Nuclear Magnetic Resonance 2

Lecture Date: February 13 th , 2008

NMR Experiments

 NMR experiments fall into some basic categories: – Basic pulse methods  Single pulse   Selective pulse or selective decoupling Solvent suppression – 2D and multi-dimensional experiments  unravel complex spectra by separation of overlapping signals, control of “mixing” between signals (to obtain more data) – Multiple resonance (heteronuclear techniques)  Are often 2D or nD sequences – Diffusion, dynamics and relaxation experiments

Common Solution-state NMR Experiments for Organic Structural Analysis Experiment

GASPE DEPT COSY HMQC HMBC NOE difference, NOESY, ROESY

Acronym

Gated-spin echo Distortionless editing by polarization transfer correlated spectroscopy

Information Provided

13 C multiplicity (C, CH, CH 2 , CH 3 ) heteronuclear multiple quantum coherence 1 H 1 H covalent bonding, 2-4 bonds 1 H 13 C covalent bonding, 1 bond heteronuclear multiple bond correlation nuclear Overhauser effect spectroscopy 1 H 13 C covalent bonding, 2-4 bonds 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

 The general scheme of 2D and multi-dimensional NMR:

Preparation Evolution (t 1 ) Mixing (t m )

Can include NOE or J coupling mixing

Detection (t 2 )

Experiment Time  2D NMR data has two frequency dimensions: FT(t 1 ) FT(t 2 )

A Simple 2D NMR Spectrum

Cross peak (“correlation”) Diagonal Peak 5 4 3 F 2 (ppm) 2 1 1 4 5 2 3 F 1 (ppm)

An Example of 2D NMR – the COSY Experiment

Correlations are observed between J-coupled protons!

(Example is a sample of sucrose in D 2 O)

Applications of NMR

 Structural analysis  Quantitative analysis  Stereochemical and conformational analysis  Solid-state analysis

Structural Analysis – 13 C NMR and Editing

13 C spectra of cholesteryl acetate: (a) continuous 1 H decopling (b) 1 H during acquisition (no NOE) (c) GASPE (APT) (d) DEPT-135

Structural Analysis: 1 H – 13 C Correlation

The 1 H 13 C HSQC analysis of clarithromycin:

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

The 1 H 13 C HMBC analysis of carvedilol:

Structural Analysis: 1 H – 15 N Correlation

The 1 H 15 N long range HMQC analysis of telithromycin:

Determination of Relative Stereochemistry

NOE difference spectroscopy

Determination of Absolute Stereochemistry

Remember the ring current effect?

Che mical Shie lding around the Be nze ne Ring

1 0.8

0.6

0.4

0.2

0 -0.2

-0.4

-0.6

-0.8

-1 4.0

12 10 8 6 4 2 0 -2 0.0

2.0

4.0

Distance from Ring Center (A)

6.0

Che mical Shie lding around the Be nze ne Ring (Expande d Vie w)

8.0

shielding (opposes field) deshielding (aligned with field) 5.0

6.0

Distance from Ring Center (A)

7.0

8.0

Above Ring In Ring Plane Above Ring In Ring Plane 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).



Determination of Absolute Stereochemistry by Mosher-Dale Method

Procedure: Derivatize a chiral alcohol with MPTA,  -methoxy  (trifluoromethyl)phenyl acetic acid  Because a phenyl group’s deshielding effects drop off more rapidly with distance than its shielding effects, protons close to a phenyl should be more shielded!

 Example: 5-nitro-2-pentanol 1 NO 2 2 4.45t

3.51q

9 H 3 CO 7.4-7.5m

8 Ph 3 2.02m

F 3 C 10 7 6 O (

S

)-MPTA-Cl => (

R

)-MPTA ester O 4 (

R

1.69m

1.26d

CH 3 11 5 H 5.15m

)-alcohol | 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 1 NO 2 3.55q

9 H 3 CO F 3 C 10 7 7.4-7.5m

8 Ph 6 O 1.35d

11 H 3 C 5 2 4.34dt

3 1.83m

4 1.62m

O (

S

)-MPTA-Cl => (

R

)-MPTA ester (

S

H 5.15m

)-alcohol | 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).

19 F Quantitative Analysis: TFA Salt Stoichiometry

Solid-state Nuclear Magnetic Resonance

 NMR in solids, like solution-state, relies on the behavior of nuclear spin energy levels in a magnetic field. However, the interactions that affect NMR spectra act differently.

No field Field = B 0 m=-1/2  E=( 

h

/2  )B 0 m=+1/2 E  In liquids , molecules reorient and diffuse quickly, leading to narrow isotropic resonances.

 In solids , the fixed orientation of individual crystallites leads to a range of resonance frequencies for anisotropic interactions.

Solid-state NMR: Magic-Angle Spinning

 The following anisotropic interactions are dependent on their orientation with respect to the large magnetic field (B 0 ): – dipolar (homo- and heteronuclear) coupling – 1st-order quadrupolar coupling – anisotropic chemical shift   These can be averaged away over time by spinning at a root of the scaling factor: The result of magic angle spinning (often combined with dipolar decoupling):

broadening P

2   cos    3 cos 2   1 E. R. Andrew, A. Bradbury, and R. G. Eades,

Nature

,

183

, 1802 (1959).

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

Cross-Polarization

 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 1 H to 13 C experiments: 90 

CW Decoupling

CP 1 H CP 13 C 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

 13 C CP-TOSS spectra of the polymorphs of SKF105517 free base 5 4 6 7 O 8 2 O H 3 C 1 9 10 NH 11 12 13 O 14 OH 15 19 3 26 21 16 17 18 20 NH 22 23 25 24  Amorphous forms generally give broadened spectra

An Example: Polymorphism in Carvedilol

 15 N SSNMR spectroscopy also shows similar effects.

5 4 6 7 O 8 2 O H 3 C 1 9 10 NH 11 12 13 O 14 OH 15 19 3 26 21 16 17 18 20 NH 22 23 25 24   Advantages: simple and easy-to-interpret spectra, valuable information about the nitrogen chemical environment Disadvantage: much lower sensitivity

Magnetic Resonance Imaging

• The basic idea: a linear magnetic field gradient imposes a linear spread of Larmor frequencies on a sample.

 0  

B

0 Gradient  0   

B

0 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.

Magnetic Resonance Force Microscopy

 A “combination” of AFM and EPR (and hopefully NMR)  Uses a nano-scale cantilever to detect spin motion induced by RF via in an magnetic field Rugar, D.; et al. Nature 2004, 430, 329–332.

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

Nuclear Spin Optical Rotation (NSOR)

 Measures NMR signals by detecting phase shifts induced in a laser beam as a the beam passes through a liquid    Gives excellent spatial resolution Currently lacks sensitivity Developed by Romalis group at Princeton Nature

2006

,

442

, 1021

Homework

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-field NMR instrumentation. Why are T 2 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?