Direct Ab initio Molecular Dynamics

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A: Kinetics, Dynamics, Photochemistry, and Excited States 2

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Activation of CO in Photo-irradiated CO-HO Clusters: Direct Ab initio Molecular Dynamics (MD) Study Hiroto Tachikawa J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b03823 • Publication Date (Web): 26 Apr 2019 Downloaded from http://pubs.acs.org on April 26, 2019

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The Journal of Physical Chemistry

2019_4_24_re-submit Activation of CO2 in Photo-irradiated CO2-H2O Clusters: Direct Ab initio Molecular Dynamics (MD) Study Hiroto Tachikawa* Division of Applied Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, JAPAN

Abstract The carbon dioxide (CO2) is one of the stable and inactive molecules which contributed to a greenhouse gas. The development of new reactions of CO2 activation, chemical fixation and conversion is a very important issue. In this report, the reactions of CO2-H2O binary clusters were investigated using a direct ab initio molecular dynamics (AIMD) method in order to find a new reaction of CO2 activation. Clusters composed of carbon dioxide and water molecules, CO2(H2O)n (n = 2-5), were utilized as a model of the binary cluster. The reaction dynamics of [CO2(H2O)n]+ following the ionization of parent neutral clusters were also investigated. Two electronic states of [CO2(H2O)n]+ were examined for direct AIMD surfaces: CO2[(H2O)n]+ (ground state) and (CO2)+(H2O)n (excited charge transfer (CT) state). After ionization of the clusters, a proton transfer (PT) reaction occurred within the (H2O)n+ moiety at the ground state, whereas the reactive HCO3 radical was formed at the CT state for OH addition to CO2+: CO2+(H2O)n → HCO3 + H+(H2O)n-1. The mechanism of the PT process and HCO3 radical formation were discussed based on theoretical results.

Corresponding author: Hiroto Tachikawa, E-mail: [email protected]

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1. Introduction Space matter is always exposed to cosmic rays and solar wind. The strong light excites and ionizes the electronic states of the molecules. The CO2 and H2O molecule covering surface of comets1-4 will cause various reactions after being ionized. The ionized H2O⁺ reacts easily with surrounding molecule (M), and proton transfer (PT) takes place H2O+ + M → M-H+ + OH. The reactions of H2O+ in pure water cluster have been extensively investigated by several groups5-31 and the reaction mechanism and product state distributions have been measured. In contrast, information on the reaction dynamics of binary (CO2-H2O)+ cluster cations is quite limited.32-33 The structures and electronic states of CO2-H2O binary clusters were investigated from theoretical and experimental perspectives. The results of IR spectroscopy and ab initio calculations suggest that the neutral CO2-H2O 1:1 complex has a T-shaped structure in which an oxygen atom of H2O binds to the carbon atom of CO2. The binding energy was calculated to be only 920 cm-1 (0.114 eV).32,34 Khan investigated the structure and binding energy of CO2(H2O)4 based on density functional theory calculations.35 A cyclic water tetramer that interacts with CO2 was obtained as the stable structure. Heinbuch et al. investigated the ionization process of CO2/H2O binary clusters by using time of flight mass spectroscopy (ionization was 26.5 eV).32 At a low CO2 concentration, protonated cluster ions H+(H2O)n were found as the dominant products. In addition, the CO2(H2O)nH+ (protonated product), CO2(H2O)n+ (un-protonated product) and (H2O)n+ (pure ionized water cluster ion) were measured via mass spectroscopy. However, the detailed reaction mechanism was not clearly understood. In the present study, the reaction dynamics of the cluster cation; CO2(H2O)n+ (n =


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2-5), following the ionization of neutral cluster CO2(H2O)n, were investigated theoretically to better understand the formation of product molecules on the surface of comets and icy planets. In this work, two electronic states, the ground and charge transfer (CT) states of ionized states of CO2(H2O)n, were examined. The primary focus is on the dependence of electronic states of vertical ionized complexes on the product states after the ionization of binary clusters.

