Ab initio CI calculations of the potential curves and nonadiabatic coupling matrix elements for collisions of protons with the ethylene molecule

Abstract

AbstractThe interaction of ethylene with a single proton has been studied theoretically by means of multireference configuration interaction calculations. Potential energy surfaces have been obtained as well as both radial and rotational nonadiabatic coupling matrix elements. Three different approaches of the proton toward the midpoint of the target molecule have been considered for eight collision channels. Along the π direction a number of deep potential minima have been found and the ground‐state proton affinity is calculated to be 7.25 eV on this basis. There is a strongly avoided crossing in this case between the higher‐energy C2H4/H+ incoming channel and the ground C2H/H singlet state, and the corresponding radial coupling matrix element has a broad shape as a result. For the approaches along the in‐plane perpendicular axes of the molecule there is a potential crossing for these two states due to their different symmetries in these orientations, from which it is clear that the incoming channel is strongly bound whereas the charge‐transfer state is repulsive. Comparison with available experimental results indicates that the computed energy differences between the electronic states are accurate to within 0.1–0.2 eV. For the approach along the CC bond there is a weakly avoided crossing between the lowest triplet state of C2H4 and the C2H state corresponding to ionization from the σ molecular orbital (MO). One of the potential minima occurring for the π approach is seen to result from a doubly excited state in which the two π electrons are transferred to a π* MO and the incoming proton, respectively, so that a relatively stable three‐center bond is formed. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003