Reaction Dynamics Following Ionization of Ammonia Dimer

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Ionization Dynamics of Ammonia Dimer on Ice Surface Hiroto Tachikawa J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b04699 • Publication Date (Web): 02 Sep 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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The Journal of Physical Chemistry

Reaction Dynamics Following Ionization of Ammonia Dimer Adsorbed on Ice surface Hiroto TACHIKAWA* Division of Applied Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, JAPAN Email: [email protected]/ Fax.

+81 11706-7897

Abstract: The ice surface provides an effective two-dimensional reaction field in the interstellar space. However, how the ice surface affects the reaction mechanism is still unknown. In the present study, the reaction of an ammonia dimer cation adsorbed both on water ice and cluster surface was theoretically investigated using direct ab-initio molecular dynamics (AIMD) combined with our own n-layered integrated molecular orbital and molecular mechanics (ONIOM) method, and the results were compared with reactions in the gas phase and on water clusters. A rapid proton transfer (PT) from NH3+ to NH3 takes place after the ionization and the formation of intermediate complex NH2(NH4+) is found. The reaction rate of PT was significantly affected by the media connecting to the ammonia dimer. The time of PT was calculated to be 50 fs (in the gas phase), 38 fs (on ice), and 28–33 fs (on water clusters). The dissociation of NH2(NH4+) occurred on an ice surface. The reason behind the reaction acceleration on an ice surface is discussed. Keywords: Proton transfer; ammonia cluster, water cluster, transition state, intermediate

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1. Introduction The mechanism of molecular evolution in interstellar space is currently an important topic in astrochemistry and astrophysics.1-5 Reaction probability of bi-molecular collision reactions increases significantly in the adsorption of the ice surface due to the increase in collision probability. Hence, the reaction probability on a 2-dimensional surface is strikingly larger than that in 3-dimensional space. The reaction of an ammonia molecule plays an important role in the initial synthesis of amino acids in molecular clouds and in space. In particular, the ionization of ammonia and related compounds caused by cosmic rays is a basic reaction in amino acid synthesis. Hence, studying the photo-reaction of ammonia on an ice surface would give important information about amino acid synthesis. In the present study, to elucidate the effects the ice surface on the reaction mechanism, the reaction of an ammonia dimer cation (NH3)2+ adsorbed on a water ice surface following its ionization were investigated using direct ab-initio molecular dynamics (AIMD)11-13 combined with our own n-layered integrated molecular orbital and molecular mechanics (ONIOM)14 methods Several molecules interacting with the ice surface have been investigated experimentally

and

theoretically.

Kawasaki

and

co-workers

investigated

photo-irradiation of ice surface at 193 nm. The dissociation of hydrogen atom was found after the photo-irradiation to the ice surface at 193 nm. The photo-dissociation spectrum was consisted of two components: fast and slow components. They revealed that a water dimer adsorbed on the ice surface correlates with the photo-dissociation of ice.15,16 Yuan et al. investigated experimentally the CO2 formation on the ice surface 17 observing an Eley-Rideal-type reaction, where CO gas molecules react by direct 3


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collisions with surface OH radicals. Yoon and Shin carried out Car–Parrinello molecular dynamics (CPMD) simulations on the heterogeneous reaction between HCl and ClONO2 on the ice surface. They showed that the Cl2 formation reaction is followed by the reaction ClONO2 + HCl → Cl2 + HNO3. A proton transfer from HCl to the ice surface noted as well. The cluster cation of ammonia molecules, (NH3)n+, have been extensively investigated from experimental and theoretical points of view.18-25 Fujii and co-workers applied infrared predissociation spectroscopy of vacuum ultraviolet-pumped ion (IRPDS-VUV-PI) to the ammonia cluster cations (NH3)n+ (n=2–4) produced by VUV photoionization in supersonic jets. It was found that (NH3)2+ has the proton transferred form composed of radical-ion complex (NH2-NH4+), whereas (NH3)4+ is consisted of a face-to-face dimer cation, (H3N-NH3)+. Yuan et al. investigated the reaction dynamics and fragmentation of ammonia cluster cation through time of flight mass spectrometry, and showed that the protonated clusters (NH3)xH+ dominate ammonia cluster mass spectra. The structures of ammonia cluster cations were also investigated using ab initio calculations. Matsuda et al. showed that the radical-ion complex is more stable than that of face-to-face dimer cation. Yuan et al. determined the structures of larger cluster cations of ammonia molecules. In previous works, using a direct AIMD method, we investigated the reaction mechanism of halocarbon CF2Cl2 on the ice surface following the electron capture and compared the results with the gas phase reaction. In the gas phase, velocity of dissociating Cl- ions was very slow, whereas the Cl- generated by dissociation on the ice surface has a higher velocity.26,27 More recently, the reaction of a water dimer cation (H2O)2+ on the ice surface, following the ionization of (H2O)2, was investigated. The 4



