Density functional calculations of the 13C NMR chemical shifts in (9,0) single-walled carbon nanotubes

The electronic structure and (13)C NMR chemical shift of (9,0) single-walled carbon nanotubes (SWNTs) are investigated theoretically. Shielding tensor components are also reported. Density functional calculations were carried out for C(30)-capped and H-capped fragments which serve as model systems for the infinite (9,0) SWNT. Based on the vanishing HOMO-LUMO gap, H-capped nanotube fragments are predicted to exhibit "metallic" behavior. The (13)C chemical shift approaches a value of approximately 133 ppm for the longest fragment studied here. The C(30)-capped SWNT fragments of D(3d)/D(3h) symmetry, on the other hand, are predicted to be small-gap semiconductors just like the infinite (9,0) SWNT. The differences in successive HOMO-LUMO gaps and HOMO and LUMO energies, as well as the (13)C NMR chemical shifts, converge slightly faster with the fragment's length than for the H-capped tubes. The difference between the H-capped and C(30)-capped fragments is analyzed in some detail. The results indicate that (at least at lengths currently accessible to quantum chemical computations) the H-capped systems represent less suitable models for the (9,0) SWNT because of pronounced artifacts due to their finite length. From our calculations for the C(30)-capped fragments, the chemical shift of a carbon atom in the (9,0) SWNT is predicted to be about 130 ppm. This value is in reasonably good agreement with experimental estimates for the (13)C chemical shift in SWNTs.     

Decay of C(60) by delayed ionization and C(2) emission: Experiment and statistical modeling of kinetic energy release

C(60) molecules highly excited in the nanosecond regime decay following ionization and dissociation by emitting a series of carbon dimers, as well as other small fragments if excitation is strong enough. The fragmentation mass spectrum and kinetic energy release of all charged fragments obtained in these experiments are interpreted within the framework of the Weisskopf theory, using a realistic Monte Carlo procedure in which the rates of all relevant decay channels are modeled using Arrhenius expressions. Comparison between the measurements and the simulated spectra allows the distribution of deposited energy to be accurately estimated. The dependence of the fragment kinetic energies on the laser fluence, found in the simulation but not observed in the experimental results, indicates that the small fragments are not necessarily emitted from small fullerenes resulting from C(60) by sequential decay. Rather, direct multifragmentation of C(60) is invoked to interpret the observed patterns. The possible role of post-ionization of neutral emitted fragments is discussed.     

Experimental measurement of the van der Waals binding energy of X-O2 clusters (X=Xe, CH3I, C3H6, C6H12)

Van der Waals binding energies for the X-O(2) complexes (X=Xe, CH(3)I, C(3)H(6), C(6)H(12)) are determined by analysis of experimental velocity map imaging data for O((3)P(2)) atoms arising from UV-photodissociation of the complex [A. V. Baklanov et al., J. Chem. Phys. 126, 124316 (2007)]. Several dissociation pathways have been observed, we focus on the channel corresponding to prompt dissociation of X-O(2) into X+2O((3)P) fragments, which is present for complexes of O(2) with all partners X. Our method is based on analysis of the kinetic energy of all three photofragments, where the O atom kinetic energy was directly measured in the experiment and the kinetic energy of the X partner was calculated using momentum conservation, along with the measured angular anisotropy for O atom recoil. We exploit the fact that the clusters are all T-shaped or nearly T-shaped, which we also confirm by ab initio calculations, along with knowledge of the transition dipole governing radiative absorption by the complex. The effect of partitioning the kinetic energy between translation along the X-O(2) and O-O coordinates on the angular anisotropy of the O atom recoil direction is discussed. Van der Waals binding energies of 110±20 cm(-1), 280±20 cm(-1), 135±30 cm(-1), and 585±20 cm(-1) are determined for Xe-O(2), CH(3)I-O(2), C(3)H(6)-O(2), and C(6)H(12)-O(2) clusters, respectively.     

Dynamic pathway model for the formation of C(60)

We present a dynamic pathway model for the formation of C(60) using the action-derived molecular dynamics simulations. We propose candidate precursors for dynamic pathway models in which carbons spontaneously aggregate due to favorable energetics and kinetics. Various planar polycyclic models are in a disadvantageous state where they cannot be trapped in the forward reaction due to their high excess internal energies. Our simulation results show that precursors either in the shape of tangled polycyclics or in the shape of open cages are kinetically favored over precursors in the shape of planar hexagonal graphite fragments. Calculated activation energies for the probable precursor models are in good agreement with experiment. Existence of chains in the models of tangled polycyclics and open cages is beneficially for the formation of C(60) molecule. Chains attached to the precursor model are energetically favorable and display lithe movements along the dynamic pathway.     

