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A-X⋯σ Interactions-Halogen Bonds with σ-Electrons as the Lewis Base Centre.

ABSTRACT: CCSD(T)/aug-cc-pVTZ//ωB97XD/aug-cc-pVTZ calculations were performed for halogen-bonded complexes. Here, the molecular hydrogen, cyclopropane, cyclobutane and cyclopentane act as Lewis base units that interact through the electrons of the H-H or C-C σ-bond. The FCCH, ClCCH, BrCCH and ICCH species, as well as the F₂, Cl₂, Br₂ and I₂ molecular halogens, act as Lewis acid units in these complexes, interacting through the σ-hole localised at the halogen centre. The Quantum Theory of Atoms in Molecules (QTAIM), the Natural Bond Orbital (NBO) and the Energy Decomposition Analysis (EDA) approaches were applied to analyse these aforementioned complexes. These complexes may be classified as linked by A-X···σ halogen bonds, where A = C, X (halogen). However, distinct properties of these halogen bonds are observed that depend partly on the kind of electron donor: dihydrogen, cyclopropane, or another cycloalkane. Examples of similar interactions that occur in crystals are presented; Cambridge Structural Database (CSD) searches were carried out to find species linked by the A-X···σ halogen bonds.

SUBMITTER: Grabowski SJ

PROVIDER: S-EPMC8434224 | biostudies-literature |

REPOSITORIES: biostudies-literature

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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.

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A-X⋯σ Interactions-Halogen Bonds with σ-Electrons as the Lewis Base Centre.

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