Intracluster ion–molecule reactions between V+ and methyl acetate or ethyl acetate clusters

International Journal of Mass Spectrometry 315 (2012) 15–21

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Intracluster ion–molecule reactions between V+ and methyl acetate or ethyl acetate clusters Dababrata Paul a , Kiryong Hong a , Tae Kyu Kim a,∗ , Jun-Sik Oh b , Kwang-Woo Jung b,∗ a b

Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea Department of Chemistry and Institute of Natural Science, Wonkwang University, Iksan, Chonbuk 570-749, Republic of Korea

a r t i c l e

i n f o

Article history: Received 9 January 2012 Received in revised form 13 February 2012 Accepted 13 February 2012 Available online 20 February 2012 Keywords: Ion–molecule reaction Vanadium Heterocluster Alkyl acetate

a b s t r a c t Intracluster ion–molecule reactions of V+ (CH3 COOR)n (R = CH3 , C2 H5 , C2 D5 ) complexes produced by the mixing of laser-vaporized plasmas and pulsed supersonic beams were investigated by reflectron time-offlight mass spectrometry. The mass spectra indicated the presence of a major sequence of cluster ions with the formula V+ (CH3 COO)(CH3 COOR)n . This sequence is attributed to the insertion of V+ into the C O bond of CH3 COOR within the heteroclusters, followed by alkyl radical (R) elimination. The observation of V+ (OR) and V+ (CH3 )(OR) ions is interpreted as arising from the insertion of V+ into the C(O) O bond of the ester group, followed by CH3 CO and CO elimination, respectively. In addition, the VO+ ion is present throughout the mass spectra, indicating that insertion of V+ into the C O bond of CH3 COOR also occurs. Within the stabilizing environs of a heterocluster, sequential insertions of VO+ ions into a second molecule produced VO+ (CH3 COO)(CH3 COOR)n and VO+ (CH3 )(OR)(CH3 COOR)n heteroclusters via reaction pathways similar to those of the V+ ion. The results of isotope-labeling experiments suggest that the reaction pathway for formation of V+ (CH3 COOH)(CH3 COOC2 H5 )n involves V+ insertion into the C O bond of ethyl acetate, followed by ␤-H atom transfer from the ethyl group. Density functional theory calculations were carried out to model the structures and binding energies of both the association complexes and the relevant reaction products. The reaction pathways and energetics of each product channel are presented. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The selective activation of chemical bonds, such as C C, C H, or C O, plays an important role in optimizing synthetic schemes in organic chemistry. The importance of transition metal ions in catalyzing the chemical reactions of various organic molecules has prompted extensive experimental and theoretical studies aimed at understanding the catalytic activity of such ions. In this context, the examination of gas phase ion–molecule reactions within heteroclusters, which avoids the complications arising from solvent environments and crystalline forces, has great potential for elucidating the details of transition metal ion catalytic activity as well as the changes in the reaction pathways as a function of cluster size [1–4]. The reactivity of a transition metal ion appears to vary depending on the electronic configuration of the metal ion [5,6]. For example, the early transition metals Sc+ , Ti+ , and V+ form strong metal–oxo bonds. Hence, reactions of these bare metal ions with acetaldehyde or acetone lead to the formation of MO+ via an oxidation pathway, where M stands for the transition metal [7–12].

∗ Corresponding authors. Tel.: +82 63 850 6208; fax: +82 63 841 4893. E-mail addresses: [email protected] (T.K. Kim), [email protected] (K.-W. Jung). 1387-3806/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ijms.2012.02.007

The reactions of Fe+ and Co+ with acetone, studied by ion cyclotron resonance (ICR) and kinetic energy release distribution (KERD) experiments, mainly produce MCO+ (M = Fe+ or Co+ ) and C2 H6 [13–15]. Reaction mechanisms have been proposed in which the carbonylation products form via oxidative insertion of the metal ion into the C C acetone bond. During the last few years, we have studied the intracluster ion–molecule reactions of Ti+ with various organic molecules (water, alcohols, ethers, and ketones) with the goal of elucidating the reactivity of Ti+ ions and the reaction pathways that lead to heterocluster complex formation [16–18]. In particular, the reaction pathways of intracluster ion–molecule reactions of Ti+ with ether clusters ((CH3 OR)n , R = CH3 , n-C3 H7 , n-C4 H9 , t-C4 H9 ) appear to vary depending on the alkyl group in the ether molecule [19]. The major sequences of heterocluster ions with the formulas Ti+ (OCH3 )m (OR)n (m = 1–3 and n = 0–2) were attributed to the insertion of Ti+ ions into the C O bonds of the ether molecules within the heteroclusters, followed by alkyl radical elimination. As the size of the alkyl group in the ether molecule increased, C H and C C bond insertions by Ti+ ions became more favorable. These results indicated that the reactivity modes of Ti+ ions (selective bond insertions and subsequent eliminations) were unique and were not found in other transition metal ions. Apart from the numerous studies of transition metal ions with ether, no experimental or theoretical

