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1H NMR Notes

by: Rachel Taylor

1H NMR Notes Chem 372

Rachel Taylor
GPA 3.5
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About this Document

These notes cover Hydrogen Nuclear Magnetic Resonance, and spin-spin splitting of signals.
Organic Chemistry
Dr. Friebe
Class Notes
Organic Chemistry, Chemistry




Popular in Organic Chemistry

Popular in Chemistry

This 6 page Class Notes was uploaded by Rachel Taylor on Sunday January 24, 2016. The Class Notes belongs to Chem 372 at Eastern Michigan University taught by Dr. Friebe in Winter 2016. Since its upload, it has received 45 views. For similar materials see Organic Chemistry in Chemistry at Eastern Michigan University.


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Date Created: 01/24/16
Nuclear Magnetic Resonance Spectroscopy 1H NMR: Determines the number and type of Hydrogen atoms in a molecule. 13 C NMR: Determines the carbon atoms present in a molecule. NMR basics:  Radio waves change the nuclear spins of elements that have nuclei with odd mass numbers.  An external magnetic field (B 0 rotates particles on their axis with or against the magnetic field. Charged particles create a magnetic field when rotated on their axis. Lower energy with nucleus aligned with magnetic field The difference of these two states=ΔE Higher energy with nucleus aligned against the magnetic field  When external energy (hv) is equal to the difference of energy states (ΔE), energy in absorbed.  Nucleus is in resonance when it absorbs RF radiation and spin flips to a higher energy state  The external magnetic field (B 0 is measured in Tesla (T). Frequency (v) is measured in Hz or MHz (1 MHz= 10 Hz). Planks constant (h) is 6.63x10 Js.4 Stronger magnetic field larger energy between spin states Higher frequency needed for resonance  TMS reference is used as a gage because it peaks at 0. Silicon is less electronegative than carbon making the electron density greater on the protons Calculating Chemical Shift: Chemical Shift (ppm)=???????????????????????????????? ????ℎ???????????????????????? ????ℎ???????????? ???????????????????????????????????? ???????????????? ???????????? ???? ???????? ????ℎ???? ???????????? ???????????????????????????????????????????????? (????????????) 1HNMR  Number: represents the number of different types of H  Position: what type of H  Intensity: relative abundance  Spin-spin splitting: information regarding neighboring environments Downfield Upfield Figure 1: NMR spectrum example Determining Equivalent protons  Protons are equivalents if they are cis or trans to the same group. Both H are cis to Cl H are cis to different atoms All H are cis to different atoms H are equivalent Both H are different. All H are different  A CH2 bonded to two different groups but the molecule does not have a stereogenic center, the protons are equivalent. To show this, replace each H with some atom Z, and if they form enantiomers, they are equivalent. Protons are enantiotopic if they are arranged like this. Enantiomers  When a CH i2 bonded to a molecule with a stereogenic center, the protons on CH are 2ot equivalent. To show this, replace each H with some atom Z and if they form diastereomers, they are not equivalent. Protons are Diastereotopic if they are arranged in this manner. Diastereomers 1H NMR position signals  Electrons shield the nucleus from B 0agnetic field with their own opposing magnetic field.  Isolated protons (H ) are only affected by 0 ; however, the electron density creates its own, opposing magnetic field minimizing the effects of B0.  Shielding shifts absorption upfield (to the right). Triple bonds shield protons with an opposing magnetic field shifting absorption upfield.  Electronegative atoms, double bonds and benzene rings increases deshielding because they pull the electrons away from the proton’s nucleus. Type of proton Chemical shift (ppm) Shielding RCH 3 ~0.9 Most shielding R 2H 2 ~1.3 R 3H ~1.7 1.5-2.5 R-CΞC-H ~2.5 2.5-4 4.5-6 6.5-8 9-10 10-12 Least shielding Table 1: Chemical shift by type of proton. Z= X, O, or N Peak intensity  Peak intensity is proportional to the number of absorbing protons.  Peak intensity is measured in proportions; integrals that are measured at 20 and 60 has a ratio of 20:60. That does not tell us the exact number of protons, but the amount relative to the other protons. Spin-Spin Splitting  Determines how many protons are on carbons near absorbing proton.  Occurs between nonequivalent protons on the same or adjacent carbons.  The number of peaks have ratios that abide by Pascal’s Triangle Ratio Row 0 1:3:3:1 1 2 1:2:1 3 4 1:1 5 6 1  Nonequivalent protons only split signals when they are three sigma bonds or less apart.  If there are more than one adjacent splitting protons, the peaks are split again. In the example below, CH 3H C2 Z 2as three different types of protons (H , Ha, abd H rescectively). H is tbe signal between H aad H , co it is split by both of them. The H sbgnal is first split into 4 peaks by Hain a 1:3:3:1 ratio. The four peaks are then split by H cn a 1:2:1 ratio. So you end up with 12 peaks in predictable ratios (as seen in the figures above).  It is important to remember that in the example above, you are only looking at the H signalb The H aignal exists upfield and the H scgnal is downfield because the electronegative Z atom deshields H c  CH 3H C2 Br2is a special case. The signals overlap making half as many peaks (6 instead of 12) show up on the NMR spectrum.  The magnitude of Trans alkenes is greater than the magnitude of Cis alkenes  OH protons are not split by other protons nor do they split other protons. Identifying molecules  By using mass spectrometry and infrared spectrometry, you can identify the molecular formula. By using H NMR spectroscopy, you can identify the relationship of the protons which will allow you to figure out the structural formula of the molecule.  Count the number of peaks. Label them (H , H a H b etc.).  Add together all the integration units (usually provided on the NMR spectrum). And divide by the number of protons (from molecular formula). This number represents the number of integration units per proton.  Take the individual integration units for each peak and divide that by the integration units per proton and round to the nearest whole number. This gives you the number of protons per signal. (3 usually means CH ,32 usually meant CH ) 2  Use splitting to determine the relationship. If there are 4 peaks, an adjacent carbon has 3 protons. If there are 3 peaks, an adjacent carbon has 2 protons, if there is 2 peaks, an adjacent carbon has 1 proton.  Use the Chemical shift data to determine the proton’s relationship with Z (X, N, or O). The more upfield the chemical shift, the more shielded the proton, the farther away it is from Z.


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