Quantum chemical study of the umbrella inversion of the ammonia molecule

Chakraborty

2006

IJMS

Abstract:In order to settle the issue of equivalence or non-equivalence of the two lone pairs of electrons on oxygen atom in water molecule, a quantum chemical study of the dipole correlation of the electronic structure of the molecule as a function of conformations generated following the normal modes of vibrations between the two extreme conformations, C 2v (∠HOH at 90º) and D ∝h (∠HOH at 180º), including the equilibrium one, has been performed. The study invokes quantum mechanical partitioning of molecular dipoles into bond moment and lone pair moment and localization of delocalized canonical molecular orbitals, CMO's into localized molecular orbitals, LMO's. An earlier suggestion, on the basis of photoelectron spectroscopy, that one lone pair is in p-type and the other is in s-type orbital of O atom of water molecule at its equilibrium shape, and also the qualitative "Squirrel Ears" structure are brought under serious scrutiny. A large number of conformations are generated and the charge density matrix, dipole moment of each conformation is computed in terms of the generated canonical molecular orbitals, CMO's and then Sinanoğlu's localization method is invoked to localize the CMO's of each conformation and the quantum mechanical hybridizations of all the bonds and lone pairs on O center are evaluated in terms of the localized molecular orbitals. Computed data demonstrate that the electronic structures i.e. two bond pairs and two lone pairs and its hybridization status of all conformations of water molecule are straightforward in terms of the LMO's. It is further revealed that the pattern of orbital hybridization changes continuously as a function of evolution of molecular shape. The close analysis of the generated LMO's reveals that one lone pair is accommodated in a pure p orbital and another lone pair is in a hybrid orbital in almost all conformations. One more important result of the Int. J. Mol. Sci. 2006, 7 72 present study is that, with the physical process of structural evolution from close angular shape to the linear transition state, the length of the σ (O-H) decreases and its strength increases as a monotone function of reaction coordinates. The bond length is shortest and the strength is largest at the transition state of structural inversion. Result of structural effect of the present study during the evolution of molecular conformations is quite consistent with the result of a very refined calculation that one physically significant feature of force field that the stretching force constants at the linear geometry are considerably larger than their equilibrium counter parts. The variation of bond strength and the hybridization of s and p orbitals on O atom center to form the σ (O-H) bond as a function of evolution of conformations is in accordance with Coulson's prediction. The total dipole moment of all conformations is partitioned into the contribution from bonds and lone pairs and correlated in terms of the computed hybridization in lone pairs. The analysis of the variation of dipole mom...

Section: Dipole Moment As a Probe Of The Electronic Structure Of Watementioning

confidence: 99%

Section: Dipole Moment As a Probe Of The Electronic Structure Of Watementioning

confidence: 99%

Section: Dipole Moment As a Probe Of The Electronic Structure Of Watementioning

confidence: 99%

See 1 more Smart Citation

Dipole Correlation of the Electronic Structures of theConformations of Water Molecule Evolving Through theNormal Modes of Vibrations Between Angular (C2v) to Linear(D∝h) Shapes

Chakraborty

2006

IJMS

“…The details can be found elsewhere [29,30,31]. The total CNDO energy of a system is: E = Σ E A + ΣΣ E AB (28) where E A is monatomic terms and E AB are diatomic terms.…”

Section: Partitioning Of Total Energy Into One-and Two-center Physicamentioning

confidence: 99%

“…Schleyer et al [28] have pointed out that while quite accurate barrier heights can be calculated, even at relatively modest levels of theory, the origin of barrier is still debatable. However, Ghosh et al [29,30] have observed that the determination of the origin of barrier to the physical process of inversion in terms of energy partitioning analysis of Fischer and Kollmar [31] is a meaningful venture.…”

Section: Introductionmentioning

confidence: 99%

Molecular Orbital and Density Functional Study of the Formation, Charge Transfer, Bonding and the Conformational Isomerism of the Boron Trifluoride (BF3) and Ammonia (NH3) Donor-Acceptor Complex

Bhattacharyya

2004

IJMS

:The formation of the F 3 B-NH 3 supermolecule by chemical interaction of its fragment parts, BF 3 and NH 3 , and the dynamics of internal rotation about the 'B-N' bond have been studied in terms of parameters provided by the molecular orbital and density functional theories. It is found that the pairs of frontier orbitals of the interacting fragments have matching symmetry and are involved in the charge transfer interaction. The donation process stems from the HOMO of the donor into the LUMO of the acceptor and simultaneously, back donation stems from the HOMO of acceptor into the LUMO of the donor. The density functional computation of chemical activation in the donor and acceptor fragments, associated with the physical process of structural reorganization just prior to the event of chemical reaction, indicates that BF 3 becomes more acidic and NH 3 becomes more basic, compared to their separate equilibrium states. Theoretically it is observed that the chemical reaction event of the formation of the supermolecule from its fragment parts is in accordance with the chemical potential equalization principle of the density functional theory and the electronegativity equalization principle of Sanderson. The energetics of the chemical reaction, the magnitude of the net charge transfer and the energy of the newly formed bond are quite consistent, both internally and with the principle of maximum hardness, PMH. The Int. J. Mol. Sci. 2004, 2 240 dynamics of the internal rotation of one part with respect to the other part of the supermolecule about the 'B-N' bond mimics the pattern of the conformational isomerism of the isostructural ethane molecule. It is also observed that the dynamics and evolution of molecular conformations as a function of dihedral angles is also in accordance with the principle of maximum hardness, PMH. Quite consistent with spectroscopic predictions, the height of the molecule's barrier to internal rotation is very small. A rationale for the low height of the barrier has been put forward in terms of the energy partitioning analysis. On the question of origin of the barrier to internal rotation, we conclude that the conformational barrier to internal rotation does not originate from a particular region of the molecule, but rather it is a result of the subtle conjoint interplay of a number of opposing effects of one-and two-center bonded and nonbonded energy terms involving the entire skeleton of the molecule.

Density functional and molecular orbital study of physical process of inversion of nitrogen trifluoride (NF₃) molecule

Int J of Quantum Chemistry

Jana

Bhattacharyya

2002

Self Cite

ABSTRACT: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 (HOMO-LUMO) gap, the global hardness and softness, and the chemical potential. The molecular conformations are generated by deforming the FNF angle through steps of 2 • 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 strange behavior with the evolution of conformations. As the molecular structure begins to distort from its equilibrium shape by opening of the 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 conformations 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 a 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 on 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 GHOSH, JANA, AND BHATTACHARYYA 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 lone-pair moment. The decrease in bond moment is overcompensated by the sharp fall of its offsetting 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 ...