Stabilisation energy of C(6)H(6)...C(6)X(6) (X = F, Cl, Br, I, CN) complexes: complete basis set limit calculations at MP2 and CCSD(T) levels

Stabilisation energies of stacked structures of C(6)H(6)...C(6)X(6) (X = F, Cl, Br, CN) complexes were determined at the CCSD(T) complete basis set (CBS) limit level. These energies were constructed from MP2/CBS stabilisation energies and a CCSD(T) correction term determined with a medium basis set (6-31G**). The former energies were extrapolated using the two-point formula of Helgaker et al. from aug-cc-pVDZ and aug-cc-pVTZ Hartree-Fock energies and MP2 correlation energies. The CCSD(T) correction term is systematically repulsive. The final CCSD(T)/CBS stabilisation energies are large, considerably larger than previously calculated and increase in the series as follows: hexafluorobenzene (6.3 kcal mol(-1)), hexachlorobenzene (8.8 kcal mol(-1)), hexabromobenzene (8.1 kcal mol(-1)) and hexacyanobenzene (11.0 kcal mol(-1)). MP2/SDD** relativistic calculations performed for all complexes mentioned and also for benzene[dot dot dot]hexaiodobenzene have clearly shown that due to relativistic effects the stabilisation energy of the hexaiodobenzene complex is lower than that of hexabromobenzene complex. The decomposition of the total interaction energy to physically defined energy components was made by using the symmetry adapted perturbation treatment (SAPT). The main stabilisation contribution for all complexes investigated is due to London dispersion energy, with the induction term being smaller. Electrostatic and induction terms which are attractive are compensated by their exchange counterparts. The stacked motif in the complexes studied is very stable and might thus be valuable as a supramolecular synthon.     

Theoretical study of M(PH3)2 complexes of C60, corannulene (C20H10), and sumanene (C21H12) (M = Pd or Pt). Unexpectedly large binding energy of M(PH3)2(C60)

DFT and MP2 to MP4(SDQ) methods were applied to M(PH3)2(C60), Pt(PH3)2(C20H10), and Pt(PH3)2(C21H12) (M = Pd or Pt, C20H10 = corannulene, and C21H12 = sumanene). The binding energy considerably fluctuates around MP2 and MP3 levels but much less upon going from MP3 to MP4(SDQ) in Pt(PH3)2(C2H4), Pt(PH3)2(C20H10), and Pt(PH3)2(C21H12). Also, the MP4(SDQ) method presents a binding energy similar to that of the CCSD(T) method in Pt(PH3)2(C2H4). Thus, it is likely that the MP4(SDQ) method is useful to evaluate binding energies of these complexes. The binding energies of Pt(PH3)2(C20H10) and Pt(PH3)2(C21H12) are evaluated to be 24.9 and 26.1 kcal/mol, respectively, by the MP4(SDQ) method and only +5.8 and -2.6 kcal/mol, respectively, by the DFT(B3LYP) method. These MP4(SDQ)-calculated binding energies of Pt(PH3)2(C20H10) and Pt(PH3)2(C21H12) are similar to that of Pt(PH3)2(C2H4), which strongly suggests that these complexes can be successfully synthesized. The binding energy of Pt(PH3)2(C60) is evaluated to be 44.8 and 45.5 kcal/mol with the ONIOM(MP4(SDQ):UFF) and ONIOM(MP4(SDQ):B3LYP) methods, respectively, and that of the Pd analogue is evaluated to be 39.9 kcal/mol with the ONIOM(MP4(SDQ):UFF) method, whereas the DFT(B3LYP), DFT(BVP86), and DFT(BPW91) methods provide much smaller binding energies. It is noted that these binding energies are much larger than those of the ethylene, corannulene, and sumanene analogues. This difference is reasonably interpreted in terms that the LUMO of C60 is at much lower energy than those of ethylene, corannulene, and sumanene. We investigated also how to separate the high level and the low level regions in the ONIOM calculation of M(PH3)2(C60) and proposed here the reasonable way to evaluate the binding energy of transition-metal complexes of C60.     

Syntheses of the C(1-6) and C(19-24) fragments of lituarines A, B, and C

[structure: see text] Lituarines A-C are marine natural products comprising a tricyclic spiroacetal bridged at C(8) and C(18) by a functionalized ester linkage conceptually obtained from a C(19-24) alcohol and a C(1-6) carboxylic acid whose oxidation level varies at C(4) and C(5). Stereoselective routes are described to compounds 26 and 27, fully functionalized ester fragments of lituarine A and lituarines B and C, respectively.     

