A high-resolution pulsed field ionization-photoelectron-photoion coincidence study of vinyl bromide

By employing the high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method, we have examined the unimolecular dissociation reaction of energy-selected C(2)H(3)Br(+) to form C(2)H(3) (+)+Br near its threshold. The analysis of the breakdown curves for C(2)H(3)Br(+) and C(2)H(3) (+) yields a value of 11.9010+/-0.0015 eV for the 0 K dissociative photoionization threshold or appearance energy (AE) for C(2)H(3) (+) from C(2)H(3)Br. This AE(C(2)H(3) (+)) value, together with the ionization energy (IE) for C(2)H(3)Br (9.8200+/-0.0015 eV) obtained by PFI-PE and threshold photoelectron (TPE) measurements, has allowed the determination of the 0 K dissociation energy (D(0)) for the C(2)H(3) (+)-Br bond to be 2.081+/-0.002 eV. The 0 K AE(C(2)H(3) (+)) from C(2)H(3)Br obtained in this study corresponds to DeltaH(f0) ( composite function )(C(2)H(3) (+))=1123.7+/-1.9 kJ/mol. Combining the latter value and the known DeltaH(f0) ( composite function )(C(2)H(3))=306.7+/-2.1 kJ/mol, we calculated a value of 8.468+/-0.029 eV for the IE(C(2)H(3)), which is in accord with the result obtained in the previous photoionization efficiency study. We have also carried out high-level ab initio calculations for the IE(C(2)H(3)) at the Gaussian-3 and the CCSD(T,full)/CBS level of theory. The CCSD(T,full)/CBS prediction of 8.487 eV for the IE(C(2)H(3)-->bridged-C(2)H(3) (+)) is in good agreement with the IE(C(2)H(3)) value derived in the present experiment. Combining the 0 K AE(C(2)H(3) (+))=11.9010+/-0.0015 eV and the IE(C(2)H(3))=8.468+/-0.029 eV yields the value of 3.433+/-0.029 eV for D(0)(C(2)H(3)-Br). We have also recorded the TPE spectrum of C(2)H(3)Br in the energy range of 9.80-12.20 eV. Members (n=5-14) of four autoionizing Rydberg series converging to the C(2)H(3)Br(+)(A (2)A(')) state are observed in the TPE spectrum. The analysis of the converging limit of these Rydberg series and the vibrational TPE bands for C(2)H(3)Br(+)(A (2)A(')) has provided more precise values for the nu(6) (+) (1217+/-10 cm(-1)) and nu(8) (+) (478+/-8 cm(-1)) modes and the IE (10.9156+/-0.0010 eV) for the formation of C(2)H(3)Br(+)(A (2)A(')) from C(2)H(3)Br.     

Heats of formation of Co(CO)(2)NOPR(3), R = CH(3) and C(2)H(5), and its ionic fragments

A joint threshold photoelectron photoion coincidence spectrometry (TPEPICO) and collision-induced dissociation (CID) study on the thermochemistry of Co(CO)(2)NOPR(3), R = CH(3) (Me) and C(2)H(5) (Et), complexes is presented. Adiabatic ionization energies of 7.36 +/- 0.04 and 7.24 +/- 0.04 eV, respectively, were extracted from scans of the total ion and threshold electron signals. In the TPEPICO study, the following 0 K onsets were determined for the various fragment ions: CoCONOPMe(3)(+), 8.30 +/- 0.05 eV; CoNOPMe(3)(+), 9.11 +/- 0.05 eV; CoPMe(3)(+) 10.80 +/- 0.05 eV; CoCONOPEt(3)(+), 8.14 +/- 0.05 eV; CoNOPEt(3)(+), 8.92 +/- 0.05 eV; and CoPEt(3)(+), 10.66 +/- 0.05 eV. These onsets were combined with the Co(+)-PR(3) (R = CH(3) and C(2)H(5)) bond dissociation energies of 2.88 +/- 0.11 and 3.51 +/- 0.17 eV, obtained from the TCID experiments, to derive the heats of formation of the neutral and ionic species. Thus, the Co(CO)(2)NOPR(3) (R = CH(3) and C(2)H(5)) 0 K heats of formation were found to be -350 +/- 13 and -376 +/- 18 kJ x mol(-)(1), respectively. These heats of formation were combined with the published heat of formation of Co(CO)(3)NO to determine the substitution enthalpies of the carbonyl to phosphine substitution reactions. Room-temperature values of the heats of formation are also given using the calculated harmonic vibrational frequencies. Analysis of the TCID experimental results provides indirectly the adiabatic ionization energies of the free phosphine ligands, P(CH(3))(3) and P(C(2)H(5))(3), of 7.83 +/- 0.03 and 7.50 +/- 0.03 eV, respectively.     

