U l t r a high-resolution fluorescence excitation spectrum of 1B1 pyrimidine in a molecular beam. Structural assignments, analysis of singlet–triplet perturbations, and implications for intersystem crossing in the isolated molecule

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Abstract
We have observed, and assigned, the fluorescence excitation spectrum of the 000 band in the 1B1←1A1 electronic transition of pyrimidine, at a resolution of ∼10 MHz. The rotational constants of the 1B1 state, the lowest excited singlet state, are A’=6352±3, B’=5853±3, and C’=3042.0±0.5 MHz. The magnitudes of these constants are not very different from those of the ground (1A1) state. However, the in‐plane a and b inertial axes in the 1B1 state are rotated by 90° with respect to those of the 1A1 state. The spectrum also exhibits numerous perturbations, evidenced by the presence of extra lines, anomalous intensities and lifetimes, and shifts of the main lines from their expected positions. The perturbations are strongly magnetic‐field dependent, demonstrating that they arise from an intramolecular coupling of the 1B1 state with nearly isoenergetic rovibronic levels of a lower triplet (3B1) state. Models are proposed to account for this behavior based on a deconvolution of the experimental spectrum and simulations of the observed Zeeman effects. The most satisfactory interpretation of the data (in the language of the zero‐order states) is obtained if it is assumed that a single rovibronic 1B1 level is spin–orbit coupled to one or a few 3B1 levels, which in turn are coupled via rotationally dependent Coriolis interactions to a dense manifold of background levels, probably those of the 1A1 state. Because the latter coupling is small, typically less than the linewidths in the spectra, it is manifested only in a K’+1 dependence of the lifetimes of selected molecular eigenstates and the reduced g values required to fit the magnetic‐field dependence of their spectra.

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