Picosecond rotational interconversion adjacent to a C═O bond studied by two-dimensional infrared spectroscopy.

Molecular conformations around the C═O group of carbonyl compounds like ketones and aldehydes play an important role in determining their reaction properties in solutions, including reaction rate, mechanism, steric structure, and chirality of products. Investigating different rotational conformers and their rapid exchange at room temperature will provide information on the rotational barrier and insights into how different rotamers may contribute to fundamental reactions in chemistry. We applied two-dimensional infrared (2D IR) spectroscopy and polarization-dependent IR transient grating technique to the study of 4,4-dimethyl-2-pentanone in CCl(4). Spectroscopic evidence showed that the internal rotation around the single carbon-carbon bond adjacent to the C═O group takes place on a picosecond time scale. DFT calculations suggested the presence of three different rotational conformations, one eclipsed and two staggered forms. Spectral simulation utilized the stochastic Liouville equation with a three-state jump model and incorporated the polarization factors that take into account the different direction of transition dipole moment in the three rotamers. The effects of the intramolecular vibrational energy redistribution process on the waiting time dependence of the 2D absorptive spectra were also included. Through comprehensive simulation of the observed spectral features, the exchange time constants between the three rotamers were determined: 5.4 ps from the eclipsed to staggered forms and 1.7 ps for the reverse direction.