Gas-Phase Reactions of Atomic Gold Cations with Linear Alkanes (C2

Article pubs.acs.org/JPCA

Gas-Phase Reactions of Atomic Gold Cations with Linear Alkanes (C2−C9) Ting Zhang,†,‡ Zi-Yu Li,† Mei-Qi Zhang,†,‡ and Sheng-Gui He*,† †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: To develop proper ionization methods for alkanes, the reactivity of bare or ligated transition metal ions toward alkanes has attracted increasing interests. In this study, the reactions of the gold cations with linear alkanes from ethane up to nonane (CnH2n+2, n = 2−9) under mild conditions have been characterized by mass spectrometry and density functional theory calculations. When reacting with Au+, small alkanes (n = 2−6) were confirmed to follow specific reaction channels of dehydrogenation for ethane and hydride transfer for others to generate product ions characteristic of the original alkanes, which indicates that Au+ can act as a reagent ion to ionize alkanes from ethane to n-hexane. Strong dependence of the chain length of alkanes was observed for the rate constants and reaction efficiencies. Extensive fragmentation took place for larger alkanes (n > 6). Theoretical results show that the fragmentation induced by the hydride transfer occurs after the release of AuH. Moreover, the fragmentation of n-heptane was successfully avoided when the reaction took place in a high-pressure reactor. This implies that Au+ is a potential reagent ion to ionize linear and even the branched alkanes.

1. INTRODUCTION Chemical ionization (CI) of saturated hydrocarbons (alkanes) can be used to identify nonpolar components in volatile organic compounds (VOCs)1 and petroleum by mass spectrometry (MS) analysis.2−6 Owing to a lack of ionizable functional groups, low basicity, or accompanying fragmentation, it is a challenging task to find an appropriate reagent ion that can react with alkanes to yield product ions characteristic of the original alkanes for the aim of chemical ionization mass spectrometry (CIMS) analysis.7−9 In the search for proper reagent ions, the reactivity of bare or ligated transition metal ions,10−14 such as Mn+,15 CpCo+ (Cp = cyclopentadienyl),16−18 ClMn2+,15 and ClMn(H2O)+,19−21 toward alkanes has attracted increasing interest. Of considerable interest here are the gold cations Au+, which are typically quite reactive and have been paid much attention, since Wilkins’s group found that Au+ could react at thermal energies with a number of hydrocarbons and alcohols.22,23 Henceforth, the property and activity of the Au+ ion have been the focus of research, such as extensive studies of gold cation complexation,24−28 the activity of Au+ toward C6F5H,29 CH3X (X = F, Cl, Br, I),30−32 NO,33 NH3,34 and CS2,35 and endothermic activation of CH4,36 H2,37 O2,38,39 and N2O.39 It is noteworthy that the ion−molecule reactions of Au+ with small alkanes follow specific reaction channels to generate product ions that can be representative of the neutral alkanes’ molecular weight,22 © XXXX American Chemical Society

which implies that the gold cation may be pursued as an appropriate reagent ion to ionize small alkanes, along with the advantage of monoisotopic element. Although the activity of some alkanes toward Au+ has been reported by previous studies, most of the reaction characteristics are still far from clear. For example, the rate constants and reaction efficiencies, the reaction mechanisms, and the reactivity of Au+ toward alkanes larger than n-heptane are unknown. Furthermore, the problem that the fragmentation occurring for larger alkanes results in the loss of molecular weight information, as an intriguing topic, has not been settled so far. To address these issues, in the present work, we further explore the reactions between Au+ and linear alkanes and try to make generalization about the possibility of Au+ as a precursor ion for ionizing linear alkanes. The reactions of Au+ with linear alkanes (C2−C9) have been characterized by using mass spectrometry and density functional theory calculations. This study includes characterization of Au+ reacting with individual and mixed alkanes, determination of their rate constants and reaction efficiencies, and exploration of reaction mechanisms. Furthermore, linear heptane, as an example, was chosen to react with Au+ in a reactor filled with high-pressure buffer gas, and Received: April 15, 2016 Revised: June 2, 2016

