Conformational study of melamine crosslinkers and spectroscopical comparison of HMMM molecules by practical measurements and quantum chemical calculations

Abstract

To understand and improve network formation processes and performance properties makes structural molecular studies critically important for the coil-coating industry. Crosslinking agents such as hexamethoxymethylmelamine (HMMM) or less methylated derivatives, e.g. methoxymethylmelamine (MMM), are often added to industrial coating formulations. For molecules with considerably fewer atoms and accordingly less rotational freedom (such as MMM) it is readily accessible to identify principal conformations. Thus, an MMM conformer study is straightforward and serves as orientation concerning HMMM conformer studies. For HMMM molecules an extensive computation method was developed to investigate the conformational distribution average and probably most likely molecular structures. Using the density functional theory (DFT) B3LYP-D3BJ method with Dunning's correlation basis set, calculations were performed to investigate the three-dimensional structural geometries of HMMM (basis set cc-pVDZ) and MMM (basis set cc-pVTZ). Beginning with 1500 conformations for HMMM and using various cut-off filters we focused on final residual 22 conformers for solvent phase calculations and 16 conformers for gas phase calculations. To the best of our knowledge, this is the first time that chemical properties for melamine crosslinkers were presented under consideration of conformational population distribution. Thus, computations of fully optimized structural geometries, energies and vibrational states indicate that preferred structural alignments for the methoxymethyl (MM) group in melamine molecules exist. Nevertheless, we suppose that as a matter of principle the MM group of MMM molecules can easily perform rotations by itself and all conformational structure geometries of MMM will exist. Concerning the MM groups position compared to triazine plane HMMM molecules have two main configurations,”3up-3down” and “4up-2down”, as dominant conformer species. Computations of weighted and averaged IR and Raman spectra of final conformations for HMMM monomers, dimers and trimers at ambient temperature are novel and in good agreement with experiment. Prediction of a final UV–Vis spectrum of top ten Boltzmann-weight and averaged monomeric HMMM conformers is in perfect agreement with practical measurement of commercially available HMMM.