2. Calculation Method The structures of neutral clusters CO2(H2O)n (n=2-4) were determined at the MP2/6-311++G(d,p) level of theory. The static structures and electronic states were calculated using the Gaussian 09 program.36 In direct ab-initio molecular dynamics (AIMD) calculations,37-39 first, the structures of the neutral clusters CO2(H2O)n (n=2-5) are optimized (MP2/6-311++G(d,p) level). Next, the trajectories on the potential energy surface of the cation state were propagated from the vertical ionization point under the condition of constant total energy (MP2/6-311G(d,p) level). Two electronic states of CO2(H2O)n+; ground and excited charge transfer (CT) states, were examined as potential energy surfaces (PESs). The trajectories of the cluster cation CO2(H2O)n+ on the ground state PES were run at the MP2/6-311G(d,p) level. The vertical ionization was assumed at time zero. The velocity Verlet algorithm was utilized with a time step of 0.1 fs. Direct ab initio MD calculations on the excited CT state PES were carried out as follows. First, the density matrix of cluster ion on the CT state at time zero was calculated by the complete active space self-consistent field (CASSCF) method with five electrons distributed in six orbitals, namely, CASSCF(5,6). The initial guess of



cluster cation was thus obtained. The trajectories for the cluster cation CO2+(H2O)n on the CT state PES were then propagated using the density matrix. We carefully monitored the total energy and electronic states during the trajectory calculation. (See Figure S1 in supporting information). It was confirmed that the drift of the total energies is kept less than 0.01 kcal/mol in all trajectory calculations. To examine the effects of the initial structure on the reaction dynamics, geometries were generated near the equilibrium point using zero-point vibration (ZPV)40 or direct AIMD calculations under a constant temperature condition (10 K),41 which is expressed as 10 K simulation. From the ZPV calculation (MP2/ 6-311++G(d,p) level), a total of ten geometrical configurations were selected and direct AIMD calculations were performed for the cluster cations. In the 10 K simulation, the structures of neutral clusters were investigated up to 1 ps using the CAM-B3LYP/ 6-311++G(d,p) method. A Nose–Hoover thermostat42,43 was applied in the trajectory calculations under thermal conditions. From the 10 K simulation, 5 geometrical configurations were selected. It should be noted that the influence of ZPV and 10 K simulations are negligibly small in the product channels, although the time of proton transfer is slightly affected by these effects. All direct AIMD calculations were performed using our own code. 3. Results 3.1. Structures of CO2(H2O)n (n=2-5) Figure 1 shows the optimized structures of neutral CO2(H2O)n complexes (n=2-5). The structure of CO2(H2O)2 has a cyclic form composed of hydrogen bonds. The oxygen atom of H2O (W1) was bound to the carbon atom of CO2. The intermolecular distance between CO2 and H2O (W1) was 2.769 Å, as the C-O distance. The water molecule (W1) was connected to W2 via a hydrogen bond (1.934 Å). The proton of W2 4 ACS Paragon Plus Environment

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was oriented to the oxygen atom of CO2 and the structure of the CO2 moiety had a slightly bent form (the angle of O-C-O was 177.3º). For n=3, the structure was also composed of a cyclic form, which is very similar to that of n=2. For n=4, the cyclic water tetramer interacts with the CO2 molecule. The moiety of CO2-W1-W4 was very similar to that of n=2. For n=5, three water molecules interact with CO2. In all cases, the oxygen atom of H2O orients toward the carbon atom of CO2 in the most stable form.

3.2. Electronic states of the CO2(H2O)n cluster at the vertical ionization points After the ionization of CO2(H2O)n, the electronic state was vertically changed from the neutral to the ionized states. Hereinafter, the electonic state of CO2(H2O)n at vertical ionized state is expressed as [CO2(H2O)n+]ver. The excitation energies and molecular charges of [CO2(H2O)3+]ver are given in Table 1. The excitation energies are classified into two groups (0-2 and 3-4 excitations). In the ground state, the NPA molecular charges on CO2, W1, W2, and W3 were calculated to be +0.017, +0.835, +0.129, and +0.019, respectively. A positive charge on W1 was larger than the others. In the 1st and 2nd excited states, a hole was localized on W2 and W3. At the 3rd excited state, the NPA molecular charges on CO2, W1, W2, and W3 were calculated to be +0.779, +0.071, +0.106, and +0.045, respectively. These results suggested that a positive charge was mainly distributed on CO2. Thus, the electronic state of [CO2(H2O)3+]ver cluster cation is drastically changed by the excitation. These results suggest that the excitation from ground to 3rd excited states is a charge transfer (CT) band from H2O(W1) to CO2 within the CO2(H2O)3+ cluster cation. Similar features were observed in all cases (n=2-5). These states are expressed as [CO2(H2O)3+]ver and 5 ACS Paragon Plus Environment


[CO2+ (H2O)3]ver. In the present study, the trajectories associated with the ground and CT state potential energy surfaces were investigated. The spatial distributions of the spin density of [CO2(H2O)3+]ver for the ground and the first excited states are illustrated in Figure S2. In the ground state, the spin densities were distributed on W1 and W2, whereas they were localized on CO2 in the CT state.

3.3. Reaction of CO2(H2O)n+ (n=3) in the ground state The time dependence of the potential energy of [CO2(H2O)3+]ver is given in Figure 2 (top), while snapshots of CO2(H2O)3+ are shown in Figure 2 (bottom). The MP2/6-311++G(d,p) optimized structure was used at time zero, while direct AIMD calculations were performed at the MP2/6-311G(d,p) level. Snapshots The electronic states of the cluster cation at the vertical ionization point of [CO2(H2O)3+]ver were calculated. Ionization occurred mainly on W1: the spin densities of CO2, W1, W2, and W3, were calculated to be 0.001, 1.002, -0.001, and -0.003 at time zero (Table S2 in SI). A hole was fully localized on H2O (W1) of the cluster cation. At time zero, the intermolecular distance was R(C-O)= 2.705 Å. After ionization, the proton transfer (PT) occurred rapidly from W1+ to W2. At 7.5 fs, the proton was located at the central position between W1 and W2: distances of H+ were calculated to be 1.415 Å from W1 and 1.361 Å from W2. PT was finished at 10.0 fs and the intermediate complex H3O--OH was formed. At 33.3 fs, the second PT occurred from W2 to W3 and a Zundal type complex, H2O (W2)--H+--H2O (W3), was formed. The CO2 molecule plays a spectator in this reaction. Potential energy 6 ACS Paragon Plus Environment

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The zero-energy level corresponds to the total energy of [CO2(H2O)3+]ver (time = 0 fs). After the ionization of CO2(H2O)n, the potential energy decreased to -35.0 kcal/mol (time = 0-10 fs). PT occurred from W1+ to W2 in this time region. The potential energy reached the lowest point at 10 fs when PT was completed. The second PT was completed at 33.3 fs. The elapsed time of PT was 10 fs (1st PT, 0-10 fs) and 20 fs (second PT, 10-33 fs), indicating that the first PT is faster than the 2nd PT. This is due to the fact that the first PT proceeds as an exothermic reaction with the energy of 20 kcal/mol, H2O+ + H2O (proton transfer) → H3O+ + OH, whereas the 2nd PT is composed of an isothermal reaction with the energy of 0 kcal/mol, H3O+ + H2O (proton transfer) → H2O + H3O+.

3.4. Reaction of CO2(H2O)n+ (n=3) in the CT state The time evolution of the potential energy of [CO2+(H2O)3]ver in the CT state is shown in Figure 3 (top). In addition, the snapshots of CO2+(H2O)n are shown in Figure 3 (bottom). The MP2/6-311++G(d,p) optimized structure was chosen in the trajectory calculation at time = 0 fs.

Snapshots The spin densities of CO2, W1, W2, and W3 in [CO2+(H2O)n]ver (n=3) were determined to be 1.002, 0.001, -0.003, and 0.000 at time zero (Table S2 in SI). A hole was localized on the CO2 of the cluster cation. The electronic structure at the CT state was significantly different from that of the ground state. After ionization, H2O (W1) gradually approached CO2+. At 65 fs, H2O (W1) collided with CO2+. At the same time, PT occurred rapidly from W1+ to W2 and the HCO3



radical was formed. The intermolecular distance between CO2+ and W1 changed from 2.704 Å (0 fs) to 1.485 Å (65 fs). At 65 fs, the proton was located in the central position between W1 and W2: the distances of H+ was 1.119 Å from W1 and 1.322 Å from W2. At 100 fs where the potential energy reached the minimum point (-61.0 kcal/mol), the HCO3 radical and a Zundal type complex; H2O(W2)--H+--(H2O)(W3), were formed. The HCO3 radical gradually dissociated (175 fs) from the water cluster cation and the radical was fully separated at 200 fs. Potential energy The zero level corresponds to the total energy of [CO2+(H2O)3]ver (time = 0 fs). Following the ionization, the potential energy decreased gradually from point a to -43.0 kcal/mol over the interval 0–48 fs. This decrease in energy was caused by the approach and collision of W1 to CO2+. At 65 fs, the energy was -50 kcal/mol when the HCO3 radical is formed (point b). After the formation of the HCO3 radical and Zundel complex (100 fs, point c), the radical gradually left the Zundal complex (175 fs, point d).

3.5. Summary of reactions of CO2(H2O)n+ (n=2-5) The results obtained for all clusters are summarized in Table 2. In the ground state PES, PT occurred in all cases. In contrast, the CT state resulted in the HCO3 radical except for n=2. In the case of n=2, a complex composed of CO2+ and H2O was formed. Time of PT is given in Table 3. The PT time was calculated as 15.5 fs (n = 2), 10.0 fs (n = 3), 15.8 fs (n = 4), and 10.8 fs (n = 5). The time of n = 4 was longer than the others (n = 3 and 5). This delay was caused by a hole delocalization over the four water molecules (W1–W4) at time zero (See Table S2 (n = 4)). The structure of water moiety 8 ACS Paragon Plus Environment

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was significantly close to that of the cyclic water tetramer (S4 symmetry), and the environments of the water molecules were similar. This specific structure caused a hole delocalization. After the structural deformation of CO2(H2O)4+, the hole was localized on W1, and then PT occurred from W1 to W2. The effects of the initial geometries on the PT time were examined for n=2-5. The results are summarized in Table 3. Direct AIMD calculations using the optimized geometry obtained using CAM-B3LYP/ 6-311++G(d,p) yielded the PT reaction and the HCO3 radical formation on the ground and CT state PESs, respectively. The PT time was calculated as 13.2 fs (n=2), 9.2 fs (n=3), 21.5 fs (n=4), and 10.0 fs (n=5). From the 10 K simulation, the average time for PT was calculated to be 9.2 fs (average of five trajectories). The effects of zero point vibration on the time evolution of potential energy of CO2(H2O)3+ are presented in Figure S3. A total of 10 trajectories were examined including ZPE. Nine trajectories were reactive while one was non-reactive. PT took place from 7.6 to 9.1 fs (average time of PT was 8.4 fs). Thus, the effects of the initial structures and ZPE were small for the present system, and the product channels were not affected by the initial structures. In addition, the effects of basis sets on the time of PT, particularly the addition of diffuse functions, were negligibly small (See Table S3).

3.6. Comparison of potential energies Figure 4 shows a comparison of the time-dependent potential energies for the ground and CT states of CO2(H2O)

3

+.

The energy difference at time zero was 52.2

kcal/mol. In the ground state PES, PT was completed within a very short time of less than 20 fs. In contrast, the reaction in the CT state PES occurred slowly because the 9 ACS Paragon Plus Environment


approach of H2O to CO2+ occurs over a long time prior to the OH addition of H2O to CO2+. The HCO3 radical formed at 60 fs after the collision of H2O to CO2+. The crossing between PESs was not found during the reaction in all cases (n=2-5). The total energies (= kinetic and potential energies) plotted as a function of time (dashed lines) indicate that both the total energies at the ground and CT states are constant during the reaction, indicating that the energy is conservative in both electronic states. The snapshots and potential energies of all systems (n=2, 4, and 5) are presented in Figures S4–S9.

