Methylene bond angle

Fig. 7.5 Distributions of bond angles found in the CSD for all sp carbons (closed line), for the methylene analogs R-CH2-R (dashed line), and for quaternary carbons with four nonhydrogen substituents R-CR2-R (dotted line).

In the molecule of 4-methylene-3-borahomoadamantane derivative 79, the structure of which was determined by X-ray analysis, the six carbon atoms of the triene system, the two boron and two silicon atoms all lie in one plane within experimental error (mean deviation 1.4 pm). The boron atoms deviate from the trigonal-planar geometry, since the sum of bond angles around the atoms is only 355.8° instead of 360°, as usually encountered in triorganoboranes. Considerable distortions of the bond angles at the terminal C-C double bond occurs in the vicinity of the boron atoms B-C(4)-C(ll) 130.60(19)° and B-C(4)-C(5) 107.38(17)° <2002CEJ1537>. 

Early spectroscopic experiments on the structure of methylene were interpreted to show that the 3Bj state was linear or nearly linear (Herzberg, 1961). However, theoretical calculations and eventually observation of the epr spectrum of triplet methylene (Bernheim et al., 1970 Wasserman et al., 1970) led to a reinterpretation of the earlier experiments (Herzberg and Johns, 1971). Now there is universal agreement that the 3Bj state of methylene is bent with a bond angle (0) of approximately 135° (see Fig. 1.)... 

The structure of At methylene has a more acute bond angle than does the 3Bt triplet. In this case, both experiment (Herzberg, 1961) and theory have been in consistent agreement. Both approaches place this bond angle at 104° (see Fig. 1). 

Fig. 2 Calculated energies of the lowest states of methylene at various bond angles. (Harrison, 1971, reproduced with permission)...

Exercise Estimate the bond angle of singlet methylene (CH2). 

The outcome is a repulsion of the C—H a-bond electrons. The methylene carbon atoms in [3.3]paracyclophane assume a chairlike conformation in solution the chair and boat forms were found to be in equilibrium 12>. The slight distortion of the molecule is reflected in the expansion of the bond angles and in dihedral angles at the bridges. The lengths of the aliphatic C—C bonds are normal (1.507 and 1.517 A). 

The calculations show that the ground state of methylene should be the triplet state (see Table 2) with a bond angle of 132—134°. This is clearly borne out by the more sophisticated calculations such as the MINDO/2, MINDO/3 and... 

Fig. 1. Molecular orbitals of methylene and their energy as a function of bond angle...

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°. 

Experimental data from 17 electron diffraction and 13 microwave spectroscopic measurements have been used in establishing this relationship, and the C-C-C bond angles ranged from 50.8° to 116.6°. One of its applications is in the structure analysis of molecules with nonequivalent methylene groups. 

Figure 2. a) Glycosidic bond angle(x) in nucleotide triphosphates b) 8-BrATP c) 8,5 -Cyclo-AMP d) Adenylyl imidodi-phosphate e) 8,y-Adenylyl methylene bisphospho-... 

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. 

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