Electron transfer reactivity in matrix-assisted laser desorption/ionization (MALDI): ionization energy, electron affinity and performance of the DCTB matrix within the thermochemical framework

DCTB [(H(3)C)(3)C-p-Ph-CH=C(CH(3))-trans-CH=C(CN)(2)] has recently advanced to the most promising matrix material for matrix-assisted laser desorption/ionization (MALDI) within material sciences. However, data that would allow the evaluation of the electron-transfer reactivity within a thermochemical framework are sparse. The present study reports the first-time determination of the ionization energy (IE) of DCTB applying photoelectron (PE) spectroscopy. The experimental IE (8.54 +/- 0.05 eV) is in excellent agreement with the theoretical value of 8.47 eV, obtained by AM1 calculations. The same level of theory determines the electron affinity (EA) as 2.31 eV. Model analytes of known thermochemistry (phenanthrene [C(14)H(10)], anthracene [C(14)H(10)] and fluorofullerene [C(60)F(46/48)]) are used to bracket the electron-transfer reactivity within DCTB-MALDI. The formation of molecular ions of these analytes either is expected or is beyond the thermochemical accessibility of the DCTB matrix.     

Zero kinetic energy photoelectron study of SO2+(X2A1) using coherent extreme ultraviolet radiation

Using our newly built extreme ultraviolet (XUV) photoelectron and photoion spectrometer, we have obtained the pulsed field ionization zero kinetic energy (ZEKE) photoelectron spectra of SO2+(X2A1)<--SO2(X1A1) by coherent XUV radiation in the energy range of 12.29-12.82 eV. The adiabatic ionization potential (IP) of SO2 is 12.3458+/-0.0002 (eV), which was determined by comparing the partially resolved rotational branch contour with the simulated one. Besides the bending vibrational mode (upsilon2) which was found to be exclusive in the photoelectron spectra (PE) reported previously, we also observed the other two modes: the symmetric stretching (upsilon1) and the antisymmetric stretching (upsilon3) vibrations. The fundamental of the symmetric stretching (upsilon(1)) is 1057 cm(-1) and the overtone of the antisymmetric stretching (2upsilon(3)) is 2494 cm(-1). The new vibrational progressions (upsilon(1)00)+, (1upsilon(2)0)+, (2upsilon(2)0)+, and (0upsilon(2)2)+ have also been observed, and these new observations suggested that the irregular structure of (0upsilon(2)0)+ assigned to the previous PE spectra should be reconsidered. The comparison of the intensities of these vibrational bands with the calculated Franck-Condon factors with harmonic approximation was also made.     

