Tunneling Governs Intramolecular Proton Transfer in Thiotropolone at Room Temperature

Abstract

The role of quantum mechanical tunneling has been found to be significant in a variety of reactions like oxidative additions and reductive eliminations in organometallic complexes, proton transfer and proton-coupled electron transfer (PCET) processes in molecules and enzymes, 3] 1,5-hydride shift reactions in hydrocarbons and free-radical polymerization. Schreiner et al. have recently reported hydrogen tunneling in hydroxymethylene to form formaldehyde. This tunneling reaction has a half-life of only 2 h and tunneling occurs through a large energy barrier. But its congeners, mercaptocarbene and selenocarbene, do not show quantum mechanical hydrogen tunneling. Recent experiments have confirmed computational predictions of heavy atom tunneling in the ring opening of radical clocks. However, experimental detection of tunneling in chemical reactions are confined mostly to low temperatures and hence, examples of tunneling at room temperature are rare. 11] In a recent paper, Yamabe and co-workers reported an extremely fast intramolecular proton transfer between the thione (1a) and the enethiol (1b) tautomers in thiotropolone (1), the sulfur analogue of tropolone in the solid state (see Scheme 1). The crystal structure of 1 showed no signatures of intermolecular hydrogen bonding with the molecules being almost perpendicular to each other. The S···O distance in 1 is (2.804! 0.001) ! and therefore much smaller than the sum of their van der Waals radii (3.2 !). The computed harmonic (anharmonic) IR stretching modes for C=S and C=O in 1a and 1b are 1120.4 cm1 (1102.0 cm1) and 1627.0 cm1 (1594.0 cm1), respectively, at the B3LYP/6-311G(d) level, while the measured values for C=S and C=O are 1088– 1109 cm1 and 1566–1600 cm1. For a detailed comparison of the important vibrational modes at various levels of theory see the Supporting Information. Anharmonicity is observed to be significant. Based on this observation, it was concluded that 1 exists in the solid state as a monomer with substantial intramolecular OH···S and SH···O H-bonding for the tautomers 1a and 1 b, respectively. Variable-Temperature (VT) NMR studies showed that the proton hops rapidly between the two tautomers with a measured DG< 6 kcal mol1 for 1aQ1b at 238 K. The computed free-energy barrier height at G2(MP2) level was 5.0 kcalmol1. The energy difference between the minima of 1 a and 1b is 3.4 kcalmol at MP2/TZVP (3.8 kcalmol1 at CCSD(T)/cc-pVTZ level on the MP2/TZVP geometry). The tunneling correction based on the Wigner approximation for an Eckart potential was estimated to be 0.8 kcalmol1 thereby leading to an effective computed free-energy barrier height of 4.2 kcalmol1. However, the C and O NMR spectra in 1 showed no temperature dependence at various temperatures of 143, 173, 213, and 298 K. In fact, even in the molten state at 333 K, the C NMR signals for 1 are similar to the low-temperature measurements. This we find interesting, since based on a classical over-the-barrier mechanism as predicted by the transition-state theory (TST), the reaction should have a large temperature dependence across a span of 190 K (from 143 K to 333 K). But, for a reaction occurring purely through quantum mechanical tunneling such a behavior is expected since the probability of tunneling is independent of the temperature. The significance of quantum tunneling in keto–enol tautomerization is well established in the literature. Jhonson et al. have studied the significance of tunneling in the hydrogen atom transfer reaction in 6,9-dimethylbenzosuberone. To understand and quantify the contribution of quantum mechanical tunneling for this reaction, we performed DFT calculations using the MPW1K hybrid functional. The success of the MPW1K functional has been established in many H-transfer reactions. The 6-31 + G(d,p) basis set was employed. Calculations were also done at MP2/TZVP level for comparison. CCSD(T)/ccpVTZ calculations at MP2/TZVP reference geometry have Scheme 1. Proton transfer reaction in thiotropolone and tropolone and their derivatives.