Moran Damian, Sukcharoenphon Kengkaj, Puchta Ralph, Schaefer Henry F, Schleyer Paul V R, Hoff Carl D
Center for Computational Quantum Chemistry, University of Georgia, Athens 30602, USA.
J Org Chem. 2002 Dec 13;67(25):9061-9. doi: 10.1021/jo0263768.
The gas phase and solvent dependent preference of the tautomerization between 2-pyridinethiol (2SH) and 2-pyridinethione (2S) has been assessed using variable temperature Fourier transform infrared (FTIR) experiments, as well as ab initio and density functional theory computations. No spectroscopic evidence (nu(S)(-)(H) stretch) for 2SH was observed in toluene, C(6)D(6), heptane, or methylene chloride solutions. Although, C(s)() 2SH is 2.61 kcal/mol more stable than C(s)() 2S (CCSD(T)/cc-pVTZ//B3LYP/6-311+G(3df,2p)+ZPE), cyclohexane solvent-field relative energies (IPCM-MP2/6-311+G(3df,2p)) favor 2S by 1.96 kcal/mol. This is in accord with the FTIR observations and in quantitative agreement with the -2.6 kcal/mol solution (toluene or C(6)D(6)) calorimetric enthalpy for the 2S/2SH tautomerization favoring the thione. As the intramolecular transition state for the 2S, 2SH tautomerization (2TS) lies 25 (CBS-Q) to 30 kcal/mol (CCSD/cc-pVTZ) higher in energy than either tautomer, tautomerization probably occurs in the hydrogen bonded dimer. The B3LYP/6-311+G(3df,2p) optimized C(2) 2SH dimer is 10.23 kcal/mol + ZPE higher in energy than the C(2)(h)() 2S dimer and is only 2.95 kcal/mol + ZPE lower in energy than the C(2) 2TS dimer transition state. Dimerization equilibrium measurements (FTIR, C(6)D(6)) over the temperature range 22-63 degrees C agree: K(eq)(298) = 165 +/- 40 M(-)(1), DeltaH = -7.0 +/- 0.7 kcal/mol, and DeltaS = -13.4 +/- 3.0 cal/(mol deg). The difference between experimental and B3LYP/6-311+G(3df,2p) [-34.62 cal/(mol deg)] entropy changes is due to solvent effects. The B3LYP/6-311+G(3df,2p) nucleus independent chemical shifts (NICS) are -8.8 and -3.5 ppm 1 A above the 2SH and 2S ring centers, respectively, and the thiol is aromatic. Although the thione is not aromatic, it is stabilized by the thioamide resonance. In solvent, the large 2S dipole, 2-3 times greater than 2SH, favors the thione tautomer and, in conclusion, 2S is thermodynamically more stable than 2SH in solution.
利用变温傅里叶变换红外（FTIR）实验以及从头算和密度泛函理论计算，评估了2-吡啶硫醇（2SH）和2-吡啶硫酮（2S）之间互变异构的气相和溶剂依赖性偏好。在甲苯、C₆D₆、庚烷或二氯甲烷溶液中未观察到2SH的光谱证据（ν(S⁻)(H)伸缩）。尽管，C(s) 2SH比C(s) 2S稳定2.61 kcal/mol（CCSD(T)/cc-pVTZ//B3LYP/6-311+G(³df,²p)+ZPE），但环己烷溶剂场相对能量（IPCM-MP2/6-311+G(³df,²p)）使2S比2SH更稳定1.96 kcal/mol。这与FTIR观察结果一致，并且与2S/2SH互变异构偏向硫酮的-2.6 kcal/mol溶液（甲苯或C₆D₆）量热焓在数量上相符。由于2S、2SH互变异构（2TS）的分子内过渡态能量比任何一种互变异构体都高25（CBS-Q）至30 kcal/mol（CCSD/cc-pVTZ），互变异构可能发生在氢键二聚体中。B3LYP/6-311+G(³df,²p)优化的C₂ 2SH二聚体能量比C₂(h) 2S二聚体高10.23 kcal/mol + ZPE，并且仅比C₂ 2TS二聚体过渡态能量低2.95 kcal/mol + ZPE。在22 - 63℃温度范围内的二聚化平衡测量（FTIR，C₆D₆）结果如下：K(eq)(298) = 165 ± 40 M⁻¹，ΔH = -7.0 ± 0.7 kcal/mol，以及ΔS = -13.4 ± 3.0 cal/(mol·K)。实验和B3LYP/6-311+G(³df,²p) [-34.62 cal/(mol·K)]熵变之间的差异是由于溶剂效应。B3LYP/6-311+G(³df,²p)核独立化学位移（NICS）分别在2SH和2S环中心上方1 Å处为-8.8和-3.5 ppm，并且硫醇是芳香性的。尽管硫酮不是芳香性的，但它通过硫代酰胺共振而稳定。在溶剂中，2S的大偶极矩比2SH大2 - 3倍，有利于硫酮互变异构体，总之，2S在溶液中热力学上比2SH更稳定。
