Warren W. Harper

REINVESTIGATION OF THE HSICL ELECTRONIC SPECTRUM : EXPERIMENTAL REEVALUATION OF THE GEOMETRY, ROTATIONAL CONSTANTS, AND VIBRATIONAL FREQUENCIES

The ground and first excited singlet states of monochlorosilylene have been reexamined using pulsed discharge jet and laser induced fluorescence techniques. HSiCl and DSiCl have been produced by an electric discharge through SiHCl3 and SiDCl3 vapor in argon. The 000 band rotational constants of four isotopomers of HSiCl and the harmonic force fields for both states have been combined to obtain the following estimates of the equilibrium structures: r′′(SiCl)=2.067(3) A, r′′(SiH)=1.525(5) A, θ′′(HSiCl)=96.9(5)°, r(SiCl)=2.040(3) A, r(SiH)=1.532(8) A, and θ(HSiCl)=118.1(5)°. Previous anomalies in the ground-state structure and the excited-state vibrational frequencies have been resolved. The radiative lifetime of the 480–400 nm (A 1A′′–X 1A) band system of HSiCl has been measured to be 432±20 ns.

Resolution of anomalies in the geometry and vibrational frequencies of monobromosilylene (HSiBr) by pulsed discharge jet spectroscopy

A detailed examination of the ground and first excited singlet electronic states of HSiBr has been carried out through analysis of the 500–400 nm band system, using pulsed discharge jet and laser‐induced fluorescence techniques. HSiBr and DSiBr have been produced by an electric discharge through SiHBr3 and SiDBr3 vapor in argon. Rotational analysis of the 000 bands yielded the structural parameters r0″(SiH)=1.518(1) A, r0″(SiBr)=2.237(1) A, θ0″=93.4(3)°, r0′(SiH)=1.497(10) A, r0′(SiBr)=2.208(2) A, and θ0′=116.4(7)°. Previous anomalies in the geometric parameters and vibrational frequencies have been resolved and the ground state bond lengths and vibrational frequencies are found to be comparable to those of SiH and SiBr. Harmonic force fields have been determined for the ground and excited states and the radiative lifetime of HSiBr has been measured to be 598±18 ns.

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

The jet-cooled B 1B2–X 1A1 spectrum of silylidene, the simplest unsaturated silylene, has been observed for the first time. H2C=Si and D2C=Si have been produced by an electric discharge through tetramethylsilane and tetramethylsilane-d12 vapor diluted in argon at the exit of a supersonic expansion. Rotational analysis of the 000 bands yielded the following substitution structures: rs″(CSi)=1.706(5) A, rs″(CH)=1.099(3) A, θs″(HCH)=114.4(2)°, rs′(CSi)=1.815(5) A, rs′(CH)=1.073(4) A, and θs′(HCH)=133.7(1)°. The electronic transition consists primarily of strong electronically allowed perpendicular bands, but a weaker system of vibronically induced parallel bands has also been assigned. Transitions involving Δv=2 changes in the ν6 (b2) mode show up prominently in the spectrum, due to a very large change in the vibrational frequency on excitation. Silylidene has very interesting excited state decay dynamics. Anomalous S2−S0 fluorescence is observed due to the very large S2−S1 energy gap. Rotational level spe...

The structure, spectroscopy, and excited state predissociation dynamics of GeH2

The spectroscopy and excited state dynamics of A 1B1 germylene (GeH2) have been investigated experimentally and theoretically. Jet‐cooled laser‐induced fluorescence spectra of GeH2 were obtained by subjecting germane (GeH4) to an electric discharge at the exit of a pulsed nozzle. The band origins of ten vibronic transitions were determined, giving values for the upper state fundamentals of ν1=783.0 cm−1 and ν2=1798.4 cm−1. Sufficient numbers of 000 band rovibronic transitions were observed to give the ground and excited state structures as r″=1.591(7) A, θ″=91.2(8)° and r′=1.553(12) A, θ′=123.4(19)°. Fluorescence lifetime measurements show that the 00,0 rotational levels decay radiatively; higher J rotational states in the 00 vibronic level decay much faster, due to a heterogeneous predissociation in the excited state. High quality ab initio studies are consistent with a model in which the lower vibronic levels of the A state predissociate through the a 3B1 state to produce Ge(3P)+H2(1Σ+g). The transit...

PULSED DISCHARGE JET SPECTROSCOPY OF DSIF AND THE EQUILIBRIUM MOLECULAR STRUCTURE OF MONOFLUOROSILYLENE

The jet-cooled laser induced fluorescence excitation spectrum of the A 1A′′−X 1A band system of DSiF has been observed using the pulsed discharge jet technique. Vibrational analysis of the spectrum yielded upper state harmonic vibrational frequencies of ω1=1322, ω2=444, and ω3=867 cm−1. Vibronic bands involving all of the upper state fundamentals of HSiF and DSiF have now been rotationally analyzed, allowing a determination of the excited state equilibrium structure as re′(SiH)=1.526±0.014 A, re′(SiF)=1.597±0.003 A, and θe′(HSiF)=115.0±0.6°. The harmonic frequencies and centrifugal distortion constants were used to obtain harmonic force fields and average (rz) structures for the ground and excited states. The ground state average structure was used to estimate the equilibrium structure of re″(SiH)=1.528±0.005 A, re″(SiF)=1.603±0.003 A, and θe″(HSiF)=96.9±0.5°.

