J. Michael McBride at 65 â An Appreciation - ACS Publications

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 6 2022-2035

Perspective J. Michael McBride at 65 - An Appreciation Mark D. Hollingsworth,*,† Jennifer A. Swift,*,‡ and Bart Kahr*,# Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, Department of Chemistry, Georgetown University, Washington, D.C. 20057, and Department of Chemistry, University of Washington, Seattle, Washington 98195 Received October 11, 2005;

Revised Manuscript Received October 14, 2005

W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: On the occasion of his 65th birthday, a retrospective of the ongoing career of J. Michael McBride (hereafter Mike), Professor of Chemistry at Yale University since 1966, is offered. An analysis of Mike’s pioneering studies of organic crystals is followed by a report on his iconoclastic approach to teaching organic chemistry. Mike’s professional colleagues are likely unaware of his innovations in the classroom, while Yale students may not know what he does in the laboratory. This essay aims to illuminate the other side of Mike for his respective audiences. Nevertheless, it is a challenge to summarize his work. Mike McBride’s scientific publications are an incomplete record of his professional life, and his presentation of organic chemistry changes with each passing year. A combination of memories, interviews of sympathetic witnesses, and our analysis of the written record give one view of the achievements of a brilliant, one-of-a-kind teacher and scholar. Mike’s Crystals Mike McBride was born on February 25, 1940, and spent his formative years in Lima, Ohio. We do not know much about Mike’s earliest infatuation with science.1 We do know that as a transfer student from the College of Wooster, he had toyed with the idea of a career in medicine until he met the great luminary Paul D. Bartlett in Chemistry 105 at Harvard. Having “screwed up the courage” to offer his services when P.D. asked for volunteers for a research project,2 Mike worked with Bartlett during his senior year. Taking P.D.’s advice that few undergraduates were likely to exhaust the teaching potential of the Harvard chemistry faculty, Mike stayed on with Bartlett for his Ph.D. as an NSF predoctoral fellow. Solvent cage control of radical reactions was all the rage in the early 1960s, so Mike settled into a traditional * Corresponding authors. Prof. Mark D. Hollingsworth, Department of Chemistry, 111 Willard Hall, Kansas State University, Manhattan, KS 66506; tel: 785-532-2727; fax: 785-532-6666; e-mail: mdholl@ ksu.edu. Prof. Jennifer A. Swift, Department of Chemistry, Georgetown University, 37th and O Streets NW, Washington, DC 20057; tel: 202687-5567; fax: 202-687-6209; e-mail: [email protected]. Prof. Bart Kahr, Department of Chemistry, Box 351700, University of Washington, Seattle, WA 98195-1700; tel: 206-616-8195; fax: 206-685-8665; e-mail: [email protected]. † Kansas State University. ‡ Georgetown University. # University of Washington.

thesis project involving kinetic and product analysis of radical pair reactions of azo compounds such as azo-3methyl-2-phenyl-2-butane (AMPB) and of peresters.3 By comparing the reactivity of the two diastereoisomers of AMPB, Mike was able to indirectly probe the steric effects of the solvent cage. In a footnote in the first section of his thesis, Mike tips his hand toward his lifelong pursuit when he writes, We should note in passing that steric retention in a solvent cage reaction can be regarded as an extension of “topochemistry.” G. M. J. Schmidt in the first of ten fine papers on topochemistry published in 1964 states the “topochemical hypothesis”: “Reaction in the solid state occurs with a minimum amount of atomic or molecular movement.” M. D. Cohen and G. M. J. Schmidt, J. Chem. Soc. 1964, 1996. This “passing remark” might suggest only a passing interest in this emerging field of solid-state organic chemistry, but nothing could be further from the truth. By his own account,4 Mike was left spellbound by the first of these articles and the opportunities they presented. Schmidt’s work on crystal packing and topochemistry had shown Mike that it might actually be possible to understand reactions in solids. Although Schmidt and co-workers were mindful of the many events that could occur between light absorption and

10.1021/cg0505362 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/02/2005

Perspective

Figure 1. Orientation dependence of EPR absorptions for a crystal of meso-AMPB after photolysis at -196 °C. Adapted from ref 6 with permission from Pure and Applied Chemistry, the IUPAC, and the author. Copyright 1967 IUPAC. See also ref 3.

product formation, they had no way of directly probing what had transpired in the interim. The topochemical hypothesis had been formulated brilliantly on the basis of starting materials and products, but Mike knew that there were many other stories to be told. And Mike likes a good story. Mike’s approach to studying reactions in crystals represents no less than a watershed in solid-state organic chemistry. Inspired by the studies of Clyde Hutchison, Jr. and Gerhard Closs on triplet states of carbenes using oriented crystal EPR,5 he recognized that studying radical pairs generated in single crystals could provide unprecedented detail about the actual mechanisms of solid-state reactions. P. D. Bartlett had the good sense to let his student run with this idea, and Mike’s EPR studies of triplet radical pairs formed in the low-temperature photolysis of single crystals of AMPB launched the field of mechanistic solid-state organic chemistry. The EPR signals from radical pairs persisted for days at liquid nitrogen temperatures, long enough to study them in many orientations and to determine the inter-radical distance and relative orientation in the crystal framework. At the time, the crystal structure of meso-AMPB was not known, but the orientation dependence of the triplet signals showed that there were two symmetry-related radical pairs in the crystal with a common inter-radical distance of 6.3 Å (Figure 1).3,6 When Mike moved from Harvard to Yale as Assistant Professor, the ascendancy of Yale as the place to do physical organic chemistry was assured.7 He quickly became the resident expert in EPR spectroscopy and collaborated with Jerry Berson on trimethylenemethane analogues8-10 and with Martin Saunders, Ron Breslow,

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and Ed Wasserman on demonstrating that the ground state of the unsubstituted cyclopentadienyl cation was a triplet.11-15 In his independent work, he set about using radical pair EPR and other techniques to study crystal lattice control of radical pair reactions in important radical initiators such as peroxides and azo compounds.16-23 Through the clever use of isotope effects (and lack thereof for rate-limiting diffusion), Mike showed that for the cyanoisopropyl radicals formed in the photolysis of azobisisobutyronitrile (AIBN), methyl group rotation is rapid on the time scale of the atom transfer involved in disproportionation, but that the rate-determining step for this reaction involved radical diffusion, not atom transfer.18 The painstaking care and unwillingness to take anything for granted that Mike displayed in these early years have been hallmarks of his career. At the low levels of photolysis used for his EPR studies (usually parts per million), Mike found that he could interpret the radical pair geometries in terms of pristine crystal structures and even model them successfully using computer simulations, but one nagging question was whether the observable intermediates could account for the products that were formed.20 Even after a few percent conversion, the likelihood that one or more of a reaction site’s six nearest neighbors had been photolyzed was already high, so Mike used isotope dilution techniques to measure product distributions at low conversions. Steady-state spin counting and isotope dilution showed that the radical pair characterized by EPR accounted for the bulk of the products formed in the reaction.24 The disproportionation products were formed at “normal” sites, but experiments combining isotope dilution and isotope effects on mixed AIBN crystals containing 3H in some molecules and both 14C and 2H in others showed that the small yield of radical coupling product must have come from abnormal (presumably damaged) sites that were not in competition with the “normal” sites that gave disproportionation. Vanity has never been one of Mike’s defining characteristics, but enthusiasm for science issfor decades, and still today, the “vanity plates” on Mike’s car have simply read, “AIBN.” Although Mike’s entry into the field of solid-state organic chemistry had started with Schmidt’s papers on topochemistry, he did not have to look very far before he found reactive crystalline systems that violated the topochemical hypothesis. One of the first of these was azobis-3-phenyl-3-pentane (APP), in which one of the disproportionation products (3-phenyl-2-pentene) was formed as a mixture of diastereomers, only one of which (the E isomer) was predicted by “least motion” ideas.22 The Z-isomer showed no isotopic enrichment or depletion in partially deuterated APP, again showing that radical diffusion, not hydrogen abstraction, controlled the rate of formation of this product. The “non-topochemical” behavior that Mike observed in APP was the tip of the iceberg. During the next three decades, Mike and his students studied the radical pairs in a variety of peroxides and azo compounds.25 With the use of oriented crystals, it was usually possible to use the electron-electron dipolar interactions (the D tensor) to determine the inter-radical separation to within 0.01-0.02 Å and the relative orientations of the radicals

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Figure 2. Reaction scheme for acetyl benzoyl peroxide (ABP) showing methyl-benzoyloxyl (MB) and methyl-phenyl (MP) pairs.

