CHEM 125A: FRESHMAN ORGANIC CHEMISTRY II - Lecture 24 - Professor J. Michael McBride
Higher-Order Effects, Dynamics, and the NMR Time Scale
Because spin-spin splitting depends on electron spin precisely at a
nucleus, splitting by a C-13 depends on its orbital’s hybridization.
"Higher-order effects" that give complex multiplets for nuclei with
similar chemical shifts can be understood in terms of the mixing of wave
functions of similar energy. Averaging of chemical shifts or spin-spin
splitting may be used to measure the rate of rapid changes in molecular
structure, such as changes in conformation or hydrogen bonding. Since
the spectroscopic time scale depends on frequency differences, averaging
is easier in NMR than in IR. A typical problem involves predicting the
NMR spectrum of a compound with diastereotopic groups. In proton
decoupling radio frequency irradiation of a particular proton can make
it cease to split the NMR signals from nearby protons.
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