Triplet diphenylcarbenes protected by trifluoromethyl and bromine groups. A triplet carbene surviving a day in solution at room temperature

Two types of diphenyldiazomethanes having two trifluoromethyl and two bromine groups at the ortho positions, either in unsymmetrical or in symmetrical fashion, that is, (2,6-dibromo-4-phenylphenyl)[4-phenyl-2,6-bis(trifluoromethyl)phenyl]diazomethane (U-1-N2) and bis(2-bromo-4-phenyl-6-trifluoromethylphenyl)diazomethane (S-1-N2), are prepared. Triplet diphenylcarbenes (U-(3)1 or S-(3)1) are generated from those precursors and are characterized by ESR, UV/vis spectroscopy at low temperature, as well as time-resolved UV/vis spectroscopy at room temperature. Those carbenes are shown to be at least 2 orders of magnitude less reactive than the most stable triplet diphenylcarbene thus far known, that is, bis(2,6-dibromo-4-phenylphenyl)carbene. It has been also shown that S-(3)1 is significantly more stable than U-(3)1 even though both have the same two kinds of substituents. It is suspected that the perpendicular alignment of the two most bulky groups is a more effective way to shield the carbenic center than the planar one. By this way, triplet substituted diphenylcarbene surviving nearly a day in solution at room temperature is realized for the first time.     

Triplet diphenylcarbenes protected by o-Aryl groups

Diphenyldiazomethanes (2a-N(2)) having phenyl groups at the ortho positions were prepared and the corresponding diphenylcarbenes (2a) photolytically generated from them were characterized not only by the traditional product analysis method but also by ESR and UV/vis spectroscopy in a rigid organic matrix at low temperature and in laser flash photolysis in solution at room temperature. Product analysis indicated that fluorenes 4a are formed almost exclusively. Fluorene is most likely produced by the attack of singlet carbene (1)2a on an ortho carbon of the phenyl substituent to generate isofluorene, followed by 1,5-H shift. Irradiation of 2a-N(2) in a 2-methyltetrahydrofuran (2-MTHF) matrix at 77 K gave ESR signals ascribable to triplet carbenes ((3)2a). UV/vis spectra of (3)2a were obtained by irradiating 2a-N(2) under identical conditions. However, laser flash photolysis (LFP) of 2a-N(2) in degassed benzene at room temperature showed transient absorption bands completely different from those observed in photolysis in the 2-MTHF matrix at 77 K. The transient band decayed in first-order with a rate constant of 7.6 x 10(4) s(-1), which was not retarded by deuterium substitution of o-phenyl hydrogens. Essentially the same transient band was observed in LFP of fluorene 4a. The LFP of 2a-N(2) in nondegassed benzene gave transient absorption bands ascribable to triplet carbene (3)2a and the corresponding carbonyl oxide. The quenching rate constant of (3)2a by oxygen and the lifetime of (3)2a in the absence of oxygen were estimated by plotting the observed pseudo-first-order rate constant of the formation of the carbonyl oxide against the concentration of oxygen. They were 1.9 x 10(7) M(-1)s(-1) and 16 micros, respectively. Similar studies with diphenyldiazomethanes having a series of substituents (4-Ph, 3,5-(CF(3))(2), and 2-F) on the ortho-phenyl rings gave essentially identical results, indicating that those substituents exhibit little effect on overall reaction pathway. From those studies, it is suggested that triplet carbenes (3)2 are also trapped by the ortho-phenyl ring to give eventually fluorenes.     

Generation, characterization, and kinetics of triplet Di[1,2,3,4,5,6, 7,8-octahydro-1,4:5,8-di(ethano)anthryl]carbene

The title carbene has been generated by photolysis of the corresponding diazo precursors and studied by spectroscopic means, i. e., electron paramagnetic resonance (EPR) and UV/vis spectroscopy in matrixes at low temperature and laser-flash photolysis in solution at room temperature, with the product analysis. The results are compared with triplet di(2,3,5,6-tetramethylphenyl)carbene, an open-chain counterpart, which revealed that bicycloalkyl groups are acting as a fairly good kinetic protector for the triplet carbene as opposed to the open-chain counterpart. The formation of all-hydrocarbon triplet carbenes having a half-life over a second under normal conditions was realized for the first time. Effects of para-substituents on the structure and reactivities of the carbene are also investigated and discussed in terms of polar and spin electronic effects.     