to within a degree or two. Although the term “mechanism” had traditionally meant a sequence of differently bonded intermediates, this work expanded the meaning of that term to include the actual three-dimensional trajectories of the fragments involved.26 And, although the concept of least motion was often useful for understanding individual steps, the radicals often moved far apart, in seemingly unpredictable ways, before coming together to give products. Determining the relative positions of localized radicals from single crystal EPR was a relatively straightforward matter, but interpreting the EPR spectra of radical pairs containing delocalized radicals required knowledge of the ground-state spin distribution of the radicals in question.27,28 Nowhere did this problem become more acute than with the benzoyloxyl radical, a centerpiece of Mike’s research for over a decade in the 1970s and early 1980s.21,29-33 The so-called doublet instability problem made reliable calculation of the ground state of delocalized radicals difficult, and for the benzoyloxyl radical, the choice appeared to be between an unsymmetrical 2A′ state and a delocalized (C2v) π radical. Mike knew well that reactions in solids could be full of surprises, but the use of the most likely spin distributions for benzoyloxyl resulted in mechanisms that defied logic. In acetyl benzoyl peroxide (ABP), for example, lowtemperature photolysis gives methyl-benzoyloxyl (MB) pairs, which can lose CO2 either thermally or photochemically to give methyl-phenyl (MP) pairs (Figure 2). Immediately after photolysis, it appeared that the methyl group had backed up from its original position by 2.4 Å (assuming that it was freer to move than the benzoyloxyl), but then it appeared to return to its original position when the benzoyloxyl radical decarboxylated to give a MP pair. In, “EPR Studies of Radical Pairs. The Benzoyloxy Dilemma,” Mike writes, “This coincidence is conceivable, but suspicious.” Even more suspicious was the nearly identical behavior of phenyl radicals in dibenzoyl peroxide, whose crystal packing was completely different from ABP.29 The benzoyloxyl dilemma and the mechanistic anomalies posed by more than a decade of research were resolved by measurement of the 17O hyperfine tensors for benzoyloxyl in both carbonyl and peroxy labeled ABP.32 Without a doubt, the ground state of the ben-

Perspective

Figure 3. Radical pair geometries (thin lines) in acetyl benzoyl peroxide (ABP) superimposed on the crystal structure (dark lines) of the starting material. The positions of the methyl radicals, first in MB and then in MP pairs, are depicted as small open circles at the lower right. The phenyl radical coincides approximately with the initial phenyl orientation in ABP. Reproduced with permission from ref 25. Copyright 1983 American Chemical Society.

Figure 4. Bis(3,3,3-triphenylpropanoyl) peroxide (TPPP) showing the chemical structures of the starting material and of the radicals before (T) and after (R) neophyl rearrangement. For certain pathways in TPPP, local stress generated by CO2, not orbital overlap, dictated the choice of rearrangement pathway.

zoyloxyl radical was 2B2, a delocalized sigma radical! Although it took a few more years for theoreticians to catch up with experiment,34,35 it was almost immediately apparent that the correct ground state provided logical mechanisms in which the benzoyloxyl radicals rotated in their planes by approximately 30° (Figure 3).25,36 The motion of the benzoyloxyl radical in ABP was so “logical” that it was possible to model it by minimizing the torques and forces on the fragments, which were treated as rigid bodies.37,38 A full-blown molecular mechanics treatment using a surface walking algorithm and EPR constraints accurately mimicked the radical geometries of both MB and MP pairs, even though the surrounding molecules were held in fixed positions.39 At first glance, not all of the mechanisms were so logical. In the photochemical reactions used to generate radical pairs from azo compounds and diacyl peroxides, as many as four fragments were generated from one precursor, often in the most tightly packed region of the crystal. Far from being simple spectators, the “inert” N2 and CO2 molecules appeared to play active roles in controlling the motions of the radical fragments. In provocative work on bis(3,3,3-triphenylpropanoyl) peroxide (TPPP), Mike followed radical pair structures through successive neophyl rearrangements and showed that in certain sites, the choice of phenyl that rearranged was controlled not by orbital overlap with its methylene target but by local stress created upon photoextrusion of the CO2 molecules (Figure 4).25,40,41 Studies of TPPP and related systems made it clear that reaction-induced stress could be a controlling feature of solid-state reactivity.42 Reasoning that repul-

Perspective


Figure 5. Schematic potential energy surface classifies solidstate reactions according to how different regions of the surface are affected by passing from the gas to solid state. Regions A, B, C, and E represent chemical effects that involve bond making or breaking, whereas regions D and F are purely physical effects that involve no bonding change. Reproduced with permission from ref 40. Copyright 1981 American Chemical Society.

sive potentials vary much more rapidly with distance that attractive ones, Mike generalized this work and created a new classification scheme for understanding how a rigid environment can influence the rate of a single step reaction (Figure 5). Although most techniques for studying solid-state reactions cannot identify (much less distinguish) such influences, the exquisitely detailed information from radical pair EPR studies allows this in many instances. It is our opinion that Mike’s intuition about how solids work is second to none. Combined with his reluctance to overinterpret his data, we take note of his challenge to the most common approach used to understand reactions in solids: One might think that the substantial lattice strain which is generated during radical pair formation would make this type of reaction a poor model for solid-state reactions in general, but it could equally well be that hypothetical reaction in an unstrained lattice is in fact the poor model. Only the first molecule to react in a perfect crystal could react in a truly strain-free environment. All molecules reacting subsequently should be more or less sensitive to strain generated by earlier events, and the type of strain effects revealed in the present work may be widespread in solid-state chemistry.40 Such provocative statements were not made in a vacuum. Earlier work in the group had shown that brief photolysis of ABP generated radical pairs that decayed in a simple, first-order fashion. More extensive photolysis, however, generated a second class of radical pairs that decayed 40 times more slowly.43 But the difference between radical pair structures in the fast and slow sites was less than 0.06 Å, or 1%. Mike rightly asks, “How can these species, whose structures are identical within much less than the amplitude of thermal vibration, react at such different rates?”25 The answer, according to Mike, and now two more decades of research on solid-state reactions, is that the mechanical properties of solids are just as important as packing when trying to interpret reactivity. Even at conversions as low as ∼0.06%, decomposition of remotely distant sites can rigidify the crystal and retard reaction rates by orders of magnitude.25,26,43,44 Many solid-state reactions, in particular single crystal to single crystal transformations, generate much less reactioninduced stress than the peroxide reactions, and others still20,22 (especially unimolecular rearrangements) seem

Figure 6. Reaction scheme in diundecanoyl peroxide (UP) showing the positions of the alpha carbons of the radicals (squares) superimposed upon the crystal structure of the starting material for the three principal intermediates detected by EPR. Dashed lines denote the bonds that are broken upon photolysis, and the arrows in pair C show the direction of the CR-Cβ bond, as determined by hyperfine splitting (hfs) anisotropy for UP-2,2,2′2′-d4 (β-UP). Reproduced with permission from ref 26. Copyright 1990 Wiley-Interscience.

to be more forgiving than the peroxide decompositions,33 which often generate four fragments from one precursor.45 So it is not surprising that solid-state chemists have gotten a lot of mileage out of interpreting solidstate reactions in terms of pristine crystal structures. But the fragmentation reactions of peroxides serve as useful counterexamples that amplify the influence of reaction-induced stress, which must be present in virtually every reacting solid. Mike’s initial model of reaction-induced long-range stress, which was based on an isotropic continuum of elasticity,26,44 was later elaborated by Luty and Eckhardt in a comprehensive “chemical pressure” model that treats the elasticity of reacting solids anisotropically.46 In part because they comprised a homologous series of materials that could be compared and functionalized, Mike turned his attention to long-chain diacyl peroxides in 1979. This seemingly small piece of territory became such a mainstay of his research throughout the 1980s and 1990s that Mike has often quoted Henry David Thoreau’s Walden (I have travelled a good deal in Concord...)47 when introducing this work in seminars.4 In fact, the radical pair reactions in some of these (in particular, diundecanoyl peroxide or UP and bis(11bromoundecanoyl) peroxide or 11-BrUP) may be known in greater detail than any other reactions. If ever there was a reaction that defied the topochemical postulate, this was it. Following low-temperature photolysis, the decyl radicals generated in UP back up in stages using screw-like motions in which each carbon replaces the next one along the chain (Figure 6). From their initial positions in the starting material, where they were 5.72 Å apart, the R-carbons reach a distance of 7.68 Å in the final radical pair structure (pair C) before collapsing to give eicosane (and other products). The detailed radical pair structures and the pathways that connect them are perhaps the most definitive evidence for the reptation mechanism proposed for relaxation of linear polymers.25,48

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Figure 7. Asymmetric stretching region (in absorbance subtraction) of the IR spectra of CO2 generated by gentle photolysis in crystals of UP. (a) Species A (occurring as two different CO2 dimers) after photolysis at ∼20 K (∼0.05% conversion). (b-d) At 20 K after warming to give radical pair B (again as two pairs), C and products. See Figure 6 for corresponding radical motions. Upon moving from a to d, the general trend from high to low frequencies parallels the relaxation of high local stress, which is probably at least 20 kbar in pair A. Adapted with permission from ref 26. Copyright 1990 Wiley-Interscience.

