Theoretical Investigation on the Stability of Negatively Charged Formic

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Theoretical Investigation on the Stability of Negatively Charged Formic Acid Clusters Leonardo Baptista,*,† Diana P. P. Andrade,‡ Alexandre B. Rocha,§ Maria Luiza M. Rocco,§ Heloisa Maria Boechat-Roberty,| and Enio Frota da Silveira⊥ UniVersidade Estadual do Rio de Janeiro, Faculdade de Tecnologia, Departamento de Quı´mica e Ambiental, RodoVia Presidente Dutra Km 298, Resende, RJ, Brazil, UniVersidade do Vale do Paraı´ba, Instituto de Pesquisa e DesenVolVimento, AV. Shishima Hifumi, 2911, UrbanoVa, 12244-000, Saõ Jose´ dos Campos, SP, Brazil, UniVersidade Federal do Rio de Janeiro, Instituto de Quı´mica, Departamento de Fı´sico-Quı´mica, AVenida Athos da SilVeira Ramos, 149 Bloco A, 4° andar CEP 21941-909 Cidade UniVersita´ria, Rio de Janeiro, RJ, Brazil, UniVersidade Federal do Rio de Janeiro, ObserVato´rio do Valongo, Ladeira Pedro Antoˆnio, 43, CEP 20080-090 Rio de Janeiro, RJ, Brazil, and Pontifıćia UniVersidade Cato´lica do Rio de Janeiro, Departamento de Fı´sica, 22451-900, Rio de Janeiro, RJ, Brazil ReceiVed: January 15, 2010; ReVised Manuscript ReceiVed: May 31, 2010

Recent experimental results on negatively charged formic acid clusters generated by the impact of 252Cf fission fragments on icy formic acid target are compared to quantum mechanical calculations. Structures for the clusters series, (HCOOH)nOH-, where 2 e n e 4, are proposed based on ab initio electronic structure methods. The results show that cluster growth does not have a regular pattern of nucleation. A stability analysis was performed considering the commonly defined stability function. Temporal behavior of the clusters was evaluated by Born-Oppenheimer molecular dynamics to check the mechanism that provides cluster stability. The evaluated temporal profiles indicate the importance of hydrogen atom migration between the formic acid moieties in maintaining the stability of the structures and the water formation due to hydrogen abstraction by the hydroxyl approach. Introduction Formic acid (HCOOH) is the simplest carboxylic acid and has been observed in many astronomical sources such as protostellar NGC 7538:IRS9,1 comets,2 meteorites,3 dark molecular clouds,4 and regions associated with stellar formation.5,6 The behavior of this molecule under exposure to high-energy radiation and/or particles is important, because it may be a key compound in the formation of more complex molecules in astrophysical medium, such as acetic acid (CH3COOH) and glycine (NH2CH2COOH). Glycine has also been observed in several astronomical sources such as comets2 and condritic meteorites.3 This simple amino acid has long been searched for in interstellar medium, but has not been unambiguously detected so far. Its recent “detection” claimed by Kuan and co-workers7 has been persuasively rebutted by Snyder et al.8 One of the purposes of astrochemistry research is to understand the evolution of complex molecular species in several different objects such as giant molecular clouds, circumstellar environments of low- and high- mass protostellar, as well as in protoplanetary disks where planets, its satellites, and comets are formed. Observations of the obscured young stellar object W 33A, from ISO-SWS (short-wavelength spectrometers onboard of the Infrared Space Observatory), have revealed two broad absorption features centered at 7.24 and 7.41 µm that correspond to formic acid (HCOOH) and formate ion (HCOO-), respectively.9 Gibb et al.10 also confirmed the detection of these species in W 33A and several other sources. Recently, Andrade * To whom correspondence should be addressed. Phone: +55-24- 33547875. Fax: +55-24- 3354-7851. E-mail: [email protected]. † Universidade Estadual do Rio de Janeiro. ‡ Universidade do Vale do Paraı´ba. § Universidade Federal do Rio de Janeiro, Instituto de Quı´mica. | Universidade Federal do Rio de Janeiro, Observato´rio do Valongo. ⊥ Pontifıćia Universidade Cato´lica do Rio de Janeiro.

