Information from stereospecific spin–spin couplings about the relative absorptivities of the hydroxyl stretching bands in 2-iodophenol solutions

1981 ◽  
Vol 59 (21) ◽  
pp. 3021-3025 ◽  
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian ◽  
Timothy A. Wildman
Keyword(s):  
Free Energy ◽  
Molecular Orbital ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Temperature Dependent ◽  
Hydroxyl Proton ◽  
Spin Coupling Constants ◽  
Cis And Trans ◽  
Spin Spin Coupling

The stereospecific spin–spin coupling constants between the hydroxyl proton and the ring protons for 2-iodophenol in various solvents yield some free energy differences between the cis and trans conformations of this molecule at 305 K. Comparison with areas of the hydroxyl stretching bands in the same or similar solvents shows that the ratio of the absorptivity coefficients for the two conformers is sensitive to solvent. It is suggested that this ratio is temperature dependent and therefore apparent enthalpy differences must be considered tentative for at least some solutions. Molecular orbital calculations are consistent with the arguments concerning the absorptivity coefficients.

INDO Molecular Orbital Calculations of Nuclear Spin–Spin Coupling Constants Over Three Bonds Between 13C and 1H in Some Simple Molecules

1973 ◽  
Vol 51 (6) ◽  
pp. 961-973 ◽  
Author(s):  
R. Wasylishen ◽  
T. Schaefer
Keyword(s):  
Dihedral Angle ◽  
Molecular Orbital ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Methyl Proton ◽  
Coupling Path ◽  
Spin Coupling Constants ◽  
Simple Molecules ◽  
Spin Spin Coupling

Molecular orbital calculations, at the INDO level, of the spin–spin coupling constants over three bonds between carbon-13 and protons are reported for a number of simple molecules. In propane the coupling depends on dihedral angle in the Karplus manner. Fluorine substituents cause changes in the computed coupling which are best described as alternating with the number of bonds intervening between the substituent and the coupled nuclei. Finer details of this phenomenon are discussed and calculations on propyllithium are performed. Replacement of a central carbon atom in propane by a heteroatom does not radically alter the computed couplings. The presence of a carbonyl group in the coupling path results in an overestimate of the magnitude of the coupling. In propene the coupling between 13C in position 1 and a methyl proton displays a maximum when the C—H bond of the methyl group lies parallel to the π orbitals. In toluene the coupling to a methyl proton is insensitive to the dihedral angle over half its range, a result of importance to structural studies. Among other molecules under consideration are methylacetylene, propionaldehyde, and the strained bicyclobutane. It is suggested that in certain instances the mean of the predictions from the INDO and CNDO/2 procedures may agree better with experiment than will the prediction from either procedure alone. Calculations on fluorobenzene and 1,2-difluorobenzene suggest that the main experimental trends of the couplings between carbon and protons within the benzene ring are reproduced. Such is perhaps not true for the five-membered heterocycles.

Mechanisms of 1H,1H; 1H,13C; and 13C,13C spin–spin coupling constants in benzyl cyanide and some derivatives. Experimental and theoretical estimates of internal rotational potentials

1986 ◽  
Vol 64 (10) ◽  
pp. 2013-2020 ◽  
Author(s):  
Ted Schaefer ◽  
Glenn H. Penner
Keyword(s):  
Molecular Orbital ◽  
Cyano Group ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Similar Data ◽  
Orbital Theory ◽  
Benzyl Cyanide ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The mechanisms of long-range spin–spin coupling constants involving the methylene protons and the 13C nucleus of the cyano group are discussed for benzyl cyanide. Analysis of the 1H nmr spectrum of benzyl cyanide-8-13C in benzene-d6 solution yields nJ(H,CH2) and nJ(H,13CN) for n = 4–6. Similar data are reported for the 2,6-dichloro and 2,6-difluoro derivatives, together with some sign determinations. nJ(13C,13CN), n = 1–5, are given for the three compounds. It is shown that all these parameters are consistent with a small barrier to internal rotation about the [Formula: see text] bond in benzyl cyanide in solution. Computations at various levels of molecular orbital theory agree that this barrier is small. The nJ(13C, 13CN) imply a stabilization in polar solvents of the conformation in which the cyano group of benzyl cyanide lies in a plane perpendicular to the benzene plane. The molecular orbital calculations indicate a predominantly twofold nature of the internal barrier, although a significant fourfold component is also present. The coupling constants cannot discern the presence of the fourfold component for benzyl cyanide nor for its 2,6-difluoro derivative. 1J(13C,13CN) is solvent dependent. A table of the computed sidechain geometries is appended.