Thermochemical studies of pyrazolide

The 351.1 nm photoelectron spectrum of 1-pyrazolide anion has been measured. The 1-pyrazolide ion is produced by hydroxide (HO(-)) deprotonation of pyrazole in a flowing afterglow ion source. The electron affinity (EA) of the 1-pyrazolyl radical has been determined to be 2.938 +/- 0.005 eV. The angular dependence of the photoelectrons indicates near-degeneracy of low-lying states of 1-pyrazolyl. The vibronic feature of the spectrum suggests significant nonadiabatic effects in these electronic states. The gas phase acidity of pyrazole has been determined using a flowing afterglow-selected ion flow tube; Delta(acid)G(298) = 346.4 +/- 0.3 kcal mol(-1) and Delta(acid)H(298) = 353.6 +/- 0.4 kcal mol(-1). The N-H bond dissociation energy (BDE) of pyrazole is derived to be D(0)(pyrazole, N-H) = 106.4 +/- 0.4 kcal mol(-1) from the EA and the acidity using a thermochemical cycle. In addition to 1-pyrazolide, the photoelectron spectrum demonstrates that HO(-) deprotonates pyrazole at the C5 position to generate a minor amount of 5-pyrazolide anion. The photoelectron spectrum of 5-pyrazolide has been successfully reproduced by a Franck-Condon (FC) simulation based on the optimized geometries and the normal modes obtained from B3LYP/6-311++G(d,p) electronic structure calculations. The EA of the 5-pyrazolyl radical is 2.104 +/- 0.005 eV. The spectrum exhibits an extensive vibrational progression for an in-plane CCN bending mode, which indicates a substantial difference in the CCN angle between the electronic ground states of 5-pyrazolide and 5-pyrazolyl. Fundamental vibrational frequencies of 890 +/- 15, 1110 +/- 35, and 1345 +/- 30 cm(-1) have been assigned for the in-plane CCN bending mode and two in-plane bond-stretching modes, respectively, of X (2)A' 5-pyrazolyl. The physical properties of the pyrazole system are compared to the isoelectronic systems, pyrrole and imidazole.     

Photoelectron spectroscopy of phosphorus hydride anions

Negative-ion photoelectron spectroscopy is applied to the PH-, PH2-, P2H-, P2H2-, and P2H3-molecular anions. Franck-Condon simulations of the photoelectron spectra are used to analyze the spectra and to identify various P2H(n)- species. The simulations employ density-functional theory calculations of molecular geometries and vibrational frequencies and normal modes, and coupled-cluster theory calculations of electron affinities. The following electron affinities are obtained: EA0(PH) = 1.027 +/- 0.006 eV, EA0(PH2) = 1.263 +/- 0.006 eV, and EA0(P2H) = 1.514 +/- 0.010 eV. A band is identified as a mixture of trans-HPPH- and cis-HPPH-. Although the trans and cis bands cannot be definitively assigned from experimental information, using theory as a guide we obtain EA0(trans-HPPH)= 1.00 +/- 0.01 eV and EA0(cis-HPPH) = 1.03 +/- 0.01 eV. A weak feature tentatively assigned to P2H3- has a vertical detachment energy of 1.74 eV. The derived gas-phase acidity of phosphine is delta(acid)G298(PH3) < or = 1509.7 +/- 2.1 kJ mo1(-1).     

End-cap effects on vibrational structures of finite-length carbon nanotubes

Vibrational structures of C60-related finite-length nanotubes, C(40+20n) and C(42+18n) (1 < or = n < or = 4), in which n is, respectively, the number of cyclic cis- and trans-polyene chains inserted between fullerene hemispheres, are analyzed from density functional theory (DFT) calculations. To illuminate the end-cap effects on their vibrational structures, the corresponding tubes terminated by H atoms C(20n)H20 and C(18n)H18 (1 < or = n < or = 5) are also investigated. DFT calculations show a broad range of vibrational frequencies for the finite-size nanotubes: high-frequency modes (1100-1600 cm(-1)) containing oscillations along tangential directions (tangential modes), medium-frequency modes (700-850 cm(-1)) whose oscillations are located on the edges or end caps, and low-frequency modes (300-600 cm(-1)) involving oscillations along the radial directions (radial modes). Broadening of the calculated frequencies is due to the number of nodes in the standing waves of normal modes in the finite-size tubes. In the capped tubes, calculated vibrational frequencies are insensitive to the number of chains (n), whereas in the uncapped tubes, most vibrational frequencies change significantly with an increase in tube length. The discrepancy in the size dependency is reasonably understood by their C-C bonding networks; the capped tubes have similar bond-length alternation patterns within the polyene chains irrespective of n, whereas the uncapped tubes have various bond-deformation patterns. Thus, DFT calculations illuminate that the edge effects have strong impacts on the vibrational frequencies in the finite-size nanotubes.     