and co-workers11 have simulated in laboratory the effects of cosmic rays and soft X-ray photons on icy formic acid (HCOOH). It was demonstrated that bombardment of energetic (∼65 MeV) heavy ions produces a greater variety of ions (mainly negative ions) than the interaction with soft X-rays photons. This indicates that the dissociation of formic acid ice by cosmic rays is highly efficient in forming important negative ions, such as formate (HCOO-). Other negative ions were also formed efficiently, such as OH-, HCO-, CO-, and CO2-. Negatives ions have been detected in Titan12 and Enceladus13 atmospheres (Saturn’s satellites) by the Cassini spacecraft. The ions were observed to fall into mass groups of 10-30, 30-50, 50-80, 80-110, and 110-200 until 10 000 mass-to-charge ratios (m/q). The authors suggested that the mass peaks up to at least m/q ) 72 may be associated with negatively charged water group clusters. It is thought that negative ions must play a key role in the ion chemistry and they may participate in the formation of organic molecules in these environments. Andrade and coworks14 have also observed a high variety of negative clusters including formate and hydroxyl ions, such as (HCOOH)jCOOH-, with 1 e j e 9 and (HCOOH)rOH-, with 4 e r e 9. This last ion clusters series seem to arise due to the reaction (HCOOH)b f (HCOOH)b-1OH- + HCO+ and their peaks show a systematic enhancement in intensity when r increases. Few clusters series have this unusual feature, for example, for Arn+ or On+ clusters the maximum yield occurs at n ) 2.15 Quantum-chemical calculations might be helpful to explain such behavior. Our previous theoretical study of ionic formic acid clusters,16 comprising an ab initio electronic structure study in conjunction with Born-Oppenheimer molecular dynamics, showed that the stability of cationic clusters are related to the hydrogen migration between the formic acid moieties. That study proposed a

10.1021/jp100425h  2010 American Chemical Society Published on Web 06/11/2010

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Figure 1. (a) TOF-PDMS spectrum showing the first clusters of the negative formic acid cluster series (HCOOH)nOH-. (b) Comparison between the series (HCOOH)rOH- and (HCOOH)jCOOH-.

different assignment for the (HCOOH)nH3O+ series as (HCOOH)n(H2O)(H+), a protonated formic acid cluster associated with one water molecule. The stability analyses provided additional information on the geometries of the clusters, which indicate an exponential decay of the cluster stability with the increase of mass, as observed in the PDMS (Plasma Desorption Mass Spectrometry) spectrum. However, as shown in the experimental work of Andrade et al.,11,14 the anionic clusters that include the hydroxyl anion had an anomalous behavior (see Figure 1). The mass/charge peaks 63, 109, 155, and 201, due to the clusters (HCOOH)nOH-, with 1 e n e 4, are very weak (close to the experimental background), and do not show exponential decay. This series had a maximum intensity corresponding to a (HCOOH)8OHcluster. The PDMS spectrum does not supply sufficient information to explain this unexpected behavior, since it can only furnish information on mass and charge of clusters. This study complements our previous theoretical investigation of ionic formic acid, with the current study focusing mainly on the (HCOOH)nOH-, where 1 e n e 4. An analysis of geometry and temporal behavior of the anionic clusters were carried out. Additionally, a stability analysis was performed in order to investigate the relationship between clusters geometry and their stability. Computational Details This study is divided in two main parts. The first includes a static approach to study the ionic formic acid clusters. The (HCOOH)nOH- series was studied, where 1 e n e 4. All geometries were fully optimized without any geometry constraint at the MP2/6-31+G(d,p) level. The structures were characterized

as minima on the potential energy surface (PES) through harmonic frequency analysis. The optimization procedures started by considering the most stable conformer of dimer, trimer, and tetramer already described in the literature17 as geometry input. The ionic clusters were obtained by adding an ion in an arbitrary position of the neutral cluster and relaxing the structure until a minimum energy conformation was found. To search for different conformers, the ion optimizations were performed starting from several estimated structures. Ionic clusters with positive formation energy (∆E, ∆E0) were dismissed. The basis set was chosen to account for the description of hydrogen bonding interactions in the clusters and to avoid the basis set superposition error, which is present in a smaller basis.18 The binding energy (∆Eb) for the (HCOOH)nOH- series, where 1 e n e 4, was evaluated by following the scheme proposed by Fernandez-Lima et al.,19 at MP2/6-31+G(d,p), and is shown in eq 1:

