Measurement of relative conformational stabilities by variable temperature photoelectron spectroscopy. A study of rotational isomerism in thioanisole

The proximate coupling constants, 5~, between ortho and methyl protons in thioanisole and 18 of its derivatives are discussed as conformational indicators. On the assumption that 5~ varies as cos40, for 0" 5 0 5 90°, 0 being the angle by which the methyl group twists out of the aromatic plane, 5~ for 0 = 0' follows as -0.43 (2) Hz from the known internal barrier in thioanisole in solution. A measurement of 5~ in meta-or para-substituted thioanisole derivatives then yields an approximate value for the twofold barrier to rotation about the Csp2-s bond. For derivatives containing an ortho substituent, 5~ yields an estimate of the torsion angle for the thiomethyl moiety. In some instances these angles are compared with those derived from long-range 'H,',c and I3c,l3c coupling constants. The size of the ortho substituent appears to have only a small effect on the magnitude of 5~. The major determinant of the latter appears to be the manner in which the substituent perturbs the mobile bond order of the csp2-s bond.Key words: spin-spin coupling constants, thioanisole derivatives; 'H NMR, thioanisole derivatives; conformations, thioanisole derivatives; conformations, torsional motion of SCH, group. On discute de l'utilitC, comme indicateurs conformationnels, des constantes de couplage approximatives, 5~, entre les protons ortho et ceux du mCthyle du thioanisole et de 18 de ses dCrivCs. En faisant l'hypothkse que les valeurs de 5~ varient en fonction du cos40, pour 0" 5 0 5 90°, oG 0 est l'angle entre le groupement mCthyle et le plan du noyau aromatique, et en se basant sur la barrikre interne connue du thioanisole en solution, 5~ pour 0 = 0" est Cgal i -O,143(2)Hz. Une mesure de 5~ pour des dCrivCs du thioanisole substituks en me'ta ou en para fournit alors une valeur approximative de la barrikre binaire i la rotation autour de la liaison csp2-S. Pour des dCrivCs contenant un substituant en ortho, la valeur de 5~ fournit une tvaluation de l'angle de torsion de la portion thiomCthyle. Dans quelques cas, on a pu comparer ces angles avec ceux dCrivCs des constantes de couplage i ? i longue distance 'H, I3c et I3C, I3c. La grosseur des substituants ortho ne semble avoir qu'une faible influence sur I'amplitude de 'J. Le facteur dkterminant de cette amplitude semble Etre la f a~o n dont le substituant perturbe l'ordre de liaison mobile de la liaison c s p k 3 .

Section: Introductionsupporting

confidence: 61%

Section: Ch7-smentioning

confidence: 99%

The proximate coupling constant, ⁵J(H,CH₃), and the torsional mobility of the thiomethyl group in some thioanisole derivatives

Schaefer¹,

Sebastian²,

Salman³

et al. 1991

“…0 2 Hz. planar (C,C,SC = O") form of thioanisole is favored by 3.5 kJ/mol over the (probably) perpendicular conformer (12). This number is close to the twofold barrier in thiophenol, suggesting 3p.. .n: conjugation as a major contributor to the stability of the planar forms.…”

Section: The Conformation Of 2-hydroxyphenyl Methyl Sulfidementioning

confidence: 70%

Conformational consequences of intramolecular hydrogen bonding by OH to the directional lone-pair of sulfur in derivatives of methyl phenyl sulfide, diphenyl sulfide, and diphenyl disulfide

Schaefer¹,

Salman²,

Wildman³

et al. 1982

Complete spectral parameters for the 1H nmr spectra of 2-hydroxyphenyl methyl sulfide, 2, 2-hydroxyphenyl phenyl sulfide, 3, bis(2-hydroxy-3-tert-butyl-5-methylphenyl) sulfide, 4, and bis(2-hydroxyphenyl) disulfide, 5, are reported for CCl4 solutions at 305 K. For 2 the parameters are consistent only with a conformation in which the C—S—C plane is roughly perpendicular to the aromatic plane. The conformational determinant is the [Formula: see text] hydrogen bond which forces the mainly 3p orbital on sulfur into the benzene plane. In 3 a similar arrangement is found about the sulfur atom, with the phenyl group lying in the C—S—C plane and therefore perpendicular to the hydroxyphenyl plane (skew conformation). In 4 two [Formula: see text] hydrogen bonds exist, yielding a gable (twist) conformation. Compound 5 exists in the axial conformation with probable C2 symmetry, the CSSC dihedral angle and the CCSS torsion angles all being near 90°. For none of the compounds is there any evidence for [Formula: see text] interactions.

“…. ..rr conjugation in thioanisole and thiophenol is not very strong, the barrier to internal rotation about the C 1 -S bond being only about 3.3 kJ/mol (22,23). The ring proton shifts reflect the extent of conjugation and are the same for the two compounds to within 0.02 ppm (24,25).…”

Section: Introductionmentioning

confidence: 97%

The planar conformation of 2-methylthiobenzaldehyde in solution

Schaefer¹,

Sebastian²

1983

. Chem. 61. 26 (1983). The 'H nmr spectral parameters are extracted for a 4 mol% solution of 2-methylthiobenzaldehyde in CCI, at 305 K. The long-rangc spin-spin coupling constants involving the aldehydic and methyl protons are consistent only with a preferred conformation in which all heavy atoms are coplanar, as are the chemical shifts of the ring and methyl protons. This conclusion contradicts previous interpretations of the dipole moment, the nmr parameters, and of the infrared data for CCI, solutions. The present data show that the 0-sytl and 0-crtlri forms of the conlpound are present in roughly equal proportions.--TED SCHAEFER et RUDY SEBASTIAN. Can. J. Chem. 61, 26 (1983). On a dCduit les paramktres spectraux de la rmn du 'H d'une solution ii 4 mol% de methyl-2 thiohenznldkhyde B 305 K. Les constantes de couplages a longue distance impliquant les protons aldehydiqucs et ceux du mkthyle sont en accord uniquement avec une conformation prCf6rentielle dans laquelle tous les atornes lourds sont coplanaires. 11 en cst dc m&me pour les deplacements chimiques des protons du cycle et du mkthyle. Cctte conclusion contredit les interpretations anterieures du moment dipolaire, des paramhtres de la rnmn et dcs donnkes infrarouges dcs solutions dans le CCI,. Les donnkes actuelles rnontrent que les forrnes 0-sytl et 0-anti du composd sont prescntes approximativernent en proportions Cgales. Introductionplanar conformation of 1. A discussion (13) of the dipole moThe stereospecificity of the long-range coupling over five ment and infrared Spectrum of 1 came to the same conclusion.