Direct determination of the ionization energies of PtC, PtO, and PtO2 with VUV radiation

Photoionization efficiency curves were measured for gas-phase PtC, PtO, and PtO2 using tunable vacuum ultraviolet (VUV) radiation at the Advanced Light Source. The molecules were prepared by laser ablation of a platinum tube, followed by reaction with CH4 or N2O and supersonic expansion. These measurements provide the first directly measured ionization energy for PtC, IE(PtC) = 9.45 +/- 0.05 eV. The direct measurement also gives greatly improved ionization energies for the platinum oxides, IE(PtO) = 10.0 +/- 0.1 eV and IE(PtO2) = 11.35 +/- 0.05 eV. The ionization energy connects the dissociation energies of the neutral and cation, leading to greatly improved 0 K bond dissociation energies for the neutrals: D0(Pt-C) = 5.95 +/- 0.07 eV, D0(Pt-O) = 4.30 +/- 0.12 eV, and D0(OPt-O) = 4.41 +/- 0.13 eV, as well as enthalpies of formation for the gas-phase molecules DeltaH(0)(f,0)(PtC(g)) = 701 +/- 7 kJ/mol, DeltaH(0)(f,0)(PtO(g)) = 396 +/- 12 kJ/mol, and DeltaH(0)(f,0)(PtO2(g)) = 218 +/- 11 kJ/mol. Much of the error in previous Knudsen cell measurements of platinum oxide bond dissociation energies is due to the use of thermodynamic second law extrapolations. Third law values calculated using statistical mechanical thermodynamic functions are in much better agreement with values obtained from ionization energies and ion energetics. These experiments demonstrate that laser ablation production with direct VUV ionization measurements is a versatile tool to measure ionization energies and bond dissociation energies for catalytically interesting species such as metal oxides and carbides.     

The dissociation kinetics of energy-selected CpMn(CO)(3)(+) ions studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy has been used to investigate the dissociation kinetics of the cyclopentadienyl manganese tricarbonyl ion, CpMn(CO)(3)(+). The ionization energy of CpMn(CO)(3) was measured from the threshold photoelectron spectrum to be 7.69 +/- 0.02 eV. The dissociation of the CpMn(CO)(3)(+) ion proceeds by the sequential loss of three CO molecules. The first and third CO loss reactions were observed to be slow (lifetimes in the microsecond range). By simulating the resulting asymmetric time-of-flight peak shapes and breakdown diagram, 0 K onsets for three product ions were determined to be 8.80 +/- 0.04, 9.43 +/- 0.04, and 10.51 +/- 0.06 eV, respectively. Combined with the adiabatic ionization energy, the three successive Mn-CO bond energies in the CpMn(CO)(3)(+) were found to be alternating with values of 1.11 +/- 0.04, 0.63 +/- 0.04, and 1.08 +/- 0.06 eV, respectively. Using a scaled theoretical Cp-Mn(+) bond energy of 3.10 +/- 0.10 eV and the combined results from theory and experiment, the 298 K gas-phase heat of formation of CpMn(CO)(3) is suggested to be -419 +/- 15 kJ/mol. Based on this value, the 298 K heats of formation of CpMn(CO)(3)(+), CpMn(CO)(2)(+), CpMnCO(+), and CpMn(+) are 325 +/- 15, 546 +/- 15, 719 +/- 15, and 938 +/- 15 kJ/mol, respectively. By scaling theoretical calculated neutral bond energies with the experimental information derived in this study, the successive Mn-CO bond energies were estimated to be 1.88, 1.10, and 1.03 eV, respectively, while the Cp-Mn bond energy was found to be 2.16 eV. Comparison between the quantum chemical calculations and experimental values shows that the loss of CO groups follows the lowest energy adiabatic path, in which electronic spin on the metal center is not conserved.     

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.     

A mass spectrometric and density functional study of the intermetallic molecules AuBe, AuMg, and AuCa

The intermetallic molecules AuBe and AuCa were identified by means of the Knudsen-Effusion Mass Spectrometry technique in the high-temperature vapors produced by vaporizing Au-Be-Ca alloys of proper composition. The gaseous equilibria AuBe(g)+Au(g)=Au(2)(g)+Be(g) and AuCa(g)+Au(g)=Au(2)(g)+Ca(g) were studied in the temperature ranges 1720-1841 K and 1669-1841 K, respectively, by monitoring the partial pressures of all the species involved. The equilibrium data were analyzed by the third-law method, obtaining for the first time the dissociation energy D(0) ( composite function) of the two intermetallic species: D(0) ( composite function)(AuBe)=234.0+/-4.0 kJ/mol; D(0) ( composite function)(AuCa)=246.7+/-4.0 kJ/mol. These values are significantly higher than the recently published D(0) ( composite function) of the species AuMg (175.4+/-2.7 kJ/mol). Furthermore, the ionization energies (IE) of AuBe, AuMg, and AuCa were obtained by measuring the electron impact ionization efficiency curves, IE(AuBe)=7.5+/-0.3 eV, IE(AuMg)=6.7+/-0.3 eV, and IE(AuCa)=5.5+/-0.3 eV. Theoretical calculations were also carried out for these species by density functional theory methods (PW91 and BP86) used in conjunction with Stuttgart relativistic effective core potentials. Both functionals were found to perform very well in reproducing experimental D(0) ( composite function), IE, and molecular parameters.     

