J Mol Model (2014) 20:2444 DOI 10.1007/s00894-014-2444-9
ORIGINAL PAPER
Theoretical investigation on the atmospheric fate of CF3C(O)OCH2O radical: alpha-ester rearrangement vs oxidation at 298 K Bhupesh Kumar Mishra
Received: 2 June 2014 / Accepted: 26 August 2014 / Published online: 11 September 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract A theoretical study on the mechanism of the thermal decomposition of CF3C(O)OCH2O radical is presented for the first time. Geometry optimization and frequency calculations were performed at the MPWB1K/6–31+G(d, p) level of theory and energetic information further refined by calculating the energy of the species using G2(MP2) theory. Three plausible decomposition pathways including α-ester rearrangement, reaction with O2 and thermal decomposition (C–O bond scission) were considered in detail. Our results reveal that reaction with O2 is the dominant path for the decomposition of CF3C(O)OCH2O radical in the atmosphere, involving the lowest energy barrier, which is in accord with experimental findings. Our theoretical results also suggest that α-ester rearrangement leading to the formation of trifluoroacetic acid TFA makes a negligible contribution to decomposition of the title alkoxy radical. The thermal rate constants for the above decomposition pathways were evaluated using canonical transition state theory (CTST) at 298 K. Keywords Alkoxy radical . Rate constant . Alpha-ester rearrangement . G2(MP2)
Introduction It is now a well documented fact that atomic chlorine derived from release of chlorofluorocarbons (CFCs) into the atmosphere is responsible for the catalytic destruction of ozone in the atmosphere [1, 2]. Recently, hydrofluoroethers (HFEs) B. K. Mishra (*) Department of Chemical Sciences, Tezpur University Tezpur, Tezpur, Assam 784 028, India e-mail: [email protected] B. K. Mishra e-mail: [email protected]
have been the focus of intense attention as replacement materials for CFCs and hydrochlorofluorocarbons (HCFCs) in applications such as heat-transfer fluids in refrigeration systems, cleaning agents in the electronics industry, foamblowing and also for lubricant deposition [3, 4]. The absence of chlorine atoms in HFEs means that such compounds would have little impact on stratospheric ozone and that they would possess negligible ozone depleting potential (ODP) [5]. Understanding the degradation mechanism of HFEs is an important area of recent research in order to determine the impact of these compounds on atmospheric pollution and global warming. Fluorinated esters (FESs) are the primary products of the atmospheric oxidation of hydrofluoroethers (HFEs) [6] For example, CF3C(O)OCH3, methyl trifluoroacetate (MTFA) can be produced from the OH-initiated oxidation of CH3OCH(CF3)2 in the atmosphere [7]. Like HFEs, FESs also undergo photochemical oxidation in the troposphere with atmospheric oxidants, OH radicals or Cl atoms in marine environments. The degradation of FESs produces environmentally harmful products like trifluoroacetic acid (TFA) and COF2. TFA detected in surface waters has no known sink apart from rainwater and this species may impact on agricultural and aquatic systems [8]. Thus, it is important to study the kinetics and mechanistic degradation pathways of FESs for complete assessment of atmospheric chemistry as well as to explore the impact of FESs on environment. Considerable attention has been paid in recent years to performing experimental and theoretical studies on the decomposition kinetics of FESs [9–19]. Blanco et al. [11] studied the product distribution of CF3C(O)OCH3 methyl trifluoroacetate (MTFA) with Cl atoms using a 1,080 L quartz-glass reaction chamber at (296±2) K and concluded that reaction of O2 is the major reaction pathway. Stein et al. [12] investigated experimentally the thermal decomposition of alkoxy radical CF3C(O)OC(O)HCF3 produced by OH and Cl-initiated
2444, Page 2 of 7
J Mol Model (2014) 20:2444
