Nonclassical Isomers Research Articles

Borole/Borapyramidane Relationship.

Boroles and borapyramidanes are classical and nonclassical constitutional isomers, respectively. It is here shown that they can indeed be interconverted. Treatment of the bis(alkynyl)B(C6F5) SMe2 adduct 3·SMe2 with HB(C6F5)2 gave borole 1·SMe2, featuring trimethylsilyl substituents in both α positions to boron, by means of a 1,1-hydroboration/alkenylboration sequence. Photolysis of the classical borole adduct 1·SMe2 resulted in rearrangement to its nonclassical structural isomer, borapyramidane 2, in high yield, which exhibits a vicinal pair of trimethylsilyl substituents at the square pyramidane base. Neutral borapyramidane 2 is a rare example of an isoster of the (CH)5+ pyramidane cation. Thermolysis of borapyramidane 2 in the presence of SMe2 at 60 °C re-formed borole 1·SMe2, which converted at 100 °C to 2,3-bis-silyl-substituted borole isomer 8·SMe2. Its photolysis also gave borapyramidane 2. Prolonged photolysis of 2 at elevated temperatures converted this to borapyramidane isomer 10 containing a pair of trimethylsilyl groups in 1,3-position at its square C4-pyramidal base. The borole and borapyramidane isomers were characterized by X-ray diffraction, and the system was analyzed by density functional theory (DFT) calculations.

Theoretical Study on the Structure and Spectral Properties of Several Classical and Non-Classical C76 Isomers and Their Newly Synthesized Derivatives.

X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra, as well as the ground-state electronic/geometrical structures of the newly discovered two non-classical isomers C2-C76(NC2) and C1-C76(NC3) with their derivatives C2-C76(NC2)(CF3)14 and C1-C76(NC3)Cl24, as well as the non-IPR(isolated pentagon rule) isomer C1-#17418C76 with its embedded metal fullerene U@C1-#17418C76 have been calculated at the density functional theory (DFT) level. The electronic structure after chlorination is significantly different in the simulated X-ray spectrum. Both XPS and NEXAFS spectra reflect obvious isomer dependence, indicating that the "fingerprint" in X-ray spectroscopy can provide an effective means for the identification of the above-mentioned fullerene isomers. Time-dependent DFT was used to simulate the ultraviolet-visible absorption spectrum of U@C1-#17418C76. The calculated results are in good agreement with the experimental consequence. This work reveals that theoretically simulated X-ray and UV-vis spectroscopy techniques can provide valuable information to help researchers explore the electronic structure of fullerenes and the identification of isomers in future experimental and theoretical fields.

(Invited) New Approaches in Computational Modeling of Fullerene/Metallofullerenes

Today, computational models are extensively applied in scientific research to address fundamental as well as practical issues in material science. Fullerenes and metallofullerenes are well recognized as potential materials for next-generation electronic devices, such as quantum computers. The rapid advance in fullerene/metallofullerene research requires improved theoretical tools to understand fundamental as well as practical applications of these new members of fullerene/metallofullerene family. In this presentation, we will describe the extension of the famous spiral program by Fowler and Manolopoulos1 to generate new non-classical isomers. We have extended this approach beyond 5 and 6 membered fullerene structures to other polygon faces, such as, heptagons. In addition, we have developed a machine learning tool based on a general relationship between the structure patterns and physical properties. Finally, we will describe our current modeling studies regarding chromatographic retention mechanism(s) of fullerene/metallofullerene molecules. Reference: Fowler, Patrick W., and D. E. Manolopoulos. An Atlas of Fullerenes. Clarendon Press-Oxford, 1995.

(Invited) Extending the Spiral Algorithm to Non-Classical Isomers and Metallofullerenes

The face-spiral algorithm proposed by Manolopoulos et al.1 has been extensively used to construct classical fullerenes, 2 and the corresponding numbering of isomers has been recommended as a systematic nomenclature. The spiral algorithm is an important help in clarifying the structures of fullerenes, endohedral fullerenes and fullerene derivatives. Recent and accumulating evidence shows that non-classical fullerene cages with heptagon(s) or square(s) may be competitive with their classical counterparts in some situations; non-classical fullerene cages may be favored parent cages of metallofullerenes and fullerene derivatives. It is desirable to make a systematic examination of the effect of heptagon(s) and/or square(s) on the structures and properties of bare fullerenes, fullerene ions, metallofullerenes and fullerene derivatives. The original spiral algorithm does not generate non-classical fullerene isomers, but can be adapted to do so. An extended spiral algorithm can selectively generate non-classical fullerenes with one or two heptagon(s) or square(s) in the size range of interest, and has been used [3-5] to identify favored non-classical fullerenes and metallofullerenes. Energetic arguments suggest that non-classical fullerene cages may play a significant role in the formation of fullerenes and metallofullerenes.

