Nuclear magnetic resonance NMR chemical shift

Proton nuclear magnetic resonance (NMR) chemical shifts of 1,2,3-thiadiazoles give another indication of the aromatic character of these compounds. Compiled in Table 4 are a number of examples of proton chemical shifts for ring-substituted 1,2,3-thiadiazoles. 

In addition to the above prescriptions, many other quantities such as solution phase ionization potentials (IPs) [15], nuclear magnetic resonance (NMR) chemical shifts and IR absorption frequencies [16-18], charge decompositions [19], lowest unoccupied molecular orbital (LUMO) energies [20-23], IPs [24], redox potentials [25], high-performance liquid chromatography (HPLC) [26], solid-state syntheses [27], Ke values [28], isoelectrophilic windows [29], and the harmonic oscillator models of the aromaticity (HOMA) index [30], have been proposed in the literature to understand the electrophilic and nucleophilic characteristics of chemical systems. 

The fourth chapter in this volume, contributed by Helmut Duddeck, is an exceptionally thorough survey of substituent effects on carbon-13 nuclear magnetic resonance (NMR) chemical shifts. Organic chemists and others who are routinely dependent on 13C NMR for structure elucidation and for information about stereochemistry will welcome the summary presented here. Although... 

A systematic development of relativistic molecular Hamiltonians and various non-relativistic approximations are presented. Our starting point is the Dirac one-fermion Hamiltonian in the presence of an external electromagnetic field. The problems associated with generalizing Dirac s one-fermion theory smoothly to more than one fermion are discussed. The description of many-fermion systems within the framework of quantum electrodynamics (QED) will lead to Hamiltonians which do not suffer from the problems associated with the direct extension of Dirac s one-fermion theory to many-fermion system. An exhaustive discussion of the recent QED developments in the relevant area is not presented, except for cursory remarks for completeness. The non-relativistic form (NRF) of the many-electron relativistic Hamiltonian is developed as the working Hamiltonian. It is used to extract operators for the observables, which represent the response of a molecule to an external electromagnetic radiation field. In this study, our focus is mainly on the operators which eventually were used to calculate the nuclear magnetic resonance (NMR) chemical shifts and indirect nuclear spin-spin coupling constants. 

The H and nuclear magnetic resonance (NMR) chemical shift of all the parent structures are fully reported in CHEC-II(1996) <1996CHEC-II(6)447>. Since then, the complete proton and carbon chemical shift assignments have been made for 2- and 3-formyl, acetyl, or methyl phenoxathiin <1996PJC36>. 

In earlier literature reports, x-ray data of a-based ceramics, the /3-like phase observed in certain silica minerals was explained by a structural model based on disordered Q -tridymite. However, others have suggested that the structure of the stabilized jS-cristobalite-like ceramics is closer to that of a-cristobalite than that of Q -tridymite, based on the 29Si nuclear magnetic resonance (NMR) chemical shifts (Perrota et al 1989). Therefore, in the absence of ED data it is impossible to determine the microstructure of the stabilized jS-cristobalite-like phase. ED and HRTEM have provided details of the ceramic microstructure and NMR has provided information about the environments of silicon atoms in the structure. Infrared spectroscopy views the structure on a molecular level. 

Rhinebarger et al. [35] and Eyring et al. [36,37] have used lithium-7 nuclear magnetic resonance (NMR) chemical shift data to determine the stability constants for crown-ether complexes of Li+ in two IL systems consisting of 55/45 mole% N-butylpyridinium chloride-aluminum chloride and l-ethyl-3-methyl-imidazolium chloride-aluminum chloride. The stability constants for... 

Normal distribution 210 Nuclear magnetic resonance (NMR) chemical shift 7... 

In the last few years, DFT has also become one of the prime methods for the study of nuclear magnetic resonance (NMR) chemical shifts in transition metal complexes and other large molecules. DFT calculations of NMR chemical shifts have been reviewed (3,4). 

The dependence of the principal components of the nuclear magnetic resonance (NMR) chemical shift tensor of non-hydrogen nuclei in model dipeptides is investigated. It is observed that the principal axis system of the chemical shift tensors of the carbonyl carbon and the amide nitrogen are intimately linked to the amide plane. On the other hand, there is no clear relationship between the alpha carbon chemical shift tensor and the molecular framework. However, the projection of this tensor on the C-H vector reveals interesting trends that one may use in peptide secondary structure determination. Effects of hydrogen bonding on the chemical shift tensor will also be discussed. The dependence of the chemical shift on ionic distance has also been studied in Rb halides and mixed halides. Lastly, the presence of motion can have dramatic effects on the observed NMR chemical shift tensor as illustrated by a nitrosyl meso-tetraphenyl porphinato cobalt (III) complex. 

Seebach et al. published seminal studies of observable halocarbenoids <1983HCA308>. They reported that the nuclear magnetic resonance (NMR) chemical shifts of a variety of a-bromolithiohydrocarbons lie markedly downfield of those of the analogous alkyllithiums, alkyl bromides and dibromides, and hydrocarbons. Eor examples, see Table 2 <1979AGE784>. The dramatic shift was ascribed to a substantial rehybridization of the carbon atom arising from... 

Nuclear magnetic resonance (NMR) Chemical shift, nuclear coupling constants, relaxation times For paramagnetic proteins enhanced chemical shift resolution, contact and dipolar shifts, spin delocalisation, magnetic coupling from temperature dependence of shifts. 

DFT calculation of the nuclear magnetic resonance (NMR) chemical shifts of pentathiane 16 and 16 isomers of the monoxides were found to be in practical agreement with the experimental data of the pentathiane and the isolated 1- and 3-oxides 17 and 18, respectively <2002BCJ319>. 

The geometries, relative stabilities, and Si nuclear magnetic resonance (NMR) chemical shifts for the pair of silacyclohexane derivatives 6 and 7 have been calculated by DFT methods <2003JOC1827>. It is found that the introduction of the /3-SiMe group stabilizes the cation by 6.7kcalmoP in a manner similar to the well-known /3-Si stabilization of carbenium ions, and slightly increases the degree of pyramidalization at the Si center. 

Quantum chemical nuclear magnetic resonance (NMR) chemical shift calculations enjoy great popularity since they facilitate interpretation of the spectroscopic technique that is most widely used in chemistry [1-11], The reason that theory is so useful in this area is that there is no clear relationship between the experimentally measured NMR shifts and the structural parameters of interest. NMR chemical shift calculations can provide the missing connection and in this way have proved to be useful in many areas of chemistry. A large number of examples including the interpretation of NMR spectra of carbocations [12], boranes [10, 13], carboranes [10, 13-15], low-valent aluminum compounds [16-18], fullerenes [19-21] as well as the interpretation of solid-state NMR spectra [22-26] can be found in the literature. 

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