Chapter 14

Nuclear Magnetic Resonance Spectroscopy

“study of interaction of light with molecules”

14.1 Proton Magnetic Resonance Spectroscopy

Two or more E states are generated when external magnetic field is applied

radio wave absorption between these states

NMR spectroscopy

Hydrogen nucleus (1H) and the isotope of carbon (13C)

1H NMR and 13C NMR

Three types of information for H NMR spectrum

Chemical shift: the carbon to which proton is attached

Multiplicity( # of the peaks in each group): neighboring hydrogens

Integral: # of hydrogens

14.2 Theory of 1H-NMRTwo spin states are generated when external magnetic field is applied ( 546 Figure 14.3)

E (E difference between the two states) = hBo/2

h = plank constant,

= magnetogyric ratio (characteristic value for each nucleus)

Bo = external magnetic field

Typical values of E of proton (reference TMS)

External magnetic field 14,000 gauss (or 1.4-tesla)

then E =10-6 kcal/mol 60 x 106 s-1 or 60 MHz

46,700 gauss (4.67-tesla) magnet 200 MHz (in this book)

70,000 gauss 300 MHz

9.33-tesla magnetic 400 MHz (in this book)

Magnetic field , resolution of NMR to get high magnetic field superconducting maget is needed

(superconducting magnet is operated using liquid helium)

Magnetic field , $

13C, 19F, 2H and 31P have nuclear spins

NMR technique can be applied

12C and 16O do not have nuclear spins

no NMR absorption

Why? Ask god!

14.3 The Chemical Shift the field required for the hydrogen to resonance

varies slightly with the chemical environment of the hydrogen

The required field difference between the hydrogen of the sample and that of TMS (tetramethylsilane)

Chemical shift () =observed position of peak (Hz)

operating frequency of instrumentX 106

ex) 200MHz ( 46,700 gauss) 1H-NMR : aceton peak at 436 Hz

(436 x 106/200x106 =2.16 or 2.16 ppm

in 300 MHz NMR, this peak will be detected at 654 Hz

548 Figure 14.4

Factors on Chemical Shift ( )

downfield: left on an NMR spectrum deshielded

increased external field higher

upfield: right on an NMR spectrum shielded

decreased external field lower

Inductive effect

Exchangeable protons: O-H/N-H (H-bonding)

O-H: 2~5 (ppm), N-H: 1~3, CO2H: ~12 (10~15)

peak position varies

Pi Electron Effect

deshielding effect higher

Chemical equivalence

CH3CH3

HH

H

HCH3

H2C

HH

H

HCH2

CH3

HH

H

H

CH3CH3

AH

H

H

CH3CH3

HH

A

H

CH3CH3

HH

H

A

CH3CH3

HA

H

H

identical

identical

identical

A

A

Protons with chemically identical environment

Three different sets of chemically equivalent protons

Three NMR absorptions

Enantiomer: identical chemical shift

Diastereomer: different chemical shiftTwo different sets of chemically equivalent protons

two NMR absorptions

CBr

H3C

H

HC

Br

H3C

H

A CBr

H3C

A

H

CBr

H3C

H

H

enantiomers

C C

H

H

H

Cl

C C

H

A

H

ClC C

H

H

A

Cl

C C

H

H

H

Cl

diasteromers

Three different sets of chemically equivalent protons

Three NMR absorptions

CH3CCH3

O one NMR absorption peak

CH3H3C CH3-O-CH2Cl CH3C-O-CH3

O

SiCH3

H3C CH3

CH3

two NMR absorption peaks

CC

CH3

H3C

HH

HBr

five NMR absorption peak

A useful summary of the chemical shifts

Estimation of the chemical shift

approximate chemical shift of hydrogens

Empirical Correlation for predicting Chemical Shifts

Shoolery’s Rule (old text book)

Ex) To the value from Table 14.1, add

0.3 ppm if CH2

0.7 ppm if CH

OH

CH2

CH2

CH3

2-5 ppm according to concentration (actual value= 2.3 ppm)

O-CH3 = 3.3 ppm, 3.3 + 0.3 (due to CH2) = 3.6 ppm

(actual value= 3.5 ppm)

