Whether tetra‐tert‐butyl‐s‐indacene is a symmetric D2h structure or a bond‐alternating C2h structure remains a standing puzzle. Close agreement between experimental and computed proton chemical ...shifts based on minima structures optimized at the M06‐2X, ωB97X‐D, and M11 levels confirm a bond‐localized C2h symmetry, which is consistent with the expected strong antiaromaticity of TtB‐s‐indacene.
The structure of tetra‐tert‐butyl‐s‐indacene is a computational challenge. Highly correlated methods and popular DFT functionals predict a bond‐delocalized D2h symmetry, but excellent agreement between experimental and computed proton chemical shifts suggests a true C2h geometry.
Computational studies for a series of low to high strain anti-Bredt alkenes suggest that those with highly twisted bridgehead double bonds and a small singlet–triplet energy gap may undergo facile ...stepwise 2 + 2 cycloadditions to furnish four membered rings. A selection of reaction substrates, including ethylene, acetylene, perfluoroethylene, and cyclooctyne are considered.
Whether tetra-tert-butyl-s-indacene is a symmetric D
structure or a bond-alternating C
structure remains a standing puzzle. Close agreement between experimental and computed proton chemical shifts ...based on minima structures optimized at the M06-2X, ωB97X-D, and M11 levels confirm a bond-localized C
symmetry, which is consistent with the expected strong antiaromaticity of TtB-s-indacene.
Long-range proton-fluorine coupling constants (
J
) are helpful for the structure elucidation of fluorinated molecules. However, their magnitude and sign can change with the relative position of ...coupled nuclei and the presence of substituents. Here, trans-4-tert-butyl-2-fluorocyclohexanone was used as a model compound for the study of the transmission of
J
. In this compound, the
J
was measured to be +5.1 Hz, which is five times larger than the remaining
J
in the same molecule (
J
= +1.0 Hz and
J
= +1.0 Hz). Through a combination of experimental data, natural bond orbital (NBO) and natural J-coupling (NJC) analyses, we observed that stereoelectronic interactions involving the π system of the carbonyl group are involved in the transmission pathway for the
J
. Interactions containing the π system as an electron acceptor (e.g., σ
→ π
and σ
→ π
) increase the value of the
J
, while the interaction of the π system as an electron donor (e.g., π
→ σ
) decreases it. Additionally, the carbonyl group was shown not to be part of the transmission pathway of the diequatorial
J
coupling in cis-4-tert-butyl-2-fluorocyclohexanone, revealing that there is a crucial symmetry requirement that must be fulfilled for the π system to influence the value of the
J
in these systems.
•13C NMR chemical shift cannot be interpreted by charge and electronegative parameters.•Iodine affect the 13C chemical shift for the methyl in ortho orientation.•DFT calculation shed light on the ...proximity effect between iodine and methyl group.•Paramagnetic and spin-orbit contributions are responsible for the 13C chemical shift.
Methyl substitution in ortho position causes a deshielding of 6–7 ppm on the 13C NMR chemical shift of the own methyl group and the carbon nucleus bonded to iodine atom (ipso) in iodobenzene-like molecules. In contrast, the carbon ipso is 3–4 ppm shielded when methyl is in para. To understand how the position of methyl substitution perturbs nuclear magnetic responses in iodobenzene and diacetoxyiodobenzene derivatives, shielding mechanisms are theoretically investigated via density functional theory calculations. We show the relative ortho position between iodine and methyl allows through-space and through-bond interactions to take place, generating additional paramagnetic currents and affecting the spin-orbit coupling propagation. Relevant paramagnetic couplings that explain the para methyl substitution behavior are also presented. Shielding mechanisms discussed here for monomethylated compounds can be summed to predict the 13C NMR chemical shift in multi methyl substituted iodine-containing compounds.
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Effects of electron-donating (R = NH
) and electron-withdrawing (R = NO
) groups on
C NMR chemical shifts in R-substituted benzene are investigated by molecular orbital analyses. The
C shift ...substituent effect in
,
, and
position is determined by the σ bonding orbitals in the aryl ring. The π orbitals do not explain the substituent effects in the NMR spectrum as conventionally suggested in textbooks. The familiar electron donating and withdrawing effects on the π system by NH
and NO
substituents induce changes in the σ orbital framework, and the
C chemical shifts follow the trends induced in the σ orbitals. There is an implicit dependence of the σ orbital NMR shift contributions on the π framework,
unoccupied π* orbitals, due to the fact that the nuclear shielding is a response property.
