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The edge-cutting design, synthesis and characterization of the so far strongest non-ionic carbon superbases is presented., A new generation of carbodiphosphoranes (CDPs), incorporating pyrrolidine, tetramethylguanidine, or tris(dimethylamino)phosphazene as substituents is introduced as the most powerful class of non-ionic carbon superbases on the basicity scale to date. The synthetic approach as well as NMR spectroscopic and structural characteristics in the free and protonated form are described. Investigation of basicity in solution and in the gas phase by experimental and theoretical means provides the to our knowledge first reported pKBH+ values for CDPs in the literature and suggest them as upper tier superbases.

The metabolism of vitamin D3 includes a parallel C-3 epimerization pathway-in addition to the standard metabolic processes for vitamin D3-reversing the stereochemical configuration of the -OH group at carbon-3 (s→α). While the biological function of the 3α epimer has not been elucidated yet, the additional species cannot be neglected in the analytical determination of vitamin D3, as it has the potential to introduce analytical errors if not properly accounted for. Recently, some inconsistent mass spectral behavior was seen for the 25-hydroxyvitamin D3 (25(OH)D3) epimers during quantification using electrospray LC-MS/MS. The present work extends that of Flynn et al. ( Ann. Clin. Biochem. 2014, 51, 352-559) and van den Ouweland et al. ( J. Chromatogr. B 2014, 967, 195-202), who reported larger electrospray ionization response factors for the 3α epimer of 25(OH)D3 in human serum samples as compared to the regular 3s variant. The present work was concerned with the mechanistic reasons for these differences. We used a combination of electrospray ionization, atmospheric pressure chemical ionization, and density functional theory calculations to uncover structural dissimilarities between the epimers. A plausible mechanism is described based on intramolecular hydrogen bonding in the gas phase, which creates a small difference of proton affinities between the epimers. More importantly, this mechanism allows the explanation of the different ionization efficiencies of the epimers based on kinetic control of the ionization process, where ionization initially takes place at the hydroxyl group with subsequent proton transfer to a basic carbon atom. The barrier for this transfer differs between the epimers and is in direct competition with H2O elimination from the protonated hydroxyl group. The hidden" site of high gas phase basicity was revealed through computational calculations and appears to be inaccessible via direct protonation."

In the present study, N-methyl-2, 4, 6- cycloheptatriene-1-imine (Troponimine) with (Me)2N- and MeO- substituents have been investigated theoretically to evaluate their suitability as a new neutral organic superbase. To introduce the aforementioned system, thermochemical parameters such as gas basicity and proton affinity have been calculated along with their pKBH+ values in acetonitrile. Results revealed that all studied troponimine derivatives are more basic than 1, 8- bis(dimethylamino)-naphthalene whose gas phase basicity of 237 kcal mol-1 is considered to be the borderline between ordinary bases and superbases. However, the most important finding is that some of the proposed troponeimines are much stronger bases than the well-established cyclopropenimine superbases. Origin of troponimine basicity is elucidated through NICS calculations which reveal that troponimines become aromatic upon protonation. AIM analyses revealed that the hydrogen bond could also contribute to the basicity if a suitable substituent is located on the adjacent of the imine group of the troponimine.

This study presents a detailed experimental investigation of charge isomers of protonated 4-quinolone antibiotics molecules formed during electrospray ionization (ESI) with proposed dissociation mechanisms after collisional activation. Piperazinyl quinolones have been previously shown to exhibit erratic behavior during tandem MS analyses of biological samples, which originated from varying ratios of two isomeric variants formed during ESI. Here, a combination of ESI-collision-induced dissociation (CID), differential ion mobility spectrometry (DMS), high resolution MS, and density functional theory (DFT) was used to investigate the underlying mechanisms of isomer formation and their individual dissociation behaviors. The study focused on ciprofloxacin; major findings were confirmed using structurally related 4-quinolones. DFT calculations showed a reversal of basicity for piperazinyl quinolones between liquid and gas phase. We provide an experimental comparison and theoretical treatment of factors influencing the formation ratio of the charge isomers during ESI, including solvent pH, protic/aprotic nature of solvent, and structural effects such as pK a and proton affinity. The actual dissociation mechanisms of the isomers of the protonated molecules were studied by separating the individual isomers via DMS-MS, which allowed type-specific CID spectra to be recorded. Both primary CID reactions of the two charge isomers originated from the same carboxyl group by charge-remote (CO(2) loss) and charge-mediated (H(2)O loss) fragmentation of the piperazinyl quinolones, depending on whether the proton resides on the more basic keto or the piperazinyl group, followed by a number of secondary dissociation reactions. The proposed mechanisms were supported by calculated energies of precursors, transition states, and products for competing pathways.

