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Three new indole diterpenoids, aculeatupenes A-C (

In a specifically designed molecular structure, two sp2-hybridized carbon atoms align their unhybridized 2pz orbitals in the same orientation to form an extremely long C2pz-C2pz σ-single bond that goes beyond 3 A in length. This new type of C-C σ-bond (coined as a fringe bond) can be made more than 1 A longer than the current experimental world record (∼1.8 A) and 300 kJ/mol weaker than the ordinary C-C σ-bond (∼330 kJ/mol). If such molecular monomers are polymerized into infinite chains, the extended long C-C saturated σ-bonding framework manifests band gaps similar to those of some conventional iconic organic-conducting polymers based on conjugated networks of unsaturated bonds.

Three new indole diterpenoids, aculeatupenes A–C (1–3), together with four known compounds (4–7), were isolated from the mycelium of Aspergillus aculeatus KKU-CT2. Their structures were established by spectroscopic evidence and absolute configurations of 1–3 were determined by comparison of their experimental and calculated ECD spectra. Compounds 1, 2, and emindole SB (4) showed weak cytotoxicity against HelaS3, KB, HepG2, MCF-7, and A549 cancer cell lines with IC50 values in the range of 11.12–67.81 μM. Compound 3 showed weak cytotoxicity against HelaS3 cell lines with an IC50 value of 17.48 μM but non-cytotoxicity against Vero cell line. In addition, compound 1 exhibited weak antibacterial activity against Bacillus cereus.

Biotransformation of β-mangostin (1) by the endophytic fungus Xylaria feejeensis GM06 afforded hexacyclic ring-fused xanthenes with an unprecedented hexacyclic heterocylic skeleton. β-Mangostin (1) was transformed to two diastereomeric pairs of enantiomers, mangostafeejin A [(-)-2a/(+)-2b)] and mangostafeejin B [(-)-3a/(+)-3b)]. The chemical structures of the transformation products were elucidated by analysis of NMR and MS data, and the structure of mangostafeejin A [(-)-2a/(+)-2b)] was confirmed by single-crystal X-ray diffraction analysis. The absolute configurations of 3a and 3b were established on the basis of calculated and measured ECD data using the ECD spectra of 2a and 2b as models. The fungal biotransformation described herein provides an effective method to convert an abundant achiral plant natural product scaffold into new chiral heterocyclic scaffolds representing expanded chemical diversity for biological activity screening.

Syn and anti dihydropyrene (DHP) are excellent thermochromes, and therefore extensively studied for their thermochromic and photochromic properties, respectively. However, they suffer from thermal decomposition due to thermal instability. In this study, we thoroughly investigated pathways for the thermal decomposition of anti- and syn- dihydropyrenes through computational methods. The decomposition pathways include sigmatropic shift and hemolytic and heterolytic (cationic and anionic) cleavages. The decomposition pathway is influenced not only by the dihydropyrene (syn- or anti-) but also by the functional groups present. For anti-dihydropyrenes, sigmatropic shift is the most plausible pathways for CN and CHO internal groups. The cascade of sigmatropic shifts is followed by elimination to deliver substituted pyrenes. For CH3- and H- dihydropyrenes, hemolytic cleavage of the internal groups is the most plausible pathway for decomposition to pyrenes. The pathway is changed to heterolytic cleavage when the internal groups on the dihydropyrenes are Cl−, Br−, and SMe−. Comparison of the activation barriers for syn (30.18 kcal mol−1) and anti (32.10 kcal mol−1) dimethyldihydropyrenes for radical pathway reveal that decomposition of syn- DHP is more facile over anti-, which is consistent with the experimental observation. The decomposition pathway for syn-dihydropyrene is also hemolytic in cleavage when the internal groups are methyl and hydrogen. Syn-dihydropyrenes (symmetrical or unsymmetrical) bearing CN group do not follow sigmatropic shift, quite contrary to the anti-dihydropyrene. The lack of tendency of the syn-dihydropyrene for sigmatropic shift is rationalized on the planarity of the scaffold. The results of the theoretical study are consistent with the experimental observations. The results here help in understanding the behavior of substituents on the dihydropyrene scaffold, which will be useful in designing new molecules with improved thermal stabilities.

