Insertion of C50 into single-walled carbon nanotubes: Selectivity in interwall spacing and C50 isomers

The structures and electronic properties of nanoscale "peapods," i.e., C(50) fullerenes inside single-walled carbon nanotubes (SWCNTs), were computationally investigated by density functional theory (DFT). Both zigzag and armchair SWCNTs with diameters larger than 1.17 nm can encapsulate C(50) fullerenes exothermically. Among the SWCNTs considered, (9,9) and (16,0) SWCNTs are the best sheaths for both D(3) and D(5h) isomers of C(50), corresponding to 0.32-0.34 nm tube-C50 distances. The orientation of C(50) inside nanotubes also affects the insertion energies, which depend on the actual tube-fullerene distances. The insertion of D(3) and D(5h) isomers of C(50) is somewhat selective; the less stable D(5h) isomer can be encapsulated more favorably inside SWCNTs at same tube-C(50) spacing. Because of the weak tube-C(50) interaction, the geometric and electronic structures of the peapods do not change greatly for the most stable configurations, but the selectivity in the interwall spacing and isomer encapsulation can be useful in separating various carbon fullerenes and their isomers.     

富勒烯衍生物C50X(X=SiH2, PH, S)的结构及稳定性的理论研究

用从头算HF/3-21G方法研究了C50的环加成衍生物C50X(X=SiH2, PH, S)所有可能的异构体的结构与稳定性, 计算结果表明, SiH2基团、PH基团与S原子在C50上环加成的优先加成位置相同, 都为C3—C4类键和C4—C4类键, 并且相应形成[5,6]-闭环和[5,5]-闭环结构的最稳定异构体; 决定C50X(X=SiH2, PH, S)各异构体稳定性的主要因素, 因加成位置以及发生加成反应的C—C键的单双键类型的不同, 可能是张力、共轭效应或者二者的共同作用. 进一步比较了C50X(X=SiH2, PH, S)与C50X(X=CH2, NH, O)的结构和稳定性等, 并总结出规律性的结论, 即加成原子的大小和加成位置C—C键的类型是影响形成开环或闭环结构的C50环加成衍生物的两种主要因素.     

Synthesis and X-ray or NMR/DFT structure elucidation of twenty-one new trifluoromethyl derivatives of soluble cage isomers of C76, C78, C84, and C90

Adding 1% of the metallic elements cerium, lanthanum, and yttrium to graphite rod electrodes resulted in different amounts of the hollow higher fullerenes (HHFs) C76-D2(1), C78-C2v(2), and C78-C2v(3) in carbon-arc fullerene-containing soots. The reaction of trifluoroiodomethane with these and other soluble HHFs at 520-550 degrees C produced 21 new C76,78,84,90(CF3)n derivatives (n = 6, 8, 10, 12, 14). The reaction with C76-D2(1) produced an abundant isomer of C2-(C76-D2(1))(CF3)10 plus smaller amounts of an isomer of C1-(C76-D2(1))(CF3)6, two isomers of C1-(C76-D2(1))(CF3)8, four isomers of C1-(C76-D2(1))(CF3)10, and one isomer of C2-(C76-D2(1))(CF3)12. The reaction with a mixture of C78-D3(1), C78-C2v(2), and C78-C2v(3) produced the previously reported isomer C1-(C78-C2v(3))(CF3)12 (characterized by X-ray crystallography in this work) and the following new compounds: C2-(C78-C2v(3))(CF3)8; C2-(C78-D3(1))(CF3)10 and C(s)-(C78-C2v(2))(CF3)10 (both characterized by X-ray crystallography in this work); C2-(C78-C2v(2))(CF3)10; and C1-C78(CF3)14 (cage isomer unknown). The reaction of a mixture of soluble higher fullerenes including C84 and C90 produced the new compounds C1-C84(CF3)10 (cage isomer unknown), C1-(C84-C2(11))(CF3)12 (X-ray structure reported recently), D2-(C84-D2(22))(CF3)12, C2-(C84-D2(22))(CF3)12, C1-C84(CF3)14 (cage isomer unknown), C1-(C90-C1(32))(CF3)12, and another isomer of C1-C90(CF3)12 (cage isomer unknown). All compounds were studied by mass spectrometry, (19)F NMR spectroscopy, and DFT calculations. An analysis of the addition patterns of these compounds and three other HHF(X) n compounds with bulky X groups has led to the discovery of the following addition-pattern principle for HHFs: In general, the most pyramidal cage C(sp(2)) atoms in the parent HHF, which form the most electron-rich and therefore the most reactive cage C-C bonds as far as 1,2-additions are concerned, are not the cage C atoms to which bulky substituents are added. Instead, ribbons of edge-sharing p-C6(X)2 hexagons, with X groups on less pyramidal cage C atoms, are formed, and the otherwise "most reactive" fullerene double bonds remain intact.     

