ORGANIC CHEMISTRY 3540

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

NMR PHENOMENON

CHEMICAL SHIFT

LOW RESOLUTION HYDROGEN-1 NMR

CARBON-13 NMR

TIME AVERAGING

NUCLEAR MAGNETIC RESONANCE

NMR PHENOMENON

SOME NUCLEI HAVE A MAGNETIC MOMENT

THE NUMBER OF THE NEUTRONS OR THE NUMBER OF PROTONS MUST BE ODD.
₁ ¹H, ₆¹³C, ₉¹⁹F HAVE NUCLEAR MOMENTS.
₆ ¹²C, ₈¹⁶O DO NOT HAVE NUCLEAR MOMENTS.
(IF BOTH THE NUMBER OF PROTONS AND THE NUMBER OF NEUTRONS ARE ODD, THE NUCLEUS ALSO HAS AN ELECTRICAL QUADRUPOLE MOMENT. SUCH NUCLEI ARE BEYOND THE SCOPE OF THIS COURSE.)

IN A MAGNETIC FIELD, B_o, NUCLEAR MOMENTS ALLIGN:

WITH THE FIELD - LOW ENERGY
AGAINST THE FIELD - HIGH ENERGY

NUCLEUS CAN ABSORB A PHOTON AND GO TO A HIGHER ENERGY STATE. (We say the "spin flips" and the nuclei are in resonance.)

PHOTON IS A FUNCTION OF:
*MAGNETIC FIELD, B_o
*NUCLEAR MOMENT

{Doesn't seem too useful to be able to tell if ₁¹H is in sample......But....}

ORIGINS OF CHEMICAL SHIFT

ELECTRONS UNDER THE INFLUENCE OF AN EXTERNAL MAGNETIC FIELD, B_o, WILL CIRCULATE!

CIRCULATING ELECTRONS PRODUCE THEIR OWN MAGNETIC FIELD, B_e!
THE INDUCED MAGNETIC FIELD IS OPPOSITE TO THE EXTERNAL MAGNETIC FIELD.
THIS EFFECT ACCOUNTS FOR THE DIAMAGNETISM OF ORGANIC MATERIALS.
FERROFLUID, MORE FERROFLUID

THE NUCLEUS IS SHIELDED BY THE INDUCED MAGNETIC FIELD.

THE EXTENT OF THE SHIELDING IS A FUNCTION OF OF THE DENSITY OF THE CIRCULATING ELECTRONS.
OR, THE EXTENT OF SHIELDING DEPENDS UPON THE INDUCTIVE EFFECTS OF ATOMS ATTACHED TO CARBON.

HYDROGENS IN DIFFERENT CHEMICAL ENVIRONMENTS ARE IN DIFFERENT MAGNETIC ENVIRONMENTS, B_o PLUS B_e, AND ABSORB A DIFFERENT ENERGY PHOTON!!!!!!

THIS PRODUCES THE CHEMICAL SHIFT!
REFERENCE COMPOUND IS TMS, TETRAMETHYLSILANE.

OBTAIN AN NMR SPECTRUM.

METHYL 2,2-DIMETHYLPROPANOATE.

FOR SIGMA BONDS, THE EXTENT OF THE SHIELDING IS A FUNCTION OF THE DENSITY OF THE CIRCULATING ELECTRONS.

OR, THE EXTENT OF SHIELDING IN A SIGMA BOND DEPENDS UPON THE INDUCTIVE EFFECTS OF ATOMS ATTACHED TO CARBON.
ETHYL BROMIDE HAS TWO PEAKS AT 3.4 AND 1.7 d.
INDUCTIVE EFFECTS, AND DESHIELDING, ARE INDEED, EXTENDED TO ADJACENT HYDROGENS.

PI SYSTEMS ARE MORE COMPLEX.

CONSIDER AN ALDEHYDE.
THE CHEMICAL SHIFT OF THE ALDHYDE HYDROGEN IN ACETALDEHYDE IS 9.83 d.
CONSIDER AN AROMATIC COMPOUND, BENZENE.
THE CHEMICAL SHIFT OF THE AROMATIC HYDROGENS IN TOLUENE IS 7.1 d.
THE CHEMICAL SHIFTS OF THE HYDROGENS IN [18]ANNULENE ARE AT 8.2 AND -1.9 d.