The elimination of superoxide anion free radical O 2 ·− by the complexes of quercetin and physiological metal ions has been investigated theoretically. It turns out that Ca(II) and Zn(II) quercetin complexes are mild and efficient reagent on O 2 ·− fixing, and Fe(II) and Ni(II) quercetin complexes are excellent reagent on O 2 ·− elimination, and Cu(II) and Co(II) quercetin complexes are mainly no effect on O 2 ·− , which is consistent with some experiments, and Cu(I) and Co(I) complexes are active on O 2 ·− elimination on some sites of –OH. Some metal ion complexes herein only have one active center on metal ion itself, and none of their OH sites is active on O 2 ·− fixing such as Cr(III) and Fe(III) and Mn(II). The possible transition states of the interaction between quercetin molecule and O 2 ·− on different OH sites also have been investigated. We get the similar transition states for different OH sites, the activation energy is heavily high from reactants to transition states, and the transition states are near to the final products, which indicates that though the quercetin molecule can eliminate O 2 ·− , metal ions quercetin complexes are quite good at it, and some complexes can eliminate more than one O 2 ·− at the same time, which may be the potential effective medicines for O 2 ·− elimination to protect β cells from O 2 ·− attacking and helpful for diabetics and other relative diseases.

A two-dimensional potential energy surface was utilized to treat the proton transfer in acetylacetone (AA) and its α-halo derivatives: α-fluoro-(FAA), α-chloro-(ClAA), and α-bromo-acetylacetone (BrAA). This potential energy function, which couples O-H stretching and in-plane bending vibrations, was acquired through ab initio calculations for a fixed skeleton geometry. The resulting potential energy surfaces were then used to calculate the proton tunneling frequencies and proton transfer barrier heights. The barrier heights (the energy difference between the saddle point and the minima) calculated at the MP2/6-31G(2d,p) level of theory for proton transfers in AA, FAA, ClAA, and BrAA are 7.2, 9.4, 6.3, and 5.9 kcal mol(-1), respectively. The theoretically predicted proton transfer barrier heights exhibit excellent linear correlations with geometrical, electronic structural, and topological parameters evaluated by the atoms-in-molecule (AIM) and natural bond orbital (NBO) analyses.

To save time and computer resources, we made an attempt to design reasonable yet simple structural indicators to identify weak chemical bonds, instead of performing numerous, tedious calculations of individual bond dissociation energies (BDEs) for all bonds within a molecule. Based on the commonly available structure-property indicators for bond strength, such as bond length (R), the Mulliken interatomic electron number (MIEN), the Wiberg bond order (WBO), and BDE, we have created two new bond-strength indicators, i.e., M = MIEN/R and K = (WBO × MIEN)/R2, which shall be directly used to efficiently identify almost all weak bonds with BDE below 350 kJ/mol. If several bonds of the same type attain the same smallest values of M or K, values of the electron density at the bond critical points (ρ c ) alone can almost always pinpoint the weakest bond from the set of weak bonds, greatly reducing the amount of efforts in carrying out the calculations of the BDEs of the corresponding bonds.

Based on Lindgren and Salomonson’s analysis on Frechet differentiability [Phys Rev A 67:056501 (2003)], we showed a specific variational path along which the Frechet derivative of the Levy-Lieb functional does not exist in the unnormalized density domain. This conclusion still holds even when the density is restricted within a normalized space. Furthermore, we extended our analysis to the Lieb functional and demonstrated that the Lieb functional is not Frechet differentiable. Along our proposed variational path, the Gâteaux derivative of the Levy-Lieb functional or the Lieb functional takes a different form from the corresponding one along other more conventional variational paths. This fact prompted us to define a new class of unconventional density variations and inspired us to present a modified density variation domain to eliminate the problems associated with such unconventional density variations.

