Conformational Studies by Dynamic NMR. 57.(1) Stereodynamics of Syn-Anti Interconversion of Disubstituted Acyl Durenes

The orthogonal syn and anti isomers, originated by the restricted rotation about the Ar-C(O)Bu(t) single bonds in 1,4-bis(2,2-dimethylpropanoyl)durene (2e), have been separated by preparative thin layer chromatography. In solution they reach an equilibrium where the syn-anti ratio depends upon the polarity of the solvent. This allowed us to assign the anti structure, which has a null dipole moment, to the least retained isomer. The free energy of activation (DeltaG) for the interconversion was found to be 22.5 kcal mol(-)(1), a value high enough for identifying these species as configurational isomers. When less hindered derivatives, also having two RCO (R = Pr(i), Et, Me) substituents in the positions 1,4 of the durene moiety, were examined, the syn and anti forms could be detected only at low temperature by means of NMR spectroscopy. The corresponding interconversion barriers (DeltaG = 13.4, 11.7, 10.9 kcal mol(-)(1), respectively) are, in fact, much lower than for R = Bu(t), indicating that in these cases we are dealing with conformational rather than with configurational isomers.     

10]Annulene: bond shifting and conformational mechanisms for automerization

We report density-functional and coupled-cluster calculations on conformation change and degenerate bond shifting in [10]annulene isomers 1-5. At the CCSD(T)/cc-pVDZ//CCSD/6-31G level, conversion of the twist (1) to the heart (2) has a barrier of 10.1 kcal/mol, compared to Ea = 16.2 kcal/mol for degenerate "two-twist" bond shifting in 1. Pseudorotation in the all-cis boat isomer (3) proceeds with a negligible barrier. The naphthalene-like isomer 4 has a 3.9 kcal/mol barrier to degenerate bond shifting. The azulene-like isomer 5 is the only species for which the nature of the bond-equalized form (5-eq) depends on the method. At the CCSD(T)/cc-pVDZ//CCSD/6-31G level, 5-eq is 1.2 kcal/mol more stable than the bond-alternating form 5-alt. Conversion of 5-eq to 4 has a barrier of 12.6 kcal/mol. Despite being significantly nonplanar, both 5-eq and the transition state for bond shifting in 4 are highly aromatic based on magnetic susceptibility exaltations. On the basis of a detailed consideration of these mechanisms and barriers, we can now, with greater confidence, rule out 4 and 5 as candidates to explain the NMR spectra observed by Masamune. Our results support Masamune's original assignments for both isolated isomers.     

Vinylphosphirane-phospholene rearrangements: pericyclic [1,3]-sigmatropic shifts or not?

The conversion of "free" and Cr(CO)(5)-complexed 2-vinylphosphiranes into 3-phospholenes via [1,3]-sigmatropic shifts was studied with density functional theory and compared with the corresponding hydrocarbon system, that is, the vinylcyclopropane-cyclopentene rearrangement. All three systems behave similarly with subtle but important differences. No intermediate was found on any of the potential energy surfaces. 2-Vinylphosphiranes have smaller rearrangement barriers than vinylcyclopropane, and those carrying the Cr(CO)(5) group have still smaller ones. The rearrangement of both anti- and syn-2-vinylphosphiranes occurs in a concerted pericyclic manner with inversion of configuration at the migrating phosphorus, requiring, respectively, 29.3 and 36.7 kcal/mol, much in contrast to the 44.6 kcal/mol demanding diradical-like process for the hydrocarbon analogue. Epimerization at the phosphorus center (syn right arrow over left arrow anti) requires approximately 32.0 kcal/mol and occurs in a single step, reflecting a diradical-like ring opening-ring closure process that can occur in both a clockwise and counterclockwise fashion. Complexation of the phosphorus center by Cr(CO)(5) results in the substantial stabilization of reagents and products and further reduces the barriers for rearrangement. The anti isomer has the lowest barrier for the [1,3]-shift (DeltaE = 20.5 kcal/mol), which is slightly less than that needed for P-epimerization and for conversion of the syn isomer, both of which are nonpericyclic processes. When a P-phenyl group is introduced, the diradical-like conversion of the syn isomer is favored over the anti isomer, in agreement with experimental reports. The influence of torquoselectivity is discussed for the rearrangements of these structures with their heavy substituents. The origin of the stabilization rendered by the Cr(CO)(5) group and its influence on the [1,3]-conversion are also analyzed. The DFT activation energies for the diradical-like [1,3]-sigmatropic shifts were verified with a multireference method.     