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fast proton transfer occurred after ionization.28 However, the effects of the ice surface on the reaction mechanism was not clearly understood. In this work, we focus our attention on the roles of the ice surface on the reaction dynamics and compare the results with the isolated reaction in the gas phase.

2. Computational details 2.1. Ab-initio calculations The ice surface was modeled by two water layers composed of 48 water molecules (H2O)m (m = 48). First, the geometry of the ice surface (H2O)48 was fully optimized using the PM3 method. Next, the ammonia dimer was optimized at the MP2/6-311++G(d,p) level. The ammonia dimer was put on the central region of the model surface (H2O)48. Finally, the whole geometry of (NH3)2(H2O)48 was optimized using ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) method.29 The level of theory was expressed by the (MP2/6-311++G(d,p): PM3). The ammonia dimer and ice model surface were calculated as high-layer and low-layer levels, respectively. The dimer is consisted of proton donor and proton acceptor ammonia molecules, which are denoted by (NH3)d and (NH3)a, respectively. The standard Gaussian 09 program package was used for all ab initio calculations.30

B. Direct AIMD calculations The trajectories of (NH3)2+(H2O)48 following the ionization of (NH3)2(H2O)48 was calculated at the (MP2/6-311++G(d,p):PM3) level under the assumption of vertical ionization from the neutral state. The trajectory calculations of (NH3)2+(H2O)48 were performed using the condition of constant total energy. The velocity Verlet algorithm 5


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was used with a time step of 0.1 fs to solve the equation of motion of the system. The drifts of total energies in all trajectory calculations were less than 0.01 kcal/mol. In addition to the optimized geometry of (NH3)2(H2O)48, twenty structures of (NH3)2(H2O)48 generated around the equilibrium point were examined in the direct AIMD calculations. The intermolecular distance between (NH3)d and (NH3)a varied in the range 3.150–3.250 Å, twenty structures of (NH3)2(H2O)48 were selected, and the trajectories of ionic system were started from the chosen geometrical onfigurations of the ammonia dimer. The range of intermolecular distance was chosen from astrophysical condition at 10 K (See, Figure S1 in supporting information (SI)). The kinetic energy and angular momentum of each atom were assumed to be zero at time zero. The sampling method used is described in previous papers.31

3. Results 3.1. Geometrical structure of neutral ammonia dimer on ice surface Figure 1 shows the structure of ammonia dimer on the model ice surface, (NH3)2(H2O)48.

The

geometry

optimization

was

performed

at

the

(MP2/6-311++G(d,p):PM3) level of theory. The ammonia dimer binds to the central region of a hexagonal site on the ice surface. The ammonia dimer was connected by three protons from the surface water molecules. The donor ammonia molecule (NH3)d was connected by one hydrogen bond (bond length 1.790 Å) from the surface water molecule, while the acceptor ammonia molecule (NH3)a was bounded by two bonds with the distance of 2.588 and 2.730 Å. The intermolecular distance between (NH3)d and (NH3)a were calculated to be RNN= 3.174 Å.

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3.2. Ammonia dimer cation on ice surface following the ionization Snapshots of the (NH3)2+(H2O)48 radical cation after the vertical ionization of the parent neutral cluster are illustrated in Figure 2. In this trajectory, the optimized structure obtained by the (MP2/6-311++G(d,p):PM3) level was used for the initial structure of the (NH3)2+(H2O)48 at time=0 fs. The positive charge and cation hole were mainly localized on a part of (NH3)d in the ammonia dimer (NH3+)d-(NH3)a. The bond lengths in the ammonia dimer were (r1, r2, RNN) = (2.188, 1.021, 3.174) in Å at time zero. After the ionization, the bent structure of (NH3+)d suddenly changed to a planar structure, leading to the bending mode excitation of (NH3+)d. The proton of (NH3)d was transferred to (NH3)a during time = 20–40 fs. The snapshot at 35 fs shows that the proton passes near the transition state region in the proton transfer (PT) reaction. The distances between the proton and (NH3)a at 35 and 38 fs were (1.348, 1.272, 2.615 Å) and (0.935, 1.670, 2.548 Å), respectively, indicating that these distances were drastically changed as a function of time. The distance of r1 was changed from r1=1.348 to 0.935 Å, while r2 was changed from 1.272 to 1.670 Å, indicating that the proton was rapidly transferred at this time region. The N–N bond length was also changed from RNN=3.174 Å (0 fs) to 2.548 Å (38 fs), indicating that the both ammonia cation and neutral ammonia require the nitrogens to approach each other for the PT to take place. The time of PT was calculated to be 37.9 fs in this trajectory. After PT, the ion-radical complex consisting of NH2(NH4+) was formed (38 fs). This complex was gradually decomposed to NH2 and NH4+ at 85–138 fs because the NH4+ ion is solvated by the water molecules on the surface. The N-N distances were varied from 3.313 Å (85 fs) to 6.212 Å (200fs). The NH2 radical abstracted a hydrogen atom from the water molecule to form an ammonia molecule (NH3): NH2 + H2O → NH3 + OH. At the final stage of the reaction 7


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(200 fs), the NH3 molecule was evaporated from the surface while the NH4+ ion was solvated by the water molecules and remained on the surface. Figure 3A shows time evolutions of bond distances (r1, r2 and RNN) of (NH3)2+ on ice surface. At time= 0 fs (before the ionization), the bond lengths were r1=1.790 Å, r2=0.974 Å, and RNN =2.790 Å. The distance of H+ from (NH3)a (r1) decreased with increasing time, whereas the distance of H+ from (NH3+)d (r2) monotonically increased after the ionization. These distances encountered each other at 35 fs where PT occurred. The intermolecular distance between (NH3+)d and (NH3)a (RNN) decreased gradually as a function of time and was minimized at 38 fs. The collision between the proton and (NH3)a occurred at the smallest point of RNN. The time propagations of the (NH3)2+(H2O)48 radical cation potential energies following ionization are given in Figure 3B. The zero level of energy corresponds to that of [(NH3)2+(H2O)48]VER at the vertical ionization point from the neutral state. After ionization, the potential energy decreased gradually at 8 fs (−12 kcal/mol) and 20 fs (−14 kcal/mol) as indicated by the arrows (a and b). These minima were caused by the bending mode excitation of (NH3+)d. Additionally, the approaching of (NH3+)d to the neutral NH3 molecule decreases the potential energy of the system. PT occurred immediately at 30–50 fs without an activation barrier. The ion-radical NH2(NH4+) complex was formed after the PT (50 fs). The potential energy vibrated strongly due to the formation of the complex and the excess energy caused by the PT reaction. The hydration of NH4+ occurred gradually at the final state of the PT reaction. The above observations suggest that the reaction is composed of the following five steps (with the approximate time for each given in parenthesis): (1) Approaching of (NH3+)d to (NH3)a, and the bending mode excitation of (NH3+)d 8



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(0–30 fs). (2) PT from (NH3+)d to (NH3)a (30–50 fs), and the formation of ion-radical NH2(NH4+) complex. (3) Separation into the NH4+ ion and the NH2 radical (50–100 fs). (4) Hydration of the NH4+ ion, and hydrogen abstraction of the NH2 radical from H2O (100–150 fs). (5) Stabilization of the NH4+ ion and evaporation of NH3 from the ice surface (>150 fs). Twenty trajectories started from the Franck-Condon (FC) region for the ammonia dimer on the ice surface. All trajectories gave similar results (Figure 2 in SI). The time of PT was distributed in the ranges 37.7-39.9 fs, while the average was 38.4 fs. The lifetime of the NH2(NH4+) complex was obtained to be ~100 fs. Figure 2 shows that the hydration of NH4+ occurred at 150–200 fs after ionization.

