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

Heteronuclear single-quantum correlation spectroscopy (HSQC) is a 2D NMR technique that reveals one-bond correlations between hydrogen and a heteronucleus. The HSQC experiment is similar to the heteronuclear correlation experiment (HETCOR) but is more sensitive. In the HSQC spectrum, the proton chemical shift is plotted on the horizontal F₂ axis, while the ¹³C chemical shift is plotted on the vertical F₁ axis. The corresponding proton and ¹³C spectra are also shown. The HSQC contour plot does not have any diagonal peaks. All the cross-peaks indicate the protons directly attached to each carbon in a molecule. Similar to the COSY spectrum, imaginary lines are drawn in horizontal and vertical directions to identify the correlated peaks.

For example, the HSQC spectrum of 2-chlorobutane has a carbon signal at 11 ppm that can be assigned to the shielded methyl group and correlated to the proton multiplet at 1.01 ppm. The carbon signal at 24.8 ppm arises from the deshielded methyl group, which correlates to the proton multiplet at 1.52 ppm. Similarly, the methylene carbon signal at 33.3 ppm and methine carbon signal at 60.4 ppm correlate to the multiplets at 1.72 and 3.96 ppm, respectively. HSQC is a very useful technique for structurally elucidating complex molecules and is routinely used in protein NMR studies.