1. Nuclear Magnetic Resonance
Requirements
Molecules must contain nuclei that have a nuclear angular momentum (or nuclear spin) quantum number I >0 which results in (2I + 1) magnetic quantum numbers (states) m = + I, I -1, I -2, …, - I +1,- I.
All nuclei with odd mass, odd atomic number or both are NMR active:
I = ½ for 11H,136C, 199F, 3115P and I = 1 for 21H, 147N
For NMR measurements magnetic nuclei must be in a magnetic field as the eigen states of m are degenerated in an non-magnetic environment.
hn = DE = g(h/2p)Bo
DE is the energy difference between the generated states and depends on the applied external magnetic field Bo and the nucleus specific gyro-magnetic ratio g. The energy differences, however, are very small for all nuclei and electromagnetic radiation of radio frequency is sufficient (5-100 MHz at a field of 2.3 T).

2. Sample Preparation and Measurement
The resonance frequency no for protons in 1H-NMR are at 60 MHz in a 1.41 T magnetic field (100 MHz at 2.34 T, 300 MHz at 7.05 T, and 500 MHz at 11.7 T.
The sample is dissolved in a solvent that does not contain the magnetic nucleus of interest and this is why deuterated solvents are used in 1H-NMR: e.g. C6D6, CDCl3,or CCl4.
An NMR tube of 5 mm diameter is typically filled with 2 mg of compound dissolved in 0.5 mL of solvent (1H-NMR) and spun in the magnetic field to average out field inhomogeneities.

3. Chemical Shift
NMR would be useless if all protons in a sample had the same resonance frequency. Fortunately, the resonance frequency is slightly varied by the chemical environment of each proton. The frequency difference n of a nucleus is measured relative to a standard (tetramethylsilane (TMS) for 1H- and 13C-NMR) and have been named chemical shift d. d is given in ppm.
d(X) = 106 Dn/n with d(TMS) = 0
Example: The protons of a methyl group have a resonance frequency of 126 Hz lower than TMS at an observation frequency of 60 MHz.
dH(CH3) = 106 (126/60*106) = 2.10 ppm

4. NMR Scale Field Strength Effect
The advantage of using the dimensionless ppm unit is that the scale is independent of the external magnetic field.
Example for 1H-NMR with TMS as reference and 300 MHz resonance frequency.
3000 Hz
0 Hz
0 ppm
10 ppm
Field Strength Effect
60 MHz
300 MHz

5. NMR Scales (1H-NMR) 0 Hz 10 ppm 300 MHz, 7.05 T
(downfield)
higher frequency-less shielded
(upfield)
lower frequency-more shielded
6000 Hz
10 ppm
0 Hz
0 ppm
600 MHz, 14.1 T

6. Chemical Shift and Molecular Structure
The resonance position of a given nucleus is determined by its shielding constant s. s is made up of four terms, the diamagnetic shielding sdia, the paramagnetic shielding spara, shielding due to neighbouring groups sintra, and shielding due to intermolecular effects sintra. s can possess positive and negative values.
neff = (g/2p)Bo(1-s)
Shielding and deshielding effects are produced by local magnetic fields generated by circulating electron densities. Thus, changes in the local electron density will influence the chemical shift.
A positive value of d is left of TMS, which as a high electron density around its protons, and means the nuclei are deshielded relative to TMS.
For example, we can deduced from the increasing ppm values in 1H-NMR given below that C is more electronegative than H:
R3CH>R2CH2>RCH3>CH4

7. Chemical Shift of 1H
DE = hn0 = gB0h/2p, n0 = gB0/2p, with Blocal = Bo(1-s)
s is the electronic shielding of a specific proton and its resonance frequency with shielding is termed chemical shift d; n0 = gB0(1-s)/2p
An increase in d means a 1H nucleus is deshielded relative to TMS and vice versa;
As d depends on electron density around nuclei, the electronegativity of atoms close to the nuclei will affect the chemical shifts;
d for H in H3CX with X = F, HO, H2N, H, Me3Si, or Li are 4.26, 3.38, 2.47, 0.23, 0.0, and -0.4 (very solvent depending), respectively;

8. The electron density and, consequently, d are not only influenced by the inductive effect but also by resonance;
Hybridization of the carbon to which a proton is attached also influences the electron density around the proton; As the proportion of s-character increases form sp3 to sp hybridization, bonding electron move closer to the carbon and the proton becomes more deshielded;