  The control and observation of reactants forming a chemical bond at the
single-molecule level is a longstanding challenge in quantum physics and
chemistry. Using a single CO molecule adsorbed at the apex of an atomic force
microscope tip together with a Cu(111) surface, the molecular bending is
induced by torques due to van der Waals attraction and Pauli repulsion. As a
result, the vertical force exhibits a characteristic dip-hump evolution with
the molecule-surface separation, which depends sensitively on the initial tilt
angle the CO axis encloses with the surface normal. The experimental force data
are reproduced by model calculations that consider the CO deflection in a
harmonic potential and the molecular orientation in the Pauli repulsion term of
the Lennard-Jones potential. The presented findings shed new light on
vertical-force extrema that can occur in scanning probe experiments with
functionalized tips.
