Nuclear Magnetic Resonance Spectroscopy Dr. Sheppard Chemistry 2412L

Nuclear Magnetic Resonance Spectroscopy

Dr. Sheppard

Chemistry 2412L

Introduction NMR 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 Spin

A 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 Field

When placed in an external field, spinning nuclei act like bar magnets

Two Energy States

The 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 nucleus to flip

Spin flip = resonance Detected and recorded by the

spectrometer as a signal

The NMR Spectrometer

Magnetic Shielding If all nuclei absorbed the same amount

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

But nuclei 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 Nuclei Magnetic field strength must be increased for a

shielded nucleus to flip at the same frequency Differences detected by machine, cause

differences in signals (chemical shift, )

Nuclei in a Molecule Depending on their chemical environment, atoms in a

molecule are shielded by different amounts

Chemically equivalent nuclei Interchanged through bond rotation or element of symmetry

Have same absorption

Chemically different nuclei have different absorption

1H-NMR Spectrum for Methanol

Tetramethylsilane

TMS is added to the sample

Since silicon is less electronegative than

carbon, TMS protons are highly shielded

Signal defined as zero

Organic protons absorb downfield (to the left)

of the TMS signal

Deuterated solvent signal

Si

CH3

CH3

CH3

H3C

Chemical Shift

Measured in parts per million

Ratio of shift downfield from TMS (Hz)

to total spectrometer frequency (Hz)

Same value for 60, 100, or 300 MHz

machine

Called the delta () scale

Delta Scale

downfield upfield

Location of Signals

More 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

NMR Spectra

13C-NMR

12C has no magnetic spin 13C has a magnetic spin, but is only 1% of

the carbon in a sample Signals are weak, get lost in noise Hundreds of spectra are taken, averaged Signal = one sharp line for each different

type of carbon

3-Pentanone

How many signals? Chemical shifts:

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

O

O

2-Butanone

How many signals? Chemical shifts?

O

O

How is 13C-NMR useful for reactions we have studied?

Zaitsev vs. non-ZaitsevCH3Br

Base

E2vs.

CH3 CH2

7 signals 5 signals

Interpreting 13C-NMR

The number of different signals indicates the number of different kinds of carbon

The chemical shift indicates the functional group

Use to support 1H-NMR analysis

1H-NMR

More info than 13C-NMR The number of signals shows how many different

kinds of protons are present

The location of the signals shows how shielded or

deshielded the proton is

The intensity of the signal shows the number of

protons of that type

Signal splitting shows the number of protons on

adjacent atoms

1H-NMR

Given 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-NMR Info about type of H giving rise to signal Strongly shielded = upfield (to the right) Less shielded = downfield (to the left) Most common shifts:

McMurry, Table 13-3

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 3.5 for N-H and 4.5 for O-H

Using chemical shifts

Given a structure, predict Use to distinguish between two structures Example:

Constitutional isomers Each with 2 sets of H’s

HO

C

CH3

O

H

C

O

O

CH3

Which isomer best fits this spectrum?

orHO

C

CH3

O

H

C

O

O

CH3

Which isomer best fits this spectrum?

orHO

C

CH3

O

H

C

O

O

CH3

Intensity of Signals The area under each peak is proportional to

the number of protons Shown by integration line

Height area under peak # H’s in set Measure height with ruler or look at graph paper Ratio of height = ratio of hydrogens

HO

C

CH3

O

H

C

O

O

CH3

So far… Determine the number of sets of equivalent

hydrogen atomsNumber of signals on spectrum

Determine the number of hydrogen atoms in

each setIntegration line

Determine general information about adjacent

groupsChemical shift ()

Next… Determine specific information about

adjacent groups

In particular, how many H atoms on the

adjacent atomsSignal splitting