Naphthol ammonia clusters

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] ... 

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... 

In contrast to ammonia clusters, the minimum size for the ESPT reaction of water clusters is much larger, e.g., n = 20 to 30 for l-NpOH-fH O). For clusters of less than 100 water molecules, fast (<60 ps) and slow (-0.5 ns) components of the naphtholate anion fluorescence (ESPT emission) have been observed [149,150]. The reaction in l-NpOH (HjO) clusters is strongly influenced by the cluster temperature (internal energies), suggesting that internal cluster motion or dynamic solvation plays a crucial role in the ESPT reaction [146]. Knochenmuss et al. have considered that solvent-solute interaction induces L / L. inversion, and after relaxation into the more... 

At the beginning of this decade, Zewail and coworkers reported a fundamental work of solvation effect on a proton transfer reaction [195]. a-naphthol and n-ammonia molecules were studied in real-time for the reaction dynamics on the number of solvent molecules involved in the proton transfer reaction from alcohol towards the ammonia base. Nanosecond dynamics was observed for n=l and 2, while no evidence for proton transfer was found. For n=3 and 4, proton transfer reaction was measured at pisosecond time scale. The nanosecond dynamics appears to be related to the global cluster behavior. The idea of a critical solvation number required to onset proton transfer... 

We have developed a model to take into account these evaporation processes (for more details, in particular kinetic equations, see ref 27) that can be both applied to phenol and naphthol, The main idea is the following the excited state decays observed correspond to evaporation of ammonia molecules after excitation of ground state proton transferred naphthol-(NH3) >6 clusters. As in the case of phenolate [31], a strong change in dipole... 

Solvation Ultrafast Dynamics of Reactions VIII. Acid-Base Reactions in Finite-Size Clusters of Naphthol in Ammonia, Water, and Piperidine, S. K. Kim, J. J. Breen, D. M. Willberg, L. W. Peng, A. Heikal, J. A. Syage, and A. H. Zewail, J. Phys. Chem. 99, 7421 (1995). 

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. 

How can the size dependent reactivity be rationalized and what are the important factors which are responsible for this cluster size reactivity Experiments have been performed with 1-naphthol associated with other molecules (Knochenmuss et al. 1988 Knochenmuss and Leutwyler 1989). Naphthol undergoes proton transfer with two molecules of piperidine. For phenol, one needs three of four ammonia or three monoethylamine (MEA) molecules for the same process. 

Figure 4-16. Gas phase proton affinities (PA in kcal mol-1) of B clusters versus /n (B = piperidine, ammonia, methanol, and water) (from Knochenmuss and Leutwyler 1989). The threshold proton affinity corresponds to the energetic limit for which excited state proton transfer occurs for 1-naphthol in small clusters.

For pro tic solvents with larger dielectric constants and stronger basicity, the La and 1Lb states are inverted and relaxation from xLb to xLa takes places but there is no proton transfer to the solvent. The fluorescence is then due to the 1LB state with a small Stokes shift. The intermediate sized water clusters (n = 10-20) belong in this category. The clusters with methanol for any size n < 10 (due to a weak basicity or a small dielectric constant) follow this mechanism. From the evaluated proton affinities (see Figure 4-16), it can be seen that for n k 10 molecules of methanol (PA 243 kcal mol-1 which corresponds to the limit for proton transfer evidence in 1-naphthol complexes with piperidine or ammonia), a proton transfer should be observed. The absence of such a transfer can be related to a cluster structure effect. 

The proton transfer reaction has been also studied in rare gas matrices, showing some discrepancies with the gas phase (Brucker and Kelley 1989a,b Crepin and Tramer 1991) in 1- and 2-naphthol with ammonia, the AH B — A -HB + transition takes place for AH (NH3) /Ar matrix-embedded clusters with n > 3 (Brucker and Kelley 1989a,b). 

The observation of a fast time decay for the n = 2 mass channel excited at the cluster origin is surprising. Isotopic substitution experiments show that the decay is due to proton tunneling, yet naphthol, which is a stronger excited state acid, does not exhibit proton transfer until it is clustered with three ammonia... 

Fig. 16. The biexponential fit to the experimental time resolved 1 + 1 ionization signal of 1 — naphthol(NH3)s cluster. Adapted from Ref. 289. See text and also Refs. 290-292. The rise time is instrument limited. The short decay of ca. 60ps is the rate of proton transfer. The slow decay, t ca. 500 ps, is probably due to reorganization of the cluster. The reeiction does not occur for fewer ammonia molecules solvating the chromophore. ...

Figure 25.6 Dynamics of theadd-basereaction in different-sized clusters of 1-naphthol in ammonia. The cluster distribution is displayed in the inset. Reproduced from Ze-wail, in Femtosecond Chemistry, Voll, 1995, with permission of John Wiley Sons Ltd...

Proton-transfer reactions have been studied in finite-size clusters involving ammonia or water as solvent molecules. One particular system falling into this category has been studied extensively, namely 1-naphthol, here referred to as AH, solvated by ammonia. 

Figure 7.5 Distribution of clusters ROH.. Bn in inert gas. Mass spectrum from time-of-flight measurements on mixtures of 1-naphthol (ROH) and ammonia (B) dispersed in helium, showing peaks corresponding to ROH.. B with n = 1 to 10. From Figure 6 of Ref. [6,a].

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