Herein we describe an easily accessible class of superbasic proton sponges based on the 1,8-bisphosphazenylnaphthalene (PN) proton pincer motif and P-alkyl substituents ranging from methyl (TMPN) to n-butyl (TBPN), isopropyl (TiPrPN) and cyclopentyl (TcyPPN). These neutral bases with a pK(BH)(+) value (MeCN) of ~30 were accessible via a Kirsanov condensation using commercially available 1,8-diaminonaphthalene, and in case of TMPN and TBPN, simple one-pot procedures starting from trisalkylphosphanes can be performed. Furthermore, the known pyrrolidinyl-substituted superbase TPPN previously synthesized via a Staudinger reaction could also be prepared by the Kirsanov strategy allowing its preparation in a larger scale. The four alkyl-substituted proton sponges were structurally characterized in their protonated form; molecular XRD structures were also obtained for unprotonated TiPrPN and TcyPPN. Moreover, we present a detailed description of spectroscopic features of chelating bisphosphazenes including TPPN and its hyperbasic homologue P2-TPPN on which we reported recently. The four alkyl-substituted superbases were investigated with respect to their basic features by computational means and by NMR titration experiments revealing unexpectedly high experimental pK(BH)(+) values in acetonitrile between 29.3 for TMPN and 30.9 for TBPN. Besides their thermodynamic basicity, we exemplarily studied the kinetic basicity of TMPN and TPPN by means of NMR-spectroscopic methods. Furthermore, the competing nucleophilic versus basic properties were examined by reacting the proton sponges with ethyl iodide. Insight into the coordination chemistry of chelating superbases was provided by reacting TMPN with trimethylaluminum and trimethylgallium to give cationic complexes of Group XIII metal alkyls that were structurally characterized.

It is shown by reliable DFT B3LYP/6-311+G(2df, p)//B3LYP/6-31G* method that pyridine and s-triazine can serve as useful building blocks in tailoring neutral superbases. The former compound is used in constructing the four-fragment macrocycles substituted by a number of NMe2 substituents placed at strategic positions on the suprastructures backbone. The calculated proton affinities are within the range of 270-290 kcal mol-1, thus belonging to the upper part of the superbasicity ladder. It is shown that proton affinities of polysubstituted derivatives follow a simple additivity rule, which facilitates prediction and interpretation of the results in multiply substituted systems. The s-triazine motif is employed in forming manxane-like systems by using guanidine fragments or 1, 1-dimethylaminoethylenes, which exhibit proton affinities spread over the lower part of the superbasicity scale (256-272 kcal mol-1). The amplified basicity in both families of compounds is a consequence of the strong cationic resonance in conjugate acids supported by stabilization provided by intramolecular hydrogen bonds (IMHB). The nature of the IMHBs is examined. It is found that IMHBs in large macrocycles constructed by pyridine moieties is of a bicentric type. They are not cationic resonance supported IMHBs in the first approximation. In contrast, protonation in one of the manxane-like systems yields the IMH bonds that are stabilized by substantial cationic resonance through the hydrogen H+ bridge. Consequently, the cationic resonance supported hydrogen bonds (CRSHBs) do exist in some protonated species. The basicity of pyridine macrocycles is examined in acetonitrile. It is found that their pKa values span a range between 26-30 units meaning that they are also strong superbases in MeCN solutions compared to the threshold given by pKa(MeCN) of DMAN (18.2 units).

