Water clusters, potential energy surfaces

Hobza, P., Bludsky, O. Suhai, S., 1999, Reliable Theoretical Treatment of Molecular Clusters Counterpoise-Corrected Potential Energy Surface and Anharmonic Vibrational Frequencies of the Water Dimer , Phys. Chem. Chem. Phys., 1, 3073. 

Goldman, N. Leforestier, C. Saykally, R. J., A firstprinciples potential energy surface for liquid water from VRT spectroscopy of water clusters, Philos. Trans. R. Soc. A 2005, 1-16. doi 10.1098/rsta.2004.1504... 

To understand the fundamental photochemical processes in biologically relevant molecular systems, prototype molecules like phenol or indole - the chromophores of the amino acids tyrosine respective trypthophan - embedded in clusters of ammonia or water molecules are an important object of research. Numerous studies have been performed concerning the dynamics of photoinduced processes in phenol-ammonia or phenol-water clusters (see e. g. [1,2]). As a main result a hydrogen transfer reaction has been clearly indicated in phenol(NH3)n clusters [2], whereas for phenol(H20)n complexes no signature for such a reaction has been found. According to a general theoretical model [3] a similar behavior is expected for the indole molecule surrounded by ammonia or water clusters. As the primary step an internal conversion from the initially excited nn state to a dark 7ta state is predicted which may be followed by the H-transfer process on the 7ia potential energy surface. 

In contrast to indole-ammonia clusters, for which the different steps of the photoinduced H-transfer reaction have been analyzed in detail, we have found no hints for such a reaction in indole(H20) clusters. Probably, like for phenol(H20) complexes the endoenergetic character of the reaction H+H2O—>H30 is responsible for the missing H-transfer process in the indole(H20) clusters. Ab initio calculations of the indole-water potential energy surfaces are under way now, to elucidate this process in the heterocluster and to understand the difference with respect to the indole-ammonia complex. 

There is another minimum on the potential energy surface of the (H20)3HC1 trihydrate corresponding to an ionic form of the cluster. The ionic minimum is characterized by a cyclic structure with the Cl ion hydrogen-bonded simultaneously to the two water molecules and the hydronium ion. It is interesting to note that the ionic form is less stable, as it is located 5.2kcal/mol above the neutral minimum. Therefore, the ionic structure of the trihydrate observed in the X-ray experiments 397 cannot directly be related to this ionic minimum. In fact, the structure observed in crystals must be considered as effective, taking into account the crystal field effects. 

Two methods are in common use for simulating molecular liquids the Monte Carlo method (MC) and molecular dynamics calculations (MD). Both depend on the availability of reasonably accurate potential energy surfaces and both are based on statistical classical mechanics, taking no account of quantum effects. In the past 10-15 years quantum Monte Carlo methods (QMC) have been developed that allow intramolecular degrees of freedom to be studied, but because of the computational complexity of this approach results have only been reported for water clusters. 

NUMERICAL RESULTS PROBING THE POTENTIAL ENERGY SURFACE OF THE WATER MOLECULE WITH THE STANDARD AND RENORMALIZED COUPLED-CLUSTER METHODS... 

Quantum chemical computations of potential energies surface sections along the reaction pathway (PEES) for interaction of typical electrophiles (halogensilanes HaSiX (X = F, Cl, Br, I), trimethylchlorosilane [48,49], acetyl chloride [51]) and nucleophiles (hydrogen halides HX (X = F, Cl, Br, I), water, aliphatic amines, aliphatic alcohols [52], amino acids [53] and substituted phenols [54]) with the silica OH group in a cluster approach using semiempirical AMI, NDDO, MNDO and MNDO/H methods were performed. Representative PEES is shown in Fig.l. 

D. B. Chesnut, Structures, energies, and NMR shieldings of some small water clusters on the counterpoise corrected potential energy surface, J. Phys. Chem. A 106, 6876-6879 (2002). 

G. Jacoby, U. Kaldor, and P. Jungwirth, Relaxation of chlorine anions solvated in small water clusters upon electron photodetachment the three lowest potential energy surfaces of the neutral CI H2O complex, Chem. Phys. Lett. 293, 309-316 (1998). (d) J. Baik, J. Kim,... 

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