Infrared Spectroscopy Exercise
This discussion is being written for instructors. It is intended to provide a starting point for the development of a fully detailed set of student instructions to fit a particular course, available hardware and available software. The exercise has been tested on a computer using a Windows 95 operating system with CAChe 3.0 software.
Draw Lewis structures for NNO, OCO, and OSO. Predict the shape of each molecule using VSEPR theory.
N2O
CO2
SO2
In infrared spectroscopy light quanta are absorbed and converted into vibrational energy. There are only a limited number of fundamental vibrational movements possible for each molecule. These are called normal modes. For a nonlinear molecule with n atoms there are 3n - 6 normal modes. For a linear molecule there are 3n - 5 normal modes. In general, each normal mode absorbs light of a different wavelength. Sometimes there are two normal modes with the same frequency because they involve identical kinds of motions oriented in different directions. Normal modes involving symmetrical motion do not absorb infrared radiation. HCl is linear with n = 2. Thus 3n - 5 = 1. It is not symmetric so we expect to see one infrared absorption peak coming from excitation of its single normal mode. You should be able to explain why N2 does not absorb infrared radiation.
Predict the number of infrared peaks to be observed for NNO, OCO, and OSO. Write out a brief explanation of the basis of your prediction.
Build NNO and optimize the geometry using the AM1 semiempirical method. In order not to bias the final geometry do NOT beautify and do NOT do a preliminary mechanics calculation. Does the geometry agree with your prediction? Calculate the infrared spectrum.
Does the number of peaks agree with your prediction? Go to the Internet to get the experimental infrared spectrum at http://webbook.nist.gov/ and compare it to the calculated spectrum. Spectra are available from NIST for NNO and OCO. The infrared spectrum of sulfur dioxide is not available from NIST.
There are two kinds of vibrational motion, bending and stretching. Predict which requires the most energy. If your software allows, examine the movement of each normal mode to see if your prediction is correct. The amount of information available about normal mode motions varies with different types of software.