Resonance estimating relative importance

We found that the replacement of one C — H unit in benzene by a nitrogen atom resulted in only small changes in the form of the spin-coupled orbitals, in the relative importance of the different modes of spin coupling, and in our estimate of the resonance... 

The degrees of planarity of the peropyrene-type PAHs appear to be determined by the balance between the loss in resonance energy when the molecules become nonplanar and the increases in energy due to decreased intramolecular steric strain and stronger solvation. The degree of planarity and the relative importance of these factors for each PAH structure can be estimated from the analytical behavior of that PAH. Conversely, these molecules can serve as excellent probes of the mechanisms of HPLC. 

Draw the important resonance structures for any molecule. Assign the relative importance of these structures and estimate the resonance stabilization energy for the molecule. 

In Section 14.3, we saw several examples of fragmentation of molecular ions to give resonance-stabilized cations. Make a list of these resonance-stabilized cations and write important contributing structures of each. Estimate the relative importance of the contributing structures in each set. 

The present study concerns the two main questions which remain after the analysis of the experimental data. First, we have derived which mechanisms, in addition to the resonant dipole-dipole interaction, may couple the infrared-active monomer vibrations in (SFe)2, (SiF4)2 and (8 4)2 and lift the degeneracy of these vibrations. In this derivation we have started from an intermolecular potential which includes the electrostatic, exchange, dispersion and induction interactions. In order to estimate the relative importance of each coupling contribution we have replaced the potential by an empirical atom-atom potential, supplemented with induction terms. From this analysis it follows... 

It is important to be able to estimate the relative importance of resonance forms in order to get an idea of the best way to represent a molecule. To do so, we can assign a weighting factor, c, to each resonance form. The weighting factor is a number that indicates the percent contribution of each resonance form to the resonance hybrid. Some guidelines for assigning weighting factors are listed below. Fortunately, this is an area in which common sense does rather well. 

The differenee in reaction rates of the amino alcohols to isobutyraldehyde and the secondary amine in strong acidic solutions is determined by the reactivity as well as the concentration of the intermediate zwitterions [Fig. 2, Eq. (10)]. Since several of the equilibrium constants of the foregoing reactions are unknown, an estimate of the relative concentrations of these dipolar species is difficult. As far as the reactivity is concerned, the rate of decomposition is expected to be higher, according as the basicity of the secondary amines is lower, since the necessary driving force to expel the amine will increase with increasing basicity of the secondary amine. The kinetics and mechanism of the hydrolysis of enamines demonstrate that not only resonance in the starting material is an important factor [e.g., if... 

A unique situation is encountered if Fe-M6ssbauer spectroscopy is applied for the study of spin-state transitions in iron complexes. The half-life of the excited state of the Fe nucleus involved in the Mossbauer experiment is tj/2 = 0.977 X 10 s which is related to the decay constant k by tj/2 = ln2/fe. The lifetime t = l//c is therefore = 1.410 x 10 s which value is just at the centre of the range estimated for the spin-state lifetime Tl = I/Zclh- Thus both the situations discussed above are expected to appear under suitable conditions in the Mossbauer spectra. The quantity of importance is here the nuclear Larmor precession frequency co . If the spin-state lifetime Tl = 1/feLH is long relative to the nuclear precession time l/co , i.e. Tl > l/o) , individual and sharp resonance lines for the two spin states are observed. On the other hand, if the spin-state lifetime is short and thus < l/o) , averaged spectra with intermediate values of quadrupole splitting A q and isomer shift 5 are found. For the intermediate case where Tl 1/cl , broadened and asymmetric resonance lines are obtained. These may be the subject of a lineshape analysis that will eventually produce values of rate constants for the dynamic spin-state inter-conversion process. The rate constants extracted from the spectra will be necessarily of the order of 10 -10 s"F... 

Resonance was introduced when it was found that there are many molecules whose properties cannot be accounted for by means of a single electronic structure of the VB type, but rather by a combination of several structures [1], Although there is an element of arbitrariness in the resonance theory, in the sense of choosing VB structures, Wheland [50] systemized the basic principles to select the important resonance structures as well as to estimate their relative contribution to the ground state of a molecule. In fact, the qualitative resonance theory enjoyed such a great success due to its convenience and usefulness that resonance has become one of the most fundamental concepts in chemical theory. 

A molecule or ion for which two or more valid Lewis structures can be drawn, differing only in the placement of the valence electrons. These Lewis structures are called resonance forms or resonance structures. Individual resonance forms do not exist, but we can estimate their relative energies. The more important (lower-energy) structures are called major contributors, and the less important (higher-energy) structures are called minor contributors. When a charge is spread over two or more atoms by resonance, it is said to be delocalized and the molecule is said to be resonance stabilized, (pp. 14-18)... 

As usual, h is introduced in case the observed property of the unsubstituted molecule, (Qfi), is not equal to zero. The steric parameter ry was estimated from van der Waals radii in various ways. For ortho substituents it was found that the contributions from inductive and resonance terms outweigh by far the steric contribution, indicating the relative unimportance of the latter in the ortho effect. By contrast in aliphatic systems the contribution from r has been found to dominate all the others. The validity of this substituent constant has to be evaluated, but the correspondence between the ry from aliphatic compounds and Eg was shown by the good correlation between them (98, 99). Since the van der Waals radii do not contain directional information, the correlation between r and Eg indicates that the Eg constants reflect the size and bulk of the substituent and lack the important property of directionality. 

Under these circumstances, it is inevitable to estimate the active conformation of a chemical by another approach. The quantitative structure-activity relationship (QSAR) O) is one of the important approaches, particularly when the target site of a biologically active compound is unknown. Although X-ray crystallography is also helpful to estimate the active conformation, it provides the conformational information in a solid phase. More important is the conformation of a chemical in solution, which can be assigned in part by spectroscopic studies. Nuclear magnetic resonance (NMR) spectroscopy has been utilized to estimate the relative orientation of each atom in a molecule (2-5). Infra-red ( IR) spectroscopy is sometimes a useful tool, especially when hydrogen bonds are present ( ). Recently,... 

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