Why Does Pivalaldehyde (Trimethylacetaldehyde) Unexpectedly Seem More Basic Than 1‐Adamantanecarbaldehyde in the Gas Phase? FT‐ICR and High‐Level Ab Initio Studies

Fourier transform ion cyclotron resonance spectroscopy (FT ICR) techniques, including collision-induced dissociation (CID) methodology, were applied to the study of the gas-phase protonation of pivalaldehyde (1) and 1-adamantanecarbaldehyde (2). A new synthetic method for 2 was developed. The experiments, together with a thorough computational study involving ab initio and density functional theory (DFT) calculations of high level, conclusively show that upon monoprotonation in the gas phase, compound 1 yields monoprotonated methyl isopropyl ketone 3. The mechanism of this gas-phase acid-catalyzed isomerization is different from that reported by Olah and Suryah Prakash for the reaction in solution. In the latter case, isomerization takes place through the diprotonation of 1.     

Why are the Ca2+ and K+ binding energies of formaldehyde and ammonia reversed with respect to their proton affinities?

The binding energies (BEs) of alkali metal monocations and alkaline-earth metal dications to a series of small oxygen and nitrogen bases have been evaluated by means of CCSD(T) calculations on B3-LYP optimized structures. These calculations were carried out both using all-electron basis sets, and additionally using an effective-core potential (ECP) to describe the inner electrons of the metal. A theoretical model aiming at analyzing the effects on the binding energy trends of electrostatic, polarization, and covalent contributions, as well as geometry distortion, was employed. From this analysis, we conclude that although the neutral-ion interaction energy for alkali and alkaline-earth metal cations is dominated by electrostatic contributions, in many cases the correct basicity trends are only attained once polarization effects are also included in the model. This is indeed the case when Ca2+ and K+ are bound to ammonia and formaldehyde. Geometry distortions triggered by polarization are also necessary, in some cases, to obtain the correct basicity trends. In addition, in particular for alkaline-earth dications, the energy associated with covalent interactions sometimes dictates the basicity trend. Our observations imply that simple models based on ion-dipole interactions, that are frequently used in the literature to explain affinity trends in ion-molecule reactions, are generally not likely to be reliable.     

Strong dissimilarities between the gas-phase acidities of saturated and alpha,beta-unsaturated boranes and the corresponding alanes and gallanes

The effect that unsaturation has on the intrinsic acidity of boranes, alanes, and gallanes, was analyzed by B3 LYP and CCSD(T)/6-311+G(3df,2p) calculations on methyl-, ethyl-, vinyl-, and ethynylboranes, -alanes and -gallanes, and on the corresponding hydrides XH3. Quite unexpectedly, methylborane, which behaves as a carbon acid, is predicted to have an intrinsic acidity almost 200 kJ mol(-1) stronger than BH3, reflecting the large reinforcement of the C--B bond, which upon deprotonation becomes a double bond through the donation of the lone pair created on the carbon atom into the empty p orbital of the boron. Also unexpectedly, and for the same reason, the saturated and alpha,beta-unsaturated boranes are much stronger acids than the corresponding hydrocarbons, in spite of being carbon acids as well. The Al derivatives also behave as carbon acids, but in this case the most favorable deprotonation process occurs at C beta, leading to the formation of rather stable three-membered rings, again through the donation of the C beta lone pair into the empty p orbital of Al. For Ga-containing compounds the deprotonation of the GaH2 group is the most favorable process. Therefore only Ga derivatives behave similarly to the analogues of Groups 14, 15, and 16 of the periodic table, and the saturated derivatives exhibit a weaker acidity than the unsaturated ones. Within Group 13, boranes are stronger acids than alanes and gallanes. For ethyl and vinyl derivatives, alanes are stronger acids than gallanes. We have shown, for the first time, that acidity enhancement for primary heterocompounds is not only dictated by the position of the heteroatom in the periodic table and the nature of the substituent, but also by the bonding rearrangements triggered by the deprotonation of the neutral acid.     

Attacking boron nucleophiles: NMR properties of five-membered diazaborole rings

This paper reports computed NMR spectral data for the diazaborole anion (C2H4B1N2-) and the corresponding neutral five-membered rings with B-H (C2H5B1N2, diazaborole) and B-Li (C2H4B1Li1N2, Li-diazaborole) bonds, which are the central moieties of newly synthesized nucleophilic organoboryl five-membered rings, recently reported by Segawa et al. (Science 2006, 314, 113). Both spin-spin coupling constants and chemical shifts were obtained using high-level ab initio calculations. These data are a necessary complement to the very scarce experimental information available.     

