Spectroscopic and theoretical studies of some 2-(2'-haloacetyl)-5-substituted: 1-Methylpyrrole, furan and thiophene

The conformational study of some 2-(2’-haloacetyl)-5-substituted five-membered heteroaromatic compounds Z-CH2C(O)(C4H2X)-Y bearing a halogen (Z = Cl or Br) at the 2’-position and (Y = NO2, H, Me or Cl) substituents at the 5-position of the heteroaromatic ring for X = NMe, O and S was performed by IR carbonyl stretching band (νCO) analysis supported by the harmonic frequency, NBO and PCM calculations on the minima and saddle-point structures obtained at the M05-2X/aug-cc-pVTZ level. The computational results predicted the presence of four stable conformers, classified by the orientation of the Xring–C–C=O (syn/anti) and the O=C–C–Z (anticlinal/ synperiplanar) moieties as ac(syn), sp(syn), ac(anti) and sp(anti). The anti/syn equilibrium controls the relative stabilities of the conformers to a large extent while the anticlinal/synperiplanar accounts for the observed trend of the IR carbonyl band components. The computed νCO frequencies and the PCM trend of the relative abundance in solvents of increasing permittivity allowed the pairs of ac(syn)/ac(anti) and sp(syn)/sp(anti) conformers to be assigned to the lower and higher νCO frequency IR carbonyl doublet components, respectively. The PCM data failed to reproduce the experimental carbonyl triplet found in chloroform. However, the additional component could be justified by assuming the formation of complexes constituted by one chloroform molecule hydrogen bonded to the conformers in a PCM chloroform continuum. The NBO and short contact analysis suggested that the syn/anti conformational equilibrium is governed mainly by electrostatic interactions, while the anticlinal/synperiplanar depends to different extents on both orbital and electrostatic interactions.