Inorganic chemists often use IR spectroscopy to evaluate bond order of ligands, and as a means of determining the electronic properties of metal fragments. Students can often be confused over what shifts in IR frequencies imply, and how to properly evaluate the information that IR spectroscopy provides in compound characterization. In this class activity, students are initially introduced to IR stretches using simple spring-mass systems. They are then asked to translate these visible models to molecular systems (NO in particular), and predict and calculate how these stretches change with mass (isotope effects, 14N vs 15N). Students are then asked to identify the IR stretch of a related molecule, N2O, and predict whether the stretch provided is the new N≡N triple bond or a highly shifted N-O single bond stretch. Students are lastly asked to generalize how stretching frequencies and bond orders are related based on their results.
Evaluate the effect of changes in mass on a harmonic oscillator by assembling and observing a simple spring-mass system (Q1 and 2)
Apply these mass-frequency observations to NO and predict IR isotopic shift (14N vs. 15N) (Q3 and 4)
Predict the identity of the diagnostic IR stretches in small inorganic molecules. (Q5, 6, and 7)
Springs, rings, stands, and masses (100 and 200 gram weights for example).
Assemble students into small groups discussions to answer the questions to the activity and collaborate.