Direct Observation of Aminyl Radical Intermediate during Single

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J. Phys. Chem. B 2007, 111, 10373-10378

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Direct Observation of Aminyl Radical Intermediate during Single-Crystal to Single-Crystal Photoinduced Orton Rearrangement Pancˇ e Naumov,*,†,‡,§ Kenji Sakurai,⊥ Masahiko Tanaka,| and Hideyuki Hara# Frontier Research Base for Global Young Researchers, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamada-oka, Osaka, 565-0871 Suita, Japan, Institute of Chemistry, Faculty of Science, Sts. Cyril and Methodius UniVersity, POB 162, MK-1000 Skopje, Macedonia, International Center for Young Scientists, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, Quantum Beam Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan, WEBRAM, SPring-8, National Institute for Materials Science, 1-1-1 Kouto, Mikazuki, Sayo, Hyogo 679-5198, Japan, and ESR DiVision, Bruker Biospin K. K., 3-21-5 Ninomiya, Tsukuba, Ibaraki 305-0051, Japan ReceiVed: June 13, 2007; In Final Form: July 2, 2007

A photoinduced analogue of the thermal Orton rearrangement reaction by which an N-chlorine atom from a side amino group is transferred to a phenyl ring was studied in the solid state. Contrary to the mixture of products obtained in solution, in the N-chloro-N-acetylaminobenzene crystals the photoreaction proceeds with complete preservation of crystallinity, affording selectively and quantitatively the para isomer of chloroacetanilide. Study of the reaction mechanism by in situ steady-state photodiffraction, a combination of photoexcitation by UV light and single-crystal X-ray diffraction analysis, provided evidence for creation of N-acetyl-N-phenylaminyl (AcPhN•) radical as a metastable reaction intermediate. The structure of the aminyl radical produced in 9.2% yield from the major disordered component in the statically 85.6:14.4 disordered crystal was directly observed for the first time. The unprecedented stability of the radical is prescribed to the solid-state cage effect, the reactive center of the radical species being locked away from the reactive target molecules. The creation of the radical and its head-to-tail chain reaction within the undulated hydrogenbonded ribbons involving the acetyl carbonyl group are employed to explain the high selectivity of the photoinduced single-crystal to single-crystal Orton rearrangement. On the basis of the change of the crystal structure and the physicochemical data, a three-center five-atom mechanism involving homolytic cleavage of the N-Cl bond followed by hydrogen abstraction by the carbonyl group is suggested for the solid-state photoinduced Orton rearrangement.

1. Introduction Acid-catalyzed halogen migration from a side chain to an aromatic ring, by which N-chloroanilines in organic solvents are converted to a mixture of p- and o-aminochlorobenzenes, is commonly known as the Orton rearrangement.1 Although the originally reported reaction was believed to be intramolecular rearrangement that proceeds by direct atom exchange, later kinetic studies proved that it is actually an intermolecular twostep rearrangement that involves bimolecular nucleophilic substitution by molecular chlorine as the rate determining step.2 In 1927, Porter and Wilbur3 briefly noted that if induced by UV light, the reaction of N-chloro-N-acetylaminobenzene (Nchloroacetanilide, CAAB) can also proceed in the solid state yielding chloroacetanilide (CA). Similarly to the reaction in solution, they suggested that the photoinduced reaction is a direct exchange between the N-halogen atom and the p-hydrogen atom of the same molecule (Scheme 1a). Despite the potential that this solid-state “photoinduced Orton rearrangement” might have * Address correspondence to this author. E-mail: [email protected], [email protected]. † Osaka University. ‡ Sts. Cyril and Methodius University. § International Center for Young Scientists, National Institute for Materials Science. ⊥ Quantum Beam Center, National Institute for Materials Science. | WEBRAM, SPring-8, National Institute for Materials Science. # Bruker Biospin K. K.