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DOI: 10.1021/acs.jpca.6b03836 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 1. TOF mass spectra for the reactions of Au+ with saturated linear alkanes (C2−C9). (b)−(i) are mass spectra for individual alkanes, and (j) is for a five-component mixture of alkanes. The reaction time is 2.53 ms for (b), 1.71 ms for (c) and (d), 3.71 ms for (e), 2.21 ms for (f), 1.82 ms for (g)−(i), and 2.42 ms for (j). Partial pressure is not given for C8 and C9 due to the low saturated vapor pressures. The relative concentration of each component of the mixed C2−C6 (j) is around 1.0 ± 0.5.

rotating and translating gold disk in the presence of helium carrier gas (99.999%) at a pressure of 8 standard atmospheres. The Au+ ions generated in a gas channel were expanded and reacted with n-C7H16 in a fast flow reactor. The instantaneous total gas pressure in the fast flow reactor was estimated to be around 40 Pa at T = 298 K. After reacting in the fast flow reactor, the reactant and product ions were detected by the reflectron TOF-MS. 2.2. Computational Methods. The density functional theory (DFT) calculations using the Gaussian 09 program46 were carried out to study the reaction mechanisms of Au+ with alkanes. The TPSS functional,47 which was demonstrated to successfully reproduce the dissociation energies of various chemical bonds including Au+−C, Au−H, C−H, C−C, and many others,48,49 was used in this study. The 6-311+G(d) basis sets50 were used for C and H atoms and D95V basis sets51 combined with the Stuttgart/Dresden relativistic effective core potentials (denoted as SDD in Gaussian software) were used for Au atom. The equilibrium geometries of reaction intermediates (IMs) and transition states (TSs) were optimized in the DFT calculations. Vibrational frequency calculations were performed to check that reaction IMs and TSs have zero and one imaginary frequency, respectively. The initial guess structures of TSs were generally obtained through relaxed potential-energy (PES) scan using single or multiple internal coordinates. To ensure that a transition state connects two appropriate local minima, the minimum-energy path was constructed by the intrinsic reaction coordinate theory.52 The zero-point vibration corrected energies (ΔH0) and some Gibbs free energies (ΔG) were reported in this work. The Rice−Ramsperger−Kassel−Marcus (RRKM)53 theory and variational transition state theory53 (VTST) were used to calculate the rate constants of passing over a transition state from a intermediate and of the AuH desorption from the corresponding intermediate in the reaction of Au+ with a given

the fragmentation was successfully avoided, which implies that Au+ is a potential candidate to ionize larger linear alkanes.

2. METHODS 2.1. Experimental Methods. The reactions of Au+ with all alkanes were performed in a homemade reflectron time-of-flight mass spectrometer (TOF-MS)40 equipped with a laser ablation cluster source,41 a quadrupole mass filter (QMF)42 and a linear ion trap (LIT) reactor.43 The details of the instrument can be found in our previous studies,40,42,43 and only a brief outline of the experiments is given below. The gold cations were generated by pulsed laser ablation of a rotating and translating gold disk with a helium carrier gas (99.999%) at a pressure of 8 standard atmospheres. The second harmonic of an Nd3+:yttrium aluminum garnet (YAG at 523 nm) laser with a pulse energy of 5−8 mJ and a repetition rate of 10 Hz was used. The atomic ions Au+ were delivered through the QMF to the LIT reactor where they were confined and thermalized through collisions with the cooling gas He (about 6 Pa) for about 0.8 ms. Then a pulse of reactant alkanes was delivered into the LIT to react with Au+ for a period of time. Longer cooling time (>0.9 ms) did not affect the reaction efficiency.43,44 The partial pressures of the reactant molecules were (0.1−2.0) × 10−3 Pa, depending on the nature of reaction systems. The temperature of the cooling gas, the reactant gas, and the LIT reactor was about 298 K. The reactant and product ions ejected from the LIT were detected by the reflectron TOF-MS. To obtain accurate rate constants, Au+ was designed to react with 0.2% alkanes seeded in a N2 gas (99.999%), whose concentration was further verified by infrared spectroscopy. Furthermore, the reaction of Au+ + n-C7H16 was also investigated by employing TOF-MS coupled with a laser ablation and supersonic expansion cluster source and a fast flow reactor, which was described in our previous work.45 Briefly, the atomic ions Au+ were pulse-generated by laser ablation of a B


