Amides. 3. Experimental and Theoretical Studies of the Effect of the Medium on the Rotational Barriers for N,N-Dimethylformamide and N,N-Dimethylacetamide

Wiberg, Kenneth B.; Rablen, Paul R.; Rush, Daniel J.; Keith, Todd A.

doi:10.1021/ja00120a006

Cited by 337 publications

(375 citation statements)

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“…8 measured values for the HS state of Fe͑II͒-based SCO complexes. 23 The present analysis assumed a planar bpy structure, but already for the GS and the DFT results for the HS state there is a twist between each pyridine moiety of the bpy ligand, which was measured to 6.4°͑LS͒ in an x-ray crystallographic study 25 and simulated to about 0°͑LS͒ and about 5°͑HS͒ in Ref. 24.…”

Section: -5mentioning

confidence: 79%

Structural analysis of ultrafast extended x-ray absorption fine structure with subpicometer spatial resolution: Application to spin crossover complexes

Gawełda¹,

Pham²,

Veen³

et al. 2009

The Journal of Chemical Physics

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We present a novel analysis of time-resolved extended x-ray absorption fine structure ͑EXAFS͒ spectra based on the fitting of the experimental transients obtained from optical pump/x-ray probe experiments. We apply it to the analysis of picosecond EXAFS data on aqueous ͓Fe II ͑bpy͒ 3 ͔ 2+ , which undergoes a light induced conversion from its low-spin ͑LS͒ ground state to the short-lived ͑ Ϸ 650 ps͒ excited high-spin ͑HS͒ state. A series of EXAFS spectra were simulated for a collection of possible HS structures from which the ground state fit spectrum was subtracted to generate transient difference absorption ͑TA͒ spectra. These are then compared with the experimental TA spectrum using a least-squares statistical analysis to derive the structural change. This approach reduces the number of required parameters by cancellation in the differences. It also delivers a unique solution for both the fractional population and the extracted excited state structure. We thus obtain a value of the Fe-N bond elongation in the HS state with subpicometer precision ͑0.203Ϯ 0.008 Å͒.

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Section: -5mentioning

confidence: 79%

Structural analysis of ultrafast extended x-ray absorption fine structure with subpicometer spatial resolution: Application to spin crossover complexes

Gawełda¹,

Pham²,

Veen³

et al. 2009

The Journal of Chemical Physics

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“…[39][40][41] These calculations do not yet lead to a consensus about the C=O bond order when molecules of solvent are included. [42][43][44][45] We had observed that for peptides similar to 1-3 the barrier to isomerization (ΔG ‡ ) correlates with the frequency of the amide I vibrational mode (υ), 26,46 as predicted from the traditional view. To search for an inductive effect on the rate of cis-trans prolyl peptide bond isomerization (eq 1), we measured these rates for 1 and 3 in dioxane 47 and 1 and 2 in water 48 by using inversion transfer NMR spectroscopy.…”

Section: Inductive Effect On the Amide I Vibrational Modementioning

confidence: 95%

Inductive Effects on the Energetics of Prolyl Peptide Bond Isomerization: Implications for Collagen Folding and Stability

1996

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The hydroxylation of proline residues in collagen enhances the stability of the collagen triple helix. Previous X-ray diffraction analyses had demonstrated that the presence of an electronwithdrawing substituent on the pyrrolidine ring of proline residues has significant structural consequences [Panasik, N., Jr.; Eberhardt, E. S.; Edison, A. S.; Powell, D. R.; Raines, R. T. Int. J. Pept. Protein Res. 1994, 44, 262-269]. Here, NMR and FTIR spectroscopy were used to ascertain kinetic and thermodynamic properties ofThe pK a 's of the nitrogen atom in the parent amino acids decrease in the order: proline (10.8) > 4(R)-hydroxy-L-proline (9.68) > 4(R)-fluoro-L-proline (9.23). In water or dioxane, amide I vibrational modes decrease in the order: 1 > 2 > 3. At 37 °C in dioxane, the rate constants for amide bond isomerization are greater for 3 than 1. Each of these results is consistent with the traditional picture of amide resonance coupled with an inductive effect that results in a higher bond order in the amide C=O bond and a lower bond order in the amide C-N bond. Further, at 37 °C in water or dioxane equilibrium concentrations of the trans isomer increase in the order: 1 < 2 < 3. Inductive effects may therefore have a significant impact on the folding and stability of collagen, which has a preponderance of hydroxyproline residues, all with peptide bonds in the trans conformation.

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“…[50][51][52][53][54] The high barriers are attributed to the stabilization caused by the n N !p* CO delocalization, and are therefore a measure of amide bond stability. Wiberg et al estimated, [55] both theoretically and experimentally, the rotational free-energy barrier in N,N-dimethylacetamide, and obtained a barrier of 15.3 AE 0.1 kcal mol À1 experimentally (with respect to TS anti ) using NMR selective inversion-recovery experiments in several solvents. They also calculated the barrier by using quantum methods (at the G2A C H T U N G T R E N N U N G (MP2) Table 1.…”

Section: Delocalization Indexes and Rotation Around The Càn Bondmentioning

confidence: 99%

Resonance Structures of the Amide Bond: The Advantages of Planarity

Mujika

Matxain

Eriksson

et al. 2006

Chemistry A European J

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Delocalization indexes based on magnitudes derived from electron-pair densities are demonstrated to be useful indicators of electron resonance in amides. These indexes, based on the integration of the two-electron density matrix over the atomic basins defined through the zero-flux condition, have been calculated for a series of amides at the B3LYP/6-31+G* level of theory. These quantities, which can be viewed as a measure of the sharing of electrons between atoms, behave in concordance with the traditional resonance model, even though they are integrated in Bader atomic basins. Thus, the use of these quantities overcomes contradictory results from analyses of atomic charges, yet keeps the theoretical appeal of using nonarbitrary atomic partitions and unambiguously defined functions such as densities and pair densities. Moreover, for a large data set consisting of 24 amides plus their corresponding rotational transition states, a linear relation was found between the rotational barrier for the amide and the delocalization index between the nitrogen and oxygen atoms, indicating that this parameter can be used as an ideal physical-chemical indicator of the electron resonance in amides.

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Amides. 3. Experimental and Theoretical Studies of the Effect of the Medium on the Rotational Barriers for N,N-Dimethylformamide and N,N-Dimethylacetamide

Cited by 337 publications

References 1 publication

Structural analysis of ultrafast extended x-ray absorption fine structure with subpicometer spatial resolution: Application to spin crossover complexes

Structural analysis of ultrafast extended x-ray absorption fine structure with subpicometer spatial resolution: Application to spin crossover complexes

Inductive Effects on the Energetics of Prolyl Peptide Bond Isomerization: Implications for Collagen Folding and Stability

Resonance Structures of the Amide Bond: The Advantages of Planarity

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