Analysis of the NMR Spin−Spin Coupling Mechanism Across a H−Bond:  Nature of the H-Bond in Proteins

2004 ◽  
Vol 108 (3) ◽  
pp. 1115-1129 ◽  
Author(s):  
Tell Tuttle ◽  
Jürgen Gräfenstein ◽  
Anan Wu ◽  
Elfi Kraka ◽  
Dieter Cremer
Keyword(s):  
Spin Coupling ◽  
Coupling Mechanism ◽  
Spin Spin Coupling

A solid-state 31P NMR study of 1:1 silver–triphenylphosphine complexes — Interpretation of 1J(107,109Ag,31P) values,

2009 ◽  
Vol 87 (7) ◽  
pp. 1090-1101 ◽  
Author(s):  
Fu Chen ◽  
Se-Woung Oh ◽  
Roderick E. Wasylishen
Keyword(s):  
Solid State ◽  
Spin Coupling ◽  
Coupling Mechanism ◽  
P Values ◽  
X Ray Crystallography ◽  
Phosphine Complexes ◽  
Nmr Study ◽  
Fermi Contact ◽  
Spin Spin Coupling ◽  
Cp Mas Nmr

High-resolution solid-state 31P NMR spectroscopy was used to investigate a series of 1:1 silver–triphenylphosphine complexes, [Ph3PAgX]n, where X is a monovalent anion and n = 1, 2, 3, 4, or ∞. The 31P CP MAS NMR spectra reveal the number of distinct phosphorus sites in these complexes as well as the |1J(109Ag,31P)| values, which range from 401 ± 10 Hz (X = N3–) to 869 ± 10 Hz (X = SO3CF3–). The data obtained here and in earlier investigations indicate that |1J(109Ag,31P)| values for silver–tertiary phosphine complexes decrease as Ag–P bond lengths increase. This experimental conclusion is supported by DFT calculations, which also indicate that the Fermi-contact mechanism is the only important spin–spin coupling mechanism for 1J(109Ag,31P) in these complexes. In addition, the crystal structure of a silver–triphenylphosphine trifluoroacetate tetramer was determined using X-ray crystallography, and the structure of a silver–triphenylphosphine chloride tetramer was reinvestigated.

Long-range proton-fluorine spin-spin coupling. Further evidence for a 'direct' mechanism

1973 ◽  
Vol 26 (12) ◽  
pp. 2659 ◽  
Author(s):  
W Adcock ◽  
SQA Rizvi
Keyword(s):  
Long Range ◽  
Chemical Shifts ◽  
Variable Temperature ◽  
Strong Support ◽  
Spin Coupling ◽  
Coupling Mechanism ◽  
Direct Mechanism ◽  
Spin Spin Coupling

A variable temperature p.m.r. study of outho- and peri-acetyl substituted fluoro-naphthalenes (including o-fluoroacetophenone) has been carried out. Further, 1-fluoro- 8-methylnaphthalene (previously unknown) has been synthesized and its p.m.r. spectra has been measured at various temperatures. The data, together with 19F substituent chemical shifts (SCS) for ortho-substituted cyanofluoronaphthalenes, provide further strong support for a through-space coupling mechanism (JMe,F).

Spin–spin coupling constants between side-chain and ring fluorine nuclei in some benzotrifluoride, benzal fluoride, and benzyl fluoride derivatives: coupling mechanisms

1979 ◽  
Vol 57 (7) ◽  
pp. 807-812 ◽  
Author(s):  
Ted Schaefer ◽  
Walter Niemczura ◽  
Chiu-Ming Wong ◽  
Kirk Marat
Keyword(s):  
Nmr Spectra ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Complete Analysis ◽  
Side Chain ◽  
Coupling Mechanism ◽  
Conformational Preferences ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Coupling Mechanisms

A complete analysis of the 1H and 19F nmr spectra of 2,5- and 3,4-difluorobenzotrifluoride, together with multiple resonance experiments, yields the signs and magnitudes of the long-range 19F,19F and 1H,19F spin–spin coupling constants. The coupling mechanisms are discussed. In particular, the coupling over six bonds, [Formula: see text], whose sign is interpretable in terms of a σ–π mechanism, is too large in magnitude when compared to [Formula: see text], and [Formula: see text] in the analogous compounds. These latter three couplings are consistent in sign and magnitude with what is known about hyperfine interaction constants. The magnitudes of [Formula: see text] are reported for 4-fluorobenzotrifluoride, 3-amino-4-fluorobenzotrifluoride, 3-nitro-4-fluorobenzotrifluoride, as are 6JpF,F values for p-fluorobenzal fluoride and p-fluorobenzyl fluoride. In contrast to 6JpH,CH and 6JpF,CH it seems unlikely that, unless its coupling mechanism becomes more precisely understood, 6JpF,CF will be a reliable indicator of conformational preferences.

