Nuclear Magnetic Resonance (NMR) Spectroscopy
Nuclear Spin and Magnetic Moments
Nuclear spin
When nucleus has odd atomic number/mass or both.
It leads to spin angular momentum (I) and magnetic moment,
which can interact with an external magnetic field – forms the
basis of NMR.
NMR ‘silent’ nuclei do not possess nuclear spin.
Example of NMR active nuclei: !!H, !"H, !#"C, !$#F, !$
"%
Al, etc.
!& !"
Example of NMR ‘silent’ Nuclei: %O, "C, etc.
Nuclear Spin and Magnetic Moments
Spin States
When the spins orient themselves in a static magnetic field, the states possible are given by
(2I + 1) (where I is the spin quantum number) with integral differences ranging from +I to −I.
Spin States and Transitions
2I + 1 rule:
For 1H, spin quantum no. I = ½.
Therefore, there energy level split into
2 states. When placed in an external
magnetic field (Bo), the nuclear
magnetic field will either aligned with
(lower energy) or oppose (higher
energy) the external magnetic field.
Resonance of a Nucleus Depends on its
Surrounding
Shouldn’t all protons then resonate at the same frequency? -- NO
Diamagnetic Shielding by Electrons The protons are shielded by electrons around them. In an
applied magnetic field, the valence electrons of the nuclei
are caused to circulate. This circulation, called a local
diamagnetic current, generates a counter magnetic field
that opposes the applied magnetic field. This is called
diamagnetic shielding. In other words, electron clouds
“shield” nuclei from the external magnetic field causing
them to resonate at slightly higher energy.
Therefore, different 1H nuclei in a molecule resonate at
different energies depending on the electronic
environments around them.
Chemical Shift
q the frequency is measured directly with respect to a standard reference substance than measuring the
absolute frequency. Tetramethylsilane (called TMS in short), (CH3)4Si, is universally used as the
reference substance.
q Chemical shift (d, measured as ppm), a field-independent parameter, is defined as following:
' ()*+,- +./0/1 21 34 5' .-6-.-17- +./0/1 21 34 νsample is the absolute resonance frequency of
d=
(+-70./+8/0/*-0-. 6.-9:-17; 21 <34 the sample and νref is the absolute resonance
frequency of a standard reference compound
δ (TMS) = 0 TMS
q It should be noted that chemical shift is independent of the applied field.
1H NMR Spectrum of Benzyl Acetate
• Position of the signals/chemical
shift – related to environment of
the nucleus
• Number of the signals – types
of nucleus
• Intensity of signals/integration –
relative amount of nuclei
TMS
• Splitting pattern – further
information of neighbouring
nuclei
←d
Chemical Shift Related Concepts and its
Dependence on Various Factors
Shielding and Deshielding
• Decreased electron density
• Electrons shield the nucleus deshields the nucleus
• Absorption shifts upfield • Absorption shifts downfield
Chemical Shift Dependence on Factors
Electronegativity:
less shielded more shielded
or or
downfield upfield
H3C-F H3C-O-CH3 (H3C)3-N H3C-CH3 (H3C)4-Si
d 4.3 d 3.2 d 2.2 d 0.9 d 0.0
Number of Electronegative atoms: Distance from Electronegative atoms:
Chemical Shift range for 1H Attached
to Different Functional Groups
Solvents for NMR
Spectroscopy
Solvents containing no 1H or
deuterated solvents:
CCl4, CDCl3, CD3COCD3, D2O
Number of Signals and their Intensity (Integration)
Chemically equivalent (identical electronic
environments) proton Sets: 3
So we see 3 signals in ethanol Intensity/Integration
q Modern NMR spectrometers automatically
calculate and plot the relative integration
values.
q The ratio of integrals to one another gives
the ratio of absorbing protons in a
spectrum.
q Note that this gives a ratio, and not the
absolute number, of absorbing protons.
q A compound of molecular formula C9H10O2 gives rise to following NMR signals. How many
protons correspond to each protons?
q Relative ratio: 54: 23: 33 = 2.35: 1: 1.43 = 4.70 : 2: 2.86 ( 5: 2: 3)
Splitting Pattern for Non-equivalent Protons
NMR spectrum of ethanol
q Signals of non-equivalent protons are split into
more than one peak when they are separated
by at least one carbon/oxydgen atom. This
phenomenon leads to, instead of simple
singlets, a more complex patterns, called
multiplets (doublet, triplet, quartet, etc.). This
is known as spin-spin coupling/splitting.
q The number of adjacent protons can be
deduced from this splitting pattern.
Splitting Pattern: Few Rules of Thumb
q Chemically equivalent protons do not exhibit spin-spin coupling to each other. Ethane
exhibits only a singlet (single line) at δ = 0.85 ppm.
q Spin-spin coupling is normally observed between nuclei bound to the same carbon
(germinal coupling) or to adjacent carbons (vicinal coupling). Coupling between protons
that are three bonds away are generally weak, and it is not generally observable beyond
that.
The coupling can occur between a
and b, a and c, b and c, b and d, and
c and d. a-d coupling is not seen.
q The resonance of a proton that has n equivalent protons on the adjacent carbon is split
into n+1 peaks (multiplicity) with a coupling constant J.
q Protons that are coupled to each other have the same coupling constant J.
Origin of Splitting Pattern
Doublet pattern
For each Ha, the spins of Hb can either align or oppose,
thus creating two energy states.
Similarly, for each Hb protons, two states are possible
due to coupling with the magnetic field generated by Ha.
Triplet and Quartet patterns
←d
Pascal’s Triangle
Coupling Constant: How to Read and Significance
How to read and determine Significance
q Can be very definitive for certain chemical environments
13C NMR Spectroscopy
q Natural abundance is 1% relative to that of proton; gyromagnetic ratio is 1/4th of
that of proton – these make 13C NMR much insensitive relative to that of 1H
NMR.
q 13C NMR signals usually cannot be integrated.
q Coupling with proton attached to a carbon can lower the intensity further, which
is why often proton-decoupled 13C NMR spectra are recorded. Each carbon
appears as one line in a spectrum.
Proton Decoupled 13C NMR Spectrum
Proton-Decoupled 13C Spectrum of 1-Propanol
1. Observe the relative intensity
of the solvent peak.
2. Can you integrate the peaks?
13C Chemical Shifts