Valence of D5h C50 fullerene

The energetic and electronic properties of D5h C50 before and after passivation by H or Cl are investigated using first-principle computational method of density functuional theory with generalized gradient approximation and local density approximation functionals. The results show that H or Cl addition can lead to energetic stabilization. Additions also increase the highest occupied molecular orbit-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of C50 fullerides and make them chemically more stable. In the series of C50H2m (m = 0 approximately 7), the Saturn-shaped D5h C50H10 has the largest HOMO-LUMO gap, which suggests that such a structure of C50H10 is a "magic-number" stable one of C50 adducts, and ten is a pseudovalence or effective valence of C50 fullerene pseudoatom. This point also is supported by the energetic properties of C50H2m series such as binding energies, etc. A minimal energy reaction pathway is constructed to get C50H10 and C50H14. Some useful experience for determining the favorable addition sites was summarized. A simple steric method is developed to predict the effective valences of classical fullerenes.     

Properties of fullerene[50] and D5h decachlorofullerene[50]: a computational study

Stimulated by the recent preparation and characterization of the first [50]fullerene derivative, decachlorofullerene[50] (Science 2004, 304, 699), we have performed a systematic density functional study on the electronic and spectroscopic properties of C(50), its anions and derivatives such as C(50)Cl(10) and C(50)Cl(12). The ground state of C(50) has D(3) symmetry with a spheroid shape, and is highly aromatic; the best D(5h)C(50) singlet is nonaromatic. Both D(3)() and D(5h)() isomers of C(50) have high electron affinities and can be reduced easily. Due to the unstable fused pentagon structural features, C(50) is chemically labile and subject to addition reactions such as chlorination, dimerization and polymerization. The equatorial pentagon-pentagon fusions of D(5h)C(50) are active sites for chemical reactions; hence, D(5h)C(50) may behave as a multivalent group. The computed IR, Raman, (13)C NMR and UV-vis spectra of the D(5h)C(50)Cl(10) molecule agree well with the experimental data. Finally, D(5h)C(50)Cl(10) is predicted to have a high electron affinity and, hence, might serve as an electron-acceptor in photonic/photovoltaic applications. The geometry and (13)C NMR chemical shifts of C(50)Cl(12) were computed to assist further isolation experiments.     

Trifluoromethyl derivatives of insoluble small-HOMO-LUMO-gap hollow higher fullerenes. NMR and DFT structure elucidation of C2-(C74-D3h)(CF3)12, Cs-(C76-Td(2))(CF3)12, C2-(C78-D3h(5))(CF3)12, Cs-(C80-C2v(5))(CF3)12, and C2-(C82-C2(5))(CF3)12

Reaction of a mixture of insoluble higher fullerenes with CF3I at 500 degrees C produced a single abundant isomer of C74(CF3)12, C76(CF3)12, and C80(CF3)12, two abundant isomers of C78(CF3)12 and C82(CF3)12, and an indeterminant number of isomers of C84(CF3)12. Using a combination of 19F NMR spectroscopy, DFT calculations, and the structures and spectra of previously reported fullerene(CF3)n compounds, the most-probable structures of six of the seven isolated compounds were determined to be specific isomers of C2-(C74-D3h)(CF3)12, Cs-(C76-Td(2))(CF3)12), C2-(C78-D3h(5))(CF3)12), Cs-(C80-C2v(5))(CF3)12), C2-(C82-C2(5))(CF3)12), and C2-(C82-C2(3))(CF3)12) containing ribbons and/or loops of edge-sharing para-C6(CF3)2 hexagons. The seventh isolated compound is a C1 isomer of C78(CF3)12 containing two such ribbons. This set of compounds represents only the second reported isolable compound with the hollow C74-D3h cage and the first experimental evidence for the existence of the hollow fullerenes C76-Td(2), C78-D3h(5), C80-C2v(5), and C82-C2(5) in arc-discharge soots.     

