Density functional and molecular orbital study of physical process of inversion of nitrogen trifluoride(NF_3)molecule

摘要

The physical process of the umbrella inversion of the nitrogen trifluoride molecule has been studied invoking the formalisms of the density functional theory,the frontier orbital theory,and the molecular orbital theory.An intuitive structure and dynamics of evolution of the transition state for the event of inversion is suggested.The physical process of dynamic evolution of the molecular conformations between the equilibrium(C_(3v))shape and the planar(D_(3h))transition state has been followed by a number of molecular orbital and density functional parameters like the total energy,the eigenvalues of the frontier orbitals,the highest occupied molecular orbital and lowest unoccupied molecular orbital,the highest occupied molecular orbital and lowest unoccupied molecular orbital,the(HOMO-LUMO)gap,the global hardness and softness,and the chemical potential.The molecular conformations are generated by deforming the angle FNF angle through steps of 2 deg from its equilibrium value,and the cycle is continued till the planar transition state is reached,and the geometry of each conformation is optimized with respect to the length of the N-F bond.The geometry optimization demonstrates that the structural evolution entails an associated slow decrease in the length of the N-F bond.The dipole moment at the equilibrium form is small and that at the transition state is zero and shows a stange behavior with the evolution of conformations.As the molecular structure begins to distort from its equilibrium shape by opening of the angle FNF angle,the dipole moment starts increasing very sharply,and the trend continues very near to the transition state but abruptly vanishes at the transition state.A rationale of the strange variation of dipole moment as a function of evolution of conformatios could be obtained in terms of quantum mechanical hybridization of the lone pair on the N atom.The pattern of charge density reorganization as a function of geometry evolution is continuous depletion of charge from the F center and piling up of charge on the N center.The continuous shortening of bond length and the pattern of variation of net charge densities no atomic sites with evolution of molecular conformations predicts that the bond moment would decrease continuously.The quantum mechanical hybridization of the lone pair of the central N atom shows that the percentage of s character of the lone-pair hybrid on the N atom decreases at a very accelerated rate,and the lone pair at the transition state is accommodated in a pure p orbital.The result of the continued destruction of asymmetry of charge distribution in the lone pair on the central N atom due to the elimination of contribution of the s orbital with evolution of molecular conformations is the sharp decrease in long-pair moment.The decrease in bond moment is overcompensated by the sharp fall of its offestting component,the lone-pair moment,resulting in a net gain in dipole moment with the evolution of molecular geometry.Since the offsetting component decreases very sharply,the net effect is a sharp rise of dipole moment with the evolution of molecular conformations just before the transition state.The lone-pair moment is zero by molecular conformations just before the transition state.The lone-pair moment is zero by molecular conformations just before the transition state.The lone-pair moment is zero by virtue of the symmetry of the pure p orbital,the lone pair of the central atom in the transition state,and the sum of the bond moments is zero by symmetry of the geometry.The barrier height is quite high at approx 65.45kcal/mol,which is close to values computed through more sophisticated methods.It is argued that an earlier suggestion regarding the development of high barrier value of NF_3 system seems to be misleading and confronting with the conclusions of the density functional theory.An analysis and a comparative study of the physical components of the one-and two-center energy terms reveals that