Unveiling the relative stability and proton binding of non-classical Wells-Dawson isomers of [(NaF6)W18O54(OH)2]7- and [(SbO6)W18O54(OH)2]9-: a DFT study.

Density functional theory calculations combined with the energy and building-block decomposition analyses have been carried out to investigate the structures, stability orders, redox potentials and proton binding of the six Baker-Figgis isomers (α, β, γ, α*, β* and γ*) of [(SbO6)W18O54(OH)2]9- {H2SbW18} and [(NaF6)W18O54(OH)2]7- {H2NaW18} anions at the level of PBEsol-D3/TZP. Both bonding energy and Gibbs free energy analyses exhibit that the two non-classical Wells-Dawson (WD) species behave quite differently from each other. The pyroanimonate {H2SbW18}, with a stability order of γ* > β* > α > α* > β > γ, is a non-classical WD species, while the hexafluoride {H2NaW18} (α > β > γ > γ* > β* > α*) is a transition intermediate between classical and non-classical WD types, possessing both non-classical ([XW18O60(OH)2]n-, X = I, Te and W) and classical [Si2W18O62]8- properties. Energy decomposition analyses (EDA) reveal that spatial arrangement (Ehost), host-guest fragment interaction energy (FIE), and structural distortion energy (DE) are three key factors governing the relative stability of isomers; among these, DE is always dominant, while FIE and Ehost are subordinated but are still important. Building-block decomposition analyses (BDA) disclose that the octahedral {MO6} units of the equatorial belt, particularly the staggered belt, are always more distorted than those of the two polar caps inside each structure. The theoretical redox potentials demonstrate that the oxidizing power increases with a trend of α < β < γ and α* < β* < γ* for both species, and the first redox potential is closely related to the energy level of the LUMO of each anion. Evaluation of the proton inclusion energies suggests that {H2NaW18} can only embed two protons, while {H2SbW18} may encapsulate four; the number of embedded protons is controlled by both the charge of the heteroatom X and the volume of the tetrahedral {O4}/{OF3} cavity.

From C(58) to C(62) and back: Stability, structural similarity, and ring current.

An increasing number of observations show that non-classical isomers may play an important role in the formation of fullerenes and their exo- and endo-derivatives. A quantum-mechanical study of all classical isomers of C58 , C60 , and C62 , and all non-classical isomers with at most one square or heptagonal face, was carried out. Calculations at the B3LYP/6-31G* level show that the favored isomers of C58 , C60 , and C62 have closely related structures and suggest plausible inter-conversion and growth pathways among low-energy isomers. Similarity of the favored structures is reinforced by comparison of calculated ring currents induced on faces of these polyhedral cages by radial external magnetic fields, implying patterns of magnetic response similar to those of the stable, isolated-pentagon C60 molecule. © 2016 Wiley Periodicals, Inc.

General rules for predicting the local aromaticity of carbon polyhedra

The aromaticity of classical and nonclassical C24 fullerene isomers and their anions were studied using the topological resonance energy (TRE) method. Local aromaticity was studied using the bond resonance energy (BRE) method. On the basis of BRE values, the contributions of different types of chemical bonds to the molecular global aromaticity were analyzed and general rules for predicting the local aromaticity of fullerenes are proposed. It was found that pentagons, heptagons, and squares preferred hexagons as neighbors rather than squares as neighbors. In the hexaanionic state, pentagon pair sites exhibit larger local aromaticity than other places in the same molecule.