C-CH3 = 0.9 ppm, 0.9 + 0.3 ( CH2) + 0.3 (CH3) = 1.5 ppm

(actual value= 1.5 ppm)

0.9 ppm (C-CH3 )

Practice

14.4 Spin Coupling (H,H Coupling)

Geminal coupling = Two-Bond Coupling

C

HHIf they are diastereotopic, then have different chemical shifts

Long-range Coupling: very small (not easily detected)

C C

H H Very thoroughly studied, both experimentally and theoretically

Vicinal coupling = Three-Bond Coupling

spin coupling: change in the local magnetic field

by nearby H atoms having different

multiplicity: Bo BH; 559 & Figure 14.6~7

splitting pattern: Jax in 0~20 Hz (typical 6); s (singlet), d

(doublet), t (triplet), q (quartet), m (multiplet), br (broad)

Differing spin relationships between HA and HB

NMR spectrum of this compound

The origin of spin-spin spliting in proton A’s NMR spectrum

Spin-spin splitting in ethyl iodide (CH3CH2I)

Splitting pattern of methyl hydrogens Splitting pattern of ethyl hydrogens

Spin-spin splitting rule: n+1 rule

C C ClCl

Cl H

HH

C C ClCl

Cl H

HH

Two neighboring

give triplet (three peaks)

n+1=3

One neighboring

give douplet two peaks

n+1=2

Pascal’s triangle

Peak number and area (height)

The Coupling Constant (J)

In 1H-NMR, peak area indicates the number of hydrogen for the corresponding absorption

See 559 Fig. 14.6~7

Proton is coupled to nonequivalent protons

14.5 Complex Coupling

Leaning

The downfield peak of the triplet is slightly larger than the upfield peak.

The quartet leans toward the triplet

As the difference in chemical shift becomes smaller, the leaning effect becomes larger. 562 Figure 14.8 vs 569 Figure 14.10

14.6 Chemical Exchange

H

H

H

HH

H

H

H

flip

NMR spectrum at RT one peak

NMR spectrum at low temp two peaks

No coupling in NMR spectrum of methanol (no splitting) unless the methanol is extremely pure

CH3 O H CH3 O H CH3 O H CH3 O H

H H

In general, H in oxygen or nitrogen are subject to the rapid exchange and do not couple to nearby H’s

SLOW CAMERA EFFECT

14.7 Deuterium

Solvent for NMR

no signal not to interfere the spectrum of the sample

CCl4 can be used poor solubility

Deuterated solvents (CDCl3, (CD3)2SO, C6D6) are used in NMR

Deuterium is invisible in NMR

1H-NMR measures E + small chemical shift

E (E difference between the two states) = hBo/2

(magnetogyric ratio) of the nucleus of deuterium is different from that of hydrogen it does not appear in 1H-NMR

carbocations by NMR: 565 Focus On

14.8 Interpretation of 1H-NMR Spectra

Step 1. Examine the general position of the peak.

Step 2. Examine the integral for the ratios of the different kinds of hydrogen.

Step 3. Examine the coupling pattern.

Step 4. Construct a tentative structure

Step 5. Determine whether all the information is consistent with this structure

14.9 13C-NMR Spectrum

Chemical shift range: 0~240 ppm

Broader than that of 1N-NMR

resolution is much higher

There is no, if any, 13C -13C coupling natural abundance: 13C 1.1% & 12C 98.9%

There is 13C –1H coupling, while this is removed by broad band decoupling technique

Then in 13C-NMR, all peaks are singlet

Other 13C-NMR Techniques

Off-resonance decoupling which allows hydrogens and carbons that are directly bonded to couple

CH3:quartet, CH2:triplet, CH:doublet, C:singlet

Old technique

DEPT-NMR: Three spectra are obtained: New technique

1. Broad band decoupled spectrum

2. DEPT 90o spectrum: only CH’s appear

3. DEPT 135o: CH’s and CH3’s appear as positive absorptions and CH2’s appears as negative absorptions

see next page

Chemical equivalence in 13C-NMR