scholarly journals Proton Magnetic Resonance Spectrum of 1-Penten-3-yne. INDO and CNDO/2 Molecular Orbital Calculations on Long-Range Spin–Spin Coupling Constants in Enynes

1972 ◽  
Vol 50 (12) ◽  
pp. 1863-1867 ◽  
Author(s):  
L. Ernst ◽  
H. M. Hutton ◽  
T. Schaefer
Keyword(s):  
Molecular Orbital ◽  
Long Range ◽  
Benzene Solution ◽  
Parent Compound ◽  
Proton Magnetic Resonance Spectrum ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The high resolution p.m.r. spectra of 1-penten-3-yne in carbon disulfide and in benzene solution are analyzed. Long-range spin–spin coupling constants are discussed in terms of σ and π electron contributions. Comparisons are made with the isomeric 2-methyl-1-buten-3-yne and the parent compound, vinylacetylene. The results of INDO and CNDO/2 molecular orbital calculations are compared to the experimental coupling constants. It is concluded that the π electron contribution to 5J in enyne systems is +0.6 to 0.7 Hz and that σ electron contributions are rather small, the transoid ("pseudo-zig–zag") being larger than the cisoid one. Observed allylic coupling constants in the propene derivative are compared with the calculated values, including those for propene and 2-cyanopropene, available in the literature.

Nuclear Magnetic Resonance Spectra, Conformations, Spin Coupling Mechanisms, and INDO Molecular Orbital Calculations for the Monofluorobenzaldehydes and some Derivatives

1971 ◽  
Vol 49 (19) ◽  
pp. 3216-3228 ◽  
Author(s):  
R. Wasylishen ◽  
T. Schaefer
Keyword(s):  
Magnetic Resonance ◽  
Molecular Orbital ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Conformational Preferences ◽  
Coupling Parameters ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Magnetic Resonance Spectra

Precise analyses of the proton and some fluorine magnetic resonance spectra in acetone solution are reported for the three monofluorobenzaldehydes as well as for 2-chloro-6-fluorobenzaldehyde and for 4-fluoro-2-nitrobenzaldehyde. The conformational dependence of the coupling parameters allows the measurement of energy differences between the O-cis and O-trans conformations. The energy differences are in better agreement with the INDO predictions than they are with energies derived from i.r. data. Di-pole moments are computed reliably and their measurement is suggested as a good guide to conformational preferences for molecules of this kind. The spin–spin coupling constants between the aldehyde proton and the ring protons and fluorine nuclei are computed for benzaldehyde and the three monofluorobenzaldehydes by the INDO and CNDO molecular orbital approximations. In many instances the agreement between calculated and observed couplings is quantitative.

On the existence of cis and trans conformational isomers in 2-methylphenol and 4-chloro-2-methylphenol

1978 ◽  
Vol 56 (13) ◽  
pp. 1788-1791 ◽  
Author(s):  
Ted Schaefer ◽  
Kalvin Chum
Keyword(s):  
Free Energy ◽  
Energy Difference ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Hydroxyl Group ◽  
Trans Form ◽  
Hydroxyl Proton ◽  
Self Association ◽  
Cis And Trans ◽  
Spin Spin Coupling

The analysis of the proton magnetic resonance spectra of 2-methylphenol and of 4-chloro-2-methylphenol in benzene-d6 and CCl4 solutions yields spin–spin coupling constants over five bonds between the hydroxyl proton and the meta ring protons. These coupling constants are related to the intramolecular equilibrium between conformers in which the hydroxyl group is oriented cis or trans to the methyl group. The free energy difference between the conformers is 0.4 ± 0.1 kcal/mol, with the trans form being the more stable. The concentration dependence of the hydroxyl proton chemical shift of the chloro compound in CCl4 suggests that dimerization is relatively unimportant and yields rough values for the extent of self-association. It is argued that the derived free energy values refer to the intramolecular equilibrium for the monomers.