oxidation of CF3C(O)OCH2CF3 (TFETFA) in a FTIR smog chamber. Three loss processes were identified at 296 K, which include reaction with O2 to form CF3C(O)OC(O)CF3, α-ester rearrangement to produce CF3CO and harmful CF3C(O)OH(TFA) and thermal decomposition pathways (C–H, C–C and C–O bonds scission). They concluded that, among the three reaction pathways, reaction with O2 is the dominant pathway. The tropospheric degradation of CF3C(O)OCH3 is initiated by attack of Cl atoms and leads to the formation of the alkyl radical CF3C(O)OCH2. The latter reacts with atmospheric O2 to produce peroxy radical, CF3C(O)OC(OO)H2. In a polluted atmosphere, the peroxy radicals thus formed may react further with other oxidizing species such as NO2 and NO, which leads ultimately to the formation of alkoxy radical CF3C(O)OCH2O. The chemistry of the alkoxy radicals thus generated has been the subject of extensive experimental and theoretical investigation as these species are interesting intermediates in the atmospheric oxidation of halogenated hydrocarbons. During the recent past, a considerable number of theoretical studies have been performed on other similar alkoxy radicals [20–31]. There are three potential pathways for decomposition of alkoxy radicals that involve an α-ester rearrangement, oxidation by molecular O2 and thermal decomposition (C–O bond scission) processes. These are represented as follows:
was chosen as it was used for developing the model functional. The hybrid meta-density functional, MPWB1K has been found to give excellent results for thermochemistry and kinetics and is known to produce reliable results [33–35]. In order to determine the nature of different stationary points on the potential energy surface, vibrational frequencies calculations were performed using the same level of theory as that used for optimization. All the stationary points were identified to correspond to stable minima by ascertaining that all the vibrational frequencies had real positive values. The transition states were characterized by the presence of only one imaginary frequency. To ascertain that the identified transition states connect reactant and products smoothly, intrinsic reaction coordinate (IRC) calculations [36] were performed at the MPWB1K/6–31+G(d, p) level. As the reaction energy barriers are very sensitive to the theoretical levels, the higherorder-correlation-corrected relative energies along with the density functional energies are necessary to obtain theoretically consistent reaction energies. Therefore, a potentially high-level method such as G2(MP2) was used for singlepoint energy calculations. The G2(MP2) [37] energy was calculated in the following manner: E½G2ðMP2Þ ¼ Ebase þ ΔEðMP2Þ þ HLC þ ZPE where, Ebase =E[QCISD(T)/6−311G(d,p)],
CF3 CðOÞOCH2 O• þ O2
→
→ CF3 CðOÞOH þ CHO•
CF3 CðOÞOCðOÞH þ HO• 2
→ CF3 CðOÞO• þ CH2 O•
ð1Þ ð2Þ ð3Þ
Using the power of quantum chemistry methods, our purpose is two-fold: (1) to gain some insight into the fate of the alkoxy radical, analyzing the mechanism of the assumed oxidation with O2 that leads to the formation of CF3C(O)OC(O)H and HO•2; and (2) to study the importance of the other pathways that these radicals may undergo. To the best of my knowledge, this is the first computational evidence of the occurrence of the α-ester rearrangement for this alkoxy radical derived from fluorinated ester leading to the formation of TFA. Thermochemical studies have been performed to analyze the stability of all species involved in these reactions.
Computational methods Geometry optimization of the reactants, transition states and products was carried out at the MPWB1K level of theory [32] using the 6–31+G(d, p) basis set. The 6–31+G(d, p) basis set
ΔEðMP2Þ ¼ E½MP2=6−311 þ Gð3df ; 2pÞ−E½MP2= 6−3
11Gðd; pÞ; and HLC (high level correction)=−0.00481nβ −0.00019nα(nα and nβ are the number of α and β valence electrons with α ≥nβ) and ZPE=zeropoint energy. Geometry and frequency calculations were performed at MPWB1K/6–31+G(d, p) level. The ZPE thus obtained was corrected with a scale factor of 0.9537 to partly eliminate systematic errors [32]. This dual level calculation G2(MP2)/ MPWB1K/6–31+G(d, p) is known to produce reliable thermochemical and kinetic data [17, 38–42]. To ascertain the accuracy of the MPWB1K results, we also performed geometry optimization and single-point energy calculation with the B3LYP method using 6-31G (d, p) and 6-311G (2df,2p) basis sets. All quantum mechanical calculations were performed with the Gaussian 09 suite of programs [43].