3.7. Potential energy curves The present calculations demonstrated that the CO2(H2O)n+ clusters causes the PT reaction in the ground state PES after the ionization. Also it was found that OH addition reaction takes place on the CT state PES. To survey the entire reactions of the CO2(H2O)n, the potential energy of this system is given in Figure 5. The values are calculated for n=3. The binding energy of CO2 to the water trimer to form a CO2(H2O)3 neutral cluster is calculated to be 9.4 kcal/mol. When the neutral cluster is vertically ionized, transient cluster cations, [CO2(H2O)3+]ver (ground state) and [CO2+(H2O)3]ver (CT state) are formed. The transient cluster cations then relax to the stable forms. In the ground state of the cation, the moiety of the water cluster was ionized, [CO2(H2O)3+]ver. After PT, the complex cation; CO2(H3O+-OH)(H2O), is formed. This complex has an energy that is 45.0 kcal/mol lower than that of the vertical ionized state (ground state). PT is then a very fast process and occurs in less than 20 fs. In the CT state of transient cation, a hole is localized on CO2. This state is spontaneously relaxed, and a reactive HCO3 radical is formed by the OH addition 10 ACS Paragon Plus Environment

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reaction: CO2+ + H2O → HCO3 + H+. The reaction channels can be expressed as follows: CO2(H2O)n + IE(1) →CO2(H2O)n+ (ground) → CO2(H2O)n + IE(2) →CO2+(H2O)n (CT) →

CO2(H3O+-OH)n-2

HCO3(H+)(H2O)n-1

(PT)

(1)

(OH addition) (2)

where IE denotes the ionization energy, and IE(1) < IE(2). The final products are strongly dependent on the electronic states and ionization energy. Simulated infrared (IR) spectra of the products from PT and OH addition are given in Figure S10. These spectra are useful in detecting the HCO3 radical in a water cluster. In the IR spectrum of the OH-addition product, the O-H stretching modes (3300–3600 cm-1) disappear, whereas a new peak of the C=O stretching mode of HCO3 appears at 1800 cm-1. 4. Discussion 4.1. Astrochemical implication Emissions of carbon dioxide are considered as one of the greenhouse gases that correlate to climate change and global warming. However, chemical activation of CO2 is still a difficult problem. In laboratory experiments, activation of CO2 is generally performed using catalytic reactions with metals.44 Recently, catalytic plasma activation was performed in a laboratory experiment.45,46 In the universe (surfaces of comets and icy planets), however, the probability of CO2 reacting with a metal is extremely low, so that the catalytic reaction of CO2 is impossible. The present calculations show that CO2 is easily converted into the activated HCO3 radical by photo-irradiation of CO2-H2O binary cluster. From these observations, it is expected that CO2+ would react with H2O to form HCO3 active radicals on the surface of icy comet and planets. The active HCO3 radicals may contribute to chemical 11 ACS Paragon Plus Environment


evolution in the universe. The importance of the HCO3 radical has been suggested in material chemistry. Goss et al. investigated the HCO3 radical on the surface of diamond and determined that this radical accepts an electron from the surface and generated a conduction hole in diamond.47 Given its ubiquity, the chemistry of HCO3 radicals may be applicable to a wide variety of fields in the future.

