Settle in and get COSY!

COrrelation SpectroscopY (COSY) is one of the most common 2D NMR experiments. Why? Because, like most 2D experiments, it helps deconvolute complex data. A COSY experiment can be useful for assigning resonances in a 1D spectrum because it shows you which proton spins that are coupled to each other.

In many blogs, we have discussed multiplicities and how splitting patterns can be used to determine connectivity. The beauty of 2D methods is that they make this information less ambiguous which leads to a more straightforward recognition of coupled protons, thereby minimizing the risk of misinterpretation.

A COSY NMR spectrum is obtained by stacking a series of 1D experiments that differ only in their ‘t1’ parameter.¹ What exactly does that mean? Let’s look at the COSY pulse program (Scheme 1):

Scheme 1. Graphically representation of a COSY pulse program, where 1 represents the interscan/relaxation delay; 2, the first pulse (p1); 3, the resting time interval (t1) where the nuclear spins begin to relax; 4, the second pulse (p2); and 5, the data acquisition over a certain time (t2).

The 2D aspect to this data is introduced by performing successive Fourier transforms (FT), first on the rows of 1D stacked data measured at various t1’s. Depending on how short (or how long) the t1 time is, the signal amplitude observed at t2 will vary. Performing a second FT on the columns generates a contour plot, showing the hills (if J > 0) or valleys (if J < 0) of each peak formed.

To illustrate this let’s look at the ethyl group fragment taken from a zoomed in COSY of ethyl cinnamate (Figure 1). Shown below is the contour plot.

Figure 1. ¹H-¹H COSY NMR (60 MHz) spectrum of ethyl cinnamate in deuterated chloroform (zoom-in).

To simplify our data interpretation, we can divide these peaks into 2 types: (1) diagonal and (2) off-diagonal or cross peaks. The diagonal peaks are effectively just a 1D spectrum and can be largely ignored for 2D data treatment. The 2 diagonal peaks show where the same signals align – the triplet with the triplet and the quartet with the quartet.

The cross peaks are where all the information is! These cross peaks show the nuclear spins that are connected through magnetization transfer – i.e., the phenomenon that affords spin-spin coupling. Because a COSY NMR spectrum is a symmetric plot each spin correlation will be observed twice – once on either side of the diagonal. From left to right across the top – we see quartet (on horizontal) aligned with triplet (on vertical) and triplet (on horizontal) aligned with quartet (on vertical).

So, these cross peaks show us correlations between bonding groups. The methylene group (CH₂) is bonded to the methyl group (CH₃), in a vicinal (3-bond coupling) relationship. Instead of “manually” looking at the spectrum like we do with a 1D experiment and noting that a quartet has 3 neighbours – i.e., is bonded to a methyl group (n + 1 = 4, so n = 3) and a triplet has 2 neighbours – i.e., is bonded to a methylene (n + 1 = 3, so n = 2) we can see the correlations!

Okay, that makes sense!  Let’s expand our view to the whole ethyl cinnamate COSY (Figure 2) – we see 8 peaks – 4 on the diagonal and 4 off-diagonal (ignoring the aromatic protons).  The 4 diagonal peaks don’t show connectivity so we can narrow our focus to the 4 cross peaks that result from magnetization transfer.  We highlighted the ethyl groups above (correlations shown in green), but we can also see the geminal (2-bond) couplings for the vinyl protons.  These are highlighted in blue – again, two signals for one bonded pair.

Figure 2. ¹H-¹H COSY NMR (60 MHz) spectrum of ethyl cinnamate in deuterated chloroform.

COSY experiments typically show up to 4 bond coupling (⁴J _HH), the nature of which can vary greatly. In this blog we used a COSY-90 experiment because it provides the highest signal-to-noise ratio (SNR). If the SNR is too high and the diagonal becomes overwhelming, a COSY-45 experiment can be run to reduce this and make cross peaks more evident. Double quantum filtered DQF-COSY experiments can remove lower couplings to only see higher order transitions.¹ If you have any questions about how COSY experiments can aid with the elucidation of your compounds, please contact us! Furthermore, for more information to highlight the benefits of using COSY, check out our infographic.

References

[1] Silverstein, R, M.; Webster, F. X.; Kiemle, D. J.; “Spectrometric Identification of Organic Compounds” 7th Ed. John Wiley & Sons Inc: USA