BILASPUR SEMINAR TOPIC SHIELDING MECHANISM CHEMICAL SHIFT AND

BILASPUR SEMINAR TOPIC – SHIELDING MECHANISM, CHEMICAL SHIFT AND SPIN –SPIN INTERATION SUBMITTED BY Dr. Meena Kshetrapal

SHIFTSHIELDING MECHANISM AND CHEMICAL SHIELDING EFFECT-from figure , In any molecule circular motion of about proton , so electrons are Spinning condition. Then its generate secondary field known as induced magnetic field When we kept them in external magnetic field then if IMF If oppoe the external magnetic field. Thus field felt by proton is decrease said to shielded proton. This mechanism are called ‘’shielding mechanism’’. In this condition high electron density on proton.

DESHIELDING EFFECT-If IMF is align the externaal magnetic field the field felt by proton increase and a lower magnetic field I applied in order to ring the nucleous resonance. the proton said to “deshielded proton”. In this condition low electron density on proton. We explain the shielding and deshielding effect through following example if apply the H 0 IS 14092 guess then molecule absorbs 60 Mz of radio frequency . due to IMF proton felt 14091. 7 guess H 0, then effect as called Shielding effect. Or due to IMF proton felt 14071. 7 guess and 0. 3 guess magnetic field, thus effect as called deshielding effect.

CHEMICAL SHIFT • Such shift(compared with a standard referrence) in the position of NMR absorption which arise due to the shielding and deshielding due of proton by the electrons are called ‘’chemical shift’’. • No. of signal and position of signal is very important role in NMR. • Position of protons help us to understand the nature of the proton in compound. • Different type of proton are present in the compound and they have different electronic environment , due to variation of electronic environment proton get absorbs at different applied magnetic field. • We are use spectra paper for representation of position of PMR signal. • Unit of chemical shift is PPM. • Arise PMR signal of shielding proton on upfield and deshielding proton on downfield.

• Chemical shift is denote by δ or Ʈ value. Where δvalue is between 0 -10δ • Ʈ value is just opposite from δ value. Ʈvalue is between 10 -0Ʈ. • So Ʈ = 10 -δ or δ = 10 -Ʈ • For measuring chemical shift of various proton in a molecule , the signal for ‘’Tetra methyl silane (TMS)’’ is taken as a reference. Reference compound : - TMS (tetra methyl silane) This difference in the absorption position of the proton with respect to. TMS signal is called “Chemical shift”. 1. 2. 3. 4. TMS Is the most convenient reference and has the following characteristics. It is miscible with almost all organic substances. It is highly volatile and it readily removed from the system. It does not take in intermolecular association with the sample. It contain 12 equivalent proton and highly shielded and δ value 0.

ϑSAMPLE- resonance frequency of sample ϑTMS - resonance frequency of TMS Factor affecting chemical shift – (1)INDUCTIVE EFFECT OR ELECTRONEGATIVITY- When proton is attach with any electronegative group/atom , then proton will be deshielded. CH 3 -F CH 3 -Cl CH 3 -Br CH 3 -I CH 4 δ Value 4. 26δ 3. 05δ 2. 68δ 2. 16δ 0. 23δ O-CH 3 N-CH 3 C-CH 3 Si-CH 3 δValue 3. 3δ 2. 2δ 0. 89δ 0δ CH-Cl 3 CH 2 -Cl 2 CH 3 -Cl CH 4 δValue 7. 24δ 5. 28δ 3. 00δ 0. 28δ

(2)STERIC EFFECT: - This effect understand by following example Electron cloud of bulky group will repell the electron cloud of proton due to electrostatic repulsion. so proton has deshielded. (3) H-BONDING: - Hydrogen atom goes for H-bonding with two electronegative atom and they will take electron cloud from proton and it will deshielded. H -O O- H H H (4) HYBRIDIZATION OF C ATTACHED WITH H ATOM: - proportion of s-character is increasing then bonding e- more close to c-atom and e- are away from h-atom , so proton are deshielded. Sp – 1: 1 (50%) high δ value - 1: 2(33%) - 1: 3(25%) low δ value (5)ANISOTROPY EFFECT: - diamagnetic anisotropy effect is that property who stay that shielding and deshielding of proton is depend on the orientationwith applied magnetic field. Foe example ethene: - here dimagnetic anisotropy due to π-e- is generate induced magnetic field. IMF is oppose to H 0 (external m. f. ) so surround field of C is diamagnetic, thus field shielded. While proton of alkene is deshielded because IMF is align to H 0.

SPIN – SPIN INTERACTION it is found that in certain molecule, a single peak is not observe , a multiplet (group of peaks) is observe. • When spin of two protons are coupled , then it is known as “spin-spin interaction”. • For spin-spin interaction minimum two sets of proton are required. • Due to spin-spin interaction between neibourhood proton , splitting of singlet will be observe. • Calculation of peak intensity by Pascal triangles rule , its called as (n+1) rule. where n = no. of neibourhood proton • Those spectra who followed (n+1) rule, as called “first order spectra”. Pascal triangle No. of n No. of peak structure intensity 1 1 1 2 doublet 1: 1 1 2 3 triplet 1: 2: 1 1 3 3 1 3 4 quartet 1: 3: 3: 1 1 4 5 4 1 4 5 quintet 1: 4: 6: 4: 1 1 5 10 10 5 1 5 6 sextet 1: 5: 10: 5: 1

• In spectra paper , diffence between o singal of TMSand centre of multiplet , as called “chemical shift”. • Due to neighourhood proton , singlet is split into multiplet. Then difference between multiplet , called as “coupling costant J”. example – H 3 C - CH 2 – Cl - 2 NMR singnal obtain a b For proton a = n+1 = 2+1 = 3 – triplet For proton b = n+1 = 3+1 = 4 - quartet peak of proton b is occurs downfield because it attach Cl group and peak of proton a is occurs upfield. • For first order spectra or Δϑ=10 J Where Δϑ=difference between two chemical shift Of two neighbourhood proton. EX((2) 2 -BUTANONE CH 3 -CH 2 -CO-CH 3 a b c for proton a=2+1 =3 triplet(upfield) for proton b=3+1 = 4 quartet(downfield) for proton c=singlet (downfield) If no. of proton is absent in the adjacent carbon then set of proton will give singlet.