HNMR Lecture Supp

7/30/2019 HNMR Lecture Supp

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54 Lecture Supplement: Proton NMR Spectroscopy

Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy

Fundamental principle: The energy required to cause nuclear spin flip is a function of

the magnetic environment experienced by the nucleus.

Charged particle (nucleus or electron) in motion creates a magnetic field. In an external magnetic field, the magnetic moment (spin) of 1H nucleus (a

proton) becomes aligned with the external field (lower energy) or against it

(higher energy).

Externalmagnetic

field

No external magnetic field With external magnetic field

Addition of energy results in nuclear spin flip:

E ~ 0.02 cal mol-1

= radio wave photons

Add energy

Relax (release energy)

Ground state

Nuclear spin parallel toexternal magnetic field

Lower energy

Excited state

Nuclear spin antiparallelto external magnetic field

Higher energy


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Lecture Supplement: Proton NMR Spectroscopy 55

What does an NMR spectrum look like?

NMR signal observed when radio waves supplied match E for nucleus underobservation in the external magnetic field; proton is said to be in resonance.

Downfield (deshielded)low magnetic field strength

Upfield (shielded)high magnetic field strength

Intensity

NMR signal

Information From NMR Spectrum

What information from the NMR spectrum is useful to deduce molecular structure?

Number of signals Position of signals (chemical shift) Relative intensity of signals (integration) Splitting of signals (spin-spin coupling)

Number of Signals

Protons in the same magnetic environment are equivalent: give same signal Nonequivalent protons give different signals Therefore number of signals = number of equivalent sets of protons in molecule

Examples:

HF versus HCl H C

H

H

Cl

H C

H

H

C

H

H

Cl H C

H

H

C

H

H

OH

H C

F

F

C

Cl

Cl

H F C

H

H

C

Cl

Cl

F


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HH

H

HH

H

HH

H

CH3H

H

Sample Spectra Verify what we have learned about equivalent protons.

CH3 C

CH3

CH3

CH2OH

H

HH

H

OH

OH

Position of Signals: The Chemical Shift

Magnetic field at nucleus influences E

Magnetic field strength

Spin

state

energy

E

spin parallel to applied magnetic field

spin antiparallel to applied magnetic field


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What contributes to magnetic field felt by nucleus?

Magnetic fields of earth and NMR spectrometer: same for all nuclei in molecule Magnetic fields provided by electrons and other nuclei in molecule: varies

between nuclei in molecule

Chemical shift

E scale that is independent of spectrometers magnetic field Chemical shift identical for a given proton regardless of spectrometer used Chemical shift unit = parts per million (ppm) Most proton chemical shifts 0-15 ppm range Represents E relative to reference proton: (CH3)4Si (tetramethylsilane; TMS) =

0.00 ppm by definition

How does chemical shift depend on molecular structure?

Molecule 1H-NMR chemical shift

(CH3)4Si 0.00 ppm

CH4

CH3I

CH3Br

CH3Cl

CH3F

CH2Cl2

CHCl3

CCl4

How does electronegativity influence chemical shift? Electron cloud shields nucleus from applied magnetic field Electronegative atoms remove electron density from proton Less electron density = less shielding = higher chemical shift


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Characteristic Proton NMR Chemical Shifts (ppm)

RCH3 0.9 RCCH 2.5 ArH 6.5 - 8

RCH2R acyclic 1.3 RNHCH3 2 3 RCHO, ArCHO 9.5 11

RCH2R cyclic 1.5 RCH2X(X = Cl, Br, I)

3.5 RNH2 1 3

R3CH 1.5 2.0 RCH2NR2 2.3 2.7 ArNH2 3 5

CH3R

R R

1.8 OCH3 3.8 R N

O

R

H

5 9

R CH3

O

2.0 2.6 OCH2 3.3 4.1 ROH 1 5

CH2R

O

2.2 3.0 R2C=CH2 5.0 ArOH 4 7

ArCH3 2.3 RCH=CR2 5.3 RCO2H 10 13

ArCH2R 2.6

The exact chemical shift for each proton type does not need to be memorized. This

table will be provided on an exam if needed.

Similar functional group = similar chemical shift.

Characteristic chemical shifts are averages for typical protons. Vary somewhat between

molecules. Example:

CH3O CH3

O

2.01 ppm

CH3

O

2.59 ppm

Other useful chemical shift observations

Chemical shift RCH3 < RCH2R < R3CH, because EN of C > EN of H Electron-withdrawing or donating effects on chemical shift decrease with

distance. Example:

CH3OH CH3CH2OH CH3CH2CH2OH

3.39 ppm 1.18 ppm3.59 ppm

0.93 ppm1.53 ppm

3.49 ppm

CH4

0.23 ppm


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Unlike IR stretching frequencies, we cannotuse characteristic chemical shiftstable to assign proton types to NMR signals. Example: 3.8 ppm not alwaysROCH3 and 2.2 not always ArCH3.Exception: 6.5 8.0 ppm usually (not always)

benzene ring protons.

