Wheeler Steven E, Schaefer Henry F
Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602, USA.
J Phys Chem A. 2009 Jun 18;113(24):6779-88. doi: 10.1021/jp9029387.
Despite the key role of the HOSO radical in the combustion of sulfur-rich fuels, the thermochemistry of this simple species is not well-established. Due to the extraordinary sensitivity of the potential energy surface to basis set and electron correlation methods in ab initio computations, there is no consensus in the literature regarding the structure of the global minimum syn-HOSO. A definitive enthalpy of formation for HOSO is presented, based on systematically extrapolated ab initio energies, accounting for electron correlation primarily through coupled cluster theory, including up to single, double, and triple excitations with a perturbative correction for connected quadruple excitations [CCSDT(Q)]. These extrapolated valence electronic energies have been corrected for core-electron correlation, harmonic and anharmonic zero-point vibrational energy, and non-Born-Oppenheimer and scalar relativistic effects. Our final recommended enthalpy of formation is Delta(f)H(0)(o)(syn-HOSO) = -58.0 kcal mol(-1). The planar anti-HOSO transition state lies 2.28 kcal mol(-1) above the syn-HOSO minimum, while predicted reaction enthalpies for H + SO(2) --> HOSO, HOSO --> OH + SO, HOSO + H --> H(2) + SO(2), and OH + HOSO --> SO(2) + H(2)O are -38.6, 68.0, -64.4, and -80.1 kcal mol(-1), respectively. We provide incontrovertible evidence for a quasi-planar structure of the syn-HOSO radical, with a remarkably flat torsional energy surface, based on CCSD(T) geometries and harmonic vibrational frequencies energies with up to quintuple-zeta quality basis sets. The energy separation between planar syn-HOSO and the nonplanar global minimum is a mere 5 cm(-1) at the cc-pV(T+D)Z CCSD(T) level of theory. Computed fundamental vibrational frequencies for syn-HOSO and syn-DOSO based on a full quartic force-field evaluated at the cc-pV(T+d)Z CCSD(T) level of theory are in agreement with available experimental data. The present results confirm a previously tentative assignment of a band at 1050 cm(-1) to the HOS bending mode.
尽管HOSO自由基在富硫燃料燃烧中起着关键作用，但其热化学性质尚未得到很好的确立。由于从头算计算中势能面对基组和电子相关方法异常敏感，文献中对于全局最小顺式-HOSO的结构尚无定论。基于系统外推的从头算能量给出了HOSO确定的生成焓，主要通过耦合簇理论考虑电子相关，包括单、双、三激发以及对连接四重激发的微扰校正[CCSDT(Q)]。这些外推的价电子能量已针对芯电子相关、谐波和非谐波零点振动能以及非玻恩-奥本海默和标量相对论效应进行了校正。我们最终推荐的生成焓为ΔfH(0)(o)(顺式-HOSO)= -58.0 kcal mol(-1)。平面反式-HOSO过渡态比顺式-HOSO最小值高2.28 kcal mol(-1)，而对于H + SO2→HOSO、HOSO→OH + SO、HOSO + H→H2 + SO2以及OH + HOSO→SO2 + H2O的预测反应焓分别为-38.6、68.0、-64.4和-80.1 kcal mol(-1)。基于CCSD(T)几何结构和谐波振动频率能量以及高达五重ζ质量基组，我们为顺式-HOSO自由基的准平面结构提供了无可争议的证据，其扭转能面非常平坦。在cc-pV(T+D)Z CCSD(T)理论水平下，平面顺式-HOSO与非平面全局最小值之间的能量间隔仅为5 cm(-1)。基于在cc-pV(T+d)Z CCSD(T)理论水平下评估的全四次力场计算的顺式-HOSO和顺式-DOSO的基本振动频率与现有实验数据一致。目前的结果证实了先前将1050 cm(-1)处的谱带初步归属于HOS弯曲模式。
