Investigation of the preferential solvation and dynamical properties of Cu+ in 18.6% aqueous ammonia solution using ab initio quantum mechanical charge field (QMCF) molecular dynamics and NBO analysis

Accepted Manuscript Investigation of the preferential solvation and dynamical properties of Cu+ in 18.6% aqueous ammonia solution using ab initio quantum mechanical charge field (QMCF) molecular dynamics study and NBO analysis

Wahyu Dita Saputri, Yuniawan Hidayat, Karna Wijaya, Harno Dwi Pranowo, Thomas S. Hofer PII: DOI: Reference:

S0167-7322(18)34394-0 https://doi.org/10.1016/j.molliq.2018.11.022 MOLLIQ 9931

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

25 August 2018 30 October 2018 2 November 2018

Please cite this article as: Wahyu Dita Saputri, Yuniawan Hidayat, Karna Wijaya, Harno Dwi Pranowo, Thomas S. Hofer , Investigation of the preferential solvation and dynamical properties of Cu+ in 18.6% aqueous ammonia solution using ab initio quantum mechanical charge field (QMCF) molecular dynamics study and NBO analysis. Molliq (2018), https://doi.org/10.1016/j.molliq.2018.11.022

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ACCEPTED MANUSCRIPT 1

Investigation of the Preferential Solvation and Dynamical Properties of Cu+ in 18.6% Aqueous Ammonia Solution using Ab Initio Quantum Mechanical Charge Field (QMCF) Molecular Dynamics Study and NBO analysis Wahyu Dita Saputri, ab Yuniawan Hidayat, ac Karna Wijaya,a Harno Dwi Pranowo*ab, Thomas S. Hoferǂd Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Gadjah Mada, Sekip Utara, Yogyakarta 55281, Indonesia b. Austrian-Indonesian Centre (AIC) for Computational Chemistry, Universitas Gadjah Mada, Sekip Utara, Yogyakarta 55281, Indonesia. c. Department of Chemistry, Faculty of Mathematics and Natural Sciences, Sebelas Maret University, Surakarta 5712612, Indonesia. d. Theoretical Chemistry Division, Institute of General, Inorganic and Theoretical Chemistry, University of Innsbruck, Innrain 80-82, A-6020 Innsbruck, Austria E-mail: [email protected] (co-corresponding author) *Corresponding author E-mail: [email protected] ǂ Co-Corresponding author E-mail: [email protected]

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a.

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ABSTRACT:

Structural and dynamical properties of Cu+ in 18.6% aqueous ammonia solution at 298.15 K have been investigated via quantum mechanical charge field molecular dynamics (QMCF MD). The QM region was set to a radius of 6.7 Å to include the first and second solvation shell. The Hartree-Fock (HF) level was applied to calculate ion-ligand and ligand-ligand interactions in the QM

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region using the LANL2DZ-ECP basis set for the ion and DZP-Dunning for the ligands. The Cu+-N and Cu+-O radial distribution functions showed maximum first shell probabilities at distances of 2.23 and 2.30 Å, respectively. Predominantly, four NH 3 molecules were found to form a tetrahedral [Cu(NH3)4]+ complex, whereas the formation of a short-lived intermediate

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[Cu(NH3)3H2O]+ complex was also observed. The mean residence times of NH 3 and H2O ligands in the first solvation shell were estimated as 14.6 ps and 1.3 ps, respectively, reflecting the strong interaction between Cu + and ammonia as well as the occurrence of rapid water exchange. The vibrational power spectrum of the Cu +-N vibration in the first solvation shell revealed a wave number

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of 252 cm-1 with corresponding force constant of 43.0 Nm-1. In addition an NBO analysis was carried out, confirming the strong electrostatic character of the Cu+-NH3 and Cu+-H2O interaction, and highlighting that the presence of H2O ligands can destabilize

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the first solvation shell.

Keywords: Cu+, ammonia, preferential, solvation, QMCF, NBO

1. Introduction Ion solvation is a phenomenon dominated by the electrostatic interaction of the central ion with surrounding ligands in solution. Experimental studies of ion solvation typically rely on spectroscopy or/and diffraction method[1,2]. In addition to these experimental approaches, ion solvation is also investigated on the microscopic level employing theoretical methods[3]. In particular molecular simulations in conjunction with quantum chemical approaches have become a reliable method to investigate the properties of ionic compounds in solutions [4–14].

