Fig. 2

Experimental centre-of-mass product velocity and scattering angle distributions. a Scheme illustrating the directions and velocities of the rectants in the centre of mass frame and defining forward and backward scattering, as well as the scattering angle θ. b Chemical structures of the neutral reactants. c–e Velocity map images for four reactions of type X⁻ + RY (R=CH₃, C₂H₅, ⁱC₃H₇, ^tC₄H₉): c: F⁻ + RCl; d: F⁻ + RI; e: Cl⁻ + RI. Each histogram consists of a sum of about 50,000 product ion events. For each nucleophile-leaving group combination, the right panel illustrates the corresponding scattering angle distribution as function of methyl substitution of the neutral reactant, where the error bars depict the wighted statistical errors for each histogram bin. A clear transition from backward to forward scattering dynamics is observed in all three cases. The energetic limits for the S_N2 and E2 channels are marked by red and white dashed circles, respectively. The experimental result for F⁻ + CH₃Cl (upper left image) has been adapted from a recent publication¹³