Conformational studies by dynamic NMR. 90. Structure and stereodynamics of the rotamers of di- and tri-alpha-naphthylphenyl derivatives

The crystal structure of 1,3,5-tris(4-methylnaphth-1-yl)benzene, 1, shows one naphthyl substituent in an anti relationship to the other two. On the other hand, low temperature (-70 degrees C) (1)H NMR spectra in solution show the presence of a second rotational conformer (rotamer) having all the three naphthyl substituents in a syn relationship. The interconversion barrier between the anti (77%) and syn (23%) rotamers of 1 was determined by line shape simulation of the temperature-dependent NMR spectra (Delta G(++) = 12.1 kcal mol(-1)). In the analogous disubstituted meta and paraderivatives, that is, 1,3- and 1,4-bis(4-methylnaphth-1-yl)benzene (2 and 3, respectively), the presence of both the anti and syn rotamers was also detected by low-temperature NMR spectroscopy. In the latter compounds, the proportions of the anti and syn forms are nearly equal, and the corresponding anti to syn interconversion barriers were found to be lower (11.4 and 11.1(5) kcal mol(-1), respectively) than those of the trisubstituted derivative 1.     

Synthesis and stereodynamics of highly constrained 1,8-bis(2,2'-dialkyl-4,4'-diquinolyl)naphthalenes

The syn and anti isomers of axially chiral 1,8-diquinolylnaphthalenes have been synthesized via Pd-catalyzed Stille coupling of 1,8-dibromonaphthalene and 2-alkyl-4-trimethylstannylquinolines. Optimization of the cross-coupling reaction allowed the preparation of highly constrained 1,8-bis(2,2'-dimethyl-4,4'-diquinolyl)naphthalene, 2, and 1,8-bis(2,2'-diisopropyl-4,4'-diquinolyl)naphthalene, 3, in 42% and 41% yield, respectively. Employing Pd(PPh(3))(4) and CuO as the cocatalysts in the coupling reaction of 1,8-dibromonaphthalene and 2-alkyl-4-trimethylstannylquinolines proved to be superior over other catalysts such as PdCl(2)(dppf), Pd(2)(dba)(3)/P(t-Bu)(3), and POPd. The C(2)-symmetric anti isomers of 2 and 3 were found to be more stable than the corresponding meso syn isomer. The ratio of the two enantiomeric anti conformers to the syn conformer was determined as 7.9:1 for 2 and 8.6:1 for 3 by NMR and HPLC analysis. The atropisomers of 2 and 3 were found to be stable to rotation about the chiral axis at room temperature and all three stereoisomers of 2 were isolated by semipreparative HPLC on a Chiralpak AD column. The diastereoisomers of 3 were separated via preferential crystallization of the anti isomers from diethyl ether. Slow syn/anti interconversion was observed for both atropisomers at enhanced temperature, and the diastereomerization and enantiomerization processes were monitored by NMR and HPLC. The Gibbs activation energy, DeltaG++, for the isomerization of 2 was determined as 116.0 (112.1) kJ/mol for the conversion of the anti (syn) to the syn (anti) isomer at 71.0 degrees C. The rotational energy barrier of 3 was determined as 115.2 (111.1) kJ/mol for the conversion of the anti (syn) to the syn (anti) isomer at 66.2 degrees C.     

