The thiolato complex [platinum(II) (bipyridine)(N,S-aminoethanethiolate)](+)Ch(-) (1) undergoes sequential reactions with singlet oxygen to initially form the corresponding sulfenato complex [platinum(II) (bipyridine)(N,S(═O)-aminoethansulfenate)](+) (2) followed by a much slower reaction to the corresponding sulfinato complex. In contrast with many platinum dithiolato complexes, 1 does not produce any singlet oxygen, but its rate constant for singlet oxygen removal (k(T)) is quite large (3.2 × 10(7) M(-1) s(-1)) and chemical reaction accounts for ca. 25% of the value of k(T). The behavior of 1 is strikingly different from that of the complex platinum(II) (bipyridine)(1,2-benzenditholate) (4). The latter complex reacts with (1)O(2) (either from an external sensitizer or via a self-sensitized pathway) to form a sulfinato complex. These two very different reactivity pathways imply different mechanistic pathways: The reaction of 1 with (1)O(2) must involve O-O bond cleavage and intermolecular oxygen atom transfer, while the reactive intermediate in complex 4 collapses intramolecularly to the sulfinato moiety.

We have applied 57Fe nuclear resonance vibrational spectroscopy (NRVS) for the first time to study the dynamics of Fe centers in Iron-sulfur protein crystals, including oxidized wild type rubredoxin crystals from Pyrococcus furiosus, and the MoFe protein of nitrogenase from Azotobacter vinelandii. Thanks to the NRVS selection rule, selectively probed vibrational modes have been observed in both oriented rubredoxin and MoFe protein crystals. The NRVS work was complemented by extended X-ray absorption fine structure spectroscopy (EXAFS) measurements on oxidized wild type rubredoxin crystals from Pyrococcus furiosus. The EXAFS spectra revealed the Fe-S bond length difference in oxidized Pf Rd protein, which is qualitatively consistent with the crystal structure.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The unusual uranium reaction system in which uranium(4+) and uranium(3+) hydrides interconvert by formal bimetallic reductive elimination and oxidative addition reactions, [(C(5)Me(5))(2)UH(2)](2) (1) ⇌ [(C(5)Me(5))(2)UH](2) (2) + H(2), was studied by employing multiconfigurational quantum chemical and density functional theory methods. 1 can act as a formal four-electron reductant, releasing H(2) gas as the byproduct of four H(2)/H(-) redox couples. The calculated structures for both reactants and products are in good agreement with the X-ray diffraction data on 2 and 1 and the neutron diffraction data on 1 obtained under H(2) pressure as part of this study. The interconversion of the uranium(4+) and uranium(3+) hydride species was calculated to be near thermoneutral (~-2 kcal/mol). Comparison with the unknown thorium analogue, [(C(5)Me(5))(2)ThH](2), shows that the thorium(4+) to thorium(3+) hydride interconversion reaction is endothermic by 26 kcal/mol.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Heteromultimetallic hydride clusters containing both rare-earth and d -transition metals are of interest in terms of both their structure and reactivity. However, such heterometallic complexes have not yet been investigated to a great extent because of difficulties in their synthesis and structural characterization. Here, we report the synthesis, X-ray and neutron diffraction studies, and hydrogen addition and release properties of a family of rare-earth/ d -transition-metal heteromultimetallic polyhydride complexes of the core structure type ‘Ln 4 MH n ’ (Ln = Y, Dy, Ho; M = Mo, W; n = 9, 11, 13). Monitoring of hydrogen addition to a hydride cluster such as [{(C 5 Me 4 SiMe 3 )Y} 4 ( μ -H) 9 Mo(C 5 Me 5 )] in a single-crystal to single-crystal process by X-ray diffraction has been achieved for the first time. Density functional theory studies reveal that the hydrogen addition process is cooperatively assisted by the Y/Mo heteromultimetallic sites, thus offering unprecedented insight into the hydrogen addition and release process of a metal hydride cluster.

