The crystal structure of the six-member heterocyclic delta-valerothiolactone (1-thiocycloalkan-2-one) compound has been determined by X-ray diffraction at low temperature revealing that its skeleton adopts a half-chain conformation The conformation around the thioester group is almost planar with an anti orientation of the C=O double bond with respect the S-C single bond [C(2)-S(1)-C(6)-O(1) = 176 26(8)degrees The skeletal parameters especially valence angles [angle C5-C6-S = 121 19(6)degrees angle O=C6-C5 = 122 25(8)degrees angle C6-S-C2 = 106 80(4)], differ from those typically found in acyclic thioester compounds symptomatic of the presence of strain effects The conventional ring strain energy was determined to be 7 5 kcal/mol at the MP2/6-311++G(d p) level of calculation within the hyperhomodesmotic model approximation Moreover the valence electronic structure was investigated by lid photoelectron spectroscopy assisted by quantum chemical calculations at the OVGF/6-311++G(d,p) level of theory The first three bands at 9 35 9 50 and 11 53 eV denote ionizations related with the n(s) no and pi(c=o) orbitals respectively demonstrating the importance of the -SC(O)- group in the outermost electronic properties