By analyzing the local free volume in the crystal,49,50 Mike was able to show that the reptation mechanism moved the termini of the radicals into a loosely packed region of the crystal. The driving force for radical motion was obviously the local stress from the CO2 molecules that had been generated in the photolysis, but what was the magnitude of this stress? One of the coauthors of this piece (M.D.H.) was lucky enough to enter Mike’s group soon after the role of the CO2 molecules was inferred from the work on TPPP. Mike’s approach was to use the asymmetric stretching frequencies of the CO2 molecules as in situ reporters of local stress and its anisotropy (Figure 7). More than 1,000 spectra later, it had become abundantly clear that in the initial stages of reaction, the local stresses could be 20 000 atm (20 kbar) or more.26,51 The effects of such high stresses were manifested in many ways,50 including relaxation of the stress by remote defects52 and large intra- and intermolecular steric isotope effects. (Deuterium labeling had been used routinely to simplify the EPR spectra of radical pairs, but this work showed that the isotopes played a more active mechanistic role.) Combined with an analysis of intermolecular vibrational coupling between CO2 molecules53 and subtle frequency shifts from nearby deuterium atoms,26,51 polarized IR spectra of oriented crystals made it possible to narrow the range

Perspective

of structures for the CO2 dimers dramatically. Although the literature had suggested that “orientational” effects might be small,54 Mike correctly predicted that it would be possible to distinguish different orientational isomers of 18OdCd16O (i.e., 18OdCd16O from 16OdCd18O) in the asymmetric reaction cavities of the peroxides. This provided yet another level of detail about the reaction, including end-for-end exchange of the CO2 molecules.26,51,55,56 In 11-BrUP, where the radicals could not easily back up into the bromine-filled interface, it was possible to trap an earlier intermediate in which the radicals were more closely spaced and the CO2 molecules were jammed together, side by side. With subsequent warming, the radicals attacked the R-hydrogens of their peroxide neighbors in a chain decomposition that churned out CO2 molecules and an R-lactone. Here, at last, was a crystalline system undergoing radical-molecule reactions that could be studied with EPR and infrared spectroscopy. And study it Mike and his students did. By our count, over 25 isotopomers of 11-BrUP and its analogues have now been synthesized, crystallized, and characterized by single-crystal EPR spectroscopy.57-60 When crystals are completely deuterated in the R-position, the chain decomposition takes a different path, with δ-hydrogen abstraction instead. All told, over a dozen intermediates were identified on this complex reaction surface, which was later proven, through an analysis of birefringence, to be localized within a single layer of this tetragonal peroxide crystal.61 The stereospecifically deuterated variants of 11-BrUP were perhaps the most revealing, since the crystals are chiral (space group P41(3)212), and the atom transfer reactions could easily distinguish between hydrogen and deuterium.62 A single crystal of a stereospecifically R-deuterated peroxide could therefore be right- or lefthanded, but, as far as the reaction was concerned, these crystals are diastereomeric by virtue of the large isotope effect on the reaction pathway. Radicals initially generated in heterochiral crystals containing the same stereospecifically R-deuterated peroxide expressed this subtle diastereoisomerism by abstracting an R-hydrogen in the P43212 crystal, but, thwarted by a deuterium in the same position in the P41212 form, abstracted a δ-hydrogen instead. Few experiments more dramatically illustrate the highly constrained reaction coordinates in single crystals; those who know the intricacies of this work (and the molecular recognition work described below) understand why Mike was awarded the Prelog Stereochemistry Medal by the Swiss Federal Institute of Technology in 1992 (Figure 8). As if chain decompositions in peroxides weren’t complicated enough, Mike turned his attention to energetic materials such as nitramines, where even gentle photolysis kicked off reaction pathways that generated a dizzying array of intermediates. By this time, however, Mike had developed all of the necessary tools to study such complex reactions. By combining EPR and IR results, it was possible to dissect pathways that included more than 50 distinct intermediates!63,64 In the high explosive RDX, for example, two pathways were disentangled. Both mechanisms began by photoconversion of a nitramine to an N-nitrite, which then decomposed to give, in one pathway, an NO/nitroxide

Perspective

Figure 8. Jack Dunitz (ETH, Zu¨rich), Mike McBride, and Meir Lahav (Weizmann Institute, Rehovot, Israel) on Long Island during the 13th International Conference on the Chemistry of the Organic Solid State (ICCOSS 13), which was held at SUNY, Stony Brook, NY. Mike gave one of his many ICCOSS keynote lectures, this time on “Following more complex radical reactions in organic crystals.” Photo courtesy of John R. Scheffer.

Figure 9. One-bond (left) and three-bond (right) cleavage pathways in the high explosive RDX. Each starts with the photoisomerization of an N-nitro group to an N-nitrite.

radical pair, or, in another, an NO/NO2 radical pair via a three-bond cleavage (Figure 9).65 In this work, the NO orbital degeneracy gave characteristic EPR spectra as a result of spin-orbit coupling. Because this coupling was extraordinarily sensitive to the crystal field, NO proved to be another vital probe of the crystal environment. In 1992, NO was proclaimed as the Molecule of the Year by Science magazine for its role in signaling, but by that time it had long been a champ in Mike’s lab. A tetramorphic explosive, HNIW, was the next energetic material to feel the full force of Mike’s 2 kilowatt arc lamp (at 4 K).66 Here, common intermediates in polymorphs were crucial in the tensor assignments. In HNIW, exchange coupling, not spin-orbit coupling, took center stage. A small interaction (0.003 cm-1) was detected between a pair of 15NO2 radicals separated by more than 9 Å.67 This energy, the difference between singlet and triplet states in a radical pair, is ordinarily very hard to predict or measure because it is smaller


than kT. Catching it at work is a testament to the power of single-crystal spectroscopy in the right hands. In the nitramines, as in his work on peroxides50,52,68 and substituted indolinospyropyrans,69-71 Mike has always been mindful of the otherwise intractable information that one could obtain by comparing singlecrystal spectral parameters (e.g., D-tensors, IR shifts of CO2) for closely related crystals, either as pure materials or as “solid solutions.” In his most recent contribution,71 which follows this Perspective, he unambiguously assigns the long-enigmatic triplet excited state of an aromatic nitro group through a “new” fragment superposition method, in which he uses the local geometric differences between nitro groups on a common framework to find the unique tensor assignment. Although this work is over 20 years old, it is as cutting edge today as it was in 1980, when it was so far ahead of its time that Mike had a hard time publishing it. Although the first 20 years of Mike’s research career are better known for the studies of reactions within single crystals, his group could never ignore the underlying importance of crystal growth. After all, singlecrystal growth must always precede single-crystal spectroscopy. By the 1980s, the study of crystal growth mechanisms from solution became an active area of inquiry in its own right in the McBride lab. Mike’s earliest work in crystal growth can be regarded in the context of “crystal engineering.” He had already demonstrated that the isolation of substituent effects on reactivity required generating homologous series of crystals in which the analysis of packing differences could be reduced to a consideration of only two dimensions instead of three. This strategy bore fruit in his study of the physical properties of a large body of fatty acid diacyl peroxide crystals, some of which were available in the lab from previous photochemical studies. By meticulously measuring the melting points of some 60 symmetrical and unsymmetrical diacyl peroxides, he was able to create a unique database reflecting the aggregate intermolecular interactions. In this way, Mike showed how melting point, a notoriously vexing physical property (easy to measure and hard to understand) could be interpreted by studying the packing at two-dimensional layer interfaces in single crystals.68,72 In fact, in Mike’s most recent lecture at a Summer School on Stereochemical Aspects of Novel Materials at the University of California, Santa Barbara, just a few weeks ago at the time of writing, he presented the digested wisdom accumulated from years of reflection upon this work, most of which was derived from an old Ph.D. thesis.73 His presentation was notable not only because it appeared as fresh as experiments a few weeks old, but because Mike was able to give the most cogent explanation of what melting point means and what to expect for the melting point in a crystal that has already undergone a structural phase transformation. Later, the group tried to engineer a homologous series of layered crystals based on the meso-hydrobenzoin scaffold.74 Although the layer motif tolerated the addition of some 4,4′-substituents, not all derivatives produced lamellar crystals. Mike characteristically asked what might be learned from this crystal engineering

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Figure 11. Schematic showing face-selective, polar ordering of guest ducks (mergansers) in a host crystal comprised mostly of mallards. This type of symmetry breaking can help explain the optical anomalies observed in crystals in Figures 10 and 12. Adapted with permission from ref 81. Copyright 1989 Wiley-VCH. Figure 10. Photomicrograph (along [001]) showing sectoring and anomalous birefringence in a mixed crystal containing 85% bis(11-bromoundecanoyl) peroxide and 15% (11-bromoundecanoyl) undecanoyl peroxide. The yellow and blue interference colors in adjacent growth sectors indicate that the symmetry has been lowered by admixing the guest and that the optical indicatrix is sequentially rotated by 90°. Used with permission from J. Michael McBride and Steven B. Bertman.

strategy gone awry. He reasoned that parallel C-X dipoles prevented the anticipated layered structure. Dilution by admixing C-CH3 derivatives regenerated the lamellar motif. Mike’s recognition of the importance of destabilizing interactions complements typical design strategies emphasizing favorable ones. Mike’s appreciation of the work from the Weizmann Institute75,76 only grew over the years with the development of crystal stereochemistry from second and third generation Israeli solid-state chemists and friends that included Meir Lahav, Les Leiserowitz, and Lia Addadi.77 When one of Mike’s students noted that his tetragonal crystals containing mixtures of peroxides were birefringent when viewed along the ostensible 4-fold axis (Figure 10),78 Mike was primed to interpret the optical anomaly as a consequence of the nonstatistical occupancy of guest molecules among crystal sites that would otherwise have the same potential energy had the crystal achieved equilibrium.79 Although all of Mike’s lectures are memorable, those in attendance can never forget his presentation on optically anomalous crystals at Prof. David Curtin’s retirement symposium at the University of Illinois (the first Midwest Organic Solid State Chemistry Symposium, June 1988). Equipped with a polarizer that he hung from the reflector of an overhead projector and birefringent models of the crystals made from Cellotape and Thomas’ English Muffin wrappers, Mike wowed the audience when he gave his polarizer a spin, and the overhead spelled, “WOW”, then “DAVE”, then “WOW”, then “DAVE” in rapid succession. Until the time of this discovery, optical crystallography had never been a major theme in Mike’s lab, but it was always practiced.30 Whereas polarized light microscopy fell out of fashion in many laboratories over the years, Mike’s students wore out his Leitz microscope until the gears were stripped bare. The McBride lab was never the place to go for fancy new equipment, perhaps

