Water clusters systems

Quantum Mechanical Studies of the Hydrogen Bond in Water Cluster Systems. 

Hydrogen-bonded clusters are an important class of molecular clusters, among which small water clusters have received a considerable amount of attention [148, 149]. Solvated cluster ions have also been produced and studied [150, 151]. These solvated clusters provide ideal model systems to obtain microscopic infonnation about solvation effect and its influence on chemical reactions. 

The ultimate reason for studying water clusters is of course to understand tire interactions in bulk water (tliough clusters are interesting in tlieir own right, too, because finite-size systems can have special properties). There has been... 

Bat ii Z and Miller R E 1996 Molecular clusters structure and dynamics of weakly bound systems J. Phys. Chem. 100 12 945-59 Liu K, Cruzan J D and Saykally R J 1996 Water clusters Science 271 929-33... 

Knowledge concerning the mechanism of hydrates formation is important in designing inhibitor systems for hydrates. The process of formation is believed to occur in two steps. The first step is a nucleation step and the second step is a growth reaction of the nucleus. Experimental results of nucleation are difficult to reproduce. Therefore, it is assumed that stochastic models would be useful in the mechanism of formation. Hydrate nucleation is an intrinsically stochastic process that involves the formation and growth of gas-water clusters to critical-sized, stable hydrate nuclei. The hydrate growth process involves the growth of stable hydrate nuclei as solid hydrates [129]. 

Liquid water is an essential component of most terrestrial chemical processes, including those of living organisms. The cooperativity of H-bonding in water clusters is therefore of primary importance for understanding the structure and dynamics of pure water, as well as a vast spectrum of aqueous solvation phenomena in biotic and abiotic systems. In the present section we examine cooperativity effects for a... 

These QM/MM calculations are in contrast to a standard evaluation of chemical shielding for gas phase water clusters where the classical point charge environment is omitted entirely. The same solvation shell criterion as before was applied, and the system was treated purely quantum mechanically. 

As it is now very well known, accurate studies of the water-water interaction by means of ab-initio techniques require the use of larger and flexible basis sets and methods which consider correlation effects [85,94-96], Since high level ab-initio post-Hartree-Fock calculations are unfeasible because of their high computational cost for systems with many degrees of freedom, Density Functional Theory, more economical from the computational point of view, is being more and more considered as a viable alternative. Recently, we have presented [97] results of structural parameters and vibrational frequencies for the water clusters (H20) , n=2 to 8, using the DFT method with gradient corrected density functionals. 

Among the recently published works, the one which showed that the cyclic structures of water clusters open up to form a linear structure above a certain threshold electric field value a was a systematic ab initio study on the effect of electric field on structure, energetics, and transition states of trimer, tetramer, and pentamer water clusters (both cyclic and acyclic) [36], Considering c/.v-butadiene as a model system, the strength and the direction of a static electric field has been used to examine the delocalization energy, the probabilities of some local electronic structures, the behavior of electron pairs, and the electronic fluctuations [37]. Another recent work performed by Rai et al. focused on the studies using the DFT and its time-dependent counterpart of effects of uniform static electric field on aromatic and aliphatic hydrocarbons [38],... 

Lee and Meisel incorporated Py, at levels of 10 M or more, into 1200 EW acid form samples that were swollen with water and with ferf-butyl alcohol. It was concluded based on the /3//1 value for water swollen samples that the Py molecules were located in the water clusters and were most likely near fluorocarbon—water interfaces. It was also concluded, based on both absorption and emission spectra, that the probes had strong interactions with the SO3 groups that were exchanged with Ag+ and Pb + cations in the case of water containing samples. Likewise, the pyrene molecules were rationalized as being surrounded by terf-butanol molecules in that case. However, excimer formation (due to the presence of adjacent pyrene molecules) in the ferf-butyl alcohol system suggested the loss of cluster morphology-... 

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. 

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