Electron density and chemical shift

In an attempt to relate calculated results to experimental findings for monomeric, lignin model compounds, preliminary work has compared theoretically determined electron densities and chemical shifts reported from carbon-13 nuclear magnetic resonance spectroscopy (62). Although chemical shifts are a function of numerous factors, of which electron density is only one, both theoretical and empirical relationships of this nature have been explored for a variety of compound classes, and are reviewed by Ebra-heem and Webb (63), Martin et al. (64), Nelson and Williams (65), and Farnum (66). 

Solvent effects on the fluorescence spectrum and the effect of protonation on electron density and chemical shift in NMR spectrum of pyridazine were investigated. 

The complexity of factors contributing to ap means that, unlike for H, heavy atom chemical shifts cannot be interpreted using concepts of local electron density, and chemical shifts for different heavy elements are interpreted differently from each other with significant reliance on comparison to known compounds. 

Recently, ab initio calculations (MP2/6-311G ) have been used to study nine nonclassical carbocations derived from cyclic hydrocarbons, including the 2-norbornyl cation. Electron density and chemical shifts, geometries, energetics, and orbital interactions within the NBO formalism were computed and analyzed. In sharp contrast to the findings by Werstiuk s group, 3c-2e bonding... 

Earlier, Mason266 studied the aza analogs of indolizine and indole and found that the electronic spectra of the former series conformed more closely to the spectrum of the indenyl anion, from which it was concluded that 7r-electron delocalization is more pronounced in this series, a result which appears to be contrary to the conclusions from MO calculations. Proton resonances in the PMR spectrum of 215 and of various aza-indolizines428 429 show that these compounds sustain a ring current, and Black, Brown, and Hcfferman,429 in a study designed to correlate electron density and chemical shift data, commented that the data also support Mason s view that jr-electron delocalization is more extensive in these compounds than in the indole series. 

The HMO 7r-electron densities and chemical shifts of the methine protons of compounds of the type (129) both decreased as a function of X in the order N > 0 = S > Se. In ref. 220 is given a correlation between the lowest HMO transition energies and the long-wavelength absorptions of benzothiazole cyanine dyes that contain the phenalene ring. 

Consideration of a series of compounds containing methyl groups illustrates clearly file influence of the electron density on chemical shift. As file electron-withdrawing ability of groups attached to the methyl group increase, progressive downfield shifts are evident and 8 values increase. Conversely TMS comes very far upheld because... 

TABLE II1-4. correlation BETWEEN ir ELECTRON DENSITIES AND CORRECTED CHEMICAL SHIFTS (108,110)... 

In addition to the obvious structural information, vibrational spectra can also be obtained from both semi-empirical and ab initio calculations. Computer-generated IR and Raman spectra from ab initio calculations have already proved useful in the analysis of chloroaluminate ionic liquids [19]. Other useful information derived from quantum mechanical calculations include and chemical shifts, quadru-pole coupling constants, thermochemical properties, electron densities, bond energies, ionization potentials and electron affinities. As semiempirical and ab initio methods are improved over time, it is likely that investigators will come to consider theoretical calculations to be a routine procedure. 

Chain processes, free radical, in aliphatic systems involving an electron transfer reaction, 23,271 Charge density-NMR chemical shift correlation in organic ions, 11,125 Chemically induced dynamic nuclear spin polarization and its applications, 10, 53 Chemiluminescence of organic compounds, 18,187... 

Figure 2, WWJ plot. Variation of differential chemical shift with s electron density and 4s electron contribution for iron 3d configurations. Adapted from Ref. 34.

The problem of the sign of AR/R for the divalent tin compounds was investigated by Lees and Flinn (16). In the relationship between the quadrupole splitting and chemical shift for the stannous compounds, two distinct correlations became apparent—compounds with a linear covalent bond, and compounds with a predominantly planar bond. Furthermore, there exists a linear relationship between the number of 5 p electrons and the chemical shift and hence the total 5 electron density. Using free tin ion wave functions in a self-consistent field calculation, they showed that the direct eflEect of adding 5 electrons is considerably... 

Nuclei in molecules are affected by the local electron density and their chemical shift, the ratio of the frequency at which an NMR transition occurs to the strength of the main field as measured by the frequency at which protons resonate, is given by ... 

Chemical shift variations among N-methyl cations have been related to electron densities and, in the case of fV-methylpyridinium ions, to Hammett substituent constants.126 Upfield shifts of H,126-128 13C,126 and 14N nuclei126 are observed when electron-donating groups increase the electron density at the quaternized site. In instances where shift differences are large, application of these correlations to molecules giving isomeric products will enable conclusive identification of product structures to be made. 

Sheinker et al.i9S 399a have studied systems 404, 405, 406, and 340. Calculations on the first three systems showed that their -electron delocalization energies (DE)20 are of the order of 0.310, which is close to the values of condensed hydrocarbons. Free-valence indices indicate that imidazo[l,5-a]benzimidazole 406 is the least stable. Reactivity toward electrophiles at positions 1 and 3, protonation at position 2, and chemical shift data (Section V,G,2) are consistent with calculated electron densities.398... 

A correlation between calculated electron densities and observed chemical shifts has been reported for derivatives of l,217 whereas Black et al.263 have... 

Figure 9 Long-range coupling constants (absolute values) and chemical shifts (p.p.m.) for thienothiapyrylium salts (measured in CF3C02D) values of electron densities in parentheses (calculated after the HMO method with h = 0.9, kc-s 0.9)...

Calculated 7r- electron densities and orbital energies, obtained for chromone using a semi-empirical PPP method, were shown to correlate well with H NMR chemical shifts and polarographic half-wave potentials (78lJC(A)53l). 

A linear correlation between 13C chemical shifts and local n electron densities has been reported for monocyclic (4n + 2) n electron systems such as benzene and nonbenzenoid aromatic ions [76] (Section 3.1.3, Fig. 3.2). In contrast to theoretical predictions (86.7 ppm per n electron [75]), the experimental slope is 160 ppm per it electron (Fig. 3.2), so that additional parameters such as o electron density and bond order have to be taken into account [381]. Another semiempirical approach based on perturbational MO theory predicts alkyl-induced 13C chemical shifts in aromatic hydrocarbons by means of a two-parameter equation parameters are the atom-atom polarizability nijt obtained from HMO calculations, and an empirically determined substituent constant [382]. 

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