Déplacements chimiques de la résonance magnétique nucléaire 13C dans les complexes carbonylés octaédriques des métaux de transition du groupe VIB. Effet écran produit par les électrons d non-liants du métal

1977 ◽  
Vol 55 (23) ◽  
pp. 4056-4060 ◽  
Author(s):  
Daniel Cozak ◽  
Ian S. Butler
Keyword(s):  
Electronic Structure ◽  
Chemical Shift ◽  
Metal Carbonyl ◽  
Shielding Effect ◽  
Chemical Shielding ◽  
Carbonyl Ligands ◽  
Résonance Magnétique ◽  
Carbonyl Bond ◽  
D Orbitals ◽  
C Nmr

The 13C nmr shielding effect at the carbonyl ligands due to the paramagnetic current in the non-bonding d orbitals of the metal in the group VIB M(CO)6 complexes has been evaluated. For these complexes, the only contribution of significant importance to the chemical shielding calculated for the carbonyl ligands is opposite to that observed experimentally. The carbonyl chemical shift produced by substitution of one of the carbonyl ligands is discussed in terms of electronic structure of the metal and the metal–carbonyl bond lengths in the M(CO)5L complexes.

Étude de la réactivité des thiényldiazines: nitration et résonance magnétique nucléaire de 13C dans H2SO4 concentré

1982 ◽  
Vol 60 (21) ◽  
pp. 2668-2674 ◽  
Author(s):  
Jean Bourguignon ◽  
Stella Chapelle ◽  
Pierre Granger ◽  
Guy Queguiner
Keyword(s):  
Electronic Structure ◽  
Electrophilic Substitution ◽  
Nmr Spectra ◽  
Competitive Reactions ◽  
Résonance Magnétique ◽  
C Nmr

The reactivity of the thienyl diazines 1 to 7 has been studied by means of the nitration reaction. In order to examine any relationship between the reactivity and the electronic structure of these compounds, we have recorded their 13C nmr spectra in CDCl3 and in concentrated sulfuric acid.The 13C results are correlated to relative reactivities observed during competitive reactions between the thienyl diazines 1 and 2, 2 and 7, 2 and 4, 2 and 6.This method is a rarely-used approach for the study of electrophilic substitution of a molecule in the reaction mixture.

Analyse par résonance magnétique nucléaire 13C de la stabilité relative des spirochromènes et des mérocyanines dérivant du méthoxy-3 nitro-5 salicylaldéhyde

1982 ◽  
Vol 60 (21) ◽  
pp. 2644-2653 ◽  
Author(s):  
Michel Maguet ◽  
Michel Le Baccon ◽  
Yves Poirier ◽  
Robert Guglielmetti
Keyword(s):  
Chemical Shift ◽  
Carbon Atom ◽  
Intermediate Position ◽  
Nmr Study ◽  
Résonance Magnétique ◽  
Limiting Value ◽  
C Nmr

Heterocycloimmonium salts generally react with 3-methoxy 5-nitro salicylaldéhyde to give spirochromenes or merocyanines. Only the latter compounds are obtained in the thiazolinic and the benzimidazolinic series. The linking of a paraffinic ring to the merocyanines allows their 13C nmr study. The comparison of their spectra with those of heterocycloimmonium salts stresses a hyperconjugative effect in the thiazolinic and the benzimidazolinic series. The benzoxazolinic series appears to be in an intermediate position: the thermodynamic stabilities of spirochromenes and merocyanines are quite similar. The limiting value of the chemical shift of the carbon atom involved in the spiroannellation of merocyanines, can be approximately determined.

A 13C and 15N nuclear magnetic resonance study of solid ammonium thiocyanate

1987 ◽  
Vol 65 (5) ◽  
pp. 941-946 ◽  
Author(s):  
Ross M. Dickson ◽  
Michael S. McKinnon ◽  
James F. Britten ◽  
Roderick E. Wasylishen
Keyword(s):  
Ammonium Thiocyanate ◽  
Nuclear Magnetic Resonance Study ◽  
Magic Angle Spinning ◽  
Magic Angle ◽  
Chemical Shielding ◽  
X Ray ◽  
Shielding Constants ◽  
Angle Spinning ◽  
Good Agreement ◽  
C Nmr

The static 13C nmr powder pattern for solid ammonium thiocyanate is analyzed to obtain the 13C chemical shielding anisotropy, 321 ± 7 ppm, and the 13C–14N dipolar splitting, 1295 ± 25 Hz. Slow magic angle spinning 15N nmr experiments are analyzed to obtain a nitrogen chemical shielding anisotropy of 415 ± 15 ppm. The 13C–14N dipolar splitting leads to an effective C—N bond length of 1.19 ± 0.01 Å, in good agreement with the value of 1.176 Å reported from accurate X-ray and neutron crystallographic studies. In solid NH4NCS absolute values of the average shielding constants [Formula: see text] and ct[Formula: see text] are 52 and 34 ppm, respectively. Comparison of calculated and observed [Formula: see text] values indicates that intermolecular interactions decrease the 13C and 15N shielding constants by approximately 10 and 30 ppm, respectively.

