Bonding character

Figure B2.4.1. Proton NMR spectra of the -dimethyl groups in 3-dimethylamino-7-methyl-l,2,4-benzotriazine, as a fiinction of temperature. Because of partial double-bond character, there is restricted rotation about the bond between the dunethylammo group and the ring. As the temperature is raised, the rate of rotation around the bond increases and the NMR signals of the two methyl groups broaden and coalesce.

When the several halides of a given element are considered, changes in bond character are also found. The fluoride is generally the most ionic with ionic character decreasing from fluoride to... 

It seems now established by NMR spectroscopic investigations that a change can take place in electronic structures and atomic configuration of the dyes depending on the polarity of the solvent. Parameters describing the transition from one single bond to more double bond character vary according to the nature of the solvent (107). 

In Table 1-9 we have collected only the 7r-bond orders calculated by allvalence-electrons methods and compared their values with those deduced from experimental bond lengths. Both data are indicative of an aromatic molecule with a large dienic character. The 2-3 and 4-5 bonds especially present a large double-bond character, whereas both C-S bonds are relatively simple. 

Increased single bond character in a carbonyl group is associated with a decreased carbon—oxygen stretching frequency Among the three compounds benzaldehyde 2 4 6 trimethoxybenzaldehyde and 2 4 6 trinitrobenzaldehyde which one will have the lowest frequency carbonyl absorption" Which one will have the highest" ... 

This IS an unusually high rotational energy barrier for a single bond and indicates that the carbon-nitrogen bond has significant double bond character as the reso nance picture suggests... 

Infrared IR spectroscopy is quite useful in identifying carboxylic acid derivatives The, carbonyl stretching vibration is very strong and its position is sensitive to the nature of IKT the carbonyl group In general electron donation from the substituent decreases the double bond character of the bond between carbon and oxygen and decreases the stretch mg frequency Two distinct absorptions are observed for the symmetric and antisym metric stretching vibrations of the anhydride function... 

In resonance terms the shorter carbon-oxygen bond distance in phenol is attrib uted to the partial double bond character that results from conjugation of the unshared electron pair of oxygen with the aromatic ring... 

Cyclobutane has less angle strain than cyclopropane (only 19.5°). It is also believed to have some bent-bond character associated with the carbon-carbon bonds. The molecule exists in a nonplanar conformation in order to minimize hydrogen-hydrogen eclipsing strain. 

Any discussion based on reactivity ratios is kinetic in origin and therefore reflects the mechanism or, more specifically, the transition state of a reaction The transition state for the addition of a vinyl monomer to a growing radical involves the formation of a partial bond between the two species, with a corre sponding reduction of the double-bond character of the vinyl group in the monomer ... 

Lithium hydride reacts vigorously with siUcates above 180°C. Therefore, glass, quart2, and porcelain containers cannot be used in preparative processes. That only traces dissolve in polar solvents such as ether reflects its significant (60—75%) covalent bond character. It is completely soluble in, and forms eutectic melting compositions with, a number of fused salts. 

Isomerization. Maleic acid is isomerized to fumaric acid by thermal treatment and a variety of catalytic species. Isomerization occurs above the 130 to 140°C melting point range for maleic acid but below 230°C, at which point fumaric acid is dehydrated to maleic anhydride. Derivatives of maleic acid can also be isomerized. Kinetic data are available for both the uncatalyzed (73) and thiourea catalyzed (74) isomerizations of the cis to trans diacids. These data suggest that neither carbonium ion nor succinate intermediates are involved in the isomerization. Rather, conjugate addition imparts sufficient single bond character to afford rotation about the central C—C bond of the diacid (75). 

Dinuclear stmctures are known for molybdenum(III) in a series of air and moisture sensitive compound containing multiple Mo—Mo bonds. Examples include Mo2(N(CH2)2)g (Fig- 5b) and Mo2(CH2Si(CH2)3)g in which there is a strong Mo—Mo bond, presumably of triple-bond character (28). 

The double-bond length in 1,3-butadiene is 0.134 nm, and the ingle-bond, 0.148 nm. Since normal carbon—carbon single bonds are 0.154 nm, this indicates the extent of double-bond character in the middle single-bond. Upon complexing with metal carbonyl moieties like Fe(CO)2, the two terminal bonds lengthen to 0.141 nm, and the middle bond shortens even more to 0.145 nm (18). 

The bonding between carbon monoxide and transition-metal atoms is particularly important because transition metals, whether deposited on soHd supports or present as discrete complexes, are required as catalysts for the reaction between carbon monoxide and most organic molecules. A metal—carbon ( -bond forms by overlapping of metal orbitals with orbitals on carbon. Multiple-bond character between the metal and carbon occurs through formation of a metal-to-CO TT-bond by overlap of metal-i -TT orbitals with empty antibonding orbitals of carbon monoxide (Fig. 1). 

Bond lengths and infrared spectra support the multiple-bond character of the M—CO bonds. Coordination of a CO molecule to a metal center can change the C—O bond order. According to the description of ( - and TT-bonding given herein, increased ( -bonding between a metal and CO results in a... 

Nonthermoprocessible Condensation Polyimides. These are obtained from condensation of aromatic dianhydrides with aromatic diamines. They are linear noncross-linked resins but their rigid chain stmcture and strongly hydrogen-bonded character leads to systems which do not melt or soften before decomposition. 

The stability of isothiazole derives from the fact that it has an aromatic delocalized ir-electron system. The NMR chemical shifts, which depend, inter alia, on ring currents, and the high stability of the molecular ions in mass spectrometry, are typical of aromatic compounds, and X-ray measurements confirm the partial double bond character of all the bonds of the ring. 

The role of IR spectroscopy in the early penicillin structure studies has been described (B-49MI51103) and the results of more recent work have been summarized (B-72MI51101). The most noteworthy aspect of a penicillin IR spectrum is the stretching frequency of the /3-lactam carbonyl, which comes at approximately 1780 cm" This is in contrast to a linear tertiary amide which absorbs at approximately 1650 cm and a /3-lactam which is not fused to another ring (e.g. benzyldethiopenicillin), which absorbs at approximately 1740 cm (the exact absorption frequency will, of course, depend upon the specific compound and technique of spectrum determination). The /3-lactam carbonyl absorptions of penicillin sulfoxides and sulfones occur at approximately 1805 and 1810 cm respectively. The high absorption frequency of the penicillin /3-lactam carbonyl is interpreted in terms of the increased double bond character of that bond as a consequence of decreased amide resonance, as discussed in the X-ray crystallographic section. Other aspects of the penicillin IR spectrum, e.g. the side chain amide absorptions at approximately 1680 and 1510 cm and the carboxylate absorption at approximately 1610 cm are as expected. 

NMR and, 3, 951 aromaticity, 3, 945 delocalization energy, 3, 959 deprotonation, 3, 972 disulfones reactions, 3, 970 double bond character, 3, 945 electronic energy levels, 3, 946 electrophilic reactions, 3, 965 electrophilic substitution, 3, 960 half-wave potential, 3, 968 NMR, 3, 952 H NMR, 3, 951 nucleophilic reactions, 3, 969 oxidation, 3, 967 oxides... 

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