UH CHEM 3331 - nuclear magnetic resonance (3 pages)

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nuclear magnetic resonance



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nuclear magnetic resonance

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Lecture number:
27
Pages:
3
Type:
Lecture Note
School:
University of Houston
Course:
Chem 3331 - Organic Chemistry
Edition:
1
Documents in this Packet
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CHEM 3331 1nd Edition Lecture 27 Outline of Last Lecture I Mass Spectrometry II Electromagnetic spectrum III Infrared Spectroscopy Outline of Current Lecture I NMR Spectroscopy II Number of different peaks III Positions of peaks IV Peak area V Peak splitting Current Lecture I NMR Spectroscopy NMR stands for nuclear magnetic resonance It analyzes the nuclei of H1 C13 N15 F19 and P31 Today we will only be focusing on hydrogen In all of these atoms they have an odd number of protons or neutrons protons They experience a magnetic spin where they rient themselves in an external magnetic field There are and states is high energy and is low energy E h 2 B0 gyromagnetic ratio B0 magnetic field B B0 Bshielding shielding 50 100Hz measured in ppm II Number of different peaks The number of peaks the number of kinds of H CH4 1 CH3CH3 1 CH3CH2CH3 2 H2C CH2 1 H2C CHF 3 CH3OH 2 benzyne 2 These hydrogens differ or rather don t differ due to symmetry being peripheral or internal relationship to F Br Cl and I III Positions of peaks These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute This depends on shielded or deshielded The scale goes from 0 12 but runs backwards The deshielded hydrogens are closest to twelve while the shielded H are closer to 0 We use TMS as our 0 point because it is the most shielded molecule Chemical shift difference from TMS peak Hz frequency of radiowaves MHz compound 4 3 CH3F 3 4 CH3OH 3 0 CH3Cl 2 7 CH3Br 2 2 CH3I 1 23 CH3CH2OH 0 94 CH3CH2CH2OH 3 0 CH3Cl 5 3 CH2Cl2 7 2 CHCl3 Alkanes 0 9 1 4 Alkenes 5 6 Alkynes 2 5 3 5 Carbonyl 2 41 IV Peak area CH3C OCH2OC CH3 3 3 peaks 3 2 9 The relative areas under the peaks are known as integrals and are equal to the ratio of different H CH3 2CHOCH CH3 2 2 12 2 6 1 CH2CH2OCH2CH3 2 6 4 3 2 1 5 1 V Peak splitting Peak splitting is determined by the number and kinds of neighbors an H has next to it There is the n 1 rule 3 neighbors 4 peaks split 2 neighbors 3 peaks split 0 neighbors 0 peaks split We can use pascals triangle to determine the integrals of the areas of each peak in the peak split The peak split ratio depends on the possibilities of spin either or It is always symmetric CH3 2CHCl has two peaks However these peaks are split Each CH3 is acted upon by the CH so they both have two 1 1 peaks The CH is acted upon by the two CH3 molecules so it has seven peaks with a 1 6 15 20 15 6 1 ratio When a H is acted upon by two or more neighbors that are not of the same kind then the peas are split differently They are split one at a time For example a benzene with Cl and Br on 1 and 3 Each hydrogen is different We will label them H1 H4 starting with the hydrogen between Cl and Br working clockwise H3 has two neighbors H2 and H4 but they are not the same because H2 is closer to Br and H4 is closer to Cl We will have H4 act on H3 first creating a doublet Then H2 will split both of these peaks into two Creating a double doublet with a 1 1 1 1 ratio This is different from a quartet which is a 1 3 3 1 ratio and the space in between peaks is much larger for the double doublet than a quartet Do some practice problems to really get the hang of this


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