Four new sesterterpenoids, ansellone B (4), phorbadione (5), secoepoxyansellone A (6), and alotaketal C (7), have been isolated from specimens of the sponge Phorbas sp. collected in British Columbia, Ansellone B (4) has an unprecedented heterocyclic skeleton featuring an oxocane ring, and secoepoxyansellone A (6) is the first example of the degraded "secoansellane" sesterterpenoid carbon skeleton. Alotaketal C (7) is an activator of cAMP signaling in HEK cells.

A first complete μSR study of the T dependences of the (reduced) muon, Aμ′(T), and proton, Ap(T), β-hyperfine coupling constants (hfcc) of the muoniated t-butyl radical is reported in the faujasitic zeolites NaY and USY, and the results are compared with similar data and with early EPR results in condensed bulk phases. The results are also compared with single-molecule UMP2 and DFT/B3LYP calculations in the bulk and in an NaY zeolite fragment of Si and O atoms with both OH- and H-capping. Muon hfcc are reported for the first time for the Mu-isobutyl radical in the bulk phase and are also compared with theory and with EPR data. The present results for the muon and proton hfcc of Mu-t-butyl in the bulk complement earlier work published elsewhere at higher temperatures but are extended here down to 5 K to facilitate comparisons with in vacuo theory at 0 K. Good fits to the data for both Aμ′(T) and Ap(T) for Mu-t-butyl are found from the calculated hfcc in both the bulk and in NaY, assuming a Boltzmann-weight...

3-Amino-1-phenyl-2-buten-1-one (APBO) was synthesized by amination of benzoylacetone (BA) and its structure was studied by X-ray crystallographic method. The geometry of APBO was also optimized by means of density functional theory (DFT) and ab initio calculations and the results were compared with the X-ray crystallographic data. The vibrational fundamentals predicted within harmonic model, calculated at the B3LYP/6-311++G**, and by anharmonic model, calculated at the B3LYP/6-31G** level, display excellent agreement with the measured data. The proposed assignments are further confirmed by observing the deuterium isotopic shifts of different bands through predictions by the same theoretical method. The theoretical results obtained for APBO were compared with those of 4-amino-3-penten-2-one (APO).

Computational methods were used to study the surprising 2004 synthesis of decamethyldizincocene, Zn2(η(5)-C5Me5)2, which was the first molecule to have a direct, unbridged bond between two first-row transition metals. The computational results show that the methyl groups of decamethylzincocene, Zn(η(5)-C5Me5)(η(1)-C5Me5), affect the transition-state stability of its reaction with ZnEt2 (or ZnPh2) through steric hindrance, and this could possibly allow a counter-reaction, the homolytic dissociation of Zn(η(5)-C5Me5)(η(1)-C5Me5) into Zn(η(5)-C5Me5)(•) and (η(1)-C5Me5)(•), to occur, and because no such steric hindrance occurs when zincocene, Zn(η(5)-C5H5)(η(1)-C5H5), is used as a reactant, its dissociation never occurs regardless of what ZnR2 reagent is used.

Borrowing ideas from Hess’s law in thermodynamics and the holographic electron density theorem, we have proposed two novel population analyses based on ionic partition of overlap distributions (IPOD). The IPOD population analyses combine the conceptual simplicity of the Mulliken and Lowdin population analyses with low computational cost and mild basis-set variation effect. More promisingly, the resultant IPOD charges are of the similar quality to the natural population analysis.

Following our recent paper on linear-expansion shooting techniques (LIST) [Wang, Y. A.; Yam, C. Y.; Chen, Y. K.; Chen, G. H. J. Chem. Phys.2011, 134, 241103], in which the direct approach (LISTd) and the indirect approach (LISTi) were proposed to accelerate the self-consistent field convergence, we discovered a highly simple solution to cure the linear-dependence problem of LISTd. The resultant method, LISTb, is a better direct approach to LIST and muscles similar performances to existing LIST methods. More promisingly, LISTb even outshines the best LIST method, LISTi, for systems involving transition-metal atoms.

