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A novel low-temperature intramolecular exchange was detected by (13)C NMR spectrometry in the Na(+) and K(+) salts of the title compound. The process causes the pairwise exchange in the dianion ring of C(2"), C(3"), and C(4") with C(8"), C(7"), and C(6"), respectively. The free energy of activation (DeltaG()(exch)) for the dipotassium salt (2(2-)/2K(+)) in THF-d(8) at 230 K is 12.6 kcal mol(-1). Two key questions are addressed: (1) Why are these carbons anisochronous and (2) what is the mechanism of exchange? NMR data for 1-cyclooctatetraenylnaphthalenedipotassium (3(2-)/2K(+)) as well as ab initio HF/3-21G(++) calculations for 3, 3(2-), and 3(2-)/2K(+) indicate that the nonequivalence is due to both slow rotation across a barrier at which the naphthalene and COT(2)(-) rings are approximately coplanar and slow inversion of the neutral COT ring. This results in the noteworthy circumstance of diastereotopic carbons being observed in a molecule that does not possess either a stereogenic or a prostereogenic center. Comparison of DeltaG()(exch) and DeltaG(++)(BS) for 2(2-)/2K(+) with the corresponding values for 2(2-)/2Na(+) and 2(2-)/2Li(+) and of DeltaG(++)(exch) with DeltaG(++) for ring inversion in 1,4-dicyclooctatetraenylnaphthalene leads to the conclusion that COT(2-) ring rotation and COT ring inversion both contribute to exchange in 2(2-)/2K(+) in a 3:1 ratio, but that exchange occurs exclusively by ring rotation in 2(2-)/2Li(+). The latter result is attributed to looser ion pairing in the dilithium (and disodium) salts.

The rate constants for bond shift (k(BS)) in phenylcyclooctatetraene (1b) and its p-nitro and p-methoxy analogues (1a and 1c, respectively) in THF-d(8) were determined by dynamic NMR spectrometry to be identical, but k(BS) is eight times greater at 280 K relative to 1b when the para substituent is cyclooctatetraenyldipotassium (2(2-)/2K(+)). These results are discussed in the context of (a) possible intrinsically small substituent effects (as determined by (13)C chemical shifts in the ground state (GS)) for 1a-c and (b) differences in steric interactions and resonance stabilization between the ground and BS transition state (TS). The latter factor was modeled by employing HF/3-21G(*) ab initio molecular orbital calculations of the GS and ring inversion TS. It is concluded that k(BS) is unchanged in 1a-c because the potentially greater pi interaction in the BS TS is counterbalanced by a greater degree of twist between the aryl and COT rings resulting from increased steric hindrance relative to the GS. However, pi interaction assumes a greater importance in the TS of 2(2-)/2K(+) owing to a decreased HOMO-LUMO energy gap compared to 1a-c, particularly when the counterions are solvated. This causes a decrease in the inter-ring twist angle and, together, these changes are responsible for the observed increase in k(BS) in 2(2-)/2K(+). The effect of substituents on a possible contribution of heavy atom tunneling to the reaction mechanism is also discussed.

Dicyclooctatetraenylmethane (1) and dicyclooctatetraenyldimethylsilane (2) in THF-d(8) at 272 K exist as mixtures of diastereomers in ratios of 1:0.8 and 1:1, respectively. Nine energy minima (four meso and five racemic conformers) were located for each compound by geometry optimization at the HF/6-31G level of theory. The effects of torsional strain, steric interactions and dynamic electron correlation were analyzed. The diastereomeric ratios for 1 and 2 were reproduced reasonably well from the total energy calculated for each conformer corrected for its conformational enthalpy and entropy contributions. The ratio of rate constants for bond shift (BS) (k(BS)(1)/k(BS)(2)) is three times greater than the corresponding ratio for ring inversion. This suggests that additional substituent effects, such as pi interactions, are operative in the transition state for BS.

The single-crystal X-ray structures of 6-(4-(dimethylamino)phenyl)-6-methylfulvene (2) and of two polymorphs of 6-(4-(dimethylamino)phenyl)-6-phenylfulvene (3(P21/c) and 3(Pca21)) have been determined and the structures of a series of 6-arylfulvenes (1−8) have been optimized at the HF/6-31G level. Analysis of these structures reveals how resonance and crystal lattice effects influence the degree of coplanarity between the aryl and fulvene rings. The torsional angles at the aryl−fulvene bonds are significantly larger in the optimized structures than in the X-ray structures. Natural bond orbital π charges and dipole moments calculated for the X-ray and optimized structures show that the crystalline environment enhances molecular polarization. Qualitative responses for second harmonic generation in powder samples have been observed in 2, 3(Pca21), and 5. Compounds 2 and 3(Pca21) have similar packing motifs despite packing in different space groups.

