Question

4. A mol lecule A2B2 has i Wavenumbers nfrared absorption and Raman al lines as in the followin Stron Very stron Weak Infra-Red Raman 3374 3287 1973 729 612 Very strong, PR contour Very stron R contour Deduce what you can about the structure of the molecule and assign the observed vibrations to particular modes whenever possible. Use the data below to help with your answers Characteristic stretching frequencies Approx frequency in cm Group Approx frequency Group in cm 2900 3030 3300 2580 3600 Alkane C-H Alkene-C-H Alkane C-C Alkene C-C Alkyne C C c-s C-O 1100 1660 2220 1100 1650 Alkyne S-H O-H C-H

Answer 1

Raman spectroscopy depends on a change in polarizability of a molecule, whereas IR spectroscopy depends on a change in the dipole moment. Symmetric stretch/bend changes the polarisibilty and hence is Raman active while asymmetric stretching/ bending is IR active. The peak at 3373 is due to alkyne C-H symmetric stretching. 3287 is assymetric stretching. The rest peaks are assigned and the molecule can be deduced from the vibrational spectra to be acetylene

337 Raman 2 329 I R Ariintue bend 下