HIDDEN ASSUMPTIONS IN THE AVERAGE STRUCTURE METHOD

Publisher Website
Google Scholar
Abstract
Common to all average structure methods are three steps: (l) Measurements are made; (2) Structural parameters are calculated; (3) A structure is drawn which exhibits parameters close to the calculated ones. Each step has a certain amount of uncertainty associated with it. as does the interpretation of the result. One particular method involving elemental analysis, number-average-molecular-weight determination and 1H- and 13C-nmr, is examined in detail. It is shown that certain of the assumptions commonly made are arbitrary and almost certainly misleading; furthermore, little has appeared in the literature to indicate that complex mixtures exhibit the chemical behaviors that their average structures would predict. Several developments are discussed which could vastly improve the average structure method. These include: (a) Application of new (and particularly multidimensional) nmr techniques; (b) A better understanding of the molecular weight distributions of heavy fractions; (c) Application of computer methods (such as random isomer generation subject to constraints, followed by statistics) to structural parameter determination; and, most important of all, (d) A systematic effort to compare the predictions of the average structure method with the results of chemical experiments.