Crucial Role of Solvent-Impacted Molecular Anionic Resonances in

May 15, 2014 - Shoushan Wang, Jinxiang Liu, Changzhe Zhang, Li Guo, and Yuxiang Bu. School of Chemistry and Chemical Engineering, Institute of ...
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Crucial Role of Solvent-Impacted Molecular Anionic Resonances in Controlling Protonation Modes in the Acetonitrile−Water Anionic Cluster Revealed by ab Initio Molecular Dynamics Simulations Shoushan Wang, Jinxiang Liu, Changzhe Zhang, Li Guo, and Yuxiang Bu* School of Chemistry and Chemical Engineering, Institute of Theoretical Chemistry, Shandong University, Jinan 250100, P. R. China S Supporting Information *

ABSTRACT: We present an ab initio molecular dynamics simulation study of a CH3CN−(H2O)40 cluster with an excess electron (EE) injected vertically in this work. Instead of surface bound or internally solvated electron, a hydrated CH3CN− is first formed as the CN transient after geometrical relaxation. The driving forces for the formation of CH3CN− are bending vibration of ∠CCN angle, which initiates transfer of an extra charge to the CH3CN LUMO, and hydration effect of the immediate water molecules, which plays a stabilizing role. Solvent thermal fluctuation can lead to different resonances (the quasi-C2-resonance versus quasi-N-resonance) from the CN transient and further cause the hydrated CH3CN− system to evolve via two distinctly different pathways featuring spontaneous proton transfer to the central C and N sites, producing two different protonation products, respectively. The solvent thermal fluctuation induced formation of hydrogen bonding with the corresponding sites (C2 versus N) is responsible for the quasi-resonances and interconversion between three resonant structures and further proton transfers featuring spontaneous transfer of a proton to C2 or to N from its interacting water molecule. The duration of CH3CN− for either of the two proton transfer processes is less than 200 fs. On the basis of experimental ESR results in which only the CH3CHN radical was found and present theoretical calculations, it is suggested that the trans-CH3CNH radical can be further converted to the CH3CHN radical via a water-mediated hydrogen atom transfer path.

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INTRODUCTION Acetonitrile (CH3CN) contains three of the four major biogenic elements (C, N, O, H) and can be synthesized experimentally from basic starting ingredients such as CH4, NH3, CO2, and H2O.1 It has been detected to be abundant in cometary comae, Titan’s atmosphere, and the interstellar medium,2−7 and is also considered to be a probable species on the primitive earth. Ionizing radiation studies of aqueous solutions of CH3CN irradiated from cosmic rays (mainly γ-ray) and ultraviolet rays have been widely performed from the prebiotic chemistry perspective.8−13 Aldehyde, oligomers, and amino acids (mainly glycine) have been identified in acidhydrolyzed (or hydrolyzed) products of the irradiated CH3CN−H2O mixtures. The CH3CN−H2O binary mixtures, therefore, play an important role in the origin of life region. Reaction mechanisms were proposed to interpret the formations of the observed products and possible intermediates.11 CH3CN monomer radical anion, CH3CN−, as a crucial intermediate was proposed according to the reaction between CH3CN molecule and hydrated electron and was considered to initiate partly the subsequent sequence of reaction events.11 However, CH3CN− was not detected experimentally owing to rapid protonation reactions with surrounding water molecules, which produced two radicals, CH3CHN and CH3CNH, which are key reactants for the formation of biological organic molecules.8,11 This underlines the highly reactive feature of CH3CN− in aqueous phase. The protonation product, the © 2014 American Chemical Society

