Supersonic solvation

If one considers the van der Waals complexes as a way to study binary collisions, the possibility of the formation of clusters of given size is a way to probe the role of the environment of other molecules on the reactivity. It is well known that solvent effects play an important role, not only in the kinetics but also in the results of chemical reaction. The study of molecular clusters in supersonic jet experiments allows step-by-step solvation of reactants as will be shown in this chapter, most of the reactions which have been studied occur when a finite number of molecules is reached—this number being often small (less than ten molecules). 

The development of the picosecond-jet technique is presented. The applications of the technique to the studies of coherence (quantum beats), photodissociation, isomerization and partial solvation of molecules in supersonic-jet beams are detailed with emphasis on the role of intramolecular energy redistribution. Experimental evidence for intramolecular threshold effect for rates as a function of excess molecular energy is given and explained using simple theory for the redistribution of energy among certain modes. Comparison with R.R.K.M. calculation is also made to assess the nature of the statistical behaviour of the energy redistribution. 

Most investigations of photoinduced electron transfer have been performed in condensed phases. Much less is known about conditions that permit the occurrence of intramolecular ET in isolated (collision-free) molecular D-A systems. A powerful method for this kind of study is the supersonic jet expansion teehnique (which was originally developed by Kantrowitz and Grey in 1951 [66]) combined with laser-induced fluorescence (LIF) spectroscopy and time-of-flight mass spectrometry (TOF-MS). On the other hand, the molecular aspects of solvation can be studied by investigations of isolated gas-phase solute-solvent clusters which are formed in a supersonic jet expansion [67] (jet cooling under controlled expansion conditions [68] permits a stepwise growth of size-selected solvation clusters [69-71]). The formation of van der Waals complexes between polyatomic molecules in a supersonic jet pro-... 

Experiments with clusters in a supersonic jet can advantageously be employed to study the effects of solvation of a chromophore on its emission spectrum. Hence it is desirable to characterize the composition and the structure of the solute-solvent clusters as precisely as possible. The cluster size distributions can be determined by TOF-MS after resonant two-photon ionization, R2PI [84, 92a,c]. This allows for a... 

The effects of the mixed supersonic expansion of CDMA with various solvent molecules (such as cyclohexane, carbon tetrachloride, acetone, acetonitrile, methanol, dichloromethane and chloroform) on the emission spectra have been investigated by Phillips and co-workers [82d[. The cluster size distribution was varied by changing the nozzle temperature and the partial pressure of the solvent. Two emission components were observed in each case. The long-wave emission was attributed to dimers (which can be isolated or solvated) and to monomer complexed with chloroform or dichloromethane (of unknown stoichiometry). On the other hand, it has been reported by Bernstein and co-workers [84] that CDMA forms with acetonitrile two kinds of 1 1 complexes of different geometry. The first cluster has a structured excitation spectrum, similar to that of the bare molecule, but blue shifted by about 252 cm . The second exhibits a broad excitation spectrum with some resolvable features between 31400 and 31 600 cm (Table 2). The complexes show different fluorescence spectra excitation into the broad absorption leads to the red-shifted emission with respect to that of the monomer (Figure 8) and of the blue ... 

As has been discussed above, molecular clusters produced in a supersonic expansion are preferred model systems to study solvation-mediated photoreactions from a molecular point of view. Under such conditions, intramolecular electron transfer reactions in D-A molecules, traditionally observed in solutions, are amenable to a detailed spectroscopic study. One should note, however, the difference between the possible energy dissipation processes in jet-cooled clusters and in solution. Since molecular clusters are produced in the gas phase under collision-free conditions, they are free of perturbations from many-body interactions or macro-molecular structures inherent for molecules in the condensed phase. In addition, they are frozen out in their minimum energy conformations which may differ from those relevant at room temperature. Another important aspect of the condensed phase is its role as a heat bath. Thus, excess energy in a molecule may be dissipated to the bulk on a picosecond time-scale. On the other hand, in a cluster excess energy may only be dissipated to a restricted number of oscillators and the cluster may fragment by losing solvent molecules. 

One of the most important motivations for the study of gaseous systems, as repeatedly hinted at, is the hope of obtaining a better connection with theory and theoretical modeling. The structure of solvated adducts and charge-transfer pairs in solution cannot be deduced directly from experimental data. In the gas phase, rota-tionally resolved spectroscopy provides information on the structure. The method also allows a much better vibrational resolution than liquid-phase spectroscopy, allowing in principle the elucidation of subtle effects such as the role of torsional motion. All of these advantages are enhanced in supersonic jets, where only a small number of quantum states are initially populated. 

Figure 18. LIF excitation spectra of tetracene seeded in a supersonic expansion of Ar, exhibiting the transition from the isolated molecule (at p = 180 torr) to the molecule solvated in an Ar cluster (p = 8300 torr). Data from Ref. 1 S3.

Cluster ions can be formed by photon or electron interaction with a neutral cluster produced in a supersonic expansion [1321. Another process restricted to clusters is ligand-switching or the replacement of one ligand for another. Often exothermic ligand-switching reactions take place at rates near the gas kinetic limit, especially for small values of n [72, 133]. Chemical-reactivity studies as a function of cluster size show a variety of trends [93, 127. 133]. Proton-transfer reactions are often unaffected by solvation, while nucleophilic-displacement reactions are often shut down by as few as one or two solvent molecules. 

Laser-induced fluorescence also offers the possibility of looking at the solvation structure in its isolated form, that is, short-range order without the continuum contribution. This has attracted keen interest in supersonic molecular beam studies of van der Waals complexes (27-28). The successive additions of n solvating molecules B form a solvated species AB. Spectral shifts and lineshape changes in the absorption of A as it becomes AB, or in the laser-induced fluorescence decay of those consecutively... 

The effect of excess vibrational energy and molecular solvation (in supersonic expansions) on the excited state decay processes [74-78]. The details of the method used to extract rate eonstants for each of the three parallel radiationless decay processes may be found in References [62] and [72]. The reversibility of the photochemical reactions that promote Sj relaxation has been established by comparing the overall quantum yield of photochemical thioketone consumption (< )(, < 10 ) with the net quantum efhciency of the photochanicaUy induced nonradiahve Sj decay processes, which have a dominant role in tiCSj - S,), a number that is much larger [58,62,73],... 

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