Computational evaluation of the evidence for tri-trans-[12]annulene

[reaction: see text] Automerization in tri-trans-[12]annulene (1) was investigated by DFT, MP2, and coupled-cluster methods. Using the highest level of theory employed here, CCSD(T)/cc-pVDZ//BHandHLYP/6-311+G(d,p), we located two low-energy pathways for degenerate conformational change from the lowest-energy conformer of 1 (1a): one with E(a) = 4.5 kcal/mol that interconverts the three inner trans hydrogens with the three outer trans hydrogens and one with E(a) = 2.7 kcal/mol that interconverts the three inner hydrogens with each other. These results are consistent with the experimental results of Oth and co-workers on [12]annulene 1a (Oth, J. F. M.; R?ttele, H.; Schr?der, G. Tetrahedron Lett. 1970, 61). The conformational exchange of the inner trans hydrogens with the outer ones is predicted to occur via a one-step process involving a C(2)-symmetric transition state and not via the D(3)-symmetric transition state (1b) that was postulated earlier. Conformer 1b was found to be a shallow minimum 6.7 kcal/mol above 1a with a barrier of 0.4 kcal/mol for conversion to 1a. Finally, GIAO-B3LYP/6-311+G(d,p) and BHandHLYP/6-311+G(d,p) computed (1)H NMR chemical shifts of 1a and three other low-lying isomers support Oth's original assignment of observed (1)H NMR peaks to 1a at both low and high temperature.     

Monosilicon-substituted cyanoacetylene: a computational study

A detailed theoretical investigation of the [H,Si,C(2),N] potential energy surfaces including 28 minimum isomers and 65 interconversion transition states is reported at the Gaussian-3//B3LYP/6-31G(d) level. Generally, the triplet species lie energetically higher than the singlet ones. The former three low-lying isomers are linear HCCNSi 1 (0.00 kcal/mol), branched SiC(H)CN 12 (7.09 kcal/mol), and bent HNCCSi 7 (14.22 kcal/mol), which are separated by rather high barriers from each other and are kinetically very stable with the least conversion barriers of 32.6-70.5 kcal/mol. Two energetically high-lying isomers HCNCSi 3 (42.99 kcal/mol) and SiC(H)NC 13 (36.05 kcal/mol) are also kinetically stable with a barrier of 49.19 and 21.42 kcal/mol, respectively. Additionally, five high-lying isomers, that is, three chainlike isomers, HCCSiN 2 (55.17), HCSiNC 6 (47.80), HSiNCC 11 (78.83), and one three-membered ring isomer HN-cSiCC 19 (51.21), and one four-membered ring isomer cSiCN(H)C 27 (50.6 kcal/mol), are predicted to each have lower conversion barriers of 12-18 kcal/mol and can be considered as meta-stable species. All of the predicted 10 isomers could exist as stable or meta-stable intermediates under suitable conditions. Finally, the structural and bonding analysis indicate that the [H,Si,C(2),N] molecule contains various properties that are of chemical interest (e.g., silylene, SiC triple bonding, and conjugate SiN triple bonding and CC triple bonding, charge-transfer specie, planar aromatic specie, cumulate double bonding). This is the first detailed theoretical study on the potential energy surfaces of the series of hydrogenated Si,C,C,N-containing molecules. The knowledge of the present monohydrogenated SiC(2)N isomerism could provide useful information for more highly hydrogenated or larger Si,C(2),N-containing species.     

Restricted rotation in 9-phenyl-anthracenes: a prediction fulfilled

The calculated phenyl rotation barrier in 9-phenylanthracene has been reported as ~21 kcal mol(-1), but experimental verification of this barrier is limited by its intrinsic symmetry. V-T NMR indicated the barrier to interconversion of the syn (C(2v)) and anti (C(2h)) rotamers of 9,10-bis(3-fluorophenyl)anthracene to be ~21 kcal mol(-1). Likewise, the V-T NMR spectra of 9-(1-naphthyl)-10-phenylanthracene reveal that the rotational barrier of the unsubstituted phenyl ring is at least 21 kcal mol(-1).     

