Study OF MOLECULAR INTERACTIONS OF SUBSTITUTED BENZENE BY 13C NMR T1 RELAXATION TIME STUDIES AND AB INITIO QUANTUM MECHANICAL CALCULATIONS

• Main Author: SIAHAAN (NIM 30503008), PARSAORAN
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/11124
• Institution: Institut Teknologi Bandung

Intermolecular interactions and molecular rotational motions are two interrelated physical phenomena, which are important to be studied in order to deeply understand and explore the properties and the usefulness of chemical systems. Intermolecular non-covalent interactions, particularly, are one aspect of interactions that is now intensively studied to complement the more matured understanding of intramolecular covalent interactions. Although it is weak, the intermolecular interactions in the solutions can lead to the molecular association between solute and solvent. The chemical properties are affected by intermolecular interactions which have definite structure and geometry. The studies of intermolecular interaction by combination of 13C NMR T1 spin-lattice relaxation experiment and ab initio quantum theory on the molecular level are relatively new research area.<p>Different substituted groups on benzene have been known to cause different of rotational motion behavior and intermolecular interaction properties. This research studied the substituted groups of phenylacetylene, Ph-C=CH, and benzonitrile, Ph-C=N, which have triple bond in the highly symmetric and polar solvent by combining 13C NMR T1 spin-lattice relaxation time experiment and ab initio quantum calculations. Hexamethylphosphoric triamide (HMPT), OP[N(CH3)2]3, is an exceptional solvent in that it has a quite large dipole moment but comparatively low relative dielectricity constant. This research studied the interrelation of rotational motion behavior and intermolecular interactions in highly diluted solution between substituted benzene molecules and solvent molecules, and to observe the effect of substituted groups of solute and types of solvent, i.e. HMPT and cyclohexane (cy). Substituted group -C=C-H and -C=N has triple bond and they are different on the =C-H and =N groups. The hydrogen atom of -C=C-H group is weakly acidic and positively charge, and the nitrogen atom of -C=C-H group is negatively charge. With those difference that the intermolecular interactions are predicted result in interaction configuration on its association. The molecular association which obtained because of the intermolecular interactions can be described with forms fa---HMPT, fa---cy, bn---HMPT and bn---cy for fa and bn, respectively.<p>The study of effect of the substructure of molecule between -C=C-H and -C=N substituted groups require the research methods which is able to observe the subtle different of properties on molecular level. By experiment, the rotational motion behavior of single molecule and its association in highly diluted solution in cy and HMPT. On the other hand, computationally the ab initio quantum theory can be used to investigate the difference of molecular properties on the substructure and its effect on the properties of the association of molecules. Using the 13C NMR T1 spin-lattice relaxation, the rotation motion of the vector reorientation of the 13C-1H nuclei dipole-dipole on each carbon atom in the molecule can be determined. Whereas, the calculation by ab initio method can determine the interaction energy and its effect on the molecular configuration and geometry which mainly influence the behavior of rotational motion of molecules.<p>This research consists of two main parts : (1) experiments-to investigate the kind of rotational motion of the solute molecules by means of 13C NMR T1 relaxation times at ambient temperature as measured by inversion recovery sequence methods, and (2) computations-to calculate the energy of intermolecular interactions by ab intio self-consistent field, Hartree-Fock (SCF-HF) quantum method. Therefore, the aims of the research were : (1) to determine the effect of substituents -C=CH and -C=N on the rotational motion behavior of phenylacetylene and benzonitrile, respectively, at high dilution in HMPT and cy through the measured 13C T1 relaxation times, and (2) to ascertain the type of molecular association fa---HMPT in comparison to the solute-solvent interaction in fa---cy, bn---HMPT and bn---cy by calculating the intermolecular interaction for various geometries.