We have studied single-walled carbon nanotubes (SWCNTs) doped with transition metal (TM) atoms with both exo and endo doping configurations. The electronic and geometric properties of these TM-doped SWCNTs were calculated within density functional theory. It was found that the endo-doped SWCNTs are less stable than the exo-doped counterparts due to the large geometric strain of the deformation in the endo-doped nanotubes. On the basis of partial charge analysis, the TM-doped SWCNTs have localized net charge distributions, whereas the spin densities in Sc-, Co-, and Cu-doped SWCNTs are delocalized over the entire nanotubes. The TM-doped SWCNTs are mostly metallic or narrow-gap semiconductive. With highly localized frontier molecular orbitals, the exo-doped SWCNTs are better electron donors than the corresponding endo-doped systems. As the dopant TM changes from Sc to Zn in the same row of the periodic table or from the top to the bottom in the same Pt group, the energy of the highest occupied crystal orbit...

The Lewis acidic character of alkyl- and arylplatinum complexes has been investigated within density functional theory with an emphasis on supramolecular structures (e.g., buckybowls, fullerenes, and nanotubes) doped with Pt atoms. By selective substitution of a single atom within a larger carbon framework with a Pt atom, the resulting species demonstrate a strong preference for interaction with small organic molecules, particularly polar substances via a Lewis acid/Lewis base complexation. Coordination with CO and maleic anhydride offers a gauge of comparing the Lewis acidity of these alkyl- and arylplatinum complexes. We have also examined the use of these organometallic species as catalysts for the Diels−Alder reaction and have found that curved π-conjugation and delocalization of electrons offer modest rate enhancements and improvements in selectivity. Preliminary results suggest that extended molecular architectures such as Pt-doped nanotubes may provide selectivities that differ from that of traditi...

The hyperfine coupling constants (HFCCs) of all the butyl radicals that can be produced by muonium (Mu) addition to butene isomers (1- and 2-butene and isobutene) have been calculated, to compare with the experimental results for the muon and proton HFFCs for these radicals reported in paper II (Fleming, D. G.; et al. J. Phys. Chem. A 2011, 10.1021/jp109676b) that follows. The equilibrium geometries and HFCCs of these muoniated butyl radicals as well as their unsubstituted isotopomers were treated at both the spin-unrestricted MP2/EPR-III and B3LYP/EPR-III levels of theory. Comparisons with calculations carried out for the EPR-II basis set have also been made. All calculations were carried out in vacuo at 0 K only. A C-Mu bond elongation scheme that lengthens the equilibrium C-H bond by a factor of 1.076, on the basis of recent quantum calculations of the muon HFCCs of the ethyl radical, has been exploited to determine the vibrationally corrected muon HFCCs. The sensitivity of the results to small variations around this scale factor was also investigated. The computational methodology employed was "benchmarked" in comparisons with the ethyl radical, both with higher level calculations and with experiment. For the β-HFCCs of interest, compared to B3LYP, the MP2 calculations agree better with higher level theories and with experiment in the case of the eclipsed C-Mu bond and are generally deemed to be more reliable in predicting the equilibrium conformations and muon HFCCs near 0 K, in the absence of environmental effects. In some cases though, the experimental results in paper II demonstrate that environmental effects enhance the muon HFCC in the solid phase, where much better agreement with the experimental muon HFCCs near 0 K is found from B3LYP than from MP2. This seemingly better level of agreement is probably fortuitous, due to error cancellations in the DFT calculations, which appear to mimic these environmental effects. For the staggered proton HFCCs of the butyl radicals exhibiting no environmental effect in paper II, the best agreement with experiment is consistently found from the B3LYP calculations, in agreement also with benchmark calculations carried out for the ethyl radical.

