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.

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.

In-plane and out-of-plane conformational preferences of the sulfhydryl group in some halothiophenol derivatives

1979 ◽  
Vol 57 (12) ◽  
pp. 1421-1425 ◽  
Author(s):  
Ted Schaefer ◽  
William J. E. Parr
Keyword(s):  
Sulfhydryl Group ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Hydroxyl Group ◽  
Recent Analysis ◽  
Phenol Derivatives ◽  
Out Of Plane ◽  
Intramolecular Hydrogen ◽  
Cis And Trans ◽  
Spin Spin Coupling

Long-range spin–spin coupling constants between sulfhydryl protons and ring protons in some halothiophenol derivatives in CCl4 solutions are reported. In contrast to the corresponding phenol derivatives, substantial amounts of out-of-plane conformers are present at 305 K for all but 2,6-dichlorothiophenol. The cis and trans conformers differ by only about 0.2 kcal/mol in free energy for 2,4-dibromothiophenol and 2,4-dichlorothiophenol, in good agreement with a recent analysis of the dipole moment observed for the latter compound. The barrier to internal rotation of the sulfhydryl group is considerably smaller than for a hydroxyl group and rough estimates are given for the barrier in a few compounds. For example, the barrier in 2,3,5,6-tetrafluorothiophenol is lower than in 2,6-dichlorothiophenol. STO-3G MO calculations overestimate the internal barrier to rotation of the sulfhydryl group, but yield charge densities for this group which indicate that a major cause of the relative weakness of its intramolecular hydrogen bonds resides in its lack of polarity.

Theoretical and experimental conformational preferences of the aldehyde and fluoroaldehyde groups in 2-methylbenzaldehyde, 2-trifluoromethylbenzaldehyde, and 4-chloro-2-methylbenzoyl fluoride. Proximate couplings

1980 ◽  
Vol 58 (22) ◽  
pp. 2364-2368 ◽  
Author(s):  
Ted Schaefer ◽  
Salman R. Salman ◽  
Timothy A. Wildman
Keyword(s):  
Free Energy ◽  
Benzene Solution ◽  
Energy Difference ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Mo Calculations ◽  
Conformational Preferences ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Anti Form

On the basis of long-range spin–spin coupling constants, the O-syn conformation of 2-methylbenzaldehyde in CCl4 solution at 305 K is favored over the O-anti form by a free energy of 0.53 kJ/mol. This number is compatible with other experiments, as well as with STO-3G MO calculations in which the geometry of the substituents is optimized. The latter yield 0.52 kJ/mol in the internal energy difference. In benzene solution, 2-trifluoromethylbenzaldehyde exists in the O-anti form to the extent of at least 95% at 305 K. In CCl4 solution at this temperature, the population of the O-syn conformer of 4-chloro-2-methylbenzoyl fluoride is likely 75% or more of the total, in semiquantitative agreement with STO-3G optimization procedures. Substantial proximate couplings exist between 1H and 19F nuclei in the sidechains of the latter two compounds and are compared with INDO MO FPT computations. These yield negative values for [Formula: see text] in 2-trifluoromethylbenzaldehyde, whereas the experimental value is 2.23 Hz.

A proton magnetic resonance estimate of the extent of intramolucular hydrogen bonding in derivatives of 2-trifluoromethyl phenol. Solvent effects

1976 ◽  
Vol 54 (14) ◽  
pp. 2243-2248 ◽  
Author(s):  
Ted Schaefer ◽  
J. Brian Rowbotham
Keyword(s):  
Free Energy ◽  
Hydrogen Bonding ◽  
Vapor Phase ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Hydroxyl Group ◽  
Spin Coupling Constants ◽  
Hydroxyl Protons ◽  
Spin Spin Coupling ◽  
Derivatives Of

