Other donor-acceptor complexes

It is even more misleading to use the unlimited flexibility of multipole (or distributed multipole) series to)5t the final tot potential. With sufficient effort, such numerical fitting is assured of success for any chosen set of data values (whether of chemical origin or not). Successful numerical fitting should not be mistaken for conceptual validity or predictive reliability. 

Hydrogen-bonded clusters are an important subset of the large number of neutral and ionic binary complexes (often called van der Waals molecules ) that can now be characterized by modern gas-phase spectroscopic and molecular beam techniques. These complexes often exhibit puzzling structural properties that seem to defy simple rationalization or prediction. As an illustrative example, we consider here the simple n-n binary complexes formed from isoelectronic closed-shell diatomic species CO (carbon monoxide) and NO (nitrosyl cation). 

Let us first try to predict CO- NO structure from the classical electrostatic perspective. A simple geometrical model of CO- - -NO angular shape is shown in (9.5), specified in terms of the angle 6 between fixed monomers, with the negative end of the CO dipole oriented toward the midpoint of the NO cation at distance R (optimized for each 0)  

The actual angular potential AE(9) is shown in Fig. 9.6, demonstrating the spectacular/a//Mrc of electrostatics-based structural reasoning. As seen in Fig. 9.6, the predicted electrostatic wells at 0 = 0°, 180° (or 180°) all turn out to be maxima, and the actual potential minima (near 0 = 55°, 135°) are oriented nearly as far as possible from predicted electrostatic angles. Of course, the angular dependence of Fig. 9.6 

As a result of such CT-induced charge delocalization, Coulomb-type repulsions within the cation are significantly reduced, conferring significant electrostatic stabilization on the complex. This additional CT-induced electrostatic stabilization provides an instructive example of the symbiotic interplay between classical and nonclassical contributions to binding energy, which intrinsically makes the separation into independent energy components somewhat problematic. 

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The investigations included donor adducts with PO [93], 34,PS [+) [94], 35, and P(NMes ) +) [95], 36. While 34 was only formulated as an intermediate species, the other donor-acceptor complexes, 35 and 36, were characterized by X-ray investigations. To complete this series it may also be compared with bis(ylide)-substituted phosphonium halides [96], 37. For these cases the donors refer to... 

Charge transfer and other donor-acceptor complexes... 

However, these observations are not proof of the role of a donor-acceptor complex in the copolymcrization mechanism. Even with the availability of sequence information it is often not possible to discriminate between the complex model, the penultimate model (Section 7.3.1.2) and other, higher order, models.28 A further problem in analyzing the kinetics of these copolyincrizations is that many donor-acceptor systems also give spontaneous initiation (Section 3.3.6.3). 

The UV spectra suggest that the equilibrium between the diazonium ion and the solvent, on the one hand, and an electron donor-acceptor complex (8.58) on the other, lies on the side of the complex. The latter may possibly exist also as a radical pair (8.60) or a covalent compound (8.59). Dissociation of this complex within a cage to form an aryl radical, a nitrogen molecule, and the radical cation of DMSO is slow and rate-determining. Fast subsequent steps lead to the products observed. 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

By contrast, the acidity of the metal salts used in these cements has a less clear origin. All of the salts dissolve quite readily in water and give rise to free ions, of which the metal ions are acids in the Lewis sense. These ions form donor-acceptor complexes with a variety of other molecules, including water, so that the species which exists in aqueous solution is a well-characterized hexaquo ion, either Mg(OH2)g or Zn(OH2)g. However, zinc chloride at least has a ternary rather than binary relationship with water and quite readily forms mixtures of Zn0-HCl-H20 (Sorrell, 1977). Hence it is quite probable that in aqueous solution the metal salts involved in forming oxysalt cements dissolve to generate a certain amount of mineral acid, which means that these aqueous solutions function as acids in the Bronsted-Lowry sense. 

In a similar way, the formation of halide complexes with other jt-acceptors in Fig. 3 are revealed by the appearance of new absorption bands in the electronic spectra to reflect the yellow to red colorations of the mixtures. The spectral data thus indicate that halide salts form well-defined electron donor/acceptor complexes with organic jt-acceptors, as typified by Eq. 2 ... 

Molecules of this type are often called donor-acceptor complexes or sometimes charge transfer complexes (because charge is transferred from the donor to the acceptor as the nonbonding electron pair of the donor atom is shared with the acceptor atom). In other words, there is a formal transfer of one electron, which is evident in the formal charges on the atoms in the complex. Once formed, however, the bond is simply a covalent bond consisting of a pair of shared electrons, whose origin is irrelevant to the nature of the... 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

The usefulness of these ideas in clearing up some old mysteries and puzzles made me think that we are now reaching a clarification of ideas which is bringing a comprehensive theory of cationic polymerisations at last within sight, if not yet within our grasp. A further reason for believing this is that advances in the fields of anionic polymerisation, of radiation chemistry, of electrochemistry, of donor-acceptor complexes, and others, have helped to define the limits within which the comprehensive theory must be constructed. It may be useful to show here just what some of these limits are. 

These figures are based on published and submitted papers. Among others, carboranes, donor-acceptor complexes of boron, ring molecules with silicon, phosphoranes, sul-fones have received much attention. The lack of gas-phase structural studied of other classes of compounds, e. g., sulfuranes, is also noteworthy. The above elements were selected as most typical. Much interest is concentrated however on other elements as well, which may have been somewhat neglected in the past. The increase of the amount of structural data is also facilitating demand for further extension of the circle of compounds studied. 

Attempted preparation of the other adamantane 1,3-diyl dications was also futile. Thus, ionization of 2-methyl-5-bromoadamantane in FS03H-SbF5/S02 gave only the monocation donor-acceptor complex (48). Even the ionization of the 5-bromo-2-adamantanone gave the carboxonium donor-acceptor complex (49)." ... 

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