CH3CHN radical, was identified experimentally by ESR spectroscopy,14 but its isomer, another protonation form (CH3CNH radical), has not been identified so far and was only speculated to be produced according to the reaction producing NC−CNH radical in irradiated aqueous solutions of cyanogen.11,15 Moreover, the isomeric feature (trans or cis or all) of the CH3CNH radical was also not determined in the proposed reaction mechanisms. Different isomers of the CH3CNH radical have different reaction properties.16 In addition, the CH3CN−H2O binary mixtures are extensively used in electrochemical field in which electron transfer (ET) reactions are always involved.17 Consequently, investigations of protonation mechanisms and the protonation products of CH3CN− reacting with waters are helpful for better understanding the prebiotic chemical evolution in irradiated aqueous media and electrochemical primary reactions occurring in the CH3CN−H2O binary solvents. The CH3CN molecule has extremely negative electron affinities (−2.84 eV/vertical versus −2.22 eV/adiabatic), which are responsible for autodissociation of CH3CN− in the gas Special Issue: International Conference on Theoretical and High Performance Computational Chemistry Symposium Received: March 27, 2014 Revised: May 7, 2014 Published: May 15, 2014 9212

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phase.18,19 However, CH3CN− has been determined experimentally in the acetonitrile β-crystal20,21 and was demonstrated theoretically to be stable in a polar environment.18,22 It is found that the extra charge on CH3CN− is nearly averagely distributed at the cyanide carbon (C2) and at the nitrogen (N).20,22 Resonance theory now is widely used to illustrate physical and chemical features of organic molecules such as acidity or basicity, dipole moment, bond length, bond order, and charge distribution and to make predictions of reaction paths and products.23−25 To interpret the extra charge distribution on CH3CN−, resonances of CH3CN− were proposed from experimental data and then supplemented from theoretical data.20,22 It is well-known that the local structure and thermal fluctuation of solvent environment can effectively influence the solvation dynamics and states of an EE.26−29 Our preliminary calculations indicate that the singly occupied molecular orbital (SOMO) of CH3CN− is a mixed one by the C−N π* orbital and −CH3 outside end Rydberg orbital, implying that the CH3CN− structure is a resonance hybrid (Figure S1, Supporting Information). However, due to the influence of solvent thermal fluctuation, excess charge distribution on CH3CN− may be modified, yielding different quasi-resonance structures (essentially resonance hybrids), and leading to stateto-state interconversion among the resonance hybrids. On the basis of resonance theory, component resonances are less stable than the parent resonance hybrid.30 Hence, the abovementioned quasi-resonances of CH3CN− may be responsible for different protonation reactions and products occurring in irradiated CH3CN−H2O binary mixtures. Unfortunately, this microscopic level information is nearly unable to obtain experimentally owing to ultrafast reaction dynamics. However, ab initio molecular dynamics (AIMD) simulations can provide the relevant dynamical information on these questions. The CH3CN−H2O clusters have been detected in the planetary ionosphere and the relevant reactions occurring in bulk solutions can be reproduced sufficiently in medium-sized clusters.31−35 In the present work, ab initio molecular dynamics (AIMD) simulations are conducted by using a CH3CN@(H2O)40 cluster, a mixed cluster containing a CH3CN and 40 H2O molecules, with an EE added vertically with a primary objective to investigate possible states and their interconversion of CH3CN− and different protonation paths and products in the water cluster. The modulating role of solvent thermal fluctuation is also explored. In addition, solvation dynamics of the vertically added EE is also studied to mimic the formation process of CH3CN−, which is probably related to that in planetary ionosphere. Three possible resonance states are observed in the early stage of the AIMD simulation trajectory: the CN transient having nearly equivalent distributions of an EE at the C2 and N sites, the quasi-C2resonance with a large proportion of EE distribution at C2, and quasi-N-resonance with a large proportion of EE distribution at N. The bending vibration of the ∠CCN angle and the hydration effect of immediate water molecules cooperatively make CH3CN− first be in the CN transient, and then two state evolutions either to the quasi-C2-resonance or to quasi-Nresonance proceed in the subsequent processes, which are controlled by solvent thermal fluctuation. Further, for each quasi-X-resonance state, proton transfer is observed from the immediate solvent H2O to its hydrogen-bonding X site, producing a stable CH3CHN radical or a metastable transCH3CNH radical. The duration of CH3CN− is only hundreds of femtoseconds (less than 200 fs) in either of the two

protonation processes. Isomerization can proceed from the trans-CH3CNH radical to the CH3CHN radical, and the watermediated hydrogen atom transfer path is predicted to be the most possible mode.