The long-range spin–spin coupling constants between hydroxyl protons and ring protons or fluorine nuclei are used to establish the conformer populations in iodine and brornine derivatives of 2-trifluoromethylphenol in C6H12, CCl4, and C6D6 solutions. The sequence Cl, [Formula: see text] is established for the so-called hydrogen bonding preferences of the hydroxyl group in 2,4,6-trisubstituted phenols, the corresponding free energy sequence being −ΔG = 1690, 1690 > 1300 > 1230 > 0 ± 200 cal/mol at 32 °C in CCl4 solution. An indirect estimate of the free energy differences in the vapor phase suggests the sequence −ΔG = 2800, 2800 > 2400 > 2300 > 1100 ± 300 cal/mol; the latter value meaning that the hydroxyl group in 4-bromo-2-trifluoromethylphenol prefers the CF3 group by this amount in the vapor phase. Benzene interacts preferentially with the OH group in this compound to the extent of 1300 cal/mol (ΔG), referenced to the vapor phase.

Comparison of the intramolecular hydrogen bonds and of the internal barrier to rotation the hydroxyl and sulfhydryl groups in 2-methoxyphenol and 2-methoxythiophenol

1979 ◽  
Vol 57 (4) ◽  
pp. 450-453 ◽  
Author(s):  
Ted Schaefer ◽  
Timothy A. Wildman
Keyword(s):  
Potential Function ◽  
Long Range ◽  
Sulfhydryl Group ◽  
Far Infrared ◽  
Energy Difference ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Hydroxyl Group ◽  
Intramolecular Hydrogen ◽  
Spin Spin Coupling

The long-range spin–spin coupling constants involving the hydroxyl proton in 2-methoxyphenol are consistent with a potential function for rotation of the hydroxyl group derived from far infrared torsion data. The internally hydrogen bonded cis conformer is the only one detected by nmr. In contrast, the long-range couplings of the sulfhydryl group in 2-methoxythiophenol indicate that the trans conformer is favored over the cis conformer by a free energy difference of 0.2 + 0.2 kcal/mol. Furthermore, the barrier to internal rotation of the sulfhydryl group is much smaller than that of the hydroxyl group in these two compounds. It is suggested that the barrier in the thiophenol derivative is 2.7 ± 0.6 kcal/mol, to be compared with a twofold component of 6 kcal/mol in the rotational potential function of the phenol analog.

Orientations of the hydroxyl and isopropyl groups in the cis and trans conformers of 2-isopropylphenol and 2-isopropyl-6-methylphenol

1981 ◽  
Vol 59 (11) ◽  
pp. 1656-1659 ◽  
Author(s):  
Ted Schaefer ◽  
Brenda M. Addison ◽  
Rudy Sebastian ◽  
Timothy A. Wildman
Keyword(s):  
Coupling Constants ◽  
Spin Coupling ◽  
Hydroxyl Group ◽  
Hydroxyl Groups ◽  
Molecular Orbital Calculations ◽  
Isopropyl Group ◽  
Phenol Derivatives ◽  
Hydroxyl Protons ◽  
Cis And Trans ◽  
Spin Spin Coupling

Measurement of the long-range spin–spin coupling constants between methine or hydroxyl protons and the ring protons in 2-isopropylphenol shows that the trans conformer is favored by 2.5 ± 0.3 kJ/mol for a 2 mol% solution in CCl4, at 305 K. In this conformer the methine C—H bond of the isopropyl group points towards the hydroxyl group. In the cis conformer, the C—H bond points away from the hydroxyl group. In 2-isopropyl-6-methylphenol the conformation in which the O—H bond lies trans to the isopropyl group is preferred by 1.2 ± 0.3 kJ/mol in free energy at 305 K in CCl4 solution. This result agrees with an additive superposition of the conformational preferences of the hydroxyl groups in the monosubstituted phenol derivatives. Molecular orbital calculations do not agree with the existence of a cis conformer.

