HCN exchange on [Cu(HCN)4]+: a quantum chemical investigation

Density functional (B3LYP, B3PW91, X3LYP, BP86, PBEPBE, PW91PW91, and M06) and ab initio (MP2, MP4sdq, CCSD, and CCSD(T)) calculations with extended basis sets (6-311+G**, TZVP, LANL2DZ+p, and SDD+p, the latter including extra polarization and diffuse functions) indicate that HCN exchange on [Cu(HCN)₄]⁺ proceeds via an associative interchange (I_a) mechanism and a D_3h transition structure {[Cu(HCN)₅]⁺}^?. The activation barrier, relative to the model complex [Cu(HCN)₄]⁺·HCN, varies modestly, depending on the computational level. Typical values are 8.0?kcal?M^?1 (B3LYP/6-311+G**), 6.0?kcal?M^?1 (M06/6-311+G**), and 4.8?kcal?M^?1 (CCSD(T)/6-311+G**//MP2(full)/6-311+G**). Inclusion of an implicit solvent model (B3LYP(CPCM)/6-311+G**) leads to an activation barrier of 5.8?kcal?mol^?1. Comparison of the HCN exchange mechanisms on [Li(HCN)₄]⁺ (limiting associative, A) and [Cu(HCN)₄]⁺ (associative interchange, I_a) reveals that π back donation in the equatorial Cu–N bonds in the transition state determines the mechanism.     

Theoretical study on reactions of cyclic-N3 with NO, NO2 and Cl2

In this article, we report our detailed mechanistic study on the reactions of cyclic-N₃ with NO, NO₂ at the G3B3//B3LYP/6-311+G(d) and CCSD(T)/aug-cc-pVTZ//QCISD/6-311+G(d)+ZPVE levels; the reactions of cyclic-N₃ with Cl₂ was studied at the G3B3//B3LYP/6-311+G(d) and CCSD(T)/aug-cc-pVTZ//QCISD/6-31+G(d)+ZPVE levels. Both of the singlet and triplet potential-energy surfaces (PESs) of cyclic-N₃ + NO, cyclic-N₃ + NO₂ and the PES of cyclic-N₃ + Cl₂ have been depicted. The results indicate that on singlet PESs cyclic-N₃ can undergo the barrierless addition–elimination mechanism with NO and NO₂ forming the respective dominant products N₂ + ¹cyclic-NON and ¹NNO(O) + N₂. Yet the two reactions on triplet PESs are much less likely to take place under room temperature due to the high barriers. For the cyclic-N₃ + Cl₂ reaction, a Cl-abstraction mechanism was revealed that results in the product cyclic-N₃Cl + Cl with an overall barrier as high as 14.7 kcal/mol at CCSD(T)/aug-cc-pVTZ//QCISD/6-31+G(d)+ZPVE level. So the cyclic-N₃ radical could be stable against Cl₂ at low temperatures in gas phase. The present results can be useful for future experimental investigation on the title reactions.     

Gas phase and solution state stability of complexes L…M, where M = Cu, Ni, or Zn and L = R−C(O)NHOH (R = H, NH2, CH3, CF3, or Phenyl)

The structure and gas-phase metal affinities (M = Cu²⁺, Ni²⁺, and Zn²⁺) of formohydroxamic acid derivatives R–C(O)NHOH (R = H, NH₂, CH₃, CF₃ and Phenyl) were studied using the B3LYP/6-311+G(d,p) method of DFT theory. In order to evaluate the conformational behavior of these systems in water, we carried out CPCM-SCRF optimization calculations at the B3LYP/6-311+G(d,p) levels of theory. The obtained optimized geometries and interaction affinities of the gas and solution phase were compared. The following order of stability was found for ionic complexes of the transition metals: Cu²⁺ > Ni(t)²⁺ > Zn²⁺. The same stability order would be expected according to the Irving–Williams order of stability constants. The high-spin complexes of the Ni²⁺ were more stable than the low-spin complexes. The solvent effect reduced the observed relative stability of individual metallic complexes of substituted hydroxamic acids.     

