H NMR Spectroscopy

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Note by abbeyjarvis, updated more than 1 year ago
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Undergraduate CHM2WS1 Note on H NMR Spectroscopy, created by abbeyjarvis on 11/21/2013.

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H NMR SpectroscopyOverview-absorbs energy corresponding to the radio frequeny portion of the electromagnetic spectrum-depends on spin states (2 states 1/2 and -1/2)-When placed in a magnetic field the two states are no longer equivalent where one is stabilised and the other destabilised.-dE is proportional to wavenumber-Spectra will differ from one machine to another dueto different field strengthsChemical shift/ppm=(v(sample)-(v(referencee) Hz/Vo(MHz)100 MHz  peak/MHz = ppm e.g. 360/100 = 3.6 ppmThis will be the same for different field strengths as the peak is shifted.

Electronegative and electropositive substituentsAs the electronegativity increases the peak is shifted downfield (to a higher ppm)As the substitution increases the peak is shifted downfield (to a higher ppm)Electronegative substituents shift peaks downfield at a greater strength than increasing the substituents do.Inductive EffectsInductive effects run through the sigma-framework. There are also mesomeric effects working through the conjugation of the p- and pi-orbitals.The protons which are closer to the direction of the negative charge are shifted to a higher ppm. The protons at the lower slightly positive charge won't be shifted as high.

ImportantEquivalent nuclei cannot couple to each otherThe size of the vicinal couplings is described using the Karplus equations which relates the magnitude of coupling to the angle between the two protons.Cis & Trans3J trans  12-16 Hz3J cis     7-11 HzWhen the protons are closer together the coupling constant is smallerPositional180 degrees from each other (both axial)   8-12 Hz1 axial and 1 equatorial                            5-9 HzBoth equatorial                                        2-6 HzThe more favourable the positions of the Hydrogen substituents the smaller the couping constant.Electronegative and electropositive substituentsethane-E+    8.4 Hzethane-Cl     7.3 Hzethane-3Cl   6.0HzAs you increase the electronegativity there is a decrease in the coupling constantBond length effects on coupling constantsGenerally the increase in bond length  decreases the  coupling constant.Ranging from 18 to 6 HzAngle strainAs the angle strain is decreases there is an increase in the coupling constant.3 membered ring( with 1 double bond) 0.5-2Hz6 membered ring (with 1 double bond) 8-10 HzMeasuring coupling constantsPeak * field strengtha-b = coupling constante.g doublet3.54ppm   3.54 x 100MHz = 354 Hz3.60ppm   3.60 x 100MHz = 360 Hz3J(a-b) = 360-354 = 6HzAs you increase the hertz scale you get the same seperation between 2 peaks which are coupling. The spread of the peaks will just change.

When a proton couples to 2 equivalent protons you get a Triplet, but if it couples to 2 inequivalent protons you get a doublet of doublets.

Overview

Factors affecting shift

Coupling

Coupling Patterns

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