Arylphosphines and dialkylbiarylphosphines react with singlet oxygen to form phosphine oxides and phosphinate esters. For mixed arylphosphines, the most electron-rich aryl group migrates to form the phosphinate, while for dialkylbiarylphosphines migration of the alkyl group occurs. Dialkylbiarylphosphines also yield arene epoxides, especially in electron-rich systems. Phosphinate ester formation is increased at high temperature, while protic solvents increase the yield of epoxide. The product distribution provides evidence for Buchwald's recent conformational model for the aerobic oxidation of dialkylbiarylphosphines.

Our single-crystal neutron diffraction results unambiguously reveal a four-coordinate H atom located in the center of a soluble organometallic tetrahedral complex [Li(THF)4][(C5Me4SiMe3)4Dy4(μ-Cl)(μ-H)8]. The core of the molecule consists of a tetranuclear cluster with one interstitial, two face-bridging and five edge-bridging hydride ligands. The four Dy–H distances to the interstitial hydride ligand are 2.249(9), 2.255(9), 2.157(12) and 2.160(12)Ǻ. The compound was prepared via the reaction of [(C5Me4SiMe3)4Dy4(μ-H)8(THF)2] with LiCl. Neutron data collected on a 3mm3 pale-yellow single crystal on the Quasi-Laue diffractometer VIVALDI at I.L.L. (Grenoble) which gave an agreement factor R=10.1% in the final structure refinement against 6947 reflections. The existence of a four-coordinate hydrogen reinforces previous results observed with a series of high-connectivity hydride ligands located at the interstitial cavities of molecular clusters. Interestingly, this structure allows us to analyze simultaneously three different types of hydride coordination in the same molecule (M2(μ2-H), M3(μ3-H), and M4(μ4-H)).

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An intensely phosphorescent Pt complex in cyclohexane is efficiently quenched by exciplex formation with extremely weak Lewis bases such as toluene and other aromatic compounds.

A four-coordinate hydrogen atom has been unambiguously located, by single-crystal neutron diffraction for the first time, in the center of the tetrahedral metal complex Y4H8(Cp')4(THF) [Cp'=C5Me4(SiMe3)]. The core of the molecule consists of a tetranuclear cluster with one interstitial, one face-bridging, and six edge-bridging hydride ligands. The compound was prepared via the reaction of YCp'(CH2SiMe3)2(THF) with gaseous H2. Neutron data were collected on a 4 mm3 crystal at the Quasi-Laue diffractometer VIVALDI at ILL (Grenoble)1a and on an 8 mm3 crystal at the SXD diffractometer at ISIS (Didcot). The final agreement factor is R = 8.9% for 4171 reflections. The existence of 4-coordinate hydrogen now completes the series of high-connectivity hydride ligands located in the interstitial cavities of molecular cluster complexes. We had previously reported the existence of 6-coordinate hydrogen in the octahedral cavity of [HCo6(CO)15]- in 1979, and 5-coordinate hydrogen in the square pyramidal cavities of [H2Rh13(CO)24]3- in 1997, also via single-crystal neutron analyses.

The synthesis, characterization, and photophysical properties are reported for a series of ((CN)-N-Lambda)Pt dyad complexes with cyclometalated ligands ((CN)-N-Lambda: F = 2-(4',6'-difluorophenyl)pyridyl, tpy = 2-(4'-methylphenyl)pyridyl, thpy = 2-(2-thienyl)pyridyl, and btp = 2-(2-benzothienyl)pyridyl). The dyads are connected with a bridging ligand, sym-tetraacetylethane (tae), that consists of two 2,4-pentadionate units covalently linked at the 3-position. Stepwise synthesis was applied to obtain both homoleptic and heteroleptic dyads that have been characterized by H-1 NMR and elemental analysis. X-ray crystallographic analysis shows a near orthogonal orientation of the two diketonate moieties in the di-tpyPt (84 degrees) and FPt-thpyPt (89 degrees). An investigation of the photophysical properties of the dyads has been carried out. For homodyads, the emission characteristics are governed by the nature of the cyclometalating ligand, allowing the emission to be tuned throughout the visible spectrum. The di-FPt and di-tpyPt dyads show a modest decrease in luminescent lifetimes compared to their mononuclear analogs. The di-FPt complex undergoes efficient self-quenching by an excimer that is presumed to have only pi-pi interactions. The heterodyads exhibit efficient intramolecular triplet energy transfer leading to the luminescence almost exclusively from the lower-energy moiety. The energy transfer is presumed to be mediated through the bridging tae ligand by a Dexter-like mechanism involving a combination of hopping and superexchange processes.

