Synthesis and characterization of carbene derivatives of Th@C3v(8)-C82 and U@C2v(9)-C82: exceptional chemical properties induced by strong actinide–carbon cage interaction

Chemical functionalization of endohedral metallofullerenes (EMFs) is essential for the application of these novel carbon materials. Actinide EMFs, a new EMF family member, have presented unique molecular and electronic structures but their chemical properties remain unexplored. Here, for the first time, we report the chemical functionalization of actinide EMFs, in which the photochemical reaction of Th@C_3v(8)-C₈₂ and U@C_2v(9)-C₈₂ with 2-adamantane-2,3′-[3H]-diazirine (AdN₂, 1) was systematically investigated. The combined HPLC and MALDI-TOF analyses show that carbene addition by photochemical reaction afforded three isomers of Th@C_3v(8)-C₈₂Ad and four isomers of U@C_2v(9)-C₈₂Ad (Ad = adamantylidene), presenting notably higher reactivity than their lanthanide analogs. Among these novel EMF derivatives, Th@C_3v(8)-C₈₂Ad(I, II, III) and U@C_2v(9)-C₈₂Ad(I, II, III) were successfully isolated and were characterized by UV-vis-NIR spectroscopy. In particular, the molecular structures of first actinide fullerene derivatives, Th@C_3v(8)-C₈₂Ad(I) and U@C_2v(9)-C₈₂Ad(I), were unambiguously determined by single crystal X-ray crystallography, both of which show a [6,6]-open cage structure. In addition, isomerization of Th@C_3v(8)-C₈₂Ad(II), Th@C_3v(8)-C₈₂Ad(III), U@C_2v(9)-C₈₂Ad(II) and U@C_2v(9)-C₈₂Ad(III) was observed at room temperature. Computational studies suggest that the attached carbon atoms on the cages of both Th@C_3v(8)-C₈₂Ad(I) and U@C_2v(9)-C₈₂Ad(I) have the largest negative charges, thus facilitating the electrophilic attack. Furthermore, it reveals that, compared to their lanthanide analogs, Th@C_3v(8)-C₈₂ and U@C_2v(9)-C₈₂ have much closer metal–cage distance, increased metal-to-cage charge transfer, and strong metal–cage interactions stemming from the significant contribution of extended Th-5f and U-5f orbitals to the occupied molecular orbitals, all of which give rise to their unusual high reactivity. This study provides first insights into the exceptional chemical properties of actinide endohedral fullerenes, which pave ways for the future functionalization and application of these novel EMF compounds.

Photochemical reaction of Th@C_3v(8)-C₈₂ and U@C_2v(9)-C₈₂ with 2-adamantane-2,3′-[3H]-diazirine (AdN₂, 1) afforded three isomers of Th@C_3v(8)-C₈₂Ad and four isomers of U@C_2v(9)-C₈₂Ad (Ad = adamantylidene), respectively.     

Interaction of Novel Anionic Gemini Surfactants with Dodecyl Sodium Sulfate in Aqueous NaCl Medium and the Performance of Their Mixtures

The interaction in two mixtures of two novel anionic gemini surfactants, sodium 2,2′-(6,6′-(ethane-1,2-diylbis(azanediyl)bis(4-(hexylamino)-1,3,5-triazine-6,2-diyl)bis(azanediyl)diethanesulfonate (C₆-2-C₆) and sodium 2,2′-(6,6′-(ethane-1,2-diylbis(azanediyl)bis(4-(octylamino)-1,3,5-triazine-6,2-diyl) bis(azanediyl) diethanesulfonate (C₈-2-C₈), and conventional anionic surfactants, sodium dodecyl sulfate (SDS), have been investigated in 0.1 M NaCl aqueous solutions. The mixed systems are C₆-2-C₆/SDS and C₈-2-C₈/SDS, and the mole factions (α_G) of geminis are 0.1, 0.3, 0.5, 0.7, and 0.9, respectively. Mixtures of both C₆-2-C₆/SDS and C₈-2-C₈/SDS exhibit synergism in surface tension reduction efficiency and mixed micelle formation. But, all mixtures except C₆-2-C₆/SDS (α_G = 0.7), C₆-2-C₆/SDS (α_G = 0.9), and C₈-2-C₈/SDS (α_G = 0.1) don't exhibit synergism in surface tension reduction effectiveness. The performances, such as wetting, emulsification, and dispersion were measured and the results showed all mixtures posses application properties.     