Interactions between atomic Pt and pristine or Stone−Wales-defective (5,5) single-walled boron nitride nanotubes (BNNTs) were studied using density functional theory (DFT) with truncated nanotube models. The binding energy of Pt on a pristine BNNT is about 20 kcal/mol with little dependency on the binding site. On the other hand, when the Stone−Wales (SW) defect is presented, the atomic Pt is preferentially inserted between the B−B bond in the SW defect region with a large binding energy of 58 kcal/mol. On an SW-defective BNNT, the atomic Pt, even placed away from the defect site, may eventually (thermodynamically) move toward the defect area until being trapped between the B−B bond, and the final adduct has decreased reactivity toward both electrophiles and nucleophiles compared with Pt adsorption to pristine BNNTs. Pt adsorption on pure or SW-defective BNNTs makes the hosting nanotube wide-gap semiconductive by introducing the valence states of the absorbed Pt into the band gap of the nanotube. In compa...

The pyridine–pyridinium cationic complexes [(Py2H) + ], with two different counterions (perchlorate and bromide), were prepared. The vibrational spectra of the complexes have been recorded. We then performed theoretical calculations to aid our understanding of the experimental data. To include the proton tunneling effect in our study, we adopted a simple one-dimensional potential energy surface (PES) for the motion of the hydrogen atom (at the center). This PES was derived from quantum mechanical calculations at the B3LYP/6-311++G �� level of theory for a fixed skeleton geometry. Based on such a PES, we computed the vibrational energy levels, from which a tunneling splitting of 487 ± 2 and 242 ± 2 cm � 1 was

A simple, practical method for treating the proton tunneling in 2-nitromalonaldehyde (NO2MA) was used. A two-dimensional potential energy surface (PES) function, which couples OH stretching and in-plane bending modes, has been constructed for motions of the hydrogen atom by the aid of quantum mechanical calculations at the MP2/6-31G** level for a fixed skeleton geometry. This PES function was used to calculate energy levels, from which a tunneling splitting of 34.5 and 3.2 cm−1 was obtained for light and deuterated NO2MA, respectively, in excellent agreement with the experimental value of 35.0 ± 15.0 and 3.0 ± 0.02 cm−1, respectively. The double-well barrier height was predicted to be 46.4 kJ/mol with respect to the midpoint between the two adjacent minima of the hydrogen motions on the PES. The hydrogen-bond strength was estimated to be over 50 kJ/mol.

The geometries, chemical shifts, aromaticity, and reactivity of finite-length open-ended armchair single-walled carbon nanotubes (SWCNTs) have been studied within density functional theory. The widely used model of capping hydrogen atoms at the open ends of a SWCNT changes the chemical activity of the SWCNT and destabilizes the frontier molecular orbitals. The edge pi-orbital of the open ends enhances both pi- and sigma-aromaticity of the first belt of hexagons of carbon atoms at the open ends. The effect of the open ends on the structure and chemical reactivity of the SWCNT reaches only the first several layers of the hexagons of carbon atoms. Additions of carbene and dichlorocarbene to the nanotube reveal that the open ends have higher reactivities than the inner regions.

Based on numerical evidence and theoretical arguments, we demonstrate that the unconventional density variations are not really unconventional and should be included in the density variational domain of any ground-state density. As a result, Lieb's Theorem 3.10 (Lieb, Int J Quantum Chem 1983, 24, 243) is reconfirmed. More specifically, the energy variation Ev is of order higher than linear order of the density variation n(r) about the ground-state density n0(r) of a system with a fixed number of electrons in Hilbert space, whereas Ev is of linear order of n(r) about the ground-state density of a system with a varied number of electrons in Fock space. The unconventional density variations are not the cause for the linear-order behavior of Ev in Fock space and the derivative discontinuity of the total energy. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem 109: 3199 -3216, 2009

The structures, stabilities, and electronic properties of the single-vacancy-defected fullerenes, C60 and C70, and the single- and double-vacancy-defected single-walled carbon nanotubes (SWCNTs) were studied within density functional theory. The isomerization barriers for the single-vacancy-defected C60 on the triplet potential energy surface (PES) are lower than those on the singlet PES. The symmetric double-vacancy-defected (10,0) SWCNT is the most stable one among the models investigated. According to the analyses of frontier molecular orbitals (FMOs), nature bond orbitals, and local density of states, introduction of vacancy on the SWCNT decreases the band gap of semiconducting SWCNT, increases the band gap of conducting SWCNT, destructs the π conjugation of the FMOs, and gives rise to enhanced chemical activity. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