The complete structure of {K(MeTHF)(3)}(2){Yb{(mu-H)(2)BC8H14}(4)} including the positions of the H atoms has been determined by single-crystal neutron diffraction on the quasi-Laue diffractometer VIVALDI at the Institut Laue-Langevin in Grenoble, France. Each [(mu-H)(2)BC8H14](-) ligand is coordinated to the Yb atom via two Yb-H-B bridges, and two out of the four [(mu-H)(2)BC8H14](-) units exhibit C-H...Yb agostic interactions. Crystallographic details: space group C2/c (Monoclinic); a = 17.62 (4), b = 19.92(4), c = 21.05 (4) A; beta = 95.62(9) angstrom: Z = 4. Final agreement factor: R(F) = 13.5% for 1668 unique reflections with l > 2 sigma(l). (c) 2008 Elsevier B.V. All rights reserved.

Long sought structural data on an f-element tuck-in complex have been obtained for the title compound 1 that contains the first example of both tuck-in and tuck-over bonding in a ligand derived from C5Me5− by metalation (see scheme). One of the features of the C5Me5− group that makes it such a desirable ligand in organometallic chemistry is the fact that it is relatively inert to the CH activation that often complicates the chemistry of C5H5− metallocene complexes.1–4 Although C5Me5− is more resistant to attack on the CH bond, its methyl groups can be metalated with some highly reactive metal species.5–10 Since these CH activated ligands arise from extremes in reactivity, they have been involved in unprecedented reactions. For example, the homogeneous CH activation of methane was first discovered with [{(C5Me5)2LuMe}n]5 based on a mechanism involving the "tucked-in"6 complex "[(C5Me5){C5Me4(CH2)}Lu]". Tuck-in6 intermediates have also played prominent roles in explaining the CH activation in complexes such as [(C5Me5)2ScMe]6 and [(C5Me5)2Th(CH2CMe3)2].11 Despite the repeated use of tuck-in complexes in mechanistic schemes involving f elements, no spectroscopic or crystallographic evidence has ever been presented to support the existence of such an intermediate in a lanthanide or actinide complex. Tuck-in complexes have been crystallographically characterized with transition metals.7, 9, 10, 12 For example, in the {(C5Me5)2TiH} system, [(C5Me5)Ti{η5:η1-C5Me4(CH2)}] (1) could be isolated.7 However, in these transition-metal tuck-in compounds, low-oxidation-state tetramethylfulvalene resonance structures can contribute to the stability of the complexes.7, 9, 10, 12, 13 With the limited oxidation states available for f elements, this is less likely. 1 Crystallographic evidence for an alternative type of C5Me5− metalation has been obtained with lanthanides in the form of "tuck-over" complexes [(C5Me5)2Ln(μ-H)(μ-η1:η5-CH2C5Me4)Ln(C5Me5)] (2; Ln=Y,14 La,15 Sm,16 and Lu17) in which the methylene group formed by CH activation is attached to a second metal atom. In some cases, double CH activation at two methyl groups resulted in C5Me3(CH2)23− ligands, for example, in [{(C5Me5)3Ln2[C5Me3(CH2)2]}2] (3; Ln=Ce18 and Sm19).13, 20) The only crystallographic evidence for this type of CH activation of a C5Me5− ligand in an actinide complex involves a methylene group attached to nitrogen not the metal. Thermolysis of the U6+–imido complex [(C5Me5)2U(NAd)2] (Ad=1-adamantyl) formed [(C5Me5)U{η1:η5-NAd(CH2C5Me4)}(NHAd)] (4,21)—a reaction that could involve a tuck-in (or tuck-over) intermediate. The only other suggestion of C5Me5− metalation in actinide chemistry is kinetic data on a multiple-pathway transformation involving formation of [(C5Me5)2Th(μ-CH2)2EMe2] from [(C5Me5)2ThR2] (R=CH2EMe3; E=C, Si).11 We report here the first crystallographically characterized tuck-in complex of an f element and the first crystallographic data on uranium tuck-in and tuck-over structures. Both are found in the same structure (Figure 1). This bonding mode has not been observed previously in