Gas-phase oxidation of Cm+ and Cm2+ --thermodynamics of neutral and ionized CmO

Fourier transform ion cyclotron resonance mass spectrometry was employed to study the products and kinetics of gas-phase reactions of Cm (+) and Cm (2+); parallel studies were carried out with La (+/2+), Gd (+/2+) and Lu (+/2+). Reactions with oxygen-donor molecules provided estimates for the bond dissociation energies, D[M (+)-O] (M = Cm, Gd, Lu). The first ionization energy, IE[CmO], was obtained from the reactivity of CmO (+) with dienes, and the second ionization energies, IE[MO (+)] (M = Cm, La, Gd, Lu), from the rates of electron-transfer reactions from neutrals to the MO (2+) ions. The following thermodynamic quantities for curium oxide molecules were obtained: IE[CmO] = 6.4 +/- 0.2 eV; IE[CmO (+)] = 15.8 +/- 0.4 eV; D[Cm-O] = 710 +/- 45 kJ mol (-1); D[Cm (+)-O] = 670 +/- 40 kJ mol (-1); and D[Cm (2+)-O] = 342 +/- 55 kJ mol (-1). Estimates for the M (2+)-O bond energies for M = Cm, La, Gd, and Lu are all intermediate between D[N 2-O] and D[OC-O] - that is, 167 kJ mol (-1) < D[M (2+)-O] < 532 kJ mol (-1) - such that the four MO (2+) ions fulfill the thermodynamic requirement for catalytic oxygen-atom transport from N2O to CO. It was demonstrated that the kinetics are also favorable and that the CmO (2+), LaO (2+), GdO (2+), and LuO (2+) dipositive ions each catalyze the gas-phase oxidation of CO to CO2 by N2O. The CmO 2 (+) ion appeared during the reaction of Cm (+) with O 2 when the intermediate, CmO (+), was not collisionally cooled - although its formation is kinetically and/or thermodynamically unfavorable, CmO 2 (+) is a stable species.     

Heats of formation of C(6)H(5)(•), C(6)H(5)(+), and C(6)H(5)NO by threshold photoelectron photoion coincidence and active thermochemical tables analysis

Threshold photoelectron photoion coincidence has been used to prepare selected internal energy distributions of nitrosobenzene ions [C(6)H(5)NO(+)]. Dissociation to C(6)H(5)(+) + NO products was measured over a range of internal energies and rate constants from 10(3) to 10(7) s(-1) and fitted with the statistical theory of unimolecular decay. A 0 K dissociative photoionization onset energy of 10.607 ± 0.020 eV was derived by using the simplified statistical adiabatic channel model. The thermochemical network of Active Thermochemical Tables (ATcT) was expanded to include phenyl and phenylium, as well as nitrosobenzene. The current ATcT heats of formation of these three species at 0 K (298.15 K) are 350.6 (337.3) ± 0.6, 1148.7 (1136.8) ± 1.0, and 215.6 (198.6) ± 1.5 kJ mol(-1), respectively. The resulting adiabatic ionization energy of phenyl is 8.272 ± 0.010 eV. The new ATcT thermochemistry for phenyl entails a 0 K (298.15 K) C-H bond dissociation enthalpy of benzene of 465.9 (472.1) ± 0.6 kJ mol(-1). Several related thermochemical quantities from ATcT, including the current enthalpies of formation of benzene, monohalobenzenes, and their ions, as well as interim ATcT values for the constituent atoms, are also given.     