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Figure 2. Variation of ion intensities with respect to the given alkane partial pressure in the reaction of Au+ with mixed hydrocarbons of equal contents (1:1). The solid lines are analytically fitted to the experimental data points according to the approximation of the pseudo-first-order reaction mechanism.

3. RESULTS AND DISCUSSION 3.1. Reaction of Au+ with Individual and Mixed Alkanes. The atomic ions Au+ were generated by laser ablation and thermalized, and then they interacted with linear alkanes ranging from ethane (C2H6) to n-nonane (n-C9H20) in an ion trap reactor43 for a period of time. The TOF mass spectra for the reactions are shown in Figure 1. All of the studied alkanes can react with Au+ under thermal collision conditions. For ethane (Figure 1b), as reported previously,22,55 dehydrogenation occurred exclusively with AuC2H4+ (m/z = 225) observed. A minor addition complex AuC2H6+ was also observed, which can be clearly seen in the mass spectrum of Figure S1. For alkanes from propane to n-hexane (Figure 1c− f), hydride transfer took place and the products peaks can be assigned as C3H7+, C4H9+, C5H11+, and C6H13+with m/z = 43, 57, 71, and 85, respectively. Note that in the reaction of Au+ with n-hexane (Figure 1f), the primary product C6H13+ was accompanied by very weak signals assigned to the alkyl cations C3H7+ and C4H9+. For alkanes larger than n-hexane (Figure 1g−i), extensive fragmentation took place apart from the hydride transfer. For n-heptane, the major observed product was the alkyl cations C4H9+ rather than C7H15+. For n-octane, the alkyl cations C4H9+ and C5H11+ were predominantly observed. For n-nonane, C4H9+, C5H11+, and C6H13+ were the

alkane. In these calculations, the energy of the initially formed reaction intermediate (E) and the energy barrier (E⧧) for each step were needed. The reaction intermediates carry vibrational energy (Evib) of the given alkane, the center of mass kinetic energy (Ek), and the binding energy (Eb) between Au+ and the given alkane, that is, E = Evib(alkane) + Ek + Eb. The values of Evib and Eb were obtained from the DFT calculations, and Ek = μυ2/2, in which μ is the reduced mass of Au+ and the given alkane, and υ is the velocity. The velocity υ was calculated by the equation υ = (3kT/m)1/2, in which k is the Boltzmann constant, T is the temperature of the ion trap reactor (∼298 K),43 and m is the mass of the Au+. The rate was calculated as k(E) = gN⧧(E − E⧧)/ρ(E)/h, in which g is the symmetry factor and is taken as 1 in the text, ρ(E) denotes the density of states of the metastable intermediates at the energy E, N⧧(E − E⧧) is the total number of states of the transition state, and h is the Planck constant. The direct count method proposed by Beyer and Swinehart54 was used for determining the number N⧧ and density ρ of states required for RRKM and VTST calculations by using the DFT-calculated vibrational frequencies under the approximation of harmonic vibrations. In the VTST calculations, the geometry of corresponding intermediate was optimized by fixing the distance between the alkyl cation and AuH at various values. C


