Chemical Physics Letters 555 (2013) 72–78

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Chemical Physics Letters

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Reaction of volatile anaesthetic desflurane with chlorine atom.Theoretical investigation

Wiktor Zierkiewicz ⇑Faculty of Chemistry, Wrocław University of Technology, Wybrze _ze Wyspianskiego 27, 50-370 Wrocław, Poland

a r t i c l e i n f o

Article history:Received 16 October 2012In final form 7 November 2012Available online 16 November 2012

0009-2614/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cplett.2012.11.011

⇑ Fax: +48 71 320 4360.E-mail address: wiktor.zierkiewicz@pwr.wroc.pl

a b s t r a c t

In this work, the mechanism of the reaction between desflurane (CF3CHFOCHF2) and Cl was investigatedat the CCSD(T)/CBS level of theory. The reaction between an anaesthetic and Cl is described by a three-step mechanism. The reaction channel where the hydrogen atom from the CHF group is abstracted isthe most favorable. The CCSD(T)/CBS electronic energy calculated for this channel is 0.47 kcal mol�1.The CCSD(T)/CBS enthalpies of three channels range between 1.17 and 5.25 kcal mol�1, which indicatesthe endothermic processes. All the steps of the reaction studied are discussed in details.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Increase of the atmospheric concentration of halogenated or-ganic compounds is partially responsible for a change of the globalclimate. They influence on the cooling mechanism of the Earth byhindering the escape of the outgoing terrestrial IR radiation intospace [1].

Halogenated ethers, such as desflurane, enflurane, isofluraneand sevoflurane are the most widely used inhaled anaestheticagents. It is known that about 20% of all clinically used volatile ana-esthetics are released to the atmosphere [2]. It has been reportedthat combined global emissions of desflurane, isoflurane and sevo-flurane are likely in the range of several kilotons per year [1].Andersen et al. [1] have estimated that the annual climate impactof global emissions of inhaled anaesthetics, is equivalent to emis-sion of CO2 by one million of cars.

Desflurane (1,2,2,2-tetrafluoroethyl difluoromethyl ether) ahighly volatile narcotic gas, has been the subject of several studies[3–7]. The atmospheric lifetime of this greenhouse gas was esti-mated at 14 years [8]. The chlorine atoms play an important rolein the atmospheric chemistry [9]. An average global concentrationof Cl was estimated to be about 104 atoms cm�3 [10].

The reactions of the Cl atom with several halogenatedethers: CF3CH2OCH3 [11], CH3CH2OCF3 [11], CF3CH2OCHF2 [12–15],CF3CH2OCClF2 [14], CF3CHFOCF3 [16,17], CF3CH2OCF3 [18] andCHFClCF2OCHF2 [9] have been studied in the literature.

It has been shown that desflurane (CF3CHFOCHF2) reacts withthe chlorine atom [8,11,19]. However, no theoretical investigationhas been performed on the mechanism of this process as yet. Inthis work, the thorough studies were performed at the CCSD(T)/

ll rights reserved.

CBS//wB97XD/6-311++G(d,p) level of theory. The results obtainedcan be important for understanding the mechanism of the reactionwhich constitutes the initial step of degradation of an anaestheticin atmosphere.

2. Theoretical methods

Full geometry optimizations followed by the calculations ofvibrational frequencies and infrared intensities were performedfor all reactants, complexes, products and transition states of twopossible channels of reaction between desflurane and the chlorineatom using the restricted and unrestricted density functionalmethod wB97XD [20] combined with the 6-311++G(d,p) basis set[21,22]. It is worth to mention that Croft et al. [23,24] have shownthat the wB97XD functional performs remarkably well in calcula-tions of stabilization energies of the chlorine atom complexes.

The electronic and interaction energies were evaluated by thecomplete basis set limit (CBS) calculations at the CCSD(T) orUCCSD(T) levels of theory [25].

