An lmprovement on the Formula for Group Electronegativity Liu Huai, Wang Qingwei, and Llu Lixin Siping Teacher's College, Siping, Jilin Province, P.R. China
Several group electronegativity methods have been proposed according to Sanderson's principle of electronegativ. last (3) was given by Steven. ity equalization ( 1 , 2 , 3 ) The
An lmprovement to Equation 1 Based on the discussion above, electronegativity should be equalized within the margin of groups belonging to the centers of positive and negative charges or within groups belonging to the symmetric centers. However, electronegativities of atoms in molecules should in general not he considered as equalized. So, we improved eq 1.
where X, is the electronegativity as equalized in Sanderson's principle; where v is the number of atoms in a particular molecule or group; where N=
v = the total number of atoms in the molecule or group
where x is the electronegativityof each atom; and where q is the partial charge on the group. The Limitations of Equation 1 The use of eq 1can cause deviations. However, eq 1is satisfactory for simple inorganic molecules and symetrical molecules such as NaC1, CzH4,etc. Equation 1 is not satisfactory for complex organic molecules or molecules that have the same atoms but different functional groups, such as CH3COOH, CH30H, etc. Hydrogen atoms of different groups in molecules have different environments that cause differential distribution of partial charges. Thus, the equalization electronegativities should be differenttoo. Furthermore, when calculating the group electronegativities using eq 1,different groups with the same composition have the same group electronegativity. Two examples are given below.
This is not acceptable because the values of group electronegativities basically represent the power of inductive effects that should decrease as the distance increases. Comparisons of Group Electronegativities (Pauling Units) Group -CH3 --CzHs -CHO -COOH -0CHO -SCN -NCS -0CN -NCO
-ON0 -NO0 -0CI dCl0 -0C102 -0C103
This work 2.28 2.29 2.64 2.84 3.00 2.67 2.78 3.07 2.98 3.33 3.30 3.29 3.36 3.38 3.40
Sanderson 2.63 2.64 2.96 3.12 3.12 2.96 2.96 3.18 3.18 3.49 3.49 3.56 3.59 3.61 3.62
where No is the number of groups in the species formula vo or the number of groups given; and where X'G is the electronegativity of group G.
where n, is the number of atoms or smaller groups in group G, v, the number of atoms or smaller groups; and where xp is the electronegativityof an atom or smaller group. Because the total charge in any system is conserved, all partial charges must be equal to q.
where q is the total number of charges in the system, S is the partial charge on each atom or group, and v is the number of atoms or groups in the system. The following example illustrates the use of eqs 2 , 3 , and 4. (Pauling electrouegativities are from ref 4). Sample Calculations for Methy Alcohol CH30H Group Electronegativitiesand the EqualizedElectronegativity
Bratsch 2.28 2.29 2.64 2.80 2.80 2.71
2.71 2.97 2.97 3.30 3.30 3.29 3.34 3.37 3.38
The Partial Charges on the Groups (5)
Volume 69 Number 10 October 1992
783
OLI'O+ =
LWO+ =
z.z
=%
Z'Z - PLP'Z
z.z
=%
Z'Z - PZE'Z
