Dipole moments, electron density mapping

Further, we have considered the equilibrium configuration or distribution of the electrons within the molecule (dipole moments, electron density maps, and, for simple molecules, the spectroscopic description of the ground electronic states of molecules). We now have left a consideration of the displaceability of these electrons from their equilibrium configurations in their ground states. We have already encountered a term which is in effect a measure of this displaceability, namely the polarizability (see section 18). Since the molecule as a whole and especially its electrons must be subject to the quantum restrictions, the electrons cannot be considered as continuously displaceable but rather as capable of existing only in a... 

More recently, a variety of other methods has been developed to describe the electronic distribution properties of drug molecules. The electron distribution in a molecule can be estimated or determined by experimental methods such as dipole-moment measurements, NMR methods, or X-ray diffraction. The latter method provides very accurate electron-density maps, but only of molecules in the solid state it cannot be used to provide maps of the nonequilibrium conformers of a molecule in a physiological solution. 

Dative bonds are often drawn in textbooks as arrows between atoms, to indicate the transfer of charge from one atom to another upon donation of a lone pair of electrons. Very early on, Bader pointed out that this description is far from reality. Electron density maps were calculated for NH3 and Sip4 and their adduct H3N S1F4 (which can also be written as H3N -Si F4). The results are shown in Figure 10.55 [50]. The pyramidal NH3 molecule has a dipole moment of 1.47 D, while the tetrahedral Sip4 has none. Upon adduct formation, the dipole moment increases dramatically to 5.61 D. The structure and dipole moment have been verified by microwave spectroscopy [51]. However, it would be erroneous to conclude that this is due to the... 

One interesting species, unknown until recently, is HOF, which was studied by Peslak et al.,611 and electron-density contour maps were described. More recently, it was studied by Kim and Sabin.512 The computed bond lengths with a near-minimal GTO basis were i (H—O) = 1.080 A, J (0—F) = 1.450 A, and 0 = 100.8°, in fair agreement with the results of a microwave investigation. Force constants were computed and the population analysis was reported. A more extensive basis set was used in a similar calculation by Ha, who computed a dipole moment of 2.72 D.513... 

Once the multipole analysis of the X-ray data is done, it provides an analytical description of the electron density that can be used to calculate electrostatic properties (static model density, topology of the density, dipole moments, electrostatic potential, net charges, d orbital populations, etc.). It also allows the calculation of accurate structure factors phases which enables the calculation of experimental dynamic deformation density maps [16] ... 

Fig. 4. The anatomy of a p-like state . Two isodensity contour maps (+ 0.01 and + 0.03 a.u." ) of the same LUMO orbital are shown side by side. Unlike the p-Uke orbitals in one-electron models, LUMO states in MQC MD-DFT and CIS models have the lobes pushed outwards between the first and the second solvation shells, with < 20% of the spin density residing inside the cavity. This results in considerable firagmentation of the diffuse part of the wavefunction. The O 2p orbitals are strongly polarized, with opposite signs of the orbitals attained by water molecules on the opposite sides of the cavity in the direction of transition dipole moment.

F ure 10.4 Bond moments and resultant dipole moments in NH3 and NF3. The electrostatic potential maps show the electron density distributions in these molecules. 

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