Calculations of chemical shifts

Parameters for the calculation of chemical shift in nmr spectra. Chemical shifts in nmr spectra. 

One point worth noting is that the theoretical calculation of chemical shifts predicts absolute chemical shifts, whereas experimental values are reported with respect to some convenient but otherwise arbitrary reference. Although theoretical chemical shift trends in a series of compounds may be compared with experimental trends, it can be useful to also calculate the chemical shift of a solid reference compound for comparison, while realizing that such a calculation will also be subject to some degree of inaccuracy. 

The ability of NMR to distinguish poly types will be greatly aided, as it was in the recent case discussed of SiC, by theoretical calculations of chemical shifts and CS A values (and for quadrupolar nuclei, NQCC values). Also, MAS-NMR may succeed in identifying low levels of polytypes or new ones that have not been seen by diffraction methods. 

These examples show that the quantum chemical calculation of nuclear spin-spin coupling constants which has been available for the general scientific community only quite recently is an additional valuable and reliable tool for the interpretation and assignment of NMR spectra of carbocations. Taken together quantum chemical ab initio calculation of chemical shifts and nuclear spin-spin coupling constants opens the possibility for a complete simulation of NMR spectra solely based on calculated carbocation structures. 

The treatment of Saika and Slichter has served as model for several subsequent calculations of chemical shifts of nuclei other than hydrogen. The most successful of these calculations have been those of Griffith and Orgel (39) and Freeman et al (35) on the shifts of complexed Co69(III). Shifts for Co59(III) in a variety of octahedrally coordinated complexes are shown in Table I. The range of 14,000 ppm in chemical shifts for these rather similarly constituted complexes is such as to suggest that a para-... 

Until quite recently, however, theoretical prediction of NMR spectral properties significantly lagged experimental work. The ultimate factor slowing theoretical work has been simply that it is more difficult to model the interactions of a wave function with a magnetic field than it is to model interactions with an electric field. Nevertheless, great progress has been made over the last decade, particularly with respect to DFT, and calculation of chemical shifts is becoming much more routine than had previously been true. 

The rest of the papers included in the volume provide a snap shot of the field at the time of the symposium. The papers by Ferraro, Sternberg, Schrecken-bach, and Webb address new methodologies to calculate NMR chemical shifts. The papers by Kuroki, Kurosu, Sakurai, and Facelli present novel calculations of chemical shifts in molecular systems of biological interest. The contributions by McDermott, Case, and Martin report important advances in the understanding of chemical shifts, and the contribution from de Dios emphasizes the importance of the local geometry on the determination of the NMR chemical shifts. The modeling of chemical shifts in inorganic compounds is discussed in detail in the contributions from Buhl, Moore, Wasylishen, Henry, Tossell, Alam, and Jameson (Chapter 23). 

Recently, Srivastava and Kumar (269) have devised another simple method for calculation of chemical shifts of X-ray absorption edges. This is illustrated in Fig. 10, where the absorption edge is indicated as a transition of a K or L core electron to the lowest part of the conduction band (Ec) above the Fermi level (EF) and the energy difference between the edge for the compound and the metal by the chemical shift 8E. The chemical shift corresponds to a shift in the bottom of the conduction bands of a metal and its compounds when they undergo chemical combination. It follows that... 

C. A. Stortz and A. S. Cerezo, The 13C NMR spectroscopy of carrageenans Calculation of chemical shifts and computer-aided structural determination, Carbohydr. Polym., 18 (1992) 237-242. 

Chini, M. G. Jones, C. R. ZampeUa, A. D Auria, M. V. Renga, B. Fiorucci, S. Butts, C. P Bifulco, G. Quantitative NMR-derived interproton distances combined with quantum mechanical calculations of chemical shifts in the stmeochemical determination of conicasterol F, a nuclear receptor ligand from theonella swinhoei, ... 

Hartmann, E., and R. Szargan (1979). SCF-Aa SW calculation of chemical shifts of x-ray and electron emission lines and relaxation properties of SLX4 molecules. Chem. Phys. Lett. 68, 175-78. 

Summary The protonation of E-l-p-anisyl-2-triisopropylsilyl-ethene with FSO3H leads to the formation of syn- and awr/-l-/>-anisyl-2-triisopropylsilyl-ethyl cations. The experimental C NMR chemical shifts and energy barrier for the /anr/-isomerization are compared with ab initio DFT quantum chemical calculations of chemical shifts and... 

Spectral assignments therefore often can be made with confidence. For multiply substituted aromatic rings, the identity of the substituents, but not their relative positions, may be known. Calculation of chemical shifts for all possible substitution possibilities and comparison with the observed positions can produce a structural assignment. 

Tables 3-1 through 3-3 contain data on which empirical calculations of chemical shifts can be made. The tables represent a fraction of the data available on the fundamental alkane, alkene, and aromatic structures. Moreover, corrections must be applied in order to avoid nonadditivity caused primarily by steric effects. Thus, three groups on a single carbon atom, two large groups cis to each other on a double bond, or any two ortho groups can cause deviations from the parameters listed in the tables. If sufficient model compounds are available, the corrections shown can be applied. Further empirical calculations are possible for any structural entity, so that the eclipsing strain in cyclobutanes, the variety of steric interactions in cyclopentanones, or the variations in angle strain in norbornanes may be taken into account.

Empirical as well as ab-initio quantum mechanical calculations of chemical shifts are likely to become an important tool for resonance assignments. Specifically, the ab-initio-type calculations are likely to prove useful in predicting conformations of polymorphs by comparing the, derived from guessed 3-D molecular structure inputs, with experimental chemical shifts. 

Values reported in chemical shift tables assume free rotation of the OH group. The anisotropy of this can be judged either from hydrogen bonded cases, from solid state NMR spectra in which the two ortho protons or carbons have become non-equivalent, or from theoretical calculations of chemical shifts (see Section n.M). 

Overviews of theoretical calculations of chemical shifts using salicylaldehydes are given . In these papers a large number of methods and basis sets are tested. 

Nowadays, NMR parameters are frequently calculated at the DFT level. 5 This approach produces better results than the HF method with comparable computational requirements. A review article describing the calculation of chemical shifts in heterocyclic compounds has recently been published. ... 

These forces which drive the system under investigation into the direction of the minimum pseudo-energy contain derivatives of the theoretical chemical shifts with respect to the coordinates. As pointed out in Section 5, the calculation of chemical shift derivatives is even more time consuming than the calculation of the chemical shifts itself. The calculations should be performed at least on the same theoretical level as the chemical shifts. If theoretical or empirical chemical shift contour maps have been worked out in advance, their derivatives can be calculated numerically. If the contour maps are constructed as a function of the dihedral angles (see Sections 6.3-6.4), only the forces with respect to these inner coordinates are readily obtained. 

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