立体拥挤二苯基卡宾分子结构的电子效应

通过对二苯基重氮甲烷的光照射产生了一系列具有对称对位取代基的三线态二(2,6-二甲苯基)卡宾.用电子顺磁共振波谱对其进行了研究.通过对不同粘度的基质(matrix)中零磁场分裂参数D和E的测定,依据电子自旋离域取代基常数σr对三线态二苯基卡宾的分子结构的取代基效应进行了分析.并通过对卡宾的热消失温度及其室温脱气苯溶液中寿命的测定,对三线态二(2,6-二甲苯基)卡宾的稳定性进行了定量考察.结果表明,对卡宾中心的自旋电子具有离域效应的取代基使三线态二(2,6-二甲苯基)卡宾采取低能稳定的直线型结构,且显示了更好的热稳定性和更长的寿命.     

Preparation of sterically congested diphenyldiazomethanes having a pyridine ligand and magnetic characterization of photoproducts of their 2:1 copper complexes

[structure: see text] To confirm whether high-spin species can be generated as a result of ferromagnetic interaction between the 3d spin of metal ions and the 2p spins of triplet carbene through the pyridyl group located remote from the carbene center, [2,6-dibromo-4-(3- and 4-pyridyl)phenyl](4-tert-butyl-2,6-dimethylphenyl)diazomethanes were prepared and the corresponding carbenes were generated either in the absence or presence of Cu(hfac)2. These were characterized by ESR and UV/vis in a matrix at low temperature, and by laser flash photolysis in solution at room temperature. These studies indicated that although both carbenes generated a fairly stable complex with copper ions, the 4-pyridyl isomer formed a high-spin species as a result of ferromagnetic interaction between the 3d spin of metal ions and the 2p spins of triplet carbene. Such an interaction in the corresponding 3-isomer is likely to be antiferromagnetic. This is further confirmed by magnetic measurements using a Superconducting Quantum Interference Device (SQUID). The results demonstrate that extension of this method will enable stable high-spin polycarbenes to be obtained.     

Preparation of bis(diazo) compounds incorporated into butadiyne and thiophene units and generation and characterization of their photoproducts

(2,6-dimethyl-4-tert-butylphenyl)(2,4,6-tribromophenyl)diazomethane(-N(2)) was found to be stable enough to survive under Sonogashira coupling reaction conditions, and aryldiazomethyl substituents were introduced at the 1,4-positions of butadiyne (4-2N(2)) and the 2,5-positions of thiophene(5-2N(2)). Irradiation of those bis(diazo) compounds generated bis(carbenes), which were characterized by using ESR and UV/vis spectroscopic techniques in a matrix at low temperature as well as time-resolved UV/vis spectroscopy in solution at room temperature. These studies revealed that both of the bis(carbenes), 4 and 5, have singlet quinoidal diradical ground states with a very small singlet-triplet energy gap of less than 1 kcal mol(-1). A remarkable increase in the lifetime of bis(carbenes), as opposed to that of the monocarbene (2), was noted and was interpreted to indicate that bis(carbenes) are thermodynamically stabilized as a result of delocalization of unpaired electrons throughout the pi net framework. In spite of the stability, both bis(carbenes) are readily trapped by molecular oxygen to afford bis(ketones). Presumably, the reaction of the upper-lying localized quintet states with oxygen is much faster than that for lower-lying states.     

Singlet vinylcarbenes: spectroscopy and photochemistry

The first direct spectroscopic characterization of singlet vinylcarbenes is reported. Several vinyl amidines were converted to the corresponding vinylchlorodiazirines through straightforward Graham oxidation. Irradiation of the diazirines at 8 K in N2 matrices gave the corresponding singlet vinylchlorocarbenes, which could be characterized by IR, UV/vis, and calculational modeling. Hence, vinylchlorocarbene, 1-methylvinylchlorocarbene, and 1-cyclopentenylchlorocarbene were all generated and investigated. The spectra are consistent with localized carbene structures compared to the very delocalized triplet vinylcarbenes. In all cases, the carbenes readily cyclize to the corresponding cyclopropenes on visible irradiation, together with H-shift in the parent vinylchlorocarbene. In the cyclopentenyl system, cyclization leads to a highly strained bicyclo[3.1.0]hexene.     

