Waals Complexes and Clusters

laser spectroscopy and quadrupole mass spectrometry were used by Fischer et al. to study vibrational predissociation of clusters of C2H4, and CsHg, but-l-ene, cis- and trans-but-2-ene, and isobutene. They obtained spectra in the range 2900—3200 cm and for C2H4 clusters predissociation was observed to result from excitation near the v-i, and vg fundamentals and the i 2 + V12 combination band. The vibrational bands were observed to have Lorentzian lineshapes with IWHM of ca. 5 cm. A homogeneous broadening mechanism was assumed and the widths were used to calculate excited-state lifetimes. Valentini and co-workers studied the predissociation of C2H4 clusters at 950 cm in a crossed laser/molecular beam apparatus. 

Schumacher, M. Kappes, K. Marti, P. Radi, M. Sdiar, and B. Schmidthalter, Ber. Bunsenges. 

Classical trajectory aitd classical time-dependent self-consistent field calculations of the dynamics of sequential dissociation processes of the type XI,(u)Y X+yu jY X+Y-M ju ) 

Quasiclassical trajectory calculation of the cross-sections for the possible dissociation channels and product energy distributions for ionization/dissociation of ArHj 

Simulation of the i.r. spectrum of ArHD from an accurate calculation of photodissociation cross-sections. Comparison with experiment Laser excitation spectrum of NaArjA H — 

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One of the motivations for studying Van der Waals complexes and clusters is that they are floppy systems with similarities to the transition states of chemical reactions. This can be taken one stage further by studying clusters that actually are precursors for chemical reactions, and can be broken up to make more than one set of products. A good example of this is H2-OH, which can in principle dissociate to fonn either H2 + OH or H2O + H. Indeed, dissociation to H2 O -t H is energetically favoured the reaction H2 + OH—> H2 O -t H is exothennic by about 5000... 

This chapter reviews several gas-phase studies involving atoms, simple molecules, van der Waals complexes and clusters. Electron-transfer reactions are central processes in a variety of scientific disciplines as outlined in a recent review by Bixon and Jortner [1]. We highlight here the current understanding in the dynamics of the gas-phase electron-transfer reactions, and end the chapter by presenting work which intends to bridge the gap between the standard knowledge of electron-transfer reactions in the gas phase and in condensed phases. 

The low translational temperature achieved in supersonic beams allows the generation and observation of loosely bound van der Waals complexes and clusters (Sect. 4.3). The collision-free conditions in molecular beams after their expansion into a vacuum chamber facilitates saturation of absorbing levels, since no collisions refill a level depleted by optical pumping. This makes Doppler-free saturation spectroscopy feasible even at low cw laser intensities (Sect. 4.4). 

The combination of molecular beam methods with laser spectroscopic techniques has brought about a large variety of new methods to study molecules, radicals, loosely bound van der Waals complexes, and clusters. This is discussed extensively in Chap. 9. 

Chapter 11 through Chapter 24 deliver experimental BDEs in inorganics, organo-metallics, ions, clusters, supramolecules, hydrogen- and surface-bonded species, van der Waals complexes, and isotopic species. 

Very few experiments providing direct information about the pathways of intramolecular energy flow in van der Waals complexes have been reported to date. In most cases studied so far, distinct channels for IVR and vibrational predissociation (VP) could not be detected separately. Among the polyatomic van der Waals complexes the cluster T Ar of s-tetrazine and argon is one of the few favourable exceptions. It exhibits several channels for... 

These improved potentials have provided a stimulus to the dynamicists and have led directly to the rapid increase in methods available to study the nuclear motion problem. Above I have considered some of these methods with particular reference to those appropriate for the large amplitude motion found in Van der Waals complexes and problems of increased dimensionality encountered in polyatomic clusters. It is hoped that the proponents of methods that I described as promising prove me right and the proponents of methods which I have expressed doubts about prove me wrong. 

Waals (vdW) complexes and clusters. On the other hand, classical simulations of the same systems are discussed these yield a deeper understanding of cluster structure and dynamics [9-14], and indicate the connection between molecular and bulk physics which arises in these systems [5-16,41]. 

It is also possible to measure microwave spectra of some more strongly bound Van der Waals complexes in a gas cell ratlier tlian a molecular beam. Indeed, tire first microwave studies on molecular clusters were of this type, on carboxylic acid dimers [jd]. The resolution tliat can be achieved is not as high as in a molecular beam, but bulk gas studies have tire advantage tliat vibrational satellites, due to pure rotational transitions in complexes witli intennolecular bending and stretching modes excited, can often be identified. The frequencies of tire vibrational satellites contain infonnation on how the vibrationally averaged stmcture changes in tire excited states, while their intensities allow tire vibrational frequencies to be estimated. 

Supramolecular aggregations are commonly referred to by a variety of terms, including adduct, complex, and van der Waals molecule. In this chapter we shall primarily employ the more neutral term cluster, which may, if desired, be qualified with the type of intermolecular interaction leading to clustering (e.g., H-bonded cluster ). General and specific types of intermolecular forces are discussed in the following sections. 

When facing a full collision process, the existence of such a partial electron transfer state is elusive and usually difficult to evidence experimentally. A major difficulty comes from its transient character. A convenient approach to stabilize the electron transfer between two encounters is to consider a complex formed with both of them. This was the case with the Au (H20) complex mentioned above. This is also the case with the neutral van der Waals complexes considered hereafter. In this case, the characterization of the charge-transfer state requires a careful experimental investigation. We review here how spectroscopy of these clusters can characterize partial electron transfers. 

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-... 

Figure 11. LIF excitation and emission spectra of jet-cooled monomer and clusters of IX [90a-c] top, monomer middle, dimer bottom, van der Waals complexes between IX and acetonitrile. The electronic origins of the excitation spectra of the latter complexes are labelled A (1 2 complex) and B (1 1). Low-intensity features of the 1 3 complex are observed in the red part of the spectrum with the high pressure of acetonitrile. The fluorescence spectra were recorded with a spectral resolution of the detection monochromator of 10-15 nm (the position of excitation is indicated by an asterisk).

The dissociation of weakly bound van der Waals complexes is a special case of unimolecular dissociation [20]. Because of the exceedingly weak coupling between the dissociation coordinate and the mode (or modes) initially excited, and the very low density of states of the energized complex, narrow resonances are the dominant features of van der Waals spectra. There are, of course, many similarities between the dynamics of physically bound and chemically bound molecules. The dissociation dynamics of these special molecules (or clusters) has been studied in great detail, both experimentally and theoretically. Exhaustive review articles are available [85-89] and therefore van der Waals molecules will not be particularly considered in this chapter. However, one must keep in mind that, as the density of states of van der Waals molecules increases, their dynamics becomes more and more comparable with the dynamics of strongly bound molecules [90,91]. 

Extensive attention is devoted to the interactions between aniline and aniline cation and neutral partners to form clusters which are investigated by recent techniques. In particular, van der Waals complexes between aniline and Ar, Kr, N2, CO, in both neutral and cationic forms, are studied by the technique of zero kinetic energy photoelectronic spectroscopy174. Theoretical studies175 on the aniline and aniline cation, free or complexed with a number of partners (as potential solvents) produce important results about the cluster geometry and the relative importance of different kinds of interactions. 

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