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02012010 - CH 310N MWF 8:00 Lecture 6 M Reading Assignment...

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CH 310N – MWF 8:00 – Lecture 6 – M 2/1/2010 Graded Homework : HW#03 deadline = 4:00 p.m. on Tu 2/2 HW#04 deadline = 2:00 p.m. on F 2/5 Reading Assignment : Section 15.1 (we will not cover Ch 14!!!) Last Time : Relationship of chemical shift to structure (continued…) Spin-spin splitting in 1 H NMR Textbook Problems : 13.6-13.9, 13.12, 13.13, 13.16-13.24, 13.27 Today : If you have an excusable conflict for ANY of the three midterm exams, the deadline to sign up to take a conflict exam is 5:00 p.m. on Friday, February 5 . 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 d 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.
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