REMPI spectroscopy and theoretical calculations of cis and trans 3-fluoro-N-methylaniline

Abstract The ab initio and density functional theory (DFT) calculations of 3-fluoro-N-methylaniline (3FNMA) reveal that two rotamers, cis and trans 3FNMA, are stable for each of the S 0 , S 1 , and D 0 states. The vibronic spectra of the two rotamers in the S 1 state have been recorded by the one-color resonant two-photon ionization (R2PI) spectroscopy. The band origins of the S 1  ← S 0 electronic transition of cis and trans 3FNMA are found to be 33 816 ± 3 and 34 023 ± 3 cm −1 . The two rotamers display similar vibrational frequencies, and the slight energy difference in some modes reflects the conformation effect due to the relative orientation of the NHCH 3 group. Besides, the trans rotamer displays more vibronic features in the low-frequency region, which are active modes mainly involving the CH 3 and the NHCH 3 groups. By the two-color R2PI spectroscopy, the adiabatic ionization energies (IEs) of cis and trans 3FNMA are determined to be 61 742 ± 10 and 61 602 ± 10 cm −1 , respectively. It is derived from the R2PI spectroscopic data that, compared with the trans rotamer, the cis rotamer is more stable by 302 ± 25 cm −1 in the S 1 state, but less stable by 45 ± 25 cm −1 in the D 0 state. With the aid of theoretical calculations, the substitution and conformation effects on the properties of 3FNMA, including the molecular structures, vibrational frequencies, and the relative stability of the two rotamers, were discussed in detail.