Nuclear Magnetic Resonance Spectroscopy

Dr. TodebushChemistry 2412L

IntroductionNMR is the most powerful technique for

organic structure determination◦ Number and type of atoms in a molecule◦ Connectivity of atoms

Used to study a wide variety of nuclei:◦ 1H◦ 13C◦ 15N, 19F, 31P

Radio-frequency radiation used to transition between energy states◦ 30 – 900 MHz◦ Transition = nuclear spin

Nuclear SpinA nucleus with an odd atomic

number or an odd mass number has a nuclear spin

The spinning charged nucleus generates a magnetic field

External Magnetic FieldWhen placed in an external field,

spinning protons act like bar magnets

Two Energy StatesThe magnetic fields of the

spinning nuclei will align either with the external field, or against the field

A photon with the right amount of energy can be absorbed and cause the spinning proton to flip

Spin flip = resonance◦ Detected and recorded by the

spectrometer as a signal

Magnetic ShieldingIf all protons absorbed the same

amount of energy in a given magnetic field, not much information could be obtained

But protons are surrounded by electrons that shield them from the external field

Circulating electrons create an induced magnetic field that opposes the external magnetic field◦Effective magnetic field

Shielded ProtonsMagnetic field strength must be increased

for a shielded proton to flip at the same frequency

Differences detected by machine, cause differences in signals (chemical shift, d)

Protons in a MoleculeDepending on their chemical environment, protons in a

molecule are shielded by different amountsChemically equivalent nuclei

◦ Interchanged through bond rotation or element of symmetry◦ Have same absorption

Chemically different nuclei have different absorption

NMR SignalsThe number of signals shows how many

different kinds of protons are presentThe location of the signals shows how

shielded or deshielded the proton isThe intensity of the signal shows the

number of protons of that typeSignal splitting shows the number of

protons on adjacent atoms

The NMR Spectrometer

The NMR Graph

Tetramethylsilane

TMS is added to the sampleSince silicon is less electronegative than

carbon, TMS protons are highly shieldedSignal defined as zeroOrganic protons absorb downfield (to

the left) of the TMS signalDeuterated solvent signal

Si

CH3

CH3

CH3

H3C

Chemical ShiftMeasured in parts per millionRatio of shift downfield from TMS

(Hz) to total spectrometer frequency (Hz)

Same value for 60, 100, or 300 MHz machine

Called the delta (d) scale

Delta Scale

downfield upfield

Location of SignalsMore electronegative

atoms deshield more and give larger shift values (downfield)

Effect decreases with distance

Additional electronegative atoms cause increase in chemical shift

Hydrogen and Carbon Chemical Shifts

13C-NMR

12C has no magnetic spin13C has a magnetic spin, but is only

1% of the carbon in a sampleSignals are weak, get lost in noiseHundreds of spectra are taken,

averagedSignal = one sharp line for each

different type of carbon

3-PentanoneHow many signals?Chemical shifts:

◦sp3 C upfield◦sp, sp2 C downfield◦C adjacent to en atom downfield

O

O

2-ButanoneHow many signals?Chemical shifts?

O

O

How is 13C useful for reactions we have studied?Zaitsev vs. non-Zaitsev

CH3Br

Base

E2vs.

CH3 CH2

7 signals 5 signals

1H-NMRMore info than 13C-NMRGiven a structure, how many

signals are expected?How many sets of H in each

molecule?CH3 CH

CH3

CH3 CH2 CH

CH3

CH3 CH3 CH2 CH2CH3 CH2 CH3

Isomers• Same molecular formula• Same IR stretches• Different NMR

Another example:CH3 C

O

CH2 C

O

O C

CH3

CH3

CH3

Chemical shifts in 1H-NMRInfo about type of H giving rise to

signalStrongly shielded = upfield (to the

right)Less shielded = downfield (to the

left)Most common shifts:

◦Wade Appendix 1A◦Wade, Table 13-3◦-CH2-O-C(O)- ranged from 3.7-4.7 ppm

Typical Values

O-H and N-H Signals

Chemical shift depends on concentration

Hydrogen bonding in concentrated solutions deshield the protons, so signal is around d3.5 for N-H and d4.5 for O-H

Using chemical shifts

Given a structure, predict dUse to distinguish between two

structuresExample:

◦Constitutional isomers◦Each with 2 sets of H’s

HOC

CH3

O

HC

O

O

CH3

Which isomer best fits this spectrum?

orHO

CCH3

O

HC

O

O

CH3

Which isomer best fits this spectrum?

orHO

CCH3

O

HC

O

O

CH3

Intensity of SignalsThe area under each peak is

proportional to the number of protonsShown by integration line

◦ Height a area under peak a # H’s in set◦ Measure height with ruler or look at graph

paper◦ Ratio of height = ratio of hydrogens

HOC

CH3

O

HC

O

O

CH3