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 S0, S1, and D0 states. The vibronic spectra of the two rotamers in the S1 state have been recorded by the one-color resonant two-photon ionization (R2PI) spectroscopy. The band origins of the S1←S0 electronic transition of cis and trans 3FNMA are found to be 33816±3 and 34023±3cm−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 NHCH3 group. Besides, the trans rotamer displays more vibronic features in the low-frequency region, which are active modes mainly involving the CH3 and the NHCH3 groups. By the two-color R2PI spectroscopy, the adiabatic ionization energies (IEs) of cis and trans 3FNMA are determined to be 61742±10 and 61602±10cm−1, respectively. It is derived from the R2PI spectroscopic data that, compared with the trans rotamer, the cis rotamer is more stable by 302±25cm−1 in the S1 state, but less stable by 45±25cm−1 in the D0 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.