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?

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 χaa, χbb-χcc and χab were determined. Interpretation of the spectroscopic constants led to the conclusion that the observed complex has Cs 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) A 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 A 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 C2v symmetry via the non-bonding electron pair on O.