Dihalogen molecules

Although energy conservation constraints dictate which VP channels are open, it is the nature of the intermolecular interactions, the density of states and the coupling strengths between the states that ultimately dictate the nature of the dynamics and the onset of IVR. These factors are dependent on the particular combinations of rare gas atom and dihalogen molecule species constituting the complex. For example, Cline et al. showed that, in contrast to He Bra, Av = 2 VP in the He Cla and Ne Cla complexes proceeds via a direct... 

The interaction of dihalogen molecules XY with different acceptors B quite often leads to vicious chemical reactions. In most cases, the 1 1 complexes are extremely short-lived. To investigate these prereactive complexes experimentally in a collision-free environment, pulsed-nozzle, Fourier-transform microwave spectroscopy has turned out to be the ideal technique. Legon and coworkers prepared a large number of these complexes and performed detailed rotational spectroscopic analyses. Several series of simple molecules... 

Efg Electric field gradient n-pair Non-bonding electron pair Tt-pair -bonding electron pair XY Generalised dihalogen molecules... 

This chapter is restricted to a discussion of halogen-bonded complexes B XY that involve a homo- or hetero-dihalogen molecule XY as the electron acceptor and one of a series of simple Lewis bases B, which are chosen for their simplicity and to provide a range of electron-donating abilities. Moreover, we shall restrict attention to the gas phase so that the experimental properties determined refer to the isolated complex. Comparisons with the results of electronic structure calculations are then appropriate. All of the experimental properties of isolated complexes B- XY considered here result from interpreting spectroscopic constants obtained by analysis of rotational spectra. 

There are two general conclusions of importance. First, the distance r(Z- X), where Z is the electron donor atom/centre in the complex B- XY, is smaller than the sum of the van der Waals radii ax and ax of these atoms. This result has been shown [179] to be consistent with the conclusion that the van der Waals radius of the atom X in the dihalogen molecule X is shorter along the XY internuclear axis than it is perpendicular to it, i.e. there is a polar flattening of the atom X in the molecule XY of the type suggested by Stone et al. [180]. This result has been shown to hold for the cases XY = CI2 [174], BrCl [175], C1F [176] and IC1 [178], but not for F2, in which the F atom in the molecule appears (admittedly on the basis of only a few examples) to be more nearly spherical [177]. 

The second conclusion concerns the difference Ar = rB...Hx(Z - X)-rB. -xy(Z X) between the Z to X distances in the two series B- HX and B- XY. Ar is positive and nearly constant for a given B and X, when XY is CI2, Br2, BrCl or ClF. Since the order of the internuclear distances is r(XY) > r(HX) for any given atom X, this result means the outer atom Y of the dihalogen molecule XY is always more distant from a given point in B for the complex B- XY than is the atom X from the same reference point in B for the complex B- HX. This second general result is relevant to the discussion of linear versus non-linear hydrogen and halogen bonds in Sect. 6. 

Studies of larger species are more complex and the difficulty in the evaluation of their potential surfaces increases with their size. Up to now accurate potentials have been obtained by inversion of spectroscopic data or through high level ab initio calculations " for several triatomic vdW systems. Thus, the interactions for such clusters are available with satisfactory accuracy, which permits the testing of various models of nonadditivity for their ability to reproduce a number of experimental observations. These facts made complexes composed of two rare-gas atoms and a dihalogen molecule especially attractive targets for the study of nonadditive forces. The first attempt to extract information on nonadditive interactions from... 

Figure 2 Energy contributions AEeistat, AEpauii and AEorb to the total interaction energy AEint in the dihalogen molecules E2 at the equilibrium bond length...

Calculations lead to a similar conclusion for the FIF -I- CI2 pair both of these structures have been subsequently observed by their infrared spectra in solid Ar and Ne". Due to the questionable existence of the H-bonded geometry, as well as the very weak interaction energy of only 2 kcal/mol or less, one can conclude that the dihalogen molecule does not act as a proton acceptor in a H-bond. 

Table 25 4. Bond Energies of Carbon-Halogen Compounds C-X and Covalent and van der Waals Radii of Dihalogen Molecules...

CI2, Bt2,12 and dihalogen compounds XX react with electron donors like amines or ethers to form complexes where the dihalogen molecules act as electron acceptors. The structure of some such complexes will be discussed in Section 18.6. Similarly the hydrogen halides HX react with electron donors to form hydrogen-bonded complexes, and the structures of some of them will be described in Section 18.7. The chapter ends with descriptions of the structures of the hydrogen-bonded H2O dimer in the gas phase, of solid ice and liquid water, and a brief account of the poly water episode. 

For a review of the gas phase structures of complexes of dihalogen molecules with electron donors see A. C. Legon, Angew. Chem. Int. Ed, 38 (1999) 2686. 

J.C. Polanyi and his co-woikers have applied their IR chemiluminescence method to several reactions of H atoms with asymptotic dihalogen molecules, e.g. 

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