This study expands the knowledge on the conformational preference of 1,3-amino alcohols in the gas phase and in solution. By employing Fourier transform infrared spectroscopy, nuclear magnetic ...resonance (NMR) spectroscopy, density functional theory (DFT) calculations, quantum theory of atoms in molecules (QTAIM), natural bond orbital (NBO) analysis, and molecular dynamics (MD), the compounds 3-aminopropan-1-ol (1), 3-methylaminopropan-1-ol (2), and 3-dimethylaminopropan-1-ol (3) are evaluated. The results show that the most stable conformation of each compound in the gas phase and in nonpolar solvents exhibited an O–H···N intramolecular hydrogen bond (IHB). Based on the experimental and theoretical OH-stretching frequencies, the IHB becomes stronger from 1 to 3. In addition, from the experimental NMR J-couplings, the IHB conformers are predominant in nonbasic solvents, representing 70–80% of the conformational equilibrium, while in basic solvents, such conformers only represent 10%. DFT calculations and QTAIM analysis in the gas phase support the occurrence of IHBs in these compounds. The MD simulation indicates that the non-hydrogen-bonded conformers are the lowest energy conformations in the solution because of molecular interactions with the solvent, while they are absent in the implicit solvation model based on density. NBO analysis suggests that methyl groups attached on the nitrogen atom affect the charge transfer energy involved in the IHB. This effect occurs mostly because of a decrease in the s-character of the LPN orbital along with weakening of the charge transfer from LPN to σ*OH, which is caused by an increase in the C–C–N bond angle.
Whether tetra‐tert‐butyl‐s‐indacene is a symmetric D2h structure or a bond‐alternating C2h structure remains a standing puzzle. Close agreement between experimental and computed proton chemical ...shifts based on minima structures optimized at the M06‐2X, ωB97X‐D, and M11 levels confirm a bond‐localized C2h symmetry, which is consistent with the expected strong antiaromaticity of TtB‐s‐indacene.
The structure of tetra‐tert‐butyl‐s‐indacene is a computational challenge. Highly correlated methods and popular DFT functionals predict a bond‐delocalized D2h symmetry, but excellent agreement between experimental and computed proton chemical shifts suggests a true C2h geometry.
The conformational preferences of 3-hydroxytetrahydropyran (1) were evaluated using infrared and nuclear magnetic resonance spectroscopic data in solvents of different polarities. Theoretical ...calculations in the isolated phase and including the solvent effect were performed, showing that the most stable conformations for compound 1 are those containing the substituent in the axial and equatorial orientations. The axial conformation is more stable in the isolated phase and in a nonpolar solvent, while the equatorial conformation is more stable than the axial in polar media. The occurrence of intramolecular hydrogen-bonded O–H···O in the axial conformer was detected from infrared spectra in a nonpolar solvent at different concentrations. Our attempt to evaluate this interaction using population natural bond orbital and topological quantum theory of atoms in molecules analyses failed, but topological noncovalent interaction analysis was capable of characterizing it.
The natural
J
-coupling
(NJC) method is applied
to analyze the Fermi contact contribution of the NMR spin–spin
coupling constant decomposing this contribution in terms of natural
localized molecular ...orbitals. We investigated the influence of the
basis set on the NJC analysis for the formyl group coupling constant
(
1
J
CHf
) of benzaldehyde derivatives.
NJC and other NBO analyses, like steric and natural Coulombic energy,
were chosen to explain the influence of electron-donating and electron-withdrawing
groups on
1
J
CHf
for some substituted
benzaldehydes (Me, OH, OMe, F, Cl, Br, I, and NO
2
). For
the
ortho
derivatives, electronegative substituents
near the C–Hf bond increase the
1
J
CHf
coupling. This effect could be related to an increase
in formyl carbon
s
character and changes in the carbon
and hydrogen natural charges. This indicates that the substituents
in
ortho
have a proximity effect on
1
J
CHf
coupling mainly of electrostatic origin
instead of the expected hyperconjugative interactions.
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