Nuclear Magnetic Resonance Spectroscopy

90 Pages·2013·5.01 MB·English

John E. McMurry
www.cengage.com/chemistry/mcmurry
Chapter 13
SSttructture DDettermiinattiion:
Nuclear Maggnetic
Resonance Spectroscopy
Revisions by Dr. Daniel Holmes – MSU
Paul D. Adams • University of Arkansas
The Use of NMR Spectroscopy
Used to map carbon-hydrogen framework of

molecules
Used to determine relative location of atoms

within a molecule
Most helpful spectroscopic technique in organic

chhemiisttry
Depends on very strong magnetic fields

Earth’s magnetic field is ~0.00006 Tesla

Refrigerator magnet is ~0.005 Tesla

MMRRII range ffrom 11.55 – 33.00 TTeslla

Largest NMR Magnet at MSU is 21.2 Tesla

The Use of NMR Spectroscopy
The Use of NMR Spectroscopy
Otto Stern, USA: Nobel Prize in Physics 1943, "for his contribution to the

ddeevveellooppmmeenntt ooff mmoolleeccuullaarr rraayy mmeetthhoodd aanndd hhiiss ddiissccoovveerryy ooff tthhee mmaaggnneettiicc
moment of the proton"
Isidor I. Rabi, USA: Nobel Prize in Physics 1944, "for his resonance method

for recording the magnetic properties of atomic nuclei"
Felix Bloch, USA and Edward M. Purcell, USA: Nobel Prize in Physics

1952, "for their discovery of new methods for nuclear magnetic precision
measurements and discoveries in connection therewith"
Richard R. Ernst, SSwitzerland: Nobel Prize in CChemistry 11999911, "ffor his

contributions to the development of the methodology of high resolution
nuclear magnetic resonance (NMR) spectroscopy
KKuurrtt WWüütthhrriicchh, SSwwiittzzeerrllaanndd:: NNoobbeell PPrriizzee iinn CChheemmiissttrryy 22000022, "ffoorr hhiiss

development of nuclear magnetic resonance spectroscopy for determining
the three-dimensional structure of biological macromolecules in solution"
Paul C. Lauterbur, USA and Peter Mansfield, United Kinggdom: Nobel Prize

in Physiology or Medicine 2003, "for their discoveries concerning magnetic
resonance imaging"
Why This Chapter?
NMR is the most valuable spectroscopic

technique used for structure determination
Through-bonds and through-space

More advanced NMR techniques are used in

biological chemistry to study protein structure
andd ffollddiing
Hadjuk et al. J. Am. Chem. Soc. 2000, 122, 7898
13.1 Nuclear Magnetic Resonance
SSppeeccttrroossccooppyy
1H or 13C nuclear spins (or any NMR active nucleus like

1155NN,3311PP,2299SSii,22HH, or 1111BB)) wiillll alliign parallllell tto or agaiinstt an
external magnetic field
Parallel orientation is lower in energy making this spin

state more populated
AAtt 2211.22 TT ((990000 MMHHzz)), tthhee eexxcceessss ppooppuullaattiioonn iiss oonnllyy

0.014%, which means there are only 140 spins out of a
million aligned with the field
13.1 Nuclear Magnetic Resonance
SSppeeccttrroossccooppyy
Radio energy of exactly correct frequency (resonance)

causes nuclleii tto fflliip iintto anttii-parallllell sttatte
Energy needed is related to molecular environment

((ppropportional to field strenggth,, B))
Frequency of transition: =-B

0
13.2 The Nature of NMR
AAbbssoorrppttiioonnss
Electrons in bonds shield nuclei from magnetic field

Different signals appear for nuclei in different

environments
The NMR Measurement
The sample is dissolved in a solvent that does not have

aa ssiiggnnaall iittsseellff ((CCDDCCll )) aanndd ppllaacceedd iinn aa lloonngg tthhiinn ttuubbee
3
The tube is placed within the magnet

The NMR Measurement
A radiofrequency pulse (10-15 s) is transmitted to the

ssaammppllee, nnuucclleeaarr ssppiinnss ‘fflliipp’ ttoo hhiigghheerr eenneerrggyy ssttaattee iiff iinn
resonance with pulse
Nuclei relax back to equilibrium, which is detected as

miicroscopiic vollttage osciillllattiions iin tthhe NNMMRR probbe
The oscillations decay over time (Free Induction Decay or

FID)
Pulses repeated many times and data summed to get

improved Signal compared to the Noise
Fourtier Transform is used to convert the FID to a

spectrum with frequency vs. intensity