Vibrational Spectra and Theoretical Calculations of cis- and trans-3-Fluoro-N-methylaniline in the Neutral (S(0)) and Cationic (D(0)) Ground States.

Abstract

The mass-analyzed threshold ionization spectra of jet-cooled cis- and trans-3-fluoro-N-methylaniline (3FNMA) were recorded by ionizing via the vibrationless 0(0) and various vibrational levels of the S1 state. The adiabatic ionization energies of cis- and trans-3FNMA are determined to be 61,742 ± 5 and 61,602 ± 5 cm(-1), respectively. In the 0-1800 cm(-1) region, most of the observed vibrations in the D0 state result from the in-plane ring deformation and substituent-sensitive modes. For the high-frequency vibration region, the infrared-ultraviolet double-resonance and autoionization-detected infrared spectroscopies were applied to investigate the N-H and C-H stretching vibrations of bare 3FNMA in the S0 and D0 states. The C-H stretching vibrational information, which we failed to obtain for the bare 3FNMA cation, is complemented by recording the infrared-photodissociation spectra of its Ar cluster cation. It is revealed that a red-shifted frequency and an enhanced intensity are observed for the N-H stretch, while blue-shifted frequencies and greatly decreased intensities are found for both aromatic and the methyl C-H stretches. The blue shift of the C-H stretches is first explained by the balance of two factors, namely, the hyperconjugative interaction and the rehybridization effect. Analysis of the vibrational frequencies reveals a correlation between the relative stability of two rotamers in different electronic states and the relative rigidity of aromatic ring, indicating a mechanism of the long-range interactions "through bond" between the substituents. The density functional theory calculations can well reproduce the vibrational spectra in both S0 and D0 states. With the experimental and theoretical data, the substitution and conformation effects on the properties of 3FNMA in the S0 and D0 states, including the molecular structures, the reactive sites of electrophilic attack, and the vibrational behaviors, were discussed in detail.