First-Order Splitting Patterns

All of this assumes that the proton in question is only coupled to other protons that are far away in chemical shift, so that its coupling pattern is simple ( first order or weak coupling ). If it is coupled to nearby peaks, distortions of the peak intensities and more complex patterns can result, and this effect is strongest at lower field strengths ( second order or strong coupling ). To state this more precisely, the 7 coupling in hertz between two spins must be much less than the chemical shift difference in hertz to see a simple first-order pattern. We could write this as 

We first divide the resonance position (3.56 ppm) into two equal peaks (1 1 ratio) by moving left 5.0 Hz (7/2) and right 5.0 Hz (7/2) (Fig. 2.5). Then each of these peaks is divided again into two equal peaks (1 1 ratio) by the 4.0-Hz coupling 2.0 Hz to the left and 2.0 Hz to the right. This results in a pattern we call a doublet of doublets or (more concisely) a double doublet (abbreviated dd ). In the literature we would report the peak like this 53.56 (dd, 10.0,4.0). To deconstruct (analyze) the pattern, we first note that all four peaks are of the same height, and because 4 is a power of 2 (22 = 4), we assume that there is no overlap of 

A triplet can be viewed as a special case of a double doublet where 7ab = 7ac. In this case the two inner peaks (peaks 2 and 3) have the same resonant frequency and combine 

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First-order splitting patterns for some common molecular structures. (Numbers in circles represent relative total areas of the multiplets)... 

Figure 9-32 High-resolulion nmr spectrum of ethyl iodide, CH3CH2I, at 60 MHz relative to TMS, 0.00 ppm. The first-order splitting pattern is seen in the well-separated three-four line pattern for the CH3—CH2 resonances. The second-order splitting is the additional fine structure superimposed on the three-four pattern.

Fig. 3.63 First order splitting patterns for aromatic protons. Data reproduced from M. Zanger (1972). The Determination of Aromatic Substitution Patterns by Nuclear Magnetic Resonance , Organic Magnetic Resonance, 4,4. Published by Heyden and Son Ltd.

There are collections of calculated spectra that can be used to match complex, first-order, splitting patterns (see references to Wiberg 1962 and Bovey 1988). Alternatively, these spin systems can be simulated on the computer of a modern NMR spectrometer. For example see the NMRSIM computer program available from Bruker BioSpin. 

There is a first-order splitting pattern common to all 3n states, independent of the physical content. All the molecule-dependent physical information is contained in the parameter Aso- These facts are, of course a consequence of the tensor properties expressed in the Wigner-Eckart theorem. 

To determine the first-order splitting pattern of an atomic state in terms of the phenomenological spin-orbit parameter Aso [Eq. 203], we utilize... 

NON-FIRST-ORDER SPLITTING PATTERNS STRONG COUPLING... 

Simple splitting patterns that are produced by the coupling of protons that have very different chemical shifts (A v J is greater than about 8 or so) are c Wed first-order splitting patterns. These can usually be interpreted by using two rules. 

Sketch the first-order splitting patterns you would expect to observe for the following spin systems (your sketches should be similar to those in Figs. 12.11 and 12.13) ... 

In Section 4.7, the first-order splitting patterns arising from coupling between nuclei with / 7 0 are described, but in some circumstances so-called second-order spectra are observed, and these are more complicated to interpret. Second-order spectra can be observed in cases where the coupling between two nuclei is greater than or comparable to their chemical shift difference (in terms of frequency). The effect is to change the... 

Examination of the NMR spectrum of anethole (Fig. 7.64a) shows (crudely) just such a four-line pattern for the ring protons. In fact, a para-disubstituted ring is easily recognized by this four-line pattern. However, the four lines do not correspond to a simple first-order splitting pattern. That is because the two protons Ha and Ha are not magnetically equivalent (Section 7.3). Protons Ha and Ha interact with each other and have finite coupling constant /aa - Similarly, Hb... 

If the user wants to make a more precise examination of the results obtained by means of the above-mentioned signal assignment program, first-order splitting pattern search will be executed. 

CHAlUHGtl The H NMR spectrum of cholesteryl benzoate (see Section 4-7) is shown as spectrum N. Although complex, it contains a number of distinguishing features. Analyze the absorptions marked by integrated values. The inset is an expansion of the signal at 6 = 4.85 ppm and exhibits an approximately first-order splitting pattern. How would you describe this pattern (Hint The peak patterns atS = 2.5,4.85, and 5.4 ppm are simplified by the occurrence of chemical shift and/or coupling constant equivalencies.)... 

Predict the first-order splitting pattern in the H NMR spectrum of 3-methyl-1-butyne. Strategy... 

In general, proton-decoupled Si, Sn, and Pb spectra are simple, and consist of single lines for each type of chemical environment for the nucleus in question in the relevant sample. Satellite resonances can be more complicated, as mentioned above, but they can usually be readily distinguished from the main peaks, even for Sn. When or P is present, first-order splitting patterns are normal. Proton-couoled spectra are not often studied, so that second-order Si, Sn, or Pb spectra have rarely been treated (see, however, 25). Enrichment with silicon-29 can lead to second-order features for symmetrical species, and an example (26) is shown in Figure 2. In such cases homonuclear decoupling is a desirable technique (22,24). 

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