NMR Spectroscopy

Nuclear Magnetic Resonance

Nuclei possess charge and can spin

Spinning charge generates magnetic dipole

Magnetic Dipoles

characterized by nuclear magnetic spin quantum number “I”.

Three cases of “I”

1.I= 0

- Have an even number of protons and an even number of neutrons

- Do not spin - hence do not interact with applied magnetic field.

Examples: ¹²C, ¹⁶O

I = ½

Nuclei are NMR visible

examples: ¹H, ¹³C (non-radioactive isotope of C, 1.06% natural abundance)

. I > 1/2

Examples: ²H and ¹⁴N – difficult to observe

diagram 1

diagram 2

Planck’s equation:

∆E = hν

ν = γB_0/2π,

∆E = γ B_0h/2π

γ Boh/2π = hν

γ = gyromagnetic ratio; a constant that depends on the magnetic moment of the nucleus.

Bullet 1

•More nuclei aligned with externally applied magnetic field or lower energy α-spin state.

•∆E = 

Energy difference between α and β-spin states

•Increasing the applied magnetic field =

Increasing ∆E between spin states

Nuclei can

absorb energy and flip from α-spin state to β-spin state.

When this happens, nucleus is said to be in RESONANCE with applied magnetic field, hence term “Nuclear Magnetic Resonance”.

The difference

in energy ΔE between two spin states corresponds to frequency in the Radiofrequency region of the EM spectrum.

downfield

left

these protons sense a larger effective magnetic field, so come into resonance at a higher frequency

upfield

these protons sense a smaller effective magnetic field, so come into resonance at a lower frequency

Example: CH₃Cl Vs CH₄

Electronegative Cl withdraws e^-1 density from the ‘C’ and ‘H’ atoms – thus DESHIELDING them

Deshielding or Lesser shielding = more of the applied magnetic field strength experienced = higher (DOWNFIELD) Frequency absorbed

Schematic of an NMR spectrometer 1

The sample is dissolved in solvent in a thin NMR tube, and placed in a magnetic field.

Schematic of an NMR spectrometer 2

In the NMR probe, the sample is rotated in a magnetic field and irradiated with a short pulse of RF radiation.

NMR tech picture

Common NMR solvents:

A Typical NMR Sample Prep:

10-20 mg compound + 0.5-0.6 mL CDCl3

NMR spectrum:

A plot of the intensity of an observed NMR signal Vs the frequency, measured relative to a reference compound; typically Tetramethylsilane (TMS)

TMS

Tert-butylmethylether

Advantages of TMS:

1. TMS HAS 12 equiv. H’s

2. NMR Signal due to protons in TMS appears as a singlet at 0 ppm (way upfield from other signals).

3. It is low-boiling and inert.

Chemical shift (Position of the Peak):

Frequency difference between the resonance observed for the nucleus and the resonance observed for the reference compound (usually TMS).

δ =

Degree of Unsaturation:

2 (No. of C atoms)  +  2  +  (No. of N atoms)  – (No. of Halogen atoms) -  (No. of H atoms)

                                      2    

For a DU ≥ 4 = 3 double bonds + 1 ring (= 1 Aromatic ring)

Size of/ area under the peak ((Integration) of NMR signals

is indicative of the relative number of identical protons in a particular environment.

Number of signals

corresponds to different groups of non-equivalent protons.

c. Position of signals

is indicative of the kind of proton/ s responsible for the signal.

d. Spin-spin splitting of signals

indicates the number of neighboring protons

Chemically equivalent protons

protons in the same environment. Equivalent protons do not split each other’s signals.

Chemically in-equivalent protons

give more than one peak.

Area under an NMR signal is

proportional to the number of absorbing protons.

Height of each step

is proportional to the area under the peak which in turn is  proportional to the number of absorbing protons.

Spin-spin splitting, occurs, peaks, # sigma bonds

•Spin-spin splitting occurs only between non-equivalent protons on the same carbon or adjacent carbons.

•A set of “n” non-equivalent protons splits the signal of a neighboring proton into (n+1) peaks.

•Splitting not generally observed for protons separated by more than 3 sigma bonds.

- Splitting by

“n” nuclei of spin quantum number “I” gives (2nI + 1) lines.

For protons where I= ½, this equation results in (n + 1) lines

Spacing (in Hz) between the lines in the multiplet = Coupling Constant (J) (units Hz)

Superscript before the symbol “J” represents the no. of inclusive bonds between the coupled nuclei

Coupled nuclei are identified by labels as subscripts after the symbol “J”

Ex. 2Jab = 2.4

Superscript before the symbol “J” represents

no. of inclusive bonds between the coupled nuclei

Coupled nuclei are identified by

labels as subscripts after the symbol “J”

Ex. 2Jab = 2.4

•OH protons

are not split and they do not split the NMR signal of adjacent protons signal appears as a singlet

Signal Multiplicities and Relative Intensities of Signal Peaks

Number of peaks  Name

                 1  singlet (s)

                 2  doublet (d)

                 3  triplet (t)

                 4  quartet (q)

                 5  quintet

                 6  sextet

                 7  septet

                >7  multiplet (m)

relative peak intensities: pascals triangle