1559T_ch10_175-19810/30/0518:09Pa9e177 EQA Keys to the Chopter·177 etic fields for Deshiclded hydroge Aod 沈 0 Chemical shift den right to left,with the usual hydrogen spectrum covering a range from to 10 ppm.Table 10-2 shows typical General regions of the nur spectrum E D B 00g5 10 Chemical shif(ppm) Region Chemical shint Hydrogen type 0-15 ppm -type hydrogen 1.5-3.0pm to carbon-containing 3.045ppm hydrogenson cabons attached toelectroegive atoms .0 ppn alkene-type hydroge四 nzene-type hydrogen -10.0Ppm ydrogens of aldehyde group NMR of this 10-5.Chemical Shift Equivalence strongly shielded ones. Deshielded hydrogens require lower external magnetic fields for resonance, whereas shielded ones require higher fields. So we have NMR spectra that have the following qualitative relationships: As described in the text, a resonance signal’s position is measured as a field-independent chemical shift, which has units of parts per million (ppm) of the total applied field, often called units. These read from right to left, with the usual hydrogen spectrum covering a range from 0 to 10 ppm. Table 10-2 shows typical chemical shifts for common types of hydrogens. There is a lot here, but for most purposes all you really need to know are the types of hydrogens that resonate in several general regions of the NMR spectrum. Region Chemical shift Hydrogen type A 0–1.5 ppm alkane-type hydrogens B 1.5–3.0 ppm hydrogens on carbons next to carbon-containing functional groups C 3.0–4.5 ppm hydrogens on carbons attached to electronegative atoms D 4.5–6.0 ppm alkene-type hydrogens E 6.0–9.5 ppm benzene-type hydrogens F 9.5–10.0 ppm hydrogens of aldehyde group With this as a basis, you are ready to start interpreting NMR spectra. For the initial problems of this type, simply count the number of signals in the spectrum and note the position of each one. Then see if you can come up with a structure that displays the correct number of signals in approximately the observed places, using the material in the text section. If you can, you have probably picked a sensible structure for the unknown molecule. 10-5. Chemical Shift Equivalence This section presents detailed procedures for determining which hydrogens in a molecule have identical chem￾ical shifts due to chemical equivalence. The simplest examples of this are the four hydrogens of methane or Deshielded hydrogens Shielded hydrogens Keys to the Chapter • 177 1559T_ch10_175-198 10/30/05 18:09 Page 177