ACCEPTED MANUSCRIPT 2 Transition metal ions, especially copper, are essential for a broad range of catalytic reactions and redox processes[15–17]. A variety of solvent molecules such as NH 3 and H2O, can directly interact with Cu + ions, and the formation of [Cu(NH 3)n]+ (n=3-8) clusters have been studied via photodissociation spectroscopy[18], indicating that the complexes [Cu(NH 3)2]+ and [Cu(NH 3)3]+ are formed in aqueous ammonia at high concentrations. The Cu⁺-ammonia interaction is also relevant in view of the toxicity of copper in biological systems and various mechanisms transferring the ion to Cu +-binding proteins[19–21] have been identified. Since copper is widespread in nature as a compound in the form of sulfides, arsenides, chlorides, and weakness of these

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carbonates, it can be directly extracted from the respective minerals. A particular

extraction techniques is linked to the amount of ammonia that is often added beyond the ideal threshold. In this case, unwanted organic species may form, influencing the extraction process in a negative way[22]. Despite the fact that several experimental studies, such as X-ray diffraction[2,23], UV-Vis spectroscopy[24], and X-ray absorption fine structure spectroscopy(EXAFS)[25] have been applied to study the properties of Cu+ in solution, its tendency to undergo spontaneous disproportionation makes experimental approaches to study structural and dynamical properties highly challenging.

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Theoretical methods on the other hand provide the possibility to access much shorter time frames, however, no simulation studies focusing on the preferential solvation of Cu + in aqueous ammonia have been carried out in the past.

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A particularly successful strategy to study ions in solution are QM/MM-based simulation approaches since they describe the critical interactions (in this case the ion and its immediate surroundings) via electronic structure theory[8,26–30]. However, if only the first solvation shell is included in the QM-zone, ion-ligand

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non-Coulombic potentials describing the interaction between the ion and solvent in the MM -region have to be provided. In many cases, the preparation of non-Coulombic potential function is a tedious and difficult procedure, and in many cases, the achievable of accuracy is limited [31]. For this reason, an extension of

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the QM-region to include both the first and the second solvation shell as realized in the Quantum Mechanical Charge Field Molecular Dynamics (QMCF MD) framework is required. In the QMCF MD approach, the radius of the QM region is adequately large and the solute molecules are at the center of the QM region, in order

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to only the Coulombic interactions between the solute and the solvent molecules in the MM zone need to be evaluated. The point charges of all MM particles are counted in the core Hamiltonian as a perturbation

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potential to achieve a more realistic description of the effect of surrounding solvent molecules on the QM region. Similar as in QM/MM methods, the QMCF approach partitions the system into a high- and low-level zone. However, since the ion-solvent interactions beyond the second shell are effectively only dependent on Coulombic contributions, the tedious construction of Cu +-H2O and Cu+-NH3 potentials can be avoided. In addition, the increase of the accuracy of this approach was also achieved by formulating an improved electrostatic embedding technique compatible with the periodic boundary condition. The QMCF MD strategy has proven to be a versatile approach for the study of ion solvation[10,11,32–38] and has been successfully applied to study the preferential solvation of various systems in water-ammonia mixtures[39,40].

The natural bond orbital (NBO) methodology is an analysis based on the quantum mechanical wave function and its practical evaluation by modern computational methods[41]. The key feature of the NBO

ACCEPTED MANUSCRIPT 3 framework is to define the atomic orbitals in the molecule environment based on the one-electron density matrix enabling to derive molecular bonding information based on the electron density. NBO also provides information about orbital interactions, bond formation, and electronic donor-acceptor properties of the involved molecules[42–45]. The numerical value correlated to a number of electron pairs localized in a bond between two atoms, the Wiberg bond index (WBI), is one of the results obtained from an NBO analysis[46]. The dominating contribution to the ion-ligand interaction in solvated systems involves the delocalization of the ionic charge,

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which is influenced by the number of coordinating ligands and the respective coordination geometry[47,48]. In this study, a combination of the QMCF molecular dynamics simulation framework and an NBO analysis are employed to investigate the preferential solvation and the associated dynamical properties of Cu ⁺ in 18.6% aqueous ammonia solution. The composition of the first solvation shell has been analysed via radial distribution functions (RDFs), coordination number distributions (CNDs), and angular distribution function (ADFs). The dynamical properties have been evaluated based on the vibrational power spectrum obtained via Fourier Transform of the associated velocity autocorrelation function (VACFs) as well as the calculation

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of mean ligand residence time (MRT) for NH 3 and H2O ligands. Together, these analyses provide a detailed description of the coordination complex, while the respective donor-acceptor stabilisation was characterized

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using NBO analysis.

