AbstractThe geometry, harmonic and anharmonic force fields, and fundamental vibrational frequencies ofcis‐ andtrans‐thiolformic acid are studiedab initioin the 4‐31G basis set. An extensive comparison is made between changes in diagonal and off‐diagonal quadratic and cubic force constants and diagonal stretching quartic constants in going from the chain to the ring structure in thiolformic acid and formic acid. The changes in the force constants are indicative of a much weaker interaction in the trans conformer between SH and OC, compared with O–H and OC, in keeping with the weaker hydrogenbonding property of the SH