Candidate Prediction For UIR Carriers In Warm Interstellar Clouds

Interstellar molecules?ISM?exist in the interstellar space as the forms of inorganic molecules and organic molecules.There are about 170 kinds of interstellar molecules being identified at present.Many complex organic molecules' transition lines have been observed in the warm and dense interstellar clouds.A lot of Unidentified lines need to be confirmed which is called U-line.Based on the First principles calculation and Density function theory,we performed a prediction and investigation of U-line candidates' structure,bonding character and infrared spectrum.Based on the sp² hybridized carbon clusters,we constructed carbon fragments whose carbon atoms range from 10 to 24.Considering the fact that the hydrogen is abundant in the cosmic environment,a proportion of ISM contain hydrogen atoms.Therefore,based on the carbon fragments calculated as carbon skeleton,hydrogen atoms are added on the molecules.As a result,the structures turn into PAHs and PAH-like molecules.Calculate their infrared spectrum at the same level.Compared the calculated infrared spectrum with the observed spectrum,the conclusions are as follows:1)By incising and optimizing the structures,we obtained stable and metastable state carbon fragments whose carbon atoms rang from 10 to 24.Among the carbon fragments,there are many molecules present peculiar and funny structures.C₁₀-1 molecule with the C2 v symmetry resembles a table-tennis paddle.In addition,the carbon fragment C₁₂-1 with the D2 h symmetry has a rectangle made up of four carbon atoms in the middle connecting two benzene rings locating on both side of the CF.By comparing with the observed infrared spectrum,the calculated line of the three CFs: C₁₀-1,C₁₅-4,C₁₉-4 own more than one UIR features.In the combined IR spectrum,there are four features that consistent with the UIR features: 7.7?m,11.2?m,12.0?m,18.9?m.Furthermore,when extract the combination wavelength ranging from 10 to 15?m to compare with the IRAS18317 IR spectrum,we find that there is an obviouscommon character in the two graphs: in 11.3?m and 12.91?m,there are two peaks;the main peak values in combination IR spectrum get close to the IRAS18317 IR spectrum.Therefore,the three Carbon Fragments: C₁₀-1,C₁₅-4,C₁₉-4 as UIR candidates provide a possibility for IRAS18317.2)By using the similar procedure,the PAH-like molecules infrared frequencies are calculated at the B3LYP/6-311+G?g,d?level and carbon fragments whose carbon atoms rang from 10-36 are constructed.Combine the possible candidates: C₂₄H₁₈,C₂₅H₁₇,C₂₈H₂₂,C₃₃H₂₃,and C₃₆H₃₀ to one IR spectrum to search for the UIR features.Combine the possible candidates: C₂₄H₁₈,C₂₅H₁₇,C₂₈H₂₂,C₃₃H₂₃,and C₃₆H₃₀ to one IR spectrum to search for the UIR features.In the combined IR spectrum,there are six features that are consistent with the UIR features: 3.3?m,6.2?m,7.7?m,8.6?m,11.0?m,13.5?m.They are generated by different candidates and different bonds vibrating modes.Briefly speaking,PAH-like molecules provide a possibility for the UIR carries.