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Issue 7, 2005
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The geometric (H/D) isotope effect in porphycene: grid-based Born–Oppenheimer vibrational wavefunctions vs. multi-component molecular orbital theory

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Abstract

Two approaches for the determination of the primary and secondary geometric isotope effect are compared for the exemplary porphyrinoid system porphycene, which has two intramolecular hydrogen bonds. A three-dimensional Born–Oppenheimer potential energy surface is calculated in terms of the symmetric and antisymmetric N–H stretching as well as a low-frequency hydrogen bond vibrational normal mode coordinate. From the respective ground-state nuclear wavefunction the quantum correction to the classical equilibrium geometry is determined. Further, geometry optimization within a full-dimensional multi-component molecular orbital (MC_MO) type calculation, which treats both the electrons and the hydrogen-bonded protons quantum mechanically, is performed. Both approaches yield geometric isotope effects, that is, upon H/D double substitution the hydrogen bonds are weakened and the respective N–N distances increase. In addition the MC_MO calculation gives a H/D isotope effect on the electronic structure, that is, the electronic wavefunction becomes more localized at the deuterium nucleus as compared with the proton case.

Graphical abstract: The geometric (H/D) isotope effect in porphycene: grid-based Born–Oppenheimer vibrational wavefunctions vs. multi-component molecular orbital theory

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Publication details

The article was received on 14 Jan 2005, accepted on 21 Feb 2005 and first published on 02 Mar 2005


Article type: Paper
DOI: 10.1039/B500620A
Citation: Phys. Chem. Chem. Phys., 2005,7, 1368-1373
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    The geometric (H/D) isotope effect in porphycene: grid-based Born–Oppenheimer vibrational wavefunctions vs. multi-component molecular orbital theory

    M. F. Shibl, M. Tachikawa and O. Kühn, Phys. Chem. Chem. Phys., 2005, 7, 1368
    DOI: 10.1039/B500620A

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