We have used density functional theory and ab initio methods to study different mechanistic possibilities for the formation of decamethyldizincocene from the reaction between decamethylzincocene and diethylzinc. Our results suggest that decamethyldizincocene could form from the combination of two pentamethylcyclopentadienylzinc radicals. More importantly, our data show that homolytic dissociation of decamethylzincocene into pentamethylcyclopentadienylzinc and pentamethylcyclopentadienyl radicals is 6.7 kcal/mol less costly than the analogous dissociation of zincocene. If such an energy difference is coupled with the fact that the activation barrier to form the half-sandwich product pentamethylcyclopentadienylethylzinc is 11.8 kcal/mol more costly than to form cyclopentadienylethylzinc, we can rationalize why dizincocene does not form experimentally.

The adsorption of small gaseous molecules to the metal center in Pt-doped (5,5) single-walled carbon nanotubes has been explored within density functional theory. A model system consisting of a single Pt atom residing in the middle of a carbon nanotube with capping H atoms is used for our investigation. For all gases studied, the overall process of adsorption was found to be exothermic, where the affinity strongly depended on the orientation of the molecule. By examining the density of states and molecular orbitals of these nanotube−adsorbate complexes in comparison to the bare Pt-doped nanotube, we show that the electronic structure of these materials is strongly influenced by the presence of gases. Hence, we propose an application of Pt-doped single-walled carbon nanotubes as gas sensors and hope to motivate experimental work in this field.

Molecular structure of 1-chloro-1,1-difluoro-pentane-2,4-dione (monochlorodifluoro-acetylacetone, CDFAA) has been investigated by means of ab initio and Density Functional Theory (DFT) calculations and the results were compared with those of 1,1,1-trifluoro-pentane-2,4-dione (trifluoro-acetylacetone, TFAA). The harmonic vibrational frequencies of the two most stable cis -enol forms were calculated at the B3LYP level of theory using 6-31G ∗∗ and 6-311++G ∗∗ basis sets. We also calculated the anharmonic frequencies at the B3LYP/6-31G ∗∗ level of theory for these two stable cis -enol isomers. The calculated frequencies and the Raman and IR intensities were compared with the experimental results. The energy difference between the two stable cis -enol forms, calculated at the MP2/6-31G ∗∗ level, is only 3.42 kJ/mol. The observed vibrational frequencies and intensities in the gas phase are in excellent agreement with the corresponding values calculated for the two most stable conformers. However, the observed IR and Raman frequencies also indicate coexisting of both tautomers in the liquid phase and in solution. According to the theoretical calculations, at the B3LYP/6-311++G ∗∗ level, the hydrogen bond strength for the most stable conformer is 54.45 kJ/mol, about 2.4 kJ/mol less than that of the corresponding conformer of TFAA. These results are in agreement with the obtained experimental data.

Molecular structure of 3-chloro-pentane-2,4-dione (known as α-chloroacetylacetone, ClAA) has been investigated by means of ab initio and Density Functional Theory (DFT) calculations and the results were compared with those of its parent, pentane-2,4-dione (known as acetylacetone, AA). The harmonic vibrational frequencies of the cis -enol form were calculated at the B3LYP level of theory using 6-311G** and 6-311++G** basis sets. We also calculated the anharmonic frequencies at B3LYP/6-31G** level of theory. The calculated frequencies and the Raman and IR intensities were compared with the experimental results. According to the theoretical calculations, at the B3LYP/6-311++G** level, the hydrogen bond strength for ClAA is 68.7 kJ/mol, about 2.3 kJ/mol stronger than that in AA. This result is in agreement with the OH/OD stretching, the OH/OD in-plane bending, and O⋯O stretching frequencies. The 1 H chemical shifts for ClAA and AA are computed at the B3LYP/6-311++G** level using the GIAO method. The calculated difference chemical shift of the enolated proton in ClAA and AA is in excellent agreement with the experimental results. Natural bond orbital (NBO) analyses indicate that, in spite of electron withdrawing nature of Cl atom, the steric effect increases the hydrogen bond strength.