Conformational studies by dynamic NMR. 94.1 cogwheel pathway for the stereomutations of durene derivatives containing the mesityl ring

The low-temperature NMR spectra of 1,4-bis(mesitoyl)durene, 1, and of 1,4-bis(mesitylethenyl)durene, 2, reveal the presence of syn and anti rotamers at the equilibrium, their relative proportions depending on the dielectric constant of the solvent. In solution the more stable rotamer of 1 is the anti whereas, in the case of 2, the more stable is the syn. Depending on the crystallization solvent employed the more (anti) and the less stable (syn) rotamers were both observed (X-ray diffraction) in the solid state of 1. On the other hand, only the less stable rotamer (anti) was found to be present in the solid state of 2. As shown by MM calculations, the syn-to-anti interconversion occurs via a correlated process (cogwheel pathway) involving the mesityl-C and durene-C bond rotations: the dynamic NMR technique yields an experimental barrier of 8.2 kcal mol(-)(1) for 1 and 13.1 kcal mol(-)(1) for 2. In the case of derivative 2 a second barrier, due to a second type of correlated rotation process (torsion), was also determined (8.6 kcal mol(-)(1)). As a consequence of the restriction of this second torsional motion the anti rotamer of 2 displays two distinguishable NMR spectra at -133 degrees C, corresponding to a pair of conformers with different symmetry (anti C(i)() and anti C(2)).     

Unusually low barrier to carbamate C-N rotation

The barrier for rotation about an N-alkylcarbamate C(carbonyl)-N bond is around 16 kcal/mol. In the case of an N-phenylcarbamate, the rotational barrier is lowered to 12.5 kcal/mol, but with N-(2-pyrimidyl)carbamates the barriers are so low (<9 kcal/mol) that the syn and anti rotamers cannot be observed as separate signals by 500 MHz NMR spectroscopy at 183 K. X-ray and computational data show that the N-(2-pyrimidyl) carbamates have C(carbonyl)-N bonds that are on average 0.03 A longer than for related N-phenylcarbamates. The computational results trace the origin of the effect to increased single bond character for the C(carbonyl)-N bond due to the increased electron-withdrawing ability of the pyrimidyl ring.     

Birch reduction of hexaphenyl- and pentaphenylbenzene and an X-ray crystallography and NMR spectroscopy study of cis- and epi-1,2,3,4,5,6-hexaphenylcyclohexane and of 2,3,5,6-tetraphenyl-1,1'-bicyclohexylidene: Cannizzaro's conundrum revisited

The Birch reduction of hexaphenylbenzene yields two isomers of 1,2,3,4,5,6-hexaphenylcyclohexane. The X-ray crystal structure of the all-cis isomer, 1, reveals that the severe steric crowding among the three axial phenyls is alleviated by a marked splaying out of those three aryl substituents relative to the positioning in a conventional chair structure. A second product, 2, was identified crystallographically and by NMR spectroscopy as the 1,3-diaxial-2,4,5,6-tetraequatorial (epi) isomer of hexaphenylcyclohexane, in which only five of the six additional hydrogen atoms are positioned on the same face of the C(6)Ph(6) precursor. A variable-temperature NMR study of the all-cis isomer 1 yielded a chair-to-chair inversion barrier of approximately 19 kcal mol(-1), which is somewhat higher than the previously reported values for all-cis-1,2,3,4,5,6-C(6)H(6)R(6) in which R=Me or CO(2)Me. The possible relevance to Cannizzaro's 1854 report of a product with the formula (C(7)H(6))(n) is discussed. By contrast, Birch reduction of pentaphenylbenzene led to the formation of 2,3,5,6-tetraphenyl-1,1'-bicyclohexylidene.     

Role of coordination geometry in dictating the barrier to hydride migration in d6 square-pyramidal iridium and rhodium pincer complexes

