NMR Lecture 5

Integration of 13 C spectra

Proton decoupler always on, no pulse delay between experiments

How do you know a 3 microsec

pulse = 22°

Effects of the pulse delay between pulses, 400 s

Proton decoupler always on (NOE)

Pulse delay 400 s; gated decoupler-on only when acquiring signal

Summary of Information obtained by examining nmr spectra

1. Chemical shift (identifies nature of nucleus)2. Area (identifies relative number of nuclei)3. Multiplicity (NOE)

2D NMR basically provides information about connectivity and proximity

Applications of 1H and 13C NMR

Molecular formula C9 H10 O

IR: 1720 cm-1

area: 5: 2: 3

CH2 CH3

O

Molecular formula C7 H14 O2

IR 1720 cm-1

1H NMR

CCH3 CH2

CCH3

O OCH3

CH33:2:3:6

F NMR of F2

CF24

CF25H

=CF3-

F

FF

F

NH2

H

Returning to Dynamic Nuclear Magnetic Resonance

For a simple exchange process coalescence /21/2

Most chemical shift differences are of the order of a few hundred Hz or less. Rate constants are of the order of few hundred sec-1. These are large in comparison to k

10-4 – 10-5

sec-1, measured by conventional kinetics but small relative to many dynamic processes occurring in molecular systems such as rotations about bonds.

Assign the spectrum and explain the coupling observed

Let’s do a thought experiment

What would the NMR spectrum of the methyl resonances of 4- dimethyl-3-hydroxycyclobutenone look like if all the methine hydrogens of this molecule had the same spin?

HO

OCH3

H

CH3

HO

OCH3

H

CH3

J13 , J15 are the same sign

J13 J15 are of opposite sign

HO

OCH3

H

CH3

J13 , J15 are the same sign

J13 , J15 are of opposite sign

What are some mechanisms by which the two methyl groups can become identical?

HO

OCH3

H

CH3

O

H

CH3

CH3

H

O

O

CH3

H

CH3

OH

H2 O

HO

OCH3

H

CH3

O

CH3

H

CH3

OH

O

O

H

CH3

H

CH3

HO

OCH3

H

CH3

O

CH3

H

CH3

OH

HO CH3

OHCH3

HO

OCH3

H

CH3

OH

O CH3

H

H3C

1,3-hydrogen shift

HO

OCH3

H

CH3

OH

O CH3

H

H3C

1,3-hydrogen shift

HO

OCH3

H

CH3

J is the same sign

J is of opposite sign

OH

O CH3

H

H3C

HO

OCH3

H

CH3

HO CH3

OHCH3

O

CH3

H

CH3

OH

HO

OCH3

H

CH3O

CH3

H

CH3

OHO

O

H

CH3

H

CH3

1,3-hydrogen shift; JAX

of the same sign O

CH3

H

CH3

OH

HO

OCH3

H

CH3

HO

OCH3

H

CH3

1,3-hydrogen shift; JAXof the opposite signs

O

CH3

H

CH3

OH

Problem Set DNMR 4 Spin system

Frequencies (Hz): 124, 132, 158 160.5

kab /k kac /k kad /k kbc /k kbd /k kcd /k

keto enol tautomerism: 0 1 1 1 1 0

enolization: 1 1 1 1 1 1

1,3 hydrogen shift JAX + 0 1 0 0 1 0

1,3 hydrogen shift JAX - 0 0 1 1 0 0