2. Methods

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The simulation conducted in this work follows the QMCF MD framework. [32,49] In this method the atomic partial charges of the MM atoms are incorporated into the quantum mechanical calculation via an additional potential in the QM Hamiltonian, achieving an electrostatic embedding of the QM zone, improving the overall

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description of the chemical system[50–53]

After the execution of the QM calculation, the QM region is further separated into two sub-zones, namely

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the core-region and the solvation layer [54,55]. Due to the increased distance between atoms in the QMcore region and the MM zone, the non-Coulombic contributions are typically beyond their cutoff distance and can be discarded. Atoms in the solvation layer, on the other hand, are in close vicinity to the MM region, and

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hence, both Coulomb and non-Coulomb contributions have to be considered. Since the solvation layer is only occupied by solvent molecules, the non-Coulomb potential of the solvent model is applied, thus leading to no further complication in the setup of the simulation The forces in the respective zones in the QMCF-MD approach are then evaluated as follows[56]

𝑀

𝐹𝐽𝑐𝑜𝑟𝑒

=

𝐹𝐽𝑄𝑀

+ ∑ 𝐼=1

𝑞𝐼𝑀𝑀 + 𝑞𝐽𝑄𝑀 𝑟𝐼𝐽2

[1 + 2

𝜀 + 1 𝑟𝐼𝐽 3 ( ) ] 2𝜀 − 1 𝑟𝑐

(1)

ACCEPTED MANUSCRIPT 4 𝑀 𝑙𝑎𝑦𝑒𝑟 𝐹𝐽

=

𝐹𝐽𝑄𝑀

+ ∑

𝑞𝐽𝑄𝑀 . 𝑞𝐼𝑀𝑀 𝑟𝐼𝐽2

𝐼=1

(2)

𝐼=1

𝑀𝑀 𝑄𝑀 𝑞𝐽 2 𝑟𝐼𝐽

𝑁 +𝑁2 𝑞𝐼

𝑀𝑀 1 𝐹𝐽𝑀𝑀 = ∑𝑀 𝐼=1;𝐼≠𝐽 𝐹𝐼𝐽 + ∑𝐼=1

𝑀

𝜀 + 1 𝑟𝐼𝐽 3 [1 + 2 ( ) ] + ∑ 𝐹𝐼𝐽𝑛𝐶 2𝜀 − 1 𝑟𝑐

[1 + 2

𝜀+1 2𝜀−1

𝑟

3

𝑁

2 ( 𝐼𝐽) ] + ∑𝐼=1 𝐹𝐼𝐽𝑛𝐶

(3)

𝑟𝑐

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The forces acting on particle J in the core region (𝐹𝐽𝑐𝑜𝑟𝑒 ) are composed of the respective quantum mechanical force plus the Coulombic interactions between particle J and all atoms in the MM region based on their respective charge. This expression includes the Baker-Watts reaction field (RF) approach[57] to account for the truncation of long-ranged Coulombic, with with ε being the associated permittivity of the 𝑙𝑎𝑦𝑒𝑟

solvent medium. The force acting on a particle in the layer region 𝐹𝐽

corresponds to the quantum

mechanical force, and all Coulombic (plus RF contribution) and non-Coulombic contributions obtained from the interaction with the MM atoms[58]. Since only solvent molecules are present in the layer-region, the non-

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Coulombic interaction potential is readily available. Thus, by increasing the QM zone to encompass two layers of solvation and separating the QM region into two further sub-zones, the application of an ion-solvent non-Coulombic potentials is not required, greatly enhancing the applicability of this general simulation

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𝑀𝑀 approach[55]. The forces for particles located in the MM region 𝐹𝐼𝐽 are obtained by summing all interactions

between the MM particles enhanced by the Coulombic forces exterted by all atoms in the QM core and QM layer region, as well as the non-Coulombic forces exterted by all atoms in the solvation layer.[35]

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To achieve smooth transitions of molecules between the solvation layer and the MM zone, a continuous switching function 𝑆(𝑟) is applied in a buffer region of 0.2 Å between the QM and MM zones [58] : − 𝐹𝐽𝑀𝑀 ) + 𝐹𝐽𝑀𝑀

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𝑙𝑎𝑦𝑒𝑟

𝐹𝐽𝑆𝑚𝑜𝑜𝑡ℎ = 𝑆(𝑟) (𝐹𝐽

(4)

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The structural properties of the Cu +-solvation complex are studied via different radial and angular distribution functions. To investigate the exchange dynamics of water and ammonia ligands of an individual solvation shell, the associated mean residence times (MRT) is evaluated, counting the number of successful

𝜏 0.5 =

𝑡𝑠𝑖𝑚 𝑁𝑎𝑣 0.5 𝑁𝑒𝑥

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exchange events via the “direct method” [59]:

(5)

with 𝑡𝑠𝑖𝑚 representing the simulation time, 𝑁𝑎𝑣 is the average number of ligands in the particular shell and 0.5 𝑁𝑒𝑥 is the number of exchanges with a minimum displacement time of t*= 0.5 ps.