2(HCOOH)nOH- f (HCOOH)n-1OH-+ (HCOOH)n+1OH∆Eb ) En-1 + En+1 - 2En

(1)

where E is the electronic energy of the cluster, corrected by the zero point vibrational energy. The binding energy provides the stability of any cluster on increasing or decreasing by one unit. This energy difference may be related to the stability of the cluster and, therefore, to their abundance measured by the mass

Stability of Negatively Charged Formic Acid Clusters

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Figure 2. Optimized geometries of formic acid dimer at MP2/6-31+G**: (a) (HCOOC)2OH-; (b) (HCOOC)3OH- conformation 1; (c) (HCOOC)3OHconformation 2; (d) (HCOOC)4OH- conformation 1; (e) (HCOOC)4OH- conformation 2.

spectrometer. Two different geometries for the formic acid pentamers were optimized in order to evaluate the binding energy of tetramer and the structures can be viewed in the Supporting Information. Additionally, the binding energy were evaluated at MP2/6-311+G(d,p) level, considering only the electronic energy and the geometry optimized at the MP2/631+G(d,p) level. The second part of this work is a Born-Oppenheimer molecular dynamics study, which was performed to investigate the mechanism that maintains the ionic clusters bonded. In these calculations, the PBE1PBE density functional with 6-31++G(d,p) basis set was used. According to Truhlar et al., this functional leads to a good description of weak interactions, such as hydrogen bond and interactions presented in charge transfer complexes, and correctly describes the behavior of the asymptotic limit.20,21 Andersen and Carter,22 studying the combustion of dimethyl ether, replaced all light hydrogen atoms in their system with deuterium atoms to minimize error in the dynamics caused by the complete neglect of quantum effects of light hydrogen. As already commented in the previous paper,16 there is no interest in evaluating any dynamic property during the simulation, since the main goal is to analyze the structural changes of ionic clusters along the temporal evolution of the system and to verify the process that stabilizes formic acid moieties. The theoretical study of cationic clusters has followed this approach and has provided the necessary insight to interpret the experimental results.

More than 10 simulations were performed for (HCOOH)2OHand (HCOOH)3OH- clusters with distinct simulation times at 56 K. The first group of simulations started from equilibrium geometries and its initial conditions were obtained from the electronic structure calculations. The input geometry was obtained at the MP2/6-31+G(d,p) level, the Hessians were evaluated during the dynamics, the vibrational energy was obtained by thermal energy sampling,23 and the simulation time was nearly 180 fs. The second group of simulations comprises the impulse dynamics of anionic clusters. The hydroxyl anion was launched over the neutral clusters with different energies and distinct orientations in order to investigate the capability to form clusters due to collisions between slow fragments. The chosen kinetic energy of the anion has to have the same magnitude of vibrational energy of the neutral cluster to avoid cluster fragmentation due to collision with a high energy anion. The velocity vector of the OH- group corresponds to kinetic energies of 0.547, 5.47 × 10-3, and 5.47 × 10-5 eV. The complete study was performed using the Gaussian 03 package.24 Results and Discussion Figure 2 illustrates the optimized geometries for anionic clusters. The neutral structure is distorted due to the presence of the hydroxyl anion. As pointed out in our previous paper for positive charged clusters, the growth does not present a regular pattern of nucleation observed in other systems.16,19,25 As stated, the cationic clusters series has some general characteristics as the clusters size increases.16 The (HCOOH)nH+

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series has one hydrogen atom added to one oxygen atom and the (HCOOH)nH3O+ series is better described as protonated formic acid clusters with one additional water molecule.16 However, the (HCOOH)nOH- clusters series has different characteristics as the clusters size increases. The dimer is formed by a carboxylate anion interacting with a neutral formic acid and a water molecule. Two different geometries were optimized for the trimer. The first has two formic acids moieties and a carboxylate interacting with one water molecule. The second has a more complex structure with a hydroxyl added to one formic acid molecule. Two structures were obtained for the anionic tetramer. The first has the basic structure of the most stable neutral conformation,17e two formic acid dimers interacting in a pattern similar to π-π stacking interaction. The optimized structure indicates the formation of C-O bond between one formic acid moiety and the hydroxyl anion. This fact can be observed by verifying that r(C11O22) ) 1.407 Å, which nearly equals the average value of typical C-O bond distance 1.404 ( 0.033 Å (considering the value range of 1.334-1.448 Å).26 The θ(O12C11O10) bond angle is 111.90°, almost the regular value observed for the θ(CCC) bond angle of tetrahedral carbon atom of propane, θ(CCC) ) 112.40°, and heptane θ(CCC) ) 112.60°.26 The second tetramer conformation has a water molecule between the neutral dimer and the formate-formic acid dimer. The hydrogen 18 interacts simultaneously with both formate structures to stabilize the cluster, and is expected to migrate between both moieties. The clusters were also optimized at PBE1PBE/aug-cc-pvtz level and the results are similar to those obtained at MP2/6-31+G(d,p) level, as can be verified in the Supporting Information. Andrade and co-workers11 proposed that the series (HCOOH)nOH- may be formed due to the internal conversion of a neutral cluster into (HCOOH)n-1OH- cluster and HCO+ fragment, as shown in the reaction scheme below.

