Nuclear Magnetic Resonance 2 doc

Lecture Date: February 13 th , 2008Nuclear Magnetic Resonance 2 NMR Experiments  NMR experiments fall into some basic categories: – Basic pulse methods – 2D and multi-dimensional experi

Lecture Date: February 13 th , 2008

Nuclear Magnetic Resonance 2

NMR Experiments

 NMR experiments fall into some basic categories:

– Basic pulse methods

– 2D and multi-dimensional experiments

control of “mixing” between signals (to obtain more data)

Common Solution-state NMR Experiments for

Organic Structural Analysis

Provided

GASPE

DEPT

Gated-spin echo Distortionless editing by polarization transfer

13 C multiplicity (C, CH, CH 2 , CH 3 )

COSY correlated spectroscopy 1 H- 1 H covalent

bonding, 2-4 bonds HMQC heteronuclear multiple

quantum coherence

1 H- 13 C covalent bonding, 1 bond HMBC heteronuclear multiple

bond correlation

1 H- 13 C covalent bonding, 2-4 bonds NOE difference,

NOESY,

ROESY

nuclear Overhauser effect spectroscopy

1 H- 1 H proximity in space, 1.8-4.5 A

Pulse Sequences

 Modern NMR involves flexible spectrometers that can

implement pulse sequences, which are designed to

extract and simplify relevant information for the

spectroscopist

 Designed to harness a property or properties of the

nuclear spin Hamiltonians

– J-coupling

– Chemical shift

– Quadrupolar coupling

– Dipolar coupling

 Or, are designed to measure a bulk effect

– Relaxation

– Diffusion

– Chemical exchange or dynamics

Basic Pulse Sequences

 A single pulse and acquire

An Example of 1D NMR

Top – 1H spectrum Middle – Selective pulse Bottom – homonuclear decoupling

Multi-dimensional NMR

Evolution (t 1 ) Detection (t 2 )

Experiment Time

Can include NOE or J-coupling mixing

A Simple 2D NMR Spectrum

Diagonal Peak

Cross peak (“correlation”)

1

2

3

4

5

1 2 3 4 5

An Example of 2D NMR – the COSY Experiment

Correlations are

observed between

J-coupled protons!

(Example is a sample

of sucrose in D2O)

Applications of NMR

 Structural analysis

 Quantitative analysis

 Stereochemical and conformational analysis

 Solid-state analysis

Structural Analysis –13C NMR and Editing

13C spectra of

cholesteryl acetate:

(a) continuous 1H

decopling

(b) 1H during

acquisition (no

NOE)

(c) GASPE (APT)

(d) DEPT-135

Structural Analysis: 1H –13C Correlation

The1H-13C HSQC

analysis of

clarithromycin:

Structural Analysis: Long-range 1H –13C Correlation

The1H-13C HMBC

analysis of

carvedilol:

Structural Analysis: 1H –15N Correlation

The1H-15N

long-range HMQC

analysis of

telithromycin:

Determination of Relative Stereochemistry

NOE difference spectroscopy

Determination of Absolute Stereochemistry

Remember the ring

current effect?

J A Dale and H S Mosher, J Am Chem Soc., 95, 512-519 (1973).

C E Johnson and F A Bovey, J Chem Phys., 29, 1012 (1958).

Ch e mica l Sh ie ld in g a ro u n d th e Be n ze n e Rin g

-2 0 2 4 6 8 10 12

Dis t ance fr om Rin g Ce nte r ( A)

Abov e Ring

In Ring Plane

Ch e mica l Sh ie ld in g a ro u n d th e Be n ze n e Rin g (Ex pa n d e d Vie w )

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Dis tance f ro m Ring C e nt e r (A)

Abov e Ring

In Ring Plane

shielding (opposes field)

deshielding (aligned with field)

Determination of Absolute Stereochemistry by

Mosher-Dale Method

-methoxy--(trifluoromethyl)phenyl acetic acid

distance than its shielding effects, protons close to a phenyl should be more

shielded!

F 3 C

O Ph

H3CO

CH 3

(R)-alcohol

NO 2

(S)-MPTA-Cl

=> (R)-MPTA ester

1 2 3 4

4.45t 2.02m 1.69m 1.26d

5.15m

3.51q 7.4-7.5m

8 9

10

11 5 6 7

| 5 J H9,F10 | = 1.2 Hz

| 3 J H11,H5 | = 6.2 Hz

| 3 J H2,H3 | = 6.9 Hz

| 4 J H2,H4 | = 0 Hz

F 3 C O Ph

H 3 CO

(S)-alcohol

H 3 C

(S)-MPTA-Cl

=> (R)-MPTA ester

1 2 3 4 4.34dt 1.83m 1.62m 1.35d

5.15m

3.55q 7.4-7.5m 8 9

10

11

5 6 7

NO 2

| 5 J H9,F10 | = 1.1 Hz

| 3 J H11,H5 | = 6.3 Hz

| 3 J H2,H3 | = 6.8 Hz

| 4 J H2,H4 | = 2.2 Hz

J A Dale and H S Mosher, J Am Chem Soc., 95, 512-519 (1973).