Di(triptycyl)carbene: a fairly persistent triplet dialkylcarbene

The title carbene was generated and characterized by matrix spectroscopy (ESR and UV/vis) and laser flash photolysis along with theoretical calculations, which revealed interesting effects of triptycyl groups on structure and reactivities of carbenes.     

DFT studies of biarylcarbenes and the carbene-biradical continuum

We use electronic structure modeling, mainly density functional methods, to characterize a variety of long-lived bisaryl triplet carbenes. The bisaryl systems have a triplet ground state, favored by a substantial delocalization of the spin into the aromatic rings. One can imagine two extreme cases of the bonding in these species, representable as valence isomers. In the first case, spin delocalization is minor and incidental; the spin is predominantly located on the central carbene carbon. In the second case, spins are separated by large distances and are found primarily on the aromatic rings; the rings are linked by an allenic bridge. The bisphenyl carbenes tend toward the first limit. They can be kinetically stabilized by ortho substituents which shield the reactive center and para substituents which protect that site which has substantial spin density. The bond angle at carbene is opened from about 142 degrees (the B3LYP/6-31G value for the parent bis(phenyl)carbene) to 160 degrees or more by these substituents. Bisanthryl carbenes illustrate the second extreme, favoring a D(2)(d)() structure and possessing a low-lying open shell singlet state. A hypothetical phenyl-9-anthrylcarbine lies between the carbine and diradical extemes. The same principle which guides the design of stabilized diphenylcarbene carbenes and substitution of reactive sites by bulky protective groups serves to stabilize the bis-9-anthrylcarbene biradical.     

Azulenylcarbenes: Rearrangements on the C11H8 Potential Energy Surface

Four isomeric azulenylcarbenes were synthesized in argon matrices by photolysis of the corresponding diazo precursors, and the photochemistry of these carbenes was studied. The carbenes and their rearranged products were characterized by IR, UV/Vis, and EPR spectroscopy, and the experimental data were compared to results from DFT calculations. While 2‐, 5‐ and 6‐azulenylcarbene show triplet ground states, 1‐azulenylcarbene exhibits a singlet ground state, in accord with theoretical predictions. The rearrangements of the azulenylcarbenes give access to a number of unusual C₁₁H₈ isomers, such as other carbenes and strained allenes.     

Reactive carbon-chain molecules: synthesis of 1-diazo-2,4-pentadiyne and spectroscopic characterization of triplet pentadiynylidene (H-C[triple bond]C-:C-C[triple bond]C-H)

1-Diazo-2,4-pentadiyne (6a), along with both monodeuterio isotopomers 6b and 6c, has been synthesized via a route that proceeds through diacetylene, 2,4-pentadiynal, and 2,4-pentadiynal tosylhydrazone. Photolysis of diazo compounds 6a-c (lambda > 444 nm; Ar or N2, 10 K) generates triplet carbenes HC5H (1) and HC5D (1-d), which have been characterized by IR, EPR, and UV/vis spectroscopy. Although many resonance structures contribute to the resonance hybrid for this highly unsaturated carbon-chain molecule, experiment and theory reveal that the structure is best depicted in terms of the dominant resonance contributor of penta-1,4-diyn-3-ylidene (diethynylcarbene, H-C[triple bond]C-:C-C[triple bond]C-H). Theory predicts an axially symmetric (D(infinity h)) structure and a triplet electronic ground state for 1 (CCSD(T)/ANO). Experimental IR frequencies and isotope shifts are in good agreement with computed values. The triplet EPR spectrum of 1 (absolute value(D/hc) = 0.6157 cm(-1), absolute value(E/hc) = 0.0006 cm(-1)) is consistent with an axially symmetric structure, and the Curie law behavior confirms that the triplet state is the ground state. The electronic absorption spectrum of 1 exhibits a weak transition near 400 nm with extensive vibronic coupling. Chemical trapping of triplet HC5H (1) in an O2-doped matrix affords the carbonyl oxide 16 derived exclusively from attack at the central carbon.     