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investigation into the mechanism and energetics of the reactions of vanadium ions with ester molecules is available in the literature. Moreover, few studies have examined the chemical reactions occurring within a heterocluster ion itself. In the present study, we investigated gas phase V+ (CH3 COOR)n (R = CH3 , C2 H5 , C2 D5 ) heterocluster systems to unravel the competitive ion–molecule reactions that occur within the ionized clusters and to identify reactive pathways not ordinarily found in bimolecular ion–molecule collisions. To probe the cluster reactivity, we examined reactions in heteroclusters produced by laser ablation and supersonic beam expansion techniques. Studying the variations in reactivity within vanadium–acetate heteroclusters as a function of alkyl radical size adds to our molecular-level understanding of the nature of V+ ion alkyl radical elimination reactions. In addition, density functional theory calculations were performed to rationalize the detailed reaction mechanism of the reaction of V+ with CH3 COOCH3 .

intrinsic reaction coordinate (IRC) method [25,26]. These calculations were carried out using the GAUSSIAN03 package [27]. Splitting between the ground (5 D,3d4 ) and lowest triplet (3 F,3d3 4s1 ) states of V+ was fairly well reproduced by B3LYP calculations. The calculated 1.09 eV splitting energy of V+ (3 F − 5 D) in our results was similar to the experimental (1.08 eV) [28] and theoretical (0.94 eV) [29] values. Geometries calculated at the B3LYP/TZV+6-311++G(d,p) level were also quite good. For example, the calculated value of re = 1.545 A˚ for VO+ compared well to the experimental value, ro = 1.561 ± 0.002 A˚ [30]. The structural data, including the bond lengths and bond angles of methyl acetate ˚ C O = 1.440 A, ˚ C(O) O = 1.352 A, ˚ (MA, CH3 COOCH3 , C O = 1.206 A, ˚ ∠COC = 116.0◦ ) were in good agreement with experC C = 1.507 A, imental results [31].

4. Results and discussion 2. Experimental The experimental details used in this work have been described previously [16,17]. Therefore, only a brief description is given here. V+ ions were generated by focusing the third harmonic (355 nm) of a Nd:YAG laser onto a rotating vanadium disk with an illumination area of diameter 0.5 mm. A pulse valve was employed to produce reactant clusters by supersonic expansion of alkyl acetate (CH3 COOR) vapor seeded in argon with a stagnation pressure of 1–3 atm. The laser-ablated V+ ions traversed the supersonic jet stream perpendicularly, 1 cm from a rotating target, where they reacted with the reactant clusters. The resulting ion complexes were then skimmed by a conical skimmer (1 mm in diameter). They then traveled to the extraction region of the reflectron time-offlight mass spectrometer (RTOFMS). The cluster ions were extracted using a high-voltage pulse applied to an extraction electrode, and the ions drifted along a field-free region (1 m in length). The ions were then reflected using double-stage reflectron plates, and they traveled an additional 64 cm. Finally, the ions were detected using a Chevron microchannel plate (MCP) detector. The mass spectra of the cluster ions were obtained using a 500 MHz digital oscilloscope. Spectral grade CH3 COOCH3 (99.5%), CH3 COOC2 H5 (99.8%), and CD3 COOCD3 (99.5%) (Aldrich Chemical) were used after several freeze–pump–thaw cycles to remove the dissolved atmospheric gases and other high vapor pressure impurities.

Methyl acetate (MA) provides a good model system for unraveling the chemical reactivity of the V+ ion with respect to insertion reactions into different chemical bonds, for example, C O, C O, C C, and C H. Fig. 1 illustrates a typical time-of-flight mass spectrum obtained from the reaction of V+ with MA. The prominent peaks that appear over the entire mass spectrum correspond to cluster ions with the formulas V+ (CH3 COO)(MA)n (denoted bn ), V+ (CH3 )(OCH3 )(MA)n (denoted cn ), V+ (OCH3 )(MA)n (denoted dn ), VO+ (MA)n (denoted en ), VO+ (CH3 COO)(MA)n (denoted fn ), and VO+ (CH3 )(OCH3 )(MA)n (denoted gn ), where n = 0, 1, 2,. . .. The formation of these cluster ions implies that V+ readily reacts with MA molecules solvated within intact V+ (MA)n (an series) heteroclusters. The presence of (MA)n H+ (mn series) cluster ions can be ascribed to intracluster protonation of the parent (MA)n + ions formed in the region in which the laser-ablated plume and the supersonic cluster beam intersect.

