Major infrared active bands

Normal vibrations related to a change in dipole moment are infrared active. Groups with large dipole moments, such as C=0 and N—H, typically have strong infrared absorptions. The majority of reported spectroscopic studies by infrared spectroscopy focus on determination of polymer molecular composition by analysis of characteristic vibrations of fiinctional groups. The power of vibrational spectroscopy, ie its selectivity and sensitivity, cannot be overestimated. With accurately defined band assignment, particularly if the transition dipoles are well established, quantitative analysis of sample anisotropy in terms of segmental orientation can be accnrately established. 

Structure of 4 symmetry. The compound (CF2)4Fe(CO)4 was originally said (130) to have C—O bands at 2160, 2120, and 2100 cm but more recently four bands at 2160, 2108,2088, and 2055 cm have been reported (112). Four bands is the number expected for an octahedral structure (C2 ) in which the CF2 groups bridge adjacent octahedral coordination positions of the iron atom. The bis(perfluoroalkyl)irontetracarbonyls, (Ri )2Fe(CO)4, show several major carbonyl stretches. Four infrared active C—O stretches would be expected for a cis isomer and one for the trans isomer. Evidently, the cis isomers are present, and the presence of the trans isomer also cannot be ruled out 155). A similar conclusion can be made about the isomeric character of the perfluoroalkyliron tetracarbonyl iodides 155). 

CFCs, which have a major role in stratospheric ozone destruction (Section 4.6.4), are also of concern because they are radiatively active in portions of the infrared spectrum that are not already strongly attenuated by water vapor, CO2, CH4, or N2O. The radiative efficiency of a mole of CFCs is about 10,000 times as much as a mole of CO2 CO2 has a radiative efficiency of approximately 1.4 x 10 W/(m ppb(v)), whereas the radiative efficiencies of CFCs are on the order of 0.3 W/(m ppb(v)) (IPCC, 2013). CFCs also have long atmospheric residence times, ranging from 45 years for CFC-11 to 1700 years for CFC-115 (CCIF2CF3) (IPCC, 2007). The locations of some CFC absorbance bands are shown in Fig. 4.48. 

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