The Determination of Molecular Quantities from Measurements on Macroscopic Systems. I

Abstract

For many molecules the determination of molecular quantities, as electric dipole moments or molar absorption coefficients, has to be based on measurements on solutions or other dense phases. The evaluation of such data is rather involved due to the interactions between neighboring molecules, and taking this into account, a general method is presented. Some quantities appropriate for the description of a macroscopic system (bulk quantities) are defined and their properties are discussed, especially partial molar quantities (PMQ's),which are well known from thermodynamics. To any PMQ some corresponding model molar quantities (MMQ's) can be introduced, each MMQ based on a particular molecular model. The MMQ's are related to the interesting molecular quantities and may allow their estimation. The values of PMQ's can be determined uniquely from experimental data, if the investigated system meets just a very few general requirements, whereas the values of the corresponding MMQ's are generally dependent on the chosen model and hence unique only, if a true model is available. As examples, approximate molecular models are introduced for MMQ's corresponding to the Gibbs energy and to quantities convenient for the evaluation of permittivity, optical absorption and electro-optical absorption measurements.

Große elektrische Feldeffekte auf die dipolare Aggregation von Merocyaninfarbstoffen

“…Geburtstag gewidmet Die Effekte externer elektrischer Felder auf makroskopische Systeme, Phasen mit chemischen Gleichgewichten eingeschlossen, sind von grundlegendem Interesse in der chemischen Thermodynamik. [1] Allgemeine thermodynamische Gleichungen zur Beschreibung von Dielektrika variabler Zusammensetzung in externen elektrischen Feldern wurden von Koenig [2] auf der Grundlage der Guggenheimschen Untersuchungen [3] entwickelt. Auch wurden dynamische Effekte gepulster elektrischer Felder untersucht und zur Untersuchung schneller Reaktionen und dielektrischer Relaxationsphänomene im Zusammenhang mit chemischen Prozessen verwendet.…”

The Determination of Molecular Quantities from Measurements on Macroscopic Systems.III. Electro-optical Absorption Measurements on Michler's Ketone

“…From the values of M, measured at varying compositions of the phase, the value of the PMQ YOG, as defined by (1.167), can be determined and for phases meeting a few requirements also the value of the corresponding MMQ vG, as was outlined in Sect. 9.5 of paper I of this series [1].…”

“…To relate the PMQ's to molecular quantities necessitates the introduction of an appropriate molecular model allowing the definition of corresponding model molar quantities (MMQ's). Some general relations between PMQ's and MMQ's pertinent to electro-optical absorption measurements have been presented in paper I of this series [1]. In this paper the relations between the MMQ's and some molecular quantities are outlined, based on a separate molecular model (SMM) described previously [1].…”

“…For a macroscopic phase meeting a few general requirements [1], it is possible to define certain bulk quantities without introducing any assumption relating to the particular molecular structure of the considered phase. Of special interest are extensive quantities and their adjoint densities, as defined by (1.21)*, since original experimental quantities directly determined with suitable measurement devices, belong mostly either to one or the other type or can easily be transformed into one.…”

“…Some general relations between PMQ's and MMQ's pertinent to electro-optical absorption measurements have been presented in paper I of this series [1]. In this paper the relations between the MMQ's and some molecular quantities are outlined, based on a separate molecular model (SMM) described previously [1]. The relations are used to determine properties of isolated molecules from electro-optical absorption measurements on solutions, as will be exemplified for Michler's ketone.…”