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1.19 PRINe orntion in the infrared region occurs due to excitation of molecule from the lower to the higher l level. Since each vibrational level 1s aSsociated With a number of closely spaced rotational levels, IR spectra may be regarded as vibrational rotational spectra. at the atams in a molecule are not ridiigy helld but keen on rihratina to and fro ahamt a