The NMR Spectrum

2b The NMR Spectrum. Simplistically, the NMR spectrum of a molecule is the sum of all of the contributions made by the individual nuclei that possess nonzero spin ( H, 13C). The spectrum itself comprises a series of peaks, the position and patterns of which are indicative of the molecule under investigation. The position (or chemical shift) of the peak arises due to nuclear shielding. The magnetic field at the nucleus is not equal to that of an applied external field because electrons around the nucleus shield it from the applied field. This difference between the applied and the actual field at the nucleus is termed nuclear shielding. 

The final location of a peak within an NMR spectrum is then a combination of these high and low field shifts and is a function of the nucleus and its environment. 

Absorption of various RFs occurs at a constant field strength. Absorption of a particular RF occurs at various field strengths. The traditional NMR spectrum is a plot of the latter—absorption vs. field strength. 

Intuitively, one may expect all hydrogen nuclei to absorb a particular RF at a particular field strength, reflecting an assumption that all hydrogen nuclei are identical. If this were true, the proton NMR spectrum of all compounds would be identical—hardly useful for any analytical characterization. Fortunately, this is not the case. 

How many major peaks can be expected in the NMR spectrum of (a) ethyl alcohol, (b) n-propyl alcohol, 

FIGURE 10.14 The NMR spectrum of ethylbenzene with integration trace. 

Second, the area under a peak is indicative of the number of hydrogens it represents. This, too, is important qualitative information, and so NMR spectrometer data systems are designed to determine 

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The effects of TIP also appear in figure B 1,11.3 and figure B 1.11.4. In the NMR spectrum, all the resonances of the sp carbons lie above 100 ppm (a usefiil general rule of thumb) because A is smaller for multiple bonds. The highest shifts are for the carbonyl C at 169 ppm and the ring C attached to oxygen at 155... 

Despite these simplifications, a typical or F NMR spectrum will nomially show many couplings. Figure BTl 1.9 is the NMR spectrum of propan-1-ol in a dilute solution where the exchange of OH hydrogens between molecules is slow. The underlymg frequency scale is included with the spectrum, in order to emphasize how the couplings are quantified. Conveniently, the shift order matches the chemical order of die atoms. The resonance frequencies of each of the 18 resolved peaks can be quantitatively explained by the four... 

No molecule is completely rigid and fixed. Molecules vibrate, parts of a molecule may rotate internally, weak bonds break and re-fonn. Nuclear magnetic resonance spectroscopy (NMR) is particularly well suited to observe an important class of these motions and rearrangements. An example is tire restricted rotation about bonds, which can cause dramatic effects in the NMR spectrum (figure B2.4.1). 

The observable NMR signal is the imaginary part of the sum of the two steady-state magnetizations, and Mg. The steady state implies that the time derivatives are zero and a little fiirther calculation (and neglect of T2 tenns) gives the NMR spectrum of an exchanging system as equation (B2.4.6)). 

Several empirical approaches for NMR spectra prediction are based on the availability of large NMR spectral databases. By using special methods for encoding substructures that correspond to particular parts of the NMR spectrum, the correlation of substructures and partial spectra can be modeled. Substructures can be encoded by using the additive model greatly developed by Pretsch [11] and Clerc [12]. The authors represented skeleton structures and substituents by individual codes and calculation rules. A more general additive model was introduced... 

Fig. 9.25 If the leucine side chain interconverts rapidly between too conprmations then the NMR spectrum will he an average of them. With a traditional refinement this leads to a structure that sirmltaiieausly tries to satisfy all restraints and is at the top of the energy barrier between the two minima.

Ojj 1.5591. The NMR spectrum corresponds to reasonably pure (CH3)2C=C=C(SCH3)2. Efforts to distil the compound led to partial dimerization. 

Note 3. The NMR spectrum of the product is misleadingly simple, with just one peak at 6 5.5 ppm ... 

Concentration in a water-pump vacuum gave the chloroallene, n 1.5980, in more than 90% yield. The NMR spectrum showed that no starting compound was present and the purity was satisfactory. Attempts to distil the allene led to extensive polymerization. 

In the NMR spectrum of cis-l,2-bis[2-diethylamino-5-nitrothiazol-4-yl] ethylene (17) (1570), the nonequivalence of olefinic protons requires that the rotation of the NO2 group be hindered. 