SCHEME 1: Suggested Reaction Mechanisms of the Photoinduced Rearrangement of N-Chloro-N-acetylaminobenzene (CAAB) to p-Chloroacetanilide (p-CA): Unimolecular (a), Bimolecular (b), and Chain (c) Reactiona

a The labels of the reactive atoms originating from the same molecule are represented with the same color.

as a convenient, inexpensive, solventless green method for the synthesis of halogenoaromatic compounds, for example, as precursors for the preparation of chloroaromatic antibiotics or anilide herbicides and pesticides, it has not received any further scientific attention.

10.1021/jp074575v CCC: $37.00 © 2007 American Chemical Society Published on Web 08/14/2007

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Figure 1. Effect of UV irradiation on the IR spectra of solid CAAB. The spectra of the pure CA isomers are shown for comparison.

Our photolysis studies showed that if CAAB in aprotic organic solvents is exposed to polychromatic continuous-wave (cw) UV light (λmax ) 365 nm) in the absence of acid, it undergoes rapid isomerization to a mixture of para, ortho, and meta isomers of CA and smaller amounts of additional products whose composition depends critically on the solvent.4 Contrary to the mixture of isomers obtained in solution, in situ and ex situ IR spectroscopic analysis (Figures 1 and S1)5 and spectral comparison with the pure CA isomers and DFT-computed vibrational spectra of the probable products (Figure S2, Table S2)6 confirmed that in the solid state the photolysis results in complete conversion to the para isomer of CA as the only product. If induced by low-intensity UV light in single crystals, the rearrangement proceeds with preservation of crystal integrity until nearly quantitative conversion has been completed after about 1 day and thus it is an irreversible single-crystal to singlecrystal (scsc) reaction. The exceptional selectivity, high yield, and scsc course of the reaction indicate stringent topochemical control over the process with minimum possible movement of reactant moleculessa common characteristic of gas-induced,7,8 thermally induced, and photoinduced9 scsc transformations, such as, for example, the extensively studied methyl group rearrangements.10 Depending on the particular molecular arrangement in the unreacted crystal, three mechanisms could account for the topochemical reaction course of CAAB at molecular level: (a) unimolecularsdirect exchange of chlorine and hydrogen atoms of the same molecule (Scheme 1a), (b) bimolecularsatom swapping between proximate stacked molecules of opposite orientations (Scheme 1b), and (c) concerted or consecutive multimolecular (chain)sreaction between suitably oriented adjacent molecules (Scheme 1c). The absence of significant intermolecular interactions of the side group or the presence of co-facial π-π-dimers in the crystal structure would favor unimolecular and bimolecular reaction, respectively, while head-to-tail hydrogen bonding is expected to promote the multimolecular mechanism. In this study, the steady-state version of the photodiffraction technique was applied to determine the intermediate state of the reaction in order to discriminate among these models. The method, which combines excitation of a crystalline sample by UV or visible light to trigger a physical process or a chemical reaction and probing of the resultant structural changes by X-ray or neutron radiation, has experienced remarkable success in elucidating structures of photoinduced intermediates and products which drive important processes, such as dimerizations and cyclizations,9 proton

