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

Abstract

The possibilities of emission from the A ~ 1 A and a ~ 3 A excited states of the triatomic halosilylenes, halogermylenes, and halostannylenes (HMX, M=Si, Ge, Sn, X = F, Cl, Br and 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 a ~ -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 A ~ -X ~ fluorescence is found to be limited by the possibility of HMX (A ~ 1 A) → H ( 2 S) + MX ( 2 ) 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 A ~ 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 A ~ state vibrational manifold and are predicted to be unobservable by LIF spectroscopy. The LIF spectrum of HSnCl is expected to consist of only 1 or 2 bands, with slightly more activity for DSnCl, precisely as has recently been found experimentally. HSnBr and DSnBr have deeper A ~ state bonding wells and their LIF spectra are thus likely to be more extensive. Although HSnI/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.

Electronic excited states of monobromosilylene molecules including the spin–orbit-coupling

“…Compared to the experimental results, 21,22 the deviation is less than 0.014 Å, 1.71 and 6.61 cm À1 for the bond lengths, the bond angle and the vibrational frequencies, respectively, for the X 1 A 0 state, and 0.07 Å, 1.31 and 9.05 cm À1 for the A 1 A 00 state. There are no experimental studies on the triplet state a 3 A 00 , and our calculations are in good agreement with previous icMRCI + Q studies by Tarroni et al 27 It is worth mentioning that the A 1 A 00 and a 3 A 00 states exhibit larger bond angles (4201) compared to the X 1 A 0 state due to the promotion of an electron from a silicon-based nonbonding s orbital to an unoccupied out-ofplane Si 3p orbital. 23 Neither experimental nor theoretical studies have been performed so far on the spin-orbit-coupled states of HSiBr.…”

“…There are no experimental studies on the triplet state a 3 A′′, and our calculations are in good agreement with previous icMRCI + Q studies by Tarroni et al . 27 It is worth mentioning that the A 1 A′′ and a 3 A′′ states exhibit larger bond angles (>20°) compared to the X 1 A′ state due to the promotion of an electron from a silicon-based nonbonding σ orbital to an unoccupied out-of-plane Si 3p orbital. 23…”

“…26 Very recently, Tarroni et al conducted a study on HSiBr using the CASSCF(10,10)/icMRCI+Q/aug-cc-pVQZ method, presenting the spectroscopic constants and potential energy curves (PECs) of the a 3 A 00 and A 1 A 00 states. 27 To date, little information is available for the electronic excited states beyond the A 1 A 00 state of HSiBr. In addition, relativistic effects including the spin-orbit-coupling (SOC) effect, which is expected to have a significant influence on the behavior of the electronic states in compounds containing heavy atoms, [28][29][30] have not been investigated for HSiBr yet.…”

“…26 Very recently, Tarroni et al conducted a study on HSiBr using the CASSCF(10,10)/icMRCI+Q/aug-cc-pVQZ method, presenting the spectroscopic constants and potential energy curves (PECs) of the a 3 A′′ and A 1 A′′ states. 27…”

Performance of EOM-CCSD(T)(a)*-Based Quartic Force Fields in Computing Fundamental, Anharmonic Vibrational Frequencies of Molecular Electronically Excited States with Application to the Ã1A″ State of :CCH2 (Vinylidene)

“…A similar but not as extreme performance is also shown in DSiF for the CcCR + QZ ν 1 D–Si stretch at 1219.8 cm –1 , which differs from the experiment by 45.5 cm –1 or 3.9%. However, previous theoretical studies with multireference methods , agree with the present coupled cluster QFFs, as shown in Table . Additionally, the CcCR rotational constants for the state of both HSiF and DSiF are well-behaved with errors of 0.50% or less, most notably with an absolute difference of 31.2 MHz for the C 0 constant of DSiF.…”