The Electronic Spectroscopy and Photophysics of Piperidine in the Vapor Phase

The ground- and lowest excited-state properties of piperidine vapor are explored with respect to understanding its absorption and fluorescence properties. A ground-state intrinsic reaction coordinate (IRC) calculation was used to model the conformational potential energy surface connecting the equatorial and axial conformers. At the MP2/6-311++G** level of theory, the equatorial conformer is more stable by 310 cm -1 than the axial conformer, and the inversion barrier height is 2033 cm -1 . Two transitions in the UV, with origins of 38 707 and 44 070 cm -1 are assigned. The S 1 ← S 0 transition ( f obs ∼ 3.2 × 10 -3 ) is Rydberg in nature, with considerable involvement of all the ring heavy atoms. A vibrational analysis of this transition shows a main progression in 640 cm -1, which is assigned as the N−H out-of-plane bending motion. The CIS-calculated equilibrium geometry of the S 1 state indicates considerable distortion of the N atom relative to the C α atoms. The one-dimensional absorption spectrum is modeled on the basis of the ground-state IRC and the corresponding vertical CAS(2,2)/MP2 surface. The IRC dependence of the transition moment was taken into account. The radiative rate constant, k r, is estimated be ca. 1.7 × 10 6 s -1 . A weak fluorescence with ν̃ max = 33 500 cm -1 is observed with a quantum efficiency, q f, of ca. 1.3 × 10 -4 . The lifetime of the S 1 state is estimated to be ca. 80 ps on the basis of q f / k r . Deuterium substitution ( d 1 and d 11) results in slight increases in q f .