All calculations for open-shell systems were based on an unre-stricted mechanism; however, for clarity, U will be omitted fromthe UwB97XD, UCCSD(T) and UMP2 abbreviations, in the remain-ing text. The CCSD(T)/CBS stabilization energy was calculated asthe sum of the MP2/CBS stabilization energy and the CCSD(T) cor-rection term [26]. The MP2/CBS energy was extrapolated from theMP2 energies evaluated at the MP2/aug-cc-pVTZ and MP2/aug-ccpVQZ levels. The extrapolation method of Helgaker et al. hasbeen used [27]. The CCSD(T) correction term (the difference be-tween the CCSD(T) and MP2 interaction energies) was determinedby using the aug-cc-pVDZ basis set [28,29].

The CCSD(T)/CBS enthalpies of formation of the species investi-gated, under standard conditions in the gas phase, were deter-mined as the sum of the CCSD(T)/CBS electronic energies and the

W. Zierkiewicz / Chemical Physics Letters 555 (2013) 72–78 73

zero-point vibrational energy (ZPE) and the thermal correction toenthalpy obtained at the wB97XD/6-311++G(d,p) level.

The transition states of the reaction channels have been foundusing a synchronous transit-guided quasi-Newton (STQN) method[30,31]. The first-order saddle point was confirmed by calculationsof vibrational frequencies (one imaginary frequency was obtained).Natural bond orbital (NBO) analysis has been performed at the DFTlevel [32,33]. For open-shell species NBO analysis was applied sep-arately to a and b spin density matrices, as described by Carpenterand Weinhold [34]. All computations were carried out with GAUSSIAN

09 set of programs [35].

3. Results and discussion

The most stable conformer of desflurane has been selected forinvestigation [7]. Its structure has been optimized at thewB97XD/6-311++G(d,p) level of theory, and is presented in Figure 1(selected structural parameters are depicted).

It has been shown that CF3CHFOCHF2 (desflurane) reacts withchlorine atom at temperature of 295 ± 4 K [8,9,18]. The rate coeffi-cient of this reaction, determined experimentally, equals to(1.0 ± 0.3) � 10�15 [8], (1.1 ± 1.9) � 10�15 [11], (1.20 ± 0.12) �10�15 cm3 molecule�1 s�1 [19] at 295 ± 4 K. In this reaction one ofthe hydrogen atoms is abstracted.

The mechanism of the reaction between desflurane and thechlorine atom can be described as a three-step mechanism:

CF3CHFOCHF2 þ Cl� ! CF3CHFOCHF2 . . . Cl� step1CF3CHFOCHF2 . . . Cl� ! TS1! ½CF3CFOCHF2� � . . . HCl step2½CF3CFOCHF2� � . . . HCl! ½CF3CFOCHF2� � þHCl step3

where the H8 atom is abstracted.And

CF3CHFOCHF2 þ Cl� ! CF3CHFOCHF2 . . . Cl� step1CF3CHFOCHF2 . . . Cl� ! TS2! ½CFÞ3CHFOCF2� � . . . HCl step2½CF3CHFOCF2� � . . . HCl! ½CF3CHFOCF2� � þHCl step3

where the H12 atom is abstracted.According to the experimental data, the [CF3CFOCHF2]� or

[CF3CHFOCF2]� radicals are the products of this reaction in a yieldof about 83% or 17%, respectively [8]. This fact suggests that theC4–H12 bond is stronger than the C2–H8 one.

As follows from Figure 1 the C2–H8 atom distance (1.093 Å) islonger than the C4–H12 distance by 0.005 Å. The calculated C-Hstretching frequencies are 3126 and 3188 cm�1 for C2–H8 andC4–H12, respectively.