Molecular structures of phosphorus compounds: Part IX. Gas-phase structure of 2,2-dimethylpropylidynephosphine, (CH3)3C-CP

The geometric structure of 2,2-dimethylpropylidynephosphine, (CH₃)₃C-CP, in the gas phase has been determined by joint analysis of electron diffraction and microwave spectroscopy data. The following parameters (r_av values) were obtained: PC₁ = 1.536(2), C₁-C₂ = 1.473(4),C₂-C₃ (methyl) = 1.543(2), C₃-H = 1.080(3) Å, ∠C₁-C₂-C₃ = 109.0(2) and ∠H-C₃H = 108.8(5)°. Error limits are 2σ values.     

Effect of ethylene glycol on micellization and micellar-catalyzed alkaline hydrolysis reaction of a cationic surfactant at 293–313 K

Changes in critical micellar concentrations (CMC’s) of gemini surfactant, α, ω-ethane bis(dimethyl cetyl ammonium bromide) (C₁₆-2-C₁₆) with different concentrations of ethylene glycol (EG) addition have been investigated by electrical conductivity method. Subsequently, alkaline hydrolysis of ethyl acetate (EA) in the presence of C₁₆-2-C₁₆ and C₁₆-2-C₁₆-EG has been studied conductometrically at 303.2 and 313.2 K, respectively. It was found that an increase in concentrations of EG added to C₁₆-2-C₁₆ aqueous solutions caused an increase in CMC’s of C₁₆-2-C₁₆, provoked by the decrease in the interfacial Gibbs energy contribution to G _M^∘. The hydrolysis of EA showed catalytic and restrained dual behavior in the presence of surfactant, it may be related to higher microviscosity and change of morphology with increased surfactant for C₁₆-2-C₁₆ at higher concentration. Addition of EG did not change microenvironment in micellar interfacial region significantly, which had less effect on gemini C₁₆-2-C₁₆ micellar catalytic efficiency.     

Crystallographic and Theoretical Investigations of Er2@C2 n (2 n=82, 84, 86): Indication of Distance-Dependent Metal–Metal Bonding Nature

Successful isolation and characterization of a series of Er-based dimetallofullerenes present valuable insights into the realm of metal–metal bonding. These species are crystallographically identified as Er₂@C_s(6)-C₈₂, Er₂@C_3v(8)-C₈₂, Er₂@C₁(12)-C₈₄, and Er₂@C_2v(9)-C₈₆, in which the structure of the C₁(12)-C₈₄ cage is unambiguously characterized for the first time by single-crystal X-ray diffraction. Interestingly, natural bond orbital analysis demonstrates that the two Er atoms in Er₂@C_s(6)-C₈₂, Er₂@C_3v(8)-C₈₂, and Er₂@C_2v(9)-C₈₆ form a two-electron-two-center Er−Er bond. However, for Er₂@C₁(12)-C₈₄, with the longest Er⋅⋅⋅Er distance, a one-electron-two-center Er−Er bond may exist. Thus, the difference in the Er⋅⋅⋅Er separation indicates distinct metal bonding natures, suggesting a distance-dependent bonding behavior for the internal dimetallic cluster. Additionally, electrochemical studies suggest that Er₂@C_82–86 are good electron donors instead of electron acceptors. Hence, this finding initiates a connection between metal–metal bonding chemistry and fullerene chemistry.     

气相色谱-质谱表征甲醇制烯烃副产汽油中的C5～C7烯烃

采用气相色谱-质谱(GC-MS)对甲醇制烯烃(MTO)副产汽油中的C5~C7烯烃进行了详细的定性表征,对MTO副产汽油中的49个单烯烃、11个二烯烃和9个环烯烃共计69个C5~C7烯烃组分在聚甲基硅氧烷柱上的保留指数进行了测定和定性确认。根据GC-MS定性分析结果建立了MTO副产汽油中C5~C7烯烃的保留指数数据库,采用气相色谱对副产汽油中C5~C7烯烃组分进行了定量分析。定量结果表明:MTO汽油以C5~C7脂肪族烯烃为主,含有少量的二烯烃和环烯烃,烷烃和环烷烃含量很少。MTO副产汽油中C5~C7烯烃的详细表征为其综合利用提供了依据。     

