And how much does it increase after IR absorption? What is the change in bond length at a natural frequency of vibration and at a given IR energy, for the C=O bond for example?
Your questions seems to involve a classical viewinstead of a quantum mechanical one. A molecule has a ground state and first excited vibration state. In the gorund state only the zero point vibration is present. The expected value of the interatomic distance can be calculated for the ground state and for the fist excited state function.
Hello! Here (http://www.nist.gov/data/PDFfiles/jpcrd4.pdf) you can find the spectral atlas of CO. On the page 18 (164 printed) you find the potential energy curve of the CO molecule. The lowest lying curve (X1Sigma+) is the ground electronic state CO. From the curve you can see, that the middle of the zero vibrational level lies at approximately 1.13 Angstrom. This is the average separation of the nuclei in the ground electronic and vibration state. If you want to know also how much this distance changes during the vibration (the semi-classical approximation), just make an vertical line through the 0 energy and measure the points where it crosses the potential energy curve. This points are in the semi-classical approximation the limits of the distance between the C and O nuclei, but the nuclei will be most probably found right in the middle (this is valid only for v=0 state). If you want the same for higher energies, let's say for v=1 state, find appropriate energy of v=1 vibration state in the text and make the vertical line at this energy. Now it is bit different than the ground state, because for higher vibrational states the nuclei are most probably found at the points, where the vertical line (v=1 energy) crosses the potential energy curve and not in the middle as in the case of v=0. My visual guess for v=1 is that the nuclei change their separation from 1.05 Angstrom - 1.22 Angstrom.
It's possible also that X-ray diffraction data on your compound can give you some clues : indeed single X-ray diffraction experiments give you the position of the atoms but also the thermal motions of the atoms (given by the anisotropic thermal ellipsoids)
perhaps this can help
http://www.christian.naether.uni-kiel.de/pdf/Course_Mexiko_4_Lecture.pdf
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