Proton Magnetic Resonance of the Methyl Derivatives of 2-Fluoropyridine. Sigma and Pi Electron Contributions to Spin–Spin Coupling Constants

1971 ◽  
Vol 49 (11) ◽  
pp. 1799-1803 ◽  
Author(s):  
J. B. Rowbotham ◽  
R. Wasylishen ◽  
T. Schaefer
Keyword(s):  
Nitrogen Atom ◽  
Proton Magnetic Resonance ◽  
Electron System ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Electron Contribution ◽  
Spin Coupling Constants ◽  
Methyl Derivatives ◽  
Spin Spin Coupling ◽  
Derivatives Of

The p.m.r. spectra of the methyl derivatives of 2-fluoropyridine are analyzed. The signs and magnitudes of the long-range spin–spin coupling constants between the methyl protons and the ring protons and between the methyl protons and fluorine are consistent with a model in which the nitrogen atom polarizes the sigma electron system but leaves the pi electron contribution to the coupling constants relatively unchanged. There are dramatic changes in the ring proton – fluorine couplings while the couplings involving the methyl protons vary little from those in the corresponding toluene derivatives. Thus the coupling over six bonds between fluorine and methyl protons is 1.25 ± 0.03 Hz in 2-fluoro-5-methyl pyridine compared to 1.15 ± 0.02 Hz in p-fluorotoluene.

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.

A proton magnetic resonance determination of the small rotational barriers about the C—Si bond in phenyl derivatives of chloro and methyl silanes

1977 ◽  
Vol 55 (3) ◽  
pp. 557-561 ◽  
Author(s):  
William J. E. Parr ◽  
Ted Schaefer
Keyword(s):  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of ◽  
Low Energy Conformations

The long-range spin–spin coupling constants between protons bonded to silicon and ring protons in C6H5SiH3, C6H5SiH2Cl, C6H5SiH2CH3, C6H5SiHCl2, and C6H5SiH(CH3)2 are determined from the proton magnetic resonance spectra of benzene solutions. A hindered rotor treatment of the barrier to internal rotation about the C—Si bond, in conjunction with the coupling constants over six bonds, allows the deduction of the low-energy conformations for C6H5SiH(CH3)2 and for C6H5SiHCl2, as well as of barriers of 1.0 ± 0.2 kcal/mol. The approach becomes less reliable for C6H5SiH2CH3 and for C6H5SiH2Cl and, particularly for the latter compound, the derived barrier is very likely an upper limit only. Ab initio molecular orbital calculations of the conformational energies are reported for C6H5SiH3, C6H5SiH2Cl, and for C6H5SiHCl2.

Methyl–Methyl Proton Spin–Spin Coupling Constants in Some Six-membered Ring Compounds. A Test for Delocalization in Borazine

1974 ◽  
Vol 52 (3) ◽  
pp. 489-496 ◽  
Author(s):  
J. Brian Rowbotham ◽  
T. Schaefer
Keyword(s):  
Electron System ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Substitution Pattern ◽  
Methyl Proton ◽  
Ring Compounds ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of

Spin–spin coupling constants over five, six, and seven bonds between protons in different methyl groups are reported for the xylenes, derivatives of benzene, pyridine, pyrimidine, pyridinium salts, p-benzoquinone, and borazine. The coupling magnitudes are characteristic of the substitution pattern in the delocalized systems. Calculations at the INDO-MO-FPT level of methyl proton couplings in N- and B-methylborazines are in agreement with the available experimental evidence in indicating a relatively weak transmission of spin density information via the presumed π electron system of borazine.

The Effect of α-Substituents on Benzylic Spin–Spin Coupling Constants. Derivatives of 2,6-Dichlorotoluene

1971 ◽  
Vol 49 (11) ◽  
pp. 1818-1822 ◽  
Author(s):  
A. F. Janzen ◽  
T. Schaefer
Keyword(s):  
Potential Energy ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Energy Calculations ◽  
Potential Energy Calculations ◽  
Electron Contribution ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of

Spin–spin coupling constants between methylene protons and ring protons over six and five bonds, [Formula: see text], are reported for α-substituted derivatives of 2,6-dichlorotoluene. Potential energy calculations suggest that the C—X bond of the CH2X group lies perpendicular to the plane of the ring. The six-bond coupling is dominated by a π electron mechanism and depends on the conformation of CH2X with respect to the aromatic plane and on the electronegativity of the substituent, X. The five-bond coupling is independent of X and it is suggested that the decrease in the π electron contribution to this coupling caused by the increasing electronegativity of X, is compensated by a corresponding increase in the sigma electron contribution.

