01292010 - CH 310N MWF 8:00 Lecture 5 F 1/29/2010 Reading...

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CH 310N – MWF 8:00 – Lecture 5 – F 1/29/2010 Graded Homework : HW#02 deadline = 2:00 p.m. TODAY!!! HW#03 deadline = 4:00 p.m. on Tu 2/2 Reading Assignment : Sections 13.11, 13.12 Textbook Problems : No new problems… Last Time : NMR theory (continued…) Equivalence of protons NMR integration Relationship of chemical shift to structure 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 . Last Time : Relationship of chemical shift to structure (continued…) Spin-spin splitting in 1 H NMR
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Relationship of Chemical Shift to Structure Earlier, we said that differences in chemical shift are caused by differences in the effective magnetic field experienced by each nucleus. Nuclei that are more strongly shielded by electrons require a larger applied field to bring them into resonance, and therefore absorb “upfield” (right side of chart). Nuclei that are deshielded require a smaller applied field to bring them into resonance, and therefore absorb “downfield” (left side of chart). A typical 1 H NMR spectrum will have absorptions between 0–10 d , and can be divided into 5 general regions (which will be useful for you to know; see the “Exam Appendix” handout for more detailed information) : C C H (vinylic) C C H (allylic or benzylic or next to carbonyl) C Y H (Y = O, N, halogen) C C H (saturated) H (aromatic)
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There are 2 general trends : Chemical shift increases as the number of nearby electronegative groups increases. Protons attached to saturated (i.e. sp 3 ) carbons absorb at higher fields as compared to protons attached to unsaturated (i.e. sp 2 , sp) carbons. Why does electronegativity affect chemical shift?
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01292010 - CH 310N MWF 8:00 Lecture 5 F 1/29/2010 Reading...

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