The mechanism of double proton transfer in dimers of uracil and 2-thiouracil--the reaction force perspective

The intermolecular double proton transfer in dimers of uracil and 2-thiouracil is studied through density functional theory calculations. The reaction force framework provides the basis for characterizing the mechanism that in all cases has been associated to a dynamic balance between polarization and charge transfer effects. It has been found that the barriers for proton transfer depend upon the nature of the acceptor atoms and its position within the seminal monomer. Actually, the change in the nature of the hydrogen bonds connecting the two monomers along the reaction coordinate may favor or disfavor the double-proton transfer.     

An experimental and theoretical investigation of gas-phase reactions of Ca2+ with glycine

The gas-phase reactions between Ca(2+) and glycine ([Ca(gly)](2+)) have been investigated through the use of mass spectrometry techniques and B3-LYP/cc-pWCVTZ density functional theory computations. The major peaks observed in the electrospray MS/MS spectrum of [Ca(gly)](2+) correspond to the formation of the [Ca,C,O(2),H](+), NH(2)CH(2) (+), CaOH(+), and NH(2)CH(2)CO(+) fragment ions, which are produced in Coulomb explosion processes. The computed potential energy surface (PES) shows that not only are these species the most stable product ions from a thermodynamic point of view, but they may be produced with barriers lower than for competing processes. Carbon monoxide is a secondary product, derived from the unimolecular decomposition of some of the primary ions formed in the Coulomb explosions. In contrast to what is found for the reactions of Ca(2+) with urea ([Ca(urea)](2+)), minimal unimolecular losses of neutral fragments are observed for the gas-phase fragmentation processes of [Ca(gly)](2+), which is readily explained in terms of the topological differences between their respective PESs.     

Major- and trace-element distribution in cigarette tobacco,ash and filters

Journal of Radioanalytical and Nuclear Chemistry - To evidence the efficiency of cigarette filter we have determined the content of 27 major and trace elements in two brands of filter and...     

On the Origin of the Enhanced Acidity of Chalcocyclopentadienes (Cyclopentadiene Chalcogenols) in the Gas Phase

The intrinsic acidity of chalcocyclopentadienes (CpXH; X=O, S, Se, Te) is investigated by high‐level G3B3 and G2 ab initio as well as B3LYP DFT calculations, which show that, independent of the nature of the heteroatom, all chalcocyclopentadienes are stronger acids in the gas phase than cyclopentadiene. However the acidity does not increase regularly down the group, and the acidity enhancement for Te derivatives is five times larger than for O derivatives, but only twice that of S‐containing compounds. The most favorable deprotonation process corresponds to loss of the proton attached to the heteroatom, with the sole exception of the 5‐substituted 1,3‐cyclopentadienes, for which the O and S derivatives are predicted to behave as carbon acids. No matter the nature of the heteroatom, the 1‐substituted 1,3‐cyclopentadienes are the strongest acids. The intrinsic acidity of all isomers, namely, 1‐substituted, 2‐substituted, and 5‐substituted 1,3‐cyclopentadienes, increases with increasing aromaticity of the anion formed on deprotonation, and therefore the Te compound is the strongest acid for the three series. However, the intrinsic acidity of chalcocyclopentadienes is not dictated by aromaticity, so that, in general, the most stable deprotonated species do not coincide with the most aromatic ones.     

Effect of Fish Bone Bioactive Peptides on Oxidative,Inflammatory and Pigmentation Processes Triggered by UVB Irradiation in Skin Cells

In the present study, we evaluated for the first time the photoprotective effect of fish bone bioactive peptides (FBBP) preparation isolated from silver carp (Hypophthalmichthys molitrix) discarded tissue using in vitro experimental models of skin cells exposed to ultraviolet B (UVB) irradiation and stressing agents. FBBP preparation was obtained by papain treatment of minced bones and centrifugal ultrafiltration, and the molecular weight (MW) distribution was characterized by size exclusion and reversed-phase high performance liquid chromatography (RP-HPLC). In vitro assessment of the effect of FBBP pretreatment in UVB-irradiated L929 fibroblasts and HaCaT keratinocytes revealed their cytoprotective activity. Their capacity to efficiently reduce reactive oxygen species (ROS) production and lipid peroxidation varied in a dose-dependent manner, and it was greater in fibroblasts. A decrease of proinflammatory cytokines secretion, in particular of tumor necrosis factor alpha (TNF-α), was found after FBBP pretreatment of THP-1-derived inflamed macrophages. Melanin production and tyrosinase activity investigated in UVB-irradiated Mel-Juso cells were lowered in direct relation to FBBP concentrations. FBBP fractions with high radical scavenging activity were separated by ion exchange chromatography, and two collagenic sequences were identified. All these results offer new scientific data on aquaculture fish bone-derived peptides confirming their ability to control the antioxidant, anti-inflammatory and pigmentation processes developed during UV irradiation of skin cells and recommend their use as valuable natural ingredients of photoprotective cosmeceutical products.