Biological Evaluation and Structure-Activity Relationships of Bis-(3-aryl-3-oxo-propyl)-methylamine Hydrochlorides and 4-Aryl-3-arylcarbonyl-1-methyl-4-piperidinol Hy-drochlorides as Potential Cytotoxic Agents and their Alkylating Ability towards Cellular Glutathione in Human Leukemic T Cells

Various bis (3-aryl-3-oxo-propyl)methylamine hydrochlorides 1 and their corresponding structural and non-classical isomers 4-aryl-3-arylcarbonyl-1-methyl-4-piperidinols 3 were evaluated against human leukemic T (Jurkat) cells and found to possess significant cytotoxicity. Among the series 1 (bis-Mannich bases) and 3 (corresponding piperidinols), compounds la, 1c and 1e showed cytotoxic potency which was approximately 1.6, 3.7 and 3.4 times that of the reference drug 5-fluorouracil, respectively. Except for compound 1d, conversion of bis-Mannich bases to their corresponding piperidinols 3a, 3b and 3e lowered the potency. Besides chloro derivative 1d, bis-Mannich bases displayed greater cytotoxicity compared with their mono-Mannich bases, series 5. Representative bis-Mannich bases (1a, 1e) and piperidinols (3a, 3e) decreased the glutathione level of Jurkat cells. Molecular modeling was utilized in order to evaluate whether the shape, size, critical volume, solvent accessible area and partition coefficient of the different compounds had contributed to the varying potencies observed. Bis-Mannich bases 1a, 1c and 1e may serve as candidate anticancer agents for future development.

Evaluation of Anticonvulsant Activities of Bis(3-aryl-3-oxo-propyl)ethylamine Hydrochlorides and 4-Aryl-3-arylcarbo-nyl-1-ethyl-4-piperidinol Hydrochlorides

Bis-Mannich bases, bis(3-aryl-3-oxopropyl)ethylamine hydrochlorides 1-4, and their corresponding structural and non-classical isomers, 4-aryl-3-arylcarbonyl-1-ethyl-4-piperidinol hydrochlorides 5-8, were synthesized. The aryl part was phenyl in 1 and 5, p-methylphenyl in 2 and 6, p-chlorophenyl in 3 and 7, and 2-thienyl in 4 and 8. The chemical stuructures of the compounds were confirmed by 1H-NMR, 13C-NMR, UV, IR and elemental analyses. Anticonvulsant activities of the compounds were evaluated by the maximum electroshock (MES) and subcutaneous pentylenetetrazole (scMet) tests in the dose range of 30-300 mg/kg. Alterations in biological activity depending on modifications in chemical structure were also followed. Compounds 1-4, 6, and 8 were toxic and caused death of the animals 20 min after the injection. Compounds 2, 3 and 6 were also neurotoxic at the 100 mg/kg dose level. While only compound 7 was active in the scMet test at 300 mg/kg within 4 h, all the compounds showed activity in the MES test at different dose levels and time periods. In conclusion, compounds 5 and 7, which were not toxic and did not show neurotoxicity, seemed to be candidate compounds to develop new anticonvulsant compounds useful in the treatment of the grand mal (compounds 5, 7) and petit mal (compound 7) epilepsies.

Nonclassical C n ( n = 30–40, 50) fullerenes containing five-, six-, seven-member rings

Classical and heptagon-contained nonclassical fullerene isomers of C 30–C 40 and C 50 are generated by Spiral algorithm and optimized using DFT methods. There are much larger number of nonclassical isomers than classical ones. Energies of isomers generally increase with the number of heptagons inside and those lowest-energy nonclassical isomers contain only one heptagon. For all fullerenes considered, there is no nonclassical isomer that energetically prevails over the lowest-energy classical counterpart. But the best nonclassical isomers are more stable than most classical ones. The validity of pentagon adjacency penalty rule (PAPR) and hexagon neighbor rule (HNR) is examined for the nonclassical fullerenes. PAPR is still roughly applicable while HNR almost breaks down. A new topological index U with better performance over PAPR and HNR is presented and its superiority is further confirmed on two subsets of C 50 isomers. This U index can be considered as an extension of PAPR to nonclassical isomers with existence of heptagons.

Theoretical study on non-classical fullerene C 54 with square(s) or heptagon(s)

To put insight into the structures and stability of non-classical fullerenes, density functional theory calculations were performed on both classical and non-classical isomers of fullerene C 54. The calculated results demonstrate that the most stable isomer is classical and the classical isomers obey the pentagon adjacency penalty rule. For non-classical isomers with one or two squares, although they have fewer fused pentagons they are markedly disadvantageous in energy and unexpectedly, have larger HOMO–LUMO gaps compared with classical ones. The gap of one isomer with two squares is unusually up to 2.54 eV, closer to 2.76 eV of IPR-C 60 at the same level of theory. For non-classical isomers with heptagon(s), the ones with a heptagon are more advantageous than the classical ones with the same number of B 55 bonds, some of them are even more advantageous than some classical isomers with fewer B 55 bonds. It is found that the pyramidization angles of vertexes involving in square are larger than other vertexes without involving in square and the pyramidization angles of vertexes shared by two pentagons and one heptagon is smaller than the pyramidization angles of vertexes shared by two pentagons and one hexagon. These findings suggest that the incorporation of square may lead to additional strain and unusual kinetic stability; and heptagon may release the strain in adjacent pentagon–pentagon bonds.