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The Journal of Physical Chemistry A ⎡ (k P + k 2P2)t R ⎤ IR = exp⎢ − 1 1 ⎥ ⎣ ⎦ kT

main products. Furthermore, the hydrogen was primarily from the middle C atom to Au+ in the isotopic labeling experiments for the reaction of Au+ with CD3CH2CH2CD3 (available in this laboratory), as an example, which is shown in Figure S2. Note that a trace amount of AuCnH2n+ + H2 was also observed for alkanes from propane to n-nonane, which is consistent with previous study.22 However, the intensities of AuCnH2n+ (n > 2) are close to the detection limit and therefore are not depicted in the mass spectra. The above results can be summarized as following reactions: Au+ + CnH 2n + 2 → AuCn H 2n+ + H 2

Au+ + CnH 2n + 2 → AuH + CnH 2n + 1+

(n = 2)

in which IR is the intensity of the reactant ions, P1 and P2 are the partial pressures of C2H6 and n-C4H10, respectively, which have the same values in our experiments, k is the Boltzmann constant (1.38 × 10−23 J/K), T is the temperature (∼298 K), tR is the reaction time (∼1.74 ms), and k1 and k2 are the pseudofirst-order rate constants for the reactions of Au+ with C2H6 and n-C4H10, respectively. Note that systematic deviations of tR (±3%), T (±2%), and P (±20%) can result in an unavoidable uncertainty for the absolute rate constant. When the experimental relative abundance curves are fitted by the analytical functions, the absolute rate constants are determined to be (0.70 ± 0.03) × 10−9 cm3 molecule−1 s−1 for k1(Au+ + C2H6) and (1.78 ± 0.05) × 10−9 cm3 molecule−1 s−1 for k2(Au+ + n-C4H10). Here, ±0.03 × 10−9 and ±0.05 × 10−9 cm3 molecule−1 s−1 are the uncertainties of the absolute rate constants k1 and k2, respectively, corresponding to one standard error in the least-squares fitting. According to the obtained rate constants, the relative krel(Au+ + C2H6) is calculated to be 0.39. When the calibrated rate constant k2(Au+ + n-C4H10) is taken as 1.02 × 10−9 cm3 molecule−1 s−1, the rate constant k1(Au+ + C2H6) can be estimated to be 0.40 × 10−9 cm3 molecule−1 s−1, which agrees well with the reported value of 0.33× 10−9 cm3 molecule−1 s−1.55 The reaction efficiency ϕ, calculated as k1/kcol, where k1 is the rate constant and kcol is the theoretical rate of collision, normalizes the rate constant and makes it possible to make a direct comparison about the rate constants among a series of alkanes. In the present work, the theoretical collision rate constant is defined by the equation kcol = 2π(e2α/μ)1/2,57,58 in which e is the charge of the Au+, α is the electric polarizability of the alkane molecule and μ is the reduced mass of the ion Au+ and neutral alkane. Table 1 lists the rate constants and reaction efficiencies for reactions of Au+ ion with alkanes from ethane to n-hexane. The

(1)

(n = 3−6) (2)

Au+ + CnH 2n + 2 → AuH + CmH 2m + Cn − mH 2n − 2m + 1+ (n = 7, m = 3; n = 8, m = 3, 4; n = 9, m = 3−5) (3)