transfer,11 creation of excited states and exciplexes,12 photoinduced phase transitions,13 and metal-ligand bond dissociation.14 2. Experimental Section 2.1. Synthesis and Crystallization. CAAB prepared by N-chlorination of acetanilide (Tokyo Chemical Industry) with purity of >99.5% ascertained by gas chromatography and nitrogen content, and mp of 365.1 K (lit. mp 364-367 K), was recrystallized under diffuse red light from various solvents and stored at 278 K. CAAB is moderately soluble in n-hexane and light petroleum ether (PE), but readily soluble in benzene, 1,2dichloroethane, CHCl3, and CH3OH. The polymorph and the composition of the recrystallized compound depend critically on the solvent composition: based on the DSC and IR data, crystallization from PE and acetone:PE (1:9) affords the pure form A of CAAB (mp 363 K), while crystallization from CHCl3: PE with ratios of 1:9 and 1:4 affords mixtures of form B CAAB (mp 354 K) and a small amount of o-CA (mp 359 K). Crystallization from CHCl3 results in complete isomerization to p-CA, while from acetone brown decay products are obtained in addition to colorless prisms of m-CA. Repeated recrystallization (CHCl3:PE) results in an increasing amount of p-CA, decreased solubility, and poor crystallinity. The identity of the product p-CA and o-CA was confirmed with X-ray diffraction analysis.15 2.2. Photoirradiation Experiments. In the photoirradiation experiments, the samples were excited with a 250 W highpressure mercury UV lamp (SP-7, Ushio) with internal heat filter. For both ex situ and in situ photolysis, pure dry single crystals and KBr-pelleted microcrystalline powder of CAAB (∼1%), respectively, were irradiated with heat-filtered UV light. The inertness of the KBr matrix was ascertained spectroscopically (Figure S1, Supporting Information), both single crystals and powders yielding pure p-CA as the only detectable product. According to the kinetics monitored by the characteristic bands of the product in the IR spectra, the reaction in single crystals proceeds several times slower than that in the powder state. Due to release of molecular chlorine and spontaneous sublimation of the photoproduct, UV-irradiated CAAB crystals stored in the dark at room temperature gradually turn opalescent, but the overall crystal integrity is preserved. In the solution-state photolysis experiments, CAAB in degassed solvents was irradiated in a quartz cell. The clear solutions turned opalescent immediately after the onset of irradiation, and brown precipitate

Watching Radicals occasionally formed. The solutions were evaporated under reduced pressure and analyzed. 2.3. Physicochemical Characterization. Cyclic DSC curves were recorded in the range 113-393 K with an EXII STAR thermal analyzer (Rigaku) at the heating rate of 5 deg/min in nitrogen gas flow, using open aluminum crucibles, pure alumina as standard, and nonpulverized crystals (∼3 mg) as samples. The FTIR spectra were recorded at room temperature from pressed KBr pellets with a Nicolet 4700 spectrometer (Thermoelectron). The ESR spectra were recorded at room temperature with an EMX-Plus ESR spectrometer (Bruker Biospin) at the microwave frequency of 9.855 GHz and power of 0.5 mW, with modulation amplitude at 2 G, using pulverized form A CAAB crystals in a quartz tube. The 13CP MAS NMR spectra were recorded with a CMX-300 spectrometer (Varian Infinity). The assignments are based on the SCF GIAO shielding tensors calculated on the B3LYP/6-31G(d,p) optimized structure of CAAB.16 2.4. X-ray Diffraction. Single-crystal X-ray diffraction data on CAAB were collected in ω-oscillation mode with a Bruker three-circle diffractometer equipped with CCD detector and N2 open type low-temperature system, using monochromatic Mo KR X-ray radiation (0.71073 Å).17 The temperature at the sample position was corrected by direct measurement with a thermocouple. The structures were solved by using direct methods18 and refined19 on F2. The thermal parameters of the major component were kept anisotropic, while the minor components were refined as isotropic models. A number of restraints and constraints were applied to treat the disorder, including similar thermal parameters for the analogous atoms of the three components, and distance and planarity constraints of the side groups. All hydrogen atoms were added as riding bodies at chemically sensible positions, and the dihedral angles of the methyl groups were allowed to refine. The single crystal was initially irradiated 2 h with heat-filtered polychromatic cw UV light from a mercury lamp in the dark, and the irradiation was continued with very weak light during the data collection. If exposed to room light, the UV-irradiated crystals gradually turn opaque, probably due to subsequent photoinduced radical reactions. Structure elucidation of the completely transformed crystal was not feasible due to the sensitiveness of the product p-CA to X-rays. The powder diffraction pattern of CAAB in the 2θ range of 0-60° was recorded with use of synchrotron X-ray radiation at beamline BL15XU of SPring-8, the 8 GeV synchrotron in Harima (Japan), using the Debye-Scherrer capillary method and imaging plate detector (400 mm length) which provided an angular resolution of 0.01°. The sample, loaded into a Ø0.3 mm Lindemann capillary and temperature controlled with a cold stream of nitrogen gas (Oxford Cryosystems 700 series Cryostream cooler), was spun at 1 s-1 during 10-s exposure to X-rays with λ ) 0.6358 Å (checked against silicon standard). The zero point was detected by exposure of the same IP to the direct beam. The background was modeled with a nineparameter shifted Chebyshev function and the peak profiles were fitted with the Howard’s pseudo-Voight function.20 The occupancy factor of the disordered structures at 200 and 300 K was refined with GSAS21 as an isotropic bimolecular model starting from the single-crystal coordinates.22 The distances were restrained to their expected single-crystal values and all atoms within the same component were constrained to have identical occupancy. 2.5. Theoretical Calculations. The theoretical calculations were performed with the Gaussian03 suite of programs.16

