Abstract:. Can. J. Chem. 66, 1641 (1988). The long-range spin-spin coupling constant over six bonds between the "F nucleus and the I3C nucleus in the side chain, 6~( C ,~) , is reported for 4--F--C6H4-X-R, where X = 0 , S and R = CH3, CH2CH3, CH(CH3)? and C(CH3),. 6~(~,~) depends on sin2 0, where 0 is the angle by which the side chain twists out-of-plane about the C, -X bond. Expectation values of sin2 0 are obtained from 6~(C,F), yielding magnitudes of the apparent twofoldbarrier to rotation about the CW2-X bond. In t… Show more
“…Interestingly, the reason that these values are all so similar is due to the existence of perpendicular rotamers of successively lower energies for the phenols 7b, 7c and 7d which compensate for the observed decrease in the phenoxyl radical energies on descending the group. 21, 28 As expected from Wright's results, the difference in deviation from planarity upon varying fused ring size is not sufficient to account for the differences in BDE values for any series of antioxidants. Therefore, other geometrical factors were considered.…”
An (RO)B3LYP/LANL2DZdp//B3LYP/LANL2DZ model for the prediction of the homolytic bond dissociation enthalpy (BDE) and adiabatic ionisation potential (IP) of phenolic antioxidants containing heavy chalcogens has been developed. The model has been used to probe the relationship between geometry, chalcogen substitution and activity for a series of alpha-tocopherol analogues of varying ring size. From this, a series of design principles for cyclic antioxidants has emerged, embodied by the compound 4-hydroxy-2,2,3,5,6-pentamethylbenzoselenete (4c). This compound is predicted to have a BDE comparable to alpha-tocopherol, and should act in a dual chain-breaking and hydroperoxide-decomposing manner, by analogy with other selenide antioxidants. The stability of chalcogen-substituted benzoxetes was considered, and the as yet unsynthesised benzotelluretes are predicted to be stable. Finally, an attempt was made to determine antioxidant mechanism by considering calculated BDE and IP data together with experimental rate data.
“…Interestingly, the reason that these values are all so similar is due to the existence of perpendicular rotamers of successively lower energies for the phenols 7b, 7c and 7d which compensate for the observed decrease in the phenoxyl radical energies on descending the group. 21, 28 As expected from Wright's results, the difference in deviation from planarity upon varying fused ring size is not sufficient to account for the differences in BDE values for any series of antioxidants. Therefore, other geometrical factors were considered.…”
An (RO)B3LYP/LANL2DZdp//B3LYP/LANL2DZ model for the prediction of the homolytic bond dissociation enthalpy (BDE) and adiabatic ionisation potential (IP) of phenolic antioxidants containing heavy chalcogens has been developed. The model has been used to probe the relationship between geometry, chalcogen substitution and activity for a series of alpha-tocopherol analogues of varying ring size. From this, a series of design principles for cyclic antioxidants has emerged, embodied by the compound 4-hydroxy-2,2,3,5,6-pentamethylbenzoselenete (4c). This compound is predicted to have a BDE comparable to alpha-tocopherol, and should act in a dual chain-breaking and hydroperoxide-decomposing manner, by analogy with other selenide antioxidants. The stability of chalcogen-substituted benzoxetes was considered, and the as yet unsynthesised benzotelluretes are predicted to be stable. Finally, an attempt was made to determine antioxidant mechanism by considering calculated BDE and IP data together with experimental rate data.
“…The conformation of aromatic ethers and thioethers is a topic of a fundamental interest . A series of NMR studies of thioethers in solution indicate the presence of both a planar ( 1 ) and a perpendicular ( 2 ) conformer (see Chart ). − For unsubstituted thioanisole, the planar conformation seems to be prevalent. 1 …”
Two-color zero kinetic energy (ZEKE) photoelectron
spectra due to the cation D0 states of thioanisole and
its
van der Waals complexes with argon (thioanisole−Ar and
−Ar2) in a supersonic jet are reported for the
first
time, together with mass-selected REMPI (resonantly enhanced
multiphoton ionization) spectra associated
with their S1 states. For bare thioanisole, the
adiabatic ionization energy (I
a) is 63 906 ±
3 cm-1 and the S1
origin is located at 34 506 ± 1 cm-1.
Low-frequency vibrational progressions due to the excitation of
the
CH3S torsion (94 cm-1) are
observed in the ZEKE photoelectron spectra. A series of hot bands
showing a
large anharmonicity is observed in a REMPI spectrum, indicating that
the CH3S torsional frequency is 68
cm-1 in the neutral ground state. An ab
initio 6-311G** calculation gives a torsional barrier of 530
cm-1
along the S−C(sp2) bond and a minimum energy for the
CH3 group rotated 90° from the aromatic
plane.