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CALCULATIONAL AND SIMULATION DETAILS AIMD simulations have been extensively used to effectively treat the formation and cleavage processes of chemical bonds,31,32 and to explore the solvation dynamics of excess electrons in liquids,28,36−39 and thus are also used here to investigate the excess electron impacted processes in the CH3CN−H2O cluster with an excess electron. The computational details are briefly described as follows. A large neutral cluster CH3CN(H2O)55 was constructed and first equilibrated for 3 ns by classical MD simulation and then was further equilibrated for 3 ps by AIMD simulation.40 A CH3CN(H2O)40 cluster was extracted from the equilibrated snapshot configuration for subsequent AIMD simulation study on excess electron binding. The Becke, Lee, Yang, and Parr (BLYP) exchange−correlation functional22,41 and the triple-ζ basis set augmented with two polarization functions optimized for condensed phase systems (molopt-TZV2P)31,42 were used to evaluate energies and forces. The combination of cluster anion CH3CN(H2O)40− including the first and second hydration shells of CH3CN and the high-level basis set molopt-TZV2P makes the computational cost acceptable and is viewed to be adequate to investigate possible protonation reactions between CH3CN− and surrounding waters.33 The system was placed in a 20 × 20 × 20 Å3 cubic box. To eliminate the two defects of DFT theory,31 dispersion interaction43 and self-interaction correction (a = 0.2, b = 0.0)44 were all considered in our AIMD simulation studies. A Nosé−Hoover chain thermostat was used to maintain the system temperature at 300 K.45 Goedecker− Teter−Hutter norm-conserving pseudopotentials46 were employed to treat core electrons of heavy atoms and a cutoff of 300 Ry was used for the auxiliary plane wave basis set. The wavelet-based Poisson solver was employed with open boundary conditions. All calculations were carried out by using the Quickstep module of CP2K package.47

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RESULTS AND DISCUSSION Several initial conformations of the CH3CN(H2O)40 cluster with an EE added vertically were used for subsequent AIMD simulation studies with the similar evolution trends and the same conclusions (see the Supporting Information, section S2). Before discussing in detail the AIMD simulation results of the CH3CN(H2O)40− cluster anion, it is necessary to examine whether or not the AIMD simulation of neutral CH3CN(H2O)55 parent cluster gets equilibrated or the selected initial snapshot configurations of the CH3CN(H2O)40 cluster are reasonable. Radial distribution functions (RDFs) of selected typical atom pairs were calculated from the equilibrium trajectory of the CH3CN(H2O)55 parent cluster (Figure S2, Supporting Information). The present AIMD simulation results are in good agreement with the previous classical molecular dynamics simulations, implying that all selected initial conformations of the CH3CN(H2O)40 cluster are reasonable.48,49 It is noted that the hydrogen bond is formed between the CH3CN N atom and surrounding water molecules with the first peak of gN‑HW(r) at around 2.1 Å, whereas no hydrogen bond is found between the C2 atom and surrounding water molecules as evidenced by the first peak of gC2‑HW(r) at around 9213

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Figure 1. Snapshots extracted from the PT-to-C trajectory. (A) Spin density distributions (isodensity: 0.0004) at different times that show the time evolution of excess electron state. (B) The corresponding cluster structures.

Figure 2. Snapshots extracted from the PT-to-N trajectory. (A) Spin density distributions (isodensity: 0.0004) at different times that show the time evolution of excess electron state. (B) The corresponding cluster structures.

