The thioformamidyl group as an α-substituent in carbocations

1995 ◽  
Vol 73 (9) ◽  
pp. 1468-1477 ◽  
Author(s):  
M. Bertone ◽  
D.L.J. Vucković ◽  
A. Cunje ◽  
C.F. Rodriquez ◽  
E. Lee-Ruff ◽  
...  
Keyword(s):  
Molecular Orbital ◽  
Ring Opening ◽  
Molecular Orbital Calculations ◽  
Transition Structure ◽  
Isodesmic Reactions ◽  
Open Structure ◽  
Methyl Group ◽  
Benzyl Cation ◽  
Stabilizing Effect ◽  
Ethyl Cation

Abinitio: molecular orbital calculations at MP2(FULL)/6-311G(d,p) or MP2(FULL)/6-31G(d,p) are reported for carbocations RR′CCHO+, RR′CCHS+, RR′CCONH2+, and RR′CCSNH2+where R and R′ are H, CH3, C-C3H5, and C6H5. Primary (R=R′=H), secondary (R=H, R′=alkyl or phenyl), and tertiary (R′=R′=CH3) ions prefer the cyclic oxiranyl or thiiranyl structure 9, with the open structure 8 being a transition structure for ring opening. Tertiary carbocations with R=R′=phenyl or cyclopropyl and the 9-thioformamidyl-9-fluorenyl cation have the open structure. Isodesmic reactions show CONH2 to be weakly stabilizing in the methyl cation, and CSNH2 has a larger stabilizing effect, roughly equivalent to that of a methyl group. An α-thioamide substituent is less stabilizing in the ethyl cation and even less stabilizing in the isopropyl cation. In C6H5CHCSNH2+ the CSNH2 group is slightly destabilizing and, by extrapolation, is more destabilizing in Ar2CCSNH2+. The rearrangement of the α-thioformamidyl-benzyl cation to benzothiophene is calculated to have a low barrier, 15.4 kcal/mol at HF/6-31G(d,p). Keywords: molecular orbitals, destabilized carbocations, rearrangement.

ENDOR Studies of X-Irradiated Single Crystals of Sulphamethoxazole

1992 ◽  
Vol 47 (9) ◽  
pp. 950-954 ◽  
Author(s):  
R. Krzyminiewski ◽  
A. Lund
Keyword(s):  
Free Radicals ◽  
Hydrogen Atom ◽  
Single Crystals ◽  
Molecular Orbital ◽  
Unpaired Electron ◽  
Room Temperature ◽  
Molecular Orbital Calculations ◽  
Methyl Group ◽  
Isoxazole Ring ◽  
Spin Densities

Abstract Single crystals of sulphamethoxazole were X-irradiated at 273 K. ESR and ENDOR spectra were obtained at 100 K. The free radicals stable at room temperature are formally formed by abstraction of a hydrogen atom from the methyl group of the molecule. The unpaired electron is delocalized in the isoxazole ring. The assignment is supported by comparisons of spin densities obtained experimentally and by semiempirical molecular orbital calculations

INDO Molecular Orbital Calculations on the Conformational Dependence of Long-range Spin–Spin Coupling of Methyl Protons in Toluene, p-Fluorotoluene, and the Xylenes

1972 ◽  
Vol 50 (12) ◽  
pp. 1852-1862 ◽  
Author(s):  
R. Wasylishen ◽  
T. Schaefer
Keyword(s):  
Molecular Orbital ◽  
Perturbation Technique ◽  
Spin Coupling ◽  
Methyl Groups ◽  
Molecular Orbital Calculations ◽  
Orbital Theory ◽  
Fluorine Nucleus ◽  
Rotational Angle ◽  
Methyl Group ◽  
Spin Spin Coupling