The rate constants for ring inversion (1) (at −31 °C) and bond shift (2) (at 19 °C) in the title compound (3) are 3−5 and 2 times the corresponding values for the 1,4 isomer due to steric hindrance in the ground state of 3.

The rate constants for bond shift (kBS) and intramolecular charge transfer (kCT) in alkali metal salts of bicyclooctatetraenyl dianions (12-) in THF-d8 were determined by dynamic NMR spectroscopy. The values of kBS and kCTincrease in the order K < Na ≤ Rb < Cs and K < Rb < Na < Cs ≪ Li, respectively. It is concluded that the K, Rb, and Cs salts react as contact ion pairs (CIPs) while the Li and Na salts react as solvent-separated ion pairs (SSIPs). Increases in kBS and kCT are observed in 12-/2K+ on going to more polar solvents (NH3/CH3NH2 and THF-d8/DMEU). Comparison with kinetic studies of four other dianions has led to the conclusion that 12-/2Na+ is the first example of a Na salt of a dianion to undergo charge transfer by SSIPs. This is attributed to the difficulty in forming a Na-bridged CIP transition structure in 12-/2Na+ that can compete with an SSIP transition structure.

1,4-Dicyclooctatetraenyl-1,3-butadiyne and p-, m-, and o-di(cyclooctatetraenylethynyl)benzene (1−4, respectively) and their dipotassium salts (12-−42-) in THF-d8 have been synthesized and studied b...

The rate constants for ring inversion (kr.i.) and bond shift (kb.s.) in 1 and 2 were determined by dynamic NMR spectrometry while the rate constants for bond shift and intramolecular charge transfe...

The free energies of activation for bond shift in the carbon group substituted cyclooctatetraenes (COT−M(CH3)3) in THF-d8 at 298 K have been determined to be 16.4, 16.2, 16.2, and 18.1 kcal/mol for M = Si, Ge, Sn, and C, respectively, and 15.6 kcal/mol for CH3−COT. These data permit an interpretation of the previously reported opposite orders for the ease of the first and second electrochemical reductions in the Si, Ge, and Sn compounds. It is postulated that the order of the first reduction potential is controlled by a decrease in overlap between the substituent and the ring π orbitals in the order Si > Ge > Sn, whereas the second reduction potential is controlled by the energy gap between the symmetric π HOMO of the COT radical anion and an interacting substituent σ* orbital of π symmetry (eσ*π − eπ), which increases in the order Sn < Ge < Si. HF/3-21G molecular orbital calculations indicate that the high barrier for t-Bu−COT primarily reflects steric effects in the transition state.

The single crystal x-ray structures of (E)-(2-arylethenyl)cyclooctatetraenes 1 and 2 have been determined. The nitro group in 2 does not influence the geometry of the COT ring relative to that of 1. The influence of various substituents on several structural parameters of the COT ring are analyzed in terms of steric/electrostatic and electronegativity effects of the substituent.

The free energies of activation for bond shift in fluoro-, chloro-, bromo- and iodocyclooctatetraene in THF- d 8 were determined by 13 C NMR spectroscopy to be 12.7, 15.0, 15.4 and 16.1 kcal/mol, respectively, at 298 K. Analysis of HF/6-31G*-optimized geometries and natural atomic charges for the ground state and ring inversion transition state for cyclooctatetraene (COT) and its monofluoro and monochloro derivatives indicates that the bond shift transition state for COT-F is stabilized (relative to COT) by electrostatic interactions between the fluorine and the flanking hydrogens, whereas those of the larger halogens are destabilized by steric interactions with the vicinal CH groups. © 1997 Elsevier Science Ltd.