Infrared vacuum-ultraviolet laser pulsed field ionization-photoelectron study of CH3Br+ (X(2)E(3/2))

By preparing methyl bromide (CH3Br) in selected rotational levels of the CH3Br(X(1)A1; v1 = 1) state with infrared (IR) laser excitation prior to vacuum-ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) measurements, we have observed rotationally resolved photoionization transitions to the CH3Br(+)(X(2)E(3/2); v1(+) = 1) state, where v1 and v1(+) are the symmetric C-H stretching vibrational mode for the neutral and cation, respectively. The VUV-PFI-PE origin band for CH3Br(+)(X(2)E(3/2)) has also been measured. The simulation of these IR-VUV-PFI-PE and VUV-PFI-PE spectra have allowed the determination of the v1(+) vibrational frequency (2901.8 +/- 0.5 cm(-1)) and the ionization energies of the origin band (85 028.3 +/- 0.5 cm(-1)) and the v1(+) = 1 <-- v1 = 1 band (84 957.9 +/- 0.5 cm(-1)).     

Photoelectron spectra and ion chemistry of imidazolide

The 351.1 nm photoelectron spectrum of imidazolide anion has been measured. The electron affinity (EA) of the imidazolyl radical is determined to be 2.613 +/- 0.006 eV. Vibrational frequencies of 955 +/- 15 and 1365 +/- 20 cm(-1) are observed in the spectrum of the (2)B1 ground state of the imidazolyl radical. The main features in the spectrum are well-reproduced by Franck-Condon simulation based on the optimized geometries and the normal modes obtained at the B3LYP/6-311++G(d,p) level of density functional theory. The two vibrational frequencies are assigned to totally symmetric modes with C-C and N-C stretching motions. Overtone peaks of an in-plane nontotally symmetric mode are observed in the spectrum and attributed to Fermi resonance. Also observed is the photoelectron spectrum of the anion formed by deprotonation of imidazole at the C5 position. The EA of the corresponding radical, 5-imidazolyl, is 1.992 +/- 0.010 eV. The gas phase acidity of imidazole has been determined using a flowing afterglow-selected ion tube; delta(acid)G298 = 342.6 +/- 0.4 and delta(acid)H298 = 349.7 +/- 0.5 kcal mol(-1). From the EA of imidazolyl radical and gas phase acidity of imidazole, the bond dissociation energy for the N-H bond in imidazole is determined to be 95.1 +/- 0.5 kcal mol(-1). These thermodynamic parameters for imidazole and imidazolyl radical are compared with those for pyrrole and pyrrolyl radical, and the effects of the additional N atom in the five-membered ring are discussed.     

A photoelectron photoion coincidence study of the vinyl bromide and tribromoethane ion dissociation dynamics: heats of formation of C2H3+, C2H3Br, C2H3Br+, C2H3Br2+, and C2H3Br3

The threshold photoelectron photoion coincidence (TPEPICO) technique has been used to measure accurate dissociative photoionization onsets of vinyl bromide and 1,1,2-tribromoethane. The reactions investigated and their 0 K onsets are C2H3Br + hnu --> C2H3+ + Br (11.902 +/- 0.008 eV); C2H3Br3 + hnu --> C2H3Br2+ + Br (10.608 +/- 0.008 eV); and (C2H3Br3 + hnu --> C2H3Br+ + 2Br (12.301 +/- 0.035 eV). The vinyl ion heat of formation (Delta(f)H degrees 298K = 1116.1 +/- 3.0 kJ/mol) has been calculated using W1 theory and used as an anchor along with the measured dissociation energies to determine the heats of formation, Delta(f)H degrees 298K, in kJ/mol, of the following bromine-containing species: C2H3Br (74.1 +/- 3.1), C2H3Br+ (1021.9 +/- 3.1), C2H3Br2+ (967.1 +/- 4.0), and C2H3Br3 (53.5 +/- 4.3). These results represent accurate and consistent experimental determinations of heats of formation for these bromine-containing species, which serve to correct the discrepancies in the literature for C2H3Br and C2H3Br+ and provide the first experimental determination for the enthalpies of formation of C2H3Br2+ and C2H3Br3.     

