Quantumchemistry, NWI-MOL406, Computer exercises, Assignment 2

In this exerise we study fumaric and maleic acid. Look up the structures, and find that these are cis-trans isomers (fumaric is "trans"). First we optimize the molecular structures at the B3LYP level. Note that for the maleic acid, the structure is still ambiguous, as there are stll several isomers possible. Try a few. We expect that the structure of maleic acid with an internal hydrogen bond is lowest in energy.

In real life, these acids crystallize in different crystal structures (kept together by intermolecular hydrogen bonds). The fumaric is the most stable. You could study the difference, and identify the structures and possible polymorphs (if any) with a spectroscopic experiment (Raman, IR, NMR, etc.). Here we connect to the latter. Calculate the isotropic NMR shielding for fumaric and maleic acid and see whether there is any difference (e.g. in the 13C shielding). We cannot do this for the crystal now, so just do it for the molecules in vacuum (and pretend they are frozen in a crystal). Also comment on what you might expect for the molecules in solution.

In brief: Optimize the possibles structure of the isomers and calculate the isotropic shielding. Investigate the effect of basis set size.

Tbe core of the report consists of: structures of the isomers, table with shieldings and a discussion. Please also see the checklist.

Added note: there appear to be some problems with the structural optimization of maleic acid for some basis sets (although, when specifying the basis set only once in the nw-input file, I cannot any more reproduce these problems). For maleic acid you can limit your study to one basis set only, i.e., restrict your basis set study to fumaric acid.
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