Baptista et al. TABLE 1: Geometric Parameters Evaluated at MP2/ 6-31+G(d,p) Levela parameter Dimer r(C1C5) r(O3H9) r(O4H9) r(O2H12) θ(C1O2H12) θ(C5O6H10) φ(C1O2H12 O7) φ (C5O6H10 O7)

4.086 1.119 1.326 2.034 120.42 122.53 8.77 8.59 Trimer-1

r(C1C5) r(C5C8) r(O2H12) r(O7H15) θ(O7H15O6) θ(O6H12O2) φ(C5H12O2C1) φ (O4H13O17H16)

4.226 3.821 1.040 1.041 172.51 173.80 -117.86 11.46 Trimer-2

r(C1C5) r(C5C9) r(O2H17) r(O7H16) θ(O7C5O6) θ(O6H17O2) φ(C5O6H17O2) φ (H13O4C5O7)

3.61635 3.83691 1.91357 1.02371 111.91361 154.22318 8.82065 63.83504 Tetramer-1

r(C1C5) r(C7C11) r(C11O22) r(O10H17) r(O12H18) r(O2H13) θ(C11O22H21) θ(O12C11O10) θ(C1C7C11) φ(C1C7C11C5) φ (C1O9C7O8)

(HCOOH)n f (HCOOH)n-1OH- + HCO+ However, the free energy of reaction, evaluated at MP2/631+G** level, is highly positive, that is, 189.15 kcal mol-1, as expected for a reaction with the formation of two ionic fragments. Therefore, the proposed reaction may not lead to the (HCOOH)nOH- series. More likely, the anionic clusters are sputtered from the solid after the impact of the fission fragments or formed in the gas phase by the encounter of an ionic fragment with neutral species. Although it is experimentally difficult to assign the structure of these clusters in the spectrum,15 the stability analysis may furnish an insight about the cluster assignment of the PDMS spectrum. The binding energies of the clusters, including those of the most stable structures, present positive values, except for n ) 4, and show that the stability of the clusters varies with the mass increase in the range 1 e n e 4, which indicates that the structures are stable and can be present in the PDMS spectrum (Table 2). The small negative binding energy observed for tetramer does not mean that the cluster is unstable, but indicates a lower stability of the cluster or that the cluster can rearrange more easily than a smaller cluster. However, it is necessary to study the high-order cluster (n g 5) to verify the intensity maximum at n ) 8, observed in the PDMS spectrum, which is computationally prohibitive at the level of theory used in the present work. The results, obtained at MP2/6-311+G(d,p), indicate that increasing the basis set does not alter the clusters stability.

value

3.800 3.649 1.407 0.980 0.990 0.993 105.27 111.96 85.17 44.27 -112.21 Tetramer-2

r(C1C5) r(C7C11) r(C11O21) r(O10H18) r(O12H17) r(O2H13) θ(C11O22H22) θ(O12C11O10) θ(C1C7C11) φ(C1C7C11C5) φ (C1O8C7O9)

3.823 15 4.260 28 3.686 77 1.307 71 1.873 54 0.996 05 76.426 39 128.932 06 73.026 70 -38.903 56 164.031 52

a

Bond distances are in Angstroms and bond angles and dihedral angles are in degrees.