The stretching frequency of the Cu–Ligand vibrational mode has been evaluated via Fourier transform of the respective velocity autocorrelation functions (VACFs) C(t).

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𝐶(𝑡) =

𝑁 ∑𝐼 𝑡 ∑𝑁 𝐽 𝑣𝐽 (𝑡𝐼 )𝑣𝐽 (𝑡𝐼 +𝑡)

(7)

𝑁 ∑𝐼 𝑡 ∑𝑁 𝐽 𝑣𝐽 (𝑡𝐼 )𝑣𝐽 (𝑡𝐼 )

with 𝑁 being the number of particles, 𝑁𝑡 corresponds to the number of time origins 𝑡𝐼 , and 𝑣𝐽 denotes a given velocity component of particle J (Eq 7). The force constant k is then evaluated from the associated wavenumber ῡ as 𝑘 = 4𝜋 2 µ(ῡ𝑐)2

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(8)

where µ and 𝑐 are reduced mass and the speed of light, respectively.

3. Simulation protocol

The QMCF-MD simulation is performed in the canonical NVT ensemble under periodic boundary conditions and minimum image convention. A cubic box with side length of 31.76 Å was used, containing

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one Cu+ ion, 184 ammonia, and 815 water molecules. This system corresponds to an 18.6% aqueous ammonia solution with an experimental density of 0.927 g/cm 3. The temperature was kept constant at 298.15 K using the Berendsen weak-coupling algorithm[60] with a relaxation time of 0.1 ps. For the description of

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the MM molecules, the flexible ammonia[61] and BJH−CF2 water[62] models were employed, each consisting of intra- and intermolecular contributions. To integrate the equations of motions a second order Adams−Bashforth predictor−corrector algorithm was employed, the respective time step was set to 0.2 fs.

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The radii of the QM-core zone was set to 3.5 Å, the layer zone ranged from 3.5 to 6.7 Å including smoothing zone of 0.2 Å. The Barker-Watts reaction field [57] was employed to account for long-range electrostatic contributions beyond the Coulombic cutoff set to 12 Å.

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The QM region was treated via ab initio Hartree–Fock (HF) which proved as a good compromise between computational cost and accuracy in previous studies[10,32,63,64].

The DZP Dunning basis

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sets[65] were assigned to all atoms of ammonia and water, in case of Cu + the relativistically corrected LANL2DZ ECP[66] basis was applied. Partial charges of any species in QM region were derived via Mulliken population analysis, which has been re-evaluated in each MD step[67]. The starting configuration was taken

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from a classical MD simulation using published pair potentials for the Cu–O and Cu-N interaction[68]. This system was equilibrated for several hundred picoseconds including heating and cooling conditions via classical MD, followed by a re-equilibration at QMCF level for 20 000 MD steps (4ps). Sampling was performed for 115 000 MD steps collecting data every fifth step, resulting in a total simulation time of 23 ps. All QM calculations were executed using TURBOMOLE 5.9[69], the visualization of trajectory was done with the VMD package.[70] The NBO analysis was carried out using Gaussian 09[71], employing the respective geometries that have been extracted from the QMCF MD simulation trajectory. The stable cluster coordinate in a specific frame time was taken from this simulation trajectories, and then this coordinate was lead to the NBO analysis. The obtained of Cu(I)-ligand coordinate was geometrically optimized at the level HF of theory using LANL2DZ ECP basis set for Cu(I) and DZP (Dunning) for solvent molecules. The Mulliken population

ACCEPTED MANUSCRIPT 6 analysis with NBO was employed, and the Wiberg bond index and stabilization energy were calculated. The structure for [Cu(NH 3)4]+ has been taken from a previous study of Cu + in pure liquid ammonia.

4. Results and discussion 4.1.

Radial distribution function (RDF)

The radial distribution functions (RDFs) of the Cu-N and Cu-O pairs, and the respective integration number is presented in Fig. 1. The Cu-N RDF displays a well-defined first shell peak ranging from 1.95 to

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2.89 Å with a peak maximum located at 2.23 Å. The integration of the first shell peak results in an average coordination number of 3.95, i.e. Cu+ is predominantly coordinated by 4 ammonia molecules. Beyond the first shell, the pair distribution continuously connects to the bulk, the formation of a second NH 3 solvation shell is not observed). In comparison with Cu+ in liquid ammonia solutions studied via experimental (e.g. EXAFS[25]) and theoretical (e.g. QMCF-MD[32]) methods (see Table 1.), Cu+-N showed a longer distance. This suggests that the solvation properties of Cu+ in aqueous ammonia solutions has a higher flexibility than the solvation character in liquid ammonia and also indicates that water molecules give contribution to the

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interaction between Cu +-ammonia that can be proven by NBO analysis below.