Figure 3. Time evolutions of relevant quantities in the PT-to-C trajectory: distances and angles (A); spin densities (B); VDE and the radius of gyration (C). The pink and green arrows position the CN transient and quasi-C2-resonance structures, respectively. Od and Hd denote the oxygen and hydrogen atoms of the reactive water molecule, AXYZ denotes the corresponding angle, and dXY denotes the corresponding distance, respectively. 9214

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Figure 4. Time evolutions of relevant quantities in the PT-to-N trajectory: distances and angles (A); spin densities (B); VDE and the radius of gyration (C). The pink arrows position the CN transient. Od and Hd denote the oxygen and hydrogen atoms of the reactive water molecule, AXYZ denotes the corresponding angle, and dXY denotes the corresponding distance, respectively.

3.2 Å due to strong hydrophobicity of the −CH3 group. These hydration structures play an important role in controlling the protonation pathways of CH3CN−. Snapshots extracted from two separate AIMD simulations of the CH3CN(H2O) 40− cluster anion with two different protonation products are depicted in Figures 1 and 2, respectively, and time evolutions of relevant quantities in the two AIMD simulations are presented in Figures 3 and 4, respectively. Analogous solvation dynamics were monitored after an EE was injected vertically into different initial configurations (Figures 1A and 2A). The spin density is distributed mainly on the cluster surface after an EE is added vertically with only tiny extra charge localizing on CH3CN. Then it gradually gathers on the CH3CN with the radius of gyration decreased continuously and eventually CH3CN− is produced (Figures 1 and 2 shows CH3CN− formation within 40 and 100 fs). The bending vibration of the ∠CCN angle and the stability requirement from the hydration effect of the immediate water environment cooperatively lead to the formation of CH3CN−. The bending vibration of the ∠CCN angle starting at tens of femtoseconds initiates transfer of a part of the extra charge to the CH3CN lowest unoccupied molecular orbital, which is a mixture of the C−N π* orbital and the −CH3 outside end Rydberg orbital. The stability requirement from the hydration effect of surrounding water molecules forces the ∠CCN angle to further bend, which conversely makes more extra charge transfer to CH3CN (Figure S3, Supporting Information; see below). Mutual promotion of the two actions elongates the C2−N distance to ca. 1.25 Å owing to the unfavorable antibonding effect of the C−N π* orbital and bends the ∠CCN bond angle to ca. 130°. A hydrated CH3CN− anion was eventually yielded with a radius of gyration around 2.0 Å for the excess electron (Figures 3A−C and 4A−C), which

is called the CN transient. However, interestingly, excess charge resonance is observed in the subsequent time evolution. To clearly distinguish these observed different states, different resonance structures of CH3CN− are defined with the following criteria. In all cases, the total spin density on CH3CN− is not less than 0.85 and the remainder is distributed in the water region. If the spin density at the N (or C2) atom is larger than 0.5, implying that the extra negative charge is mainly located at the C2 (or N) atom, the quasi-C2-resonance (or the quasi-Nresonance) is assigned to CH3CN−, whereas if the spin density at the −CH3 group, C2, and N atom are all smaller than 0.5, meaning that the extra negative charge is almost distributed averagely at the three local sites, the CN transient is assigned to CH3CN− (Scheme 1). CH3CN, HCN, and CO2 all have linear structures, and the bending vibration of related bond angles of HCN and CO2 in polar solvents plays a crucial role in Scheme 1. Schematic Representation of Three Resonance States of the CH3CN− Radical Aniona

a

Note: the size of red arcs denote the relative negative charge distributuons, and the size of blue arcs denote the relative spin density distributuons at the three local spaces (−CH3, C2, N). The CN transient: no spin density (ρ) at any local place exceeds 0.5. The quasiC2-resonance: ρN > 0.5. The quasi-N-resonance: ρC2 > 0.5. 9215

