Chemical shift models

In our preliminary communication ( 1) we reported a pKa of 11.7 0.1 for Tlj. Utilizing the same 31p nmr technique we find pKa = 10.3 0.2 for While the structure of the conjugate base of 2, phosphoranide can be unambiguously assigned (see chemical shift models yy, and ) (11) the same structural assessment is not as satisfying for the conjugate base of (or. Phosphine derivatives (12) have 5lp NMR chemical shifts similar... 

A number of pyrans, including 3-hydroxy-tetrahydropyran (both axial conformer, 29 and equatorial conformer, 30), 2-methoxy-tetrahydropyran 33, 3-methyl-tetrahydropyran 32, and several 4-substituted tetrahydropyrans, along with 2-methyl-l,3-dioxolane and the rigid cyclic ethers 7-oxabicyclo[2.2.1]heptane and 1,8-cineole, were studied extensively by NMR. These empirical results, in conjunction with the literature data for a variety of acyclic and cyclic ethers, were used to examine the reliability of O-substituent chemical shift models in these systems. The empirical data correlate well with predictions made from the model and it is concluded that ethereal oxygen substituent chemical shifts are due to both steric and electrostatic terms <1998J(P2)1751>. 

A more detailed inspection of the descriptors in these chemical shift models reveals some interesting trends. A complete discussion of the individual... 

Finally, to test the applicability of the chemical shift models to compounds similar to those of the reference set, chemical shifts can be simulated for the prediaion set compounds. The NMR chemical shifts for the external prediaion compounds were entered and stored. Compounds 33—42 contain 88 carbon atom centers which were divided into 5 subsets for shift simulation. These subsets contain 18 primary, 27 secondary, 28 tertiary without... 

Modeling of Chemical Shift Modeling NMR spectroscopy chemical shifts is a theoretical subject aimed at the quantum mechanical links between the... 

In liquids and solutions a chemical shift model should ideally account for the dynamical disordering of the solvent structures. This calls for models that are based on a decomposition of the intermolecular contributions to the shift and a parameterization of these contributions in terms of solvent stmcture, for example, atom-atom distribution functions. Such models should ideally account for the dependence of shift on temperature and pressure. From the distribution functions the shifts can be derived as well as the full photoelectron spectral function, including shift, width, and asymmetry, upon condensation. A basic assumption is that photoionization is vertical, meaning that both initial and final states can be associated with the same nuclear conformation. This approximation is well grounded considering the time scales between the photoelectron process and the rearrangement of the solvent molecules, which means that the solvent is not in equilibrium with respect to the final state. A common assumption behind such models is also that the internal solute nuclear motion is decoupled from external forces. This means that the spectral function/ can be written as a convolution of internal and external parts,/ and/ / respectively,... 

Ah initio methods are applicable to the widest variety of property calculations. Many typical organic molecules can now be modeled with ah initio methods, such as Flartree-Fock, density functional theory, and Moller Plesset perturbation theory. Organic molecule calculations are made easier by the fact that most organic molecules have singlet spin ground states. Organics are the systems for which sophisticated properties, such as NMR chemical shifts and nonlinear optical properties, can be calculated most accurately. 

The conformational characteristics of PVF are the subject of several studies (53,65). The rotational isomeric state (RIS) model has been used to calculate mean square end-to-end distance, dipole moments, and conformational entropies. C-nmr chemical shifts are in agreement with these predictions (66). The stiffness parameter (5) has been calculated (67) using the relationship between chain stiffness and cross-sectional area (68). In comparison to polyethylene, PVF has greater chain stiffness which decreases melting entropy, ie, (AS ) = 8.58 J/(molK) [2.05 cal/(molK)] versus... 

Table 15 contains the C chemical shifts of some selected indazoles. The major difference between indazoles and isoindazoles lies in the chemical shifts of carbons C-3 and C-7a. The substituent chemical shifW (SCS) induced by the substituent in position 3 have been discussed using an empirical model (770MR(9)716). The model that gives the best results, AS = OS + + c and 3i are the Swain-Lupton parameters and 5 is the Schaefer... 

Interpolation methods based on N chemical shifts require the use of the general equations.Those reported in the previous edition (76AHCSl,p. 29, see also 82JOC5132) have been slightly modified for the present purpose. We call / x the observed average property, and the property of the individual tautomers (A or B), / ma and / mb a corresponding property that can be measured (in a model compound or in the solid state) or calculated theoretically, and P and / b the correction factors defined as P = -... 

The measurement of correlation times in molten salts and ionic liquids has recently been reviewed [11] (for more recent references refer to Carper et al. [12]). We have measured the spin-lattice relaxation rates l/Tj and nuclear Overhauser factors p in temperature ranges in and outside the extreme narrowing region for the neat ionic liquid [BMIM][PFg], in order to observe the temperature dependence of the spectral density. Subsequently, the models for the description of the reorientation-al dynamics introduced in the theoretical section (Section 4.5.3) were fitted to the experimental relaxation data. The nuclei of the aliphatic chains can be assumed to relax only through the dipolar mechanism. This is in contrast to the aromatic nuclei, which can also relax to some extent through the chemical-shift anisotropy mechanism. The latter mechanism has to be taken into account to fit the models to the experimental relaxation data (cf [1] or [3] for more details). Preliminary results are shown in Figures 4.5-1 and 4.5-2, together with the curves for the fitted functions. 

From the NMR data of the polymers and low-molecular models, it was inferred that the central C—H carbons in the aliphatic chain in polymer A undergo motions which do not involve the OCH2 carbons to a great extent. At ambiet temperatures, the chemical shift anisotropy of the 0(CH2)4 carbons of polymer A are partially averaged by molecular motion and move between lattice positions at a rate which is fast compared to the methylene chemical shift interaction. 

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