Syntheses of the olefin hydride complexes [(POCOP)M(H)(olefin)][BAr(f)(4)] (6a-M, M = Ir or Rh, olefin = C(2)H(4); 6b-M, M = Ir or Rh, olefin = C(3)H(6); POCOP = 2,6-bis(di-tert-butylphosphinito)benzene; BAr(f) = tetrakis(3,5-trifluoromethylphenyl)borate) are reported. A single-crystal X-ray structure determination of 6b-Ir shows a square-pyramidal coordination geometry for Ir, with the hydride ligand occupying the apical position. Dynamic NMR techniques were used to characterize these complexes. The rates of site exchange between the hydride and the olefinic hydrogens yielded ΔG(++) = 15.6 (6a-Ir), 16.8 (6b-Ir), 12.0 (6a-Rh), and 13.7 (6b-Rh) kcal/mol. The NMR exchange data also established that hydride migration in the propylene complexes yields exclusively the primary alkyl intermediate arising from 1,2-insertion. Unexpectedly, no averaging of the top and bottom faces of the square-pyramidal complexes is observed in the NMR spectra at high temperatures, indicating that the barrier for facial equilibration is >20 kcal/mol for both the Ir and Rh complexes. A DFT computational study was used to characterize the free energy surface for the hydride migration reactions. The classical terminal hydride complexes, [M(POCOP)(olefin)H](+), are calculated to be the global minima for both Rh and Ir, in accord with experimental results. In both the Rh ethylene and propylene complexes, the transition state for hydride migration (TS1) to form the agostic species is higher on the energy surface than the transition state for in-place rotation of the coordinated C-H bond (TS2), while for Ir, TS2 is the high point on the energy surface. Therefore, only for the case of the Rh complexes is the NMR exchange rate a direct measure of the hydride migration barrier. The trends in the experimental barriers as a function of M and olefin are in good agreement with the trends in the calculated exchange barriers. The calculated barriers for the hydride migration reaction in the Rh complexes are ～2 kcal/mol higher than for the Ir complexes, despite the fact that the energy difference between the olefin hydride ground state and the agostic alkyl structure is ～4 kcal/mol larger for Ir than for Rh. This feature, together with the high barrier for interchange of the top and bottom faces of the complexes, is proposed to arise from the unique coordination geometry of the agostic complexes and the strong preference for a cis-divacant octahedral geometry in four-coordinate intermediates.     

Off-center oxygen-arene interactions in solution: a quantitative study

[reaction: see text] Triptycenes with C1-MeO/RCOO (R = H, Me, Et, i-Pr, CF3) and C9-XC6H4CH2 (X = Me, H, F, CN, CF3) have been prepared to determine lone pair-arene interactions in the off-center configuration. The ratios of the syn and anti conformers were determined by low-temperature NMR spectroscopy. The syn conformer allows the attached arene and the MeO/ester to interact with each other while the anti conformer does not. The free energies of interaction have been derived from the syn/anti ratios. Compound 7 in the ester series with X = H and R = CF3 is the only compound that shows a slightly repulsive interaction (0.08 kcal/mol). Compound 2e in the MeO series with X = CF3 exhibits an attractive interaction (-0.47 +/- 0.05 kcal mol). All other compounds show smaller attractive interactions.     

Conformational analysis and rotational barriers of alkyl- and phenyl-substituted urea derivatives

Potential energy surfaces (PES) for rotation about the N-C(sp(3)) or N-C(aryl) bond and energies of stationary points on PES for rotation about the C(sp(2))-N bond are reported for methylurea, ethylurea, isopropylurea, tert-butylurea, and phenylurea, using the B3LYP/DZVP2 and MP2/aug-cc-pVDZ methods. The analysis of alkylureas reveals cis and (less stable) trans isomers that adopt anti geometries, whereas syn geometries do not correspond to stationary points. In contrast, the analysis of phenylurea reveals that the lowest energy form at the MP2 level is a trans isomer in a syn geometry. The fully optimized geometries are in good agreement with crystal structure data, and PESs are consistent with the experimental dihedral angle distribution. Rotation about the C(sp(2))-N bond in alkylureas and phenylurea is slightly more hindered (8.6-9.4 kcal/mol) than the analogous motion in the unsubstituted molecule (8.2 kcal/mol). At the MP2 level of theory, the maximum barriers to rotation for the methyl, ethyl, isopropyl, tert-butyl, and phenyl groups are predicted to be 0.9, 6.2, 6.0, 4.6, and 2.4 kcal/mol, respectively. The results are used to benchmark the performance of the MMFF94 force field. Systematic discrepancies between MMFF94 and MP2 results were improved by modification of several torsional parameters.     