any metal complex of ligands derived from C5Me5− to our knowledge. Molecular structure of 7 (thermal ellipsoid drawn at the 50 % probability level). X-ray crystallography revealed the structure of complex 7 (Figure 1). A doubly-metalated [μ-η5:η1:η1-C5Me3(CH2)2]3− ligand is engaged in both tuck-in and tuck-over binding to uranium. The large thermal ellipsoids in the C1–C5 ring are likely the result of multiple ring orientations. However, resolution limits in the X-ray data were not adequate to propose a disordered model. Consequently, the disorder was treated as high thermal activity. Although the bridging hydride ligands were not located in the X-ray crystal structure, their presence was established by other means. Attempts to prepare a diamagnetic thorium analog according to Equation (1) were unsuccessful. The infrared spectrum of 7 displayed a broad band centered at 1164 cm−1 in the region typical for U-H-U stretching modes. This absorption band is similar to those observed for both 5 and 6,22, 24 which give rise to broad bands centered at 1188 and 1176 cm−1, respectively. Attempts to synthesize a deuterium analog of 7 were thwarted by the fact that neither [{(C5Me5)2UD2}2] nor [{(C5Me5)2UD}2] were accessible due to hydride exchange with the C5Me5− rings.24 The infrared spectrum of the product made according to Equation (1), but from precursors synthesized from [(C5Me5)2UMe2] and D2, was identical to that of 7. The presence of hydrides in 7 was probed by measuring the gas evolution during the synthesis of 7 from 622 by means of a Toepler pump. Only one equivalent of dihydrogen per two uranium atoms was obtained as shown in Equation (1). Two equivalents of dihydrogen would be expected, if a U3+ complex without hydride ligands was formed instead of 7. The reactions of 7 with phenol and C6H5OD gave further support for the presence of hydrides. Reaction of 7 with phenol yields H2 and the bis(phenoxide) complex [(C5Me5)2U(OPh)2] (8) in 92 % yield [Eq. (3)]. The identity of 8 was confirmed by X-ray crystallography (Figure 2). Reaction of 7 with C6H5OD gave HD28 and a product that had 2D NMR resonances at 3.1 ppm consistent with the presence of deuterium in place of hydrogen in C5Me5− rings of 8. Molecular structure of 8 (thermal ellipsoid drawn at the 50 % probability level). The structure is similar to those of [(C5Me5)2U(EPh)2] (E=S, Se)26 except the U-O1-C21 and U-O2-C27 angles (174(1) and 172(1)°) are larger.27 The structure of 7 contains two uranium atoms separated by 3.7917(5) Å, a distance intermediate between the U⋅⋅⋅U distances in tetravalent 5 (3.606(6) Å) and trivalent 6 (3.8530(7) and 3.8651(7) Å).22 The larger distance in 7 vs 5 is consistent with the more extensive bridging structure that includes the tuck-over unit. The presence of the two "tuck" units in the C11–C15 ring led to significant variation of the UC bond lengths to that ring, although the ring planarity is not affected and the CC distances are equivalent within the error limits. The ring carbon atoms attached to the methylene groups, C11 and C15, have the shortest U1Cring distances (2.422(6) and 2.436(7) Å), for C12 and C14 these distances are 2.709(6) and 2.783(8) Å, and the ring carbon most distant from the methylene groups has a U1C13 bond length of 2.935(7) Å. The U2C16 tuck-over linkage (2.640(1) Å) is longer than typical UCalkyl single bonds (for example, 2.414(7) and 2.424(7) Å for UCMe in [(C5Me5)2UMe2]29). This is consistent with the long LnC bonds of the CH2 tuck-over groups observed in lanthanide complexes.14–19 The U1C16 distance (2.722(8) Å) is in the broad range of UC distances, and hence C16 may also transfer electron density to U1. A similar situation is seen in the lanthanide tuck-over complexes.14–19 The U1C20 distance, the first structurally