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Figure 5. Radial distribution functions (g(r)) of the C2 and N atoms with respect to water hydrogen atoms in three different cases. The blue curves represent g(r) from the equilibrium trajectory of neutral CH3CN(H2O)55 parent cluster. The red curves represent g(r) from the trajectory of the charged cluster before the PT-to-N reaction. The black curves represent g(r) from the trajectory of the charged cluster before the PT-to-C reaction.

effectively trapping and stabilizing an EE.37,50,51 In neat liquid CH3CN, the vertically added excess electron is trapped and stabilized by a core CH3CN molecule with its ∠CCN angle bent to ca. 162°.28 Relative to CH3CN, H2O has a smaller size, faster average solvation time (260 fs), and larger solvation energy.52 Therefore, it is possible that bending of the ∠CCN angle can effectively trap and stabilize an excess electron in water environment, producing CH3CN−. Potential energy curves of CH3CN− evolving with the ∠CCN angle were scanned in the both the gas phase and PCM-model water (Figure S3, Supporting Information). In the gas phase, CH3CN− is quite unstable with the bending of ∠CCN angle. On the contrary, solvation with the PCM-model water immediately stabilizes CH3CN− and the stability of CH3CN− increases along with the bending of the ∠CCN angle up to 130°. It is apparent that the formation of CH3CN− is a spontaneous process in the PCM-model water. Interestingly, the structure related to the minimum point of the potential energy curve of CH3CN− in the PCM-model water is in good agreement with the one solvated in liquid CH3CN.18,22 Consequently, it is reasonable that CH3CN− instead of surface bound or internally solvated electron is eventually produced in the CH3CN(H2O)40− cluster anion. The CN transient of CH3CN− only lasts for 20−30 fs and then may be converted to the quasi-C2-resonance or quasi-Nresonance depending on the influence of solvent thermal fluctuation. Different quasi-resonances of CH3CN− induce different polarizations to the immediate water environment and further cause different protonation reactions and protonation products. Two kinds of proton transfers are observed in the subsequent time evolution. That is, after a short duration of the formed CH3CN−, proton transfer (PT) to C2 (PT-to-C), producing the most stable product, the CH3CHN radical, occurs when CH3CN− is in the quasi-C2-resonance, whereas the PT to N (PT-to-N), producing a metastable product, the trans-CH3CNH radical, is also observed when CH3CN− is in the quasi-N-resonance. Detailed results and discussions of the two PT reactions are presented below. PT-to-C Reaction. After duration of about 20 fs in the CN transient, the state of CH3CN− was converted to the quasi-C2resonance with the spin density mainly located at N due to the influence of solvent thermal fluctuation (Scheme 1). The duration of quasi-C2-resonance (20−30 fs) is comparable to that of the CN transient and state interconversion of CH3CN− between the CN transient and quasi-C2-resonance was observed with the tendency of favoring the quasi-C2-resonance

