Probing the Chemistry of P-Bearing Molecules in Interstellar Environments and other Extraterrestrial Environments
Authors: Mancini Luca , Rosi Marzio , Balucani Nadia , Skouteris Dimitrios , Codella Claudio , Ceccarelli Cecilia 
Published: October 2020

Phosphorus is one of the most important elements in biochemistry together with carbon, oxygen, hydrogen and nitrogen. Therefore, P-bearing compounds with some prebiotic potential and their possible formation pathways in extraterrestrial environments are attracting a lot of interest.&#160;In recent years, phosphorus has been clearly identified in the in the coma of comet 67P/Churyumov-Gerasimenko (1), while only a bunch of P-bearing molecules (namely PO, PN, CP, C2P and HCP) have been observed in the gas phase of circumstellar envelopes around evolved stars (2-7) and only two simple species have been detected in star forming regions, that is, PO and PN (8-11). If we focus only on solar-type star forming regions, only two detections are available, that is PN and PO toward the shocked region L1157-B1 (12) and the Class I protostar B1-b (13). Phosphorus chemistry in the conditions of the interstellar medium is poorly understood and the interstellar reservoir of this element is strongly debated. Since the first experimental work on ion-molecule reactions by Anicich and coworkers (14), PO has been indicated as the main reservoir of phosphorus and HPO+ as its major precursor. PO can also be transformed to PN by gas-phase chemistry (15). Thirty years ago, a series of theoretical investigation on ion-molecule reactions has been performed by Largo et al. (16-18) in order to explain the formation of P-O, P-N, P-C bonds. In spite of those efforts, the chemistry of interstellar phosphorus and its connections to the P-compounds detected in small bodies of the Solar System remains mostly unexplored and poorly characterized. For this reason, we have undertaken a systematic investigation of possible gas-phase formation routes of simple P-molecules by means of electronic structure and kinetic calculations. This approach is made necessary by the fact that P is a difficult species to deal with in laboratory experiments.In this work we present a new theoretical analysis of the reaction P+ + H2O and P+ + NH3 at a higher level of theory than those employed by Largo et al. in 1991. More specifically, we make use of DFT calculations for geometry optimization and frequency analysis coupled to a CCSD(T) reevaluation of the energy for each identified stationary point of the reaction potential energy surface. The data coming from electronic structure calculations will be used to perform a kinetic analysis using a Rice-Ramsperger-Kassel-Marcus (RRKM) code implemented for this purpose in order to derive the rate coefficients and branching ratios. A possible formation mechanism is proposed for the formation of both PO and PN. In the figure, the potential energy surface for the reaction of P+ with a water molecule is reported. This process leads to the formation of the POH+ ion which can later transfer a proton to molecules like NH3 which have a large proton affinity.&#160;&#160;&#160;[1] Altwegg K. et al., Prebiotic chemicals&#8212;amino acid and phosphorus&#8212;in the coma of comet 67P/Churyumov-Gerasimenko, Sci. Adv., 2016, 2 : e1600285.[2] Turner, B. E. &amp; Bally, J., Detection of Interstellar PN: The First Identified Phosphorus Compound in the Interstellar Medium, 1987, ApJL, 321, L75.[3] Ziurys L.M., Detection of Interstellar PN: The First Phosphorus-bearing Species Observed in Molecular Clouds,&#160; 1987, ApJL, 321, L81.[4] Guelin M. et al., Free CP in IRC +10216, 1990, ApJL, 230, L9-L11.[5] Ag&#250;ndez M. et al., Discovery of Phosphaethyne (HCP) in Space: Phosphorus Chemistry in Circumstellar Envelopes, APJ, 662, ApJ, 2, L91-L94.[6] Tenenbaum E.D. et al., Identification of Phosphorus Monoxide (X2&#928;r) in VY Canis Majoris: Detection of the First PO Bond in Space, 2007, ApJ, 666, 1, L29-L32.&#160;[7] Halfen D.T. et al.,Detection of the CCP Radical (X2&#928;r) in IRC +10216: A New Interstellar Phosphorus-containing Species, 2008, ApJL, 667, 2, L101.[8] Rivilla V.M. et al., Phosphorus-bearing molecules in the Galactic Center, 2018, MNRAS, 475, 1, L30-L34.[9] Rivilla V.M et al., ALMA and ROSINA detections of phosphorus-bearing molecules: the interstellar thread between star-forming regions and comets, 2020, MNRAS, 492, 1, 1180-1198.[10] Rivilla V.M., The first detections of the key prebiotic molecule PO in star-forming regions, 2018, IAU Symposium, 332,&#160; 409-414.[11] Rivilla V.M., The First Detections of the Key Prebiotic Molecule PO in Star-forming Regions, 2016, ApJ, 826, 2, 161.[12] Lefloch B. et al., Phosphorus-bearing molecules in solar-type star-forming regions: first PO detection, 2016, MNRAS 462, 3937&#8211;3944.[13] Bergner J.B. et al.,&#160; Detection of Phosphorus-bearing Molecules toward a Solar-type Protostar, 2019, The ApJ Lett., 884, 2, L36.[14] Thorne L.R et al., The chemistry of Phosphorus in Dense Interstellar Clouds, 1984, ApJ, 280, 139-143.[15] Millar T.J. et al., An efficient gas phase synthesis for interstellar PN, 1987, Mon. Not. R. Aster. Soc., 229, 41-44.[16] Largo A. et al., Theoretical Studies of Possible Processes for the Interstellar Production of Phosphorus Compounds. Reaction of P+ with Water, 1991, J. Phys. Chem., 95, 5443-5445.[17] Largo A. et al., Theoretical Sttudies of Possible Processes for the Interstellar Production of Phosphorus Compounds. Reaction of P+ with Methane, 1991, J. Phys. Chem., 95, 6553-6557.[18] Largo A. et al., Theoretical Sttudies of Possible Processes for the Interstellar Production of Phosphorus Compounds. Reaction of P+ with Ammonia, 1991, J. Phys. Chem., 95, 170-175.