On the structure and relative stability of C50 fullerenes

The complete set of 271 classical fullerene isomers of C50 has been studied by full geometry optimizations at the SAM1, PM3, AM1, and MNDO quantum-chemical levels, and their lower energy structures have also been partially computed at the ab initio levels of theory. A D(5h) species, with the least number of pentagon adjacency, is predicted by all semiempirical methods and the HF/4-31G calculations as the lowest energy structure, but the B3LYP/6-31G* geometry optimizations favor a D3 structure (with the largest HOMO-LUMO gap and the second least number of adjacent pentagons) energetically lower (-2 kcal/mol) than the D(5h) isomer. To clarify the relative stabilities at elevated temperatures, the entropy contributions are taken into account on the basis of the Gibbs energy at the HF/4-31G level for the first time. The computed relative-stability interchanges show that the D3 isomer behaves more thermodynamically stable than the D(5h) species within a wide temperature interval related to fullerene formation. According to a newly reported experimental observation, the structural/energetic properties and relative stabilities of both critical isomers (D(5h) and D3) are analyzed along with the experimentally identified decachlorofullerene C50Cl10 of D(5h) symmetry. Some features of higher symmetry C50 nanotube-type isomers are also discussed.     

C59XH（X=N,B）与1,3-丁二烯Diels-Alder环加成反应的区域选择性

应用半经验的AMI和密度泛函B3LYP/6-31G*方法对1,3-丁二烯与C59XH(X=N,B)Diels-Alder环加成反应的区域选择性进行理论研究,选择一些有代表性的C59XH(X=N,B)的6-16键探讨环加成反应的机理.1,3-丁二烯与C59NH进行的Diels-Alder反应,随着加成位置远离C59NH的N原子,活化能越来越低,但都比1,3-丁二烯与C60相应反应的活化能高.与此相反,对于1,3-丁二烯与C59BH进行的环加成反应.加成位置最靠近B原子的2,12/r-和2,12/f-过渡态的势垒最低,并且比1,3-丁二烯与C60进行环加成反应的活化能约低18 kJ·mol-1,其产物也是热力学最稳定的.与C60相应的反应相比,C59NH和C59BH中N和B原子不同的电子性质对其邻位双键进行Diels-Alder环加成反应的活性产生了不同影响,前者使反应活性降低,后者使反应活性增强.     

Modeling of configurations and third-order nonlinear optical properties of C(36) and C(34)X(2) (X=B,N)

Using the ab initio method, the geometrical structures of C(36) and the X (B,N)-doped isomers C(34)X(2) have been optimized. On the basis of the optimized structures, then, the third-order nonlinear optical polarizabilities gamma in the different optical processes of the third-harmonic generation, electric-field induced second-harmonic generation and degenerate four-wave mixing, and two-photon absorption (TPA) cross sections delta are calculated by using TDB3LYP method coupled with the sum-over-states method. The calculated results show that the one-photon allowed excitation process dominate the two-photon excitation process for C(36)-D(6h), whereas the two-photon allowed excitation process dominate the one-photon excitation process for C(36)-D(2d) and C(34)X(2) (B,N). It is found that the largest resonant TPA peaks of dopant fullerenes have a blueshift and the TPA cross sections have an enhancement compared with those of the parent fullerenes of isomers C(36)-D(6h) and C(36)-D(2d).     

Theoretical Study of the Molecular Electrostatic Potential of C_(50)

1 INTRODUCTION All fullerenes made so far obey the isolated pentagon (IPR)[1], which governs the stability of fullerenes comprising hexagons and exact 12 pen- tagons. Smaller fullerenes, which violate the IPR, are predicted to have high instability and especially difficult isolation due to their condensed pentagons and increased strain. The production of smaller fullerene C36 has been reported[2]. However, the definite characterization of a C36-based solid is in doubt. Moreover, variou…     

D(5h) C50X10: saturn-like fullerene derivatives (X=F, Cl, Br)

Based on the recently reported D(5h) C(50), the geometries and stabilities of its Saturn-like derivatives C(50)X(10) (X=F, Cl, Br) have been investigated by DFT method. Compared with C(50), the equatorial carbon atoms in C(50)X(10) have been saturated by halogens and change to sp(3) hybridization to release the large angle strain. Because the equatorial carbon atoms have been taken out of the pi system by the halogens "ring," the C(50)X(10) system has been split into two well-delocalized conjugated annulene subunits, and then the electronic stabilization has been enhanced.     

