peak, researchers can identify which proton multiplet correlates to which carbon signal, enabling precise structural assignment.

In an HSQC contour plot, the horizontal F2 axis displays proton chemical shifts while the vertical F1 axis displays carbon-13 chemical shifts. Cross-peaks appear where correlated proton and carbon signals intersect. For example, in 2-chlorobutane, a shielded methyl carbon at 11 ppm correlates to a proton multiplet at 1.01 ppm, while a deshielded methyl carbon at 24.8 ppm correlates to a proton multiplet at 1.52 ppm.

HSQC reveals one-bond C–H correlations, allowing researchers to map which protons are attached to which carbons throughout a molecule. This direct connectivity information is invaluable for determining molecular structure, particularly in protein NMR studies where complex three-dimensional structures require detailed assignment of all C–H bonds. HSQC's sensitivity and clarity make it routinely used in structural analysis.

To identify correlated peaks in an HSQC spectrum, draw imaginary horizontal and vertical lines from each cross-peak, similar to the method used in COSY spectra. These lines intersect the corresponding proton and carbon signals on their respective axes. This visual technique allows researchers to trace each cross-peak back to its associated chemical shifts, confirming the C–H correlation and enabling accurate structural assignment.

The 2-chlorobutane HSQC spectrum demonstrates how different carbon environments produce distinct chemical shifts that correlate to specific proton signals. The shielded methyl carbon at 11 ppm correlates to protons at 1.01 ppm, the deshielded methyl carbon at 24.8 ppm correlates to protons at 1.52 ppm, and the methylene and methine carbons at 33.3 and 60.4 ppm correlate to protons at 1.72 and 3.96 ppm respectively, illustrating systematic C–H assignment.