• NMR is absorption spectrometry.
• Permits identification of atomic configuration.
• Radiofrequency waves are used in NMR.RF waves have long wavelength so less energy.
• Range of RF is from 30 KHZ TO 30 MHZ.
• Felix Bloch and Edward Purcell received Nobel Prize for NMR discovery.
• NMR mechanism includes change in spin orientation on exposure to magnetic field.
• Spin quantum number(I) describes angular momentum.
• Total spin quantum number depends on the number of nucleons and symmetry of charge distribution.
• Precessional frequency-No. of spin carried out by the nucleus in the presence of magnetic field per second.
Thus when magnetic field applied the nucleus spins with precessional frequency.
Before application of magnetic field it was moving with random frequency.
• When no magnetic field, only one energy level. but in presence of magnetic field 2 energy levels with aligned and opposite orientations of the nuclear spins.
• Precessional frequency is directly proportional to field strength(ν)ν ά Bo
• Remember-
When mag.field=1.4 tesla,ν=60MHZ.
When mag.field=2.3 tesla,ν=100MHZ.
When mag.field=7.0tesla,ν=300MHZ.
When mag.field=14.1Tesla,ν=600MHZ.
1 Tesla=104 Gauss.so 1.4 Tesla=14000 Gauss.
Imp-Tesla is measure of magnetic flux density not field strength.
• Resonance occurs when,
Applied frequency from RF oscillator=precissional frequency of spinning nucleus.
Relaxation process of nucleus
• Spin-lattice or longitudinal relaxation-gives energy to lattice so system gets heated.
• Spin-spin or transverse relaxation-give energy to nearby nuclei so no net change in energy of system.
REMEMBER:spin-lattice relaxation time (T1)determines the rate at which absorption of RF can occur.
Rememeber : relaxation process follow first order process and T1 is the reciprocal of first order rate constant.
T2 is spin-spin relaxation time.
Remember-if T1 and T2 small,broad absorption lines,if they r large sharp abs. lines.
FOR non-viscous solids T1 is large so sharp lines in NMR spectrum.
FOR 14N nucleus T1 is small due to quadrapole so broad signals in 14N NMR.
Internal standards
• TMS(tetramethyl silane)(0.01-1%) – universally accepted,used only in organic solvents.TMS is used in 1H and 13C NMR.
• Sodium salt of 3-(trimethylsilyl)-propane sulfonic acid and DDS is used in aqueous solvents.
• CFCl3 for 19F NMR.
• H3PO4 for 31P NMR.
• NH3(absolute standard) and nitromathane(mostly used) for 14N NMR.
• Water for 17O NMR.
Solvents for NMR
• CDCl3 mostly used.
• Should not contain hydrogen (aprotic solvents)
• For polar solutes-D2O,DMSO-d6,CCl2,CS2 ,CF3.
• Compound solubility should be atleast 10%.
• Permits identification of atomic configuration.
• Radiofrequency waves are used in NMR.RF waves have long wavelength so less energy.
• Range of RF is from 30 KHZ TO 30 MHZ.
• Felix Bloch and Edward Purcell received Nobel Prize for NMR discovery.
• NMR mechanism includes change in spin orientation on exposure to magnetic field.
• Spin quantum number(I) describes angular momentum.
• Total spin quantum number depends on the number of nucleons and symmetry of charge distribution.
• Precessional frequency-No. of spin carried out by the nucleus in the presence of magnetic field per second.
Thus when magnetic field applied the nucleus spins with precessional frequency.
Before application of magnetic field it was moving with random frequency.
• When no magnetic field, only one energy level. but in presence of magnetic field 2 energy levels with aligned and opposite orientations of the nuclear spins.
• Precessional frequency is directly proportional to field strength(ν)ν ά Bo
• Remember-
When mag.field=1.4 tesla,ν=60MHZ.
When mag.field=2.3 tesla,ν=100MHZ.
When mag.field=7.0tesla,ν=300MHZ.
When mag.field=14.1Tesla,ν=600MHZ.
1 Tesla=104 Gauss.so 1.4 Tesla=14000 Gauss.
Imp-Tesla is measure of magnetic flux density not field strength.
• Resonance occurs when,
Applied frequency from RF oscillator=precissional frequency of spinning nucleus.
Relaxation process of nucleus
• Spin-lattice or longitudinal relaxation-gives energy to lattice so system gets heated.
• Spin-spin or transverse relaxation-give energy to nearby nuclei so no net change in energy of system.
REMEMBER:spin-lattice relaxation time (T1)determines the rate at which absorption of RF can occur.
Rememeber : relaxation process follow first order process and T1 is the reciprocal of first order rate constant.
T2 is spin-spin relaxation time.
Remember-if T1 and T2 small,broad absorption lines,if they r large sharp abs. lines.
FOR non-viscous solids T1 is large so sharp lines in NMR spectrum.
FOR 14N nucleus T1 is small due to quadrapole so broad signals in 14N NMR.
Internal standards
• TMS(tetramethyl silane)(0.01-1%) – universally accepted,used only in organic solvents.TMS is used in 1H and 13C NMR.
• Sodium salt of 3-(trimethylsilyl)-propane sulfonic acid and DDS is used in aqueous solvents.
• CFCl3 for 19F NMR.
• H3PO4 for 31P NMR.
• NH3(absolute standard) and nitromathane(mostly used) for 14N NMR.
• Water for 17O NMR.
Solvents for NMR
• CDCl3 mostly used.
• Should not contain hydrogen (aprotic solvents)
• For polar solutes-D2O,DMSO-d6,CCl2,CS2 ,CF3.
• Compound solubility should be atleast 10%.
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