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310NSp0801252008

# 310NSp0801252008 - CH 310N MWF 8am Lecture 5 F Reading...

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CH 310N – MWF 8am – Lecture 5 – F 1/25/2008 Graded Homework : HW#01 deadline = 3:00 p.m. on Mon 1/28 HW#02 deadline = 3:00 p.m. on Thu 1/31 Last time : 1 H NMR (continued…) NMR integration Relationship of chemical shift to structure Spin–spin splitting and the (n+1) rule Textbook Problems : 13.10, 13.15 (part c only), 13.16 through 13.19, 13.24, 13.26, 13.28, 13.30 Reading Assignment : Sections 13.12, 13.13, 15.1 Today : 1 H NMR (continued…) Coupling constants & multiplicative splitting 13 C NMR

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Coupling Constants The distance between peaks in a multiplet is called the coupling constant ( J ). Unlike chemical shifts which we report using the δ scale, J values are always reported in units of Hz J values generally range between 0–18 Hz. Exact value of J depends on geometry. J = 6–8 Hz for coupled protons in acyclic alkanes. J is the same for both sets of protons that are coupled. J is independent of spectrometer field strength. If J = 7 Hz, it will be 7 Hz on a 60 MHz instrument, a 300 MHz instrument, a 500 MHz instrument, etc… Coupling constants become extremely useful when interpreting complex NMR spectra – If 2 multiplets have the same J value, they are probably related…the protons causing each multiplet are probably neighbors within the molecule. Some general rules : (keep these in mind as you start to work NMR problems) 1) Equivalent protons (homotopic or enantiotopic) do not show spin-spin splitting. 2) Signal for a proton with n neighboring protons will be split into a multiplet containing n + 1 peaks, with coupling constant J. Coupling is most common between protons on adjacent carbons. “Long range” coupling can occur through π bonds, however (more later…) 3) Two groups of protons coupled to each other have the same J .
It is useful to know the relative intensities of the lines in the multiplets that are commonly observed in a 1 H NMR spectrum. They can be predicted using Pascal’s triangle : # Equivalent Adjacent Protons Multiplet Ratio of Intensities 0 singlet 1 1 doublet 1 : 1 2 triplet 1 : 2 : 1 3 quartet 1 : 3 : 3 : 1 4 quintet 1 : 4 : 6 : 4 : 1 5 sextet 1 : 5 : 10 : 10 : 5 : 1 6 septet 1 : 6 : 15 : 20 : 15 : 6 : 1

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310NSp0801252008 - CH 310N MWF 8am Lecture 5 F Reading...

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