3.3. Ionization dynamics of (NH3)2 in the gas phase Figure 4A shows the structure of the ammonia dimer in the gas phase. All calculations were carried out at the MP2/6-311++G(d,p) level. The N-H bond of (NH3)d orients toward the nitrogen atom of (NH3)a. The lengths of hydrogen bonds were calculated to be RNN= 3.261 Å and r1= 2.260 Å. The N-H bond length of (NH3)d was r2=1.018 Å. The highest occupied molecular orbital (HOMO) is illustrated in Figure 4B. HOMO is mainly composed of the non-bonding (NH3)d orbital. The molecular charges on (NH3)d and (NH3)a were close to zero, indicating that the charge transfer does not take place in the neutral state. The spatial distribution of spin density on the ammonia dimer cation [(NH3)d(NH3)a+]ver at the vertical ionization point from the neutral state is given in Figure 4C. The unpaired electron was mainly distributed on 9


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(NH3)d. The spin densities on the (NH3)d and (NH3)a were 0.996 and 0.004, respectively, while molecular charges on the (NH3)d and (NH3)a molecules were +0.964 and +0.036, respectively. From the results, the ammonia dimer is schematically expressed by [(NH3)d+–(NH3)a]ver at the vertical ionization point. The snapshots for the ammonia dimer cation following the ionization of the (NH3)2 neutral state are given in Figure 5. Time evolutions of interatomic distances (r1, r2, and RNN), and potential energy of (NH3)2+ in gas phase are given in Figure S3, and all snapshots are illustrated in Figure S4. The calculation was carried out at the MP2/6-311++G(d,p) level. At time zero, the ammonia dimer was vertically ionized from (NH3)2. The atomic distances for r1, r2, RNN in Å were 2.260, 1.018, and 3.261, respectively, at time zero. The hole was localized on the (NH3+)d molecule. Immediately, (NH3+)d and (NH3)a gradually approached each other. At time = 46 fs, the proton of (NH3+)d was located at the central region between (NH3)d and (NH3)a with distances of 1.348, 1.283, and 2.616 Å. This region corresponds to a transition state (TS) for PT from (NH3+)d to (NH3)a. The intermolecular distance was RNN = 2.616 Å at 46 fs. These results indicate that the intermolecular nitrogens must approach each other in PT. The first PT process was completed at 53 fs (1.039, 1.496, 2.535 Å), and the ion-radical complex NH2(NH4+) was formed as the product at 174 fs. The complex was observed to have a large amplitude motion corresponding to the intermolecular vibrational mode between NH4+ and NH2. However, the complex did not decompose because the excess energy of NH2(NH4+) formed by the PT reaction did not reach the energy level of the dissociation limit for NH4+ + NH2. Consequently, the complex remained as a long lived intermediate.

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Table 1. Time of proton transfer (PT in fs) and the lifetime (in fs) of intermediate complex NH2(NH4+) in gas phase and on ice surface. The calculations of direct AIMD were

performed

at

the

MP2/6-311++G(d,p)

(in

gas

phase)

and

(MP2/6-311++G(d,p):PM3) (on ice surface) levels.

field

time of PT / fs

In gas phase

50.3

On ice surface

37.9 (38.4) a

lifetime ∞ ~(100-150) fs

a

Average value of 20 trajectories

3.4. Ionization dynamics of the ammonia dimer on water clusters In this section, the effects of water clusters on the ionization dynamics of (NH3)2 were examined following the procedure outlined in the sections above. The optimized structures of (NH3)2(H2O)n (n = 1-12) were given in Figures 6 and 7. The geometry optimizations were carried out at the MP2/6-311++G(d,p) level. The intermolecular distances of (NH3)2 on the water clusters (RNN) are given in Table 2 together with the value for the ammonia dimer in the gas phase. The distances were distributed in the range of 3.105-3.155 Å. These values are slightly shorter than that of (NH3)2 in the gas phase. In case of (NH3)2 in the gas phase (n = 0), the distance was calculated to be RNN = 3.261 Å. Thus, it was found that the intermolecular distance of (NH3)2 is shortened in the adsorption to the water clusters. The molecular charges and the spin densities on (NH3)d and (NH3)a of [(NH3)2(H2O)n+]ver at the vertical ionization point are given in Table 2. The molecular charges on (NH3)d and (NH3)a in the gas phase were +0.964 and +0.036, respectively, 11


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while the spin densities on (NH3)d and (NH3)a were 0.996 and 0.004, respectively. The hole was completely localized on (NH3)d in the gas phase. Spatial distributions of highest occupied molecular orbitals (HOMOs) of (NH3)2(H2O)n (n=1-12) are illustrated in Figures S5 and S6. HOMOs are mainly localized on the (NH3)2 moiety of (NH3)2(H2O)n, indicating that the ionization takes place from the (NH3)2 moiety. As a typical case of (NH3)2(H2O)n, we discuss the electronic states using the results of n = 6. The molecular charges on (NH3)d and (NH3)a were +0.882 and +0.040, respectively, while the spin densities on (NH3)d and (NH3)a in [(NH3)2(H2O)n+]ver were +0.938 and +0.008, respectively. Similarly to the ionization state of the ammonia dimer in the gas phase, the hole is mainly localized on the moiety of (NH3)d in [(NH3)2(H2O)n]ver+ at the vertical ionization point. However, the magnitude of hole localization in (NH3)2+ on the cluster is weaker than that of the gas phase. The hole is slightly delocalized on (NH3)a. The electronic states of (NH3)2 on the water cluster were different from that of the gas phase. The highest occupied molecular orbitals (HOMOs) of (NH3)2 (H2O)n (n=1-4, 6, 8, and 12) are illustrated in supporting information. Snapshots of the (NH3)2+(H2O)6 radical cation following vertical ionization of the parent neutral cluster are illustrated in Figure 8. Potential energy and bond distances of (NH3)2+(H2O)6 following the ionization is given in Figure S7. This sample trajectory was started from the optimized structure of (NH3)2+(H2O)6 at time= 0 fs. The AIMD calculation was carried out at the MP2/6-311G(d,p) level. The spin densities on (NH3)d and (NH3)a were 0.943 and 0.008, respectively, indicating that a hole was mainly localized on (NH3)d in the ammonia dimer (NH3+)d-(NH3)a at time zero. The bond distances in the moiety of the ammonia dimer were (r1, r2, RNN) = (2.167, 1.023, 3.152 Å) at time zero. After the ionization, the proton of (NH3)d was rapidly transferred to 12



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(NH3)a. The structure at time = 24.8 fs showed that the proton moves around the transition state (TS) for the PT reaction. The intermolecular distances were shortened from RNN = 3.152 to 2.808 Å in PT. The PT reaction was almost completed at 29.7 fs. The ion-radical complex was not decomposed after the ionization of (NH3)2+(H2O)6. The similarly direct AIMD calculations were carried out for all systems (n = 1-5). The dynamics features obtained for n = 1-5 were essentially similar to that of n = 6. The results for n = 1-6 are summarized in Table 3 together with those for the gas phase (n =0). The time of PT was centered around 27.6 fs (n = 2 and 5) and 32.9 fs (n = 1). These results revealed that the time of PT in the water cluster is shorter than that in the gas phase. Namely, the PT rate is accelerated by the ice surface.

3.5. Comparison of the reaction time of PT for different media As shown in previous sections, PT takes place after the ionization of the ammonia dimer in all three situations discussed: on the ice surface, on the water clusters, and in the gas phase. The time of PT in the (NH3)2+ dimer cation was calculated to be 50.3 fs in the gas phase. In contrast, the time of PT on the ice surface and on the water cluster was 37.9 fs (38.4 fs, average) and 28-33 fs, respectively. These values are plotted in Figure 9. The linear relation between RNN and time of PT was obtained. The short hydrogen bond results in the fast PT. To elucidate the differences in the PT time, AIMD calculations were carried out at the MP2 method with 6-311G(d,p) and 6-311++G(d,p) basis sets. The intermolecular distance (RNN) between (NH3)d and (NH3)a in the gas phase was changed from 2.70 to 3.70 Å. The time of PT is plotted in Figure 9 as a function of RNN. The time of PT increased with increasing RNN. This is due to the fact that more time is necessary for 13


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NH3+ and NH3 to approach each other in the case of longer nitrogen separation. Figure 9 also shows that the peak appears at RNN = 2.90 Å, indicating that PT takes more time at RNN = 2.90 Å. In this region, the direction of the N-H bond is largely deviated from the axis of the hydrogen bond (N-H--N) at the first collision of (NH3+)d and (NH3)a. Hence, PT in the first collision does not take place if (NH3+)d collides with (NH3)a. PT takes place in the second collision. The same calculations were carried out at the MP2/6-311G(d,p) level. The result is shown using a dotted line (Figure 9). The shape of time at PT was similar to that of MP2/6-311++G(d,p) calculation. As clearly seen in Figure 9, the time of PT is dependent on the intermolecular distance (length of the hydrogen bond between ammonia molecules). Namely, the rate of PT is accelerated by the short hydrogen bond. This is due to the fact that the intermolecular distance of the ammonia dimer is influenced by the media. The short hydrogen bond gives the fast PT. Additionally, the values for the ice and water clusters deviated from the curves of the gas phase. For example, the PT rates on the clusters shifted 10-15 fs to the left (relative to those of the gas phase). This increase in rates is caused by the change of the (NH3)2+ electronic states of (NH3)2+ due to the differences in the media.

3.6. Potential energy surface of (NH3)2 Potential energy curve for the PT reaction following the ionization of (NH3)2 in the gas phase is given in Figure S8. The calculations were carried out using MP2, CCSD, QCISD and MP4SDQ methods with the 6-311++G(d,p) basis set. The ionization potentials of (NH3)2 were calculated to be 9.84 eV (MP2) and 9.73 eV (CCSD). These values are in reasonably agreement with the experiments (9.54-9.95 eV).24 The vertical 14



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ionization point, [(NH3)2+]ver, was +34.7 kcal/mol higher in energy than the equilibrium point of radical-ion complex, NH2-NH4+, (+36.0 kcal/mol in CCSD). The radical-ion complex has two forms as well as the water dimer cation.32 These are “proton transferred from”, expressed by NH2-NH4+, and “face-to-face form”, (H3N-NH3)+. The latter form has a N-N bond, and the energy level was +5.9 kcal/mol (MP2) and +7.7 kcal/mol (CCSD) higher than the radical-ion complex. These isomers are connected by a transition state (TS), and the barrier heights from NH2-NH4+ to (H3N-NH3)+ were calculated to be +18.3 kcal/mol (MP2) and +17.4 kcal/mol (CCSD). The intrinsic reaction coordinate (IRC) from the vertical ionization point to the radical ion-complex calculated at the MP2/6-311++G(d,p) level is given in Figure S8. The potential energy surface is composed of purely steep slope without barrier. Namely, the trajectory from [(NH3)2+]ver reaches directly to the radical-ion complex. The face-to-face form is not correlated to the PT reaction. The relative energies calculated by MP2 method were in good agreement with those of CCSD calculations.

3.7. Effects of zero point energies (ZPEs) on the reaction mechanism of gas phase and ice surface Importance of zero point energy (ZPE) on the reaction mechanism has been pointed out by several groups.33-40 Especially, the effect of ZPE becomes larger in the reaction with activation barrier. In previous sections, ZPE was neglected in the initial condition of [(NH3)2+]ver. Hence, it should be noted that the present model is limited in non ZPE case, and is based on single trajectory simulation. In this section, the effects of ZPE on the time of PT in (NH3)2+ and (NH3)2+(H2O)48 were preliminary examined. First, the ZPE simulations of neutral systems, (NH3)2 and 15


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(NH3)2(H2O)48, were carried out at the MP2/6-311++G(d,p) and (MP2/6-311++G(d,p): PM3) levels. Second, the geometries and velocities of atoms were selected from the ZPE simulations. Next, direct AIMD calculations were carried out for cation systems. The results are given in Table 4.

Table 4. Time of proton transfer (PT in fs) in gas phase and on ice surface obtained from direct AIMD calculations including zero point energies (ZPEs) .

field

time of PT / fs

In gas phase

45.9a

On ice surface

18.0b

a


b


The time of PT was distributed in the ranges 41.0-56.3 fs in case of (NH3)2+ in gas phase, while the average value was calculated to be 45.9 fs. In case of (NH3)+ on ice surface, the time of PT was distributed in 9.2-36.7 fs, and the average time of PT was 18.0 fs. Inclusion of ZPE slightly changed the time of PT in gas phase. However, the effects of ZPE were smaller than the medium one. This is due to the fact that the PT reaction in ammonia dimer cation has no activation barrier, and potential energy surface is composed of a purely steep slope (Figure S8). Snapshots of sample trajectory of (NH3)2+(H2O)48 are given in Figure S9. Inclusion of ZPE accelerated the time of PT on ice surface. This is caused by the ammonia dimer with short N-N bond length generated on the ice surface.

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4. Discussion 4.1 Conclusion From the present calculations, it was revealed that (1) the ice surface accelerates the proton transfer reaction and (2) it also accelerates the dissociation of intermediate complex, expressed by NH4+(NH2) → NH4+ + NH2. In contrast, the complex NH4+(NH2) does not dissociate in the gas phase. The acceleration of ice surface (i.e, fast proton transfer) is originated from the short hydrogen bond of the ammonia dimer on ice. The calculations showed that the N-N bond lengths of (NH3)2 were 3.174 Å (ice) and 3.261 Å (in gas phase). The length in ice surface was 0.1 Å shorter than gas phase. Thus, the specific feature observed on the ice surface is caused by the short hydrogen bond.

4.2. Comparison with water dimer on ice surface In a previous study,28 we applied a similar technique to the water dimer cation on the ice surface. Following ionization, PT occurred from H2O+ to H2O and the intermediate complex was formed as expressed by H2O+-H2O → H3O+-OH. The dissociation of intermediate complex H3O+(OH) was found on the ice surface. The process of PT in the water dimer is qualitatively similar to that of (NH3)2 on the ice surface. Namely, the PT reaction is accelerated by the ice surface in both dimers. However, the time scale is slightly different for the two reactions. In the water dimer cation, the time of PT on the ice surface was three times faster than the gas phase. The time of PT was 10 fs (on ice) and 28 fs (in the gas phase). The hydrogen bond lengths of the water dimer were calculated to be ROO = 2.914 Å (gas 17


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phase) and 2.760 Å (ice surface). The difference in the hydrogen bond length (ROO = 0.154 Å) before and after adsorption of the water dimer to the ice surface is greater than that of ammonia dimer (RNN = 0.087 Å). The greater acceleration of PT seen for the water dimer is caused by the shorter hydrogen bond on the ice surface. In the present study, a similar conclusion was obtained for the ammonia dimer on the ice surface, although the magnitude of PT acceleration for the ammonia dimer is smaller than that of the water dimer. Most critically, the ice surface modifies the reaction mechanism and consequently speeds up the reaction for hydrogen bonded dimers in general.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

Acknowledgments. The author acknowledges partial support from JSPS KAKENHI Grant Number 15K05371 and MEXT KAKENHI Grant Number 25108004.

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spectroscopy

of

ammonia

cluster

cations

(NH3)n

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(35) Cotton, S. J.; Miller, W. H., The symmetrical quasi-classical model for electronically non-adiabatic processes applied to energy transfer dynamics in site-exciton models of light-harvesting complexes, J. Chem. Theory Comput. 2016, 12, 983−991. (36) Sibert III, E. L.; Hynes, J. T.; Reinhardt, W. P., Classical dynamics of highly excited CH and CD overtones in benzene and perdeuterobenzene. J. Chem. Phys. 1984, 81, 1135-1144. (37) Lu, D.-h.; Hase, W. L. Classical trajectory calculation of the benzene overtone spectra. J. Phys. Chem. 1988, 92, 3217-3225. (38) Wyatt, R. E.; Iung, C.; Leforestier, C., Quantum dynamics of overtone relaxation in benzene. ii. 16 mode model for relaxation from CH(v=3)., J. Chem. Phys. 1992, 97, 3477-3486. (39) Stock, G., Phys. Rev. Lett. 2009,102, 118301. (40) Lu, D.-h.; Hase, W. L. Classical mechanics of intramolecular vibrational energy flow in benzene. v. effect of zero point energy motion. J. Chem. Phys. 1989, 91, 7490-7497.

