Chemistry 125: Lecture 59March 21, 2011

Precession and MRINMR Spectroscopy

Chemical Shift This

For copyright notice see final page of this file

90° RF Pulse and the “Rotating Frame”

Applied Magnetic Field

Precessing proton gives riseto constant vertical field

Will rotating horizontal field generate 100 MHz RF signal?No, because there are many

precessing protons with all possible phases.

Consider a “rotating frame” in which the observer orbits at 100 MHz - protons seem to

stand still as if no applied field.

(just long enough to rotate all nuclear spin axes by 90°).

Fast precession(~100 MHz)

Slow precession

(~0.1 MHz)

Horizontal fields cancel.

Subsequent precession generates100 MHz RF signal in lab frame.

100 MHz RFin lab frame

Until “relaxation”reestablishes equilibrium.

and rotating horizontal field.

Pulse a very weak magnetic field fixed in this rotating frame

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

that reports theirlocal magnetic field.

MRI:locating protons

within body using non-uniform field

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

MRI: find protons in body(e.g. fluid H2O) Bz

wrap in several miles of special wire at 4K

~1.5 Tesla(15,000 Gauss)

Superconducting Solenoidprotons precess at 63 MHz

So there are protons in the body, but where?

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

Establish a temperature gradientand listen with a stopwatch.

P. LeMone(2007)

subtract from Bz on left

MRI: find protons in body(e.g. fluid H2O)

add to Bz near head

const Hz

63.10 MHz

63.05 MHzconst B

z

subtract from Bz

near feet

dB z/dz

Bz

add to Bz on right

dBz/dx

~1.5 Tesla(15,000 Gauss)

protons precess at 63 MHz40 mT/mm

(~30 ppm/mm)

63.00 MHz

Superconducting Solenoid

Four analogous top/bottom coils

establish

These three gradients allow slicing in all directions to construct a 3Dtomograph.

dBz/dySo there are protons in the body,

but where?

Functional MRI:locating protons

whose signal strength is being fiddled with

BOLD Imaging

SubjectFasting

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

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

Subjectrecently

fed

DifferenceMap

wit

h pe

rmis

sion

of

Dr.

Tony

Gol

dsto

ne, I

mpe

rial

Col

lege

minus

Cell activity increasesblood oxygen supply,

speeds relaxation.

NMR:locating protonswithin molecules

using uniform field?

HO-CH2-CH3http://www3.wooster.edu/chemistry/is/brubaker/nmr/nmr_spectrum.html

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”

Bo

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

In the late 1950s chemistry departments began buying NMR spectrometers with fields homogeneous enough to determine molecular structures from chemical shifts (and spin-spin

splittings). With multi-user equipment, it was a challenge to keep the fields sufficiently homogeneous to obtain sharp lines.

At SUNY-Stony Brook in 1972 physical chemist Paul Lauterbur would take over the departmental machine nightly and destroy the field inhomogeneity.

By establishing gradients in different directions he located two 1 mm tubes of H2O within a 5 mm tube of D2O, and published this “zeugmatogram” in Nature in 1973. 30 years later he shared the Nobel Prize in Physiology or Medicine for inventing MRI.

Do Not Touch These Gradient Knobs!!! or this one!

have 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 puzzle out molecular structures, abandoned organic chemistry to take up fundamental research on quantum theory

(and later became a professional studio photographer).

Magnetic Resonance Spectrometers

(and X-ray Diffractometers)

Some of theMagnetic Resonance

Spectrometersin Yale's

Chemistry Department

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)

New 1000 MHz (23.5 Tesla) NMR Spectrometer

NHFML - Florida State Universitynow 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

d (ppm) 012345678

2 3 3

in CDCl3 solventat 5.9T (250 MHz)

CH3COCH2CH3

O

0.029 ppm× 250 MHz

7.3 Hz

CHCl3

“Low” Resolution(~0.3 ppm)

High Resolution(~3 ppb, sample spun)

?

Triplet(1:2:1)

Quartet(1:3:3:1)

Peak Width~3 ppb

7.3

7.37.37.3

Chem 220 NMR problem 7

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

The Chemical Shift

Chemical Shift and Shieldinghighelectrondensity

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.

d (ppm) 01234567891011

AlkylR-H H

C CH CH X

X = O, Hal, NRC

CH

O

RC H

ORC

OH

O

R-OH(depends on conc, T)

d+

d-

Diamagnetism from Orbiting

Electrons

Bapplied

End of Lecture 59March 21, 2011

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