To check the mechanism that provides the stability of anionic clusters, temporal behavior of (HCOOH)2OH- and (HCOOH)3OH- clusters was simulated by Born-Oppenheimer molecular dynamics. The stability of optimized clusters structures during the time evolution and the possibility to form clusters due to collision of two slow fragments were analyzed. The animated trajectory can be seen in the Supporting Information section. The molecular dynamics simulation, starting from equilibrium geometries, was performed with trajectory time greater than 700 fs and can also be seen in the Supporting Information section.



TABLE 2: Stability Analysis Performed for (HCOOH)nOH- Clusters, Where 1 e n e 4, at MP2/ 6-31+G(d,p) levela

values of kinetic energies. However, in all simulations, hydrogen migration occurred between the formic acid moieties, which maintain the stability of dimeric structure. The dimeric structure was broken due to the anion approach over the y axis 5.47 × 10-3 eV of kinetic energy (Figure 1 of Supporting Information section). As expected, the OH approach led to the formation of a water molecule, which was observed after 119 fs of simulation. All slow collisions broke the dimeric structure, thereby indicating that the approach over the y axis is ineffective to form stable clusters. The approach over the z axis (Figure 5) was the most effective to form anionic clusters. The anion approach over this axis favors the achievement of the geometry previously optimized (Figure 2a), which can prevent the lost of dimeric structure. The hydrogen abstraction was observed for all kinetic energies considered, and the former water molecule translated around the pair formate-formic acid, which is stable due to the migration of a hydrogen atom between the two formic acid moieties. Figure 5 presents the simulation over the z axis approach, considering 5.47 × 10-5 eV of kinetic energy; hydrogen abstraction occurred 192 fs. After the hydrogen abstraction, the ion-dipole interaction between the pair formate-formic acid induced a hydrogen migration between the two moieties, which maintains the dimeric structure. Since the value of 5.47 × 10-5 eV of kinetic energy is the most probable value that may lead to stable clusters, the trimer simulations were conducted considering only this kinetic energy for OH-. Figures 6 and 7 show the impulse dynamics over the three axes considered, and as can be seen, the hydrogen abstraction was observed in all cases. The approach over the x axis indicates the formation of a water molecule plus a cluster formed between two formic acids monomers and a formate anion (Figure 2 of Supporting Information section). The cluster is stable during the entire simulation due to the hydrogen migration between the formic acids moieties. The relation between stability and hydrogen migration may be verified analyzing the simulation over the y and z axis (Figures 6 and 7). Both simulations had a rapid hydrogen abstraction by the OH- group leading to water formation. After the y axis approach (Figure 6), the formic acids moieties move in order to optimize the long-range interaction. A hydrogen migration in 88 fs occurred in all course of simulation, maintaining the trimer structure stable. However, in the approach over the z axis, the hydrogen migration was not observed and the trimer structure was divided into two separated basic structures: a neutral formic acid dimer and a formate anion (Figure 7). Additionally, the simulation over the y axis showed the water molecule moving around the trimer formic acid-formate, which indicates the formation of the expected anionic cluster due to the collision between two slow fragments in the gas phase. The current simulation reveals the following characteristics of the anionic clusters: (1) The most correct structure that represents the anionic clusters is a single water molecule interacting with a formic acid-formate structure. (2) Stable structures had a hydrogen migration between the formic acid moieties. (3) Collisions between hydroxyl anion and a neutral cluster can form new clusters with a general molecular formula (HCOOH)nHCOO-, which constitutes the series observed in the PDMS spectrum, and has a higher desorption yield when compared to the (HCOOH)nOH- series.

cluster

binding energy

monomer dimer trimer tetramer

37.47 (39.58) 8.12 (7.68) 25.95 (22.20) -2.80 (1.53)

a Values between parentheses were obtained at MP2/ 6-311+G(d,p) level considering the geometry optimized at MP2/ 6-31+G(d,p). Values in kcal mol-1.

Figure 3. Main axes for the OH- approach for the formic acid cluster.

