Nuclear Magnetic Resonance 1 pot

Lecture Date: February 11 th , 2008Nuclear Magnetic Resonance 1 Nuclear Magnetic Resonance – Chapter 19 of Skoog, et al.. – Handout: “What SSNMR can offer to organic chemists” – Nuclear

Lecture Date: February 11 th , 2008

Nuclear Magnetic Resonance 1

Nuclear Magnetic Resonance

– Chapter 19 of Skoog, et al

– Handout: “What SSNMR can offer to organic chemists”

– Nuclear spin transitions, in the 5-900 MHz range

– Magnetic resonance imaging (MRI)

The Electromagnetic Spectrum

What is NMR?

frequencies of nuclear magnetic systems are

investigated

spectroscopy, in which resonant radiation is absorbed by

an ensemble of nuclei in a sample, a process causing

detectable emissions via a magnetically induced

electromotive force

A Abragam, The Principles of Nuclear Magnetism, 1961, Oxford: Clarendon Press.

Things that can be learned from NMR data…

– Which atoms/functional groups are present in a molecule

– How the atoms are connected (covalently bonded)

– Conformation

– Stereochemistry

– (Better known as MRI)

History of NMR

concepts of electron and nuclear spin

study 1 H and 7 Li NMR with a resonance

method, but fails because of relaxation

(Stanford) observe 1 H NMR in 1 kg of parafin at

30 MHz and in water at 8 MHz, respectively

Bloch

record 13 C spectra

(ETH) for FT and 2D NMR

Lauterbur and P Mansfield for MRI

P C Lauterbur F Bloch

E M Purcell R R Ernst

Photographs from www.nobelprize.org

Nuclear Magnetism

created by the nucleons

(protons and neutrons) inside

the atomic nucleus

– The magnetic moment is

proportional to the current flow

through the “nuclear loop”

to a distant charge center

N

S

From http://education.jlab.org

Basic NMR Theory

align or oppose this field.

magnetic moments of the nuclei,

which themselves are caused by

the internal structure of the

nucleus Two nuclear properties

stand out:

– Spin (1/2 for 1 H, 13 C, etc…)

– Gyromagnetic ratio

in the lower energy state

(determined by a Boltzmann

distribution).

per trillion!

Nuclear Spin

magnetization is observed to

“precess” at the Larmor frequency

(usually several hundred MHz):

ratio







2

0 0

B



0

 

angular (rad/s) linear (Hz, cycles/s)

B0

Elements Accessible by NMR

Figure from UCSB MRL website

White = only spin ½ Pink = spin 1 or greater (quadrupolar) Yellow = spin ½ or greater

Pulsed vs Continuous-Wave NMR

radio-frequency experiments

(e.g several kilohertz), absorption of RF by sample is

monitored

– Historically first method for NMR

– Poor sensitivity

– Still used in lock circuits

applied to the sample, and the response is monitored

– Much more flexible (pulse sequences followed from this…)

– Short pulses can excited a range of frequencies

NMR Theory: The Rotating Frame

oscillate at or near this same frequency

analysis and understanding

Frame rotating at the Larmor frequency

(hundreds of MHz)

Frame is now still

eye

x

y

Spin Systems

that the governing interactions can be separated and

treated individually

– Experimentally, this results in spectral simplification (in that

transitions are not hopelessly entangled) and also allows for

detailed manipulations (pulse sequences) to extract information

the nuclear spin Hamiltonians

to “spin systems” Examples of spin systems:

– Several 1 H nuclei (i.e hydrogen) within 2 or 3 covalent bonds of

each other

– A 1 H nucleus attached to a 13 C nucleus

NMR Theory: RF Pulses

z

x

y

Drawing depicts a 90opulse

z

x

y

length and phase

Drawing depicts a 180opulse

NMR Theory: RF Pulses and Spin Echoes

An RF pulse:

Two pulses:

echo

(delays and extra

pulse)

Actually not “solid”,

contains RF

frequencies

Selection Rules

+/- 1) are allowed by angular

momentum rules (which govern

spins in NMR)

directly detected in NMR

experiments

double-quantum states (or

zero-quantum, triple-zero-quantum, etc…),

let them evolve with time, then

convert them back to SQ states

for observation





Energy levels for two coupled spins showing SQ (single quantum) transitions in green and forbidden ZQ (zero quantum) and DQ (double quantum) transitions in red

SQ

X X

NMR Theory: T1Relaxation

relaxation (re-establishment

of Boltzmann equilibrium)

by spins interacting with the

“lattice”

quickly FT experiments can

be repeated for signal

averaging

provide useful data on

molecular motions

x

z

y

relaxation (dephasing of

coherence) by spins

interacting with each other

magnetization can be kept

in the x-y plane

(FWHH) of the NMR

signals:

x

z

y

* 2 2

/

1

1

T





NMR Theory: The Chemical Shift

shield are circulated by the big

magnetic field, inducing smaller

fields.

basis of NMR as an analytical

tool

available for 1 H, 13 C, 15 N, 29 Si,

TPPO

PbSO4

x

y z

ref ref x

ppm







( )  106 

Above: the chemical shift in solids is not a single peak!