A Computational Study of Chair–Twist Energy Differences and the Chair–Chair Conformational Interconversion in Tetrahydro-2H-thiopyran (Thiacyclohexane,Thiane)

Ab initio molecular orbital theory with the LANL2DZ, 3-21G, 6-31G(d), 6-31+G(d), 6-31+G(d,p), 6-311+G(d,p),6-31G(2d), 6-31G(3d), and 6-311G(d,p) basis sets and density functional theory (B3P86, B3LYP, B3PW91) have been used to calculate the structures, relative energies, enthalpies, entropies, and free energies of the chair, 1,4-twist, and 2,5-twist conformers of tetrahydro-2H-thiopyran (tetrahydrothiopyran, thiacyclohexane, thiane, pentamethylene sulfide). All levels of theory calculated similar energy values and the effect of basis sets on the calculated energies was small. The chair conformer of tetrahydro-2H-thiopyran was 5.27 kcal/mol more stable than the 1,4-twist conformer, which was slightly more stable (0.81 kcal/mol) than the 2,5-twist conformer. The chair–1,4-twist and chair–2,5-twist free energy differences ( G°_{c – t}) were 5.44 and 5.71 kcal/mol, respectively. Intrinsic reaction coordinate [IRC, minimum-energy path (MEP)] calculations connected the transition state between the chair and the 2,5-twist conformers. This transition state is 9.73 kcal/mol higher in energy than the chair conformer and the energy differences between the chair and the 1,4-boat and 2,5-boat transition states were 8.07 and 6.38 kcal/mol, respectively. Stereoelectronic hyperconjugative interactions were observed in the chair, 1,4-twist, and 2,5-twist conformers of tetrahydro-2H-thiopyran. The stereoelectronic hyperconjugative effects in the chair conformer of tetrahydro-2H-thiopyran have been compared to those in the respective chair conformers of tetrahydro-2H-pyran, tetrahydro-2H-selenane, and tetrahydro-2H-tellurane.     

Thermal isomerization of acetylnitrene: a quantum-chemical study

The electronic structure and pathways of thermal isomerization of formylnitrene and acetylnitrene were studied by the B3LYP/6-311G(d,p) density functional method and ab initio G2(MP2,SVP) computational procedure using the geometries obtained from B3LYP calculations. According to G2 calculations, both nitrenes have singlet ground states while the energies of the corresponding triplet states are 2.8 and 5.7 kcal mol^–1 higher. For acetylnitrene, the activation barrier to the nitrene isocyanate isomerization was estimated at 28.9 kcal mol^–1 (G2). Calculations revealed no pathway for single-step isomerization of nitrene into cyanate in both systems. The formation of methyl cyanate from isocyanate is thermodynamically unfavorable (E = 26.5 kcal mol^–1) and requires a high activation barrier (89.4 kcal mol^–1) should be overcome. Based on the results obtained, the pathways of transformation of nitrene formed in thermal decomposition of acetyl azide (Curtius rearrangement) were analyzed.     

Theoretical Study of Gas-Phase Acidities of Selected Angiotensin-Converting Enzyme Inhibitors

The structure and gas-phase acidities of six members of large angiotensin-converting enzyme (ACE) inhibitor family [cilazaprilat, silanediol, fosinoprilat, AcSDKP, angiotensin_I (terminal part), and RXP_407] have been studied using the ONIOM Becke3LYP/6-311+G(d,p):HF/3-21G^∗ method. The investigated ACE inhibitors are weak acids with calculated acidity of about 1270–1650 kJ mol⁻¹. Of acids studied the highest gas-phase acidity (1273 kJ mol⁻¹) possesses experimental ACE inhibitor RXP_407. This drug, according to the computed pK_a value (3.2), is also in water solution the most acidic compound of the ACE inhibitors investigated.     

Molecular Structure and Conformational Preferences of Trimethyl Phosphorotrithioite,P(SMe)<Subscript>3</Subscript>, Evaluated by Gas-Phase Electron Diffraction and Quantum Chemical Calculations

Free P(SMe)₃ molecule was studied by gas electron diffraction (GED) and by B3PW91/6-311+G* (DFT) and MP2/6-31+G* calculations. Each conformer is characterized by three dihedral angles τ(CSPlp), where lp denotes the direction of the lone electron lone pair on the P atom. DFT calculations indicate that the most stable conformer is an anti,gauche+,gauche- (ag+g-) conformer of C _s symmetry; the next are the ag+g+ (ΔE = 2.5 kJ mol⁻¹), g+g+g+ (ΔE = 5.2 kJ mol⁻¹), and aa+g+ (Δ E = 12.5 kJ mol⁻¹) conformers. The MP2 calculations give the similar order, with the relative energies of 0.3, 4.3, and 10.6 kJ mol⁻¹, respectively. The experimental GED data agree well with the presence of only two conformers: χ(ag+g+) = 80(20)% and χ(ag+g-) = 20(10)%.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 5, 2005, pp. 742–750.Original Russian Text Copyright © 2005 by Belyakov, Khramov, Baskakova, Naumov.     

