CHEM 2211 1st Edition Lecture 13 Outline of Last Lecture I II III IV Brief intro to HNMR Shielding effects of NMR Number of Signals in HNMR Chemical Shif Outline of Current Lecture I II III IV V VI Integrating NMR signals Splitting signal is described by the N 1 rule What causes splitting Coupling constant identifies coupled protons Diastereotopic hydrogens are not equivalent Time dependency of NMRs Current Lecture I II III IV Integrating the NMR signals A The signals produced by the hydrogens of a compound vary in height because the area under each signal is proportional to the number of protons producing the signal B On older graphs the integral is shown to be an integral trace The height of each step is the approximate number of hydrogens C Modern graphs impose the integral number onto the graph above the corresponding peak Splitting of signal is described by the N 1 rule A An explanation for the multiple peaks at point son the graph B Caused by the protons bonded to neighboring carbons C N 1 rule is as follows number of hydrogens on non equivalent neighboring carbons 1 number of peaks on graph What causes splitting A The peaks represent the number of ways that the protons can align with or against the magnetic field B Refer to figures 15 15 and 15 16 in the book for the most accurate representation of this concept Coupling constant identify coupled protons V VI A J denotes the coupling constant and is defined as the distance between the peaks of a split NMR signal B Equivalent hydrogens have the same J value C J values are constant despite a change in magnetic field and frequency Diastereotopic hydrogens are not equivalent A Carbon bonded to two hydrogens and two other nonequivalent groups have enantiotropic hydrogens because changing one of the hydrogens to another group will result in a pair of enantiomers This is called a prochiral center B A carbon attached to two hydrogens and is a part of an asymmetric center has diastereotopic hydrogens because replacing one of the hydrogens creates a diastereomers Time dependence of NMR spectroscopy A Each graph represents an average of the molecule s conformers because the molecules are constantly changing
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