Suppose two people are talking in a noisy room and one is trying to hear the other. The overall result is a spectrum such as the one shown in Figure 9-23. Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful and widely used techniques in chemical research for investigating structures and dynamics of molecules. Principal. Unless special precautions are taken, integrals usually should not be considered accurate to better than about $$5\%$$. The objective is to assign a structure to the compound $$\ce{C_4H_8O_3}$$ whose nmr spectrum is shown in Figure 9-30 and whose infrared spectrum shows prominent bands at $$2900 \: \text{cm}^{-1}$$, $$1750 \: \text{cm}^{-1}$$, $$1000 \: \text{cm}^{-1}$$, and $$1100 \: \text{cm}^{-1}$$. This book shall give its readers an overview about the NMR techniques used in pharmaceutical applications and help the method to become accepted as the most significant analytical tool in the pharmacopoeia. This is because the valence electrons around a particular nucleus and around neighboring nuclei respond to the applied magnetic field so as to shield the nucleus from the applied field. For example, the proton chemical shifts of the methyl halides (Table 9-4) show decreasing shielding, hence progressively low-field chemical shifts with increasing halogen electronegativity $$\left( \ce{F} > \ce{Cl} > \ce{Br} > \ce{I} \right)$$: The effect of electronegativity on a more remote proton as in is expected to be smaller as more bonds intervene. If an external magnetic field is applied, an energy transfer is possible between the base energy to a higher energy level (generally a single energy gap). The $$\ce{C_9H_{10}}$$ protons are coupled to each other, not to A, B, or C. The five-proton signal at $$7.28 \: \text{ppm}$$ is typical of a phenyl group, $$\ce{C_6H_5}$$, and the one-proton signals at $$5.35$$ and $$5.11 \: \text{ppm}$$ are in the region for alkenic protons, . For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. When this happens, the resulting spectrum can be quite complex, as our next example shows. To confirm the assignment, the chemical shifts should be checked (Table 9-4). Cavanagh, Fairbrother, Palmer, and Skelton Protein NMR spectroscopy Principles and practice Academic press, 1996. These may seem odd units for magnetic field strength but because resonance occurs at $$\nu = \gamma H$$, either frequency units ($$\text{Hz}$$, radians $$\text{sec}^{-1}$$) or magnetic field units (gauss) are appropriate. If we can go to a sufficiently low temperature to make interconversion of the conformations slow (on the order of 10 times per second), then we will expect to see the three different chemical shifts $$H_\text{A}$$, $$H_\text{B}$$, and $$H_\text{C}$$ with intensities corresponding to the actual populations of the conformations at the sample temperature. The chemical shifts of the presumed $$\ce{CH_3}$$ groups are at $$3.70 \: \text{ppm}$$ and $$3.35 \: \text{ppm}$$. Consequently the $$H_\text{A}$$ and $$H_\text{B}$$ hydrogens are nonequivalent and would be expected to have different chemical shifts. The $$\ce{^{13}C}$$ data indicate clearly that warfarin is not $$15$$ in solution but is a mixture of two diastereomers ($$16$$ and $$17$$, called cyclic hemiketals) resulting from addition of the $$\ce{-OH}$$ group of $$15$$ to the $$\ce{C=O}$$ bond: This is one example of the power of $$\ce{^{13}C}$$ nmr to solve subtle structural problems. \Sigma_Y \tag { 9-4 } \ ) 9-33 usually follow simple rules possible from base energy higher., but probably the easiest is to hold the magnetic properties of certain nuclei what kinds of information obtained from. 14 } \ ] integral is not the only way that spin-coupling interactions can be put in the center the! 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