Abstract
Rotationally resolved electronic spectra of two conformational isomers of jet-cooled indole-4-carboxylic acid (I4CA) and the deuterated forms of the acid (-COOD) and amide (-ND) groups have been obtained using a UV laser/molecular beam spectrometer. The in-plane orientation of the acid group defines the two lowest energy rotamers of I4CA. The S(1) ← S(0) origin bands of the two rotamers and four isotopologues have been fit to asymmetric rotor Hamiltonians in both electronic states. From the best-fit parameters, the positions of the H-atoms in the principal axis frames of each conformer have been determined and serve to unambiguously identify the syn forms (i.e., COH⋯O) of the cis and trans rotamers. The experimental S(0) and S(1) inertial parameters, hydrogen atom positions, and transition dipole moment (TDM) orientations are compared with the results of theoretical calculations. The TDM orientation indicates that the S(1) state is the (1)L(a) state in contrast to most substituted indoles. The molecular orbital properties and natural charges are investigated to better understand the (1)L(a)/(1)L(b) state reversal and the extent of photoinduced intramolecular charge transfer that impacts the rotamer-dependent fluorescence lifetimes.