Bond moments addition

The assumption of bond moment additivity mentioned above leads to expressions for d/Xg/dSt in terms of a relatively small number of parameters. For suppose that... 

Actually the assumption of bond moment additivity does not hold even wdthin 10 per cent, but the procedure just described serves as a framework for the calculation of dfig/dSt on the basis of more elaborate models, in which niteraction effects between the bonds would presumably b< taken into consideration. 

Several other molecular orbital models have been applied to the analysis of VCD spectra, primarily using CNDO wave functions. The nonlocalized molecular orbital model (NMO) is the MO analog of the charge flow models, based on atomic contributions to the dipole moment derivative (38). Currents are restricted to lie along bonds. An additional electronic term is introduced in the MO model that corresponds to s-p rehybridization effects during vibrational motion. 

The work which is reviewed here provides accurate structural data from micro-wave and radiofrequency spectroscopy of relatively small molecule, hydrogen bonded complexes. Its role has been to provide information concerning the stereochemistry and electronic properties — electric dipole moments and nuclear hyperfine interactions — characteristic of hydrogen bonds. The experiments are done on gas phase samples, often in molecular beams, which eliminates environmental perturbations of the hydrogen bonds. In addition, the small molecules used are amenable to ab initio calculations 7 9) and thus the results are extremely useful as criteria for the accuracy of these calculations. Finally, the results are useful to construct models of more complex systems in chemistry and biology involving hydrogen bonds 4). 

Dichlorobenzene. Compare your experimental results with the values computed from a vector addition of carbon-chlorine bond moments obtained from the dipole moment of monochlorobenzene (5.17 X 10 ° C m, or 1.55 D). If they do not agree, suggest possible physical reasons for the disagreement. 

Anesthetics with appreciable dipole moments. In addition to interacting with dispersion and induction forces they can enter electrostatic interactions of the ion-dipole or dipole-dipole type. Such molecules are likely to associate with the polar sites in lipids and proteins. N2O is an example. Anesthetics with no appreciable dipole moment but high bond moments might also be put into this category. 

FIGURE 20.6 Moments of inertia for selected classes of molecular structures. In each case, m represents the sum of all the atomic masses in the molecule, (a) Diatomic molecules have a single moment of inertia, uniquely related to the bond length, (b) Linear triatomic molecules also have a single moment of inertia that can be determined from rotational spectra. Because / depends on two bond lengths, additional information is required to determine both. This is accomplished by comparing rotational spectra of isotopically substituted molecules. 

Integrated i.r. intensity data have been used to calculate bond moments and their derivatives [453] the calculated dipole moment was -0.93 D (3.10 x 10" ° C m), compared with an experimental value (Stark effect upon the microwave spectrum) of -0.95 D (3.17 x 10-30 c m) [1215]. The original vibrational intensity data for COFj [981] has now been corrected [1363], and the CNDO/2 calculations [1827] of the dipole moment derivatives have been revised [289,292,1363,1417] many of the bond moment derivatives (the squares of which are approximately proportional to band intensities) were shown to be transferable [1675]. Bond dipole moments have also been calculated by ab initio methods (with a STO 5-31G basis set) [1941]. In addition, the dipole moment derivatives have been calculated under the MINDO/3 formalisms [1586], but little reliance can be placed on the results obtained. 

D. The calculations confirm that the negative end of the dipole is towards phosphorus. - The calculated dipole moments of phosphole (1.9 D) and pyrrole (2.0 D) are similar, and, unlike furan, the positive ends of the dipoles are towards the heteroatoms. Dipole moments have been used, in combination with results from other methods of study, to estimate the preferred conformations of the dichloride (179), of the phosphites (180), and of triarylphosphine oxides. The use of dipole moments to aid stereochemical studies of compounds has been reviewed. Additive polarizability parameters should not be used in the calculations, and it has been recommended that data should be obtained from model compounds containing identical environments for the phosphorus atoms. The sensitivity of bond moments to structural changes has been studied perfluoroalkyl groups lower the phosphoryl bond moment, and the P—N bond moment is very sensitive to the valence state of the phosphorus atom. The conformational analyses of phospho-nates, phosphonamides, silyl phosphates, and a number of dioxaphosphori-nans (181) - have been reported. The P—Se bond moment has been estimated to be 1.24 D. The zwitterionic structure (182) was identified by its high dipole... 

The bond moment idea falls into the latter category of quantities. It was a very early idea of the chemist that each bond should be assigned a characteristic dipole moment, such that the total molecular electric dipole moment would be obtained by simple vector addition ios-i9) is justified, as has been seen, in so far as... 

---