Theoretical study on the reaction pathways of HFCO + H2O

Journal of Molecular Structure: THEOCHEM 858 (2008) 88–93

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Theoretical study on the reaction pathways of HFCO + H2O Wu-Hung Tsai, Jia-Jen Ho * Department of Chemistry, National Taiwan Normal University, 88, Section 4, Tingchow Road, Taipei 116, Taiwan

a r t i c l e

i n f o

Article history: Received 21 January 2008 Received in revised form 21 February 2008 Accepted 22 February 2008 Available online 29 February 2008 Keywords: Ab initio Reaction mechanism HFCO H2O

a b s t r a c t For the reaction of methanoyl fluoride with water, both optimized structures and vibrational wavenumbers of reaction intermediates, transition structures and product complexes were calculated and characterized with theory at the MP2/6-311++G(d,p) level. Including a catalytic path and concerted and stepwise hydrolysis paths, possible reaction mechanisms were also investigated. The catalytic reaction of HFCO yielding HF and CO has the smallest activation barrier, 29.6 kcal/mol, whereas for the concerted hydrolysis 33.0 kcal/mol is required to overcome the barrier to form transoid HCOOH + HF, which is less than for the stepwise counterpart, 42.0 kcal/mol. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Methanoyl fluoride HFCO is produced in the decomposition of alkoxy radical CF3CFHO [1–3], which is formed during the atmospheric photo-oxidation of 1,1,1,2-tetrafluoroethane (hydrofluorocarbon 134a, HFC-134a) in the presence of NOx. This fluorocarbon [4–6] is a prospective substitute for HCFCl2 used in automotive airconditioning systems. As methanoyl fluoride is a major product in the degradation of hydrochlorofluorocarbons (HCFC), and as its subsequent photo-dissociation has important consequences for the protective terrestrial ozone layer, the dissociation and potential energetics of the title reactions provoked our interest. Moore’s group performed stimulated-emission-pumping (SEP) experiments on the dissociation of vibrationally excited HFCO on the ground S0 surface [7–11]. Saito et al. observed the thermal decomposition of HFCO behind shock waves over a temperature range 1160–1480 K, and measured the second-order rate coefficient [12]. Weiner and Rosenfeld [13] used an ultraviolet laser to effect the photolysis of HFCO; according to their interpretation the reactant was excited to produce H + FCO, or F + HCO, and, eventually, H + F + CO, showing features distinct from those of thermal reactions. For the reaction of atom F with HCO, Donaldson and Sloan [14] interpreted their results as indicating the existence of an enduring HFCO intermediate. Performing photo-dissociation of HFCO at 193 nm, Lee et al. [15] detected products with fragmentation-translational spectra utilizing a tunable vacuum-ultraviolet beam from a synchrotron for ionization; the F-elimination channel HFCO ? HCO + F dominates, with a branching ratio 0.66, and about 17% of HCO

* Corresponding author. Tel.: +886 2 29309085; fax: +886 2 2934249. E-mail address: [email protected] (J.-J. Ho). 0166-1280/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2008.02.023

decomposes further to H + CO; the H-elimination channel HFCO ? FCO + H has a branching ratio 0.28 and about 21% of FCO decomposes further to F + CO. In determining the potentialenergy surface for the ground state, several quantum-chemical calculations [16–26] yielded satisfactory agreement on the geometry of the transition structure. Goddard et al. [16] found a barrier height approximately 47 kcal/mol, similar to the result 46.9 kcal/ mol of Kamiya and Morokuma [17]. Francisco et al. [19] found a barrier height 43.2 kcal/mol and an energy 9.0 kcal/mol for the dissociation of HFCO. On fitting the calculated data at 3855 geometries of ground-state potential surface to a ‘global’ analytic function, Wei and Wyatt [20] obtained a barrier height 48 kcal/mol and heat of reaction 5.6 kcal/mol. The various routes for dissociation of HFCO are: HFCO ! H þ FCO

ð1Þ

! F þ HCO

ð2Þ

! HF þ CO

ð3Þ

! FCOH

ð4Þ

! HCOF

ð5Þ

Channel 3 has the least energy among these paths. Tropospheric degradation of the proposed CFC leading to the formation of HFCO might be followed by hydrolysis [27,28]. Williams et al. [29–33] investigated theoretically the mechanism and catalysis for a carbonyl addition in which an addition of H2O to HXCO yields CHX(OH)2 (in which X = H, Cl, F), which then decomposes preferentially by 1,2-elimination of HX to produce methanoic acid. These authors proposed this reaction Scheme 1.