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Figure 2. (A, B) Color-coded slices in the N-chloro-N-acetyl plane of the difference Fourier density of nonirradiated (A, scale: Fmin/max ) -0.3/0.9 e‚Å-3) and irradiated (B, scale: Fmin/max ) -0.5/0.6 e‚Å-3) CAAB crystals refined as a single-molecule model (white lines) with anisotropic thermal parameters and “riding” hydrogens. The electron density increases from blue to red and white colored regions. The residual peaks joined by red and black lines correspond to the minor component (primed labels) and photoinduced radical (doubly primed labels). (C, D) ORTEP-style diagrams (30% probability level) of the refined disordered structure before (C) and after (D) the irradiation.

CAAB, p-CA, and six additional model molecules were fully DFT-optimized, and the respective harmonic (all-real) frequencies were calculated on the resulting geometries to confirm minima for the stationary points. The choice of the method B3LYP/6-31G(d,p) was selected as a compromise between cost and accuracy, based on its success in reproducing similar structures, energies, and substituent effects. A similar, but unrestricted series of calculations with the same basis set was performed for CAAB, the five model radicals, and the respective dehydrogenated species. The energies calculated during the single-point calculations were zero-point corrected. 3. Results and Discussion CAAB, prepared by chlorination of acetanilide, was screened for polymorphs by recrystallization from various solvents. DSC (Figure S3, Supporting Information) and IR data ascertained the existence of two stable polymorphs, A and B (mp 363 and 354 K, respectively), obtained by slow evaporation, of which the former was selected for further study on the basis of its superior crystallinity (the crystals of form B were of insufficient quality for X-ray diffraction analysis). The initial structure was determined from single-crystal X-ray diffraction data collected from a light-protected CAAB crystal at 270 K.23 The 3D difference Fourier map of the unconstrained anisotropic singlemolecular model revealed a surplus of electron density due to pronounced disorder (particularly apparent around the N-chloroN-acetyl group C1N1Cl1C7O1C8 in Figure 2A), which was accounted for by including a secondary mirror isomer (C1′N1′Cl1′C7′O1′C8′, Figure 2C). The chlorine atoms and

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Figure 3. ESR spectrum of CAAB excited at λmax ) 365 nm (left) and schematic of the photolysis reaction of creation of the N-acetylN-phenylaminyl (AcPhN•) radical (right).

