Selected Reference Net Atomic Charges

For example, the charge-NMR shift correlation [Eq.(5.9)] gives n=— 4.4122 for p = 30.12 me. Using this result and the formula given for the methane carbon atom (Table 5.2), we get the useful formula 

There is one particular n that merits special attention. It reflects simple customary ideas The electron-attracting power of otherwise similar atoms decreases as their electron populations increase, thus opposing charge separation. This concept views local charge variations as events occurring most reluctantly, suggesting that the carbon atoms found in alkanes should be very similar to one another, conceivably differing as little as possible from one another. 

For a set of alkanes, each one containing two different carbons with net charges and qs, this constraint amounts to minimizing the sum (qr — qsf over the set. Using the formulas of Table 5.2, it is found that 

The negative n value means that the alkane carbon net charges are positive. This relatively important C -H polarity is in line with the view that hydrogen is certainly more electronegative than carbon, as Mulliken and Roothaan [110] and others [111-113] have pointed out. 

Here we examine the carbon net charges of ethane and ethylene, obtained from SCF and configuration interaction calculations, corrected by means of the appropriate p, determined for n = —4.4122. Remember that the same value of p applies to both ethane and ethylene, as n is solely determined by the effectiveness of the inductive effects. Equation (5.15) is used to get p, namely, p = 138.68 me in 4-31G + Cl calculations and thus, from Eq. (5.10), the corresponding carbon charges of ethane and ethylene (see Table 5.5). 

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