Chemistry Reference
In-Depth Information
3.6.2 NMR and ESR/EPR
The calculation of NMR parameter has been studied extensively; see [
3
,
73
] for
general overviews. In 2001, Sebastiani and Parrinello implemented the NMR
chemical shift calculation in the plane wave AIMD code CPMD [
74
]. From this
implementation it was possible to treat extended systems within periodic boundary
conditions, i.e., the method was applicable to crystalline and amorphous insulators
as well as to liquids. The problem of the position operator was solved by the use of
maximally localized Wannier functions. Several benchmark calculations showed
good agreement with experimental values.
A linear scaling, tested with up to 3,000 basis functions, was implemented in
Q-Chem by Ochsenfeld et al. in 2004 [
75
]. The calculations were dependent on a
Hartree-Fock formalism and test calculations with more than 1,000 atoms made.
In 2009, the calculation of the NMR chemical shifts and EPR
g
tensors was
extended to the Gaussian and plane wave code CP2k [
76
]. Weber et al. performed
several test calculations with good agreement with experimental results. Addition-
ally, the NMR shifts in isolated as well as hydrated adenine were calculated.
3.6.3 EXAFS
Near-edge X-ray absorption spectra calculations at the DFT level were also carried
out in the framework of AIMD [
77
-
81
]. Several test calculations have been carried
out: water and CO with different basis sets and core-hole potentials, the C, O, and N
K-edges in (CH
3
)
2
CO, CH
3
COH, and C
5
H
5
N, as well as water and CH
3
OH dimers
for the sensitivity to weak intermolecular interactions. For the basis set dependence
the 6-31G
**
, 6-311G
**
, 6-311++G(2d,2p), 6-311++G(3fd,2dp), Iglo-III, Roos-
ADZ-ANO, Roos-ATZ-ANO, aug-cc-pVDZ, aug-cc-pVTZ, aug-cc-pVQZ, and
aug-cc-pV5Z basis sets were compared, and it was observed that the EXAFS
spectra significantly varied with the basis set in number of signals, signal position,
as well as signal shape. Even with the largest basis set the experimental O K-edge in
water was only marginally described by the BLYP exchange functional. The same
was found for CO. For the dependence on the core-hole potential, a comparison for
H
2
and CO molecules with the aug-cc-pV5Z basis set and the BLYP functional
were made. Using full core-hole potentials, the entire spectrum was shifted by
several eV to higher energies and, similar to the basis set choice, the choice of the
functional largely influenced the spectrum. Despite these deficiencies, EXAFS
calculations of (CH
3
)
2
CO, CH
3
COH, and C
5
H
5
N showed a resemblance between
theoretical and experimental spectra for the different atoms, and therefore an
alignment depending on these calculations was possible [
80
]. Weaker interactions
were investigated at water-water and methanol-methanol dimers. In both
calculations the weak hydrogen bonds significantly changed the spectra for the
acceptor and the donor in accordance with chemical intuition and experiment,
allowing for an assignment of the experimental results to different coordinations
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