methyl carbon atoms of the two mirror components are coplanar and nearly co-incident, whereas the two phenyl rings are inclined at about 14° to each other. The refined populations of the major and minor components at 270 K are 85.6% and 14.4%, respectively.24 The occupational nondegeneracy is probably due both to steric factors related to asymmetry of the side group and to differences in the intermolecular interactions. Both carbonyl components are weakly hydrogen bonded to ohydrogen atoms of two phenyl rings and to p-hydrogen atom of a third phenyl ring (Figure S4, Supporting Information). The disorder is similar to that observed in crystals of other simple substituted benzenes.24,25 No evidence of dynamic process could be obtained between 123 and 350 K from the variabletemperature 13C CP MAS NMR spectrum (Figure S5, Supporting Information). The static nature of the disorder in the interval 200-300 K, where the DSC results show the absence of phase transitions, was confirmed by the temperature-resolved powder diffraction pattern of CAAB recorded by using synchrotron X-ray radiation (Figure S6, Supporting Information); within the expected uncertainty, the fractions of the minor component at 200 (15.5%) and 300 K (13.8%) are identical with its fraction in a single crystal at 270 K before the irradiation (14.4%). Excitation of CAAB crystals by exposure to UV radiation results in a strong ESR signal at 3511 G structurized into six or seven subbands (Figure 3). The resonance is stable in air at least for several hours at room temperature, but gradually loses its structure with prolonged isothermal aging. The signal was evidence of homolysis of the N1-C7 bond and creation of notably stable N-acetyl-N-phenylaminyl (AcPhN•) radical. The structure of the irradiated CAAB was studied with in situ X-ray photodiffraction on a single crystal at 270 K subjected to very weak cw UV excitation in order to maintain the equilibrium concentration of the radical.26 In addition to the minor component C1′N1′Cl1′C7′O1′C8′, the difference Fourier map of the irradiated crystal contained excess electron density around the major group C1N1Cl1C7O1C8 corresponding to a tertiary, photoinduced component C1′′N1′′C7′′O1′′C8′′ (Figure 2B). The photoinduced component lacks N and p-chlorine atoms and was identified as the AcPhN• radical detected by ESR spectroscopy. Except for the absence of chlorine atoms,27 the conformations of AcPhN• and the major CAAB component are similar (Figure 2D). At 270 K, the populations of the major, minor, and radical components in the structure of the partially reacted crystal refined as a mixed isotropic-anisotropic model were 77.5%, 13.3%, and 9.2%, respectively. The two most apparent reasons for the low yield of the radical are the temporal and spatial averaging of its occupancy over the crystal volume and the time span for data collection. Additional contributions may arise from nonuniform irradiation, recombination to CAAB, and depletion

Figure 4. (A) Undulated head-to-tail C(Ph)-H‚‚‚O(carbonyl) hydrogenbonded chains in the unreacted crystal of CAAB. (B, C) Molecular packing in the crystal of CAAB enriched with the photoinduced AcPhN• radical (represented as red ball-and-stick model) viewed along the a-axis (B) and b-axis (C). The major and minor components are presented as ochre and brown stick models, respectively. The major and minor components at the radical site and the radical components at all other sites are omitted for clarity.

due to partial conversion to the product. The occupancy factors of the three components in the mixed crystal, the electron density, and the single set of signals in the ESR spectra suggest that most of the AcPhN• radical is created from the major component. However, the quantitative yield observed in the IR spectra entails reactivity of the minor component. The unfavorable orientation of the side group of the minor component with the chlorine atom pointing away from the p-hydrogen atom (Figure 4A) requires reorientation to the position of the major component before the photolysis. Thermally activated flipping of the side group can be advocated that equilibrates the ratio of the major and the minor components in the crystal after the population of the former has been depleted by photolysis. Free aminyl radicals R2N• are generally very reactive species, which, when prepared synthetically or electrochemically, can be stabilized as radical crystals by steric protection, or by mitigation of the nitrogen electron deficiency by electronic effects or metal coordination.28 The persistence of the AcPhN• radical produced photochemically in the single crystal of CAAB is unprecedented. To the best of our knowledge, the observation of AcPhN• represents the first direct evidence of the structure of the sterically unprotected aminyl radical. According to the calculated relative order of reaction enthalpies (kJ mol-1) of the homolytic dissociation R2NH f RN• + H• (UB3LYP/631G(d,p) level, details of the optimized structures are listed in

Watching Radicals

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TABLE 1: Selected Intermolecular Distances (d) and Angles (∠) between the AcPhN• Radical at (1 - x, 1 - y, 1 - z) and the Surrounding Molecules in the Radical-Enriched CAAB Crystal N•---HsC H

ringa

comp.

d(N•---H)/Å

d(N•---C)/Å

∠/deg

H4 H4′ H6 H6′ H6 H6′ H2 H2′

Ph4 Ph4′ Ph3 Ph3′ Ph1 Ph1′ Ph2 Ph2′

major minor major minor major minor major minor

4.248(4) 4.468(4) 4.185(3) 4.265(3) 4.569(4) 4.322(4) 4.564(3) 4.779(3)