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[36] to confirm the connection between the transition structure and the designated intermediates. We calculated atomic charges from a natural population analysis (NPA) [37,38] using the NPA options in the program. 3. Result and discussion

Scheme 1.

According to this Scheme 1, they obtained an activation energy 43.1 kcal/mol for reaction HFCO + H2O ? CHF(OH)2, relative to HFCO + H2O, but neither detailed potential-energy surface nor correlated path was reported. In this work we report the possible reaction paths and the potential-energy surfaces for the reaction HFCO + H2O from our calculations at the MP2/6-311++G(d,p) level; we discuss the possible products, intermediates and transition structures, and whether the path is concerted or stepwise. 2. Computational methods We performed the quantum-chemical calculations with the Gaussian 03 program package [34]. We optimized the stationary points on the potential-energy surfaces for the HFCO + H2O reactions at the MP2 [35] level, using basis sets 6-311++G(d,p) with increased accuracy of polarized split-valence and diffuse functions for heavy and hydrogen atoms. To characterize the optimized structures as local minima or transition structures, we conducted a vibrational analysis at the same level of theory; correction for zero-point energy (ZPE) was performed at the same level. We also performed calculations of the intrinsic reaction coordinate (IRC)

The reason that we adopted this level and basis set was that we applied it to calculate both an optimized structure of HFCO at 298 K, shown in Fig. 1 and agreeing with measured structural data (bond lengths and angles) [39], and an activation energy 46.8 kcal/ mol of the barrier for HFCO ? HF + CO, agreeing with 49 ± 4 kcal/ mol obtained in experiments by Choi and Moore et al. [9]. Our calculated relative energy of the first transition structure with respect to the reactant is +42.1 kcal/mol, in agreement with a value 43.1 kcal/mol calculated by Francisco and Williams et al. [33] at the MP4SDTQ/6-311G**//MP2/6-311G** level. We are therefore confident of the quality of our calculations at this level for a reaction of this type. 3.1. Path 1: H2O acts as a catalyst in HFCO + H2O ? HF + CO + H2O The calculated sequence of reactions on the potential-energy surfaces with respect to the reaction coordinates is HFCO þ H2 O ! 1IM ! 1TS1 ! 1IM1 ! HF þ CO þ H2 O Table 1 presents calculated energies, relative energies and vibrational wavenumbers for these species; Fig. 1 shows the potentialenergy surface. The calculated optimized structures are drawn in Fig. 2, and the atomic charges of NPA appear in Table 2. The reactants form first an adduct 1IM, at 3.2 kcal/mol; atom H3 then shifts to atom O5 and atom H6 to F4. For transition structure 1TS1 the barrier has a height 32.8 kcal/mol; formation of product complex 1lM1, the most stable compound on the calculated potential-energy surfaces, releases 15.7 kcal/mol. From the calculated distances for bonds between the atoms and from the atomic charges in the transition structures and the intermediate complexes, formation of the products is readily rationalized. In the product complex 1IM1 shown in Fig. 2, the distances are H3–O5 0.964 Å, and F4–H6 0.934 Å, indicating the ready formation of bonds H3O5 and F4H6;

50.0

40.0

E (kcal/mol)

30.0

20.0

10.0

0.0

-10.0 O

C H

-20.0

Reaction Coordinate Fig. 1. Potential-energy profile of three reaction paths; values (kcal/mol) in parentheses pertain to reactants HFCO + H2O.