The MP2(fc)/6-311+G**//HF/6-31G* and B3LYP/6-311+G**//HF/6-31G* calculations show that N-[3-(dimethylamino)propyl] and N-[4-(dimethylamino)butyl] derivatives of 2,3-diamino-cycloprop-2-ene-1-imine (CPI) possess high gas phase proton affinities (PA) and pronounced basicities in acetonitrile (MeCN). Tris[N-dimethylaminopropyl(-butyl)]-CPI derivatives achieve superbasic PA and pKa values. The reason behind these remarkable features is the stability of the resulting conjugate acids. They are stabilized by aromatization of the three-membered ring accompanied by the cationic resonance between the cyclopropyl ring and amino nitrogen atoms of the side chains, and, last but not least, by the cooperative multiple intramolecular hydrogen bonding effect. The latter is particularly enhanced in tris-substituted CPIs. It was found that the N-[4-(dimethylamino)butyl] side chains contribute more to the stability of the conjugate acids than those of the corresponding compound with a propyl chain, because they undergo a somewhat larger relaxation upon protonation. The studied compounds might be helpful in building up a dense ladder of strong bases and superbases, which could contribute to the development of the chemistry of superacids and superbases. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

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.

Multiple aminopropyl substitutions of the initial N,N′,N″-trimethylguanidine lead to high intrinsic absolute proton affinities (APA) and basicities culminating in APA = 268.4 kcal mol−1 (1 kcal = 4.184 kJ) in N,N′,N″-tris(3-aminopropyl)guanidine (9). The reason behind a high proton affinity is identified as a strong cationic resonance in the central guanidine moiety and the strength of the intramolecular hydrogen bonding (IMHB), which is enhanced upon protonation. A cooperative IMHB effect in 9 and 9H+ realized by three N(sp3)H···N(sp3) H-bridges is estimated to be as high as 14.7 and 33.0 kcal mol−1, respectively. It follows that the IMHB effect contributes 18.3 kcal mol−1 to the absolute proton affinity of 9, which is a respectable amount. The basicity of 9 in acetonitrile is estimated to be very high, as evidenced by the corresponding pKa = 28.8. A derivative of 9, N,N′,N″-tris(3-dimethylaminopropyl)guani-dine (10), assumes an even higher superbasic proton affinity of 275 kcal mol−1 due to an additional relaxation effect caused by the methyl groups. The corresponding pKa(MeCN) = 29.4. Copyright © 2002 John Wiley & Sons, Ltd.

The role of intramolecular hydrogen bonding (IMHB) in determining the proton affinities and basicities of some bis(tetramethylguanidine)systems was examined. For this purpose a series of molecular backbone moieties serving as carriers of the bis(tetramethylguanidine)crowns were explored. It was found that the best backbones are provided by phenanthrene and 9,10-dihidrophenanthrene, giving rise to proton affinities as large as 268.2 and 266.8 kcal mol1−, respectively. The corresponding pKa values in acetonitrile are 29.0 and 28.8, implying that these two compounds [6(bs)and 5(bs)] are candidates for powerful superbases. Their intramolecular hydrogen bond strengths are ≈19 kcal mol1−, which result inter alia from the partial protonation of the vis-a-vis guanidine group.

Imines are an interesting but relatively poorly-characterized class of organic compounds: for example, is the gas phase ion-derived C7H7N species observed by Nibbering et al. benzaldimine or troponimine? Proton affinities are important quantities for the understanding of organic compounds. We have investigated the structure, energetics and proton affinities of benzaldimine, troponimine, and their carbonyl and olefinic analogs (benzaldehyde, tropone, styrene, heptafulvene) using G2(MP2) computational theory. We find the benzenoid species (benzaldimine, benzaldehyde, styrene) to be more stable by 20–25 kcal mol−1 than their nonbenzenoid isomers (troponimine, tropone, heptafulvene). From our calculations and analysis of literature data, we derive the enthalpy of formation and proton affinity of benzaldimine to be 39 ± 3 and 220 ± 3 kcal mol−1 respectively and conclude Nibbering's C7H7N is, in fact, this species.

