Chemistry 125: Lecture 59March 22, 2010

NMR Spectroscopy Chemical Shift and Spin-Spin Coupling

This

For copyright notice see final page of this file

NMR:locating protonswithin molecules

using uniform field?

HO-CH2-CH3http://www.wooster.edu/chemistry/is/brubaker/nmr

Oscilliscope Trace(1951)

The “Chemical” Shift

2.48 ppm

Fractional difference in applied field 0.00000248 !

Requires very high uniformity of field

to avoid “MRI”

H

Listen at fixed frequency.Tune H to “hear” precession.

Dr. Lauterbur became interested in possible biological applications of nuclear magnetic resonance after reading a paper in 1971 by Raymond V. Damadian, who described how some cancerous tissues responded differently to the magnetic fields than normal tissue.

Until then, most scientists placed the samples in a uniform magnetic field, and the radio signals emanated from the entire sample. Dr. Lauterbur realized that if a non-uniform magnetic field were used, then the radio signals would come from just one slice of the sample, allowing a two-dimensional image to be created.

The nuclear magnetic resonance machine at SUNY was shared among the chemistry professors, and the other professors needed to perform their measurements in a uniform magnetic field. Dr. Lauterbur had to conduct his work at night, returning the machine to its original settings each morning.

i.e. one particular frequency

Some of theMagnetic Resonance

Spectrometersin Yale's

Chemistry Departmenthave put classical structure proof

by chemical transformation (and even IR!) out of business.

One Yale “natural products” organic professor, whose research used chemical transformations to determine molecular structures, abandoned organic chemistry to take up fundamental research on quantum theory

(and later became a professional studio photographer).

500 MHz

500 MHz

600 MHz

600 MHz

800 MHz

~83 = 512times assensitive

as 100 MHz(not to mentionthe chemical

shift advantage discussed below)

*

1) Boltzmann factor2) Energy quantum3) Electronics sensitivity

*

EPR (Electron Paramagnetic Resonance)

(for Free Radicals with SOMOs)e magnet is 660x H+!

EPR (Electron Paramagnetic Resonance)9 GHz

~3000 Gauss(0.3 Tesla)

NHFML - Florida State University VarianAssociates

New 900 MHz (21 Tesla) NMR spectrometers

NHFML now has a pulsed field NMR at 45 Tesla(there is no charge for use, but you have to have a great experiment

HO-CH2-CH3http://www.wooster.edu/chemistry/is/brubaker/nmr

Oscilliscope Trace(1951)

1 2 3

Area(integral)

Which peak is which set of

protons? number of protons, because

they are so similar

(not like IR)

http://www.wooster.edu/chemistry/is/brubaker/nmr

2.9 1

1955Advertisement

1) O3 2) H2O2

C-OHHO-COO

cis-caronic acid

1:1

Structural proof by chemical degradation

(venerable)

3:1

?

?

OO O

O

O

O O

O

H CC

H

http://www.wooster.edu/chemistry/is/brubaker/nmr

Advantage of “similarity” of protons(unlike IR where various modes have very different

changes in dipole moment, and thus very different signal strengths)

Higher Resolution Shows Splitting

1959

Ethyl Acetate

averages field inhomogeneities

1959

A 90° pulse makesspinning nuclei (1H, 13C) “broadcast” a frequency

that tells theirLOCAL magnetic field.

Components ofEffective Magnetic Field.

Inhomogeneous ~ 30,000 G for MRI CAT scan. (4 G/cm for humans, 50 G/cm for small animals)

Applied Field:

Homogeneous for Chemical NMR Spectroscopy (spin sample)

Molecular Field:Net electron orbiting - “Chemical Shift” (Range ~12 ppm for 1H, ~ 200 ppm for 13C)

Nearby magnetic nuclei - “Spin-Spin Splitting” (In solution JHH 0-30 Hz ; JCH 0-250 Hz)

Beffective

Bmolecular (diamagnetic)

Bapplied

Chemical Shift and ShieldingCH3

SiCH3H3C

H3C

highelectrondensity

shielded

upfield

high e- densitylow chemical shift

low frequency

deshielded

downfield

low e- densityhigh chemical shift

high frequency

CH3C C-H ?! ???

TMS

Beffective

Bmolecular (diamagnetic)

Bapplied

Note: Electron orbiting to give B is driven by B; so B B.

Cf. Table 15.4 p. 720

ZERO!Suppose molecule

undergoes rotational averaging.

average over

sphere

average around circle

1/r3 Electrons Orbiting

Other Nuclei

Diamagnetism from Orbiting

Electrons

Bapplied

PPM

Ignore them!

ZERO!

average over

sphere

Electrons Orbiting

Other Nuclei

Unless orbiting depends on molecular orientation

Bapplied

Diamagnetic“Anisotropy”

(depends on direction)

NOT

Diamagnetic AnisotropyBenzene “Ring Current”

B0 can only drive circulation about a path to which it is perpendicular.

If the ring rotates so that it is no longer perpendicular to B0,

the ring current stops.15.30

Aromaticity: PMR Chemical Shift Criterion

HCCl3

TMS

-4.23

14 electrons(43 + 2)

DIAMAGNETIC ANISOTROPY!

?

DIAMAGNETIC ANISOTROPY

8 H 2 H

TMS10 electrons

(distorted)

Diamagnetic AnisotropyAcetylene “Ring Current”

Warning!This handy picture of

diamagnetic anisotropy due to ring current

may well be nonsense!

(Prof. Wiberg showed it to be nonsense for 13C.)

The H nuclei of benzene lie outside the orbital path

when there is ring current. (B0 augmented; signal shifts downfield).

The H nuclei of acetylene lie on the orbital axis

when there is ring current. (B0 diminshed;

signal shifts upfield).

15.32

Spin-Spin Splitting

15.36.jpg

Three peaks from four different sets of

molecules in the sample.

~1:2:1 Triplet

15.37.jpg

~1:3:3:1 Quartet

Isotropic JH-H is mediated by

bonding electrons(through-space part is anisotropic,

averaged to zero by tumbling)

End of Lecture 59March 22, 2010

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