Nuclear Magnetic Resonance (NMR) for beginners

Overview NMR is a sensitive, non-destructive method for elucidating the structure of organic molecules Information can be gained from the hydrogens (proton NMR, the most common), carbons ( 13 C NMR) or (rarely) other elements

Spin States All nuclei have a spin state (I ) Hydrogen nuclei have a spin of I = ±½ (like electrons) Spin number gives number of ways a particle can be oriented in a magnetic field: 2I + 1

Spin States In the absence of a magnetic field the spin states are degenerate The “spinning” nucleus generates its own magnetic field

Spin States In a magnetic field the states have different energies BoBo B’  B’ 

Spin states in a magnetic field Energy difference linearly depends on field strength  = magnetic moment of H (  N or x J/T)

Spin states in a magnetic field Even in a very large field (1-20T) the energy difference is small (~0.1cal/mol)

Spin states in a magnetic field A small excess of protons will be in the lower energy state These can be promoted to the higher state by zapping them with EM radiation of the proper wavelength Wavelength falls in the radio/TV band (frequency of MHz)

Spin states in a magnetic field Stronger magnetic field necessitates shorter wavelength (higher frequency) After low energy protons are promoted to the higher energy state they relax back to the lower state

Making NMR work Not all protons absorb at the same field values Either magnetic field strength or radio frequency must be varied Frequency/field strength at which the proton absorbs tells something about the proton’s surroundings

Making NMR work

Sample must be spun to average out magnetic field inhomogeneity

NMR data collection Continuous wave data collection (CW): –Magnetic field value is varied –Intensity of emitted RF compared to RF at detector –Absorption is plotted on graph

NMR data collection CW NMR of isopropanol

NMR data collection Pulsed Fourier transform data collection: –Short bursts of RF energy are shot at sample –Produces a decay pattern –FT done by computer produces spectrum

Simple FT FID and spectrum

More complex FT FID and spectrum

Even more complex FT FID

FT NMR Spectrum

Pulsed FT NMR of isopropanol

Chemical shift Protons in different environments absorb at different field strengths (for the same frequency) Different environment = different electron density around the H

Chemical shift positions High field, shielded Low field, deshielded Reference (tetramethylsilane) PPM of applied field (  ) from reference

Chemical shift positions

NMR reference Tetramethylsilane ((CH 3 ) 4 Si) Advantages: –Makes one peak –12 equivalent H, so little is needed –Volatile, inert, soluble in organic solvents –Absorbs upfield of hydrogens in most organic compounds

Shielding/deshielding Electron density affects chemical shift Electrons generate a magnetic field opposed to the applied field H in high electron density absorbs upfield (toward TMS, 0ppm) from H in low electron density

Shielding/deshielding Effect of electronegativity: electronegative atom nearby removes electron density and causes deshielding TMS protons are extremely shielded because Si is electropositive compared to C

Shielding/deshielding Few protons absorb upfield of TMS Alkyl groups are electron donating, so alkanes absorb around 0-2ppm (  ) Hydrogens near electronegative atoms are deshielded Absorption is around 3-4 

Anisotropy “Anisotropy”: any characteristic that varies with direction (asymmetric) Applied to the shielding/deshielding characteristics of electrons in some systems

Anisotropy Aromatic hydrogens are in the deshielding region of the magnetic field generated by circulating electrons

Typical chemical shifts

Spin-spin coupling Magnetic field felt by a proton is affected by the spin states of nearby protons – either shielding or deshielding Case 1: neighboring single protons These H can either be the same or opposite spins – equal probability Makes doublets of two equal peaks at both absorptions

NMR spectrum of dichloroacetaldehyde

Coupling constants Separation between peaks is the “coupling constant” Symbol: J Measured in Hz It is the same for both coupled protons

Spin-spin coupling Case 2: Single proton next to a pair Single proton splits the pair into a doublet Spin state possibilities for pair: BoBo      Equal energy  Integration ratio: 1:2:1

Spin-spin coupling Single proton is split into a triplet Any group of n protons will split its neighbors into n + 1 peaks Intensity follows Pascal’s triangle (Fibonacci series)