because no instrument manufacturer could ever imagine the kinds of experiments that would be required in his group. Weissenberg cameras hummed until long after they qualified as museum pieces. Mike’s students lined up daily to use his ingenious EPR goniometer, a highschool wood shop project if ever there was one. When students from the Berson and Wiberg groups tried to lord it over the McBride students by claiming that their equipment was S.O.A. (aka state of the art), the McBrideans learned from Mike that they should pay no attention. And there were extra kudos from Mike if masking tape (or later, duct tape) played an integral part in making your Rube Goldberg contraption work!80 The basic concepts of symmetry reduction in mixed single crystals were masterfully illustrated in Mike’s Angewandte Chemie essay with a cartoon crystal made of ducks.81 Most chemists might not know the difference between a mallard and a merganser, but Mike’s marriage since 1964 to Flo, an extraordinary, awardwinning birdwatcher and educator, most likely prompted this ingenious melding of ornithology and crystallography. Flo was the videographer in the interviews with Jack Dunitz and Les Leiserowitz that we share as web-enhanced objects (see Figure 13 below). The duck schematic (Figure 11) illustrates how faceselective, polar ordering of guest ducks (mergansers) occurs in a host crystal comprised of mallards. Although the lattice appears to have P2 symmetry, it is locally asymmetric. Because of the repulsive crests at the backs of their heads, the guest ducks will not enter the crystal headfirst. On the top face, for example, the merganser is attached to a step edge right side up and is eventually locked into place by the host duck that comes in from behind it. Likewise, mergansers that attach to the bottom face go in upside down. On the right face, however, the merganser is not locked into place and is therefore expelled before overgrowth. Thus, in the top and bottom growth sectors of the crystal (see diagonal lines), the guest sublattice is polar; in the right and left sectors, guests are depleted. An “impurity” could be a distinct compound (merganser) or alternatively a misaligned molecule, as is the case in Figure 12. The duck analogy works less well with disorder in pure crystals, but if one imagines the elementary units as duck dimers, it can be extended. In either case, nonstatistical incorporation can break symmetry and lead to optical anomalies.

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that critical etch pit dimensions in certain directions were requisite before dissolution in other directions were activated. Because none of this work is yet in the open literature, we will here adopt the policy of less is more; these stories are best told by Mike.

Figure 12. Photomicrograph (along [001]) of an optically anomalous crystal of 1,5-dichloro-2,3-dinitrobenzene. Images of this sort served as the inspiration for the cover art by Susan Oviatt in this issue of Crystal Growth & Design (2005, Vol. 5, issue 6). Here, the phenomenology is comparable to that in Figure 10. However, in this case, the “impurity” is the dichlorodinitrobenzene molecule in minor orientations. The ratio of orientations can vary from site to site on the growth surfaces. The rich optical banding is a consequence of uncontrolled growth.

It soon became clear that comparable observations had been analyzed at length by many of the great crystallographers of the previous century. Their struggles were unfortunately buried by the ascendance of X-ray crystallography. By studying the development of crystal science in the 19th century, a variety of substances were identified whose optical properties preserved detailed histories of crystal growth mechanisms,82 including many minerals and several organics such as 1,5dichloro-2,3-dinitrobenzene, the inspiration for this special issue’s cover art (Figure 12). One of Mike’s highly cited papers83 was prompted by the request to evaluate a report that stirred solutions of sodium chlorate deposit homochiral crystals.84 By making a movie of the sodium chlorate crystallization, Mike established the veracity of the result but further showed that secondary nucleation was the simplest explanation of the “spontaneous generation of chirality.” Intrigued by the possibility of such chiral induction in the natural world, Mike, armed with a shovel and a senior football tackle, set off for the “Herkimer Diamond Mine” in St. Johnsville, NY. There they collected several dozen geodes containing thousands of quartz crystals. Back in the lab, these crystals were etched in acid; the resultant hemihedral pits were used to assign their individual handedness and to discern whether local enantiomorphous biases might also be attributed to secondary nucleation. Mike had been thrilled by the appearance of scanning probe microscopes since they first appeared in the mid1980s. In 1991, he put aside his affection for custombuilt equipment and purchased a commercial atomic force microscope, thus enabling a “direct” analysis of crystal growth mechanisms. His students seized the first new piece of equipment to enter the lab in years. Here too, he took best advantage of dissolution.85 Studies of benzoin were especially intriguing because they showed

Mike’s Webpage Those scientists who have sat through Mike’s breathtaking lectures can only imagine that he is a favorite among students and would not be the least bit surprised to learn that he has won not only local awards for his teaching86 but also national acclaim, including the 1996 Catalyst Award from the Chemical Manufacturers Association and a Camille and Henry Dreyfus TeacherScholar Award (1972-1977). Mike currently holds the Richard M. Colgate Professorship, intended for distinguished teachers of Yale freshman. One need only examine the evolving webpage (http://classes.yale.edu/ chem125a/) that he has prepared for his Chemistry 125 (Freshman Organic Chemistry) students to discover that he is the most attentive, incisive, and imaginative organic chemistry professor around. The number of handcrafted pages that currently occupy the Chemistry 125 website must exceed 1000. It is surely as large as Morrison and Boyd, but oh so much more fun. Here, we will review some of the Chemistry 125 website, emphasizing where possible those aspects that might be of particular interest to crystallographers and solid-state chemists. Start clicking the Chemistry 125 links and you will immediately sense Mike’s appreciation of history. The website is loaded with portraits of long-dead scientists, original translations of seminal articles from past centuries, and mediations of vigorous debates, some consequential, others ridiculous. But this is not history as window dressing on a traditional course. It is serious, original scholarship representing nothing less than a revolution in the teaching of organic chemistry, or any science for that matter. The analyses of original documents, the pivot on which Mike balances a contemporary understanding of organic chemistry, is his answer to a challenge of the great essayist, Lewis Thomas, who, in a 1982 editorial much admired by Mike, insisted that we must change the way that science is presented. “We might begin”, Thomas suggested, “by looking at the common ground that science shares will all disciplines, particularly the humanities and with social and behavioral sciences. For there is indeed a common ground. It is called bewilderment.”87 Mike is out to bewilder the heck out of his studentss and thereby produce scientists out of each and every one. Bewildering surely is Mike’s choice to begin his course with a discussion of Samuel Pepys, a famous English diarist who knew little of science and less of mathematics. “Off-the-mark” topics continually pop up in Chemistry 125; they shake students to life. They are never arbitrary, but their integral function only becomes apparent with time, after which Mike is revealed as a master of course composition. Pepys is remembered for his skill of close observation. No detail was too small to escape his attention. What better model than a careful diarist for the reconstruction of a science on what was actually observed, in chronological order? And yet that is only one of the many functions of Pepys. With Pepys, a witness to the execution of Charles I, Mike cleverly

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inserts his students into the drama that will unfold by virtue of the fact that two of the regicides who ordered the death of the King, Whalley and Goffe, fled across the Atlantic and hid in a cave on West Rock, an outcropping that can be seen by clambering atop Sterling Chemistry Laboratory. Like a Swiss army knife, Pepys has other uses still. As an adult, he struggled to teach himself multiplication and even enlisted the assistance of Isaac Newton in his attempt to understand the laws of probability. Newton, then, is used to reintroduce students to inverse square force laws that he considered not only for gravity but to explain the attractions within and between molecules. Amazingly, Newton was able to argue in the 17th century against inverse square forces in chemistry. So then, asks Mike, what is the chemical force law? What are the functions? Could there be a more basic question that is never asked in introductory chemistry classes? Instead, students get stable octets and Lewis dot structures without any justification, without deference to the forces between real particles. In fact, Lewis structures argue against inverse square force laws because they violate Earnshaw’s theorem, a principle of electrostatics, that states that local energy minima and maxima cannot exist among particles whose interactions are proportional to r-2. Mike first became a serious student of Earnshaw’s theorem when he encountered the Levitron, a counterintuitive toy. His fascination with this device seemed over-the-top at the time, especially because it was a distraction from “chemistry.” No one but Mike could foresee the first lesson in Freshman Chemistry in a floating magnet. Any instructor aware of Earnshaw’s theorem, and determined to minimize Lewis structures in favor of a rigorous understanding of quantum chemistry, might part company with Lewis as precocious but misguided. Not Mike. He knows that Lewis and his contemporaries were aware of the inadequacy of classical electrostatics as the basis of chemical structure. They went ahead nonetheless with ad hoc force laws and the simplest electronic configurations. Talk about bewildered! And yet, some things survive from this miasma, despite their limitations, as the grammar of chemistry. After all, Mike illustrates that we can see bonds (Figure 13)88,89 and lone-electron pairs in electron difference density mapss three cheers for Lewissbut we do not always see them where we expect them.90,91 The latter qualification is illustrated in the 1981 X-ray analysis of a bicyclobutane derivative that shows a striking absence of electron density between bridgehead atoms that we would nonetheless expect from Lewis theory.90 Who but Mike’s colleague, Ken Wiberg, could have foretold that just a year later it would be possible to prepare one of those “bondless” molecules ([1.1.1]propellane) and store it in a bottle at room temperature?92 Of course, there has got to be a more complete theory of bonding, but how did we come to know it? The driving force behind Mike’s unique presentation of organic chemistry is his quest to answer the question, “How do YOU know?” Mike, ever scholarly, cites his 3-year-old son John for stressing the importance of examining how it is that we have come to know what we know. The particular topics, each one of which relentlessly returns to this question, are organized by the four “Big Cs”: Composition, Constitution, Config-