4.929(5) 4.940(5) 4.685(3) 4.700(3) 5.194(4) 4.959(4) 5.322(4) 5.556(4)

132.93(4) 115.46(4) 117.34(3) 112.53(4) 127.99(4) 128.94(4) 141.39(4) 143.63(3)

O---HsC H4 H4′ H6 H6′

ringa

comp.

d(O---H)/Å

d(O---C)/Å

∠/deg

Ph4 Ph4′ Ph3 Ph3′

major minor major minor

2.902(3) 3.130(3) 5.548(5) 5.626(5)

3.764(4) 3.823(4) 5.910(5) 5.914(5)

154.85(5) 132.74(5) 108.60(3) 103.58(3)

a Symmetry operators: (x, y, z) (Ph1), (-1 + x, y, z) (Ph2), (x, 1 + y, z) (Ph3), (1 - x, 0.5 + y, 0.5 - z) (Ph4).

Table S3 in the Supporting Information),16 Ph2N• (0) < PhHN• (26.9) < AcPhN• (34.0) < HN• (93.0) < AcHN• (108.8), the phenyl groups stabilize while the acetyl groups are expected to destabilize the aminyl radical. As the molecular nonplanarity of the AcPhN• radical in the crystal of CAAB prevents significant stabilization by electron donation from the phenyl ring, the most probable reason behind its stability is the absence of close contacts of the reactive nitrogen (d(N•‚‚‚H) > 4.15 Å, Table 1). The reactive center of the photoinduced radical is locked in the lattice far from the potential target molecules, so that the recombination reaction and a reaction involving the carbonyl group, as will be discussed below, seem to be the only pathways for its decay. The latent reactivity of the radical can be visually observed: although that UV irradiated CAAB crystals are initially transparent, if stored in the dark for several days, the crystal faces that were exposed to the direct UV light gradually acquire opalescence while the overall crystal integrity is preserved. The exceptional selectivity of the photoinduced rearrangement of CAAB can be explained by considering the supramolecular structures of the nonreacted and radical-enriched crystals (Figure 4, Table 1). If it is assumed that the photoinduced radical AcPhN• at (1 - x, 1 - y, 1 - z) in the mixed crystal is surrounded only by nonreacted, thermally disordered CAAB molecules, the reactive N-acetylamino group of AcPhN• approaches the o-hydrogen atoms of the phenyl rings at (x, y, z) (Ph1), (-1 + x, y, z) (Ph2), and (x, 1 + y, z) (Ph3), and the p-hydrogen atom of the phenyl ring at (1 - x, 0.5 + y, 0.5 z) (Ph4). Abstraction of the o-hydrogen from Ph1 and Ph2, which are positioned quasiperpendicular to the plane of the side group of AcPhN•, is unlikely to occur due to their unfavorable disposition relative to the reactive nitrogen. If the C-H‚‚‚O hydrogen-bonding geometry is considered, the o-hydrogen atom of Ph3 and the p-hydrogen atom of Ph4 seem equally preferred for abstraction. The observed high selectivity of the solid-state reaction with 100% preference of the p-hydrogen atom suggests that its course is actually directed by the disposition of the carbonyl group of AcPhN•, which is significantly closer, hydrogen bonded, and oriented at a less acute angle to the p-hydrogen of Ph4 than to the o-hydrogen of Ph3. The structure thus indicates that the hydrogen-chlorine exchange does not proceed directly, but via mediation by the carbonyl group, which