It is shown, by utilizing a reliable model at the MP2 level of theory, that the imino group attached to the cyclopropene moiety exhibits a high proton affinity (PA). The reason behind the appreciable PA is identified as significant aromatization of the three-membered ring spurred by the protonation. Further amplification of the PA can be achieved by NH2 substitutions at the CC endo double bond, since amino groups stimulate aromatization of the cyclopropene fragment. Additionally, they release some of their lone pair electron density thus contributing to a uniform distribution of the positive charge over the entire molecular system. This effect is even more pronounced if the diaminoiminocyclopropene system is substituted by sizable alkyl groups. The aromatic stabilization in the protonated 2,3-diaminocyclopropeneimine, estimated by the corresponding homodesmic reaction, seems to be as high as 59.5 kcal/mol. Very potent organic bases can be obtained by designing polycyclic systems possessing two or more cyc...

The absolute proton affinities (APA) of 20 α-amino acids, as obtained by the MP2(fc)/6-311+G∗∗//HF/6-31G∗ + ZPVE(HF/6-31G∗) and the scaled Hartree–Fock (HFsc) models, are presented. It is shown that the α-NH2 group is protonated in all but four cases: lysine (K), proline (P), histidine (H), and arginine (R). There is a good overall agreement with experimental data measured by the kinetic method. However, there are some notable exceptions such as glutamine (Q) and lysine (K), where strong hydrogen bonds in the protonated forms occur. It is suggested that the present results and theoretical models employed could be useful for resolving such experimental ambiguities. Furthermore, it appears that the HFsc model provides an efficient tool for elucidating APAs of artificial α-AAs, derivatives of natural α-AAs and their oligomers.

The problem of the intramolecular proton transfer isomerism in arginine, leading to conventional neutral and zwitterionic forms of this compound, is addressed by high level theoretical models. It is shown that arginine has two neutral and two zwitterionic isomers implying that there exist two additional unconventional isomers, which have not been identified so far. It appears also that the most stable neutral isomer is energetically more favourable than both zwitterions, which implies that the former should be preferred in the gas phase. Examination of atomic charges obtained by the electron density partitioning techniques reveals that the charge distributions of neutral and zwitterionic isomers are not as widely different as expected. This finding is counterintuitive, since it contradicts the classical notion of chemical bonding and a customary picture of zwitterions involving two local complementary fragments possessing unit charges of opposite sign. The true distribution of the electron density is more uniform and quite similar to that of the neutral form. The proton affinity of arginine is estimated to be 249 kcal mol–1. Hence, it follows that arginine is a very basic compound although it belongs to a family of 20 fundamental α-amino acids. A very high proton affinity is interpreted in terms of the resonance effect spurred by protonation in the guanidine moiety and by a strong hydrogen bonding taking place in the protonated form.

The spatial structure of biphenyl 1 is studied by the semiempirical AM1 and ab initio HF/6-31G* and MP2(fc)/6-31G* theoretical models. The resulting bond distances are in good agreement with the X-ray structure. The calculated dihedral angle is in accordance with the value observed by the electron diffraction technique (theta = 45°). Its large value is a compromise between the steric hindrance effect and the pi-electron conjugation. The estimated barrier heights for the internal rotation is very low however. Theoretical values are in accordance with the available experimental evidence. The calculated proton affinity (PA) obtained by the scaled (AM1)sc model and by using the MP2 level of theory compares very well with the experimental value. It is by some 13 kcal/mol higher than the reference PA value of benzene, because of the strong resonance interaction between two phenyl rings. The increase in the pi-electron conjugation energy triggered by protonation overcomes the steric repulsion between H atoms thus decreasing the twist angle by some 20 °. The apical carbon atom, placed para to coannular CC bond, is most susceptible to the proton attack. On the other hand, the PA values for ipso and meta carbons are substantionally lower since an amplification of the inter-ring conjugation interaction is then precluded. The PA increments for the CH3 group and F atom monosubstituted biphenyls are determined by using the (AM1)sc approach. They are employed in estimating proton affinities of a number of polysubstituted biphenyls applying a very simple additivity formula based on the independent substituent approximation (ISA). It is shown that the performance of the additivity rule is very good. Variation in the PA of substituted biphenyls is rationalized in the terms of the conjugation effect and repulsion between hydrogen atoms or substituents attached to the vis-a-vis ortho positions of the neighbouring rings (steric effect). Finally, the proton affinity of fluorene possessing a "frozen" planar biphenyl moiety is calculated and compared with that of the paradigmatic Mills-Nixon (MN) system - indan. It is found that PA values of the former compound are determined by the MN and resonance effects, the latter being predominant. The most basic site in fluorene is the C(4) atom where both effects act in a synergistic way.

The spatial and electronic structure of the parent 2-tetrazene and its tetramethyl- and ethyl derivatives are examined by an ab initio approach. It is shown by using the MP2(fc)/6-311+G ∗∗ //HF/6-31G ∗ + ZPVE (HF/6-32G ∗ ) model that alkylated tetrazenes exhibit strong intrinsic basicity as evidenced by their high absolute proton affinities. The latter is due to the appreciable electron density concentration at the terminal nitrogen atoms, the increased II-electron delocalization in conjugate acids and the electron density relaxation effect occurring upon protonation.

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. HMPN is by nearly 12 orders of magnitude more basic than Alder's classical 1,8-bis(dimethylamino)naphthalene (DMAN). The title compound, HMPN, is prepared and fully characterized. The spatial structure of HMPN and its conjugate acid is determined by X-ray technique and theoretical DFT calculations. It is found that monoprotonated HMPN has an unsymmetrical intramolecular hydrogen bridge (IHB). This cooperative proton chelating effect renders the bisphosphazene more basic than Schwesinger's set of "monodentate" P1 phosphazene bases. The density functional calculations are in good accordance with the experimental results, providing some complementary information. They conclusively show that the high basicity of HMPN is a consequence of the high energy content of the base in its initial neutral state and the intramolecular hydrogen bonding in the resulting conjugate acid with contributions to proton affinity of 14.1 and 9.5 kcal/mol, respectively.

The synthesis of novel N, N', N'' tris(3 dimethylaminopropyl)-guanidine 1 is described and the X-ray structure of its hexafluorophosphate salt measured (1H· PF6). The hydrogen bonding in protonated 1 and in 1H· PF6 is also discussed.

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. Unlike in other protonated amino acids (glycine, proline, phenylalanine, tyrosine, and tryptophan) studied earlier, the exchange rate of the carboxyl group in protonated histidine is slower than that of the amino group. The most stable conformers and the enthalpies of neutral and protonated histidine and its methyl ester are calculated at the G3(MP2) level of theory. It is shown that strong intramolecular hydrogen bonding between the amino group and the imidazole ring nitrogen sites is responsible for the stability and specific properties of the protonated histidine. It is found that the proton fluctuates between the amino and imidazole groups in the protonated form across an almost vanishing barrier. Proton affinity (PA) of histidine calculated by the G3(MP2) method is 233.2 and 232.4 kcal mol(-1) for protonation at the imidazole ring and at the amino group nitrogens, respectively, which is about 3-5 kcal mol(-1) lower than the reported experimental value.