Perspective

Figure 13. Electron difference density contour map for the central portion of 1,1,3,3-tetraphenylbutatriene showing perpendicular orientations of adjacent π-bonds. Contour interval 0.1 e Å-3: zero contour broken, negative contours dotted. Reproduced with permission from ref 88. Copyright 1975 American Chemical Society. W Videos (used with permission from Florence S. McBride) of Mike McBride interviewing W Leslie Leiserowitz and W Jack Dunitz on aspects of bonding revealed by electron difference density maps are available in QuickTime 7 format. Annotated transcripts are available at the Chemistry 125 website (http:// classes.yale.edu/chem125a/).

uration, and Conformation. How did we come to know what atoms are contained within molecules? How did we come to know how atoms are connected to one another? How did we come to know the metric relationships within molecules? How did we learn to distinguish between left and right? Organic chemistry was a tar pit until Liebig invented a sufficiently accurate method of combustion analysis that distinguished between complex carbon compounds with similar empirical formulas. Improving C/H analysis might seem like a prosaic achievement to sophisticated students of the 21st century, but imagine the gravitas that is added when Mike shows the students where Liebig’s CO2 collector is carved in stone relief on the building in which they are receiving their lecture. And, just to prove that he is not content with an artist’s rendering, Mike offers to the class a slide show of his pilgrimage to an original Kaliapparat in Liebig’s lab in Giessen. Liebig fared less well when it came to Constitution. He missed the prize lying behind the common analysis of silver fulminate and silver cyanate, the discovery of isomerism. It went to Berzelius, a self-described “poor crystallographer” who was nevertheless good enough to have reclassified all minerals in terms of their chemical composition. His pejorative self-characterization probably referred to his facility with a goniometer. He knew better than anyone alive what was inside crystals, their chemical composition, but did not consider himself a crystallographer because he was not quick or sure with the characterization of their outsides. In fact, Chemistry 125 students learn how it is that we owe the science of stereochemistry and molecular chirality to the optical goniometer, and they are introduced to Groth’s Chemische Kristallographie,93 a tribute to this simple device. After many decades in which X-ray crystallographers emphasized the insides of crystals at the expense of their outsides, nanoscience is driving a renewed interest in crystal shape with the AFM serving as a contact goniometer. Mike has had a blast with his AFM during the past 15 years. He is keen to show students what can be learned by “seeing” atoms, but he is even more eager to demonstrate to them that the organic chemists

Perspective

of the 19th century were able to determine the structures of organic compounds without scanning probe microscopes, NMR spectrometers, or X-ray diffractometers. Mike believes that the determination of the structures of organic compounds is one of the greatest achievements of humankind. Like an architecture professor describing a great cathedral by looking at the antecedent structures, plans for the existing church, plans discarded, and seams between architects and between centuries, Mike McBride does not deliver organic chemistry. He reassembles it. Wo¨hler was one of the original assemblers whose preoccupation was Constitution. Like Liebig’s improved analyses, Wo¨hler’s urea synthesis was a spark plug for organic chemistry. But Wo¨hler thought he had found a better path through the thickets. He said that Liebig “limits himself much too much to minutiae and puts a greater weight on the correctness of his analyses than on the plausibility of how Nature might have organized the construction of a compound.”94 Wo¨hler’s urea synthesis was a serendipitous result of his study with Berzelius of the isomerism of silver cyanate. Mike shows, following the paper of Dunitz et al.,95 that the very first solid-state chemical reaction was the collaborative decomposition of ammonium cyanate by Wo¨hler...and Liebig.96 The Liebig/Wo¨hler/Berzelius correspondence/collaboration fleshed-out by Mike is fascinating. Berzelius argues to Wo¨hler that Liebig should have “his hand slapped” for carrying out “quick and dirty” experiments while they continue their productive collaboration. (Mike never has a bad word to say about anyone, and it must be for him a psychic relief when long-dead figures go at it.) The torturous birth of “radical” theory, following the observation of the persistence of the C7H5O fragment throughout a variety of chemical transformations, helped sort out the Constitution question.97 Here, Mike could have worked in his own experience with the benzoyloxyl radical. He does not. One of the things that he admired about his own mentor, P. D. Bartlett, was that he made it “impossible to tell from [his organic chemistry lectures] that he himself had made important contributions, or even knew, much less trained, the individuals whose work he discussed.”2 As Mike rebuilds organic structural chemistry, he restores many favorite episodes that standard texts have long since abandoned, such as Fischer’s proof of the relative configuration of the sugars. In fact, graduate students working through monthly cumulative exams forever lived in fear that Mike might ask them to “Reproduce Fischer’s proof” as he is once rumored to have done. Of course, the greatest triumphs in organic structure determination accompany the greatest missteps. In another sphere, this idea is emphasized by Michael Jordan, who said, “I missed 9,000 shots during my career.” Mike, a basketball fan, surely understands MJ’s wisdom. In teaching science too, the hits and misses likewise must live together. Mike’s most requested paper during the olden days, when reprints were still requested, was “The Hexaphenylethane Riddle”, a masterful account of how it could be that the most distinguished chemists of the day collectively assigned the wrong structure to the triphenylmethyl dimer, despite a mountain of chemical evidence98 pointing to the


correct methylenecyclohexadiene formulation already clearly articulated.99 In this essay, Mike catches generations of textbook authors with their pants down. Many have contended that Gomberg, on seeing his solution of trityl chloride turn yellow, knew he had generated a free radical. In fact, Gomberg never mentioned the color of his solution. As his synthesis predates quantum chemistry, he could not have known that conjugated free radicals had small HOMO-LUMO gaps well matched to the energies of visible photons. Here, we see the development of an implicit history that results from one author projecting his experience onto past events that are then promulgated by copycats. This is unfortunate, in Mike’s view, because it drains the bewilderment out of the story of discovery. “Completion of Koerner’s Proof that the Hydrogens in Benzene are Homotopic: An Application of Group Theory”100 is another gem that arose from Mike’s analysis of the development of structural organic chemistry. His realization that the logic Koerner used to establish that benzene had a regular hexagonal structure was slightly incomplete prompted this group theoretical inquiry. As a gag, at the annual holiday party, the graduate students gave Mike a purported 1H NMR spectrum of benzene. But it had two peaks, not one! Mike tackles Conformation in a lesson on how we know that DNA is a double helix. Mike shows how this understanding was achieved through his remarkably lucid analysis of Rosalind Franklin’s famous Photo 51. He begins by considering the optical transform of a helical light bulb filament. Proceeding to the more structured scattering pattern of DNA in Franklin’s photo, Mike builds DNA from the evidence for his students, no small number of them these days likely to pursue molecular biology. He shows which features of this smudge tell us that DNA is not only a helix like a light bulb filament, but a double helix complete with major and minor grooves, with characteristic diameter and base pair spacings, though not strictly determined. One of the most delightful excursions on the Chemistry 125 website is Mike’s essay on the “Direct Observation of Atoms through Clairvoyance”. This is the story of a band of so-called theosophists, who, through the exercise of clairvoyant powers, could minimize the conception of themselves so as to make small things such as atoms seem large. So effective were the theosophists that their publications were filled with sketches of atoms of the elements drawn from nature, just as John James Audubon’s were filled with pictures of birds. Atoms revealed to the theosophists were elaborate assemblages of ultimate particles. In hindsight, they resemble atomic orbitals. Those who know Mike may first assume that he is merely getting carried away with the hilarious story of a bunch of kooks. Mike is attracted to oddball characters, whether they be mad inventors, the fictional creations of Kurt Vonnegut, or the eccentric denizens of lower Manhattan in the work of Joseph Mitchell. But the serious purpose of this excursion is revealed when he provides the following assessment: What of today’s conscientious but naı¨ve chemistry student, who is being taught on the basis of the authority of a text or lecturer, rather than on the basis of direct evaluation of experimental evidence? To this student the [atomic pictures of the

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clairvoyant chemists] may seem as plausible as the atomic orbital diagrams of a conventional chemistry text. Why not? In truth they are equally far-fetched, in the sense that most textbooks neither discuss orbitals realistically nor support them with physical (or theoretical) evidence. At the risk of using hyperbolic language, this is radical. Organic textbook authors, suggests Mike, are quite often little more than theocratic authorities, expecting acquiescence without first doing the hard work to earn the student’s trust and acceptance. Mike does this hard work. Not only does he explain how Schro¨dinger’s equation works and why its relevance to electronic structure is limited to single electron wave functions, but he shows the form of the functions (for hydrogen atoms) and permits his students to carry out experimental quantum mechanical investigations on the computer using a wave function plotting program. Mike offers all this so that he can give credible explanations of the chemistry that is to follow on the basis of the theory of atoms and molecules without relying on the empirical organic model with which we have learned to predict (justify?) chemical and physical properties. (A caution: elbow grease is not enough. Mike is able to do what he does in a freshman organic chemistry class because he has a highly motivated clientele101 and because he is one of the most gifted lecturers on the planet.) This introduction to quantum mechanics allows Mike to provide a rigorous interpretation of the Bu¨rgi-Dunitz angle.102-104 His students come away with much more mechanistic insight than is given in the typical “Nucleophilic Addition to Carbonyls” chapter, effortlessly imparted because they understand what orbitals are and whence they come.105 The Chemistry 125 website is also Mike’s travel logs Palermo, Giessen, Zu¨rich, Vienna, Ann Arbor, Cambridge, Stockholm, Kirkintolloch (Scotland)shave digital camera will travel for students. No archive too obscure, no bronze plaque too insignificant, no gravesite too creepy. Mike is always preparing for his course because he is always prepared to add to it. The result is personal, which only makes it that much more impressive. Mike has created something that is not like anything else. And, because he succeeds so spectacularly in his purpose, his very personal website is anything but idiosyncratic. J. Michael McBride is circumspect. Like his colleague from another century, J. Willard Gibbs, he has an “innate tendency to avoid an expression of opinion on anything where he had not thoroughly explored all the implications or where for any reason he was not entirely aware of his ground.”106 Nevertheless, in his reinvention of organic chemistry, as evidenced on the Chemistry 125 website, we can discern important aspects of his character. You can almost hear Mike cheering for Moses Gomberg, who politely refused to engage in war work because teaching duties at the University of Michigan would be unfulfilled. Mike’s habitual sense of fairness spills out in his speculations about the distributive laws that govern heat in blackbody radiators and the money in the pockets of citizens of nations, large and small. We can only fantasize about how history might have turned out

