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Issue 29, 2010
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Ab initio anharmonic vibrational frequency predictions for linear proton-bound complexes OCH+CO and N2H+–N2

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Abstract

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 Schrödinger 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∞h transition state barrier (Δ = 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 (ΔZPE) and thermal vibrational excitation (ΔEvib) at 300 K, the single and double dissociation energies in these complexes are in excellent agreement with thermochemical gas phase ion data.

Graphical abstract: Ab initio anharmonic vibrational frequency predictions for linear proton-bound complexes OC–H+–CO and N2–H+–N2

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Publication details

The article was received on 10 Feb 2010, accepted on 01 Jun 2010 and first published on 29 Jun 2010


Article type: Paper
DOI: 10.1039/C002774J
Citation: Phys. Chem. Chem. Phys., 2010,12, 8311-8322
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    Ab initio anharmonic vibrational frequency predictions for linear proton-bound complexes OCH+CO and N2H+–N2

    K. Terrill and D. J. Nesbitt, Phys. Chem. Chem. Phys., 2010, 12, 8311
    DOI: 10.1039/C002774J

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