In the Classroom

Visualizing the Positive-Negative Interface of Molecular Electrostatic Potentials as an Educational Tool for Assigning Chemical Polarity

Konrad Schonborn,€ Gunnar Host,*€ and Karljohan Palmerius Division of Media and Information Technology, Department of Science and Technology (ITN), Linkoping€ University, Campus Norrkoping,€ SE-601 74 Norrkoping,€ Sweden *[email protected]

According to Atkins and Beran (1), “A polar molecule is a A molecule is nonpolar if it generates: molecule with a nonzero electric dipole moment. Conversely, a nonpolar molecule has a zero electric dipole moment.” Predicting I. one closed and rotationally symmetrical isosurface and/or molecular polarity relies on combining an interpretation of the II. more than one isosurface, of which, each exhibits rotational overall shape of a molecule with the directions of the dipole symmetry with one or more of the other isosurfaces. moments arising from charge separation. Although understanding polar polarity is closely linked to constructing fundamental chemical A molecule is if it generates: knowledge about solubility and intermolecular forces, students III. any isosurface(s) that do not conform to either I or II. find it challenging to assign polarity to molecules (2). To help in interpreting the polarity of a molecule, charge For instance, benzene and SF4Cl2 molecules yield symmet- separation can be visualized by mapping the electrostatic poten- rical isosurfaces (colored in green, Figure 2), enabling a nonpolar tial at the van der Waals surface using a color gradient (e.g., assignment as per rule I and II, respectively. In contrast, the Figure 1, left). Another method indicates positive and negative isosurface displayed in Figure 3 (left) shows the separation regions of the electrostatic potential by displaying blue and red between the negative and positive regions of electrostatic poten- isosurfaces, respectively (e.g., Figure 1, right). Although these tial surrounding an H2O molecule. This visualized information, visualizations capture the molecular charge distribution effi- in conjunction with rule III, can be employed to predict that the ciently, using them to deduce overall polarity requires students molecule is polar. Similarly, the polarity of more complex to engage in the potentially demanding process of interpreting molecules, such as adenine in Figure 3 (right), can be assigned the relative positions of electron-rich and electron-poor areas. As based on the topographical information together with the a supplement to such techniques, we present a visual tool that applicable rule(s) (e.g., adenine is polar as per rule III). could help students assign polarity by exploiting the unique There may be clear pedagogical benefits of using the method topography of the interface between negative and positive to assign molecular polarity. First, visualizing such isosurfaces regions of electrostatic potential surrounding a molecule. Specifi- may provide students with an unconventional yet powerful cally, the tool renders the electrostatic potential isosurface(s) of a conceptualization of certain properties of the dipole moment. molecule obtained when the isovalue is set at 0. We propose that Here, the overall shape and orientation of the isosurface(s) may overall polarity can then be assigned by applying the following rules impart a visual appreciation of the alignment of a dipole moment of interpretation. (e.g., Figure 3 (left), in the case of H2O, the dipole moment

Figure 1. Two conventional methods for visualizing the charge separation in a molecule (adenine), showing the electrostatic potential at the van der Waals surface (left), and regions of positive (blue) and negative (red) electrostatic potential (right).

1342 Journal of Chemical Education Vol. 87 No. 12 December 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. _ 10.1021/ed100417c_ Published on Web 09/28/2010_ In the Classroom

Figure 2. A planar molecule (benzene, left) and an octahedral molecule (SF4Cl2, right) assigned as nonpolar through application of rule I and II, respectively.

Figure 3. A bent molecule (H2O, left) and a more complex planar molecule (adenine, right) assigned as polar through application of rule III. Arrows represent approximate dipole moment vectors. vector and the isosurface are aligned perpendicularly). Second, currently evaluating the perceptual power of the method and its whereas conventional methods (e.g., Figure 1) require students to effect on students' conceptual understanding of polarity. For this mentally integrate the relative positions of positive and negative communication, we have purposefully chosen relatively simple regions, assignment of polarity via a singular interfacial topography molecules to illustrate the method, but invite interested col- becomes a “one-step” rather than a “multi-step” task. Third, use of leagues to contact us with other structures for rendering. the visualization tool might stimulate students to reflect upon “ - ” polarity in terms of a polar nonpolar continuum ,wherean Literature Cited increase in isosurface symmetry implies an increase in nonpolar properties of the molecule and vice versa. 1. Atkins, P. W.; Beran, J. A. General Chemistry, 2nd ed.; Scientific To our knowledge, no workers have applied such isosurfaces American Books: New York, 1992; p 329. as an educational tool for visualizing molecular polarity. We are 2. Furio, C.; Calatayud, M. L. J. Chem. Educ. 1996, 73, 36.

1343 r 2010 American Chemical Society and Division of Chemical Education, Inc. _ pubs.acs.org/jchemeduc _ Vol. 87 No. 12 December 2010 _ Journal of Chemical Education