The structure of a DMSO–water mixture from Car–Parrinello simulations

doi:10.1016/S0009-2614(02)01377-5

Chemical Physics Letters

Volume 364, Issues 5–6, 16 October 2002, Pages 497-502

https://doi.org/10.1016/S0009-2614(02)01377-5 Get rights and content

Abstract

The mixture of DMSO–water was studied with Car–Parrinello simulation techniques. A threefold coordination at the DMSO oxygen, methyl group hydrogen–oxygen atom contacts, Rüssel-structures and 1-DMSO–3-H₂O clusters are observed. The H(DMSO)–O(H₂O) and the H(DMSO)–H(H₂O) radial distribution functions are almost identical. For different far water the angular distribution displays a dialectic character: close water orient with the oxygens to the methyl hydrogen far waters vice versa. The emerging picture for the dynamical behavior of the methyl groups is that of a near-hydrophilic–far-hydrophobic, where the hydrophobic character is probably dominant. A detailed mechanism of one such an event is given.

Introduction

Many experimental quantities of the DMSO–water mixture expose a non-additive behavior [1] mostly at n_DMSO=0.2–0.3. The special features of the phase diagram in this region led to the believe that 1-DMSO–3-H₂O and 1-DMSO–2-H₂O clusters would be responsible [1]. In order to find an explanation molecular dynamics simulations based on classical pair potentials were undertaken [2], [3], [4], [5], [6], [7], [8], [9]. These studies elucidated cluster of the 1-DMSO–2-H₂O and 2-DMSO–1-H₂O type but no 1-DMSO–3-H₂O cluster were found. The absence of such a 1-DMSO–3-H₂O cluster has prompted this study of the n_DMSO=0.25 system with the Car–Parrinello simulation technique. The Car–Parrinello method has the advantage of calculating the intermolecular forces on the fly, i.e., no pre-made interaction model is involved. Polarization effects are explicitly included and the pairwise additivity as used in the traditional molecular dynamics simulations is circumvented. Previous static calculations of different clusters indicated many-body effects up to 30% [10]. Influences of the approximations made in the previous molecular dynamics simulation on calculated quantities were first systematically studied by Huber and co-workers [11] with models obtained from ab initio quantum chemical calculations.

The aim of this study is to reveal the structure of the DMSO–water mixture in order to help to understand the non-additive behavior of this system.

Section snippets

Methodology

The simulation was undertaken employing a Car–Parrinello [12] ansatz, where the electronic structure was calculated within the Kohn–Sham formulation of density functional theory (DFT) in combination with the BLYP functional [13], [14]. In order to investigate the performance of BLYP few static calculations with Turbomole[15] and the TZVP basis on the (H₂O)₂, the (DMSO)₂ and the DMSO–water dimer are given in Table 1. B3LYP and MP2 data are from [10]. Only for the DMSO-dimer energy a considerable

Microscopic pictures

The first insight into the microscopic structure can be gained by simply looking at cluster-snapshots of the simulation (see Fig. 1). All hydrogen bonded molecules are shown in the picture. If a O–H(DMSO) distance is shorter than 2.5 Å the oxygen donating molecule is also depicted. If necessary the important DMSO is marked with an arrow. Most often the DMSOs are involved in a very expanded hydrogen bond network, e.g. clusters 4 and 5 in Fig. 1. This is in accordance with [8], where the authors

Radial pair distribution function

Further understanding is provided by the radial pair distribution function in Fig. 2. For each picture the solid line gives the hydrogen-acceptor–hydrogen function and the dashed dotted line shows the hydrogen–hydrogen function (all intramolecular contributions are omitted). The hydrogen in (A) is from the H₂ molecule (dissolved in ammonia), in (B) it is from the water and in (C) it is from the DMSO methyl groups.

Thus, in (A) of Fig. 2 two functions of a H₂ molecule prepared in situ in ammonia,

Angular distribution

In Fig. 3 the angular distribution of the H(CH₃)–O(H₂O) vector with the OH-vector of the water molecule is depicted for different close water molecules. If $cos α≈−1 (α=180°)$ the hydrogens of the water molecule point away from the methyl groups, and if $cos α≈1 (α=0°)$ the hydrogens of the water orient towards the hydrogen atom of the methyl group. The long dashed line includes all water if their oxygen is further away than 2.7–4.0 Å from the methyl hydrogen, the dashed-dotted line shows the

Conclusion

In our study, we observed what was indicated in static isolated cluster calculations [10] already: O–H(CH₃) contacts, 1-DMSO–3-H₂O cluster and a coordination number greater than two for the DMSO oxygen. In accordance with the traditional molecular dynamics simulation we see the elongation of the water hydrogen bonds and we find a very involved hydrogen bond network. An interesting feature was reflected in the average orientation of the water at the methyl hydrogen: very close to the methyl

Acknowledgements

The authors would like to thank Prof. B.A. Hess and Dr. M. Reiher for giving B.K. the opportunity to calculate the BLYP data from Table 1 and Michael Odelius for helpful discussion. A generous allocation of computer time by the Zentrum für Informatikdienste der Universität Zürich is acknowledged. B.K. furthermore would like to thank the DFG for financial support.

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The structure of a DMSO–water mixture from Car–Parrinello simulations

Abstract

Introduction

Section snippets

Methodology

Microscopic pictures

Radial pair distribution function

Angular distribution

Conclusion

Acknowledgements

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