HACA's Heritage: A Free-Radical Pathway to Phenanthrene in Circumstellar Envelopes of Asymptotic Giant Branch Stars.
The hydrogen-abstraction/acetylene-addition (HACA) mechanism has been central for the last decades in attempting to rationalize the formation of polycyclic aromatic hydrocarbons (PAHs) as detected in carbonaceous meteorites such as in Murchison. Nevertheless, the basic reaction mechanisms leading to the formation of even the simplest tricyclic PAHs like anthracene and phenanthrene are still elusive. Here, by exploring the previously unknown chemistry of the ortho-biphenylyl radical with acetylene, we deliver compelling evidence on the efficient synthesis of phenanthrene in carbon-rich circumstellar environments. However, the lack of formation of the anthracene isomer implies that HACA alone cannot be responsible for the formation of PAHs in extreme environments. Considering the overall picture, alternative pathways such as vinylacetylene-mediated reactions are required to play a crucial role in the synthesis of complex PAHs in circumstellar envelopes of dying carbon-rich stars.
Department of Chemistry, University of Hawai'i at Manoa, Honolulu, HI, 96822, USA.
Tao
0000-0003-4101-2385
T
Yang
Department of Chemistry, University of Hawai'i at Manoa, Honolulu, HI, 96822, USA.
Ralf I
0000-0002-7233-7206
RI
Kaiser
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA.
Tyler P
TP
Troy
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA.
Bo
B
Xu
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA.
Oleg
0000-0003-2068-4991
O
Kostko
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA.
Musahid
0000-0003-1216-673X
M
Ahmed
Department of Chemistry and Biochemistry, Florida International University, Miami, FL, 33199, USA.
Alexander M
AM
Mebel
Department of Physics, Samara National Research University, Samara, 443086, Russian Federation.
Marsel V
MV
Zagidullin
Department of Physics, Samara National Research University, Samara, 443086, Russian Federation.
Valeriy N
VN
Azyazov
circumstellar envelopes; gas phase chemistry; hydrogen-abstraction/acetylene-addition (HACA); mass spectrometry; polycyclic aromatic hydrocarbons