Proton magnetic resonance spectra of 3-fluorotoluene and 2-chloro-5-fluorotoluene. The sign and coupling mechanism of

1978 ◽  
Vol 56 (17) ◽  
pp. 2233-2236 ◽  
Author(s):  
Ted Schaefer ◽  
Werner Danchura ◽  
Walter Niemczura
Keyword(s):  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Spin Coupling ◽  
Coupling Mechanism ◽  
Coupling Constant ◽  
Electron Component ◽  
Electron Contribution ◽  
Full Analysis ◽  
Spin Spin Coupling ◽  
Magnetic Resonance Spectra

A full analysis of the proton magnetic resonance spectra of 3-fluorotoluene and of 2-chloro-5-fluorotoluene, as 10 mol% solutions in CS2, demonstrates that the long-range spin–spin coupling constant over five bonds between methyl protons and fluorine-19 is negative. The coupling mechanism consists of a large positive σ electron component and a negative π electron component. The negative sign of the π electron contribution arises from a spin density in the 2pz orbital at carbon-3, which is opposite in sign to that of the spin densities at C-2 and C-4. Combined with positive hyperfine interaction constants, QCCH and QCF, the consequence is a negative π electron component.

The negative 7J(CHO, CH3) in 4-methylbenzaldehyde. Ionicity of the carbonyl bond

1992 ◽  
Vol 70 (10) ◽  
pp. 2555-2557
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian
Keyword(s):  
First Principles ◽  
Valence Bond ◽  
Spin Coupling ◽  
Coupling Mechanism ◽  
Unexpected Result ◽  
Coupling Constant ◽  
Valence Bond Structure ◽  
Carbonyl Bond ◽  
Spin Spin Coupling ◽  
Benzaldehyde Derivatives

The spin–spin coupling constant over seven bonds between the formyl and methyl protons in 4-methylbenzaldehyde is −0.030 Hz in CS2/C6D12/TMS, and (−)0.035 Hz in acetone-d6, solutions at 297 K. This unexpected result is rationalized in terms of a spin–spin coupling mechanism attributed to the importance of a valence bond structure with an ionic carbonyl bond. The result again emphasizes the sensitivity to substituent perturbations of the six-bond coupling constant in quasi-planar benzaldehyde derivatives. It can have either sign and presents a challenge to its computation from first principles.

Sigma and Pi Electron Contributions to Long-range Spin–Spin Coupling Constants in the Methyl Derivatives of the Fluoropyridines

1972 ◽  
Vol 50 (14) ◽  
pp. 2344-2350 ◽  
Author(s):  
J. B. Rowbotham ◽  
T. Schaefer
Keyword(s):  
Nitrogen Atom ◽  
Long Range ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Coupling Mechanism ◽  
Methyl Group ◽  
Spin Coupling Constants ◽  
Methyl Derivatives ◽  
Spin Spin Coupling ◽  
Derivatives Of

Seven methyl derivatives of the 3- and 4-fluoropyridines are synthesized and their p.m.r. spectra are analyzed. The nuclear spin–spin coupling constants are compared with previous results for the four methyl derivatives of 2-fluoropyridine. A model in which the nitrogen atom polarizes primarily the σ electron system of the ring, leaving the π electron contribution to the coupling mechanism relatively unaffected, qualitatively accounts for the large majority of the coupling constants. For example, the coupling over six bonds between methyl protons and a fluorine nucleus, [Formula: see text] is the same whether the fluorine atom or the methyl group is placed ortho to the nitrogen atom and is little different from its value in p-fluorotoluene. The model is consistent with significant σ electron contributions to long-range couplings over four and five bonds from methyl protons to fluorine nuclei or ring protons. Evidence is adduced for resonance structures in which fluorine conjugates with nitrogen or with ring carbon atoms. An earlier suggestion, that hyperconjugation of the methyl group with nitrogen is necessary to the interpretation of the observed couplings, is dropped. Instead, a substantial polarization of the σ electron core near C-2 and -6 is invoked but apparently does not extend appreciably beyond these atoms in the ring.

The carbonyl group as a reluctant transmitter of hyperconjugative or σ–π spin–spin coupling interactions in derivatives of benzaldehyde, acetophenone, and benzophenone

1986 ◽  
Vol 64 (9) ◽  
pp. 1859-1863 ◽  
Author(s):  
Ted Schaefer ◽  
James Peeling ◽  
Glenn H. Penner ◽  
Alberta Lemire ◽  
Reino Laatikainen
Keyword(s):  
Opposite Sign ◽  
Coupling Constants ◽  
The Other ◽  
Spin Coupling ◽  
Coupling Mechanism ◽  
State Information ◽  
Spin Polarizability ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of

Unlike their counterparts in anisole or toluene derivatives, the six-bond spin–spin coupling constants between para ring protons or 19F nuclei and protons or 13C nuclei in the sidechain of derivatives of benzaldehyde, acetophenone, and benzophenone can apparently contain components of opposite sign, at least for the fluorine derivatives. The σ–π components are much smaller in magnitude than in toluene derivatives, leading to very small or unobservable coupling constants. Consequently they are of limited use in conformational analysis. INDO MO FPT computations and their modifications are examined as to the reasons for the small σ–π magnitudes. Although the spin polarizability of the 2pz orbital on oxygen appears to play an important role in the transmission of nuclear spin state information, the computations do not account for a 19F coupling mechanism that appears to be significant for planar conformations. On the other hand, spin–spin coupling constants over five formal bonds to meta protons are sizeable and stereospecific.

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