科研之友
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摘要
Ab initio anharmonic transition frequencies are calculated for strongly coupled (i) asymmetric and (ii) symmetric proton stretching modes in the X-H+-X linear ionic hydrogen bonded complexes for OCHCO+ and N2HN2+. The optimized potential surface is calculated in these two coordinates for each molecular ion at CCSD(T)/aug-cc-pVnZ (n = 2-4) levels and extrapolated to the complete-basis-set limit (CBS). Slices through both 2D surfaces reveal a relatively soft potential in the asymmetric proton stretching coordinate at near equilibrium geometries, which rapidly becomes a double minimum potential with increasing symmetric proton acceptor center of mass separation. Eigenvalues are obtained by solution of the 2D Schrodinger equation with potential/kinetic energy coupling explicity taken into account, converged in a distributed Gaussian basis set as a function of grid density. The asymmetric proton stretch fundamental frequency for N2HN2+ is predicted at 848 cm(-1), with strong negative anharmonicity in the progression characteristic of a shallow "particle in a box'' potential. The corresponding proton stretch fundamental for OCHCO+ is anomalously low at 386 cm(-1), but with a strong alternation in the vibrational spacing due to the presence of a shallow D-infinity h transition state barrier (Delta = 398 cm(-1)) between the two equivalent minimum geometries. Calculation of a 2D dipole moment surface and transition matrix elements reveals surprisingly strong combination and difference bands with appreciable intensity throughout the 300-1500 cm(-1) region. Corrected for zero point (Delta ZPE) and thermal vibrational excitation (Delta E-vib) at 300 K, the single and double dissociation energies in these complexes are in excellent agreement with thermochemical gas phase ion data.
- 出版日期2010
