Ab Initio Molecular Dynamics Study of the Reaction of U+

Article pubs.acs.org/JPCA

Ab Initio Molecular Dynamics Study of the Reaction of U+ and U2+ with H2O in the Gas Phase: Direct Classical Trajectory Calculations Peng Li,† Wenxia Niu,‡ Xiaofeng Tian,§ Tao Gao,*,† and Hongyan Wang∥ †

Institute of Atomic and Molecular Physics, Sichuan University, Chengdu, 610065, China College of Physical Science and Technology, Sichuan University, Chengdu 610065, China § College of Nuclear Technology and Automation Engineering, Chengdu University of Technology, Chengdu, 610059, China ∥ School of Physical Science and Technology, Southwest Jiaotong University, Chengdu, 610031, China ‡

S Supporting Information *

ABSTRACT: The gas phase reactions of U+ and U2+ with H2O were investigated using an ab initio molecular dynamics method. All of the information along the minimum energy path were calculated with density functional theory (DFT) and coupled cluster methods. For U+ with H2O, the molecular dynamics simulations yield a branching ratio of 86% for the H2 elimination channel to 14% for the H atomic elimination channel in agreement with the quadruple ion trap mass spectrometry (QIT/MS) experimental ratio of 91% to 9%. In the case of U2+ + H2O, there is a crossing of the potential energy surfaces (PES) after the first transition state. Crossing seams between the PES and possible spin inversion processes were studied by means of the intrinsic reaction coordinate (IRC) approach. For U2+ with H2O, all trajectories are corresponds to H atom elimination channel, this is consistent with the Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) experimental results. The chemical bonding evolution along the reaction pathways was discussed by using topological methodologies of the electron localization function (ELF).

■

the first one, in agreement with the earlier low-energy ion-beam experimental studies.11 In contrast, in FTICR-MS experiments, the formation of UOH+ was not observed.2 Jackson et al.10 have demonstrated that QIT-MS/FTICR-MS contrasting results are consequence of the different pressures used in the two techniques (10−6 Torr for FTICR-MS, 10−3 Torr in the case of QIT-MS experiments). FTICR-MS conditions results, therefore, in H atomic elimination channel in contrast to QIT-MS, which results in three-body processes. For the reaction of the U2+ with H2O, similar reaction products were observed at the QIT-MS the experiments, although reliable reaction rate constants could not be determined for this reaction because significant ion losses were observed during the course of the reactions. On the other hand, the FTICR-MS12 experimental data indicate that U2+ reacts with H2O to produce only UOH2+, it is possible to conclude that reaction 4 should be an exothermic process and that the reason for the lack of detection of reactivity is the slowness of the reaction.

INTRODUCTION During the last few decades, reactions of actinide cations with the ordinary molecules in the air have attracted considerable attention.1−9 Most of these studies have been mainly analyzed the distinctive electronic structures, the reactivity, and bonding of the actinide ions, which in the absence of perturbing factors correlate directly with their energetics. Particularly interesting is the analysis of the pivotal role of the 5f electrons in chemistry. As is well-known, the 5f electrons are relatively high in energy and spatially extended for the early actinides, resulting in high oxidation states and direct participation of the 5f electrons in bonding.3 The majority of the particular chemistry of the actinides can be owing either directly or indirectly to the changing character of the 5f electrons. Therefore, numerous research has been performed on the actinide cations reaction with small molecules both experimentally1−5 and theoretically.6−9 Experimentally (quadruple ion trap mass spectrometry, QIT/ MS),10 the following reaction products were detected during the reaction of U+ with H2O: +

+

U + H 2O → UO + H 2 +

+

U + H 2O → UOH + H

(1) (2)

(3)

U2 + + H 2O → UO2 + + H 2

(4)

Received: January 18, 2013 Revised: March 25, 2013 Published: April 30, 2013

The QIT-MS results indicate that both reactions are exothermic with a branching ratio of about 10:1 in favor of © 2013 American Chemical Society

U2 + + H 2O → UOH2 + + H

3761

dx.doi.org/10.1021/jp4006247 | J. Phys. Chem. A 2013, 117, 3761−3770

The Journal of Physical Chemistry A

Article

the TS toward the products in our calculations. The rotation sampling temperature was 300 K. A step size of 0.25 amu1/2 bohr was used for integrating the trajectories. The energy and angular momentum were conserved to better than 10−5 hartree and 10−9 ℏ, respectively.