Rationale for the opposite stereochemistry of the major monoadducts and interstrand crosslinks formed by mitomycin C and its decarbamoylated analogue at CpG steps in DNA and the effect of cytosine modification on reactivity

Mitomycin C (MMC) is a potent antitumour agent that forms a covalent bond with the 2-amino group of selected guanines in the minor groove of double-stranded DNA following intracellular reduction of its quinone ring and opening of its aziridine moiety. At some 5'-CG-3' (CpG) steps the resulting monofunctional adduct can evolve towards a more deleterious bifunctional lesion, which is known as an interstrand crosslink (ICL). MMC reactivity is enhanced when the cytosine bases are methylated (5 MC) and decreased when they are replaced with 5-F-cytosine (5FC) whereas the stereochemical preference of alkylation changes upon decarbamoylation. We have studied three duplex oligonucleotides of general formula d(CGATAAXGCTAACG) in which X stands for C, 5MC or 5FC. Using a combination of molecular dynamics simulations in aqueous solution, quantum mechanics and continuum electrostatics, we have been able to (i) obtain a large series of snapshots that facilitate an understanding in atomic detail of the distinct stereochemistry of monoadduct and ICL formation by MMC and its decarbamoylated analogue, (ii) provide an explanation for the altered reactivity of MMC towards DNA molecules containing 5MC or 5FC, and (iii) show the distinct accommodation in the DNA minor groove of the different covalent modifications, particularly the most cytotoxic C1α and C1β ICLs.     

Synthesis of (C5Me5)2(C5Me4H)UMe, (C5Me5)2(C5H5)UMe, and (C5Me5)2UMe[CH(SiMe3)2] from cationic metallocenes for the evaluation of sterically induced reduction

To probe the correlation of unusual (C5Me5)(1-) reactivity with steric crowding in complexes such as (C5Me5)3UMe and (C5Me5)3UCl, slightly less crowded (C5Me5)2(C5Me4H)UX analogues (X = Me, Cl) were synthesized and their reactivity was evaluated. The utility of the cationic precursors [(C5Me5)2UMe](1+), 1, and [(C5Me5)2UCl](1+), 2, in the synthesis of (C5Me5)2(C5Me4H)UMe, 3, and (C5Me5)2(C5Me4H)UCl, 4, was also explored. Since the use of precursor [(C5Me5)2UMe][MeBPh3], 1a, is complicated by the equilibrium between 1a and (C5Me5)2UMe2/BPh3, the reactivity of [(C5Me5)2UMe(OTf)]2, 1b, (OTf = O3SCF3) prepared from (C5Me5)2UMe2 and AgOTf, was also studied. Both 1a and 1b react with KC5Me4H to form 3. Complex 4 readily forms by addition of KC5Me4H to [(C5Me5)2UCl][MeBPh3], generated in situ from (C5Me5)2UMeCl and BPh3. Complex 1b was preferred to 1a for the synthesis of (C5Me5)2(C5H5)UMe, 5, and (C5Me5)2UMe[CH(SiMe3)2], 6, from KC5H5 and LiCH(SiMe3)2, respectively. Complex 6 is the first example of a mixed alkyl uranium metallocene complex. Sterically induced reduction (SIR) reactivity was not observed with 3-6 although the methyl displacements from the (C5Me5)(1-) ring plane for 3 are the closest observed to date to those of SIR-active complexes. The (1)H NMR spectra of 3 and 4 are unusual in that all of the (C5Me4H)(1-) methyl groups are inequivalent. This structural rigidity is consistent with density-functional theory calculations.     

