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Shock Wave and Theoretical Modeling Study of the Dissociation of CHF I. Primary Processes 2

2

Carlos J. Cobos, Klauss Hintzer, Lars Soelter, Elsa Tellbach, Arne Thaler, and Juergen Troe J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05854 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry

Shock Wave and Theoretical Modeling Study of the Dissociation of CH2F2 I.

Primary Processes

C. J. Cobosa, K. Hintzerb, L. Sölterc, E. Tellbachc, A. Thalerb, and J. Troec,d* a

INIFTA, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CONICET, Casilla de Correo 16, Sucursal 4, La Plata (1900), Argentina b

c

Dyneon GmbH, Gendorf, D-84508 Burgkirchen, Germany

Institut für Physikalische Chemie, Universität Göttingen, Tammannstrasse 6, D-37077 Göttingen, Germany

d

Max-Planck-Institut für Biophysikalische Chemie, Am Fassberg 11, D-37077 Göttingen, Germany

May 2017 to be published in The Journal of Physical Chemistry A *Email: [email protected] ACS Paragon Plus Environment


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Abstract

The unimolecular dissociation of CH2F2 leading to CF2 + H2, CHF + HF, or CHF2 + H, is investigated by quantum chemical calculations and unimolecular rate theory. Modeling of the rate constants is accompanied by shock wave experiments over the range 1400 – 1800 K monitoring the formation of CF2. It is shown that the energetically most favorable dissociation channel leading to CF2 + H2 has a higher threshold energy than the energetically less favorable one leading to CHF + HF. Falloff curves of the dissociations are modeled. Under the conditions of the described experiments, the primary dissociation CH2F2 → CHF + HF is followed by a reaction CHF + HF → CF2 + H2. The experimental value of the rate constant of the latter indicates that this reaction does not proceed by an additionelimination process involving CH2F2 * intermediates as assumed before.

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I. Introduction

Fluoromethanes CHxF4-x (x = 0, 1, 2, 3, 4) dissociate either by simple bond fission or by complex elimination processes, the former characterized by loose, the latter by rigid activated complexes. Depending on the conditions, the reactions proceed as single- or multi-channel unimolecular reactions. Whereas the dissociations of CH4 (see, e.g., refs. 1 and 2) and CF4 (see, e.g., refs. 3 and 4) decompose by single C-H and C-F bond fissions, respectively, the situation is less clear for the mixed cases (x = 1, 2, 3). The present work tries to improve this situation by investigating one of these systems, i.e. the dissociation of CH2F2. Before we start to consider this reaction, we have a look at the two other mixed systems.

Ab initio calculations of potential energy surfaces for the dissociation of CH3F in ref. 5 were combined with calculations of limiting high-pressure rate constants of various reaction channels, e.g., the spin-allowed reactions

(1)

CH3F → H + CH2F

∆° = 416 kJ mol-1

(2)

CH3F → CHF + H2

∆° = 377 kJ mol-1

(3)

CH3F → F + CH3

∆° = 456 kJ mol-1

(unless stated otherwise, the enthalpies given in this article are from ref. 6; for more recent quantumchemical calculations, see ref. 7). Pre-exponential factors of the limiting high-pressure rate constants for the thermal bond fissions (1) and (3) were found to be close to 1016 s-1, in contrast to 1015 s-1 for the elimination reaction (2). Using such values in simulations of larger mechanisms, however, one has to take into account that the reactions generally will be in their falloff range. In addition, the spinforbidden channel CH3F → 3CH2 + HF (∆° = 346 kJ mol-1 ), which dominates thermal dissociation experiments,8,9 has to be taken into account. Finally, as all channels are coupled, multi-channel unimolecular rate theory has to be employed.10

The situation appears much simpler for the dissociation of CHF3 for which experiments and theoretical modeling in the falloff range of the reaction are available.11,12 The experiments here have indicated a dominance of the elimination process

(4)

CHF3 → CF2 + HF

∆° = 220 kJ mol-1


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(with a pre-exponential factor of the limiting high-pressure rate constant near 1015 s-1). The bond fissions

(5)

CHF3 → CF3 + H

∆° = 438 kJ mol-1

(6)

CHF3 → CHF2 + F

∆° = 528 kJ mol-1

are energetically much less favorable and have not been detected.

The situation is controversial for the dissociation of CH2F2 which is the subject of the present article. The reaction has been studied before in flow system experiments.11 No evidence for the energetically most favorable pathway

(7)

CH2F2 → CF2 + H2

∆° = 251 kJ mol-1

was found in this work and the reaction was suggested to proceed by

(8)

CH2F2 → CHF + HF

∆° = 320 kJ mol-1

In contrast to this, reaction (7) in ref. 13 was assumed to be the major channel and this reaction was implemented into the simulation of the complex mechanism of the reaction between C3F6 and H atoms. The bond fission reactions

(9)

CH2F2 → CHF2 + H

∆° = 425 kJ mol-1

(10)

CH2F2 → CH2F + F

∆° = 493 kJ mol-1

involve higher energies and so far were not considered. It is the goal of the present work to shed more light on the dissociation channels of CH2F2. Like in earlier work from our laboratory (see, e.g., refs. 4, 12, and 14), we model the reaction by quantum-chemical calculations and unimolecular rate theory. Our theoretical work is accompanied by shock wave experiments monitoring the intense UV absorption of CF2. As CF2 can be formed either directly by the primary reaction (7) or through secondary reactions following reaction (8), we hope to clarify the situation. The analysis of CF2 profiles, in addition, provides access to secondary reactions. These results help to improve the extensive database15 which is used in technical applications of hydrofluorocarbons.