Dehydrogenation corresponding to reaction 1 occurs for the reaction of Au+ with ethane. Larger alkanes (C3−C6) can react with the Au+ through the major channel of hydride abstraction corresponding to reaction 2. Alkanes larger than n-hexane (C7−C9) start to fragment extensively into small alkyl cations when reacting with Au+. In an effort to model practical samples experimentally, Au+ was allowed to react with an artificial mixture containing five linear alkanes (C2−C6) (Figure 1j). The expected alkyl product ions (R−H+) for all the alkanes were present in the mixture; fragment ions or other product ions were not produced. This result represents that Au+ can be an appropriate reagent ion to ionize linear alkanes from ethane to hexane. 3.2. Rate Constant and Reaction Efficiency. The rate constants for the reactions of Au+ ion with tested alkanes from ethane to n-hexane were measured. To acquire reliable relative rate constants for the reactions of Au+ with C2−C6, the rate constant k1(Au+ + n-C4H10) was used as the reference. Then mixed gases composed of n-C4H10 and each of other alkanes with molar ratio of 1:1 were prepared as reactant gases. In this case, the rate constant for each alkane can be obtained through the relative values with respect to k1(Au+ + n-C4H10) when Au+ ions react with the mixed alkanes. Note that the rate constant k1(Au+ + n-C4H10), in the same way (as seen in Figure 2e), was estimated to be 1.02 × 10−9 cm3 molecule−1 s−1 after calibration by the absolute rate constant k1(Au+ + n-C4H8), which has been reported as 1.1 × 10−9 cm3 molecule−1 s−1 with the reaction efficiency of 100%.56 Here, only the rate constant analysis for the reaction of Au+ with mixed C2H6 and n-C4H10, as an example, is discussed below. The details about the reactions of Au+ with other alkanes will not be further discussed. Figure 2a shows a plot of the kinetic data including the signal variation of the reactant and product ions in the reaction of Au+ with mixed C2H6 and n-C4H10 with respect to the partial pressures of C2H6 or n-C4H10. The relative intensity of the reactant ion Au+ decreases and those of product ions including AuC2H4+ and C4H9+ increase gradually as the partial pressure of reactant molecules increases. Because the number of alkanes is much larger than that of Au+ ion in the linear ion trap, pseudofirst-order kinetics is assumed for the ion−molecule reactions. Thus, the relative intensity IR of the reactant ion Au+ could be well fitted by the following equation:

Table 1. Products, Pseudo-First-Order Rate Constants, and Reaction Efficiencies for Reactions of Au+ with Linear Alkanes (C2−C6)a reagent C2H6 C3H8 n-C4H10 n-C5H12 n-C6H14

product +

Au(C2H4) + H2 AuH + C3H7+ AuH + C4H9+ AuH + C5H11+ AuH + C6H13+

krelb

k1

kcolc

ϕ

0.39 0.71 1.00 1.25 1.96

0.40 0.72 1.02 1.28 2.00

0.97 0.98 1.00 1.02 1.04

0.41 0.73 1.02 1.25 1.92

Rate constants including k1 and kcol have units of 10−9 cm3 molecule−1 s−1. bkrel is the relative rate constant respect to the reaction of Au+ with n-C4H10 in the unit of 1. ckcol is the theoretical collision rate constant of collision. a

reaction efficiencies increase with alkyl chain length increasing, which is contributed to the polarizability increasing with chain length. With the polarizability increasing, the complexation energy increases for the ion/molecule complex and the total vibrational energy also increases for each alkane, which is supported by the DFT study (seen in Table 2). The higher stabilization of ion/molecule complex and higher internal energy of each alkane, lead to a longer lifetime of the complex and a higher probability for the hydride transfer to occur.18 The overall reaction efficiencies range from 0.41 to 1.92. Note that D


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The Journal of Physical Chemistry A Table 2. Calculated Vibrational Energy of C2−C9 and Relative Energies of the Encounter Complexes (EC), Transition States (TS), Product Complexes (PC), and Products for Hydrogen Transfer in Reactions of Au+ with Linear Alkanes (C2−C9) R

