What is IR? What does it measure? Which bonds are typically IR active? Which are

Question

What is IR? What does it measure? Which bonds are typically IR active? Which are not? Which absorb strongly? Which are weak? What does it tell you about a compound? Can it be used to conclusively identify a structure? What does an IR spectrum look like? Where is the baseline? What is an absorbance? What are the typical wavelengths? Wavenumbers? Where is the fingerprint region? What does "conjugation" mean? How does this affect an IR frequency? Why? Which molecules form hydrogen bonds? How does hydrogen bonding affect IR frequency? Is water a good solvent for a sample to be analyzed by IR? Why or why not? You will be provided with a correlation table to assist in interpretation of IR spectra. You should be familiar with the correlation table, and which types of bonds typically absorb at which wavenumbers.

Explanation / Answer

The wavelengths covered are typically between 2.5 to 50 m which correspond to wavenumbers of 4000 to 200 cm-1. The region below 1500cm-1 is called the finger print region as it contains a lot of complex absorptions corresponding to bending vibrations and overtones and can be used as a fingerprint for a given compound.

5. A conjugated system in organic chemistry consists of alternating single and double bonds such that the pi-electrons are delocalized in a molecule (eg. C=C-C=C-C=C). As a result of conjugation the C=C bond in the given example would be weaker than an isolated C=C bond since the electrons of the double bond are delocalised in the former case. Thus the C=C bond of a conjugated system will absorb at a lower frequency that an isolated double bond.

6. Molecules which have hydrogen atom bonded to a highly electronegative atom like oxygen, nitrogen and fluorine will form hydrogen bonds. Presence of H-bonding causes a shift toward lower frequency of all functional groups that are involved in hydrogen bonding and also the peaks are generally broadened.

7. Water should not be used as a solvent for IR spectroscopy. This is because water absorbs very strongly in the IR region and hence will overshadow the peaks of the sample. Another problem is that the sample holders for IR spectroscopy are made of KBr or NaCl and water tends to react with them.

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