Theoretical study on structures and stability of C4P isomers

The structures, energetics, spectroscopies, and stabilities of doublet C(4)P isomeric species are explored at the DFT/B3LYP, QCISD, and CCSD(T) (singlet-point) levels. A total of 12 minimum isomers and 27 interconversion transition states are located. At the CCSD(T)/6-311G(2df)//QCISD/6-311G(d)+ZPVE level, the lowest-lying isomer is a floppy CCCCP 1 (0.0 kcal/mol) mainly featuring a cumulenic structure |C=C=C=C=P*|, which differs much from the analogous C4N radical (|*C-C[triple bond]C-C[triple bond]N|). The quasi-linearity and the low bending mode of 1 are in contrast to the previous prediction. The second energetically followed isomer PC-cCCC 3 (14.9 kcal/mol) possesses a CCC ring-bonded to CP. The two low-lying isomers are separated by a high-energy ring-closure/open transition state (26.5 kcal/mol) and thus are very promising candidates for future laboratory and astrophysical detection. Furthermore, four high-energy isomers, that is, two bent isomers CCPCC 2 (68.4 kcal/mol) and CCPCC 2' (68.5 kcal/mol) and two cagelike species 10 (56.0 kcal/mol) and 11 (67.9 kcal/mol), are also stabilized by considerable barriers. The present work is the first detailed potential energy survey of CnP clusters and can provide useful information for the investigation of larger CnP radicals and for understanding the isomerism of P-doped C vaporization processes.     

3,3-Sigmatropic Shifts of Allyl Group Along Cyclopentadiene Ring Perimeter

Reversible non-degenerate 3,3-sigmatropic shifts of the allyl group along the perimeter of the five-membered ring occurring with energy barriers ΔG°≠ = 28.5–30.2 kcal/mol (o-dichlorobenzene-d4) have been detected in the allyl derivatives of 5-methyl-1,2,3,4-tetramethoxycarbonylcyclopentadiene by NMR method. Using DFT B3LYP/6-311++G(d,p) method, it has been shown that degenerate migrations of the allyl group in the related 5-allyl-1,2,3,4,5-pentamethoxycarbonylcyclopentadiene should occur via 3,3-sigmatropic shift through transition states with conformation of a six-membered ring (chair or boat, with close barriers ΔG°≠ = 27.4 or 27.7 kcal/mol, respectively). The simulated higher barrier of alternative 1,5-sigmatropic shifts of the allyl group (ΔG°≠ = 30.8 kcal/mol) indicates the energy preference of the migrations via 3,3-shifts.     

NH tautomerism in the natural chlorin derivatives

The NH tautomerism of five Mg-free chlorophyll a and b derivatives 2-6 was studied utilizing NMR spectroscopy and molecular modeling. The results from the dynamic NMR measurements of the chlorins revealed that substituent effects contribute crucially to the free energy of activation (DeltaG(double dagger)) in the NH tautomeric processes. An intermediate tautomer for the total tautomeric NH exchange in a chlorin was observed for the first time, when the (1)H NMR spectra of chlorin e(6) TME (3) and rhodin g(7) TME (4) (TME = trimethyl ester) were measured at lower temperatures. The lower energy barriers (DeltaG(1)(double dagger)) obtained for the formation of the intermediate tautomers of 3 and 4, assigned to the N(22)-H, N(24)-H trans-tautomer, were 10.8 and 10.6 kcal/mol, respectively. The energy barrier (DeltaG(2)(double dagger) value) for the total tautomeric NH exchange in the five chlorins was found to vary from 13.6 kcal/mol to values higher than 18 kcal/mol. The lowest DeltaG(2)(double dagger) value (13.6 kcal/mol) was obtained for rhodochlorin XV dimethyl ester (2), which was the only chlorophyll derivative lacking the C(15) substituent. In the case of chlorins 4 and 5, the steric crowding around the methoxycarbonylmethyl group at C(15) raised the DeltaG(2)(double dagger) activation free-energy to 17.1 kcal/mol. However, the highest energy barrier with DeltaG(2)(double dagger) > 18 kcal/mol was observed for the NH exchange of pyropheophorbide a methyl ester (6), possessing the macrocycle rigidifying isocyclic ring E. Our results demonstrate that the steric strain, arising either from the steric crowding around the bulky substituent at C(15) or the macrocycle rigidifying isocyclic ring E, slows down the NH tautomeric process. We suggest that deformations in the chlorin skeleton are closely connected to the NH tautomeric exchange and that the exchange occurs by a stepwise proton-transfer mechanism via a hydrogen bridge.     

