Predicting NMR Properties

Another way of obtaining information about the distribution of electrons is by computing the polarizability. This property depends on the second derivative of the energy with respect to an electric field. We ll examine the polarizability of formaldehyde in Chapter 4. 

Gaussian jobs report the CPU time used and the sizes of their scratch files upon completion. Here is the data for our formaldehyde job  

We win run this job on methane at the Hartree-Fock level using the 6-31G(d) basis our molecule specification is the result of a geometry optimization using the B3LYP Density Functional Theory method with the same basis set. This combination is cited 

Exploring Chemistry with Electronic Structure Methods 

Here is the predicted shielding value for the carbon atom in methane  

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A particularly good selection of physical properties may be spectra, because they are known to depend strongly on the chemical structure. In fact, different types of spectra carry different kinds of structural information, NMR spectra characterize individual carbon atoms in their molecular environment. They therefore correspond quite closely to fragment-based descriptors, as underlined by the success of approaches to predict NMR spectra by fragment codes (see Section 10.2.3). 

In order to do so, you will need to perform Hartree-Fock NMR calculations using the 6-311+G(2d,p) basis set. Compute the NMR properties at geometries optimized with the B3LYP method and the 6-31G(d) basis set. This is a recommended model for reliable NMR predictions by Cheeseman and coworkers. Note that NMR calculations typically benefit from an accurate geometry and a large basis set. 

In Exercise 3.5, we predicted the NMR properties of benzene and calculated the relative shift for the carbon atom with respect to TMS. In this exercise, we will compare those results with ones computed using other basis sets. 

Determine the effect of basis set on the predicted chemical shifts for benzene. Compute the NMR properties for both compounds at the B3LYP/6-31G(d) geometries we computed previously. Use the HF method for your NMR calculations, with whatever form(s) of the 6-31G basis set you deem appropriate. Compare your results to those of the HF/6-311+G(2d,p) job we ran in the earlier exercise. How does the basis set effect the accuracy of the computed chemical shift for benzene ... 

The physicochemical properties of the feeds were determined by appropriate ASTM methods. One of the key objectives of this study was to evaluate the possibility of using H-NMR spectra to predict these properties. 

A feasibility study on the application of H-NMR petroleum product characterization to predict physicochemical properties of feeds and catalyst-feed interactions has been performed. The technique satisfactorily estimates many feed properties as well as catalyst-feed interactions to forecast products yield. There are, however, limitations that have to be understood when using the H-NMR method. The technique, in general, is not capable either to estimate the level of certain contaminants such as nitrogen, sulfur, nickel, and vanadium when evaluating feed properties or the effect of these contaminants on products yields while testing catalyst-feed interactions. 

As might be expected, NMR calculations that ignore electron correlation often give poor results, especially for molecules which typically require a correlated treatment in order to predict other properties accurately. For example, a good description of multiple bonds and lone pairs generally requires a correlated method. Thus, RHF NMR predictions for molecules such as CO and acetonitrile are poor (20). Furthermore, it has recently been shown that isotropic chemical shift calculations at the RHF level are unreliable for benzenium (21) and related carbenium ions which we often encounter in catalysis. 

Modem methods based on density-functional theory (DFT) can describe relative activation barriers of organometallic reactions, i.e. relative reactivities, as well as the transition-metal NMR chemical shifts of the reactant complexes involved. It is thus possible to reproduce or rationalize observed correlations between these properties or to predict new ones. NMR/reactivity correlations that could be reproduced theoretically ("intrinsic correlations") are summarized. Newly predicted NMR/ reactivity correlations are discussed for the ethylene polymerization with V(=0-X)R3 or V(=Y)R3 catalysts. When X or Y are varied (X = A1H3, Li+, SbF5, H+ Y = NH, O, S, Se), both... 

The QM theory of chemical shielding was originally developed many years ago [22,23], but only later have ab initio methods and density functional theories (DFT) been reliably used for the prediction of NMR properties of isolated molecular systems, and finally of solvated systems. The latter step has been achieved by extending the gas-phase theoretical methods to continuum solvation models (see Ref. [11] for a sufficiently updated list of papers). 

The database is updated on an annual basis with new data extracted from the literature. This database is also the foundation of data supporting the prediction algorithms that are required to predict NMR spectral properties for chemical structures not contained within the database. 

Lanthanide ions continue to be very widely used as shift reagents for substrate molecules (Section III.F). There is, however, considerable interest in the NMR properties of the lanthanide complexes themselves. The isotropic shifts of the latter are invariably dominated by dipolar interactions between the lanthanide ion and the nucleus under question. Bleaney (156) has predicted that the dipolar shift will be dominated by a component which varies as T around room temperature whereas Horrocks et a/. (157,158) predict a more complex temperature dependence. Hill et al. (159) have examined the temperature dependence of some tetraethylammonium tetrakis-N,N-diethyldithiocarbamato-lanthanate(iii) salts, Et4N Ln(dtc)4, in an attempt to clarify the situation. The observed temperature dependences are complex and both contact and dipolar contributions had to be considered in the form ... 

A number of approaches have been proposed for the computation of NMR properties in the framework of DF approaches [86-91]. Here we will make explicit reference to the GIAO model, which appears particularly effective [92,93]. It has been recently pointed out that the MP2 method predicts chemical shifts which are closer to experiment than those obtained using DF approaches, including the B3LYP model [93]. It is so natural to include, as a stringent test, the computation of chemical shieldings. In particular, we have selected some examples in order to investigate the different possible hybridizations of carbon atoms. We have next added the ozone molecule, which is a particularly difficult test for NMR properties [90,92], The results are collected in table XIII. 

Arguably the most important observable magnetic property in molecular chemistry is the chemical shift in NMR. Consequently, ring current models have been tested according to their performance in predicting NMR chemical shifts. Pople, in 1956, was the first to apply ring current approach to the calculation of NMR shifts [41]. 

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