Studies On Excited And Ionic Spectroscopy Of Bromofluorobenzene And Methylanisole

The research on vibronic and cation spectra of aromatic compounds has very important significance in the field of atmospheric environmental chemistry, combustion chemistry, environment monitoring and astrochemistry. The experimental results and theoretical calculations on the first electronically excited S1state and the cation ground Do state will help us to obtain the information about the geometry structure, vibrational frequency, excited energy, ionization energy, molecular orbital and symmetry of the molecules in the S1state and the Do state, and provide experimental basis for further research on the chemical reaction mechanism. In the present paper, the resonance-enhanced mutiphoton ionization (REMPI) and the mass-analyzed threshold ionization (MATI) techniques have been used to study the excited and ionic spectra of bromofluorobenzene and methylanisole. The following are the main results:1. The S1←S0electronic transition of o-, m-, and p-bromofluorobenzene were investigated by using one-color resonant two-photon ionization (1C-R2PI) technique. The first electronic excitation energies of o-, m-, and p-bromofluorobenzene were measured to be36987,36961, and36223cm-1, respectively. The vibrational bands observed in the REMPI spectra were assigned with the help of ab initio and density functional theory (DFT) calculations. The excitation energies and the vibrational bands of these molecules were determined by the combined action of resonant effect and inductive effect. In addition, comparing the REMPI spectrum of p-bromofluorobenzene and the REMPI spectrum of Br atom which come from the photodissociation of p-bromofluorobenzene, the potential barrier height (the energy of the crossing points between the bound S1state and a repulsive state relative to the S1minimum) of p-bromofluorobenzene in the S1state was determined to be lower than2815cm-1(0.35eV).2. The S1←S0electronic transition and threshold ionization of the cis and trans rotamers of m-methylanisole were investigated by using1C-R2PI and MATI techniques. The first electronic excitation energies were measured to be36049±2and36117±2cm-1, and the adiabatic ionization energies were determined to be64859±5and65110±5cm-1for cis-and trans-m-methylanisole, respectively. Results from the ab initio and DFT calculations provide satisfactory interpretation for our experimental findings on the difference of the transitional energies of the cis and trans rotamers and help us assign the vibronic and cation spectra obtained in the present experiments. The observed active vibrations of both rotamers of m-methylanisole in the electronically excited S1and cationic ground Do states mainly involve the methyl torsion, in-plane ring deformation, and substituent-sensitive bending vibrations. The present experimental and theoretical results show that the geometry of the cation in the Do state is somewhat different from that of the neutral in the S1state for cis-and trans-m-methylanisole. In addition, the strengths of the through-space substituent-substituent and substituent-ring interactions are found to follow the order:S0<S1<D0.3. The S1←S0electronic transition and threshold ionization of trans-o-methylanisole were investigated by using1C-R2PI and MATI techniques. The excitation energy of the S1←S0transition and the adiabatic ionization energy of this molecule were measured to be36364±2and64755±5cm-1, respectively. The vibronic and cation spectra of trans-o-methylanisole were assigned with the help of ab initio and DFT calculations. The red-shift of excitation energy and ionization energy with respect to those of anisole and toluene indicates that the interaction of the CH3and OCH3group with the ring is stronger in S1and Do states than that in the So state. Results from the calculations and the observed vibronic and cation spectra implies that the molecular geometry, symmetry, and vibrational coordinates of the cation changed too much during the D0←S1ionization process.