Structure and ground and first electronic excited state vibrational modes of the ethyl-p-aminobenzoate conformers Original Research Article

A combined theoretical and experimental study of ethyl-p-aminobenzoate (EAB) molecule to substantiate the number of conformers, identify their electronic 000 transitions, vibrational bands, structures and ionisation energies is reported. The experimental study is mainly based on pump–probe laser techniques on the sample molecule prepared in a supersonic expansion beam and includes one-colour and two-colour resonance enhanced multiphoton ionisation (REMPI, R2PI) hole burning and dispersed emission spectroscopies. The spectra are rationalised with the help of density functional theory (DFT) computations at the B3LYP/6-31+G* level. EAB R2PI spectrum is reported for the first time and the trans and gauche conformers ionisation energy thresholds have been measured to be 7.828 and 7.826 eV, respectively. The small energy difference of the trans and gauche 000 transitions is suggested to be originated in the different contribution of the ethyl group orbitals to the conformer highest and lowest occupied molecular orbital states. Ground and first electronic excited states vibrational spectra of both conformers have been assigned with the help of the computed DFT vibrational wave numbers. The most intense bands of the vibrational spectra are identified as belonging to ring vibrations: 10a, 17b, 1+skeletal vibration, 5, 17a, 18b among others. Most vibrational modes in both conformers appear at close wave numbers and only the low energy modes, in the region below 400 cm−1, should be regarded as the characteristic fingerprint of each conformer. No long vibrational progressions have been observed.