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Nature and Strength of Lewis Acid/Base Interaction in Boron and Nitrogen Trihalides.

ABSTRACT: We have quantum chemically investigated the bonding between archetypical Lewis acids and bases. Our state-of-the-art computations on the X₃ B-NY₃ Lewis pairs have revealed the origin behind the systematic increase in B-N bond strength as X and Y are varied from F to Cl, Br, I, H. For H₃ B-NY₃ , the bonding trend is driven by the commonly accepted mechanism of donor-acceptor [HOMO(base)-LUMO(acid)] interaction. Interestingly, for X₃ B-NH₃ , the bonding mechanism is determined by the energy required to deform the BX₃ to the pyramidal geometry it adopts in the adduct. Thus, Lewis acids that can more easily pyramidalize form stronger bonds with Lewis bases. The decrease in the strain energy of pyramidalization on going from BF₃ to BI₃ is directly caused by the weakening of the B-X bond strength, which stems primarily from the bonding in the plane of the molecule (?-like) and not in the ? system, at variance with the currently accepted mechanism.

SUBMITTER: Rodrigues Silva D

PROVIDER: S-EPMC7756781 | biostudies-literature | 2020 Dec

REPOSITORIES: biostudies-literature

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Nature and Strength of Lewis Acid/Base Interaction in Boron and Nitrogen Trihalides.

Rodrigues Silva Daniela D de Azevedo Santos Lucas L P Freitas Matheus M Fonseca Guerra Célia C Hamlin Trevor A TA

Chemistry, an Asian journal 20201021 23

We have quantum chemically investigated the bonding between archetypical Lewis acids and bases. Our state-of-the-art computations on the X3 B-NY3 Lewis pairs have revealed the origin behind the systematic increase in B-N bond strength as X and Y are varied from F to Cl, Br, I, H. For H3 B-NY3 , the bonding trend is driven by the commonly accepted mechanism of donor-acceptor [HOMO(base)-LUMO(acid)] interaction. Interestingly, for X3 B-NH ...[more]

PMID: 33015969

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Project description:The high sensitivity of silole- and silafluorene-containing polymers for detecting organic nitro, nitrate, and nitramine explosives cannot be solely attributed to favorable analyte-polymer hydrophobic interactions and amplified fluorescence quenching due to delocalization along the polymer chain. The Lewis acidity of silicon in conjugated poly(silafluorene-vinylene)s is shown to be important. This was established by examining the (29)Si NMR chemical shifts (Delta) for the model trimer fragment of the polymer CH(3)-silafluorene-(trans-C(2)H(2))-silafluorene-(trans-C(2)H(2))-silafluorene-CH(3). The peripheral and central silicon resonances are up-field from a TMS reference at -9.50 and -18.9 ppm, respectively. Both resonances shift down-field in the presence of donor analytes and the observed shifts (0 to 1 ppm) correlate with the basicity of a variety of added Lewis bases, including TNT. The most basic analyte studied was acetonitrile and an association constant (K(a)) of 0.12 M(-1) was calculated its binding to the peripheral silicon centers using the Scatchard method. Spin-lattice relaxation times (T(1)) of 5.86(3) and 4.83(4) s were measured for the methyl protons of acetonitrile in benzene-d(6) at 20 degrees C in the absence and presence of the silafluorene trimer, respectively. The significant change in T(1) values further supports a binding event between acetonitrile and the silafluorene trimer. These studies as well as significant changes and shifts observed in the characteristic UV-Vis absorption of the silafluorene group support an important role for the Lewis acid character of Si in polymer sensors that incorporate strained silacycles. The nitro groups of high explosives may act as weak Lewis-base donors to silacycles. This provides a donor-acceptor interaction that may be crucial for orienting the explosive analyte in the polymer film to provide an efficient pathway for inner-sphere electron transfer during the electron-transfer quenching process.

Dataset Information

Nature and Strength of Lewis Acid/Base Interaction in Boron and Nitrogen Trihalides.

Publications

Nature and Strength of Lewis Acid/Base Interaction in Boron and Nitrogen Trihalides.

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