The Cu-O RDF shows a very low probability in the region of the first shell, indicating that the Cu +-NH 3 interaction is the dominating contribution in the formation of the solvation complex. Only a faint indication of

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the first shell peak is visible, the respective peak distance being 2.30 Å. Integration of the Cu-O RDF up to the border between the first shell and the bulk results in a low average coordination number of 0.3. This non integer value implies that in the majority of configurations registered along the simulation only ammonia

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ligands are present in the first solvation layer (see also section Ligand Dynamic below). Similar as in the case of the Cu-N RDF the formation of a second solvation shell cannot be detected in the Cu-O RDF. The observed solvation structure can be explained by the strong Cu-NH3 interaction on the

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one hand and the strong hydrogen-bond capabilities of water molecules on the other hand 8. Both, the Cu– O and the Cu-N RDF show nonzero values in the region between the first solvation shell and the bulk, which

4.2.

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points towards the occurrence of ligand exchange reactions.

Coordination number distribution (CND)

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Since the coordination numbers obtained from the RDFs represent averages over the entire simulation time, a CND analysis was carried out to monitor the occurrence of individual coordination patterns. The CND of ligands in the first solvation shell is presented in Fig. 2, employing radial criteria of 3.35 and 3.14 Å in case of NH 3 and H2O, respectively. The dominant coordination number in case of NH 3 registered in 71.8% of all sampled configurations is 4, followed by coordination numbers 3 and 5 with occurr ences of 26.5 and 1.8%, respectively. In case of water a coordination number 0 (i.e. no water being present in the first shell) has been registered in 69.3% of the configurations, followed by coordination numbers 1 and 2 having an occurrence of 29.8 and 0.9%, respectively. The combined CND considering NH 3 and H2O reveals a consistent, four-fold coordination occurring with a probability of 83.35%, with only small occurrences of coordination numbers 3 (5.22%); 5 (10.77%); and 6 (0.67%), respectively.

ACCEPTED MANUSCRIPT 7 In order to obtain further information on the instantaneous composition of the first solvation shell, a time series of the respective first shell coordination numbers (CNs) of H 2O and NH 3 as well as the associated sum has been prepared (Fig. 3). It can be seen that aside from short time fluctuations, a substitution of an NH3 ligand by H2O took place after 17.5 ps. The resulting [Cu(NH 3)3(H2O)]+ species is rather short-lived and attempts to reverse the structure to the more stable [Cu(NH 3)4]+ complex (see NBO analysis below) are observed in the range from 21 to 23 ps. The time series of the combined coordination number of NH 3 plus H2O remains effectively close to four, displaying only short-time fluctuation of ±1 in good agreement with the

4.3.

Angular Distribution Function (ADF)

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CND-results.

Due to the presence of two different types of ligand molecules in the first solvation shell, the distribution of the associated ligand-ion-ligand angles was of particular interest. Figure 4(A) and (B) depict the N-Cu+-N and N-Cu+-O ADFs considering all molecules located within the first solvation shell. The N-CuN ADF displays a distinct peak at 108.5°, which agrees well with the expected angle of 109.5° corresponding

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to an ideal tetrahedral arrangement. In addition, a distinct shoulder close to 120° is visible, which points towards the occurrence of an intermediate trigonal-planar coordination of NH 3 ligands (i.e. n=3). Figure 4(B) shows the ADF for all N-Cu+-O combinations registered within the first shell. A similar overall

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shape as in case of the N-Cu+-N case is observed. However, the peak maxima are shifted to slightly lower angles of 91° and 111°. In addition, the N-Cu+-O ADF shows larger fluctuations compared to the N-Cu+-N case, being the result of the weaker ion-water interaction, presumably leading to increased mobility and exchange dynamics of the water molecules.

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Individual snapshots of the [Cu(NH 3)4]+, [Cu(NH 3)4H 2O]+ and [Cu(NH3)3H 2O]+ complexes observed in

4.4.

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the QMCF MD simulation are shown in Fig 5(A) to (C).