Results and discussion The detailed thermodynamic calculations performed at G2 (MP2), MPWB1K and B3LYP levels for reaction enthalpies and free energies associated with reaction channels (1–3) are listed in Table 1. Thermal corrections to the energy at 298 K were included in the determination of these thermodynamic functions. Free energy values show that two reactions (1 and 2) are exergonic (ΔrG0298
ORIGINAL PAPER
Theoretical investigation on the atmospheric fate of CF3C(O)OCH2O radical: alpha-ester rearrangement vs oxidation at 298 K Bhupesh Kumar Mishra
Received: 2 June 2014 / Accepted: 26 August 2014 / Published online: 11 September 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract A theoretical study on the mechanism of the thermal decomposition of CF3C(O)OCH2O radical is presented for the first time. Geometry optimization and frequency calculations were performed at the MPWB1K/6–31+G(d, p) level of theory and energetic information further refined by calculating the energy of the species using G2(MP2) theory. Three plausible decomposition pathways including α-ester rearrangement, reaction with O2 and thermal decomposition (C–O bond scission) were considered in detail. Our results reveal that reaction with O2 is the dominant path for the decomposition of CF3C(O)OCH2O radical in the atmosphere, involving the lowest energy barrier, which is in accord with experimental findings. Our theoretical results also suggest that α-ester rearrangement leading to the formation of trifluoroacetic acid TFA makes a negligible contribution to decomposition of the title alkoxy radical. The thermal rate constants for the above decomposition pathways were evaluated using canonical transition state theory (CTST) at 298 K. Keywords Alkoxy radical . Rate constant . Alpha-ester rearrangement . G2(MP2)
Introduction It is now a well documented fact that atomic chlorine derived from release of chlorofluorocarbons (CFCs) into the atmosphere is responsible for the catalytic destruction of ozone in the atmosphere [1, 2]. Recently, hydrofluoroethers (HFEs) B. K. Mishra (*) Department of Chemical Sciences, Tezpur University Tezpur, Tezpur, Assam 784 028, India e-mail: [email protected] B. K. Mishra e-mail: [email protected]
have been the focus of intense attention as replacement materials for CFCs and hydrochlorofluorocarbons (HCFCs) in applications such as heat-transfer fluids in refrigeration systems, cleaning agents in the electronics industry, foamblowing and also for lubricant deposition [3, 4]. The absence of chlorine atoms in HFEs means that such compounds would have little impact on stratospheric ozone and that they would possess negligible ozone depleting potential (ODP) [5]. Understanding the degradation mechanism of HFEs is an important area of recent research in order to determine the impact of these compounds on atmospheric pollution and global warming. Fluorinated esters (FESs) are the primary products of the atmospheric oxidation of hydrofluoroethers (HFEs) [6] For example, CF3C(O)OCH3, methyl trifluoroacetate (MTFA) can be produced from the OH-initiated oxidation of CH3OCH(CF3)2 in the atmosphere [7]. Like HFEs, FESs also undergo photochemical oxidation in the troposphere with atmospheric oxidants, OH radicals or Cl atoms in marine environments. The degradation of FESs produces environmentally harmful products like trifluoroacetic acid (TFA) and COF2. TFA detected in surface waters has no known sink apart from rainwater and this species may impact on agricultural and aquatic systems [8]. Thus, it is important to study the kinetics and mechanistic degradation pathways of FESs for complete assessment of atmospheric chemistry as well as to explore the impact of FESs on environment. Considerable attention has been paid in recent years to performing experimental and theoretical studies on the decomposition kinetics of FESs [9–19]. Blanco et al. [11] studied the product distribution of CF3C(O)OCH3 methyl trifluoroacetate (MTFA) with Cl atoms using a 1,080 L quartz-glass reaction chamber at (296±2) K and concluded that reaction of O2 is the major reaction pathway. Stein et al. [12] investigated experimentally the thermal decomposition of alkoxy radical CF3C(O)OC(O)HCF3 produced by OH and Cl-initiated
2444, Page 2 of 7
J Mol Model (2014) 20:2444
