Are defect models consistent with the entropy and specific heat of glass formers?

We show that pointlike defect model of glasses cannot explain the thermodynamic properties of glass formers, as for example, the excess specific heat close to the glass transition, contrary to the claim of Garrahan and Chandler [Proc. Natl. Acad. Sci. U.S.A. 100, 9710 (2003)]. More general models and approaches in terms of extended defects are also discussed.     

The reaction of nitrogen atoms with methyl radicals: Are spin-forbidden channels important?

A computational study of the N(4S) + CH3 reaction has been carried out. The reactants approach through an attractive potential surface leading to an intermediate, H3CN, whose formation does not involve any barrier. In agreement with the experimental results, the dominant channel for this reaction is H2CN+H. The theoretically estimated rate coefficient for the overall process at 298 K is 9.1 x 10(-12) cm3 s(-1) molecule(-1), which is nearly 1 order of magnitude lower than the experimental result, but also much larger than those computed for the reactions of ground-state nitrogen atoms with halomethyl radicals. The analysis of the singlet potential energy surface, and the corresponding computational kinetic study, shows that for the reaction of excited nitrogen atoms with methyl radicals, the preferred product from the kinetic point of view is also H2CN+H, but in this case production of HCN is significant (with branching ratios around 0.185). According to our calculations, spin-forbidden processes are highly unlikely for the N(4S) + CH3 reaction. However, further evolution of the preferred products, H2CN+H, might explain the experimental observation of hydrogen cyanide as a minor product in this reaction.     

ChemInform Abstract: Is CrF6 Octahedral? Experiment Still Suggests “Yes!”

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

Are the pyrazolines formed from the reaction of [60]fullerene with alkyl diazoacetates unstable?

[60]Fullerene-fused pyrazolines 1 were prepared by the reaction of C₆₀ with alky diazoacetates under the solid-state high-speed vibration milling conditions as well as in toluene solution. Pyrazolines 1 were stable in refluxing toluene and its thermolysis process in 1,2-dichlorobenzene was investigated, the decomposition rates and activation energies of pyrazolines 1 were obtained. The current work demonstrated that the liquid-phase reaction of C₆₀ with alkyl diazoacetates undergoes via 1,3-dipolar cycloaddition pathway at room temperature, or proceeds via carbene mechanism at a temperature of refluxing toluene, thus clarifies the previous ambiguity of its reaction mechanism.     

Reaction of Phenyl-Substituted o-Quinodimethanes with Nitric Oxide. Are Benzocyclobutenes Suitable Precursors for Nitric Oxide Cheletropic Traps?

In order to elucidate the potential of substituted o-quinodimethanes as reagents for the trapping of nitric oxide (NO) in biological systems, the reaction of alkoxyl- and alkyl-substituted 7,8-diphenyl- and 7,7,8-triphenyl-o-quinodimethanes with nitric oxide in solution was investigated by ESR spectroscopic and UV/vis stopped-flow techniques. Photolytic decarbonylation of 1,3-diphenyl- and 1,1,3-triphenylindan-2-ones gave the corresponding phenyl-substituted benzocyclobutenes as the major products and low photostationary concentrations of o-quinodimethanes. During 266-nm laser flash photolysis (LFP) of 1,3-dimethoxy-1,3-diphenylindan-2-one and 1-methoxy-1,3,3-triphenylindan-2-one in acetonitrile, species absorbing in the 400-600 nm range were produced, which were attributed to configurational isomers of the corresponding 7,7,8,8-substituted o-quinodimethanes. The isomeric o-quinodimethanes decayed at significantly different rates, indicating a strong influence of the relative orientation of the terminal substituents on their stability. Reaction of the raw photolysates of the 2-indanones with NO produced strong ESR spectra of the corresponding cyclic nitroxide radicals, isoindolin-2-oxyls. The nitroxide radicals were generated in a two-phase process, the first, rapid phase being attributed to the reaction of NO with the photolytically formed o-quinodimethanes and the second, slow phase reflecting the reaction with small amounts of o-quinodimethanes, generated by thermal ring opening of the phenyl-substituted benzocyclobutenes and probably a direct reaction of NO with the benzocyclobutenes. The kinetics of both steps, as evaluated by stopped-flow UV/vis and ESR spectroscopy, revealed a strong dependence of the rate constants of the o-quinodimethane + NO reaction on the substitution pattern of the o-quinodimethanes, with rate constants spanning a range of 10-4000 M(-)(1) s(-)(1). The rate constants ((0.4-7.5) x 10(-)(4) s(-)(1)) for the reaction of NO with the 7,7,8,8-tetrasubstituted benzocyclobutenes are much less influenced by the substitution pattern. The utility of phenyl-substituted benzocyclobutenes as "reservoirs" for o-quinodimethane-type nitric oxide traps is discussed.     

Are structures with Al-H bonds represented in the photoelectron spectrum of Al3O4H2-?

