Jet spectroscopy, structure, anomalous fluorescence, and molecular quantum beats of silylidene (H2C=Si), the simplest unsaturated silylene

Abstract

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The Rotational Spectra of H2CCSi and H2C4Si

“…Although silicon analogs of many pure carbon clusters (C n ) and acetylenic free radicals (C n H) have recently been characterized by high-resolution spectroscopy (1,2), only H 2 CSi, the silicon analog of vinylidene (H 2 CC:), has been reported in the gas phase (3,4). Because small silicon-bearing molecules such as SiS, SiC, and SiC 2 , and the cumulene carbenes H 2 C 3 (5), H 2 C 4 (6), and H 2 C 6 (7) are abundant in the circumstellar shells of evolved carbon stars (8), H 2 CCSi and longer silacumulenes are plausible candidates for astronomical detection.…”

The excited electronic states of H2CSi

“…Table III gives the predicted excitation energies obtained in this work, and compares the 1 B 2 -1 A 1 excitation energy with the experimental T 0 . 19 The agreement is excellent; our best method, CISDϩQ TZ(2d f ,2pd), differs from experiment by only 0.2 kcal/mol. Notice, too, that the singlet and triplet A 2 states differ in energy by only 3 kcal/mol, while the singlet and triplet B 2 states differ in energy by 28 kcal/ mol.…”

“…This frequency is anomalously low, ϳ265 cm Ϫ1 , in the ground state, 13,17 and increases to 731 cm Ϫ1 in the 1 B 2 state. 19 Our theoretical predictions are 327 cm Ϫ1 in the ground state, and 795 cm Ϫ1 in the excited state. Thus, theory is about 60 cm Ϫ1 above the experimental value for each state.…”

“…13 A most exciting development, and the inspiration for this theoretical study, is Harper, Waddell, and Clouthier's observation of the jet-cooled B 1 B 2 -X 1 A 1 spectrum of silylidene. 19 In their study, rotational analysis of the spectra of both the parent and perdeuterated compound yielded the substitution structure of H 2 CvSi in the ground and excited state. Their study also posed several questions about the structure and vibrational frequencies of silylidene.…”

Heavy atom nitroxyl radicals. II: Spectroscopic detection of H2As=O, the prototypical arsenyl free radical

“…Well-known examples of molecules that exhibit anomalous fluorescence in violation of Kasha's rule include azulene and several of the thiocarbonyl compounds ͑XYC= S͒. 34 More recently, we have discovered 35,36 that H 2 C = Si and H 2 CGe also fluoresce from the B 1 B 2 excited state, with a variety of interesting photophysical properties, including single rotational level fluorescence decays that exhibit molecular quantum beats. The occurrence of anomalous emission from all these molecules is attributable to their unique energy level structure in which the second excited electronic state is far removed from the first, so that coupling between the two excited states is minimal.…”