Shielding, NMR

The methods listed thus far can be used for the reliable prediction of NMR chemical shifts for small organic compounds in the gas phase, which are often reasonably close to the liquid-phase results. Heavy elements, such as transition metals and lanthanides, present a much more dilficult problem. Mass defect and spin-coupling terms have been found to be significant for the description of the NMR shielding tensors for these elements. Since NMR is a nuclear effect, core potentials should not be used. 

Note that the inner carbon atoms are the first two atoms listed for each compound. The predicted NMR shielding values will appear in the output in the same order as the atoms are listed in the molecule specification section. 

The mixed derivative of an external and an internal magnetic field (nuclear spin) is the NMR shielding eonstant, [Pg.239]

Since HF calculations have a tendency to underestimate the N—N and the C—N bond lengths in triazoles [98JPC(A)620, 98JPC(A) 10348], the structural parameters should be computed at least at the DFT or MP2 levels. This is particularly true if electron-donating substituents are attached to the ring. Nitrogen NMR shielding tensors were computed for a set of methylated triazoles and tetrazoles but will be discussed in the context of tetrazoles (cf. Section IV,B). 

The nitrogen NMR shieldings in methylated triazoles and tetrazoles were computed at the GIAO/HF/6-31-I--I-G level and more recently using multireference... 

The correlation analysis of spectroscopic properties in terms of a,- and crR-type parameters has been very important. Substituent effects on 19F NMR shielding in... 

This subject has been associated with the development of the or and type scales almost from the start74 (see Section II.B), but the first paper in which sulfinyl and sulfonyl groups played a part appears to have been one by Taft and coworkers in 196367. The main object of this paper was to study the effect of solvent on the inductive order by 19F NMR measurements on a large number of mcta-substituted fluorobenzenes in a great variety of solvents. The relationship between the NMR shielding parameter and selected systems as equation 10 ... 

The three F-nmr shielding sets (nos. 15, 16, 17) are fitted with uniquely better precision by the 0% scale. The SD for each of these sets is smaller by factors of two to three times than that achieved with any other parameters (Table VIII). This result is consistent with the fact that these sets have much larger X values (1.5 to 4.2) than do the reaction series (.7 to 1.0) and... 

Evidence has been presented from F-nmr shielding effects in m- and p-substituted fluorobenzenes (10) that solvent interaction modifies the O/ values of a number of the substituents of Table V (specifically NMc2, NH2, NHCOMe, OMe, CH3SO, SF3, CF3SO, CF3, SO3ME, CN, CO2R, NO2 and MeCO). The indicated changes in Oj between hydrocarbon and weakly protonic solvents are... 

Adamo, C., Barone, V., 1998c, Toward Chemical Accuracy in the Computation of NMR Shieldings The PBE0 Model , Chem. Phys. Lett., 298, 113. 

Helgaker, T., Jaszunski, M., Ruud, K., 1999, Ab Initio Methods for the Calculation of NMR Shielding and Indirect Spin-Spin Coupling Constants , Chem. Rev., 99, 293. 

Malkin, V. G., Malkina, O. L., Salahub, D. R., 1993, Calculations of NMR Shielding Constants by Uncoupled Density Functional Theory , Chem. Phys. Lett., 204, 80. 

Rauhut, G., Puyear, S., Wolinski, K., Pulay, P., 1996, Comparison of NMR Shielding Calculated from Hartree-Fock and Density Functional Wave Functions Using Gauge-Including Atomic Orbitals , J. Phys. Chem., 100,... 

Schreckenbach, G., Ziegler, T., 1997a, Calculation of NMR Shielding Tensors Based on Density Functional Theory and a Scalar Relativistic Pauli-Type Hamiltonian. Application to Transition Metal Complexes , Int. J. 

Schreckenbach, G., 1999, The 57Fe NMR Shielding in Ferrocene Revisited. A Density-Functional Study of Orbital Energies, Shielding Mechanisms, and the Influence of the Exchange-Correlation Functional , J. Chem. Phys., 110, 11936. 

Wiberg, K. B., 1999, Comparison of Density Functional Theory Models Ability to Reproduce Experimental 13C-NMR Shielding Values , J. Comput. Chem., 20, 1299. 

In the first, quantum mechanics can be used to calculate the NMR coupling constant between two nuclear spins in the molecular environment or the NMR shielding constant. Jensen [8] in Chapter 10 of the reference provides a comprehensive introduction to the methods for calculating these molecular properties. The NMR coupling constant, KAB, is related to the second derivative of the energy with respect to the internal magnetic moments, /u, arising from the nuclear spin of the two atoms, A and B. 

Table 1.29Si NMR shielding constants a and chemical shifts 6 in ppm and their localized contributions for SiX4 (I) and XSifNH (II)...

Case58 investigated the effect of ring currents on NMR shielding constants by means of the DFT calculations. The studied rings included the ones commonly found in proteins and nucleic acids. The shielding constants were calculated for methane molecule placed in several positions relative to the ring. The calculations provided data needed to derive structural parameters from measured chemical shifts in proteins and nucleic acids. 

A more complete list of early applications of QM/MM methods to enzymatic reactions can be found elsewhere [18, 35, 83, 84], Gao [85] has reviewed QM/MM studies of a variety of solution phenomena. QM/MM methods have also been used to study the spectra of small molecules in different solvents [86] and electrochemical properties of photosynthetic reaction centers within a protein environment [87-89], An approach has also been developed for calculation of NMR shielding tensors by use of a QM/ MM method [90]. 

The everlasting need for better experimental tools and theoretical methods to explore the field of carbocation chemistry had also a significant influence on the further development of ab initio methods for the calculation of NMR shielding and indirect spin-spin coupling constants. Carbocation chemistry thus serves as a forerunner for a close integration of experimental and computational approaches in all areas of chemistry. 

Structures and nomenclature for the most important five-membered monocycles with one or more heteroatoms are depicted in Scheme 1. The aromaticity scale in five-membered heterocycles has been long debated.97-101 The decreasing order of aromaticity based on various criteria is (DRE values in kcal/ mol) benzene (22.6) > thiophene (6.5) > selenophene > pyrrole (5.3) > tellurophene > fur an (4.3). Pyrrole and furan have comparable ring strains (Scheme 38). The aromaticity of furan is still controversial 100 the NMR shielding by ring current estimated it at about 60% of the aromaticity of benzene, and other methods reviewed earlier102 estimated it at less than 20%. 

TABLE 12. Nitrogen NMR shieldings of a nitro group in substituted nitrobenzenes50 (0.25 M solutions)... 

The correlation analysis of spectroscopic properties in terms of o/ and a -type parameters has been very important. Substituent effects on 19F NMR shielding in fluorobenzenes have been studied in great detail by Taft and colleagues90,96 97. For <5 linear regression on a/ is on the whole satisfactory, but a term in a with a small coefficient is sometimes introduced. The correlation analysis of <5°, however, requires terms in both o/ and os-type parameters, with rrR being widely applicable. Many new values of these parameters have been assigned from fluorine chemical shifts. In recent years there has also been extensive use of correlation analysis of 13C NMR data98,99. 

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