Final answer:
The three parameters used to describe an IR band are wavenumber, % transmittance, and the shape of the absorption peak.
Step-by-step explanation:
When analyzing an infrared (IR) spectrum, the three parameters used to describe an IR band are wavenumber, transmittance, and the shape of the absorption peak. The wavenumber, measured in cm⁻¹, defines the position of the absorption band and is inversely proportional to wavelength. It indicates the number of wave cycles in one centimeter. The % transmittance measures how much light is transmitted through the sample at each frequency, with 100% indicating no absorption. Finally, the shape of the absorption peak, which can be broad, sharp, or somewhere in between, helps characterize the type of bond and molecular environment contributing to that particular IR absorption.
Different functional groups in molecules absorb IR radiation at characteristic wavenumbers. For instance, alkynes show IR absorbance peaks in the range of 2100-2250 cm⁻¹, while terminal alkenes can be identified by their absorbance at about 3300 cm⁻¹. Understanding these parameters helps in identifying the molecular structure of the substance being studied.