HSQC: One-Bond Heteronuclear Correlation

Introduction

The ¹³C spectrum of an organic molecule tells you how many different carbons there are, and the ¹H spectrum tells you about the protons, but a 1D spectrum cannot say which proton sits on which carbon. HSQC provides exactly that link. Every peak in an HSQC spectrum pairs a proton shift with the shift of the carbon it is directly bonded to. Because it is sensitive and clean, HSQC is usually the first heteronuclear 2D experiment run on an unknown.

Core explanation

HSQC exploits the large one-bond coupling between a proton and its carbon, ¹J(CH). For sp³ carbons it is about 125–135 Hz, for sp² carbons about 155–170 Hz, and for terminal alkynes around 250 Hz. Couplings over two or three bonds are much smaller, below about 10 Hz. Delays in the pulse sequence are set to match ¹J, typically 1/(4 × 145 Hz) ≈ 1.7 ms, so only directly bonded pairs transfer magnetisation efficiently.

The sequence starts on the protons. An INEPT block transfers proton polarisation to the attached carbon, creating carbon single-quantum coherence. This carbon coherence evolves during t₁ at the ¹³C chemical-shift frequency, which becomes the F1 axis. A proton 180° pulse in the middle of t₁ removes the effect of C–H coupling, so each carbon gives a single line in F1. A reverse INEPT block then sends the magnetisation back to the proton, which is detected during t₂, usually with broadband carbon decoupling so each proton appears as a singlet in F2. The out-and-back route — proton to carbon to proton — is why HSQC is called an inverse or proton-detected experiment.

Proton detection gives a large sensitivity advantage. The signal-to-noise ratio of an NMR experiment scales roughly with γ excited × γ detected^(3/2). Because the proton magnetogyric ratio is about four times that of ¹³C, starting and finishing on protons gives, in principle, an enhancement of around 30-fold over direct ¹³C observation with the same number of ¹³C nuclei. That matters because ¹³C is only about 1.1% abundant: in HSQC the 98.9% of protons bonded to ¹²C must be suppressed, which modern instruments do with pulsed field gradients that select only coherence pathways passing through ¹³C.

The spectrum has ¹H along F2 and ¹³C along F1. There is no diagonal. Each CH, CH₂ and CH₃ group gives a peak; quaternary carbons, carbonyl carbons without H, and carbons bearing no protons do not appear at all. A CH₂ group whose two protons are diastereotopic , for example next to a stereocentre, gives two peaks at different proton shifts but the same carbon shift — an immediate signature of a non-equivalent methylene pair. Protons on oxygen or nitrogen give no peak with carbon, so OH and NH signals are absent from a ¹H–¹³C HSQC; ¹H–¹⁵N HSQC is used for those.

In multiplicity-edited HSQC , an extra delay makes CH₂ peaks appear with opposite phase to CH and CH₃ peaks, usually drawn in different colours. This replaces the separate DEPT-135 experiment and gives multiplicity information with carbon resolution spread across two dimensions.

HSQC also spreads overlapping proton signals according to their carbon shifts. Protons overlapping at 1.3 ppm in the 1D spectrum may be separated if their carbons differ by even 0.5 ppm, since carbon shifts cover a much larger range.

Step-by-step reasoning

1. Read each HSQC peak's coordinates as (δH, δC). 2. Use colour or sign to classify it as CH₂ or CH/CH₃. 3. Identify diastereotopic pairs by two proton shifts at one carbon shift. 4. Compare with the 1D ¹³C spectrum: carbons missing from HSQC are quaternary or protonless. 5. Label protons and carbons together for later HMBC work.

Visual explanation

Draw the proton spectrum along the top and the carbon spectrum down the side. Each spot sits where a vertical line from a proton meets a horizontal line from its carbon. A carbonyl at 170 ppm in the side spectrum has no spot on its horizontal line.

Real-world analogy

HSQC is like a building's directory that lists each resident beside their flat number. It tells you who lives where, but not who lives next door. Empty flats — quaternary carbons — never appear in the list.

Real-world example

Natural-product chemists isolating a new compound of perhaps 1 mg run HSQC early because it is sensitive enough to see every protonated carbon in a few hours, while a direct ¹³C spectrum of so little material could take a day or more.

Why?

Why does HSQC show only one-bond correlations? The transfer delays are tuned to ¹J(CH) of about 145 Hz. Couplings over two or three bonds are only a few hertz, so during those short delays they build almost no transferable magnetisation.

Common misconception

"Every carbon appears in HSQC." Only carbons carrying at least one proton appear. Quaternary carbons, C=O without H and nitrile carbons are invisible, so an HSQC count underestimates the total carbon count.

Worked example

Question: An edited HSQC of butan-2-ol shows peaks at (3.8, 69), (1.45, 32), (1.18, 23) and (0.92, 10). The peak at (1.45, 32) has opposite sign to the others. Assign each.

Reasoning: CH–O is deshielded: (3.8, 69) is C-2 H. Opposite sign identifies CH₂: (1.45, 32) is C-3. The methyl on C-2 is next to the CH–O: (1.18, 23) is C-1. The terminal methyl on C-3: (0.92, 10) is C-4.

Answer: C-2 69 ppm/3.8 ppm; C-3 32/1.45 (CH₂); C-1 23/1.18; C-4 10/0.92.

Quick check

1. Why does a carbonyl carbon at 175 ppm in a ketone never appear in a ¹H–¹³C HSQC spectrum? Answer: It has no directly attached proton, so there is no one-bond coupling to transfer magnetisation.

Exam focus

State that HSQC correlates each proton with its directly bonded carbon via ¹J(CH). Explain inverse detection and its sensitivity advantage. Use edited HSQC to identify CH₂ groups and recognise diastereotopic methylene protons.

Advanced insight

HMQC, an older alternative, uses multiple-quantum coherence during t₁, so proton–proton couplings broaden F1 peaks. HSQC keeps single-quantum carbon coherence and gives sharper carbon lines. Non-uniform sampling and pure-shift variants can push HSQC resolution far enough to separate carbons differing by a few hundredths of a ppm, valuable for complex mixtures.

Summary

HSQC transfers magnetisation from proton to directly bonded carbon and back, recording carbon shifts in F1 and proton shifts in F2. Delays tuned to ¹J(CH) select one-bond pairs. Proton detection makes it sensitive. Protonless carbons do not appear, edited HSQC separates CH₂ from CH/CH₃, and diastereotopic protons show as two peaks on one carbon.

Practice questions

1. What is a typical value of ¹J(CH) for an sp³ carbon, and why does it matter for HSQC? Answer: About 125–135 Hz; the transfer delays are tuned to this large coupling so that only one-bond pairs correlate. 2. An HSQC shows two proton peaks at 2.6 and 2.9 ppm on the same carbon at 42 ppm. What does this indicate? Answer: A CH₂ group with diastereotopic, non-equivalent protons, often next to a stereocentre. 3. Why is HSQC more sensitive than a direct ¹³C spectrum? Answer: It starts with and detects proton magnetisation, which has a much larger magnetogyric ratio than ¹³C. 4. How can you find the quaternary carbons of a molecule using HSQC? Answer: Compare the ¹³C spectrum with the HSQC; carbons present in the ¹³C spectrum but absent from HSQC carry no protons.