4.2. Comparison with previous studies A similar OH addition reaction was identified in the SO2(H2O)n+ cluster cation after the ionization of the parent neutral state.48 In the case of SO2, the addition reaction occurs in the ground state PES of SO2(H2O)n+. This is because the ionization energy of SO2 is lower than that of H2O (12.35 eV vs. 12.62 eV) and ionization occurs preferentially on SO2 instead of the H2O clusters. In the case of CO2, the ionization energy is 13.78 eV, which is larger than that of H2O. The ground state PES of CO2(H2O)n+ leads to the PT product, and the CT state yields the OH addition product (HCO3 radical). Heinbuch et al. investigated the ionization process of binary clusters composed of CO2 and H2O by utilizing time of flight mass spectroscopy (photon energy = 26.5 eV).32 At low CO2 concentration, several cluster ions were observed as ionic products in the mass spectrum: H+(H2O)n, CO2(H2O)nH +, CO2(H2O)n+, and (H2O)n+. However, the isolated HCO3 radical was not observed in the spectrum because the HCO3 radical has no charge. The HCO3 radical binding to water cluster cation may be hidden in the spectrum.


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4.3. Conclusions The results obtained in this investigation can be summarized as follows. (1) The ground state of the cation radical of CO2(H2O)n (n=2-5) at the vertical ionization point is expressed as CO2[(H2O) n+], where a hole is localized on the water cluster; the CT state is expressed as (CO2+)(H2O)n, where a hole is localized on CO2. (2) After ionization, PT occurs within the H2O cluster in the ground state. This process is very fast and is completed within 20 fs. (3) In contrast, the HCO3 reactive radical is formed in the CT state. The products are selected by the ionization state of the CO2(H2O)n cluster. CO2 is known as an inactive molecule. This study demonstrated that CO2 is activated by photoreaction in a water cluster and is easily converted into the active HCO3 radical.

Acknowledgments. The author (HT) acknowledges partial support from Grant Number 18K05021 (JSPS KAKENHI).

SI The SI is available in ACS website.

Time dependent potential energies at the ground and CT states of CO2(H2O)3+, NPA molecular

charges on CO2 and water molecules (W1-W5) of [CO2(H2O)n+]ver (n=2-5), spatial distribution of spin densities on the cluster cation of [CO2(H2O)n+]ver (n=3), the effects of zero point energy on time of proton transfer (PT) in [CO2(H2O)n+]ver (n=3), the result of direct AIMD calculation of CO2(H2O)n+ (n=2, 4, and 5), and IR spectra of product cations are provided in the SI.



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Table 1. Excitation energies of [CO2(H2O)3+]ver (Eex in eV) and molecular charges on CO2 and water molecules (W1, W2, and W3) of [CO2(H2O)3+]ver calculated using the SAC-CI method with 6-311++G(d,p) basis set.

molecular charge state

Eex / eV

CO2

W1

W2

W3

ground (0)

0.00

0.017

0.835

0.129

0.019

1st

0.22

0.010

0.118

0.830

0.042

2nd

0.81

0.023

-0.004

0.050

0.931

3rd

1.62

0.779

0.071

0.106

0.045

4th

1.69

0.967

-0.003

0.025

0.011

5th

1.74

0.246

0.390

0.285

0.080

Table 2. Summary of direct AIMD calculations. Products from ground and CT state potential energy surfaces after ionization of the neutral parent clusters. PT and HCO3 mean proton transfer product and HCO3 radical formation. The direct AIMD calculations were performed at the MP2/6-311G(d,p) level based on the initial geometries calculated at the MP2/6-311++G(d,p) level. n

ground state

CT state

2

PT

(CO2)+-(H2O)2 complex

3

PT

HCO3

4

PT

HCO3

5

PT

HCO3


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Table 3. Time of PT (in fs) calculated using the direct AIMD method at the MP2/6-311 level. A: initial geometries were calculated at the MP2/6-311++G(d,p) level, B: initial geometries were calculated at the CAM-B3LTP/6-311++G(d,p) level, 10 K sim: initial geometries were generated using 10 K simulation at the CAM-B3LYP/6-311++G(d,p) level, and ZPV: initial geometries were generated using zero point energy vibration (ZPV) at the MP2/6-311++G(d,p) level. n