Negative-ion photoelectron spectroscopy, gas-phase acidity, and thermochemistry of the peroxyl radicals CH(3)OO and CH(3)CH(2)OO

Methyl, methyl-d(3), and ethyl hydroperoxide anions (CH(3)OO(-), CD(3)OO(-), and CH(3)CH(2)OO(-)) have been prepared by deprotonation of their respective hydroperoxides in a stream of helium buffer gas. Photodetachment with 364 nm (3.408 eV) radiation was used to measure the adiabatic electron affinities: EA[CH(3)OO, X(2)A' '] = 1.161 +/- 0.005 eV, EA[CD(3)OO, X(2)A' '] = 1.154 +/- 0.004 eV, and EA[CH(3)CH(2)OO, X(2)A' '] = 1.186 +/- 0.004 eV. The photoelectron spectra yield values for the term energies: Delta E(X(2)A' '-A (2)A')[CH(3)OO] = 0.914 +/- 0.005 eV, Delta E(X(2)A' '-A (2)A')[CD(3)OO] = 0.913 +/- 0.004 eV, and Delta E(X(2)A' '-A (2)A')[CH(3)CH(2)OO] = 0.938 +/- 0.004 eV. A localized RO-O stretching mode was observed near 1100 cm(-1) for the ground state of all three radicals, and low-frequency R-O-O bending modes are also reported. Proton-transfer kinetics of the hydroperoxides have been measured in a tandem flowing afterglow-selected ion flow tube (FA-SIFT) to determine the gas-phase acidity of the parent hydroperoxides: Delta(acid)G(298)(CH(3)OOH) = 367.6 +/- 0.7 kcal mol(-1), Delta(acid)G(298)(CD(3)OOH) = 367.9 +/- 0.9 kcal mol(-1), and Delta(acid)G(298)(CH(3)CH(2)OOH) = 363.9 +/- 2.0 kcal mol(-1). From these acidities we have derived the enthalpies of deprotonation: Delta(acid)H(298)(CH(3)OOH) = 374.6 +/- 1.0 kcal mol(-1), Delta(acid)H(298)(CD(3)OOH) = 374.9 +/- 1.1 kcal mol(-1), and Delta(acid)H(298)(CH(3)CH(2)OOH) = 371.0 +/- 2.2 kcal mol(-1). Use of the negative-ion acidity/EA cycle provides the ROO-H bond enthalpies: DH(298)(CH(3)OO-H) = 87.8 +/- 1.0 kcal mol(-1), DH(298)(CD(3)OO-H) = 87.9 +/- 1.1 kcal mol(-1), and DH(298)(CH(3)CH(2)OO-H) = 84.8 +/- 2.2 kcal mol(-1). We review the thermochemistry of the peroxyl radicals, CH(3)OO and CH(3)CH(2)OO. Using experimental bond enthalpies, DH(298)(ROO-H), and CBS/APNO ab initio electronic structure calculations for the energies of the corresponding hydroperoxides, we derive the heats of formation of the peroxyl radicals. The "electron affinity/acidity/CBS" cycle yields Delta(f)H(298)[CH(3)OO] = 4.8 +/- 1.2 kcal mol(-1) and Delta(f)H(298)[CH(3)CH(2)OO] = -6.8 +/- 2.3 kcal mol(-1).     

Polymorphism in Br2 clathrate hydrates

The structure and composition of bromine clathrate hydrate has been controversial for more than 170 years due to the large variation of its observed stoichiometries. Several different crystal structures were proposed before 1997 when Udachin et al. (Udachin, K. A.; Enright, G. D.; Ratcliffe, C. I.; Ripmeester, J. A. J. Am. Chem. Soc. 1997, 119, 11481) concluded that Br2 forms only the tetragonal structure (TS-I). We show polymorphism in Br2 clathrate hydrates by identifying two distinct crystal structures through optical microscopy and resonant Raman spectroscopy on single crystals. After growing TS-I crystals from a liquid bromine-water solution, upon dropping the temperature slightly below -7 degrees C, new crystals of cubic morphology form. The new crystals, which have a limited thermal stability range, are assigned to the CS-II structure. The two structures are clearly distinguished by the resonant Raman spectra of the enclathrated Br2, which show long overtone progressions and allow the extraction of accurate vibrational parameters: omega(e) = 321.2 +/- 0.1 cm(-1) and omega(e)x(e) = 0.82 +/- 0.05 cm(-1) in TS-I and omega(e) = 317.5 +/- 0.1 cm(-1) and omega(e)x(e) = 0.70 +/- 0.1 cm(-1) in CS-II. On the basis of structural analysis, the discovery of the CS-II crystals implies stability of a large class of bromine hydrate structures and, therefore, polymorphism.     

