Reasons broad unresolved bands

This section consists of a compilation of references pertaining to He(Ia) and He(IIa) gas-phase PE spectra of saturated, closed-shell hydrocarbons C H (Table 1). (With few exceptions, such PE spectra consist of broad, strongly overlapping, and usually unresolved bands. For this reason, only a few PE spectra will be shown explicitly in Section V.)... 

Al ough rotational frequencies of the whole molecule are not infrared active, they often couple with the stretching and bending vibrations in the molecule to give additional fine structure to these absorptions, thus further complicating the spectrum. One of the reasons a band is broad rather than sharp in the infrared spectrum is rotational coupling, which may lead to a considerable amount of unresolved fine structure. 

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