Rotationally resolved S1-S0 electronic spectra of 2,6-diaminopyridine: a four-fold barrier problem

A comparison of the electronic properties of the nitrogen-containing rings aniline, 2-aminopyridine, and 2,6-diaminopyridine (26DAP) shows that the potential energy surface of the molecule is significantly affected as more nitrogen atoms are added to the system. High resolution, rotationally resolved spectra of four vibrational bands in the S(1)-S(0) electronic transition of 26DAP were obtained in order to explain these changes. The zig-zagging inertial defects point to a double minimum excited state potential energy surface along the coupled amino group inversion vibrational mode, which becomes a four-fold well (and barrier) problem when the existence of two nearly degenerate isomers is taken into account. Assuming that the molecules are in the lower energy, opposite-side configuration, ab initio calculations were performed using the MP2/6-31G** level of theory to create a potential energy surface modeling the simultaneous antisymmetric NH(2)-inversion mode. The calculated potential energy surface shows a ground electronic state barrier to simultaneous NH(2) inversion of ~220 cm(-1), and a fit to experimental vibrational energy level spacings and relative intensities produces an excited electronic state barrier of ~400 cm(-1). The ground state barrier is less than that in aniline, but the excited state barrier is larger.