13.4 Nuclear Shielding and H Chemical Shift Not only is pulsed FT-NMR the best method for obtaining proton spectra, it is the only practical method for many other nuclei, including C. It also makes possible a large number of sophisticated techniques that have revolutionized NMR spectroscopy 13.4 NUCLEAR SHIELDING AND TH CHEMICAL SHIFTS Our discussion so far has concerned H nuclei in general without regard for the envi- other atoms--carbon, oxygen, nitrogen, and so on-by covalent bonds. The electrons in magnetic field of the elec the protons. Alone, a proton would feel the full strength of the external field, but a pro- gen bond opposes the - hydro these bonds, indeed all the electrons in a molecule, affect the magnetic environment of trons in the carbo ton in an organic molecule responds to both the external field plus any local fields within resulting magnetic field the molecule. An external magnetic field affects the motion of the electrons in a mole- experienced by the proton and the carbon is slightly cule, inducing local fields characterized by lines of force that circulate in the opposite less than se direction from the applied field(Figure 13. 6). Thus, the net field felt by a proton in a ess than %o- molecule will always be less than the applied field, and the proton is said to be shielded All of the protons of a molecule are shielded from the applied field by the electrons, but some are less shielded than others Sometimes the term"deshielded is used to describ this decreased shielding of one proton relative to another. The more shielded a proton is, the greater must be the strength of the applied field in order to achieve resonance and produce a signal. A more shielded proton absorbs rf radiation at higher field strength (upfield) compared with one at lower field strength (downfield). Different protons give signals at different field strengths. The dependence of the resonance position of a nucleus that results from its molecular environment is called its chemical shift. This is where the real power of NMR lies. The chemical shifts of various protons in a molecule can be different and are characteristic of particular struc tural features the igure 13.7 shows the H NMR spectrum of chloroform(CHCl3)to illustrate how ology just developed applies to a real Instead of measuring chemical shifts in absolute terms, we measure them with respect to a standard--tetramethylsilane( CH3)4 Si, abbreviated TMS. The protons of Tms are more shielded than those of most organic compounds, so all of the signals in a sam- ple ordinarily appear at lower field than those of the TMS reference. When measured this chapter is an ele using a 100-MHz instrument, the signal for the proton in chloroform(CHCI,), for exam- pane tearing ay modeling con. pIe, appears 728 Hz downfield from the TMS signal. But since frequency is proportional tains models of (CHa) Si and to magnetic field strength, the same signal would appear 1456 Hz downfield from TMS (CHa)aC in which the greate on a 200-MHz instrument. We simplify the reporting of chemical shifts by converting and hydrogens of TMS is appar need not actually be present in the sample, nor even appear in the spectrum in order to tential and in the calculated o them to parts per million(ppm) from TMS, which is assigned a value of 0. The TMs ent be serve as a reference Chemical shift(8)= position of signal- position of TMS peak x 106 spectrometer frequency Thus, the chemical shift for the proton in chloroform is: 1456Hz-0Hz 200×10°Hz ×10=728pm When chemical shifts are reported this way, they are identified by the symbol 8 and are independent of the field strength. Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteNot only is pulsed FT-NMR the best method for obtaining proton spectra, it is the only practical method for many other nuclei, including 13C. It also makes possible a large number of sophisticated techniques that have revolutionized NMR spectroscopy. 13.4 NUCLEAR SHIELDING AND 1 H CHEMICAL SHIFTS Our discussion so far has concerned 1 H nuclei in general without regard for the envi￾ronments of individual protons in a molecule. Protons in a molecule are connected to other atoms—carbon, oxygen, nitrogen, and so on—by covalent bonds. The electrons in these bonds, indeed all the electrons in a molecule, affect the magnetic environment of the protons. Alone, a proton would feel the full strength of the external field, but a pro￾ton in an organic molecule responds to both the external field plus any local fields within the molecule. An external magnetic field affects the motion of the electrons in a mole￾cule, inducing local fields characterized by lines of force that circulate in the opposite direction from the applied field (Figure 13.6). Thus, the net field felt by a proton in a molecule will always be less than the applied field, and the proton is said to be shielded. All of the protons of a molecule are shielded from the applied field by the electrons, but some are less shielded than others. Sometimes the term “deshielded,” is used to describe this decreased shielding of one proton relative to another. The more shielded a proton is, the greater must be the strength of the applied field in order to achieve resonance and produce a signal. A more shielded proton absorbs rf radiation at higher field strength (upfield) compared with one at lower field strength (downfield). Different protons give signals at different field strengths. The dependence of the resonance position of a nucleus that results from its molecular environment is called its chemical shift. This is where the real power of NMR lies. The chemical shifts of various protons in a molecule can be different and are characteristic of particular struc￾tural features. Figure 13.7 shows the 1 H NMR spectrum of chloroform (CHCl3) to illustrate how the terminology just developed applies to a real spectrum. Instead of measuring chemical shifts in absolute terms, we measure them with respect to a standard—tetramethylsilane (CH3)4Si, abbreviated TMS. The protons of TMS are more shielded than those of most organic compounds, so all of the signals in a sam￾ple ordinarily appear at lower field than those of the TMS reference. When measured using a 100-MHz instrument, the signal for the proton in chloroform (CHCl3), for exam￾ple, appears 728 Hz downfield from the TMS signal. But since frequency is proportional to magnetic field strength, the same signal would appear 1456 Hz downfield from TMS on a 200-MHz instrument. We simplify the reporting of chemical shifts by converting them to parts per million (ppm) from TMS, which is assigned a value of 0. The TMS need not actually be present in the sample, nor even appear in the spectrum in order to serve as a reference. Chemical shift ()  106 Thus, the chemical shift for the proton in chloroform is:   106 7.28 ppm When chemical shifts are reported this way, they are identified by the symbol  and are independent of the field strength. 1456 Hz 0 Hz 200  106 Hz position of signal position of TMS peak spectrometer frequency 13.4 Nuclear Shielding and 1 H Chemical Shifts 493 0 C H FIGURE 13.6 The induced magnetic field of the elec￾trons in the carbon–hydro￾gen bond opposes the exter￾nal magnetic field. The resulting magnetic field experienced by the proton and the carbon is slightly less than 0. The graphic that begins this chapter is an electrostatic potential map of tetramethylsi￾lane. Learning By Modeling con￾tains models of (CH3)4Si and (CH3)4C in which the greater electron density at the carbons and hydrogens of TMS is appar￾ent both in the electrostatic po￾tential and in the calculated atomic charges. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website