Chemical reaction jet spectroscopy, molecular structure, and the bending potential of the à 1A″ state of monofluorosilylene (HSiF)

The jet‐cooled laser induced fluorescence excitation spectrum of the A 1A″–X 1A′ band system of HSiF has been observed with the chemical reaction jet technique. Vibrational analysis of the spectrum gave upper state fundamental vibrational frequencies of ν1=1547 cm−1, ν2=558 cm−1, and ν3=857 cm−1. Seven bands in the spectrum were recorded at high resolution and rotationally analyzed, providing excited state molecular constants. The upper state vibrational and rotational bending levels were fitted to a semirigid bender model to obtain the equilibrium geometry and the potential energy barrier to linearity. Due to correlations in the parameters, it was necessary to fix the bond angle at the ab initio value of 114.5°. The resulting fitted model yielded re(Si–F)=1.602 A, re(Si–H)=1.548 A with a potential energy barrier to linearity of 9130 cm−1.

The electronic spectra of jet-cooled HGeCl and HGeBr

Laser-induced fluorescence spectra of jet-cooled HGeCl, DGeCl, HGeBr, and DGeBr have been obtained using the pulsed discharge technique. The germylenes were produced by an electric discharge through argon seeded with the appropriate monohalogermane (H3GeX or D3GeX). All three excited state vibrational frequencies have been obtained for each of the four molecules. Analysis of the rotational structure of the 000 bands of the A 1A″–X 1A′ band system allowed the determination of r0 structures in the ground and excited states, with the bond angles constrained to previous ab initio values. The results for HGeCl are: r0″(Ge–Cl)=2.171(2) A, r0″(H–Ge)=1.592(1) A, θ0″(HGeCl)=94.3°, r0′(Ge–Cl)=2.146(15) A, r0′(H–Ge)=1.613(2) A, θ0′(HGeCl)=114.5°; and for HGeBr: r0″(Ge–Br)=2.329(12) A, r0″(H–Ge)=1.598(6) A, θ0″(HGeBr)=93.9°, r0′(Ge–Br)=2.308(1) A, r0′(H–Ge)=1.615(1) A, and θ0′(HGeBr)=116.3°. The radiative lifetimes of the A 1A″ states of HGeCl and HGeBr have been measured to be 548±19 ns and 736±24 ns, respectively.

Ab initio MO study of selected aluminum and boron chlorides and fluorides: Comparison with 11B NMR spectra of a tetrachloroborate melt

Molecular geometries were fully optimized for AlCl3, AlCl4‐, Al2Cl6, Al2Cl7‐, AlF3, AlF4‐, Al2F6, Al2F7‐, BCl3, BCl4‐, B2Cl6, B2Cl7‐, BF3, BF4‐, B2F6, and B2F7‐, as well as a few mixed halogen species, at the Hartree‐Fock (HF) level, using basis sets from STO‐3G to 6–311 + G(d). In some cases geometries were also optimized at the MP2 level. Where possible, the computed geometries were compared to known structures from electron or X‐ray diffraction. The agreement between these was quite good for the neutral species, and somewhat poorer for the anions. Vibrational frequencies were calculated for all species at the HF level with the largest basis set. The geometries were characterized as minima or transition structures. Various formation reaction enthalpies were calculated; these compare well with known values. More extensive calculations on the BF3/BF4‐ system indicate the structures and enthalpies are nearly converged with respect to basis set size and level of correlation treatment. The previously unknown species B2Cl7‐ is predicted to be energetically stable on the basis of the calculations. Some features of the 11B NMR spectra of room temperature melts consisting of mixtures of boron trichloride with 1‐methyl‐3‐ethylimidazolium chloride are presented. These features suggest that these melts may contain small amounts of B2Cl7‐ as an intermediate in an exchange reaction.

THE ELECTRONIC SPECTRUM OF MONOIODOSILYLENE (HSII) REVISITED

The A 1A″–X 1A′ spectra of jet-cooled HSiI and DSiI have been studied using the pulsed discharge technique, using H3SiI and D3SiI as precursors. The excited state vibrational frequencies have been determined and the literature value of ν1′ substantially revised. Although a reliable excited state equilibrium structure was unattainable, the rotational constants of the 000 bands gave the structural parameters r0″(Si–I)=2.463(1) A, r0″(Si–H)=1.534(1) A, θ0″(HSiI)=92.4(1)°, r0′(Si–I)=2.436(1) A, r0′(Si–H)=1.515(5) A, and θ0′(HSiI)=114.9(2)°. The radiative lifetime of the 000 band has been measured to be 1230±30 ns. Trends in the structural parameters, vibrational frequencies, and their changes on electronic excitation for the monohalosilylenes have been discussed.

Spectroscopic detection and characterization of iodogermylene (HGel)

Monoiodogermylene has been detected for the first time using pulsed discharge and laserinduced fluorescence techniques. HGeI and DGeI were produced by an electric discharge through argon seeded with H3GeI or D3GeI. Although the vibronic structure in the spectra was very limited, all three excited state vibrational frequencies have been obtained for both isotopomers. Analysis of the partially resolved rotational structure of the 000 bands gave the following approximate r0 structures, with the bond angles constrained to previous ab initio values: r0″(Ge–I)=2.525(10) A, r0″(H–Ge)=1.593(15) A, θ0″(HGeI)=93.5°, r0′(Ge–I)=2.515(10) A, r0′(H–Ge)=1.618(15) A, and θ0′(HGeI)=116.2°. The fluorescence lifetime of monoiodogermylene in the lowest rovibronic levels is 1.515±0.004 μs and shows significant variations on deuteration and with rotational and vibrational level.