Contributing structures estimating relative importance

Infra-red, microwave, and X-ray photoelectron spectroscopy Infra-red and ultra-violet spectroscopy has been widely used for investigating the structure of intermolecularly hydrogen-bonded complexes in the solid state (Novak, 1974) and in solution (Zundel, 1976, 1978 Clements et al., 1971a,b,c Pawlak et al., 1984). By analysing the infra-red spectra of equimolar liquid mixtures of amines with formic or acetic acid, the relative importance of structures [10] and [11] was estimated (Lindemann and Zundel, 1977). It was proposed that [10] and [11] make equal contributions to the observed structure of the complex when the p -value of the carboxylic acid is approximately two units lower than that of the protonated amine. 

There are a number of other criteria that we can use to estimate relative stabilities, and hence relative importance, of contributing structures. One of these has to do w ith (a) electronegativity and location of charge. 

Quantitative structure-activity relationships QSAR. The QSAR approach pioneered by Hansch and co-workers relates biological data of congeneric structures to physical properties such as hydrophobicity, electronic, and steric effects using linear regression techniques to estimate the relative importance of each of those effects contributing to the biological effect. The molecular descriptors used can be 1-D or 3-D (3D-QSAR). A statistically sound QSAR regression equation can be used for lead optimization. 

The following preferences will help you estimate the relative importance of the various contributing structures. In fact, we can rank structures by the number of these preferences they follow. Those that follow the most preferences contribute the most to the hybrid, and any structure that violates all four of these preferences can be ignored. 

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. 

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 indicates the percent contribution of each resonance form to the overall structure. Some guidelines for assigning weighting factors are summarized below. 

Contributions in this section are important because they provide structural information (geometries, dipole moments, and rotational constants) of individual tautomers in the gas phase. The molecular structure and tautomer equilibrium of 1,2,3-triazole (20) has been determined by MW spectroscopy [88ACSA(A)500].This case is paradigmatic since it illustrates one of the limitations of this technique the sensitivity depends on the dipole moment and compounds without a permanent dipole are invisible for MW. In the case of 1,2,3-triazole, the dipole moments are 4.38 and 0.218 D for 20b and 20a, respectively. Hence the signals for 20a are very weak. Nevertheless, the relative abundance of the tautomers, estimated from intensity measurements, is 20b/20a 1 1000 at room temperature. The structural refinement of 20a was carried out based upon the electron diffraction data (Section V,D,4). 

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. 

It is important to mention that from the analysis of the elastic and dielectric measurements and of the A and B molecular field constants the relative contribution of the acoustical and optical vibrations to the structural ordering could be estimated. 

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