SCHEME 2: Simplified Reaction Mechanism of the Atom Exchange Reaction in CAAB Suggested on the Basis of the Photodiffraction Data for the AcPhN• Radical as Intermediate

assists the abstraction of the p-hydrogen atom by formation of a bifurcated hydrogen bond. On the basis of the change of the crystal structure and physicochemical data, a three-center five-atom mechanism involving homolytic cleavage of the N-Cl bond, accompanied by hydrogen abstraction by the carbonyl group, can be suggested (Scheme 2). The lack of ESR signals other than the one from AcPhN• implies that the abstraction of the hydrogen is a heterolytic process. The carbonyl group, in its hydroxyl form, then transfers the proton to the nitrogen atom, while the proton is replaced by the chlorine atom. The head-to-tail molecular arrangement within the undulated hydrogen-bonded ribbons (Figure 4A) provides the possibility for consecutive or simultaneous radical-mediated chlorine-hydrogen atom exchange between neighboring molecules along the hydrogen-bonded chain. Such rationale corresponds to the chain radical-mediated reaction model (Scheme 1c), disfavoring the unimolecular and bimolecular mechanisms to explain the topotactic control observed for this scsc reaction. The structure of the radicalenriched crystal represents the first direct evidence for the origin of the remarkable efficiency and selectivity of the photoinduced Orton rearrangement in the solid state. Acknowledgment. We thank Dr. T. Fujita, Dr. Y. Uemura, Dr. Y. Katsuya, Mr. D. Nomoto, and Dr. S. W. Ng for their help. This study was performed through Special Coordination Funds for Promoting Science and Technology from the MEXT of the Japanese Government. Supporting Information Available: Experimental IR spectra of CAAB (Figure S1), Theoretical spectra of CAAB and the model compounds (Figure S2), DSC curves (Figure S3), plot of molecular packing of CAAB (Figure S4), 13C CP MAS NMR spectra (Figure S5), measured and fitted powder X-ray diffraction patterns of CAAB (Figures S6 and S7), list of intermolecular distances (Table S1), list of Cartesian coordinates and intramolecular parameters of CAAB and the optimized model compounds, radicals, and the respective closed-shell species (Tables S2 and S3), and complete ref 16. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Bender, G. Ber. 1886, 18, 293-295. (b) Chattaway, F. D.; Orton, K. J. P. J. Chem. Soc. 1899, 75, 1046-1048. (c) Ramamurthy, V.; Venkatesan, K. Chem. ReV. 1987, 87, 433-481. (2) Fujii, T. ReV. Phys. Chem. Jpn. 1974, 44, 38-55 and references cited therein. (3) Porter, C. W.; Wilbur, P. J. Am. Chem. Soc. 1927, 49, 2145-2149. (4) Depending on the solvent, in addition to CA, acetanilide, diacetylhydrazobenzene, and chlorinated solvents have been previously isolated: Hodges, F. W. J. Chem. Soc. 1933, 240-246. (5) The few minor spectral differences between p-CA produced photochemically in the crystal of CAAB and p-CA in the crystal of the pure compound are due to different geometry around the N1-C7 bond, which is locked in cis and trans conformations, respectively.

