J. Michael McBride

Mechanical Stress and Reactivity in Organic Solids

Organic single crystals provide an ideal model for studying the factors that influence chemical processes in structured media. Reaction trajectories are well defined and reveal the influence of spontaneous mechanical stresses equivalent to tens of thousands of atmospheres. Analysis of molecular and crystal structures helps to explain both local mechanical properties, which influence reactions, and bulk properties such as melting point, compressibility, and surface energy.

Physical chemistry: Did life grind to a start?

Many solids can adopt two mirror-image crystal forms, and often grow as mixtures of both. A curious mechanism of crystal growth might explain why some mixtures convert into one form when subjected to grinding.

Enantiomer-Specific Oriented Attachment: Formation of Macroscopic Homochiral Crystal Aggregates from a Racemic System†

Lets get together: Racemic samples of d- and l-enantiomorphous NaBrO3 (or NaClO3) crystals aggregate with nearly complete enantioselection. Centimeter-sized enantiopure megacrystals are often produced, and these can be sorted easily.

Metastability in Supersaturated Solution and Transition towards Chirality in the Crystallization of NaClO3

Reports on spontaneous deracemization of crystal mixtures of achiral or racemizing compounds (Viedma deracemization) are arousing strong interest not only because of their potential applications but also for understanding spontaneous emergence of chirality in chemical scenarios. The systems that may undergo such deracemizations involve compounds that crystallize as enantiopure crystals (racemic conglomerates) but are achiral or racemize rapidly in solution. The experimental conditions that promote such deracemizations are wet grinding of a racemic conglomerate in contact with its saturated solution and, if necessary, addition of a catalyst to accelerate racemization of the compound in the liquid phase. The results cannot be explained by dynamic kinetic resolution, because the assumption of a single parent crystal does not apply when a mixture of enantiomeric crystals is ground. Moreover, they cannot be explained by a second-order dynamic kinetic resolution process, nor by enantioselective inhibition of crystal growth under the influence of a chiral minor component, because of the absence of a second chiral compound. For systems such as NaClO3 and 1,1’-binaphthyl, there are reports of crystallizations that, under the action of strong stirring, yield polycrystalline mixtures of composition near to homochirality. Such results have been interpreted by the one-single-parent-crystal assumption together with a crystal growth dominated by secondary nucleation processes; the strong stirring creates new seeds by fracturing crystals. However, a recent report that describes a homochiral outcome in the crystallization of NaClO3 from boiling solutions suggests that the deracemization might also occur at the level of subcritical clusters. Notice that chiral amplification by secondary nucleation by no means precludes amplification at the level of subcritical clusters. In fact, some results in these previous reports indicate that the enantiomeric excess of crystals increases with the initial supersaturation, which, as we will show, is consistent with a significant role of the metastable stage preceding primary nucleation. Several mechanisms have been proposed to explain Viedma deracemization. However, it is important to understand the thermodynamic aspects and constraints that allow such deracemizations to occur. As we have previously discussed, in the initial stagnant conditions the system is in a chemical equilibrium that is determined solely by interactions between individual crystals and the solvated chemical compound. Solid-to-solution interaction does not distinguish between the enantiomorphic crystals. Under these conditions the enantiomorphic solids are thermodynamically identical, and the system is defined by one component in addition to the solvent; there is only one solid phase. This is the long-accepted chemical interpretation in respect to the available degrees of freedom that explain the experimental chemical behavior of these systems. Under Viedma deracemization conditions, we proposed that erosion of the crystals would lead to clusters that, in their interaction with other crystals or with each other, would recognize each other as different thermodynamic phases, that is, an enantiomeric discrimination would occur between the two enantiomorphic solid phases. Consequently, under constant grinding the system is taken out of equilibrium and evolves towards a final stationary state that, according to the published reports, is homochiral or nearly homochiral. Note, however, that certain experimental conditions may lead to a racemic stationary state of different composition than the stagnant conditions, or even to oscillations. Customary failure to publish “negative” results inhibits evaluating the range of possibilities. We have now conducted experiments to determine whether, in the crystallization of NaClO3 from boiling solutions, deracemization can occur at the level of subcritical clusters during the metastable stage previous to crystal growth. Our starting assumption, according to a previous report, is that in these systems during the highly concentrated, supersaturated metastable stage, that is, before crystal growth, an enantiomeric excess of chiral subcritical clusters [*] Dr. Z. El-Hachemi, Dr. J. Crusats, Prof. J. M. Rib Department de Qu mica Org nica and Institute of Cosmos Science (ICC), Universitat de Barcelona (IEEC-UB) c. Mart i Franqu s 1, 08028-Barcelona Catalonia (Spain) Fax: (+ 34)93-339-7878 E-mail: zelhachemi@ub.edu jmribo@ub.edu