Ionization energies of LinX (n = 2, 3; X = Cl, Br, I) molecules

Molecules of Li(n)X (n = 2, 3; X = Cl, Br, I) were examined with a magnetic sector mass spectrometer by surface ionization using a triple rhenium filament impregnated with fullerene (C60). The ionization energies obtained for Li(2)Cl, Li(2)Br and Li(2)I molecules are 3.8 +/- 0.1, 3.9 +/- 0.1 and 4.0 +/- 0.1 eV, respectively. The first ionization energy of Li(2)Cl is documented, while there are no literature data for the ionization energies of Li(2)Br and Li(2)I. The molecules of Li(3)Cl, Li(3)Br and Li(3)I were detected experimentally for the first time with ionization energies of 4.0 +/- 0.1, 4.1 +/- 0.1 and 4.1 +/- 0.1 eV, respectively. The ionization energies of Li(n)X (n = 2, 3; X = Cl, Br, I) are in correlation with the theoretical prediction of their hyperlithiated configurations.     

Infrared-vacuum ultraviolet pulsed field ionization-photoelectron study of C2H4(+) using a high-resolution infrared laser

The infrared (IR)-vacuum ultraviolet (VUV)-pulsed field ionization-photoelectron (IR-VUV-PFI-PE) spectrum for C2H4(X1A(g), v11 = 1, N'(Ka'Kc') = 3(03)) in the VUV range of 83,000-84,800 cm(-1) obtained using a single mode infrared laser revealed 24 rotationally resolved vibrational bands for the ion C2H4(+)(X2B(3u)) ground state. The frequencies and symmetry of the vibrational bands thus determined, together with the anharmonic frequency predictions calculated at the CCSD(T)/aug-cc-pVQZ level, have allowed the unambiguous assignment of these vibrational bands. These bands are mostly combination bands. The measured frequencies of these bands yield the fundamental frequencies for v8+ = 1103 +/- 10 cm(-1) and v10+ = 813 +/- 10 cm(-1) of C2H4(+)(X2B(3u)), which have not been determined previously. The present IR-VUV-PFI-PE study also provides truly rovibrationally selected and resolved state-to-state cross sections for the photoionization transitions C2H4(X1A(g); v11, N'(Ka'Kc')) --> C2H4(+)(X2B(3u); vi+, N+(Ka+Kc+)), where N'(Ka'Kc') denotes the rotational level of C2H4(X1A(g); v11), and vi+ and N+(Ka+Kc+) represent the vibrational and rotational states of the cation.     

Vacuum ultraviolet pulsed-field ionization-photoelectron study of H2S in the energy range of 10-17 eV