Natural bond orbital (NBO) analysis

The application of an NBO analysis in recent studies on K + and Rb+ in ammonia solution[37,39] provided

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an additional characterization of the ion-solvent interactions in addition to the data obtained from the QMCF simulation. In this work, a similar analysis has been applied to four different model systems, namely

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[Cu(NH3)4]+ in pure liquid and aqueous ammonia as well as [Cu(NH 3)3H 2O]+ and [Cu(NH3)4H2O]+ in aqueous ammonia. The respective Wiberg bond indices as well as the individual stabilisation energies are listed in Tables 2-5, respectively.

As expected, the Wiberg bond indices (WBI) are close to zero, highlighting the electrostatic nature of the ion-ligand interactions. However, while in the cases of Cu+-NH3 interactions WBI values in the range of about 0.05 to 0.17 are obtained, the Cu +-H2O value is found as 0.015. This finding is in line with the common classification of water being a hard ligand compared to the NH 3, which is often regarded as soft.[72,73] The data provided by the NBO analysis agrees well with the conclusions drawn from the RDF and CND data. In addition, the NBO calculations provide further information on the stabilisation energy of the ionligand interaction. It can be seen that the stabilisation observed in the [Cu(NH3)4]+ cases (for both pure as well as aqueous ammonia, Tables 2 and 3) is lower compared to the [Cu(NH 3)3H 2O]+ and [Cu(NH3)4H2O]+

ACCEPTED MANUSCRIPT 8 system (Tables 4 and 5). Especially, the Cu+-H2O interaction is notably weaker compared to its NH 3 counterpart. The NBO results clearly show that the presence of H 2O in the first shell results in an overall destabilisation of the solvation complex.

4.5.

Ligand dynamics

To evaluate the dynamics of ligand exchange reactions between the first shell and the bulk, the mean residence times (MRTs) of ammonia and water ligands within the first shell were calculated via the ‘‘direct

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method”.[59] An MRT value of 14.6 ps was estimated for ammonia ligands from 23 ps of simulation time, with the first exchange being observed after 13 ps of simulation time. The MRT value obtained for water ligands migrating to/from the first solvation shell was calculated to be 1.3 ps. This value has to be interpreted in view of the very low occurrence of H 2O molecules in the first solvation shell. Despite several attempts of water ligands to enter the first solvation shell over the course of the simulation (see Fig. 3), only a few of these attempts have been successful. However, even in these comparably rare cases, the overall mean lifetime of the first shell water molecule is low since the more stable [Cu(NH 3)4]+ solvation complex is quickly

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re-established. The dynamics of the exchange processes of all registered ligand exchange events are illustrated in the form of ion-ligand distance plots in Fig. 6(A) for H 2O and 6(B) for NH 3. The power spectrum of the Cu–N interactions in the first solvation shell depicted in Figure 7 is obtained

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via VACFs and subsequent Fourier transform (FT). The highest intensity of the Cu-N vibrational mode was found at 252 cm-1, with a corresponding force constant of 43.0 Nm-1. This value compares well with the frequencies obtained for Cu + in liquid ammonia[32], and this frequency value is exactly the same as observed for Ag+ in aqueous ammonia.[40] In comparison, the associated wave number observed for the

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tetraaminecopper(II) complex[74] was reported as 420cm -1, which is only a factor of 1.67 larger than in the Cu(I) case.

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Overall the dynamical analysis of the Cu⁺ ion in 18.6% aqueous ammonia solution highlight that the first solvation shell is preferentially occupied by four strongly bound ammonia ligands in a tetrahedral geometry,

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whereas a direct interaction of water ligands with Cu + is not preferred.

5. Conclusion

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The QMCF MD simulation approach has proven as a highly suitable method to investigate the preferential solvation and dynamical properties of Cu⁺ in 18.6% aqueous ammonia solution, being a comparably challenging target system from the experimental point of view. The simulation results give clearly highlight the preference to form a tetrahedral [Cu(NH 3)4]+ solvation complex, with an average Cu +-N distance of 2.23 Å. Although the formation of a [Cu(NH 3)3(H 2O)]+ complex was also observed within the simulation time, this species is comparably short-lived, and the tetrahedral ammonia complex was quickly re-formed. For both, H 2O and NH3, no preferential orientation beyond the first sol shell was observed, which can be attributed to the strong binding of ammonia to the ion, presumably resulting in an effective shielding of the central charge.

ACCEPTED MANUSCRIPT 9 The strong nature of the Cu +-NH3 interaction is also highlighted by an NBO analysis, while in the Cu +H2O case a significantly lower Wiberg index and stabilisation energy have been found.

