Dynamics and fragmentation of van der Waals and hydrogen bonded cluster cations: (NH₃)_n and (NH₃BH₃)_n ionized at 10.51 eV

A 118 nm laser is employed as a high energy, single photon (10.51 eV/photon) source for study of the dynamics and fragmentation of the ammonia borane (NH₃BH₃) cation and its cluster ions through time of flight mass spectrometry. The behavior of ammonia ion and its cluster ions is also investigated under identical conditions in order to explicate the ammonia borane results. Charge distributions, molecular orbitals, and spin densities for (NH₃BH₃)_n and its cations are explored at both the second-order perturbation theory (MP2) and complete active space self-consistent field (CASSCF) theory levels. Initial dissociation mechanisms and potential energy surfaces for ionized NH₃BH₃, NH₃, and their clusters are calculated at the MP2/6-311++G(d,p) level. Protonated clusters (NH₃)_xH⁺ dominate ammonia cluster mass spectra: our calculations show that formation of (NH₃)_n−1H⁺ and NH₂ from the nascent (NH₃)_n⁺ has the lowest energy barrier for the system. The only common features for the (NH₃)_n⁺ and (NH₃BH₃)_n⁺ mass spectra under these conditions are found to be NH_y⁺ (y = 0,…,4) at m/z = 14–18. Molecular ions with both ¹¹B and ¹⁰B isotopes are observed, and therefore, product ions observed for the (NH₃BH₃)_n cluster system derive from (NH₃BH₃)_n clusters themselves, not from the NH₃ moiety of NH₃BH₃ alone. NH₃BH₂⁺ is the most abundant ionization product in the (NH₃BH₃)_n⁺ cluster spectra: calculations support that for NH₃BH₃⁺, an H atom is lost from the BH₃ moiety with an energy barrier of 0.67 eV. For (NH₃BH₃)₂⁺ and (NH₃BH₃)₃⁺ clusters, a B^δ+⋯H^δ−⋯^δ−H⋯^δ+B bond can form in the respective cluster ions, generating a lower energy, more stable ion structure. The first step in the (NH₃BH₃)_n⁺ (n = 2, 3) dissociation is the breaking of the B^δ+⋯H^δ−⋯^δ−H⋯^δ+B moiety, leading to the subsequent release of H₂ from the latter cluster ion. The overall reaction mechanisms calculated are best represented and understood employing a CASSCF natural bond orbital description of the valence electron distribution for the various clusters and monomers. Comparison of the present results with those found for solid NH₃BH₃ suggests that NH₃BH₃ can be a good hydrogen storage material.