Use of isosbestic point wavelength shifts to estimate the fraction of a precursor that is converted to a given product

An isosbestic point is observed in overlaid spectra when a chromophoric pre cursor is converted to a product with a different spectrum, so that it is o ften assumed that an isosbestic point occurs only when the precursor is qua ntitatively converted to a single product. We show experimentally and by co mputer simulations that more complex reactions also exhibit isosbestic poin ts and that the wavelength of the isosbestic point may change. Such wavelen gth changes will occur if either (i) the molar absorbtivity of the precurso r changes or (ii) the fraction of the precursor that is converted to multip le products changes. In the latter case, the isosbestic wavelength and mola r absorbtivities of the precursor and product can be used to calculate the fraction of the precursor that is converted to products from the relationsh ip, f = is an element of (Precursor)(M)/is an element of (Product)(M), wher e f is the fractional conversion, is an element of (Precursor)(M) is the mo lar absorbtivity of the precursor, and is an element of (Product)(M) is the molar absorbtivity of the product.