NBO analysis has revealed that the occupancies of the bondingr(C2–H8) and r(C4–H12) orbitals are very similar (1.9872 and1.9875 e, respectively), while occupancies of the antibonding

Figure 1. Atom numbering and selected structural parameters of the optimizedstructure of the most stable conformer of desflurane. Distances are in angstroms,angles are in degrees. Calculation performed at the wB97XD/6-311++G(d,p) level oftheory.

sigma orbitals differ from each other. The occupancy of r�(C2–H8) is 0.0321 e, while that of the r�(C4–H12) orbital is 0.0422 e,which is surprising. Thus, despite the fact that the occupancy ofthe r�(C4–H12) orbital is larger than that of the r�(C2–H8), theC4–H12 bond length is shorter than C2–H8.

In desflurane, the C2 and C4 carbon atoms have different natu-ral charges, C2 has a small positive charge, 0.441 e, while the C4atom accommodates a large charge, 0.864 e. This is caused by dif-ferent environments of the C2 and C4 atoms (see Figure 1). A nat-ural charge on the H8 atom (0.186 e) is smaller than that on C2, by0.255 e. A natural charge on the H12 atom (0.148 e) is smaller thanthat on C4, by 0.716 e. Thus, the larger difference between thecharges on the H and C atoms corresponds to a stronger C–H bond.

The CCSD(T)/CBS//wB97XD/6-311++G(d,p) calculated bond dis-sociation energies (BDEs) for C2–H8 and C4–H12 are 104.83 and108.91 kcal mol�1, respectively.

To test the level of calculations, the value of BDE of the O–Hbond in the water molecule has been computed and compared tothe literature data. The experimental value of BDE of the O–H bondin water is 118.1 [36] and 117.9 kcal mol�1 [37]. BDE calculated atthe CCSD(T)/CBS//wB97XD/6-311++G(d,p) level, in this work, is118.36 kcal mol�1, while the BDE values obtained at the MC-QCISD//BH&H-LYP, MC-QCISD//B3LYP and MC-QCISD//MP2 are119.83, 120.19 and 120.04 kcal mol�1, respectively [17]. Thus, thevalue of BDE of the O–H bond in water, obtained at the theoreticallevel applied in this work, almost reproduces the experimentalvalue.

3.1. Energy profile of the reaction

Figure 2 presents the CCSD(T)/CBS//wB97XD/6-311++G(d,p)electronic energy profile of the title reaction (the ZPE correctionshave been obtained at the wB97XD/6-311++G(d,p) level). Thesum of the energies of the reactants (isolated desflurane and Cl)was set to be zero for reference. The reaction between desfluraneand the chlorine atom has three reaction channels: abstraction ofthe H8 atom (channel 1 is marked by the dotted black line),abstraction of the H12 atom (channel 2 and 3 are marked by thedashed orange and green lines, respectively). As it is seen from thisfigure, the products of the reaction channels 2 and 3 are the same.

The channel 1 is energetically more favorable than the others.The CCSD(T)/CBS calculated electronic energy corrected for ZPE(DECCSD(T)

corr) for channel 1 is 0.47 kcal mol�1, while the corre-sponding value obtained for two other channels is larger,4.58 kcal mol�1 (see Figure 2). The enthalpies of the reaction chan-nels calculated at the CCSD(T)/CBS//wB97XD/6-311++G(d,p) levelare 1.17 kcal mol�1 (channel 1) and 5.25 kcal mol�1 (channels 2and 3). Thus, all reactions are endothermic.

Figure 2 shows that for all channels, in the first step, the com-plexes between desflurane and the chlorine atom (I, II and III in Fig-ure 2) are formed with the binding energies (DECCSD(T)

corr) of �1.93,�1.45, and�1.37 kcal mol�1, respectively. Is worth to mention thatvalues of the energy of the first step of the reaction betweenCF3CHFOCF3 and the chlorine atom, calculated at the MC-QCISD//BH&H-LYP/6-311G(d,p) level, was �1.99 kcal mol�1 [17].