Conflicts of interest Declarations of interest: none

Acknowledgements

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The authors dedicate this article to the memory of Dr. rer. nat. Ria Armunanto. W.D. Saputri acknowledges a Ph.D. scholarship issued by the Ministry of Research, Technology and Higher Education of the Republic of Indonesia (1511/E4.4/2015). Research reported in this publication was jointly supported by the ASEANEuropean Academic University Network (ASEA-UNINET), the Austrian Federal Ministry of Science, Research and Economy, and the Austrian Agency for International Cooperation in Education and Research (OeAD-GmbH). The authors are also grateful to Niko Prasetyo for his technical help at the initial stage of the

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simulation.

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CE

PT

ED

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Copper(I) in the Gas Phase, J. Am. Chem. Soc. 101 (1979) 7127–7129.

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[78]

ACCEPTED MANUSCRIPT 15

Table 1 Structural properties of Cu + in liquid ammonia as well as pure and aqueous ammonia solution, with rCu-N and rCu-O, corresponding to the associated Cu-N and Cu-O distances respectively, CN is the coordination number in the first shell.

Method

Cu+ in 18.6% aqueous ammonia Cu+ in 18.6% aqueous ammonia Cu+ in ammonia, gas phase

QMCF MD Simulation

2.23

QMCF MD Simulation

2.30

High-pressure mass spectrometry Infrared Spectroscopy

Vaporized Cu+ ions with neutral ammonia clusters [Cu(N^N)(P^P)]+

Infrared Photodissociat ion + DFT calculation XRD

2.07

Vaporized Cu+ ions with neutral water clusters

Infrared Photodissociat ion + DFT calculation UV-Vis Spectroscopy Interfaced HF/MM+3bd Monte Carlo Simulation

2.10

This work

1

3

This work

[75] [76] [25]

4

[77]

-

[23]

2

[77]

3-4

[78]

2.20

6

[68]

2.15

4

[32]

ED

2.23

-

PT

CE

AC

QMCF MD Simulation

4

NU

2.004

1 Cu+ in liquid ammonia (1000 molecules)

Ref.

3

EXAFS

1 Cu+ in liquid ammonia (215 molecules)

CN

3-5

Cu+ in liquid ammonia

Cu+ in aqueous ammonia

rCu-o (Å)

MA

Cu+ in ammonia, gas phase

rCu-N (Å)

SC RI PT

System

ACCEPTED MANUSCRIPT 16

Table 2 NBO analysis results for [Cu(NH3)4]+ in pure liquid ammonia (structure taken from Ref.

27).

System

Wiberg index

Second order perturbation theory analysis of Fock

[Cu(NH3)4]

Cu-N (in liquid

matrix in NBO basis donor-acceptor NBO

+

ammonia

Donor

solutions)

NBO

Acceptor NBO

Stabilisation energy in

LP* (Cu)

BD (N-H)

LP* (Cu)

LP (N)

LP* (Cu)

BD (N-H)

LP* (Cu)

LP (N)

LP* (Cu)

BD (N-H)

LP* (Cu)

61.93

LP (N)

LP* (Cu)

198.66

77.19

228.45 68.45

205.09 55.44

184.64

NU

MA

0.1424

LP (N)

ED

Cu-NH3 (4)

0.1412

LP* (Cu)

PT

Cu-NH3 (3)

0.1671

BD (N-H)

CE

Cu-NH3 (2)

0.1671

AC

Cu-NH3 (1)

SC RI PT

kJ mol-1

ACCEPTED MANUSCRIPT 17 Table 3 NBO analysis results for [Cu(NH 3)4]+ in 18.6% aqueous ammonia

System [Cu(NH3)4

of ]+

Wiberg index

Second order perturbation theory analysis of Fock

Cu-N

matrix in NBO basis donor-acceptor NBO

(in

aqueous

Donor NBO

ammonia

Acceptor

Stabilisation

NBO

energy in kJ mol -1

60.0404

LP (N)

LP* (Cu)

213.2166

BD (N-H)

LP* (Cu)

40.75216

LP (N)

LP* (Cu)

159.9962

BD (N-H)

LP* (Cu)

51.3377

LP (N)

LP* (Cu)

209.4092

BD (N-H)

LP* (Cu)

34.2669

NU

0.1198

LP* (Cu)

LP* (Cu)

MA

LP (N)

ED

Cu-NH3 (4)

0.1544

BD (N-H)

PT

Cu-NH3 (3)

0.1272

CE

Cu-NH3 (2)

0.148

AC

Cu-NH3 (1)

SC RI PT

18.6%)

160.2890

ACCEPTED MANUSCRIPT 18 Table 4 NBO analysis results for [Cu(NH 3)4H2O]+ in 18.6% aqueous ammonia

of

Wiberg index

Second order perturbation theory analysis of

[Cu(NH3)4H2

Cu-N

Fock matrix in NBO basis donor-acceptor NBO

O]+

aqueous

(in

Donor NBO

ammonia

Acceptor

Stabilisation

NBO

energy in kJ mol1

18.6%)

