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In its use of complex, real-world problems to introduce concepts and motivate learning in an active and cooperative learning environment, problem-based learning provides a powerful alternative to the passive lecture tradition in introductory science courses in biology, physics, and chemistry.The past decade has seen a national call for change in both the how and the what of college science teaching (science classes are taught using fifty-minute, content-driven lectures. Abstract concepts and principles are often presented first and only later illustrated with idealized examples that may be far removed from the students' personal experiences or interests. Memorization of facts and algorithmic problem solving are stressed, rather than conceptual understanding. The new information transferred to the student is assumed to fall into a preexisting framework with all the proper connections, automatically supplanting any contradictory ideas the student may already hold (Carey, 1986). Grade competition keeps students isolated; typical end-of-chapter, plug-and-chug exerdises foster knowledge without conceptual understanding (Mazur, 1992).In short, the structure of traditional science courses erects numerous roadblocks to students becoming actively involved in their own learning. Encouraging students to remain in this passive role in the classroom has the further unfortunate effects of promoting rote learning, obscuring rhe differences between high school and college thinking, and riveting intellectually immature students to a naive view of knowledge and its acquisition (

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1391cyclophane.',2 The lengths of dipoles were considered to be standard C-X bond distances when X was a single atom. When X was a group of atoms, the dipole length was considered to be the maximum extension of the group as projected on the C-X bond axis. The dipole lengths used were C-H (1.09 A), C-CN (2.62 A), C-Br (1.94 A), and C02R (2.99 A). The position of the ionizable hydrogen was fixed at 1.45 A beyond the carboxyl carbon on the extension of the C-C bond in accordance with the suggestion of Kirkwood and We~theimer.~ Bond or group moments were assessed from data tabulated by S m~t h~~ for the dipole moments of substituted methanes in benzene solution. It is suggested that these values be used as they present a better interpre-(29) C. P. Smyth, "Dielectric Behavior and Structure", McGraw-Hill, New York, 1955, pp 244-270. tation than those based on gas-phase measurement^.^^ I t has been assumed that the bond moment of the C-H bond is 0.4 with hydrogen as the negative t e r m i n~s .~ S~m t h~~ devotes considerable discussion to this point and concludes that popular opinion supports 0.4 as the magnitude, but the controversy continues over the direction.The effective dielectric (DE) was calculated by the spherical cavity model of Kirkwood and Westheimer: assuming an external dielectric of 32 and an internal dielectric of 2. The center of the dipole was placed 1.5 8, below the surface of the cavity, according to the suggestion of T a n f~r d .~~ This procedure fixes the cavity dimensions so that the effective dielectric may be calculated from the tabulations of Westheimer, Jones, and Lad.7dAbstract: The synthesis and characterization of a series of tetraarylporphyrins which bear covalently attached alkyl and aryl mercaptans designed to serve as axial ligands are described. The coordination chemistry of the iron(I1) complexes of these "mercaptan-tail" porphyrins has been investigated by IH N M R , IR, and electronic absorption spectroscopy, magnetic circular dichroism, and magnetic susceptibility measurements. Ferrous complexes of the alkyl mercaptan-tail porphyrins appear to remain four-coordinate, intermediate spin (S = 1) in solution. The situation is less clear in the case of appended aryl mercaptans and a "tail-on/tail-off" equilibrium is implicated. In the presence of carbon monoxide, however, binding of thiol trans to CO is observed in both the alkyl and aryl cases. By the addition of an appropriate base, six-coordinate mercaptide-Fe(I1)-CO complexes can be generated; these reproduce quite well the characteristic absorption and MCD spectra of cytochrome P-450, suggesting that such compounds are indeed viable models for the active site of cytochrome P-450.