H

O

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Table 1 Calculated imaginary vibrational wavenumber/cm1 (Fimg) of transition states, energy (E/hartree), zero-point energy (ZPE), total energy (Etot, E + ZPE), relative energy (Erel/ kcal/mol) for intermediates, transition structures and products in reaction system HFCO + H2O 23IM 2TS1 2IM1 HCOOH(t) HCOOH(t)+HF Path3 31TS1 3IM1 3TSr 3IMc 3TS2c 3IM2c 3TS2t 3IM2t HCOOH(c)+HF HCOOH(c)+HF HCOOH(t) HCOOH(t)+HF

289.634455 289.576101 289.650373 189.362298 289.641191

0.044795 0.043980 0.046652 0.034137 0.043702

289.589660 289.532121 289.603721 189.328161 289.597489

3.1 33.0 12.0

289.561712 289.634649 289.628377 289.629686 289.573338 289.646789 289.579588 289.653848 189.354902 289.633795 189.362298 289.641191

0.044142 0.050414 0.049465 0.049932 0.044916 0.046585 0.045173 0.046912 0.033736 0.043301 0.034137 0.043702

289.517570 289.584235 289.578912 289.579754 289.528422 289.600204 289.534415 289.606936 189.321166 289.590494 189.328161 289.597489

42.1 0.3 3.6 3.1 35.3 9.7 31.5 14.0

787.9

8.0 1761.3 331.5 1407.0 1403.2

3.7 8.0

Reaction energy (E), Gibbs free energy (G) and transition state vibration frequency for the reaction of HFCO + 2H2O at calculation level MP2/6-311++G(d,p).

the angle F4H6O5 is 178.0°, implying H-bonding to exist between F4H6 and O5. That the bond distance C1-O2 is 1.138 Å indicates also the ready formation of molecule CO. The atomic charge on atom H3 (Table 2) that alters according to 0.143 ? 0.3471 ? 0.493 in the process 1IM ? 1TS1 ? 1IM1 indicates that the original CAH bond becomes an OAH bond, in which the atomic charge on H becomes more positive. For 1IM1 to form the final products HF, H2O and CO requires 8.6 kcal/mol. In summary, at the level MP2/6311++G(d,p), the calculated barrier for the reaction HFCO + ! H2O ? HF + CO + H2O is only 29.6 kcal/mol, which is much less than that for HFCO ? HF + CO. H2O therefore acts catalytically in the former reaction. 3.2. Path 2: The hydrolytic concerted reaction of HFCO+H2O ? HCOOH(t) + HF In this path the sequence and species in the reactions on the potential-energy surfaces are HFCO þ H2 O ! 23IM ! 2TS1 ! 2IM1 ! HCOOHðtÞ þ HF in which HCOOH(t) represents transoid HCOOH shown in Fig. 2. Atom O of nucleophilic H2O attacks the carbon atom of HFCO; simultaneously, one atom H of H2O transfers to atom F of HFCO, which causes the breaking of bond CAF and the formation of bond HAF, producing eventually HCOOH(t) and HF. The initial adductcomplex 23IM at 3.1 kcal/mol is almost as stable as 1IM, shown in Fig. 2; that dihedral angle F4C1O5H6 = 10.8° is nearly planar facilitates the attack of atom O5 toward C1, and the transfer of atom H6 from O5 to F4. The transition barrier in the formation of product complex 2IM1, at 12.0 kcal/mol, via transition structure 2TS1 is 36.1 kcal/mol, greater than for path 1; 2IM1 eventually decomposes to HCOOH(t) + HF. The dihedral angle of H3C1O2O5 in 2IM1 is 179.6°, essentially planar, and that of H3C1O5H7 is 179.2°; 2IM1 is clearly an adduct of trans-HCOOH with HF. The reaction HFCO+H2O ? HCOOH(t) + HF is exothermic by 8.0 kcal/mol according to calculation at the MP2/6-311++G(d,p) level.