Perspective

differently had Mike happened in 1967 upon a certain scion of a famous Yale family, passed-out on the tomb of Willard Gibbs in Grove Street Cemetery, a desecration indeed. Chances are, Mike would have offered him, as he would anyone, a helping hand. In 1975, Mike said, “Much of the organization, presentation, and philosophy of my courses has been shamelessly lifted from [Bartlett’s] Chem 105 [at Harvard].”2 Thirty years later, there is no way that this can be true. Fortunately, it can sometimes take a lifetime to get over a great teacher. Happy Birthday, Mike. Acknowledgment. We thank J. R. Scheffer, S. B. Bertman, and J. M. McBride for their unpublished images. Special thanks go to Florence S. McBride for permission to use her videos in this piece. M.D.H. is grateful to Pharmascience, Inc. for an unrestricted grant during preparation of this manuscript. J.A.S. is grateful for support from the National Science Foundation Division of Materials Research (DMR-0093069) and the Camille and Henry Dreyfus Foundation. B.K. is grateful for support from the National Science Foundation Division of Chemistry (CHE-0349882) and from the National Science Foundation Center on Materials and Devices for Information Technology Research (CMDITR), DMR-0120967. Appendix In an attempt to be fair, we have written this perspective without mentioning the names of the students who contributed to the many aspects of the research that we have discussed, but Mike would be the first to want us to acknowledge the dedication and high Table 1. Members of the McBride Groupa 1966 1967 1968 1969

1970 1971 1972

1973 1974 1975

1976

1978 1979 1980 1981 1982 1983

Donald S. Malament* Nathan J. Karch* Karen J. Skinner* Charles L. Lerman James R. Broach Marvin Lasky* Annette B. Jaffe* Michael J. Tremelling* Edward T. Koh§ Robert J. Blaskiewicz Edwin P. Slisz Jo-David Fine Richard P. Pankratz** Bonnie L. Whitsel* Jan B. Wollack Edward N. Maurer Rodger S. Miller** Howard S. Hochster David J. Bishop** Terrence J. McCormick Chukwuka U. Mba Matthias R. Gisler** Ethan A. Lerner E. Oliver Jones IV Lawrence Rissman Michael W. Vary* Donald W. Walter* Carol L. Reichel Robert Hirsch Brigitte E. Segmuller* Stephen R. Wasserman Aaron H. Goldberg Ronald A. Merrill* Dorothy Levene Mark D. Hollingsworth* Bruce A. Weber* David E. Mills* Philip A. Cole Claire L. Pettiette Joseph L. Pont

1984

1985 1986 1987 1988

1989 1991

1992

1993 1996

1997 1998 1999 2000 2002 2003 2004

Donna Z. Cioffi* Simon K. Kearsley** Steven J. Melly Georgianne Valli Xu Wu Feng* Aaron M. Panner Marko Yakovlevitch Steven B. Bertman* Thomas C. Semple** Ellen Whalen John P. Toscano III* William B. Hetzel III Randall L. Carter* Martin M. Vassas, Jr.* Bart E. Kahr** Robert L. La Duca, Jr. Sarah H. Tolbert Michael C. Biewer* Lev R. Ryzhkov** Curtis A. Hastings Elana Gordis Jennifer A. Swift* Lee M. Nagao* Sarah L. Clodfelter* Frederick Howard Matthew F. Weinschenk* Abhijit A. Patel Kevin L. Pate* John Whipple Nir Goldman Rajib Pal Benjamin P. Negin Miki E. Kunitake L. Kraig Steffen# Thomas J. Gniadek Andrew Beenken John Goeltz Pia Sorensen Joshua H. Baraban

a The symbols are as follows: * ) graduate student; ** ) postdoctoral associate; # ) visiting faculty member; § ) high school student; no symbol ) undergraduate.

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standards that his group members have shared with him through the years. Here, we present in Table 1 a list of past and present members of the McBride group, with dates of matriculation. References (1) Kurt Vonnegut made Mike’s hometown famous when he imagined a 1950s Lima science fair aspirant who was determined to show the “important services that acids...were performing every day for Humanity”, but whose acid-laden car was disastrously rear-ended while leaving his driveway. With acids out of the hunt, the state competition in Cleveland was contested between two displays “about crystals and how they grow and why they grow.” Needless to say, Mike survived Lima with his love of crystals intact. See Vonnegut, K. Hocus Pocus; Berkley Books: New York, 1991; pp 31-38. (2) McBride, J. M. In P.D. and the Bartlett Group at Harvard 1934-1974; McBride, J. M., Ed.; P. D. Bartlett: Cambridge, MA, 1975; pp 207-211. (3) McBride, J. M. Caged radical pairs and A further study of the norbornane peresters. Ph.D. Thesis, Harvard University, Cambridge, MA, 1966. This incorporates Bartlett, P. D.; McBride, J. M. A Further Study of the exo- and endo-2carbo-t-butylperoxynorbornanes. J. Am. Chem. Soc. 1965, 87, 1727-1733. (4) McBride, J. M. The Gerhard M. J. Schmidt Prize and Memorial Lecture; The Weizmann Institute of Science: Rehovot, Israel, March 6, 1990. (5) Brandon, R. W.; Closs, G. L.; Hutchison, C. A., Jr. Paramagnetic resonance in oriented ground-state triplet molecules. J. Chem. Phys. 1962, 37, 1878-1879. (6) Bartlett, P. D.; McBride, J. M. Configuration, conformation and spin in radical pairs. Pure Appl. Chem. 1967, 15, 89107. (7) McBride, J. M. Chemistry. In Science at Yale; Altman, S., Ed.; Yale University: New Haven, CT, 2002. (8) Berson, J. A.; Bushby, R. J.; McBride, J. M.; Tremelling, M. 2-Isopropylidenecyclopentane-1,3-diyl. Preparation, properties, and reactions of a distorted trimethylenemethane. Direct evidence for a triplet reaction. J. Am. Chem. Soc. 1971, 93, 1544-1546. For years, Mike and Jerry shared weekly group meetings. (9) Platz, M. S.; McBride, J. M.; Little, R. D.; Harrison, J. J.; Shaw, A.; Potter, S. E.; Berson, J. A. Triplet ground states of trimethylenemethanes. J. Am. Chem. Soc. 1976, 98, 5725-5726. (10) Vary, M. W.; McBride, J. M. 2,3-Diazo-7-isopropylidenebicyclo[2.2.1]hept-2-ene, C8H12N2. Cryst. Struct. Commun. 1980, 9, 85-90. (11) Saunders, M.; Berger, R.; Jaffe, A.; McBride, J. M.; O’Neill, J.; Breslow, R.; Hoffman, J. M., Jr.; Perchonock, C.; Wasserman, E.; Hutton, R. S.; Kuck, V. J. Unsubstituted cyclopentadienyl cation, a ground-state triplet. J. Am. Chem. Soc. 1973, 95, 3017-3018. (12) He also used DSC in his collaboration with Harry Wasserman on strain energies of cyclophanes and their thermal adducts13 and single-crystal X-ray diffraction with Fred Ziegler on the stereochemistry of synthetic intermediates.14-15 (13) McBride, J. M.; Keehn, P. M.; Wasserman, H. H. Thermochemistry of syn- and anti-[2.2](1,4)naphthalenophane and dibenzoequinine. Strain energies. Tetrahedron Lett. 1969, 47, 4147-4150. (14) Vary, M. W.; McBride, J. M.; Piwinski, J. J.; Ziegler, F. E. (()-(1R*,2R*,3R*)-3-acetyl-2-vinylcyclohexan-1-ol, C10H16O2. Cryst. Struct. Commun. 1979, 8, 807-814. (15) Vary, M. W.; McBride, J. M.; Cady, M. A.; Ziegler, F. E. p-Bromophenyl urethane of (()-(3R,1′R)-1-hydroxymethyl2-(1-methylpentyl)-3-methylcyclohex-1-ene, C21H30NO2Br. Cryst. Struct. Commun. 1979, 8, 799-805. (16) Malament, D. S.; McBride, J. M. R,R′-Dichloroazoalkanes. I. Synthesis: Stereospecificity and side reactions. The crystal structure of 1,1′-dichloro-1,1′-diphenyl-1,1′-azopropane. J. Am. Chem. Soc. 1970, 92, 4586-4593. (17) Malament, D. S.; McBride, J. M. R,R′-Dichloroazoalkanes. II. The mechanism of stereospecific synthesis and substitution. J. Am. Chem. Soc. 1970, 92, 4593-4598.

