Chemistry Notes for B.Sc. and M.Sc.

  The effect which changes the intensity of one spin system (either same or different kind of nuclei like H or C) when the transition of another spin system (another nuclei like H) is perturbed from equilibrium population (via saturation or relaxation processes) by irradiating with a second tunable radiofrequency pulse applied at the precessional frequency of latter nuclei. The NOE effect will be homonuclear when both the nuclei are same (like H and H) and will be heteronuclear when both the nuclei will be different (like C and H). It may be positive as well as negative and its magnitude depends on the gyromagnetic ratios of the nuclei involved in dipolar coupling. It will be positive when the gyromagnetic values of both the nuclei are positive and is given by below equation:   NOE max  = ½ (γ irr /γ obs ) While the total intensity = 1 + NOE max The nuclei which are irradiated with the second radiofrequency is represented as S and its gyromagnetic ratio as γ irr . While t...

NMR-Spectroscopy Principle  NMR spectroscopy involves the absorption of electromagnetic radiation in the radiofrequency region and the principle of NMR spectroscopy revolves around the non- zero value of nuclear spin i.e. I   S. No. Number of Protons Number of Neutrons Nuclear Spin (I) examples 1 even even 0 6 C 12 , 8 O 16 2*             odd even Half integer (1/2, 3/2, 5/2) 1 H 1 , 9 F 19 , 15 P 31 3**             even odd Half integer (1/2, 3/2, 5/2) 6 C 13 4***              odd odd Full integer 7 N 14 *This is possible when both atomic number and mass...

Gross selection rule   tells whether a molecule will be active or inactive in a particular Spectroscopy.   Specific selection rule tells about the allowed changes in the magnitude of energy when it is active in a particular Spectroscopy.     Rotational or Microwave Spectroscopy Gross selection rule: permanent dipole moment  Specific selection rule: ∆J = +1 or -1 ( for rigid rotor) 2. Vibrational or IR Spectroscopy Gross selection rule: change in dipole moment during vibrations Specific selection rule: ∆v = +1 or -1 ( for harmonic oscillator) 3. Raman Spectroscopy Gross selection rule: change in polarizability  Specific selection rule:   ∆J = 0, +2 or -2   ∆v = +1 or -1  4. Electronic or UV-Visible Spectroscopy Gross selection rule: No stringent rule Specific selection rule: ∆s = 0  and ∆l = +1 or -1 5. Nuclear Magnetic  Resonance or Radio waves Spectroscopy Gross selection rule: nuclear spin should be no zero  Specific se...