Microwave spectra for 11 isotopomers of bis(η 5 -cyclopentadienyl)tungsten dihydride ((C 5 H 5 ) 2 WH 2 ) were recorded in the 5-14 GHz region using a Flygare-Balle-type pulsed beam spectrometer. Spectra arising from four tungsten isotopomers of both the (C 5 H 5 ) 2 WH 2 and (C 5 H 5 ) 2 WHD species and three W isotopomers for the (C 5 H 5 ) 2 WD 2 complex have been measured. The ∼250 b-type transition frequencies assigned for these near-prolate asymmetric top molecules were accurately described (σ fit ) 2-4 kHz) using the rotational parameters A, B, and C and one centrifugal distortion constant, ∆ J . The small value obtained for ∆ J indicates a fairly rigid structure. From a least-squares fit using the resulting 33 rotational constants to obtain the molecular structure, we were able to determine the W-H bond length, r(W-H) ) 1.703(2) Å, the H-W-H bond angle, ∠(H-W-H) ) 78.0(12)°, the W-Cp centroid distance, r(W-Cp) ) 1.940(8) Å, the angle made by the Cp centroids with tungsten, ∠(Cp-W-Cp) ) 155(2)°, and the average C-C bond length, r(C-C) ) 1.429(8) Å. The hydrogen atom separation is r(H-H) ) 2.14(2) Å, indicating that this is clearly a "classical dihydride" rather than an "η 2 -dihydrogen" complex. The WH 2 moiety parameters determined from Kraitchman's equations (r(W-H) ) 1.682(2) Å, ∠(H-W-H) ) 78.6(2), r(H-H) ) 2.130(2) Å) agree well with the least-squares results. Furthermore, the r e parameters obtained from DFT calculations agree well with the experimental r 0 structural parameters. To our knowledge, this work marks the first microwave study of a bent-metallocene complex. The present measurements were made with a pulsed-beam Fourier transform spectrometer employing a homodynetype detection system, and this configuration is described. This homodyne system greatly simplifies the microwave circuit, with no apparent loss in sensitivity.

Single perylene bisimide molecules deposited onto Al(2)O(3) (0001) and investigated under controlled ultrahigh vacuum conditions display fluorescence intermittency behavior characteristic of an interfacial charge transfer process. Remarkably, even though the molecules are deposited on a crystalline surface with reduced disorder, power-law-distributed bright and dark periods are observed. These data can be understood based on activated formation of localized small polaron states in Al(2)O(3) (0001). We present a kinetic scheme capable of explaining the occurrence of power-law distributions for both "on" and "off" periods for single molecules on the sapphire substrate. These findings represent a first step toward understanding interfacial charge transfer processes under controlled conditions on crystalline surfaces and at the single molecule level.

Single vibronic level dispersed fluorescence spectra of jet-cooled HGeCl and DGeCl have been recorded by laser excitation of selected bands of the A 1A"-X 1A' electronic transition. Twenty-six ground state vibrational levels of HGeCl and 42 of DGeCl were measured, assigned, and fitted to standard anharmonicity expressions, which allowed all the harmonic frequencies to be determined for both isotopomers. A normal coordinate least squares analysis obtained by fitting the harmonic frequencies yielded reliable values for five of the six force constants. The ground state effective rotational constants and force field data were combined to calculate average (rz) and approximate equilibrium (re z) structures, with re z(GeH)=1.586(1) A, re z(GeCl)=2.171(2) A, and the bond angle fixed at our CCSD(T)/aug-cc-pVTZ ab initio value of 93.9 degrees . Comparisons show that the derived bond lengths are consistent with those of the appropriate diatomic molecules in their ground electronic states and the bond angle is similar to that of germylene (GeH2). A Franck-Condon simulation of the vibrational intensities in the 0(0) (0) band emission spectrum of HGeCl using ab initio force field data shows good agreement with experiment, lending credence to the vibrational analysis of the observed spectra.

The Ã 1A″–X̃ 1A′ electronic spectra of jet-cooled HPO and DPO have been studied using the techniques of pulsed discharge jet, laser-induced fluorescence, and wavelength resolved emission spectroscopy. All of the vibrational frequencies in the ground and excited states have been obtained for both isotopomers and vibrational force fields have been determined for both states. Rotational analysis of the high-resolution 000 band spectrum of DPO has yielded the first rotational constants of the deuterated species. By combining the rotational constants of DPO with literature values for the rotational constants of HPO, we have derived reliable structures of HPO in the combining states with estimated equilibrium values of r″(PH)=1.4578(6) Å, r″(PO)=1.4801(1) Å, θ″=104.62(7)°, r′(PH)=1.4671(26) Å, r′(PO)=1.5579(6) Å, and θ′=97.4(4)°. The decrease in the bond angle on n–π* electronic excitation is contrary to predictions based on Walsh diagrams. A quantitative ab initio study shows that the variation of the orbital energies with bond angle differs in the ground and excited states of HPO, and these differences account for the anomalous change in bond angle on electronic excitation.