height 45.2 kcal/mol, to another intermediate structure 3IM1, HFCO + H2O ? 23IM ? 3TS1 ? 3IM1, then splits in two ways: 3IM1 ! 3TS2t ! 3IM2t ! HCOOHðtÞ þ HF ðorÞ

ð3aÞ

3IM1 ! 3TSr ! 3IMc ! 3TS2c ! 3IM2c ! HCOOHðcÞ þ HF

ð3bÞ

As the dihedral angle H7O5C1O2 of 23IM is 11.8°, approaching planarity, atom O5 of H2O can attack atom C1, with atom H7 of H2O simultaneously shifting to atom O2 of HFCO. During this process, transition structure 3TS1 appears before formation of intermediate 3IM1. A subsequent rotation with a small barrier, 3.3 kcal/mol, via 3TSr enables formation of another intermediate, 3IMc, for which bond H7AO2 rotates to the other side. With the elongation of bond H7AO2 and an increased interaction between atoms H7 and F4, another transition state 3TS2c occurs before formation of intermediate 3IM2c at 9.7 kcal/mol; the barrier for this step is 32.2 kcal/mol. 3IM2c can decompose further to the final products, cisoid HCOOH + HF, which reaction is exothermic by 3.7 kcal/mol. Another possible stepwise path after intermediate 31M1 is via 3(a), in which atom H6 of H2O shifts to atom F4, causing breaking of bonds C1AF4 and O5AH6 and the formation of bond C1AO5. Transition structure 3TS2t has a barrier of height 31.2 kcal/mol in the formation of stable intermediate 31M2t, at 14.0 kcal/mol, which can decompose to form transoid HCOOH + HF, which reaction is exothermic by 14.3 kcal/mol. In structure 3TS2t, the dihedral angle H6O5C1F4 in the four-membered ring is 10.0°, appearing to be more nearly planar than that in 3IM1, 47.1°; angle H3C1O2H7 in 3TS2t is 176.0°, thus larger than that, 169.1°, in 3IM1, which facilitates transfer of a proton and the formation of 3IM2t. The distance between atoms C1AO5 decreases from 1.383 Å in 3IM1 to 1.295 Å in 3TS2t, and that between atoms C1AF4 increases from 1.395 Å in 3IM1 to 1.888 Å in 3TS2t, indicating a strengthened bond C1AO5 and the breaking of bond C1AF4. The dihedral angle H3C1O5H7 in 3IM2t is 180.0°, indicating a transoid conformation; the length of bond C1AO5 decreases to 1.213 Å, and of F4AH6 to 0.930 Å, both much smaller than in 3IM1, indicating almost complete formation of these bonds. 3IM2t eventually dissociates to HF + HCOOH(t) on absorbing 6.0 kcal/mol. In path (3b), dihedral angle H7O2C1F4 rotates from 74.5° in 3IM2 to 58.9° in 3IMc via transition structure 3TSr with barrier of height 3.3 kcal/ mol; in 3IMc, F4C1H3 forms a mirror plane that bestows on structure 3IMc symmetry Cs. The transfer of atom H7 to F4 continues, causing formation of double bond C1AO2 and cleavage of bond C1AF4. Transition structure 3TS2c has a barrier of height 32.2 kcal/mol before formation of product complex 3IM2c. Atoms C1, O2, H7 and F4 in 3TS2c form an almost planar four-membered ring, dihedral angle H7O2C1F4 = 4.8°, which facilitates the transfer of H7 to atom F4, and the internuclear distance in C1AF4 increases to 1.900 Å, nearly a broken bond, whereas the distance in F4AH7 decreases to 1.317 Å, and C1AO2 to 1.283 Å, indicating the gradual formation of these two bonds. For product complex 3IM2c the dihedral angle H3C1O5H6 is 0.0°, indicating formation of a cisoid HCOOH structure. The distance of bond F4AH7 decreases further to 0.932 Å, and C1AO2 to 1.207 Å, also implying the complete formation of these two bonds. The further dissociation of 3IM2c to HF and HCOOH(c) still requires 6.0 kcal/ mol. In summary, at the MP2/6-311++G(d,p) level of calculation, the reaction HFCO + H2O ? HF + HCOOH(c) is exothermic by 3.7 kcal/mol.

