Atomic ordinalnumber (fi), which is coloured for non-carbon atom in molecular topological graph, appears solity for every non-hydrogen atom in molecule. A novel connectivity index (m^F) basedon the adjacency matric and fi is proposed in this paper. 1^F among m^F has excellent selectivity for alkanes and its derivatives. Satisfactory linear and curve equations between 1^F and - △fHθ^m of 42 alkyl derivatives (such as F, Cl, Br, I, OH, NH2, and SH) are developed as follows: - △fHθ^m=-131.98+101.761^7, γ=0.9884; - △fHθ^m - =- 24355+27.371^F^1.75. γ=0.9956. Their correlativities are all even more ideal than that in the reported literature which γ is equal to 0.2930.

Key words: ALKYL, ENTHALPY OF FORMATION, QUANTITATIVE STRUCTURE ACTIVITY RELATIONSHIP