The use of Koopmans'theorem (KT) for the calculation of scaled energies of π * negative ion resonance states has been investigatcd by ab initio molecular theory as a function of basis set. HF/D95v energies calculated for HF/6-31G-optimized geometries have been found to correlate with experimental electron attachment energies for 56 π * negative ion states in 39 alkenes, polyenes, and benzenoid hydrocarbons with a correlation coefficient of 0.989 and with average and largest absolute errors of 0.11 and 0.32 eV, respectively. An equally good correlation was obtained at the same basis set for MP2/6-31G * -optimized geometries. Various factors that influence the quality of KT correlations are analyzed

Dicyclooctatetraenylethyne, (E)-1,2-dicyclooctatetraenylethylene and 1,4-dicyclooctatetraenylbenzene (3a-c) have been prepared through a convergent synthesis by the palladium-catalyzed coupling of 1,2-bis(tributylstannyl)ethyne, (E)-1,2-bis(tributylstannyl)ethylene, and 1,4-bis(tributystannyl)benzene (2a-c), respectively, with bromocyclooctatetraene (1). Modified Stille coupling conditions with tri(2-furyl)phosphine and triphenylarsine as ligands were employed. Alkynylcyclooctatetraenes, including 3a, have been prepared by the palladium/copper-catalyzed coupling of terminal alkynes with 1. Bridged dicyclooctatetraenes 3a and 3c were also prepared through a linear synthesis starting from 2,6-cyclooctadien-1-one

The microwave spectra of the normal and the 2-13C isotopic species of 1-cyanobicyclo[1.1.0]butane have been observed and assigned, leading to the following rotational constants: (normal), A=8807.202 ± 0.004, B=2924.334 ± 0.002, C=2509.322 ± 0.002 and (isotope), A=8608.85 ± 0.85, B=2902.88 ± 0.02, and C=2478.56 ± 0.02 MHz. Measurements of the second-order Stark effect led toμT=4.08 ± 0.01 D. Based on the available microwave data coupled with geometryoptimizedab initio molecular orbital structures for cyanocyclopropane and 1-cyanobicyclo[1.1.0]-butane, a molecular structure for the latter molecule is proposed. Analysis of the dipole moments of these molecules and of bicyclo[1.1.0]butane has led to the conclusion that the bicyclobutyl group is a better electron donor than is cyclopropyl. In addition, a simple frontier molecular orbital model is not sufficient for explaining all of the structural changes that occur on substituting cyano at the bridgehead of bicyclo[1.1.0]butane.

Bridged dicyclooctatetraenes have been synthesized by the palladium-catalyzed coupling of distannanes with bromocyclooctatetraene by employing the ligands tri(2-furyl)phosphine and triphenylarsine. The coupling of bromocyclooctatetraene with terminal alkynes is compared to couplings with alkynylstannanes for the preparation of alkynylcyclooctatetraenes.

Dicyclooctatetraenylethyne, (E)-1,2-dicyclooctatetraenylethylene and 1,4-dicyclooctatetraenylbenzene (3a-c) have been prepared through a convergent synthesis by the palladium-catalyzed coupling of 1,2-bis(tributylstannyl)ethyne, (E)-1,2-bis(tributylstannyl)ethylene, and 1,4-bis(tributystannyl)benzene (2a-c), respectively, with bromocyclooctatetraene (1). Modified Stille coupling conditions with tri(2-furyl)phosphine and triphenylarsine as ligands were employed. Alkynylcyclooctatetraenes, including 3a, have been prepared by the palladium/copper-catalyzed coupling of terminal alkynes with 1. Bridged dicyclooctatetraenes 3a and 3c were also prepared through a linear synthesis starting from 2,6-cyclooctadien-1-one

The free energies of activation for bond shift in fluoro-, chloro-, bromo- and iodocyclooctatetraene in THF- d 8 were determined by 13 C NMR spectroscopy to be 12.7, 15.0, 15.4 and 16.1 kcal/mol, respectively, at 298 K. Analysis of HF/6-31G*-optimized geometries and natural atomic charges for the ground state and ring inversion transition state for cyclooctatetraene (COT) and its monofluoro and monochloro derivatives indicates that the bond shift transition state for COT-F is stabilized (relative to COT) by electrostatic interactions between the fluorine and the flanking hydrogens, whereas those of the larger halogens are destabilized by steric interactions with the vicinal CH groups. © 1997 Elsevier Science Ltd.

