By organizing an extracurricular seminar, based on the analysis of the intermolecular dispersion action in normal alkanes, a new topological index (named "Intermolecular Interaction Index (IMI)") was proposed to express the intermolecular dispersion force. The IMI has an excellent linear relationship with the boiling point (T[subscript b]) of normal alkanes containing carbon atoms C[subscript 2]-C[subscript 40] (standard error only 0.87 K). For T[subscript b] of branched alkane isomers, only the addition of a parameter [delta]AOEI ("average odd-even index difference") is needed to establish the correlation equation. The seminar activity promotes students' ability for molecular structure-property reasoning and provides students with a preliminary understanding of the molecular graph, topological index, and principle of development of quantitative structure-property relationship (QSPR) models.