Gas-phase interaction of thiophene with the Ti(8)C(12)(+) and Ti(8)C(12) met-car clusters

The reactivity of the Ti(8)C(12)(+) met-car cation toward thiophene was investigated using density functional theory (DFT) and mass selective ion chemistry. It is shown that the experimentally observed mass spectrum can be well described by the DFT calculations. In contrast to the weak bonding interactions seen for thiophene on a TiC(001) surface, the Ti(8)C(12)(+) met-car cation is able to interact strongly with up to four thiophene molecules with the cluster staying intact. In the most stable conformation, the thiophene molecules bond to the four low-coordinated Ti(0) sites of Ti(8)C(12)(+) via a eta(5)-C,S coordination. The stability and the activity of the Ti(8)C(12)(+) met-car is observed to increase with an increasing number of attached thiophene molecules at the Ti(0) sites, which is associated with a significant transfer of electron density from thiophene to the cluster. The additional electron density on the Ti(8)C(12)(+) cation cluster, however, is not sufficient to cleave the C-S bonds of thiophene and the dissociation reaction of thiophene is predicted to be a highly activated process. By contrast, DFT calculations for the neutral Ti(8)C(12) met-car predict that the dissociation reaction leading to adsorbed S and C(4)H(4) fragments is energetically favorable for the first thiophene molecule. The binding behavior for subsequent addition of thiophene molecules to the neutral met-car is also presented and compared to that of the cation.     

Intrinsic conformational preferences of C(alpha,alpha)-dibenzylglycine

The intrinsic conformational preferences of C (alpha,alpha)-dibenzylglycine, a symmetric alpha,alpha-dialkylated amino acid bearing two benzyl substituents on the alpha-carbon atom, have been determined using quantum chemical calculations at the B3LYP/6-31+G(d,p) level. A total of 46 minimum energy conformations were found for the N-acetyl- N'-methylamide derivative, even though only nine of them showed a relative energy lower than 5.0 kcal/mol. The latter involves C 7, C 5, and alpha' backbone conformations stabilized by intramolecular hydrogen bonds and/or N-H...pi interactions. Calculation of the conformational free energies in different environments (gas-phase, carbon tetrachloride, chloroform, methanol, and water solutions) indicates that four different minima (two C 5 and two C 7) are energetically accessible at room temperature in the gas phase, while in methanol and aqueous solutions one such minimum (C 5) becomes the only significant conformation. Comparison with results recently reported for C (alpha,alpha)-diphenylglycine indicates that substitution of phenyl side groups by benzyl enhances the conformational flexibility leading to (i) a reduction of the strain of the peptide backbone and (ii) alleviating the repulsive interactions between the pi electron density of the phenyl groups and the lone pairs of the carbonyl oxygen atoms.     

Hydrogen atom transfer reactions of C2-, C4-, and C6-: bond dissociation energies of linear H-C2n- and H-C2n (n = 1, 2, 3)

The reactions of C2-, C4-, and C6- with D2O and ND3 and of C4- with CH3OH, CH4, and C2H6 have been investigated using guided ion beam tandem mass spectrometry. Hydrogen (or deuterium) atom transfer is the major product channel for each of the reactions. The reaction threshold energies for collisional activation are reported. Several of the reactions exhibit threshold energies in excess of the reaction endothermicity. Potential energy calculations using density functional theory show energy barriers for some of the reactions. Dynamic restrictions related to multiple wells along the reaction path may also contribute to elevated threshold energies. The results indicate that the reactions with D2O have the smallest excess threshold energies, which may therefore be used to derive lower limits on the C-H bond dissociation energies of the C2nH- and C2nH (n = 1-3) linear species. The experimental lower limits for the bond dissociation energies of the neutral radicals to linear products are D0(C2-H) >or= 460 +/- 15 kJ/mol, D0(C4-H) >or= 427 +/- 12 kJ/mol, and D0(C6-H) >or= 405 +/- 11 kJ/mol.     

Synthesis of the C(18)-C(34) fragment of amphidinolide C and the C(18)-C(29) fragment of amphidinolide F

A convergent synthesis of the C(18)-C(34) fragment of amphidinolide C and the C(18)-C(29) fragment of amphidinolide F is reported. The approach involves the synthesis of the common intermediate tetrahydrofuranyl-β-ketophosphonate via cross metathesis, Pd(0)-catalyzed cyclization, and hydroboration-oxidation. The β-ketophosphonate was coupled to three side chain aldehydes using a Horner-Wadsworth-Emmons (HWE) olefination reaction to give dienones, which were reduced with l-selectride to give the fragments of amphidinolide C and F.     