FT-IR, FT-Raman vibrational spectra and molecular structure investigation of 2-chloro-4-methylaniline: A combined experimental and theoretical study

In this work, the experimental and theoretical vibrational spectra of 2-chloro-4-methylaniline (2Cl4MA, C₇H₈NCl) were studied. FT-IR and FT-Raman spectra of 2Cl4MA in the liquid phase have been recorded in the region 4000–400 cm⁻¹ and 3500–50 cm⁻¹, respectively. The structural and spectroscopic data of the molecule in the ground state have been calculated by using Hartree-Fock (HF) and density functional method (B3LYP) with the 6-31G(d), 6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p) and 6-311G(d), 6-311G(d,p), 6-311+G(d,p), 6-311++G(d,p) basis sets. The vibrational frequencies have been calculated and scaled values have been compared with experimental FT-IR and FT-Raman spectra. The observed and calculated frequencies are found to be in good agreement. The complete assignments were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. The DFT-B3LYP/6-311++G(d,p) calculations have been found more reliable than the ab initio HF/6-311++G(d,p) calculations for the vibrational study of 2Cl4MA. The optimized geometric parameters (bond lengths and bond angles) were compared with experimental values of aniline and p-methylaniline molecules.     

Formation of a rhodium(II) monohydrido complex derived from wilkinson's complex RhCl(PPh3)3 in the interlamellar spaces of montmorillonite and catalytic hydrogenation of cyclohexene

The interaction of molecular hydrogen with [Rh(PPh₃)₃]⁺ (1a) “immobilized” in the interlamellar spaces of montmorillonite resulted in the formation of a monohydrido complex, [Rh^IIH(PPh₃)₃] (2a), characterized by electrochemical data of the clay-loaded electrode, IR, EPR and hydrogen absorption studies. Heterogenized homogeneous catalytic hydrogenation of cyclohexene catalysed by 1a was investigated in the temperature range 283–313 K. The order of reaction with respect to cyclohexene and hydrogen concentration is fractional and first order with respect to catalyst concentration. Thermodynamic parameters ΔH⁰ and ΔS⁰ corresponding to the formation of the monohydrido species were found to be 18 kcal mol⁻¹ and 61 e.u., respectively. The activation enthalpy, ΔH^‡, and entropy, ΔS^‡, for the hydrogenation of cyclohexene by the Rh^II—H complex in clay are more negative by about 2 kcal mol⁻¹ and 7 e.u. compared to Wilkinson's catalyst, RhCl(PPh₃)₃ (1), in homogeneous solution.     

Molecular structure and decomposition mechanism of peracetic acid esters AcOOR (R = Me,Bu<Superscript>t</Superscript>)

The molecular structure and conformational mobility of methyl and tert-butyl esters of peracetic acid AcOOR (R = Me (1), Bu^t (2)) were studied by the ab initio MP4(SDQ)//MP2(FC)/6-31G(d,p) method and density functional B3LYP/6-31G(d,p) approach. The B3LYP calculated equilibrium conformations of the molecules are characterized by the C-O-O-C torsion angles of 93.6° (1) and 117.0° (2). Structural features of the molecules under study and a distortion of tetrahedral bond configuration at the C_α atom were explained using the natural bonding orbital approach. The standard enthalpies of formation of AcOOMe (−328.5 kJ mol⁻¹) and AcOOBu^t (−440.4 kJ mol⁻¹) were determined using the G2 and G2(MP2) computational schemes and the isodesmic reaction approach. The transition state of AcOOMe decomposition into AcOOH and formaldehyde was calculated (E _a = 122.8 kJ mol⁻¹). The thermal effects of homolytic decomposition of the peroxy esters following a concerted mechanism (Me^· + CO₂ + ^·OR) and simple homolysis of the peroxide bond (AcO^· + ^·OR) were found to be 97.5±0.3 and 155.1±0.3 kJ mol⁻¹, respectively. At temperatures below 400 K, the most probable decomposition mechanism of peroxy esters 1 and 2 involves simple homolysis of the O-O bond.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2021–2027, October, 2004.     