3. Theoretical Density functional theory was used for geometry optimization and frequency calculations of all involved species for the reaction of V+ with methyl acetate. All geometries were optimized without symmetry constraints using Becke’s three-parameter hybrid functional [20] combined with the Lee, Yang, and Parr [21,22] correlation functional (B3LYP) with Ahlrichs TZV [23] for V+ and 6311++G(d,p) for other atoms (C, H, and O). We calculated the triplet and quintet potential energy surfaces (PES) for the ion–molecule reaction of V+ with CH3 COOCH3 . These two states are quite close in energy and may interconvert during the reaction process [9,24]. The molecular geometries (reactants, products, intermediates, and transition states) of both electronic states were fully optimized at the B3LYP/TZV+6-311++G(d,p) levels of theory. Vibrational frequency calculations were carried out at the same level of theory to estimate all stationary points as either minima or transition states and to calculate the zero-point energy (ZPE) for all reported geometries. All transition states presented in this work were identified by a single imaginary frequency and were confirmed using the

Fig. 1. Mass spectrum of heterocluster ions formed by laser-ablated V+ and methyl acetate (MA) clusters. an : V+ (MA)n ; bn : V+ (CH3 COO)(MA)n ; cn : V+ (CH3 )(OCH3 )(MA)n ; dn : V+ (OCH3 )(MA)n ; en : VO+ (MA)n ; fn : VO+ (CH3 COO)(MA)n ; gn : VO+ (CH3 )(OCH3 )(MA)n ; mn : (MA)n H+ .

D. Paul et al. / International Journal of Mass Spectrometry 315 (2012) 15–21

The intact cluster ions V+ (MA)n are formed either from the association reaction between V+ and MA clusters or from clustering of neutral MA onto an ionic core: V+ + (MA)p → [V+ (MA)p ]‡ → V+ (MA)n + (p–n)MA

(1a)

V+ + (MA)q → V+ (MA)q + (n–q)MA → V+ (MA)n

(1b)

The evaporation of several MA molecules in reaction (1a) dissipates both the excess kinetic energy of the laser-ablated V+ ions and the excess energy produced during the exothermic ion–molecule association reaction [3]. Vanadium has a lower ionization energy than MA (IE = 6.74 eV for V [32,33] and 10.25 eV for MA [34]). Hence, once the complex is formed, the positive charge on the heterocluster is expected to reside on the V atom. Evidence supporting this charge distribution has been gathered in a number of beam experiments designed to study the formation of metal ion–organic molecules or metal–rare gas clusters [35,36]. The reaction products can be divided into two groups: (i) V+ containing clusters (bn , cn , and dn ) and (ii) VO+ -containing clusters (en , fn , and gn ). The prominent reaction pathways for the formation of V+ (CH3 COO)(MA)n ions are characterized by insertions of a V+ ion into the C O bond (methoxy group) of an MA molecule within the parent V+ (MA)n clusters, followed by CH3 elimination. V+ (MA)m → [CH3 COO V+ CH3 ](MA)m−1 → V+ (CH3 COO)(MA)n + CH3 + (m − n − 1)MA V+

(2)

V+

CH3 ] intermediate formed by insertion can The [CH3 COO dissociate internally, and the V+ (CH3 COO) ion is produced via methyl radical elimination. In this reaction, CH3 COO behaves more like a tightly bound group than as a solvating ligand species. The binding energies of Mg+ OCH3 (67.35 kcal/mol) and Co+ OCH3 (>69 kcal/mol) are found to be much higher than those of Mg+ CH3 OH (37.7 kcal/mol) and Co+ CH3 OH (35.28 kcal/mol) [37,38]. Bonding in the CH3 COO V+ ion is thus likely to resemble covalent bonding rather than an electrostatic interaction. Another intracluster ion–molecule reaction channel of V+ (MA)n heterocluster ions involves insertion of the V+ ion into the C(O) O (ester group) bond of an MA molecule. Because V+ (OCH3 )(MA)n and V+ (CH3 )(OCH3 )(MA)n ions are the dominant products observed in the present experiments, it is likely that, within the heterocluster ions, insertion of V+ into the C(O) O bond to form the [CH3 C(O) V+ OCH3 ] intermediate is the favored reaction channel among the possible insertion routes (i.e., C O, C O, C C, and C H bonds). V+ (MA)m → [CH3 C(O) V+ OCH3 ]‡ (MA)m−1 → V+ (OCH3 )(MA)n + CH3 CO + (m − n − 1)MA

(3)

V+ (MA)m → [CH3 C(O) V+ OCH3 ]‡ (MA)m−1 → V+ (CH3 )(OCH3 )(MA)n + CO + (m − n − 1)MA

(4)