Thermochemistry of N-N containing ions: a threshold photoelectron photoion coincidence spectroscopy study of ionized methyl- and tetramethylhydrazine

The unimolecular dissociation reactions of the methylhydrazine (MH) and tetramethylhydrazine (TMH) radical cations have been investigated using tandem mass spectrometry and threshold photoelectron photoion coincidence spectroscopy in the photon energy ranges 9.60-31.95 eV (for the MH ion) and 7.74-29.94 eV (for the TMH ion). Methylhydrazine ions (CH3NHNH2(+*)) have three low-energy dissociation channels: hydrogen atom loss to form CH2NHNH2(+) (m/z 45), loss of a methyl radical to form NHNH2(+) (m/z 31), and loss of methane to form the fragment ion m/z 30, N2H2(+*). Tetramethylhydrazine ions only exhibit two dissociation reactions near threshold: that of methyl radical loss to form (CH3)2NNCH3(+) (m/z 73) and of methane loss to form the fragment ion m/z 72 with the empirical formula C3H8N2(+*). The experimental breakdown curves were modeled with Rice-Ramsperger-Kassel-Marcus theory, and it was found that, particularly for methyl radical loss, variational transition state theory was needed to obtain satisfactory fits to the data. The 0 K enthalpies of formation (delta(f)H0) for all fragment ions (m/z 73, m/z 72, m/z 45, m/z 31, and m/z 30) have been determined from the 0 K activation energies (E0) obtained from the fitting procedure: delta(f)H0[(CH3)2NNCH3(+)] = 833 +/- 5 kJ mol(-1), delta(f)H0 [C3H8N2(+*)] = 1064 +/- 5 kJ mol(-1), delta(f)H0[CH2NHNH2(+)] = 862 +/- 5 kJ mol(-1), delta(f)H0[NHNH2(+)] = 959 +/- 5 kJ mol(-1), and delta(f)H0[N2H2(+*)] = 1155 +/- 5 kJ mol(-1). The breakdown curves have been measured from threshold up to h nu approximately 32 eV for both hydrazine ions. As the photon energy increases, other dissociation products are observed and their appearance energies are reported.     

Dissociative photoionization of mono-, di- and trimethylamine studied by a combined threshold photoelectron photoion coincidence spectroscopy and computational approach

Energy selected mono-, di- and trimethylamine ions were prepared by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). Below 13 eV, the main dissociative photoionization path of these molecules is hydrogen atom loss. The ion time-of-flight (TOF) distributions and breakdown diagrams for H loss are analyzed in terms of the statistical RRKM theory, which includes tunneling. Experimental evidence, supported by quantum chemical calculations, indicates that the reverse barrier along the H loss potential energy curve for monomethylamine is 1.8 +/- 0.6 kJ mol(-1). Accurate dissociation onset energies are derived from the TOF simulation, and from this analysis we conclude that Delta(f)H degrees (298K)[CH(2)NH(2)(+)] = 750.4 +/- 1.3 kJ mol(-1) and Delta(f)H degrees (298K)[CH(2)NH(CH(3))(+)] = 710.9 +/- 2.8 kJ mol(-1). Quantum chemical calculations at the G3, G3B3, CBS-APNO and W1U levels are extensively used to support the experimental data. The comparison between experimental and ab initio isodesmic reaction heats also suggests that Delta(f)H degrees (298K)[N(CH(3))(3)] = -27.2 +/- 2 kJ mol(-1), and that the dimethylamine ionization energy is 8.32 +/- 0.03 eV, both of which are in slight disagreement with previous experimental values. Above 13 eV photon energy, additional dissociation channels appear besides the H atom loss, such as a sequential C(2)H(4) loss from trimethylamine for which a dissociation mechanism is proposed.     

TPEPICO spectroscopy of vinyl chloride and vinyl iodide: neutral and ionic heats of formation and bond energies

The 0 K dissociative ionization onsets of C2H3X --> C2H3(+) + X (X = Cl, I) are measured by threshold photoelectron-photoion coincidence spectroscopy. The heats of formation of C2H3Cl (Delta H(f,0K)(0) = 30.2 +/- 3.2 kJ mol(-1) and Delta(H f,298K)(0) = 22.6 +/- 3.2 kJ mol(-1)) and C2H3I (Delta(H f,0K)(0) = 140.2 +/- 3.2 kJ mol(-1) and Delta(H f,298K)(0) = 131.2 +/- 3.2 kJ mol(-1)) and C- X bond dissociation enthalpies as well as those of their ions are determined. The data help resolve a longstanding discrepancy among experimental values of the vinyl chloride heat of formation, which now agrees with the latest theoretical determination. The reported vinyl iodide heat of formation is the first reliable experimental determination. Additionally, the adiabatic ionization energy of C2H3I (9.32 +/- 0.01 eV) is measured by threshold photoelectron spectroscopy.     