In the next step, the reaction proceeds via transition states, TS1for channel 1 and TS2 for the channels 2 and 3. As follows from Fig-ure 2, the values of the DECCSD(T)

corr of the barrier height is5.58 kcal mol�1 for channel 1, and 8.09 kcal mol�1 for the otherchannels. The MC-QCISD//BH&H-LYP/6-311G(d,p) + ZPE calculatedvalues of the TS for the reaction between CF3CHFOCF3 and Cl was5.39 kcal mol�1 [17].

Finally, in this step the complexes between the desflurane rad-ical and HCl molecule are formed with the DECCSD(T)

corr equal to�1.37, 2.68, and 4.60 kcal mol�1 for IV, V, VI, respectively. The

Figure 2. Potential energy surface (PES) for reaction of desflurane with the chlorine atom. Electronic energies were calculated at the CCSD(T)/CBS//wB97XD/6-311++G(d,p)level, and corrected for ZPE obtained at the wB97XD/6-311++G(d,p) level. The sum of the energies of the reactants (isolated desflurane and the chorine atom) was set as zerofor reference. Channel 1 is marked by a dotted black line, channel 2 by a dashed orange line and channel 3 by a dashed green line. (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

74 W. Zierkiewicz / Chemical Physics Letters 555 (2013) 72–78

energy of the analogous step of the CF3CHFOCF3 + Cl reaction was�2.08 kcal mol�1 [17].

In the last step of the reaction, these complexes dissociate intothe isolated radical (VII for channel 1, VIII for channels 2 and 3) andthe HF molecule. In the next section, the steps of the reaction stud-ied will be discussed in details.

3.2. Step 1 – formation of desflurane. . .Cl complex

The first step of the reaction studied is the formation of a com-plex between desflurane and the chlorine atom. The wB97XDinvestigation of the potential energy surface (PES) has revealedthe presence of three minima (complexes I, II and III), which corre-spond to three structural isomers of the [CF3CHFOCHF2. . .Cl]� com-plex. Figure 3 shows the structures of these complexes.

According to the CCSD(T)/CBS results the absolute value of theelectronic energy of the complex (I) is larger than these of the com-plexes (II) and (III) by 0.32 and 0.38 kcal mol�1, respectively.

As follows from Figure 3, the chlorine atom in the complex (I)interacts with the F9 and H8 atoms of desflurane. The Cl. . .F9(3.24 Å) and Cl. . .H8 (2.99 Å) distances are slightly longer thanthe sum of the corresponding van der Waals radii (3.22 and2.95 Å, respectively). The C2F9 bond distance is elongated undercomplexation by 0.004 Å, while the C2H8 bond distances is chan-ged by less than 0.001 Å. The interaction between the Cl and H8atoms can be confirmed by the decrease of the C2H8 stretching fre-quency, by 7 cm�1. The C2F9Cl and C2H8Cl angles are 95.8� and118.3�, respectively.

In complexes (II) and (III) the chlorine atom interacts with theH12 atom and the F11 or F10 atom, respectively (see Figure 2).In both complexes the Cl. . .H12 distances (3.22 and 3.06 Å) arelarger than the sum of the corresponding van der Waals radii.The Cl. . .F11 distance in complex (II) and the Cl. . .F10 in (III) arealso longer than 3.22 Å. It is worth to mention that the Cl. . .O3 dis-

tance in complex (II) is 3.76 Å (sum of the corresponding van derWaals radii is 3.27 Å). What is interesting, in the (II) complex theC4F11 bond is contracted by �0.003 Å with respect to that in iso-lated desflurane, while the C4F10 bond length in the (III) complexis elongated by 0.006 Å under complexation.

Despite the fact that in both complexes considered, the C4H12bond is contracted by 0.001 Å, no change in the C–H stretching fre-quency was observed.

The CCSD(T)/CBS calculated interaction energies for complexes(I), (II) and (III) are �2.01, �1.60 and �1.45 kcal mol�1, respec-tively. The fact that the chlorine atom is stronger bound in complex(II) than in (III) can be explained as consequence of an additionalinteraction between Cl and the O3 in the former complex.

The CCSD(T)/CBS calculated electronic energies and enthalpiesof the first step of three channels of the title reaction are presentedin Table 1.

As follows from the data presented in Table 1, all of these reac-tions are exothermic. The values of the DECCSD(T)

corr are �1.93,�1.45 and �1.37 kcal mol�1 for reaction channels 1, 2 and 3,respectively.

3.3. Step 2 – formation of desflurane radical and HCl complex

In the second step of the reaction the complex between the des-flurane radical and HCl molecule is formed. In this step the interac-tion of the chlorine atom with the H8 or H12 atoms leads to thechemically activated intermediate which decomposes by cleavageof the C–H bond and the formation of the HCl molecule bondedto the desflurane radical. This step is the rate limiting step of thereaction mechanism.

Figure 4 shows the structures of the transition states TS1 andTS2 optimized at the B97XD/6-311++G(d,p) level. It should bestressed that TS1 is the transition state of the channel 1, whileTS2 is the transition state of the other two reaction channels (see

Figure 3. Structures of desflurane complexes with the chlorine atom optimized at the wB97XD/6-311++G(d,p) level. Selected distances are marked by a dot line. Distancesare in angstroms, angles are in degrees.

Table 1Electronic energies and enthalpies of step 1 of reaction CF3CHFOCHF2 + Cl (inkcal mol�1). Results from the CCSD(T)/CBS//wB97XD/6-311++G(d,p) calculations.

Channel DECCSD(T) DECCSD(T)corr

a DHCCSD(T) b

1 �2.01 �1.93 �1.742 �1.59 �1.45 �1.273 �1.45 �1.37 �1.16

a Corrected for ZPE obtained at the wB97XD/6-311++G(d,p) level.b Corrected for ZPE and thermal correction to enthalpy, obtained at the wB97XD/

6-311++G(d,p) level.

W. Zierkiewicz / Chemical Physics Letters 555 (2013) 72–78 75

Figure 2). Both structures of the transition states were confirmedby calculations of vibrational frequencies. One imaginary fre-quency has been obtained, �1059 and �831 cm�1 for TS1 andTS2, respectively. Each of the imaginary frequencies correspondsto the normal mode, which can be described as the motion of theH atom between the C and Cl atoms.

As is seen in Figure 4, in the case of TS1, the C2. . .H8 distance(1.389 Å) is elongated by 0.296 Å (27%) in comparison to the equi-librium bond length. The H8. . .Cl distance in this transition stateamounts to 1.470 Å which is longer by 15% than the bond lengthin isolated HCl (1.283 Å). In the case of TS2, the C4. . .H12 bond(1.443 Å) is elongated by 0.355 Å (33%) in comparison to the equi-librium distance, while the H12. . .Cl atom distance (1.426 Å) is 11%longer than the H–Cl bond length in isolated hydrogen chloride. InTS1, an elongation of the C2–H8 bond leads to the contraction ofthe C2–O3 bond by �0.037 Å. In TS2, an analogous elongation ofthe C4–H12 bond causes the contraction of the O3–C4 bond by�0.029 Å.

This step leads to the intermediate. The values of the barrierheight calculated at the CCSD(T)/CBS level (the electronic energycorrected for ZPE) are 5.58 and 8.09 kcal mol�1 for channel 1 andthe other channels, respectively (see Figure 2).

In TS1 and TS2, the spin population on the chlorine atom is0.415 and 0.358 e, respectively. The remaining spin population isdistributed over the desfurane molecule. A high spin density hasbeen found on the C2 atom in TS1 (0.476 e), and on the C4 atomin TS2 (0.526 e).

The structures of the complexes IV, V and VI which are the prod-ucts of the second step of the title reaction are shown in Figure 4.

As follows from Figure 4, in the complex (IV) there is a hydrogenbond between hydrogen chloride and desfurane radical. The dis-tance between H8 and F10 (2.23 Å) is smaller than the sum ofthe corresponding van der Waals radii (2.67 Å). The ClH8F10 angleequals to 152.2�. The CCSD(T)/CBS interaction energy in this com-plex is �2.92 kcal mol�1. The binding energy of the complex (V) isslightly smaller and equals to �2.90 kcal mol�1. This complex isalso stabilized by hydrogen bond between HCl and the radical.The H12. . .O3 distance is 2.17 Å (the sum of the van der Waals radiiof H and O is 2.72 Å). As follows from Figure 4, in the (VI) complexthe H12. . .C4 distance (2.29 Å) is shorter than the sum of the cor-responding van der Waals radii by �0.61 Å. The binding energyof this complex is �1.07 kcal mol�1.

In the complexes discussed, the spin population is distributedover the desfurane radical. In the complex (IV), the highest spindensity has been found on the C2 atom (0.824 e). In the case ofthe complexes (V) and (VI) the C4 atom shows the highest spindensity (0.823 e and 0.777 e, respectively).

In Table 2, the electronic energies and enthalpies of the secondstep of CF3CHFOCHF2 + Cl reaction are collected.

Inspection of Table 2 reveals that all of these reactions are endo-thermic. The values of the DHCCSD(T) are 1.17, 4.71 and6.54 kcal mol�1 for reaction channels 1, 2 and 3, respectively. Thecorresponding values of the DECCSD(T)

corr are 0.56, 4.13 and5.97 kcal mol�1.

3.4. Step 3 – dissociation of the complexes between the desfluraneradical and HCl

The last step of the title reaction is the dissociation of the com-plexes between the desflurane radical and HCl. The optimizedstructures of the [CF3CFOCHF2]� and [CF3CHFOCF2]� radicals areshown in Figure 5 (along with some selected geometricalparameters).

In the case of the radical (VII), the highest spin density has beenfound on the C2 atom (0.824 e), while in the radical (VIII) it is onthe C4 atom (0.823 e).

The electronic energies and enthalpies of the last step of reac-tion studied are collected in Table 3. According to these resultsthe dissociation reactions in channels 1 and 2 are endothermic,

Figure 4. Structures of the transition states TS1 and TS2, and desflurane complexes with chlorine radical optimized at the wB97XD/6-311++G(d,p) level. Selected distancesare marked by a dot line. Distances are in angstroms, angles are in degrees.

Table 2Electronic energies and enthalpies of step 2 of reaction channels of CF3CHFOCHF2 + Cl

76 W. Zierkiewicz / Chemical Physics Letters 555 (2013) 72–78

while the reaction in channel 3 is slightly exothermic. The valuesof the DHCCSD(T) are 1.74, 1.81 and �0.13 kcal mol�1, respectively.

(in kcal mol�1). Results from the CCSD(T)/CBS//wB97XD/6-311++G(d,p) calculations.

Channel DECCSD(T) DECCSD(T)corr

a DHCCSD(T)b

1 3.91 0.56 1.172 7.57 4.13 4.713 9.43 5.97 6.54

a Corrected for ZPE obtained at the wB97XD/6-311++G(d,p) level.b Corrected for ZPE and thermal correction to enthalpy obtained at the wB97XD/

6-311++G(d,p) level.

3.5. Rate constant of the reaction

The rate constant of the title reaction was calculated from thefollowing equation:

kðTÞ ¼ kBT

hc0 expð�DG#=RTÞ ð1Þ

where kB is the Boltzmann constant (1.380649 � 10�23 J K�1); T thetemperature; h the Planck constant (6.6260696 � 10�34 J s�1); c0

the standard concentration (2.68719 � 1019 molecules cm�3); DG#

the difference between Gibbs free energy of the transition stateand the sum of the G of the reactants (isolated desflurane and thechlorine atom); R is the gas constant (1.985876 cal K�1 mol�1).

