FT-NMR

Fundamentals Nuclear spin Spin quantum number – ½ Nuclei with spin state ½ are like little bar magnets and align with a B field. Can align with (++) or against (+-) B Small energy gap between + and – spin alignment (NMR insensitive/Boltzman dist) Can probe difference with RW

(NMR insensitive/Boltzman dist) Small population difference between +1/2 and -1/2 state It is the small excess of nuclei in the -1/2 that produce NMR signal

Common NMR nuclei Protons, 1 H 13 C 15 N 19 F 31 P Sensitivity depends on natural isotopic abundance and   E =  ћB 0, bigger magnet, greater sensitivity

Precession of nuclear dipoles

The basis of the NMR experiment Chemical Shift; Nuclei in different bonding environments have different  Es (electron density). Spin-Spin Splitting; Adjacent nuclei split the signal into multiplets in a predictable fashion.

Chemical Shift Shielding –Electrons have spin, produce local B environments –Protons in different electronic environments experience different B (B m +B e ), different precessing frequencies,  E = h  –Chemical shift proportional to size of magnet –ppm {(  -  0 )/  0 }*10 6

Spin-Spin Coupling Adjacent nuclei have a 50/50 chance of being spin up (+1/2) or spin down (-1/2) Each produce a small magnetic field that is either with or against B 0 1 adjacent proton CHOCH3 –CH 3 is a doublet at frequencies   -  a,   +  a (equal intensity), 1:1 –CH is a quadruplet 

Splitting Patterns J values Quadruplet                         Triplet        Multiplets ¼ ½ ¼ ¾ 1 ½ ¾ ¾ 1 ½ ¾ ¼ ½ ¼

Precession of nuclear dipoles

FT pulse Radiofrequency generator –A short, intense pulse generates a magnetic field in the x-y plane (excites all nuclei) –M 0 of the nuclei interacts with the magnetic field produced by the pulse. –Tips M 0 off axis Θ =  B1  p  p – length of pulse, 90  pulse

Vector illustration

Relaxation T 1 spin-lattice (relaxing back to precessing about the z axis) T 2 spin-spin (fanning out)

Induced current in coil After pulse, nuclei begin to precess in phase in the x-y plane Packet of nuclei induce current in RF coil Relaxation is measured by monitoring the induced coil → FID (→ FT) NMR spectrum

FID

Noise reduction and increasing resolution Apodization: Multiply the free-induction decay (FID) by a decreasing exponential function which mathematically suppresses the noise at long times. Other forms of apodization functions can be used to improve resolution or lineshape.. Zero filling

13 C NMR 13 C frequency Different tuning folk Broadband Decoupling of 1 H No spin-spin coupling NOE effect Assignments based on chemical shift Wider frequency range

Obtaining a 13 C NMR Spectrum 1 H Broadband decoupling –Gives singlet 13 C peaks, provided no F, P, or D present in the molecule) –Continuous sequence of pulses at the 1 H frequency causes a rapid reversal of spin orientation relative to the B 0, causing coupling to 13 C to disappear

1 H channel 13 C channel Broadband Decoupling

H 3 C 4 -C 3 H=C 2 H-C 1 OOH C-1 C-3 C-2 solvent C-4

13 C Chemical Shifts Reference is TMS, sets 0 ppm A range of 200 ppm Chemical shifts can be predicted –Empirical correlations –Ex. Alkanes  i = n  + 9.4n  – 2.5n  + 0.3n  + 0.1n  + Sij 2-methylbutane  i = * *2 – 2.5* = 22.0 (22.3)

Signal averaging 13 C experiment generally take longer than 1 H experiments because many more FIDs need to be acquired and averaged to obtain adequate sensitivity. NOE effect (enhancement/reduction in signal as a result of decoupling) 1H1H 1H1H 13 C N1N1 N4N4 N3N3 N2N2 N2N2 N1N1 N3N3 N4N4 W2W2 W1W1

NOE effect W 2 (Enhancement) dominates in small molecules Relevant for all decoupling experiments

Other more complex 1D Experiments 1 H NOE experiment Inversion Recovery Experiment; Determination of T 1 J modulated Spin Echo INEPT Experiment DEPT Experiment

Targeted 1 H Spin Decoupling Continuous irradiation at a frequency ( 2 ) that corresponds to a specific proton in the molecule during the 1 H NMR experiment All coupling associated with the protons corresponding to 2 disappears from the spectrum

  channel 1 H channel 1 H targeted decoupling (NOE)

TMS

NOE- nuclear Overhauser effect Saturation of one spin system changes the equilibrium populations of another spin system NOE effect can be positive or negative. In small molecules it is usually positive

Selective Heteronuclear Decoupling Saturate at a specific frequency Multiplets collapse reveal connectivity

More Complex NMR Pulse Sequences J-Modulated Spin Echo experiment –Cq and CH 2 down and CH 3 and CH up DEPT experiment –  = 45 , 90 , 135  –CH3 [DEPT(90)], CH2 [DEPT(45)-DEPT(135)], CH [DEPT(45)+DEPT(135)-0.707DEPT(90)] 2D-NMR –Het. 2D J resolved/Homo 2D J resolved – 1 H- 1 H COSY – 1 H/ 13 C HETCOR

CH and CH 3 C q and CH 2

DEPT DEPT(90) CH 3 DEPT(45) – DEPT(135) CH 2 DEPT(45)+DEPT(135) DEPT(90) CH 13 C decoupled spectra