1342 Journal of Chemical Education

_Vol. 87 No. 12 December 2010

_pubs.acs.org/jchemeduc

_r2010 American Chemical Society and Division of Chemical Education, Inc.

10.1021/ed100417c Published on Web 09/28/2010

In the Classroom

Visualizing the Positive-Negative Interface ofMolecular Electrostatic Potentials as an EducationalTool for Assigning Chemical PolarityKonrad Sch€onborn, Gunnar H€ost,* and Karljohan PalmeriusDivision of Media and Information Technology, Department of Science and Technology (ITN),Link€oping University, Campus Norrk€oping, SE-601 74 Norrk€oping, Sweden*gunnar.host@liu.se

According to Atkins and Beran (1), “A polar molecule is amolecule with a nonzero electric dipole moment. Conversely,a nonpolarmolecule has a zero electric dipolemoment.”Predictingmolecular polarity relies on combining an interpretation of theoverall shape of a molecule with the directions of the dipolemoments arising from charge separation. Although understandingpolarity is closely linked to constructing fundamental chemicalknowledge about solubility and intermolecular forces, studentsfind it challenging to assign polarity to molecules (2).

To help in interpreting the polarity of a molecule, chargeseparation can be visualized by mapping the electrostatic poten-tial at the van der Waals surface using a color gradient (e.g.,Figure 1, left). Another method indicates positive and negativeregions of the electrostatic potential by displaying blue and redisosurfaces, respectively (e.g., Figure 1, right). Although thesevisualizations capture the molecular charge distribution effi-ciently, using them to deduce overall polarity requires studentsto engage in the potentially demanding process of interpretingthe relative positions of electron-rich and electron-poor areas. Asa supplement to such techniques, we present a visual tool thatcould help students assign polarity by exploiting the uniquetopography of the interface between negative and positiveregions of electrostatic potential surrounding amolecule. Specifi-cally, the tool renders the electrostatic potential isosurface(s) of amolecule obtained when the isovalue is set at 0. We propose thatoverall polarity can then be assigned by applying the following rulesof interpretation.

A molecule is nonpolar if it generates:

I. one closed and rotationally symmetrical isosurface and/orII. 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 symmet-rical isosurfaces (colored in green, Figure 2), enabling a nonpolarassignment as per rule I and II, respectively. In contrast, theisosurface displayed in Figure 3 (left) shows the separationbetween the negative and positive regions of electrostatic poten-tial surrounding an H2O molecule. This visualized information,in conjunction with rule III, can be employed to predict that themolecule is polar. Similarly, the polarity of more complexmolecules, such as adenine in Figure 3 (right), can be assignedbased on the topographical information together with theapplicable rule(s) (e.g., adenine is polar as per rule III).

There may be clear pedagogical benefits of using the methodto assign molecular polarity. First, visualizing such isosurfacesmay provide students with an unconventional yet powerfulconceptualization of certain properties of the dipole moment.Here, the overall shape and orientation of the isosurface(s) mayimpart 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 derWaals surface (left), and regions of positive (blue) and negative (red) electrostatic potential (right).

r2010 American Chemical Society and Division of Chemical Education, Inc.

_pubs.acs.org/jchemeduc

_Vol. 87 No. 12 December 2010

_Journal of Chemical Education 1343

In the Classroom

vector and the isosurface are aligned perpendicularly). Second,whereas conventional methods (e.g., Figure 1) require students tomentally integrate the relative positions of positive and negativeregions, assignment of polarity via a singular interfacial topographybecomes a “one-step” rather than a “multi-step” task. Third, use ofthe visualization tool might stimulate students to reflect uponpolarity in terms of a “polar-nonpolar continuum”, where anincrease in isosurface symmetry implies an increase in nonpolarproperties of the molecule and vice versa.

To our knowledge, no workers have applied such isosurfacesas an educational tool for visualizing molecular polarity. We are

currently evaluating the perceptual power of the method and itseffect on students' conceptual understanding of polarity. For thiscommunication, we have purposefully chosen relatively simplemolecules to illustrate the method, but invite interested col-leagues to contact us with other structures for rendering.

Literature Cited

1. Atkins, P. W.; Beran, J. A. General Chemistry, 2nd ed.; ScientificAmerican Books: New York, 1992; p 329.

2. Furi�o, C.; Calatayud, M. L. J. Chem. Educ. 1996, 73, 36.

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.