BARC/PUB/2020/0610

 
 

The electronic absorption spectrum of Anisole studied by photoabsorption spectroscopy and quantum chemical calculations

 
     
 
Author(s)

Shastri, A.; Das, A. K.; Rajasekhar, B. N.
(A&MPD)

Source

Journal of Quantitative Spectroscopy and Radiative Transfer, 2020. Vol. 242: Article no. 106782

ABSTRACT

Photoabsorption studies on anisole (C6H5OCH3) are performed in the UV–VUV region (4.5–11.8 eV) using synchrotron radiation; the absorption spectrum in the region > 7.8 eV is reported here for the first time. Quantum chemical calculations are carried out on ground and excited states of anisole using the DFT and TDDFT methodologies and compared with experimental spectra in order to interpret the observed features in terms of the transitions involved, their valence/Rydberg nature and relative intensities. In order to validate the ground state spectral parameters, the infrared spectrum is recorded and assigned in the 600–3500 cm−1 region; a few new vibrational bands observed in the 1700–2700 cm−1 region are assigned to overtone and combination modes. The electronic absorption spectrum in the 4.5–5.0 eV region shows an extensive vibrational progression belonging to the first valence (ππ) transition, while the 5.0–7.0 eV region is dominated by a broad and intense band peaking at ~6.6 eV which is assigned to two strong overlapping valence transitions originating from the HOMO–1 orbital. Additionally, throughout the 5.0–11.8 eV region, diffuse bands are observed which are attributed to Rydberg series (ns, np and nd) converging to the first four IPs of anisole. Theoretically simulated potential energy curves of ground and first few excited singlet and triplet states help in achieving a better understanding of the spectral features and the UV induced photodissociation mechanisms of anisole. The present studies suggest that photon excitation at 248 nm is likely to result in direct dissociation along a repulsive excited state (3s) in contrast to earlier studies which have attributed it to internal conversion to the ground state.

 
 
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