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Naumov et al. Bagley, K. A.; Coppens, P. J. Am. Chem. Soc. 2002, 124, 9241-9248. (d) Nakai, H.; Mizuno, M.; Nishioka, T.; Koga, N.; Shiomi, K.; Miyano, Y.; Irie, M.; Breedlove, B. K.; Kinoshita, I.; Hayashi, Y.; Ozawa, Y.; Yonezawa, T.; Toriumi, K.; Isobe, K. Angew. Chem., Int. Ed. 2006, 45, 6473-6476. (15) Basic crystallographic data for p-CA and o-CA. p-CA (colorlesss plate from CH3Cl:PE mixture): orthorhombic, Pna21, Z ) 4, a ) 9.7059(21) Å, b ) 12.7461(28) Å, c ) 6.5270(14) Å, R ) β ) γ ) 90°. Note: the crystal is sensitive to X-rays. o-CA (colorless prism from CH3Cl:PE ) 1:4): orthorhombic, Pbca, Z ) 4, a ) 10.5675(18) Å, b ) 9.4516(16) Å, c ) 16.5098(28) Å, R ) β ) γ ) 90°. The complete structural data are available upon request from the corresponding author. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; et al. Gaussian03, Revision B.05; Gaussian, Inc.: Wallingford CT, 2004 (the complete reference is provided in the Supporting Information). (17) SAINT-Siemens Area Detector Integration and SMART-Siemens Molecular Analysis Research Tool; Siemens Analytical X-ray Instruments Inc.: Madison, WI, 1996. (18) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (19) Sheldrick, G. M. E. SHELXL-97, Structure refinement program; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (20) Howard, C. J. J. Appl. Crystallogr. 1982, 15, 615-620. (21) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS), Los Alamos National Laboratory report LAUR 86-748, 2004. (22) 300 K: a ) 7.1458 Å, b ) 6.8789 Å, c ) 17.4071 Å, R ) 90°, β ) 91.50°, γ ) 90°, V ) 832.46 Å3, wRp ) 2.55%, Rp ) 1.98%. 200 K: a ) 7.0342 Å, b ) 6.8376 Å, c ) 17.3368 Å, R ) 90°, β ) 91.78°, γ ) 90°, V ) 832.46 Å3, wRp ) 3.08%, Rp ) 2.33%. (23) Crystal data for nonirradiated crystal: C8H8ClNO, Mr ) 169.60, T ) 270 K, λ ) 0.71073 Å, monoclinic, P21/c, a ) 7.0616(12) Å, b ) 6.8269(11) Å, c ) 17.276(3) Å, β ) 91.781(3)°, V ) 832.5(2) Å3, Z ) 4, Fcalc ) 1.353 Mg‚m3, µ ) 0.397 mm-1, F(000) ) 352, crystal size 0.35 × 0.30 × 0.27 mm, Θ range ) 2.36-26.39°, reflections (collected/unique) 6039/ 1569, Rint ) 0.0250, GoF ) 1.076, R1 ) 0.0641, wR2 ) 0.1713. (24) According to a neutron diffraction study, the occupancy of the minor component in the crystal of the structurally related molecule p-chlorophenylformamide is 13.2%, which is close to the value observed for CAAB: Tam, C. N.; Cowan, J. A.; Schultz, A. J.; Young, V. G.; Trouw, F. R.; Sykes, A. G. J. Phys. Chem. 2003, B107, 7601-7606. (25) (a) Tozuka, Y.; Yamamura, Y.; Saito, K.; Sorai, M. J. Chem. Phys. 2000, 112, 2355-2360. (b) Yasuda, N.; Uekusa, H.; Ohashi, Y. J. Mol. Struct. 2003, 647, 217-222. (26) Crystal data for irradiated crystal: C8H8ClNO, Mr ) 169.60, T ) 270 K, λ ) 0.71073 Å, monoclinic, P21/c, a ) 7.096(6) Å, b ) 6.889(7) Å, c ) 17.388(16) Å, β ) 91.51(2)°, V ) 849.7(13) Å3, Z ) 4, Fcalc ) 1.326 Mg‚m3, µ ) 0.389 mm-1, F(000) ) 352, crystal size 0.41 × 0.36 × 0.05 mm, Θ range ) 2.34-26.19°, reflections (collected/unique) 4106/ 1551, Rint ) 0.0789, GoF ) 0.765, R1 ) 0.0589, wR2 ) 0.1373. (27) Due to the disorder, the electron density corresponding to the detached chlorine atom was smeared in the difference map, and no attempt was made to model it with a single set of xyz coodinates. (28) (a) Miura, Y.; Kurokawa, S.; Nakatsuji, M.; Ando, K.; Teki, Y. J. Org. Chem. 1998, 63, 8295-8303. (b) Miura, Y.; Tomimura, T.; Teki, Y. J. Org. Chem. 2000, 65, 7889-7895. (c) Miura, Y.; Tomimura, T. Chem. Commun. 2001, 627-628. (d) Bu¨ttner, T.; Geier, J.; Frison, G.; Harmer, J.; Calle, C.; Schweiger, A.; Scho¨nberg, H.; Gru¨tzmacher, H. Science 2005, 307, 235-238.