The significant contribution of the V+ (OCH3 ) + CH3 CO product channel can be rationalized by the direct bond cleavage of the V+ C bond in the [CH3 C(O) V+ OCH3 ] intermediate. V+ (CH3 )(OCH3 ) + CO products arise from the [CH3 C(O) V+ OCH3 ] intermediate by CH3 migration to form an [O C V+ (CH3 )(OCH3 )] intermediate, followed by elimination of CO. The mechanism of this reactive pathway will be described in detail in the context of the calculated results in the following section. Among the reaction products, VO+ -containing clusters include VO+ (MA)n , VO+ (CH3 COO)(MA)n , and VO+ (CH3 )(OCH3 )(MA)n ions. These products are attributed to the insertion of V+ ions into to C O bond of the MA molecules within heteroclusters.

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V+ (MA)m → [(CH3 )(OCH3 )C V+ O]‡ (MA)m−1 → VO+ (MA)n + C3 H6 O + (m − n − 1)MA

(5)

The formation of VO+ via reaction (5) is not surprising considering that V+ binds very strongly to oxygen atoms [39]. The initial interaction site between the V+ and MA molecules within the heterocluster is envisioned to be the more basic site of the oxygen atom. Bonding in the association complex presumably involves formation of a dative bond requiring electron donation from the oxygen atom of the carbonyl group of MA to the 3d orbital of the V+ ion. The V+ ion then inserts into the C O bond to form a [(CH3 )(OCH3 )C V+ O] intermediate, after which the VO+ ion is formed by elimination of C3 H6 O. A similar mechanism was proposed to describe the oxidation pathway for the reaction of Ti+ with CH3 CHO, which occurred through a C O bond insertion process [9,11]. This mechanism is also in reasonable agreement with the results of Allison and Ridge [10], who described the TiCln O+ product channel for ion–molecule reactions of chlorotitanium ions TiCln + (n = 1–3) with CH3 COCH3 . In our recent study, we also observed that the reaction of Ti+ with CH3 COCH3 clusters is dominated exclusively by the insertion of a Ti+ ion into the C O bond to produce TiO+ (CH3 COCH3 )n clusters [40]. Within the stabilizing environs of a heterocluster, insertion of a VO+ ion into a second MA molecule produces VO+ (CH3 COO)(MA)n (fn series) and VO+ (CH3 )(OCH3 )(MA)n (gn series) heteroclusters via similar reaction pathways, (2) and (4), respectively. To interpret the reaction mechanism, we calculated the ground state structures and relative energies of the possible V+ MA complexes and related reaction products at the B3LYP/TZV+6311++G(d,p) level using the GAUSSIAN03 package [27], which includes Becke’s three-parameter nonlocal hybrid exchange functional and the nonlocal correlation functional of Lee, Yang, and Parr [21]. Recent studies have shown that reactions between transition metal ions and molecules are affected by the electronic state and kinetic energy of the transition metal ion [41,42]. While we cannot exclude all possible reactions of the electronically excited V+ ions, we believe that the reaction patterns observed in the current experiments mainly arise from ground-state reactions. This is because laser-ablated V+ ions are likely to be efficiently quenched by collisions with the supersonic CH3 COOR/Ar beam. In addition, the failure to observe any reaction products from the endothermic reaction channels, even at high laser fluence, supports our hypothesis that V+ ions in high-lying states make no appreciable contributions to the above reactions. The electronic state of the reactant V+ ion that correlated with the reaction product formations is the 5 D (3d4 ) state, which is the lowest quintet state having four d electrons. Nevertheless, in the calculations we considered both the ground quintet (5 D) and excited triplet (3 F) states of V+ ions for the ion–molecule reaction of V+ with MA because the chemical reactivity of transition metal ions with organic molecules is greatly influenced by the spin state of the metal ion. Two possible V+ MA encounter complexes (referred to as CP1 and CP2) are found as the optimized ground state structures. CP1 corresponds to the case in which the V+ ion interacts with the oxygen lone pair orbitals of the methoxy group, which is a feasible route for C O and C(O) O bond insertions. CP2 represents the case in which the V+ ion approaches the O atom of the carbonyl group, thereby activating the C O bond. The reaction pathway in Fig. 2, V+ + MA, suggests that the V+ ion can insert into the C O bond of the methoxy group within CP1 followed by CH3 -elimination to produce the V+ (CH3 COO) ion. Fig. 2a shows the potential energy surface of the V+ insertion reaction into the C O bond of MA, where the superscript denotes the spin multiplicity. IM and TS represent the intermediate and transition state, respectively. ZPE corrections were included in calculations of the reaction energies. The optimized geometries of the reactants, products, intermediates, and

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Fig. 3. Potential energy diagrams for V+ (CH3 )(OCH3 ) and V+ (OCH3 ) formation channels of V+ insertion into the C(O) O bond of CH3 COOCH3 . See supplementary data, Fig. S1, for bond lengths and angles of the optimized structures.

