Bond lengths measurement

X-ray crystallographic studies are available on the ethoxy compound 1858 and on N,N-dimethylcyclopent[c]azepin-3-amine(19).59 The former is a fully conjugated, planar, 14k, non-benzenoid aromatic system, while the latter is virtually planar and, on the basis of bond length measurements, is best represented as the dipolar mesomer 19B. 

Fig. 22 Plot of C-OX bond length versus Hammett a for the aromatic substituent Y for triphenylmethyl ethers and esters [108]. The error bars represent two standard deviations in the bond-length measurements. Reprinted with permission from Edwards et al. (1986a). Copyright 1986 American Chemical Society.

Parallel with this work went the development of quantum theory, with the valence bond and molecular orbital treatment of aromatic molecules. Theoretical predictions were verified by accurate bond length measurements and excellent agreement was finally obtained, particularly in the much studied naphthalene-anthracene series. At this stage it could be claimed that the structure of planar aromatic systems was well understood. 

This kind of microwave spectroscopy is the best technique available for determining the structure of small molecules in the gas phase. Microwave frequencies can be measured with extremely high accuracy, permitting bond length measurements literally... 

A harmonic oscillator can only change its vibrational quantum number by one when it absorbs a photon (Av = 1) therefore, the only frequencies which can be absorbed are near the classical vibrational frequency co = -JkJJi. The absorption will also change the rotational quantum number (Ay = 1). In practice, this means that the infrared spectrum of a small molecule has rotational structure, which permits bond length measurement as well as force constant measurement (Figure 8.6). 

This apparent discrepancy disappeared after the resonance hybrid theory was developed benzene is regarded as a hybrid of the two structures shown above. The structure of benzene is often represented by either of the figures shown at the left, in which the dashed or solid circle represents a half bond , so that the bond order of each C-C bond is 1.5. Bond length measurements are entirely consistent with this interpretation they are almost exactly halfway between the values found in compounds known to contain single and double bonds. 

Improved precision of bond-length measurements has been achieved, and in some cases it was possible to show that two or four slightly different 1,2-dithiolium units can exist in the same crystal lattice. Com-... 

The resulting rour-mcmbercd ring is not si uarc, and it appears from bond length measurements that there tire tiUerntiting single and triple Mo—Mo bonds. Tetranuclear cluster units (rhomboid il Moj). connected by oxygen atoms and forming infinite chains, are found in Ba, uMo Oi. ... 

In sulfoxides (1), the nature of the sulfur-oxygen bond remains somewhat doubtful. It is sometimes depicted as a single bond (S O or S+—O ), which emphasises the polar character, or as a double bond (S=0). In this book, the double-bonded structure is preferred on the basis of bond length measurements (see Chapter 1, p. 10). 

Bird index calculated from statistical deviation of bond orders from bond length measurements higher values imply higher aromaticity, on a scale of 0—100 <8571409). 

Thus the molecule contains three bonds, one being of the a type, and two being tt bonds. It is found from bond length measurements, that the molecule probably contains a carbon-oxygen triple bond, so the molecular... 

Comment In (a), structure II is also consistent with bond length measurements, which show one shorter (152 pm) phosphorus-oxygen bond and three longer (157 pm) ones. 

X-ray crystallographic [126] and electron diffraction studies [127] on biphenylene indicate that formula (I) provides the best single representation of this compound. For example the bond lengths measured by the electron diffraction study are as follows (A) ... 

Bond length measurements on the phosphazene P3N3(F4)N PF2 NPFjNPFj suggest it has predominately a structure with A cr, and A o P atoms [38]. 

Bond length measurements on this type of compound indicate double bonds (8.267) as found in stable phosphenes (Chapter 6.19). 

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