A novel topological index Y~X of alkyl derivatives based on the molecular topological thoery is introduced in this paper. The index, which is very easy to calculate and also has good diserimination between alkyl derivatives, is well correlated to the standard enthalpy of formation of several of alkyl derivatives M~nL(n=1,2,3; L=F, OH, Cl, NH~2, Br, SH, I) with good correlation coefficients (R≥ 0.999). The calculated values are in good agreement with the experimental data. This method possesses the advantage of easy computation and clear physical significance. The predicted values of Y~X are also satisfactory.

Key words: ENTHALPY OF FORMATION, ALKYL COMPOUND