A

B

10 K sim

ZPV

2

15.5

13.2

--

--

3

10.0

9.2

9.2

8.4

4

15.8

21.5

--

--

5

10.8

10.0

--

--



Figure Captions

Figure 1. The optimized structures and geometrical parameters of neutral CO2(H2O)n (n = 2-5). The bond distances and angles are in Å and degrees, respectively. The calculations were carried out at the MP2/6-311++G(d,p) level. Figure 2. Result of direct AIMD calculations for CO2(H2O)n+ (n=3) in the ground state. (Upper) Time dependence of the potential energy of CO2(H2O)3+ after ionization. The lettered arrows indicate the positions of the lettered structures drawn out below. (Lower) Snapshots of CO2(H2O)3+. The direct AIMD calculations were performed at the MP2/6-311G(d,p) level. Bond distances are given in Å. Figure 3. Result of direct AIMD calculation of CO2+(H2O)n (n=3) at the charge transfer (CT) state. (Upper) Time dependence of the potential energy of CO2+(H2O)3 after ionization. The lettered arrows indicate the positions of the lettered structures drawn out below. (Lower) Snapshots of CO2+(H2O)3. Figure 4. Comparison of time dependent potential energies in the ground and CT states of CO2(H2O)3+. The calculations were performed at the MP2/6-311G(d,p) level. Dashed lines indicate the total energies (= kinetic + potential energies) plotted as a function of time. Figure 5. Schematic illustration of potential energy curves of neutral system CO2(H2O)3 and radical cation CO2(H2O)3+ systems (ground and CT states). The direct AIMD calculations were carried out at the MP2/6-311++G(d,p) level. The values of relative energies (in kcal/mol) were calculated for n=3.


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References (1) Filacchione, G.; De Sanctis, M. C.; Capaccioni, F.; Raponi, A.; Tosi, F.; Ciarniello, M.; Cerroni, P.; Piccioni, G.; Capria, M. T.; Palomba, E.; et. al., Exposed Water Ice on the Nucleus of Comet 67P/Churyumov-Gerasimenko. Nature 2016, 529, 368-372. (2) Sivaraman, B.; Venkataraman, V.; Kalyaan, A.; Arora, S.; Ganesh, S. Exposed Amorphous Water Ice on Comet 49P/Arend-Rigaux. Adv. Space Res. 2015, 56, 2428-2431. (3) McKay, A. J.; Kelley, M. S. P.; Cochran, A. L.; Bodewits, D.; DiSanti, M. A.; Russo N. D.; Lisse, C. M. The CO2 Abundance in Comets C/2012 K5 (LINEAR), and 290P/Jager as Masured with Spitzer. Icarus, 2016, 266, 249-260. (4) Wang, F.R.; Schmidhammer, U.; Larbre, J.P. ; Zong, ZZ. ; Marignier, J.L.; Mostafavi, M.

Time-dependent Yield of the Hydrated Electron and the Hydroxyl

Radical in D2O: A Picosecond Pulse Radiolysis Study, Phys. Chem. Chem. Phys., 2018, 20, 15671-15679. (5) Douberly, G. E.; Walters, R. S.; Cui, J.; Jordan, K. D.; Duncan, M. A. Infrared Spectroscopy of Small Protonated Water Clusters, H+(H2O)n (n=2-5): Isomers, Argon Tagging, and Deuteration. J. Phys. Chem. A 2010, 114, 4570-4579. (6) Guha, S.; Neogi, S. G.; Chaudhury, P. Study of Structure and Spectroscopy of Water-Hydroxide Ion Clusters: A Combined Simulated Annealing and DFT-Based Approach. J. Chem. Sci. 2014, 126, 659-675. (7) Jerzy, M. Theoretical Investigation of the Reaction Paths of the Aluminum Cluster Cation with Water Molecule in the Gas Phase: A Facile Route for Dihydrogen Release. J. Phys. Chem. A 2015, 119, 8683-8691. (8) Klyne, J.; Schmies, M.; Fujii, M.; Dopfer, O. Stepwise Microhydration of Aromatic Amide Cations: Formation of Water Solvation Network Revealed by Infrared Spectra of Formanilide+-(H2O)n Clusters (n

Direct Ab initio Molecular Dynamics

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