MOLECULAR INTERACTION BETWEEN BENZONITRILEAND HEXAMETHYLPHOSPHORIC TRIAMIDEBY 13CNMR T1RELAXATIONTIME STUDIES ANDAS INITIOQM CALCULATIONS: EXTENDED INVESTIGATION

ABSTRACT It has been obtained the anisotropy ratio a = Tdortho-,meta-13C)/Tdpara-13Co)f dilute solutions of bn change from 1.7 in fa solution and 1.5 in neat bn to 1.0 in HMPT. Thus the anisotropyratio comes out to be a = 1. In HMPT, obviously, solvent molecules cluster around Ph-C=N in such a way, that non-covalent interactions lead to isotropic reorientational motion like a spherical molecule. To conform with the T1times, a layer arrangement with at least two HMPT molecules per Ph-C=N seems to be likely. From computational quantum calculations of non-covalent intermolecular interactions and Mie potential analysis, the solute-solventmolecularpairs in bn.. .HMPThave almost equal interaction energies for the ortho, meta, and para configuration and the layered configurations are energeticallypermitted Keywords: 13CT1spin-lattice relaxation times, ab initio quantum chemistry calculations, intermolecular interactions, isotropic and anisotropic rotational motio

MOLECULAR INTERACTION BETWEEN BENZONITRILE AND HEXAMETHYLPHOSPHORIC TRIAMIDE BY ¹³C NMR T₁ RELAXATION TIME STUDIES AND AB INITIO QM CALCULATIONS: EXTENDED INVESTIGATION

It has been obtained the anisotropy ratio a = T1(ortho-,meta-13C)/T1(para-13C) of dilute solutions of bn change from 1.7 in fa solution and 1.5 in neat bn to 1.0 in HMPT. Thus the anisotropy ratio comes out to be a = 1. In HMPT, obviously, solvent molecules cluster around Ph-CºN in such a way, that non-covalent interactions lead to isotropic reorientational motion like a spherical molecule. To conform with the T1 times, a layer arrangement with at least two HMPT molecules per Ph-CºN seems to be likely. From computational quantum calculations of non-covalent intermolecular interactions and Mie potential analysis, the solute-solvent molecular pairs in  bnּּּHMPT have almost equal interaction energies for the ortho, meta, and para configuration and the layered configurations are energetically permitted    Keywords: 13C T1 spin-lattice relaxation times, ab initio quantum chemistry calculations, intermolecular interactions,  isotropic and anisotropic rotational motio

INVESTIGATION OF MOLECULAR INTERACTION BETWEEN PHENYLACETYLENE AND HEXAMETHYLPHOSPHORIC TRIAMIDE BY ¹³C NMR T₁ RELAXATION TIME STUDIES AND AB INITIO QM CALCULATIONS

Intermolecular interactions and molecular translational and rotational mobility are key factors in molecular material sciences, e.g. liquid crystals. One of the important substructures is given by phenylacetylene, Ph-CºCH. Its rotational behavior in its pure form and in high dilution in hexamethylphosphoric triamide OP[N(CH3)2]3 (HMPA) has been studied by means 13C NMR T1 relaxation times at ambient temperature as measured by the inversion recovery method. HMPA is an exceptional solvent in that is has a quite large dipole moment but comparatively low relative dielectricity constant. From the molecular shape Ph-CºCH is expected to exhibit anisotropic rotational diffusion which in fact can be deduced from the measured set of T1 values of the ortho, meta and para carbon nuclei in the neat liquid as well as in the HMPA solution. This expected result rules the dominance of a linearly molecules pair Ph-CºCH...HMPA along their dipole moment axes as anticipated in view of the large HMPA dipole moment. In order to conform with the T1 data, a linear arrangement of Ph-CºCH via the interaction between its weakly acidic H-atom with negatively charge O-atom of HMPA molecules seems to lead to such an anisotropic rotational motion. This hypothesis is supported by ab initio QM calculations which come out with higher interaction energy for linear orientation than other geometries. These ab initio calculations were performed with the basis set of RHF/6-31G(d) for the single molecules of Ph-CºCH and HMPA as well as for their various geometries of the molecules pair. Molecular dynamics simulations need to be performed for further confirmation.   Keywords: Relaxation Times, HMPA, pheylacetylene, ab initio, intermolecular interaction, rotational diffusio