Equilibrium bond lengths, harmonic frequencies, electron affinities, and dissociation energies (as appropriate) were determined for I, I-,I2, I2 -, and I3 - by CCSD(T) and four DFT methods (BLYP, BPW91, B3LYP, and B3PW91) using a basis set consisting of a relativistic effective core potential and a triple-zeta plus 2df polarization functions (ECP-TZ(2df)) for the valence electrons. Comparison of the DFT results with the CCSD(T) results and available experimental information indicates that the B3PW91 approach does particularly well describing the bonding in these species. B3PW91/ECP-TZ(2df) calculations of I5 - in linear (D ∞ h ) and bent (C 2 v ) geometries indicate that the linear structure is a low-energy transition state lying only about 0.1−0.2 eV above the C 2 v global minimum-energy structure. Harmonic frequencies and infrared and Raman intensities are calculated for both structures and compared to available experimental information.
The reactions of B+ + nH2 to produce BH2 +(H2) n - 1 have been studied by high-level ab initio techniques. The reaction mechanism and associated activation energy is found to depend dramatically on the number of H2 molecules present. For n = 1, the reaction proceeds stepwise: first breaking the H2 bond and forming one BH bond followed by forming the second BH bond. This process has an activation energy of about 57 kcal/mol. For n = 2, the reaction proceeds via a pericyclic mechanism though a planar cyclic transition state where two H2 bonds are broken while simultaneously two BH bonds and one new H2 bond are formed. The activation energy for this process decreases dramatically from the n =1 value to only about 11 kcal/mol. For n = 3, the reaction proceeds through a true insertion mechanism; however, the actual insertion occurs late in the reaction after over 75% of the exothermicity has been realized. The addition of the third H2 molecule decreases the activation energy to only about 3.4 kcal/mol. For n = 4, the reaction mechanism is essentially identical to that of the n = 3 case. However, the fourth H2 causes the activation energy to increase by about 2 kcal/mol relative to the n = 3 case because the additional H2 molecule causes one of the other three H2 molecules to be slightly further away from the boron ion in the transition state geometry. The computational results are compared with the experimental results of Kemper, Bushnell, Weis, and Bowers (J. Am Chem. Soc. 1998 , 120, xxxx) and are in full agreement with the experimental conclusion that the n = 3 electrostatic cluster ion is the most reactive. On the basis of a comparison of experimentally determined magnitude and isotopic dependence of the activation energies with the computed adiabatic reaction barriers, it is suggested that the observed reaction rate may be dominated by a nonclassical tunneling contribution.
The stability of the ammonium radical (NH4) was determined from measurements of the kinetic energy released in its fragmentation products following formation in a fast electron capture process: NH4++Na → NH4*+Na+. Scattering profiles for heavy (NH3) and light (H) dissociation products were obtained from beam measurements with 5–16 keV NH4+ ions. The existence of a predissociative barrier in the radical is inferred from edge structure and scattering continua in H atom profiles. The radical is bound with respect to a potential minimum but all of the isotopic species NH4, NH3D, NH2D2, and NHD3 undergo rapid loss of H atoms and have ground states lying above their dissociation limits. The radical ND4 has unusual stability with its ground state lying close to or below its dissociation limit. Dissociative lifetimes for stable and unstable states of ND4 differ by at least two orders of magnitude. The possible significance of these observations on the interpretation of optical transitions involving the ground states of NH4 and ND4 are discussed. From these measurements the ionization potential of 4.73±0.06 eV for NH4 is calculated. The stabilities of CH3NH3 and CH3ND3 radicals and their dissociative pathways have also been investigated.
Born-Oppenheimer (BO) potentials for the ground and first-excited electronic states of He2+ are determined using high level ab initio techniques for internuclear separations R of 1.2-100 bohrs and accurately fit to analytical functions. In the present formulation, the BO potentials are nuclear mass independent, and the corresponding BO approximation is obtained by ignoring four terms of the full rovibronic Hamiltonian. These four Born-Oppenheimer correction (BOC) terms are as follows: (1) mass polarization, (2) electronic orbital angular momentum, (3) first derivative with respect to R, and (4) second derivative with respect to R. In order to enable an exact rovibronic calculation, each of the four BOC terms are computed as a function of R, for the two electronic states and for their coupling, without any approximation or use of empirical parameters. Each of the BOC terms is found to make a contribution to the total energy over at least some portion of the range of R investigated. Interestingly, the most significant coupling contribution arises from the electronic orbital angular momentum term, which is evidently computed for the first time in this work. Although several BOC curves exhibit a nontrivial dependence on R, all are accurately fit to analytical functions. The resulting functions, together with the BO potentials, are used to compute exact rovibronic energy levels for 3He 3He+,3He 4He+) and 4He 4He+. Comparison to available high quality experimental data indicates that the present BOC potentials provide the most accurate representation currently available of both the low- and high-lying levels of the ground electronic state and the bound levels of the excited state.
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