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Issue 19, 2008
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Scope and limitations of the SCS-MP2 method for stacking and hydrogen bonding interactions

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Abstract

Fluorobenzenes are π-acceptor synthons that form π-stacked structures in molecular crystals as well as in artificial DNAs. We investigate the competition between hydrogen bonding and π-stacking in dimers consisting of the nucleobase mimic 2-pyridone (2PY) and all fluorobenzenes from 1-fluorobenzene to hexafluorobenzene (n-FB, with n = 1–6). We contrast the results of high level ab initio calculations with those obtained using ultraviolet (UV) and infrared (IR) laser spectroscopy of isolated and supersonically cooled dimers. The 2PY·n-FB complexes with n = 1–5 prefer double hydrogen bonding over π-stacking, as diagnosed from the UV absorption and IR laser depletion spectra, which both show features characteristic of doubly H-bonded complexes. The 2-pyridone·hexafluorobenzene dimer is the only π-stacked dimer, exhibiting a homogeneously broadened UV spectrum and no IR bands characteristic for H-bonded species. MP2 (second-order Møller–Plesset perturbation theory) calculations overestimate the π-stacked dimer binding energies by about 10 kJ/mol and disagree with the experimental observations. In contrast, the MP2 treatment of the H-bonded dimers appears to be quite accurate. Grimme’s spin-component-scaled MP2 approach (SCS-MP2) is an improvement over MP2 for the π-stacked dimers, reducing the binding energy by ∼10 kJ/mol. When applied to explicitly correlated MP2 theory (SCS-MP2-R12 approach), agreement with the corresponding coupled-cluster binding energies [at the CCSD(T) level] is very good for the π-stacked dimers, within ±1 kJ/mol for the 2PY complexes with 1-fluorobenzene, 1,2-difluorobenzene, 1,2,4,5-tetrafluorobenzene, pentafluorobenzene and hexafluorobenzene. Unfortunately, the SCS-MP2 approach also reduces the binding energy of the H-bonded species, leading to disagreement with both coupled-cluster theory and experiment. The SCS-MP2-R12 binding energies follow the SCS-MP2 binding energies closely, being about 0.5 and 0.7 kJ/mol larger for the H-bonded and π-stacked forms, respectively, in an augmented correlation-consistent polarized valence quadruple-zeta basis. It seems that the SCS-MP2 and SCS-MP2-R12 methods cannot provide sufficient accuracy to replace the CCSD(T) method for intermolecular interactions where H-bonding and π-stacking are competitive.

Graphical abstract: Scope and limitations of the SCS-MP2 method for stacking and hydrogen bonding interactions

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Supplementary files

Article information


Submitted
29 Nov 2007
Accepted
24 Jan 2008
First published
18 Feb 2008

Phys. Chem. Chem. Phys., 2008,10, 2758-2766
Article type
Paper

Scope and limitations of the SCS-MP2 method for stacking and hydrogen bonding interactions

R. A. Bachorz, F. A. Bischoff, S. Höfener, W. Klopper, P. Ottiger, R. Leist, J. A. Frey and S. Leutwyler, Phys. Chem. Chem. Phys., 2008, 10, 2758
DOI: 10.1039/B718494H

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