Molecular structure and vibrational frequencies of 4,4,4-trifluoro-1-phenyl-1,3-butanedione, known as trifluorobenzoylacetone (TFBA), have been investigated by means of density functional theory (DFT) calculations. The results were compared with those of benzoylacetone (BA), acetylacetone (AA), and trifluoroacetylacetone (TFAA). Comparing the calculated and experimental band frequencies and intensities suggests coexisting of both stable cis-enol conformers in comparable proportions in the sample. The energy difference between the two stable chelated enol forms is negligible, 0.96 kcal/mol, calculated at B3LYP/6-311++G** level of theory. The molecular stability and the hydrogen bond strength were investigated by applying the natural bond orbital (NBO) theory and geometry calculations. The theoretical calculations and spectroscopic results indicate that the hydrogen bond strength of TFBA is between those of TFAA and AA, considerably weaker than that of BA.

Fourier transform infrared and Fourier transform Raman spectra of 3,4-diacetyl-2,5-hexanedione, known as tetraacetylethane (TAE) or α,α′-bis-acetylacetone, and its deuterated analogue have been obtained. Density functional theory (DFT) B3LYP and BLYP calculations have been carried out with the purpose of understanding the vibrational spectra of this compound and its deuterated analogue. The calculated geometrical parameters show a very strong hydrogen bond, compared with its parent molecule, acetylacetone (AA), with an O⋯O distance of 2.464–2.505 A. This bond length is about 0.05–0.06 A shorter than that for AA. According to the theoretical calculations, TAE has an asymmetric structure with a hydrogen bond strength of about 17.3 kcal/mol per bond (calculated with 6-311++G∗∗ basis set), about 1.4 kcal/mol more than that for AA. This increase in the hydrogen bond strength is consistent with the frequency shifts for OH/OD stretching, OH/OD out-of-plane bending, and O⋯O stretching modes and downfield proton chemical shift upon substitution of α-H atom with acetylacetone radical. The geometries of keto–keto and some of the enol–keto tautomers were also fully optimized and compared with the enol–enol tautomer. To investigate the effect of acetylacetone radical on the hydrogen bond strength, the charge distributions, steric effects, and Wiberg bond orders in TAE and AA were studied by the Natural Bond Orbital (NBO) method for optimized model compounds at B3LYP/6-31G∗∗ level of theory. The results of NBO analysis indicate that the steric effect is the main factor for increasing the hydrogen bond strength in TAE compared with that in AA.

Molecular structure and vibrational frequencies of 1,3-diphenyl-1,3-propanedione, known as dibenzoylmethane (DBM), have been investigated by means of density functional theory (DFT) calculations. The results were compared with those of benzoylacetone (BA) and acetylacetone (AA), the parent molecule. IR and Raman spectra of DBM and its deuterated analogue were clearly assigned.The calculated hydrogen bond energy of DBM is 16.15 kcal/mol, calculated at B3LYP/6-311++G** level of theory, which is 0.28 kcal/mol more than that of AA. This result is in agreement with the vibrational and NMR spectroscopy results. The molecular stability and the hydrogen bond strength were investigated by applying the Natural Bond Orbital analysis (NBO) and geometry calculations. The theoretical calculations indicate that the hydrogen bond in DBM is relatively stronger than that in BA and AA.

