In the Classroom
Visualizing the Positive-Negative Interface of Molecular Electrostatic Potentials as an Educational Tool for Assigning Chemical Polarity € nborn, Gunnar Ho € st,* and Karljohan Palmerius Konrad Scho Division of Media and Information Technology, Department of Science and Technology (ITN), Link€ oping University, Campus Norrk€ oping, SE-601 74 Norrk€ oping, Sweden *
[email protected] According to Atkins and Beran (1), “A polar molecule is a molecule with a nonzero electric dipole moment. Conversely, a nonpolar molecule has a zero electric dipole moment.” Predicting molecular polarity relies on combining an interpretation of the overall shape of a molecule with the directions of the dipole moments arising from charge separation. Although understanding polarity is closely linked to constructing fundamental chemical knowledge about solubility and intermolecular forces, students find it challenging to assign polarity to molecules (2). To help in interpreting the polarity of a molecule, charge separation can be visualized by mapping the electrostatic potential at the van der Waals surface using a color gradient (e.g., Figure 1, left). Another method indicates positive and negative regions of the electrostatic potential by displaying blue and red isosurfaces, respectively (e.g., Figure 1, right). Although these visualizations capture the molecular charge distribution efficiently, using them to deduce overall polarity requires students to engage in the potentially demanding process of interpreting the relative positions of electron-rich and electron-poor areas. As a supplement to such techniques, we present a visual tool that could help students assign polarity by exploiting the unique topography of the interface between negative and positive regions of electrostatic potential surrounding a molecule. Specifically, the tool renders the electrostatic potential isosurface(s) of a molecule obtained when the isovalue is set at 0. We propose that overall polarity can then be assigned by applying the following rules of interpretation.
A molecule is nonpolar if it generates: I. one closed and rotationally symmetrical isosurface and/or II. more than one isosurface, of which, each exhibits rotational symmetry with one or more of the other isosurfaces.
A molecule is polar if it generates: III. any isosurface(s) that do not conform to either I or II.
For instance, benzene and SF4Cl2 molecules yield symmetrical isosurfaces (colored in green, Figure 2), enabling a nonpolar assignment as per rule I and II, respectively. In contrast, the isosurface displayed in Figure 3 (left) shows the separation between the negative and positive regions of electrostatic potential surrounding an H2O molecule. This visualized information, in conjunction with rule III, can be employed to predict that the molecule is polar. Similarly, the polarity of more complex molecules, such as adenine in Figure 3 (right), can be assigned based on the topographical information together with the applicable rule(s) (e.g., adenine is polar as per rule III). There may be clear pedagogical benefits of using the method to assign molecular polarity. First, visualizing such isosurfaces may provide students with an unconventional yet powerful conceptualization of certain properties of the dipole moment. Here, the overall shape and orientation of the isosurface(s) may impart a visual appreciation of the alignment of a dipole moment (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).
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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, whereas conventional methods (e.g., Figure 1) require students to mentally integrate the relative positions of positive and negative regions, assignment of polarity via a singular interfacial topography becomes a “one-step” rather than a “multi-step” task. Third, use of the visualization tool might stimulate students to reflect upon polarity in terms of a “polar-nonpolar continuum”, where an increase in isosurface symmetry implies an increase in nonpolar properties of the molecule and vice versa. To our knowledge, no workers have applied such isosurfaces as an educational tool for visualizing molecular polarity. We are
r 2010 American Chemical Society and Division of Chemical Education, Inc.
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currently evaluating the perceptual power of the method and its effect on students' conceptual understanding of polarity. For this communication, we have purposefully chosen relatively simple molecules to illustrate the method, but invite interested colleagues to contact us with other structures for rendering. Literature Cited 1. Atkins, P. W.; Beran, J. A. General Chemistry, 2nd ed.; Scientific American Books: New York, 1992; p 329. 2. Furio, C.; Calatayud, M. L. J. Chem. Educ. 1996, 73, 36.
pubs.acs.org/jchemeduc
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Vol. 87 No. 12 December 2010
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Journal of Chemical Education
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