The structure of 1,2,3,3-tetrachloro-4,5-dimethyl spiro-[2.3] -hexa-1,4-diene

An electron diffraction study of 1,2,3,3-tetrachloro-4,5-dimethyl spiro[2.3]-hexa-1,4-diene (Spiro) in the gas phase, confirmed the atom connectivities and led to a structure with roughly tetrahedral bonding about the spiro-carbon atom. The internuclear distances were obtained with moderate precision. The optimum least squares fitting of the experimental molecular scattering function gave a model with C_s symmetry. The r_g values are: (C₃-C₆) = 1.54 (.10) Å (assumed), (C₂-C₃) = 1.52 (.02) Å, <CC >_av= 1.35, (.010) Å, (C₅-C₆) = 1.46 (.02) Å, (C₄-C₇) = 1.52 (.03) Å, (C-Cl) = 1.69 (.03) Å and (-C-Cl) = 1.77 (.03) Å. The angle (θ) between the C₃-C₆ bond and the cyclopropene plane is 127.5°. The large value for <CC >_av is compared with the X-ray crystal structure result, and with the structures of other cyclopropenes and cyclobutenes.     

Preferential Formation of Mono-Metallofullerenes Governed by the Encapsulation Energy of the Metal Elements: A Case Study on Eu@C2n (2n=74–84) Revealing a General Rule

Encapsulating one to three metal atoms or a metallic cluster inside fullerene cages affords endohedral metallofullerenes (EMFs) classified as mono-, di-, tri-, and cluster-EMFs, respectively. Although the coexistence of various EMF species in soot is common for rare-earth metals, we herein report that europium tends to prefer the formation of mono-EMFs. Mass spectroscopy reveals that mono-EMFs (Eu@C_2n) prevail in the Eu-containing soot. Theoretical calculations demonstrate that the encapsulation energy of the endohedral metal accounts for the selective formation of mono-EMFs and rationalize similar observations for EMFs containing other metals like Ca, Sr, Ba, or Yb. Consistently, all isolated Eu-EMFs are mono-EMFs, including Eu@D_3h(1)-C₇₄, Eu@C_2v(19138)-C₇₆, Eu@C_2v(3)-C₇₈, Eu@C_2v(3)-C₈₀, and Eu@D_3d(19)-C₈₄, which are identified by crystallography. Remarkably, Eu@C_2v(19138)-C₇₆ represents the first Eu-containing EMF with a cage that violates the isolated-pentagon-rule, and Eu@C_2v(3)-C₇₈ is the first C₇₈-based EMF stabilized by merely one metal atom.     

Isolation and Crystallographic Characterization of Two,Nonisolated Pentagon Endohedral Fullerenes: Ho3N@C2(22010)-C78 and Tb3N@C2(22010)-C78

Purified samples of Ho₃N@C₂(22010)-C₇₈ and Tb₃N@C₂(22010)-C₇₈ have been isolated by two distinct processes from the rich array of fullerenes and endohedral fullerenes present in carbon soot from graphite rods doped with Ho₂O₃ or Tb₄O₇. Crystallographic analysis of the endohedral fullerenes as cocrystals with Ni(OEP) (in which OEP is the dianion of octaethylporphyrin) shows that both molecules contain the chiral C₂(22010)-C₇₈ cage. This cage does not obey the isolated pentagon rule (IPR) but has two sites where two pentagons share a common C−C bond. These pentalene units bind two of the metal ions, whereas the third metal resides near a hexagon of the cage. Inside the cages, the Ho₃N or Tb₃N unit is planar. Ho₃N@C₂(22010)-C₇₈ and Tb₃N@C₂(22010)-C₇₈ use the same cage previously found for Gd₃N@C₂(22010)-C₇₈ rather than the IPR-obeying cage found in Sc₃N@D_3h-C₇₈.     

The preparation and properties of metallofluorene complexes of transition metals.

Metalluorene complexes (π-C₅H₅)M(CO)(C₁₂H₈) (IVa)-(IVc) and (π-C₅H₅) (CO)(C₁₂F₈) (IVd)-(IVf) (M = Co, Rh and Ir) have been prepared from reactions of the appropriate (π-cyclopentadienyl) carbonylmetal diiodides with 2,2′-dilithhiolbiphenyl (IIa) and 2,2′-dilithiooctafluorobiphenyl (IIb), respectively The triphenylphoshine substitution reactions of cobalt compounds (IVa) and (IVd) have also been studied. Reaction of (IIa) and (IIb) with norbornadieneplatinum dichloride result in the preparation of metallocyclic platinum compounds (π-C₇H₈) and (π-C₇H₈)Pt(C₁₂H₈). A reaction of (IIb) with zirconocene dichloride produces (π-C₅H₅)₂Zr(C₁₂F₈), the first example of a ziconium-containing metalofluorene.     