Fig. 2. (a) Potential energy surface of the V+ insertion reaction into the C O bond of CH3 COOCH3 . (b) Geometries of the reactants, products, intermediates, and transition states of the C O bond insertion pathway, optimized at the B3LYP/TZV+6311++G(d,p) level (bond lengths are given in Å and angles are given in◦ ).

transition states of the C O bond insertion pathway are represented in Fig. 2b. Initially V+ ion approaches the O atom of the methoxy group without crossing a barrier and forms an association complex CP1 IM1 . The CP1 5 IM1 intermediate is predicted to be more stable than the separated reactants by 32.24 kcal/mol. The triplet state association complex CP1 3 IM1 is less stable than the quintet state CP1 5 IM1 by 26.55 kcal/mol. Fig. 2b shows that the V+ ion is positioned a distance of 2.095 A˚ from the O atom in the CP1 5 IM1 intermediate. Binding to V+ lengthens the adjacent C(O) O and ˚ respectively, thereby actiH3 C O bonds by 0.089 A˚ and 0.038 A, vating both C O bonds. The next reaction step is V+ insertion into the O CH3 bond of MA to produce the [CH3 COO V+ CH3 ] intermediate (CP1 5 IM6 ) via a three-membered ring structure (CP1 5 TS16 ). Relatively stable formation of a four-membered ring

structure, CP1 5 IM7 , by the approach of V+ to the carbonyl O atom results in fragmentation to produce V+ (CH3 COO) + CH3 . Energetically, CP1 5 IM7 and CP1 3 IM7 constitute the most stable species along the quintet and triplet surfaces, respectively. The exothermicity of this reaction pathway, −27.08 kcal/mol, suggests that this reaction is rather favorable, as observed in the current mass spectrum (bn series). As an alternative reaction pathway for formation of V+ containing clusters, the V+ ion can also insert into the C(O) O bond of the ester group in an MA. Fig. 3 shows the potential energy diagram for V+ (CH3 )(OCH3 ) and V+ (OCH3 ) formation channels. The geometrical parameters (bond lengths and angles) of the optimized structures are given in Fig. S1, supplementary data. After the formation of an association complex (CP1 5 IM1 ), the V+ (OCH3 ) ion (dn series) is expected to form via simultaneous C(O) O and C(O) V+ bond ruptures of the three-membered ring structure (CP1 5 TS12 ). The substantial elongation of a C(O) O bond in the CP1 5 IM1 inter˚ to a bond length comparable to that in the mediate (1.441 A) ˚ makes this reaction pathway CP1 5 TS12 transition state (1.922 A) favorable. Along the PES, 4 V+ OCH3 + 2 OC2 H3 formation channel is calculated to be −1.61 kcal/mol on the quintet surface. Alternatively, the V+ ion in CP1 5 TS12 approaches the carbonyl O atom while simultaneously breaking the C(O) O bond to produce the CP1 5 IM2 intermediate. Because the three-membered ring structure CP1 5 IM2 is unstable (Fig. S1, supplementary data), the V+ ion again attacks the carbon atom of the methyl radical attached to the carbonyl group and forms the CP1 5 IM3 intermediate via the CP1 5 TS23 transition state with a barrier of 20.87 kcal/mol. It is mentionable that the CP1 3 IM2 intermediate is located at Erel = −62.98 kcal/mol, representing the deepest point along with a barrier 32.07 kcal/mol at the triplet surface. The calculation results indicate that V+ (CH3 )(OCH3 ) product channels are exothermic (−8.77 kcal/mol and −32.50 kcal/mol) for both spin states. Hence the elimination of CO from CP1 5 IM3 produces the V+ (CH3 )(OCH3 ) ion (cn series) as a favorable pathway. This is consistent with a view that activation of methane by a transition metal oxide permits CH3 migration to form a molecular complex between the metal atom and the methanol [43,44]. The observation of dominant VO+ -containing cluster ions (en , fn , and gn series) across the entire mass spectrum reveals the effective oxidation of the V+ ion. To interpret the formation mechanism of the VO+ ion via an oxidation pathway, we calculated potential

D. Paul et al. / International Journal of Mass Spectrometry 315 (2012) 15–21

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Fig. 4. Potential energy surfaces corresponding to the V+ insertion into the C O bond of CH3 COOCH3 . See supplementary data, Fig. S2, for bond lengths and angles of the optimized structures.

