a low- or medium-resolution infrared spectrum of a complex molecule and 

 deduce what functional groups and/or bonds the molecule possesses. This type 

 of analysis utilized for interstellar spectra would not yield specific molecules, 

 but would indicate the types of complex molecules present. Any organic mole- 

 cule would be expected to show a C-H stretch at 3.38 to 3.51 jum, an olefinic 

 hydrocarbon could show a C=C stretch at 5.95 to 6.17 jum, an alcohol could 

 show an O-H stretch at 2.75 to 2.79 jum, etc. A special area of investigation that 

 may hold unusual promise is the spectra of complex molecules in the far infrared 

 (over 50 yum or less than 6 THz); far-infrared spectra correspond to low- 

 frequency vibrational transitions, which are often more precise indicators of the 

 specific species than are the higher-frequency vibrations. Whether this specificity 

 would remain under low resolution is unclear at present in large part because of 

 a lack of spectroscopic data in this region of the spectrum. The best-studied 

 far-infrared spectrum is probably that of methanol, in which it arises from a 

 so-called torsional motion. Torsional spectra of many other simple species still 

 remain to be studied in detail in the laboratory. 



It has been suggested that the well-known diffuse bands, seen in the visible 

 and ultraviolet as starlight passes through diffuse interstellar clouds, are due to 

 complex gas-phase organic hydrocarbons, specifically C n species with n > 5. 

 Although it is unclear how complex molecules can be efficiently synthesized 

 under the low-density conditions of diffuse clouds, it is worthwhile to pursue 

 this line of reasoning. The claim is that long-chain C n molecules might be able 

 to withstand photodissociation via the interstellar radiation field that penetrates 

 into diffuse, but not dense, clouds. Instead, the molecules manage to utilize the 

 energy they receive from an absorbed photon to internally convert from one 

 state to another until they re-emit the excess energy and avoid dissociation. The 

 internal conversion process leads to a finite width for the excited states of the 

 molecules, which is supposed to cause the diffuseness of the visible bands. 

 Also, as discussed earlier, a new variant of this claim has been made that links 

 the diffuse bands with polycyclic aromatic hydrocarbons (PAHs) and suggests 

 the width of the features to be due to the unresolved rotational fine structure. 

 These claims should be investigated in some detail because if they are at all valid, 

 they represent a manner of specifically detecting complex species via line width. 

 What must be understood are the precise spectra of these species, their photo- 

 dissociation rates in the diffuse interstellar medium, and the source or sources 

 of the large spectral widths. 



Theoretical models of interstellar chemistry in the future should include 

 significantly more complex molecules than have heretofore been the case. This 

 is especially true of gas-phase models because many ion-molecule laboratories 

 are currently measuring relevant rate coefficients. The semidetailed approach to 

 chemical modeling offers the best opportunity for making predictions of the gas- 

 phase abundances of complex species. 



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