Acetylene, interatomic distances

The interatomic distance values given are stated to hold for hexahalogen, rym-trihalogen and -dihalogen benzenes. We expect the distances in these different compounds to be different, however and investigation of these and other halogen compounds is under way in this Laboratory. De Laszlo has also reported values of C-Br and C-I distances in ethylenic and acetylenic compounds in a letter to Nature, 130, 474 (1936). 

Values found for interatomic distances and bond angles in the thirteen hydrocarbons studied are given in Table XIV. The carbon-carbon singlebond distance is found to have the constant value 1.54 = = 0.02 A., being unaffected by the presence of an adjacent double bond or benzene nucleus (provided that it does not form part of a conjugated system). The carbon-carbon double-bond distance in allene and acetylene has the value 1.34 A. This is 0.04 A. less than that formerly given by the table of covalent radii, which has accordingly been revised. The effect of the revision on the bond distance-resonance curve is discussed. 

The C—C and C—B interatomic distances in carboranes can also be related to the coordination numbers of the skeletal atoms. Two factors tend to make these distances shorter than the B —B distances in comparable boranes the preference of the carbon atoms for sites of low coordination number and the greater electronegativity of carbon than boron, which increases the electron density in the region of the carbon atoms and so strengthens the bonds that they form. Table IX lists some C—C distances for closo- and wido-carboranes 13, 20, 21, 26, 98,121,168) and metal-acetylene 50, 58,112) complexes, relating them... 

Decreases in interatomic distances in the bromine and iodine derivatives of ethylene and benzene have also been reported,64 with magnitudes about the same as for the chlorine derivatives. In diehloro-acetylene,65 dibromacetylene, and diiodacetylene,66 in which the halogens are conjugated with a triple bond, there are decreases of about... 

The disordered nature of the amorphous phase has prevented the direct determination of the structural parameters by the usual structural techniques such as X-ray diffraction. Solid-state NMR can give us those parameters even for amorphous samples. Yannoni and Clark [14] applied nutation NMR spectroscopy, which is specifically designed to measure interatomic distances, to determine the bond lengths in both the cis- and tran -polyacetylenes. They used polyacetylene polymerized from a mixture of 4% doubly C-enriched acetylene and doubly depleted acetylene. In Fig. 7.6, the observed and simulated proton decoupled nutation spectra of the cis sample are shown. The sharp peak in the centre arises from the isolated nuclei in the sample. The remainder of the spectrum is a Pake-doublet arising from the dipolar coupling of adjacent nuclei in the polyacetylene. The best fit to the observed spectrum corresponds to a distribution of bond length with... 

C-2b c In the cationic Ag(I) arylacetylide complexes [Ag3(dppm)3L]2+ (111) and [Ag3(dppm)3L2] + (112) the trinuclear silver cluster has triangular pyramidal or bipyramidal geometry, with acetylenic C atoms in apical positions. Interatomic distances, 111 Ag—Ag... 

The molecular dimensions in dicyanoacetylene, also shown in the figure, are of interest. The difference between the different C-C bonds is clearly reflected in the difference in interatomic distances, but it is noteworthy that the single bond distance of 1 37 A is very considerably shorter than the standard C-C single-bond distance of 1 54 A. Very nearly the same distance is found also in cyanogen and in cyano-acetylene, and it is clear that in these molecules the simple formulation is an inadequate representation of the configuration. In the particular case of dicyanoacetylene we must assume that there is appreciable resonance between the ordinary configuration... 

Therefore, comparison of interatomic distances leads to the conclusion that acetylenes are more strongly bonded to the metal than olefins. This is also confirmed by theoretical considerations. Acetylene complexes containing a small... 

In line with van t Hoff s stereochemistry and the orientation of elliptic orbits, covalent bonds could be represented by tetrahedra that touch in apical, edgewise and facial mode, involving one, two or three electron pairs in an interaction, also shown in Fig. 2. This theory predicts increased bond strength with increasing bond order, but fails to account quantitatively for observed internuclear distances. For example, this model predicts the interatomic distances in methane and acetylene in the ratio of 3 1. 

---