Possible interstellar formation of phosphorus analogue of hydrazoic acid: A computational study on the reaction between HN and PN Priya Bhasi, Zanele P. Nhlabatsi, Sanyasi Sitha ⇑ Department of Chemistry, University of Johannesburg, PO Box 524, Auckland Park, Johannesburg 2006, South Africa article info Article history: Received 6 November 2015 Received in revised form 9 December 2015 Accepted 9 December 2015 Available online 17 December 2015 Keywords: Interstellar ISM PN NH HPNN Hydrazoic acid abstract Using computational calculations we have shown the reaction between the NH and NP. We have analysed both the singlet and triplet potential energy surfaces for the reaction and found that in both the surfaces, the reaction is exothermic in nature. Based on this, we suggest that the species like HNNP, HNPN and HPNN (Phosphorous analogues of the Hydrazoic acid) can possibly be detected in the interstellar med- ium. We hope these molecules may be able to act as precursors for many phosphorus containing prebiotic molecules. We have also found that many isomers have the signature NAN linkage and suggest that they can be viewed as potential candidates as reservoir for the N 2 in interstellar medium. Also, we have shown how unlike the N 2 which has very large dissociation energy and zero dipole moment; its analogue NP shows some interesting chemistry owing to its comparatively low dissociation energy and small dipole moment. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Till date almost 70 species have been detected in ISM, which contains one or more nitrogen atom(s) in their skeletons [1]. This clearly indicates that nitrogen bearing molecules are very much crucial to understand the active chemistry exhibited in the ISM [2–11]. The molecular form of the nitrogen, i.e., N 2 molecule poses many difficulties in terms of its detection as well as estimation of its abundances in the ISM and this is because, N 2 is a homonuclear diatomic molecule with zero dipole moment [8,9,12–18]. Other characteristic of N 2 is the strong triple bond (bond dissociation energy of approximately 225.96 kcal/mol [19]) a hindrance to its active chemistry. In the ISM one interesting molecule detected is PN (Phosphorus mononitride) [20–22], (which is also the first phosphorus containing species detected in ISM) which is analogous to N 2 (as phosphorus belongs to the same nitrogen group) and thus a similar chemical behaviour like that of N 2 can also be expected for the molecule PN. Interestingly the bond dissociation energy for the molecule PN is approximately 174.6 kcal/mol, which is around 50 kcal/mol lower compared to N 2 [23] and also has a non- zero dipole moment of approximately 2.5 Debye [24]. Reduction in the dissociation energy compared to the counterpart homonuclear N 2 and a non-zero dipole moment are the main factors with which PN can be expected to show some interesting chemistry in the ISM. Thus, one can say that some interesting reactions of PN with other molecules or fragments available in the interstellar medium can be expected. Till date studies related to interstellar reactions of PN are very few [25,26] and this is despite the fact that it has been detected in the interstellar medium long before [20–22]. Similarly, the other important fragment detected in the interstellar medium is HN [27,28]. So, in this work we have carried out a computational study of the potential energy surface (PES) analysis of the neutral– neutral reaction between NH and NP. Such a study is going to give a reasonable idea about the chemical reactivity behaviour of PN. 2. Computational methods All the calculations have been carried out using Gaussian soft- ware package [29]. We have used various methods like, B3LYP/ 6-31++G(3df,2pd), B3LYP/aug-cc-pvdz, MP2/6-31++G(3df,2pd), G3 and G3B3 to completely analyze the reaction potential energy surface. The true minima and the transition states were confirmed from analysis of their frequencies by ensuring that all frequencies were positive for the minimum, with there is only one imaginary frequency for the transition state. We have also carried out the analysis of the displacement vectors for the imaginary frequency to ascertain that the TS is a structurally true TS and also confirmed by the IRC analysis. All the thermodynamic quantities were calcu- lated from the zero point energy (ZPE) corrected energies of the stationary points in the PES. Results from the B3LYP method are http://dx.doi.org/10.1016/j.comptc.2015.12.004 2210-271X/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: ssitha@uj.ac.za (S. Sitha). Computational and Theoretical Chemistry 1078 (2016) 129–137 Contents lists available at ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc