An ab initio quantum mechanical charge field molecular dynamics simulation of hydrogen peroxide in water

Computational and Theoretical Chemistry 980 (2012) 15–22

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An ab initio quantum mechanical charge field molecular dynamics simulation of hydrogen peroxide in water Syed Tarique Moin 1, Thomas S. Hofer, Bernhard R. Randolf, Bernd M. Rode ⇑ Theoretical Chemistry Division, Institute of General, Inorganic and Theoretical Chemistry, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria

a r t i c l e

i n f o

Article history: Received 27 May 2011 Received in revised form 29 September 2011 Accepted 2 November 2011 Available online 16 November 2011 Keywords: QMCF MD Stability of hydrogen peroxide Hydrogen bonds Radial distribution functions Mean residence times H-bond dynamics

a b s t r a c t An ab initio quantum mechanical charge field molecular dynamics (QMCF MD) simulation was performed on a single molecule of hydrogen peroxide immersed in water to investigate its stability in aqueous solution, since pure hydrogen peroxide is very unstable. The structural parameters such as radial distribution functions (RDFs), coordination number distributions (CNDs) and angular distribution functions (ADFs) indicate the existence of 4 hydrogen bonds between hydrogen peroxide and water molecules, with both molecules acting as hydrogen bond donors and hydrogen bond acceptors. The overall hydration shell consists of 6 water molecules surrounding the hydrogen peroxide molecule. The analysis of the hydrogen bond dynamics verified the presence of strong hydrogen bonds compared to pure water, thus confirming the stabilization of hydrogen peroxide in aqueous solution. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen peroxide (H2O2) is an ubiquitous molecule and wellknown for its bactericidal and oxidizing properties. It plays significant roles in a number of biological [1–4] and chemical processes [5,3,6] including atmospheric processes [7–9]. Pure H2O2 is unstable in acidic and basic solutions as well as in the presence of transition metals. Water–hydrogen peroxide system has gained much attention of experimentalists and theoreticians whereas only few data are available for pure H2O2 due to difficulties encountered in its experiment hardly. Various experimental and theoretical studies were reported on hydrogen peroxide–water clusters which describe their structures, energetics and thermodynamic properties [10–13]. H2O2 forms hydrogen bonds with organic molecules and halides whereas strong electrostatic interactions contribute in the complex formation between H2O2 and alkali cations [14]. Experimental and theoretical methods were applied on the gas phase chemistry of protonated hydrogen peroxide ([HOOH]H+) to study proton transfer in H2O2 [15,16]. The rotational or torsional barrier of hydrogen peroxide was investigated by applying a number of theoretical methods on hydrogen peroxide and hydrogen peroxide–water complexes [17–19]. The aqueous solution of H2O2 is an important condensed system which motivates to investigate

interactions between hydrogen peroxide and water molecules which are thought to be responsible for the stabilization of hydrogen peroxide in water. In the past various theoretical studies were reported on small H2O2–(H2O)n complexes in the gas phase [20] and in a dielectric continuum [21,19] which provided information about hydrogen bonding between H2O2 and H2O. The interaction of H2O2 with ice surface models [22], dissociation of H2O2 in supercritical water [23,24] and H2O2 adsorption at the air/water interface [25] were also investigated by using quantum chemical methods and classical simulations. A combined quantum/classical force field based molecular dynamics simulation was carried out to evaluate the properties of hydrogen peroxide in aqueous solution [26]. Physical and chemical properties of hydrogen peroxide in water can only be explained via investigation of solvent influences on the H2O2 molecule. The advance ab initio quantum mechanical charge field molecular dynamics (QMCF MD) simulation method [27–29] has already been successfully applied to address problems related to structure and dynamics of hydrogen bonded systems [31,30]. The purpose of this study was to explore the structural and dynamical behavior of hydrogen peroxide in aqueous solution as well as its stabilization in water, expected to be achieved by hydrogen bonds.

2. Methods ⇑ Corresponding author. Tel.: +43 512 507 5129; fax: +43 512 507 2714. E-mail address: [email protected] (B.M. Rode). Permanent address: Dr. Panjwani Center for Molecular Medicine and Drug Research, International Center for Chemical and Biological Sciences, University of Karachi, Pakistan.

2.1. Simulation method

1

2210-271X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.comptc.2011.11.006

An advance quantum mechanical/molecular mechanical hybrid molecular dynamics (QM/MM MD) scheme known as ab initio

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quantum mechanical charge field molecular dynamics (QMCF MD) [27–29], was developed to investigate the structure and dynamics of a variety of solutes in aqueous solutions [31,32]. The QMCF MD formalism does not require any non-Coulombic potential functions for solute–solvent interactions but only for the solvent–solvent interactions. The simulation box consists of a core zone and a layer zone, which constitute the high level region (QM region) containing an H2O2 molecule alongwith the full solvation layer while the remaining part of the system making the MM region consists of solvent particles only. The interactions of solute with solvent particles are treated by quantum mechanics automatically including all polarization, charge transfer and many-body effects during simulation whereas with the particles present beyond the QM region are treated by Coulombics. The interactions between particles within the solvation shell has an additional non-Coulombic contribution and the force field potentials (flexible BJH-CF2 water model [33,34]) were employed for solvents in the MM region for the energy and force evaluation. This approach needs solute atoms to be near the QM center and whenever solute moves towards the vicinity of QM/MM interface, non-Coulombic potential functions are needed which are typically supplied only for the solvation layer. The radius of the QM zone depends on the size of solute and the radius of about 5–6 Å is considered sufficient for the QM region as realized by the extended QM/MM MD simulations [35,36]. The increased size of the QM region allows to neglect the non-Coulombic contribution of the solute–solvent potential, thus eventually can be omitted. Another improved implementation to the QMCF formalism is related to the description of hydrogen bonds crossing the QM/MM interface. More details relevant to the methodical improvement can be found in references [27–29]. The ab initio Hartree–Fock (HF) level of theory with Dunning double n plus polarization (DZP) basis sets [37,38] were considered the best compromise between accuracy and computational cost [39,40]. Mulliken population analysis [41] was performed for particles in the QM region in every step of the simulation to derive partial atomic charges which are dynamically changed along the simulation. The integration of point charges assigned to atoms in the MM region that dynamically change their positions are done via a perturbation term into the core Hamilton operator:

V0 ¼

M X qJ r J¼1 iJ

ð1Þ

where M is the number of atoms in the MM region, qJ are the partial charges of atoms of the flexible BJH-CF2 [33,34] water. The intramolecular flexibility of the water model ensures smooth transitions of molecules between QM and MM region by applying a smoothing function S (r) [27]. The level of theory and basis sets must be validated prior to perform the ab initio QMCF MD simulation of a system. For this purpose, average binding energies were calculated from the energy minimization of gaseous hydrogen peroxide–water and hydrogen peroxide–water–water complexes with and without polarizable continuum model (PCM) at different levels of theory using DZP basis sets for O and H atoms (cf. Table 1). Binding energies obtained at the HF level are lower than the correlated methods such as MP2

and CCSD whereas values obtained using B3LYP functionals are larger in most cases that are also supported by the structural features evaluated for these complexes in particular too short H-bond distance, thus showing its rigidity (cf. Table 2). To include the electronic exchange correlation effects, hybrid density functional such as PBE0 was also employed for the binding energy calculations of the clusters. The PBE0 values are more deviating compared to CCSD methods and even than B3LYP functional (see Table 1). Fig. 1 shows the possible configuration of the complexes obtained from HF optimization calculations, though numerous configurations can exist besides these illustrated configurations. Application of PCM taking the solvent influence into account affects the structure of these hydrates involving H-bonds without much affecting the gas phase intramolecular features of H2O2 itself at each level of theory employed with above mentioned basis sets. The use of the HF method along with DZP basis sets was considered therefore, appropriate compared to DFT methods such as B3LYP functionals which yield too rigid geometries for the H-bonded systems [42– 44] as shown by short H-bond distances obtained for all these hydrates as well. The inclusion of electron correlations is expected to fine-tune the evaluation of structure and dynamics of this Hbonded system. However, the associated computational effort renders the application of these approaches unfeasible at the present moment. Attempts were also made to obtain an improved description of hydrogen bonding for liquids in condensed phase by using hybrid density functional such as PBE0 which include indeed electronic exchange effects as in HF and correlation effects at the DFT level. Molecular dynamics simulations of liquid water utilizing different hybrid density functionals were performed at the state point of 350 K to evaluate structure, dynamical and electronic properties of liquid water [45]. Analysis of hydrogen bonding indicated that hybrid density functionals give slightly smaller number of hydrogen bonds than at pure density functionals but similar H-bond population. Guidon et al. [46] also reported MD simulations of bulk liquid water using different density functionals such PBE, PBE0. Strictly speaking about the H-bonds, O–H stretching frequency of liquid water was calculated considering the involvement of OH in H-bond formation. The results would suggest a smaller population of strong hydrogen bonds in simulations based on hybrid density functionals. Another study involves molecular dynamics simulations employing both semilocal (PBE) and hybrid (PBE0) exchange and correlation functionals of heavy water D2O at different temperatures ranging from 370 to 470 K [47]. It was shown that the PBE0 functional gave an improved agreement of structural and vibrational properties of liquid water with experimental results than PBE and in general with semilocal functionals. However, several issues related to the description of both structural and vibrational properties of liquid remain to be addressed including the determination of the equilibrium density of water. The H-bond description of one system thus could be improved but could be varied for another system using same hybrid density functionals. 2.2. Simulation protocol The system for the QMCF MD simulation consisted of a preequilibrated cubic box of 999 water molecules containing a single

Table 1 Binding energies (kcal/mol) of gas phase hydrogen peroxide hydrates with and without PCM calculated at HF, MP2, CCSD, B3LYP and PBE0 levels of theory. Binding energies (kcal/mol) Hydrogen peroxide hydrates

(a) H2O2  H2O (b) O2H2  OH2 (c) H2O  H2O2  H2O

Gas phase

PCM

HF

MP2

CCSD

B3LYP

PBE0

HF

MP2

CCSD

B3LYP

PBE0

3.9 8.0 12.1

5.4 8.3 16.7

4.9 7.6 15.0

5.1 9.6 18.6

5.37 9.9 18.5

2.3 4.6 6.7

4.0 6.9 11.7

3.6 6.1 10.1

4.1 7.7 13.6

2.76 0.401 3.2

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Table 2 HF, MP2, CCSD and B3LYP optimized geometries of gas phase hydrogen peroxide mono- and dihydrate with and without PCM; distances and angles are given in Å and degree, respectively. H2O2 hydrates

Method

Geometrical features O–H

O–O

O–H

OH2O2  Hwat

HH2O2  Owat

OH2O2  Hwat–Owat

OH2O2–HH2O2  Owat

HF MP2 CCSD B3LYP

0.95 0.97 0.97 0.97

1.39 1.47 1.46 1.45

0.95 0.97 0.97 0.97

2.17 2.04 2.08 1.99

– – – –

166.3 167.2 166.2 168.5

– – – –

(b) O2H2  OH2

HF MP2 CCSD B3LYP

1.00 0.97 0.97 0.99

1.40 1.47 1.46 1.46

1.00 0.98 0.97 0.97

– – – –

1.65 1.88 1.92 1.86

– – – –

171.0 149.3 149.0 145.9

(c) H2O  H2O2  H2O

HF MP2 CCSD B3LYP

0.95 0.97 0.97 0.97

1.39 1.46 1.46 1.45

0.96 0.99 0.98 1.00

2.05 1.89 1.94 1.82

1.89 1.76 1.80 1.70

156.4 160.3 159.9 162.1

165.7 169.2 168.6 169.9

HF MP2 CCSD B3LYP

0.95 0.97 0.97 0.97

1.39 1.93 1.46 1.45

0.95 0.97 0.97 0.97

2.06 1.93 1.98 1.89

– – – –

177.9 175.0 175.5 179.1

– – – –

(b) O2H2  OH2

HF MP2 CCSD B3LYP

0.96 0.99 0.98 0.99

1.39 1.46 1.46 1.45

0.95 0.97 0.97 0.97

– – – –

1.86 1.76 1.79 1.72

– – – –

179.2 178.4 178.8 178.6

(c) H2O  H2O2  H2O

HF MP2 CCSD B3LYP

0.95 0.97 0.97 0.97

1.39 1.46 1.46 1.45

0.96 0.99 0.98 1.00

2.15 1.95 2.00 1.87

1.86 1.73 1.78 1.68

153.7 158.7 158.1 160.5

170.5 173.2 172.9 172.8

Gas phase (a) H2O2  H2O

PCM (a) H2O2  H2O

molecule of hydrogen peroxide whose initial configuration was obtained from the HF-optimized hydrogen peroxide-dihydrate structure. The density of the periodic simulation cube with a side length of 31.05 Å corresponds to the density of pure water determined at 298 K (0.997 g/cm3). The equations of motion were integrated utilizing the Adams–Bashforth predictor–corrector algorithm with a time step of 0.2 fs. The simulation was performed in the canonical NVT ensemble by weakly coupling [48] the system to a heat reservoir at 298 K employing a relaxation time of 0.2 ps. The cutoff for long-range Coulombic interactions was set to 15.0 Å and the reaction field method was applied to account for the influence of the solvent beyond the cutoff distances. Cutoff distances of 5.0 and 3.0 Å were employed for O–H and H–H non-Coulombic interactions between water molecules, respectively. The radii for the QM core and layer zone were set to 3.5 and 6.2 Å. The smoothing region between QM and MM region was applied in the range between 6.0 and 6.2 Å. The system was equilibrated for 10 ps and the production simulation was then carried out for 10 ps to sample trajectories. 3. Results and discussion 3.1. Structure

Fig. 1. HF optimized configurations of H2O2–(H2O)n (whereas n = 1, 2) with geometrical feature; distances and angles are given in Å and degrees.

The structural features of hydrogen peroxide (H2O2) were found to be influenced by solvent molecules which are shown by the intramolecular geometry of H2O2 in water obtained from the QMCF MD simulation. The results obtained from various experiments [49–51] show a good agreement between experimental studies and the QMCF MD simulation as summarized in Table 3. Fig. 2a and b illustrate the distribution of the O–O–H angle (h) and the H–O–O–H dihedral angle (/) in hydrogen peroxide and their respective maxima were found at 103.5° ± 18.5 and 95.3°, respectively. A distinct characteristic of H2O2 was the thermal variation

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Table 3 Structural features of hydrogen peroxide molecule in different crystals and water obtained from experiments and QMCF MD simulation, respectively. Methods/crystals

O–O distance (Å)

O–O–H angle (h)

H–O–O–H dihedral angle (/)

X-ray crystal [49] Neutron diffraction [50] (NH2)2H2O2 [51] QMCF MD simulation

1.49 1.45 1.46 1.40 ± 0.075

97° 103° 101.5° 103.5° ± 18.5

94° 9° 106° 95.3°

Fig. 2. (a) O–O–H angle (h) and (b) H–O–O–H dihedral angle (/) distribution of H2O2 in aqueous solution (inset: Fluctuations in the dihedral angle of hydrogen peroxide along the simulation).

in its dihedral angle (defined in the range of 0–360°) evaluated along the simulation (inset Fig. 2b). In the dihedral distribution two distinct peaks at 95.3° and 240° were observed which describe the possible existence of several transitions between two different conformers found in aqueous solution of H2O2 i.e. cisoid and transoid transition state (TS). The probability of finding ciscoid

TS was null (/ = 0°) but during the simulation time interconversion phenomena between these two conformers can not be ruled out in the aqueous solution (cf. Fig. 2b). The observed fluctuation in the dihedral angle indicates that the system restored its original conformational state. The reason behind non-equivalent peaks observed in the dihedral distribution was limited sampling time.

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S.T. Moin et al. / Computational and Theoretical Chemistry 980 (2012) 15–22

However, no lasting conformational change was observed within the simulation time, as following the transoid route the H2O2 returned to its initial conformational state after each transoid transition and this finding is consistent with a theoretical result evaluated by means of combined quantum/classical force field simulation [26]. These variations were attributed to the geometry of the solute and the configuration of water molecules forming the hydration layer which influences the structure and dynamics of H2O2 in aqueous solution. The hydration structure of hydrogen peroxide was elucidated by radial distribution functions (RDFs) between atomic pairs of H2O2 and water molecules. Fig. 3a illustrates the RDF of O-atoms

12.5

2.5

2

10

1.5

7.5

1

Integration

5

0.5

2.5

2

3

4

5

7

6

8

9

10

Distance (Å)

b

3

1.5 OA − Hwat OB − Hwat

2

0.5

1

g (r)

Integration

1

0

0 1

2

3

4

5

6

7

8

9

10

Distance (Å) 3

3 HA − Owat HB − Owat

2.5

g (r)

2

2

1.5

1.5

1

20

1.5

15

1

10

1

0.5

0

2 2.5

Integration

c

0.5

1

2

3

4

5

6

7

8

9

10

0.5

Integration

g (r)

OA − Owat OB − Owat

g (r)

a

of hydrogen peroxide with water oxygens (O–Owat RDF) which shows a broad peak between 2.4 and 3.7 Å with a mean distance of 3 Å. The O–Owat mean distance according to the peak maximum also indicates the existence of hydrogen bonds between solute and solvent molecules. Furthermore, O  Hwat and H  Owat RDFs were also plotted for each oxygen and hydrogen of H2O2 to evaluate the presence of hydrogen bonds between hydrogen peroxide and water molecules (cf. Fig. 3b and c). Fig. 3b displays the O  Hwat RDF showing a shoulder peak from 1.6 to 2.6 Å with a maximum at 2.1 Å and integration of the RDF up to the minimum yielded only one hydrogen bound to each oxygen atom of H2O2. Similarly, one hydrogen bond is formed between each hydrogen of H2O2 and water molecules deduced from the integration of a sharp peak visualized between 1.5 and 2.4 Å in the H  Owat RDF. The hydration layer around the whole H2O2 molecule was analyzed by the RDF plot between the centroid ðCH2 O2 ; using O–O bond) of hydrogen peroxide and O-atoms of water molecules (CH2 O2 —Owat RDF) shown in Fig. 4. This RDF plot shows a broad peak in the range between 2.5 and 4.1 Å. The respective integration yielded 6 water molecules which surround the H2O2 molecule. The coordination number distribution (CND) was evaluated for water molecules surrounding H2O2 using a shell boundary according to the CH2 O2 —Owat RDF (see Fig. 4). Fig. 5 presents the distribution of water molecules located in the hydration shell, which exhibits a variation from 4 to 10 water molecules bound to H2O2 with a maximum of 6. The number of H-bonds between hydrogen peroxide and water molecules were also assessed by the CND analysis performed between atoms of hydrogen peroxide and water molecules. Fig. 6a displays the distribution of water hydrogens making H-bonds with each O-atom of hydrogen peroxide. The distribution for the O  Hwat bond confirms that only one hydrogen atom is bonded with each oxygen of H2O2 and this observation also coincides with the maximal probability distribution for the number of H-bonds formed between water oxygens and H-atom of H2O2 (cf. Fig. 6b). Consequently two hydrogen bonds exist at each –OH group of hydrogen peroxide, thus accumulating up to a total of four H-bonds. Fig. 7 presents a snapshot of the hydration shell surrounding hydrogen peroxide showing both types of hydrogen bonding. The normalized angular distribution functions (ADFnorm) were calculated for O  Hwat–Owat and O–H  Owat angles formed between hydrogen peroxide and water molecules. The criteria used to define hydrogen bond distances for the ADF analysis were adopted from the minima shown in OH2 O2    Hwat and HH2 O2    Owat g(r) functions (see Fig. 3). Fig. 8a and b display the ADF plots with maxima at 180° showing the linearity characteristic

5

0

Distance (Å) Fig. 3. Radial distribution functions between (a) each oxygen of H2O2 and oxygens of water molecules, (b) each oxygen of H2O2 and hydrogens of water molecules and, (c) each hydrogen of H2O2 and oxygens of water molecules; OA/HA and OB/HB denote two oxygens and hydrogens of H2O2.

0

0 2

3

4

5

6

7

8

9

10

CEN - Owat (Å) Fig. 4. Radial distribution function of centroid ðCH2 O2 ) of the hydrogen peroxide with oxygen atoms of water molecules.

