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REPLACE THIS Photolytic Sources of HCN and HNC and Isomerization Dynamics BOX WITH YOUR ORGANIZATION’S HIGH RESOLUTION LOGO Jonathan Smith*, Andrei Dzekola, Drew Behrendt. Department of Chemistry Temple University

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ABSTRACT

C) Figure 4: Final populations Hydrogen Cyanide HNC and HCN are observed (HCN) and Hydrogen Isocyanide (HNC) resulting from photolysis of vinylcyanide [3]. spectroscopically in the interstellar Figure 1: Final relative populations of Hydrogen Cyanide (HCN) and Figure 2: Structures of Hydrogen Cyanide (HCN) (A), medium. This pair of isomers are used Hydrogen Isocyanide (HNC) under at excess energy E and the % of different Hydrogen Isocyanide (HNC) (B) and its relative Transition as dense gas tracers in molecular vinylcyanide photolysis reaction channels I-VII [3]. State (C). clouds, are a that have great potential The first step taken using the WebMO software is Geometry as a temperature probe because of Optimization of the isomer molecules in order to allow transition sate INTRODUCTION calculations. One of the first things to do after a successful geometry temperature dependent, isomer-specific optimization is to calculate vibrational frequencies listed in “Table 4 ”. formation and destruction pathways [2]. The negative frequency correspond to the motion of hydrogen between Previous observations of the HNC/HCN Molecular Reaction Dynamics is a field within physical carbon and nitrogen that is displayed in the transition state of the chemistry, studying why chemical reaction occur, how to predict “Figure 2”. These early exploratory calculation is carried out with a abundance ratio show that the ratio semi-empirical approach to be later refined with planned ab initio and Figure 5: Modeled HNC/HCN ration vs two their behavior and how to control them. Photolysis, or a chemical possible H+HNC barrier height expressed in K DFT calculations. This approach also allows us to quickly find possible Figure 3: Schematic potential decreases with increasing temperature, process by which molecules are broken down into smaller units with varied densities as a function of time in years energy diagram for vinycyanide something that standard astrochemical through the absorption of light is the principle upon which many transition states from photolysis of cyanide containing compounds under conditions typical in the Orion Molecular possibly or confirmed to be present in the interstallar medium. leading to HCN formation [3]. models cannot reproduce. At this stage astrophysical compounds such as HNC and HNC are being studied Cloud 1. [4] [2]. The HCN and HNC molecules are abundant in interstellar clouds The energy difference between the product (Hydrogen of the research, the focus is placed on and the isomerization process proceeds via a hydrogen atom Isocyanide) and its relative transition state gives the Activation Energy CONCLUSIONS use of semi-empirical computational transfer mechanism displayed in “Figure 2”. which is used in order to determine the reverse rate constant using On earth, gaseous hydrogen cyanide (HCN) is an important RRKM. Clearly, with equilibrium reverse constant of Observed amount of HNC in dense molecular clouds is roughly equal in analysis upon the two isomers to abundance to HCN, which is not the case under room temperature equilibrium intermediate in the combustion reactions of hydrocarbon flames 1.710 1/) it is safe to conclude that isomerization of HCN confirm previously tested effects of containing a nitrogen source. By contrast, the HNC isomer, a to HNC is too slow, and thermodynamically too unfavorable to take conditions, leading to a conclusion that there is a different mechanism occurring in temperature and pressure on the compound of much lower stability (higher energy)[5]. Such a low place at room temperature without the aid of energetic collisions and is space that results in production of HNC and isomerization. It is supported by “Figure less likely at 10K which is typical in the interstellar medium.. 1”, in which final relative populations of HCN/HNC is nearly equal at conditions close activation energy and the reverse rate stability of HNC at earth conditions, makes the system a great isomerization model for study under laboratory conditions. to those in space (dense state) when resulting from photolysis of vinylcyanide [3]. constant for the transition state and test The abundance ratio of HNC/HCN from photolysis of Table 1: Computational Heat of Formation, Entropies and Enthalpies Furthermore, the results in “Figure 1” is illustrated in graphical form in “Figure 4” to for Hydrogen Cyanide, Hydrogen Isocyanide, as well as its give a better gradual illustration. The results computed in this research, “Table 1” out possible CN contantaining photolytic vinylcyanide varies from one reaction pathway to another in Transition State. apparent relation with excess energy in “Figure 1”. In order to provide similar values to published research by Zahra Homayoon in “Figure 1” sources. Compound PM3 Heat of illustrating fairly good precision for methods used. explain these variations, isomerization is invoked in the model which Name Formation Enthalpy Entropy yields expected results, being inefficient at low excess energy but Similar behavior of relative concentration of HCN to HNC is displayed in becomes more relevant at high excess energy. Hydrogen Cyanide (HCN) 32.9 kcal/mol 2168.7 cal/mol 48 cal/mol*K “Figure 5” done by a different research team [4]. In the same way, the data is Hydrogen Isocyanide consistent with regards to relative concentrations of HCN to HNC as the ratio gets The relative population and internal energy content of (HNC) 56.6 kcal/mol 2281.9 cal/mol 48 cal/mol*K HCN/HNC can be deduced by examining the population and energy closer 1 as temperature increase. HCN (Trans. State) 108.1 kcal/mol 2390.4 cal/mol 52 cal/mol*K content of the corresponding coproduct acetylene in the The data provided illustrate a possibility of a different plausible photolytic photodissociation reaction [7]. Table 2: Vibrational Frequencies for Hydrogen Cyanide (HCN). mechanism occurring in Dense State (In Space) that could be discovered by further Due to existence of CH CN and CH NC, and HCN and HNC in research. Investigation of vinylcyanide, cyanoallene, and methycyanoformate as 3 3 Vibration Frequencies for HCN space [6], suggests that other nitrile–isonitrile pairs await detection HCN/HNC photoltic precursors is underway with hopes of finding transition states and a plausible and related production mechanism to be discovered. leading to varied HNC/HCN ratios. Mode Frequency (cm-1) IR Intensity