Structures and stability of isomers of [C,N,N,P] system

Interstellar species have been of interest to chemists because of their unusual structures and reactivities, such as CN, NP, CP, and SiN, which have been identi-fied in interstellar medium[1―4] and well characterized for the formation, structures, spectr…     

Selected Ion Flow Tube Study of the Gas-Phase Reactions of CF(+), CF(2)(+), CF(3)(+), and C(2)F(4)(+) with C(2)H(4), C(2)H(3)F, CH(2)CF(2), and C(2)HF(3)

We study how the degree of fluorine substitution for hydrogen atoms in ethene affects its reactivity in the gas phase. The reactions of a series of small fluorocarbon cations (CF(+), CF(2)(+), CF(3)(+), and C(2)F(4)(+)) with ethene (C(2)H(4)), monofluoroethene (C(2)H(3)F), 1,1-difluoroethene (CH(2)CF(2)), and trifluoroethene (C(2)HF(3)) have been studied in a selected ion flow tube. Rate coefficients and product cations with their branching ratios were determined at 298 K. Because the recombination energy of CF(2)(+) exceeds the ionization energy of all four substituted ethenes, the reactions of this ion produce predominantly the products of nondissociative charge transfer. With their lower recombination energies, charge transfer in the reactions of CF(+), CF(3)(+), and C(2)F(4)(+) is always endothermic, so products can only be produced by reactions in which bonds form and break within a complex. The trends observed in the results of the reactions of CF(+) and CF(3)(+) may partially be explained by the changing value of the dipole moment of the three fluoroethenes, where the cation preferentially attacks the more nucleophilic part of the molecule. Reactions of CF(3)(+) and C(2)F(4)(+) are significantly slower than those of CF(+) and CF(2)(+), with adducts being formed with the former cations. The reactions of C(2)F(4)(+) with the four neutral titled molecules are complex, giving a range of products. All can be characterized by a common first step in the mechanism in which a four-carbon chain intermediate is formed. Thereafter, arrow-pushing mechanisms as used by organic chemists can explain a number of the different products. Using the stationary electron convention, an upper limit for Δ(f)H°(298)(C(3)F(2)H(3)(+), with structure CF(2)═CH-CH(2)(+)) of 628 kJ mol(-1) and a lower limit for Δ(f)H°(298)(C(2)F(2)H(+), with structure CF(2)═CH(+)) of 845 kJ mol(-1) are determined.     

Conformational analysis of the sixteen C(4)-C(6) and C(4)-C(8) linked dimers of (+)-catechin and (−)-epicatechin

A conformational analysis has been performed for sixteen dimers of (+)-catechin and/or (?)-epicatechin using molecular mechanics (MM2). Monomer units are linked by 4α-6, 4α-8, 4β-6, and 4β-8 bonds. THe four possible combinations of (+)-catechin and/or(?)-epicatechin are used for each bonding pattern. The objectives are characterization of (1) the two rotational isomers at the bond between the two monomer units and (2) the conformations of the heterocyclic rings. There is a twofold rotation about the bond between monomer units. Differ4ences in the energies at the two minima range from a few tenths of a kcal/mol to several kcal/mol, depending on the dimer Heterocyclic rings occupy a range of conformations that can be described as half chairs with varying degrees of distoration toward C(2) or C(3) sofas. The more frequent distortion is toward the C(2) sofa. Interconversion between most of the heterocyclic ring conformations can be obtained by coordinated motion of C(2) and C(3), over a range of about 40 pm, with respect to the mean plane of the fused aromatic ring system.     

Towards Stereoselective Synthesis of the C(31)–C(39) and C(20)–C(27) Fragments of Phorboxazole A

The stereoselective synthesis of the C(31)–C(39) and C(20)–C(27) fragments of phorboxazole A ( 1 ) was achieved from commercially available and inexpensive D ‐mannitol. Crimmins aldol reaction and a decarboxylative Claisen‐type reaction are the key steps for the C(31)–C(39) fragment, and L ‐proline‐catalyzed aldol reaction, Sharpless asymmetric epoxidation, and epoxide ring opening reaction with Gilman's reagent are the key steps for the C(20)–C(27) fragment of phorboxazole.     

