Mechanical aspects of molecular rods

The mechanical response of three types of molecular rods to various loads was computationally examined. These involved stretching, compression and several modes of bending. The rods studied were (i) polyynes ranging from C4 to C20 (1), (ii) [2]-[4]staffanes (2) and (iii) short oligomers consisting of the bicyclo[2.2.2]octane unit (3). In spite of the inhomogeneity of the rods, a good to excellent compliance with the laws of mechanical engineering was obtained. Thus, all three systems obeyed Hook's law for the serial combination of individual springs. Upon compression the force - first derivative of the energy with respect to the displacement - increases linearly until buckling of the rod is reached. The critical buckling point shows a very good correlation with the inverse square of the length of the rod. In the compression mode, the rod acquires the shape of the sine function, as expected.