(Figure 3B). This state interconversion between the two resonance hybrids can be illustrated indirectly by time evolution of the distances between the center of spin density distribution and C2 as well as N (dEc‑C2 and dEc‑N, Figure S4, Supporting Information). dEc‑C2 is much smaller than dEc‑N in the CN transient due to considerable contribution of spin density from −CH3 group, and then dEc‑N is decreased rapidly to be comparable to dEc‑C2 within 20 fs owing to spin density transfer to N, implying the formation of quasi-C2-resonance (Figure 3B). Figure 3C shows the individual averaged vertical detachment energies (VDEs) which can distinguish different resonance hybrids of CH3CN− from each other. The VDE of CH3CN− is ∼3.8 eV in the CN transient and ∼4.2 eV in the quasi-C2-resonance with an average of ∼4 eV in water environment. A larger VDE of the quasi-C2-resonance than the CN transient comes from stronger electron affinity of ptype molecular orbital of the C2 relative to the antibonding mixed molecular orbital. Interestingly, the VDE curve of the CH3CN(H2O)40− cluster anion is similar to the spin density curve of the N atom with a good linear correlation (Figure S5, Supporting Information). The VDE evolution can be used as auxiliary criterion for the identification of quasi-C2-resonance. CH3CN− only lasts for about 150 fs and then transfer of a proton to the C2 atom starts at ca. 190 fs and completes within 30 fs, producing the CH3CHN radical (Figures 1B and 3A−C). State interconversion from the CN transient to the quasi-C2resonance is responsible for the PT-to-C reaction. Preliminary preparation for the PT-to-C reaction is completed in the formation and subsequent time evolution of the CN transient. Surrounding water molecules are polarized and reoriented when the surrounding reactive water molecules become close to the C2 due to increase of spin density of the C2 site and the influence of solvent thermal fluctuation. The C2−Hd distance is continuously decreased while the ∠OdHdC2 angle is gradually increased and finally a weak hydrogen bond between the C2 and Hd of the reactive water molecule is formed (Figure 3A− B). Meanwhile, the ∠C1C2Hd angle is gradually increased to be about 120°. In the first formation and subsequent time evolution of the quasi-C2-resonance, the augmented negative charge at the C2 further polarizes and rearranges the reactive water molecule. The C2−Hd distance is further decreased and the ∠OdHdC2 angle is consistently increased. Approach of the reactive water molecule can be observed obviously from radial distribution functions of the C2 atom relative to the water hydrogen atoms in three different cases (Figure 5A). The first peak around 2.0 Å in the PT-to-C case is attributed to the Hd of 9216

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the reactive water molecule. The ∠C1C2Hd angle fluctuates at 120°, which is consistent with 117° of the CH3CHN radical optimized in the gas phase (Figure 3A).16 In the state interconversion between the CN transient and quasi-C2resonance, the weak hydrogen bond formed between the C2 and Hd of the reactive water is adjusted constantly and the ∠C1C2H d angle fluctuates steadily around 120°. The spontaneous PT-to-C reaction is triggered in the process of forming the quasi-C2-resonance once again after a weak hydrogen bond between the C2 and Hd is well formed (with an ∠OdHdC2 angle larger than 170°) and completes within 30 fs, producing the most stable CH3CHN radical (Figures 1B, 3A− C and S6 (Supporting Information) ca. 5.11 kcal/mol more stable than the metastable trans-CH3CNH radical in the PCMmodel water). Therefore, the solvent thermal fluctuation induced formation of hydrogen bonding with the C2 site is responsible for the quasi-C2-resonance and further the spontaneous PT-to-C reaction. Moreover, the protonation product, the CH3CHN radical, can be further stabilized by forming hydrogen bonds with surrounding waters in the aqueous phase. Consequently, the PT-to-C reaction toward the formation of the most stable product, the CH3CHN radical, can be viewed to be thermodynamically favored. PT-to-N Reaction. Besides aforementioned state interconversion between the CN transient and quasi-C2-resonance, an alternative state conversion channel of CH3CN− starting from the CN transient is also observed. In a parallel AIMD simulation of the CH3CN(H2O)40− cluster anion (Figures 2A and 4A−C), CH3CN− in the CN transient state is also formed first. dEc‑C2 is also smaller than dEc‑N when CH3CN− is in the CN transient on the basis of the same discussion as in the PTto-C case (Figure S7, Supporting Information). An averaged VDE of the CN transient is ∼3.8 eV, in agreement with that in the PT-to-C reaction trajectory (Figures 3C and 4C). However, the CN transient only lasts for about 30 fs and then the spin density on CH3CN− starts to transfer to the C2 with a trend of forming the quasi-N-resonance in which the spin density is mainly located at the C2 due to the influence of solvent thermal fluctuation (Scheme 1, Figure 4B,C). The duration of CH3CN− is less than 50 fs and the PT-to-N reaction is immediately triggered in the conversion process of the two CH3CN− resonance hybrids. The current PT-to-N mechanism is clearly different from that of the PT-to-C reaction. The reactive water molecule is the one that already hydrogen-bonds to the N in the initial conformation (Figure 4A). This pre-existing hydrogen bond becomes more compact (with the N−Hd distance reduced to ca. 1.8 Å) due to increase of spin density of the N whereas the increase of spin density of the C2 only slightly polarizes and attracts the surrounding water molecules but without a weak hydrogen bond formed in the formation and subsequent time evolution of the CN transient. This difference can be clearly observed from RDFs of the C2 and N with respect to water hydrogen atoms in three different cases (Figure 5A,B). The first peak around 2.8 Å indicates that it does not form a weak hydrogen bond between the C2 and surrounding water molecules. Meanwhile, the ∠C2NHd angle is gradually decreased to about 118°, which is in good agreement with 117° of the trans-CH3CNH radical optimized in the gas phase (Figure 4A).16 After necessary conditions (preexisting hydrogen bond and ∠C2NHd angle) are well established, the spontaneous PT-to-N reaction is immediately triggered in the state conversion of CH3CN− from the CN transient to quasi-N-resonance with the formation of a

