Thermal Charge Transfer Methods

In the thermal charge transfer methods the electron affinity of a molecule is determined by bracketing the electron affinity of a test species between that of two species with known electron affinities. When the reaction studied is AB —) + CD AB + CD(—) direct charge transfer, the relative electron affinities 

In 1983 the HPMS TCT method for determining molecular electron affinities was introduced. It is based on the same fundamental concepts as the ICR method, but the absolute value was anchored to the electron affinity of benzophenone obtained from the ECD. Later, the scale was anchored to the electron affinity of SO2. At present about 300 electron affinities of organic molecules have been determined by the TCT and/or ECD methods [1,3, 55-59]. The TCT method has been used to measure Ea between 0.50 eV for nitromethane and 3.2 eV for tetracyano-ethylene, as shown in Chapter 10. 

Many TCT values were determined by both the ion cyclotron resonance (ICR) and high-pressure mass spectrometry (HPMS) variants [3, 55-59]. In the ICR method AG values are calculated from measured ion ratios and known concentrations at a single temperature and AH obtained by making assumptions about AS. For example, in the above study of naphthoquinone and benzoquinone, the entropy changes are assumed to be the same [56]. In the HPMS method measurements are made as a function of temperature so that AH and AS can be obtained (AH = AG + T AS). Good agreement between the independently obtained values 

The kinetic method for determining the direction of charge transfer by colli-sional ionization is based on the competitive dissociation of mass-selected electron 

This method was first applied to relative electron affinities of substituted nitro-benzenes. All but one of these has been measured by HPMS TCT studies. However, the Ea of s-butyl nitrobenzene has only been determined by collisional ionization and is still listed in the NIST tables as 2.17(20) eV. This value is referenced to a high value for nitrobenzene and should be about 1 eV lower [60]. The electron affinities of aromatic hydrocarbons have been reported using the collisional ionization method. The value for biphenylene is larger than that obtained from half-wave reduction potentials. The values for pyrene, anthracene, and c-CgHg are consistent with other reported values, but the values for benzanthracene, coronene, and benzo[ghi]perylene are significantly lower than the largest precise value and are attributed to excited states. 

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In the second half of the twentieth century four experimental techniques were developed to measure Ea the equilibrium, the photon, the beam, and the thermal charge transfer methods [4, 18-29]. In addition to the above reactions observed in the ECD and NIMS, negative ions can be formed in other reactions and complementary energetics and kinetics determined. The ECD and NIMS results have been integrated and compared with data obtained from other studies [4] ... 

Speedy and accurate desktop computers and modern programs such as HYPERCHEM place quantum mechanical calculations within the reach of any experimental chemist. The CURES-EC procedure simulates equilibrium methods of measuring electron affinities by calculating the difference between the optimized forms of the anion and neutral. The READS-TCT determination of charge densities in anion complexes simulates thermal charge transfer experiments. The effect of... 

The pulsed source method, despite several limitations, appears to be a very useful technique for studying ion-molecule reactions at thermal energies. Although the studies to have date been limited primarily to simple hydrogen transfer reactions, the technique should also prove useful for studying charge transfer and hydride ion transfer reactions at thermal energies. 

At shorter distances, particularly those characteristic of H-bonded and other charge-transfer complexes, the concepts of partial covalency, resonance, and chemical forces must be extended to intramolecular species. In such cases the distinction between, e.g., the covalent bond and the H-bond may become completely arbitrary. The concept of supramolecular clusters as fundamental chemical units presents challenges both to theory and to standard methods of structural characterization. Fortunately, the quantal theory of donor-acceptor interactions follows parallel lines for intramolecular and intermolecular cases, allowing seamless description of molecular and supramolecular bonding in a unified conceptual framework. In this sense, supramolecular aggregation under ambient thermal conditions should be considered a true chemical phenomenon. 

Many spectroscopic methods have been employed for the investigation of such systems For example, wide-band, time-resolved, pulsed photoacoustic spectroscopy was employed to study the electron transfer reaction between a triplet magnesium porphyrin and various quinones in polar and nonpolar solvents. Likewise, ultrafast time-resolved anisotropy experiments with [5-(l,4-benzoquinonyl)-10,15,20-triphenylpor-phyrinato]magnesium 16 showed that the photoinduced electron transfer process involving the locally-excited MgP Q state is solvent-independent, while the thermal charge recombination reaction is solvent-dependent . Recently, several examples of quinone-phtha-locyanine systems have also been reported . 

Excitation of the charge transfer band is a powerful method to ionize the CT complex, especially when the ionization is not thermally accessible. Several examples are shown below (12, 13). 

The method of proton or nuclear magnetic resonance has been applied to the compounds (C5H6)2ReH and Fe(C5H6)2 (213, 132). Furthermore, attempts were made by spectrophotometric methods to establish the formation of a thermodynamically stable molecular complex of fer rocene (H). According to these measurements, ferrocene and iodine together in solution show no charge transfer complex, but are merely in thermal equilibrium with ferricinium triiodide. 

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