The experimental investigation mentioned above unquestionably provides precious information on the gas phase reactivity of uranium cations with H2O. However, limitations still exist in terms of detailed information about the reaction mechanisms. As a consequence, theoretical studies capable of making reliable predictions of the properties of actinide reactions are of main importance. The reactions of U+ and U2+ with H2O were first investigated by density functional theory to study O−H activation by uranium ions, and the potential energy surfaces of U+ and U2+ with water have been mapped by several DFT methods.6 The main objective of this work is to give insight into the dynamics of the reaction of U+ and U2+ with H2O, with a special interest into the analysis of the topological features of all of the involved species and the description of the bonding evolution along the reaction pathways. The molecular dynamics of the reaction are investigated using direct classical trajectory calculations.

■

RESULTS AND DISCUSSION In both reaction paths, more than one spin state was taken into account. In the case of U+ with H2O, we have considered the importance of quartet, sextet, and doublet spin states. The quartet state 4I9/2 (the [Rn] 5f3 7s2 configuration) is the ground state (GS) of the U+ bare cation. The two low-lying sextet and

■

COMPUTATIONAL DETAILS On the basis of the previous studies of the reaction of U cations with water,6,10,12 three different approaches were used to optimize the structures of the minima and transition states on the potential energy surface (PES) for the current reaction system. First, the B3LYP,13,14 PW91PW91,15 TPSSTPSS,16 and BMK17 methods were performed, along with Stuttgart/Bonn relativistic effective core (SDD)18 for U atoms (32 valence electrons) and the 6-311++G(d,p) basis set for O and H atoms. These calculations were carried out using the Gaussian09 programs.19 For the reaction pathway analysis we have ensured that every transition structure has only one imaginary frequency and examined by performing IRC (Intrinsic Reaction Coordinate) calculations. Second, calculations were performed by using the relativistic two component zero-order regular approximation (ZORA) with both scalar relativistic (SR) together with the PW91 functionals and the type TZP basis set for U (frozen core, 12 valence electrons) and the TZ2P basis set for H2O (O 1s frozen) as implemented in the ADF package.20 Finally, coupled cluster CCD calculations with the SDD for U and the 6-311++G(d,p) basis set for O and H atoms were accomplished using the Gaussian09 programs. The Multiwfn21 package was used to analyze the bonding evolution along the reaction pathway. The description of the bond changing is based on the analysis of the electron localization function (ELF)22,23 which provides a description of the chemical bond for almost all classes of compounds. It exhibits maxima at the most probable positions of localized electron pairs, and each special position is surrounded by a basin in which there is an increased probability of finding an electron pair. These basins correspond to the qualitative electron pair domains of the VSEPR model.24,25 The larger the electron localization is in a region, the more likely the electron motion is confined within it. These studies have provided very useful pieces of information on the bonding evolution during reaction. The direct trajectory calculations were carried out at the PW91/SDD level, which was determined by compromise of computational costs and accuracy. We employed the Born− Oppenheimer molecular dynamics (BOMD)26 method implemented in the Gaussian09 package. This method uses a fifthorder polynomial fitted to the energy, gradient, and Hessian at each time step.27 The Hessians are updated for five steps before being recalculated analytically. Trajectories were started from

Figure 1. Structures and selected geometric parameters of stationary points on the UOH2+ potential energy surface optimized at the B3LYP/SDD, PW91/SDD, TPSS/SDD, BMK/SDD, and PW91/ ZORA-SR levels of theory (from top to bottom rows, respectively). Bond distances are in Å, and angles are in degrees. 3762



Article

Figure 2. Potential energy profile for the isomerization and dissociation of UOH2+ computed at the PW91/SDD levels.

Table 1. Relative Energies (kcal/mol) of the Stationary Points on the Quartet UH2O+ Potential Energy Surface at the PW91/ SDD Level a

B3LYP PW91a TPSSa BMKa PW91b CCDa PW91c d

1

TS1

2

TS2

3

UO++H2

UOH++H

HUO++H

36.939 38.384 53.274 40.675 45.147 36.746 44.079 4.081

54.623 51.343 60.019 63.968 53.416 76.759 52.321 3.882

0.000 0.000 0.000 0.000 0.000 0.000 0.000 3.756

28.775 18.794 27.645 9.713 16.142 28.199 18.635 3.758

5.063 −1.363 −0.830 −15.591 −5.710 −9.416 −0.961 3.758

53.552 21.211 56.653 −8.860 18.524 7.457 7.731

58.097 57.515 62.314 31.180 64.669 87.158 54.362

75.832 59.665 61.799 54.027 50.542 45.166

a

SDD for U and 6-311++G(d, p) for H and O atoms. bFrozen core TZP for U and TZ2P for H and O atoms, ZORA-SR results. cPW91/SDD ref (6). dExpectation value of S 2 for the quartet surface at the PW91/SDD level of theory.

doublet states are6L11/2 [Rn] 5f3 6d1 7s1 and 2I9/2 [Rn] (5f3 7s2), respectively.8,28 For U2+ + H2O we have studied quintet and triplet states. The quintet 5I4 (electronic configuration [Rn] 5f4) is the GS of the U2+ cation. The lowest-energy excited state is the triplet state 3H4 ([Rn] 5f2 7s2).8,28 A. U+ + H2O Reaction. Structures and Energetics. The optimized geometrical parameters of all of the involved stationary points at their lowest energy spin states are shown in Figure 1 for a number of levels of theory. Figure 2 demonstrates the relative energies of these stationary structures in the doublet, sextet, and quartet states at the PW91/SDD level of theory. Results of Michelini and co-workers6 indicated that, for the reaction of U+ with water, the reaction path evolves along the quartet surface from the formation of the initial U+− OH2 complex to the UOH+ + H and UO+ + H2 products at the B3LYP/SDD level of theory. Our calculations confirm that the quartet state stays as the ground state all along the reaction pathway (see Figure 2). Beyond the U+−OH2 complex, the quartet spin surface has very little spin contamination. Therefore, we will focus our study on the quartet spin surface for U+−OH2 isomerization and dissociation reaction pathway to the products. Relative energies for the stationary points along the paths have been calculated at several levels of theory, and the results are listed in Table 1. This range of calculations which contain pure GGA functional, meta-GGA, and couple-

cluster calculations together with relativistic effective core potentials (RECP) should give reliable results on geometries and energetic about the dissociation reaction of UOH2+. The studies by Michelini and co-workers6 have shown that + U −OH2, 1, is in a deep potential minimum below reactants. After rising to the first transition state (TS1), a hydrogen shift from the O atom to the U+ cation along with the O−H bond breaking, leading to a hydrido-metal-hydroxy intermediate HU(OH)+, 2. Then, the second hydrogen migration from oxygen to U+ yields the subsequent insertion intermediate. This reaction mechanism is similar with the reactions of Th+ and Th2+ with H2O.9 Our results indicate that the stable initial complex UOH2+, 1, is about 38.38 kcal/mol higher than HU(OH)+ and TS1 is 51.34 kcal/mol higher than HU(OH)+ at the PW91/SDD level, which was consistent with the Michelini’s calculation (about 44.08 and 52.32 kcal/mol higher than HU(OH)+, respectively). For the dehydrogenation channel calculated at the PW91/SDD level, a barrier of 18.79 kcal/mol needs to be overcome first to reach H2UO+, which is also consistent with the Michelini’s result of 18.63 kcal/mol. For H atom elimination channel, the exothermicity of this process is lower than that of the dehydrogenation reaction which agrees with the experimental conclusion that the formation of UO+ is thermodynamically favored.10,11 We have taken into account the possible formation of the dihydride 3763



Article

Figure 3. ELF shaded surface map with projection and contour line map of stationary points on the U+ with H2O reaction pathway, a shaded surface map with projection; b contour line map.

uranium oxo ion, the (H)2UO+ isomer; however, our calculations indicate that these isomers are higher energy that are not involved in the dehydrogenation paths. The possible formation of HUO+ isomer was also considered. This process may take place from the HUOH+ intermediate by cleavage of

the O−H bond. However, we cannot be located the transition state in this case, and the scan calculations performed varying the O−H bond length indicate that the oxygen−hydrogen bond breaking process is intrinsic transition barrierless. This is quite different from the formation of HThO+ in the Th+ + H2O 3764



Article

reaction pathways, using the analysis of the ELF function and the more traditional Mayer 2-center bond order approach. The ELF shaded surface with projection map and contour line map corresponding to the lowest-energy minima and transition states of pathways are shown in Figure 3. The ELF topological analysis of the complex U−OH2 indicates that there is no covalent bond formation between the fragments, as evidenced by the absence of a disynaptic valence basin between uranium and the water oxygen atom (see Figure 3). The U−O bond has a slightly lower population mostly due to a diminishing of the uranium contribution to that basin. Therefore, the interaction between U+ with H2O in the first complex corresponds physically to an electrostatic interaction. These results agree with the description of the bonding found by using the Mayer 2-center bond order analysis, which indicates only the presence of the O−H bonds of water. The ELF topological analysis of TS1 structure shows that in this case the first O−H bond breaking takes place at this stage of the reaction. This fact is evidenced by the weakened of the V(O,H1) basin (see Figure 3) which is replaced by a trisynaptic V(H1,U,O) basin, and also supported by the bond order analysis. The formation of the TS1 involves only small changes of the valence basin populations, which are in line with the small changes of the bond lengths. In the case of intermediate HU(OH)+, the ELF analysis shows the lack of V(O,H1) disynaptic valence basins, giving place to the formation of a disynaptic V(U,H1) basin. This is an indication of the broken of the first O−H1 bond at this stage and the formation of a U−H1 covalent bond. Meanwhile, the second O−H2 bond is forming a hydroxyl. In TS2, the hydroxyl is broken but the O−H2 bond is not yet completely broken, conversely, H1−H2 bond is beginning to form. This feature can be proven by the existence of the V(O,H2) basin with a very low electron population and a disynaptic V(H1,H2) basin with signs of growth. This is an indication of the strong weakening of this bond (3b of Figure 3). The ELF analysis of the H2UO+ shows the lack of two disynaptic valence basins, V(H1,U) and V(O,H2). This indicates that the U−H1 and O−H2 covalent bonds are completely broken at this step, giving place to the formation of a disynaptic V(H1,H2) basin, with an electron population very close to two electrons. It is evident that the H2 bond is already formed and that the structures can be considered as formed by two fragments, UO+ and H2. The ELF description of the H2UO+ is supported by the bond order analysis, which shows that the bond order of H1−H2 is comparable to the corresponding values for the free H 2 molecule. (The corresponding bond orders are available in the Supporting Information). Dynamics. The rate constants of two critical steps at room temperature, that is, 1→2 and 2→3, (denoted as Kij for reaction i→j) are computed with Rice−Ramsberger−Kassel− Marcus (RRKM) theory.30,31 The formation of the initial U − OH2+complex is quite exothermic, and a conservative estimate is that TS1 lies at least 20 kcal/mol below the reactants, U+ + H2O.6 Since an additional 51.34 kcal/mol is released in the formation of the intermediary product form TS1, using the vibrational data in Table S1 and an initial energy of 71.34 kcal/ mol, we computed the rate constant K12 = 4.2 × 1012 s−1; for critical step 2→3, using the same method, we calculated K23 = 9.4 × 1010 s−1. The rate constants predicted by RRKM indicate that the critical step 1→2 was occur quickly than the 2→3 one. This conclusion is consistent with the theoretical analysis. Because TS1 has two low frequency vibrations, while the TS2

Figure 4. Snapshots of typical trajectories from the MD simulations: (a) UOH2+ → UO+ + H2; (b) UOH2+ → UOH+ + H. (The corresponding movies are available in the Supporting Information.)

reaction. The formation of HThO+ could place from the HThOH+ intermediate by cleavage of the O−H bond, and it it involves an intrinsic transition barrier height of around 47.1 kal/mol at the PW91/SDD level.29 As is well-known, some DFT methods perform well in predicting energies, but the results can be sensitive to the functional.29 As can be seen from Table 1, compared with the CCD calculation, the values for other levels of theory are within the trusted range. For product 3, B3LYP/SDD provides higher energy than other methods. TPSS/SDD shows better agreement in the dissociation energies of products but gives a higher relative energy for U+−OH2 complex. When compared to CCD/SDD, BMK/SDD underestimates all of the relative energies except for complex 1. The PW91/SDD calculations are in good agreement with PW91/ZORA-SR and CCD/SDD results and are consistent with the Michelini’s6 results. Therefore, PW91/SDD has been chosen for the ELF study and the trajectory calculations. The geometries for the various stationary points in Figure 1 show relatively little dependence on the level of theory. For the minima, most bond lengths agree to within 0.1 Å, and angles agree to ± 0.2°, except for the ∠HUO bond angle in 3. The vibrational frequencies for the stationary points calculated at the PW91/SDD level are listed in Table S1(see the Supporting Information). Bonding Evolution Analysis. To have a deeper understanding of the reaction mechanisms, we have analyzed the bonding evolution of each of the species involved in the 3765



Article

Figure 5. Potential energy relative speed and distance between products corresponding time evolution: (a and b) UOH2+ → UO+ + H2; (c and d) UOH2+ → UOH+ + H.

quite effectively produces toward the formation of the dehydrogenation products. Two representative snapshots along the BOMD trajectory and the corresponding time evolution of the potential energy are presented in Figures 4 and 5. It is seen that the time to reach intermediary product 2 from TS1 is relatively short, but dissociation of 2 takes a longer time because it is a deep well on the potential energy surface, and longer times are needed for these trajectories to dissociate. For H2 molecular elimination channel, the intermediary product 2 is formed from TS1 in about 21 fs, and the product 3 is formed about 107 fs. When in case of H atom elimination channel, the time to reach product 3 from intermediary product 2 is relatively long (about from 24 to 230 fs). B. U2+ + H2O. Structures and Energetics. In the case of the reaction of U2+, we have studied two different spin states: quintet and triplet. The optimized geometrical parameters of the involved stationary points at their lowest energy spin states are shown in Figure 6. The geometrical parameters obtained at the different levels are very close, with the major deviation being less than 0.2 Å for distances and 5° for angles. The Potential energy profiles for isomerization and dissociation of U2+−OH2 complex are shown in Figure 7. In that figure we report the relative energies of all the species with respect to the HU(OH)2+ ground state.

did not have any (see Table S1), this will result in a much higher density of states at the available energy. With the vast additional energy and the small number of vibrational modes, the reaction may take place more rapidly.8 It is also confirmed from the following molecular dynamics calculations. To obtain a better description of the branching ratio, the molecular dynamics of the U+ + H2O reaction were simulated using ab initio classical trajectory calculations. As previously mentioned, the PW91/SDD/6-311++G(d,p) level of method is suitable and efficiently for the foregoing computation, so we still using this theory for simulating the molecular dynamics of UOH2+. About 200 trajectories obviously do not allow quite good quality statistical sampling, but it must be kept in mind that the computational cost of the direct trajectory calculations is high even at the PW91/SDD/6-311++G(d,p) level. In spite of this limitation, we believe that the present calculations give important information on the reaction dynamics. Because the U+ + H2O reactant is at least 20 kcal/mol above the TS1, the trajectories were started at the TS1 with 20 kcal/ mol extra energy distributed among the vibration modes. The H2 molecular elimination channel is dominant with a branching ratio of 86% compared to 14% for the H atom elimination channel which were in agreement with the experimental ratio of 91% to 9%.10 From the present BOMD simulation, it is therefore concluded that the U+ (4I9/2) + H2O (X̃ 1A1) reaction 3766



Article

spin state take place. The rest of the reaction evolves along the triplet spin surface. Therefore, the spin crossing occurs before the system has surmounted the transition state. Our theoretical calculation confirm that the thermodynamically favored isomerization and dissociation of UOH2+ correspond to the dehydrogenation process, and the products UOH2+ + H have been found to be higher in energy (see Figure 7). This is consistent with the result of Michelini et al.6 In Table 2 the relative energies (kcal/mol) of the stationary points on the triplet UOH22+ at all the studied levels of theory are gathered, and the corresponding vibrational frequencies are listed in Table S2 (see the Supporting Information). For the H atom elimination process, we have also considered the possible formation of HUO2+ by cleavage of the O−H bond from the HUOH2+ intermediate. We were unable to localize that transition state in the case of this reaction, and the scan calculations performed varying the H−O bond length indicate that the oxygen−hydrogen bond breaking process is intrinsic transition barrierless. This is similar with the formation process of the HThO2+ in the reaction for Th2+ + H2O.9 ELF analysis shows that the bonding evolution of U+ + H2O and U2+ + H2O reactions are very similar, with the only exception of HUOH2+ species, which is a linear and σ-like region along the U−O axis with the increase of ELF values of the U−O and U−H′ (the one beside the U atom). A detailed description is available as Supporting Information, which also offers the ELF shaded surface map with projection and contour line map in Figure S1. Potential Surface Topology and Crossing Seam. As discussed in the previous section, the inversion of spin multiplicity changed from the ground quintet state to the triplet state in the region after the TS1. Although several algorithms32−35 have been developed for the location of surface crossings, the potential energy of the system that consists of four atoms has more than six internal degrees of freedom, and this makes it difficult to perform a detailed inspection for a crossing point. Therefore, we chose an approach suggested by Yoshizawa, 36 Li and Zhang, 37 and Guo et al. 38 for

Figure 6. Structures and selected geometric parameters of stationary points on the UOH22+ potential energy surface optimized at the B3LYP/SDD, PW91/SDD, TPSS/SDD, BMK/SDD, and PW91/ ZORA-SR levels of theory (from top to bottom rows, respectively). Bond distances are in Å, and angles are in degrees.

As can be seen in Figure 7, the reaction starts at the quintet reactant ground state, and only after the system overcomes the first transition state does an intersystem crossing to the triplet

Figure 7. Potential energy profile for the isomerization and dissociation of UOH22+ computed at the PW91/SDD levels. 3767



Article

Table 2. Relative Energies (kcal/mol) of the Stationary Points on the Triplet UH2O2+ Potential Energy Surface a

