LInk organic, inorganic, & physical chemistry

~ from molecular interactions to chemical reactions ~

Module B : Fundamentals of chemical reaction

So far, we have focused on the case where molecules interact weakly with each other. However, of cause, there are cases where molecules interact more strongly by exchanging electrons (charge transfer; CT). In this Module B, we set "charge-transfer complexes" as target subjects, that are not fully chemically reactive, but exhibit signs of chemical reactions. Chemical reactions are processes that create new molecules through the formation and breaking of chemical bonds. Therefore, they are always accompanied by a change in the number of electron occupancies in the bonding and anti-bonding orbitals, i.e., a charge transfer. While kinetic analysis of chemical reactions requires a certain amount of skill, both experimentally and theoretically, charge-transfer complexes are relatively easy to follow, even for undergraduates. Specifically, we are taking up two types of subjects including CT: a model for simple practice, and a more realistic CT complex of I2 and amines that you can observe in a wet experimental laboratory course.

  • Module B-1: Substituent effect in CT complexes

The amounts of CT and strength of CT interaction depend on the orbitals of the molecules. Here, we focus on a set of relatively small model CT complexes (Fig. B-1) and discuss how we can quantify the electron-donating/withdrawing nature of the molecules. Using geometry optimization, wave function, and atomic charge analyses, you'll be able to understand the basics of CT in a quantitative manner.

Fig. B-1 Model complexes.
[B(R1)3…N(R2)3 (R1,2 = H, F, CH3)]

Exercise

  • The accuracy of CT description depends on the density functional. Using CAM-B3LYP, estimate the CT amounts.

  • Compare the frontier orbitals.

Input Sample

Download Gaussian Inputs for Module B-1

  • Module B-2: Understanding color change upon mixing of I2 and amine: An observable CT case

One of the difficulties in studying chemical reactions is that it is difficult to directly and experimentally observe molecules reacting. As a result, in many cases, when studying chemical reactions, many students tend to think that they have to memorize the reaction mechanism.

Here, we will attempt to understand the signs of chemical reactions from the tie-up of experiment and theory, using CT complexes as the subject matter, in which "color," a physical property that we can observe with the naked eye, changes. In order to quantitatively understand and control more applied cases in which chemical reactions proceed and new substances are formed, it is necessary to gain a deep understanding of systems that allow us to closely examine what is happening when reagents are mixed, rather than suddenly conducting time-series analysis of chemical reactions that progress from moment to moment.

In this module B-2, we focus on I2...triethylamine complex (Fig. B-2,3). You will acquire basic skills related to chemical reaction analysis. Through this module, you will acquire basic skills in quantum chemical calculations for molecular interaction analysis and excited state calculations.

Fig. B-2 An observation example of color change by forming CT complex.

Fig. B-3 Molecular orbital description of the CT interaction.

Exercise

  • Calculate UV-Vis absorption spectra at the TD-CAM-B3LYP level of theory.

  • Assign the absorption bands and explain CT.

Input Sample

Continued with ECP data

Download Gaussian Inputs for Module B-2