Nonglyceride lipids of the seed coats ofArtemisia absinthium andOnopordum acanthium

The seed coats ofArtemisia absinthium andOnoporum acanthium have been shown to contain paraffinic hydrocarbons (C₃₁-C₂₅ and C₃₁-C₂₇, respectively), wax esters (C₃₂-C₂₃ and C₅₁-C₃₃), acetyltriterpenols (six types, five types), and acyltriterpenols with a predominance of palmitoyltriterpenols (esters of higher saturated unbranched C_32:0-C_12:0 fatty acids and triterpenols with a molecular weight of 426 belonging to the ursene group).Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 154–157, March–April, 1981.     

Synthetic studies on bafilomycin A1: stereoselective synthesis of the C12-C17 fragment and its coupling with the C1-C11 subunit

Stereoselective addition of (E)-1-lithio-2-tributylstannylethylene on a chiral cyclic di-t-butylsilyleneketal C₁₄-C₁₇ aldehyde afforded the required Felkin-Anh adduct for the synthesis of the C₁₂-C₁₇ fragment of bafilomycin A₁, the configuration of which was assigned unambiguously. After appropriate coupling with the enantiopure C₁-C₁₁ fragment, the C₁₂-C₁₇ subunit obtained here can be used for the study of the 16-membered macrolide formation either by an acyl activation or an intramolecular Stille reaction. Intermolecular esterification of the 15-OH with an acyl activation of the carboxylic acid of the C₁-C₁₁ fragment, in modified Yamaguchi's conditions, affords here an intermediate for examining an intramolecular Stille coupling.     

Reactions of metal carbonyl derivatives X. The synthesis and reactivity of some dinuclear derivatives of iron containing both bridging carbonyls and bridging phosphido or sulphido groups

The tertiary phosphine π-C₅H₅Fe(CO)₂P(C₆H₅)₂ reacts with a suspension of Fe₂(CO)₉ in benzene to give the dinuclear complex π-C₅H₅Fe₂P(C₆H₅)₂(CO)₆. This compound is also obtained by nucleophilic attack of [π-C₅H₅Fe(CO)₂]⁻ on Fe(CO)₄-[P(C₆H₅)₂Cl] in tetrahydrofuran. Irradiation of a benzene solution of π-C₅H₅Fe₂-P(C₆H₅)₂(CO)₆ with ultraviolet light affords π-C₅H₅Fe₂P(C₆H₅)₂(CO)₅ which contains both a bridging carbonyl and a bridging phosphido group. The unstable bridged sulphido derivatives π-C₅H₅Fe₂SR(CO)₆ (R = CH₃ and C₆H₅) and π-C₅H₅Fe₂(t-C₄H₉S)(CO)₅ are similarly obtained employing π-C₅H₅Fe(CO)₂SR as ligand. The reactions of π-C₅H₅Fe₂P(C₆H₅)₂(CO)₅ with tertiary phosphines and phosphites yield three types of products depending on the reaction conditions and the ligand involved. Examples include π-C₅H₅Fe₂P(C₆H₅)₂(CO)₄P(C₆H₅)₃, a mono-substituted derivative of π-C₅H₅Fe₂P(C₆H₅)₂(CO)₅, and π-C₅H₅Fe₂P(C₆H₅)₂(CO)₅P(C₂H₅)₃ and π-C₅H₅Fe₂P(C₆H₅)₂(CO)₄[P(OCH)₃)₃]₂, mono- and bis-substituted derivatives of π-C₅H₅Fe₂P(C₆H₅)₂(CO)₆, respectively. The reaction of π-C₅H₅Fe₂P(C₆H₅₂(CO)₅ with (C₆H₅)₂PCH₂P(C₆H₅)₂ in benzene under reflux affords [π-C₅H₅Fe₂P(C₆H₅)₂(CO)₄](C₆H₅)₂PCH₂P(C₆H₅)₂ in which the ditertiary phosphine bridges two iron atoms.     