(18) McBride, J. M. Rotational diffusion control of radical disproportionation in the solid-state photolysis of azobisisobutyronitrile. J. Am. Chem. Soc. 1971, 93, 6302-6303. (19) Jaffe, A. B.; Malament, D. S.; Slisz, E. P.; McBride, J. M. Solvent steric effects. III. Molecular and crystal structures of azobisisobutyronitrile and azobis-3-cyano-3-pentane. A structural deuterium isotope effect. J. Am. Chem. Soc. 1972, 94, 8515-8521. (20) Jaffe, A. B.; Skinner, K. J.; McBride, J. M. Solvent steric effects. II. The free-radical chemistry of azobisisobutyronitrile and azobis-3-cyano-3-pentane in viscous and crystalline media. J. Am. Chem. Soc. 1972, 94, 8510-8515. (21) Karch, N. J.; McBride, J. M. Solvent steric effects. VII. Lattice control of free radicals from the photolysis of crystalline acetyl benzoyl peroxide. J. Am. Chem. Soc. 1972, 94, 5092-5093. (22) Skinner, K. J.; Blaskiewicz, R. J.; McBride, J. M. Solvent steric effects. VI. Solid-state photolysis of azobis-3-phenyl3-pentane. Crystal-lattice control of the stereochemistry of radical disproportionation. Isr. J. Chem. 1972, 10, 457-470. (23) Tremelling, M. J.; McBride, J. M. Solvent steric effects. V. Azobis-2-methyl-3-phenyl-2-butane. The absolute configuration of some derivatives of 2-methyl-3-phenylbutane. J. Org. Chem. 1972, 37, 1073-1075. Correction. J. Org. Chem. 1973, 38, 4217. (24) Jaffe, A.; McBride, J. M. Isotope studies of the reaction sites in AIBN photolysis. Solid Org. State 1974, 3, 16-22. (25) McBride, J. M. The role of local stress in solid-state radical reactions. Acc. Chem. Res. 1983, 16, 304-312. (26) Hollingsworth, M. D.; McBride, J. M. Photochemical mechanism in single crystals: FTIR studies of diacyl peroxides. In Adv. Photochem.; Volman, D. H., Hammond, G. S., Gollnick, K., Eds.; Wiley-Interscience: New York, 1990; Vol. 15, pp 279-379. (27) McBride, J. M. Overlap and the prevalence of banana bonds in free radicals and carbenes. J. Am. Chem. Soc. 1977, 99, 6760-6762. (28) Reichel, C. L.; McBride, J. M. Bent bonds in the bridgehead triptycyl radical. J. Am. Chem. Soc. 1977, 99, 6758-6760. (29) McBride, J. M.; Vary, M. W.; Whitsel, B. L. EPR studies of radical pairs. The benzoyloxy dilemma. ACS Symp. Ser. (Organic Free Radicals) 1978, 69, 208-223. (30) McBride, J. M.; Gisler, M. R. Isotopic and optical studies of the decomposition of crystalline dibenzoyl peroxide. Mol. Cryst. Liq. Cryst. 1979, 52, 121-132. (31) Vary, M. W.; McBride, J. M. Single-crystal EPR studies of radical pairs in dibenzoyl peroxide. Mol. Cryst. Liq. Cryst. 1979, 52, 133-144. (32) McBride, J. M.; Merrill, R. A. 2B2 Benzoyloxy, a delocalized σ radical. J. Am. Chem. Soc. 1980, 102, 1723-1725. Due to a printing error, the experimental angle between unique hfs eigenvectors (105.7°) was not given explicitly here. It matches the INDO LUMO for 2B2 of 110° much better than those of the other three candidates for the ground state (0°, 18°, and 35°). (33) Karch, N. J.; Koh, E. T.; Whitsel, B. L.; McBride, J. M. An X-ray and electron paramagnetic resonance structural investigation of oxygen discrimination during the collapse of methyl-benzoyloxy radical pairs in crystalline acetyl benzoyl peroxide. J. Am. Chem. Soc. 1975, 97, 6729-6743. (34) Peyerimhoff, S. D.; Skell, P. S.; May, D. D.; Buenker, R. J. Configuration interaction study of the three lowest electronic states in the formyl and acetyl radicals. J. Am. Chem. Soc. 1982, 104, 4515-4520. (35) Feller, D.; Huyser, E. S.; Borden, W. T.; Davidson, E. R. MCSCF/CI investigation of the low-lying potential energy surfaces of the formyloxyl radical, HCO2•. J. Am. Chem. Soc. 1983, 105, 1459-1466. (36) McBride, J. M.; Vary, M. W. Radical pairs in crystalline dibenzoyl peroxide. Evidence for triplet ground states. Tetrahedron 1982, 38, 765-775. This article contains a particularly lucid rationale for Hund’s Rule. (37) Merrill, R. A. The benzoyloxyl radical, a 2B2 ground state. Ph.D. Thesis, Yale University, New Haven, CT, 1986. (38) See Gavezzotti, A.; Bianchi, R. Potential-energy calculations and packing analysis for molecular motions in reactive diacyl peroxide crystals. Chem. Phys. Lett. 1986, 128, 295299 for an analysis of free volume in this reaction cavity.

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(39) Kearsley, S. K.; McBride, J. M. Using molecular mechanics to model the movement and relaxation of radical pairs created by photolysis of single crystals of acetyl benzoyl peroxide (ABP). Mol. Cryst. Liq. Cryst. 1988, 156, 109-122. (40) Walter, D. W.; McBride, J. M. Neophyl rearrangements in crystalline bis(3,3,3-triphenylpropanoyl) peroxide. 2. Structural studies by X-ray and EPR. The mechanistic role of inert molecules. J. Am. Chem. Soc. 1981, 103, 7074-7084. (41) Walter, D. W.; McBride, J. M. Neophyl rearrangements in crystalline bis(3,3,3-triphenylpropanoyl) peroxide. 1. Dynamic studies by EPR. J. Am. Chem. Soc. 1981, 103, 70697073. (42) See Gavezzotti, A. Packing analysis in reactive crystals. The decomposition of bis(3,3,3-triphenylpropanoyl) peroxide in the solid state. Tetrahedron 1987, 43, 1241-1251 for a free volume analysis of this system. (43) Whitsel, B. L. The kinetics and structure of radical pairs: acetyl benzoyl peroxide. Ph.D. Thesis, Yale University, New Haven, CT, 1977. (44) Hollingsworth, M. D.; McBride, J. M. Infrared studies of long-range stress in solid-state peroxide photoreactions. Mol. Cryst. Liq. Cryst. 1988, 161, 25-41. (45) McBride, J. M. Radical-pair reactions in organic single crystals, In Static, Kinematic and Dynamic Aspects of Crystal and Molecular Structure; Riva, L., Ed.; 18th International School of Crystallography: Erice, Italy, 1991; pp 101-106. (46) Luty, T.; Eckhardt, C. J. General theoretical concepts for solid state reactions: Quantitative formulation of the reaction cavity, steric compression, and reaction-induced stress using an elastic multipole representation of chemical pressure. J. Am. Chem. Soc. 1995, 117, 2441-2452. (47) Thoreau, H. D. Walden; Random House: New York, 1937; p 4. (48) Segmuller, B. E. Diundecanoyl peroxide: EPR study and product analysis. Ph.D. Thesis, Yale University, New Haven, CT, 1982. (49) McBride, J. M. Analysis of local free volume in lamellar crystals: an aid for understanding radical mobility in solids. Mol. Cryst. Liq. Cryst. 1983, 96, 19-31. (50) McBride, J. M.; Segmuller, B. E.; Hollingsworth, M. D.; Mills, D. E.; Weber, B. A. Mechanical stress and reactivity in organic solids. Science 1986, 234, 830-835. (51) Hollingsworth, M. D. Infrared studies of carbon dioxide dimers as a probe of local stress in solid state peroxide reactions. Ph.D. Thesis, Yale University, New Haven, CT, 1986. (52) Hollingsworth, M. D.; McBride, J. M. Specific long-range effects on relaxation of local stress during a solid-state reaction. J. Am. Chem. Soc. 1985, 107, 1792-1793. (53) Hollingsworth, M. D.; McBride, J. M. Coupling of CO2 asymmetric stretching in dimers photogenerated within long-chain diacyl peroxide single crystals. Chem. Phys. Lett. 1986, 130, 259-264. (54) Bernstein, E. R. Site effects in isotopic mixed crystals-site shift, site splitting, orientational effect and intermolecular Fermi resonance in the vibrational spectrum of benzene. J. Chem. Phys. 1969, 50, 4842-4856. (55) Pate, K. L. Probing molecular mobility of reaction-generated carbon dioxide in photolyzed acyl peroxide single crystals: an FT-IR investigation. Ph.D. Thesis, Yale University, New Haven, CT, 2000. (56) Later we discuss at length Mike’s extraordinary performance as an undergraduate instructor; he was no slouch at graduate mentoring either. In 1987, he was honored (along with MDH) for the work described in this paragraph with the American Chemical Society’s Nobel Laureate Signature Award for Graduate Education in Chemistry. (57) Mills, D. E. Bis(11-bromoundecanoyl) peroxide: remote substituent effects on a solid-state reaction. Ph.D. Thesis, Yale University, New Haven, CT, 1986. (58) Feng, X. W. Mechanistic ESR study in the solid state: radical reactions in crystals of bis(11-bromoundecanoyl) peroxide and its analogs. Ph.D. Thesis, Yale University, New Haven, CT, 1989. (59) Biewer, M. C. Investigation of radical motion in the singlecrystal photolytic decomposition mechanism of (11-bromoundecanoyl)(decanoyl) peroxide. Ph.D. Thesis, Yale University, New Haven, CT, 1995.

