Carbon Atom as an Extremely Strong Nucleophilic and Electrophilic Center: Dendritic Allenes Are Powerful Organic Proton and Hydride Sponges.

Gas-phase proton affinities (PAs) and hydride affinities (HAs) of organic bases possessing an allene moiety and substituted with methyl, dimethylamino, cyano, and vinyl substituents were examined with the B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d) model. It was shown that a number of superbases and hyperbases can be obtained, as well as the potent hydride sponges. Methyl or dimethylamino substituents increased the proton affinity of the parent molecule, and the cyano substituents increased its hydride affinity.

Hydrogen bond dynamics of histamine monocation in aqueous solution: Car-Parrinello molecular dynamics and vibrational spectroscopy study.

Hydration of histamine was examined by infrared spectroscopy and Car-Parrinello molecular dynamics simulation. Histamine is a neurotransmitter and inflammation mediator, which at physiological pH conditions is present mainly in monocationic form. Our focus was on the part of vibrational spectra that corresponds to histamine N-H stretching, since these degrees of freedom are essential for its interactions with either water molecules or transporters and receptors.

Polycyclic croissant-like organic compounds are powerful superbases in the gas phase and acetonitrile--a DFT study.

A new type of the π-electron polycyclic organic superbases is proposed exhibiting very high proton affinities in the gas phase spanning the range from 280 to 323 kcal mol(-1) and the pK(a) up to 41.4 units in the MeCN solutions. Their salient feature is formation of the (H(+))-bridge upon protonation and a strong cationic resonance effect in the corresponding conjugate acids.

Polycyano derivatives of some organic tri- and hexacyclic molecules are powerful super- and hyperacids in the gas phase and DMSO: computational study by DFT approach.

B3LYP/6-311+G(2df,p)//B3LYP/6-31+G(d) calculations convincingly show that tricyclic organic compounds 1 and 2 offer scaffolds suitable for tailoring powerful neutral organic acids. Their cyanation at all C(sp(2))-H positions provide superacids as evidenced by their enthalpies of deprotonation ΔHacid(1bCN) = 257.2 and ΔHacid(2bCN) = 250.

Superbasicity of a bis-guanidino compound with a flexible linker: a theoretical and experimental study.

The bis-guanidino compound H(2)C{hpp}(2) (I; hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) has been converted to the monocation [I-H](+) and isolated as the chloride and tetraphenylborate salts. Solution-state spectroscopic data do not differentiate the protonated guanidinium from the neutral guanidino group but suggest intramolecular "-N-H..

The isomerization barrier in cyanocyclobutadienes: an ab initio multireference average quadratic coupled cluster study.

The energy profiles of the isomerization of mono, di-, and tetracyano-substituted cyclobutadienes (CBDs) are computed at the multireference average quadratic coupled cluster/complete active space self-consistent field level of theory. It was found that the energy barrier heights for the automerization reaction are 2.6 (tetracyano-CBD), 5.

Interpretation of Brønsted acidity by triadic paradigm: a G3 study of mineral acids.

Deprotonation enthalpies and the gas-phase acidities of 24 inorganic acids are calculated by using composite G3 and G2 methodologies. The computed values are in very good accordance with available measured data. It is found that the experimental DeltaH(acid) values of the FSO(3)H and CF(3)SO(3)H are too high by some 6 and 7 kcal mol(-1), respectively.

Hyperstrong neutral organic bases: phosphazeno azacalix[3](2,6)pyridines.

The gas-phase basicities and pKa values in acetonitrile of tris(phosphazeno)-substituted azacalix[3](2,6)pyridine and some related compounds are examined by DFT computational method. It is shown that the hexakis(phosphazeno) derivative of azacalix[3](2,6)pyridine is a hyperstrong neutral base as evidenced by the absolute proton affinity (APA) of 314.6 kcal mol(-1) and pKa(MeCN) of 37.

Derivatives of azacalix[3](2,6)pyridine are strong neutral organic superbases: a DFT study.

The gas phase basicities and pKa values in acetonitrile of azacalix[3](2,6)pyridine and its derivatives are determined by the B3LYP DFT method. It is found that all compounds of this series are neutral organic superbases. The proton attacks the inner pyridine N(sp2) atom, thus forming a bifurcated intramolecular hydrogen bond.

Hydride affinities of some substituted alkynes: prediction by DFT calculations and rationalization by triadic formula.

Hydride affinities (HAs) of the ethynes substituted by a wide range of different substituents are considered by using the B3LYP methodology. The computed values are in fair agreement with available experimental data, which are unfortunately scarce. The trend of changes of the HAs is rationalized by trichotomy formula.

Automerization reaction of cyclobutadiene and its barrier height: an ab initio benchmark multireference average-quadratic coupled cluster study.

The problem of the double bond flipping interconversion of the two equivalent ground state structures of cyclobutadiene (CBD) is addressed at the multireference average-quadratic coupled cluster level of theory, which is capable of optimizing the structural parameters of the ground, transition, and excited states on an equal footing. The barrier height involving both the electronic and zero-point vibrational energy contributions is 6.3 kcal mol(-1), which is higher than the best earlier theoretical estimate of 4.

Clar's sextet rule is a consequence of the sigma-electron framework.

A number of condensed PAHs are examined to identify the underlying reasons governing empirical Clar's rule taking benzene as a limiting case. It is found that the so-called Clar's structures are the only minima on the MP2(fc) potential energy hypersurfaces, meaning that other conceivable valence isomers are nonexistent. The influence of the electron correlation energies to the stability of Clar's structures is substantial with predominating influence of the sigma-electrons.

