CH Mode Mixing Determines the Band Shape of the Carboxylate Symmetric Stretch in Apo-EDTA, Ca²⁺–EDTA, and Mg²⁺–EDTA

Abstract

The infrared spectra of EDTA complexed with Ca 2+ and Mg 2+ contain, to date, unidentified vibrational bands. This study assigns the peaks in the linear and two-dimensional infrared spectra of EDTA, with and without either Ca 2+ or Mg 2+ ions. Two-dimensional infrared spectroscopy and DFT calculations reveal that, in both the presence and absence of ions, the carboxylate symmetric stretch and the terminal CH bending vibrations mix. We introduce a method to calculate participation coefficients that quantify the contribution of the carboxylate symmetric stretch, CH wag, CH twist, and CH scissor in the 1400−1550 cm −1 region. With the help of participation coefficients, we assign the 1400−1430 cm −1 region to the carboxylate symmetric stretch, which can mix with CH modes. We assign the 1000−1380 cm −1 region to CH twist modes, the 1380−1430 cm −1 region to wag modes, and the 1420−1650 cm −1 region to scissor modes. The difference in binding geometry between the carboxylate− Ca 2+ and carboxylate−Mg 2+ complex manifests as new diagonal and cross-peaks between the mixed modes in the two complexes. The small Mg 2+ ion binds EDTA tighter than the Ca 2+ ion, which causes a redshift of the COO symmetric stretches of the sagittal carboxylates. Energy decomposition analysis further characterizes the importance of electrostatics and deformation energy in the bound complexes.

Vibrational anisotropy decay resolves rare earth binding induced conformational change in DTPA

“…Such structural change affects the vibrational relaxation in two main ways. (1) As discussed in previous work on EDTA complexes, 30,31 the IVR process to lower energy modes becomes slower. (2) In addition, the conformation change associated with metal bound DTPA leads to an increase in the inter-site coupling as the carboxylate groups approach each other.…”

“…Previous work has demonstrated that the high frequency vibrational modes of carboxylate groups can be used to probe conformational and structural changes in the condensed phase. [28][29][30][31][32][33][34][35] In the context of previous PAC ligands, the ultrafast vibrational dynamics of EDTA's carboxylate groups have been extensively studied by the Baiz group 31 and the Garret-Roe group. 30 Using mid-IR pump-probe spectroscopy, the Baiz group measured the vibrational lifetime of the carboxylate groups in free and metal bound EDTA.…”

“…[28][29][30][31][32][33][34][35] In the context of previous PAC ligands, the ultrafast vibrational dynamics of EDTA's carboxylate groups have been extensively studied by the Baiz group 31 and the Garret-Roe group. 30 Using mid-IR pump-probe spectroscopy, the Baiz group measured the vibrational lifetime of the carboxylate groups in free and metal bound EDTA. 31 They observed a net slowdown in the relaxation dynamics of the asymmetric stretching modes for metal bound EDTAs, which was attributed to poor coupling between the asymmetric and the symmetric stretch of the carboxylates.…”

Expanding the Transuranic Metal–Organic Framework Portfolio: The Optical Properties of Americium(III) MOF-76

“…Here, the vibrational modes of the BTC ligands may serve as sensitive probes of the coordination and bonding environments. Changes in the carboxylate vibrational frequencies, for instance, can indicate subsequent increases/decreases in the charge transfer between the BTC ligand and the Am 3+ metal center …”

Relation Between Bond Angle and Carbon–Oxygen Stretching Frequencies in CO2–Containing Compounds

“…In the case of the antisymmetric stretching mode, these peaks are typically in a region with low spectral congestion, making them easier to identify than many other features in the fingerprint region of the spectrum. The positions of the carboxylate stretching signatures are sensitive to subtle changes in complex structure and charge distribution, making them both challenging to interpret and desirable to model. − Here, we present a simple and generalizable relationship between the frequencies of the O–C–O stretching modes and the O–C–O bond angle (θ OCO ).…”