energy surfaces for V+

insertion into the C O bond of the encounter complex (see Fig. 4). The geometrical parameters (bond lengths and angles) of the optimized structures are given in Fig. S2, supplementary data. CP2 5 IM1 corresponds to the case in which V+ is positioned a distance of 1.974 A˚ from the O atom of carbonyl group. In our recent work, it was found that CP2 5 IM1 is more stable than CP1 5 IM1 by 16.82 kcal/mol [45], suggesting that the former complex is the more favorable of the two structures. Both quintet and triplet states can form collinear complexes with MA stabilized by 50.05 kcal/mol and 35.70 kcal/mol relative to the reactants, respectively, without encountering a reaction barrier. Ongoing from the free MA to CP2 5 IM1 , the C O bond is length˚ It is likely that this bond weakening ultimately ened by 0.042 A. leads to the rupture of C O bond. Once the encounter complex (CP2 5 IM1 ) is formed, V+ insertion into the C O bond produces the intermediate CP2 3 IM2 with spin-orbit coupling. Fig. 4 shows that there is an intersystem crossing (ISC) between CP1 5 TS12 and CP1 3 IM2 . Similar to the recent calculations of Ti+ + CH3 COCH3 reaction [40,46], we focus on the ISC point between CP2 5 IM1 and CP2 3 IM2 , because this ISC point is directly related to the most favorable reaction pathway of the direct metal-ion insertion. It is unlikely that the reactions proceed on the quintet PES, because the barriers height between CP2 5 IM1 and CP2 5 IM2 is too high (80.73 kcal/mol). The ISC has been reported in the reactions of Ti+ with small organic molecules such as H2 O, CH4 , and CH3 CHO [9,38]. Calculations by Rue et al. have also suggested coupling between the quintet and triplet states of the isoelectronic V+ (CS2 ) system [47]. Therefore, we conclude that the reaction of 5 V+ with MA predominantly occurs on the triplet PES after 3 IM2 formation. Because the four-membered ring structure is unstable, the CP2 3 IM2 can rearrange to CP2 3 IM3 intermediate via CP2 3 TS23 transition state with a small barrier of 2.72 kcal/mol (Fig. S2, supplementary data). CP2 3 IM3 intermediate then undergoes ␣-H atom transfer from the CH3 group to the ␤-C atom (CP2 3 IM4 ), followed by CH2 CHOCH3 elimination, to produce a VO+ ion (en series). The overall reaction energies were calculated to be −16.02 kcal/mol for 3 VO+ + CH CHOCH product channel, in good agreement with the 2 3 experimental observations of the dominant VO+ -containing cluster ions. Therefore, our results indicate that the VO+ formation pathway occurs through five elementary steps: association complexation, intersystem crossing, V+ insertion into the C O bond,

Fig. 5. Mass spectrum of heterocluster ions formed by laser-ablated V+ and ethyl acetate (EA) clusters. an : V+ (EA)n ; bn : V+ (CH3 COO)(EA)n ; Cn : V+ (CH3 )(OC2 H5 )(EA)n /V+ (CH3 COOH)(EA)n ; dn : V+ (OC2 H5 )(EA)n ; en : VO+ (EA)n ; fn : VO+ (CH3 COO)(EA)n ; Gn : VO+ (CH3 )(OC2 H5 )(EA)n /VO+ (CH3 COOH)(EA)n ; mn : (EA)n H+ .

hydrogen transfer, and fragmentation. Furthermore, it is noted that the peaks corresponding to VO+ (CH3 COO)(MA)n (fn series) and VO+ (CH3 )(OCH3 )(MA)n (gn series) ions present significant contributions to the mass spectrum. This result indicates that the VO+ ion within the VO+ (MA)n cluster can undergo subsequent C O and C(O) O insertion reactions with a solvating methyl acetate moiety, as discussed above. The presence of the CH3 CH2 group in ethyl acetate (EA) is expected to alter the cluster reaction pathways away from those involving methyl acetate because ethyl acetate possesses ␤-H atoms that can induce intramolecular H atom migration. Fig. 5 shows a typical mass spectrum of ionic species produced from the reaction of V+ with EA clusters seeded in 1.5 atm Ar. The dominant cluster ions belong to the series of heterocluster ions, V+ (CH3 COO)(EA)n (labeled bn ), produced from V+ insertions into the C O bond of ethoxy group. The observation of VO+ (EA)n (labeled en ) and VO+ (CH3 COO)(EA)n (labeled fn ) ions indicates that oxidative insertion of V+ ions into the C O bond produces the VO+ ion. Within the stabilizing environs of a heterocluster, the sequential insertions of VO+ ions into a second molecule produce VO+ (CH3 COO)(EA)n heteroclusters via reaction pathways similar to those involving the V+ ion. Compared to the case of the V+ + MA system, the observation of an abnormally intense peak corresponding to the V+ (CH3 )(OC2 H5 ) ion (m/e 111 and denoted c0 ) is quite surprising because it is expected to arise from the insertion of V+ into the C(O) O bond, followed by CO elimination. V+ (EA)m → [CH3 C(O) V+ OC2 H5 ]‡ (EA)m → V+ (CH3 )(OC2 H5 )(EA)n + CO + (m − n − 1)EA

