Potential energy surfaces for ion-molecule reactions. Intersection of the 3A2 and 2B1 surfaces of NH +2

Abstract

The N⁺+ H₂ system is one of the few ion-molecule reactions for which detailed molecular beam studies have been carried out. To complement this experimental research, we have performed a theoretical study of two of the low-lying NH⁺₂ potential energy surfaces. The intersection and avoided intersection (for C_s geometries) of the lowest ³A₂ and ³B₁ surfaces allows a pathway by which the ground state of NH⁺₂ may be accessed without a potential barrier. The electronic structure calculations employed a double zeta plus polarization basis set, and correlation effects were taken into account using the newly developed Vector Method (VM). To test the validity of this basis, additional self-consistent-field studies were performed using a very large contracted gaussian basis N(13s 8p 3d/9s 6p 3d), H(6s 2p/4s 2p). The ³A₂ surface, on which N⁺ and H₂ may approach, has a surprisingly deep potential minimum, ∼60 kcal mol^–1, occurring at r_e(NH)∼ 1.26 Å and θ_e(HNH)∼ 43°. Electron correlation is responsible for about 15 kcal of this well depth, which appears fairly insensitive to extension of the basis set beyond the double zeta plus polarization level. The line of intersection (or seam) of the ³A₂ and ³B₁ surfaces is presented both numerically and pictorially. The minimum energy along this seam occurs at ∼51 kcal below separated N⁺+ H₂. Thus for sufficiently low energies one expects N⁺— H₂ collisions to provide considerable “complex formation”. Further molecular beam experiments at such low energies (< 0.5 eV) would be of particular interest.