<p>In HMPT, the 13C T1 spin-lattice relaxation times of phenylacetylene and benzonitrile was reduced four times approximately compared to those in pure form, whereas in cy they were almost similar. It can be concluded that the intermolecular interaction between phenylacetylene and benzonitrile with HMPT were extremely stronger than with cy. The 13C T1 values of carbon nuclei C2,6 (ortho), C3,5(meta), C4(para) of benzonitrile in HMPT were almost similar, ak=1. However, in fa, the 13C T1 values of C2,6 (ortho) and C3,5(meta) are similar and different with C4(para), ak=1.<p>From the molecular shape, Ph-C=CH was expected to exhibit anisotropic rotational behaviour which have been proved from the measured set of T1 values of the ortho, meta and para carbon in the neat liquid as well as in the HMPA and cy solution. This expected results indicate the dominance of a linearly molecules association fa---HMPA along their dipole moment axes as anticipated in observed large HMPA dipole moment. In order to conform with the T1 data, a linear arrangement of Ph-C=CH via the interaction between its weakly acidic H-atom with negatively charged O-atom of HMPA molecules seems to lead to such an anisotropic rotational motion. Similarly, from the molecular shape, Ph-C?N was expected to exhibit anisotropic rotational diffusion which in fact have been proved from the measured set of T1 values of the ortho, meta and para carbon nuclei in the neat liquid as well the cyclohexane solution. In HMPA, on the contrary, Ph-C=N T1 values indicate almost perfect isotropic reorientational motion showing spherically shaped molecules, ak=1. This unexpected results indicate the dominance of a linear configuration fa---HMPA. In order to conform with the T1 data, a layered arrangement of Ph-C=N via the aromatic ring electrons with at least two HMPA molecules seems to lead to such an isotropic rotational motion.<p>The experimental results showed that the fa and bn have the distinctly different interaction behavior in the HMPT solvent. The linearly molecules association fa---HMPA hypothesis were supported by ab initio QM calculations which come out with a higher interaction energy for linear configuration than other geometries. The layered configurations fa---HMPT were energetically forbidden. Whereas the molecular association of bn---HMPT have almost similar interaction energy for ortho, meta, and para configurations. However, the layered configurations were energetically permitted. These ab initio calculations were performed with the theory and basis set of RHF/6-311G(d,p) and BSSE correction for the single molecules as well as for their various configuration of the molecular association.<p>Interaction energies analysis by Mie potential and harmonic oscillator approach which were calibrated to the calculated ab initio shows that the fa---HMPT and bn---HMPT association were stable. This fact can be explained by its vibrational energy levels and vibrational constant. The values of, the depth of the well of Mie potential, for fa---HMPT linier, ortho, meta, and para are -18.62,-6.90, -10.12, and -10.50 kJ/mol, respectively, the ground state energies are 0.87, 0.47, 0.57, and 0.58 kJ/mol, the vibrational constant are 81.44, 23.35, 34.63, and 36.34 Nm-1. For bn---HMPT ortho, meta, and para are -18.69, -16.78, and 16.85, respectively, the ground state energies are 0.78, 0.62, and 0.63 kJ/mol, and vibrational constant are 64.91, 47.80, and 41.85 Nm-1. Whereas the values of for fa---cy and bn---cy association compared with fa---HMPT and bn---HMPT were much smaller, so that there were no energy levels of vibrations for their association.<p>This research could be continued to obtain the more complete understanding and or more quantitative about the relationship between intermolecular interactions and rotational motion behaviour that can be measured by T1 spin-lattice relaxation, as well as to extend the study of the more complex molecules. Eventually, T1 measurements are expected to be used to study interaction in larger molecules (DNA, protein, lipid, cellulose, carbohydrate, and other complex molecules in biological systems or synthetic polymers) and in condensed matter (liquid crystals, graphite, silica), to complement the developping computational approach and ab initio simulations. Molecular dynamics study, based on force-field or ab initio, will enrich our understanding. In the future, those various research are expected to contribute to the spectrum of methods that can be used to study non-covalent interaction that will benefit of our understanding of biomolecular behaviour in living cells, supramolecules, nanoparticles, as well as larger condensed systems. <br /> <br /> <br />