(6)

Alternatively, the [CH3 COO V+ C2 H5 ] intermediate formed from V+ insertion into the ethoxy C O bond of ethyl acetate can also produce a V+ (CH3 COOH) ion via ␤-H atom migration from the ethyl group.

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V+ (EA)n → [CH3 COO V+ C2 H5 ]‡ (EA)m → V+ (CH3 COOH)(EA)n + C2 H4 + (m − n − 1)EA

(7)

Unfortunately, the two different pathways listed above cannot be distinguished by mass spectrometry because the V+ (CH3 )(OC2 H5 ) and V+ (CH3 COOH) ions have identical mass m/e 111. Similarly, the Gn series peaks in Fig. 5 correspond to VO+ (CH3 )(OC2 H5 )(EA)n and/or VO+ (CH3 COOH)(EA)n ions produced from the VO+ insertion into the C(O) O and C O bonds of EA within the heteroclusters, respectively. It is noted that evidence for the CH3 COOH formation pathway is not found in the reaction of V+ with MA clusters. Therefore, the current results demonstrate that the [CH3 COO V+ C2 H5 ] intermediate may decompose to V+ (CH3 COOH) + C2 H4 by H-atom transfer from the ␤-carbon of the ethyl group. The isotope substitution experiments with deuterated ethyl acetate provide additional evidence that is important for understanding H atom migration within heteroclusters. We employed CD3 COOC2 D5 (d-EA) to resolve the overlapping ion peaks with the same mass but different chemical composition, such as the peaks corresponding to V+ (CH3 )(OC2 H5 )(EA)n and V+ (CH3 COOH)(EA)n , or to VO+ (CH3 )(OC2 H5 )(EA)n and VO+ (CH3 COOH)(EA)n , in the mass spectrum shown in Fig. 5. In the low-mass region (Fig. S3, supplementary data), the intensity of V+ (CD3 )(OC2 D5 ) (m/e 119, denoted c0 ) and VO+ (CD3 )(OC2 D5 ) (m/e 135, denoted g0 ) ions is much lower than that of V+ (CD3 COOD) (m/e 115, denoted c0 ) and VO+ (CD3 COOD) (m/e 131, denoted g0 ) ions. The pronounced CD3 COOD formation pathway clearly demonstrates that the C O bond insertion channel of the V+ /VO+ ions, followed by ␤-D atom migration, is more favorable than the C(O) O bond insertion channel. As the cluster size increases, however, these two reactive channels show similar contributions, i.e., the intensities of the V+ (CD3 COOD)(d-EA) (m/e 211, denoted c1 ) and VO+ (CD3 COOD)(dEA) (m/e 227, denoted g1 ) ions are comparable to the intensities of the V+ (CD3 )(OC2 D5 )(d-EA) (m/e 215, denoted c1 ) and VO+ (CD3 )(OC2 D5 )(d-EA) (m/e 231, denoted g1 ) ions, as shown in the high mass region. These results imply that the reactivity of the V+ /VO+ ion diminishes with increasing solvation by d-EA molecules. The apparent quenching of D atom migration is attributed to increased stabilization of V+ (d-EA)n cluster ions with an increase

