QTAIM and the stress tensor describe a torsion phi, 0.0 degrees <= phi < 25.0 degrees, of the C4-C7 torsional bond that affects the H center dot center dot center dot H bonding interactions linking the two phenyl rings of para-substituted biphenyl, C12H9X, X = N(CH3)(2), NH2, CH3, CHO, CN, NO2 and C12H9-Y, Y = SiH3, ZnCl, COOCH3, SO2NH2, SO2OH, COC1, CB3. For all values of X and Y, shorter H center dot center dot center dot H bond-path lengths corresponded to higher values of both the stiffness and torsion co. The onset of a phase transition-like behavior is found by the stress tensor stiffness. The atomic basins of the H H interactions are affected by the para-substituent groups. (C) 2016 Elsevier B.V. All rights reserved.