Does the radical Me3O√ exist in the gas phase? A joint experimental and theoretical study

A combination of experiment and theory indicate that when Me₃O⁺ undergoes vertical Franck–Condon one-electron reduction, it does not produce a stable Me₃O√ species, but decomposes to give MeOMe and Me√. Theoretical calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311G(d) level of theory indicate that the symmetrical radical Me₃O√ is not a stable species. However, there is a stable doublet neutral corresponding to a van der Waals complex MeOMeMe, a species which requires only 4.5 kJ mol⁻¹ of excess energy to cause decomposition to MeOMe and Me√.     

Production of hyperlithiated Li2F by a laser ablation of LiF–Li3N mixture

Lithium-excess binary clusters Li_nF_n−1 (n=2–9) were detected by photoionization time-of-flight mass spectrometry in a supersonic cluster beam generated by a laser ablation of a solid mixture of lithium fluoride and nitride. Laser power dependence of the Li₂F⁺ signal intensity has indicated that the ionization energy of the hyperlithiated Li₂F molecule is lower than 4.66 eV. The theoretical vertical ionization energy obtained by the CCSD(T)/6-311+G(d)//B3LYP/6-311+G(d) calculations are 4.47 eV. No nitrogen-containing clusters were detected. The absence of Li₄N is ascribed to the exothermicity of the reaction, 2Li₃N→N₂+Li₆.     

Low temperature IR spectroscopy and photochemistry of matrix-isolated α-pyridil

α-Pyridil [(C₆H₄NO)₂] has been isolated in low temperature argon and xenon matrices and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6–311++G(d,p) calculations. Calculations predicted the existence of three different conformers exhibiting skewed conformations around the intercarbonyl bond and the two C₅H₄NC(O) fragments nearly planar. The two higher energy forms, TCG and CCSk were estimated theoretically to be, respectively, 21.0 and 35.1 kJ mol⁻¹ higher in energy than the most stable form, TTG. In consonance with the relatively high energies predicted by the calculations for the two less stable conformers of α-pyridil, only the most stable conformer was found spectroscopically to be present in the studied matrices. Infrared spectra obtained for the neat low temperature amorphous and crystalline states reveals that the TTG conformer is also the sole conformer present in these phases. UV irradiation (λ > 235 nm) of matrix-isolated α-pyridil led to its isomerization into unusual molecular species bearing Hückel-type pyridine (aza-benzvalene) rings.     

Theoretical study on structures and stability of SiCP2 isomers

The structures, energetics, spectral parameters and stability of the singlet SiCP₂ isomers are explored at the density functional theory and ab initio levels. Eight isomers connected by ten interconversion transition states are located at the CCSD(T)/6-311G(2d)//B3LYP/6-311G(d)level. The kinetically stable isomers and their relevant interconversion transition states are further refined at CCSD(T)/6-311+G(2df)//QCISD/6-311G(d) level. At QCISD/6-311G(d) level, one four-membered ring isomer cSiPCP and two linear structures PSiCP, SiCPP possess considerable kinetic stability (more than 15 kcal/mol). The valence bond structures of three kinetically stable SiCP₂ isomers are analyzed. The similarities and discrepancies in structure, energy and stability between SiCP₂ and its analogous C₂P₂, Si₂P₂, SiCN₂ and CSiNP molecules are also discussed. The predicted structures and spectroscopic properties are expected to be informative for the identification of the SiCP₂ in the laboratory and space.     

Synthesis and vibrational study of platinum(II) and palladium(II) complexes of glyoxilic acid oxime

New platinum(II) and palladium(II) complexes of glyoxilic acid oxime (gao) have been prepared and characterised by infrared (4000–150 cm⁻¹) and Raman (4000–200 cm⁻¹) spectra. The gao acts as bidentate ligand bonding through the oxime nitrogen and carboxyl oxygen atoms to form neutral bis-chelate square-planar complexes. The lowest energy conformer of the gao ligand (ectt) was selected among 16 theoretically possible conformers on the basis of ab initio calculations at HF/3-21G*, HF/6-31G* and HF/6-311** levels of the theory from which structural parameters and conformational stabilities have been obtained. A complete vibrational assignment of the gao was performed for the lowest energy ectt conformer on the basis of ab initio optimised parameters and normal coordinate analysis calculations (PED). NCA calculations of the complexes studied were also performed.     

Proton affinities of ketones, vicinal diketones and α-keto esters: a computational study

The proton affinities of seven different ketones, vicinal diketones, and α-keto esters (acetophenone, 2,2,2-trifluoroacetophenone, 2,3-butanedione, 1-phenyl-1,2-propanedione, methyl pyruvate, ethyl benzoylformate, and ketopantolactone) have been evaluated theoretically using the conventional ab initio HF and several post-HF methods (MP2, MP4, CCSD), density functional methods with the B3LYP hybrid functional, as well as some ab initio model chemistries [CBS-4M, G2(MP2), and G3(MP2)//B3LYP]. The chemical compounds studied are frequently used substrates in the asymmetric hydrogenation over chirally modified platinum catalysts where the protonation properties of the chiral modifier and the substrates are of great interest. In most cases, the proton affinities (PAs) evaluated with the CCSD/6-311+G(d,p)//B3LYP/TZVP and G2(MP2) methods are in good agreement with the existing experimental ones. However, the previously reported PA of 2,3-butanedione seems to be too high by 10-15 kJ mol⁻¹. The B3LYP/TZVP//B3LYP/TZVP and MP2/6-311+G(d,p)//B3LYP/TZVP model chemistries predict proton affinities that are systematically higher and lower than the experimental PAs, respectively. If proton affinities are evaluated as the average of the PAs calculated with these two theoretical methods a very good agreement with the experimental results is obtained. The mean absolute deviation (MAD) from experiment of this combination method for the PAs of 13 test molecules is 4.0 kJ mol⁻¹. For 9 molecules composed only of first-row atoms the MAD is 2.5 kJ mol⁻¹. The B3LYP/TZVP//B3LYP/TZVP and MP2/6-311+G(d,p)//B3LYP/TZVP methods provide significant savings in computational time and disk space compared to the CCSD/6-311+G(d,p)//B3LYP/TZVP and G2(MP2) models. Therefore, it is suggested that if no experimental or highly accurate theoretical data is available (due to computational cost), the proton affinities of similar compounds as investigated in this paper, can be evaluated with the combination method. For the studied molecules, this method gives the following PAs (in kJ mol⁻¹): 788 (2,3-butanedione, exptl 802); 798 (2,2,2-trifluoroacetophenone, exptl 799); 811 (ketopantolactone); 813 (methyl pyruvate); 825 (1-phenyl-1,2-propanedione); 862 (acetophenone, exptl 861); 865 (ethyl benzoylformate).     

Molecular structure, barriers to internal rotation, and the enthalpy of formation of cumene hydroperoxide PhCMe2OOH: a DFT study

Conformational analysis of cumene hydroperoxide PhCMe₂OOH (1) has been carried out using the density functional methods B3LYP/6-31G(d,p) and B3LYP/6-311+G(3df,2p). Ignoring rotation of methyl groups, molecule 1 has seven conformers differing in orientation of the — CMe₂OOH fragment relative to the benzene ring and in mutual position of atoms in this fragment. The molecular structures, relative energies, and statistical distribution of the conformers were determined, and intramolecular rotational barriers were estimated. The enthalpies of formation of all conformers of molecule 1 were calculated using two approximations with inclusion of zero-point vibrational energy and temperature correction. Calculations using the isodesmic reaction (IDR) scheme made it possible to reduce the systematic error of the determination of the enthalpy of reactions. The total enthalpy of formation of compound 1 calculated with inclusion of statistical distribution of rotamers equals −19.7±3.6 kcal mol⁻¹. The combination of the B3LYP/6-31G(d,p) approximation and the IDR scheme gives fairly accurate results (relative error is ±0.4 kcal mol⁻¹) as compared to those obtained with the extended basis set 6-311+G(3df,2p). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1157–1164, June, 2008.     