Preparation of a copper ion complex of sterically congested diphenyldiazomethanes having a pyridine ligand and characterization of their photoproducts

To show that persistent high-spin polycarbenes can be realized by utilizing hetero spin systems, two diphenyldiazomethanes having pyridyl groups, i.e., bis{4-(4-pyridyl)-2,6-dimethylphenyl}diazomethane (4,4'-DPy-1-N(2)) and {2,4-di(4-pyridyl)-6-bromophenyl}(2,6-dimethyl-4-tert-butylphenyl)diazomethane (2,4-DPy-1-N(2)), were prepared. Triplet carbenes, 4,4'-DPy-1 and 2,4-DPy-1, generated by photolysis of the corresponding diazomethanes were characterized by spectroscopic means (ESR and UV/vis in matrix at low temperatures and laser flash photolysis in solution at room temperature). The results showed that they were fairly persistent. Magnetic properties of the photoproducts from a 1:1 complex between DPy-1-N(2) and Cu(hfac)(2) (hfac = hexafluoroacetylacetonate) were characterized by ESR and a superconducting quantum interference device (SQUID) magneto/susceptometer. The field dependences of magnetization for the complexes, expressed by using M versus H/T plots, were analyzed in terms of the Brillouin function to be S = 6.80 (F = 0.60) for the 1:1 complex of 4,4'-DPy-1 and Cu(hfac)(2) and S = 3.71 (F = 0.73) for the 1:1 complex of 2,4-DPy-1 and Cu(hfac)(2) at 2.0 K. Thus, it has been demonstrated that a high-spin species is actually generated in the photoproducts and that the complexed carbenes showed significant stability.     

Electronic stabilization of ground state triplet carbenes

Based on systematic ab initio (CCSD(T)/cc-pVDZ) studies of substituent effects, we present a concept for the construction of electronically stabilized triplet ground state carbenes with singlet-triplet energy separations (DeltaEST) exceeding that of methylene. Sterically demanding and conjugating substituents were excluded from the selection of model compounds under investigation, as these either destabilize both the singlet and the triplet states or delocalize unpaired spins away from the carbene carbon. Negative partial charges on the carbene center allow for the prediction of the electronic stabilization of substituted carbenes. To decrease carbene reactivity, we chose beta-substituents with strong polar bonds. Among them, highly electronegative elements such as fluorine and oxygen enlarge the DeltaEST value with respect to hydrogen, while chlorine does not due to p-orbital participation.     

Photochemistry of furyl- and thienyldiazomethanes: spectroscopic characterization of triplet 3-thienylcarbene

Photolysis (λ > 543 nm) of 3-thienyldiazomethane (1), matrix isolated in Ar or N(2) at 10 K, yields triplet 3-thienylcarbene (13) and α-thial-methylenecyclopropene (9). Carbene 13 was characterized by IR, UV/vis, and EPR spectroscopy. The conformational isomers of 3-thienylcarbene (s-E and s-Z) exhibit an unusually large difference in zero-field splitting parameters in the triplet EPR spectrum (|D/hc| = 0.508 cm(-1), |E/hc| = 0.0554 cm(-1); |D/hc| = 0.579 cm(-1), |E/hc| = 0.0315 cm(-1)). Natural Bond Orbital (NBO) calculations reveal substantially differing spin densities in the 3-thienyl ring at the positions adjacent to the carbene center, which is one factor contributing to the large difference in D values. NBO calculations also reveal a stabilizing interaction between the sp orbital of the carbene carbon in the s-Z rotamer of 13 and the antibonding σ orbital between sulfur and the neighboring carbon-an interaction that is not observed in the s-E rotamer of 13. In contrast to the EPR spectra, the electronic absorption spectra of the rotamers of triplet 3-thienylcarbene (13) are indistinguishable under our experimental conditions. The carbene exhibits a weak electronic absorption in the visible spectrum (λ(max) = 467 nm) that is characteristic of triplet arylcarbenes. Although studies of 2-thienyldiazomethane (2), 3-furyldiazomethane (3), or 2-furyldiazomethane (4) provided further insight into the photochemical interconversions among C(5)H(4)S or C(5)H(4)O isomers, these studies did not lead to the spectroscopic detection of the corresponding triplet carbenes (2-thienylcarbene (11), 3-furylcarbene (23), or 2-furylcarbene (22), respectively).     

A singlet aryl-CF3 carbene: 2-benzothienyl(trifluoromethyl)carbene and interconversion with a strained cyclic allene

Matrix-isolated 2-benzothienyl(trifluoromethyl)carbene was generated by irradiation of the corresponding diazirine, and characterized by IR and UV/vis spectroscopy, in situ trapping, and DFT modeling. Experiments and calculations indicate that the carbene is a ground-state singlet, in contrast to previously characterized aryl(trifluoromethyl)carbenes. The carbene could be interconverted photochemically with a ring-opened thioquinomethide and a highly strained cyclic allene.     

