Abstract:

Simple models of Quantum Chemistry (particle in a box, harmonic oscillator, rigid rotor) are included by most of current Chemistry Degree curricula. Usually, they are taught at the beginning of a physical chemistry course or ending an introduction to quantum mechanics enclosed within a course on physics. As a consequence, many students look at them as they were not more than textbook cases with relatively simple solutions for the time-independent Schrödinger equation. That is, these models seem to be just opportunities we have to apply the postulates of Quantum Mechanics at a rather low mathematical cost. Thus, it is not strange we realise most of our students include these simple models as part of those artificial and useless concepts they are commanded to learn along their university days. In other words, simple models are not more than another one of those difficult trials they have to overcome to get their degree, with no connection to real chemistry. When this believing is complemented by poor mathematical abilities, something that, unfortunately, is becoming more and more common among Chemistry students, a very bad learning attitude towards these models and further concepts of quantum chemistry is set.

We believe this problem can be avoided if we introduce the simple models of Quantum Chemistry highlighting the practical interest they have for modelling molecular motions and how these movements modify molecular energy. As variations of molecular energy can be traced in many cases through spectroscopy these models will open us a wide horizon in real chemistry. Of course, we believe that repeating these sentences in the class room is not enough to modify the attitude of our students. Nevertheless, asking them to solve numerical exercises involving well known molecules (especially if solutions reveal significant information about them) can be very encouraging and take the students to a different point of view on this subject.

Here, we report exercises involving (and even combining) particle in a box, harmonic oscillator and rigid rotor. They were proposed to our second year of Chemistry degree students. Some of the exercises were designed as homework the students had to give back before certain deadlines. Other exercises are shown as typical examination questions. In these cases, the date and the type of exam where they were included are displayed as an additional warning about their importance. Moreover, some of them were solved and discussed in detail in class room.

The exercises also aim to other objective: let our students leave the ball and stick model (BSM) they assume for molecules, thinking on them as rigid entities with a given shape.

This action was complemented modifying the classical approach to simple models. Thus before the detailed study of the first one (particle in a box) we talked about molecular energies and the terms in which they are usually split into. At this point we describe together the three simple models to approximate kinetic, rotation, and vibration molecular energies, and even talk about electronic molecular energies. Finally, we indicate that exam results have improved after using this educational approach.

Keywords:

Quatum Chemistry, Physical Chemistry, Molecule Motion.