Vib

EC

TS

PC

P

C2H6 C3H8 n-C4H10 n-C5H12 n-C6H14 n-C7H16 n-C8H18 n-C9H20

0.02 0.05 0.08 0.11 0.15 0.19 0.23 0.27

−1.39 −1.68 −1.89 −1.97 −2.10 −2.12 −2.15 −2.20

−1.19 −1.48 −1.61 −1.72 −1.76 −1.80 −1.83 −1.84

−1.56 −1.83 −1.92 −2.01 −2.34 −2.38 −2.43 −2.46

0.07 −0.95 −1.11 −1.25 −1.39 −1.45 −1.54 −1.59

the values are larger than 1 for larger alkanes (C4−C6), which may be attributed to the fact that the Langevin theory57 generally underestimates the real theoretical collision rate constant. The high reaction efficiencies also suggest that Au+ possesses high reactivity toward alkanes (C2−C6) to act as the reagent ion to ionize linear alkanes for further characterization and quantification. 3.3. Reaction Mechanism. To interpret the experimental results above, the DFT calculations were performed for the reaction mechanisms and important results are summarized in Figures 3−5 and Table 2. Other possible pathways are shown in Figures S3−S8. It can be seen that the reaction of each alkane with Au+ is thermodynamically and kinetically favorable. The detailed mechanisms are discussed in the text below. Mechanism for Dehydrogenation. The lowest potentialenergy profile and the relevant structural information for the reaction of Au+ + C2H6 are shown in Figure 3a. The initial barrier-free interaction of C2H6 and Au+ gives rise to the encounter complex I1 with a binding energy of 1.38 eV. The complex is characterized by an interaction between the Au+ ion with H atom and a concomitant elongation of C−H bond. Then the reaction proceeds through oxidative addition (I1 → TS1 → I2) with a small barrier of 0.19 eV, affording a more stable complex I2 with the formation of Au−H and Au−C bond and the cleavage of the activated C−H bond. Here, the ability of Au+ to undergo hydride abstraction and to insert into C−H bonds requires that the ion has a vacant orbital that can accept an electron pair. Such a vacant orbital can be the gold 6s orbital that has decreased energy due to the relativistic effect.48,59−61 To generate a H2 molecule, hydrogen atom transfer from the CH3 moiety to the gold atom occurs, resulting in the formation of additional Au−H and Au−C bonds and the release of additional energy (0.65 eV). In the stable complex I3, the distance of H−H is 80 pm, which is very close to the H−H bond length (76 pm) in the free hydrogen molecule, suggesting that H2 formation is basically complete. The H2 molecule is then evaporated and the product ion AuC2H4+ is formed. It is noteworthy that complex I2 cannot dissociate directly into AuH + C2H5+ because the reaction is endothermic by 0.07 eV, which agrees well with the experimental observations. Mechanism for Hydride Transfer. Figure 3b describes the optimal pathway for the reaction of C3H8 + Au+. Similar to the reaction of C2H6 + Au+, the approaching of Au+ activates one H atom bonded to the middle C atom in C3H8, leading to the insertion of Au+ into the activated C−H bond (I4 → TS3 → I5) and the rotation of Au−C single bond (I5 → TS4 → I6). The intermediate I6 here can serve as the branch point for

Figure 3. DFT-calculated potential-energy profiles and structures for reactions of Au+ with C2H6 (a) and C3H8 (b). The ZPE-corrected energies (ΔH0 in eV) with respect to the separated reactants and some bond lengths (in pm) are shown. The Gibbs free energies of P3 and TS5 are given in parentheses.

product formation. On the one hand, the AuH can be directly released with C3H7+ formation (I6 → P3). On the other hand, the species can undergo the H atom transfer from terminal C atom to Au atom (I6 → TS5 → I7) and the liberation of H2 with AuC3H6+ formation (I7 → P4). Apparently, the H2 elimination is thermodynamically more favorable than the hydride transfer process, and the energy of TS5 (−0.98 eV) is even lower than that of P3 (−0.95 eV) by about 0.03 eV in terms of the ΔH0 values. RRKM theory53 and RRKM-based VTST53 were used to estimate the absolute rate constant of traversing TS5 from intermediate I6 and the dissociation rate (kd) of I6 → AuH + C3H7+. The calculated results show that the value of kd (6.37 × 1010 s−1) is larger than the rate of internal conversion I6 → TS5 (2.27 × 109 s−1) by one order of magnitude, indicating that the AuH loss rather than the H2 elimination is preferred for the reaction of Au+ with C3H8. It is noteworthy that hydride abstraction proceeds through a “loose” transition state and H2 elimination through TS5 involves a “tight” transition state. Moreover, the separated product P3 has larger entropy than that of the transition state TS5. When the entropy contribution (−TΔS) to the free energy is taken into account, the energy of TS5 (−0.68 eV) is higher than that of P3 (−1.05 eV) by 0.37 eV, which is similar to the previously studied reaction of MgO+ + CH4.62 So the decay of complex I6 by hydride transfer is favored over passage through the energetically lower TS5. To further understand the mechanism of C−H bond activation, the hydride transfer from alkanes (C4−C9) was E


