High-resolution rotational coherence spectroscopy of para-cyclohexylaniline

Abstract

A high-resolution rotational coherence spectroscopy (RCS) investigation of para-cyclohexylaniline (pCHA) was performed with a solid-state picosecond laser setup, which allowed for the determination of rotational constants with unprecedented precision for a RCS experiment. The technique of time-resolved fluorescence depletion was used for the RCS measurements. The unique structural features of pCHA enabled the determination of both ground and excited state rotational constants. Three different sets of recurrences were observed in the spectrum and assigned to K″-, K′-, and J″-type transients. From a detailed analysis by a grid search procedure based on the numerical simulation of RCS spectra and a nonlinear least-squares fitting routine the following rotational constants for the ground state were obtained: A″=2406.5±0.6 MHz, (B+C)″=714.9±0.4 MHz. For the electronic excited state two different sets of constants were found to fit the experimental data within the reported uncertainties: set (I) A′=2343.6±1.3 MHz, (B+C)′=714.4±1.7 MHz and set (II) A′=2346.3±1.3 MHz, (B+C)′=719.3±2.1 MHz. From additional information set (II) was found preferable for the description of the excited state. Furthermore, the fluorescence lifetime and the alignment of the transition dipole moment in the molecular frame were obtained from the fit procedure. For a structural characterization of pCHA we performed ab initio calculations of the electronic ground and excited state using HF/6-31G(d) and CIS/6-31G(d) levels of theory, respectively. These results were compared with the experiments and used to investigate the dependence of the rotational constants on characteristic intramolecular coordinates.