2D NMR spectroscopy

The COSY (Correlated Spectroscopy) experiment

The COSY experiment is the oldest and most well-known (shift correlated) 2D methods.

The aim is to uncover the scalar spin-spin coupling based (via bonds) Relations between cores.

Particularly useful is the inclusion of a COSY spectrum when overlapping multiplets and partial higher order spectra occur in the 1D NMR spectrum.

COSY pulse sequences

There are many different COSY pulse sequences, it is also between heteronuclear (eg, C, H-COSY) and homonuclear COSY spectra (eg, H, H-COSY) differ.

The most common method is the H, H-COSY experiment , which will be explained in more detail below.

In its simplest form, the COSY of two 90 °-pulses only in the evolution time t is ₁ are separated:

Fig.1

H, H-COSY spectrum

In H, H-COSY spectrum are on both axes, the 1 H chemical shifts plotted; In principle, both the axes 1 to see H-NMR spectra. Thus, there is a symmetric to the diagonal diagram.

1 H-NMR spectrum of acetylsalicylic acid

Fig.2

H, H-COSY spectrum of acetylsalicylic acid

In the spectrum, only the range from 7.0 to 8.2 ppm is applied, because only here HH scalar couplings can be expected.

Fig.3

There are two types of signals:

• Diagonal signals: join the coordinates δ _a δ _a (in core A), δ _b δ _b (in core B) ... on, but play no role in the evaluation of the couplings between different cores, since it is only the signal of a nucleus is. The diagonal with all its signals corresponding to the 1D H-NMR spectrum.

EXAMPLE

Acetylsalicylic acid

7.13 ppm / 7.13 ppm = δ ₂ δ ₂ (H atom 2)

7.34 ppm / 7.34 ppm = δ ₄ δ ₄ (H atom 4)

7.61 ppm / 7.61 ppm = δ ₃ δ ₃ (H atom 3)

8.11 ppm / 8.11 ppm = δ ₅ δ ₅ (H atom 5)

• Cross signals: These signals are based on the scalar spin-spin coupling and are suitable for the evaluation of spectra of enormous importance.

EXAMPLE

Acetylsalicylic acid

7.13 ppm / 7.61 ppm (δ ₂ δ ₃ ), and 7.61 ppm / 7.13 ppm (δ ₃ δ ₂ ) - vicinal coupling between the H-atoms 2 and 3

7.34 ppm / 7.61 ppm (δ ₄ δ ₃ ) and 7.61 ppm / 7.34 ppm (δ ₃ δ ₄ ) - vicinal coupling between the H-atoms 4 and 3

7.34 ppm / 8.11 ppm (δ ₄ δ ₅ ) and 8.11 ppm / 7.34 ppm (δ ₅ δ ₄ ) - vicinal coupling between the H-atoms 4 and 5

In general it can be seen in the COSY spectrum each scalar coupling between two nuclei at four signals (two cross and two diagonal peaks) resulting connected a square; in the following example, the vicinal coupling between the H atoms is highlighted 3 and 4.

Fig.4

With good resolution of the COSY spectrum, the coupling constants can be determined from the fine structure of the cross and diagonal signals, but this is rarely done because of the 1-D H-NMR spectra is easily possible.