Benzene dimer Binding energy

Figure 10.4 compares binding in the water dimer and benzene dimers with that in the water-benzene dimer. The energy components were calculated by EFP... 

Figure 8. Radiative association rate constants measured for dimerization of methyl-substituted benzene ions. The binding energies derived from these measurements are shown in Table 5.

Table 5. Binding Energies Ei, for the Methyl-Substituted Benzene Dimer Cations Based on Application of the Generic Standard Hydrocarbon Model to the Radiative Association Kinetic Data. I obs is the Efficiency per Collision of Radiative...

All such reactions are characterized by a lowering of the cross section as the internal energy of the adduct is increased. The lifetime of the benzene dimer ion, as well as those of similar adduct ions, is very sensitive to its internal energy since it is very loosely bound (it has a binding energy of 8 kcal/mole). Increasing the product-ion internal energy by vibrational excitation of the reactant readily promotes the dissociation back into reactants. 

It was recognized early on that the binding energy of the benzene dimer is quite sensitive to computational method. The potential energy surface of 77 is completely repulsive at the HF level, as well as with any DFT that does not include a dispersion correction, such as B3LYP. " Furthermore, the weakly bound benzene dimer is susceptible to basis set superposition error and counterpoise corrections are likely to be necessary. ... 

There are only a few gas-phase experiments of the benzene dimer. Some experiments point to a T-shape dimer while others ° implicate a sandwich or more likely a displaced stracture. Hole-burning experiments find evidence for multiple different structures. The experimental estimates of the binding energy of 77 are 1.6 0.2 and 2.4 0.4 kcal mol ... 

TABLE 3.30 Binding Energy (kcai moi of the Benzene Dimer 77... 

Figure 3.16 Models of nonaromatic analogs of the benzene dimer. The value within parentheses is the binding energy in kilocalorie per mole computed at (a) SCS-MP2/TZVPP and (b) CCSD(T)/AVTZ.

Class C clusters (ions) tend to have strong bonds in excess of 2000 cm i if they are open shell. An example is the benzene neutral dimer which is bound by 565 cm C while the benzene ionic dimer is bound by 5300 cm (Grover et al., 1989 Krause et al., 1991). On the other hand, closed-shell species are much more weakly bound. For instance, the average binding energy of a methanol monomer to the Cs+(CH30H) cluster is below 1000 cm (Liu and Lisy, 1988 Draves et al., 1990). 

TABLE 10.2 Intermolecular distances (angstroms) and binding energies (kcal/mol) in the water-benzene dimer... 

EFPl and general EFP methods were extensively used to investigate non-covalent interactions in clusters and liquids. For example, EFPl water potential was used to characterize structures and binding energies in water clusters and liquid water [31-33]. General EFP method was employed in studies of alcohol-water clusters and mixtures [34-35] and solvation of ions [36-37], benzene and substituted benzene dimers [14, 38], water-benzene complexes [39], intermolecular interactions in st3mene clusters [40] and DNA base pairs [5,41]. 

EFP interaction energies in the water dimer, benzene dimers, and water-benzene dimers are compared in Fig. 5.1 [39]. Interaction in the water dimer is dominated by the Coulomb term (—8.6 Real/ mol), whereas the polarization and dispersion components are almost 10 times weaker. Contrarily, dispersion forces (—4.9 Real/ mol) determine binding in the parallel-displaced benzene dimer. Interestingly, the two structures of the benzene-water dimer and the T-shaped benzene dimer exhibit significant contributions from... 

Diedrich et al. could demonstrate with calculations on the dimers of methane, ammonia, and water, as well as the benzene dimer, that DMC performs very well on the whole range of interactions from pure dispersive to mainly electrostatic. " They used pseudopotentials and HF orbitals. With a similar approach, Korth et al. calculated the full S22 test set of dimers and the pairs of nucleic adds both in the Watson-Crick and the stacked conformation. The benchmark calculations revealed a mean absolute deviation for the binding energy of only 0.68kcal/mol. Very accurate results for the parallel displaced benzene dimer were obtained by Sorella et al. who obtained a binding energy of 2.2kcal/mol. These authors used their AGP approach with a Jastrow function and carefully optimized wave function parameters. 

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