CHARACTERISTIC PROTON CHEMICAL SHIFTS.

BRIEF PROTON CHEMICAL SHIFTS

CHEMICAL SHIFT VALUES AND RELATIVE NUMBER OF PROTONS CAN BE USED TO DETERMINE STRUCTURE, EVEN OF ISOMERS.

EXAMPLES.

NOTE: WE HAVE BEEN DOING LOW RESOLUTION PROTON NMR.

IN HIGH RESOLUTION PROTON NMR, WE WILL SEE HYDROGENS ON ADJACENT CARBON ATOMS "SPLITTING" SIGNALS (PEAKS) INTO MULTIPLE PEAKS WHICH PROVIDES ADDITIONAL DATA FOR STRUCTURE DETERMINATION.

WILL CONSIDER THIS NEXT SEMESTER.

CARBON-13 VS. HYDROGEN-1 NMR

PROBLEMS:

CARBON-12 DOESN'T HAVE A MAGNETIC MOMENT BUT CARBON-13 DOES.
ONLY 1.1% OF THE CARBON ATOMS ARE CARBON-13!
RESULTED IN A VERY NOISY, ALMOST USELESS SPECTRUM!
SOLUTION TO NOISE - DIFFERENT ELECTRONICS, ETC.
NOISE IS RANDOM. TAKE MANY (HUNDREDS OR THOUSANDS) SPECTRA AND USE COMPUTER TO AVERAGE SPECTRA.
THE NOISE IS MINIMIZED. BUT, IT TAKES 5-10 MINUTES TO OBTAIN A CONVENTIONAL SPECTRUM..........
USING MODERN TECHNIQUES CAN DO THOUSANDS OF SPECTRA IN A SHORT TIME AND GET A VERY GOOD SPECTRUM WITH VERY SHARP PEAKS!

PEAKS ARE NOT QUANTITATIVE!

HOWEVER, MOST OF THE TIME YOU DO NOT HAVE EQUIVALENT CARBONS!

AND SINCE SIGNALS ARE SHARP, YOU GENERALLY SEE A SIGNAL FOR EACH CARBON (BUT NOT ALWAYS!).

CHEMICAL SHIFTS.

MOST C-13 PEAKS ARE 0-220 d DOWNFIELD FROM TMS.

CARBON BONDED TO ELECTRONEGATIVE ATOMS ARE DESHIELDED AND APPEAR DOWNFIELD.

sp³ CARBON GENERALLY 0-90 d.

sp² CARBON GENERALLY 100-170 d.

CARBONYL COMPOUNDS (C=O) WAY DOWNFIELD AT 170-220 d.

CHARACTERISTIC CARBON-13 CHEMICAL SHIFTS!

MODERN TECHNIQUES CAN ALSO TELL HOW MANY HYDROGENS ARE ATTACHED TO EACH CARBON.

USE DATA TO CORRELATE PEAKS WITH SPECIFIC CARBON ATOMS OR SOLVE STRUCTURE.

DETERMINE THE STRUCTURE OF:

SAMPLE 1, C₆H₁₂.

SAMPLE 2, C₈H₁₆.

SAMPLE 3, C₈H₈O₂.

TIME AVERAGING

ABSORPTION OF INFRARED ENERGY (VIBRATION) IS INSTANTANEOUS (10^-13 SECONDS).

ABSORPTION OF RADIO FREQUENCY (NMR) IS SLOW (10^-3 SECONDS).

TWO RAPIDLY (AT LEAST 10³ TIMES PER SECONDS) INTERCONVERTING SPECIES WILL GIVE A TIME AVERAGED SPECTRUM.

SLOWING THE PROCESS DOWN (DROP TEMPERATURE) CAN GIVE THE SPECTRUM OF EACH.

AXIAL AND EQUATORIAL HYDROGENS OF CYCLOHEXANE.
CAN DETERMINE ACTIVATION ENERGY FROM SUCH DATA.

HIGH RESOLUTION PROTON AND C-13 NMR SPECTRA ARE AVAILABLE AT THE National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan. YOU MAY HAVE DIFFICULTY UNDERSTANDING THE HIGH RESOLUTION PROTON NMR SPECTRA.