Unexpected substituent effects in offset pi-pi stacked interactions in water

We have measured the rotational barriers of meta- and para-substituted N-benzyl-2-(2-fluorophenyl)pyridinium bromides in aqueous solution by dynamic NMR as a model system for offset-stacking interactions in proteins. Because the benzyl ring can stack with the 2-fluorophenyl ring in the offset conformation in the ground state, but not the transition state, the rotational barrier reflects the magnitude of the stacking interaction. Only a small (0.1 kcal/mol) change in rotational barrier was found for para substituents relative to hydrogen. A much larger energy difference was found for electronegative meta substituents (up to 0.66 kcal/mol for CF3). Evidence suggests that this is due at least in part to an electrostatic interaction between electron-poor hydrogens on one ring with the electronegative substituents on the other ring.     

Hydrogen atom mediated Stone-Wales rearrangement of pyracyclene: a model for annealing in fullerene formation

We have investigated the Stone-Wales (SW) rearrangement of pyracyclene (C(14)H(12)) using quantum mechanical molecular modeling. Of particular interest in this study is the effect of an added hydrogen atom on the barriers to SW rearrangement. Hydrogen atoms are found in high abundance during combustion, and their effect upon isomerization of aromatic compounds to more stable species may play an important role in the combustion synthesis of fullerenes. We have calculated the barriers for the SW rearrangement in pyracyclene using density functional theory B3LYP/6-31G(d) and B3LYP/6-311G(d,p). Two mechanisms have been investigated: (i) a mechanism with two identical transition states of C(1) symmetry and a cyclobutyl intermediate and (ii) a mechanism with one transition state containing an sp(3) carbon (J. Am. Chem. Soc. 2003, 125, 5572-5580; Nature 1993, 366, 665-667). We find that the barriers for these mechanisms are 120.0 kcal mol(-1) for the cyclobutyl mechanism and 130.1 kcal mol(-1) for the sp(3) mechanism. Adding a hydrogen atom to the internal bridge carbon atoms of pyracyclene reduces the barrier of the cyclobutyl mechanisms to 67.0 kcal mol(-1) and the sp(3) mechanism to 73.1 kcal mol(-1). The bonding of carbon atoms in pyracyclene is similar to those found in isomers of C(60), and the barriers are low enough so that these reactions can become significant during fullerene synthesis in flames. Adding hydrogen atoms to the external bridge atoms on pyracyclene produces a smaller reduction in the SW barrier and adding hydrogen atoms to nonbridge external carbon atoms results in no reduction of the barrier.     

Structural characterization of anti- and syn-allyltricarbonyliron bromide: rotational spectra, quadrupole coupling, and density functional calculations

Rotational transitions for two distinct structural isomers of allyltricarbonyliron bromide have been clearly observed in the cold molecular beam of a pulsed-beam Fourier transform microwave spectrometer. Rotational transitions exhibiting quadrupole splitting patterns for each isomer were measured for the 79Br and 81Br isotopomers. Both isomers are accidental near-prolate symmetric tops. The measured rotational constants for the 79Br isotopomer are A(anti) = 920.6148(2) MHz, B(anti) = 582.8866(12) MHz, C(anti) = 581.3027(12) MHz, A(syn) = 919.5055(1) MHz, B(syn) = 584.1865(1) MHz, and C(syn) = 581.6392(1) MHz. Analysis of the isotopic substitution data and possible transition assignments indicates that these molecules have Cs symmetry. Both isomers are found to have a dipole component along the a axis. However, the anti isomer has a "c" type dipole component, whereas a "b" dipole component is found for the syn isomer. It was found necessary to carefully analyze both rotational constants and the quadrupole coupling data in order to determine the correct assignment of dipole moment components for each isomer. This change in dipole assignments implies that there is a switch of inertial axes upon isomerization resulting from a subtle shift of the allyl center of mass coordinates, upon reorientation of the allyl ligand. The X-ray and DFT calculated structures for the anti isomer are in excellent agreement with the present data. No previous structural data for the syn isomer were available, and the present analysis strongly supports the expected conformation.