Origin of the 29Si NMR chemical shift in R3Si–X and relationship to the formation of silylium (R3Si+) ions

2020 ◽  
Vol 49 (45) ◽  
pp. 16453-16463 ◽  
Author(s):  
Winn Huynh ◽  
Matthew P. Conley
Keyword(s):  
Chemical Shift ◽  
Chemical Shifts ◽  
29Si Nmr ◽  
Surface Oxygen ◽  
Nmr Chemical Shift ◽  
Dft Methods ◽  
Nmr Chemical Shifts

The origin in deshielding of 29Si NMR chemical shifts in R3Si–X, where X = H, OMe, Cl, OTf, [CH6B11X6], toluene, and OX (OX = surface oxygen), as well as iPr3Si+ and Mes3Si+ were studied using DFT methods.

scholarly journals Is hydrogen electronegativity higher than Pauling’s value? New clues from the 13C and 29Si NMR chemical shifts of [CHF3] and [SiHF3] molecules

2019 ◽  
Vol 91 (10) ◽  
pp. 1679-1686
Author(s):  
Michele Benedetti ◽  
Federica De Castro ◽  
Antonella Ciccarese ◽  
Francesco Paolo Fanizzi
Keyword(s):  
Chemical Shift ◽  
Chemical Shifts ◽  
29Si Nmr ◽  
Nmr Chemical Shift ◽  
Nmr Chemical Shifts

Abstract We previously demonstrated that the δ NMR chemical shift of central NMR active atoms (A), in simple halido [AXn] (A=C, Si, Ge, Sn, Pb, Pt; Xn = combination of n halides, n = 4 or 6) derivatives, could be directly related to X radii overall sum, Σ(rL). Further correlation have also been observed for tetrahedral [AX4] (A=C, Si; X4 = combination of four halides) compounds where the X Pauling electronegativities sum, $\Sigma (\chi _L^{{\rm{Pau}}}),$ exceeds a specific value (≈12.4). In this work, we focused on these latter systems considering the H vs. X substitution. The analysis of the literature reported δ(13C) and δ(29Si) NMR chemical shift for the mono hydrogenated derivatives and in particular for [CHF3] and [SiHF3], characterized by the lowest Σ(rL) and the highest $\Sigma (\chi _L^{{\rm{Pau}}}),$ suggests a revised value for the H electronegativity ranking with respect to Pauling’s.

scholarly journals Carbon-13 NMR Chemical Shift of Methyl Group: A Useful Parameter for Structural Analysis of C-Methylated Flavonoids

2006 ◽  
Vol 1 (11) ◽  
pp. 1934578X0600101
Author(s):  
Pawan K. Agrawal ◽  
Chandan Agrawal ◽  
Shravan Agrawal
Keyword(s):  
Structural Analysis ◽  
Chemical Shift ◽  
Chemical Shifts ◽  
Nmr Chemical Shift ◽  
Methyl Group ◽  
Nmr Chemical Shifts ◽  
Nmr Study ◽  
Pterocarpus Santalinus ◽  
Group A ◽  
C Nmr

The 13C NMR resonances corresponding to the C-Me group of C-6 and/or C-8 C-methylated-flavonoids absorb between 6.7–10.0 ppm and typically between 6.7–8.7 ppm. A comparative 13C NMR study reflects that the 13C NMR chemical shifts reported for 6-hydroxy-5-methyl-3′,4′,5′-trimethoxyaurone-4-O-α-L-rhamnoside from Pterocarpus santalinus and 8-C-methyl-5,7,2′,4′- tetramethoxyflavanone from Terminalia alata are inconsistent with the assigned structures, and therefore need reconsideration.

Accurate and cost-effective NMR chemical shift predictions for proteins using a molecules-in-molecules fragmentation-based method

2020 ◽  
Vol 22 (47) ◽  
pp. 27781-27799
Author(s):  
Sruthy K. Chandy ◽  
Bishnu Thapa ◽  
Krishnan Raghavachari
Keyword(s):  
Chemical Shift ◽  
Quantum Chemical ◽  
Chemical Method ◽  
Chemical Shifts ◽  
Cost Effective ◽  
Quantum Chemical Method ◽  
Solvation Model ◽  
Nmr Chemical Shift ◽  
Nmr Chemical Shifts

We have developed a two-layer Molecules-in-Molecules (MIM2) fragmentation-based quantum chemical method including an efficient solvation model for the prediction of NMR chemical shifts with a target accuracy of ∼0.30 ppm for 1H and ∼2–3 ppm for 13C.

29Si NMR chemical shifts in pertrimethylsilylated 1,6-anhydro-β-D-glucopyranose derivatives. Empirical assignment from Hammett type dependence of the chemical shifts

1983 ◽  
Vol 48 (9) ◽  
pp. 2503-2508 ◽  
Author(s):  
Jan Schraml ◽  
Jaroslav Včelák ◽  
Miloslav Černý ◽  
Václav Chvalovský
Keyword(s):  
Chemical Shift ◽  
Carbon Atom ◽  
Silicon Atom ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
Polar Effect ◽  
29Si Nmr ◽  
Nmr Chemical Shifts ◽  
Trimethylsilyl Groups

29Si chemical shift of Si-3 silicon atom of the trimethylsiloxy group attached to C(3) carbon atom in 1,6-anhydro-β-D-glucopyranose derivatives correlates linearly with the sum of Taft polar constants σσ of substituents R2 and R4 on C(2) and C(4) carbon atoms. Quality of this correlation allows assignment of Si-3 line in the spectra of derivatives containing two or three trimethylsilyl groups in the molecule. The shielding order δ(Si-4) < δ(Si-3) found in 1,6-anhydro-2,3,4-O-tris(trimethylsilyl)-β-D-glucopyranose is the same as recently found in other pyranose derivatives with the same configuration of substituents but the order is reversed by strong polar effect of the substituent in 1,6-anhydro-2-O-p-toluenesulphonyl-3,4-O-bis(trimethylsilyl)-β-D-glucopyranose. This finding warns against indiscriminate use of empirical assignment rules based on simple shielding order without considering possible substituent effects.

