Water-benzene dimer

In this example application, the predictions of EFP and accurate ah initio methods are compared with experimental values for the water-benzene dimer, with a focus on the question, what is the nature of the water-benzene interaction ... 

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

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 10.4 Binding in water dimer, benzene dimer, and water-benzene dimer by EFP. All values are in kcal/mol. 

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... 

The attack on the aromatic nucleus by hydroxyl radicals is probably analogous to that by phenyl and methyl radicals, Eq. (34a,b). Evidence that the first step is the addition of hydroxyl radical to benzene, rather than abstraction of a hydrogen atom, has recently been adduced from a study of the radiolysis of water-benzene mixtures. The familiar addition complex may undergo two reactions to form the phenolic and dimeric products respectively, Eq. (34a,b). Alternative mechanisms for the formation of the dimer have been formulated, but in view of the lack of experimental evidence for any of the mechanisms further discussion of this problem is not justified. 

Jorgensen, W. L. Severance, D. L., Aromatic-aromatic interactions free energy profiles for the benzene dimer in water, chloroform and liquid benzene, J. Am, Chem. Soc. 1990, 112, 4768 1774. 

FIGURE 3.4 (See color insert following page 302.) The prototypical noncovalent interactions between (a) water dimer, (b) Ar dimer, and (c) benzene dimer. 

Many applications of new force fields and new QM/MM methods of necessity focus on agreement with experimental or otherwise calculated results. Also in this section we will first show that DRF indeed gives a reliable model for static and response potentials and can lead to QM/MM—or even completely MM calculations—that are as good as, e.g., SCF calculations. To that end we point at some results for simple systems like the water and benzene dimers, and the three- and four-body interactions in several systems. 

While the water dimer serves as the prototype system for studying hydrogen-bonding interactions, the benzene dimer serves this same purpose for tt-tt stacking. The latter may also be relevant for technological applications, but it is mainly associated with... 

In the foregoing sections we have shown that the parameterization of DRF, based on good quality QM calculations (large basis set, correlated wave functions, multipole derived charges and polarizabilities fitted to experiment) on monomers, gives for two difficult and extreme examples, i.e., the water and benzene dimers, results good enough to define it as default for QM/MM and MM calculations. 

It was thus of interest to determine whether a transannular addition could occur for the cyclo-octenyl radical. The cyclo-octenyl radical has been prepared from its parent halohydrocarbon in several matrices in the rotating cryostat. At 77°K the radical was stabilized in all of the matrices but when the samples were warmed reaction took place. The e.s.r. spectra showed that the main reaction in matrices of water, benzene or camphane was hydrogen abstraction adjacent to the double bond to give a cyclic allyl radical (9) rather than the bicyclic radical. However, in a matrix of bicyclopentadiene dimer cyclization occurs to give the bicyclic radical and in a matrix of adamantane both allylic and bicyclic radicals were formed. 

J. Gao, Supercritical hydration of organic compounds. The potential of mean force for benzene dimer in supercritical water, J. Am. Chem. Soc. 115, 6893-6895 (1993). 

Recently, considerable effort has been devoted to obtaining quantum mechanical intermolecular potentials suitable for fluid simulations. For some examples see 75 for (HF)2 76 for (N2)2 77-80 for (CO)2 81 for the benzene dimer 82 for the SiH4 dimer 83 for water-argon and water-methane potentials 84 for the formamide dimer and N,N-dimethylformamide 85 for lithium iodide in dimethylsulfoxide and 86 for Ni2+ in aqueous solution. Rather than discuss each of these studies, here we will focus on a few important developments that we anticipate could alter the capacity or approach to development... 

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]. 

Figure 5.1 EFP total interaction energies and energy components [kcal/mol] in water dimer, two benzene-water dimers, and two benzene dimers. ZPE-corrected interaction energies of the dimers are also shown.

The thermodynamic free energy perturbation route was used by Gao [228] in the analysis of the benzene dimer formation in supercritical water at 673K and 35MPa, via NPT Monte Carlo simulation of TIP4P water model and OPLS benzene. The resulting PMF indicated no local water density increase around benzene, as if benzene behaved as a weakly repulsive solute [187]. In fact, the calculated association constant was approximately three times smaller than that corresponding to water at ambient conditions. 

Gao, J. (1993) Supercritical Hydration of Organic Compounds. The Potential of Mean Force for Benzene Dimer in Supercritical Water, Journal of the American Chemical Society 115, 6893-6895... 

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. 

Interaction energy components for the water dimer, benzene dimer, and the CI2--OH2 complex. The water dimer is in its equilibrium geometry, the benzene dimer is in the parallel stacked geometry with center-of-mass separation 3.7A, and the Cl2"-OH2 complex Is In a symmetrical complex with all atoms in a plane and Cl-Cl 0 collinear with r(CI-"0)= 2.755 A. All energies are reported InkJmor ... 

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