Quantum chemical study of the umbrella inversion of the ammonia molecule

The physical event of the umbrella inversion of ammonia has been studied in detail by application of the formalisms of frontier orbital theory, the de nsity functional theory, the localized molecular orbital method, and the en ergy partitioning analysis. An intuitive structure for the transition state and dynamics of the physical process of structural reorganization prior to inversion have been suggested. The computation starts with the CNDO/2 equi librium geometry, and thereafter the cycle proceeds through all the conform ations of ammonia obtained by varying the angle HNH angle in steps of 2 deg rees from its equilibrium value up to the transition state. The geometry of each conformation is optimized with respect to the length of the N-H bond. The glimpses of the charge density reorganization during the movement of t he molecule from equilibrium conformation toward the transition state is co mputed in terms of dipole moment and the quantum mechanical hybridizations of bond pair and lone pair of N atom through the localized molecular orbita ls (LMOs) of all the conformations. Results demonstrate that as the geometr y of the molecule begins to evolve through the reorganization of structure, the N-H bond length and the dipole moment begin to decrease, and the trend continues up to the transition state. The dipole moment of the molecule at the suggested transition state is zero. The computed nature of quantum mec hanical hybridization of bond pair and lone pair of the N atom as a functio n of reaction coordinates of the angle HNH angles reveals that the percenta ge of s character of the lone pair hybrid decreases and that of the bond pa ir hybrid forming the sigma(N-H) bond increases during the process of geome try reorganization from the equilibrium share to the transition state. The rationale of the zero dipole moment of the transition state for inversion i s not straightforward from its point-group symmetry, hut is self-evident fr om its electronic structure drawn in terms of LMOs. The electronic structur e of the transition state, which may be drawn in terms of the LMOs, seems t o closely reproduce its suggested intuitive structure. The pattern of varia tion of dipole moment and nature of the changes of the percentage of the s character in the lone pair hybrid creating dipole with the evolution of geo metry during the physical process of structural reorganization for the inve rsion are found to be nicely correlated according to the suggestion of Coul son. The profiles of the increasing strength of the N-H bond and the increa sing percentage of s character of the bond pair hybrid of N atom forming th is bond as a function of reaction coordinates are also found to be correlat ed in accordance with the suggestion of Coulson. The profile of global hard ness as a function of reaction coordinate seems to demonstrate that the dyn amics of the evolution of the molecular structure from equilibrium shape to the transition state following the course of suggested mode of structural reorganization conforms to the principle of maximum hardness (PMH). The pro files of parameters like the energies of highest occupied and lowest unoccu pied molecular orbital (HOMO and LUMO), the gap in energy between HOMO and LUMO, the global hardness, the global softness, and chemical potential as a function of reaction coodinates of a continuous structural evolution from equilibrium shape to the transition state mimic the potential energy diagra m of the total energy. Both the frontier orbital parameters and the density functional quantities are found to be equally effective and reliable to mo nitor the process of necessary structural reorganization prior to the inver sion of molecules. An energy partitioning analysis demonstrates that the origin of barrier has no unique single source rather as many as four mutually exclusive but inte racting one and two-center energy terms within the molecule entail the orig in and the height of the barrier. From a close analysis of the results, it seems highly probable that the necessary structural reorganization prior to umbrella inversion of ammonia most realistically occurs following the cour se of normal modes of vibration of the molecule. (C) 2000 John Wiley & Sons , Inc.