CHAPTER THIRTEEN Spectroscopy with this as background, we will now discuss spectroscopic techniques individu ally. NMR, IR, and UV-VIS spectroscopy provide complementary information, and all are useful. Among them, NMR provides the information that is most directly related to molecular structure and is the one we shall examine first 13.3 INTRODUCTION TO H NMR SPECTROSCOPY Nuclear magnetic resonance spectroscopy depends on the absorption of energy when the nucleus of an atom is excited from its lowest energy spin state to the next higher one. We should first point out that many elements are difficult to study by NMr, and some can't be studied at all. Fortunately though, the two elements that are the most common in organic molecules(carbon and hydrogen) have isotopes(H andC) capable of giv g NMR spectra that are rich in structural information. A proton nuclear magnetic res f protons was first detected nance(H NMR) spectrum tells us about the environments of the various hydrogens in a molecule; a carbon-13 nuclear magnetic resonance (C NMR)spectrum does the same (Stanford). Purcell and Bloch for the carbon atoms. Separately and together H andC NMR take us a long way shared the 1952 Nobel Prize toward determining a substance's molecular structure. We'll develop most of the general principles of NMr by discussing H NMR, then extend them toC NMR. The C NMR discussion is shorter, not because it is less important than H NMR, but because many of the same principles apply to both techniques. Like an electron, a proton has two spin states with quantum numbers of + and -2. There is no difference in energy between these two nuclear spin states; a proton is just as likely to have a spin of +3 as -=. Absorption of electromagnetic radiation can only occur when the two spin states have different energies. A way to make them different is to place the sample in a magnetic field. A proton behaves like a tiny bar mag net and has a magnetic moment associated with it(Figure 13.3). In the presence of an external magnetic field o, the state in which the magnetic moment of the nucleus is aligned with o is lower in energy than the one in which it opposes il ￠|< (a) No external magnetic field (b) Apply external magnetic field o FIGURE 13.3(a)In the absence of an external magnetic field, the nuclear spins of the protons re randomly oriented. (b)In the presence of an external magnetic field o, the nuclear spins are oriented so that the resulting nuclear magnetic moments are aligned either parallel or antiparallel to o. The lower energy orientation is the one parallel to o and there are more nuclei that have this orientation Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteWith this as background, we will now discuss spectroscopic techniques individu￾ally. NMR, IR, and UV-VIS spectroscopy provide complementary information, and all are useful. Among them, NMR provides the information that is most directly related to molecular structure and is the one we shall examine first. 13.3 INTRODUCTION TO 1 H NMR SPECTROSCOPY Nuclear magnetic resonance spectroscopy depends on the absorption of energy when the nucleus of an atom is excited from its lowest energy spin state to the next higher one. We should first point out that many elements are difficult to study by NMR, and some can’t be studied at all. Fortunately though, the two elements that are the most common in organic molecules (carbon and hydrogen) have isotopes (1 H and 13C) capable of giv￾ing NMR spectra that are rich in structural information. A proton nuclear magnetic res￾onance (1 H NMR) spectrum tells us about the environments of the various hydrogens in a molecule; a carbon-13 nuclear magnetic resonance (13C NMR) spectrum does the same for the carbon atoms. Separately and together 1 H and 13C NMR take us a long way toward determining a substance’s molecular structure. We’ll develop most of the general principles of NMR by discussing 1 H NMR, then extend them to 13C NMR. The 13C NMR discussion is shorter, not because it is less important than 1 H NMR, but because many of the same principles apply to both techniques. Like an electron, a proton has two spin states with quantum numbers of  and . There is no difference in energy between these two nuclear spin states; a proton is just as likely to have a spin of  as . Absorption of electromagnetic radiation can only occur when the two spin states have different energies. A way to make them different is to place the sample in a magnetic field. A proton behaves like a tiny bar mag￾net and has a magnetic moment associated with it (Figure 13.3). In the presence of an external magnetic field 0, the state in which the magnetic moment of the nucleus is aligned with 0 is lower in energy than the one in which it opposes 0. 1 2 1 2 1 2 1 2 490 CHAPTER THIRTEEN Spectroscopy (a) No external magnetic field (b) Apply external magnetic field 0 0 FIGURE 13.3 (a) In the absence of an external magnetic field, the nuclear spins of the protons are randomly oriented. (b) In the presence of an external magnetic field 0, the nuclear spins are oriented so that the resulting nuclear magnetic moments are aligned either parallel or antiparallel to 0. The lower energy orientation is the one parallel to 0 and there are more nuclei that have this orientation. Nuclear magnetic resonance of protons was first detected in 1946 by Edward Purcell (Harvard) and by Felix Bloch (Stanford). Purcell and Bloch shared the 1952 Nobel Prize in physics. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website