1D NMR Spectroscopy

Chemistry 355

Fall 2010

Note: Most figures are from Silverstein, Webster and Kiemle, 7/e, while others

are from the Web. While sources are not cited for space limitations, the author

claims no credit or copyright for any of these unless specifically noted.

Principles of Magnetic Resonance

Nuclear spin (angular momentum)

Based on nuclear particles (each has spin )

Must have odd number of protons, neutrons, or both

I = 0; 12C not NMR active

I = ; 1H, 13C, 15N, 31P, 19F, 29Si

I = 1; 2H, 14N

I = 5/2; 17O

Principles of Magnetic Resonance

Spins orient in a magnetic field

Small proportion

DE = (hg/2p)B0

Radio frequency low energy transition

At 298K and 2.35 T (100 MHz) only 0.00041% of protons in a given sample

are excitable.

Difference is greatest at low temperature

Principles of Magnetic Resonance

Spins precess about the magnetic field at a

characteristic frequency

Larmor frequency

n1 = (g/2p)B0

Principles of Magnetic Resonance

(a,b) - By irradiating with Rf energy, we can flip the magnetic moment of a nucleus or nuclei (move it from

vertical)

(c, d) - As the nucleus recovers, it will emit its Larmor frequency as a

damped oscillation called a free

induction decay (FID)

(d)

Rf

pulse

FID

Immediately

After

pulse

Before

pulse

Magnets Field 1 T = 10,000 gauss Earth 0.3 gauss Permanent magnets

0.1 T (5 MHz) to 2.35 T (100 MHz) Limited by bulk and materials

Electromagnet Normal Superconducting

4.7 T (200 MHz) to 22.3 T (950 MHz)

Cryocooled

Frequency Proportional to field Different for nuclei Spectrometer frequency usually

expressed for 1H

60 MHz at 1.4 T 400 MHz at 9.4 T 950 MHz at 22.3 T 13C frequency that of 1H

The NMR Spectrometer

Magnet Mini radio station

Transmitter Receiver Amplifier Digitizer (FT) Computer (FT)

transmitter receiver

Sweep

Generator

amplifier

preamplifier

The Magnet and

Spectrometer work

together, but are

independent.

Any magnet can be used with either pulsed FT or continuous wave NMR

The Basic 1D NMR Experiment Continuous wave

Sweep frequency

Nuclei ring at their resonant frequency

First spectrum 1951

The Basic 1D NMR Experiment Fourier transform

All frequencies pulsed simultaneously

(Heisenberg)

Listen to all sing back at once - Free Induction

Decay (FID)

Fourier transform

Time to Frequency

Greater sensitivity

Time averaging

The Basic 1D NMR Experiment

The Effect of Field

Coupling frequency is a constant (Hz)

Higher frequency = more Hz/ppm

Higher field gives

Higher resolution

Greater sensitivity

0123PPM

0123PPM

0123PPM

0123PPM

0123PPM

60 MHz

100 MHz

200 MHz

400 MHz

800 MHz

-1012345PPM

The Effect of Field

012345PPM

012345PPM

60 MHz

400 MHz

800 MHz

HO

Chemical Shift - d Position on frequency scale

Expressed in ppm to make it field independent

Influenced by Electron density about atom

Electronegative atoms deshield

Silicon shields

Organometallics

Anisotropic effects Double bonds deshield

Triple bonds shield

Correlation charts

Best discussion in Carey

Integration Area under peak is

proportional to relativenumbers of atoms

Accuracy to ~1-5%

Depends strongly on acquisition parameters Relaxation time

Proton integrates

Carbon does not (usually)

1H-1H Through-bond (J) Coupling Coupled via electron spins

Distance between peaks in Hz First two peaks is always 1st J

Up to five bonds away

2-bond (geminal)

3-bond (vicinal)

4-bond (allylic)

Same both ways (J1,2 = J2,1)

OH, NH solvent dependent, exchangeable

Pascals triangle n + 1 rule

Only holds if all J identical

Pentet (p)

(sxt)

(spt)

(o)

(n)

0123PPM

7 Hz

7 Hz

CH3CH2OH

1H-1H Through-bond (J) Coupling Magnitudes vary 0-20 Hz

2J>=3J>4J>5J

Free rotation ~7-10 Hz

For hindered rotation J depends on dihedral angle

Karplus relationship Largest at 0o, 180o

Smallest around 80o-90o

Different relationship for 2, 3, and 4 bond

NH2

H

NH2

H

NH2

NH2

NH2

NH2

NH2

NH2

H

NH2

H

NH2

180o

Large J30o

Small J

1H-1H Through-bond (J) Coupling Nonstandard couplings doublet of doublets (dd)

11 Hz

18 Hz

1H-1H Through-bond (J) Coupling

Nonstandard couplings doublet of quartets (dq)

5 Hz

8 Hz

1H-1H Through-bond (J) Coupling

Nonstandard couplings (dqd)

1H-1H Through-bond (J) Coupling

Look for basic patterns to get couplings

1H-X Through-bond (J) Coupling Other magnetically active nuclei

13C, 15N, 31P, 19F, 29Si

Metals 6Li, 51V, 195Pt, 199Hg

13C NMR Same arguments apply as with 1H

Pulse, observe FT for spectrum Larger Sweep width Less sensitivity

1.11% 13C

Chemical shift Electronegative atoms Anisotropic effects

No integration Except quant 13C Long pulse delay

Coupling with protons Decoupling

13C NMR Decoupling

Irradiate 1H while acquiring 13C

Can be continuous or pulsed

Inverse gated decoupling

DEPT Editiing Distortionless Enhancement through

Polarization Transfer Irradiate 1H while acquiring 13C

Can be continuous or pulsed

Pulse widths

45o CH, CH2, CH390o CH only

135o CH2 up, CH3, CH down

Normal

Decoupled13C

DEPT 90

DEPT 135

DEPT Editing Distortionless Enhancement through

Polarization Transfer Irradiate 1H while acquiring 13C

Can be continuous or pulsed

Pulse angle q

45o CH, CH2, CH390o CH only

135o CH2 up, CH3, CH down

Normal

Decoupled13C

DEPT 90

DEPT 135

Summary of Information from NMR

NMR 1H

Chemical Shift what kind of protons

Integration how many protons and in what groups

Couplings

how many neighbors (n+1)

Which neighbors (J values)

13C How many carbons (normal decoupled)

How many protons (coupled or DEPT)

2D spectra direct connectivity (9/21)

General Notes on Structure Elucidation of

Synthetic Materials from Spectroscopy Some techniques yield lots of information. Others may not yield as much but may give critical information.

NMR protons, carbons. connectivity IR functional groups MS mol. wt., structural fragments UV-Vis absorbing moieties, esp. aromatics (Characteristic lmax)

Use all the information you can get Reaction info

Compare to starting material to see what has changed Compare to expected data for expected product based on structure Look at possible alternate products based on the reactants/reagents used Be on the lookout for impurities

Starting material and co-reactants Solvents Workup reagents/solvents Cleaning solvents and grease

These are all interactive. You may need to go back and forth a few times to settle on a logical structure

Any structure you propose should agree with all characterization data.

Spectroscopy Homework

(Silverstein) 1.1-1.3

2.2

3.1, 4.1 No Pople notation

Label equivalent proton groups

Label equivalent carbons

3.10 a Determine splitting pattern and coupling constants

3.4, 4.4 a-d Give structure and assign resonances for proton, carbon, IR and mass spectra to the best of your ability

Due 9/13