An Ab Initio and Density Functional Theory Study of Keto−Enol Equilibria of Hydroxycyclopropenone in Gas and Aqueous Solution Phase

Abstract

Keto-enol tautomerism in hydroxycyclopropenone (2-hydroxy-2-cyclopropen-1-one) has been studied using ab initio methods, the B3LYP functional of density functional theory, as well as complete basis set (CBS-QB3 and CBS-APNO) and G3 methods. Absolute and relative energies were calculated with each of the methods, whereas computations of geometries and harmonic frequencies for hydroxycyclopropenone and 1,2-cyclopropanedione were computed in the gas phase but were limited to HF, MP2 and CCSD levels of theory, and the B3LYP functional, in combination with the 6-31++G** basis set. Using the MP2/6-31++G** gas phase optimized structure, each species was then optimized fully in aqueous solution by employing the polarizable continuum model (PCM) self-consistent reaction field approach, in which HF, MP2 and B3LYP levels of theory were utilized, with the same 6-31++G** basis set. In both gas and aqueous solution phases, the keto form is higher in energy for all of the model chemistries considered. The presence of the solvent, however, is found to have very little effect on the bond lengths, angles and harmonic frequencies. From the B3LYP/6-31++G** Gibbs free energy, the keto-enol tautomeric equilibrium constant for 2-hydroxy-2-cyclopropen-1-one<==> 1,2-cyclopropanedione is computed to be K(T)(gas) = 2.35 x 10(-6), K(T)(aq) = 5.61 x 10(-14). It is concluded that the enol form is overwhelmingly predominant in both environments, with the effect of the solvent shifting the direction of equilibrium even more strongly in the favor of hydroxycyclopropenone. The almost exclusive nature of this species is attributed to stabilization resulting from aromaticity. Confirmation is provided by comparison of the simulated vibrational spectra of hydroxycyclopropenone with the measured infrared spectrum in an argon matrix.

Computational study of keto–enol equilibria of tropolone in gas and aqueous solution phase

“…For instance, comparing B3LYP/6-311++G ** with G3 results for the ratios of the conformer (1) to (2), they are 3.57 · 10 8 :1 and 7.31 · 10 7 :1, respectively, reflecting Gibbs free energy differences between these two species of 10.73 and 11.67 kcal mol À1 as given in Table 3. It is instructive to compare the results obtained in this investigation with recent calculations of the tautomeric equilibrium constant for hydroxycyclopropenone [8] and phenol [6,7]. For the 2-hydroxy-2-cyclopropen-1-one « 1,2-cyclopropanedione system K T (g) was computed to be 2.35 · 10 À6 and K T (aq) = 5.61 · 10 À14 at the B3LYP/6-31++G ** theoretical level.…”

“…In a number of chemical systems, however, this trend is not followed. Examples of recently studied [1][2][3][4][5][6][7][8] molecules in which the enol structure is observed to predominate include hydroxycyclopropenone [8] and phenol [6,7]. Ab initio calculations of the tautomeric equilibrium constant on the latter are found to be in excellent agreement with experiment, and confirm the exclusive presence of phenol, while high-level computations on the former species in both gas and aqueous solution phase predict that equilibrium is directed toward the enol rather than to 1,2-cyclopropanedione.…”

Tautomeric Equilibria in 3‐Amino‐1‐(2‐aminoimidazol‐4‐yl)prop‐1‐ene, a Central Building Block of Marine Alkaloids

“…This includes calculations for all conformers with Becke's B98 hybrid functional [14,15] in combination with the 6-31G(d) basis set, the MP2(FC)/6-31+G(2d,p)//B98/6-31G(d) level recently identified as a reliable method for the calculation of proton and methyl cation affinities of N-and P-bases, [11] and the G3MP2B3 compound method developed by Curtiss et al [15,16] The G3MP2B3 level is considered to be the most accurate in this series of methods. [16,17] All methods agree in that tautomers I and III, which include the aromatic imidazole ring system in its standard tautomeric form, are of almost identical stability, while tautomers II and IV are much less stable. At G3MP2B3 level tautomers II and IV are predicted to be less favorable than III by 21.8 kJ/mol and 22.9 kJ/mol, respectively.…”

“…Combination of these energies with thermochemical corrections obtained at B98/6-31G(d) level yield enthalpies described as "H 298 [MP2(FC)/6-31+G(2d,p)//B98/6-31G(d)]" in the text. For the four best conformers obtained at MP2(FC)/6-31+G(2d,p)//B98/6-31G(d) level refined relative enthalpies have been calculated using the G3(MP2)B3 compound method developed by Curtiss et al [15][16][17] For the sake of consistency identical B98/6-31G(d) geometries were used in the MP2(FC)/6-31+G(2d,p) and G3(MP2)B3 calculations. Solvent effects have been calculated using the PCM continuum solvation model in its IEF-PCM incarnation [20,[22][23][24] in combination with UAHF radii.…”

Computational study of tautomerism and aromaticity in mono‐ and dithio‐substituted tropolone

“…This has been rationalized on the basis of aromaticity featuring in the delocalized enolate ring structure, and is often simply understood using Hückel theory2 and the concept of resonance stabilization. These species have been studied extensively, both experimentally3–9 and theoretically,10–14 and not only in the context of tautomerism, but also to elucidate their properties and characteristics—both chemical and physical. These include the three‐membered ring system hydroxycyclopropenone,13, 15–17 a photochemical precursor to hydroxyacetylene, the six carbon ring compound phenol,11, 12 and tropolone,7–9, 14, 18–27 a seven‐membered cyclic conjugated ketone.…”