The 2-D NMR Spectrum

A 2-D NMR spectrum is obtained after carrying out two Fourier transformations on a matrix of data (as opposed to one Fourier transform on an array of data for a 1-D NMR spectrum). A 2-D NMR spectrum will generate cross peaks that correlate information on one axis with information on the other usually, both axes are chemical shift axes, but this is not always the case. 

2-D pulse sequence contains four parts instead of three. The four parts of the 2-D pulse sequence are relaxation, evolution, mixing, and detection. The careful reader will note that preparation has been split into two parts evolution and mixing. 

Evolution involves imparting phase character to the spins in the sample. Mixing involves having the phase-encoded spins pass their phase information to other spins. Evolution usually occurs prior to mixing and is termed tj (not to be confused with Tj the relaxation time ), but in some 2-D NMR pulse sequences the distinction is blurred, for example in the correlation spectroscopy (COSY) experiment. Evolution often starts with a pulse to put some magnetization 

1- D NMR pulse sequence. A series of delays and RF pulses culminating in the detection, amplification, mixing down, and digitization of the FID. 

Preparation. The placement of magnetization into the xy plane for subsequent detection. 

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If we wish to include peak-picking information above the 1-D NMR spectrum, we may choose to reduce the vertical intensity of the plot to allow room for that extra information (either in Hz or ppm, depending on need). 

We can summarize what we have just discovered the 2-D gCOSY spectrum allows us to determine more precisely the chemical shifts of resonances that overlap in the 1-D NMR spectrum. 

The most basic NMR experiment is the one-pulse proton experiment.23-25 Proton chemical shifts typically range from 0 to 10 ppm, so the spectral width should be set at least this large. A good approach is to set the spectral width to a larger value, such as 15 ppm, to identify the actual limits of the resonances observed for a given sample. Then the spectral width can be reset to a smaller value specific to the sample. Acquisition parameter values determined for the 1-D proton spectrum can be used as a guideline for other proton-detected experiments, including the proton dimension of two-dimensional experiments. 

D NMR spectrum. A linear array showing amplitude as a function of frequency, obtained by the Fourier transformation of an array with amplitude as a function of time. 

To appreciate the importance of concentration, suppose that we find a four-scan 1-D NMR spectrum obtained from a 20mM sample of one compound (compound 1) requires 30s of instrument time to obtain a signal-to-noise ratio of 100 1 for an uncoupled methyl resonance. We may then wonder what signal-to-noise ratio we wUl obtain for a similarly uncoupled methyl resonance arising from a minor component in the same sample (compound 2) that is present with a concentration of 1.4mM. 

When we attempt to measure chemical shifts, we may encounter a poor S/N in the directly-detected 1-D NMR spectrum. Therefore, we may have to resort to the use of indirect detection methods—especially the H-detected HMBC experiment—to obtain the chemical shifts of atoms lacking directly attached H s. 

NMR data can be shovm in a number of different formats. In some cases, a table containing shifts, multiplicities, integrals, etc. may suffice to describe the information contained in a simple 1-D NMR spectrum. In other cases, however, we may wish to show a colored contour plot to help capture the complex nature of a particular 2-D NMR spectrum. 

Once the resonances of G2-OH and G4-OH were assigned, the complexation chemistry of [PtCU] to these dendrimers was studied using Pt NMR. This isotope has a magnetogyric ratio 0.855 times that of G and a natural abundance of 33.8%. As a consequence, Pt NMR is almost 20-fold more sensitive than NMR. The Pt 1 D-NMR spectrum of an equilibrated mixture of K2[PtCl4] and G2-OH dendrimer in aqueous solution is shown in Figure 68. Resonances were observed for [PtC ]" (-1617 ppm) and species where C was replaced by one-, two-, and three tertiary amine nitrogens from the G2-OH dendrimer (at -1878, -2133, and... 

Figure 2.21. HFI NOE difference spectra (b, c) and FIFI NOESY diagram (d) of a-pinene (1) with /-/ NMR spectrum (a) for comparison [(CD3)2CO, 10% v/v, 25 °C, 200 MHz, section from <5 = 0.85 to 2.34 ]. Vertical arrows in (b) and (c) indicate the irradiation frequencies in the HH NOESY plot (d), cross-signals linked by a dotted line show the NOE detected in (c)...

Figure 1.37 The effect of line broadening (LB) multiplication on the appearance of H-NMR spectra. (a IJB = 10) and (b LB = 5) H-NMR spectra recorded after multiplying the FIDs by positive LB values. (c LB = 0). The same H-NMR spectrum recorded without line broadening. (d LB = -2) Sharper signals are obtained when the FID is multiplied by negative LB values. 

A multiplet centered at 8 4.90 was assigned to the C-16 proton, which exhibited COSY interactions with the C-15 methylene and C-17 methine protons. Since an acetoxy group was inferred from IR absorptions and the presence of a singlet at 8 1.90 in the ID H-NMR spectrum, C-16 seemed to be a plausible site of its substitudon in ring D of the skeleton (fragment III). 

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