Phenol ammonia cluster

In this section, we discuss the photoinduced hydrogen transfer from phenol to water and ammonia in phenol-water and phenol-ammonia clusters, respectively, as a representative model of excited-state chromophore-to-solvent hydrogen transfer reactions. 

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].

W. Siebrand, M. Z. Zgierski, J. K. Smedarchina, M. Vener and J. Kaneti, The structure of phenol-ammonia clusters before and after proton transfer. A theoretical investigation, Chem. Phys. Lett., 266 (1997) 47-52. 

S. Ishiuchi, M. Saeki, M. Sakai and M. Fujii, Infrared dip spectra of photochemical reaction products in a phenol/ammonia cluster examination of intracluster hydrogen transfer, Chem. Phys. Lett., 322 (2000) 27-32. 

P. Bering, Structure and vibrations of the phenol-ammonia cluster, J. Chem. Phys. 102, 9197-9204 (1995). (c) S. Tanabe, T. Ebata, M. Fujii, and N. Mikami, OH stretching vibrations of phenol-(H20) (n = 1-3) complexes observed by IR-UV double-resonance spectroscopy, Chem. Phys. Lett. 215, 347-352 (1993). (d) D. Michalska, W. Zierkiewicz, D. C. Bien ko, W. Wojciechowski, and T. Zeegers-Huyskens, Troublesome vibrations of aromatic molecules in second-order Moller-Plesset and density functional theory calculations infrared spectra of phenol and phenol-OD revisited, J. Phys. Chem. A 105, 8734-8739 (2001). 

The molecular mechanism of photoacidity of phenol has been investigated theoretically by Sobolewski and coworkers [100-102]. The ab initio calculations on phenol-ammonia cluster predicted that the increased acidity of excited phenol is not due to a property of the optically excited jot state, but rather arises from the nonadiabatic interaction of the jiji state with an optically dark state of jta character. The jot potential energy function is crossed by the jta function, and the jro energy is strongly stabilized when the proton moves from the chromophore to the solvent (ammonia) as illustrated in Figure 2.6 [100]. Hence, they consider that jia state plays a key role in the ESPT reaction of phenol-ammonia cluster. 

FIGURE 2.6 Calculated potential energy profiles associated with proton transfer reaction in phenol-ammonia cluster. (From Domcke, W. and Soholewski, A.L., Science, 2003, 202, 1693. With permission.)... 

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. 

This article will deal more with naphthol-ammonia clusters but the case of phenol-ammonia can be quickly summarized here. The dynamical process observed in the excited state of phenol has long been attributed to an excited state proton transfer [1-7] ... 

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],... 

As in isolated phenol and in phenol-ammonia/water clusters, the OH bond is broken homolytically in 7HQ-A3, resulting in the transfer of a hydrogen atom rather than proton transfer. As found for phenol-A /W and naphthol-A /W clusters, ammonia is a better hydrogen acceptor than water. Excited-state hydrogen transfer processes are thus strongly favoured in an ammonia environment. 

O. David, C. Dedonder-Lardeux and C. Jouvet, Is there an excited state proton transfer in phenol (or l-naphthol)/ammonia clusters , hit. Rev. Phys. Chem., 21 (2002) 499-523. 

In these two examples of phenol and naphthol-ammonia clusters, we have evidenced the role played by evaporation phenomena that are expected to be present in a lot of clusters pump-probe studies and ean not only hide but replace the reactive processes. Since absorption... 

We here report a nonadiabatic electron wavepacket study on the excited state reaction in phenol that is hydrogen-bonded with ammonia clusters Ph OH (NH3) PhO [H (NHs) ] (7.42)... 

Phenol has been interesting many chemists as a model system to study how photo-excited bases in DNA can deactivate to the ground state without resulting in mutation. Not only in such biology-oriented studies, this molecule shows many other interesting features when put in surrounding molecules, clusters, and solvents. Small ammonia clusters are frequently used in place of water-molecule clusters, because ammonia is more proton-attractive than water, and very extensive studies have been performed... 

Fig. 7.25 Spatial distribution of some of the relevant molecular orbitals in phenol with three membered ammonia cluster. (Reprinted with permission from K. Nagashima et al., J. Phys. Chem. A 116, 11167 (2012)).

Fig. 7.26 Selected static properties of phenol-(NH3)3. (a) The potential energy surfaces for the nine low-lying excited states, one-dimenional projection in OH distance with the other degrees of freedom frozen at the ground state geometry shown in panel (b). The height of the potential barrier of the lowest excited state is about 0.0029 hartree (0.8 eV). (c) and (f) the bond order of OH and NH. (d) and (g) the Mulliken charge for the site of PhO, the transferring proton site (trH), the total ammonia cluster (AMC), and ammonia molecule AMI that is hydrogen-bonded to phenol, (e) and (h) unpaired electron population at the same sites as in panel (d) and (g). Panels (c), (d), and (e) are for the first excited state, while (f), (g), and (h) exhibit for the second excited state. (Reprinted with permission from K. Nagashima et al., J. Phys. Chem. A 116, 11167 (2012)).

To further characterize the electronic properties of the first and second excited states, let us examine the quantities related to electronic density. Panel (c), (d), and (e) of Fig. 7.26 exhibit the bond order at OH bond, the Mulliken charge of some selected parts, and the number of unpaired electrons of the first excited state of phenol and three membered ammonia cluster as a function of the bond length of OH. Likewise panel (f), (g), (h) show the similar quantities for the second lowest excited state, which strongly couples nonadiabatically with the first excited state. In panel (a) to (h) of Fig. 7.27 are displayed the similar graphs for phenol and five membered ammonia cluster. 

Fig. 7.28 (Center) The energy level of molecular orbitals formed in the interaction of phenol and three membered ammonia cluster at the geomtry after proton-nucleus is shifted to the ammonia site. (Left) The MO levels for PhO-H at the same geometry but without the ammonia cluster. (Right) Those of the ammonia cluster without phenol. (Reprinted with permission from K. Nagashima et al., J. Phys. Chem. A 116, 11167 (2012)).

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