Infrared Studies of Long-Range Stress in Solid-State Peroxide Photoreactions

Abstract When crystals of di(undecanoyl) peroxide are photolyzed at low temperature to conversions greater than about 0.06%, in situ FTIR spectroscopy of the CO2 product shows numerous v3 bands which are not observed after less extensive photolysis. The shift of these bands to high frequency suggests the influence of stress fields generated by distant reaction sites. The influence becomes more pronounced upon annealing to about 140K, but disappears after annealing above 280K. This behavior is discussed in terms of relaxation through both rapid elastic deformation and stepwise, thermally activated plastic deformation. Isotope labeling shows that the new CO2 vibrations are not tightly coupled to one another. An analysis based on random decomposition shows that the perturbing defects lie well beyond the first coordination sphere of the probe CO2 molecule and probably cooperate in influencing subsequent reactions. These results suggest caution in interpreting solid state product distributions, even at very lo...

Analysis of Local Free Volume in Lamellar Crystals: An Aid for Understanding Radical Mobility in Solids

Abstract In crystals with two short axes it is easy to partition free volume into local components by plotting a profile of the free areas of planes parallel to the plane defined by the short axes. Local free volume is used to analyze radical motion in bis-undecanoyl peroxide. It is relevant to melting-point alternation in long-chain compounds and to packing in a lipid bilayer.

Radical pair in crystalline dibenzoyl peroxide evidence for triplet ground states

Abstract Zero-field splitting (zfs) tensors and g tensors are reported for two phenyl-benzoyloxyl radical pairs (PB5K and PB15K) and two benzoyloxyl-benzoyloxyl pairs (BB10K and BB25K) in photolyzed single crystals of dibenzoyl peroxide. The g tensors show that in three of the pairs a benzoyloxyl radical has undergone in-plane rotation by about 40°. In PB15K this motion relieves steric repulsion from a CO 2 molecule, and it is suggested that motion in the BB pairs relieves repulsion between radical oxygens in a ground-state triplet radical pair. Spin-orbit coupling is necessary to explain the zfs of PB5K. This explanation requires that the triplet of the electronically excited radical pair lie below the corresponding singlet by an amount which is significant in comparison to the excitation energy. The source of singlet-triplet splitting is discussed briefly and simple VB calculations are shown not to support the above interpretation, although the STO3G and 4-31G basis sets employed are very likely inadequate. Atomic coordinates are reported for crystalline dibenzoyl peroxide.

Steady-state optical spin polarization from a spiropyran at 105 K. Possible evidence for a diradical

Abstract When irradiated at 105 K a single crystal of 3′,3′-dimethyl-6-methoxy-8-nitrospiro-[2H-1-benzopyran-2,2′-indoline], gave a triplet-state EPR spectrum which was strongly polarized in the steady state. D, g, and ϰ (intersystem crossing) tensors were diagonal in different frames. Excitation migration was very slow, suggesting that the short-lived (

Interpreting Substituent Effects on the Crystal Packing of Long-Chain Diacyl Peroxides. The Crystal Structures of D1 (11 Bromoundecanoyl) Peroxide and Di (Undecanoyl) Peroxide

Abstract Although crystals of di(11-bromoundecanoyl) peroxide and di(undecanoyl) peroxide have different space groups (P43212 and C2221), the molecules pack in almost identical layers. They differ only in the nature of stacking across interfaces involving the terminal groups. Because the 90° twist about the O-O bond locks neighboring molecules together within the layer, each peroxide shows a single solid phase from 5K to the melting point. Analysis of the stacking pattern in terms of the six possible orientational relationships suggests special stability for an L-shaped motif of C-Br-Br-C. Other substituents create different stackings of the same layer structure to give three crystal classes and five space groups among 14 compounds. Unsymmetrical peroxides are useful both for forcing a variety of substituted chains (particularly odd-even homologues) to pack with identical layer structures, and for controlling the stacking pattern. Because structural differences are localized in the vicinity of the substit...

Coupling of CO2 asymmetric stretching in dimers photogenerated within long-chain diacyl peroxide single crystals

Low-temperature photolysis of single crystals of long-chain diacyl peroxides produces long-lived aggregates of two alkyl radicals separated by two CO2 molecules. IR spectroscopy of asymmetric stretching modes in CO2 dimers formed within unlabeled and 13C-labeled crystals reveals intermolecular Davydov or resonant coupling of 0.5–9.0 cm−1 for three differently arranged pairs. Frequency and intensity patterns are in quantitative agreement with first-order perturbation theory.