Determination of chemical shifts in 6-condensed syringylic lignin model compounds

Holzforschung ◽  
2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Lucas Lagerquist ◽  
Jani Rahkila ◽  
Patrik Eklund
Keyword(s):  
Chemical Shift ◽  
Methoxy Group ◽  
Chemical Shifts ◽  
Model Compounds ◽  
Correlation Peak ◽  
Lignin Model Compounds ◽  
Benzylic Alcohols ◽  
Lignin Model ◽  
C Nmr

Abstract A small library of 6-substituted syringyl model compounds with aliphatic, carboxylic, phenylic, benzylic alcohols and brominated substituents were prepared. The influence of the substituents on the chemical shifts of the compounds was analyzed. All of model compounds showed a characteristic increase in the 13C NMR chemical shift of the methoxy group vicinal to the substitution. This 13C NMR peak and its corresponding correlation peak in HSQC could potentially be used to identify 6-condensation in syringylic lignin samples.

scholarly journals Protein structure refinement using a quantum mechanics-based chemical shielding predictor

2016 ◽  
Author(s):  
Lars A. Bratholm ◽  
Jan H. Jensen
Keyword(s):  
Quantum Mechanics ◽  
Simulated Annealing ◽  
Chemical Shift ◽  
Force Field ◽  
Chemical Shifts ◽  
Protein Structures ◽  
Empirical Methods ◽  
Structural Refinement ◽  
Chemical Shielding ◽  
Chemical Shift Prediction

The accurate prediction of protein chemical shifts using quantum mechanics (QM)-based method has been the subject of intense research for more than 20 years but so far empirical methods for chemical shift prediction have proven more accurate. In this paper we show that a QM-based predictor of protein backbone and CB chemical shifts (ProCS15, PeerJ 2016, 3:e1344) is of comparable accuracy to empirical chemical shift predictors after chemical shift-based structural refinement that removes small structural errors. We present a method by which quantum chemistry based predictions of isotropic chemical shielding values (ProCS15) can be used to refine protein structures using Markov Chain Monte Carlo (MCMC) simulations, relating the chemical shielding values to the experimental chemical shifts probabilistically. Two kinds of MCMC structural refinement simulations were performed using force field geometry optimized X-ray structures as starting points: Simulated annealing of the starting structure and constant temperature MCMC simulation followed by simulated annealing of a representative ensemble structure. Annealing of the CHARMM structure changes the CA-RMSD by an average of 0.4 Å but lowers the chemical shift RMSD by 1.0 and 0.7 ppm for CA and N. Conformational averaging has a relatively small effect (0.1 - 0.2 ppm) on the overall agreement with carbon chemical shifts but lowers the error for nitrogen chemical shifts by 0.4 ppm. If a residue-specific offset is included the ProCS15 predicted chemical shifts have RMSD values relative to experiment that are comparable to popular empirical chemical shift predictors. The annealed representative ensemble structures differs in CA-RMSD relative to the initial structures by an average of 2.0 Å, with >2.0 Å difference for six proteins. In four of the cases, the largest structural differences arise in structurally flexible regions of the protein as determined by NMR, and in the remaining two cases, the large structural change may be due to force field deficiencies. The overall accuracy of the empirical methods are slightly improved by annealing the CHARMM structure with ProCS15, which may suggest that the minor structural changes introduced by ProCS15-based annealing improves the accuracy of the protein structures. Having established that QM-based chemical shift prediction can deliver the same accuracy as empirical shift predictors we hope this can help increase the accuracy of related approaches such as QM/MM or linear scaling approaches or interpreting protein structural dynamics from QM-derived chemical shift.

13C NMR CHEMICAL SHIFT CALCULATION FOR THE 1, 1'-DISUBSTITUTED FERROCENE COMPOUNDS

1986 ◽  
Vol 2 (01) ◽  
pp. 73-76
Author(s):  
Li Cun ◽  
◽  
Wu Weixiong ◽  
Pen Xin ◽  
Chao Qirong ◽  
...  
Keyword(s):  
Chemical Shift ◽  
Nmr Chemical Shift ◽  
Ferrocene Compounds ◽  
Shift Calculation ◽  
C Nmr
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