SPECTROSCOPY EXAM FULLY SOLVED #11
What is spectroscopy? - correct answer This uses electromagnetic radiation to quantify
the energy differences between possible states of an unknown compound by vibration,
rotations, and other molecular properties. This can help us identify different carbon
backbones or functional groups so that we can identify that unknown molecule.
* You only need a small amount of sample but you need fancy equipment.
Infrared spectroscopy - correct answer This uses IR to measure vibrations between
molecules to see bonds, bond bending, stretching and to be able to identify what
functional groups are present.
What are the absorbance and wavenumber of IR spectroscopy and what is the
significance of these ranges? - correct answer The IR is typically in wavelegnths of 750
nm - 1 mm.
The absorbances at from 25,000 to 2,500 nm and the wavenumbers 4000 cm-1 to 400
cm-1. Within this range, the molecule will be put into vibration (either symmetrical bend
or stretch or asymmetrical bend or stretch).
In order to be able to tell what vibration is happening, there has to be a change in the
dipole so a symmetrical stretch for example won't show you anything because it's not
changing the dipole.
Wavenumber = - correct answer 1/wavelength
What are wavenumbers (cm-1) analogous to? - correct answer Frequency - remember f
= c/wavelength.
What is the IR fingerprint region? - correct answer 1,500 to 400 cm-1 causes really
specific and complex vibrations and so experts can tell exactly what is there at those
ranges.
What are the three wavenumbers you need to be able to identify on the IR spectra for
test day? - correct answer OH at 3300 cm-1 (broad peak).
NH at 3000 cm-1 (sharp peak).
COOH at 1700 cm-1 (sharp peak).
In IR spectroscopy, what is the effect of increased bonds to the carbon (e.g. OH
compared to COOH)? - correct answer You'll get increased frequency with increase
bonds to the carbon.