B3LYP PW91a TPSSa BMKa PW91b PW91c CCDa e

1

TS1

2

TS2

3

UO2+ + H2

UOH2+ + H

HUO2+ + H

6.245 2.448 5.144 14.003 5.207 7.462 9.414 3.001

20.791 24.026 24.509 f 25.346 25.416 26.333 2.740

0.000 0.000 0.000 0.000 0.000 0.000 0.000 2.042

29.879 23.555 26.812 f 14.627 25.365 21.389 2.028

−6.907 −6.224 −7.948 −15.704 −16.823 −6.843 −25.097 2.017

8.202 9.128 6.102 −2.365 20.222 7.939

23.829 28.419 28.656 6.760 29.061 33.057

95.573 83.798 86.565 105.680 74.515 89.259

a

SDD for U and 6-311++G(d, p) for H and O atoms. bFrozen core TZP for U and TZ2P for H and O atoms, ZORA-SR results. cPW91/SDD, ref 6. Expectation value of S2 for the quartet surface at the PW91/SDD level of theory. fCannot be located.

e

Figure 8. Potential energies along the IRC (a) along the triplet IRC and (b) along the quintet IRC.

quintet and the triplet potential energy surfaces. This crossing seam is the most critical aspect in this reaction pathway because the molecular system should change its spin multiplicity from the quintet state to the triplet state near this crossing region, leading to a significant decrease in the barrier height of intermediate 2 from −34.31 to −54.43 kcal/mol at the PW91/ SDD level. We performed single point computations of the quintet state as a function of the structural change along the IRC of the triplet state. Figure 8a displays the computed potential energy profiles of the triplet and the quintet states, respectively, as a function of the structural change along the triplet IRC. As can be seen, crossing seam point CP1 is located at s = −0.43 (near TS1) with an energy of −552.737 hartree. The complex at this point has a C1 geometry, in which the U− O bond distance is 2.052 Å. According to the methodology of Yoshizawa and Li and co-workers, CP1 is the energy maximum crossing point between the triplet and the quintet potential energy surfaces. In Figure 8b, the solid and the dotted lines indicate computed potential energy profiles of the triplet and the quintet states, respectively, as a function of structural change along the quintet IRC. Another crossing point CP2 is found at s = −2.607 with an energy of −552.744 hartree. At point CP2 the complex has a Cs structure, in which the U−O bond distance is 1.961 Å. this is the energy-minimum crossing point. Therefore, there is a crossing seam between CP1 and CP2. The reaction system is most likely to change its spin multiplicity from the quintet to the triplet states in this crossing region. Dynamics. In the case of U2+ + H2O, we have tried to find a similar dynamics to that found for U+, in which the H2

Figure 9. Snapshots of typical trajectories from the MD simulations of HU(OH)2+ → UOH2+ + H. (The corresponding movies are available in the Supporting Information.)

approximately locating the crossing points of two PESs in quintet and triplet states. The main idea for this method is to perform a series of single-point computations of one spin state along the IRC of the other spin state and vice versa. Such an analysis will help locate energy crossing points. As shown in Figure 7, crossing seams exist after the TS1, between the 3768



Article

Figure 10. Potential energy relative speed and distance between products corresponding time evolution of HU(OH)2+ → UOH2+ + H.

■

molecular elimination channel and the H atom elimination channel are clearly simulated. However, all of the attempts have failed; this may because the formation of the transition state (TS1) provokes a crossing between the triplet and quintet spin surfaces. The spin of potential energy surface multiplicity must be specified in BOMD calculation. In order to avoid the crossing regions of impact of the molecular simulation, the trajectories were started at the intermediary product 2 with 20 kcal/mol extra energy. Our calculations indicate that all trajectories are corresponds to H atom elimination channel, that is, in our simulations the only reaction product is UOH2+. This is consistent with the FTICR-MS experimental results11 and does not correspond exactly with the QIT-MS studies10 in which UOH2+ and UO2+ both were detected as reaction products. Representative snapshots along the BOMD trajectory and the corresponding time evolution of the potential energy are presented in Figures 9 and 10. It can be seen that the time to reach product 3 from intermediary product 2 is about 143 fs.

■

ASSOCIATED CONTENT

S Supporting Information *

Vibrational frequencies and Mayer 2-center bond orders of the stationary points, movies of the trajectories in Figures 4 and 9, and the full citation for ref 19. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86 28 85405234. E-mail address: [email protected]. cn. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are very grateful to Miss Sobereva for many helpful discussions. Our thanks are also due to the reviewers for the valuable suggestions on improving our paper. Computer time made available by the Center of High Performance Computing at Physics discipline of Sichuan University is gratefully acknowledged.

CONCLUSION

In summary, the present structure calculations, the reaction path based on the intrinsic reaction coordinate, and ELF analyses as well as direct dynamics calculations provide much detail of the reactions. Crossing seams between the PES of U2+ case were studied by means of the IRC approach. The ELF analysis provides a clear description of the reactions between U+, U2+, and water. The processes of U+ and U2+ activating the O−H bond in the reactions were demonstrated. The stabilization of the initial complex does not imply the formation of a chemical bond between the U2+ and H2O, and the ELF values between U2+ and O is slower lower than the value found in the case of U+. The U+ (U2+) − OH2 complex is characterized by high ionic character between the terminal O and the U atom. Ab molecular dynamics simulations for the U+ + H2O couple produce a branching ratio of 86% for the H2 elimination channel to 14% for the H atomic elimination channel in agreement with the experimental ratio of 91% to 9%. In the case of U2++H2O, all trajectories are corresponds to H atom elimination channel, this is consistent with the FTICR-MS experimental results and does not correspond exactly with the QIT-MS studies in which UOH2+ and UO2+ were detected both reaction products.

■

REFERENCES

(1) Cornehl, H. H.; Heinemann, C.; Marc, alo, J.; Pires de Matos, A.; Schwarz, H. The “Bare” Uranyl(2+) Ion, UO22+. Angew. Chem., Int. Ed. Engl. 1996, 35, 891−894.. (2) Cornehl, H. H.; Wesendrup, R.; Diefenbach, M.; Schwarz, H. A Comparative Study of Oxo-Ligand Effects in the Gas-Phase Chemistry of Atomic Lanthanide and Actinide Cations. Chem.Eur. J. 1997, 3, 1083−1090. (3) Gibson, J. K. Gas-phase Chemistry of Actinide Ions: Probing the Distinctive Character of the 5f Elements. Int. J. Mass Spectrom. 2002, 214, 1−21. (4) Liang, B.; Hunt, R. D.; Kushto, G. P.; Andrews, L. Reactions of Laser-Ablated Uranium Atoms with H2O in Excess Argon: A Matrix Infrared and Relativistic DFT Investigation of Uranium Oxyhydrides. Inorg. Chem. 2005, 44, 2159−2168. (5) Gibson, J. K.; Marçalo, J. New Developments in Gas-Phase Actinide Ion Chemistry. Coord. Chem. Rev. 2006, 250, 776−783. (6) Michelini, M. C.; Russo, N.; Sicilia, E. How Can Uranium Ions (U+, U2+) Activate the O-H Bond of Water in the Gas Phase? Angew. Chem., Int. Ed. 2006, 45, 1095−1099. (7) Michelini, M. C.; Russo, N.; Sicilia, E. Gas-Phase Chemistry of Actinides Ions: New Insights into the Reaction of UO+ and UO2+ with Water. J. Am. Chem. Soc. 2007, 129, 4229−4239. 3769



Article

(30) Steinfeld, J. I.; Francisco, J. S.; Hase, W. L. Chemical Kinetics and Dynamics; Prentice-Hall: Upper Saddle River, NJ, 1999; p 231. (31) Marcus, R. A.; Rice, O. K. The Kinetics of the Recombination of Methyl Radicals and Iodine Atoms. J. Phys. Chem. 1951, 55, 894−908. (32) Koga, N.; Morokuma, K. Determination of the Lowest Energy Point on the Crossing Seam Between Two Potential Surfaces Using the Energy Gradient. Chem. Phys. Lett. 1985, 119, 371−374. (33) Yarkony, D. R. Systematic Determination of Intersections of Potential Energy Surfaces Using a Lagrange Multiplier Constrained Procedure. J. Phys. Chem. 1993, 97, 4407−4412. (34) Bearpark, M. J.; Robb, M. A.; Schlegel, H. B. A Direct Method for the Location of the Lowest Energy Point on a Potential Surface Crossing. Chem. Phys. Lett. 1994, 223, 269−274. (35) Harvey, J. N.; Aschi, M.; Schwarz, H.; Koch, W. The Singlet and Triplet States of Phenyl Cation. A Hybrid Approach for Locating Minimum Energy Crossing Points B Between Non-interacting Potential Energy Surfaces. Theor. Chem. Acc. 1998, 99, 95−99. (36) (a) Yoshizawa, K.; Shiota, Y.; Yamabe, T. Intrinsic Reaction Coordinate Analysis of the Conversion of Methane to Methanol by An Iron−Oxo Species: A Study of Crossing Seams of Potential Energy Surfaces. J. Chem. Phys. 1999, 111, 538−545. (b) Yoshizawa, K.; Kagawa, Y. Reaction Pathways for the Oxidation of Methanol to Formaldehyde by an Iron−Oxo Species. J. Phys. Chem. A 2000, 104, 9347−9355. (37) Zhang, G. B.; Li, S. H. Dehydrogenation of Methane by GasPhase Os+: A Density Functional Study. Organometallics 2003, 22, 3820−3830. (38) Guo, Z.; Ke, Z. F.; Phillips, D. L.; Zhao, C. Y. Intrinsic Reaction Coordinate Analysis of the Activation of CH4 by Molybdenum Atoms: A Density Functional Theory Study of the Crossing Seams of the Potential Energy Surfaces. Organometallics 2008, 27, 181−188.

(8) Alikhani, M. E.; Michelini, M. C.; Russo, N.; Silvi, B. Topological Analysis of the Reaction of Uranium Ions (U+, U2+) with N2O in the Gas Phase. J. Phys. Chem. A 2008, 112, 12966−12974. (9) Mazzone, G.; Michelini, M. C.; Russo, N.; Sicilia, E. Mechanistic Aspects of the Reaction of Th+ and Th2+ with Water in the Gas Phase. Inorg. Chem. 2008, 47, 2083−2088. (10) Jackson, G. P.; King, F. L.; Goeringer, D. E.; Duckworth, D. C. Gas-Phase Reactions of U+ and U2+ with O2 and H2O in a Quadrupole Ion Trap. J. Phys. Chem. A 2002, 106, 7788−7794. (11) Armentrout, P. B.; Beauchamp, J. L. Reactions of U+ and UO+ with O2, CO, CO2, COS, CS2 and D2O. Chem. Phys. 1980, 50, 27−36. (12) Gibson, J. K.; Haire, R. G.; Santos, M.; MarHalo, J.; Pires de Matos, A. Oxidation Studies of Dipositive Actinide Ions, An2+ (An = Th, U, Np, Pu, Am) in the Gas Phase: Synthesis and Characterization of the Isolated Uranyl, Neptunyl, and Plutonyl Ions UO22+(g), NpO22+(g), and PuO22+(g). J. Phys. Chem. A 2005, 109, 2768−2781. (13) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (14) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (15) Perdew, J. P.; Burke, K.; Wang, Y. Generalized Gradient Approximation for the Exchange-Correlation Hole of a Many-Electron System. Phys. Rev. B 1996, 54, 16533−16539. (16) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Climbing the Density Functional Ladder: Nonempirical Meta−Generalized Gradient Approximation Designed for Molecules and Solids. Phys. Rev. Lett. 2003, 91, 146401. (17) Boese, A. D.; Jan, M. L. M. Development of Density Functionals for Thermochemical Kinetics. J. Chem. Phys. 2004, 121, 3405−3416. (18) Kuchle, W.; Dolg, M.; Stoll, H.; Preuss, H. Energy-Adjusted Pseudopotentials for the Actinides. Parameter Sets and Test Calculations for Thorium and Thorium Monoxide. J. Chem. Phys. 1994, 100, 7535−7542. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (20) Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. Chemistry with ADF. J. Comput. Chem. 2001, 22, 931−967. (21) Lu, T.; Chen, F. Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580−592. (22) Becke, A. D.; Edgecombe, K. E. A Simple Measure of Electron Localization in Atomic and Molecular Systems. J. Chem. Phys. 1990, 92, 5397−5403. (23) Savin, A.; Nesper, R.; Wengert, S.; Fassler, T. R. ELF: The Electron Localization Function. Angew. Chem., Int. Ed. Engl. 1997, 36, 1808−1832. (24) Gillespie, R. J.; Robinson, E. A.; Gilbert, N. Lewis and the Chemical Bond: The Electron Pair and the Octet Rule from 1916 to the Present Day. J. Comput. Chem. 2007, 28, 87−97. (25) Lu, T.; Chen, F. Meaning and Functional Form of the Electron Localization Function. Acta Phys.-Chim. Sin. 2011, 27, 2786−2792. (26) Bakken, V.; Millam, J. M.; Schlegel, H. B. Ab Initio Classical Trajectories on the Born−Oppenheimer Surface: Updating Methods for Hessian-Based Integrators. J. Chem. Phys. 1999, 111, 8773−8777. (27) Millam, J. M.; Bakken, V.; Chen, W.; Hase, W. L.; Schlegel, H. B. Ab Initio Classical Trajectories on the Born−Oppenheimer Surface: Hessian-Based Integrators Using Fifth-Order Polynomial and Rational Function Fits. J. Chem. Phys. 1999, 111, 3800−3805. (28) Blaise, J.; Wyart, J.-F. International Tables of Selected Constants. Energy Levels and Atomic Spectra of Actinides, Vol. 20, Tables of Constants and Numerical Data, Paris, 1992, taken from http://www.lac.u-psud.fr/Database/Contents.html. (29) Zhou, J.; Schlegel, H. B. Ab Initio Molecular Dynamics Study of the Reaction between Th+ and H2O. J. Phys. Chem. A 2010, 114, 8613−8617. 3770


Ab Initio Molecular Dynamics Study of the Reaction of U+

Recommend Documents