46.6098

BD (N-H)

LP* (Cu)

LP (N)

LP* (Cu)

BD (N-H)

LP* (Cu)

LP (N)

LP* (Cu)

BD (N-H)

LP* (Cu)

LP (N)

LP* (Cu)

139.2854

BD (O-H)

LP* (Cu)

4.8116

LP (N)

LP* (Cu)

16.1084

157.3184 15.8992 62.2579 33.1791

122.6749 41.1706

NU

0.0146

LP* (Cu)

MA

Cu-H2O (5)

0.1108

LP (N)

ED

Cu-NH3 (4)

0.0988

LP* (Cu)

PT

Cu-NH3 (3)

0.0601

BD (N-H)

CE

Cu-NH3 (2)

0.1175

AC

Cu-NH3 (1)

SC RI PT

System

ACCEPTED MANUSCRIPT 19 Table 5 NBO analysis results for [Cu(NH 3)3H2O]+ in 18.6% aqueous ammonia

of Fock matrix in NBO basis donoracceptor NBO

ammonia 18.6%)

Donor

Acceptor

Stabilisation

NBO

NBO

energy in kJ mol -1

BD (N-H)

LP* (Cu)

56.8187

LP (N)

LP* (Cu)

192.0874

BD (N-H)

LP* (Cu)

70.7514

LP (N)

LP* (Cu)

250.32872

BD (N-H)

LP* (Cu)

76.6509

LP (N)

LP* (Cu)

218.2374

BD (O-H)

LP* (Cu)

27.4888

LP (N)

LP* (Cu)

0.1356

0.1646

0.1480

0.0695

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aqueous

NU

Cu-H2O (4)

(in

MA

Cu-NH3 (3)

Second order perturbation theory analysis

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Cu-NH3 (2)

Cu-N

index

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Cu-NH3 (1)

Wiberg

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[Cu(NH3)3H2

of O]+

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System

116.8173

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Figure 1 - Cu+-N (black) and Cu+-O (red) radial distribution function (solid) and the respective running integration (dashed) obtained from the QMCF MD simulation of Cu+ in 18.6% aqueous ammonia.

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Figure 2 - Coordination number distribution of ammonia and water molecules in the first solvation shell of Cu + in 18.6% aqueous ammonia (left) and the respective sum (right) obtained from the QMCF MD simulation.

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Figure 3 - Time series of the instantaneous coordination number of NH3 (black), H2O (red) and the respective sum (green) for Cu+ in 18.6% aqueous ammonia obtained from the QMCF MD simulation

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Figure 4 - Distribution of (A) the N-Cu+-N and (B) the N-Cu+-O angles registered in the first solvation shell of Cu+ in 18.6% aqueous ammonia obtained from the QMCF MD simulation

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(C)

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Figure 5 - Snapshots depicting (A) the tetrahedral [Cu(NH3)4]+ complex, (B) a H2O molecule entering the first solvation shell and (C) the [Cu(NH3)3(H2O)]+ complex. (Colours: Cu+ green; N blue; O red; H white)

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Figure 6 - Time evolution of Cu+–Ligand distance for (A) water oxygen and (B) ammonia nitrogen atoms involved in ligand exchange events registered along the QMCF MD simulation (blue). The dashed lines represent the boundaries of 3.35 Å (NH3) and 3.14 Å (H2O) separating the first solvation shell from the bulk. Brown solid lines correspond to non-exchanging ligands.

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Figure 7 - Power spectrum of the first shell Cu+-N interaction in 18.6% aqueous ammonia obtained via Fourier transform of the associated velocity autocorrelation function. The dashed line represents the vibrational frequency of 252 cm-1

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HIGHLIGHTS

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The preferential of tetrahedral [Cu(NH3)4]+ complex is higher than [Cu(NH3)3(H2O)]+ [Cu(NH3)3(H2O)]+ is short-lived, and the tetrahedral ammonia was quickly re-formed. The strong binding Cu+-ammonia is an effective shielding of the central charge. The strong nature of the Cu+-NH3 interaction is highlighted by an NBO analysis. The Cu+-H2O case a significantly lower Wiberg index and stabilisation energy.

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Investigation of the Preferential Solvation and Dynamical Properties of Cu+ in 18.6% Aqueous Ammonia Solution using Ab Initio Quantum Mechanical Charge Field (QMCF) Molecular Dynamics Study and NBO analysis