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In this study, we explore computationally the Methylidyne Addition-Cyclization-Aromatization (MACA) mechanism as a viable, barrierless pathway for phenalene synthesis under low-temperature conditions. Through electronic structure calculations and Rice\u2013Ramsperger\u2013Kassel\u2013Marcus (RRKM) statistical methods, we demonstrate that the reaction of 1-vinylnaphthalene (C10H7C2H3) with the methylidyne radical (CH) leads to the formation of 1H-phenalene via a bimolecular reaction, a process that is exoergic and without entrance barrier. The MACA mechanism facilitates the growth of the aromatic carbon backbone via a [5 + 1] ring annulation, providing a new insight into PAH formation in cold molecular clouds. Notably, the MACA mechanism has previously been shown to form indene (C9H8), which was detected in TMC-1 as well, via a [4 + 1] annulation, demonstrating its potential to produce a variety of complex PAHs by addition of a five- and six-membered ring to a benzene moiety via [4 + 1] and [5 + 1] annulation, respectively. This work highlights the importance of barrierless, exoergic reactions involving MACA in the synthesis of complex aromatic molecules in space, expanding our physicochemical understanding of carbon-rich chemistry in cold molecular clouds.<\/jats:p>","DOI":"10.1021\/acs.jpca.6c03215","type":"journal-article","created":{"date-parts":[[2026,8,4]],"date-time":"2026-08-04T13:15:49Z","timestamp":1785849349000},"page":"6422-6429","source":"Crossref","is-referenced-by-count":0,"title":["Formation of 1\n                    <i>H<\/i>\n                    -Phenalene\n(C13H10) in the Taurus Molecular Cloud via Methylidyne\nAddition-Cyclization-Aromatization (MACA)"],"prefix":"10.1021","volume":"130","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-7233-3133","authenticated-orcid":true,"given":"Alexander M.","family":"Mebel","sequence":"first","affiliation":[{"name":"Florida International University , , , ,","place":["Miami, Florida, United States, 33199"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7233-7206","authenticated-orcid":true,"given":"Ralf I.","family":"Kaiser","sequence":"additional","affiliation":[{"name":"University of Hawai\u2032i at Ma\u0305noa , , , ,","place":["Honolulu, Hawaii, United States, 96822"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"316","published-online":{"date-parts":[[2026,7,29]]},"reference":[{"key":"2026081308270789500_cit1","doi-asserted-by":"publisher","DOI":"10.3847\/2041-8213\/abfd3a","article-title":"Discovery of the\npure polycyclic aromatic hydrocarbon indene (c-C9H8) with GOTHAM observations of TMC-1","volume":"913","author":"Burkhardt","year":"2021","journal-title":"Astrophys. 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