We propose a new density functional for the evaluation of the total electronic energy by subtracting the Roothaan energy, i.e. the Hartree energy of the density residual, from the Hohenberg-Kohn-Sham (HKS) functional, which is normally used in self-consistent Kohn-Sham (KS) density functional theory (DFT) calculations. Because of the positive semi-definite nature of the Roothaan energy, the resulting Wang-Zhou (WZ) functional always produces a total energy lower than that from the HKS functional and usually converges to the exact total energy from below. Following the same spirit of the Zhou-Wang-λ (ZWλ) functional in the recently proposed orbital-corrected orbital-free (OO) DFT method (Zhou and Wang, J Chem Phys 2006, 124, 081107), we linearly mix the WZ functional with the HKS functional to allow further systematic error cancellations. The resulting Wang-Zhou-α (WZα) functional is compared with the ZWλ functional in OO-DFT calculations for systems within different chemical environment. We find that the optimal value of α for the WZα functional is more stable than that of λ for the ZWλ functional. This is because the WZ functional remedies the oscillatory convergence behavior of the Harris functional and renders the direct evaluation of α for the WZα functional more plausible in the application of the linear-scaling OO-DFT method for large systems. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem 107: 2995-3000, 2007

The Staudinger reaction of phosphane and azide has been investigated by Atom-centered Density Matrix Propagation (ADMP) approach to ab initio molecular dynamics (AIMD) in combination with molecular orbital analysis within density functional theory. At room temperature, the reaction pathway with the cis initial attack dominates the Staudinger reaction. Electrostatic interaction, charge transfer, and covalent overlap are responsible for the initial attack and for the system to overcome the initial reaction barrier. The rotation of PH3 and PH vibrations facilitate the isomerization of the system from cis intermediate to the last transition state, which indicates that small substituent groups on phosphane can facilitate the last stage of the Staudinger reaction. During the course of the reaction, the change of the average polarizability correlates positively to the change of the potential energy of the system, which clearly suggests that polar solvents can facilitate the overall reaction by stabilizing all transition states and reducing all reaction barriers.

Complete conformational analysis of all possible keto and enol forms of thenoyltrifluoroacetone (TTFA) was carried out using density functional theory with the B3LYP functional and the 6-31G**, 6-311G**, and 6-311++G** basis sets. In addition, the geometries and energies of the four most stable chelated conformers and their corresponding open structures were obtained at the MP2/6-31G** level of theory. The energy differences between the four stable chelated enol conformers, in the gas phase, calculated at the B3LYP levels are negligible. However, calculations at the MP2 level indicate that the B2 conformer (the hydroxyl group in the - CF 3 side) is significantly more stable than others, in agreement with the X-ray diffraction results. The calculated intramolecular hydrogen bond (IHB) energy E HB and the strength of the bond have been compared, and an imperfection in the prevalent method of evaluating E HB has been perceived. The IHB of TTFA was compared with those in several β-dicarbonyls.

The ozonization at the vacancy defect site of the single-walled carbon nanotube has been studied by static quantum mechanics and atom-centered density matrix propagation based ab initio molecular dynamics within a two-layered ONIOM approach. Among five different reaction pathways at the vacancy defect, the reaction involving the unsaturated active carbon atom is the most probable pathway, where ozone undergoes fast dissociation at the active carbon atom at 300 K. Complementary to the experiments, our work provides a microscopic understanding of the ozonization at the vacancy defect site of the single-walled carbon nanotube.

[Chemical reaction: See text] Conjugated, shape-persistent macrocycles based on [3 + 3] Schiff-base condensation are of interest for supramolecular materials. In an effort to develop new discotic liquid crystals based on these compounds, a series of macrocycles with peripheral alkoxy groups of varying length have been prepared. The synthesis and mechanism of formation have been probed by isolation of oligomeric intermediates. A single-crystal X-ray diffraction study of one macrocycle revealed a nonplanar, strongly hydrogen-bonded structure. To our surprise, even with very long substituents, the macrocycles were not liquid crystalline. This has been rationalized by ab initio calculations that indicate the macrocycles are undergoing rotation of the dihydroxydiiminobenzene rings that may not allow a stable discotic liquid crystalline phase. These results provide new insight into the formation and properties of these large macrocycles and may provide guidance to developing stable liquid crystalline materials in the future.