NMR

What are the types of NMR

Carbon NMR - C13

Proton NMR - H1

What is NMR based on and why is it important?

Nuclei Spin

• Allows them to interact w/ an external magnetic field

When a sample of NMR sensitive nuclei are aranged within an electric field, how do they orient themselves?

• The align themselves with the poles of the external magnetic field so they they’re either aligned parallel or antiparallel to the earth’s external field

Which nuclei orientation is more energetic?

Antiparallel > Parallel

• Parallel is slightly lower in energy, making their spin oreientation slightly favored

When an oriented nuclei is hit w/ EMR of the right frequency, what occurs?

Energy absorption

• Causes nuclei to change from lower energy spin state to the higher energy state

• When this occur, the magnetic nuclei are said to be in resonance w/ the applied radiation = NMR

What does the exact frequency needed for nuclei resonance depend on?

• The strength of the external field

• The identity of the nucleus

• The Electronic environment of the nucleus

What is the relationship between the strength of an external field, and the energy needed for nuclei to be put in resonance

If a strong magnetic field is applied, the energy difference between the two states is larger, and higher frequency radiation is required for a spin flip

If a weaker field is applied, less energy is required to effect the transition between nuclear spin states

When an external field hits a molecule, what do the electrons do?

The surrounding electrons set up tiny magnetic fields of their own that act in opposition to the applied field so the effective fieled actually felt is weaker

Called Shielding

What does nuclei shielding depend on?

The electornic environment a C or H is in

• all picked up on NMR

• Allows us to derive structural information

Can H and C be obserbed at once?

No, they molecules require different amounts of energy to splin flip

How to operate a basic NMR

1. An organic sample is dissolved in a suitable solvent (CDCl3, no hydrogens)

2. Sample place between poles of a magnet causing nuclei to arrange in one of two possible orientations

3. Sample shined w/ rf energy

4. Frequency of irridation is constant with strength of applied dfield being varied so each nuclei comes into resonance at slighlty different field strengths

How long does it take to read IR vs NMR

IR measures vibrations so its pretty instant, NMR is much slower and blurry

What are peaks on the left and right side of the chart?

• Left part of chart is lowfield/downfield

• Right part of chart is upfield/highfield

• Downfield shifts require a lower field strength for resonance —> Less shielding

• Upfield require stronger field strength for resonance

What are all NMR peaks in reference to

A reference point: TMS: (CH3)4Si

• utilized for both H and C13

• It’s chem shift is 0, and other absorptions occur downfield

How is the problem of low natural abundance solved with NMR?

• Overcome by using signal averaging and FT NMR

What’s Signal avearging

• many individual runs added together by a computer than averaged to improve clarity of an NM

  • low abundant isotope have weak signals so they’re cluttered with random background electric noise

  • Still takes a while to formulate as one NMR takes 5 - 10 minutes

What is FT NMR

• A modern NMR that is able to collect signals pretty instantly

• The use of fourier transfers only takes a few seconds to record entire spectrim vs minutes

What are the characteristics of C NMR

• Resonances are between 0 and 220 ppm downfield

• Carbons atom’s chemical shift is affected by the EN of nearby atom, more downfield causing deshieding and for a carbon to come into resonance at a lower field

What is DEPT (Distortionless enhancement by polarization transfer)

• method to determine hydrogen # attached to each carbon in a molecule distinguishing between ch3, ch2, ch and quaternary carbons

Where do most H1 chem shifts fall?

Within 0 to 10 delta range

Since any element can be in an NMR, what are the requirements for it to be NMR active

Isotope must either:

1. Odd atomic number

2. Or even number z with odd mass number

What’s the method to determine if two peaks are chemically equivalent or distinct on C NMR?

1. Draw molecule

2. See if a C position leads to a constintional isomer

3. If names the same = equivalent, if names different nonequivalent

For C NMR does chemically distinct carbons = peaks? How bout H NMR

Carbon yes, hydrogen no peaks are usually n + 1

What are the 4 types of carbons

1* = ch3

2* = ch2

3* = ch

4* = c

If given dept data, our different classes of carbon data will point different directions, what directions will they point

1. CH and CH3 peaks point upward

2. CH2 peaks point downward

3. C peaks are missing altogether

What are the differences in H - NMR

1. Peak Integration

2. Spin - Sping Splitting -

What is peak integration

• The area under each peak is proprotional to the number of protons the resonance has

What is the range for proton NMR

Delta 10, ppm 15

In H NMR, what does the height of peaks represent?

Ratio of hydrogen # between different h’s

What do we need to know about spin splitting?

Occurs on adjacent, nonequivalent carbons

We won’t get the peaks we expect.

• Multiple peaks

Types:

• Singlet = no splitting, one peak observed

• Doublet = split into 2 peaks

• Triplet = 3

• Quartet = 4

• Multiplet = 5+

• Broad = no sharp peaks, just a broad hump

What are the rules for spin splitting

1. Label two adjacent carbons A & B

2. If there are m protons on carbon A, the resonance for the protons on carbon B with at least be m+1

3. Vice versa for carbon B

• essentially look at your atom in question, and count number of adjacent H’s and add 1, 3 bonds away

How do you solve combination spectroscopy problems:

1. Look at mass spectrum, find M+

   • try to find molecular formula that adds to this. First guess, assume CxHyO, if that doesnt work add another O

   • Partial credit on exam

2. Look at IR spectrum to determine funct. groups present and ones absent

3. Look at C NMR to count # of distinct carbons

   • If there are fewer carbons in your formula, there must be some kind of symmetry to the molecule

4. Finally look at H NMR, count # of distinct H’s, look for Ch3 and CH2 pieces

   • Start to piece molecule together