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.

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.

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.

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.

The three conformations of 3,5-dichloro-2-hydroxythiophenol in solution

1981 ◽  
Vol 59 (22) ◽  
pp. 3204-3207
Author(s):  
Ted Schaefer ◽  
Richard P. Veregin ◽  
David M. McKinnon
Keyword(s):  
Hydrogen Bond ◽  
Free Energy ◽  
Chemical Shifts ◽  
Coupling Constants ◽  
The Other ◽  
Spin Coupling ◽  
Ring Plane ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Energy Preference

The long-range spin–spin coupling constants for the sidechain protons in 3,5-dichloro-2-hydroxythiophenol show that the compound exists as a mixture of three conformers in CCl4 solution at 305 K. The conformer, in which the S—H bond is held roughly perpendicular to the ring plane by an [Formula: see text] hydrogen bond, is 13% abundant. The other two conformers, of roughly equal proportions, contain an [Formula: see text] hydrogen bond. One of these has the S—H bond cis to the OH group, the other has it trans. The chemical shifts of the SH proton and of H-6 are in agreement with these conclusions. The free energy preference of the [Formula: see text] over the [Formula: see text] bond is 1140 ± 100 cal/mol at 305 K. The five-bond coupling between the sidechain protons is negative and very likely involves proximate interactions via lone pairs on oxygen and/or sulfur.

Structural Characterization of Natural Products By Means of Quantum Chemical Calculations of NMR Parameters: New insights

2021 ◽  
Author(s):  
Fabio Luiz Paranhos Costa ◽  
Ana Carolina Ferreira de Albuquerque ◽  
Rodolfo Goetze Fiorot ◽  
Luciano Morais Lião ◽  
Lucas Haidar Martorano ◽  
...  
Keyword(s):  
Natural Products ◽  
Structural Characterization ◽  
Quantum Chemical ◽  
Chemical Shifts ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Nmr Parameters ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The calculation of NMR parameters for natural products was pioneered by Bifulco and coworkers in 2002. Since then, modelling 1H and 13C chemical shifts and spin-spin coupling constants for this...

THE ANALYSIS OF NUCLEAR MAGNETIC RESONANCE SPECTRA: III. PYRIDINE AND DEUTERATED PYRIDINES

1957 ◽  
Vol 35 (12) ◽  
pp. 1487-1495 ◽  
Author(s):  
W. G. Schneider ◽  
H. J. Bernstein ◽  
J. A. Pople
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
High Resolution ◽  
Chemical Shifts ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Calculated Spectrum ◽  
Proton Resonance ◽  
Nuclear Magnetic Resonance Spectra ◽  
Spin Coupling Constants

The proton resonance spectra of pyridine, 2,6-pyridine-d2, 3-pyridine-d1, and 4-pyridine-d1 have been obtained for the pure liquids under conditions of high resolution. The spectra have been analyzed as proton groupings of AB2X2, AB2, perturbed ABX, and B2X2 respectively. The spin-coupling constants obtained from analysis of the simpler spectra of the deuterated molecules were used to suggest trial solutions for the analysis of the complicated AB2X2 spectrum of pyridine. A final set of chemical shifts and spin-coupling constants derived for pyridine give satisfactory agreement between the observed and calculated spectrum.

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