Sandra A. Guerrero

Conformational studies of α-substituted carbonyl compounds. Part 1. Conformation and electronic interaction in hetero-substituted acetones by infrared and ultraviolet spectroscopy

νCO Frequencies and intensities and n→π* transition energies were measured for some heterosubstituted ketones (XCH2COMe : X = F, Cl, Br, I, NMe2, OMe, SMe, or SEt) and compared with the corresponding unsubstituted ketone. The stability of the gauche-conformers is discussed in terms of hyperconjugative interactions between σC – X and πCO orbitals and repulsive interactions between CO and C – X dipoles. The carbonyl frequency shifts, induced by inductive (ΔνI), field (ΔνF), and hyperconjugative effects (ΔνH), are estimated separately. A close relationship is shown to exist between the ΔνH values and the energies of the n→π* transition.

Conformational and electronic interaction studies of α-substituted carbonyl compounds. Part 9. ω-Hetero-substituted acetophenones

νCO frequencies and intensities, n→π*CO transition energies and α-methylene carbon chemical shifts were measured for some ω-hetero-substituted acetophenones (φCOCH2X: X = F, OMe, NMe2, NEt2, Cl, Br, SEt, and I), and their conformations were estimated with the help of molecular-mechanics calculations. The stability of the gauche rotamers is discussed in terms of hyperconjugative interactions between σCX and πCO* orbitals. The carbonyl frequency shifts of the cis rotamers are interpreted as due to the substituent field effect, while the corresponding shifts for the gauche rotamers have been ascribed to an interplay of the inductive and hyperconjugative effects. The abnormal negative νCOgauche shifts and the n→π*CO band bathochromic shifts of the ω-fluoro- and ω-methoxy-acetophenones, and the observed shielding effect on the α-methylene carbon atom of the studied acetophenones, are discussed in terms of π*CO/σCX and π*CO/X1p orbital overlapping. The higher stabilization of the ω-ethylthioacetophenone gauche rotamer, the largest negative νCOgauche shift, and the lower shielding effect for its methylene carbon have been interpreted as due to the simultaneous occurrence of π*CO/σCS and πCO/σCS orbital interactions.

Conformational and electronic interaction studies of 2-fluoro-substituted N, N-dimethylacetamides

Abstract The ν CO IR analysis of the 2-fluoro-substituted N,N -dimethylacetamides RC(O)NMe 2 [RFCH 2 ( 1 ), F 2 CH ( 2 ) and F 3 C ( 3 )] and of the parent N,N -dimethylacetamide [RCH 3 ( 4 )] in solvents of increasing polarity suggests the occurrence of Fermi resonance for 2 , 3 and 4 and of cis – gauche rotational isomerism for 1 . The 1st overtone analysis along with ab initio calculations at MP2/6−31+G(d,p) level are in agreement with the existence of the cis and gauche conformers for 1 and indicate the occurrence of cis – gauche and gauche – gauche conformers for 2 and of a single cis – gauche – gauche rotamer for 3 . The stabilisation of the gauche rotamer with respect to the cis rotamer for 1 and of the gauche – gauche rotamer over the cis – gauche rotamer for 2 , along with the unique cis – gauche – gauche rotamer for 3 is discussed in terms of a −I inductive effect of the CF n group, a Repulsive Field Effect between the CO and C−F dipoles and the orbital interaction between one or two fluorine 2p lone pairs (in the gauche conformation) and the π CO ∗ orbital. Eigenvector analysis shows that the fluorine 2p orbital coefficient at the LUMO increases progressively, going from the cis – gauche – gauche rotamer of 3 to the cis – gauche rotamer of 2 , to the gauche – gauche rotamer of 2 and to the gauche rotamer of 1 . Carbonyl frequency gauche shifts (Δ ν ) are in line with this trend and support the π CO ∗ / n F orbital increases in the same direction.

CONFORMATIONAL AND ELECTRONIC INTERACTION STUDIES OF α-SUBSTITUTED CARBONYL COMPOUNDS. XIV. α-(ARYLSULFONYL)-p-SUBSTITUTED ACETOPHENONES[1]

Abstract The νCO IR analysis of α-(p-phenylsulfonyl)-p-substituted acetophenones X-⊘C(O)CH2S(O)2⊘-Y 1–8, being X and Y = NO2, H and OMe substituents, supported by Molecular Mechanics data of the α-methylsulfonylacetophenone (model compound), indicates the existence of the cis/gauche rotational isomerism. The less polar gouche 2 rotamer is the more stable (conc. ≅ 90%) and the more polar cis rotamer is the less stable one along the series. Compounds 4 (X = Y = H) and 3 (X = H, Y = OMe) present another less stable and rather polar gauche 1 rotamer. The almost constant carbonyl gauche 2 shifts (Δνg2) together with the quasi-invariability of the cis/gauche ratios, when X varies and Y is fixed, indicates a very nearly constant summing up of π∗CO/[sgrave]C-SCO2 and π∗CO/[sgrave]C-SCO2 orbital interactions. The progressive stabilization of the gauche 2 rotamer when X is fixed and Y changes from NO2 to OMe substituents, along with the parallel decrease of the positive sulfonyl asymmetric frequency shifts (ΔνSO2(a...