oxidation of CF3C(O)OCH2CF3 (TFETFA) in a FTIR smog chamber. Three loss processes were identified at 296 K, which include reaction with O2 to form CF3C(O)OC(O)CF3, α-ester rearrangement to produce CF3CO and harmful CF3C(O)OH(TFA) and thermal decomposition pathways (C–H, C–C and C–O bonds scission). They concluded that, among the three reaction pathways, reaction with O2 is the dominant pathway. The tropospheric degradation of CF3C(O)OCH3 is initiated by attack of Cl atoms and leads to the formation of the alkyl radical CF3C(O)OCH2. The latter reacts with atmospheric O2 to produce peroxy radical, CF3C(O)OC(OO)H2. In a polluted atmosphere, the peroxy radicals thus formed may react further with other oxidizing species such as NO2 and NO, which leads ultimately to the formation of alkoxy radical CF3C(O)OCH2O. The chemistry of the alkoxy radicals thus generated has been the subject of extensive experimental and theoretical investigation as these species are interesting intermediates in the atmospheric oxidation of halogenated hydrocarbons. During the recent past, a considerable number of theoretical studies have been performed on other similar alkoxy radicals [20–31]. There are three potential pathways for decomposition of alkoxy radicals that involve an α-ester rearrangement, oxidation by molecular O2 and thermal decomposition (C–O bond scission) processes. These are represented as follows:
was chosen as it was used for developing the model functional. The hybrid meta-density functional, MPWB1K has been found to give excellent results for thermochemistry and kinetics and is known to produce reliable results [33–35]. In order to determine the nature of different stationary points on the potential energy surface, vibrational frequencies calculations were performed using the same level of theory as that used for optimization. All the stationary points were identified to correspond to stable minima by ascertaining that all the vibrational frequencies had real positive values. The transition states were characterized by the presence of only one imaginary frequency. To ascertain that the identified transition states connect reactant and products smoothly, intrinsic reaction coordinate (IRC) calculations [36] were performed at the MPWB1K/6–31+G(d, p) level. As the reaction energy barriers are very sensitive to the theoretical levels, the higherorder-correlation-corrected relative energies along with the density functional energies are necessary to obtain theoretically consistent reaction energies. Therefore, a potentially high-level method such as G2(MP2) was used for singlepoint energy calculations. The G2(MP2) [37] energy was calculated in the following manner: E½G2ðMP2Þ ¼ Ebase þ ΔEðMP2Þ þ HLC þ ZPE where, Ebase =E[QCISD(T)/6−311G(d,p)],
CF3 CðOÞOCH2 O• þ O2
→
→ CF3 CðOÞOH þ CHO•
CF3 CðOÞOCðOÞH þ HO• 2
→ CF3 CðOÞO• þ CH2 O•
ð1Þ ð2Þ ð3Þ
Using the power of quantum chemistry methods, our purpose is two-fold: (1) to gain some insight into the fate of the alkoxy radical, analyzing the mechanism of the assumed oxidation with O2 that leads to the formation of CF3C(O)OC(O)H and HO•2; and (2) to study the importance of the other pathways that these radicals may undergo. To the best of my knowledge, this is the first computational evidence of the occurrence of the α-ester rearrangement for this alkoxy radical derived from fluorinated ester leading to the formation of TFA. Thermochemical studies have been performed to analyze the stability of all species involved in these reactions.
Computational methods Geometry optimization of the reactants, transition states and products was carried out at the MPWB1K level of theory [32] using the 6–31+G(d, p) basis set. The 6–31+G(d, p) basis set
ΔEðMP2Þ ¼ E½MP2=6−311 þ Gð3df ; 2pÞ−E½MP2= 6−3
11Gðd; pÞ; and HLC (high level correction)=−0.00481nβ −0.00019nα(nα and nβ are the number of α and β valence electrons with α ≥nβ) and ZPE=zeropoint energy. Geometry and frequency calculations were performed at MPWB1K/6–31+G(d, p) level. The ZPE thus obtained was corrected with a scale factor of 0.9537 to partly eliminate systematic errors [32]. This dual level calculation G2(MP2)/ MPWB1K/6–31+G(d, p) is known to produce reliable thermochemical and kinetic data [17, 38–42]. To ascertain the accuracy of the MPWB1K results, we also performed geometry optimization and single-point energy calculation with the B3LYP method using 6-31G (d, p) and 6-311G (2df,2p) basis sets. All quantum mechanical calculations were performed with the Gaussian 09 suite of programs [43].
Results and discussion The detailed thermodynamic calculations performed at G2 (MP2), MPWB1K and B3LYP levels for reaction enthalpies and free energies associated with reaction channels (1–3) are listed in Table 1. Thermal corrections to the energy at 298 K were included in the determination of these thermodynamic functions. Free energy values show that two reactions (1 and 2) are exergonic (ΔrG0298