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Figure 4. DFT-calculated potential-energy profiles and structures for the reaction of Au+ + n-C7H16. The solid and dashed lines are for early and late AuH loss, respectively. The ZPE-corrected energies (ΔH0 in eV) with respect to the separated reactants and some bond length (in pm) are shown. The Gibbs free energies of P5, P5-TS3, TS7, and P7-TS1 are given in parentheses.

of AuH, the fragmentation of generating C3H6 can take place through several steps including two steps of hydrogen migration (I9 → TS7 → I10→ TS8 → I11), the cleavage of C−C bond (I11 → TS9 → I12) to liberate a C3H6 molecule (I12 → P7), a further hydrogen migration (P7 → P7-TS1 → P7-I1), and the final dissociation into AuH + C4H9+ (P7-I1 → P6). When the different reaction pathways for early and late AuH release are compared, the energy of the critical transition state P5-TS3 (−0.30 eV) in the pathway of early AuH release is lower than that of P7-TS1 (−0.22 eV) in the pathway of late AuH release. The Gibbs free energy of P5-TS3 (−0.48 eV) is also lower than that of P7-TS1 (−0.46 eV). Furthermore, the direct dissociation into C7H15+ + AuH (P5 of −1.58 eV) is entropically more favorable than the first H transfer (TS7 of −1.24 eV). Thus, it can be concluded that n-C7H16 would fragment into small alkyl cations after the process of AuH loss. To demonstrate why a minor fragmentation into small alkyl cations occurs in the reaction of Au+ + n-C6H14, the reaction pathway was also investigated and shown in Figure 5. The reaction mechanism is similar to that of the Au+ + n-C7H16 reaction. However, the energy of P8-TS3 is as high as −0.08 eV, which does not favor the cleavage of a C−C bond. Furthermore, considering the entropy contribution (−TΔS)

also calculated and the energies of important stationary point in the lowest-energy pathway are listed in Table 2. As larger alkanes (C4−C9) are identified to have behavior similar to that of C3H8 when reacting with Au+, the detailed mechanisms will not be further discussed. However, there still exists a distinct property that alkanes larger than n-hexane start to fragment extensively into a small alkyl cation, whose mechanisms are also calculated and discussed in the text below. Mechanism for Fragmentation. Although the fragmentation of alkyl cations originated from larger alkanes has been proposed to be induced by hydride transfer,22 there is no clear evidence to confirm whether the fragmentation occurs before or after AuH loss so far. To address this issue, Figure 4 depicts the lowest-energy pathway from the hydride transfer to the fragmentation in the reaction of Au+ + n-C7H16. Similarly, the insertion of Au+ into one of the middle C−H bonds (I8 → TS6 → I9) leads to the hydride transfer to reach I9. Then, AuH is released to generate C7H15+ (I9 → P5). The subsequent fragmentation occurs from C7H15+ through three steps of hydrogen migration (P5 → P5-TS1 → P5-I1 → P5-TS2 → P5I2 → P5-TS3 → P5-I3), leading to the simultaneous cleavage of C−C bond to produce C3H6 + C4H9+ (P5-I3 → P6). Alternatively, after the formation of I9 and before the release F


