Nitrogen Substituted PAHs - Heterocyclic Aromatic Amines.

Produced coincidentally during the same nuleosynthetic cycle late in the stellar lifecycle, the elements carbon, oxygen, and nitrogen are present in similar (i.e. same order of magnitude) abundance in the circumstellar outflows which characterize the PAH formation zones. In carbon-rich objects (those with C/O > 1), the oxygen is effectively incorporated into the carbon monoxide, CO, that forms very early on in the process. It is the carbon that is left over after CO formation that serves as the feedstock for the formation of PAHs and other carbon-rich materials. The fate of the nitrogen in this process is less clear. Nitrogen-dominated compounds (e.g. NH3, N2H2, NOx, etc.) are not abundant and therefore cannot account for the lion's share nitrogen thrown off by the dying star. While it has also been established the missing nitrogen cannot be found in the form of nitrogen-containing functional groups ( e.g. -CN, -NH2, etc.) attached to a PAH core [Infrared Spectra of Substituted Polycyclic Aromatic Hydrocarbons, S. R. Langhoff, C. W. Bauschlicher, Jr., D. M. Hudgins,S. A. Sandford, and L. J. Allamandola, J.Phys.Chem.A, 102, 1632 (1998)], the contribution from PAHs which incorporate one or more N atoms into their carbon skeleton has yet to be thoroughly investigated. Some examples of these so-called aza-PAHs, properly known as heterocyclic aromatic amines, can be seen in the table below.

In principle, polycyclic aromatic compounds which incorporate nitrogen in their skeletal structures are attractive candidates from a chemical standpoint because substitution of a N atom for a CH group in an aromatic ring can be accomplished without the loss of aromaticity in that ring and with only a modest loss in stability. That notwithstanding, information regarding the infrared spectroscopic properties of aza-PAHs is required to critically assess what role - if any - this class of aromatic compound may play in the astrophysical problem. The aza-PAHs which are the subjects of our study are listed in the table below.

 The Inventory of Aza-PAHs Currently Under Study in the Astrochemistry Laboratory.

Compound
Formula
Structure
1) Quinoline
C9H7N
2) Isoquinoline
  
3) Acridine
C13H9N
4) 2-azapyrene
C15H9N
5) 1-azabenz[a]anthracene
C17H11N
6) 2-azabenz[a]anthracene
  
7) 1-azachrysene
C17H11N
8) 2-azachrysene
  
9) 4-azachrysene
  
10) Dibenz[a,h]acridine
C21H13N
11) Dibenz[a,j]acridine
  

Samples 1 - 3 were purchased from Sigma-Aldrich

Samples 4 - 11 were purchased from the NCI Chemical Carcinogen Reference Standard Repository.

Updated January 2001.