Activation and Thermodynamic Parameter Study of the Heteronuclear CO···H-N Hydrogen Bonding of Diphenylurethane Isomeric Structures by FT-IR Spectroscopy Using the Regularized Inversion of an Eigenvalue Problem Nicolas Spegazzini,* ,† Heinz W. Siesler, ‡ and Yukihiro Ozaki † † Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Gakuen 2-1, Sanda, Hyogo 669-1337, Japan ‡ Department of Physical Chemistry, University of Duisburg-Essen, D 45117 Essen, Germany ABSTRACT: The doublet of the ν(CO) carbonyl band in isomeric urethane systems has been extensively discussed in qualitative terms on the basis of FT-IR spectroscopy of the macromolecular structures. Recently, a reaction extent model was proposed as an inverse kinetic problem for the synthesis of diphenylurethane for which hydrogen-bonded and non- hydrogen-bonded CO functionalities were identified. In this article, the heteronuclear CO···H-N hydrogen bonding in the isomeric structure of diphenylurethane synthesized from phenylisocyanate and phenol was investigated via FT-IR spectroscopy, using a methodology of regularization for the inverse reaction extent model through an eigenvalue problem. The kinetic and thermodynamic parameters of this system were derived directly from the spectroscopic data. The activation and thermodynamic parameters of the isomeric structures of diphenylurethane linked through a hydrogen bonding equilibrium were studied. The study determined the enthalpy (ΔH = 15.25 kJ/mol), entropy (TΔS = 14.61 kJ/mol), and free energy (ΔG = 0.6 kJ/mol) of heteronuclear CO···H-N hydrogen bonding by FT-IR spectroscopy through direct calculation from the differences in the kinetic parameters (δΔ ‡ H, -TδΔ ‡ S, and δΔ ‡ G) at equilibrium in the chemical reaction system. The parameters obtained in this study may contribute toward a better understanding of the properties of, and interactions in, supramolecular systems, such as the switching behavior of hydrogen bonding. ■ INTRODUCTION Supramolecular polymer chemistry has become a major field of research in recent years. 1 Supramolecular polymers consist of relatively low molecular weight species that are able to assemble spontaneously into higher-ordered structures through designed motifs that form reversible noncovalent bonds based on hydrogen bonding interactions. 2-4 Self-assembled polymers are usually stimuli-responsive, 5 and the strength of supramolecular interactions between constituent monomers in the system can often be precisely controlled by the application of a suitable external perturbation (to avoid a second time stimulus) such as heat 6 or light. 7 This, in turn, enables specific physical properties of the polymers, such as tensile strength, to be modulated rapidly. Upon removal of the external stimulus, the properties of the material return to those of its original state. This switchable behavior of supramolecular polymers has been demonstrated in applications as diverse as adhesives, coatings, and most recently, healable materials. 8 Thus, hydrogen bonding is important in many fields because it directly affects the final properties of many materials and physicochemical processes. The strengths of hydrogen bonds span a broad range: the very strong have covalent character, whereas the weak have energies slightly above van der Waals interactions. 9 Hydrogen bonds can be generally described as “the hydrogen is an attractive interaction between the hydrogen f rom a group XH and an atom or a group of atoms Y, in the same or dif ferent molecule(s), where there is evidence bond formation”. 10 A thorough understanding of the nature and strength of hydrogen bonding in a system can provide insight into its fundamental chemical and biochemical processes (e.g., DNA structure, protein folding, and enzyme interactions). 11 Furthermore, such understanding is essential in the design of sophisticated materials that use hydrogen bond motifs to build supramolecular structures. 12 A number of homonuclear (CO···H-O and N···H-N) and heteronuclear (CO···H-N) hydrogen bonding motifs have been developed and applied to the preparation of supramolecular materials. 13,14 Some of the most elegant and successfully employed motifs in supramolecular polymerization are the quadruple hydrogen-bonding ureidopyrimidone developed by Meijer and co-workers, 15,16 the chain folding polyimide, and the telechelic polyurethane based on a simple hydrogen-bonding interaction. 17 Weaker binding motifs (multiple hydrogen bonding) lead to potentially lower depolymerization temperatures (akin to Received: December 12, 2011 Revised: June 15, 2012 Published: June 26, 2012 Article pubs.acs.org/JPCA © 2012 American Chemical Society 7797 dx.doi.org/10.1021/jp211968s | J. Phys. Chem. A 2012, 116, 7797-7808