CHAPTER THIRTEEN Spectroscopy 87.28 ppm Downfield Decreased shielding Upfield Increased shielding CTMS) 80 ppm Chemical shift(8, ppm) FIGURE 13.7 The 200-MHz 'H NMR spectrum of chloroform(HCCl3). Chemical shifts are mea red along the x-axis in parts per million (ppm) from tetramethylsilane as the reference, wh is assigned a value of zero. PROBLEM 13.3 The 'H NMR signal for bromoform(CHBr3) appears at 2065 Hz when recorded on a 300-MHz NMR spectrometer. (a) What is the chemical shift of this proton?(b)Is the proton in CHBr3 more shielded or less shielded than the NMR spectra are usually run in solution and, although chloroform is a good sol vent for most organic compounds, it's rarely used because its own signal at 8 7.28 ppm would be so intense that it would obscure signals in the sample. Because the magnetic properties of deuterium(D=H)are different from those of H, CDCl3 gives no sig nals at all in an H NMR spectrum and is used instead. Indeed, CDCl3 is the most com- monly used solvent in H NMR spectroscopy. Likewise, D2O is used instead of H2O for water-soluble substances such as carbohydrates 13.5 EFFECTS OF MOLECULAR STRUCTURE ON TH CHEMICAL SHIFTS Nuclear magnetic resonance spectroscopy is such a powerful tool for structure determi Problem 13.3 in the preced- nation because protons in different environments experience different degrees of shield- on the ing and have different chemical shifts. In compounds of the type CH3X, for example, chemical shift difference be. the shielding of the methyl protons increases as X becomes less electronegative. Inas Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsitePROBLEM 13.3 The 1 H NMR signal for bromoform (CHBr3) appears at 2065 Hz when recorded on a 300-MHz NMR spectrometer. (a) What is the chemical shift of this proton? (b) Is the proton in CHBr3 more shielded or less shielded than the proton in CHCl3? NMR spectra are usually run in solution and, although chloroform is a good sol￾vent for most organic compounds, it’s rarely used because its own signal at  7.28 ppm would be so intense that it would obscure signals in the sample. Because the magnetic properties of deuterium (D 2 H) are different from those of 1 H, CDCl3 gives no sig￾nals at all in an 1 H NMR spectrum and is used instead. Indeed, CDCl3 is the most com￾monly used solvent in 1 H NMR spectroscopy. Likewise, D2O is used instead of H2O for water-soluble substances such as carbohydrates. 13.5 EFFECTS OF MOLECULAR STRUCTURE ON 1 H CHEMICAL SHIFTS Nuclear magnetic resonance spectroscopy is such a powerful tool for structure determi￾nation because protons in different environments experience different degrees of shield￾ing and have different chemical shifts. In compounds of the type CH3X, for example, the shielding of the methyl protons increases as X becomes less electronegative. Inas- 494 CHAPTER THIRTEEN Spectroscopy Problem 13.3 in the preced￾ing section was based on the chemical shift difference be￾tween the proton in CHCl3 and the proton in CHBr3 and its relation to shielding. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 Chemical shift (δ, ppm) Tetramethylsilane (TMS) δ 0 ppm H±CCl3 δ 7.28 ppm Upfield Increased shielding Downfield Decreased shielding FIGURE 13.7 The 200-MHz 1 H NMR spectrum of chloroform (HCCl3). Chemical shifts are mea￾sured along the x-axis in parts per million (ppm) from tetramethylsilane as the reference, which is assigned a value of zero. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website