Triplet diphenylcarbenes protected by (trimethylsilyl)ethynyl groups

(2,4,6-Tribromophenyl)(4-tert-butyl-2,6-dimethylphenyl)diazomethane (1a) was shown to be stable enough to survive Sonogashira coupling reaction conditions at an elevated temperature and gave not only a para-monosubstituted product, (4-trimethylsilylethynyl-2,6-dibromophenyl)(4-tert-butyl-2,6-dimethylphenyl)diazomethane (1b), but also a disubstituted one, [2,4-bis(trimethylsilylethynyl)-6-bromophenyl](4-tert-butyl-2,6-dimethylphenyl)diazomethane (1c), and a trisubstituted product, [2,4,6-tris(trimethylsilylethynyl)phenyl](4-tert-butyl-2,6-dimethylphenyl)diazomethane (1d). Triplet diphenylcarbenes (DPCs) generated by photolysis of those ethynylated diphenyldiazomethanes were characterized by ESR and UV-vis spectroscopies at low temperature and laser flash photolysis techniques in solution at room temperature. Although ESR data indicated that ethynyl groups at the ortho positions are likely to stabilize triplet DPCs both sterically and electronically more effectively than o-bromine groups, kinetic studies suggested that the stability of triplet DPCs is not increased by o-ethynyl groups, as opposed to o-bromine groups. It is likely that triplet DPCs decay by interacting with the o-ethynyl groups.     

The beta effect of silicon in phenyl cations

Irradiation of chloroanisoles, phenols, and N,N-dimethylanilines bearing a trimethylsilyl (TMS) group in the ortho position with respect to the chlorine atom caused photoheterolysis of the Ar-Cl bond and formation of the corresponding ortho-trimethylsilylphenyl cations in the triplet state. The beta effect of silicon on these intermediates has been studied by comparing the resulting chemistry in alcoholic solvents with that of the silicon-free analogues and by computational analysis (at the UB3LYP/6-311+G(2d,p) level in MeOH). TMS groups little affect the photophysics and the photocleavage of the starting phenyl chlorides, while stabilizing the phenyl cations, both in the triplet (ca. 4 kcal/mol per group) and, dramatically, in the singlet state (9 kcal/mol). As a result, although triplet phenyl cations are the first formed species, intersystem crossing to the more stable singlets is favored with chloroanisoles and phenols. Indeed, with these compounds, solvent addition to give aryl ethers (from the singlet) competed efficiently with reduction or arylation (from the triplet). In the case of the silylated 4-chloro-N,N-dimethylaniline, the triplet cation remained in the ground state and trapping by pi nucleophiles remained efficient, though slowed by the steric bulk of the TMS group. In alcohols, the silyl group was eliminated via a photoinduced protiodesilylation during the irradiation. Thus, the silyl group could be considered as a directing, photoremovable group that allowed shifting to the singlet phenyl cation chemistry and was smoothly eliminated in the same one-pot procedure.     