Photoelectron spectra of Al3O4H2- clusters formed by reactions of Al3O3- with water molecules have been interpreted recently in terms of dissociative absorption products with hydroxide and oxide anions that are coordinated to aluminum cations. Alternative isomers with Al-H bonds have lower energies, but barriers to hydrogen migrations that break O-H bonds and create Al-H bonds are high. Ab initio electron propagator calculations of the vertical electron detachment energies of the anions indicate that the species with hydrides cannot be assigned to the chief features in the photoelectron spectrum. Therefore, the previously studied dissociative absorption products are the structures that are most likely to be probed in the photoelectron spectra.     

Are recent water models obtained by fitting diffraction data consistent with infrared/Raman and x-ray absorption spectra?

X-ray absorption (XA) spectra have been computed based on water structures obtained from a recent fit to x-ray and neutron diffraction data using models ranging from symmetrical to asymmetrical local coordination of the water molecules [A. K. Soper, J. Phys.: Condens. Matter 17, S3273 (2005)]. It is found that both the obtained symmetric and asymmetric structural models of water give similar looking XA spectra, which do not match the experiment. The fitted models both contain unphysical structures that are allowed by the diffraction data, where, e.g., hydrogen-hydrogen interactions may occur. A modification to the asymmetric model, in which the non-hydrogen-bonded OH intramolecular distance is allowed to become shorter while the bonded OH distance becomes longer, improves the situation somewhat, but the overall agreement is still unsatisfactory. The electric field (E-field) distributions and infrared (IR) spectra are also calculated using two established theoretical approaches, which, however, show significant discrepancies in their predictions for the asymmetric structural models. Both approaches predict the Raman spectrum of the symmetric model fitted to the diffraction data to be significantly blueshifted compared to experiment. At the moment no water model exists that can equally well describe IR/Raman, x-ray absorption spectroscopy, and diffraction data.     

Ruthenium‐Catalyzed C?C Coupling of Fluorinated Alcohols with Allenes: Dehydrogenation at the Energetic Limit of β‐Hydride Elimination

Ruthenium(II) complexes catalyze the C C coupling of 1,1‐disubstituted allenes and fluorinated alcohols to form homoallylic alcohols bearing all‐carbon quaternary centers with good to complete levels of diastereoselectivity. Whereas fluorinated alcohols are relatively abundant and tractable, the corresponding aldehydes are often not commercially available because of their instability.     

Are metal-CO molecules linear? ScCO,TiCO, VCO,and CrCO cases studied in MRCI method compared with DFT method

The equilibrium geometries for the low-lying high-spin electronic states of ScCO, TiCO, VCO, and CrCO molecules were studied with the ab initio method. Restricted Hartree-Fock, multireference configuration interactions, and perturbation calculations were done employing flexible basis sets. We found the equilibrium geometries of the ScCO, TiCO, and VCO molecules to be linear, which is in agreement with a recent density functional calculation. In contrast, we found the CrCO molecule linear while the density functional calculation found it to be bent. As the CrCO molecule is very weakly bonded, we think that this molecule needs further studies with higher-level methods. © 1997 John Wiley & Sons, Inc.     

Are Monophospha(III)amidines and -guanidines with Ionizable Hydrogens Tautomeric? Towards a Deeper Understanding of Two Related Hetero-element Functional Groups

This paper presents definitive structural evidence for N,P(III)-monophosphaamidines in P=C and N=C isomeric forms from a combination of new syntheses, single-crystal X-ray diffraction (SC-XRD), solid-state NMR and FTIR. Evidence is also provided for C-amino-(σ²,λ³)-phosphaalkene and C-(σ³,λ³)-phosphinoimine tautomerism in solution using multi-nuclear NMR methods. Synthesis and SC-XRD structure determination of a trisubstituted N,N’,P(III)-monophosphaguanidine is presented, the first structure of a phospha(III)guanidine with two ionizable H atoms. The structural evidence is convincing for an N=C geometry, resulting in both N−H and P−H in the molecule. A detailed computational investigation using DFT methods is presented, with the goal of understanding the tautomeric structure preferences both at the fundamental level (parent molecules with all substituents on the heteroatoms being hydrogen) and using the full structures containing the very bulky 2,6-diisopropylphenyl (Dipp) substituents employed in this study. Arguments are espoused for treating phospha(III)amidines and -guanidines as new types of functional groups, similar to but distinct from the familiar organic analogues. Limited reactivity studies and a voltammetry study of one phospha(III)amidine are included with the supporting information.     

β‐Peptides with Different Secondary‐Structure Preferences: How Different Are Their Conformational Spaces?