NMR effective radius of hydrogen in XIV group hydrides evaluated by NMR spectroscopy

2017 ◽  
Vol 46 (41) ◽  
pp. 14094-14097 ◽  
Author(s):  
M. Benedetti ◽  
F. De Castro ◽  
A. Ciccarese ◽  
F. P. Fanizzi
Keyword(s):  
Nmr Spectroscopy ◽  
Chemical Shift ◽  
Chemical Shifts ◽  
Effective Radius ◽  
Nmr Chemical Shift ◽  
Ionic Radii ◽  
Nmr Chemical Shifts

In the [ABrnIm] (A = C, Si, Ge, Sn; n + m = 4) compounds, with the heavier halido ligands bonded to the central IV group elements, the 13C, 29Si, 73Ge and 119Sn NMR chemical shifts were found to be linearly related to the bonded halides ionic radii overall sum, ∑(rh). The 207Pb NMR chemical shift of the unstable [PbH4] hydride could be calculated.

scholarly journals Spektroskopische Untersuchungen an Cyclopentadienyl-dinitrosylvanadium-Komplexen CpV(NO)2L (L = Lewis-Base) / Spectroscopic Studies with Cyclopentadienyl Dinitrosylvanadium Complexes CpV(NO)2L (L = Lewis Base)

1982 ◽  
Vol 37 (5) ◽  
pp. 614-619 ◽  
Author(s):  
Max Herberhold ◽  
Herbert Trampisch
Keyword(s):  
Chemical Shift ◽  
Chemical Shifts ◽  
Spectroscopic Studies ◽  
Lewis Base ◽  
Nmr Chemical Shift ◽  
Nmr Chemical Shifts ◽  
Lewis Bases ◽  
H Nmr ◽  
Ligand Atom

Displacement of the CO ligand in CpV(NO)2CO (Cp = η5-cyclopentadienyl) by various Lewis bases (L) in solution leads to a series of (28) complexes CpV(NO)2L which were characterised by the 51V NMR chemical shift, the 13C and 1H NMR chemical shifts of the cyclopentadienyl ring, and by the NO stretching frequencies of the two nitrosyl ligands. The chemical shift δ(51V) varies over the range of ca. -1300 and -500 ppm depending on the nature of L, whereas δ(13C) of the cyclopentadienyl ring varies only between 98 and 102 ppm. The shielding of the 51V and 13C nuclei decreases as the electronegativity of the ligand atom bound to the metal increases in the order P <S <N <O

scholarly journals Carbon Atoms Speaking Out: How the Geometric Sensitivity of 13C Chemical Shifts Leads to Understanding the Colour Tuning of Phycocyanobilin in Cph1 and AnPixJ

Molecules ◽  
2020 ◽  
Vol 25 (23) ◽  
pp. 5505
Author(s):  
Sascha Jähnigen ◽  
Daniel Sebastiani
Keyword(s):  
Chemical Shift ◽  
Chemical Shifts ◽  
Explanatory Power ◽  
Nmr Chemical Shift ◽  
Correlation Pattern ◽  
Covalent Bonds ◽  
Absorption Frequency ◽  
13C Chemical Shifts ◽  
Nmr Chemical Shifts ◽  
Colour Tuning

We present a combined quantum mechanics/molecular mechanics (QM/MM) molecular dynamics–statistical approach for the interpretation of nuclear magnetic resonance (NMR) chemical shift patterns in phycocyanobilin (PCB). These were originally associated with colour tuning upon photoproduct formation in red/green-absorbing cyanobacteriochrome AnPixJg2 and red/far-red-absorbing phytochrome Cph1Δ2. We pursue an indirect approach without computation of the absorption frequencies since the molecular geometry of cofactor and protein are not accurately known. Instead, we resort to a heuristic determination of the conjugation length in PCB through the experimental NMR chemical shift patterns, supported by quantum chemical calculations. We have found a characteristic correlation pattern of 13C chemical shifts to specific bond orders within the π-conjugated system, which rests on the relative position of carbon atoms with respect to electron-withdrawing groups and the polarisation of covalent bonds. We propose the inversion of this regioselective relationship using multivariate statistics and to apply it to the known experimental NMR chemical shifts in order to predict changes in the bond alternation pattern. Therefrom the extent of electronic conjugation, and eventually the change in absorption frequency, can be derived. In the process, the consultation of explicit mesomeric formulae plays an important role to qualitatively account for possible conjugation scenarios of the chromophore. While we are able to consistently associate the NMR chemical shifts with hypsochromic and bathochromic shifts in the Pg and Pfr, our approach represents an alternative method to increase the explanatory power of NMR spectroscopic data in proteins.

The halogen effect on the 13C NMR chemical shift in substituted benzenes

2018 ◽  
Vol 20 (16) ◽  
pp. 11247-11259 ◽  
Author(s):  
Renan V. Viesser ◽  
Lucas C. Ducati ◽  
Cláudio F. Tormena ◽  
Jochen Autschbach
Keyword(s):  
Chemical Shift ◽  
13C Nmr ◽  
Chemical Shifts ◽  
Nmr Chemical Shift ◽  
Substituted Benzenes ◽  
Nmr Chemical Shifts ◽  
Halogen Effect ◽  
C Nmr ◽  
Group Effects

X (F, Cl, Br, I) and R (NH2, NO2) group effects on 13C NMR chemical shifts are explained by π and σ orbitals, respectively.

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