Consider three possible conformations for the H2O2 molecule: planar trans, plana
ID: 494750 • Letter: C
Question
Consider three possible conformations for the H2O2 molecule: planar trans, planar cis, and nonplanar gauche, with the H-O-O bond angle about 95o in each case.
(a) How many vibrational normal modes does H2O2 have?
(b) Determine the point group of each structure.
(c) For each structure, determine the symmetries of each of the normal modes and whether each mode is infrared or Raman allowed.
(d) The experimental infrared spectrum of H2O2 shows fundamental transitions at 870, 1370, 2869, and 3417 cm-1. The Raman spectrum shows fundamental transitions at 877, 1408, 1435, and 3407 cm-1. Assume that the 870 and 877 cm-1 bands arise from the same normal mode and that the 3417 and 3407 cm-1 bands also arise from the same normal mode. Which of the three structures is/are consistent with the experimental data?
Explanation / Answer
ANS a) Linear molecules has 3N-5 normal modes and non linear has 3N-6 normal modes , where N= no. of atoms present in a molecule.
H2O2 is a non linear molecule hence it a bent so we will use 3N-6
N = 4
using the formula -
3(4) -6 = 12 - 6 = 6
there are 6 normal modes.
b) there will be C2v in cic structure and C2h in trans the case of hydrogen perocide it belongd to C2 in gauche .
c) The infrared absorption spectrum of gaseous H2O2 has bands at 870 cm1 , 1370 cm1 , 2869 cm1 , and 3417 cm1 . The Raman spectrum of of the same sample has bands at 877 cm1 , 1408 cm1 , 1435 cm1 , and 3407 cm1 .
(i) Trans structure has centre inversion and no vibration mode so it can be said that it can be infrared or Raman active both .
(ii) gauche has all the nodes IR as well as Raman active
(iii) Cishas 5 modes which is IR active and allother are Raman active.