An experimental study of the intersystem crossing and reactions of C2(X1Sigmag+) and C2(a3Pi(u)) with O2 and NO at very low temperature (24-300 K)

A low-temperature gas-phase kinetics study of the reactions and collisional relaxation processes involving C2(X1Sigma(g)+) and C2(a3Pi(u)) in collision with O2 and NO partners at temperatures from 300 to 24 K is reported. The experiments employed a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus to attain low temperatures. The C2 species were created using pulsed laser photolysis at 193 nm of mixtures containing C2Cl4 diluted in N2, Ar, or He carrier gas. C2(X1Sigma(g)+) molecules were detected via pulsed laser-induced fluorescence in the (D1Sigma(u)+ <-- X1Sigma(g)+) system, and C2(a3Pi(u)) molecules were detected via pulsed laser-induced fluorescence in the (d 3Pi(g) <-- a 3Pi(u)) system. Relaxation of 3C2 by intersystem crossing induced by oxygen was measured at temperatures below 200 K, and it was found that this process remains very efficient in the temperature range 50-200 K. Reactivity of C2(X1Sigma(g)+) with oxygen became very inefficient below room temperature. Using these two observations, it was found to be possible to obtain the C2(X1Sigma(g)+) state alone at low temperatures by addition of a suitable concentration of O2 and then study its reactivity with NO without any interference coming from the possible relaxation of C2(a3Pi(u)) to C2(X1Sigma(g)+) induced by this reagent. The rate coefficient for reaction of C2(X1Sigma(g)+) with NO was found to be essentially constant over the temperature range 36-300 K with an average value of (1.6 +/- 0.1) x 10(-10) cm3 molecule(-1) s(-1). Reactivity of C2(a3Pi(u)) with NO was found to possess a slight negative temperature dependence over the temperature range 50-300 K, which is in very good agreement with data obtained at higher temperatures.     

MP2 theory investigation on the halides of D6hC36:C36Xn (X=F,Cl,Br; n=2,4,6,12)

An investigation of C(36)X(n) (X=F,Cl,Br; n=2,4,6,12) formed from the initial C(36) fullerene with D(6h) symmetry has been performed using the MP2 theory. Their equilibrium structures, reaction energies, strain energies, lowest unoccupied molecular orbital-highest occupied molecular orbital (LUMO-HOMO) gap energies, and aromaticities have been studied. The calculation results showed that those addition reaction were highly exothermic and C(36)X(n) were more stable than C(36). Moreover, from the view of thermodynamics it should be possible to detect C(36)X(6). The LUMO-HOMO gap energies of C(36)X(n) were higher than D(6h)C(36) and the redox characteristics of C(36)X(n) were weaker comparing to D(6h)C(36). The analyses of pi-orbital axis vector indicated that the chemical reactivity of C(36) was the result of the high strain, and the nucleus independent chemical shifts research showed that the stabilities of the C(36)X(6) were correlative with the conjugation effect.     

Combustion synthesis and electrochemical properties of the small hydrofullerene C50H10

The hydrofullerene C(50)H(10) is synthesized by low-pressure benzene-oxygen diffusion combustion. The structure of C(50)H(10) is identified through NMR, mass spectrometry, and IR and Raman spectroscopy as a D(5h) symmetric closed-cage molecule with five pairs of fused pentagons stabilized by ten hydrogen atoms. UV/Vis and fluorescence spectrometric analyses disclose its optical properties as comparable with those of its chloride cousin (C(50)Cl(10)). Cyclic and square-wave voltammograms reveal that the first reduction potential of C(50)H(10) is more negative than that of C(50)Cl(10) as well as C(60), with implications for the utilization of C(50)H(10) as a promising electron acceptor for photovoltaic applications.     

Reaction dynamics of phenyl radicals (C6H5) with propylene (CH3CHCH2) and its deuterated isotopologues

The reactions between phenyl radicals (C6H5) and propylene (CH3CHCH2) together with its D6- and two D3-isotopologues were studied under single collision conditions using the crossed molecular beams technique. The chemical dynamics inferred from the center-of-mass translational and angular distributions suggests that the reactions are indirect and initiated by an addition of the phenyl radical to the alpha-carbon atom (C1 carbon atom) of the propylene molecule at the =CH2 unit to form a radical intermediate (CH3CHCH2C6H5) on the doublet surface. Investigations with D6-propylene specified that only a deuterium atom was emitted; the phenyl group was found to stay intact. Studies with 1,1,2-D3- and 3,3,3-D3-propylene indicated that the initial collision complexes CH3CDCD2C6H5 (from 1,1,2-D3-propylene) and CD3CHCH2C6H5 (from 3,3,3-D3-propylene) eject both a hydrogen atom via rather loose exit transition states to form the D3-isotopomers of cis/trans-1-phenylpropene (CH3CHCHC6H5) (80-90%) and 3-phenylpropene (H2CCHCH2C6H5) (10-20%), respectively. Implications of these findings for the formation of polycyclic aromatic hydrocarbons (PAHs) and their precursors in combustion flames are discussed.     

