What do the terms 'stiffer' and 'soften' in infrared spectroscopy mean? For example, do they make the frequency larger or smaller?
Stiffer means higher force constant of the oscillator means higher frequency: http://en.wikipedia.org/wiki/Harmonic_oscillator
Any molecule has different vibrational modes with lower frequency (soft modes) and higher frequency (stiff modes)
If it refers to bonds in molecules, above posts seem to have given the answer to your question. However, if it refers to Infra-red spectroscopy, I would like to know in what context and where exactly you encounter these words in relation to IR spectroscopy. It will be appropriate if can write the statement as such.
Addendum: Soft mode word is commonly used in relation to solids exhibiting structural phase transition and frequency of such a mode decreases as the solid approaches the transition temperature and becomes nearly zero at transition point or close to it. I wonder if you came across the use of soft and stiffer modes in this context. Note that the frequencies of such modes fall in far infra-red spectrum.
This is analogous to stiffness in springs; a stiffer spring tends to oscillate at a higher frequency.
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