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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. February 2002, Volume 1, Number 2, 64-69 |
Ab initio HF and MP2 Study of the Atomic Charges in Some XY42-
Tetrahedral Anions - A Route to Model Hydrogen Bonding Proton Acceptor Abilities
Bojan Soptrajanov, Ljupco Pejov, and Mira Trpkovska
Internet Electron. J. Mol. Des. 2002, 1, 64-69
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Abstract:
The problem of predicting the proton acceptor abilities of various
species, as well as the understanding of the variety of factors
influencing these abilities is of substantial importance for the
interpretation of the IR spectra of hydrogen-bonded molecules in
solid state. The proton acceptor abilities of some XY42- anions
(SO42-, SeO42-, MoO42-,
WO42-, CrO42-
and BeF42-) were studied
using ab initio quantum chemical methods and IR spectroscopy.
The geometries of the anionic species were optimized at both HF
SCF and MP2 (frozen core) levels of theory. The charge
distribution for the optimized geometries was investigated using
both the Mulliken scheme and the natural population analysis
(NPA). Quantum-chemically computed partial atomic charges
on Y atoms within various XY42- tetrahedral species correlate
well with the spectroscopically measured frequencies of the
proton-donor ammonia molecule rocking modes. The results
obtained using the quantum chemical methods were compared to
the experimental spectroscopic results for several
[Cu(NH3)4]XY4 and
[Cu(NH3)4]XY4.H2O species. Conclusions
regarding the proton acceptor abilities of the XY42- anions were
mainly derived on the basis of the experimentally obtained IR
frequencies of the ρ(NH3) modes. The atomic charges on the Y
atoms in a series of tetrahedral XY42- species derived from ab
initio HF/LANL2DZ and MP2/LANL2DZ densities with
Mulliken as well as NPA charge-assignment schemes give a
correct order of proton acceptor abilities of these anions,
according to the spectroscopically obtained positions of the
ρ(NH3) modes in a series of structurally similar
[Cu(NH3)4]XY4
and [Cu(NH3)4]XY4.H2O compounds.
The ab initio (both HF and
MP2) results are practically completely in line with the
experimental observations, while the much simpler
electronegativity-based approach fails to explain some of the
spectroscopically observed trends.
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