characterized bond length between an f element and the carbon atom of a CH2 tuck-in group, is 2.564(1) Å. Hence, this bond is longer than a terminal alkyl bond, as expected for bridging ligands, but it is not as long as for a methylene bridge attached to a second metallocene. In comparison, the TiCH2 distance in 1 is very similar to a Ti3+Calkyl bond.7 The isolation of 7 raises several basic questions about the reactivity of UH groups. Although it is reasonable that a UH group in 6 can metalate a methyl group in C5Me5−, as has previously been observed with LnH bonds,14, 16 a sigma-bond metathesis with elimination of H2 would lead to a trivalent [(C5Me5)U(C5Me4CH2)] moiety from one of the {(C5Me5)2UH} units in dimeric 6. It is not clear how or why a second metalation would occur at the C5Me4CH22− ligand to form the observed tuck-in and tuck-over structure. It is possible that the hydride ligands in 7 are formed by reduction of H2 with U3+ ions in the same way that hydride ligands in 5 are formed from H2 and 6 in the reverse of the equilibrium in Equation (1). Hence, as the C5Me5− rings are being metalated, the H2 produced in this process may react with the U3+ centers before it leaves the metal coordination sphere. Clearly, the chemistry of uranium metallocene hydrides has some extra dimensions that have not yet been fully explored. New uranium hydrides are accessible that combine hydride and alkyl functionality. In addition, double CH activation is possible in this class of compounds to form tuck-in and tuck-over structures in a single complex. In any case, the long sought crystallographic evidence for the postulated f element tuck-in intermediates has been obtained and the existence of both tuck-in and tuck-over structures for actinides has been established. 7: In a glovebox, a green-brown solution of a 1:1 mixture of 5 and 6 (248 mg, 0.244 mmol) in toluene (10 mL) was heated to 110 °C for 3 minutes with frequent venting. The mixture was allowed to cool slowly to room temperature, and solvent was removed under reduced pressure to yield a green oil. A concentrated solution in toluene (3 mL) produced dark green crystals of 7 (188 mg, 0.185 mmol, 75 %) after 2 days at −35 °C. 1H NMR (C6D6, 298 K): δ=−23.9 (br s, 30 H, C5Me5, Δν1/2=600 Hz), −2.6 ppm (s, 15 H, C5Me5, Δν1/2=8 Hz), [−0.3 (s, 2 H), 1.5 (s, 3 H), 1.1 (s, 2 H), C5Me5(CH2)2]. 1H NMR (C6D6, 343 K): δ=−19.3 (s, 30 H, C5Me5, Δν1/2=70 Hz), −7.7 (s), −1.5 (s, 15 H, C5Me5, Δν1/2=11 Hz), 3.4 (s), 1.2(s), 0.4(s). 13C NMR (C6D6, 343 K): δ=−47.5 (C5Me5), −58.6 (C5Me5), 126.0 (C5Me5), 129.7 ppm (C5Me5). IR (KBr): =2966 (m), 2903 (vs), 2854 (vs), 2722 (w), 1434 (m), 1377 (m), 1164 (s), 1020 (s), 903 (m), 799 (m), 586 (m) cm−1. Elemental analysis calcd for C40H60U2: C 47.24, H 5.95, U 46.81; found: C 47.58, H 5.91, U 46.43. 7: A sealable Schlenk flask fitted with a Teflon stopper was charged with 6 (499 mg, 0.490 mmol) in toluene (20 mL). After four freeze-pump-thaw cycles, the solution was heated to 110 °C for 20 minutes. The reaction mixture was frozen in liquid nitrogen and evacuated using a Toepler pump equipped with a U-trap cooled in liquid nitrogen. The non-condensable gas was collected (9.8 μmol, 0.93 equiv) and subsequently analyzed by 1H NMR spectroscopy in C6D6 to be H2 (single resonance at 4.46 ppm). The solvent from the reaction mixture was evaporated to dryness yielding 7 as a dark brown crystalline material (0.469 g, 94 %). 8: In a glovebox, a solution of PhOH (56 mg, 0.59 mmol) in toluene (3 mL) was added to a stirred solution of dark green 7 (152 mg, 0.150 mmol) in toluene (5 mL). The solution immediately turned dark orange. After the mixture was stirred overnight, the solution was evaporated to dryness to yield 8 as a dark orange crystalline powder (192 mg, 92 %). Crystals of 8 suitable for X-ray diffraction studies were grown at −35 °C from a concentrated toluene solution. 