The fluorescence excitation spectra and dispersed fluorescence spectra of trans‐stilbene have been recorded and analyzed. Vibrational assignments for the eight low‐frequency modes have been made for both the S0 and S1(π,π∗) electronic states, and these differ substantially from those of previous workers. Two‐dimensional kinetic and potential energy calculations were carried out in order to determine the potential energy surfaces for the two phenyl internal rotations ν37 and ν48. The function V(ϕ1,ϕ2)= 1/2V2(2+cos 2ϕ1+cos 2ϕ2)+V12 cos 2ϕ1 cos 2ϕ2 +V12′ sin 2ϕ1 sin 2ϕ2, with V2=1550 cm−1, V12=337.5 cm−1, and V12′ = 402.5 cm−1 for the S0 state and with V2=1500 cm−1, V12=−85 cm−1, and V12′ = −55 cm−1 for the S1(π,π∗) state fits the observed data (nine frequencies for S0 and six for S1) extremely well. The barriers to simultaneous internal rotation of both phenyl groups are given by twice the V2 values. The fundamental frequencies for these torsions are ν37=9 cm−1 and ν48=118 cm−1 for the S0 state and ν37=35 cm−1 and ν48=110 cm−1 for the S1 excited state. The third torsion ν35, which is the internal rotation about the C=C bond, was assigned at 101 cm−1 for the S0 state based on a series of overtone frequencies (202, 404 cm−1, etc.). For S1, ν35=99 cm−1 based on observed frequencies at 198, 396 cm−1, etc.
Kinetic energy calculations were also carried out for this mode, and a one‐dimensional potential energy function of the form V(θ)=1/2V1(1−cos θ)+1/2V2(1−cos 2θ)+1/2V4(1−cos 4θ) was utilized to reproduce the frequencies for the ground state. For the excited state, an additional V8 term was added in order to fit the data for the trans potential energy well. The data indicate that the trans→twist barrier for the S1 state is higher than 1400 cm−1. However, a somewhat revised frequency assignment would be compatible with a barrier of 1250 cm−1, which is close to the value of 1200 cm−1 determined from dynamics studies.