Cotton Effect in Copper-Proline Complexes in the Visible Region

Abstract

Optical rotatory dispersion (ORD) is an effective and relatively inexpensive method for probing both molecular structural and electronic properties. A typical ORD experimental setup in a student laboratory course utilizes the spectrally broad output of a tungsten lamp. Unfortunately its spectral output does not overlap with most of the electronic resonances in common molecular systems in a chemistry lab. Therefore it is difficult to demonstrate to students a representative Cotton effect. In this article we suggest taking advantage of the visible d–d electronic transition of Cu2+. This allows us to contrast the normal ORD response of d- and l-proline in aqueous solution with the strong Cotton effect observed when these amino acids are complexed with a metal cation. This experiment covers a number of topics in organic and inorganic chemistry and in optical spectroscopy. We provide the details of the complex preparation and discuss the instrumental basics for the ORD data measurements. This experiment is suitable for a second-year undergraduate laboratory course in physical chemistry.

Cu(Proline)2 Complex: A Model of Bio-Copper Structural Ambivalence

“…The preparation and characterization of the optical absorption and optical rotatory dispersion of the Cu(L-proline) 2 complex was reported in a previous contribution [24]. We collect steady-state IR spectra of complexes in deuterium oxide using a Nicolet 6700 FTIR spectrometer, Thermo Fisher Scientific Co., Madison, WI, USA.…”

“…Furthermore, we compute ORD spectra applying the Kramers-Kronig transform for the modelled CD resonances [44,45]. We attune the spectral width of the Lorentzian line-shapes to match the shape of the experimentally detected ORD spectrum [24].…”

“…Having performed a structural analysis using electronic optical responses, it is important to review the IR properties of the system, and mechanisms of the inhomogeneity of the IR absorption of carbonyl modes of the Cu(L-proline) 2 , in particular [24]. In Figure 2c, we compare the IR absorption of the modelled structures computed by DFT calculations with FTIR spectrum for the prepared Cu(L-proline) 2 complex.…”

“…In this contribution, following an early suggestion to approach the understanding of structure and reactivity in proteins using simple complexes [23], we test experimental and theoretical approaches to explore the structural and dynamic properties of a Cu(L-proline) 2 complex, the synthesis of which and preliminary optical characterizations were reported previously [24]. Early efforts in synthesis and characterization of Cu 2+ complexes with amino acids indicated sensitivity of the optical activity of d-d transitions to the nature of the ligands [25,26].…”

“…Here, it is important to stress that optical spectroscopy techniques are particularly helpful in the characterization of the structure and dynamics in liquids at room temperature. In this respect, it is interesting to note that infrared (IR) absorption of the carbonyl of a Cu(L-proline) 2 complex in solution revealed a line-shape that is broadened at the higher frequency side [24], which could be due to: (1) a frequency split between symmetric and antisymmetric carbonyl modes upon harmonic coupling; (2) the presence of complexes in both cis and trans geometries [32]; (3) possible variance about the asymmetry of the ring structures; (4) variances of interactions with the solvent. Due to this complexity, we review electronic and IR responses using the same level of theory and describe both structural and dynamic properties of the Cu(L-proline) 2 complex.…”