A novel approach in analyzing aromaticity by homo- and isostructural reactions: an ab initio study of fluorobenzenes.

The influence of fluorine substitutions on the stability of benzene is examined by using the Hartree-Fock (HF) and MP2 models. It is conclusively demonstrated that homodesmotic reactions based on the open-chain zigzag polyenes are unsatisfactory. A comparison of the intramolecular interactions of educts and products shows that they are not well balanced.

Gas-phase structure of protonated histidine and histidine methyl ester: combined experimental mass spectrometry and theoretical ab initio study.

Gas-phase H/D exchange experiments with CD3OD and D2O and quantum chemical ab initio G3(MP2) calculations were carried out on protonated histidine and protonated histidine methyl ester in order to elucidate their bonding and structure. The H/D exchange experiments show that both ions have three equivalent fast hydrogens and one appreciably slower exchangeable hydrogen assigned to the protonated amino group participating in a strong intramolecular hydrogen bond (IHB) with the nearest N(sp2) nitrogen of the imidazole fragment and to the distal ring NH-group, respectively. It is taken for granted that the proton exchange in the IHB is much faster than the H/D exchange.

Periodic trends in bond dissociation energies. A theoretical study.

Bond dissociation energies (BDEs) of all possible A-X single bonds involving the first- and second-row atoms, from Li to Cl, where the free valences are saturated by hydrogens, have been estimated through the use of the G3-theory and at the B3LYP/6-311+G(3df,2pd)//B3LYP/6-31G(2df,p) DFT level of theory. BDEs exhibit a periodical behavior. The A-X (A = Li, Be, B, Na, Mg, Al, and Si) BDEs show a steady increase along the first and the second row of the periodic table as a function of the atomic number Z(X).

High basicity of phosphorus-proton affinity of tris-(tetramethylguanidinyl)phosphine and tris-(hexamethyltriaminophosphazenyl)phosphine by DFT calculations.

It is shown by approximate but reliable DFT calculations that the title compounds represent very strong superbases in gas phase and MeCN. In particular, tris-(hexamethyltriaminophosphazenyl)phosphine has a proton affinity, PA, of 295.5 kcal mol(-1) and records a pKa(MeCN) of 50 +/- 1 units.

1,8-Bis(hexamethyltriaminophosphazenyl)naphthalene, HMPN: a superbasic bisphosphazene "proton sponge".

It is shown that a combination of Schwesinger's phosphazene base concept and the idea of the disubstituted 1,8-naphthalene spacer, first introduced by Alder in paradigmatic 1,8-bis(dimethylamino)naphthalene (DMAN), yields a new superbase, HMPN, which represents the up to date most basic representative of this class of "proton sponges", as evidenced by the theoretically estimated proton affinity PA = 274 kcal/mol and the measured pK(BH+) (MeCN) 29.9 +/- 0.2.

Extremal acidity of Rees polycyanated hydrocarbons in the gas phase and DMSO--a density functional study.

It is shown by using the density functional theory (DFT) calculations that Rees tricyclic[10]annulene 1 and its benzo-annelated derivative 2, substituted by CN groups at all peripheral sp2 carbon positions, represent hyperstrong neutral superacids in the gas-phase and DMSO.

Hydride affinities of borane derivatives: novel approach in determining the origin of lewis acidity based on triadic formula.

The problem of intrinsic Lewis acidities of simple boron compounds (BH3-mXm, m = 0-3, X = F, Cl, Br, CH3, and OH) is assessed by their gas-phase hydride affinities (HAs). A simple and intuitively appealing picture of the interaction process including detachment of an electron from the hydride ion H-, capture of the pruned electron to the investigated Lewis acid (LA), and subsequent formation of the homolytic chemical bond between two newly created radicals is proposed. It enables transparent and straightforward dissection of the initial and final state effects, which taken together with the intermediate relaxation stabilization determine the trend of changes in the hydride affinities.

The origin of aromaticity: important role of the sigma framework in benzene.

The physical nature of aromaticity is addressed at a high ab initio level. It is conclusively shown that the extrinsic aromatic stabilization energy of benzene E(ease)B, estimated relative to its linear polyene counterpart(s), is very well-reproduced at the Hartree-Fock (HF) level. This is a consequence of the fact that the contributions arising from the zero-point vibrational energy (ZPVE) and electron correlation are rather small.

In search of ultrastrong Brønsted neutral organic superacids: a DFT study on some cyclopentadiene derivatives.

An efficient but reasonably accurate B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) computational procedure showed that pentasubstituted cyclopentadienes such as (CN)5C5H, (NO2)5C5H, and (NC)5C5H containing strongly electron-withdrawing groups are neutral organic superacids of unprecedented strength. The boldface denotes the atom attached to the cyclopentadiene framework. All of them exhibit prototropic tautomerism by forming somewhat more stable structures with C=NH, NO2H, and N=CH exocyclic fragments, respectively.

On the bond-stretch isomerism in the benzo[1,2:4,5]dicyclobutadiene system--an ab initio MR-AQCC study.

Bond-stretch isomerism in benzo[1,2:4,5]dicyclobutadienle (BDCB) has been investigated using the MR-AQCC/6-31G(d) method, a high-level multireference ab initio approach including size-extensivity corrections. The applied theoretical approach includes both nondynamical and dynamical electron correlation effects. Full MR-AQCC geometry optimizations of localized (1) and delocalized (3) isomers as well as the transition structure (TS) have been determined using D2h, symmetry restriction.