Characteristic Bond Lengths in Free Molecules

CFC s, global warming potential, 14-34 to 35 CFC s, various properties, 6-133 to 135 Characteristic Bond Lengths in Free Molecules, 9-48... 

The conformation parameter a (=A/Af, where Af is A of a hypothetical chain with free internal rotation) for cellulose and its derivatives lies between 2.8-7.5 2 119,120) and the characteristic ratio ( = A2Mb//2, where Ax is the asymptotic value of A at infinite molecular weight, Mb is the mean molecular weight per skeletal bond, and / the mean bond length) is in the range 19-115. These unexpectedly large values of a and Cffi suggest that the molecules of cellulose and its derivatives behave as semi-flexible or even inflexible chains. For inflexible polymers, analysis of dilute solution properties by the pearl necklace model becomes theoretically inadequate. Thus, the applicability of this model to cellulose and its derivatives in solution should be carefully examined. 

The results of x-ray diffraction study of Z-2-(2-acyl-l-phenylvinyl)pyrroles [507,510] show the increased length of the C=0 bond (1.244-1.249 A) as compared to that characteristic of a free carbonyl bond (1.208 A) in the -C=C-C=0 fragment. The C=C bond in molecule also gets longer (from 1.325 A to 1.372-1.378 A) and the =C-C= bond is shortened (from 1.486 A to 1.443-1.448 A). These changes in the lengths of bonds may be ascribed to the delocalization of electron density along the 7-membered cycle formed due to intramolecular interaction N-H---0. 

---