Double bond length

All the carbon-oxygen bonds are found to be of equal length and intermediate between carbon-oxygen single and double bond length. 

Because the carbons that are singly bonded m one resonance form are doubly bonded m the other the resonance description is consistent with the observed carbon-carbon bond distances m benzene These distances not only are all identical but also are intermediate between typical single bond and double bond lengths... 

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). 

Organic thionylamines have planar, cis structures (9.9) in the solid state and in solution, as determined by X-ray crystallography and N NMR spectroscopy, respectively. The gas-phase structures of the parent compound HNSO and MeNSO have been determined by microwave spectroscopy. The S=N and S=0 double bond lengths are 1.51-1.52 and 1.45-1.47 A, respectively. The bond angle [Pg.168]

CC bond distances in localized allylic systems have been held at 1. 5A and 1.3A (typical of CC single and double bond lengths, respectively), and at 1.4A for delocalized systems. 

A large body of experimental evidence confirms that covalent bonds have characteristic distances depending on bond type. Carbon-carbon single and double bond lengths are around 1.54A and 1.32A, respectively, while partial double bond distances, e.g., in benzene, are about 1.40A. 

Ab initio calculations on H2Si=NH (HF/6-31G ) show a planar molecular geometry with a SiN double bond length of 157.6 pm and an Si=N—H angle of 125.2°. 

In an early electron diffraction investigation of the structure of 2-methylpropene (isobutylene), Bartell and Bonham (1960) found that the three terminal carbon atoms are arranged in an almost perfect equilateral triangle around the central carbon despite the considerable difference in the single and double bond lengths (Figure 5.4). This result led Bartell to suggest that the terminal carbon atoms are close-packed around the central carbon atom. He then... 

The successful synthesis and isolation of a series of heavy ketones (r r2M = X M = Si, Ge, Sn, Pb X = S, Se, Te) using kinetic stabilization (vide infra) and the remarkable progress in the field of theoretical calculations prompted chemists to perform computational calculations on the a and it bond energies as well as on the single and double bond lengths of H2M=X at the higher level of theory.14... 

Table 1. Double bond lengths X=X [A] p-p(x) overlap integrals S ionization potentials IP [eV] of the dissociation products XH2 T-bond strengths Erot [kcal mol 1] from the barrier of rotation, calculated at the CASSCF(2,2)/6-31G + ZPE (zero point energy) level of theory.

We have recently investigated X-ray structure of 3-methoxycarbonyl-[l,4,2] diazaphospholo[4,5-a]pyridine as first example of molecular structure determination for [l,4,2]diazaphosphole ring [84], The ester substituent lies strictly in the molecular plane with carbonyl group in the trans orientation with the formal C=P bond. Endocyclic P-N and P-C bonds are averaged between respective single and double bond lengths. 

There have now been four experimental determinations of a silicon-carbon double bond length. The first of these was a gas phase electron diffraction study of 1,1-dimethylsilene (173). This study was the subject of much controversy since the experimentally determined bond length, 1.83 A, was much longer than the one predicted by ab initio calculations (1.69-1.71 A, see below) (159). Since the calculations were carried out at a relatively high level of theory and the effects of electron correlation on determining the Si=C bond length were considered, the validity of the data extracted from the electron diffraction study is in serious doubt. 

The second determination of a silicon-carbon double bond length came from the X-ray crystal structure of l,l-bis(trimethylsilyl)-2-(trimethyl-siloxy)-2-(l-adamantyl)-l-silaethene (1) (122). Again, the experimentally... 

At the HF level, the value of the C=C bond length is clearly underestimated. The inclusion of electron correlation at different levels of calculation leads to values in closer agreement with experiment. The value of the C—C bond length is less sensitive to the inclusion of electron correlation. As a consequence of this fact, the CC bond alternation (the difference between CC single and double bond lengths) is overestimated at the HF level. The inclusion of dynamical electron correlation through MP calculations corrects this error. A very similar result is obtained at the CASSCF level of calculation31. 

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