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Kenneth Leopold, Principal Investigator

Computational Studies of Partially Bound and Hydrogen Bound Complexes

These researchers are running high-level ab initio calculations on several molecules under investigation in their laboratory and predicting the structures and bond energies for systems they hope to study in the near future. These molecules include HCN-HCN-BF3, HCN-HCN-SO3, HCN-B(CH3)3, CH3CN-B(CH3)3, (CH3)3P-BF3, and (CH3)3P-SO3. For the first two of these, extensive microwave data have already been obtained and interest is now in energetics and solvation effects. For the remaining species, structure predictions are being obtained.

The main focus of this work involves a novel class of molecules for which the term "partially bound" has been coined. The distinguishing feature of these systems is that they contain an atom-atom linkage that is neither a weak intermolecular attraction nor a bone fide chemical bond. Thus, new insights into the intermediate regime between bonded and non-bonded interactions are gained. The work may be described as representing fundamental studies of molecular structure and bonding.

The researchers are using high-resolution microwave spectroscopy to characterize partially bonded systems. Although this program is primarily experimental, the work has, on occasion, benefitted from collaborations with theorists both at the University of Minnesota and abroad. These have all involved high-level ab initio calculations that have significantly enhanced the ability to interpret experimental data.

High-level calculations support the experimental program in several ways. The first is in structure prediction. The frequencies of microwave absorptions by gas phase molecules depend strongly on molecular structure. Therefore, spectral search time is dramatically reduced if predicted values of structural parameters are available in advance. This is especially relevant for partially bound species, since the usual rules for estimating bond lengths and bond angles in conventional systems do not apply. The second way is in energetics. While microwave spectroscopy provides detailed information about molecular and electronic structure, it reveals nothing about bond energies. However, bond energies are vital to an understanding of all chemical interactions and may now be reliably obtained using modern quantum mechanical techniques. With the combination of spectroscopic and theoretical results, these researchers can provide a more complete description of partially bound systems than is available from either theory or experiment alone. The final way involves solvation. Structure and bonding of partially bound molecules have been shown to be extraordinarily sensitive to the presence of a local environment. In other laboratories, the self consistent reaction field model has been shown to be a crude but effective means of modeling the observed medium effects in some cases. Thus, computational methods can be used to make predictions about the response of the molecules studied to a condensed environment.

This information is available in alternative formats upon request by individuals with disabilities. Please send email to alt-format@msi.umn.edu or call 612-624-0528.

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