Surface‐enhanced Raman scattering of M2–pyrazine–M2 (M = Cu, Ag, Au): Analysis by natural perturbation orbitals and density functional theory functional dependence

• Published in: Journal of computational chemistry 2020-06, Vol.41 (17), p.1628-1637
• Main Authors: Miyamoto, Masaya, Hada, Masahiko
• Format: Article
• Language: English
• Subjects: Copper; Density functional theory; Dependence; DFT functional dependence; Gold; Molecular orbitals; NPO; Perturbation methods; pyrazine; Raman intensity; Raman spectra; SERS; Silver
• ISSN: 0192-8651; 1096-987X
• DOI: 10.1002/jcc.26205

Here, we propose a new method to analyze various electronic properties of molecules based on natural perturbation orbitals (NPOs). We applied the proposed method to chemical enhancement of the surface‐enhanced Raman scattering (SERS) intensity of M2–pyrazine–M2 (M = Cu, Ag, Au) complexes. The SERS intensity can be effectively decomposed into the contributions of four NPO pairs (1σ–1σ*, 2σ–2σ*, 1π–1π*, and 2π–2π*), so NPO analysis makes the SERS intensity much easier to understand than by conventional canonical molecular orbitals. Moreover, we analyzed the dependence of the density functional theory functional on the SERS intensity. For the Ag2–pyrazine–Ag2 complex, the BP86 functional overestimates the Raman intensity by about 23 times compared with coupled‐cluster singles and doubles level of theory, while the CAM‐B3LYP functional gives moderately accurate values. This overestimation arises from the inaccuracy of the energy derivative along the normal vibrational mode. Natural Perturbation Orbitals (NPO) Analysis is newly proposed. This analysis shows that the surface‐enhanced Raman scattering intensity of Ag2–pyrazine–Ag2 arises from the contributions of this NPO pairs (1π–1π*).