Probably benzenering protons

3.7 ppm notnecessarily OCH3

2.2 ppm notnecessarily ArCH3

TMS0.00 ppm

Relative Intensity of Signals: Integration

Beers Law: amount of energy absorbed or transmitted moles of stuff present For NMR, amount of radio energy proportional to area under peak in spectrum

(area measurement = integration)

Relative intensities of signals proportional to relative number of equivalentprotons

Integrals do not always correspond to exact number of protons

Sample NMR Spectra Verify what we have learned so far about equivalent protons,

chemical shifts and integration.

H C

H

H

OH4.19 ppm: integral = 1.0 (1 H)

3.41 ppm: integral = 3.0 (3 H)

Smallest integral often set = 1


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Proton count = 6 : 6Integration = 1 : 1

Integration gives ratio of

protons, which is often notthe exact number of protons.

Splitting of NMR Signals: Spin-Spin Coupling

1

H-NMR spectrum of CH3CH2Br: more complex than we might have predicted!

CH3CH2Br

Four lines

a quartet

Three lines

a triplet


1.68 ppm: integral = 1.5 (3H)

What is the origin of this extra complexity?

Ha C C Hb

Ha, Hb parallel

Ha, Hb antiparallel

Different spin flip energies (E) for Ha

two chemical shifts for Ha Ha signal is

split into two lines (a doublet)

Spin-spin coupling: spin of one nucleus influences the spin of another nucleus

Splitting: effect on NMR spectrum caused by spin-spin coupling

Coupling constant(J): spacing between lines in a splitting pattern J


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What happens to splitting when there is more than one neighbor?

Example: Ha has two neighbors (Hb and Hc)Ha C C Hc

Hb

equalenergy

Two neighborsparallel to Ha

One parallel, one

antiparallel to Ha

Two neighbors

antiparallel to Ha

Three possible neighbor spin

states; Ha signal split into three

lines (a triplet) in 1:2:1 ratio

a triplet

In general: the signal for a proton with n neighbors is split into n+1 lines.

No neighbors singlet; one neighbor doublet; two neighbors triplet;

three neighbors quartet; four neighbors pentet, etc.

Rules and Restrictions for Proton-Proton Spin-Spin Coupling

What are the restrictions on spin-spin coupling? What constitutes a coupling neighbor?

1. Only nonequivalent protons couple.

Ha C

Hb

H

C

Hc

H

C

Hd

H

C

H

H

H

X

X

Hb couples with Hc.

Hb and Ha do not couple because they are equivalent.

Hc and Hd do not couple because they are equivalent.

2. Protons separated by more than three single bonds usually do not couple.

Ha C

Hb

C C

Hc

Hd

X Ha can couple with HbHa can couple with HcHa cannot couple with Hd


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Pi bonds do not count toward the bond limit, but the coupling constants may be too

small to be resolved.

C

Hb

Ha

C

C

Hc

Hd

Ha can couple with Hb, Hc and Hd, butJad may be small.

CH2CH3

CH3

H

X

X

The benzene ring coupling club: a benzene ring

blocks some coupling that otherwise should beobserved (i.e,Jis too small).

All nonequivalent benzene ring protons may couple with each other, but the coupling

constants may be too small to be resolved.

CH3

OCH3Hb

Hc

Ha

Hd

Ha, Hb, Hc and Hd all couple with each other, butJad may be small.

3. Signals for O-H and N-H protons are usually singlets. (Splittingof O-H or N-H protons is observed only in highly pure samples,

and is very rare.)H2N C

H

H

C

H

H

OH

Singlets

Sample Spectra Verify what we have learned about equivalency, chemical shifts,

integration and splitting.

1H-NMR (ppm):

3.39 (triplet, integral = 1.0)

1.87 (sextet, integral = 1.0)

1.03 (triplet, integral = 1.5)


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1H-NMR (ppm):3.79 (septet, integral = 1.0)

1.31 (doublet, integral = 6.0)

1

H-NMR (ppm):5.66 (multiplet, integral = 1.0)

1.98 (multiplet, integral = 2.0)

1.16 (multiplet, integral = 2.0)

Multiplet: a splitting patternthat is too complex to

decipher.

Non-First Order Splitting (such as a doublet of doublets)

Normal splitting patterns occur when coupling constants of neighboring protons are

equal. Example: The CH of CHCH2 is a triplet whenJab = Jac. When Jab Jac, more

complex patterns arise, such as a doublet of doublets.

C C

Ha Hb

Hc

J

Jab =Jacnormal triplet

JabJacdoublet of doublets

For this course, we will assume that coupling constants are usually equal, and that

normal splitting patterns will usually be observed. Exceptions are plentiful.


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1H-NMR (ppm):5.83 (doublet of doublets,

integral = 1.0)

Doublet of doublets = | | | |_

4.93 - 4.82 (multiple, integral

= 2.0)

1.00 (singlet, integral = 9.0)

H

O 1H-NMR(ppm):


7.87 - 7.56 (multiplet, integral

= 5.0)

1H-NMR (ppm):7.38 - 7.00 (multiplet, integral

= 1.7)


Splitting Jab seems to

disappear as Ha Hbchemical shift difference

becomes smaller.


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How do we decude molecular structure from NMR spectra?

Formula: C9H12

1H-NMR: 7.40 7.02 ppm (multiplet, integral = 5); 2.57 ppm (triplet, integral = 2); 1.64ppm (sextet, integral = 2) and 0.94 ppm (triplet, integral = 3).

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