27Al quadrupole interaction in zeolites loaded with probe molecules--a quantum-chemical study of trends in electric field gradients and chemical bonds in clusters.
← Research streamDensity functional theory calculations in zeolite clusters found that protonation forming a Brønsted acid site weakens a corresponding Al–O bond and is associated with a large EFG and a calculated 27Al QCC of 18.2 MHz. Adsorption of probe molecules via hydrog…
Study snapshot
Abstract
The electric field gradient (EFG) has been calculated in zeolite clusters at the aluminium site surrounded by four SiO4 tetrahedra. Density functional theory (DFT) with the 6-31G** basis set has been employed. Formation of a Brønsted acid site by protonation of one oxygen atom of the AlO4 tetrahedron perturbs the coordination of aluminium, i.e., the corresponding Al-O bond is considerably weaker than in the unprotonated case. This leads to a large EFG, and the calculated quadrupole coupling constant (QCC) for 27Al is 18.2 MHz. Different probe molecules were adsorbed on the Brønsted site. The hydrogen bond formed between the acid proton and the probe molecule weakened the zeolitic O-H bond. For conservation of the overall bond order of the oxygen atom, its bonds to the neighboring tetrahedral atoms (Si, Al) become stronger. As a consequence, the perturbation of the AlO4 tetrahedron and the EFG at the aluminium position decrease depending on the strength of the hydrogen bond. Perturbation of an oxygen atom of the AlO4 tetrahedron by accepting a hydrogen bond from the base molecule also affects the corresponding Al-O bond order. A linear correlation is found between the calculated QCC constants for 27Al and the Al-O bond orders of the oxygen atoms which are perturbed by protonation or by hydrogen bonds. A geometrical shear strain parameter and a simple electrostatic point charge model are less successful at predicting the trends in EFG which clearly shows the importance of the chemical bonds.
AI evidence extraction
Structured fields below were generated from the paper’s abstract and metadata to make the evidence easier to inspect. Automated extraction may be incomplete or incorrect; verify important details against the original paper.
Main findings
Density functional theory calculations in zeolite clusters found that protonation forming a Brønsted acid site weakens a corresponding Al–O bond and is associated with a large EFG and a calculated 27Al QCC of 18.2 MHz. Adsorption of probe molecules via hydrogen bonding reduced perturbation of the AlO4 tetrahedron and decreased the EFG/QCC depending on hydrogen bond strength, with a reported linear correlation between calculated 27Al QCC values and Al–O bond orders of perturbed oxygen atoms.
Outcomes measured
- Electric field gradient (EFG) at aluminium site in zeolite clusters
- 27Al quadrupole coupling constant (QCC)
- Al–O bond order changes with protonation and hydrogen bonding
- Correlation between QCC and Al–O bond orders
Limitations
- Computational cluster modeling (DFT) rather than experimental measurements
- Specific basis set and model assumptions may limit generalizability to real zeolite materials
- No biological/health outcomes or EMF exposure parameters reported
View raw extracted JSON
{
"study_type": "in_vitro",
"exposure": {
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"outcomes": [
"Electric field gradient (EFG) at aluminium site in zeolite clusters",
"27Al quadrupole coupling constant (QCC)",
"Al–O bond order changes with protonation and hydrogen bonding",
"Correlation between QCC and Al–O bond orders"
],
"main_findings": "Density functional theory calculations in zeolite clusters found that protonation forming a Brønsted acid site weakens a corresponding Al–O bond and is associated with a large EFG and a calculated 27Al QCC of 18.2 MHz. Adsorption of probe molecules via hydrogen bonding reduced perturbation of the AlO4 tetrahedron and decreased the EFG/QCC depending on hydrogen bond strength, with a reported linear correlation between calculated 27Al QCC values and Al–O bond orders of perturbed oxygen atoms.",
"effect_direction": "unclear",
"limitations": [
"Computational cluster modeling (DFT) rather than experimental measurements",
"Specific basis set and model assumptions may limit generalizability to real zeolite materials",
"No biological/health outcomes or EMF exposure parameters reported"
],
"evidence_strength": "insufficient",
"confidence": 0.74,
"peer_reviewed_likely": "yes",
"keywords": [
"electric field gradient",
"EFG",
"zeolite clusters",
"DFT",
"6-31G**",
"Brønsted acid site",
"protonation",
"hydrogen bond",
"probe molecules",
"27Al",
"quadrupole coupling constant",
"QCC",
"Al-O bond order"
],
"suggested_hubs": []
}AI-extracted fields are generated from the abstract and metadata and may be incomplete or incorrect. This material is for informational purposes and is not medical advice.
Identifiers and source
DOI: 10.1016/s0926-2040(97)00056-8
PubMed: 9477447
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