Nuclear Magnetic Resonance

Describe NMR spectroscopyAnalyse spectraNuclear Magnetic Resonance1How does it work?2

How does it work?3The sample is positioned in the magnetic field and excited via pulsations in the radio frequency input circuit. The realigned magnetic fields induce a radio signal in the output circuit which is used to generate the output signal. Fourier analysis of the complex output produces the actual spectrum. Nuclear Magnetic Resonance Spectroscopy41H or 13C nucleus spins and the internal magnetic field aligns parallel to or against an aligned external magnetic fieldParallel orientation is lower in energy making this spin state more populatedRadio energy of exactly correct frequency (resonance) causes nuclei to flip into anti-parallel state Energy needed is related to molecular environment (proportional to field strength, B)13.2 The Nature of NMR Absorptions5Electrons in bonds shield nuclei from magnetic fieldDifferent signals appear for nuclei in different environments

The NMR Measurement6The sample is dissolved in a solvent that does not have a signal itself and placed in a long thin tubeThe tube is placed within the gap of a magnet and spunRadiofrequency energy is transmitted and absorption is detectedSpecies that interconvert give an averaged signal that can be analyzed to find the rate of conversionCan be used to measure rates and activation energies of very fast processesChemical Shifts7The relative energy of resonance of a particular nucleus resulting from its local environment is called chemical shift NMR spectra show applied field strength increasing from left to rightLeft part is downfield is upfieldNuclei that absorb on upfield side are strongly shielded.Chart calibrated versus a reference point, set as 0, tetramethylsilane [TMS]Measuring Chemical Shift8Numeric value of chemical shift: difference between strength of magnetic field at which the observed nucleus resonates and field strength for resonance of a referenceDifference is very small but can be accurately measuredTaken as a ratio to the total field and multiplied by 106 so the shift is in parts per million (ppm)Absorptions normally occur downfield of TMS, to the left on the chartCalibrated on relative scale in delta () scaleIndependent of instruments field strength9

Characteristics of 13C NMR Spectroscopy10Provides a count of the different types of environments of carbon atoms in a molecule13C resonances are 0 to 220 ppm downfield from TMS (Figure 13-7)Chemical shift affected by electronegativity of nearby atoms O, N, halogen decrease electron density and shielding (deshield), moving signal downfield.sp3 C signal is at 0 to 9; sp2 C: 110 to 220 C(=O) at low field, 160 to 22011Spectrum of 2-butanone is illustrative- signal for C=O carbons on left edge

1H NMR Spectroscopy and Proton Equivalence12Proton NMR is much more sensitive than 13C and the active nucleus (1H) is nearly 100 % of the natural abundanceShows how many kinds of nonequivalent hydrogens are in a compoundTheoretical equivalence can be predicted by seeing if replacing each H with X gives the same or different outcomeEquivalent Hs have the same signal while nonequivalent are differentThere are degrees of nonequivalenceChemical Shifts in 1H NMR Spectroscopy 13Proton signals range from 0 to 10

Lower field signals are Hs attached to sp2 C

Higher field signals are Hs attached to sp3 C

Electronegative atoms attached to adjacent C cause downfield shift

Integration of 1H NMR Absorptions: Proton Counting14The relative intensity of a signal (integrated area) is proportional to the number of protons causing the signalThis information is used to deduce the structureFor example in ethanol (CH3CH2OH), the signals have the integrated ratio 3:2:1For narrow peaks, the heights are the same as the areas and can be measured with a ruler

Spin-Spin Splitting in 1H NMR Spectra15Peaks are often split into multiple peaks due to interactions between nonequivalent protons on adjacent carbons, called spin-spin splittingThe splitting is into one more peak than the number of Hs on the adjacent carbon (n+1 rule)The relative intensities are in proportion of a binomial distribution and are due to interactions between nuclear spins that can have two possible alignments with respect to the magnetic fieldThe set of peaks is a multiplet (2 = doublet, 3 = triplet, 4 = quartet)Simple Spin-Spin Splitting16An adjacent CH3 group can have four different spin alignments as 1:3:3:1This gives peaks in ratio of the adjacent H signalAn adjacent CH2 gives a ratio of 1:2:1The separation of peaks in a multiplet is measured is a constant, in Hz J (coupling constant)

Rules for Spin-Spin Splitting17Equivalent protons do not split each otherThe signal of a proton with n equivalent neighboring Hs is split into n + 1 peaksProtons that are farther than two carbon atoms apart do not split each other

Exercise18Carboxylic acids (RCO2H) react with alcohols (ROH) in the presence of an acid catalyst. The reaction product of propanoic acid with methanol has the following spectroscopic properties. Propose a structure.

MS: M+ = 88IR: 1735cm-11HNMR : 1.11 (3H,t); 2.32 (2H,qrt);3.65 (3H,s)13CNMR : 9.3, 27.6, 51.4, 174.6

Exercise19Nitriles (RCN) react with Grignard reagents (RMgBr). The reaction product from 2-methylpropanenitrile with methylmagnesium bromide has the following spectroscopic properties. Porpose a structure.

MS: M+ = 86IR: 1715cm-11HNMR : 1.05 (6H,d); 2.12 (3H,s);2.67 (1H,sept)13CNMR : 18.2, 27.2, 41.6, 211.2