A: Molecular Structure, Quantum Chemistry, General Theory

Electronically Excited States of Vitamin B12: Benchmark Calculations Including Time-Dependent Density Functional Theory and Correlated ab Initio Methods

Karina Kornobis†, Neeraj Kumar†, Bryan M. Wong‡, Piotr Lodowski§, Maria Jaworska§, Tadeusz Andruniów, Kenneth Ruud, and Pawel M. Kozlowski*†
† Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, United States
‡ Materials Chemistry Department, Sandia National Laboratories, Livermore, California 94551, United States
§ Department of Theoretical Chemistry, Institute of Chemistry, University of Silesia, Szkolna 9, PL-40 006 Katowice, Poland
Institute of Physical and Theoretical Chemistry, Department of Chemistry, Wroclaw University of Technology, 50-370 Wroclaw, Poland
Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Tromsø, 9037 Tromsø, Norway
J. Phys. Chem. A, 2011, 115 (7), pp 1280–1292
DOI: 10.1021/jp110914y
Publication Date (Web): January 31, 2011
Copyright © 2011 American Chemical Society
*Phone (502) 852-6609; fax (502) 852-8149; e-mail pawel@louisville.edu.
General Physical Chemistry

Abstract

Abstract Image

Time-dependent density functional theory (TD-DFT) and correlated ab initio methods have been applied to explore the electronically excited states of vitamin B12 (cyanocobalamin or CNCbl). Different experimental techniques have been used to probe the excited states of CNCbl, revealing many issues that remain poorly understood from an electronic structure point of view. Due to its efficient scaling with size, TD-DFT emerges as one of the most practical tools that can be used to study the electronic properties of these fairly complex molecules. However, the description of excited states is strongly dependent on the type of functional used in the calculations. In the present contribution, the choice of a proper functional for vitamin B12 was evaluated in terms of its agreement with both experimental results and correlated ab initio calculations. Three different functionals, i.e., B3LYP, BP86, and LC-BLYP, were tested. In addition, the effect of the relative contributions of DFT and HF to the exchange-correlation functional was investigated as a function of the range-separation parameter, μ. The issues related to the underestimation of charge-transfer excitation energies by TD-DFT were validated by the Λ diagnostic, which measures the spatial overlap between occupied and virtual orbitals involved in the particular excitation. The nature of the low-lying excited states was also analyzed based on a comparison of TD-DFT and ab initio results. Based on an extensive comparison with experimental results and ab initio benchmark calculations, the BP86 functional was found to be the most appropriate in describing the electronic properties of CNCbl. Finally, an analysis of electronic transitions and reassignment of some excitations were discussed.

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History

  • Published In Issue February 24, 2011
  • Article ASAPJanuary 31, 2011
  • Received: November 15, 2010
    Revised: December 22, 2010

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