Chem 125 Lecture 63Preliminary

4/1/08Projected material

This material is for the exclusive use of Chem 125 students at Yale and may not

be copied or distributed further.

It is not readily understood without reference to notes from the lecture.

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

local magnetic field.

MRI:locating protons

within body using non-uniform field

MRI

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

How to find Crickets,if you can’t see them:

Establish a temperature gradientand listen with a stopwatch.

X-Ray Tomographywww.colorado.edu/physics/2000/tomography/final_rib_cage.html

MRI: find protons in body

z-gradient

Main Field (z)

x-gradient

y-gradient

radio antenna

(e.g. fluid H2O)

Functional MRI:locating protons

whose signal strength is being fiddled with

BOLD Imaging

http://www.csc.mrc.ac.uk/d/file/LargeFigs/Bell/fMRI.jpg

Cell activity increasesblood oxygen supplyincreases relaxation

Functional MRI (fMRI)e.g. Blood Oxygen-Level

Dependent (BOLD) ImagingSpatial Resolution ~1 mmTemporal Resolution 2 sec

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 ppmFractional

difference in applied field 0.00000248 ! Requires very high

uniformity of field

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 “natural products” organic chemist turned to quantum theory,

another to photography.

500 MHz

500 MHz

600 MHz

600 MHz

800 MHz

~83 = 512times assensitive

as 100 MHz

(not to mentionchemical shift

advantage)

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?

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

2.9 1

1955

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

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

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

N.B. The orbiting to give B is driven by B; so B B.

ZERO!

average over

sphere

average around circle

1/r3 Electrons Orbiting

Other Nuclei

Diamagnetism from Orbiting

Electrons

Bapplied

PPM

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.

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.)

In acetylene C-H is parallel to B0 when there is ring current (B0 diminshed;

shifted upfield).

In benzene they are perpendicular

(B0 augmented; shifted downfield).

End of Lecture 63April 1, 2009