metastable trans-CH3CNH radical (Figures 2B, 4A−C, and S8, Supporting Information). Therefore, conversion of the resonance hybrids of CH3CN− from the CN transient to quasiN-resonance is responsible for the PT-to-N reaction. Surprisingly, a cis-CH3CNH radical corresponding to another minimum of energetic profile obtained in the gas phase is not observed in our AIMD simulations of the CH3CN(H2O)40− cluster anion.16 The effective hydrogen bond region between the CH3CN N atom and surrounding water molecules in the trajectory of the neutral CH3CN(H2O)55 parent cluster and bending of the ∠CCN angle are employed as a preliminary interpretation. Water molecules forming effective hydrogen bonds with the CH3CN N atom are mainly from the region surrounded by the dashed lines (Figure S9, Supporting Information). It is easier for these water molecules to fill up the cavity previously occupied by the CH3CN N atom to maintain the pre-existing hydrogen bonds or make an exchange of hydrogen bonds among different water molecules in this region in the formation process of the CN transient, which involves bending of the ∠CCN angle. This action can be definitely evidenced by time evolution of the N−Hd distance displayed in Figure 4A. Moreover, the trans-CH3CNH radical is thermodynamically more stable than the cis-CH3CNH radical (about 3.29 kcal/mol more stable in the PCM-model water). Thus, the trans-CH3CNH radical rather than its isomer, the cisCH3CNH radical, is eventually produced in the PT-to-N process. Conversion of Protonation Products. Two protonation products, the CH3CHN radical and trans-CH3CNH radical, are observed in the present AIMD simulations of the CH3CN(H2O)40− cluster anion. However, as above-mentioned, only the CH3CHN radical was definitely observed experimentally by ESR spectroscopy whereas the other protonation product, the trans-CH3CNH radical, was merely suggested and speculated as a crucial intermediate in the reaction events occurring in irradiated aqueous solutions of CH3CN.11 Besides the forward dimerization of the trans-CH3CNH radical, it is impossible to exclude the possibility that the trans-CH3CNH radical can be also converted to the CH3CHN radical16 due to deposit of excess energy in the irradiated CH3CN−H2O binary mixtures. The free energy barrier needed for conversion of the two protonation products is too high to overcome in water for AIMD simulation without adding excess energy to the simulated system. Thus, the trans-CH3CNH radical was observed as a final product in the current AIMD simulations of the CH3CN(H2O)40− cluster anion. Three possible conversion paths from the AIMD simulated product, the trans-CH3CNH radical, to the practical product, the CH3CHN radical, are proposed. All transition states are subject to intrinsic reaction coordinate calculations to confirm the connection between the reactants and products. The first one is a proton coupled electron transfer path. Activation free energies are calculated to be 37.5 kcal/mol in the gas phase and 38.9 kcal/ mol with the consideration of hydration effect using the PCM model (Figure S10, Supporting Information). The second one is a water-mediated double proton cooperative transfer path. Free energy barriers are calculated to be 33.7 kcal/mol in the gas phase and 31.1 kcal/mol with the consideration of hydration effect using the PCM model (Figure S11, Supporting Information). The third one is a water-mediated hydrogen atom transfer path. Activation free energies are calculated to be 24.2 kcal/mol in the gas phase and 22.2 kcal/mol by considering PCM-model hydration effect (Figure S12, Support9217