Concerning the nonexistence of the intramolecular hydrogen bond in 2-nitrothiophenol

1977 ◽  
Vol 55 (21) ◽  
pp. 3732-3735 ◽  
Author(s):  
Ted Schaefer ◽  
William J. E. Parr
Keyword(s):  
Hydrogen Bond ◽  
Free Energy ◽  
Intramolecular Hydrogen Bond ◽  
Energy Difference ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Free Energy Difference ◽  
Spin Coupling Constants ◽  
Intramolecular Hydrogen ◽  
Spin Spin Coupling

The long-range spin–spin coupling constants between the sulfhydryl proton and the ring protons in 2-nitrothiophenol in CDCl3 and C6D6 solutions suggest the presence of two conformers in which the S—H bond prefers the benzene plane. The conformer in which the S—H bond lies trans to the nitro group is favoured over the cis conformer by a free energy difference of 0.5 ± 0.2 kcal/mol at 305 K. Apparently any intramolecular hydrogen bond is very weak compared to that in 2-nitrophenol.

Computations and measurements of the structures and conformations of 2- and 3-fluorobenzaldehyde in the gas and in solution

1990 ◽  
Vol 68 (2) ◽  
pp. 339-345 ◽  
Author(s):  
Ted Schaefer ◽  
Craig S. Takeuchi
Keyword(s):  
Free Energy ◽  
Chemical Shifts ◽  
Geometry Optimization ◽  
Energy Difference ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Solvent Mixtures ◽  
Mo Calculations ◽  
Trans Form ◽  
Free Energy Difference

Precise 1H and 19F nuclear magnetic resonance chemical shifts and spin–spin coupling constants are reported for 4 mol% solutions of 2-fluorobenzaldehyde (2FB) and 3-fluorobenzaldehyde (3FB) in CS2/C6D12/TMS/C6F6 and acetone-d6/TMS/C6F6 solvent mixtures at 300 K. A small amount of the O-cis conformer of 2FB is present even in the nonpolar solvent mixture, corresponding to a free energy difference of 7.6(3) kJ/mol between the planar O-cis and O-trans conformers. In the polar solvent, this number decreases to 4.5(2) kJ/mol. The O-cis and O-trans conformers of 3FB have very similar abundances in the two solvent mixtures, the former being favored by a free energy difference of 0.38(4) kJ/mol in the nonpolar medium and, unexpectedly, considering its highly polar nature, by only 0.27(4) kJ/mol in the polar environment. STO-3G MO computations, with geometry optimization, of the internal rotational potentials of 2FB and3FB confirm the planarity of the O-cis conformer of 2FB, that is, that the planar form is more stable than a somewhat twisted conformer. 6-31G MO calculations for the four planar conformers yield structures of potential use in the fitting of rotational spectra in the vapor. Extrapolation of the 1H nmr data for 2FB implies a free energy difference of 11.5 ± 0.6 kJ/mol at 300 K in the vapour favoring the O-trans form, midway between the two theoretical estimates. The present experiments and computations are compared with the latest rotational, vibrational, and electronic spectra and with other assessments of the relative conformer stabilities in the vapor and in solution. Keywords: NMR for 2- and 3-fluorobenzaldehyde; conformations in solution and vapor, for 2- and 3-fluorobenzaldehyde; MO calculations, 2- and 3-fluorobenzaldehyde.

199Hg-1H Spin-spin coupling in vinyl and ethynyl mercurials

1964 ◽  
Vol 17 (11) ◽  
pp. 1204 ◽  
Author(s):  
PR Wells ◽  
W Kitching
Keyword(s):  
Coupling Constants ◽  
Spin Coupling ◽  
Spin Coupling Constants ◽  
Cis And Trans ◽  
Spin Spin Coupling

The proton magnetic spectra of cis and trans-2-chlorovinylmercuric chloride, bis(trans-2-chlorovinyl)mercury, and methylethynylmercury have been determined. Large variations in the 199Hg-1H spin-spin coupling constants are observed for different hybridization states and different geometries.

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