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

The vibrational structure of the X̃A11−ÃB11 and ÃB11−B̃A11 band systems of GeH2∕GeD2 based on global potential energy surfaces

Transition probabilities were evaluated for the X (1)A(1)-A (1)B(1) and A (1)B(1)-B (1)A(1) systems of GeH(2) and GeD(2) to analyze the X-->A-->B photoexcitation. Franck-Condon factors (FCFs) and Einstein's B coefficients were computed by quantum vibrational calculations using the three-dimensional potential energy surfaces (PESs) of the X (1)A(1), A (1)B(1), and B (1)A(1) electronic states and the transition dipole moments for the X-A and A-B systems. The global PESs were determined by the multireference configuration interaction calculations with the Davidson correction and the interpolant moving least squares method combined with the Shepard [Proceedings of the 1968 23rd ACM National Conference (ACM, New York, 1968)] interpolation. The barriers to linearity correcting the spin-orbit interaction are evaluated to be 22,000 cm(-1) for the X state, 6300 cm(-1) for the A state, and 560 cm(-1) for the B state. The obtained FCFs for the X-A and A-B systems indicate that the bending mode is strongly enhanced in the excitation since the equilibrium bond angle greatly varies within the three states. The photoexcitation and fluorescence spectra calculated for the X-A system agree well with the observed spectra. The theoretical lifetimes for lower vibrational levels of the A and B states were calculated from the fluorescence decay rates for the A-X, B-A, and B-X emissions, and the lifetimes for the A state are in good agreement with the observed values except those affected by predissociation.

Vibrational spectra of germanium-carbon clusters in solid Ar: identification of the nu4(sigma(u)) mode of linear GeC5Ge

The linear GeC(5)Ge cluster has been detected in Fourier transform infrared spectra observed when the products from the dual laser evaporation of carbon and germanium rods were trapped in solid Ar at approximately 10 K. Comparison of (13)C isotopic shift measurements with the predictions of density functional theory calculations at the B3LYP/cc-pVDZ level confirms the identification of the nu(4)(sigma(u)) mode of GeC(5)Ge at 2158.0 cm(-1).

Reversible Valence Equilibrium Reactions in Main Group Compounds. A Theoretical Study

The potential energy surface for the intramolecular reaction of singlet state RR'E=ERR' (E = C, Si, Ge, Sn, and Pb) has been explored using density functional theory. All the stationary points, including the unsymmetrical reactant (R'R(2)E-ER'), the transition state, the symmetric product (R'RE=ERR'), and the monomer (R'RE) were completely optimized at the B3LYP/LANL2DZdp level of theory. Our theoretical findings suggest the following: (1) Both double-bonded RR'C=CRR' and RR'Si=SiRR' species are true minima on their potential energy surfaces and should be the only compounds existing at all temperatures. (2) The germanium system will occur either in the dimeric R(2)R'Ge-GeR' and RR'Ge=GeRR' structures or the monomeric RR'Ge structure, depending on the temperature. (3) If the size of the substituent (R) is small, then the unsymmetrical single-bonded R(2)R'Sn-SnR' molecule can exist at low temperatures. At room temperature, the unsymmetrical R(2)R'Sn-SnR' species can exist in equilibrium with its RR'Sn monomer. (4) The unsymmetrical R(3)Pb-PbR compound may be kinetically stable at low temperatures. On the other hand, it is predicted that both the unsymmetrical R(3)Pb-PbR and the symmetric R(2)Pb=PbR(2) species will spontaneously dissociate into R(2)Pb monomers at room temperature. Our theoretical results are in good agreement with available experimental observations (J. Am. Chem. Soc. 2003, 125, 7520), and the results obtained allow a number of predictions to be made.

Organogermanium Reactive Intermediates. The Direct Detection and Characterization of Transient Germylenes and Digermenes in Solution

Diphenylgermylene (Ph2Ge) and its Ge=Ge doubly bonded dimer, tetraphenyldigermene (6a), have been characterized directly in solution for the first time by laser flash photolysis methods. The germylene is formed via (formal) cheletropic photocycloreversion of 3,4-dimethyl-1,1-diphenylgermacyclopent-3-ene (4a), which is shown to proceed in high chemical (>95%) and quantum yield (phi = 0.62) by steady-state trapping experiments with methanol, acetic acid, isoprene, and triethylsilane. Flash photolysis of 4a in dry deoxygenated hexane at 23 degrees C leads to the prompt formation of a transient assigned to Ph2Ge (lambda(max) = 500 nm; epsilon(max) = 1650 M(-1) cm(-1)), which decays with second-order kinetics (tau approximately 3 micros), with the concomitant growth of a second transient species that is assigned to digermene 6a (tau approximately 40 micros; lambda(max) = 440 nm). Analogous results are obtained from 1,1-dimesityl- and 1,1-dimethyl-3,4-dimethylgermacyclopent-3-ene (4b and 4c, respectively), which afford Mes2Ge (tau approximately 20 micros; lambda(max) = 560 nm) and Me2Ge (tau approximately 2 micros; lambda(max) = 480 nm), respectively, as well as the corresponding digermenes, tetramesityl- (6b; lambda(max) = 410 nm) and tetramethyldigermene (6c; lambda(max) = 370 nm). The results for the mesityl compound are compared to the analogous ones from laser flash photolysis of the known Mes2Ge/6b precursor, hexamesitylcyclotrigermane. The spectra of the three germylenes and two of the digermenes are in excellent agreement with calculated spectra, derived from time-dependent DFT calculations. Absolute rate constants for dimerization of Ph2Ge and Mes2Ge and for their reaction with n-butylamine and acetic acid in hexane at 23 degrees C are also reported.

The insertion of germylene into the H—H bond; rate constant limits and thermochemistry. Ab initio and DFT calculations on the reactions of GeH2 and SiH2 with H2

The technique of laser flash photolysis in the gas-phase has been used to set limits on the rate constants for the bimolecular reaction of germylene (GeH2) with deuterium (D2) at both ambient and elevated temperatures (585 K). These limits show that the activation energy for the insertion of GeH2 into the H—H bond is at least 19 (±6) kJ mol–1. Thermochemical arguments place the activation energy approximately in the range 63–84 kJ mol–1. DFT B3LYP/6-311++G(3df,2pd) and ab initio QCISD(T)/6-311G++(3df,2pd)//QCISD/6-311G(d,p) calculations have been carried out on the potential energy surfaces of reactions ZH2 + H2 [Formula: see text] ZH4 (Z= Ge, Si). Both methods predict the same mechanisms for germylene and silylene insertion which include formation of loose prereaction complexes and transition states of similar structure. The prereaction complex is only about half as strong in the case of germylene (ΔH (298 K) = –9 (–11) kJ mol–1) as in the case of silylene (ΔH (298 K) = –16 (–21) kJ mol–1) (QCISD values cited with B3LYP values in parentheses). The differences in activation energies are even more significant. Germylene insertion has a very high barrier of 58 (56) kJ mol–1 compared to that of silylene 13 (6) kJ mol–1. Calculated activation parameters for both reactions are in reasonable consistency with experimental results. Reasons for the enhanced H—H insertion barrier for germylene compared with silylene are discussed.Key words: laser flash photolysis, germylene, silylene, deuterium, activation energy, thermochemistry, ab initio calculation, DFT B3LYP calculation.