How TOCSY extends COSY’s reach in 2D NMR

2D NMR experiments can provide a wealth of information to aid in the structural elucidation of chemical compounds. Of the many 2D NMR experiments available, the homonuclear COSY (COrrelation SpectroscopY) sequence is one of the most popular and is used to identify scalar couplings between nuclear spins. An example is shown below:

Figure 1. gCOSY (100 MHz) spectrum of 2-propoxybenzaldehyde in deuterated chloroform.

Focusing on the alkyl region (δ = 0 – 5 ppm), we observe an off-diagonal cross peak between the sextet (H₂) and the triplet (H₁). Also, a separate cross peak is between the triplet (H₃) and sextet (H₂) which is expected as this corresponds to the propoxy fragment of the compound.

A closely related 2D experiment is the total correlation spectroscopy (TOCSY) experiment. The main difference to highlight between the two experiments is that COSY is suited for identifying direct couplings (typically ²J – ³J), whilst TOCSY provides a broader view of all protons within a spin system (>²J). The difference between the two experiments is shown below:

Figure 2. gTOCSY (100 MHz) spectrum of 2-propoxybenzaldehyde in deuterated chloroform.

The same cross peaks observed in the COSY spectrum also appear in the TOCSY spectrum (shown in blue). However, the key difference between these two experiments is highlighted by the green correlation, which indicates ⁴J coupling between the two triplets (H₁ and H₃).

TOCSY compliments COSY for analyzing organic molecules as both experiments can be used together to build a clearer picture of the proton connectivity in a molecule. Please don’t hesitate to contact us if you have any questions about the COSY and TOCSY experiment or if you want to see how our instrument can be incorporated into your workflow!