Conformational studies of α-substituted carbonyl compounds. IV: Electronic interaction in 2-thiasubstituted carbonyl systems. ω-Ethylthio-p-substituted acetophenones

Abstract The carbonyl stretching frequencies and the apparent molar absorptivities are reported for some ω-ethylthio-p-substituted acetophenones and for the corresponding unsubstituted acetophenones. In the ω-ethylthio-series the solvent effect indicates the presence of the cis/gauche rotational isomerism, being the gauche rotamer predominant in the whole series (ca. 90%). In the ω-ethylthioacetophenones, on going from electron-attracting to electron-donating substituents, the progressive increase in the carbonyl cis shifts (Δνc) is attributed to the increasing Field Effect between C[dbnd]O and C[sbnd]S dipoles, and the progressive decrease in the carbonyl gauche shifts (Δνg) is due to the deacreasing contribution of the hyperconjugative interaction between πCO and σc[sbnd]s orbitals. The U.V. spectra of the ω-ethylthioacetophenones display a n → π∗CO band which is bathochromically shifted in relation to the corresponding acetophenones, indicating the occurrence of the hyperconjugative interaction between...

Hyperconjugative interactions in halogen-substituted carbonyls: ultraviolet photoelectron spectroscopy of ω-halogenoacetophenones

The HeI and HeII photoelectron spectra of the para-substituted ω-halogenoacetophenones Y-C6H4COCH2X [Y = NO2, CN, H, OCH3, N(CH3)2; X = H, Cl, Br, I] are assigned on the basis of the composite-molecule approach, substituent effects, and intensity variations observed on changing the wavelenght of the ionizing radiation. Ab initio minimal basis set calculations using the pseudo-potential method have been used to calculate the most stable conformation of acetophenone and its ω-bromo and ω-iodo derivatives. The first band of the iodine derivatives shows peculiar energy and intensity variations on changing the para-substituent. These findings have been ascribed to an easier interaction of the iodine lone-pair orbitals with the substrate owing to their lower ionization energy values and larger size with respect to the other halogens.

Conformational and electronic interaction studies of α-substituted carbonyl compounds. VI: p-substituted α-phenylthioacetones

Abstract The analysis of the v co bands in the I. R. spectrum of p-substituted α-phenylthioacetones indicates a cis/gauche rotational isomerism. The slight solvent effect on the αc/αg ratios on going from a non polar solvent to a polar one, for the title compounds, as well as the minute sensitivity of the v co (cis) frequencies on going from electron-attracting to electron-donating substituents are interpreted on the grounds of the Field (F) and Inductive (I) Effects, which act in opposition in the cis rotamer, practically cancelling each other. The decreasing cis/gauche population ratio on going from electron-attracting to electron-donating substituents suggests the Occurrence of the hyperconjugative interaction in the gauche rotamers of the title compounds. The progressive shielding effect on the methylene and carbonyl carbon atoms in the 13C NMR spectra, on going from electron-donating to electron-attracting substituents is ascribed at least in part, to the “Reserve Polar Effect”.

CONFORMATIONAL AND ELECTRONIC INTERACTION STUDIES OF α-SUBSTITUTED CARBONYL COMPOUNDS. XI. ω-(p-PHENYLTHIO)-p-SUBSTITUTED ACETOPHENONES

Abstract The analysis of the vco bands in the I.R. spectra of ω-(p-phenylthio)-p-substituted acetophenones indicates the presence of cislgauche rotational isomerism. The decreasing cislgauche ratio on going from electron-attracting to electron-donating substituents at the phenylthio group in each p-substituted ace-tophenone series is discussed in terms of the π∗co/σc-s hyperconjugative interaction in the gauche rotamers. The carbonyl cis shifts (δvc ) of the title compounds are interpreted as due to an interplay of Field (F) and Inductive (I) Effects. The carbonyl gauche shifts (δvs ) trend in the whole series has been ascribed to the variation of the extension of the π∗co/σc-s hyperconjugative interaction. The lower Non-Additivity Effect (NAE) of the methylene carbon of the title compounds in relation to the corresponding (ω-bromo and ω-iodo acetophenones together with the lowest NAE of the methylene carbon for the (ω-(p-nitrophenylthio)-p-nitroacetophenone (3) suggests the simultaneous occurrence of the...

Spectroscopic Evidences for the Hyperconjugative Interaction in 2-Bromosubstituted Acetophenones

Abstract Several 2-bromo-4′-substituted acetophenomes were investigated by vibrational and electronic spectroscopies. The solvent effect on the bands assigned to μCO and μC-Br together with the comparison between the fundamental and overtone μCO frequencies indicate a cis/gauche rotational isomerism. The progressive.

Preferred conformations in the solid state of some α-(p-phenylsulfinyl)-p-substituted acetophenones

Information on the geometrical structures of alpha-(p-phenylsulfinyl)-p-substituted acetophenones X-PhC(O)CH2S(O)Ph-Y [X = OMe, Y = H (1); X = NO2, Y = OMe (2); X = OMe, Y = NO2 (3); IUPAC names: (1) 4-methoxyphenyl phenylsulfinylmethyl ketone; (2) 4-nitrophenyl 4-methoxyphenylsulfinylmethyl ketone; (3) 4-methoxyphenyl 4-nitrophenylsulfinylmethyl ketone] have been obtained from X-ray diffraction analyses. A comparison of these results with those previously obtained from X-ray diffraction and ab initio computations of alpha-methylsulfinylacetophenone, PhC(O)CH2S(O)Me, indicated that (1) and (2) adopt in the crystal a cis1 conformation and (3) assumes a quasi-gauche geometry. The stabilization of these conformations in the crystal is discussed in terms of the dipole moment coupling, Coulombic and intramolecular charge transfer interactions between the oppositely charged atoms of the C==O and S==O dipoles. The p-substituted benzene ring is quasi-coplanar with the sulfinyl group for (1) and (3), but is quasi-perpendicular for (2). Conjugation and repulsion between the sulfinyl sulfur lone pair and the pi-benzene ring seem to be responsible for the observed geometries.