Cis–Trans Conformational Dependence of the Carboxylate pK in Glycylsarcosine

1974 ◽  
Vol 52 (12) ◽  
pp. 2348-2352 ◽  
Author(s):  
R. A. Morton ◽  
S. S. Danyluk
Keyword(s):  
Chemical Shift ◽  
Theoretical Calculations ◽  
Chemical Shifts ◽  
Molecular Models ◽  
Amino Groups ◽  
Cis And Trans ◽  
In Cis ◽  
Cis Isomer

A study is reported of the influence of pD upon the 220 MHz proton n.m.r. spectrum of the dipeptide glycylsarcosine (Gly-Sar) in D2O at 23 °C. Variation of pD produces characteristic changes in chemical shifts for Gly-Sar and an analysis of the pD–chemical shift curves shows that the carboxyl pK differs significantly in cis and trans conformers, having values of 3.01 ± 0.03 and 3.39 ± 0.03, respectively. From a consideration of molecular models and theoretical calculations, it is concluded that the pK difference arises from a closer juxtaposition of terminal carboxyl and amino groups in the cis isomer.

scholarly journals Bethe-Salpeter Study of the Optical Absorption of trans and cis Azobenzene-Functionalized Metal-Organic Frameworks using Molecular and Periodic Models

2021 ◽  
Author(s):  
Aseem Rajan Kshirsagar ◽  
Claudio Attaccalite ◽  
Xavier Blase ◽  
Jing Li ◽  
Roberta Poloni
Keyword(s):  
Optical Absorption ◽  
Gas Phase ◽  
Metal Organic Framework ◽  
Molecular Models ◽  
Gw Approximation ◽  
Periodic Models ◽  
Metal Organic ◽  
Reasonable Description ◽  
Cis And Trans ◽  
In Cis

The optical absorption spectra of the azobenzene-functionalized metal-organic framework, PCN-123, are calculated in cis and trans configurations using the Bethe-Salpeter equation (BSE) formalism and the GW approximation using periodic and non-periodic models. In the visible, near-UV and mid-UV region the optical excitations in the MOF are associated with the azobenzene functionalities and this results in spectral features similar to the case of the gas phase azobenzene and the azo-functionalized ligand. The most noticeable difference is the significantly more intense S<sub>1</sub> band for cis in the MOF as compared to the free molecules which points to a faster and more complete cis→trans isomerization in the framework, with strong implications for the design of MOFs with high photoconversion efficiencies. Consistent with these findings, all the molecular models employed to represent the MOF, including the smallest, are found to yield a reasonable description of the low energy optical spectra (between 2 and 5 eV) of the periodic framework, with the exception of the stronger S<sub>1</sub> band of cis in the MOF, a feature that we attribute to a limitation of the fragment model to correctly represent the wavefunction of the extended framework.

Proton chemical shifts and electron densities in aromatic and heteroaromatic molecules. I. Procedure and chemical shift corrections; Applications to azines

1967 ◽  
Vol 20 (7) ◽  
pp. 1305 ◽  
Author(s):  
PJ Black ◽  
RD Brown ◽  
ML Heffernan
Keyword(s):  
Chemical Shift ◽  
Electric Field ◽  
Electron Distribution ◽  
Theoretical Calculations ◽  
Chemical Shifts ◽  
Lone Pair ◽  
Excess Charge ◽  
Heteroaromatic Compounds ◽  
Electric Field Dependence ◽  
Proton Chemical Shifts

In this series, the observed proton chemical shifts of heteroaromatic compounds are used to obtain estimates of the electron distribution in these systems. These estimates are deduced from the measured shifts after applying corrections for the shielding contributions arising from ring currents, magnetic anisotropy of neighbouring atoms, and the electric field from the lone-pair dipoles on heteroatoms, in a manner closely related to that of Gil and Murrell. The corrected chemical shifts are then used to deduce an ?experimental? π-electron distribution via the electric field dependence of the proton shifts on the excess charge at all ring positions. However, in most cases, there are not enough experimental shifts to determine uniquely the excess charge at each ring atom, and it is necessary to assume that some inaccessible charges are close to values obtained from theoretical calculations. With these limitations, a comparison is made between the excess charges deduced in the above fashion from chemical shift data on mono- and poly-cyclic azines and diazines and those obtained by theoretical calculations using the BJ VESCF method. It is emphasized that this interpretation is tentative and that the extent of σ-electron polarization is not well understood at present.

Orientation of the methoxyl group in cis- and trans-2-methoxy-4-methyl-1,3-dioxanes and related compounds as inferred by 1H and 13C nmr spectroscopy. The significance of the "exo-anomeric" effect

1982 ◽  
Vol 60 (9) ◽  
pp. 1067-1072 ◽  
Author(s):  
Vanga S. Rao
Keyword(s):  
Dynamic Equilibrium ◽  
Chemical Shifts ◽  
13C Nmr Spectroscopy ◽  
Methoxyl Group ◽  
Anomeric Effect ◽  
1H And 13C Nmr ◽  
Nuclear Overhauser ◽  
Cis And Trans ◽  
In Cis ◽  
Related Compounds

Carbon-13 chemical shifts and 13C—1H couplings are reported for some substituted 2-methoxy-1,3-dioxanes and related tetrahydropyran derivatives. These data, together with nuclear Overhauser enhancement experiments, provided evidence for a favored orientation of the methoxyl group. From an analysis of the 13C chemical shifts and the magnitude of vicinal 13C—1H coupling between —OCH3 and H-2, a possible dynamic-equilibrium orientation for the methoxyl group is proposed, and an evaluation of the significance of a "generalized exo-anomeric effect" is presented.

