Bond lengths in ethylene

Problem 6.4 Predict (a) the geometry of ethylene, H,C=CH, (f>) the relative C-to-C bond lengths in ethylene and ethane (c) the relative C—H bond lengths and bond strengths in ethylene and ethane (d) the relative bond strengths of C—C and C==C. ... 

Each of the carbon-hydrogen sigma bonds is formed by overlap of an sp2 hybrid orbital on carbon with the Is orbital of a hydrogen atom. The C—H bond length in ethylene (1.08 A) is slightly shorter than the C—H bond in ethane (1.09 A) because the sp2 orbital in ethylene has more s character (one-third, v) than an sp3 orbital (one-fourth, v). The s orbital is closer to the nucleus than the p orbital, contributing to shorter bonds. 

They have used model of lone pairs for water. It would be interesting to check the result CC bond length in ethylene and acetylene, and O-H bond length and H-O-H angle for water. The results have been collected in Table 19. 

Although we cannot experimentally observe a ir bond directly (all we can observe are the positions of the atoms), the structure of ethylene provides strong support for its presence. First, the C—C bond length in ethylene (1.34 A) is much shorter than in compounds with C—C single bonds (1.54 A), consistent with the presence of a stronger C=C double bond. Second, all six atoms in C2H4 lie in the same plane. The 2p orbitals that make up the ir bond can achieve a good overlap only when the two CH2... 

K = double bond force constant in ethylene cr = single bond force constant in ethane d = double bond length in ethylene s = single bond length in ethane (10)... 

Fig. 2. Time-evolution of the methyl/ethyl C-C distances for both the zirconocene and the corresponding titanocene catalyst. The two curves starting at around 3.2 A represent the distance between the methyl carbon atom and the nearest-by ethylene carbon atom in the zirconocene-ethylene and the titanocene-ethylene complex, respectively. The two curves starting at around 1.35 A reflect the ethylene internal C-C bond lengths in the two complexes.

Ethylene, bond angles in, 16 bond lengths in, 16 bond strengths in, 16 electrostatic potential map of, 74, 147... 

Table I. Bond lengths (in A) and bond angles (in degrees) for the ethylene molecule...

The units by which crystallographers describe interatomic distances are Angstrom units (A = 10 8 cm.). Normal values for carbon-carbon interatomic distances are 1.34 A for a double bond (as in ethylene) and 1.54 A (as for-diamond) for a single bond. In a truly aromatic compound (such as benzene) the C-C bond length, as mentioned above, is 1.39 A. C-C-C angles are 109.5° for a tetrahedral carbon atom (sp3) and 120.0° for a trigonal carbon atom (sp2). 

TABLE 1. Twist angles ( ) and bond lengths in some tetrasubstituted ethylenes... 

Figure 4-6. Plane wave convergence of the carbon-carbon bond length in the ethylene and butadiene molecules, from the simulations with the CPMD program13 (Troullier-Martins pseudopotentials,1415 time step 4 a.u., fictitious mass 400 a.u., unit cell 12 A x 12 A x 12 A)...

The radius of charge circulation in the twisted molecule is not specified but should be of the order of the C—C bond length. In fact, using r = 117 pm, the calculated energy of 270 kJmol-1 agrees with the accepted 7r-bond strength in ethylene. 

Introduction of the bulky phenyl substituent to yield CH3 considerably distorts the n-n conjugation of the naphthyl moiety with the ethylene bond of the pyran ring but does not essentially affect the structure of the molecule. The conformation and the distribution of the bond lengths in CH3 and CH2 are similar (Table 5). 

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