Nonclassical fullerenes with a heptagon violating the pentagon adjacency penalty rule

Nonclassical fullerenes with heptagon(s) and their derivatives have attracted increasing attention, and the studies on them are performing to enrich the chemistry of carbon. Density functional theory calculations are performed on nonclassical fullerenes C(n) (n = 46, 48, 50, and 52) to give insight into their structures and stability. The calculated results demonstrate that the classical isomers generally satisfy the pentagon adjacency penalty rule. However, the nonclassical isomers with a heptagon are more energetically favorable than the classical ones with the same number of pentagon-pentagon bonds (B(55) bonds), and many of them are even more stable than some classical isomers with fewer B(55) bonds. The nonclassical isomers with the lowest energy are higher in energy than the classical ones with the lowest energy, because they have more B(55) bonds. Generally, the HOMO-LUMO gaps of the former are larger than those of the latter. The sphericity and asphericity are unable to rationalize the unique stability of the nonclassical fullerenes with a heptagon. The pyramidization angles of the vertices shared by two pentagons and one heptagon are smaller than those of the vertices shared by two pentagons and one hexagon. It is concluded that the strain in the fused pentagons can be released by the adjacent heptagons partly, and consequently, it is a common phenomenon for nonclassical fullerenes to violate the pentagon adjacent penalty rule. These findings are heuristic and conducive to search energetically favorable isomers of C(n), especially as n is 62, 64, 66, and 68, respectively.

Electronic structures, stabilities, and spectroscopies of the fullerene derivatives C 68X 4 (X = H, F, Cl)

A systematic first-principle study based on the density functional theory has been performed on the electronic structures and stabilities of fullerene C 68 and its derivatives C 68X 4 (X = H, F, Cl). By searching the 6332 classical and 43 nonclassical isomers of C 68, the ground state is found to be the classical isomer 6290 bearing C 2 symmetry. However, after attaching X atoms to the active sites of C 68, the heptagon-containing nonclassical derivatives C 68(c)-2 C 68H 4, C 68(c)-2 C 68F 4, and C 68(c)-3 C 68Cl 4 are predicted to be the most stable among all derivatives. The chemical deriving could affect the electronic structures distinctly, and enhance the stability of fullerene C 68. The Mulliken charge populations of the most stable C 68X 4 are calculated, showing that different X atoms added to C 68 will cause remarkably different charge populations. The IR, Raman, and NMR spectra of the most stable C 68X 4 are also calculated and presented to facilitate future experimental identification.

Electronic Structure and Bonding in Neutral and Dianionic Boradiphospholes: R′BC2P2R2 (R=H, tBu, R′=H, Ph)

Classical and non-classical isomers of both neutral and dianionic BC(2)P(2)H(3) species, which are isolobal to Cp(+) and Cp(-), are studied at both B3LYP/6-311++G(d,p) and G3B3 levels of theory. The global minimum structure given by B3LYP/6-311++G(d,p) for BC(2)P(2)H(3) is based on a vinylcyclopropenyl-type structure, whereas BC(2)P(2)H(3)(2-) has a planar aromatic cyclopentadienyl-ion-like structure. However, at the G3B3 level, there are three low-energy isomers for BC(2)P(2)H(3): 1) tricyclopentane, 2) nido and 3) vinylcyclopropenyl-type structures, all within 1.7 kcal mol(-1) of each other. On the contrary, for the dianionic species the cyclic planar structure is still the minimum. In comparison to the isolobal Cp(+) and H(n)C(n)P(5-n)(+) isomers, BC(2)P(2)H(3) shows a competition between pi-delocalised vinylcyclopropenyl- and cluster-type structures (nido and tricyclopentane). Substitution of H on C by tBu, and H on B by Ph, in BC(2)P(2)H(3) increases the energy difference between the low-lying isomers, giving the lowest energy structure as a tricyclopentane type. Similar substitution in BC(2)P(2)H(3)(2-) merely favours different positional isomers of the cyclic planar geometry, as observed in 1) isoelectronic neutral heterodiphospholes EtBu(2)C(2)P(2) (E = S, Se, Te), 2) monoanionic heterophospholyl rings EtBu(2)C(2)P(2) (E = P(-), As(-), Sb(-)) and 3) polyphospholyl rings anions tBu(5-n)C(n)P(5-n) (n = 0-5). The principal factors that affect the stability of three-, four-, and five-membered ring and acyclic geometrical and positional isomers of neutral and dianionic BC(2)P(2)H(3) isomers appear to be: 1) relative bond strengths, 2) availability of electrons for the empty 2p boron orbital and 3) steric effects of the tBu groups in the HBC(2)P(2)tBu(2) systems.

Hydrogen and silyl bridges in group 13 and 14 atom containing molecules

AbstractGroup 13 and group 14 atom containing small hydrides were investigated by quantum chemical methods to characterize their possible nonclassical bridged isomers. A robust set of methods were used like NBO analysis, topological analysis of ELF, AIM theory, several bond indices (Mayer bond order, fuzzy atoms bond order, Wiberg bond index, atom–atom overlap weighted NAO bond order), and geometry. The hydrogen bridges can be interpreted by 2 electron – 3 center bonds in NBO analysis, trisynaptic X1‐Hb‐X2 valence cores in ELF, and two X‐Hb bond critical points in AIM theory. The bond orders of the X‐Hb bonds are always around half of that in a classical bond, and the sum of the bond orders for the two bridging bonds is always around one. Bonding interaction between the pillar X1‐X2 atoms exists in several cases, suggested by NBO analysis, ELF, the geometry and the bond orders. In the AIM picture, however, the X1‐X2 pillar bond or one of the X‐Hb bridging bond is missing. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Octacoordination of the nitrogen atom in M4NO 4 + systems (M = Li, Na, K)

The electronic and geometric structures of M4NO4+ compounds (M = Li, Na, K) in classical and nonclassical isomeric forms were studied by the ab initio (MP2(full)/6-311+G**) and density functional theory (B3LYP/6-311+G**) methods. For all M atoms, structurally stable nonclassical isomers were found with octacoordinated nitrogen atoms and tetracoordinated oxygen atoms. For Li4NO4+, the classical structure with the tetracoordinated nitrogen atom is energetically more stable, whereas nonclassical structures with the octacoordinated nitrogen center are more stable for Na4NO4+ and K4NO4+.

Ab initio calculation of carbon clusters. II. Relative stabilities of fullerene and nonfullerene C24

Chemical stabilities of six low-energy isomers of C24 derived from global-minimum search are investigated. The six isomers include one classical fullerene (isomer 1) whose cage is composed of only five- and six-membered rings (56-MRs), three nonclassical fullerene structures whose cages contain at least one four-membered ring (4-MR), one plate, and one monocyclic ring. Chemical and electronic properties of the six C24 isomers are calculated based on a density-functional theory method (hybrid PBE1PBE functional and cc-pVTZ basis set). The properties include the nucleus-independent chemical shifts (NICS), singlet-triplet splitting, electron affinity, ionization potential, and gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO) gap. The calculation suggests that the neutral isomer 2, a nonclassical fullerene with two 4-MRs, may be more chemically stable than the classical fullerene (isomer 1). Analyses of molecular orbital NICS show that the incorporations of 4-MRs into the cage considerably reduce paratropic contributions from HOMO, HOMO-1, and HOMO-2, which are mainly responsible for the sign change in NICS from positive for isomer 1 (42) to negative (-19) for isomer 2, although C24 clusters satisfy neither 4N+2 nor 2(N+1)2 aromaticity rule. Anion photoelectron spectra of four cage isomers, one plate, one monocyclic ring, and one tadpole isomer, as well as three bicyclic ring isomers are calculated. The simulated photoelectron spectra of mono- and bicyclic rings (with C1 symmetry) appear to match the measured HOMO-LUMO gap (between the first and second band in the experimental spectra) [S. Yang et al., Chem. Phys. Lett. 144, 431 (1988)]. Nevertheless, the nonclassical fullerene isomers 3 and 4 apparently also match the measured vertical detachment energy (2.90 eV) reasonably well. These results suggest possible coexistence of nonclassical fullerene isomers with the mono- and bicyclic ring isomers of C24(-) under the experimental conditions.