Dicyclooctatetraenylmethane (1) and dicyclooctatetraenyldimethylsilane (2) in THF-d8 at 272 K exist as mixtures of diastereomers in ratios of 1:0.8 and 1:1, respectively. Nine energy minima (four meso and five racemic conformers) were located for each compound by geometry optimization at the HF/6-31G* level of theory. The effects of torsional strain, steric interactions and dynamic electron correlation were analyzed. The diastereomeric ratios for 1 and 2 were reproduced reasonably well from the total energy calculated for each conformer corrected for its conformational enthalpy and entropy contributions. The ratio of rate constants for bond shift (BS) (kBS(1)/kBS(2)) is three times greater than the corresponding ratio for ring inversion. This suggests that additional substituent effects, such as π interactions, are operative in the transition state for BS.

Deviations from the pairing theorem previously noted for the linear polyenes are explained on the basis of 1,3-π,π interactions in the negative ion states. Such interactions also contribute significantly to the stabilization of the π * −2 negative ion state of (Z)-relative to (E)-1,3,5-hexatriene. Koopmans' theorem (KT) does not explain the equal π * −3 electron affinities of (E)- and (Z)-hexatriene but CI-singles calculations closely approximate the relative experimental values. Scaled KT and Δe SCF calculations lead to the predictions that (a) the crossover between unbound and bound first π * negative ion states in the linear polyenes occurs between hexatriene and octatetraene, (b) the first π * negative ion states of fulvene and bicyclo[1.1.0]but-1(3)-ene are bound and (c) that of cyclobutadiene is unbound.

The microwave spectra of the normal and the 2-13C isotopic species of 1-cyanobicyclo[1.1.0]butane have been observed and assigned, leading to the following rotational constants: (normal), A=8807.202 ± 0.004, B=2924.334 ± 0.002, C=2509.322 ± 0.002 and (isotope), A=8608.85 ± 0.85, B=2902.88 ± 0.02, and C=2478.56 ± 0.02 MHz. Measurements of the second-order Stark effect led toμT=4.08 ± 0.01 D. Based on the available microwave data coupled with geometryoptimizedab initio molecular orbital structures for cyanocyclopropane and 1-cyanobicyclo[1.1.0]-butane, a molecular structure for the latter molecule is proposed. Analysis of the dipole moments of these molecules and of bicyclo[1.1.0]butane has led to the conclusion that the bicyclobutyl group is a better electron donor than is cyclopropyl. In addition, a simple frontier molecular orbital model is not sufficient for explaining all of the structural changes that occur on substituting cyano at the bridgehead of bicyclo[1.1.0]butane.

Polarization of the π-electron systems of indenyllithium (In −Li+) and indenylpotassium (In −K+), which is generally indicative of the degree and tightness of contact ion pairing, was investigated by 13C NMR spectroscopy. The effect of changing solvent is about two thirds as great for In −K+ as for In −Li+. The indenyl anion is more strongly polarized with Li+/dimethyl sulfoxide than with Li+/ammonia, whereas the reverse is observed for In −K+. Complexation of In −K+ in tetrahydrofuran (THF) by cryptand(2,2,2) results in a π-electron distribution identical with that of In −Li+ in hexamethylphosphoramide, indicative of solvent- or agent-separated ion pairs or free ions. Small but significant cation–anion interactions are observed for In −K+ in hexamethylphosphoramide and for In −N+Me4 in THF.

The Kratschmer–Huffman carbon‐arc method of preparing fullerenes has been used to generate carbon‐coated transition metal (TM) and TM‐carbide nanocrystallites. The magnetic nanocrystallites were extracted from the soot with a magnetic gradient field technique. For TM=Co the majority of nanocrystals exist as nominally spherical particles, 0.5–5 nm in radius. Hysteretic and temperature‐dependent magnetic response, in randomly and magnetically aligned powder samples frozen in epoxy, correspond to fine particle magnetism associated with monodomain TM particles. The magnetization exhibits a unique functional dependence on H/T, and hysteresis below a blocking temperature TB. Below TB, the temperature dependence of the coercivity can be expressed as Hc=Hc0[1−(T/TB)1/2], where Hc0 is the 0 K coercivity.

Carbon‐coated gadolinum and holmium carbide nanocrystallites have been generated using a modification of the Huffman‐Kratschmer carbon arc process. Bulk amounts of these particles were isolated from the other by‐products using a magnetic field gradient. Transmission electron microscopy revealed the presence of 10–50 nm diameter crystallites coated with numerous graphitic layers. The nanocrystallite phases were identified as Gd2C3 and Ho2C3, respectively, by x‐ray and electron diffraction. Magnetization measurements were performed using a superconducting quantum interference device magnetometer between ±5 T at temperatures ranging from 4 to 200 K. The magnetization curves were shown to scale as a function of H/T. The RE3+ sites in RE2C3 have C3 site symmetry. For Gd2C3 the universal curve was fit with a Brillouin function consistent with the Gd3+ free‐ion ground‐state values of J=7/2 and g=2. The 5I8 Ho3+ free‐ion ground state is split, presumably due to a C3 symmetry crystal field. Consequently, for Ho2C3...