1 856 0.6277 REFERENCES 2 856 0.6275 1. Materials by Dr. Kalju Kahn, Department of Chemistry and Biochemistry, UC Santa Barbara. 3 2286 0.9566 4 3298 0.9125 (2010). 2. Anton I. Vasyunin, Karin I. Oberg,, and Dawn Graninger. "The HNC/HCN Ratio in Star Table 3: Vibrational Frequencies for Hydrogen Isocyanide (HNC). Forming Regions." Cornell University, 21 Apr. 2014. Web. 03 Dec. 2016 3. Zahra Homayoon. " Ab Initio and RRKM Study of the HCN/HNC Elimination Channels from Vinyl Vibration Frequencies for HNC Cyanide - The Journal of Physical Chemistry A (ACS Publications). N.p., 2011. Web. 01 Feb. METHODS 2017. Mode Frequency (cm-1) IR Intensity 4. The HCN/HNC Abundance Ratio Toward Different Evolutionary Phases of Massive Star FormationMihwa Jin et al. 2015 The Astrophysical Journal Supplement Series 219 2 Chemical reactions occur by the rearrangement of nuclear configurations 1 614 0.8347 5. TALBI, Dahbia, ELLINGER, Yves. Isomerization versus hydrogen exchange reaction in the from the reactant state to the product state. Chemical reaction dynamics 2 614 0.8347 HNCHCN conversion. Chemical physics letters, 1996, vol. 263, no. 3-4, p. 385-392 3 2170 2.6424 include molecules that have a lot of energy that follow a path that involves 6. Reggie L. Hudson Journal of Chemical Education 2006 83 (11), 1611 DOI: 10.1021/ed083p1611 4 3579 0.7807 7. Michael J. Wilhelm, Jonathan M. Smith, Hai-Lung Dai. (2016) Note: Reconstructing interferograms high energy configurations, while in contrast the molecules with less improves spectral SNR. The Journal of Chemical Physics 145:3. Online publication date: 23-Aug- energy will follow the path that involves configurations of lower energy [1]. Table 4: Vibrational Frequencies for Hydrogen Cyanide Transition State. 2016. The WebMO Computational Chemistry Interface hosted on the Google Cloud platform is used in order to generate the heats of formation Vibration Frequencies for HNC CONTACT for Hydrogen Cyanide, Hydrogen Isocyanide, as well as its Transition State Transition State in order to compute the activation energy and reverse rate constant for the Andrei Dzekola: [email protected] reaction taking place as well as for acetylene and other photolysis Mode Frequency (cm-1) IR Intensity coproducts. First calculations are done assuming STP conditions and then Drew Behrendt: [email protected] 1 -1427 0.4392 compared to conditions more similar to space. 2 1756 0.9605 Dr. Jonathan Smith: [email protected] 3 2124 2.3018

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