A 2-methylthio substituent decreases the basicity of thiazole pK = 2.52) by 0.6 pK unit (269). The usual bathochromic shift associated with this substituent in other heterocycles is also found for the thiazole ring (41 nm) (56). The ring protons of thiazole are shielded by this substituent the NMR spectrum of 2-methylthiothiazole is (internal TMS, solvent acetone) 3.32 (S-Me) 7.3 (C -H) 6.95 (Cj-H) (56, 270). Typical NMR spectra of 2-thioalkylthiazoles are given in Ref. 266. 

Polar solvents shift the keto enol equilibrium toward the enol form (174b). Thus the NMR spectrum in DMSO of 2-phenyl-A-2-thiazoline-4-one is composed of three main signals +10.7 ppm (enolic proton). 7.7 ppm (aromatic protons), and 6.2 ppm (olefinic proton) associated with the enol form and a small signal associated with less than 10% of the keto form. In acetone, equal amounts of keto and enol forms were found (104). In general, a-methylene protons of keto forms appear at approximately 3.5 to 4.3 ppm as an AB spectra or a singlet (386, 419). A coupling constant, Jab - 15.5 Hz, has been reported for 2-[(S-carboxymethyl)thioimidyl]-A-2-thiazoline-4-one 175 (Scheme 92) (419). This high J b value could be of some help in the discussion on the structure of 178 (p. 423). 

The 4-Hydroxy-thiazoles are characterized by infrared absorption near 1610 cm (KBr) (3) or 1620 to 16.S0cm (CCI4) (8), indicating a strongly polarized carbonyl group. H-5 resonates near 5.6 ppm in the NMR spectrum like similar protons in other mesoionic compounds (3). Two fragmentations of the molecular ion are observed in the mass spectra. The first involves rupture of the 1,2 and 3,4 bonds with loss of C2R 0S . In the second, the 1,5 and 3,4 bonds are cleaved with elimination of C2R 0. ... 

Annulene satisfies the Huckel (4n+2) tt electron rule for aromaticity and many of its proper ties indicate aromaticity (Section 11 20) As shown in Figure 13 10a [18]annulene contains two different kinds of protons 12 he on the ring s periphery ( out side ) and 6 reside near the middle of the molecule ( inside ) The 2 1 ratio of outside/inside protons makes it easy to assign the signals in the NMR spectrum The outside protons have a chemical shift 8 of 9 3 ppm which makes them even less shielded than those of benzene The six inside protons on the... 

Thus separate signals will be seen for the protons at C 1 C 2 C 3 and C 4 Bar ring any accidental overlap we expect to find four signals in the NMR spectrum of 1 bromobutane... 

Describe the appearance of the NMR spectrum of each of the following compounds How many signals would you expect to find and into how many peaks will each signal be splif ... 

At first glance splitting may seem to complicate the interpretation of NMR spectra In fact It makes structure determination easier because it provides additional information It tells us how many protons are vicinal to a proton responsible for a particular signal With practice we learn to pick out characteristic patterns of peaks associating them with particular structural types One of the most common of these patterns is that of the ethyl group represented m the NMR spectrum of ethyl bromide m Figure 13 15... 

The NMR spectrum of isopropyl chloride (Figure 13 17) illustrates the appearance of an isopropyl group The signal for the six equivalent methyl protons at 8 1 5 is split into a doublet by the proton of the H—C—Cl unit In turn the H—C—Cl proton signal at 8 4 2 IS split into a septet by the six methyl protons A doublet-septet pattern is char acteristic of an isopropyl group... 

We know from Chapter 3 that the protons m cyclohexane exist m two different envi ronments axial and equatorial The NMR spectrum of cyclohexane however shows only a single sharp peak at 8 1 4 All the protons of cyclohexane appear to be equivalent m the NMR spectrum Why" ... 

The NMR spectrum on the other hand is very simple a separate distinct peak IS observed for each carbon... 

Figure 13 23 compared the appearance of the H and NMR spectra of 1 chloropentane and drew attention to the fact each carbon gave a separate peak well separated from the others Let s now take a closer look at the NMR spectrum of 1 chloropentane with respect to assigning these peaks to individual carbons... 

The NMR spectrum of 1 bromo 3 chloropropane contains 1 peaks at 8 30 8 35 and 8 43 Assign these signals to the appropriate carbons J... 

FIGURE 13 24 The NMR spectrum of m cresol Each of the seven carbons of m cresol gives a separate peak Integrating the spectrum would not provide useful information because the intensities of the peaks are so different even though each one corresponds to a single carbon... 

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