Li Xiao, Gewirth Andrew A
Department of Chemistry and Fredrick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
J Am Chem Soc. 2003 Sep 24;125(38):11674-83. doi: 10.1021/ja0363112.
Thiocyanate (SCN) adsorption on an Au electrode is examined using surface-enhanced Raman scattering (SERS) measurements, along with detailed density functional theory (DFT) calculations. Both the calculation and the spectroscopic measurements show that three different geometries are adopted by SCN adsorption in the potential region studied (0.0 V <or= E <or= 1.2 V vs NHE). At low potential both N-bound and S-bound forms are found, at intermediate potentials around the potential of zero charge (E(pzc)) the S-bound form dominates, and at high potentials SCN associates with the Au surface via a bridging geometry. The Stark slope observed for the C-N stretch in the N- and S-bound forms is positive, while the bridge form exhibits a negative Stark slope. DFT calculations show that the potential-dependent reorientation of SCN arises from overlap of specific SCN-based orbitals with the Fermi level of the Au surface; as the Fermi energy changes, different orbital overlaps are found. The calculations also provide an explanation for the different signs of the Stark slope observed for different geometries. For the end-on adsorption, the lowest unoccupied molecular orbital (LUMO) associated with the C-N bond is antibonding, while the corresponding LUMO in the bridge form is bonding.
利用表面增强拉曼散射（SERS）测量以及详细的密度泛函理论（DFT）计算，研究了硫氰酸盐（SCN）在金电极上的吸附情况。计算和光谱测量均表明，在所研究的电位区域（相对于标准氢电极，0.0 V≤E≤1.2 V）中，SCN吸附采用了三种不同的几何构型。在低电位下，发现了氮结合和硫结合两种形式，在接近零电荷电位（E(pzc)）的中间电位下，硫结合形式占主导，而在高电位下，SCN通过桥连几何构型与金表面缔合。在氮结合和硫结合形式中观察到的C-N伸缩振动的斯塔克斜率为正，而桥连形式则表现出负的斯塔克斜率。DFT计算表明，SCN的电位依赖性重新取向源于特定SCN基轨道与金表面费米能级的重叠；随着费米能的变化，会发现不同的轨道重叠。计算结果还为不同几何构型下观察到的斯塔克斜率的不同符号提供了解释。对于端基吸附，与C-N键相关的最低未占据分子轨道（LUMO）是反键的，而桥连形式中相应的LUMO是成键的。
