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

Abstract

: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.

A Quest for the Origin of Barrier to the Internal Rotation ofHydrogen Peroxide (H2O2) and Fluorine Peroxide (F2O2)

“…Although, more recently, there are also claims of partitioning the total SCF energy into one-and two center components in ab initio methods [20], but such fuzzy atom dissection of total energy appears to calculate the total one-and two-center interaction energies only and decomposition of such energy terms into meaningful physical components are, probably, yet to be furnished [20]. We have recently demonstrated that the Pople's energy partitioning scheme can be invoked in the significant elucidation of the origin of the barrier to the physical process of inversion [21,22] and intramolecular rotation [23,24], and such an effort is a meaningful venture. In the present venture, we shall invoke the energy partitioning scheme of Pople in the elucidation of the origin of barrier to internal rotation of hydrogen peroxide (H 2 O 2 ) and its analogue fluorine peroxide (F 2 O 2 ).…”

Computer simulation of internal rotation around B-N's connection trifluoroammineboron (III)

“…Исторически первым соединением такого рода, полученным еще в начале XIX века, является трифтороамминбор, BF 3 ←NH 3 [2]. Он обладает этаноподобной структурой, которая в последние десятилетия активно изучалась методами рентгеноструктурного анализа [3], микроволновой спектроскопии [4,5], дифракции нейтронов и ЯМР [6,7], а также с помощью расчетных DFT и ab initio приближений [6,[8][9][10]. Сама молекула трифтороамминбора характеризуется заторможенным вращением; при этом минимуму на поверхности потенциальной энергии (ППЭ) отвечает шахматный конформер, а переходному состояниюзаслоненная форма [3][4][5][6][7][8][9][10].…”

“…Таким образом, экспериментальные факты весьма противоречивы. В то же время расчетное значение высоты барьера внутреннего вращения в трифтороамминборе невелико; в рамках DFT приближения при 0 К оно составляет 0.73 ккал/моль [9], а при использовании метода МР2 -1.17 ккал/моль (408 см -1 ) [11]. При этом число методов, использованных для расчета относительной стабильности конформеров трифтороамминбора, весьма ограничено.…”

“…Результаты расчета изолированной молекулы трифтороамминбора в настоящей работе подтверждают известные данные об относительной выгодности шахматной формы (табл.). Однако сравнение экспериментальной величины барьера вращения вокруг В←N связи для газовой фазы (1.6 ккал/моль [5]) с расчетной ΔG 298 ≠ за исключением приближения HF/aug-cc-pVDZ свидетельствует, как и данные литературы [9,11], о заниженной расчетной оценке этого параметра. С другой стороны все использованные приближения заметно завышают длину связи B←N (r B←N ), экспериментальная величина которой составляет 1.60 Å для твердого состояния [3] и 1.59 Å для газообразного [5].…”

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

“…The bond energy can be conveniently evaluated invoking the energy partitioning technique of Fischer and Kollmar [43]. We have already demonstrated that the energy partitioning approach is a meaningful venture in locating the origin of barrier to the physical process of inversion [36,37] and intra molecular rotation [44,45] of molecules. We have, therefore, taken up a detailed study of the dipole correlation of the electronic structure of the conformations, including the equilibrium one, of water molecule between angular to linear shapes.…”