Modeling NMR chemical shifts

Modeling NMR Chemical Shifts in Polymers and Amorphous Matter... 

Modeling NMR chemical shifts gaining insights into... 

Modeling NMR chemical shifts gaining insights into structure and environment / Julio C. Facelli, Angel C. de Dios, editors. 

Schreckenbach G, Wolff SK, Ziegler T (1999) In Facelli JC, de Dios AC (eds.) Modeling NMR chemical shifts. (ACS symposium series 732) American Chemical Society, Boston, MA, pp 101-114... 

Julio C. Facelli and Angel C. De Dios, Modeling NMR Chemical Shifts. Gaining Insights into Structure and Environment, ACS Symposium Series 732, American Chemical Society, Washington, DC, 1999. 

Alam T (1999) Ab initio calculations of NMR chemical shielding anisotropy tensors in phosphates The effect of geometry on shielding. In Modeling NMR Chemical Shifts. ACS Symposium Series Vol. 732, Facelli JC and De Dios AC (eds), p 320-334... 

An area of research where effort is currently being directed is the study of metal complexation in aqueous solutions and on mineral surfaces (see Hochella and White 1990 and references therein). The reason that research is focusing on details of the molecular structure in this type of system is that scientists have begun to realize that speciation can play a dramatic role in the mobility and toxicity of elements in the environment (see Brown et al. 1999 for a review). Molecular modeling has the potential to make an impact on this field in a variety of ways (see Rosso, Rustad or Sherman, this volume), one of which is in modeling vibrational spectra to help interpret observed spectra. When model vibrational frequencies can be combined with model NMR chemical shifts (see Tossell, this volume) and compared with experimental spectra, complex problems may be more easily understood. 

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... 

Bienati, M., Adamo, C., Barone, V., 1999, Performance of a New Hybrid Hartree-Fock/Kohn-Sham Model (B98) in Predicting Vibrational Frequencies, Polarisabilities and NMR Chemical Shifts , Chem. Phys. Lett., 311, 69. 

Fig. 4 Proposed defect cluster model in as-made zeolites with quaternary ammonium cations as structure directing agents (SDAs) hydrogen bond distances of 1.68 A are determined experimentally from the H NMR chemical shift of 10.2 ppm X and Y are atoms not further specified in the SDA the interaction between the SDA and the SiO- group is assumed based on bond valence arguments (see text)...

The comparative study of the experimental NMR spectra of Zs-1-cyclopropyl -2-(triisopropylsilyl)ethyl cation (17) and the computational model structure E-1-cyclopropyl-2-(trimethylsilyl)ethyl cation (18) demonstrates another application of calculations of 3H and 13C NMR chemical shifts and nuclear spin-spin coupling constants. In particular vicinal3./(11,11) spin-spin coupling constants are useful for... 

The GIAO-MP2/TZP calculated 13C NMR chemical shifts of the cyclopropylidene substituted dienyl cation 27 show for almost all carbon positions larger deviations from the experimental shifts than the other cations 22-26. The GIAO-MP2/TZP method overestimates the influence of cr-delocalization of the positive charge into the cyclopropane subunit on the chemical shifts. Electron correlation corrections for cyclopropylidenemethyl cations such as 27 and 28 are too large to be adequately described by the GIAO-MP2 perturbation theory method and higher hierarchies of approximations such as coupled cluster models are required to rectify the problem. 

The endo conformation was confirmed by comparison with calculated 13C NMR chemical shifts (GIAO-MP2/TZP/DZ) of MP2/6-31G(d,p) optimized model... 

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