Download CODECS 2013 Workshop. San Lorenzo de El Escorial, Madrid, 18th

CODECS 2013 Workshop. San Lorenzo de El Escorial, Madrid, 18th –22nd April, 2013
An AAAA-DDDD quadruple hydrogen bond array – bonding, Free
Energy Surface and NMR spectra
Andrzej Bil
Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14 50-383 Wroclaw, Poland
Physikalisch Chemisches Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057
Zürich
[email protected]
Attractive arrays are intermolecular complexes formed by two monomers interacting
through multiple hydrogen bonds. Such materials are interesting as they may serve as building
blocks for supramolecular polymers where nets of monomers are held together by reversible and
highly directional non-covalent interactions. The binding strength of the complex depends on the
number of hydrogen bond involved as well as on the Donor-Acceptor pattern adopted by the net
of bonds. Recently, two cation molecules adopting AAAA-DDDD pattern have been
synthesized1,2, while one of them has an unusually high association constant Ka > 3x1012 M-1.1
Here we report the results of the theoretical studies of this complex including the interaction
energy decomposition, Free Energy Surface exploration, thermodynamics of the complex and
competing structures and NMR spectra simulation.
Unusual stability of the complex (Eint = -60.0 kcal mol-1) is related to its positive charge,
as an interaction energy of its neutral analogous is only -25.8 kcal mol-1. The interaction energy
of the charged complex decays much slower with the intermolecular distance than for the neutral
one, which is a factor influencing the mechanism of dissociation of the complex.
NMR spectrum calculated for an equilibrium structure has some features qualitatively
different from the experimental one, which may indicate some dynamic processes in the solution.
We applied metadynamics approach to Free Energy Surface exploration to find structures which
can compete with the equilibrium structure at room temperature and therefore influence the
experimental spectra. Here we present NMR spectra averaged over the structures available
within the thermal boost.
Free energy of the complex formation ΔG(300K) calculated from the partition function is
as large as 38 kcal mol-1, which is too large comparing the experimental estimation. One of the
possible factors influencing the calculated free energy are limitations of the harmonic model
applied to a system where high anharmonicity of modes related to N···H hydrogen bonds can be
expected. Here we discuss how anharmonic effects influence vibrational and rotational
contributions to the partition function of the complex, on the basis of Simple Perturbation
Theory3,4Body of the abstract. It can have several sections.
References
1. Barry A. Blight, Christopher A. Hunter, David A. Leigh, Hamish McNab, Patrick I. T. Thomson,
Nature Chemistry, 3, 244 (2011).
2. Jörg Taubitz, Ulrich Lüning, Aust. J. Chem., 62, 1550 (2009)
3. Donald G. Truhlar, Alan D. Isaacson, J. Chem. Phys., 94, 357 (1991)
4. Julien Bloino, Malgorzata Biczysko, Vincenzo Barone J. Chem. Theory Comput., 8, 1015 (2012)