Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstrasse 36, 10623 Berlin, Germany.
Institut des Sciences Moleculaires d'Orsay, UMR-CNRS 8214, and Centre Laser de l'Université Paris Sud/LUMAT FR 2764, Batiment 106, l'Université Paris Sud 11, 91405 Orsay Cedex, France.
J Chem Phys. 2014 Mar 28;140(12):124314. doi: 10.1063/1.4869341.
Vibrational and electronic photodissociation spectra of mass-selected protonated benzaldehyde-(water)n clusters, [BZ-(H2O)n]H(+) with n ≤ 5, are analyzed by quantum chemical calculations to determine the protonation site in the ground electronic state (S0) and ππ() excited state (S1) as a function of microhydration. IR spectra of [BZ-(H2O)n]H(+) with n ≤ 2 are consistent with BZH(+)-(H2O)n type structures, in which the excess proton is localized on benzaldehyde. IR spectra of clusters with n ≥ 3 are assigned to structures, in which the excess proton is located on the (H2O)n solvent moiety, BZ-(H2O)nH(+). Quantum chemical calculations at the B3LYP, MP2, and ri-CC2 levels support the conclusion of proton transfer from BZH(+) to the solvent moiety in the S0 state for hydration sizes larger than the critical value nc = 3. The vibronic spectrum of the S1 ← S0 transition (ππ()) of the n = 1 cluster is consistent with a cis-BZH(+)-H2O structure in both electronic states. The large blueshift of the S1 origin by 2106 cm(-1) upon hydration with a single H2O ligand indicates that the proton affinity of BZ is substantially increased upon S1 excitation, thus strongly destabilizing the hydrogen bond to the solvent. The adiabatic S1 excitation energy and vibronic structure calculated at the ri-CC2/aug-cc-pVDZ level agrees well with the measured spectrum, supporting the notion of a cis-BZH(+)-H2O geometry. The doubly hydrated species, cis-BZH(+)-(H2O)2, does not absorb in the spectral range of 23 000-27 400 cm(-1), because of the additional large blueshift of the ππ() transition upon attachment of the second H2O molecule. Calculations predict roughly linear and large incremental blueshifts for the ππ() transition in [BZ-(H2O)n]H(+) as a function of n. In the size range n ≥ 3, the calculations predict a proton transfer from the (H2O)nH(+) solvent back to the BZ solute upon electronic ππ(*) excitation.
经量子化学计算分析，对质量选择的质子化苯甲醛-（水）n 团簇（[BZ-（H2O）n]H(+)，n ≤ 5）的振动和电子光解光谱进行了研究，以确定基态（S0）和ππ（）激发态（S1）中质子化位置随微水合作用的变化。n ≤ 2 的[BZ-（H2O）n]H(+)的 IR 谱与 BZH(+)-(H2O)n 型结构一致，其中过量质子定位于苯甲醛上。n ≥ 3 的团簇的 IR 谱被分配给结构，其中过量质子位于（H2O）n 溶剂部分，BZ-（H2O）nH(+)。B3LYP、MP2 和 ri-CC2 水平的量子化学计算支持 S0 态下质子从 BZH(+)转移到溶剂部分的结论，对于水合尺寸大于临界值 nc = 3 的情况。n = 1 团簇的 S1 ← S0 跃迁（ππ（））的振动态谱在两个电子态中均与顺式-BZH(+)-H2O 结构一致。在单个 H2O 配体水合作用下，S1 跃迁的 S1 原点的大蓝移为 2106 cm(-1)，表明 BZ 的质子亲和力在 S1 激发后显著增加，从而强烈破坏了与溶剂的氢键。在 ri-CC2/aug-cc-pVDZ 水平上计算得到的绝热 S1 激发能和振动态结构与测量的光谱吻合良好，支持顺式-BZH(+)-H2O 几何结构的概念。由于第二个 H2O 分子附加的大蓝移，双水合物种顺式-BZH(+)-(H2O)2 在 23000-27400 cm(-1)的光谱范围内不吸收，因为 ππ（）跃迁的附加大蓝移。计算预测，[BZ-（H2O）n]H(+)中 ππ()跃迁随 n 的函数呈大致线性和大的增量蓝移。在 n ≥ 3 的尺寸范围内，计算预测在电子ππ（*）激发后，质子从（H2O）nH(+)溶剂转移回 BZ 溶质。