A computational study of conformational interconversions in 1,4-dithiacyclohexane (1,4-dithiane)

Ab initio molecular orbital theory with the 6-31G(d), 6-31G(d,p), 6-31+G(d), 6-31+G(d,p), 6-31+G(2d,p), 6-311G(d), 6-311G(d,p), and 6-311+G(2d,p) basis sets and density functional theory (BLYP, B3LYP, B3P86, B3PW91) have been used to locate transition states involved in the conformational interconversions of 1,4-dithiacyclohexane (1,4-dithiane) and to calculate the geometry optimized structures, relative energies, enthalpies, entropies, and free energies of the chair and twist conformers. In the chair and 1,4-twist conformers the C-Hax and C-Heq bond lengths are equal at each carbon, which suggest an absence of stereoelectronic hyperconjugative interactions involving carbon-hydrogen bonds. The 1,4-boat transition state structure was 9.53 to 10.5 kcal/mol higher in energy than the chair conformer and 4.75 to 5.82 kcal/mol higher in energy than the 1,4-twist conformer. Intrinsic reaction coordinate (IRC) calculations showed that the 1,4-boat transition state structure was the energy maximum in the interconversion of the enantiomers of the 1,4-twist conformer. The energy difference between the chair conformer and the 1,4-twist conformer was 4.85 kcal/mol and the chair-1,4-twist free energy difference (deltaG degrees (c-t)) was 4.93 kcal/mol at 298.15 K. Intrinsic reaction coordinate (IRC) calculations connected the transition state between the chair conformer and the 1,4-twist conformer. This transition state is 11.7 kcal/mol higher in energy than the chair conformer. The effects of basis sets on the 1,4-dithiane calculations and the relative energies of saturated and unsaturated six-membered dithianes and dioxanes are also discussed.     

Geometry of intramolecular silicon complexes: an ab initio estimation of the sensitivity to the effect of medium. (Aroyloxymethyl)trifluorosilanes

The effect of medium on the geometry of the (benzoyloxymethyl)trifluorosilane (1) molecule was studied by the HF and MP2 methods with the 6-31G(d), 6-311G(d), and 6-311+G(d,p) basis sets, as well as using the Onsager SCRF model, the PCM approach, and the data of X-ray diffraction study. Molecule 1 has a low complexation energy (5.4 kcal mol^–1 according to MP2(Full)/6-31G(d)+ZPE calculations and 6.8 kcal mol^–1 according to IR spectroscopy data), while its geometric parameters are the least sensitive to the effects of medium among all hypervalent silicon compounds studied to date. Nevertheless, the results obtained revealed a pronounced deformation of the SiO coordination bond in 1 on going from the gas phase to the polar solution and crystal. This serves as a theoretical substantiation of the hypothesis that substantial changes in the IR and NMR spectral characteristics of the (O—Si)-chelate (aroyloxymethyl)trifluorosilanes upon variation of external factors are due to geometric reasons. Analysis of the electron density distribution according to Bader indicates that the SiO bond in molecule 1 can be treated as a covalent bond of high polarity.     

Synthesis and Theoretical Investigation of a 1,8‐Bis(bis(diisopropylamino)cyclopropeniminyl)naphthalene Proton Sponge Derivative

We report herein the synthesis and characterization of a new proton sponge derivative, 1,8‐bis(bis(diisopropylamino)cyclopropeniminyl)naphthalene 4 (DACN), as well as its bis‐protonated counterpart 6 . A crystal structure of 6 is presented, along with variable temperature ¹H NMR data on the BF₄^? salt ( 6?BF₄ ). DFT calculations were performed to investigate the structure of the monoprotonated species 7 and to gain insight into the structural and electronic nature of all three species. The proton affinity (PA) of 4 , calculated at the B3LYP/6‐311G++(d,p)//B3LYP/6‐31G(d,p) level, taking into account thermal corrections from the B3LYP/6‐31G(d,p) method, was 282.3 kcal mol^?1, while its pK_a was estimated at 27.0. NICS calculations were performed to examine the changes in aromaticity within these systems upon each successive protonation. Lastly, homodesmotic reaction schemes were used in order to estimate the factors contributing to the strong PA predicted for 4 .