Studies of Weakly Bound Complexes

R M. Felker, P. M. Maxton, and M. W. Schaeffer, Nonlinear Raman studies of weakly bound complexes and clusters in molecular beams. Chem. Rev. 94, 1787 1805 (1994). 

Raman Studies of Weakly Bound Complexes and Clusters in Molecular Beams. 

Chandra, A. K., Nguyen, M. T., 1998, A Density Functional Study of Weakly Bound Hydrogen Bonded Complexes , Chem. Phys., 232, 299. 

As discussed in detail in other chapters of this volume, an important application of weakly bound complexes is in the steric control of chemical reactions (Shin et al. 1991a,b). In these studies, a molecular complex is used as a precursor to provide some control over the reactant geometry. The reaction is initiated by UV... 

A quite different approach to radiofrequency, microwave and infrared spectroscopy is that known as Fourier transform (FT) spectroscopy. As we shall see, this method of recording the spectra of transient molecular species is particularly appropriate in combination with the use of pulsed gas nozzles. For this reason it has proved to be a powerftd technique for the study of weakly bound dimer complexes formed in supersonic gas expansions. It has, however, also been used for the study of diatomic molecules, both... 

From the sophisticated measurements performed in Felker s laboratory it seems that the various versions of ionization-detected. stimulated Raman techniques (with mass analysis) have great capabilities in the high resolution vibrational spectroscopy of weakly bound complexes and clusters. One expects that IDSRS will become increasingly productive in the studies of cluster ground-state structure and dynamics. 

The method of FTMW spectroscopy has been used to study a large number of weakly bound complexes and clusters. One main goal in these studies is to determine the structure and the intermolecular dynamics, that is, the degree of large amplitude vibrational motions, in these systems, and ultimately to characterize the intermolecular interaction potential as a function of separation and orientation. These are aspects of these studies that are of interest to the inorganic chemist. 

A different class of noble gas compounds bound by van der Waals interactions are small molecules NgX, where X is either an atom or a diatomic or triatomic species such as HF, HCl, HBr, CIF, NO, HCN, CI2, Brj. The experimental and theoretical techniques for studying such weakly bound complexes and the concepts and theoretical models to understand them have recently been reviewd in several articles and monographs [151-154] and will not be discussed here. It should be mentioned, however, that detailed structural and dynamical... 

As indicated above, the study of the vibrational predissociation dynamics of weakly bound complexes is in a very fruitful period owing to the overlap which now exists between experiment and theory. Future developments in theory most certainly will be in the area of polyatomic systems where the role of open vibrational channels are likely to be important. 

Prior to this recent activity, little had been known about the intermolecular potential between the hydroxyl radical and argon, though a wealth of information on interaction potentials has been derived from spectroscopic studies of weakly bound vdW complexes in closed shell systems. Complexes of the hydroxyl radical with argon represent a model case for determining the interaction potential in an open-shell system since OH-Ar is both experimentally observable and theoretically tractable. 

Interaction-induced absorption (as the new features were called early on [353]) has stimulated considerable interest. For a long time, explanations were attempted in terms of weakly bound (02)2 polarization molecules (that is, van der Waals molecules), but some of the early investigators argued that unbound collisional pairs might be responsible for the observed absorption. More recently, a study of the temperature dependence of the induced intensities has provided evidence for the significance of collisional complexes. The idea of absorption by collisionally interacting, unbound molecular pairs was, however, not widely accepted for decades. 

Methods are also available in some cases for the direct study of productively bound enzyme-substrate complexes under unreactive conditions. These include the use of a substrate that is weakly reactive because it is at a pH at which the enzyme is largely inactive due to a residue being in the wrong ionic state... 

From the systematic work described in the previous sections several features can be outlined. First, hybrid B3LYP and gradient-corrected PWP functionals predict the same qualitative behaviour in the M (Sc, Ti, Ni and Cu) + C02 reactions, but quantitative differences are often found. With respect to the reactivity of the different transition-metal atoms studied here, it has been shown that Cu andNi give weakly bound complexes with C02 while Ti and Sc are able to form MC02 stable complexes and OMCO insertion products. In the first case, the insertion occurs without any energy harrier while in the second, a small barrier of 6 kcaPmol is found. 

To focus the review, it is necessary to pass over some topics which are timely, relevant, and could be included were it not for space limitations. One of these is the area of electron photodetachment as applied to the study of the Franck-Condon region of the neutral PES accessed from the corresponding anion. This approach has been shown to provide information about the transition state region of several bimolecular reactions. This work was pioneered by Neumark and coworkers and an excellent review is already available (Manolopoulos et al. 1993 Neumark 1992). Another noteworthy area is state-to-state studies of vibrational predissociation in weakly bound complexes. Miller and coworkers have made impressive advances in which fully state and angle-resolved product distributions have been obtained (Bemish et al. 1994 Block et al. 1992 Bohac et al. 1986, 1992a,b Bohac and Miller 1993a,b Dayton et al. 1989), and these results have been used to bring theory and experiment into accord. The present review is limited to cases in which ultraviolet photodissociation of a complexed moiety initiates reaction. 

---