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Table 2. Intermolecular distance (RNN in Å) of (NH3)2 in gas phase (n=0) and on the water clusters of neutral cluster (NH3)2(H2O)n (n=1-12), the binding energies (E(bind) in kcal/mol) of (NH3)2 to the water clusters, molecular charges on (NH3)d and (NH3)a of [(NH3)2(H2O)n]+ at the vertical ionization point from the neutral cluster, and the spin densities on (NH3)d and (NH3)a of [(NH3)2(H2O)n]+ at the vertical ionization point.

Charge

Spin density

n

RNN

E(bind)

(NH3)d

(NH3)a

(NH3)d

(NH3)a

0

3.261

----

0.964

0.036

0.996

0.004

1

3.134

11.8

0.912

0.055

0.958

0.003

2

3.114

16.9

0.564

0.371

0.610

0.357

3

3.105

13.9

0.872

0.046

0.938

0.009

4

3.154

12.3

0.875

0.046

0.936

0.009

5

3.113

13.1

0.871

0.043

0.935

0.008

6

3.152

12.0

0.882

0.040

0.938

0.008

8

3.155

13.7

0.882

0.057

0.922

0.010

12

3.141

14.1

0.889

0.060

0.919

0.009

Table 3. Time of PT in ammonia dimer cation in gas phase (n=0) and on water clusters (NH3)2+(H2O)n (n=1-6). The direct AIMD calculations were carried out at the MP2/6-311G(d,p) level.

a

n

Time of PT / fs

0

49.4 (50.3)a

1

32.9

2

27.6

3

29.5

4

30.4

5

27.6

6

29.7

Time of PT calculated at the MP2/6-311++G(d,p) level.

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Figure Captions

Figure 1. (Color online). Optimized structures and geometrical parameters of model system (ammonia dimer on ice surface, (NH3)2(H2O)m (m=48)) calculated at the ONIOM (MP2/6-311++G(d,p): PM3) level. Right panel shows an expanded view of structure around ammonia dimer. Bond lengths and angles are in Å and degrees, respectively.

Figure 2. (Color online). Snapshots of ammonia dimer radical cation on ice surface (NH3)2+(H2O)48 after vertical ionization from neutral state calculated as a function of time. Bond distances are in Å.

Figure 3. (Color online). Time evolutions of (A) bond distances, and (B) potential energy of the system after vertical ionization from neutral state calculated as a function of time.

Figure 4. (Color online). (A) Optimized structures of neutral ammonia dimer calculated at the MP2/6-311++G(d,p) level. (B) HOMO of (NH3)2, and (C) spin density on the (NH3)2+ at the vertical ionization point.

Figure 5. (Color online). Snapshots of ammonia dimer radical cation in gas phase after vertical ionization from neutral state of (NH3)2. Bond distances are in Å.

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Figure 6. (Color online). (A) Optimized structures of neutral ammonia dimer on water clusters, (NH3)2(H2O)n (n=1-4), calculated at the MP2/6-311++G(d,p) level.

Figure 7. (Color online). (A) Optimized structures of neutral ammonia dimer on water clusters, (NH3)2(H2O)n (n=5, 6, 8, and 12), calculated at the MP2/6-311++G(d,p) level.

Figure 8. (Color online). Snapshots of ammonia dimer radical cation on water cluster (n=6) after vertical ionization from neutral state of (NH3)2(H2O)6. Bond distances are in Å.

Figure 9. (Color online). Time of proton transfer (PT) in the ammonia dimer cation in gas phase, on water clusters and on ice surface. Real and dotted lines indicate the rates of PT of ammonia dimer cation in gas phase calculated as a function of RNN distance in (NH3)2 in gas phase.

Figure 10. (color online). Schematic illustration of potential energy curve of (NH3)2+ following the ionization of (NH3)2. Ip means ionization potential (in eV) of (NH3)2. The values indicate the relative energies (in kcal/mol) calculated at several levels of theory.

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Figure 6.

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Figure 7

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Figure 8

34



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Figure 9.

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Graphical Abstract

1


Reaction Dynamics Following Ionization of Ammonia Dimer

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