C68 Fullerene Isomers, Anions, and their Metallofullerenes: Charge‐Stabilizing Different Isomers

The complete set of 6332 classical isomers of the fullerene C(68) as well as several non-classical isomers is investigated by PM3, and the data for some of the more stable isomers are refined by the DFT-based methods HCTH and B3LYP. C(2):0112 possesses the lowest energy of all the neutral isomers and it prevails in a wide range of temperatures. Among the fullerene ions modeled, C(68) (2-), C(68) (4-) and C(68) (6-), the isomers C(68) (2-)(C(s):0064), C(68) (4-)(C(2v):0008), and C(68) (6-)(D(3):0009) respectively, are predicted to be the most stable. This reveals that the pentagon adjacency penalty rule (PAPR) does not necessarily apply to the charged fullerene cages. The vertical electron affinities of the neutral C(s):0064, C(2v):0008, and D(3):0009 isomers are 3.41, 3.29, and 3.10 eV, respectively, suggesting that they are good electron acceptors. The predicted complexation energy, that is, the adiabatic binding energy between the cage and encapsulated cluster, of Sc(2)C(2)@C(68)(C(2v):0008) is -6.95 eV, thus greatly releasing the strain of its parent fullerene (C(2v):0008). Essentially, C(68) fullerene isomers are charge-stabilized. Thus, inducing charge facilitates the isolation of the different isomers. Further investigations show that the steric effect of the encaged cluster should also be an important factor to stabilize the C(68) fullerenes effectively.

Structures, Stabilities, and Electronic and Optical Properties of C62 Fullerene Isomers

The 2385 classical isomers and four nonclassical isomers of fullerene C62 have been studied by PM3, HCTH/3-21G//SVWN/STO-3G, B3LYP/6-31G(d)//HCTH/3-21G, and B3LYP/6-31G(d)//B3LYP/6-31G(d). The Cs:7mbr isomer, with a chain of four adjacent pentagons surrounding a heptagon, is predicted to be the most stable isomer, followed by C2v:4mbr which is 3.15 kcal/mol higher in energy. C2:0032 with three pairs of adjacent pentagons is the most stable isomer in the classical framework. To clarify the relative stabilities of C62 isomers at high temperatures, the entropy contributions are taken into account on the basis of the Gibbs energy at the B3LYP/6-31G(d) level. Analyses reveal that Cs:7mbr prevails in a wide temperature range. The vibrational frequencies of the five most stable C62 fullerene isomers are also predicted at the B3LYP/6-31G(d) level, and the simulated IR spectra show important differences in positions and intensities of the vibrational modes for different isomers. The nucleus-independent chemical shift and the density of states of the three most stable isomers show that the square in C2v:4mbr and the adjacent pentagons in Cs:7mbr and C2:0032 possess high chemical reactivity. In addition, the electronic spectra and second-order hyperpolarizabilities are determined by means of ZINDO and the sum-over-states mode. The intensity-dependent refractive index gamma(-omega; omega, omega, -omega) at omega = 2.3305 eV of Cs:7mbr is very large because of resonance with the external field. The second-order hyperpolarizabilities of the five most stable isomers of C62 are predicted to be larger than those of C60.

Protonated acetylene revisited

The potential energy surface for protonated acetylene has been re-examined with large basis sets and highly correlated methods. The energy difference of 3.6–3.8 kcal/mol between the classical structure and non-classical (bridged) structure computed with CCSD (T)/cc-pVQZ, CCSD(T)/6-311+G(3df,2pd), BD(T)/cc- pVQZ, BD(T)/6-311+G(3df,2pd) and CBS-APNO methods is in very good agreement with the best previous calculations, 3.7–4.0 kcal/mol. In contrast, BLYP, B3LYP, PW91, PBE and TPSS density functional methods do rather poorly, yielding −0.52. 0.29, 1.81, 2.16 and 0.62 kcal/mol, respectively, with the 6-311+G(3df,2pd) basis. MP2 calculations predict the classical structure to be a transition state; however, frequency calculations at the CCSD/6-311+G(3df,2pd) level of theory show that the classical structure is a local minimum. CCSD(T), BD(T) and CBS-APNO energy calculations along the MP2/6-311+G(3df,2pd) reaction path indicate that the classical structure is a shallow local minimum separated from the non-classical structure by a very small barrier of 0.11–0.13 kcal/mol. Because the barrier for proton exchange between the non-classical isomers via the classical structure is broad and nearly flat at the top, the tunneling splitting should be reduced, possibly accounting for the 15% difference between the calculated and experimental barrier heights.