Experimental and Theoretical Estimates of Sigma and Pi Electron Contributions to Long-range Spin–Spin Coupling Constants in Coumarin and its Methyl Derivatives

1973 ◽  
Vol 51 (6) ◽  
pp. 953-960 ◽  
Author(s):  
J. B. Rowbotham ◽  
T. Schaefer
Keyword(s):  
Valence Bond ◽  
Electron System ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Orbital Theory ◽  
Coupling Path ◽  
Spin Coupling Constants ◽  
Methyl Derivatives ◽  
Spin Spin Coupling

A full analysis of the p.m.r. spectra of coumarin and five of its methyl derivatives allows the assignment of σ and π electron components to the inter-ring proton–proton spin–spin coupling constants. Magnitudes of up to 0.3 Hz are observed over a coupling path of eight bonds. The σ and π components can be combined on the basis of the appropriate valence bond structures to predict closely the observed parameters in naphthalene, suggesting only weak participation of the ether oxygen atom in the π electron system of coumarin. Calculations of the inter-ring coupling constants at the INDO and CNDO/2 levels of molecular orbital theory are helpful in the interpretation of the observed data, the agreement with experiment being quantitative in many instances. Comparison of the observed coupling constants in coumarin and styrene with the INDO results for an assumed planar form of the latter suggests that styrene exists in a nonplanar conformation.

Proton Magnetic Resonance Studies of Rotational Isomerism in Halotoluenes VIII. Stereospecific 1H–9F Coupling Constants and Conformations of some Fluoro and Chloro Derivatives of Benzalchloride

1971 ◽  
Vol 49 (12) ◽  
pp. 2033-2036 ◽  
Author(s):  
H. M. Hutton ◽  
J. B. Rowbotham ◽  
B. H. Barber ◽  
T. Schaefer
Keyword(s):  
Proton Magnetic Resonance ◽  
Rotational Isomerism ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Side Chain ◽  
Fluorine Nucleus ◽  
Spin Coupling Constants ◽  
Chloro Derivatives ◽  
Spin Spin Coupling ◽  
Derivatives Of

In solution the 2-fluoro-5-chloro-, 2-fluoro-6-chloro-3-nitro-, 2-fluoro-6-chloro-5-nitro-, and 2,4,5-trichloro-benzalchlorides prefer conformations in which the C— bond of the side-chain lies in the plane of the aromatic ring. This C—H bond eclipses that ortho C—X bond (X = H, F, Cl) in which X is smaller than Y = H, F, Cl in the C—Y bond, also ortho to the dichloromethyl group. The long-range spin–spin coupling constants between the proton in the side-chain and the ring protons or fluorine nucleus are stereospecific. In particular, the coupling over four bonds between the side-chain proton and the ring fluorine is −0.3 Hz when the C—H and C—F bonds are arranged cis to each other but is −2.5 Hz when these bonds have a transoid planar arrangement.

scholarly journals Signs of proton–proton and proton–fluorine spin–spin coupling constants in 2-fluoro-3-methylpyridine. Sigma and pi contributions to coupling in a nitrogen heterocycle

1969 ◽  
Vol 47 (9) ◽  
pp. 1507-1514 ◽  
Author(s):  
T. Schaefer ◽  
S. S. Danyluk ◽  
C. L. Bell
Keyword(s):  
Nitrogen Atom ◽  
Long Range ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Nitrogen Heterocycle ◽  
Triple Resonance ◽  
Triple Resonance Experiments ◽  
Proton Proton ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The signs of all proton–proton and proton–fluorine spin–spin coupling constants in 2-fluoro-3-methylpyridine have been determined by double and triple resonance experiments. The signs of the longrange coupling constants, JH,CH3 and JF,CH3 are the same as in fluorotoluene derivatives. Their magnitudes are consistent with the assumption that the nitrogen atom primarily polarizes the σ bonds in the molecule, leaving the π contribution to the long-range coupling relatively unaffected.

Proton spin–spin coupling constants and intramolecular hydrogen bonding in bromine derivatives of 1,3-dihydroxybenzene

1976 ◽  
Vol 54 (14) ◽  
pp. 2228-2230 ◽  
Author(s):  
Ted Schaefer ◽  
J. Brian Rowbotham
Keyword(s):  
Hydrogen Bond ◽  
Hydrogen Bonding ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Hydroxyl Groups ◽  
Oh Groups ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of

The conformational preferences in CCl4 solution at 32 °C of the hydroxyl groups in bromine derivatives of 1,3-dihydroxybenzene are deduced from the long-range spin–spin coupling constants between hydroxyl protons and ring protons over five bonds. Two hydroxyl groups hydrogen bond to the same bromine substituent in 2-bromo-1,3-dihydroxybenzene but prefer to hydrogen bond to different bromine substituents when available, as in 2,4-dibromo-1,3-dihydroxybenzene. When the OH groups can each choose between two ortho bromine atoms, as in 2,4,6-tribromoresorcinol, they apparently do so in a very nearly statistical manner except that they avoid hydrogen bonding to the common bromine atom.

One-bond CC spin—spin coupling constants in derivatives of benzene. Non-linearity of 1J(CC) vs substituent electronegativity

1993 ◽  
Vol 49 (11) ◽  
pp. 1613-1619 ◽  
Author(s):  
Krystyna Kamieńska-Trela ◽  
Andrzej Dabrowski ◽  
Henryk Januszewski
Keyword(s):  
Coupling Constants ◽  
Spin Coupling ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of
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