C50(D5h)衍生物-异质富勒烯C48P2的分子行为理论研究

用半经验的AM1和MNDO方法以及密度泛函B3LYP/3-21G方法对C₅₀(D_5h)的衍生物C₄₈P₂的所有可能的异构体进行了系统的理论研究. 优化了稳定构型, 计算了生成热、HOMO-LUMO能级差、NICS值、红外光谱及电子光谱, 并与C₄₈X₂(X=B, N)的分子行为进行了比较. 计算结果表明: (1) C₄₈P₂的最稳定异构体是异构体C₄₈P₂-78, 该异构体对应于赤道位置六元环内的1,4-取代产物; (2) 决定C₄₈P₂异构体稳定性的主要因素是碳笼的张力, 而稳定性和芳香性之间没有明显的相关性; (3) 相对较稳定的C₄₈P₂异构体的HOMO-LUMO能级差比C₅₀的HOMO-LUMO能级差大; (4) 计算出的红外光谱以及电子光谱可以供实验参考; 计算出的NICS 值也可以用来表征C₄₈P₂各异构体. (5) C₄₈P₂与C₄₈X₂(X=B, N)具有相同的取代选择性规律及稳定性决定因素, 并且相对较稳定的异构体均具有比C₅₀本体大的HOMO-LUMO能级差.     

Theoretical study of cycloaddition reactions of heavy carbenes with C60

The potential energy surfaces for the cycloaddition reaction Me2X:+C60-->Me2X(C60) (X=C, Si, Ge, Sn, and Pb) have been studied at the B3LYP/LANL2DZ level of theory. It has been found that there are two competing pathways in these reactions, which can be classified as a [6,5]-attack (path 1) and a [6,6]-attack (path 2). It was found that, given the same reaction conditions, the cycloaddition reaction of C60 via a [6,6]-attack is more favorable than that via a [6,5]-attack, both kinetically and thermodynamically. A qualitative model that is based on the theory of Pross and Shaik has been used to develop an explanation for the reaction barrier heights. As a result, our theoretical investigations suggest that the singlet-triplet splitting DeltaEst(=Etriplet-Esinglet) of the 6 valence electron Me2X: and C60 species can be used as a guide to predict their reactivity toward cycloaddition reactions. Our model results demonstrate that the reactivity of heavy carbene cycloaddition to C60 decreases in the order Me2C:>Me2Si:>Me2Ge>Me2Sn:>Me2Pb:. As a consequence, we show that electronic effects play a decisive role in determining the energy barriers as well as the reaction enthalpy.     

Methyl displacements from cyclopentadienyl ring planes in sterically crowded (C5Me5)3M complexes

The displacements of the methyl substituents away from the metal and out of the cyclopentadienyl ring plane are compared in sterically crowded (C(5)Me(5))(3)M complexes vs sterically normal f-element complexes in an attempt to evaluate the utility of this parameter in predicting unusual (C(5)Me(5))(1-) ring reactivity. The out-of-plane displacements of 16 sterically crowded tris(cyclopentadienyl) complexes of general formula (C(5)Me(5))(3)M, (C(5)Me(4)R)(3)M (R = Et, (i)Pr, (t)()Bu, SiMe(3)), (C(5)Me(5))(3)MX (X = anion), and (C(5)Me(5))(3)ML (L = neutral ligand) are compared with [(C(5)Me(5))(2)U](2)(C(6)H(6)), (C(5)Me(5))(2)Sm(PC(4)H(2)(t)Bu(2)), and 33 representative examples of f-element bis(cyclopentadienyl) complexes with normal cyclopentadienyl behavior and coordination numbers ranging from 6 to 10. In general, the methyl displacement values of sterically crowded complexes overlap with those in the other complexes, which demonstrates that the basis of the structural distortions is complex. However, if the most extreme out-of-plane displacement in each of the sterically crowded complexes is examined vs the analogous maximum out-of-plane displacement in less crowded systems, there appears to be a basis for predicting cyclopentadienyl reactivity.