Quantification of molecular aromaticity as a predictive factor of astrophysical significance

¹ Facultad de Ingenieria y Arquitectura, Universidad Central de Chile, Santa Isabel 1186, Santiago, Chile
e-mail: luis.alvarez@ucentral.cl
² NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA
³ Department of Chemistry & Biochemistry, University of Mississippi, MS 38677-1848, USA
⁴ Universidad Autónoma de Chile, Facultad de Ingenieria, Núcleo de Astroquímica & Astrofísica, Av. Pedro de Valdivia 425, Providencia, Chile
e-mail: natalia.inostroza@uautonoma.cl

Received: 2 February 2022
Accepted: 21 February 2022

Abstract

Context. This study reports the index of aromaticity calculated by numerical integration of the magnetically-induced current density for cyclic hydrocarbon molecules both known to exist in astrophysical media as well as those proposed to exist.

Aims. This study promotes the ring current strength (RCS) value for quantifying aromaticity as a means of predicting astrophysical detectability.

Methods. Density functional theory (DFT) calculations at the B3LYP/aug-cc-pVTZ level provide optimized structures and the wave-functions needed to provide the RCS values for the molecules analyzed.

Results. The known interstellar molecules examined c-C₃H₂, c-(O)C₃H₂, c-C₃HC₂H, o-benzyne, benzonitrile, 1-cyano and 2-cyanonaphthalene all have RCS values of 9.9 nA T⁻¹ (nanoampere per Tesla) or above. The known antiaromatic species have RCS values of less than 0.0 nA T⁻¹ as expected. Several proposed interstellar molecules likely will not persist if they form due to low RCS values including c-(C)C₃H₂. Other species such as p-benzyne and c-HCNN⁺ have high RCS values of 19.9 nAT⁻¹ and 14.4nAT⁻¹, respectively.

Conclusions. Cyclic hydrocarbons previously observed in astrophysical media have high RCS values. Those with low or negative RCS values have yet to be observed implying that such a metric can indicate astrophysical significance.