Abstract: | The derivative of molecular orbitals (MO) energies with respect to a bond length (dynamic orbital force DOF]) is used to estimate the bonding/antibonding character of valence MOs along this bond, with a focus on lone pair MOs, in a series of small molecules: AH (A = F, Cl, Br), AH2 (A = O, S, Se), AX3 (A = N, P, As; X = H, F), and H2CO. The HOMO DOF agrees with the calculated variation of bond length and force constant in the corresponding ground state cation, and of bond length variation by protonation. These results also agree with available experimental data. It is worthy to note that the p‐type HOMOs in AH and AH2 are found bonding. The lone pair MO is bonding in NH3, while it is antibonding in PH3, AsH3, and AF3. |