Tungsten-183 chemical shifts in cis- and trans-bis(tributylphosphine)tetracarbonyltungsten

1971 ◽  
Vol 10 (1) ◽  
pp. 206-207 ◽  
Author(s):  
Paul Joseph Green ◽  
Thomas Henderson Brown
Keyword(s):  
Chemical Shifts ◽  
Cis And Trans ◽  
In Cis

The 19F Nuclear Magnetic Resonance Spectra of Octahedral Fluorothiocyanato-N- and Fluorocyanato-N-Titanates(IV)

1974 ◽  
Vol 52 (4) ◽  
pp. 667-673 ◽  
Author(s):  
Philip A. W. Dean ◽  
Brian J. Ferguson
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Chemical Shift ◽  
Stable Equilibrium ◽  
Chemical Shifts ◽  
Mixed Species ◽  
Nuclear Magnetic Resonance Spectra ◽  
Donor Ability ◽  
The Individual ◽  
Cis And Trans ◽  
Back Donation

The low temperature 19F n.m.r. spectra of the mixtures TiF62−−TiX62− (X = Cl−, Br−, NCS−, NCO−) in sulfur dioxide solution have been examined. When X = Cl− or Br− rapid fluorine exchange is occurring even at the lowest accessible temperature, but when X = NCS− or NCO− the spectra of the individual mixed species TiF6−xXx2− are observed. In the latter two cases the solutions also contain fluorine-bridged dimers in stable equilibrium. The 19F chemical shifts of TiF6−xXx2− (X = NCS− or NCO−) and of some related mixed complexes are reasonably represented by the equation δ = pC + qT where C and T are constants characteristic of the substituent X and p and q are the number of ligands X cis and trans respectively to the resonating fluorine. The constants T appear to be a measure of the π-donor ability of the ligand X in these complexes, supporting the idea that the chemical shift of a fluorine in fluorotitanates is determined largely by the extent to which fluorine-to-titanium back donation occurs.

scholarly journals Bethe-Salpeter Study of the Optical Absorption of trans and cis Azobenzene-Functionalized Metal-Organic Frameworks using Molecular and Periodic Models

2021 ◽  
Author(s):  
Aseem Rajan Kshirsagar ◽  
Claudio Attaccalite ◽  
Xavier Blase ◽  
Jing Li ◽  
Roberta Poloni
Keyword(s):  
Optical Absorption ◽  
Gas Phase ◽  
Metal Organic Framework ◽  
Molecular Models ◽  
Gw Approximation ◽  
Periodic Models ◽  
Metal Organic ◽  
Reasonable Description ◽  
Cis And Trans ◽  
In Cis

The optical absorption spectra of the azobenzene-functionalized metal-organic framework, PCN-123, are calculated in cis and trans configurations using the Bethe-Salpeter equation (BSE) formalism and the GW approximation using periodic and non-periodic models. In the visible, near-UV and mid-UV region the optical excitations in the MOF are associated with the azobenzene functionalities and this results in spectral features similar to the case of the gas phase azobenzene and the azo-functionalized ligand. The most noticeable difference is the significantly more intense S<sub>1</sub> band for cis in the MOF as compared to the free molecules which points to a faster and more complete cis→trans isomerization in the framework, with strong implications for the design of MOFs with high photoconversion efficiencies. Consistent with these findings, all the molecular models employed to represent the MOF, including the smallest, are found to yield a reasonable description of the low energy optical spectra (between 2 and 5 eV) of the periodic framework, with the exception of the stronger S<sub>1</sub> band of cis in the MOF, a feature that we attribute to a limitation of the fragment model to correctly represent the wavefunction of the extended framework.

Solvent, concentration, and temperature dependance of the platinum–phosphorus nuclear magnetic résonance coupling constants in cis- and trans-[PtCl2(PBu3n)2]

1976 ◽  
Vol 54 (17) ◽  
pp. 2733-2738 ◽  
Author(s):  
Keith R. Dixon ◽  
Martin Fakley ◽  
Alan Pidcock
Keyword(s):  
Chemical Shifts ◽  
Specific Model ◽  
Coupling Constants ◽  
Solvent Concentration ◽  
Reaction Field ◽  
Magnetic Resonance Coupling ◽  
Resonance Coupling ◽  
Temperature Dependance ◽  
Cis And Trans ◽  
In Cis

Studies of the effects of solvent, concentration and temperature on the 31P nmr parameters of cis- and trans-[PtCl2(PBun3)2] are reported. The coupling constants, 1JPt–P are markedly dependent upon all three variables but, in contrast to previous studies on non-coordination compounds, very little corresponding variation in chemical shifts is found. Results are discussed in terms of a solvent cavity model using reaction field, Stark and Macrae effects but it is concluded that a more specific model involving competitive solvation at platinum gives a better interpretation of the data.

Long-range deuterium-induced isotope effects on 1H chemical shifts in cis- and trans-stilbene

1997 ◽  
Vol 410-411 ◽  
pp. 9-12
Author(s):  
P. Novak ◽  
Z. Meić ◽  
D. Vikić-Topić ◽  
H. Sterk
Keyword(s):  
Long Range ◽  
Chemical Shifts ◽  
Isotope Effects ◽  
Trans Stilbene ◽  
Cis And Trans ◽  
In Cis
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