Acetylene molecular geometry

Another interesting example is provided by the equilibrium structure of substituted diacetylenes. The semiexperimental approach allows us to compare their structures as obtained at the same level of accmacy. Such a comparison is reported in Thorwirth et al. [121] where the authors compared and discussed the molecular geometries of diacetylene itself and various substituted diacetylenes/acetylenes. These are summarized in Table 6.6. The main conclusion that can be drawn is that the HCCC fragment remains almost unchanged by substitution of one hydrogen of HCCCCH. From Table 6.6 the effects of conjugation (i.e., 7t-electron delocalization)... 

Practice working with your Learning By Modeling software Construct molecular models of ethane ethylene and acetylene and compare them with respect to their geometry bond angles and C—H and C—C bond distances... 

Molar absorptivity. 502 Molecular ion (M+), 410 Molecular mechanics. 130 Molecular model, dopamine, 930 acetaminophen, 29 acetylene, 18 adenine, 67 adrenaline, 323 alanine, 28, 1016 alanylserine, 1028 rr helix, 1039 p-aminobenzoic acid, 25 anti periplanar geometry, 387 a recoline, 79 aspartame, 29 aspirin. 17 ball-and-stick, 61 /3-pleated sheet, 1039 p-bromoacetophenone, 449 bromocyclohexane, 121 butane, 80... 

Following his self-consistent field MO calculations on acetylene, Burnelle (1964) examined the role of excited states and molecular vibrations in determining 88 of additions. He found that, when a proton is brought close to acetylene, the energy of the trans-hent structure falls below that of the linear form. For similar addition to ethylene, Bumelle (1965) found that the first stable intermediate derived from the 90° twisted form of ethylene. Since such a geometry could only lead to SS = 0—there is no preferred orientation for attack— this particular model was less successful for ethylene than for acetylene. 

A two-dimensional Httckel molecular orbital (HMO) theory approach to acetylenic systems yielded n-bond orders of P = 0.894 for the central C—C bond and P = 1.788 for the C C triple bonds in 1,3-butadiyne (209, 210). For comparison, P = 1 for ethylene and P = 2 for acetylene. A different criterion for determining the relative strengths of chemical bonds was used by Politzer and Ranganathan (17). Starting from STO-3G geometries and force constants, they calculated a bond order of 1.34 for the central C-C bond in diacetylene. This corresponds to a bond dissociation energy of 150 kcal/mol [211], which compares with bond orders and bond dissociation energies of 1.14 and 88 kcal/mol for ethane and 1.85 and 163 kcal/mol for ethylene. 

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