Attempted synthesis of 1,8-dicyclooctatetraenylnaphthalene (1) by the palladium(0)-catalyzed coupling of 1,8-dibromonaphthalene with cyclooctatetraenyltrimethylstannane afforded a single unsymmetrical isomer of 1 in 88% yield. Two-dimensional NMR methods and spectral synthesis were employed to assign the structure of the isomer (2). AM1 geometry-optimized structures of 2 and its isomers showed that the unexpected unsymmetrical structure of 2 results from the minimization of repulsive inter-ring H-H interactions. Compound 2 is postulated to arise via tandem [2 + 2] cycloaddition and 6 pi --4 pi + 2 sigma electrocyclization reactions of 1.

Carbon-coated gadolinium carbide nanocrystallites are generated by a process based on the Kratschmer-Huffman carbon-arc method of preparing fullerenes, and a novel magnetic-field-gradient separation technique is used to separate them. This separation of nanocrystalline byproducts of the carbon arc process is a prerequisite for many of the proposed applications of these materials. While the data presented pertain to ${\mathrm{Gd}}_{2}$${\mathrm{C}}_{3}$, this method is generally applicable to any paramagnetic or ferromagnetic compound. Structural characterization by x-ray and electron diffraction and high-resolution transmission electron microscopy reveal the presence of a single gadolinium-containing (${\mathrm{Gd}}_{2}$${\mathrm{C}}_{3}$) phase and excess carbon. The carbide phase exists as 10--50 nm spherical particles. SQUID magnetometry shows paramagnetic response attributed to ${\mathrm{Gd}}^{3+}$ ions.

The microwave spectrum of the normal isotopic species of isocyanocyclopropane and its three unique mono‐substituted 13C isotopic varieties have been investigated in their natural isotopic abundances. For the normal isotopic species, Stark modulation and RFMDR techniques lead to the following rotational constants: A=15 827.303±0.009 MHz, B=3734.004±0.002 MHz, and C=3515.892±0.002 MHz. These data, along with those for the three isotopic species, lead to the following heavy atom structural parameters: C1C2=1.521±0.007 A, C2C3=1.513±0.005 A, C1N=1.377±0.008 A, and C≡N=1.176±0.005 A. To within a precision of ±1°, the C–N≡C group is linear and is inclined at an angle of 123.4±0.6° to the plane of the ring. The structural parameters are compared to those of related molecules and interpreted in terms of a frontier molecular orbital model for the π conjugation of the cyclopropyl ring.

Electron affinities and electron attachment energies associated with the π* orbitals of several p-alkylanilines (alkyl = Me, Et, i-Pr, and t-Bu) have been determined in the gas phase by electron transmission spectroscopy and have been analyzed by ab initio molecular orbital calculations at the STO-3G and 6-31G levels of basis set. The results lead to the conclusions that (a) the 2A2 negative ion state in toluene lies below the 2B1 state, (b) the 2A2 state is stabilized in toluene relative to benzene by bonding overlap between the ortho carbon (C2(2pπ)) and the Me H(1s) orbitals in the LUMO, (c) the 2B1 state is stabilized on adding Me groups to toluene to form t-butylbenzene through reduced antibonding interactions between C1(2pπ) and Me C(2pπ) orbitals, and (d) the lowest resonance in t-butylbenzene is extensively overlapped with the second resonance and may involve strong mixing of the 2A2 and 2B1 states.

Microwave spectra have been observed and analyzed for four monosubstituted isotopic forms of vinyl isocyanide. From the resulting rotational constants the heavy-atom structure is determined as follows: C1C2=1.338(4) A, C2N=1.379(6) A, NC3=1.174(6) A, ∠ C1C2N=122.5(7)°, ∠ C2NC3=178.2(12)°. The structural parameters are compared to the experimental values of several related molecules and to ab initio results at the HF/6-31G* and MP2/6-31G* levels. The experimental and theoretical results permit an evaluation of the relative conjugating ability of vinyl versus cyclopropyl and cyano versus isocyano.