Phenol-water clusters

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. 

Phenol-water clusters are good models for the investigation of the photoinduced elementary processes occurring in living matter. Intracluster hydrogen transfer processes in phenol-water (Ph-W) complexes have extensively been studied in recent years, see refs [11-14] for reviews. Phenol-ammonia (Ph-A) clusters also have served as easily accessible and versatile models of intracluster hydrogen transfer dynamics [14,16]. It has been inferred by several authors that intracluster proton transfer occurs more readily in Ph-A clusters than in Ph-W clusters, but it has been a matter of debate whether the hydrogen or proton transfer occurs in the S excited state, or in the cluster cation, or in both [12,14],... 

Figure 3.33 PE profiles of the electronic ground state (circles), the lowest 1 tttt state (squares) and the lowest 17rcr state (triangles) of (a) the phenol-water cluster and (b) the phenol-ammonia cluster as a function of the hydrogen transfer coordinate, calculated with the CASPT2 method [32].

CHARGE FLUCTUATIONS OF THE HYDROGEN BOND NETWORK AND PROTON TRANSFER ASSISTED BY THE SOLVENT IN PHENOL-WATER CLUSTERS... 

Born-Oppenheimer Molecular Dynamics of Proton Transfer in Phenol-Water Clusters... 

The proton transfer in ionized phenol-water clusters is strongly dependent on the number of water molecules and their specific organization, i.e., the PT is a process assisted by the solvent [72], Most of the theoretical studies of PT in [C6H50H-(H20) ],+ clusters were focused on the structure, vibrational, [79,80,81] and energetic aspects [77,78]. However, much less is known on the dynamics of PT. 

Proton (Deuterium) Transfer in Phenol-Water Clusters and Fluctuations of the HB Network... 

Born-Oppenheimer molecular dynamics simulations for neutral and ionized phenol-water clusters are reported. The results for [C6H50D-(H20)4],+ illustrate how the PT dynamics is coupled to fluctuations of the solvent. The kinetics of PT/recombination in [C6H50D-(H20)4] + clusters is related to strong fluctuations of the electrostatic field of the water molecules and this relationship points out the relevance of investigating the electronic properties of the HB network for understanding chemical reaction in solution. 

Breden, G., Meerts, W. L., Schmitt, M., and Kleinermanns, K., High resolution UVspectroscopy of phenol and the hydrogen bonded phenol-water cluster, J. Chem. Phys. 104,972-982 (1996). 

Experimental frequencies for phenol and phenol-water clusters are taken from References 704 and 705. See also Table 10 for the phenol vibrational modes. Calculated frequency at the HE/6-31G(d,p) and MP2/6-31G(d,p) (in parentheses) levels (cf. Table 10). 

It is well known that water-mediated interaction stabilizes structure of biomolecules [1, 138, 247-250]. Therefore, several model molecular systems have been chosen to probe the water-mediated interactions in biomolecules and a large amount of experimental and theoretical work has been published over the years on this subject [78, 138, 251-258]. Since phenol is the simplest aromatic alcohol resembling chromophore of an aromatic amino acid, hydration of phenol molecules has been studied to understand H-bonding and solute-solvent interaction in biological systems. Several experimental and theoretical calculations have been made on the phenol-water clusters [259-273]. Recently, we have made a comprehensive analysis on structure, stability, and H-bonding interaction in phenol (P1-4), water (W1-4), and phenol-water (PmW (w = 1-3, n = 1-3, w + n < 4)) clusters using ab initio and DFT methods [245]. In this section, electronic structure calculations combined with AIM analysis on phenol-water clusters are presented. 

Figure 5. Molecular topographies of phenol, water, and phenol-water clusters as obtained from theoretical electron density. (Reproduced with permission from Ref. 245 American Chemical Society, 2005.)...

Table S. Electron density (p(rc)) and Laplacian of electron density (V p(r )) for various phenol-water clusters...

As pointed out by Popelier et al. the first moment and atomic volume of H atom involved in the H-bonding decreases by 0.02-0.05 a.u. and 5-9 a.u., respectively, in different phenol-water clusters. It is evident from the above illustrations that electron density topography analysis clearly elicits H-bonded interactions in microsolvated and water-mediated clusters [245]. 

R. Parthasarathi, V. Subramanian, and N. Sathyamurthy, Hydrogen bonding in phenol, water, and phenol-water clusters, J. Phys. Chem. A 109, 843-850 (2005). 

H. Watanabe and I. Iwata, Theoretical studies of geometric structures of phenol-water clusters and their infrared absorption spectra in the O-H stretching region, J. Chem. Phys. 105, 420-431 (1996). 

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