Doubly ¹⁵N-substituted diazenylium: THz laboratory spectra and fractionation models

¹ Dipartimento di Chimica “Giacomo Ciamician”Università di Bologna, via F. Selmi 2, 40126 Bologna, Italy
e-mail: luca.dore@unibo.it
² Centre for Astrochemical Studies, Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85749 Garching bei München, Germany
e-mail: bizzocchi@mpe.mpg.de
³ Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale del Risorgimento 4, 40136 Bologna, Italy
⁴ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: eva.wirstrom@chalmers.se
⁵ Astrochemistry Laboratory and The Goddard Center for Astrobiology, NASA Goddard Space Flight Center, Greenbelt, MD 20770, USA

Received: 16 September 2016
Accepted: 28 April 2017

Abstract

Context. Isotopic fractionation in dense molecular cores has been suggested as a possible origin of large ¹⁴N/¹⁵N ratio variations in solar system materials. While chemical models can explain some observed variations with different fractionation patterns for molecules with –NH or –CN functional groups, they fail to reproduce the observed ratios in diazenylium (N₂H⁺).

Aims. Observations of doubly ¹⁵N-substituted species could provide important constraints and insights for theoretical chemical models of isotopic fractionation. However, spectroscopic data are very scarce.

Methods. The rotational spectra of the fully ¹⁵N-substituted isopologues of the diazenylium ion, ¹⁵N₂H⁺ and ¹⁵N₂D⁺, have been investigated in the laboratory well into the THz region by using a source-modulation microwave spectrometer equipped with a negative glow discharge cell. An extended chemical reaction network has been used to estimate what ranges of ¹⁵N fractionation in doubly ¹⁵N-substituted species could be expected in the interstellar medium (ISM).

Results. For each isotopologue of the H- and D-containing pair, nine rotational transitions were accurately measured in the frequency region 88 GHz–1.2 THz. The analysis of the spectrum provided very precise rest frequencies at millimeter and sub-millimeter wavelengths, useful for the radioastronomical identification of the rotational lines of ¹⁵N₂H⁺ and ¹⁵N₂D⁺ in the ISM.