The correct value of isomer shift (in Mossbauer spectra) and its explanation for Fe(II)-TPP and Fe(III)-TPP respectively from the following are: (TPP= tetraphenylporphyrinate)
\[ \begin{array}{l} \text{(A) } 0.52\ \mathrm{mm\,s^{-1}} \\ \text{(B) } 0.45\ \mathrm{mm\,s^{-1}} \\ \text{(C) Increase in s electron density} \\ \text{(D) Decrease in s electron density.} \end{array} \]
(a)(A) and (D); (B) and (C)
(b)(A) and (C); (B) and (C)
(c)(B) and (D); (A) and (D)
(d)(B) and (D); (A) and (C)
Answer
Answer: A ✓ checked by 4AB · confidence medium
The source book printed B; on checking, A is correct — see the explanation.
Explanation
Fe(II)-TPP has the larger isomer shift (0.52 mm s⁻¹): the extra d electron shields the s electrons and lowers s-electron density at the nucleus. Fe(III)-TPP has the higher s density, so the smaller shift (0.45) (a).