in the degree of solvation. Another possibility is that the d-EA molecules surrounding the metal ion create an energy barrier in the reaction pathway for D atom migration. Within such a tightly packed solvent cage, the D atom transfer capabilities are greatly reduced due to geometrical constraints. D atom transfer is expected to decrease with increasing cluster size. The consequence is that D atom migration reactions may be suppressed in sufficiently large clusters. Scheme 1 provides a summary of the reaction pathways of V+ + CH3 COOR (R = CH3 , C2 H5 ), along with the calculated reaction energies. The reaction pathways can be divided into three categories: (i) C O bond activation, (ii) C(O) O bond activation, and (iii) C O bond activation. The reactions of the encounter complex CP1 are initiated by V+ insertion into the C O bond of an acetate molecule to form a [CH3 COO V+ R] intermediate. Hence, a V+ (CH3 COO) ion is expected to form via V+ C bond rupture. The present mass spectrometry results, in which V+ (CH3 COO) cluster ions were observed as significant products, are consistent with the calculation results, indicating that this product channel is thermodynamically favorable because of its high exothermicity (−27.08 kcal/mol). Alternatively, This intermediate could undergo ␤-H atom migration from the C2 H5 group of ethyl acetate to the O atom, followed by C2 H4 elimination, to produce the V+ (CH3 COOH) ion. As a competing pathway, the V+ ion can insert into the C(O) O bond of the ester group within CP1 to form [CH3 C(O) V+ OR]. This intermediate could produce the V+ (OR) ion via direct C(O) V+ bond cleavage. In addition, the [CH3 C(O) V+ OR] intermediate could produce V+ (CH3 )(OR), followed by transfer of a CH3 group attached to the carbonyl group and elimination of CO. The enthalpy changes for the V+ (OR) and V+ (CH3 )(OR) product channels indicate that these are exothermic reactions (−1.61 kcal/mol and −8.77 kcal/mol, respectively) and, hence, are thermodynamically favorable. The reaction involving the encounter complex CP2 is initiated by V+ insertion into the C O bond of the parent molecule to form the [(CH3 )(OR)C V+ O] intermediate, followed by CH2 CHOR elimination to produce VO+ . The observation of dominant VO+ products in this study, as well as the exothermicity of the associated reaction pathway (−16.02 kcal/mol), suggest

Scheme 1. Summary of observed reaction channels of V+ + CH3 COOR (R = CH3 , C2 H5 ) along with the calculated reaction energies (kcal/mol) for the corresponding ion–molecule reactions of V+ + (MA)n system. * Denotes reaction channel available to ethyl acetate only.

D. Paul et al. / International Journal of Mass Spectrometry 315 (2012) 15–21

that this reaction appears energetically feasible. This mechanism is analogous to the Ti+ + CH3 COCH3 → TiO+ + CH2 CHCH3 reaction, in which the oxidation channel proceeds from a [O Ti+ C(CH3 )2 ] intermediate via H atom migration to form a [O Ti+ C(H)CH2 (CH3 )] transition state [7–11]. Within the stabilizing environs of a heterocluster, the insertion of a VO+ ion into a second acetate molecule produces VO+ (CH3 COO), VO+ (CH3 )(OR), and VO+ CH3 COOH ions via similar reaction pathways, denoted (i) and (ii) in Scheme 1. It should be noted that the reactions observed in the current experiments are not under thermodynamic control. Rather, they are likely under (i) kinetic control, so the fastest reactions dominate or (ii) the pathways are determined by the initial V+ binding site. In these cases, the product branching ratios are not determined by the exothermicity, but rather by transition state energies and entropies, ease of intersystem crossing and even perhaps longrange V+ · · ·MA interactions. Further experimental and theoretical investigations focusing on the structures of the observed reaction products are needed to shed more light on V+ ion-containing alkyl acetate clusters. 5. Conclusions Competitive insertion reactions of V+ within V+ (CH3 COOR)n (R = CH3 , C2 H5 ) heterocluster ions were studied using a combination of laser ablation and supersonic beam expansion. The mass spectrum exhibited peaks corresponding to two types of reaction products: (a) V+ -containing clusters (bn , cn , and dn series) and (b) VO+ -containing clusters (en , fn , and gn series). The primary reactions produced a major sequence of V+ (CH3 COO)(CH3 COOR)n ions, attributed to the insertion of V+ ions into the C O bond of a parent molecule followed by V+ C bond rupture. The observation of V+ (OR)(CH3 COOR)n and V+ (CH3 )(OR)(CH3 COOR)n ions among the reaction products is interpreted as resulting from V+ insertion into the C(O) O bond to form an [CH3 C(O) V+ OR] intermediate. Direct C(O) V+ bond cleavage produces an V+ (OR) ion. The CO elimination channel proceeds from the intermediate by CH3 migration to form an [O C V+ (CH3 )(OR)] intermediate, which then loses CO to produce the V+ (CH3 )(OR) ion. Observation of the VO+ (CH3 COOR)n ion is understood as resulting from intersystem crossing to the triplet state after a C O insertion reaction, followed by H atom transfer and CH2 CHOR elimination. In studies of the origin of the C2 H4 elimination, isotope-labeling experiments led us to conclude that ␤-H atom migration from the ethyl group within the [CH3 COO V+ C2 H5 ] intermediate plays an important role in producing a V+ (CH3 COOH) ion. Density functional theory calculations were performed to rationalize the reaction pathways and energetics of the proposed mechanism. Acknowledgements This work was supported by the Basic Science Research Program 2009-0068446 and 2010-0006570 through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology. One of the authors (K.-W. Jung) also gratefully acknowledges the Basic Science Research Program (2010-0008852) through the NRF funded by the Ministry of Education, Science and Technology.

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