HCH angle

Fig. 5. a) A methylene group, showing the a and p orbitals (HCH angle 120°). b) The most symmetric approach of methylene to ethylene, c) Level diagram for this approach. Classification of S and A refers to the plane through the midpoint of the double bond, d) Less symmetric a approach. 

For the study of the dimerization of CH2, the 12-dimensional surface was reduced to a 6-dimensional one by imposing a fixed geometry on the two fragments. It was soon realized that the two carbon atoms always choose to lie in a plane bisecting both HCH angles. 

The values obtained were >=0.6844 cm i and E =0.0034 cm. A species with a greater motional freedom was assigned to the values of D =0.6634 cm and <0.002. From these data an HCH angle of 136° was deduced, in excellent agreement with the latest calculations. However, the values of Shell which are practically identical with the above, were interpreted differently. Similiar data obtained for HCD and DCD are additional evidence that the bond angle of triplet methylene is in fact 136°. 

Triplet methylene is known to be bent with a bond angle of approximately 136°. This is closely reproduced by all Hartree-Fock models (except for STO-3G which yields a bond angle approximately 10° too small), as well as local density models, BP, BLYP, EDFl and B3LYP density functional models and MP2 models. Semi-empirical models also suggest a bent structure, but with an HCH angle which is much too large. 

To find the contributions of the internal coordinates, C—H bond lengths and HCH angles, to these vibrational modes we first use the set of four C—H bond lengths as the basis for a representation, obtaining TCH shown below. 

The distance between the two propellane subunits in lw is 1.494 A, somewhat shorter than the corresponding distance in bicyclopropyl (1.517 A)11. The HCH angle in the methylene group is 116 ° in la. The PhCPh angle is somewhat less in the diphenyl derivative lm, 113.1° and 114.3 ° in the two inequivalent molecules in the crystal. 

Walsh orbitals6 are the MOs corresponding to the cyclopropane C-C bonds. We begin by supposing that carbon orbitals are sp2 hybrid. Indeed, many studies show that cyclopropane is more like ethylene than propane or cyclobutane. For example, it adds bromine slowly, whereas cyclobutane is inert. Hydrogenation of cyclopropane occurs at 120°and cyclobutane at 200°. Asymmetric CH2 stretching modes appear in the IR spectrum of cyclopropane at 3050 and in that of ethylene at 3080 cm-1. Methyl cyclopropyl ketone shows a carbonyl stretch at 1695 cm-1. Finally, the HCH angle is 116°6 in ethylene, and 118° in cyclopropane. 

GVB calculations on CHa have also been reported,449 so a value of 134 + 2° seems well established. Other SCF-CI calculations on the low-lying states have been reported,450 and it now seems clear that the HCH angle in the ground state is 134 2°. 

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