Final answer:
IR spectroscopy identifies molecular structures by observing characteristic vibrations of atoms within molecules as they absorb specific frequencies of IR radiation. It reveals the presence of functional groups and monitors chemical reactions, though a complete molecular structure often requires additional analytical methods.
Step-by-step explanation:
IR (infrared) spectroscopy is a powerful analytical technique used to determine the molecular structure of a compound by observing the vibration and rotation of its atoms. Infrared spectroscopy works on the principle that molecules absorb frequencies that are characteristic of their structure. When IR radiation is passed through a sample, certain frequencies are absorbed by the sample, and these absorptions are measured as a spectrum. Each type of bond within a molecule will absorb IR at a particular range of frequencies, known as absorption bands. For instance, a carbonyl group usually shows absorption in the range of 5.11 - 5.18 x 10¹³ Hz. If you were to monitor the reaction converting cyclohexanone to cyclohexanol, IR spectroscopy could indicate whether the ketone was successfully transformed into an alcohol by the presence or absence of these characteristic absorption bands in the IR spectrum. Therefore, IR spectroscopy is used not only to identify functional groups in a molecule but can also be used to monitor the progress of chemical reactions. While IR spectroscopy alone cannot always determine the complete structure of a molecule, it is invaluable when used in conjunction with other analytical methods, such as NMR or mass spectrometry, for determining the presence of specific functional groups and thus deducing structural information.