Quantumchemistry, NWI-MOL406, Computer exercises, Assignment 2
Probably this problem is now properly described.
In this exercise 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 maleic acid, the structure is still ambiguous, as there are still several isomers possible. Try a few. We expect
that the structure of maleic acid with an internal hydrogen bond is lowest in energy. For fumaric acid there is also some ambiguity (rotation over the single bonds).
Pls keep close to the structure on the wiki link, i.e. as on
this picture.
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 and quadrupolar parameters Cq and η 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). The quadrupolar parameters can be calculated with "efieldgrad" and
require conversion to Cq depending on the isotope. Assume you are dealing with 17O and 2H for the quadrupolar parameters and 13C for the shielding.
Also comment on what you might expect for the molecules in solution.
In all calculations we use B3LYP. We also want to investigate the
the effect of basis set quality, in particular on the NMR parameters. You can restrict that to the single structure of fumaric acid (see wiki above) in which you go to (at least) 6-311++G**. For the structures of maleic acid 3-21+G* is sufficient (of course, you need to do fumaric also with the same basis set).
In brief: Optimize the structure of fumaric acid (see above) and some structures of maleic acid (including the structure with internal hydrogen bond).
For the optimized structures, calculate isotropic shielding (report on C and H) and electric field gradients (report on 17O and 2H) of/at the individual nuclei. Investigate the effect of basis set size only for the single fumaric acid structure considered.
The principal values of the electric field gradient tensor are ordered by convention as:
|Vzz| > |Vxx| > |Vyy| .
To get Cq (frequency units) you need to convert from Vzz as:
Cq = e Q Vzz/h .
The quadrupole moment can be obtained from
P. Pyykkö, Mol. Phys. 116, 1328 (2017).
The core of the report consists of: structures of the isomers, table with shieldings, quadrupolar parameters and a discussion. Please also see the checklist.
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