Intensive Δv = 0 ionization transitions are
observed in the ZEKE photoelectron spectra via several S1
vibronic
levels. Concerning the thioanisole−Ar and −Ar2 vdW
complexes, the shifts in I
a amount to −117 and
−231
cm-1, respectively, with respect to
thioanisole, while the shifts in the S1 origin are −51
and −100 cm-1,
respectively. The vdW bending modes (b
x
and
b
y
) in the S1 states are resolved in
the REMPI spectrum. The
vdW b
x
mode is observed up to v = 5
in the ZEKE photoelectron spectrum due to
thioanisole−Ar.
“…' The barriers to rotation about the CS~*-S bond of C,H,SR in solution are 5.4, 1.9, -1.4, and 18.8 kJ/mol, respectively (16), for the sequence R = CH, to C(CH,),, a negative number indicating that the planar conformation is least stable, whereas the free energy differences favoring the 0-trans conformer are 0.9, 3.1 , and 5.9 kJ/mol in the sequence R = CH,, CH., CH,, CH(CH,),. Allowance of, say, a 5-10% error in all these numbers (18,26) emphasizes the similarity in the range of the two energy sequences and, by implication, the relatively weak S --0 interaction.…”
Section: Spectral Analysesmentioning
confidence: 99%
“…In the latter, the 3p lone pair lies mainly in the ring plane and repels the C=O bond. On the basis of I3c nmr chemical shifts and spin-spin coupling constants (16) it is estimated that the perpendicular conformer of tert-butyl phenyl sulfide is about 18 kJ/mol more stable than the planar.…”
. Can. J. Chem. 70, 2375Chem. 70, (1992. The 'H nuclear magnetic resonance spectral parameters are reported for 4.0 mol% solutions of 2-(4-chlorophenylthio) benzaldehyde in CS2/C6D,, and acetone-d6 at 300 K. In CS, the 0-syn conformer is 36% abundant, rising to 50% in acetone-d6. 'These abundances are compared to those of the 0-syn and 0-trans conformers of 2-(alkylthio) benzaldehydes in CCl, solution. On the basis of coupling constants and chemical shifts it is concluded that the skew conformer of the title compound is very likely the one of minimum energy in both solutions. In the skew conformer the plane of the 4-chlorophenyl group lies perpendicular to the CSC plane and also to that of the other aromatic moiety. It is suggested that the S . . . 0 interaction is rather weak and that the population of the 0-syn conformer is controlled by the orientation of the mainly 3p lone-pair orbital on sulfur. At best, the S . -. O interaction is attractive only when the 3p orbital lies perpendicular to the plane of the formyl group. The skew conformation of the title compound is contrasted to the skew conformation of 2-hydroxyphenyl phenyl sulfide in which, however, the role of the two aromatic planes is reversed; the 3p orbital now lies in or near the plane of the phenyl group COH due to an attractive 0-H . . . Operant 2 300 K, on a determine les parametres spectraux en rksonance magnetique nucleaire du 'H du 2-(4-chlorophCnylthio) benzaldehyde en solution a 4 mol% dans le CS,/C,D,, et dans llacCtone-d6. L'abondance du conformkre 0-syn est de 36% dans le CSZ et elle atteint 56% dans l'acktone-d6. On a compare ces abondances 2 celles des conformkres 0-syn et 0-trans des 2-(alkylthio) benzaldkhydes en solution dans le CC1,. En se basant sur les constantes de couplage et sur les dkplacements chimiques, on arrive a la conclusion que le conformkre gauche du composk mentionne dans le titre est trks probablement celui qui posskde I'Lnergie minimale dans les deux solutions. Dans le conformkre gauche, le plan du groupe 2-chlorophCnyle se trouve dans un plan perpendiculaire a celui du CSC et Cgalement au plan de l'unitC aromatique. On suggkre que l'interaction S . . . O est plut8t faible et que la population du conformkre 0-syn est principalement contrBlCe par I'orientation du doublet libre de I'orbitale 3p du soufre. Cette interaction S . . . 0 n'est optimale que lorsque le plan des orbitales 3p est perpendiculaire 2 celui du groupe formyle. La conformation gauche du compose mentionnk dans le titre est 2 l'oppose de la conformation gauche du sulfure de phCnyle et de 2-hydroxyphknyle dans lequel, cependant, le rBle des deux plans de la portion aromatique est inverse; I'orbitale 3p se trouve maintenant dans le plan ou prks du plan du groupe phCnyle COH 9 cause d'une interaction attractive OH. . .3p.[Traduit par la redaction]
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