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PT-to-N trajectories. This material is available free of charge via the Internet at http://pubs.acs.org.

ing Information). From a thermodynamical perspective, the trans-CH3CNH radical is most probably converted to the CH3CHN radical by a water-mediated hydrogen atom transfer path. Certainly, further clarification is needed for this possible conversion.

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Corresponding Author

*Y. Bu: e-mail, [email protected]; tel, +86-531-88365740.

CONCLUSION The formation process and two kinds of subsequent state conversions of CH3CN− in a CH3CN(H2O)40 cluster with an EE injected vertically are studied in the present work. Bending vibration of ∠CCN angle initiates transfer of the extra negative charge to the CH3CN LUMO orbital. Stability requirement from the hydration effect of immediate water environment forces the ∠CCN angle to further bend which conversely makes more extra charge transfer to CH3CN. Mutual promotion of these two actions cooperatively leads to the formation of CH3CN− valence anion instead of a surface bound or internally solvated electron. Three resonance structures are observed in our AIMD simulations. Upon the formation of CH3CN−, it is first in the CN transient with duration of 20−30 fs. Surrounding water molecules are polarized and adjusted to approach the C2 and N atoms of CH3CN− owing to increase of spin density and reduction of atomic partial charges at the two atoms in the formation and subsequent time evolution of the CN transient. Two different state evolution channels of CH3CN− starting from the CN transient separately to the quasi-C2-resonance or to the quasi-N-resonance are observed due to the influence of solvent thermal fluctuation. A weak hydrogen bond between the C2 and Hd atom of the reactive water formed in the duration of CN transient is responsible for the quasi-C2-resonance, and it is strengthened in the state interconversion process of CH3CN− between the CN transient and quasi-C2-resonance. Upon establishment of necessary conditions (forming well hydrogen bond and suitable ∠C1C2Hd angle), the PT-to-C reaction spontaneously occurs in the quasi-C2-resonance structure and the most stable product, the CH3CHN radical, is produced. On the other hand, the spontaneous PT-to-N reaction is directly triggered when the CN transient is converted to the quasi-N-resonance and necessary conditions (the pre-existing hydrogen bond and a suitable ∠C2NHd angle) are established, producing a metastable trans-CH3CNH radical. In a word, different quasiresonances of CH3CN− separately control different protonation paths and products. The geometrical isomer, the cis-CH3CNH radical, was not observed in our AIMD simulations of the CH3CN(H2O)40− cluster anion. The duration of CH3CN− in either protonation reaction process is no more than 200 fs. On the basis of experimental ESR results and theoretical calculations in the present work, the simulation product (trans-CH3CNH radical) can be converted to the practical product (CH3CHN radical) by a water-mediated hydrogen atom transfer path.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21373123, 20633060, and 20973101) and NSF (ZR2013BM027) of Shandong Province. A part of the calculations were carried out at National Supercomputer Center at Jinan, Shanghai Supercomputer Center, and High-Performance Supercomputer Center at SDU-Chem.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The calculated data including scanned potential energy curves of CH3CN− in the gas phase and PCM-model water, some AIMD simulated trajectories and time evolution curves of some physical quantities (distance, spin density, VDE, the radius of gyration, etc.), radial distribution functions of the equilibrated systems, isomerization mechanisms, singly occupied molecular orbitals and structural parameters, structures and relative free energies, and snapshots extracted from the PT-to-C and the 9218

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