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Figure 5. DFT-calculated potential-energy profiles and structures for the reaction of Au+ + n-C6H14. The ZPE-corrected energies (ΔH0 in eV) with respect to the separated reactants and some bond lengths (in pm) are shown. The Gibbs free energies of P8-TS3 are given in parentheses.

to the free energy, the energy of P8-TS3 (−0.28 eV) is higher than that of P5-TS3 (−0.48 eV) in the reaction of Au+ + nC7H16. Therefore, it agrees well with the experimental observation of minor fragmentation in the reaction of Au+ + n-C6H14 and extensive fragmentation in the reaction of Au+ + nC7H16. In the reaction of Au+ + n-C7H16, assuming that the evaporation of AuH does not take away energy of the reaction, the rate of P5-I2 → P5-TS3 on the basis of RRKM theory was estimated to be 3.14 × 106 s−1, whereas the theoretical collision rate that a cluster experiences with the 6 Pa buffer He gas in the ion trap reactor is about 1 × 106 s−1. Because the value of internal conversion is larger than the collision rate, the generated P5-I2 is able to surpass the barrier for the cleavage of C−C bond. Once the pressure of the buffer is high enough to have a collision rate of 3.14 × 106 s−1, P5-I2 can be cooled and the C−C bond may no longer break. To this end, a new experiment employing a TOF-MS coupled with a laser ablation and supersonic expansion cluster source and a fast-flow reactor has been conducted for the reaction of Au+ + n-C7H16, and the result is shown in Figure 6b. It is noteworthy that the pressure of the buffer gas in the fast-flow reactor was as high as 40 Pa in the experiment. According to the mass spectra shown in Figure 6, different from the results in ion trap experiment, C7H15+ rather than C4H9+ was generated as products, which indicates that C7H15+ was cooled by the collision of high-pressure He gas. In addition, another key factor for the different behavior is that the reactants were cooled ions in the fast flow reactor experiments but were thermalized before the reaction in the ion trap. Thus, the atomic cations Au+ is predicted to be an appropriate reagent ion to ionize larger linear alkanes (>C6) in a proper reactor filled with high pressure of buffer gas.

Figure 6. TOF mass spectra for the reactions of Au+ with n-C7H16 in an ion trap (a) and in a fast flow reactor (b). (a) replots Figure 1g. The n-C7H16 pressure and reaction time for (b) were estimated to be 0.64 Pa and 60 μs, respectively.

exothermic reactions with all given linear alkanes. Dehydrogenation occurred exclusively for ethane and hydride abstraction was the major reaction channel for alkanes larger than ethane. It has been identified that Au+ is an interesting chemical precursor to ionize small alkanes from ethane to n-hexane with high reaction efficiencies. For alkanes larger than n-hexane, the extensive fragmentation occurred with small alkyl cations being produced in the low-pressure ion trap experiment, which prevents their individual identifications. According to the DFT results, the fragmentation induced by the hydride abstraction, occurs after the process of AuH release. Further RRKM calculations and experimental results reveal that C7H15+ tends not to fragment when n-C7H16 reacts with Au+ in a reactor filled with high-pressure buffer gas. Thus, the atomic cation Au+ is predicted to be an appropriate reagent to ionize larger linear

4. CONCLUSION The gas-phase reactions of Au+ with alkanes (C2−C9) have been investigated by mass spectrometry and DFT calculations. The atomic ions Au+, prepared by laser ablation, underwent G


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alkanes (>C6) and even the branched alkanes in the highpressure reactor, which is being investigated in this laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b03836. Figures about additional mass spectra and DFTcalculated reaction mechanisms (PDF)

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AUTHOR INFORMATION

Corresponding Author

*S.-G. He. E-mail: [email protected]. Phone: +86-1062568330. Fax: +86-10-62559373. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Nos. 21325314, 21273247, and 91545122), the Major Research Plan of China (No. 2013CB834603), and CAS (No. PY201501).

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REFERENCES

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Gas-Phase Reactions of Atomic Gold Cations with Linear Alkanes (C2

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