Experimental and theoretical investigations on the vibrational and electronic spectra of 1,3-dibromobenzene.

We report here a comprehensive spectroscopic study of the vibrational and electronic spectra of 1,3 dibromobenzene. The vibrational spectrum is studied using Fourier transform infrared and Raman techniques and analyzed using density functional theory (DFT) calculations incorporating anharmonicity effects via the second-order vibrational perturbation theory method, resulting in a set of consistent assignments for all the fundamentals and several overtone and combination bands. The electronic spectrum is studied using synchrotron radiation based photoabsorption spectroscopy spanning the spectral region 1150-3000 Å (86 956-33 333 cm-1), for which the spectrum in the 1150-1700 Å (86 956-58 823 cm-1) region is reported here for the first time. The electronic absorption spectrum is richly structured and comprises valence, Rydberg, and charge transfer excitations, along with distinct vibronic features. Quantum defect analysis is used to assign Rydberg series converging to the first four IPs of 1,3 dibromobenzene, while charge transfer and valence transitions are assigned using theoretical calculations at the TDDFT/CAMB3LYP/aug-cc-pVTZ level. The extensive vibrational bands accompanying the first valence transition are analyzed and assigned using Franck-Condon factor calculations incorporating the Herzberg-Teller effect. In addition, time dependent DFT studies of excited state potential energy curves yield some new insights into the role of internal conversions and intersystem crossings in the UV photodissociation dynamics of 1,3 dibromobenzene.