The goal of this study is to shed light on the charge-transfer (CT) mechanism of surface-enhanced Raman scattering (SERS) by considering the properties of CT excited states. The calculations have been done by means of an excited-state gradient approximation for a pyridine molecule interacting with a silver cluster, and provided a satisfactory improvement in comparison to previous work. The effect of electrode potential on the SERS-CT spectra has been modelled theoretically by applying an external electric field for selected CT transitions and the enhancement of the ν and ν modes and a decline in the intensity of the ν mode under a negative electric field (which is directed toward the cluster) have been observed. These results match well with the experimental studies and also explain the effect of electrode potentials on the patterns of spectra, as experimental evidence of the CT mechanism. Finally, this study demonstrated that the excited state vector gradient can be used as a distinguishing factor to explain the SERS selection rules.

The unique plasmonic character of silver and gold nanoparticles has a wide range of applications, and tailoring this property by changing electronic and geometric structures has received a great deal of attention. Herein, we study the role of the quantum properties in controlling the plasmonic excitations of gold and silver atomic chains and rods. The influence of relativistic effects, scalar as well as spin–orbit, on the intensity and energy of plasmonic excitations is investigated. The intensity quenching and the red shift of energy in the presence of relativistic effects are introduced via the appearance of d orbitals directly in optical excitations in addition to the screening of s-electrons by mixing with the occupied orbitals. For the linear gold system, it will be demonstrated that by increasing the length the relativistic behavior declines and the contribution of d orbitals to the plasmonic excitations evidently decreases. Furthermore, silver atoms are doped in gold chains and rods (with two different arrangements) to realize how gold–silver interactions decrease the relativistic effects and enhance the intensity of collective excitations. Finally, to strengthen the plasmonic behavior of gold, the elongation of chain and doping with suitable atoms such as silver (with the classical plasmonic behavior) can be introduced as the manipulating ways to control the influence of scalar-relativistic and spin–orbit effects and, consequently, reinforce the plasmonic properties.

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Manganese is involved as a cofactor in the activation of numerous enzymes as well as the oxygen-evolving complex of photosystem II. Full understanding of the role played by the Mn(2+) ion requires detailed knowledge of the interaction modes and energies of manganese with its various environments, a knowledge that is far from complete. To bring detailed insight into the local interactions of Mn in metallopeptides and proteins, theoretical studies employing first-principles quantum mechanical calculations are carried out on [Mn-amino acid](2+) complexes involving all 20 natural α-amino acids (AAs). Detailed investigation of [Mn-serine](2+), [Mn-cysteine](2+), [Mn-phenylalanine](2+), [Mn-tyrosine](2+), and [Mn-tryptophan](2+) indicates that with an electron-rich side chain, the most stable species involves interaction of Mn(2+) with carbonyl oxygen, amino nitrogen, and an electron-rich section of the side chain of the AA in its canonical form. This is in sharp contrast with aliphatic side chains for which a salt bridge is formed. For aromatic AAs, complexation to manganese leads to partial oxidation as well as aromaticity reduction. Despite multisite binding, AAs do not generate strong enough ligand fields to switch the metal to a low- or even intermediate-spin ground state. The affinities of Mn(2+) for all AAs are reported at the B3LYP and CCSD(T) levels of theory, thereby providing the first complete series of affinities for a divalent metal ion. The trends are compared with those of other cations for which affinities of all AAs have been previously obtained.