Barreto Rafael C, Coutinho Kaline, Georg Herbert C, Canuto Sylvio
Instituto de Física, Universidade de São Paulo, Caixa Postal 66318, 05315-970, São Paulo, Brazil.
Phys Chem Chem Phys. 2009 Mar 7;11(9):1388-96. doi: 10.1039/b816912h. Epub 2009 Jan 15.
A combined and sequential use of Monte Carlo simulations and quantum mechanical calculations is made to analyze the spectral shift of the lowest pi-pi* transition of phenol in water. The solute polarization is included using electrostatic embedded calculations at the MP2/aug-cc-pVDZ level giving a dipole moment of 2.25 D, corresponding to an increase of 76% compared to the calculated gas-phase value. Using statistically uncorrelated configurations sampled from the MC simulation, first-principle size-extensive calculations are performed to obtain the solvatochromic shift. Analysis is then made of the origin of the blue shift. Results both at the optimized geometry and in room-temperature liquid water show that hydrogen bonds of water with phenol promote a red shift when phenol is the proton-donor and a blue shift when phenol is the proton-acceptor. In the case of the optimized clusters the calculated shifts are in very good agreement with results obtained from mass-selected free jet expansion experiments. In the liquid case the contribution of the solute-solvent hydrogen bonds partially cancels and the total shift obtained is dominated by the contribution of the outer solvent water molecules. Our best result, including both inner and outer water molecules, is 570 +/- 35 cm(-1), in very good agreement with the small experimental shift of 460 cm(-1) for the absorption maximum.
采用蒙特卡罗模拟和量子力学计算相结合的顺序方法，分析了水中苯酚最低π-π*跃迁的光谱位移。使用MP2/aug-cc-pVDZ水平的静电嵌入计算来考虑溶质极化，得到的偶极矩为2.25 D，与计算得到的气相值相比增加了76%。利用从蒙特卡罗模拟中采样得到的统计不相关构型，进行第一性原理的尺寸扩展计算以获得溶剂化显色位移。然后分析蓝移的起源。在优化几何结构和室温液态水中的结果均表明，当苯酚作为质子供体时，水与苯酚之间的氢键会导致红移；当苯酚作为质子受体时，则会导致蓝移。对于优化后的团簇，计算得到的位移与质量选择自由射流膨胀实验结果非常吻合。在液态情况下，溶质-溶剂氢键的贡献部分抵消，得到的总位移主要由外部溶剂水分子的贡献决定。我们包含内部和外部水分子的最佳结果是570±35 cm⁻¹，与460 cm⁻¹的吸收最大值的小实验位移非常吻合。