The aim of this study was to investigate the behavior of the protonated paralytic shellfish poisons saxitoxin (STX) and neosaxitoxin (NEO) in the gas-phase after ion activation using different tandem mass spectrometry techniques. STX and NEO belong to a group of neurotoxins produced by several strains of marine dinoflagellates. Their chemical structures are based on a tetrahydropurine skeleton to which a 5-membered ring is fused. STX and NEO only vary in their substituent at N-1, with STX carrying hydrogen and NEO having a hydroxyl group at this position. The collision-induced dissociation (CID) spectra exhibited an unusually rich variety and abundance of species due to the large number of functional groups within the small skeletal structures. Starting with triple-quadrupole CID spectra as templates, linked ion-trap MSn data were added to provide tentative dissociation schemes. Subsequent high-resolution FTICR experiments gave exact mass data for product ions formed via infrared multiphoton dissociation (IRMPD) from which elemental formulas were derived. Calculations of proton affinities of STX and NEO suggested that protonation took place at the guanidinium group in the pyrimidine ring for both molecules. Most of the observed parallel and consecutive fragmentations could be rationalized through neutral losses of H2O, NH3, CO, CO2, CH2O and different isocyanate, ketenimine and diimine species, many of which were similar for STX and NEO. Several exceptions, however, were noted and differences could be readily correlated with reactions involving NEO's additional hydroxyl group. A few interesting variations between CID and IRMPD spectra are also highlighted in this paper.

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.

The problem of the absolute proton affinity (APA) of some polyguanides is addressed by the MP2(fc)/6-311+G//HF/6-31G theoretical model. It is shown that the linear chain polyguanides exhibit increased basicity as a function of the number of guanide subunits. However, the saturation effect yields an asymptotic APA value of 254 kcal/mol. Branched polyguanides on the other hand have higher APAs than their linear counterparts. The largest proton affinity is found in a doubly bifurcated heptaguanide, being as high as 285 kcal/mol, thus potentially representing one of the strongest organic bases. Finally, it is found that all polyguanides protonate at imino nitrogen atoms, since they are apparently susceptible the most to the proton attack. The origin of their very high intrinsic basicity is traced down to a dramatic increase in the resonance interaction of the corresponding conjugate bases. For instance, the increase in the resonance energy in the protonated guanidine is estimated to be in a range of 24-27 kcal/mol, which is higher than the aromatic stabilization in benzene. The proton affinity of some polycyclic guanides including Schwesinger proton sponge and porphine is briefly discussed.

The problem of protonation of benzene (B) and cyclobutadiene (CB) is addressed by the ab initio G2 benchmark computations. It appears that the σ-complexes are the most favourable protonated forms in both cases. The proton affinity (PA) of (CB) is higher by 45 kcal/mol than that of B, which is rationalized by a decrease in the antiaromatic destabilization of the four-membered ring and stabilization by the allylic cation formation upon protonation accompanied by the ring puckering, which enables an effective additional homoconjugative interaction. The barrier for the hydrogen ring walk in B is estimated to be 8.6 kcal/mol leading to seven equivalent hydrogens on the NMR time scale. In contrast, this barrier in CB is 43.6 kcal/mol implying that the ring walk is precluded in this cationic system. The ring flipping inversion barrier of the σ-protonated CB is estimated to be 7 kcal/mol thus being in good accordance with experiment. The striking structural features of σ and π-protonated configurations of B and CB involve degrees of pyramidalization and tetrahedral distortions of carbon atoms, which are briefly discussed. Some practical models, alternative to the G2 scheme, designed for studying the PA of aromatic and antiaromatic compounds are examined and recommended.

The proton affinity (PA) of some extended pi?electron systems involving guanidine and cyclopropeneimine structural motifs is explored by the theoretical MP2 and HF_SC models. It is shown that some of the studied molecules should exhibit higher gas phase basicity than Schwesinger proton sponge, which is considered to be one of the most powerful organic superbases. The origin of the increased basicity is traced down to a dramatic resonance effect triggered by the protonation. It is interesting to note that examined compounds possess higher PAs than their polyguanide counterparts. The reason behind it is a well established fact that the three?membered rings undergo aromaticization in the conjugate acids by forming aromatic moiety. The important role of substituents in determining high inherent basicities is underscored.