A quantum chemical study on the formation of phosphorus mononitride Rommel B. Viana b , Priscila S.S. Pereira a , Luiz G.M. Macedo c , André S. Pimentel a, * a Departamento de Química, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente 225, Gávea, 22453-900 Rio de Janeiro, RJ, Brazil b Departamento de Química e Física Molecular, Instituto de Química de São Carlos, Universidade de São Paulo, Av. Trabalhador São Carlense, 400 CP 780, 13560-970 São Carlos, SP, Brazil c Laboratório de Simulação Computacional, Departamento de Química, Universidade Estadual Paulista (UNESP), Bauru, SP 17033-360 Brazil article info Article history: Received 8 June 2009 Accepted 29 July 2009 Available online 3 August 2009 Keywords: Planetary chemistry Nitrogen chemistry Quantum chemical calculations Phosphorus chemistry Potential energy surface Spin-forbidden reactions abstract The chemical mechanism of the 1 PN formation was successfully studied by using the CCSD(T)/6- 311++G(3df,3pd) level of theory. The 1 NH 3 + 3 PH and 4 P + NH 3 reaction paths are not energetically favor- able to form the 1 PN molecule. However, the 3 NH + 3 PH, 4 N+ 3 PH 3 , 4 N+ 3 PH, 4 P+ 3 NH, and 4 P+ 2 NH 2 reac- tion paths to form the 1 PN molecule are only energetically favorable by taking place through specific transition states to form the 1 PN molecule. The NH 3 + 3 PH, 4 N+ 1 PH 3 , NH 3 + 4 P, and 4 N+ 2 PH 2 reactions are spin-forbidden and the probability of hopping for these reactions was estimated to be 0 by the Lan- dau–Zener theory. This is the first detailed study on the chemical mechanism for the 1 PN formation. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Phosphorus mononitride (PN) is a diatomic molecule isovalent and analogous with molecular nitrogen that is important in the interstellar medium (10 < T < 10,000 K) [1] and in the atmospheres of Jupiter and Saturn (temperatures ranging from 80 to 1000 K and 1< P < 10 12 bar) [2]. PN has been detected or assigned as a poten- tial constituent of some dense interstellar clouds [3–6], Thorne et al. [3] is an experimental and modeling study that predates the discovery of PN in the interstellar medium. Ziurys [4] notes that the large abundance for phosphorus nitride is not predicted by ion–molecule chemistry and suggests that high-temperature processes are responsible for the synthesis of PN [7,8]. The most likely PN source may be the chemical reactions between NH x and PH y (x, y = 0–3), which have been detected or assigned in the inter- stellar medium [9–11]. The first spectroscopic evidence for the gas- eous PN in laboratory was found in 1933 [12]. Later, the heat of PN formation was experimentally determined for the first time in 1954 [13] and confirmed in 60s [14–16]. In 70s, the spectrum of phosphorus nitride has been observed in the optical and ultraviolet regions by both emission and absorption techniques [17,18]. Final- ly, ab initio electronic structure methods have been used to study the PN molecule [19–25]. The PN formation in laboratory has been obtained by thermal decomposition of phosphorus nitride (P 3 N 5 ) [26], reaction of phos- phorus atoms and nitric oxide (NO) [27–30], and reaction of phos- phorus atom and azide radical (N 3 ) [31]. As P 3 N 5 ,N 3 , and high amounts of NO do not exist in the interstellar medium and giant planets, it would be interesting to propose a chemical mechanism for the PN formation in theses environments. Unfortunately, it is difficult to experimentally study the PN formation due to the high reactivities of N and P species and the unlikely isolation of the tar- get chemical reaction in the interstellar and planetary conditions. Ab initio quantum chemical calculations may offer an alternative way for the understanding of the chemistry of PN formation in a wide variety of astronomical regions. The chemical mechanisms described in this manuscript would require that the process should have no barrier above the energy of the PH x and NH x fragments for cold regions (T < 200 K). Otherwise, the abstraction reactions may also take place in hot regions. Also, the corresponding PH x and NH x fragments should be present in non-zero concentration in both cold and hot regions. The aim of this work is to demonstrate the PN formation in a PH x and NH y reacting system (x, y = 0–3). To the best of our knowl- edge, little has been done for this class of reactions using theoret- ical calculations or experimental methods [27,28,32]. The location of the transition states for the PH x and NH y reacting system has not been reported in the literature. In view of the lack of information for this system, the purpose of this study is therefore to present the theoretical characterization of this reactional system. This pa- per is intended to provide a better description of this important reaction system, e.g., describe the transition states and intermedi- ates. It reports ab initio quantum chemical calculations to compute 0301-0104/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2009.07.008 * Corresponding author. E-mail address: a_pimentel@puc-rio.br (A.S. Pimentel). Chemical Physics 363 (2009) 49–58 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys