Kinetics Studies of the Reactions of Main Fourth-Period Monocations

Article pubs.acs.org/JPCA

Kinetics Studies of the Reactions of Main Fourth-Period Monocations (Ga+, Ge+, As+, and Se+) with Methyl Fluoride Carmen Barrientos,† Víctor Manuel Rayón,† Antonio Largo,† José Á ngel Sordo,‡ and Pilar Redondo*,† †

Departamento de Química Física y Química Inorgánica, Facultad de Ciencias, Universidad de Valladolid, 47011 Valladolid, Spain Laboratorio de Química Computacional, Departamento de Química Física y Analítica, Facultad de Química, Universidad de Oviedo, 33006 Oviedo, Spain

‡

ABSTRACT: Thermodynamics and kinetics theoretical studies on the gas-phase reactions of fluoromethane with main fourth-period monocations (Ga+, Ge+, As+, and Se+) have been carried out. Density functional theory (in particular mPW1K functional) was employed in the description of the potential energy surfaces, and refinement of the energies were done at the CCSD(T) level. The reaction rate constants were estimated using variational/conventional microcanonical transition state theory. From a thermodynamic viewpoint, the fluorine abstraction product is predicted for Ga+ and Ge+, whereas for As+ and Se+ the elimination product, MCH2+ (M = As, Se) + HF, is the preferred one. Nevertheless, the most favorable channel for the reactions of CH3F with Ga+ and Se+ cations present a net activation barrier. In the case of Ga+, the reaction proceeds via an addition channel forming the adduct complex, CH3FGa+, whereas for Se+ no reaction is found, in agreement with the experiments. The predicted reaction rate constants are in reasonable good agreement with the experimental values available. Apart from the harpoon-like mechanism, our results suggest that an oxidative addition mechanism seems to play a relevant role. cations, Ca+ and Ge+, addition with late-transition-metals cations and Ga+, HF elimination for As+, and no reaction for K+ and Se+. Recently, we have carried out theoretical studies of the potential energy surfaces (PESs) and kinetics for the reactions of CH3F with Ca+,15 alkaline-earth cations,16 and first-row transition metal (Sc+, Ti+, V+, Zn+).17 F-abstraction is shown to take place through a harpoon-like mechanism with a [M···FCH3]+ intermediate and a [M+2···F−···CH3] chargetransfer transition state. Such a mechanism was suggested in previous experimental studies for the reactions of Ca+,7 and lanthanide monocations6 with CH3F. The kinetics calculations in the framework of statistical theories (energy and total angular momentum resolved microcanonical transition state theory)15−17 for the different cations showed the relative importance of other channels, apart from the “inner” bottleneck (charge-transfer transition state), to control the kinetics of these reactions, such as the “outer” bottleneck (entrance channel) as well as contribution from the exit channel. The theoretically predicted thermal rate constants for these reactions at 295 K agreed reasonably well with the experimental values reported by Zhao et al.5 As a continuation of our project to give theoretical support for the great deal of experimental data available for this type of reactions, our aim in this study is to analyze the thermodynamics and kinetics of the reactions of fluoromethane with main fourth-period monocations (Ga+, Ge+, As+, and Se+).

I. INTRODUCTION One of the most active research fields in the last years has been selective covalent bond activation mediated by atoms or ions, due mainly to its potential applications in synthetic and catalytic processes.1−4 In this context, halogenated methane is an interesting substrate for reactions with different cations, as they constitute a simple gas-phase model that can provide an opportunity for analyzing the competition between C−H and C−X bond activation. In particular, the interaction of atoms or ions with fluoromethane has been the subject of several experimental and theoretical studies.5−12 We should point out that, due to the strong bond dissociation energy (BDE) of C−F single bond, the largest among the C−X ones, selective functionalization of C−F bonds is anything but an easy task. Apart from its relevance in synthetic chemistry, carbon− fluorine bond activation attracts a great deal of interest as a consequence of the adverse influence of fluorocarbons and mixed fluorohalocarbons in the mechanisms of stratospheric ozone depletion13 or as green-house gases.14 A systematic study, at room temperature, of the reactions of CH3F with 46 atomic transition-metal and main-group cations (fourth-period atomic cations from K+ to Se+, fifth-period atomic cations from Rb+ to Te+ (excluding Tc+), and sixthperiod atomic cations from Cs+ to Bi+) using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer has been reported by Zhao et al.5 Reaction products are shown to include F atom transfer, CH3F addition, HF elimination, and H2 elimination. The major product characterized in the reaction of CH3F with fourth-period cations was F atom transfer for the early transition-metal © 2013 American Chemical Society

Received: June 6, 2013 Revised: July 10, 2013 Published: July 16, 2013 7742

dx.doi.org/10.1021/jp405601y | J. Phys. Chem. A 2013, 117, 7742−7753

The Journal of Physical Chemistry A

Article

calculations were performed with the Gaussian09 package of programs.27 A detailed description of the kinetics model that we have used can be found in our previous study on the reaction between Ca+ and CH3F.15 Here, only the most important features will be summarized. A general schematic representation of the energy profile for the kinetics calculations is shown in Scheme 1. To deal with this type of processes, Mozurkewich

The major products obtained experimentally for these cations, with the only exception of Ge+,5 are different from F atom transfer products reported for early transition-metal cations and Ca+. In a recent work,18 the PESs for the channels evolving from the low-lying electronic states of the reactants in the M+ + CH3F (M = Ge, As, Se, Sb) reactions have been studied at the CASSCF-MRMP2 level. The formation of AsCH2+ from the reaction of As+ + CH3F in the ground state has also been studied19 employing the B3LYP functional followed by CCSD(T) single-point calculations on the resulting stationary points. However, it should be stressed that there are no kinetics studies available for these reactions, and consequently, a direct comparison with Zhao et al.’s experimental data5 is not possible. It is the main goal in the present article to fill the gap between quantum predictions and kinetics measurements by employing an appropriate theoretical kinetics model. In this work, we present a systematic density functional theory (DFT) theoretical study where we combine the characterization of the PESs for the M+ + CH3F (M = Ga, Ge, As, and Se) reactions and kinetics calculations allowing us to compute rate constants to be compared with available experimental data. The present systematic study does provide valuable mechanistic information about a number of reaction paths leading to different products, thus complementing the experimental information available and help throwing additional light on the mechanistic aspects of these processes.

Scheme 1. Schematic Representation of the General Energy Profile Employed for Kinetics Calculations

and Benson developed a kinetics formulation within the context of the Rice−Ramsperger−Kassel−Marcus (RRKM) theory.28 The main assumption in the Benson−Mozurkewich model is that the pressure is sufficiently low that the intermediates do not undergo any collisions (with the total energy, E, and total angular momentum, J, conserved). Since Zhao et al.5 employed relatively low pressures in their experiments (P = 0.35 Torr), eq 1 should work reasonably well. Under steady-state conditions, the global (observed) rate constant can be expressed as

II. COMPUTATIONAL DETAILS In our previous study on the reaction of Ca+ with fluoromethane,15 we have tested different DFT methods in order to establish the most reliable one for (a) a qualitative description of the PESs through practical theoretical methodologies, i.e., levels of theory requesting affordable computational costs, and (b) provide suitable kinetics information, i.e., rate constants to be contrasted with the experimental values. On the basis of our results, we have selected for fully exploring the PESs the modified Perdew−Wang-1-parameter functional (mPW1K),20 which has been specifically developed for kinetics studies. In conjunction with this functional, we have used Ahlrichs’ triple-ζ bases, TZVPP.21 The most relevant stationary points on the PESs were recalculated employing the Ahlrichs’ quadruple-ζ bases, QZVPP,21 followed by single-point calculations at the CCSD(T) (coupled cluster single and double excitation model augmented with a noniterative treatment of triple excitations) level,22 in order to refine the electronic energies. In the CCSD(T) calculations, we checked the T1 diagnostic,23 which was found in all cases well below the value of 0.04,24 thus supporting the validity of the CCSD(T) approach in the present case. Geometry optimizations were carried out using a tight convergence criterium and an ultrafine grid for numerical integrations. The stationary points on the PESs have been characterized by checking the negative eigenvalues of the analytical Hessian (zero for local minima and one for first-order saddle points corresponding to transition structures). In order to confirm that the transition states correspond to the desire process, intrinsic reaction coordinate (IRC)25 calculations were carried out. Both the zero-point energy and the thermal contributions to enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) have been estimated within the ideal gas, rigid rotor, and harmonic oscillator approximations.26 A temperature of 298.15 K and a pressure of 1 atm have been considered. All the electronic structure and statistical thermodynamics

kglobal

( = ×

2πμkBT −3/2 h2

)

hQ R

∞

∑∫ J=0

∞

Vmax

dEW1(E , J )

W2(E , J )· W3(E , J ) W2(E , J )· W3(E , J ) + W1(E , J )[W2(E , J ) + W3(E , J )]

× e−E / RT

(1)

where Vmax is the largest value from among the energy barriers associated with the energy profile (Scheme 1), QR represents the product of the partition functions of reactants, where the center of mass motion partition function has been factored out, and Wi(E,J) (i = 1−3) functions are the sum of states at energy lower than E and angular momentum J corresponding to the different transition structures TSi (see below). Besides, T is the temperature, R, kB, and h are the gas, Boltzmann, and Planck constants, and μ is the reduced mass. Reaction symmetry factors have been included in the sums of states, Wi(E,J), which have been computed by means of the Forst algorithm.29 For processes where no transition structure (first-order saddle point) was found on the PESs, such as the entrance and exit channels (see Scheme 1), we have adopted the E,J-resolved microcanonical variational transition state theory (μVTST) in its vibrator formulation.30,31 More specifically, a distinguishedcoordinate path (DCP)32,33 was constructed for each channel, and subsequently, projected frequencies were obtained for each point on that path. As variational transition state theory at the CCSD(T) level does not represent a practical level to carry out systematic studies like the one presented here, the mPW1K level of theory has been chosen to carry out the kinetics calculations. As we did in our previous studies,15−17 a two transition state model has been adopted (2-TS model) in the cases of Ge+ and Se+. We have explicitly considered an inner (tight) transition state located in the neighborhood of the firstorder saddle point and an outer (loose) transition state 7743



Article

Figure 1. Energy profile (in kcal/mol) for the Ga+(1S)/Ga+(3P) + CH3F reaction. The relative (taking the reactants as a reference) adiabatic potential energies (ΔU0 = ΔU + ZPE) at the mPW1K/TZVPP, mPW1K/QZVPP (in parentheses), and CCSD(T)/QZVPP//mPW1K/QZVPP (in bracket) levels are given in black and the corresponding Gibbs free energies in blue [ΔG].

elimination, are clearly endothermic processes. Only from the first excited state, 3P, (located 124.4 kcal/mol above the ground state at the mPW1K/TZVPP level) one obtains exothermic products. The most stable products on the triplet PES are GaCH2+(3B1) + HF, whereas, on the singlet PES, fluorine abstraction is preferred. If we consider the ground state of Ga+(1S), the reaction starts with the barrierless approach of gallium cation to the fluorine atom of CH3F, leading to a rather stable structure GaFCH3+(1A1) that, passing through the transition structure TS2(1A), gives the intermediate FGaCH3+(1A1), where the insertion of the gallium into the C−F bond has taken place. From this intermediate, formation of CH3 + GaF+(2Σ) is a direct process that does not involve a transition state (path 1a). In addition, via transition state TS5(1A′), one obtains HFGaCH2+(1A′), that through a direct process yields GaCH2+(1A1) + HF (path 1b). The formation of both endothermic products, CH3 + GaF +(2Σ) and GaCH2+(1A1) + HF, on the singlet PES involves quite large energy barriers. Two similar paths, 3a and 3b (Figure 1), are found if we start the reaction from the excited state of gallium 3 P. In this case, the formation of GaCH2+(3B1) is the most exothermic process. Both paths are barrier free. On both PESs (singlet and triplet), we have located an additional path, involving two transition structures, similar to what we will discuss later for the reaction with Ge+, leading to GaCH2+. In order to simplify the graphic, we have not included them in Figure 1 as they imply larger energy barriers than paths 1b and 3b, and therefore, they do not play a significant role in this case.

controlling the entrance channel. In the case of the reaction of the arsenic and gallium cations with methyl fluoride, a third transition state corresponding either to an additional inner (tight) transition state or to exit channel, respectively, was also considered (3-TS model). Our own routines were employed to carry out the kinetics calculations.15

III. RESULTS AND DISCUSSION We have investigated the most relevant stationary points on the PESs for the reactions between CH3F and Ga+, Ge+, As+, and Se+ cations. The obtained results including adiabatic potential energies (ΔU0 = ΔU + zero-point energy) and Gibbs free energies (ΔG) are shown in Figures 1−4. It is well-known that more than one electronic state may be involved in reactions catalyzed by organometallic substrates.34 Therefore, PESs corresponding to the ground and low-lying excited states of cations were investigated. In the case of the reaction with Ge+, only the doublet ground state PES is presented because the quartet PES should not be involved in the products formation (4P state for Ge+ is calculated to be about 136.55 kcal/mol above the 2P ground state, at the CCSD(T)/QZVPP level). Table 1 shows the global thermal rate constants and their limiting components, kinner, kouter, and kexit, for the four reactions calculated at different temperatures. The corresponding Arrhenius plots are collected in Figure 5. A. Ga+ + CH3F Reaction. Figure 1 shows a simplified profile for the reaction between Ga+ and CH3F. It should be noted that, focusing the Ga+ ground state, 1S, the different channels analyzed, namely, fluorine abstraction and HF 7744


2.83 × 10

3.38 × 10

8.90 × 10

−09

−09

4.17 × 10

3.80 × 10

3.54 × 10

295

335

375

415

−27

3.19 × 10

6.91 × 10

9.30 × 10

−09

−09

−09

3.16 × 10

3.03 × 10

2.91 × 10

455

495

535

575

7745

−20

3.85 × 10

9.37 × 10

2.08 × 10

−09

−09

2.43 × 10

2.39 × 10

2.36 × 10

815

855

895

935

−19

8.07 × 10

1.44 × 10

2.43 × 10

−09

−09

−09

2.29 × 10

2.27 × 10

2.25 × 10

2.23 × 10−09

975

1015

1055

1095

3.91 × 10−18

−18

−18

4.24 × 10−19

2.32 × 10−09

−19

−20

1.42 × 10−20

775

−09

4.60 × 10

2.53 × 10

735

−21

−21

2.48 × 10−09

1.29 × 10

−09

2.59 × 10

3.02 × 10

2.65 × 10

655

695

5.76 × 10

−09

2.73 × 10

615

−09

−23

−09

−22

8.54 × 10−24

2.81 × 10−09

−25

−26

7.96 × 10−29

3.33 × 10−09

−31

−33

−36

255

2.64 × 10−40

8.16 × 10

5.48 × 10

215

−46

−09

8.25 × 10

−09

6.80 × 10

175

−54

−66

4.69 × 10−09

1.70 × 10

−09

9.25 × 10

135

kinner

−09

kouter

T (K) kexit

−67

−80

−101

−42

−46

−32

−34

−25

−26

−28

−22

−23

−20

−21

7.33 × 10−20

2.66 × 10

8.91 × 10

2.71 × 10

−21

7.42 × 10−22

1.80 × 10

3.84 × 10

6.99 × 10

−24

1.07 × 10−24

1.33 × 10

1.31 × 10

9.66 × 10

5.11 × 10

−29

1.81 × 10−30

3.89 × 10

4.54 × 10

2.45 × 10

−36

4.90 × 10−39

2.65 × 10

2.45 × 10

1.89 × 10

−51

3.87 × 10−58

2.79 × 10

1.53 × 10

1.43 × 10

Ga+

−101

8.63 × 10−20

3.16 × 10

−20

1.06 × 10

−20

3.25 × 10

−21

8.94 × 10−22

2.19 × 10

−22

4.67 × 10

−23

8.57 × 10

−24

1.32 × 10−24

1.65 × 10

−25

1.62 × 10

−26

1.21 × 10

−27

6.41 × 10

−29

2.27 × 10−30

4.91 × 10

−32

5.75 × 10

−34

3.11 × 10

−36

6.22 × 10−39

3.37 × 10

−42

3.11 × 10

−46

2.41 × 10

−51

4.92 × 10−58

3.55 × 10

−67

1.95 × 10

−80

1.82 × 10

kglobal −08

6.64 × 10−09

6.70 × 10

−09

6.78 × 10

−09

6.86 × 10

−09

6.95 × 10−09

7.04 × 10

−09

7.14 × 10

−09

7.26 × 10

−09

7.38 × 10−09

7.52 × 10

−09

7.67 × 10

−09

7.83 × 10

−09

8.01 × 10

−09

8.22 × 10−09

8.45 × 10

−09

8.72 × 10

−09

9.03 × 10

−09

9.40 × 10−09

9.87 × 10

−09

1.05 × 10

−08

1.13 × 10

−08

1.24 × 10−08

1.42 × 10

−08

1.71 × 10

−08

2.23 × 10

kouter −06

7.68 × 10−08

8.11 × 10

−08

8.59 × 10

−08

9.13 × 10

−08

9.75 × 10−08

1.04 × 10

−07

1.12 × 10

−07

1.21 × 10

−07

1.32 × 10−07

1.44 × 10

−07

1.58 × 10

−07

1.74 × 10

−07

1.93 × 10

−07

2.15 × 10−07

2.42 × 10

−07

2.73 × 10

−07

3.13 × 10

−07

3.62 × 10−07

4.24 × 10

−07

5.07 × 10

−07

6.22 × 10

−07

7.89 × 10−07

1.05 × 10

−06

1.50 × 10

−06

2.39 × 10

kinner

Ge+

−08

4.31 × 10−09

4.34 × 10

−09

4.38 × 10

−09

4.42 × 10

−09

4.47 × 10−09

4.52 × 10

−09

4.58 × 10

−09

4.65 × 10

−09

4.73 × 10−09

4.81 × 10

−09

4.91 × 10

−09

5.02 × 10

−09

5.15 × 10

−09

5.29 × 10−09

5.45 × 10

−09

5.65 × 10

−09

5.88 × 10

−09

6.16 × 10−09

6.50 × 10

−09

6.96 × 10

−09

7.58 × 10

−09

8.46 × 10−09

9.79 × 10

−09

1.20 × 10

−08

1.60 × 10

kglobal

−08

−08

−08

−08

−08

−09

−09

−09

−09

−09

−09

−09

−09

−09

6.82 × 10−09

6.90 × 10

6.98 × 10

7.07 × 10

−09

7.17 × 10−09

7.28 × 10

7.40 × 10

7.53 × 10

−09

7.68 × 10−09

7.85 × 10

8.03 × 10

8.23 × 10

8.46 × 10

−09

8.72 × 10−09

9.02 × 10

9.36 × 10

9.76 × 10

−09

1.02 × 10−08

1.08 × 10

1.16 × 10

1.26 × 10

−08

1.41 × 10−08

1.63 × 10

1.99 × 10

2.65 × 10

kouter

−06

−06

−07

−07

−07

−07

−07

−07

−07

−08

−07

−08

−08

6.12 × 10−08

6.61 × 10

7.17 × 10

7.79 × 10

−08

8.51 × 10−08

9.32 × 10

1.02 × 10

1.13 × 10

−07

1.25 × 10−07

1.40 × 10

1.56 × 10

1.75 × 10

1.98 × 10

−07

2.25 × 10−07

2.57 × 10

2.96 × 10

3.44 × 10

−07

4.03 × 10−07

4.80 × 10

5.82 × 10

7.23 × 10

−07

9.28 × 10−07

1.25 × 10

1.81 × 10

2.91 × 10

−06

kinnerTS2

−09

−10

−10

−10

−10

−10

−10

−10

−10

−10

−10

−10

3.60 × 10−10

3.59 × 10

3.58 × 10

3.58 × 10

−10

3.59 × 10−10

3.62 × 10

3.66 × 10

3.72 × 10

−10

3.79 × 10−10

3.89 × 10

4.01 × 10

4.16 × 10

4.35 × 10

−10

4.59 × 10−10

4.88 × 10

5.24 × 10

5.70 × 10

−10

6.28 × 10−10

7.04 × 10

8.07 × 10

9.50 × 10

−10

1.16 × 10−09

1.50 × 10

2.07 × 10

−09

−09

kinnerTS5 3.20 × 10

As+

−10

−09

−09

−10

−10

−10

−10

−10

−10

−10

−10

−10

−10

−10

2.86 × 10−10

2.85 × 10

2.84 × 10

2.84 × 10

−10

2.85 × 10−10

2.86 × 10

2.88 × 10

2.92 × 10

−10

2.96 × 10−10

3.02 × 10

3.09 × 10

3.18 × 10

3.30 × 10

−10

3.43 × 10−10

3.60 × 10

3.81 × 10

4.07 × 10

−10

4.40 × 10−10

4.83 × 10

5.39 × 10

6.17 × 10

−10

7.30 × 10−10

9.03 × 10

1.20 × 10

1.75 × 10

kglobal

−08

−08

−08

−08

−08

−09

−09

−09

−09

−09

−09

−09

−09

−09

6.30 × 10−09

6.37 × 10

6.44 × 10

6.52 × 10

−09

6.61 × 10−09

6.71 × 10

6.81 × 10

6.93 × 10

−09

7.06 × 10−09

7.21 × 10

7.37 × 10

7.55 × 10

7.76 × 10

−09

8.00 × 10−09

8.27 × 10

8.59 × 10

8.98 × 10

−09

9.45 × 10−09

1.01 × 10

1.08 × 10

1.19 × 10

−08

1.35 × 10−08

1.58 × 10

1.97 × 10

2.69 × 10

kouter

−28

−32

−38

−21

−22

−17

−18

−15

−16

−16

−14

−15

−14

−14

1.08 × 10−13

7.79 × 10

5.44 × 10

3.68 × 10

−14

2.40 × 10−14

1.51 × 10

9.05 × 10

5.17 × 10

−15

2.79 × 10−15

1.42 × 10

6.67 × 10

2.89 × 10

1.13 × 10

−16

3.93 × 10−17

1.18 × 10

2.96 × 10

5.94 × 10

−19

8.94 × 10−20

9.22 × 10

5.72 × 10

1.74 × 10

−23

1.86 × 10−25

3.94 × 10

5.42 × 10

4.28 × 10

kinner

Se+

1.08 × 10−13

7.76 × 10−14

5.43 × 10−14

3.67 × 10−14

2.40 × 10−14

1.50 × 10−14

9.03 × 10−15

5.16 × 10−15

2.79 × 10−15

1.41 × 10−15

6.66 × 10−16

2.88 × 10−16

1.13 × 10−16

3.92 × 10−17

1.18 × 10−17

2.96 × 10−18

5.94 × 10−19

8.94 × 10−20

9.22 × 10−21

5.72 × 10−22

1.74 × 10−23

1.86 × 10−25

3.94 × 10−28

5.42 × 10−32

4.28 × 10−38

kglobal

Table 1. Kinetic Rate Coefficients in cm3 molecule−1 s−1 for the M+ + CH3F Reactions (M = Ga, Ge, As, and Se) Predicted at the MPW1K/QZVPP Level of Theory

The Journal of Physical Chemistry A Article



Article

kglobal for the reaction of Ga+(1S) with methyl fluoride computed a 295 K is very small, 2.41 × 10−51 cm3 molecule−1 s−1 (Table 1), thus indicating that the global process cannot take place. The Arrhenius plot of this constant collected in Figure 5 shows the typical behavior of a process that proceeds with a net activation barrier. Looking at its limiting components, kexit makes the large contribution to kglobal for low and moderate temperatures. As temperature increases, kexit increases faster than kinner making both contributions similar for high temperatures. Finally, the contribution of kouter to kglobal is negligible. An addition complex, GaFCH3+, is observed in the experimental study of Zhao et al.5 where the reaction is expected to be termolecular with helium atoms acting as a third body. To provide more information about this reaction pressure-dependent studies (both experimental and theoretical) should be done. Nevertheless, this is beyond the scope of the present research. In addition, Zhao et al.5 provide a result for the capture or collision rate coefficient (5.02 × 10−9 cm3 molecule−1 s−1), which is in a very good agreement with our calculated value for kouter, 4.17 × 10−9 cm3 molecule−1 s−1 at 295 K. B. Ge+ + CH3F Reaction. The results obtained for the doublet PES of the reaction between Ge+ and methyl fluoride are collected in Figure 2. The most favorable product from a thermodynamic point of view is the formation of GeF+(1S) + CH3, located −25.3 kcal/mol below the reactants at the CCSD(T)/QZVPP//mPW1K/QZVPP level. While formation of GeCH2+(2B1) + HF is also clearly an exothermic process

The most favorable channel from reactants in its ground state (1S) is path 1a: 1 1 CH3F + Ga +(1S) → GaFCH3+(A 1) → TS2(A) 1 + 2 → FGaCH3+(A 1) → CH3 + GaF ( Σ)

In this path, the transition structure lies in energy clearly above the reactants (ΔU0 = 34.1 kcal/mol at CCSD(T)/QZVPP// mPW1K/QZVPP level), and the potential well associated with the encounter complex, GaFCH3+(1A1), is rather deep (ΔU0 = −20.0 kcal/mol at CCSD(T)/QZVPP//mPW1K/QZVPP level). The stabilization of this structure arises from a weak electrostatic interaction between Ga+ and methyl fluoride. The high energetic barrier found for the step GaFCH3+(1A1) → FGaCH3+(1A1) (about 54.1 kcal/mol at CCSD(T)/QZVPP// mPW1K/QZVPP level) makes thermal activation of methyl fluoride extremely improbable and does explain the experimental observation of the adduct complex at room temperature.5 As we can see in Figure 1, products are clearly above both reactants and TS2(1A), and the exit channel is expected to play an important role in this process. Therefore, we carry out kinetics calculations adopting a three transition state model (3-TS model).15 We have explicitly considered an inner (tighter) transition state located in the neighborhood of the first-order saddle point, TS2(1A), an outer (entropic) transition state controlling the entrance channel, and a third exit barrier (associated with the endothermicity of the FGaCH3+(1A1) → CH3 + GaF+(2Σ) process) controlling the exit channel. The

Figure 2. Energy profile (in kcal/mol) for the Ge+(2P) + CH3F reaction. The relative (taking the reactants as a reference) adiabatic potential energies (ΔU0 = ΔU + ZPE) at the mPW1K/TZVPP, mPW1K/QZVPP (in parentheses), and CCSD(T)/QZVPP//mPW1K/QZVPP (in bracket) levels are given in black and the corresponding Gibbs free energies in blue [ΔG]. 7746



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transition state controlling the entrance channel and an inner (tighter) transition state located in the neighborhood of the first-order saddle point. We have not included kexit, related to the exit channel, because reaction products are well below reactants, and consequently, its consideration will not change appreciably the global rate constant (kglobal). The Arrhenius plots of kglobal and its limiting components (Figure 5) show that kinner varies with temperature as a prototypical barrierless process,35 becoming lower as the temperature increases. However, kouter becomes lower as the temperature increases. Of course, the entropic contribution is responsible for that behavior. Since the inner barrier is so low, kinetics is fully controlled by the outer bottleneck, and the computed values for kglobal and kouter are very close, as can be seen in Table 1. The global rate constant for this reaction is 7.58 × 10−9 cm3 molecule−1 s−1 calculated at 295 K, which should be compared with the experimental value reported by Zhao et al.,5 (5.80 ± 1.74) × 10−10 cm3 molecule−1 s−1 at the same temperature. As emphasized elsewhere,36 relatively small errors in the theoretical estimate of ΔG do produce considerable deviations in the predicted rate constants because of their exponential dependence on ΔG. A trivial calculation tells us that an error of 1 kcal/mol in ΔG (a goal that according to our own experience37−39 is not been achieved in todays’s theoretical thermodynamic predictions for even small sized systems40,41) gives rise to a discrepancy in about 1 order of magnitude in the estimated rate constant. As we have already mentioned, a CASSCF-MRMP2 study of the potential energy surface for the reaction of Ge+ with methyl fluoride has been carried out.18 Where the analyzed fluorine abstraction process shows a similar behavior to that reported in our study. However, the relative energies reported for intermediates, transition structures, and products differ from our calculations. In all cases, their relative energies are larger than our values (at least 5 kcal/mol). In particular, the transition state, TS2, is located 2.4 kcal/mol above the reactants, whereas from our results this transition structure is located 10.0 kcal/mol below the reactants at the CCSD(T)/QZVPP// mPW1K/QZVPP level. According to the argument in the previous paragraph, the energy of the transition state is critical for the computation of rate constants. We have estimated kglobal considering their computed energy barrier, and the obtained value at 295 K is 8.38 × 10−13 cm3 molecule−1 s−1, which is rather small when compared with the experimental value, (5.80 ± 1.74) × 10−10 cm3 molecule−1 s−1.5 Therefore, the experimental value for the rate constant is not compatible with the existence of a net energy barrier, and TS2 should be located below the reactants as it is shown in our results. Finally, we will briefly comment on the possible mechanisms for the Ge+(2P) + CH3F process. Reactions between monocations and fluorocarbons can take place basically through two different mechanisms: (i) single electron transfer (SET), or harpoon-like mechanism, and (ii) insertion−elimination.42,43 In our previous studies on the reactions between alkaline-earth and first-row transition metal monocations with methyl fluoride,15−17 the fluorine abstraction process seems to takes place mainly through a harpoon-like mechanism. In such a mechanism the key step is the electron transfer from metal cation to fluorine through inner transition state TS2, so a correlation between the second ionization energy (SIE) and the ability of C−F bond activation should be expected. However, we also emphasized the relative importance of the outer and exit channels, apart from the inner bottleneck, to control the

(−11.8 kcal/mol, at the mPW1K/TZVPP level); nevertheless, the hydrogen elimination product is clearly endothermic (15.3 kcal/mol at the mPW1K/TZVPP level). We have analyzed the profile of the different exothermic channels. The most stable F atom transfer product is reached in a similar way to that reported for the Ga+ reaction (path a). The first step is the direct formation of the electrostatic complex GeFCH3+(2A′), followed by germanium insertion into the C−F bond leading to the most stable intermediate, FGeCH3+(2A′) (located −54.2 kcal/mol below the reactants at the mPW1K/TZVPP level). This isomerization takes place through a transition state TS2(2A′), which is clearly below the reactants (−10.0 kcal/mol at the CCSD(T)/QZVPP// mPW1K/QZVPP level). From FGeCH3+(2A′), formation of CH3 + GeF+(1Σ) is a barrierless endothermic process. The whole process (path a) is thus clearly exothermic, exergonic, and barrier free. The formation of the exothermic product, GeCH2+(2B1) (Figure 2) can take place through two different routes: (a) starting from the most stable intermediate, FGeCH3+(2A′), an hydrogen atom migration from carbon to fluorine through transition state TS5(2A′) leads to c-HFGeCH2+(2A″), which directly dissociates into products. As can be seen in Figure 2, TS5 is located below the reactants (ΔUo = −6.6 kcal/mol at the mPW1K/TZVPP level), and the global process (path b in Figure 2) is exothermic and exergonic; and (b) starting from FGeCH3+(2A′), the migration of a hydrogen atom from carbon to germanium through transition state TS3(2A) yields GeHFCH2+ (2A″) from which the subsequent migration of the hydrogen bonded to germanium to fluorine atom gives t-HFGeCH2+(2A″) involving transition state TS4(2A″). Finally, from this latter intermediate, formation of GeCH2+(2B1) + HF is a barrierless process. The whole path (path c in Figure 2) involves two transition states clearly located above the reactants (ΔU0 = 7.9 and 22.4 kcal/mol for TS3 and TS4, respectively, at the mPW1K/TZVPP level), and therefore, it does not represent any competitive alternative. However, by comparing paths a and b (Figure 2), we conclude that the most favorable evolution of FGeCH3+(2A′) intermediate is the direct dissociation (path a) into products, CH3 + GeF+(1Σ). Indeed, formation of the GeCH2+(2B1) + HF (path b) is a less exothermic process that must also overcome the barrier associated with TS5(2A′) (about −6.6 kcal/mol). Therefore, according to our results for the PES of the reaction of Ge+ with methyl fluoride (Figure 2), the most favorable process corresponds to path a: CH3F + Ge+(2 P) → CH3FGe+(2A) → TS2(2A ′) → CH3GeF+(2A ′) → CH3 + GeF+(1Σ)

The experimental study of this reaction5 shows that GeF+ is the only product obtained, in full agreement with our theoretical analysis. It should be noted however that our calculations predict a rather strong stability for the FGeCH3+ (2A′) complex (ΔU0 = −54.2 kcal/mol at the MPW1K/TZVPP level, as shown in Figure 2). This result suggests that, as in the case of the As+ + CH3F reaction (see next section), such a complex should be experimentally detected. Further experimental studies should help elucidate this important point. Bearing in mind the above considerations, we have focused on path a to estimate the kinetics constants. A two transition state model has been adopted (2-TS model) in our kinetics calculations, where we have considered an outer (looser) 7747



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Figure 3. Energy profile (in kcal/mol) for the As+(3P)/As+(1P) + CH3F reaction. The relative (taking the reactants as a reference) adiabatic potential energies (ΔU0 = ΔU + ZPE) at the mPW1K/TZVPP, mPW1K/QZVPP (in parentheses), and CCSD(T)/QZVPP//mPW1K/QZVPP (in bracket) levels are given in black and the corresponding Gibbs free energies in blue [ΔG].

kinetics of these processes.15−17 However, our previous studies also suggested that contributions other than charge transfer might be significant.16 We have found a correlation between our previous kinner results obtained for Sc+ (3.16 × 10−7 cm3 molecule−1 s−1), Ti+(1.33 × 10−8 cm3 molecule−1 s−1), V+(4.55 × 10−10 cm3 molecule−1 s−1), Zn+(7.63 × 10−30 cm3 molecule−1 s−1), and Ga+(2.83 × 10−36 cm3 molecule−1 s−1) and their SIE: Sc+ (295 kcal/mol), Ti+(313 kcal/mol), V+(338 kcal/mol), Zn+(414 kcal/mol), and Ga+(473 kcal/mol).44 However, for Ge+ with a relatively large SIE, 367 kcal/mol,44 we have obtained an unexpected large kinner value of 6.22 × 10−7 cm3 molecule−1 s−1. This fact suggests that this reaction should not take place through a harpoon-like mechanism. A Mulliken population analysis shows that the net electron charge from Ge+ to F through transition state TS2 is relatively small (the partial charges of Ge+, F, and CH3 group change from 0.737, −0.249, and 0.512 electrons in GeFCH3+ to 1.073, −0.324, and 0.251 electrons, respectively, in FGeCH3+), similarly to what we have reported for the Sc+, Ti+, and V+ corresponding complexes17 and in sharp contrast with what we had found in the case of the Ca+, Sr+, and Ba+ complexes.15,16 In our previous work,16 we suggested that although charge transfer seems to be dominant, other contributions (electrostatic, dispersion, and/or induction contributions other than charge transfer) should play a role.

However, focusing on the TS2 geometries, we note that Ge+ and As+ cations (see next section) (as Sc+, Ti+, and V+)17 favor a lateral approach of M+ (the C−F−M angles for As+ and Ge+ are 89.61° and 144.57° at the mPW1K/QZVPP level, respectively), thus contrasting with the frontal attack observed in our calculations on the Ca+, Sr+, and Ba+ reactions where a linear C−F−M+ disposition is obtained.15,16 This fact strongly suggests that in the former cases where an oxidative addition is electronically plausible (existence of two valence electrons available42) such an insertion−elimination mechanism43 might also be operative according to Ge•+ + CH3F → [Ge•+ ··· FCH3] → TS2 → [F−Ge•+−CH3] → GeF+ + •CH 3

where TS2 represents a three-membered ring structure. C. As+ + CH3F Reaction. The reaction between As+ and CH3F is analyzed on the triplet and singlet potential energy surfaces, which correlate with the ground and the first excited state of As+ (3P, and 1P, respectively). Our computed results are collected in Figure 3. On the triplet surface, the most favorable product from a thermodynamic point of view, located −21.4 kcal/mol below the reactants at the CCSD(T)QZVPP// mPW1K/QZVPP level, is the formation of AsCH2+(3A2) + HF. The molecular hydrogen elimination process is exothermic by −4.1 kcal/mol at the mPW1K/TZVPP level, whereas fluorine abstraction giving AsF+(2Π) + CH3 is a slightly endothermic 7748



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one (ΔU0 = 1.8 kcal/mol at the mPW1K/TZVPP level). On the singlet potential energy surface, the most exothermic product is also AsCH2+ but in its singlet state, 1A1, located 74.4 kcal/mol below the excited reactants, at the CCSD(T)QZVPP//mPW1K/QZVPP level. Taking into account the most exothermic processes, we have considered different channels giving AsCH2+ on the triplet and singlet PESs. This is the major product found experimentally for this reaction.5 We have also included the paths yielding AsF+ formation for comparison with the reactivity of the other bare cations. First, we will consider the reaction starting from the ground state of As+(3P). The interaction between the arsenic cation and the fluorine atom of fluoromethane is attractive because of the ion-induced dipole potential, thus no transition state was found in the exothermic formation of AsFCH3+(3A″). From this intermediate, the reaction evolves toward the formation of the insertion complex FAsCH3+(3A″) through the first-order saddle point TS2(3A″). Once the insertion complex is reached, direct formation of methyl radical and AsF+(2Π) is an endothermic and barrierless process (Figure 3, path 3a). In addition, the insertion complex FAsCH3+(3A″) can also progress through two different paths (Figure 3, paths 3b and 3c) giving the HF elimination product. Both paths are similar to those found for the formation of GeCH2+ (Figure 2 paths b and c). As in the case of the reaction with Ge+, the path 3b, which implies one step, is the most favorable. Transition states TS3(3A) and TS4(3A) involved in path 3c are located higher in energy (ΔU0 are 8.5 and 12.3 kcal/mol at the mPW1K/TZVPP level, respectively) than the corresponding transition state of path 3b, TS5(3A″), located −6.1 kcal/mol at the mPW1K/TZVPP level. Therefore, on the triplet potential energy surface, the most favorable path is 3b leading to AsCH2+(3A2). Another possibility we evaluated is that the reaction starts from the excited state of As+(1P). As can be seen in Figure 3, three similar paths involving the same steps as those described for the triplet PES are found (paths 1a, 1b, and 1c). The most significant difference is that on this surface TS3(1A) is located below TS5(1A) (ΔU0 = −40.5 and −25.9 kcal/mol at the mPW1K/TZVPP level, respectively), but TS4(1A′) is located above the reactants, CH3F + As+(3P), (2.3 kcal/mol at the mPW1K/TZVPP level). Consequently, path 1b is also the most favorable one on this surface. However, the most stable product obtained through path 1b, AsCH2+, is reached on its singlet state, 1A1. We should emphasize that, for the studied reactions of four-row cations with CH3F,15,17 arsenic cation is the first one where the most favorable product is not the fluorine abstraction. In this case, it is more favorable the evolution from C−F insertion intermediate toward the formation of AsCH2+ than the C−As bond cleavage. From Figure 3, we can see that the most stable intermediate is the C−F insertion one on the singlet surface, FAsCH3+(1A′) (ΔU0 = − 79.9 kcal/mol at the mPW1K/TZVPP level). This gives the possibility of an intersystem crossing between triplet and singlet PESs along the paths (3b and 1b) leading to the most stable products, AsCH2+ + HF. This fact has been previously analyzed18 and the minimum-energy crossing point between both surfaces was located below the reactants (12.0 kcal/mol), thus suggesting that such an intersystem crossing could take place. However, the spin−orbit coupling at MCQDPT level shows that these states do not interact at this point. Bearing in mind these findings, the most favorable route for the reaction of As+(3P) with CH3F is path 3b:

CH3F + As+(3P) → CH3FAs+(3A ″) → TS2(3A ″) → CH3AsF+(3A ″) → TS5(3A ″) → HFAsCH 2+(3A ″) → AsCH 2+(3A 2) + HF

which has been the pathway we considered for the kinetics calculations. It should be noted that kexit (dissociation) is not included neither in Table 1 nor in Figure 5 because the reaction products are well below the reactants and the exit channel does not affect the kglobal computation.15 Therefore, a three transition state model has been considered for this reaction (3-TS model). We have considered two inner transition states located in the neighborhood of the first order transition structure TS2(3A″) and TS5(3A″) and an outer transition state (entropic bottleneck) controlling the entrance channel. The numerical rate constants for kglobal and its limiting components are collected in Table 1, and their respective Arrhenius plots in Figure 5. Figure 5 shows that kinner,TS2 and kinner,TS5 rate constants change with temperature as a prototypical barrierless process.35 They become lower as the temperature increases. kouter decreases at higher temperatures. kglobal is mostly controlled by kinner,TS5 with slight contributions from kouter and kinner,TS2 at the lower temperatures. The global rate constant evaluated at 295 K is 6.17 × 10−10 cm3 molecule−1 s−1 in a quite good agreement with the experimental one given by Zhao et al. (9.1 × 10−10 cm3 molecule−1 s−1).5 According to our remarks when analyzing the Ge+ + CH3F rate constant in the preceding section, the energy barriers are accurately described at the mPW1K/QTVPP level in this case. The relatively high stability of the electrostatic addition complex, AsFCH3+(3A″) (ΔU0 = −40.5 kcal/mol at the CCSD(T)/QZVPP//mPW1K/QZVPP level) for this reaction justifies the 3% of the experimental product distribution observed for this complex.5 As mentioned previously, pressure-dependent studies (both experimental and theoretical) are required to get a deeper insight into the product distribution. As remarked in the case of the Ge+ + CH3F reaction, a harpoon-like mechanism is not compatible with the relatively large SIE of As (429 kcal/mol).44 The existence of other stabilizing contributions together with the suitability of the As+ ground state electronic configuration (4s24p2) for insertion into a single bond (oxidative addition) might well play a relevant role in the As+ + CH3F reaction. The quite lateral approach of As+ to the CH3F fragment in TS2 (see Figure 3), leading to the formation of a three-membered ring structure, seems to support an insertion−elimination mechanism. The main difference with respect to Ge+ is that formation of AsF+ is energetically less favorable than GeF+, thus making formation of AsCH2+ the most stable product for the reaction with As+. The previous studies reported for this reaction18,19 (at the B3LYP level followed by CCSD(T) single-points calculations on the stationary points on the triplet energy PES19 and CASSCF-MRMP2 level for the triplet and singlet PESs18) present a similar description for the As+ + CH3F reaction. However, in both cases, the energy barriers are higher than in our predictions. As we have already commented for the case of the Ge+ + CH3F reaction, the barriers found for TS5 in these works (28.519 and 8.4 kcal/mol18) are not compatible with the experimental rate constant.5 D. Se+ + CH3F Reaction. The results for the PES of the reaction between Se+ and methyl fluoride on quartet and doublet states are collected in Figure 4. The different products 7749



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Figure 4. Energy profile (in kcal/mol) for the Se+(4S)/Se+(2D) + CH3F reaction. The relative (taking the reactants as a reference) adiabatic potential energies (ΔU0 = ΔU + ZPE) at the mPW1K/TZVPP, mPW1K/QZVPP (in parentheses), and CCSD(T)/QZVPP//mPW1K/QZVPP (in bracket) levels are given in black and the corresponding Gibbs free energies in blue [ΔG].

reached from the ground state of the reactants, Se+(4S) + CH3F, are clearly endothermic. The most favorable product, SeCH2+(4A″) + HF, is located 12.5 kcal/mol above the reactants at the CCSD(T)/QZVPP//mPW1K/QZVPP level. The same product [SeCH2+(2B2) + HF] is also the most stable one on the doublet PES, but in this case, it is clearly exothermic (−42.5 kcal/mol at the CCSD(T)/QZVPP//mPW1K/QZVPP level). In Figure 4, the paths leading to the F abstraction product are not included because it is a clearly endothermic process (ΔU0 = 33.2 kcal/mol at the CCSD(T)/QZVPP//mPW1K/ QZVPP level). We can see that formation of SeCH2+(4A″) from ground state reactants implies a huge energy barrier (TS2(4A″), which is located 31.2 kcal/mol above the reactants at the CCSD(T)/QZVPP//mPW1K/QZVPP level). If the reaction starts from Se+(2D), we can reach the most stable product, SeCH2+(2B2) + HF, through two different paths (analogous to those found in the reaction with As+) with transition states located below the excited reactants. However, the most stable intermediate is a doublet multiplicity state; therefore, an intersystem crossing between quartet and doublet PESs must be taken into consideration. Since experiments show the reaction starts from the Se+ in its ground state,5 the most favorable path could be schematized as

where an intersystem crossing occurs between SeFCH3+(4A″) and TS2(2A). This path also involves an energetic barrier (ΔU0 = 16.3 kcal/mol for TS2(2A) at the CCSD(T)/QZVPP// mPW1K/QZVPP level). In order to give an estimation of the kinetics constant in the most favorable situation (upper values), we have considered a crossing probability equal to one. In the calculations of kinetic constants, we have explicitly considered an outer transition state controlling the entrance channel and an inner transition state located in the neighborhood of the first-order saddle point, TS2(2A) (2-TS model). We have not consider explicitly TS5(2A) in our estimation because TS2(2A) is clearly the bottleneck. The results for the global rate constants and its limit components are included in Table 1, and their corresponding Arrhenius plots in Figure 5. The reported kglobal is very low, 1.74 × 10−23 cm3 molecule−1 s−1 at 295 K, and it is fully controlled by kinner. This value is in agreement with the experimental results reported by Zhao et al. where Se+(4S) was observed to be nonreactive, k < 1 × 10−13 cm3 molecule−1 s−1.5 These authors argue that the failure to observe an addition product can be attributed to the weaker bonding with CH3F, which will decrease the rate of addition by diminishing the lifetime of the intermediate adduct ion, thus leading to adduct dissociation. In that sense, our results show that ΔU0 for the addition complex is −15.3 kcal/mol at the CCSD(T)/QZVPP//mPW1K/QZVPP level. This value is considerably higher than the value computed for the Ga+ complex (−20.0 kcal/mol at the CCSD(T)/QZVPP//mPW1K/QZVPP level) where the addition product is obtained. It seems clear that theoretical predictions do overestimate the stabilizations of the Se+/ CH3F and Ga+/CH3F adducts. In addition, it is interesting to note

CH3F + Se+( 4S) → SeFCH3+(4A ″) → TS2(2A) → FSeCH3+(2A ″) → TS5(2A) → HFSeCH 2+(2A) → SeCH 2+(2 B2) + HF 7750



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Figure 5. Arrhenius plots (k given in cm3 molecule−1 s−1 and temperature in K) for the global rate constant, kglobal (purple) and its main limiting components: kinner,TS2 (red), kinner,TS5 (yellow), kouter(blue), and kexit (green).

of the addition product for the reaction of Ga+ and the nonreactivity in the case of the Se+ can be attributed to the respective values of ΔU0 for the addition complex (−20.0 and −15.3 kcal/mol at the CCSD(T)/QZVPP//mPW1K/QZVPP level, respectively). Both ΔU0 values are expected to overestimate the real stabilization of the addition complexes according to very recent benchmarks on the accuracy of computational thermochemistry predictions.39,40 Therefore, our results are in agreement with the experimental evidence. According to our results for the potential energy surface of the reaction of Ge+ with methyl fluoride, the most favorable process corresponds to HF elimination products. The global rate constant for this process, 7.58 × 10−9 cm3 molecule−1 s−1 calculated at 295 K agrees reasonably well with the experimental value reported by Zhao et al.,5 (5.80 ± 1.74) × 10−10 cm3 molecule−1 s−1, bearing in mind today’s standards of accuracy in the estimate of energy barriers. However, the relatively large SIE, 367 kcal/mol44 for Ge+ and an unexpected large kinner value of 6.22 × 10−7 cm3 molecule−1 s−1 indicate that this reaction takes place through an oxidative addition mechanism. In the reaction of As+ with CH3F, the global rate constant for the formation of AsCH2+ evaluated at 295 K is 6.17 × 10−10 cm3 molecule−1 s−1 in a quite good agreement with the experimental value reported by Zhao et al. (9.1 × 10−10 cm3 molecule−1 s−1).5 The dominant contribution to kglobal is kinner,TS5 with slight contributions from kouter and kinner,TS2 at the lower temperatures. The main difference with respect to the reaction with Ge+ is that formation of AsF+ is energetically less favorable than GeF+, in full agreement with the experimental observation.

that the M−F distance in these complexes is 2.192 Å (Ga) and 2.474 Å (Se). Consequently, greater stabilization forces are expected in the case of the GaFCH3+ complex. Indeed, a shorter distance implies larger electrostatic (M+···F−) contributions.

IV. CONCLUSIONS The modified Perdew−Wang-1-parameter functional (mPW1K) in conjunction with the TZVPP and QZVPP basis sets were employed in the description of the potential energy surfaces for the reactions of CH3F with Ga+, Ge+, As+, and Se+ cations. Refinement of the energies were done at the CCSD(T)/QZVPP level on the mPW1K/QZVPP optimized geometries. In general, from the study of the potential energy surfaces of the reactions included in this work, we can observe that the results obtained at mPW1K level with both bases, TZVPP and QZVPP, are very close. The energies calculated at the CCSD(T) level are quite similar to those predicted by the mPW1K DFT functional. The most favorable product from a thermodynamic point of view changes along the series. The fluorine abstraction product is predicted for Ga+ and Ge+, the same result that we found for Ca+,15 whereas for As+ and Se+ the elimination product, MCH2+ (M = As, Se) + HF, is the lowest-lying one. Nevertheless, from the analysis of their respective potential energy surfaces, we conclude that the most favorable paths for fluorine abstraction in the case of Ga+ and formation of SeCH2+ in the case of Se+ both present net activation barriers (ΔU0 = 34.1 and 16.3 kcal/mol at the CCSD(T) level, respectively). In both reactions, the calculated global rate constants, kglobal, are very small, in agreement with the experimental fact that no M+F product is detected.5 In addition, the experimental observation 7751



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From a mechanistic viewpoint, our main conclusion is that the two types of structures found until the present, corresponding to the inner transition structures, namely, linear (Ca+, Sr+, Ba+) and three-membered ring (Sc+, Ti+, V+, Ge+, As+), do suggest that besides a harpoon-like mechanism, the possibility of an insertion−elimination process seems to play a role in the case where an oxidative addition is electronically plausible.

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AUTHOR INFORMATION

Corresponding Author

*(P.R.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank Dr. Adrián Varela-Á l varez (Emory University) for his collaboration in the implementation of the software required to carry out the kinetics calculations reported in this work. This research has been supported by the Ministerio de Educación y Ciencia of Spain (Grant QCT2010-16864)

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REFERENCES

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Kinetics Studies of the Reactions of Main Fourth-Period Monocations

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