Methane activation by nickel cluster cations, Ni+n (n=2-16): reaction mechanisms and thermochemistry of cluster-CHx (x=0-3) complexes

The kinetic energy dependences of the reactions of Ni+(n) (n=2-16) with CD(4) are studied in a guided ion beam tandem mass spectrometer over the energy range of 0-10 eV. The main products are hydride formation Ni(n)D+, dehydrogenation to form Ni(n)CD+(2), and double dehydrogenation yielding Ni(n)C+. These primary products decompose at higher energies to form Ni(n)CD+, Ni(n-1)D+, Ni(n-1)C+, Ni(n-1)CD+, and Ni(n-1)CD+(2). Ni(n)CD(2) (+) (n=5-9) and Ni(n-1)CD(2) (+) (n > or =4) are not observed. In general, the efficiencies of the single and double dehydrogenation processes increase with cluster size. All reactions exhibit thresholds, and cross sections for the various primary and secondary reactions are analyzed to yield reaction thresholds from which bond energies for nickel cluster cations to C, CD, CD(2), and CD(3) are determined. The relative magnitudes of these bond energies are consistent with simple bond order considerations. Bond energies for larger clusters rapidly reach relatively constant values, which are used to estimate the chemisorption energies of the C, CD, CD(2), and CD(3) molecular fragments to nickel surfaces.     

Stereoselective syntheses of the C(1)-C(9) fragment of amphidinolide C

Stereoselective syntheses of the C(1)-C(9) fragments 18 and 28 of amphidinolide C have been developed. The first-generation sequence involves a diastereoselective chelate-controlled [3 + 2]-annulation reaction of 6 and 7, while the second-generation synthesis involves an intramolecular hetero-Michael cyclization of 8.     

Electronic spectra of heteroatom-containing isoelectronic carbon chains C(2n)S and C2(n)Cl+ (n=1-5)

Structures and stabilities of carbon chains C(2n)S and C2(n)Cl+ (n=1-5) in their ground states have been investigated by the density functional theory and the coupled cluster approach using single and double substitutions. The complete active space self-consistent-field method has been used for geometry optimization of selected excited states in both series. Calculations show that both C(2n)S (n=1-5) and C2(n)Cl+ (n=3-5) have linear structures in the triplet ground state 3Sigma-, while C2Cl+ and C4Cl+ have nonlinear structures in the ground state 3A". The vertical transition energies and emission energies by the multiconfigurational second-order perturbation theory in linear clusters C(2n)S and C2(n)Cl+ exhibit similar size dependences. In comparison with the available experimental observations, the predicted excitation energies for the allowed 2 3Sigma- <--X 3Sigma- transitions have an accuracy of no more than 0.24 eV. Spin-orbit coupling configuration interaction calculations indicate that the spin-forbidden 2 1Sigma+<--X 3Sigma- transition in these species has an oscillator strength with the magnitude of 10(-4)-10(-5), and they may be observable experimentally.     

HMBC-RELAY: a combined technique for the differentiation of simultaneously detected 2J(C,H) and (n)J(C,H) connectivities

We present a new pulse sequence that detects simultaneously (n)J(C,H) and 2J(C,H) connectivities. The corresponding coherences are created along independent pathways and therefore can be separated into two different subspectra. One spectrum is to show all (n)J(C,H) connectivities and the other is to show exclusively 2J(C,H) connectivities. In contrast to the previously published 2J/(n)J experiment, this sequence detects the 2J(C,H) connectivities via a C,H,H-RELAY pathway leading to an intensification of the 2J(C,H) signals. Strictly, the 2J(C,H) spectrum does not show 2J(C,H) but 3J(H,H) coupling interactions within 13CH(k)-12CH(l) fragments. Therefore, 2J(C,H) signals can appear even if the corresponding 2J(C,H) coupling constant is zero.     

Theoretical analysis of the hydrogen bond of imidazolium C(2)-H with anions

The intermolecular interaction energies of ion pairs of imidazolium-based ionic liquids were studied by MP2/6-311G level ab initio calculations. Although the hydrogen bond between the C(2) hydrogen atom of an imidazolium cation and anion has been regarded as an important interaction in controlling the structures and physical properties of ionic liquids as in the cases of conventional hydrogen bonds, the calculations show that the nature of the C(2)-H...X interaction is considerably different from that of conventional hydrogen bonds. The interaction energies of the imidazolium cation with neighboring anions in the four crystals of ionic liquids were calculated. The size of the interaction is determined mainly by the distance between the imidazolium ring and anion. The calculated interaction energy is nearly inversely proportional to the distance, which shows that the charge-charge interaction is the dominant interaction in the attraction. The orientation of the anion relative to the C(2)-H bond does not greatly affect the size of the interaction energy. Calculated interaction energy potentials of 1,3-dimethylimidazolium tetrafluoroborate ([dmim][BF(4)]) complexes show that the C(2)-H bond does not prefer to point toward a fluorine atom of the BF(4). This shows that the C(2)-H...X hydrogen bond is not essential for the attraction.