The DG# calculated at the CCSD(T)/CBS//wB97XD/6-311++G(d,p)level are 12.62 kcal mol�1 and 14.33 kcal mol�1 for channel 1 andthe other channels, respectively. The calculated (from Eq. (1)) rate

constants of the reactions (at 296 K) are 0.11� 10�15 cm3 mole-cule�1 s�1 (channel 1) and 6.02� 10�18 cm3 molecule�1 s�1 (chan-nels 2 and 3). Therefore, the reaction channel 1 is about 20 timesfaster than the others. Most recently, Andersen et al. determinedexperimentally the rate constant of the reaction between desfluraneand chlorine atom to be equal to (1.0 ± 0.7)� 10�15 cm3 mole-cule�1 s�1 at 296 K [8]. Thus, taking into account the experimentaluncertainty, the calculated rate constant of the reaction channel 1

Figure 5. Structures of desflurane radicals optimized at the wB97XD/6-311++G(d,p) level. Distances are in angstroms, angles are in degrees. In parentheses are the differencesbetween the corresponding values in desflurane radical and desflurane monomer.

Table 3Electronic energies and enthalpies of step 3 of reaction channels of reactionCF3CHFOCHF2 + Cl (in kcal mol�1). The results from the CCSD(T)/CBS//wB97XD/6-311++G(d,p) calculations.

Channel DECCSD(T) DECCSD(T)corr

a DHCCSD(T)b

1 2.81 1.84 1.742 2.86 1.90 1.813 0.86 �0.02 �0.13

a Corrected for ZPE obtained at the wB97XD/6-311++G(d,p) level.b Corrected for ZPE and thermal correction to enthalpy obtained at the wB97XD/

6-311++G(d,p) level.

W. Zierkiewicz / Chemical Physics Letters 555 (2013) 72–78 77

only slightly underestimates the experimental value. Is worth tomention that values of the k(T) of the reaction betweenCF3CHFOCF3 and the chlorine atom, calculated with the ICVT/SCTapproach (improved canonical variational transition state theoryincorporating the small curvature tunneling) also slightly underes-timated the experimental value [17].

4. Conclusions

The most important conclusions are the following:

(1) The mechanism of the reaction between desflurane and thechlorine atom can be described as a three-step mechanism:first, formation of the desflurane���Cl complexes, next, forma-tion of the complexes between the desflurane radical andHCl, and finally, dissociation of these complexes into the iso-lated radical and hydrogen chloride.

(2) Three reaction channels were identified, one for the abstrac-tion of the hydrogen atom from the CHF group, and thetwo others for the hydrogen atom abstraction from the CHF2

group. The CCSD(T)/CBS calculated values of the enthalpiesof these three channels range between 1.17 and 5.25kcal mol�1, which indicates the endothermic processes.

(3) The most energetically favorable reaction channel is thatwhere the hydrogen atom from the CHF group is abstracted.The CCSD(T)/CBS electronic energy corrected for ZPE(DECCSD(T)

corr), calculated for this channel is 0.47 kcal mol�1.The energy of the other two channels is equal to4.58 kcal mol�1.

(4) The second step of the reaction (in all three channels) pro-ceeds via the transition state. This is the rate limiting stepof the reaction mechanism. The value of the barrier height

(DECCSD(T)corr) of the most energetically favorable reaction

is 5.58 kcal mol�1 and 8.09 kcal mol�1 for the channel 2and 3, respectively.

(5) The rate constants of the reactions were calculated usingGibbs free energies obtained at the CCSD(T)/CBS//wB97XD/6-311++G(d,p) level. The calculated rate constant of thereaction channel 1 is 0.11 � 10�15 cm3 molecule�1 s�1. Thereaction channel 1 is about 20 times faster than the others.The calculated rate constant of the reaction channel 1slightly underestimates the experimentally determinedvalue.

Acknowledgments

The work was financed by a statutory activity subsidy from thePolish Ministry of Science and Higher Education for the Faculty ofChemistry of Wrocław University of Technology. A generous com-puter time from the Wroclaw Supercomputer and NetworkingCenter as well as Poznan Supercomputer and Networking Centeris acknowledged.

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