The molecular structures of mono-substituted cl-cyclohexene by gas-phase electron diffraction

The molecular structures of mono-substituted chlorocyclohexene are determined by gas-phase electron diffraction. The structural parameters are obtained by applying leastsquares analysis to the molecular scattering intensities. The bond distances (r_g) and bond angles are: (1) 1-Cl-cyclohexene: C₁C₂ = 1.336 ± 0.006 Å. C₂-C₃ = 1.500 ± 0.009 Å, C₃-C₄ = 1.533 ± 0.010 Å, C₄-C₅ = 1.537 Å, C₅-C₆ = 1.527 ± 0.010 Å, C₁-C₆ = 1.504 ± 0.009 Å. C-Cl = 1.747 ± 0.005 Å, C-H_av = 1.138 ± 0.010 Å, ∠Cl-cc = 126.3 ± 0.5°, ∠C₆C₁C₂ = 123.9 ± 0.8°. ∠C₁C₂C₃= 124.6 ± 0.8°, ∠C₄C₃C₂ = 111.8 ± 1.2° and ∠-C₅C₆C₁ = 111.3 ± 1.1°; (2) 3-Cl-cyclohexene: C₁=C₂ = 1.336 Å, C₂-C₃ = 1.501 ± 0.010 Å, C₃-C₄ = 1.513 ± 0.008 Å, C₄-C₅ = 1.542 Å, C₅-C₆, = 1.516 ± 0.007 Å, C₁-C₆ = 1.505 ± 0.006 Å, C-C1 = 1.801 ± 0.005 Å, C-H_av = 1.120 ± 0.008 Å, ∠C₆C₁C₂ = 123.2 ± 1.0°, ∠C₁C₂C₃ = 124.1 ± 1.7°, ∠C₅C₆C₁ = 113.0 ± 1.3°, ∠C₂C₃C₄ = 112.5 ± 1.5° ∠ClC₃C₂ = 110.3 ± 0.8°, ∠H-C=C = 123.0 ± 3.0° and ǒH-C-C = 109.5 ± 2.0°, with a mixture of 55% axial and 45% equatorial conformers; (3) 4-Cl-cyclohexene: C₁=C₂ = 1.336 Å, C₂-C₃ = 1.507 ± 0.007 Å, C₃-C₄ = 1.516 ± 0.008 Å, C₄-C₅ = 1.544 Å, C₅-C₆ = 1.523 ± 0.010 Å, C₁- C₆ = 1-507 Å, C-Cl = 1.799 ± 0.005 Å, C-H_av = 1.116 ± 0.005 Å, ∠C₆C₁C₂ = 123.3 ± 1.5°, ∠C₅C₆C₁ = 113.0 ± 1.0°, ∠C₂C₃C₄ = 112.3 ± 1.0°, ∠ClC₄C₃ = 110.2 ± 2.0°, ∠H-CC = 117.1 ± 4.5° and ∠H-C-C = 109.5 ± 1.0°, with a mixture of 45% axial and 55% equatorial conformers.     

Gas phase structures of the isomers of benzene,V. (dewar-benzene) by electron diffraction

The parent hydrocarbon, Dewar-benzene, has been studied by gas phase electron diffraction analysis. Assignment of C_2v symmetry gave excellent agreement between the experimental and theoretical data. The structural parameters obtained were in good agreement with previous electron diffraction structures of substituted derivatives of the Dewar-benzene series. The structural parameters with error limits are (cf. Fig. 2): r(C₃-C₆) = 1.574 ± 0.005 Å r(C₂-C₃) = 1.524 ± 0.002 Å, r(C₁-C₂) = 1.345 ± 0.001 Å, r(C₃-C₉) = 1.134 ± 0.004 Å, r(C₁-C₇) = 1.124 ± 0.004 Å, ∠C₁C₆C₅ = 116.7 ± 0.6°, ∠C₃C₆C₁ = 85.7 ± 0.2°, ∠C₆C₃C₉ = 108.0 ± 3.0°, ∠C₃C₂C₈ = 126.7 ± 2.5°, and α = 117.25 ± 0.6°. The angle γ was assumed to be 0°.     

