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Journal of Chemical Physics / Volume 134 / Issue 6 / ARTICLES / Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

J. Chem. Phys. 134, 064325 (2011); http://dx.doi.org/10.1063/1.3535595 (12 pages)

Density functional theory and Raman spectroscopy applied to structure and vibrational mode analysis of 1,1′,3,3′-tetraethyl-5,5′,6,6′-tetrachloro- benzimidazolocarbocyanine iodide and its aggregate

Metin Aydin1, Özge Dede1, and Daniel L. Akins2

1Department of Chemistry, Faculty of Art and Sciences, Ondokuz Mayıs University, 55139 Samsun, Turkey
2Center for Analysis of Structures and Interfaces (CASI), Department of Chemistry, The City College of The City University of New York, New York, New York 10031, USA

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(Received 1 September 2010; accepted 17 December 2010; published online 14 February 2011)

We have measured electronic and Raman scattering spectra of 1,1′,3,3′-tetraethyl-5,5′,6,6′-tetrachloro-benzimidazolocarbocyanine iodide (TTBC) in various environments, and we have calculated the ground state geometric and spectroscopic properties of the TTBC cation in the gas and solution phases (e.g., bond distances, bond angles, charge distributions, and Raman vibrational frequencies) using density functional theory. Our structure calculations have shown that the ground state equilibrium structure of a cis-conformer lies ∼200 cm−1 above that of a trans-conformer and both conformers have C2 symmetry. Calculated electronic transitions indicate that the difference between the first transitions of the two conformers is about 130 cm−1. Raman spectral assignments of monomeric- and aggregated-TTBC cations have been aided by density functional calculations at the same level of the theory. Vibrational mode analyses of the calculated Raman spectra reveal that the observed Raman bands above 700 cm−1 are mainly associated with the in-plane deformation of the benzimidazolo moieties, while bands below 700 cm−1 are associated with out-of-plane deformations of the benzimidazolo moieties. We have also found that for the nonresonance excited experimental Raman spectrum of aggregated-TTBC cation, the Raman bands in the higher-frequency region are enhanced compared with those in the nonresonance spectrum of the monomeric cation. For the experimental Raman spectrum of the aggregate under resonance excitation, however, we find new Raman features below 600 cm−1, in addition to a significantly enhanced Raman peak at 671 cm−1 that are associated with out-of-plane distortions. Also, time-dependent density functional theory calculations suggest that the experimentally observed electronic transition at ∼515 nm (i.e., 2.41 eV) in the absorption spectrum of the monomeric-TTBC cation predominantly results from the π → π* transition. Calculations are further interpreted as indicating that the observed shoulder in the absorption spectrum of TTBC in methanol at 494 nm (i.e., 2.51 eV) likely results from the ν″ = 0 → ν′ = 1 transition and is not due to another electronic transition of the trans-conformer—despite the fact that measured and calculated NMR results (not provided here) support the prospect that the shoulder might be attributable to the 0−0 band of the cis-conformer.

© 2011 American Institute of Physics

Article Outline

  1. INTRODUCTION
  2. COMPUTATIONAL METHODS AND EXPERIMENTAL SYSTEM
    1. Calculations
    2. Experimental
  3. RESULTS AND DISCUSSION
    1. Description of structure
    2. Charge distribution
    3. Electronic structure
      1. Characterization of TTBC by UV-visible spectroscopy
      2. Assignment of the monomeric band of TTBC by the DFT calculation
    4. Vibrational mode assignment
    5. Aggregation induced Raman bands
    6. Theoretical basis for AERS
  4. CONCLUSION

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KEYWORDS and PACS

Keywords

density functional theory, ground states, molecular configurations, organic compounds, Raman spectra, vibrational states

PACS

  • 33.20.Fb

    Raman and Rayleigh spectra (including optical scattering)

  • 31.15.E-

    Density-functional theory

  • 33.20.Tp

    Vibrational analysis

  • 33.15.Mt

    Rotation, vibration, and vibration-rotation constants

  • 33.15.Bh

    General molecular conformation and symmetry; stereochemistry

ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1063/1.3535595

PUBLICATION DATA

ISSN

0021-9606 (print)  
1089-7690 (online)

Publisher

$abstract.society.value is a Member of CrossRef American Institute of Physics

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