On the choice of optimal methodology for calculation of C-13 and H-1 NMR isotropic chemical shifts in cagelike systems. Case studies of adamantane, 2-adamantanone, and 2,4-methano-2,4-dehydroadamantane
D. Vikic-topic et L. Pejov, On the choice of optimal methodology for calculation of C-13 and H-1 NMR isotropic chemical shifts in cagelike systems. Case studies of adamantane, 2-adamantanone, and 2,4-methano-2,4-dehydroadamantane, J CHEM INF, 41(6), 2001, pp. 1478-1487
Citations number
68
Categorie Soggetti
Chemistry
Journal title
JOURNAL OF CHEMICAL INFORMATION AND COMPUTER SCIENCES
The C-13 and H-1 isotropic chemical shift values computed at HF, BLYP, B3LY
P, and MPW1PW91/6-311+G-(2d.p) levels of theory, for the BLYP and B3LYP/6-3
1G(d,p) optimized geometries of adamantane, 2-adamantanone, and 2,4-methano
-2,4-dehydroadamantane ([3.1.1] propellane) are reported and compared with
the experimental data. Except for the "inverted" carbon atoms and some of t
heir nearest neighbors, the HF values are superior over the DFT ones, when
the isotropic shifts with respect to TMS are in question. However, in case
of the relative shafts computed with respect to the most deshielded center
within the molecule, the DFT methods yield significantly better agreement w
ith the experimental data than the HF method, the hybrid DFT methods being
superior over "pure" DFT ones. The most probable reason for these findings
may be the cancellation of errors arising from the inappropriate descriptio
n of the paramagnetic contributions to the overall shielding tensor within
the Kohn-Sham approach when an internal standard (within a molecule) is cho
sen, instead of an external one. Almost excellent linear correlation was fo
und between the calculated and experimental relative shift values, which is
significantly superior at DFT levels than at HF level, further proving the
more systematical nature of errors in predicting the second-order magnetic
response properties at DFT levels of theory. Among all DFT methods employe
d, the MPW1PW91 showed the best performance, in line with the significantly
improved long-range behavior of this functional, as compared to the B3LYP
one.