ImedteneexeegYteeetih,edgatedepenas Magsct pede 90 MH 88 ow出 用用兰兰用用 ernceoruh Many nuclei undergo magnetic resonance. ALNMR Activity and Nataral Abundance of Selected Nudei 8peacSna0,5o0-Gmgnat,h —AMH l28erreheneRpetoscopvcanterentate 10-4 Using NMR Spectra to Analyze Molecular hteNeReetouagCoCotZ250e6epr08n00 nRg24 33 The difference in energy between the α and β states depends directly on the external magnetic field strength, H0. 21,150 G 90 MHz 42,300 G 180 MHz 70,500 G 300 MHz The actual energy difference is small. At 300 MHz, the energy difference for a proton is about 3 x 10-5 kcal mol-1. Because the energy difference is so small and the equilibrium between the two states is so fast, the numbers of nuclei in the two states are nearly equal, however, a slight excess will be in the α state because of the external magnetic field. When electromagnetic radiation having the same energy as energy difference strikes the nucleus, the electromagnetic radiation is absorbed and the slight excess of nuclei in the α state is reduced. Many nuclei undergo magnetic resonance. In general, nuclei composed of an odd number of protons (1H and its isotopes, 14N 19F, and 31P) or an odd number of neutrons (13 C) show magnetic behavior. If both the proton and neutron counts are even (12C or 16O) the nuclei are non-magnetic. In a hypothetical scan of CH2ClF in a 70,500-G magnet, the following spectrum would be observed: High-resolution NMR spectroscopy can differentiate nuclei of the same element. In the NMR spectrum of ClCH2OCH3 at 70,500-G from 0 to 300 MHz, one peak would be observed for each element present. Using high-resolution NMR spectroscopy, the region around each of these peaks can be expanded and additional spectral details can be observed. Using NMR Spectra to Analyze Molecular Structure 10-4 The position of an NMR absorption of a nucleus is called its chemical shift. Chemical shifts depend upon the electron density around a nucleus and are thus controlled by the structural environment of the nucleus. The NMR chemical shifts provide important clues for determining the molecular structure of a chemical compound