The conformational spaces accessible to two β‐hexapeptides in MeOH at 298 K and 340 K are investigated by molecular‐dynamics simulation with an atomistic model of both solute and solvent. The structural properties of these peptides have been previously studied by NMR in MeOH at room temperature. The experimental data could be fitted to a model (P)‐12/10‐helix for one of the peptides and a model hairpin with a ten‐membered H‐bonded turn for the other. The goal of the present work is to determine whether the conformational spaces accessible to these two peptides of seemingly different conformational properties contain any common regions. In other words, to what extent are the evident differences found at the macroscopic level also present at the microscopic structural level? It is found that, for the two peptides studied, the conformational spaces sampled in the respective simulations show significant overlap.     

Are C-H groups significant hydrogen bonding sites in anion receptors? Benzene complexes with Cl-, NO3-, and ClO4-

Theoretical calculations,examination of crystallographic data, and experimental binding energies suggest that even in the absence of electron-withdrawing substituents, simple arenes such as benzene form hydrogen bonds with anions that can exceed 50% of the strength of those formed by O-H and N-H groups. Thus, when present in a receptor, even moderately acidic C-H groups can significantly enhance the anion binding affinity and they should be considered as additional binding sites within the host cavity.     

ChemInform Abstract: Theoretical Study of Cyclic Si5H+ 5 Structural Isomers. Are There Any Analogies with the Corresponding Carbon Species?

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

Are Two Metal Ions Better than One? Mono- and Binuclear α-Diimine-Re(CO)3 Complexes with Proton-Responsive Ligands in CO2 Reduction Catalysis

Here, the reduction chemistry of mono- and binuclear α-diimine-Re(CO)₃ complexes with proton responsive ligands and their application in the electrochemically-driven CO₂ reduction catalysis are presented. The work was aimed to investigate the impact of 1) two metal ions in close proximity and 2) an internal proton source on catalysis. Therefore, three different Re complexes, a binuclear one with a central phenol unit, 3 , and two mononuclear, one having a central phenol unit, 1 , and one with a methoxy unit, 2 , were utilised. All complexes are active in the CO₂-to-CO conversion and CO is always the major product. The catalytic rate constant k_cat for all three complexes is much higher and the overpotential is lower in DMF/water mixtures than in pure DMF (DMF=N,N-dimethylformamide). Cyclic voltammetry (CV) studies in the absence of substrate revealed that this is due to an accelerated chloride ion loss after initial reduction in DMF/water mixtures in comparison to pure DMF. Chloride ion loss is necessary for subsequent CO₂ binding and this step is around ten times faster in the presence of water [ 2 : k^Cl(DMF)≈1.7 s⁻¹; k^Cl(DMF/H₂O)≈20 s⁻¹]. The binuclear complex 3 with a proton responsive phenol unit is more active than the mononuclear complexes. In the presence of water, the observed rate constant k_obs for 3 is four times higher than of 2 , in the absence of water even ten times. Thus, the two metal centres are beneficial for catalysis. Lastly, the investigation showed that the phenol unit has no impact on the rate of the catalysis, it even slows down the CO₂-to-CO conversion. This is due to an unproductive, competitive side reaction: After initial reduction, 1 and 3 loose either Cl⁻ or undergo a reductive OH deprotonation forming a phenolate unit. The phenolate could bind to the metal centre blocking the sixth coordination site for CO₂ activation. In DMF, O−H bond breaking and Cl⁻ ion loss have similar rate constants [ 1 : k^Cl(DMF)≈2 s⁻¹, k^OH≈1.5 s⁻¹], in water/DMF Cl⁻ loss is much faster. Thus, the effect on the catalytic rate is more pronounced in DMF. However, the acidic protons lower the overpotential of the catalysis by about 150 mV.     

Stereochemistry in the reaction of 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) with beta,beta dimethyl-p-methoxystyrene. Are open biradicals the reaction intermediates?

The reaction of 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) with beta,beta-dimethyl-p-methoxystyrene (1) in chloroform affords four adducts: the ene, two stereoisomeric [4 + 2]/ene diadducts, and a minor product that is probably the double Diels-Alder diadduct. In methanol, only one regioisomeric methoxy adduct is formed. The stereochemistry of the reaction was examined by specific labeling of the anti methyl group of 1 as CD(3). In chloroform, the ene adduct is formed with >97% synselectivity, while the [4 + 2]/ene diadducts are formed with 20% loss of stereochemistry at the methyl groups. In methanol, the methoxy adducts are formed with almost complete loss of stereochemistry. A mechanism involving open biradicals is inconsistent with the experimental results. It is likely that the reaction proceeds through the formation of an aziridinium imide and an open zwitterionic intermediate. The aziridinium imide leads to the formation of the ene adduct. The open zwitterion, which has sufficient lifetime to rotate around the C-C bond, leads to the formation of a [4 + 2] cycloadduct, which reacts with a second molecule of MTAD in an ene-type mode to afford two stereoisomeric [4 + 2]/ene diadducts. In methanol, solvent captures the zwitterionic intermediate and forms the methoxy adduct. The relative distribution of the products in chloroform depends on the reaction temperature. Lower temperatures favor the ene reaction (entropically favorable), whereas at higher temperatures the [4 + 2]/ene diadducts become the major products.