The A (1)A(2) states of H(2)CGe and D(2)CGe have been explored for the first time by A-X laser-induced fluorescence (LIF) spectroscopy of the orbitally forbidden S(1)-S(0) transition and stimulated emission pumping (SEP) and wavelength resolved fluorescence studies of the allowed B-A electronic transition. Medium-resolution SEP studies gave the excited A state nu(2), nu(3), nu(4), and nu(6) vibrational frequencies for H(2)C(74)Ge and D(2)C(74)Ge. The 4(1) and 6(1) levels and higher combination and overtone states are strongly Coriolis coupled, which perturbs the rotational subband structure, limiting the accuracy of the determination of the vibrational frequencies. High-resolution SEP studies of the B-A 0(0) (0) band have allowed us to determine the rotational constants of the A state of H(2)C(74)Ge, from which we were able to calculate an approximate r(0) structure with the CH bond length constrained to the ground state value. The zero-point level of D(2)C(74)Ge is substantially perturbed, most plausibly by interaction with an excited vibrational level of the nearby triplet (a (3)A(2)) state.

Single vibronic level dispersed fluorescence spectra of jet-cooled HGeBr, DGeBr, HGeI, and DGeI have been obtained by laser excitation of selected bands of the A (1)A(")-X (1)A(') electronic transition. The measured ground state vibrational intervals were assigned and fitted to anharmonicity expressions, which allowed the harmonic frequencies to be determined for both isotopomers. In some cases, lack of a suitable range of emission data necessitated that some of the anharmonicity constants and vibrational frequencies be estimated from those of HGeClDGeCl and the corresponding silylenes (HSiX). Harmonic force fields were obtained for both molecules, although only four of the six force constants could be determined. The ground state effective rotational constants and force field data were combined to calculate average (r(z)) and approximate equilibrium (r(e) (z)) structures. For HGeBr r(e) (z)(GeH)=1.593(9) A, r(e) (z)(GeBr)=2.325(21) A, and the bond angle was fixed at our CCSD(T)/aug-cc-pVTZ ab initio value of 93.6 degrees . For HGeI we obtained r(e) (z)(GeH)=1.589(1) A, r(e) (z)(GeI)=2.525(5) A, and bond angle=93.2 degrees . Franck-Condon simulations of the emission spectra using ab initio Cartesian displacement coordinates reproduce the observed intensity distributions satisfactorily. The trends in structural parameters in the halogermylenes and halosilylenes can be readily understood based on the electronegativity of the halogen substituent.

Single vibronic level emission spectra of jet-cooled HSiI and DSiI have been recorded by laser excitation of selected bands of the Ã1A″–X̃1A′ electronic transition. The data have been used to derive the ground state harmonic frequencies and anharmonicities for both isotopomers. A normal coordinate analysis of the harmonic frequencies yielded reliable values for five of the six force constants. Using previously determined ground state rotational constants and the force field data, average (rz) and approximate equilibrium (rez) structures were calculated, with rez(SiH)=1.5151(2) Å, rez(SiI)=2.4610(1) Å, and θez(HSiI)=92.5(1)°. A comparison of trends in the structural parameters and vibrational frequencies of the monohalosilylenes shows that the bond angle increases significantly and the Si–H bond length decreases slightly with heavier halogen substitution. These trends have been rationalized based on the inductive effect and the electronegativity of the halogen substituent.

The 1(01)-0(00) (9-10 GHz) and 2(02)-1(01) (18-19 GHz) rotational transitions of HSi 79Br and HSi 81Br have been measured in a pulsed discharge jet expansion to an experimental uncertainty of approximately 1 kHz using Fourier transform microwave spectroscopy. The data have yielded an effective rotational constant, the centrifugal distortion constant Dj, the bromine nuclear quadrupole coupling constants, and the bromine nuclear spin-molecular rotation interaction parameter for both isotopomers. The derived parameters have been compared to their values calculated ab initio, and the nuclear quadrupole coupling tensor has been used to investigate the Si-Br bond, giving a sigma bond ionic character of 0.60, a pi bond character of 0.22, and a total Si-Br ionic character of 0.38. These bond characteristics have been compared to trends in other halosilylenes, silanes, and the analogous carbenes.