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S.T. Moin et al. / Computational and Theoretical Chemistry 980 (2012) 15–22 25

Percent probability

20

15

10

5

0 2

4

8

6

10

12

Coordination Number Fig. 5. Coordination number distribution of water ligands surrounding the hydrogen peroxide molecule using its centroid ðCH2 O2 Þ.

a

50

OA − Hwat OB − Hwat

40

20

Fig. 7. Snapshot from QMCF simulation of H2O2 in water, illustrating the hydration shell along with multiple hydrogen bonds (colour scheme: O = red, H = white; Donor and Acceptor H-bonds are highlighted in green and blue color, respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

10

3.2. Dynamics

Probability

30

0 0

1

2

3

4

Number of hydrogen bond

b

80

HA − Owat HB − Owat

Probability

60

40

20

0 0

1

2

Number of hydrogen bond Fig. 6. Coordination number distributions of water ligands forming hydrogen bond with (a) each oxygen atom and (b) each hydrogen atom of hydrogen peroxide; OA/ HA and O B/HB denote two oxygens and hydrogens of H2O2.

associated with the ideal hydrogen bond. The high probability distribution for O–H  Owat angle than O  Hwat–Owat angle indicates that the HH2 O2    Owat bond is stronger than OH2 O2    Hwat bond.

The fluctuation in Mulliken partial charges of hydrogen peroxide atoms were monitored throughout the simulation and is shown in Fig. 9. The partial charge on the oxygen atoms fluctuated between 0.32 and 0.5 with an average value of 0.41 whereas the average partial atomic charge on hydrogen was 0.40 ± 0.07. The charge plots indicate the symmetrical distribution of charges on H2O2 atoms which were expected because of the simultaneous formation of donor and acceptor types of hydrogen bonds. These data also indicate that no charge transfer effect between H2O2 and surrounding solvent molecules occurs. The fluctuation in the atomic charges of H2O2 atoms however, corresponds to its polarizability due to difference in the electronegativity of atoms, which in turn also polarizes the solvent molecules in proximity of the solute, eventually leading to electrostatic dipole–dipole interactions. Mean ligand residence times (MRT, s) were evaluated for water molecules involved in the hydrogen bond formation with H2O2 molecule as given in Table 4. The direct method [52] was utilized to determine the MRT values for exchange events persisting longer than 0.5 ps as well as for all exchange processes (t⁄ = 0.0 ps), represented as s0.5 and s0.0. The number of attempts required to obtain one lasting exchange event was calculated by the coefficient, Rex which is the ratio between the number of all transitions occurring through a shell boundary and   the number of changes persisting 0:5 longer than 0.5 ps N 0:0 ex =N ex . The MRT value for water ligands making multiple hydrogen bonds with the hydrogen peroxide molecule was calculated as 1.37 ps, which is slightly higher than the MRT value of pure water given as 1.30 ps [53] (cf. Table 4), which apparently indicates a stable hydration shell. The Rex value of 9.6 also supports in interpreting the strength of hydration shell which is further explored by the individual hydrogen bond dynamics leading to the stability of H2O2 in the aqueous solution.

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Table 4 Dynamics properties of water molecules in the hydration shell surrounding the hydrogen peroxide molecule and of pure water as obtained from QMCF MD simulations; N 0:5 ex = Number of accounted exchange events persisiting for 0.5 ps, N 0:0 ex = Number of all transitions through a shell boundary, s = Mean residence time, Rex = Number of border-crossing attempts needed to produce one longer lasting change in the hydration shell.

H2O2 hydration shell Pure water [53]

Fig. 8. Normalized angular distribution functions (ADFnorm) between atoms of hydrogen peroxide atoms and water molecules forming hydrogen bond; OA/HA and OB/HB denote two oxygens and hydrogens of H2O2.

-0.3

(a)

av. = − 0.41

-0.35

0:5 N ex =10 ps

N 0:0 ex =10 ps

s0.5 (ps)

s0.0 (ps)

Rex

27 20

280 131

1.37 1.30

0.14 0.192

9.6 6.5

distances of 2.5 Å corresponding to the bond distance for strong hydrogen bond and O  Hwat–Owat and O  H  Owat angles P120°. The MOLVISION programme [54] was used to determine the dynamics of H-bond dynamics based on the 10 ps trajectory. It was analyzed that each hydroxyl (–OH) group of hydrogen peroxide forms in average 1.9 hydrogen bonds with water molecules in their neighborhood. Therefore, the dynamics of these hydrogen bonds were further explored by plotting the number of hydrogen bond versus simulation time. Fig. 10a and b illustrate the time evolution of OH2 O2    Hwat and HH2 O2    Owat bond, respectively, at each hydroxyl group of H2O2. The average lifetime of these H-bonds equally resulted as 1.1 ps for each OH2 O2    Hwat and HH2 O2    Owat bond, which are considerably higher than the lifetime of hydrogen bonds between water molecules [53,55,56]. It can also be seen in Fig. 10 that the OH2 O2    Hwat bonds fluctuated more than HH2 O2    Owat bonds. The presence of both types of H-bonds correlates well with the infrared spectrum of hydrogen peroxide–water binary complex studied in argon matrix [10] which shows H-bond formation between H2O2 and water molecule with each acting as hydrogen bond donor and acceptor. Besides the existence of hydrogen bond in the binary complex of hydrogen peroxide and water molecules, the Raman spectra of H2O2 in condensed phase demonstrated the role of solvents in the hydration of H2O2 leading to extensive hydrogen bonding and an increased degree of order in the aqueous solutions of hydrogen peroxide [57]. The results obtained from the QMCF MD simulation verify the stability of hydrogen peroxide in aqueous solutions due to multiple hydrogen bond formation between H2O2 and water molecules. The values of hydrogen bond lifetime are even larger than water molecules itself, thus indicating the strong association of hydrogen peroxide and water molecules whereas slightly weaker hydrogen bonds

-0.45 -0.5

3 0

0.5

2

4

6

8

10

(b) av. = 0.40

0.45 0.4

Number of H-bond

Mulliken partial charge

-0.4

(a)

2 1 0 0

1

2

3

4

5

6

7

8

9

10

6

7

8

9

10

Time [ps]

0.35 2

4

6

8

10

Simulation time [ps] Fig. 9. Fluctuations in the mulliken charges of oxygen and hydrogen atoms of hydrogen peroxide along the simulation.

The obvious reason for the stabilization of hydrogen peroxide by water is the multiple hydrogen bonds between hydrogen peroxide and solvent molecules, each acting as hydrogen bond donor and acceptor (see Fig. 7). The criteria used to evaluate dynamics of these hydrogen bonds were maximal O  Hwat or H  Owat

2

Number of H-bond

0

(b)

1

0 0

1

2

3

4

5

Time [ps] Fig. 10. Number of hydrogen bonds between (a) each oxygen atom of H2O2 and water hydrogens (O  Hwat bond) and (b) each hydrogen atom of H2O2 and water oxygens (H  Owat bond).

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(12–15%) were reported for the pure and anhydrous H2O2 than those in water as examined by the Raman spectra [58]. Thus, weak hydrogens bonds could be responsible for instability of pure hydrogen peroxide and H-bonds therefore play a significant role for the stabilization of hydrogen peroxide in aqueous solution.

[17] [18] [19] [20] [21]

4. Conclusion

[23] [24] [25]

The structure and dynamics of hydrogen peroxide in water investigated via the ab initio QMCF MD simulation indicate the existence of strong hydrogen bonds in aqueous solution of hydrogen peroxide compared to that of pure H2O2 and pure water. Both H2O2 and water molecules act as hydrogen bond donors and acceptors, thus leading to a strong association of hydrogen peroxide with water molecules. These strong hydrogen bonds are therefore held responsible for the stabilization of hydrogen peroxide in aqueous solution, which is important for the biological and chemical processes including hydrogen peroxide in aqueous environment. The existence of multiple hydrogen bonds between H2O2 and water molecules also demonstrate a correlation with the reported experimental results over H-bonds present between solute and solvents. The results obtained from the simulation provide a good picture for the stability of hydrogen peroxide in aqueous solutions via Hbonds formation and to understand the role of solvents on the structure and dynamics of hydrogen peroxide. Acknowledgments

[22]

[26] [27] [28]

[29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]

Financial support for this work by the Austrian Science Foundation (FWF) and an Austrian Technology Grant (BMWF/RFTE) for Syed Tarique Moin are gratefully acknowledged.

[40]

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