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Vibrational Sum-Frequency Generation Activity of a 2,4-Dinitrophenyl Phospholipid Hybrid Bilayer: Retrieving Orientational Parameters from a DFT Analysis of Experimental Data

Authors

  • Dr. Dan Lis,

    1. Laboratory of Lasers and Spectroscopies (LLS), Research Centre in Matter and Radiation Physics (PMR), Namur Research Institute for Life Sciences (NARILIS), University of Namur (UNamur), 5000 Namur (Belgium)
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  • Dr. Julien Guthmuller,

    1. Faculty of Applied Physics and Mathematics, Gdansk University of Technology, Narutowicza 11/12, 80233 Gdansk (Poland)
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  • Prof. Dr. Benoît Champagne,

    1. Laboratory of Theoretical Chemistry (LCT), University of Namur (UNamur), 5000 Namur (Belgium)
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  • Dr. Christophe Humbert,

    1. Université Paris-Sud,Laboratoire de Chimie Physique, CNRS, Bâtiment 201 P2, 91405 Orsay (France)
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  • Dr. Bertrand Busson,

    1. Université Paris-Sud,Laboratoire de Chimie Physique, CNRS, Bâtiment 201 P2, 91405 Orsay (France)
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  • Dr. André Peremans,

    1. Laboratory of Lasers and Spectroscopies (LLS), Research Centre in Matter and Radiation Physics (PMR), Namur Research Institute for Life Sciences (NARILIS), University of Namur (UNamur), 5000 Namur (Belgium)
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  • Dr. Francesca Cecchet

    Corresponding author
    1. Laboratory of Lasers and Spectroscopies (LLS), Research Centre in Matter and Radiation Physics (PMR), Namur Research Institute for Life Sciences (NARILIS), University of Namur (UNamur), 5000 Namur (Belgium)
    • Laboratory of Lasers and Spectroscopies (LLS), Research Centre in Matter and Radiation Physics (PMR), Namur Research Institute for Life Sciences (NARILIS), University of Namur (UNamur), 5000 Namur (Belgium)
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Abstract

The vibrational nonlinear activity of films of 2,4-dinitrophenyl phospholipid (DNP) at the solid interface is measured by sum-frequency generation spectroscopy (SFG). Hybrid bilayers are formed by a Langmuir–Schaefer approach in which the lipid layer is physisorbed on top of a self-assembled monolayer of dodecanethiol on Pt with the polar heads pointing out from the surface. The SFG response is investigated in two vibrational frequency domains, namely, 3050–2750 and 1375–1240 cm−1. The first region probes the CH stretching modes of DNP films, and the latter explores the vibrational nonlinear activity of the 2,4-dinitroaniline moiety of the polar head of the lipid. Analysis of the CH stretching vibrations suggests substantial conformational order of the aliphatic chains with only a few gauche defects. To reliably assign the detected SFG signals to specific molecular vibrations, DFT calculations of the IR and Raman activities of molecular models are performed and compared to experimental solid-state spectra. This allows unambiguous assignment of the observed SFG vibrations to molecular modes localized on the 2,4-dinitroaniline moiety of the polar head of DNP. Then, SFG spectra of DNP in the 1375–1240 cm−1 frequency range are simulated and compared with experimental ones, and thus the 1,4-axis of the 2,4-dinitrophenyl head is estimated to have tilt and rotation angles of 45±5° and 0±30°, respectively.

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