A Novel/Old Modification of the First Zagreb Index

In the seminal paper [I. Gutman, N. Trinajsti, Chem. Phys. Lett. 1972, 17, 535-538], it was shown that total electron energy (E ) of any alternant hydrocarbon depends on the sum of the squares of the degrees of the corresponding molecular graph. Nowadays, this sum is known as the first Zagreb index. In the same paper, another molecular descriptor was proved to influence E , but that descriptor was never restudied explicitly. We call this descriptor as modified first Zagreb connection index and denote it by ZC . In this paper, chemical applicability of the molecular descriptor ZC is tested for the octane isomers. Some basic properties of ZC are also established here. Furthermore, the alkanes with maximum and minimum ZC values are determined from the class of all alkanes having fixed number of carbon atoms.