C60-C70-C6H5CH3 crystal solvates: Structure and thermal stability

Differential scanning calorimetry (DSC) and X-ray diffraction were used to study the thermal stability and structure of C₆₀-C₇₀-C₆H₅CH₃ crystal solvates synthesized at room temperature. The decomposition of the crystal solvates generates C₆₀-C₇₀ solid solutions with hexagonal close-packed (hcp) structure.     

(η4-cycloocta-1,5-diene)(η6-cycloocta-1,3,5-triene)ruthenium(0): mechanistic study on carbonylation and synthesis of (η4-cycloheptatriene)tricarbonylruthenium

The carbonylation of Ru(η⁴-C₈H₁₂)(η⁶-C₈H₁₀) (I) (C₈H₁₂ = cycloocta-1,5-diene, C₈H₁₀ = cycloocta-1,3,5-triene) occurs readily at room temperature and one atmosphere pressure of carbon monoxide. The initial product is Ru(CO)-(η⁴-C₈H₁₂)(η⁴-C₈H₁₀) (II); its formation was monitored by IR spectroscopy and shown to be first order with respect to I. Further reaction with CO produces Ru(CO)₃(η⁴-C₈H₁₂). In the presence of C₇H₈ (C₇H₈ = cycloheptatriene) two major products are formed Ru(CO)₃(η⁴-C₇H₈) and Ru(CO)₂(η⁶-C₇H₈).     

Polyfluoroorganotrifluoroborates and -difluoroboranes: interesting materials in fluoroorgano and fluoroorgano-element chemistry

The aimed introduction of the polyfluoroorgano groups (4-C₅F₄N), C₆F₁₃C₂H₄, and C₂F₅ into methoxy group-containing boron electrophiles is reported. The new compounds obtained after transformations K[(4-C₅F₄N)BF₃], (4-C₅F₄N)BF₂, K[C₆F₁₃C₂H₄BF₃], C₆F₁₃C₂H₄BF₂, K[(C₂F₅)₂B(OMe)₂], and K[(C₂F₅)₂BF₂] were isolated and characterised. Additionally some of their precursors as there are Li(4-C₅F₄N), Li[(4-C₅F₄N)B(OMe)₃], (4-C₅F₄N)B(OH)₂ and the by-products Li[(4-C₅F₄N)₂B(OMe)₂], (4-C₅F₄N)₂BOH, and K[(4-C₅F₄N)₂BF₂] are described. The usefulness of polyfluoroorganodifluoroboranes for introducing polyfluoroorgano groups into hypervalent FEF bonds is demonstrated by the synthesis of [C₆F₅(4-C₅F₄N)I][BF₄] and [p-FC₆H₄(trans-CF₃CFCF)I][BF₄].     

The solubility of C<Subscript>70</Subscript> in <Emphasis Type="Italic">n</Emphasis>-alkanols-1 C<Subscript>1</Subscript>-C<Subscript>11</Subscript> over the temperature range 20–80°C

The isothermal (20°C) solubility of fullerene C₇₀ in solvents of the homologous series of nonbranched saturated alcohols C₁-C₁₁ and polythermal (20–80°C) solubility of fullerene C₇₀ in solvents of the homologous series of nonbranched saturated alcohols C₄-C₁₁ were studied. The corresponding solubility diagrams are presented and characterized.     

The electronic structure and reactivity of small ring compounds—VII : The CMR spectra of 1-cyanobicyclo[1.1.0]butane

The 25·2 MHz CMR (FFT) spectra of 1-cyanobicyclo [1.1.0]butane are reported both with and without hydrogen decoupling. Chemical shifts are discussed in terms of dipolar resonance contributions to the ground state and the unsaturated nature of the bicyclobutane ring. The C-H coupling constants, including directly bonded and long range interactions, are reported. It is suggested that the large long range H_x′-C₂ coupling constant is a result of a favorable interaction between the H_x′-C₄ bond and C₂ bond and C₂ and that the H₃-cyano carbon coupling constant results from a 0° dihedral angle and to a small extent, if at all, from a favorable geometry. The C₁-C₂ (22 Hz) and C₁-C₃ (16 Hz) ¹³C-¹³C coupling constants are presented, compared with the calculated values, and discussed in terms of hybridization. The possibility of a negative value for the C₁-C₃ coupling constant is also considered. It is pointed out that using coupling constant-hybridization relationships and extrapolating from several known to an unknown hybridization around a carbon atom can, in many cases, lead to a sizeable error in calculated hybridization.