The conformational dependence of the nuclear spin–spin coupling from methyl protons to ring protons, to the fluorine nucleus, and to protons of other methyl groups in toluene, p-fluorotoluene, and the xylenes is computed by the finite perturbation technique in the INDO approximation of molecular orbital theory. The calculated coupling over six bonds to the proton in the para position agrees quantitatively with experiment and its predicted dependence on the rotational angle of the methyl group supports a commonly assumed π electron mechanism for the transmission of spin information between the nuclei. Similar remarks apply to the fluorine nucleus in p-fluorotoluene. The couplings over five and four bonds to the protons in the meta and ortho positions display a more complex angular dependence and the former can be interpreted in terms of a dominant σ electron mechanism. The coupling between protons in different methyl groups in the ortho and meta xylenes is calculated as rather larger than the values observed in their derivatives and in the main shows the behavior expected from a π electron mechanism. Those conformations of ortho xylene in which the coupled protons are in close proximity yield computed values plausibly attributable to "direct" and/or "through-space" mechanisms. The preferred conformation of the methyl group in toluene is predicted to have a C—H bond eclipsing the plane of the aromatic ring and the calculated barriers to rotation of 0.013 kcal/mol in toluene and of 0.014 kcal/mol in p-fluorotoluene are in quantitative accord with microwave data.

A theoretical study of adduct formation between methanolate and Propan-2-one

1981 ◽  
Vol 34 (6) ◽  
pp. 1189 ◽  
Author(s):  
JC Sheldon
Keyword(s):  
Hydrogen Bond ◽  
Oxygen Atom ◽  
Ab Initio ◽  
Molecular Orbital ◽  
Theoretical Study ◽  
Adduct Formation ◽  
Molecular Orbital Calculations ◽  
Carbonyl Carbon ◽  
Methyl Group ◽  
Methoxide Anion

Ab initio molecular orbital calculations at the STO-3G level of approximation predict that the methoxide anion bonds through its oxygen atom to form complexes with acetone in at least three different ways: (i) A tetrahedral adduct at the carbonyl carbon (ΔE -262 kJ mol-1). (ii) A hydrogen-bond complex with a single hydrogen of one methyl group (- 100 kJ mol-1). (iii) A symmetrical bidentate hydrogen-bond complex with a hydrogen from each acetone methyl group (- 143 kJ mol-1).

Gaussian molecular orbital calculations of hyperconjugation in the ethyl cation

1971 ◽  
Vol 11 (2) ◽  
pp. 196-197 ◽  
Author(s):  
L.J. Massa ◽  
S. Ehrenson ◽  
M. Wolfsberg ◽  
C.A. Frishberg
Keyword(s):  
Molecular Orbital ◽  
Molecular Orbital Calculations ◽  
Ethyl Cation

The Correlation of 13C Chemical Shifts of Steroids with Atomic Charges Calculated by the Extended Hückel Molecular Orbital Method

1974 ◽  
Vol 29 (8) ◽  
pp. 1172-1178
Author(s):  
Helmar Repmann
Keyword(s):  
Molecular Orbital ◽  
Chemical Shifts ◽  
Linear Regression Analysis ◽  
Orbital Method ◽  
Molecular Orbital Calculations ◽  
Atomic Charges ◽  
Methyl Group ◽  
Extended Hückel ◽  
Hückel Theory ◽  
Steroidal Hormones

13C chemical shifts of several of the sterols and steroidal hormones which have been investigated by Reich et al. (J. Amer. Chem. Soc. 91, 7445 [1969]) are compared with the atomic charges derived from extended Hückel molecular orbital calculations for these molecules. The linear regression analysis of 126 observations for aliphatic carbons gives a correlation coefficient of 0.81, and a standard error of estimate of 8.2 ppm. The largest discrepancies arise for carbons in the neighbourhood of methyl groups, which cannot be attributed to deficiencies of the extended Hückel theory. The corrections for carbons in the α, β, and γ positions to a methyl group amount to + 11.75, - 6.44, and +4.45 ppm, respectively. The results are discussed in relation to the structural analysis of steroids.

Sign in / Sign up

Export Citation Format

Share Document