In your chemistry courses and labs, you have frequently made use of various \"sp
ID: 502460 • Letter: I
Question
In your chemistry courses and labs, you have frequently made use of various "spectroscopic" tools to identify the molecular structure of unknown compounds. You are probably familiar with the idea that molecules can attain a higher energy state by absorbing light: (no more photon Molecule in Molecu ground state excited state Because different molecules absorb light of different frequencies, one can obtain spectra that show how much light is absorbed by a particular compound as a function of the frequency (or wavelength) of the light. Spectroscopic data like these are sometimes called "molecular fingerprints" because each compound has a unique absorption spectrum by which its identity can be discovered. Spectroscopy is an incredibly powerful tool for investigating the physical structures of objects we cannot see with our normal vision! For example, here's an infrared absorption spectrum for an ester: CHCOCH But what determines the molecular fingerprint? Why is it that some molecules absorb strongly at a particular frequency of light while others do not? Let's try to answer these questions... 1. As we discussed in Dipole Radiation, a dipole that oscillates emits electromagnetic radiation (light). But how does the dipole start oscillating in the first place? 2. The natural frequency of a harmonic oscillator is given by In modeling the molecule as a harmonic oscillator, what changes to the molecule would change the values of k and m, and therefore the natural frequency? Could your reasoning explain why C o and C-O absorb light at different wavelengths?Explanation / Answer
Molecular fingerprint is determined by the bonds or functional groups present in the molecule. Each bond has a different resonance frequency, so each molecule asbsorbs certain frequencies that others do not.
1. Dipole starts oscillating when its frequency is resonant with the frequency of radiation.
2. If a bond becomes stronger, the force constant will increase and that will increase the value of k, increasing the natural frequency. If the mass of atoms participating in the bond is increased, the reduced mass (m) will increase lowering the natural frequency.For C=O, k has higher value than for C-O, so C=O will absorb at higher frequency or lower wavelength.
6. If the frequency of light is equal to natural frequency of the molecule, then it will absorb the light and go to the higher vibrational state. The light will be absorbed. If the frequency is higher or lower then there is no interaction of the molecule and light.
8. The IR spectrometer converts the frequencies into digital information showing lowering of percentage transmittance at wavenumbers (or frequencies) where there is absorption. This gives rise ot the peaks in absorbance or drops in transmittance in the IR spectrum.