A Guarna, E O Occhiato, L M Spinetti, M E Vallecchi, and D Scarpi, Tetrahedron, 51, 1775-1788 (1995).

19F Quantitative Analysis: TFA Salt Stoichiometry

Solid-state Nuclear Magnetic Resonance

on the behavior of nuclear spin energy

levels in a magnetic field However, the

interactions that affect NMR spectra act

differently

 In liquids, molecules reorient and

diffuse quickly, leading to narrow

isotropicresonances

 In solids, the fixed orientation of

individual crystallites leads to a range

of resonance frequencies for

anisotropicinteractions

E

m=+1/2

m=-1/2

No field Field = B 0

E=(h/2)B0

Solid-state NMR: Magic-Angle Spinning

time by spinning at a root of the

scaling factor:

(often combined with dipolar

decoupling):

 cos  3 cos 1

2







P broadening

E R Andrew, A Bradbury, and R G Eades, Nature, 183, 1802 (1959).

I J Lowe Phys Rev Lett 2, 285 (1959).

are dependent on their orientation

with respect to the large magnetic

field (B0):

– dipolar (homo- and heteronuclear)

coupling

– 1st-order quadrupolar coupling

– anisotropic chemical shift

 Cross-polarization is an example of a double resonance experiment

– Two resonances, typically two different nuclei, are excited in a

single experiment.

 Cross-Polarization combined with MAS (CP-MAS):

– Enhancement of signal from “sparse” spins via transfer of

polarization from “abundant” spins

– The “Hartmann-Hahn condition” allows for efficient energy

transfer between the two spins, usually via dipolar interactions

– The basic CP pulse sequence for 1H to 13C experiments:

1 H

CP

90

CW Decoupling

E O Stejskal and J D Memory “High Resolution NMR in the Solid State,” Oxford University Press, New York (1994).

A Pines, M G Gibby and J S Waugh J Chem Phys., 59, 569 (1973).

An Example: Polymorphism in Carvedilol

 13C CP-TOSS spectra of the polymorphs of SKF105517 free base

NH O

OH

O

H 3 C 1 2 3 4 5 6 7

8 9 10 11 12 13 14

15 16

An Example: Polymorphism in Carvedilol

 15N SSNMR spectroscopy also shows similar effects.

 Advantages: simple and easy-to-interpret spectra, valuable information

about the nitrogen chemical environment

 Disadvantage: much lower sensitivity

NH O

NH O

OH

O

H3C 1 2 3 4 5 6 7

8 9 10 11 12 13 14

15 16 17 18 19 20

23 24 25 26

Magnetic Resonance Imaging

• The basic idea: a linear magnetic field gradient imposes a

linear spread of Larmor frequencies on a sample.

Figure from S W Homans, A Dictionary of Concepts in NMR, Oxford, 1989.

For more details, see P G Morris, NMR Imaging in Medicine and Biology, Oxford University Press, 1986.

0

0  B

Magnetic Resonance Force Microscopy

Rugar, D.; et al Nature 2004, 430, 329–332.

R Mukhopadhyay, Anal Chem 2005, 449A-452A.

AFM and EPR (and

hopefully NMR)

cantilever to detect

spin motion

induced by RF via

in an magnetic field

Nuclear Spin Optical Rotation (NSOR)

induced in a laser beam as a the beam passes

through a liquid

19-10 19-15

Optional Homework

Also, please answer one of these based on the article you chose to read:

MRI: Describe the basic action of a field gradient on a sample Also

describe how the spin-warp imaging method obtains a 2D image, and

why it is similar to conventional 2D NMR

Solid-state NMR: What effect(s) in solid-state NMR spectra allow for the

analysis of hydrogen-bonding?

NMR-MOUSE: List the differences and similarities between unilateral

low-field NMR and traditional high-low-field NMR instrumentation Why are T2

measurements so analytically useful with this technique?

MRFM: Briefly describe the current AFM-derived devices used to detect

electron spins What advances need to be made to take the technique

forward to nuclear spins?