Reactions of carbenes in solidified organic molecules at low temperature

Studies on reactions of carbenes in reactive organic glasses at low temperatures clearly reveal that solution results and liquid phase mechanistic rules cannot be readily extrapolated to matrix conditions. Thus, the usual course of reaction of a carbene with an alkene in solution results in the formation of a cyclopropane for both the singlet and triplet states although a one-step addition possible for singlet carbene produces the cyclopropane stereospecifically and a stepwise pathway with the triplet state affords two possible stereoisomers of the cyclopropane. In a sharp contrast, the formal insertion products into the allylic C-H bonds of alkenes are produced at the expense of the cyclopropane when carbene is generated in alkene matrix at low temperature. Similar results are obtained in the reaction with alcohols, where the C-H insertion products are formed in low temperature alcoholic matrices at the expense of the O-H insertion products which are predominant products in the reaction with alcoholic solution at ambient temperature. The ¹³C labelling experiments as well as deuterium kinetic isotope effects suggest that these C-H insertion products are most probably produced from the triplet carbene, not from the singlet, by abstraction of H atom from the matrix followed by the recombination of the resulting radical pairs. Kinetic studies using ESR and laser flash photolysis techniques demonstrate that the mechanism of a H-atom transfer reaction changes from a completely classical process in a soft warm glass to a completely quantum mechanical tunneling process in a cold hard glass. Thus, as the reaction temperature is lowered, the classical reaction rate decreases, and eventually becomes much slower than decay by hydrogen atom tunneling. The members of the radical pairs which usually diffuse apart in a fluid solution are not able to diffuse apart owing to the limited diffusibility within a rigid matrix and therefore recombine with high efficiency to give the CH “insertion” products. A rather surprising and intriguing difference between the C-H insertion undergone by singlet carbenes in fluid solution at ambient temperatures and one by triplet carbenes in matrix at low temperature is noted. Thus, a marked increase in the primary and secondary C-H insertion over the tertiary is observed in the matrix reaction indicating that triplet carbenes tend to abstract H from less crowded C-H bonds. This is interpreted to indicate that the distance between carbenic center and tunneling H becomes important in H atom tunneling process. More surprisingly, the C-H insertion by triplet carbene by the abstraction-recombination mechanism in a rigid matrix proceeds with retention of the configuration, suggesting that the solid state prevents motion of the radicals to the extent that does not allow racemization to occur. Reactions with heteroatom substrates such as ethers, amines, alkyl halides and ketones are also subject to the matrix effects and the C-H insertion products increase at the expense of singlet carbene reaction products resulting from the interaction with the heteroatoms. Stereoselectivities of cyclopropanation to styrenes are also shown to be affected by the matrix effects. t-Butyl alcohol matrix is shown to be unreactive toward carbenes and thus can be used as a “solvent” in matrix carbene reactions presumably due to a large inert guest cavity provided by bulky tertiary alcohol which binds a molecular aggregate inside it. H atom tunneling in the matrix is also shown to compete with very efficient intramolecular migration of hydrogen to the carbenic center. Migration aptitude as well as stereochemistry are also found to be subject to the matrix effects.     

Preparation of oligodiazo compounds by using the suzuki coupling reaction and characterization of their photoproducts

[9-[10-(4-tert-Butyl-2,6-dimethyl)phenyl]anthryl](4-bromo-2,6-dimethylphenyl)diazomethane was found to be stable enough to survive under Suzuki coupling conditions and underwent mono-, di-, and trisubstitution with benzene mono-, di-, and triboronic acids to afford benzene derivatives having one, two, and three diazo units, respectively. The products from irradiation of those diazo compounds were characterized by ESR and SQUID measurements, which revealed that triplet, quintet, and septet ground states were formed from mono-, bis-, and tris(diazo) compounds, respectively. The stability of those high-spin species was estimated by temperature-dependent ESR and UV/vis measurements as well as laser flash photolysis, which indicated that all three species are stable up to 160 K and have a half-life of a few seconds in solution at room temperature. The finding unequivocally shows that a precursor diazo unit can basically be handled as a building block to construct ploydiazo compounds and that persistent triplet carbenes, even though they greatly lose typical reactivity as a triplet carbene, still retain electronic properties and act as a spin source when aligned properly in the pi-electron frameworks to generate a high-spin molecule with remarkable thermal stability.     

Ferromagnetic spin alignment in head-to-tail coupled oligo(1, 4-phenyleneethynylene)s and Oligo(1,4-phenylenevinylene)s bearing pendant p-phenylenediamine radical cations

The intramolecular and long-range ferromagnetic coupling between p-phenylenediamine radical cations in head-to-tail coupled oligo(1, 4-phenyleneethynylene)s and oligo(1,4-phenylenvinylene)s between neighbors and next-nearest neighbors is described. UV/vis/near-IR experiments show that the radical cations are localized in the pendant p-phenylenediamine units of the conjugated oligomers. The ESR spectra of these oligo(1,4-phenyleneethynylene) and oligo(1, 4-phenylenvinylene) di(radical cation)s are consistent with those of a triplet state. A linear behavior is observed for the doubly integrated ESR intensity of the DeltaM(s) = +/-1 and DeltaM(s) = +/-2 signals with the inverse temperature (I approximately 1/T), consistent with Curie's law. This behavior indicates a triplet ground-state diradical with a large triplet-singlet energy gap or possibly a degeneracy of singlet and triplet states.