According to the dynamics, the dimer is stable during the complete simulation time and has a hydrogen migration between the two formic acids moieties. The same behavior is observed for both trimer geometries, where the mean structures remain stable during the time of simulation. Migration of two hydrogen atoms related to the central formic acid monomer is observed. This was also verified in the previous study of cationic formic acid clusters and was considered the main factor for cluster stability.16 Additionally, Figures 2a and 2b show that the initial structure changes in time passing through different conformers. However, it is important to remark that the mean structure of a water molecule bonded to a formic acid cluster, which includes the formate anion, was preserved. Notwithstanding, the cluster was produced by the sputtering of stables structures from icy formic acid or was formed in the plasma originated by the impact of a fission fragment (FF), whose structures are stable and could be detected by mass spectrometer. The induced ion desorption process starts with the H+ emission in a picosecond time range after the FF impact into the target surface. Then, light and newly formed chemical species, such as OH-, CnHm+, and HCO, are emitted.11 Abundant desorption of neutral and intact target molecules follows, up to the final step of track relaxation. In the expanding plasma, cluster formation may occur due to the collision of two slow fragments in the gas phase. To simulate this process, the current analysis considers the impact of OH- on a neutral dimer or a trimer with different orientations and kinetic energy. Figure 3 shows the three main axes of the OH- approach, called x, y, and z. Considering the anionic dimer formation, an important issue to note is that the hydrogen abstraction is independent of energy and orientation of OH- of the neutral dimer. In all cases, the abstraction of one hydrogen atom was verified. The approach over the x axis leaded to the abstraction of hydrogen bonded to carbon atom and the approach over the y and z axis resulted in the abstraction of the hydrogen bounded to oxygen atom. The anion approach over the x axis with 5.47 × 10-5 eV of kinetic energy (Figure 4) resulted in a stable structure after the hydrogen abstraction, which occurred after 112 fs of simulation. After the abstraction, the water molecule translated around the dimer formate-formic acid, without the complete separation from the cluster structure. The water molecule, formed during the impact, completely dissociated in simulations with high

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Figure 4. Snapshots of the (HCOOH)2OH- cluster during the simulation at 56 K. Hydroxyl approach over the x axis and 5.47 × 10-5 eV of kinetic energy. Bottom: Changing of the r(CC) bond distance on the course of simulation (PBE1PBE/6-31++G(d,p)).

The Born-Oppenheimer molecular dynamics provides an insight about the maximum of abundance observed in the (HCOOH)nOH- series in the PDMS spectrum. The simulation showed that the hydroxyl approach always leads to hydrogen

abstraction with water formation. Because the number of hydrogen atoms increases with the neutral cluster size, the probability to form the expected anionic clusters may increase with the cluster mass. However, the neutral cluster abundance



Figure 5. Snapshots of (HCOOH)2OH- cluster during the simulation at 56 K. Hydroxyl approach over the z axis and 5.47 × 10-5 eV of kinetic energy. Bottom: Changing of the r(CC) and r(O7H10) bond distance on the course of simulation (PBE1PBE/6-31++G(d,p)).

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Figure 6. Snapshots of (HCOOH)3OH- cluster during the simulation at 56 K. Hydroxyl approach over the y axis and 5.47 × 10-5 eV of kinetic energy. Bottom: Changing of the r(C1C5), r(C5C8), and r(C5O17) bond distance on the course of simulation (PBE1PBE/6-31++G(d,p)).

has an exponential decay with the mass increase. Therefore, an increasing of intensity in the PDMS spectra can be expected as the anionic clusters grow, passing through a structure that has a maximum abundance, and a further decrease in the abundance of clusters.14

Conclusions Emission of negatively charged formic acid clusters generated by the impact of 252Cf fission fragments into an icy formic acid target have been studied by quantum mechanical calculations.



Figure 7. Snapshots of (HCOOH)3OH- cluster during the simulation at 56 K. Hydroxyl approach over the z axis and 5.47 × 10-5 eV of kinetic energy. Bottom: Changing of the r(C1C5), r(C5C8), and r(C1O17) bond distance on the course of simulation (PBE1PBE/6-31++G(d,p)).

The combination of time-independent calculations with the Born-Oppenheimer molecular dynamics provides the structures for the clusters formed in the PDMS spectra and furnishes insights about the cluster stability and mechanism of its formation. Experimentally, the clusters were assigned as (HCOOC)nOH-, however the time-independent study in conjunction with the Born-Oppenheimer molecular dynamics shows that the most correct assignment is (HCOOC)n(H2O)HCOO-, since all the simulation and optimized structures formed a water molecule due to hydrogen abstraction by hydroxyl anion. An important characteristic observed in this combined study is the absence of a regular pattern of nucleation, as noted previously in the cationic cluster of formic acid.16 The cluster may be formed due to sputtering of a stable anionic cluster of icy formic acid or in the plasma generated