3.3. Path 3: The hydrolytic stepwise reaction of HFCO + H2O ? HCOOH(t) (or HCOOH(c)) + HF

4. Conclusion

This reaction might proceed stepwise. Adduct-complex 23IM first passes via a much higher transition structure 3TS1, barrier

Based on calculations all at the level MP2/6-311++G(d,p), our results are summarized as follows.

W.-H. Tsai, J.-J. Ho / Journal of Molecular Structure: THEOCHEM 858 (2008) 88–93

91

Fig. 2. Optimized structures (bond length/Å angstroms and angle/deg) of reactants, products, intermediates and transition structures of paths 1, 2 and 3, calculated at the MP2/6-311++G(d,p) level. aRef. [39].

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Fig. 2 (continued)

1. The barrier of the catalytic reaction HFCO + H2O ? HF + CO + H2O is calculated to be 29.6 kcal/mol, and the heat of reaction is 7.1 kcal/mol, with respect to reactants HFCO + H2O.

2. The barrier of the concerted reaction HFCO + H2O ? 23IM ? 2TS1 ? 2IM1 ? HCOOH(t) + HF, is calculated to have a height 33.0 kcal/mol, and heat of reaction is +8.0 kcal/mol.

W.-H. Tsai, J.-J. Ho / Journal of Molecular Structure: THEOCHEM 858 (2008) 88–93 Table 2 Calculated atomic charges for intermediates and transition structures by natural population analysis NPA charge

C1

O2

H3

F4

O5

H6

H7

Path 1 1IM 1TS1 1IM1

0.893 0.798 0.573

0.604 0.514 0.566

0.143 0.371 0.493

0.435 0.742 0.609

0.939 0.965 0.955

0.472 0.560 0.584

0.469 0.492 0.479

Path 2 23IM 2TS1 2IM1

0.926 0.926 0.828

0.620 0.611 0.644

0.115 0.172 0.115

0.428 0.683 0.588

0.933 0.873 0.794

0.470 0.567 0.576

0.470 0.501 0.507

Path 3 23IM 3TS1 3IM1 3TS2t 3IM2t 3TSr 3IMc 3TS2c 3IM2c

0.926 0.957 0.908 0.980 0.855 0.908 0.903 0.979 0.840

0.620 0.860 0.743 0.689 0.727 0.756 0.735 0.793 0.685

0.115 0.118 0.103 0.153 0.120 0.087 0.081 0.136 0.101

0.428 0.438 0.455 0.706 0.602 0.453 0.457 0.716 0.604

0.933 0.856 0.771 0.820 0.723 0.739 0.735 0.681 0.716

0.470 0.505 0.480 0.577 0.581 0.468 0.471 0.497 0.483

0.470 0.573 0.478 0.505 0.497 0.485 0.471 0.578 0.581

3. The calculated first and second barriers in, and the heat of, the stepwise reaction HFCO + H2O ? 23IM ? 3TS1 ? 3IM1 ? 3TS2t ? 3IM2t ? HCOOH(t) + HF are 42.0, 31.5 and +8.0 kcal/ mol, respectively. 4. For another stepwise reaction in this sequence HFCO + H2O ? 23IM ? 3TS1 ? 3IM1 ? 3TSr ? 3IMc ? 3TS2c ? 3IM2c ? HCOOH(c) + HF, the calculated first transition barrier, rotational barrier, second transition barrier, and reaction heat are 42.0, 3.6, 35.3 and 3.7 kcal/mol, respectively. 5. Among all reaction mechanisms that we calculated, the H2O catalytic process producing HF + CO has the smallest reaction barrier, whereas the stepwise hydrolysis leading to cisoid HCOOH + HF has the greatest barrier.

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