Vacuum ultraviolet pulsed-field ionization-photoelectron (PFI-PE) spectra of H(2)S have been recorded at PFI-PE resolutions of 0.6-1.0 meV in the energy range of 10-17 eV using high-resolution synchrotron radiation. The PFI-PE spectrum, which covers the formation of the valence electronic states H(2)S(+) (X (2)B(1), A (2)A(1), and B (2)B(2)), is compared to the recent high-resolution He I photoelectron spectra of H(2)S obtained by Baltzer et al. [Chem. Phys. 195, 403 (1995)]. In addition to the overwhelmingly dominated origin vibrational band, the PFI-PE spectrum for H(2)S(+)(X (2)B(1)) is found to exhibit weak vibrational progressions due to excitation of the combination bands in the nu(1) (+) symmetric stretching and nu(2) (+) bending modes. While the ionization energy (IE) for H(2)S(+)(X (2)B(1)) obtained here is in accord with values determined in previously laser PFI-PE measurements, the observation of a new PFI-PE band at 12.642+/-0.001 eV suggests that the IE for H(2)S(+)(A (2)A(1)) may be 0.12 eV lower than that reported in the He I study. The simulation of rotational structures resolved in PFI-PE bands shows that the formation of H(2)S(+)(X (2)B(1)) and H(2)S(+)(A (2)A(1)) from photoionization of H(2)S(X (1)A(1)) is dominated by type-C and type-B transitions, respectively. This observation is consistent with predictions of the multichannel quantum defect theory. The small changes in rotational angular momentum observed are consistent with the dominant atomiclike character of the 2b(1) and 5a(1) molecular orbitals of H(2)S. The PFI-PE measurement has revealed perturbations of the (0, 6, 0) K(+)=3 and (0, 6, 0) K(+)=4 bands of H(2)S(+)(A (2)A(1)). Interpreting that these perturbations arise from Renner-Teller interactions at energies close to the common barriers to linearity of the H(2)S(+) (X (2)B(1) and A (2)A(1)) states, we have deduced a barrier of 23,209 cm(-1) for H(2)S(+)(X (2)B(1)) and 5668 cm(-1) for H(2)S(+)(A (2)A(1)). The barrier of 23 209 cm(-1) for H(2)S(+)(X (2)B(1)) is found to be in excellent agreement with the results of previous studies. The vibrational PFI-PE bands for H(2)S(+)(B (2)B(2)) are broad, indicative of the predissociative nature of this state.     

Probing the structural and electronic properties of Ag(n)H(-) (n = 1-3) using photoelectron imaging and theoretical calculations

Structural and electronic properties of silver hydride cluster anions (Ag(n)H(-); n = 1-3) have been explored by combining the negative ion photoelectron imaging spectroscopy and theoretical calculations. The photoelectron spectrum of AgH(-) exhibits transitions from AgH(- 2)Σ(+) to AgH (1)Σ(+) and AgH (3)Σ(+), with the electron affinity (EA) 0.57(3) eV. For Ag(2)H(-), the only observed transition is from Ag(2)H(-) (C(∞v)) (1)Σ(+) to Ag(2)H (C(2v)) (2)A(') and the electron affinity is 2.56(5) eV. Two obvious electron bands are observed in photoelectron imaging of Ag(3)H(-), which are assigned to the transitions from Ag(3)H(-) (C(2v)-T, which means C(2v) geometry with top site hydrogen) (2)B(2) to Ag(3)H (C(2v)-T) (1)A(1) and Ag(3)H (C(2v)-T) (3)B(2). The electron affinity is determined to be 1.61(9) eV. The Ag-H stretching modes in the ground states of AgH and Ag(2)H are experimentally resolved and their frequencies are measured to be 1710(80) and 1650(100) cm(-1), respectively. Aside from the above EAs and the vibrational frequencies, the vertical detachment energies to all ground states and some excited states of Ag(n)H (n = 1-3) are also obtained. Theoretical calculations reproduce the experimental energies quite well, and the results are used to assign the geometries and electronic states for all related species.     

Characterization of a shallow-bound 0g+ valence state of I2 using emission from the D 0u+(3P2) and F' 0u+(1D2) ion-pair states populated by amplified spontaneous emission

A range of vibrational levels of the D 0(u)(+)((3)P(2)) and F' 0(u)(+)((1)D(2)) ion-pair states of I(2) is shown to be easily generated by amplified spontaneous emission (ASE) from their more accessible partners, E 0(g)(+)((3)P(2)) and f' 0(g)(+)((1)D(2)), in sufficient concentration for dispersed fluorescence studies of the D 0(u)(+)((3)P(2)) --> 0(g)(+)(bb) and F' 0(u)(+)((1)D(2)) --> 0(g)(+)(bb) transitions to be carried out. T(0) (J = 49) of this shallow-bound 0(g)(+)(bb) valence state is unambiguously determined and an improved R(e) value of 3.952 +/- 0.005 A is obtained from optimizing the fit of the intensities of the vibrational progressions in the 0(g)(+)(bb) state, and T(e) is found to be 27311.3 +/- 2 cm(-1), leading to D(e) = 442.0 +/- 2 cm(-1).     

