Alkanes atomic charges

Rao and Singh32 calculated relative solvation free energies for normal alkanes, tetra-alkylmethanes, amines and aromatic compounds using AMBER 3.1. Each system was solvated with 216 TIP3P water molecules. The atomic charges were uniformly scaled down by a factor of 0.87 to correct the overestimation of dipole moment by 6-31G basis set. During the perturbation runs, the periodic boundary conditions were applied only for solute-solvent and solvent-solvent interactions with a non-bonded interaction cutoff of 8.5 A. All solute-solute non-bonded interactions were included. Electrostatic decoupling was applied where electrostatic run was completed in 21 windows. Each window included 1 ps of equilibration and 1 ps of data... 

In this equation, q = q C ) + ( 2)] is one-half the sum of the charges of the electron-richest bonded C atoms in each alkane molecule, expressed by means of the appropriate formulas given in Table 7.1. The net atomic charges qc and qc are both positive because n < 0. A decreasing ratio qc/q°c thus means that a carbon atom gains electronic charge with respect to that of ethane. Figure 7.1 indicates that the IPs of the alkanes are lowered as the joint electron population of the electron-richest pair of atoms increases. 

Comparison of adiabatic ionization potentials (IP) of normal and branched alkanes with carbon net charges, which indicates a lowering of the IPs with increasing electron population of the electron-richest bonded pair of carbon atoms in the molecule [170]. A monotonic correlation (which turns out to be linear and remarkably accurate) is possible only with atomic charges adjusted for n = 4.4083 and the corresponding p given by Eq. (5.14). 

Atomic Charges—Methodology and Application to Alkanes, Aldehydes, Ketones, and Amides. 

We should, however, like to call attention to the fact that the various methods may lead to significant differences in the calculated atomic charges. The example of the charges of the alkanes is typical 72) However, it seems that use of a definition of charges based on Eq. (20), with factors X adapted to the basis, will sensibly lower the discrepemcies... 

More specific discussion will focus on relationships between molecular structure and physical properties for the class of compounds as a whole. In this chapter a brief summary of the kinds of forces that attract molecules to each other is presented. Alkanes, lacking charged atoms or highly polarized bonds, do not exhibit either ionic or dipolar forces. As nonpolar molecules, alkane molecules are attracted to each other by only the rather weak London forces. These can be understood fairly simply. In even a totally unpolarized bond, the electrons are always moving. Even though the average location of the electron pair is exactly half-way between the atoms, at any particular moment in time, the electrons may be closer to one atom or the other ... 

The mechanism of IPT-catalyzed conversion of alkanes (RH) involves the photoinduced charge transfer in the photoactive WVI = O group in the octahedral moiety (A) leading to the formation of a reactive electron-deficient radicallike specie (B) capable of abstracting H atom from organic substrates ( denotes an excited state) ... 

On the basis of these results we embarked on a systematic study on the synthesis of vinyl cations by intramolecular addition of transient silylium ions to C=C-triple bonds using alkynyl substituted disila alkanes 6 as precursors.(35-37) In a hydride transfer reaction with trityl cation the alkynes 6 are transformed into the reactive silylium ions 7. Under essentially nonHnucleophilic reaction conditions, i.e. in the presence of only weakly coordinating anions and using aromatic hydrocarbons as solvents, the preferred reaction channel for cations 7 is the intramolecular addition of the positively charged silicon atom to the C=C triple bond which results in the formation of vinyl cations 8-10 (Scheme 1). 

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