Bent-back angles

Remarkable results were obtained with pentaerythritol tetra-palmitate. This symmetrical body gave a film of exceptional rigidity, so that it withstood a force at one end of 5 6 dynes per cm. without any support at the other end. When first put on the area was 100 A, per molecule and was reducible by compression to 80 A., or about four times the area of a single closely packed chain. The four chains must therefore lie parallel, and two of them must be bent back through a large angle. 

TABLE 2. Bent-back (tilt) angles in metallatetrahedranes... 

Fig. HI.2. Plastic welding practice, (a) Some typical welds. (b) Technique for feeding rod into weld. With the rod bent back into a 45° angle, it is easy to heat and the rod is forced into the seam. The torch should be fanned between the rod and the parts being welded, with somewhat greater concentration on the latter, (c) An edge seal accomplished on a hot plate.

In complexes of symmetrical fran-s-olefins where the positions of all four substituents have been determined, it is possible for one set of trans substituents to be bent back more than the other set. This twist of the CR2 groups about the C=C bond would not be obvious from an inspection of the a and /3 angles. This is true in compound XIX (Table I) where the CFs groups are bent back more than the F atoms 7 = 132(1)° and 120(1)° for F and CF3, respectively. It is difficult to ascribe this effect solely to either steric or electronic causes. It seems reasonable that certain groups bend back more for electronic reasons, but in this instance it is the bulkier group that is bent back more. It is not possible to predict on electronic grounds whether F or CF3 should bend back more. 

While the rotation barriers for highly symmetric complexes such as Mo(CO)(H2)(dppe)2 (7) can be extremely low (0.6 kcal/mol), they are not in fact zero. This is because X-ray data reveal that the structure of the MP4 fragment is distorted with at least one of the P-M-P axes bent back away from the H2 (with which it is nearly parallel). The distortions rehybridize M orbitals such that the overlap with [Pg.184]

The iron complex has a square-planar cyclobutadiene ring with C—C distances averaging 1.46 A and internal bond angles 90°+ 1°. The substituent phenyl groups are bent back out of the plane of the cyclobutadiene ring away from the iron by an average of 11 ° and are all twisted about the... 

We think that a long thin blowpipe, as in Figure 83,1, but bent at the end through a greater angle—about 150°—would be very useful for working on complex apparatus. It would enable the back of tubing to be worked from the front. 

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