Search results for "Chemical shift"
showing 10 items of 205 documents
17O NMR chemical shifts of 3-(substituted methylene)-(Z)-1(3H)-isobenzofuranones: correlations with IR stretching wavenumbers and AM1 charges
1998
Evaluation of the factors affecting direct polarization solid state (31)P-NMR spectroscopy of bulk soils
2008
Summary 31 P-NMR spectroscopy on bulk soils is a powerful tool for the identification of the different phosphorus forms in soils and for the evaluation of the dynamics of soil P. Up to now the majority of the papers dealt with liquid state 31 P-NMR spectroscopy on soluble soil organic substances. Only few papers were addressed to the study of the different phosphorus forms directly in bulk soils. In the present paper, some organic and inorganic phosphates of known structures, which are likely to be present in soil systems, were studied by direct polarization (DP) magic angle spinning (MAS) 31 P-NMR spectroscopy in order to understand the electronic factors responsible for chemical shifts of…
Synthesis and spectroscopic characterizations of both 1-ethyl-4,8-dihydro-10-methoxy-3-methyl-8-r1-6-r2-dipyrazolo[3,4-b:4′,3′-f]-[1,5]diazocin-5(1H)…
1995
The non-selective methylation of compounds 3a-d using ethereal diazomethane, allowed the synthesis of isomers 4 and 5 which were useful intermediates for the preparation, by a simple approach, of the title compounds 7 and 9. A complete assignment of the chemical shifts to the carbon atoms of the compounds 7 and 9 was performed by different nmr experiments, such as DEPT and XHDEPT for onebond CH correlations and COLOC experiments for long-range C-H correlations.
Iodine(I) and Silver(I) Complexes of Benzoimidazole and Pyridylcarbazole Derivatives
2021
The synthesis of iodine(I) complexes with either benzoimidazole or carbazole-derived sp 2 N -containing Lewis bases is described, as well as their corresponding silver(I) complexes. The addition of elemental iodine to the linear two-coordinate Ag(I) complexes produces iodine(I) complexes with a three-center four-electron (3c-4e) [N — I — N] + bond. The 1 H and 1 H- 15 N HMBC NMR studies unambiguously confirm the formation of the complexes in all cases via the [N — Ag — N] + → [N — I — N] + cation exchange, with the 15 N NMR chemical shift change between 94 to 111 ppm when compared to the free ligand. The single crystal X-ray crystallographic studies on four I + complexes revealed highly sym…
Substituent Effects on the [N-I-N](+) Halogen Bond
2016
We have investigated the influence of electron density on the three-center [N-I-N](+) halogen bond. A series of [bis(pyri din e) io dine](+) and [1,2-bis ( (pyridin e-2-71 ethynyl)b e nze n e)io dine](+) BF4- complexes substituted with electron withdrawing and donating functionalities in the para-position of their pyridine nitrogen were synthesized and studied by spectroscopic and computational methods. The systematic change of electron density of the pyridine nitrogens upon alteration of the para-substituent (NO2, CF3, H, F, Me, OMe, NMe2) was confirmed by N-15 NMR and by computation of the natural atomic population and the pi electron population of the nitrogen atoms. Formation of the [N-…
Benzen i metan jako wzorce przesunięcia chemicznego 1 H i 13 C NMR w obliczeniach teoretycznych
2020
NMR spectroscopy belongs to the most versatile techniques used in chemical laboratory for testing the presence of various compounds and elucidation of their structure. In case of more complex natural products chemical intuition and experience applied to analysis of experimental spectra is often supported by theoretical modeling of NMR spectra. However, theoretical predictions should be reliable and the errors associated with the approximations inherent to the existing methods minimized. In this respect, systematic errors, present in calculation of isotropic nuclear magnetic shieldings of the studied molecule need to be minimized. Conversion of nuclear shieldings of the studied molecule to t…
Comparison of different theory models and basis sets in the calculation of 13C NMR chemical shifts of natural products.
2004
The influence of the calculation method in mimicking experimental 13C NMR chemical shifts of 15 low-polarity natural products singularly containing 10–20 carbon atoms was investigated by employing different quantum chemistry approaches and basis sets, both in the preliminary geometry optimizations and in the following single-point 13C GIAO calculations of the NMR chemical shifts. The geometries of the involved species were optimized at the PM3, HF, B3LYP and mPW1PW91 levels whereas the 13C NMR parameters were determined at the HF, B3LYP and mPW1PW91 levels. Different combinations of basis sets were also tested. The consistency and efficiency of the considered combinations of geometry optimi…
Four-Component Relativistic DFT Calculations of C-13 Chemical Shifts of Halogenated Natural Substances
2015
We have calculated the (13) C NMR chemical shifts of a large ensemble of halogenated organic molecules (81 molecules for a total of 250 experimental (13) C NMR data at four different levels of theory), ranging from small rigid organic compounds, used to benchmark the performance of various levels of theory, to natural substances of marine origin with conformational degrees of freedom. Carbon atoms bonded to heavy halogen atoms, particularly bromine and iodine, are known to be rather challenging when it comes to the prediction of their chemical shifts by quantum methods, due to relativistic effects. In this paper, we have applied the state-of-the-art four-component relativistic density funct…
Acidity and basicity of primaryN-phenylnitramines: catalytic effect of protons on the nitramine rearrangement
2002
Para-substituted N-phenylnitramines were prepared either by oxidation of diazonium salts or by nitration under alkaline or acidic conditions. Isotopic [15N-NO2] labelling indicated that the bands characteristic of the N-nitro group appear in the 1318–1323 and 1585–1607 cm−1 regions. In the nitrogen NMR spectra, the nitramino group gives two resonances at −193 ± 3 (NH) and −32 ± 3 ppm (NO2). The chemical shifts in proton and carbon NMR spectra are predictable, based on increments and the additivity rule. The spectral data indicate the lack of conjugation between the nitramino group and another substituent bound to the ring. It seems to contradict the well-known fact that substituents strongl…
Noble Gases as Magnetic Probes in Fullerene Chemistry
2016
Since the early 1990s of the previous century, the use of magnetic active noble gas atom to indirectly probe the size and symmetry of fullerene cage has been reviewed. Molecular modeling of nuclear shielding (and chemical shift) using gauge-including atomic orbital (GIAO) approach at several levels of theory, including Hartree–Fock (HF), density functional theory (DFT), and coupled cluster (CC), is discussed. The advantage of using GIAO calculation to predict and closely reproduce experimental 3He and 129Xe NMR spectra is shown.