How is the Observation of High-Order Overtones and Combinations Elucidated by the Charge-Transfer Mechanism in SERS?

The charge-transfer chemical mechanism is responsible for altering the molecular spectral pattern and providing valuable insights into the properties of adsorbates. The impact of charge transfer becomes more pronounced in SERS spectra when CT states can gain intensity through vibronic coupling with high-intensity excitations. Experimental SERS spectra of diamino molecules, such as 4,4'-diaminostilbene (DAS) and 4,4'-diaminotolane (DAT), featuring bright CT transitions, have been compared to dipyridyl compounds, such as 1,2-bis(4-pyridyl) ethylene (BPE) and 1,2-di(4-pyridyl) acetylene (DPA), characterized by nearly dark CT excitations. This comparison aims to elucidate the effect of CT transitions on the presence of overtones and combination bands. We explain this distinction using Albrecht's formalism for resonance Raman spectroscopy within the framework of path integral time-dependent density functional theory considering the Herzberg-Teller corrections. It is worth noting that the energy gap between the highest occupied metallic orbital and the lowest unoccupied molecular orbital in diamino derivatives is noticeably smaller than in compounds featuring two pyridyl rings. The high-intensity SERS-CT spectra for diamino derivatives, primarily driven by the Albrecht A term, were acquired and used to elucidate the experimental observation of high-order modes with a significant Huang-Rhys factor. Conversely, the absolute intensity of SERS-CT for dipyridyl compounds is at least 106 times smaller than that for diamines, and the C term makes a significant contribution, explaining the silent overtones.

Influence of nitrogen doping in sumanene framework toward hydrogen storage: A computational study.

Two conditions are important to obtain appropriate substances for hydrogen storage; high surface area and fitting binding energy (BE). Doping is a key strategy that improves BE. We investigated hydrogen adsorption onto twenty six nitrogen disubstituted isomers of sumanene (C19N2H12) by MP2/6-311++G(d,p)//B3LYP/6-31+G(d) and M06-2X/6-31+G(d) levels of theory. Effect of nitrogen doping in different positions of sumanene was checked. To obtain better BE, basis set superposition error (BSSE) and zero point energy (ZPE) corrections were used. Anticipating of adsorption sites and extra details about adsorption process was done by molecular electrostatic potential (MEP) surfaces. Various types of density of state (DOS) diagrams such as total DOS (TDOS), projected DOS (PDOS) and overlap population DOS (OPDOS) and natural bond orbital (NBO) analysis were used to find better insight on the adsorption properties. In addition of temperature depending of the BE, HOMO-LUMO gap (HLG), dipole moment, reactivity and stability, bowl depth and natural population analysis (NPA) of the isomers were studied. A physisorption mechanism for adsorption was proposed and a trivial change was seen. Place of nitrogen atoms in sumanene frame causes to binding energy increases or decreases compared with pristine sumanene. The best and the worst isomers and category of isomers were suggested.