• Bulletin of the Korean Chemical Society
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• v.19 no.8
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• pp.847-851
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• 1998

13C, 15N, and 29Si NMR chemical shifts have been computed for selected X-substituted silatranes (X=Cl, F, H, CH3) using Gauge-Including Atomic Orbitals (GIAO) and Continuous Set of Gauge Transformations (CSGT) at the Hartree-Fock level of theory. The isotropic 13C chemical shifts are largely insensitive to substituent-induced structural changes. In this study, the isotropic 13C chemical shifts GIAO and CSGT calculations at the HF/6-31G and HF/6-31G* levels are sufficiently accurate to aid in experimental peak assignments. The isotropic 13C chemical shifts X-substituted silatranes at HF/6-31G* level are approximately 4 ppm different from the experimental values. In contrast, the isotropic 15N and 29Si chemical shifts and the chemical shielding tensors are quite sensitive to substituent-induced structural changes. These trends are consistent with those of the experiment. The 15N chemical shift parameters demonstrate a very clear correlation with Si-N distance, especially when we use the polarization function. Changes in anisotropy, 3a as well as in the 15N isotropic chemical shifts are due primarily to changes in the value of a.. But in case of "Si the correlations are not as clean as for the 15N chemical shift.

• Bulletin of the Korean Chemical Society
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• v.18 no.9
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• pp.981-985
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• 1997

13C, 15N, and 29Si NMR chemical shifts have been computed for selected X-substituted silatranes (X=Cl, F, H, CH3) using Gauge-Including Atomic Orbitals (GIAO) at the Hartree-Fock level of theory. The isotropic 13C chemical shifts are largely insensitive to substituent-induced structural changes. In this study, the isotropic 13C chemical shifts between 1-methyl- and 1-hydrogensilatranes by GIAO-SCF calculation at the HF/6-31G level are very similar. But the results of 1-chloro- and 1-fluorosilatranes are about 4 ppm different from the experimental values. In contrast, the isotropic 15N and 29Si chemical shifts and the chemical shielding tensors are quite sensitive to substituent-induced structural changes. These trends are consistent with those of the experiment. The isotropic 15N chemical shift demonstrates a very clear correlation with Si-N distance. But in case of 29Si the correlations are not as clean as for the 15N chemical shift; the calculated variation in the 29Si chemical shift is much larger.

• Bulletin of the Korean Chemical Society
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• v.29 no.11
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• pp.2205-2208
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• 2008

• Bulletin of the Korean Chemical Society
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• v.14 no.4
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• pp.500-506
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• 1993

$^1H$ and $^{13}C$ NMR spectra for pyridine, ${\beta}$-and ${\gamma}$-picoline, pyrazine, and 4,4'-bipyridyl complexed with paramagnetic undecatungstocobalto(II)silicate and undecatungstonickelo(II)silicate anions are reported. For these complexes the ligand exchange is slow on the NMR time scale and the pure resonance lines have been observed at room temperature. The isotropic shifts in nickel complexes can be interpreted in terms of contact shifts by ${\sigma}$-electron delocalization. Both contact and pseudocontact shifts contribute to the isotropic shifts in cobalt complexes. The contact shifts, which are obtained by subtracting the pseudocontact shifts from the isotropic shifts, require both ${\sigma}$-and ${\pi}$-electron delocalization from the cobalt ion. Slow ligand exchange has also allowed us to identify the species formed when bidentate ligands react with the heteropolyanions. Pyrazine forms a 1 : 1 complex, while 4,4'-bipyridyl forms both 1 : 1 and dumbbell-shaped 1 : 2 complexes.

• Bulletin of the Korean Chemical Society
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• v.20 no.6
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• pp.667-671
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• 1999

A series of the para-substituted 7-aryl-norbornadienyl cation (3) was prepared in FSO3H/SO2ClF solution at -100℃ and their 13C NMR shifts were measured at -80℃. The plots of the chemical shifts (changes in chemical shifts) of cationic carbon, ΔδC+, aganist our calculated new substituent constant, σc+s, gave an excellent correlation (r=0.994), with a ρc+ value of 12.63. These results indicate that the new substituent constants, σc+s, can be successfully employed to compare the relative charge demand of the carbonium ion by π-and πσ-participation.

• YAKHAK HOEJI
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• v.32 no.3
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• pp.164-169
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• 1988

$^{13}C$ NMR chemical shifts for 18 ${\alpha}-susbstituted$ toluenes at high dilution in $CCl_4$ solution have been determined. Substituents are as follows: H, Me, Et, n-Pr, iso-Pr, Ph, F, Cl, Br, $NH_2$, NHMe, $NMe_2$, OH, OMe, OCOMe, $CO_2Me$, $CO_2Et$, CN. Those chemical shifts of the methylene carbon of the toluene and the ${\alpha}-carbon$ of the n-butane systems are correlated well. (r=.975, slope=.962)

• YAKHAK HOEJI
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• v.33 no.3
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• pp.203-205
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• 1989

A method is presented for calculating the $^{13}C$ chemical shifts produced in liquid solution by referenced relative to RF frequency. The method is useful to get the real variations of chemical shifts in magnetic field by eliminating the affects of the variation of a reference substance.

• Bulletin of the Korean Chemical Society
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• v.19 no.8
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• pp.836-840
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• 1998

The intrinsic and equilibrium isotope effects on the 13C NMR chemical shift of the cyclooctanone-2-D were investigated. Equilibrium constants and changes in the free energies, enthalpy, entropy, which are derived from the temperature dependence of the isotope shifts, are reported for this isotopomer.

• Bulletin of the Korean Chemical Society
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• v.13 no.6
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• pp.595-599
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• 1992

The temperature dependence of C-13 chemical shifts are observed for the cyclooctanone arylhydrazones. The temperature-dependent chemical shifts for these derivatives are explained by postualating the existence of two equilibrating structures. In addition, the assignment between the $^{13}C$ signals of methylene carbon pairs can be done by application of the ${gamma}$ -substituent effect.

• Bulletin of the Korean Chemical Society
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• v.18 no.10
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• pp.1090-1093
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• 1997

The solvent effects on the isotropic NMR shifts in conformationally rigid ligands such as quinuclidine, pyridine, and 4-aminopyridine coordinated to the paramagnetic polyoxometalate, [SiW11CoⅡO39]6- (SiW11Co), are reported. For these complexes the ligand exchange is slow on the NMR time scale and pure 1H NMR signals have been observed at room temperature. The signals for the SiW11Co complexes are shifted upfield whe dimethyl sulfoxide-d6 (DMSO) is added to a D2O solution. The isotropic shifts are separated into contact and pseudocontact contributions by assuming that the contact shifts are proportional to the isotropic shifts of the same ligands coordinated to [SiW11NiⅡO39]6-. It is shown that both the contact and pseudocontact shifts decrease (the absolute values of the pseudocontact shifts increase), when D2O is replaced by DMSO. It is suggested that D2O, a strong hydrogen bond donor, withdraws electron density from [SiW11CoⅡO39]6-, increasing the acidity of the cobalt ion toward the axial ligand. When D2O is replaced by DMSO, the acidity of the cobalt ion in SiW11Co decreases, weakening the Co-N bond. Then both the contact and pseudocontact shifts are expected to decrease in agreement with the observed solvent effects.