Review of spectroscopy

IR: vibrational transitionsUV-Vis - electronic transitionsNMR: magnetic transitions (radio frequency)

5.1A

Some atoms act as tiny magnets

if placed in a magnetic field, will align with or against

Nuclear Magnetic Resonance spectroscopy

-1/2 spin state is slightly higher E: Slightly > 50% of atoms are in +1/2 state

5.1A

Angular frequency of precession:

Values of are in radio frequency range

varies with strength of B0!

The resonance condition:

If sample is hit with radiation of frequency = , we have resonance condition: spin flip! That frequency of radiation is absorbed.

Protons in different chemical environments have different resonance frequencies!

Key concept: chemical equivalence/nonequivalence

5.2

These molecules have only one ‘set’ of protons

5.2

equitorial and axial protons are NMR-equivalent

5.2

These molecules have two ‘sets’ of protons

5.2

These molecules have several different ‘sets’ of protons

5.2

diastereotopic protons are non-equivalent

Enantiotopic/homotopic protons have equivalent resonance frequencies

5.2

Practice: How many sets of protons? (ie. how many 1H-NMR signals?

The 1H-NMR experiment

• Put sample in strong external magnetic field• Ha protons precess at a, Hb protons at b

• Hit sample with radiation in Rf range of frequencies Ha protons absorb radiation at a, undergo ‘spin flip’ Hb protons absorb radiation at b, undergo ‘spin flip’

• Detector records which frequencies were absorbed, and intensity of each absorbance

5.3A

We must use solvents without protons!

(Used to use CCl4 commonly, but it’s carcinogenic)

5.3A

5.3B

The chemical shift

(refer to 1H-NMR spectrum of methyl acetate)

TMS (tetramethyl silane) is used as a standard: set the resonance frequencies of these 12 equivalent protons equal to zero

• Record the resonance frequencies of the protons in your sample relative to TMS protons, expressed as ppm

• eg. 7.1 T magnetic field, TMS protons resonate at 300,000,000 Hz (300 MHz)

• Ha protons resonate at 300,000,621 Hz, which is 2.07 ppm higher than TMS

protons

• Hb protons resonate at 300,001,104 Hz, which is 3.68 ppm higher than TMS

protons

Remember: Resonance frequencies vary with strength of B0!

but . . . when expressed in terms of ppm relative to TMS, the number does not change - this is why we use ‘chemical shift’ rather than Hz (or wavelength)

example: A proton has a chemical shift of 4.50 ppm.

a)What is its resonance frequency, expressed in Hz, in a 300 MHz instrument (ie an instrument with a 7.1 Tesla magnet, where TMS protons resonate at 300 MHz)?

b)What is its chemical shift expressed in Hz?

c) What is its resonance frequency in a 100 MHz instrument?

Chemical shift is abbreviated by

Higher chemical shifts are said to be ‘downfield’

Most protons in organic compounds have chemical shifts from 0-12 ppm relative to TMS

TMS is (usually) no longer added to sample: resonance frequency of deuterium in solvent is used as the actual reference point (but 0 ppm is still defined as TMS signal)

Signal integration

The area under a 1H-NMR signal (integrations) corresponds to how many protons cause the signal

In methyl acetate, the Ha signal and Hb signal both represent three protons. Thus, the area under these signals is (approx) equal

In p-xylene, Ha corresponds to six protons, Hb to four. Ratio of peak integrations is 6 to 4, or 1.5 to 1

5.3C

The basis for magnetic non-equivalence

(why do different protons have different chemical shifts?)

The shielding effect: nearby electrons create small magnetic fields in opposition to B0. These are called ‘induced fields’.

5.4A

Electronegative atoms pull electrons away from nearby protons

Protons are ‘deshielded’, experience stronger Beff

Stronger Beff means higher resonance frequency: higher (downfield) chemical shift

The deshielding effect

5.4A

Deshielding effect drops off with distance:

5.4A

5.4A

Now we can ‘assign’ peaks on the methyl acetate spectrum

Diamagnetic anisotropy

isotropy = ‘sameness’anisotropy = ‘difference’

Why is benzene chemical shift so far downfield?. . . more than just normal deshielding!

5.4B

Field from a magnet is anisotropic

at point A, you sense a field pushing northat point B, you sense a field pushing south

5.4B

6 aromatic electrons form ring current, opposed to B0

But for benzylic protons, the ring current field is aligned with B0 - makes Beff stronger! Strong deshielding effect.

5.4B

Extreme case:

outer protons are 8.9 ppm

inner protons are -1.8 ppm (upfield of TMS signal!)inside the ring, aromatic ring current is strongly shielding

(exercise 5.5)

Similar argument for vinylic, aldehyde protons

Hydrogen-bonding protons (amines, alcohols, phenols) have variable variable chemical shifts, often >4 ppm.

H-bonding patterns effect chemical shift

Often slightly broad peaks (see spectrum next slide)

(figure from section 5.6A)

5.5A

Ha

Spin-spin coupling

Ha signal in 1,1,2-trichloroethane is a doublet ‘split’ by Hb

5.5A

Hb signal is a triplet

‘split’ by Ha

Ha and Hb are coupled - their spins interact

5.5A

Splitting is seen between protons that are separated by three bonds or less

n neighbors leads to n + 1 sub-peaks

H-bonded protons generally do not show coupling

5.5A

ethyl acetate

5.5A

More examples of spin-spin coupling

5.5A

Coupling constants

J is expressed in Hz, not ppm - does not depend on strength of B0!

5.5B

Notice: same value of J!Function of interaction between Ha and Hb

Common coupling constants

5.5B

5.5C

Complex coupling

methyl acrylate

5.5C

5.5A

5.5C

(exercise 5.9 asks you to construct a splitting diagram for Hb)

5.5C

Often the ‘n+1 rule’ holds even when protons are non-equivalent. . . if J values are close

5.5C

Hc is split into a sextet (5 neighbors)

5.5C

Sometimes there is too much overlapping: just call it a multiplet (m)

13C-NMR Spectroscopy

Differences from 1H-NMR:

•Only ~ 1% of carbons are 13C - much weaker signal

•Integration not meaningful: signal intensities vary (eg. carbonyl carbon signals are very weak)

•Where TMS protons resonate at 300 MHz, 13C resonates at 75 MHz

•Chemical shift range is wider - more than 200 ppm

5.6A

Broadband decoupling: turns off C-H splitting, so we see only singlets

5.6A

Distortionless Enhancement by Polarization Transfer experiment tells us how many protons are attached

5.6A

another example:

5.6A

1H-NMR spectra can get very messy . . .

5.6A

. . . but decoupled 13C-NMR spectra usually don’t have overlapping peaks (much wider spectrum!)

13C-enrichment in biochemical studies

5.6B

5.7A

Problem: solve a structure from scratchMS: molecular ion peak at m/z = 92Combustion analysis: 52.0% carbon, 38.3% chlorine, 9.7% hydrogen(0.52)(92) = 47.8g carbon in one mole compound: this means there are 48/12 = 4 carbons(0.383)(92) = 35.2 g chlorine in one mole compound: 35.2/35.45 = 1 chlorine(0.097)(92) = 8.9 g hydrogen in one mole compound: 8.9/1 = 9 hydrogensC4H9Cl IHD = 0 (no rings or multiple bonds)

figure out fragments!

5.7A

5.7A

the solution:

5.7

Another example

5.7

fragments:

5.7

the solution:

5.8

phosphorylated compounds

5.8

5.8

31P-NMR spectroscopy