Rotational spectrum of thiophene···HCl Does thiophene act as an aromatic π-type electron donor or an n-type electron donor in hydrogen-bond formation?

Abstract

The rotational spectra of the four isotopomers [³²S]-thiophene···H³⁵Cl, [³²S]-thiophene···H³⁷Cl, [³²S]-thiophene···D³⁵Cl and [³⁴S]-thiophene···H³⁵Cl of a complex formed between thiophene and hydrogen chloride have been observed by using a pulsed-nozzle, Fourier-transform microwave spectrometer. Rotational constants, centrifugal distortion constants and Cl-nuclear quadrupole coupling constants χ_aa, χ_bb-χ_cc and χ_ab were determined. Interpretation of the spectroscopic constants led to the conclusion that the observed complex has C_s symmetry, with the Cl atom of HCl lying almost directly above the centre of mass of the thiophene ring but with the H atom of HCl pointing at the π-electron density near to the S atom. The S···H–Cl nuclei are almost collinear [ϑ=0.9(6)°] but the relatively large distance r(S···H)=2.7474(29) Å indicates that the S···H interaction is weak. The angle φ between the C₂ axis of thiophene and the S···H internuclear line was found to be 64.53(16)°. The distance r(*···Cl)=3.693 Å from the centre of mass (*) of the thiophene ring to Cl and the angle (S*Cl)=98.9° are very similar in magnitude to the corresponding quantities in thiophene···Ar. Indeed, a comparison of thiophene···Ar, thiophene···HCl, benzene···Ar and benzene···HCl revealed a strong family relationship between the geometries of these four complexes. It is concluded that the non-bonding electron pair carried by S in thiophene is so weakly nucleophilic that when thiophene forms a hydrogen bond with HCl it does so via the aromatic π-electron system. In this respect, thiophene resembles benzene and is in stark contrast to its oxygen analogue, furan, with which HCl forms a hydrogen-bonded complex of C_2v symmetry via the non-bonding electron pair on O.