Typical13C NMR Chemical Shielding

Note –17O NMR requires labeling or concentrated solutions,

and suffers from large solution-state linewidths (caused by

quadrupolar relaxation)

NMR Theory: The Chemical Shift

effects:

Dailey et al., J Am Chem Soc., 77, 3977 (1955).

Correlation of 1 H Chemical Shift and Group Electronegativity for CH 3 X Compounds

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0.0 1.0 2.0 3.0 4.0 5.0

Re lativ e Ch e mical Shift ( )

NMR Theory: The Chemical Shift

Figure from http://www.chemlab.chem.usyd.edu.au/thirdyear/organic/field/nmr/ans02.htm

NMR J-Coupling

 The J-coupling is an effect in which

nuclear magnetic dipoles couple to

each other via the surrounding

electrons

 The effect is tiny but detectable!

 Typical J-values

– 2-4 J HH can range from –15 to +15 Hz

and depends on the number of

bonds, bond angles, and torsion

angles

– 1 J CH can range from 120 to 280 Hz,

but typically is ~150 Hz in most

organics

– 2-4 JCHranges from –15 to +15 Hz

and depends on effects similar to

the 2-4 JHH

 The narrow ranges that certain 1 H and 13 C

J-coupling values fall into make spectral

editing and heteronuclear correlation

experiments possible!!!

J-Coupling: Effects on NMR Spectra

– Large difference in frequency

  >> J

“weak”

can be visualized using

Pascal’s triangle (see text)

– Small difference in frequency

Figure simulated in Bruker Topspin 2.0 DAISY module

Inspired by S W Homans, A Dictionary of Concepts in NMR, Oxford 1989, p297.

J-Coupling: Effects on NMR Spectra

monofluorobenzene

– ortho-coupling

– meta-coupling

– para-coupling

between 1 H and 19 F:

– As above (ortho, meta,

and para).

appears as a doublet of

triplets of triplets (ttd)

shown)

para

ortho meta

Structural and Conformational Analysis

assemble portions of a molecule

– In this case, the J-coupling is simply detected in a certain range

and its magnitude is not examined closely

stereochemistry of organic/organometallic/biochemical

systems in solution

– In this case, the J-coupling is measured e.g to the nearest 0.1 Hz

and analyzed more closely

W A Thomas, Prog NMR Spectros., 30 (1997) 183-207.

J-Coupling: Angle Effects

effects of bond and torsion

angles on J-coupling

(torsion) angles, 4 and

5-bond angles

In[1]:=J@q_D:= 4.22 Cos@qD2

+ - 0.5 Cos@qD+ 4.5

In[3]:=Plot@J@qD,8q, 0, p< D

6

7

8

9

Out[3]= … Graphics … Dihedral angle (radians)

Dipolar Coupling

between the moments of two spin-1/2

nuclei

– One spin senses the other’s orientation directly

through space

the internuclear distance between the

spins:

NMR) have the form:

1 2

cos 3

ear Heteronucl

3

8 r

2 0







 



Dipolar Coupling

m 0 = 4 p ´ 10 - 7 ;

g I = 6.728 ´ 10 7 ;

g S = - 2.712 ´ 10 7 ;

R@r_D: = Jg I g S

The dipolar coupling is therefore 1.332 kHz.

nucleus 1.32 angstroms apart?

The Nuclear Overhauser Effect

instantaneous dipolar coupling in an NMR or EPR

experiment

graduate student at UC Berkeley in 1953

electron spin resonance in a metal, the nuclear spins

would be polarized 1000 times more than normal!!!

The Nuclear Overhauser Effect

between the moments of two spin-1/2 nuclei.

averaged away in solution-state NMR by rapid

molecular tumbling.

 However, the dipolar interaction can still play a role via in solution-state NMR via dipolar cross-relaxation mechanisms, better known as the nuclear Overhauser

NMR Spectrometer Design

NMR Magnets

Resonance

LC

r

1





nuclear spin transitions – this circuit is part of the probe

Resonant Circuits in Probes

NMR Probe Design

designed to efficiently

produce an

inductance (~W) and

detect the result (<

mW)

NMR Electronics

Further Reading

Research”, Pergamon 1987

1961

of Nuclear Magnetic Resonance in One and Two

Dimensions”, Oxford, 1987

Resonance”, Springer-Verlag, 1996