Jawaharlal Nehru Technological University Hyderabad B.Pharmacy IV ADVANCED INSTRUMENTATION TECHNIQUES Previous Question Paper (September-2021 - R17)

Chemical shift is the variation in the resonance frequency of a nucleus relative to a standard (typically TMS) caused by the electronic environment. It is expressed in parts per million (ppm) and is defined by the formula:

The electronic environment is influenced by shielding and deshielding effects:

1. Shielding: High electron density around a nucleus (e.g., in alkanes) shields it from the external magnetic field, shifting the signal upfield to lower ppm values.
2. Deshielding: Electronegative atoms (e.g., O, N, halogens) withdraw electron density, exposing the nucleus to the external field. This shifts the signal downfield to higher ppm values.
3. Anisotropy: Pi-electron systems like benzene rings create local magnetic fields that deshield protons in the plane of the ring, resulting in a significant downfield shift (typically 7.2 ppm).

For example, in ethanol (CH_3CH_2OH), the methyl protons are shielded (~1.2 ppm), the methylene protons are deshielded by the oxygen atom (~3.7 ppm), and the hydroxyl proton is highly deshielded (~4.5 ppm) due to electronegativity and hydrogen bonding.

The fragmentation of ethyl butanoate (CH_3CH_2CH_2COOCH_2CH_3, m/z 116) involves characteristic rearrangements and cleavages:

1. McLafferty Rearrangement (m/z 88): Ethyl butanoate undergoes a six-membered transition state rearrangement involving the γ-hydrogen of the butanoate chain. This results in the loss of ethene (C_2H_4, mass 28) from the butanoate moiety, leaving behind the enol radical cation CH_2=C(OH)OCH_2CH_3 at m/z 88.
2. Double McLafferty Rearrangement (m/z 60): The ion at m/z 88 can undergo a second rearrangement involving the ethyl group on the oxygen, leading to the loss of a second molecule of ethene (mass 28), resulting in the ion CH_2=C(OH)OH at m/z 60.
3. Alpha-Cleavage (m/z 71): Cleavage of the bond between the carbonyl carbon and the ethoxy group (C-O bond) leads to the formation of the propylacylium ion [CH_3CH_2CH_2C=O]+ at m/z 71. This is a highly stable resonance-stabilized cation, explaining its high abundance in the mass spectrum.