: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Recently, there has been growing interest in research into the role of plant-derived antioxidants in food and human health. The beneficial influence of many foodstuffs and beverages including fruits, vegetables, tea, coffee, and cacao on human health has been recently recognized to originate from their antioxidant activity. For this purpose, the most commonly methods used in vitro determination of antioxidant capacity of food constituents are reviewed and presented. Also, the general chemistry underlying the assays in the present paper was clarified. Hence, this overview provides a basis and rationale for developing standardized antioxidant capacity methods for the food, nutraceutical, and dietary supplement industries. In addition, the most important advantages and shortcomings of each method were detected and highlighted. The chemical principles of these methods are outlined and critically discussed. The chemical principles of methods of 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulphonate) radical (ABTS·+) scavenging, 1,1-diphenyl-2-picrylhydrazyl (DPPH·) radical scavenging, Fe3+–Fe2+ transformation assay, ferric reducing antioxidant power (FRAP) assay, cupric ions (Cu2+) reducing power assay (Cuprac), Folin-Ciocalteu reducing capacity (FCR assay), peroxyl radical scavenging, superoxide anion radical (O
 2
 ·−
 ) scavenging, hydrogen peroxide (H2O2) scavenging, hydroxyl radical (OH·) scavenging, singlet oxygen (1O2) quenching assay and nitric oxide radical (NO·) scavenging assay are outlined and critically discussed. Also, the general antioxidant aspects of main food components were discussed by a number of methods which are currently used for detection of antioxidant properties food components. This review consists of two main sections. The first section is devoted to main components in the foodstuffs and beverages. The second general section is some definitions of the main antioxidant methods commonly used for determination of antioxidant activity of components in the foodstuffs and beverages. In addition, there are given some chemical and kinetic basis and technical details of the used methods.

Antioxidant activity of food constituents: an overview

Structural Aspects of Aromaticity

Computational Prediction of 1H and 13C Chemical Shifts: A Useful Tool for Natural Product, Mechanistic, and Synthetic Organic Chemistry

The earlier compiled self-consistent spectrophotometric basicity scale in acetonitrile (AN) was expanded to range from 3.8 to 32.0 pKa units, that is 28 orders of magnitude. Altogether 54 new relative basicity measurements (ΔpKa measurements) were carried out and 37 new compounds were introduced to the scale (it now includes altogether 89 bases). The relative basicity of any two bases in the scale can be obtained by combining at least two independent sets of measurements. Multiple overlapping measurements make the results more reliable. The overall consistency (as defined earlier) of the measurements is s = 0.03 pKa units. Thorough analysis of all of our experimental data (ΔpKa values of this and earlier works) and experimental pKa data in AN available in the literature (works from the groups of Coetzee and Padmanabhan, Kolthoff and Chantooni, Jr., the Schwesinger group, Bren' et al. and some others, altogether 19 papers) was carried out. On the basis of this analysis the anchor point of the scalepyridine...

Extension of the self-consistent spectrophotometric basicity scale in acetonitrile to a full span of 28 pKa units: unification of different basicity scales.

Basicity of some organic superbases in acetonitrile.

A magnificent edifice called acid/base chemistry is rooted in a single proton, small in size but of enormous importance. The upper floors of this building are reserved for biochemistry, while the penthouse belongs to life science. The edifice will grow into a sky scraper-like in the 21st century. Evidence provided by this Review shows that the coalescence of experimental and theoretical/computational work is not only highly desirable for that purpose, but, in fact, greatly needed.

Advances in Determining the Absolute Proton Affinities of Neutral Organic Molecules in the Gas Phase and Their Interpretation: A Theoretical Account

The spatial and electronic structure of the very strong neutral organic bases bis(tetramethylguanidino)naphthalene (TMGN), 4,5-bis(tetramethylguanidino)fluorene (TMGF) and some related compounds are explored by ab initio computational methods. Their affinity towards the proton is scrutinized both in the gas phase and in solution in acetonitrile. The protonation at the most basic center (the imine nitrogen) yields asymmetric and relatively strong intramolecular hydrogen bonds (IHB). It is found that the angular strain effect and steric repulsion practically vanish in TMGN which implies that its high absolute proton affinity (APA) has its origin in the inherent basicity of the guanidine fragment and a relatively strong IHB in [TMGN]H(+). The nonbonded repulsions in TMGF are higher than in TMGN, which in conjunction with a slightly stronger IHB in the corresponding conjugate acid makes it more basic: APA(TMGF)>APA(TMGN). An interesting new phenomenon is observed in both TMGN and TMGF: the proton triggers the resonance stabilization not only in the directly bonded guanidine moiety, but also in the other guanidine fragment which is more distant from the proton, albeit in a less pronounced manner. The latter feature is termed a partial protonation. This supports the hydrogen bonding and contributes to the IHB stabilization. Convincing evidence is presented that the solvent effect in acetonitrile is determined by two antagonistic factors: 1) the intrinsic (gas phase) proton affinity and 2) the size effect which is given by the ratio between the positive charge in molecular cation (conjugate acid) and the magnitude of the molecular surface. The resulting pK(a) values are given by an interplay of these factors.

The Proton Affinity of the Superbase 1,8‐Bis(tetramethylguanidino)naphthalene (TMGN) and Some Related Compounds: A Theoretical Study

1,8-Bis(dimethylethyleneguanidino)naphthalene (DMEGN), the second example of a peralkyl guanidine “proton sponge” based on the 1,8-naphthalene backbone, was prepared and fully characterized. The crystal structure analysis of monoprotonated DMEGN reveals an unsymmetrical intramolecular hydrogen bridge. A decrease in the basicity with respect to the noncyclic parent 1,8-bis(tetramethylguanidino)naphthalene was found. Nevertheless, a new proton sponge provides a new crossbar in the ladder of highly basic neutral organic compounds. A detailed theoretical study of DMEGN and related cyclic guanidines explains this surprising experimental result. Homodesmotic reactions reveal that the intramolecular hydrogen bond contributes effectively 10 kcal/mol to proton affinity of DMEGN.

Zvonimir B. Maksić

Papers

Basicity of some organic superbases in acetonitrile.

Advances in Determining the Absolute Proton Affinities of Neutral Organic Molecules in the Gas Phase and Their Interpretation: A Theoretical Account

The Proton Affinity of the Superbase 1,8‐Bis(tetramethylguanidino)naphthalene (TMGN) and Some Related Compounds: A Theoretical Study

1,8-bis(dimethylethyleneguanidino)naphthalene: tailoring the basicity of bisguanidine "proton sponges" by experiment and theory.

Towards the absolute proton affinities of 20 α-amino acids