Benzoic acid, molecular models

Nearly all theories to date predict that IETS intensities should be proportional to n, the surface density of molecular scatterers. Langan and Hansma (21) used radioactively labeled chemicals to measure a surface concentration vs solution concentration curve ( Fig. 10 ) for benzoic acid on alumina using the liquid doping technique. The dashed line in Fig. 10 is a 2 parameter fit to the data using a simple statistical mechanical model by Cederberg and Kirtley (35). This model matched the free energy of the molecule on the surface with that in solution. The two parameters in this model were the surface density of binding sites ( 10" A )... 

Fig. 5. Surface diffusion of the rigid rodlike molecule 4-trans-2-(pyrid-4-yl-vinyl) benzoic acid on Pd(110). In (a) and (b) two consecutive STM images taken at 361 K are shown which demonstrate the 1-dim motion. Arrows indicate molecules whose position changed circles mark fractionally imaged molecules moving under the STM tip in the course of the measurement, (c) Model for the flat adsorption geometry explaining the two observed molecular orientations in the STM data. The length of the molecule is 12.5 A. (d) Arrhenius plot of single molecule hopping rates [75].

Molecular model of benzoic acid, CgHjCOOH. Benzoic acid occurs widely in nature, particularly in berries. It finds broad use as a preservative in foods, fats, and fruit juices as a mordant for dying fabric and as a standard in calorimetry and in acid/base analysis. 

The two models have recently come into sharp contrast since they have both been applied to the analysis of the same INS spectrum of benzoic acid [51,53]. The molecular structure of this system is shown in Fig. 9.13. These are the only INS data on O-H-O bonds to be fully analysed using the phonon assisted tunnelling model. However, it is probable that the INS spectra of most systems could be subjected to a similar analysis. 

Figure 3.37 Illustration of the calculated (a) and observed (b) morphologies of benzamide, doped with benzoic acid. (Reproduced with permission from A.S. Myerson (1999), Molecular Modeling Applications in Crystallization, 1st ed., Cambridge University Press.)...

Describe the bonding in benzoic acid using the localized electron model combined with the molecular orbital model. 

Fairly recenfly there have been various other theoretical treatments of substituent effects, e.g. the correlation analysis of substituent effects on the acidity of benzoic acid by the AMI method and direct prediction of linear free-energy substiment effects from 3D structures using comparative molecular field analysis, the relevant data set being 49 substituted benzoic acids . Very recently Russian workers have presented a new model for the inductive effect, in an extremely detailed communication in three parts . The approach appears to be very successful in rationalizing a large amount of relevant experimental data. 

Table 6.7 Molecular properties of benzoic acid derivatives on a model carbon phase, k values are capacity ratios where ki values were measured on an octadecyl-bonded silica gel phase, k2 the molecular form, and fcji the ionized form. ks values were measured on an octadecyl-bonded polyvinylalcohol phase, k4 and ks were measured on a polystyrene gel (Hitachi 3013 and 3011), and ke values were measured on an ODS silica gel (410ODS). ...

A practical model phase for calculating the molecular interaction energy of benzoic acid derivatives was an allq lsilane-bonded polysiloxane, shown in... 

Figure 7.3 Model anion exchanger (hejg lguanidine) and molecular (benzoic acid) interaction.

Table 11 Molecular properties of some benzoic acid derivatives on a model carbon phase. FSl, VWl, HBl, and ESI represent the energy value of the final (optimized) structure, the van der Waals energy, the hydrogen-bonding energy, and the electrostatic energy (kcal mol ) of the complexes between a model carbon phase and a benzoic acid derivative. Log 2 values from ref. 11, log represents the capacity ratios of the molecular form, log 2i represent the ionized form, values from ref. 12. Reproduced by permission of Elsevier, ref. 15.

Table 12 Molecular interaction energy values of benzoic acid derivatives with two model phases. 2 represents a butyl-bonded silica phase, 3 represents a pentyl-bonded silica phase. Reproduced by permission of Elsevier, ref. 15.

A great many different types of molecular descriptors have been employed in the formulation of QSPRs and QSARs [170]. In early pK, studies simple empirical adjustments were employed to represent the influences of structural changes on the pK values of reference compounds [182]. A more systematic approach was proposed by Louis P Hammett, who proposed a linear free energy model for the effects ol substituents on the dissociation of benzoic acid [183-185]. Hammett established the general relationship... 

Murray, Politzer, and their co-workers have developed several descriptors based on features of the molecular electrostatic potential surface (EPS) that can be used to characterize a variety of chemical and physical properties, including pK s [26,199,231]. In studies of the acidities of substituted azoles and anilines they showed that values of the most negative surface potentials (Vrma) and the minimum local ionization energy on the molecular surface (Is,min) showed strong correlations (r 0.97) with the pK s of these compounds. Later, Ma et al. [27] found that Is,jjn and several other EPS descriptors provided good models of the pK variations in substituted phenols and benzoic acids. Sakai and co-workers [232] have shown that Vmin yields an excellent fit (r = 0.996) for the aqueous pK s of a set of 22 amines. These studies demonstrate that features of the molecular electrostatic potential surfaces of acids can offer useful guides for pK, estimatioa... 

The processes of crystallization and crystal growth, like many other processes in chemistry, are controlled by thermodynamic and kinetic factors. Thermodynamics will dictate the preferred, lowest energy form, but the rate at which this is achieved will depend on the processes involved in the molecular attachment kinetic factors. In the simplest model, the molecules are placed at the points of lowest energy on the ideal lattice structure. It is usually assumed that the entity that is being attached is a single molecule however, it could also be a dimer or a cluster of molecules. In certain situations, for instance growth of benzoic acid from a non-polar solvent, the entity which may be involved is a dimer or higher order cluster ... 

---