COMPUTATIONAL CHEMISTRY MODELS OF HYDROCARBON CHAINS TESTED.

Computational chemistry is increasingly being used to model complex molecular systems to aid the development of new pharmaceuticals, chemicals, and materials. The ability of computational chemistry to contribute to these design efforts is dependent on the availability of experimental data to validate the modeling. Hydrocarbon rich systems, such as organic polymers, self-assembled monolayers, and the lipid bilayers critical to cellular membranes, are, in principal, ideal systems for the application of computational chemistry models since the numbers of electrons are small and the force fields are simple.

As part of an effort to provide validation data for the modeling of hydrocarbon rich systems, researchers at NIST have undertaken a systematic study of the microwave rotational spectra of substituted alkanes, such as 1-pentene, 1-hexene, 1-octene, and 1-dodecene. The spectra reveal a plethora of conformational isomers, differing solely by rotation about the C-C single bonds. Comparisons of the experimental data show significant discrepancies with computational chemistry models. The origin of these differences is being examined but most likely originates with the relatively poor quality of molecular-mechanics force fields. The researchers note that the ability to precisely control the conformational relaxation in the cold molecular beam may provide a tool to investigate the complex conformational reorientation required by a protein to fold.