after the FF impact into an icy target. Slow collision between the hydroxyl and neutral formic acid cluster may lead to the (HCOOH)n(H2O)HCOO- cluster, if the hydroxyl has enough low kinetic energy and the correct orientation when it approaches the neutral cluster. Two other processes may be observed due to hydroxyl colliding into neutral clusters: (1) According to the orientation and kinetic energy of hydroxyl, the neutral cluster may be broken into smaller fragments after the hydrogen abstraction, or (2) a (HCOOH)nHCOO- cluster can be formed after hydrogen abstraction, which would lead to a different series observed in the PDMS spectrum. Two geometries were optimized with the hydroxyl ion added to a formic acid moiety, forming a tetra-coordinated carbon atom. This structure was not observed in the simulation studies, indicating that the water formation is the most favorable way

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to form an anionic cluster due to slow collisions between neutral formic acid clusters and the hydroxyl. As observed in the stability analysis and simulations, the clusters stability is related to the mass increase and hydrogen migration between the formic acid moieties. Independent of the type of charge of the cluster, the hydrogen migration is an important feature in the stability of the cluster. It is important to mention that, in all simulations, the hydroxyl approach led to abstraction of a hydrogen atom and the formation of one water molecule. Acknowledgment. The authors thank Professor Graciela Arbilla for furnishing the computational capability and to LNLS, CNPq, FAPERJ, and CAPES for supporting this project. Supporting Information Available: Animated trajectory of all Born-Oppenheimer molecular dynamics and the geometry of clusters optimized at PBE1PBE/aug-cc-pvtz. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Schutte, W. A.; Tielens, A. G. G. M.; Whittet, D. C. B.; Boogert, A.; Ehrenfreund, P.; de Graauw, T.; Prusti, T.; van Dishoeck, E. F.; Wesselius, P. Astron. Astrophys. 1996, 315, L333. (2) Crovisier, J.; Bockeleé-Morvan, D.; Colom, P.; Biver, N.; Despois, D.; Lis, D. C. Astron. Astrophys. 2004, 418, 1141. (3) Briscoe, J. F.; Moore, C. B. Metic. 1993, 28, 330B. (4) Ehrenfreund, P. E.; Charnley, S. Annu. ReV. Astron. Astrophys. 2000, 38, 427. (5) Liu, S. Y.; Girard, J. M.; Remijan, A.; Snyder, L. E. Astrophys. J. 2002, 576, 255. (6) Ehrenfreund, P.; Schutte, W. A. AdV. Space Res. 2000, 25, 2177. (7) Kuan, Y. J.; Charnley, S. B.; Huang, H. C.; Tseng, W. L.; Kisiel, Z. Astrophys. J. 2003, 593, 848. (8) Snyder, L. E.; Lovas, F. J.; Hollis, J. M.; Friedel, D. N.; Jewell, P. R.; Remijan, A.; Ilyushin, V. V.; Alekseev, E. A.; Dyubko, S. F. Astrophys. J. 2005, 619, 914. (9) Schutte, W. A.; Boogert, A. C. A.; Tielens, A. G. G. M.; Whittet, D. C. B.; Gerakines, P. A.; Chiar, J. E.; Ehrenfreund, P.; Greenberg, J. M.; van Dishoeck, E. F.; de Graauw, T. Astron. Astrophys. 1999, 343, 966. (10) Gibb, E. L.; Whittet, D. C. B.; Boogert, A. C. A.; Tielens, A. G. G. M. Astrophys. J. Sup. 2004, 151, 35. (11) Andrade, D. P. P.; Boechat-Roberty, H. M.; da Silveira, E. F.; Pilling, S.; Iza, P.; Martinez, R.; Farenzena, L. S.; Homem, M. G. P.; Rocco, M. L. M. J. Phys. Chem. C 2008, 112, 11954. (12) Coates, A. J.; Crary, F. J.; Lewis, G. R.; Young, D. T.; Waite, J.H., Jr.; Sittler, E. C., Jr. Geophys. Res. Lett. 2007, 34, L22103. (13) Coates, A. J.; Jones, G. H.; Lewis, G. R.; Wellbrock, A.; Young, D. T.; Crary, F. J.; Johnson, R. E.; Cassidy, T. A.; Hill, T. W. Icarus 2010, 206, 618.

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JP100425H

Theoretical Investigation on the Stability of Negatively Charged Formic

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