Nuclear Magnetic Resonance Spectroscopy

NMR Spectroscopy Method for determining the structure of organic molecules interpretation sample preparation data acquisition

3 NMR Instrument

Electrons shield H from external magnetic field. This results in small difference in the position of absorptions.

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Theory 1 H and 13 C have atomic spin numbers of 1/2 Have two energy levels in an applied magnetic field aligned against the field: higher energy aligned with the field: lower energy Difference in energy is quantized Radio frequency region electromagnetic radiation absorbed Frequencies absorbed depend on the environment of the nuclei and the applied magnetic field strength

Interpretation of Proton NMR Spectra Structure from the spectrum spectra of compounds with unknown structures structure determination usually requires complementary spectral data Spectrum from the structure used to verify products compare to published spectra computer model of hypothetical spectrum

Signals Due to Equivalent Nuclei Nuclei--chemically and magnetically equivalent replacing any equivalent atom of a grouping with another atom gives the same compound atoms are often equivalent by fast rotation Ethyl Iodide has two proton absorption signals from two sets of equivalent protons

60 MHz Proton NMR Spectrum of Ethyl Iodide

250 MHz Proton NMR Spectrum of Ethyl Iodide

The Chemical Shift Results from the effective magnetic field around nuclei depends on structure hybridization electronegativity of neighboring atoms Each absorption signal in the NMR spectrum can be assigned to a particular structural feature Inconsistencies sometimes occur Tables of chemical shift values are generally representative

Question: if u have a proton signal at 125 Hz in 60 MHz spectrometer, what's the position of this signal if we use 300MHz apparatus? δ = 125/60 = 2.1 ppm

Chemical Shift Data

Inductive effect influence chemical shift

Types of Hydrogens on Carbon

Splitting Patterns and Coupling Constants Helpful in determining types of hydrogen on carbons adjacent to the carbon with the protons in resonance Chemically equivalent protons on neighboring carbons typically give a pattern of lines containing one more line than their total This splitting pattern of lines represents the absorption signal for proton(s) in resonance Peak intensities follow the binomial expansion

Pascal’s Triangle

Coupling Constants (cont.) The peaks at 1.83 ppm and 3.20 ppm have been expanded and plotted on the left side of the spectrum (line spacing in the expanded region is equivalent because the protons are coupled)

Coupling Constants Quartet-triplet combination strong indicator for ethyl group The spacing between lines in an absorption signal caused by protons on a neighboring carbon is called the coupling constant (constants are measured in Hertz) The spacing between the lines in Hertz will be the same for coupled groups on adjacent carbons

abc δ 0.96 No. of neighboring protons2 Splitting (n+1)t(3)

abc δ No. of neighboring protons Splitting (n+1)

δ No. of neighboring protons Splitting (n+1)

δ No. of neighboring protons Splitting (n+1)

δ No. of neighboring protons Splitting (n+1)

28 Propose a structure for the compound of molecular formula C 4 H 10 O whose proton NMR spectrum follows. Solved Problem 3 Solution

29 Solved Problem (Continued) Solution (Continued)

Integrals of Absorption Signals Height of stair-step line is proportional to the area under the signal line The area of each absorption signal is directly proportional to the number of hydrogen atoms causing that signal

Interpreting NMR Spectra Use a combination of approaches to interpret a spectrum Complimentary spectral data can provide a molecular formula and identify some functional groups on a structure NMR spectra permit a back-and-forth analysis to correlate molecular structure with absorption signals A structure is verified when all of the absorption signals and patterns are accounted for

Deuterium D = 2 H - an isotope of 1 H invisible in 1 H NMR used in solvents CDCl 3 (deuteriochloroform) (perdeuterioacetone)

Why we use TMS as a reference? 1. It contains 12 equivalent protons so it gives strong signal 2. Its protons are highly shielded in comparison with organic cpd, so its signal occurs at higher field than other protons 3. inert 4. volatile 5. soluble in most organic solvents

Filtration of NMR Samples Through a Pasteur Pipette Can also use two pipettes and gravity filtration

Two-Dimensional COSY NMR Spectrum

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