Experimental and theoretical estimates of the internal rotational barrier of benzyl fluoride in the vapor phase

1990 ◽  
Vol 68 (4) ◽  
pp. 581-586 ◽  
Author(s):  
Ted Schaefer ◽  
Christian Beaulieu ◽  
Rudy Sebastian ◽  
Glenn H. Penner
Keyword(s):  
Molecular Orbital ◽  
Vapor Phase ◽  
Phenyl Group ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Basis Sets ◽  
Molecular Orbital Calculations ◽  
Stable Conformer ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The twofold barrier to rotation about the [Formula: see text] bond in benzyl fluoride is deduced from the long-range 1H,1H; 1H,19F; and 13C,19F nuclear spin–spin coupling constants in solution. The barrier changes from 3.2(2) kJ/mol in the polar solvent, acetonitrile-d3, to 0.7(2) kJ/mol in the nonpolar environment provided by cyclohexane-d12. In all solutions the conformer of greatest stability has the C—F bond in a plane perpendicular to that of the phenyl group. Extrapolation of the barrier to the vapor phase, using a simple reaction field model, indicates that the most stable conformer for the free (unclustered) molecule is now that with the C—F bond in the phenyl plane and that the barrier to internal rotation is 1.1(7) kJ/mol. Molecular orbital calculations with the basis sets STO-3G, 4-21G, 4-31G, 6-31G, and 6-31G* all predict the latter conformer as that of lowest energy. However, they disagree significantly among themselves as to the height of the internal barrier. The complete geometries are given for both conformers, as computed with the 6-31G basis, and the side-chain geometries are tabulated for the planar and perpendicular conformers, as given by all the bases. Keywords: benzyl fluoride, internal rotational potential; 13C,19F spin–spin coupling constants in benzyl fluoride; benzyl fluoride, molecular orbital computations.

scholarly journals The para substituent dependence of the internal rotational barrier in benzenethiol

1977 ◽  
Vol 55 (3) ◽  
pp. 552-556 ◽  
Author(s):  
Ted Schaefer ◽  
William J. E. Parr
Keyword(s):  
Ab Initio ◽  
Internal Rotation ◽  
Molecular Orbital ◽  
Rotational Barrier ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Fluorine Nucleus ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

On the basis of the observed spin–spin coupling constants between the sulfhydryl and ring protons and a hindered rotor treatment of the twofold barrier to internal rotation in a series of para substituted benzenethiol derivatives, it is argued that V2 is essentially zero in p-amino-benzenethiol and is 2.5 ± 0.2 kcal/mol in p-nitrobenzenethiol; having intermediate values for the methoxy, fluoro, methyl, and bromo derivatives in solution. The results are based on an assumed relationship between the four-bond and the fictitious six-bond couplings to the sulfhydryl proton. The conclusions are consistent with the observed magnitudes of the couplings over six and seven bonds, respectively, between the sulfhydryl proton and the fluorine nucleus and the methyl protons in the appropriate derivatives; as well as with the coupling between the sulfhydryl and methyl protons in 4-bromo-3-methylbenzenethiol. The experimental barriers are compared with ab initio molecular orbital calculations of their substituent dependence.

scholarly journals Nuclear magnetic resonance and molecular orbital studies of the conformational preferences of the fluoromethyl group in some benzylfluoride derivatives

1976 ◽  
Vol 54 (8) ◽  
pp. 1322-1328 ◽  
Author(s):  
Ted Schaefer ◽  
J. Brian Rowbotham ◽  
William J. E. Parr ◽  
Kirk Marat ◽  
Alexander F. Janzen
Keyword(s):  
Magnetic Resonance ◽  
Molecular Orbital ◽  
Fluorine Atom ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Dipole Interactions ◽  
Conformational Preferences ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The proton magnetic resonance spectra of some benzylfluoride derivatives yield long-range spin–spin coupling constants between ring protons and protons or fluorine nuclei in the fluoromethyl group. In conjunction with the eigenfunctions for a hindered twofold rotor, the couplings over six bonds are used to show that in 3,5-dichlorobenzylfluoride in solution the C—F bond prefers the benzene plane by 260 ± 50 cal/mol; in close agreement with ab initia and MINDO/3 molecular orbital calculations. The latter method suggests that in a conformation in which the C—F bond lies in a plane perpendicular to the benzene ring, the C – C – F angle reduces to 107.2° and the C – C – H angles become 116.1°, perhaps due to increased conjugation of the C—F bond or fluorine atom with the π electrons of the ring. The observed barrier is presumably a delicate balance between steric interactions, hyperconjugation or p–p conjugation effects, and dipole–dipole interactions between polarized bonds.

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