Influence of ligand polarizability on the reversible binding of O2 by trans-[Rh(X)(XNC)(PPh3)2] (X = Cl, Br, SC6F5, C2Ph; XNC = xylyl isocyanide). Structures and a kinetic study

The complexes trans-[Rh(X)(XNC)(PPh 3) 2] (X = Cl, 1; Br, 2; SC 6F 5, 3; C 2Ph, 4; XNC = xylyl isocyanide) combine reversibly with molecular oxygen to give [Rh(X)(O 2)(XNC)(PPh 3) 2] of which [Rh(SC 6F 5)(O 2)(XNC)(PPh 3) 2] ( 7) and [Rh(C 2Ph)(O 2)(XNC)(PPh 3) 2] ( 8) are sufficiently stable to be isolated in crystalline form. Complexes 2, 3, 4, and 7 have been structurally characterized. Kinetic data for the dissociation of O 2 from the dioxygen adducts of 1- 4 were obtained using (31)P NMR to monitor changes in the concentration of [Rh(X)(O 2)(XNC)(PPh 3) 2] (X = Cl, Br, SC 6F 5, C 2Ph) resulting from the bubbling of argon through the respective warmed solutions (solvent chlorobenzene). From data recorded at temperatures in the range 30-70 degrees C, activation parameters were obtained as follows: Delta H (++) (kJ mol (-1)): 31.7 +/- 1.6 (X = Cl), 52.1 +/- 4.3 (X = Br), 66.0 +/- 5.8 (X = SC 6F 5), 101.3 +/- 1.8 (X = C 2Ph); Delta S (++) (J K (-1) mol (-1)): -170.3 +/- 5.0 (X = Cl), -120 +/- 13.6 (X = Br), -89 +/- 18.2 (X = SC 6F 5), -6.4 +/- 5.4 (X = C 2Ph). The values of Delta H (++) and Delta S (++) are closely correlated (R (2) = 0.9997), consistent with a common dissociation pathway along which the rate-determining step occurs at a different position for each X. Relative magnitudes of Delta H (++) are interpreted in terms of differing polarizabilities of ligands X.     

Direct identification of propargyl radical in combustion flames by vacuum ultraviolet photoionization mass spectrometry

We have developed an effusive laser photodissociation radical source, aiming for the production of vibrationally relaxed radicals. Employing this radical source, we have measured the vacuum ultraviolet (VUV) photoionization efficiency (PIE) spectrum of the propargyl radical (C(3)H(3)) formed by the 193 nm excimer laser photodissociation of propargyl chloride in the energy range of 8.5-9.9 eV using high-resolution (energy bandwidth = 1 meV) multibunch synchrotron radiation. The VUV-PIE spectrum of C(3)H(3) thus obtained is found to exhibit pronounced autoionization features, which are tentatively assigned as members of two vibrational progressions of C(3)H(3) in excited autoionizing Rydberg states. The ionization energy (IE = 8.674 +/- 0.001 eV) of C(3)H(3) determined by a small steplike feature resolved at the photoionization onset of the VUV-PIE spectrum is in excellent agreement with the IE value reported in a previous pulsed field ionization-photoelectron study. We have also calculated the Franck-Condon factors (FCFs) for the photoionization transitions C(3)H(3) (+)(X;nu(i),i = 1-12)<--C(3)H(3)(X). The comparison between the pattern of FCFs and the autoionization peaks resolved in the VUV-PIE spectrum of C(3)H(3) points to the conclusion that the resonance-enhanced autoionization mechanism is most likely responsible for the observation of pronounced autoionization features. We also present here the VUV-PIE spectra for the mass 39 ions observed in the VUV synchrotron-based photoionization mass spectrometric sampling of several premixed flames. The excellent agreement of the IE value and the pattern of autoionizing features of the VUV-PIE spectra observed in the photodissociation and flames studies has provided an unambiguous identification of the propargyl radical as an important intermediate in the premixed combustion flames. The discrepancy found between the PIE spectra obtained in flames and photodissociation at energies above the IE(C(3)H(3)) suggests that the PIE spectra obtained in flames might have contributions from the photoionization of vibrationally excited C(3)H(3) and/or the dissociative photoionization processes involving larger hydrocarbon species formed in flames.