Molecular geometry, dipole moment

Semiempirical methods, on the other hand, utilize minimum basis sets to speed up computations, and the loss in rigor is compensated by the use of experimental data to reproduce important chemical properties, such as the heats of formation, molecular geometries, dipole moments, and ionization potentials (Dewar, 1976 Stewart, 1989a). As a result of their computational simplicity and their chemically useful accuracy, semiempirical methods are widely used, especially when large molecules are involved (see, for example, Stewart, 1989b Dewar et al., 1985 Dewar, 1975). 

Similarity is one of the fundamental concepts in chemistry as well as in biochemistry. Recently, several monographs dealt with a special kind of similarity, with molecular similarity [1, 2]. The concept of molecular similarity makes it possible to compare and classify the isolated molecules based on their individual properties, such as molecular geometry, dipole moment, charge distribution, etc. From the mathematical point of view, the relation that two molecules are similar to each other by some of their properties is an equivalency relation on the set of isolated molecules. 

Two theoretical analyses of physicochemical parameters of carbon-containing molecules have been effected. " C(ls) core-electron binding energies, calculated from atomic charges obtained by an electronegativity equilization procedure, and enthalpies of formation, molecular geometries, dipole moments, and first ionization potentials, derived from an improved version (MINDO/3) of the MINDO semi-empirical SCF MO treatment," have all been shown to be in excellent agreeement with experimentally derived values. The results of a theoretical study... 

Quantum chemistry applies quantum mechanics to problems in chemistry. The influence of quantum chemistry is evident in all branches of chemistry. Physical chemists use quantum mechanics to calculate (with the aid of statistical mechanics) thermodynamic properties (for example, entropy, heat capacity) of gases to interpret molecular spectra, thereby allowing experimental determination of molecular properties (for example, molecular geometries, dipole moments, barriers to internal rotation, energy differences between conformational isomers) to calculate molecular properties theoretically to calculate properties of transition states in chemical reactions, thereby allowing estimation of rate constants to understand intermolecular forces and to deal with bonding in solids. 

Dipole Moments In a diatomic molecule the difference in the electronegativities of bonding atoms results in a polar bond and a dipole moment. The dipole moment of a molecule made up of three or more atoms depends on both the polarity of the bonds and molecular geometry. Dipole moment measurements can help us distinguish between Afferent possible geometries of a molecule. 

Relationship Between Molecular Geometry and Dipole Moment Formula Molecular Geometry Dipole Moment ... 

Energy, geometry, dipole moment, and the electrostatic potential all have a clear relation to experimental values. Calculated atomic charges are a different matter. There are various ways to define atomic charges. HyperChem uses Mulliken atomic charges, which are commonly used in Molecular Orbital theory. These quantities have only an approximate relation to experiment their values are sensitive to the basis set and to the method of calculation. 

Contributions in this section are important because they provide structural information (geometries, dipole moments, and rotational constants) of individual tautomers in the gas phase. The molecular structure and tautomer equilibrium of 1,2,3-triazole (20) has been determined by MW spectroscopy [88ACSA(A)500].This case is paradigmatic since it illustrates one of the limitations of this technique the sensitivity depends on the dipole moment and compounds without a permanent dipole are invisible for MW. In the case of 1,2,3-triazole, the dipole moments are 4.38 and 0.218 D for 20b and 20a, respectively. Hence the signals for 20a are very weak. Nevertheless, the relative abundance of the tautomers, estimated from intensity measurements, is 20b/20a 1 1000 at room temperature. The structural refinement of 20a was carried out based upon the electron diffraction data (Section V,D,4). 

The MM2 force field3 is probably the most extensively parameterized and intensively used force field to date. It reproduces a variety of molecular properties such as geometry, dipole moments, conformational energies, barriers to rotation and heats of formation. Of particular importance for calculations of amines is that MM2 treats lone pairs on sp3 nitrogens (and oxygens) as pseudo atoms with a special atom type and parameters. A closely related force field, MM2 7, was derived from MM2 by Osawa and Jaime. MM2 uses the same potential functions as MM2, but employs a different set of parameters in an attempt to better reproduce barriers to rotation about single C—C bonds. 

Molecular electronic dipole moments, pi, and dipole polarizabilities, a, are important in determining the energy, geometry, and intermolecular forces of molecules, and are often related to biological activity. Classically, the pKa electric dipole moment pic can be expressed as a sum of discrete charges multiplied by the position vector r from the origin to the ith charge. Quantum mechanically, the permanent electric dipole moment of a molecule in electronic state Wei is defined simply as an expectation value ... 

This area is a development in the usage of NDDO models that emphasizes their utility for large-scale problems. Structure-activity relationships (SARs) are widely used in the pharmaceutical industry to understand how the various features of biologically active molecules contribute to their activity. SARs typically take the form of equations, often linear equations, that quantify activity as a function of variables associated with the molecules. The molecular variables could include, for instance, molecular weight, dipole moment, hydrophobic surface area, octanol-water partition coefficient, vapor pressure, various descriptors associated with molecular geometry, etc. For example, Cramer, Famini, and Lowrey (1993) found a strong correlation (r = 0.958) between various computed properties for 44 alkylammonium ions and their ability to act as acetylcholinesterase inhibitors according to the equation... 

Fig. 11 Relative size and orientation of dipole moment vectors of the ground state black) and the excited states JMLCT (bJA ), 3IL (b3A"), and 3MLCT (a3A") of [Re(Etpy)(CO)3(bpy)]+, projected onto the optimized ground-state molecular structure. Dipole moment vectors originate in the center of charge calculated using Mulliken population analysis. They lie in the molecular symmetry plane. (Calculated by TD-DFT G03/PBE0/vacuum at the optimized ground state geometry.) Reproduced with permission from [76]...

These polarity descriptors combine charge and geometry. Dipole moments are used to model dipole-monopole, dipole-dipole, dipole-induced dipole, and other interactions. Both molecular dipole (fi) as well as bond dipole moments may be defined for neutral molecules. A bond dipole moment due to atoms k and / separated by distance, rki, can be defined as (]i-The topographic electronic index defined in Eq. [12] is another measure (index) of polarity.The sum extends over the number of bonded atoms, N. ... 

Table 3.2 Ab initio optimized geometries, dipole moments, and intramolecular energies, AE, of 1,2 dimethoxyethane and free energies of hydration, pff, at 25° C predicted from a continuum solvent model supported by molecular...

Cox, R.A., Tyndall, G.S. Rate constants for the reactions of CH3O2 with HO2, NO and NO2 using molecular modulation spectrometry. J. Chem. Soc., Faraday Trans. 2(76), 153-163 (1980) Cremer, D., Gauss, I, Kraka, E., Stanton, J.F., Bartlett, R.J. A CCSD (T) investigation of carbonyl oxide and dioxirane. Equilibrium geometries, dipole moments, infrared spectra, heats of formation and isomerization energies. Chem. Phys. Lett. 209, 547-556 (1993)... 

Pulay P, FogarasI G, Pang F and Boggs J E 1979 Systematic ab initio gradient calculation of molecular geometries, force constants and dipole moment derivatives J. Am. Chem. Soc. 101 2550... 

---