In this paper, a new method of calculating delta (δ_i~z) of bonding atoms in molecules is proposed. Based on the δ_i~z a novel molecular connectivity index ~mX~z is set up as ~mX~z = ∑(δ_i~z·δ _j~z·δ_k~z……)~(-0.5), in which ~0X~z amd ~1X~z are defined as ~0X~z = ∑(δ_i~z)~(-0.5) and ~1X~z = ∑(δ_i~z·δ_j~z)~(-0.5) . Correlation between the~0X~z, ~1X~z and the boiling points for aliphatic unsaturated hydrocarbons is studied. The results show that the topological index has good structure-property correlativity. Relationships between the three molecular structure parameters of Xz, Xz and N (N refers to the number of carbon atoms in molecules), and the boiling points for 80 alkenes, 39 alkynes and 169 aliphatic unsaturated hydrocarbons were examined, respectively, and the following three variable regression equations were obtained: for alkenes, log(779.13 - bp) = 2.822433 - 0.0133346 ~0X~z - 0.0638379 ~1X~z + 0.0111229 N (R = 0.99895, F = 202783.65, s=3.36); for alkynes, log(797.47 - bp) = 2.809912 - 0.0108374 ~0X~z - 0.0864540 ~1X~z+ 0. 02330287V (R = 0.99935, F = 98657.36, s = 3.65); and for aliphatic unsaturated hydrocarbons, log (741.26 -bp)= 2.779526 + 0.0194388 ~0X~z- 0.0519158 ~1Z~z-0.0211047 N (K=0.99467, F = 82387.26, s = 7.74) . These experimental formulae can be used to predict the boiling points of the hydrocarbons.

Key words: molecular connectivity index;BOILING POINTS;ALKENE, ALKANE;ALKYNE