Question

The EPR spectrum of a cyclo-tetrasilane radical anion is shown below. a) Explain the observed coupling. (Hint: ¹H: I=1/2, 99% abundant; ¹³C: I=1/2, 1.0% abundant; ¹⁴N: I=1. 99.9% abundant; ²⁹Si: I=1/2, 5.0% abundant; not all the expected lines may be observed.)

b) Based on your explanation of the observed coupling in part a), explain the relative intensities of the lines in this spectrum (Hint: You may need to construct a Pascal's triangle for a nucleus with I>1/2).

Answer 1

a) The main sets of lines in this EPR spectrum is from the nitrogen coupling to the single electron from the radical delocalised on the 4 silicon atoms. This is the nuclei with the abundance of 99.9%.

There are 8 nitrogens

So the number of splitting lines that should be observed is given by the formula

(2 x N x I) +1; where I=1 and N=8 = 17 lines

There is the possibility of hyperfine splitting from the I=1/2 of the 1H also as this is also the nuclei with 99.9% abundance and there are examples in literature for such CYCLIC POLYSILANES showing hyperfine splitting due to the 1H.

b) Pascals triangle for I=1 of 14 N with abundance 99.9%

7 1 : 7 : 27 : 61 : 120 : 196 : 257 : 257 : 196 : 120 : 61 : 27 : 7 : 1

8 1 : 8 : 34 : 88 :181 :316 :453 : 514 :453 : 316 : 181 : 88 : 34 : 8 : 1

A can be seen the most intense peak is 514 with respect to the first which is 1 and so in the figure only 15 lines are clearly visible with the first and last lost in the signal to noise of the spectrum.