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On the basis of the more favorable energetics, one would have expected a dominance of reaction (7) over reaction (8). However, both reactions are elimination processes such that rigid activated complexes with high energy barriers have to be overcome and the energetics argument does not hold. This called for quantum-chemical calculations of energy profiles and transition state properties. Furthermore, the reactions were expected to be in their falloff ranges such that modeling by unimolecular rate theory became necessary. Finally, as the number of possible secondary reactions is large and their rate constants are partly unknown, theoretical work on such reactions also appeared desirable. The results of such calculations are published in a separate article.16 In the following, theoretical modeling of the three dissociation processes (7) – (9) will be described, before experimental results are shown. We found that the recorded CF2 profiles could be simulated in a surprisingly simple manner by means of reaction (8) and the secondary bimolecular reaction

(11)

∆° = −69 kJ mol-1

CHF + HF → CF2 + H2

The properties of this reaction in relation to those of reactions (7) and (8) are discussed at the end. We found that reaction (11) does not proceed as an addition – elimination process with CH2F2* intermediates, such as assumed before.15


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II. Modeling of Unimolecular Dissociation Reactions

In view of the uncertainties about the primary dissociation channels of CH2F2, i.e. reactions (7) – (10), we started this work with quantum-chemical calculations of energy parameters of the reaction channels. A variety of quantum-chemical methods were applied, such as ab initio composite level CBS-APNO calculations (equivalent to MP2/infinite basis set calculations with geometry optimization at the QCISD/6-311G(d,p) level and frequencies at the HF/6-311G(d,p) level, scaled by 0.925), G4 calculations (equivalent to CCSD(T)/infinite basis set calculations with geometry optimization at the B3LYP/6-31G(2df) level), and CCSD(T)/6-311++G(3df,3pd)//CCSD/6-311++G(d,p) calculations. The Gaussian 09 software was employed for all calculations (for more details, see refs. 17 and 18). Table 1 compares reaction enthalpies ∆° and barrier heights ∆ for the different reaction channels obtained by the various methods. The reaction enthalpies are in satisfactory agreement with values from the thermochemical tables of ref. 6 as well as the calculations of ref. 7. The until now unknown barrier heights calculated for reactions (7) and (8) show an interesting feature: while ∆° is smaller for reaction (7) than for reaction (8), the opposite is true for ∆. As ∆ is larger than ∆° for reactions (7) and (8), both reactions are elimination processes with rigid activated complexes whose properties are further considered below. Although ∆° for reaction (9) is larger than ∆ for reactions (7) and (8), it cannot a priori be neglected, as this reaction is a simple bond fission with a loose activated complex and, hence, a larger pre-exponential factor of the high-pressure dissociation rate constant. Its properties are also calculated, see below. On the other hand, because of its large ∆° reaction (10) is not expected to make a substantial contribution and is omitted from the remainder of the analysis. Fig. 1 compares the energetics of reactions (7) – (10). Inspecting the potential energy curves of the breaking bonds, reactions (9) and (10) were shown to be of simple bond fission character, see below.

We next determined rate constants for the rigid activated complex reactions (7) and (8). In order to characterize the full temperature- and pressure-dependence of the rate constants, i.e. the falloff curves of the reactions, limiting high- and low-pressure rate constants and broadening factors of the falloff curves were determined.19-21 Activated complex parameters for reactions (7) and (8), as calculated on the G4 level, are summarized in Table 2 (transition state structures are illustrated in the Supporting Information). Limiting high-pressure rate constants were then obtained by conventional transition state theory as , = 3.7 ∙ 10 exp$−400.4 kJ mol-1 ⁄R& ( s-1 and *, = 1.6 ∙ 10+ exp$−342.7 kJ mol-1 ⁄R&( s-1



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yielding pre-exponential factors typical for elimination processes. The calculation of limiting lowpressure rate constants was done using the formalism of ref. 19. It requires the guess of a reasonable value for the average energy transferred per collision 〈∆-〉, for which, like in our analysis12 of the dissociation of CHF3, 〈∆-〉⁄hc was taken as - 60 cm-1 for M = [Ar]. We obtained , ≈ 2Ar5 2.5 ∙ 1067& ⁄1500 K9:. exp$−427.9 kJ mol-1 ⁄R& ( cm3 mol-1 s-1 and *, ≈ 2Ar59.4 ∙ 10; 7&⁄1500 K9:.?@A, ≈ 0.217&⁄1500 K9:.?@A,* ≈ 0.207& ⁄1500 K9:.

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