Perspective (60) In fact, in (11-bromoundecanoyl)(decanoyl) peroxide (11BrUDP), all but one of the 16 stereospecifically R- or β-deuterated isotopomers were characterized by EPR; this work identified 11 different radical species (five radical pairs and six other radical intermediates) on this pathway.59 (61) Pate, K. L.; McBride, J. M. Using crystal optics to demonstrate single-layer localization of a solid-state chain reaction. Helv. Chim. Acta 2000, 83, 2352-2362. (62) Feng, X. W.; McBride, J. M. Hydrogen atom transfer in photolyzed (S,S)-bis(13-bromo-2-deuteriotridecanoyl) peroxide: control of regiospecificity by crystal chirality. J. Am. Chem. Soc. 1990, 112, 6151-6152. (63) Toscano, J. P., III. The low-temperature photochemistry of nitramine single crystals. Ph.D. Thesis, Yale University, New Haven, CT, 1992. (64) Ryzhkov, L. R.; Toscano, J. P. Crystal lattice effects on the orientation and orbital degeneracy of nitric oxide trapped in nitramine single crystals. Cryst. Growth Des. 2005, 5, 2066-2072. (65) McBride, J. M.; Toscano, J. P. Observation of reaction intermediates in crystalline nitramines. In Proceedings of Workshop on Desensitization of Explosives and Propellants; Rijswijk, Netherlands, 1991. (66) Ryzhkov, L. R.; McBride, J. M. Low-temperature reactions in single crystals of two polymorphs of the polycyclic nitramine 15N-HNIW. J. Phys. Chem. 1996, 100, 163-169. (67) Ryzhkov, L. R.; McBride, J. M. Structure, motion, and exchange coupling of 15NO2/15NO2 radical pairs occupying adjacent solvent cavities of R-HNIW, a nitramine hydrate. J. Am. Chem. Soc. 1997, 119, 4826-4833. (68) McBride, J. M.; Bertman, S. B.; Semple, T. C. Structural effects on surfaces within layered crystals. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 4743-4746. (69) McBride, J. M.; Evans, G. T. Steady-state optical spin polarization from a spiropyran at 105 K. Possible evidence for a diradical. Chem. Phys. Lett. 1975, 36, 41-45. (70) Vary, M. W. Part I: Localized nitro n,π* triplets: the triplet states of four nitroindolinobenzopyrans. Part II: EPR studies of radical pairs in crystalline dibenzoyl peroxide. Ph.D. Thesis, Yale University, New Haven, CT, 1979. (71) Vary, M. W.; McBride, J. M. Using spiropyran fragment superposition to assign the EPR spectrum of localized triplet excitation on a distorted aromatic nitro group. Cryst. Growth Des. 2005, 5, 2036-2042. (72) McBride, J. M.; Bertman, S. B.; Cioffi, D. Z.; Segmuller, B. E.; Weber, B. A. Interpreting substituent effects on the crystal packing of long-chain diacyl peroxides: the crystal structures of di(11-bromoundecanoyl) peroxide and di(undecanoyl) peroxide. Mol. Cryst. Liq. Cryst. 1988, 161, 1-24. (73) Bertman, S. B. Solid substituent effects: systematic investigation of the influence of molecular interaction on bulk properties of organic solids. Ph.D. Thesis, Yale University, New Haven, CT, 1990. (74) Swift, J. A.; Pal, R.; McBride, J. M. Using hydrogen-bonds and herringbone packing to design interfaces of 4,4′-disubstituted meso-hydrobenzoin crystals. The importance of recognizing unfavorable packing motifs. J. Am. Chem. Soc. 1998, 120, 96-104. (75) McBride, J. M. Crystal polarity: a window on ice nucleation. Science 1992, 256, 814. (76) Gavish, M.; Wang, J.-L.; Eisenstein, M.; Lahav, M.; Leiserowitz, L. The role of crystal polarity in R-amino acid crystals for induced nucleation of ice. Science 1992, 256, 815-818. (77) Addadi, L.; Berkovitch-Yellin, Z.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. A link between macroscopic phenomena and molecular chirality: Crystals as probes for the direct assignment of absolute configuration of chiral molecules. Top. Stereochem. 1986, 16, 1-85. (78) McBride, J. M.; Bertman, S. B. Using crystal birefringence to study molecular recognition. Angew. Chem., Int. Ed. Engl. 1989, 28, 330-333. (79) See Hollingsworth, M. D. Crystal engineering: from structure to function. Science 2002, 295, 2410-2413 for a brief overview. (80) Tape was often necessary. In no small measure because of Mike’s excitement about doing science, his students would often work into the wee hours of the night, long after the

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machine shops were closed. On Friday afternoons at 4:00 p.m., if there were a few liters of liquid helium left over from a magnet fill, some McBride group student would invariably be in line for it, hoping to spend another Friday night testing Mike’s hypotheses on the EPR spectrometer. McBride, J. M. Symmetry reduction in solid solutions: A new method for materials design. Angew Chem., Int. Ed. Engl. 1989, 28, 377-379. Kahr, B.; McBride, J. M. Optically anomalous crystals. Angew. Chem., Int. Ed. Engl. 1992, 31, 1-26. McBride, J. M.; Carter, R. L. Spontaneous resolution by stirred crystallization. Angew. Chem., Int. Ed. Engl. 1991, 30, 293-295. Kondepudi, D. K.; Kaufman, R. J.; Singh, N. Chiral symmetry breaking in sodium chlorate crystallization. Science 1990, 250, 975-977. Swift, J. A. Probing intermolecular interactions: an atomic force microscopy and X-ray investigation of crystal growth and dissolution processes. Ph.D. Thesis, Yale University, New Haven, CT, 1997. These include the Dylan Hixon ‘88 Prize for Teaching Excellence in the Natural Sciences at Yale College (1992), the DeVane Medal for Distinguished Scholarship and Teaching (1994), and the Yale Science and Engineering Award for Meritorious Service to Yale (1998). Thomas, L. The art of teaching science. New York Times Mag. March 14, 1982; pp 89-92. Berkovitch-Yellin, Z.; Leiserowitz, L. Electron density distribution in cumulenes. Low temperature x-ray study of tetraphenylbutatriene. J. Am. Chem. Soc. 1975, 97, 56275628. Berkovitch-Yellin, Z.; Leiserowitz, L. Electron density distribution in cumulenes: an X-ray study of tetraphenylbutatriene at 20 °C and -160 °C. Acta Crystallogr. 1977, B33, 3657-3669. Chakrabarti, P.; Seiler, P.; Dunitz, J. D.; Schluter, A. D.; Szeimies, G. Experimental evidence for the absence of bonding electron density between inverted carbon atoms. J. Am. Chem. Soc. 1981, 103, 7378-7380. Dunitz, J. D.; Schweizer, W. B.; Seiler, P. X-ray study of the deformation density in tetrafluoroterephthalodinitrile: weak bonding density in the C,F-bond. Helv. Chim. Acta 1983, 66, 123-133.

Crystal Growth & Design, Vol. 5, No. 6, 2005 2035 (92) Wiberg, K. B.; Walker, F. H. [1.1.1]Propellane. J. Am. Chem. Soc. 1982, 104, 5239-5240. (93) Groth, P. A. Chemische Krystallographie; Verlag von Wilhelm Engelmann: Leipzig, 1906-1919; Vol. I-V. (94) Wallach, O. Briefwechsel zwischen J. Berzelius und F. Wo¨ hler; Engelmann: Leipzig, 1901; p 291; cited on Chemistry 125 website. (95) Dunitz, J. D.; Harris, K. D. M.; Johnston, R. L.; Kariuki, B. M.; MacLean, E. J.; Psallidas, K.; Schweizer, W. B.; Tykwinski, R. R. New light on an old story: The solid-state transformation of ammonium cyanate into urea. J. Am. Chem. Soc. 1998, 120, 13274-13275. (96) Liebig, J. v.; Wo¨hler, F. Ann. Phys. Leipzig, Ser. 2 1830, 20, 369. Cited in ref 95. (97) Wo¨hler, F.; Liebig, J. v. Ann. Pharm. 1832, 3, 249. Cited on the Chemistry 125 website. (98) Skinner, K. J.; Hochster, H. S.; McBride, J. M. o- and p-Semibenzene dimers of benzylic radicals. Autoxidation of quinoid dimers. J. Am. Chem. Soc. 1974, 96, 4301-4306. (99) McBride, J. M. The hexaphenylethane riddle. Tetrahedron 1974, 30, 2009-2022. (100) McBride, J. M. Completion of Koerner’s proof that the hydrogens of benzene are homotopic. An application of group theory. J. Am. Chem. Soc. 1980, 102, 4134-4137. (101) This has included Mike’s daughter Anne, who is now an Assistant Professor of Biology and Biochemistry at Bowdoin College. (102) Bu¨rgi, H. B.; Dunitz, J. D. From crystal statics to chemical dynamics. Acc. Chem. Res. 1983, 16, 153-161. (103) Bu¨rgi, H. B.; Dunitz, J. D.; Lehn, J. M.; Wipff, G. Stereochemistry of reaction paths at carbonyl centers. Tetrahedron 1974, 30, 1563-1572. (104) Bu¨rgi, H. B.; Dunitz, J. D.; Shefter, E. Chemical reaction paths. IV. Aspects of O‚‚‚CdO interactions in crystals. Acta Crystallogr. 1974, B30, 1517-1527. (105) McBride, J. M. A rationale for the shape of simple Hu¨ckel orbitals. J. Chem. Educ. 1974, 51, 471. (106) Wheeler, L. P. Josiah Willard Gibbs: The History of a Great Mind; Yale University Press: New Haven, 1948; p 141.

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