Which triatomic monohalosilylenes, monohalogermylenes, and monohalostannylenes (HMX) fluoresce or phosphoresce and why? An ab initio investigation

The possibilities of emission from the Ã1A′′ and ã3A′′ excited states of the triatomic halosilylenes, halogermylenes, and halostannylenes (HMX, M = Si, Ge, Sn; X = F, Cl, Br, I) have been explored in a series of extensive ab initio calculations. The triplet states are found to have deep bonding wells supporting an extensive manifold of vibrational levels, which could give rise to observable triplet–singlet phosphorescence.The ã–X˜ band systems of the halosilylenes are calculated to occur at the red edge of the visible and are likely to be very weak. In contrast, the HGeX and HSnX triplet–singlet spectra are shifted 1000–2000 cm−1 to the higher energy and are expected to be significantly stronger due to increased spin–orbit coupling, making the spectra viable targets for experimental investigations. The ÖX˜ fluorescence is found to be limited by the possibility of HMX (Ã1A′′) → H (2S) + MX (2Π) dissociation in the excited state, leading to the expectation that HGeF is unlikely to be detectable by laser-induced fluorescence (LIF) spectroscopy. The HSiX and HGeX species with known LIF spectra are found to have deeper à state bonding wells and minimal or no calculated barriers to dissociation. It is generally found that the intensity in their LIF spectra tails off due to a diminution of vibrational overlap rather than the abrupt opening of a dissociation channel. Few of the HSnX species are known experimentally. HSnF and DSnF are found to dissociate very low down in the à state vibrational manifold and are predicted to be unobservable by LIF spectroscopy. The LIF spectrum of HSnCl is expected to consist of only one or two bands, with slightly more activity for DSnCl, precisely as has recently been found experimentally. HSnBr and DSnBr have deeper à state bonding wells, and their LIF spectra are thus likely to be more extensive. Although HSnI and DSnI are calculated to have deep bonding wells with respect to the H + MX dissociation, predictions are complicated by the existence of a global small bond angle minimum and the opening of a second SnH + I dissociation channel