Computational study of structural properties of lithium cation complexes with carbamate-modified disiloxanes

Abstract

Lithium cation solvation structures [Li(S)(n=1-4)](+) with ligands of cyclic or noncyclic carbamate-modified disiloxanes are optimized at B3LYP level of theory and compared to their corresponding simplified carbamates and to the organic carbonates ethylene carbonate (EC) and dimethyl carbonate (DMC). The electrostatic potentials (ESP) of these investigated carbonyl-containing solvents are mapped on the electron density surface. The maximum ESP is located at the C=O-oxygen, whereas the disiloxane functionality represents an unpolar residue. Natural Bond Orbitals (NBO) analysis reveals strong n(N) →π(C[double bond, length as m-dash]O) donor-acceptor interactions in carbamates which outrun dipolar properties. As a result, higher total binding energies (ΔE(B)) for solvation of Li(+) in carbamates (-148 kcal mol(-1)) are found than for carbonates (-137 kcal mol(-1)). Furthermore, the disiloxane moiety with its Si-O bond is stabilized by n(O) →σ*(Si-C) hyperconjugation that provides additional electron density to a nearby SiCH3 methyl group thus supporting an additional SiCH2-H...Li(+) coordination. The formation of all investigated solvation structures is exothermic. Owing to steric hindrance of noncyclic carbonyl-containing ligands and the bulky disiloxane functionality, the solvation structure [Li(S)3](+) is the preferred structure according to Gibbs free energy ΔG(B) results.

Conformation and EPR characterization of rigid, 310‐helical peptides with TOAC spin labels: Models for short distances

“…The J couplings in the present study can be compared to those of the Ala/TOAC peptides described by Hanson et al 10,30,31 that are also in the 3 10 -helix conformation. The Ala-based HEXA 1,4 peptide shows less of the five-line component spectrum than the Aib-based HEPTA 3,6 . Also, in the Alabased HEXA 1,5 peptide the five-line component is almost absent, whereas it is prominent in the corresponding Aibbased peptide HEXA 1,5 (for the relative TOAC positions see Table III).…”

“…The spectra of the HEPTA 3,6 and the HEXA 1,5 , peptides (class I) are simulated with the same J value, but with different lineshape parameters for the five-line component. The narrowline component which contributes 3% in the case of HEPTA 3,6 and 13% for HEXA 1,5 has parameters close to the respective monoradical peptide (Table II).…”

“…Three of the spectra of the biradical peptides differ from those of the monoradical peptides: HEPTA 3,6 , HEXA 1,5 , and OCTA 2,7 . The spectra of HEPTA 3,6 and HEXA 1,5 peptides have a five-line component that shows that J > A N /2, and in the OCTA 2,7 spectrum the low-field line has a smaller amplitude, that is, a larger linewidth, than the spectrum of OCTA 7 . The NONA 2,8 and NONA 2 spectra are identical within the experimental noise, and also at the higher spectral resolution of W-band EPR ( Figure 2) the two compounds have almost identical spectra.…”

“…The first possibility is that in a fraction of the peptides one of the nitroxides is chemically degraded. For HEPTA 3,6 , the narrow-line contribution is as low as 3 %, and nitroxide degradation in that order cannot be categorically excluded. However, for HEXA 1,5 this contribution is 13 %, making this interpretation definitely not likely.…”

“…Most electron paramagnetic resonance (EPR) methods for distance determination require freezing the sample. [1][2][3][4][5][6] To address this problem, in the present work we investigate spin-spin interactions in model peptide systems in liquid solution. Such peptides have to be rigid to serve as useful model compounds.…”