1H NMR (500 MHz, C6D6): δ=3.19 (s, 30 H, C5Me5, Δν1/2=5 Hz), 3.9 (t, 2 H, 3JH,H=8 Hz, p-H), 1.8 (t, 4 H, 3JH,H=8 Hz, m-H), −13.6 ppm (d, 4 H, 3JH,H=8 Hz, p-H). 13C NMR (125 MHz, C6D6): δ=−32.8 (C5Me5), 141.1 (C5Me5), 125.1 (m-Ph), 104.1 (o-Ph), 108.1 ppm (p-Ph). IR (KBr): =2972 (m), 2907 (m), 2857 (m), 1588 (vs), 1489 (vs), 1475 (vs), 1377 (w), 1252 (vs), 1276 (vs), 1160 (s), 1065 (m), 998 (m), 873 (vs), 863 (vs), 754 (vs), 691, 604 (s) cm−1. C,H analysis calcd for C32H40O2U: C 55.33, H 5.80; found: C 55.62, H 5.50. In a similar experiment, 7 (12 mg, 0.012 mmol) in C6D6 was added to a J-Young tube containing a frozen slurry of (Et3NH)BPh4 (10 mg, 0.024 mmol) in C6D6. The J-Young tube was capped immediately and a color change from brown-green to brown was observed. 1H NMR spectroscopy showed quantitative conversion of starting material to the previously characterized [(C5Me5)2U]BPh425 and H2, which exhibited a 1H NMR resonance at 4.46 ppm. Compound 7 crystallizes in the space group P with a=10.5198(17), b=11.0156(17), c=16.281(3) Å, α=89.529(3), β=81.943(3), γ=80.842(3)°, V=1844.0(5) Å3, Z=2, ρcalcd=1.828 Mg m−3, R1=0.0436 [I > 2σ(I)], wR2=0.1114, GOF=1.047. Compound 8 crystallizes in the space group P with a=9.409(2), b=9.667(2), c=17.020(4) Å, α=99.753(4), β=96.714(4), γ=108.070(4)°, V=1426.6(6) Å3, Z=1, ρcalcd=1.617 Mg m−3, R1=0.078 [I > 2σ(I)], wR2=0.184. Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Poster Sessions (iBIX)", which is now constructing at Materials and Life-science Facility (MLF) of J-PARC.STARGazer has several functional components; 1) peak search from the raw data, 2) determination of the UB matrix, 3) finding the Bravais lattice, 4) refinement of the UB matrix, 5) calculate the intensities of all Bragg reflections, and 6) data visualization.The algorithms of crystallographic fundamental functions of those components referred the algorithms of program ISAW, which is a data processing software package developed on Argonne National Laboratory.In addition, STARGazer has some additional functions optimized for the measurement of protein crystals on the iBIX; real-space indexing technique to find UB matrix, refinement of the detector position simultaneously in UB matrix refinement, and finding the Bragg reflections which are overlapping with neighboring reflections.In the near future, a function to deconvolute the overlapping Bragg reflections will be added.STARGazer was developed based on a software library "Manyo-Lib", which is a framework software for data analysis at MLF developed by J. Suzuki and co-workers.Each component of STARGazer works independently as a part of Manyo-Lib, and users of other instruments in MLF and other pulsed neutron facilities can easily use the components for their data processing.

Two chiral tetraphenylenes, 2,15-dideuteriotetraphenylene (7) and 2,7-dimethyltetraphenylene (15) were synthesized and resolved to address the tetraphenylene inversion barrier problem. Neutron diffraction investigation of enantiopure 7 showed that the molecule retained its chirality integrity during its synthesis from enantiopure precursors and therefore rules out the possibility of the tetraphenylene framework possessing a low-energy barrier to inversion. Thermal study on 15 and tetraphenylene 1 further revealed that their inversion barriers were not overcome up to 600 degrees C, at which temperature these compounds underwent skeletal contraction into triphenylene with activation energies of 62.8 and 58.2 kcal/mol, respectively. This result is supported by computational studies which yielded an inversion barrier of 135 kcal/mol for tetraphenylene as a consequence of the peri-hydrogen repulsions at its planar conformation.