Experimental coupling constants

The presence of e-p coupling provides an additional channel for the relaxation of phonon states leading to an increase in phonon linewidths [13]. In addition, the modes shift in frequency due to the coupling. The magnitude of the induced shift A w(e p) has been given by the approximate formula [14,15] 

The foregoing discussion implies that in order to understand the doping behaviour observed in the optical spectra of the fullerides, we must consider both bonding and vibronic effects. A simple method of separating these two contributions has been given in Ref. [7] and is outlined here for the sake of completeness. The shift in mode frequency is expressed as sum of two contributions  

For the e-p contribution, we wish to find a simple dependence on molecular charge that reflects the fact that there is no e-p coupling in C60, C 0 and C -. Thus, we write 

Combining equations (5)-(7), we arrive at the simple doping expression 

This situation was later remedied by Ceulemans et al. [18] who used an extended-Wu potential employing six force constants. For consistency, we shall refer to this model as the 6k model. The model builds upon the 4k model by adding in two extra force constants corresponding to two interaction constants k5 and k6, as shown in Fig. 7. Such a model can correctly predict the frequencies of the flg modes. Using the model, the authors obtained values for the six force constants by minimising the error between predicted and observed mode frequencies. As can be seen in Fig. 7, a much better fit to the data is obtained in this way. 

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Commercial implementations of this general approach are ACD/I-Lab [36], Specinfo (Chemical Concepts) [37], WINNMR (Bruker), and KnowItAll (Bio-Rad) [38]. Figure 10.2-3 shows the workspace generated by ACD/I-Lab after predicting a H NMR spectrum. ACD calculations are currently based on over 1 200 000 experimental chemical shifts and 320 000 experimental coupling constants [36]. 

CALCULATED AND EXPERIMENTAL COUPLING CONSTANTS FOR VARIOUS NITROTHIAZOLES (285)... 

For each molecule (isomer A and isomer B), obtain dihedral angles for the following pairs of vicinal hydrogens Hg-Hy, H Hg, Hy-Hn, and Hg-Hyaxiai- Use the Karplus equation to estimate coupling constants for each pair, and then compare your predictions to the experimental coupling constants (see above). Which molecule is artemisin acetate and which is 6-epiartemisin acetate ... 

The radical cation of benzene has been produced by y irradiation of benzene adsorbed on silica gel (62, 63). The seven-line spectrum shown in Fig. 24 is expected for a molecule having six equivalent protons. An experimental coupling constant of 4.4 G, compared to a value of 3.75 G for the negative ion, gives strong support for attributing the spectrum to a positive ion. The g value is also consistent with this assignment. 

Prasad and coworkers60 studied the ESR spectra of cation radicals of dienes, trienes, tetraenes and pentaenes formed in CFCI3 matrix by X-ray irradiation. The structures of the resulting cation radicals were deduced by comparing the experimental coupling constant to those derived from INDO calculation. The unpaired spin density decreases with increasing chain length. 

The proton nmr behaviour of the norbornyl ion provides a wealth of information which, however, dso appears to be of limited value in the structural problem. The cation has been observed by H-nmr in many solutions containing SbFs as well as in GaBr3—SO2 Qensen and Beck, 1966). At —80° in SbFs—SO2 it exhibits 3 peaks, 5T86 (area 6), 2 82 (area 1) and 5 01 (area 4). The assignments and experimental coupling constants are shown in Figure 5 (Olah et al., 1970). The 5-01 p.p.m. peak indicates equivalence of the four protons, which is caused by rapid 6,1,2-hydride shifts. 

Table 3 collects the values of the experimental coupling constants involving the 33S nucleus so far reported in the literature. 

Comparison of calculated and experimental coupling constants and the s characters of the atoms involved in the coupling (158)... 

The values of A.v, were obtained from experimental coupling constants for mono-substituted ethanes,45 and the values tabulated for 39 substituents covered a range from —0.92 Hz for a lithium substituent (electron donating), through 0.00 Hz for H, to 13.9 Hz for OEtV BI, (electron withdrawing). However, the authors commented that the variation of the As, values defied a consistent explanation, and at that time had not been totally correlated with any other set of substituent constants or properties of the groups.45... 

Table 6. Experimental /-coupling constants of purine, its N-Me derivatives, and the...

Grayson and Sauer computed the coefficients in a Karplus-type equation of the spin-spin coupling constants for a series of rotated ethane geometries. The coupling constants were calculated at the SCF, SOPPA, and SOPPA-CCSD levels and compared with results of previous calculations and experimental data. It was found that the coefficients in the Karplus equation calculated at the SOPPA-CCSD level are in good agreement with coefficients derived from experimental coupling constant data or results of MCSCF calculations. 

ACD/H-NMR from Advanced Chemistry Development (ACD) Labs calculates H-NMR spectra under any basic frequency. The system uses 3D molecular structure minimization and Karplus relationships to predict proton-proton coupling constants. The software recognizes spectral differences among diastereotopic protons, cis-trans isomers of alkenes, syn-anti isomers of amides, oximes, hydrazones, and nitrosa-mines. The base data set includes more than 1,000,000 experimental chemical shifts and 250,000 experimental coupling constants. To quantify intramolecular interactions in new organic structures and to predict their chemical shifts, ACD/HNMR uses an algorithm based on intramolecular interaction parameters to quantify intramolecular interactions in new organic structures and to predict their chemical shifts. 

Table 6. Some calculated" and experimental coupling constants V( Si,X) [Hz]... 

At room temperature and in solutions, free radicals are too reactive, and their concentrations too low, to be conveniently detected by ESR spectroscopy. To observe and quantify transient radicals, it is customary to transform the unstable species into stable nitroxide radicals with the spin-trapping technique. Spin-trapping of a macroalkyl radical with pentamethylnitrosobenzene, for instance, can be represented schematically by Eq. (33). The structure of the parent radical could be deduced from the ESR spectrum of the spin adducts, using standard rules of spin coupling and experimental coupling constants. 

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