Dissociation kinetics of energy-selected Cp(2)Mn(+) ions studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy has been used to investigate the dissociation kinetics of the manganocene ion, Cp(2)Mn(+) (Cp = eta(5)-cyclopentadienyl). The Cp loss reaction was found to be extremely slow over a large ion internal energy range. By simulating the measured asymmetric time-of-flight peak shapes and breakdown diagram, the 0 K thermochemical dissociation limit for CpMn(+) production was determined to be 9.55 +/- 0.15 eV. A CpMn(+)-Cp bond energy of 3.43 eV was obtained by combining this CpMn(+) + Cp dissociation limit with the Cp(2)Mn adiabatic ionization energy of 6.12 +/- 0.07 eV. Combining the measured onset with known heats of formation of Cp and Mn(+), the Cp-Mn(+) bond energy was determined to be 3.38 +/- 0.15 eV. These results lead to 298 K heats of formation of Cp(2)Mn(+) and CpMn(+) of 863 +/- 7 and 935 +/- 16 kJ/mol, respectively. Finally, by combining these results with a previous measurement of the CpMn(CO)(3) --> CpMn(+) + 3CO + e(-) dissociation limit, we arrive at a new value for Delta(f)H degrees (298K)(CpMn(CO)(3)) of -424 +/- 17 kJ/mol.     

Threshold photoelectron-photoion coincidence spectroscopy: dissociation dynamics and thermochemistry of Ge(CH3)4, Ge(CH3)3Cl, and Ge(CH3)3Br

Threshold photoelectron-photoion coincidence spectroscopy (TPEPICO) has been used to investigate the gas-phase ionic dissociation energies and thermochemistry of Me4Ge and Me3GeX, (Me = methyl; X = Cl, Br) molecules. The 0 K dissociation onsets for these species have been measured from the breakdown diagram and the ion time-of-flight distributions, which were modeled with the statistical RRKM theory and DFT calculations. The measured 0 K dissociative photoionization onsets were as follows: Me3Ge+ + Me (9.826 +/- 0.010 eV); Me3Ge+ + Cl (10.796 +/- 0.040 eV); Me3Ge+ + Br (10.250 +/- 0.011 eV); Me2GeCl+ + Me (10.402 +/- 0.010 eV); and Me2GeBr+ + Me (10.333 +/- 0.020 eV). These onsets were used to obtain new values for delta(f)H(degrees)298 (in kJ/mol) of the neutral molecules Me3GeCl (-239.8 +/- 5.7) and Me3GeBr (-196.5 +/- 4.3), and also for the following ionic species: Me3Ge+ (682.3 +/- 4.1), Me2GeCl+ (621.1 +/- 5.8), and Me2GeBr+ (657.8 +/- 4.7).     

Mass spectrometric and computational study of SnPb in the gas phase

The SnPb molecule has been identified in a Knudsen effusion mass spectrometry experiment. The direct dissociation reaction and two isomolecular exchange reactions involving the Sn(2) and Pb(2) molecules have been studied, in the 1426-1705 K range of temperatures, using both second and third law procedures. The D(degree)0(SnPb,g) has been derived, for the first time, as (122.6+/-4.0) kJ mol(-1). Density functional and ab initio calculations up to the coupled clusters level of theory were also performed. In addition, the anion dissociation energy D(degree)0(SnPb(-),g) of (179.2+/-4.2) kJ mol(-1) was determined using the D(degree)0(SnPb,g) mass spectrometric value derived in this investigation and literature data.     

Potential energy surface for activation of methane by Pt(+): a combined guided ion beam and DFT study

A guided-ion beam tandem mass spectrometer is used to study the reactions of Pt(+) with methane, PtCH(2)(+) with H(2) and D(2), and collision-induced dissociation of PtCH(4)(+) and PtCH(2)(+) with Xe. These studies experimentally probe the potential energy surface for the activation of methane by Pt(+). For the reaction of Pt(+) with methane, dehydrogenation to form PtCH(2)(+) + H(2) is exothermic, efficient, and the only process observed at low energies. PtH(+), formed in a simple C-H bond cleavage, dominates the product spectrum at high energies. The observation of a PtH(2)(+) product provides evidence that methane activation proceeds via a (H)(2)PtCH(2)(+) intermediate. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies in eV (kJ/mol) of D(0)(Pt(+)-H) = 2.81 +/- 0.05 (271 +/- 5), D(0)(Pt(+)-2H) = 6.00 +/- 0.12 (579 +/- 12), D(0)(Pt(+)-C) = 5.43 +/- 0.05 (524 +/- 5), D(0)(Pt(+)-CH) = 5.56 +/- 0.10 (536 +/- 10), and D(0)(Pt(+)-CH(3)) = 2.67 +/- 0.08 (258 +/- 8). D(0)(Pt(+)-CH(2)) = 4.80 +/- 0.03 eV (463 +/- 3 kJ/mol) is determined by measuring the forward and reverse reaction rates for Pt(+) + CH(4) right harpoon over left harpoon PtCH(2)(+) + H(2) at thermal energy. We find extensive hydrogen scrambling in the reaction of PtCH(2)(+) with D(2). Collision-induced dissociation (CID) of PtCH(4)(+), identified as the H-Pt(+)-CH(3) intermediate, with Xe reveals a bond energy of 1.77 +/- 0.08 eV (171 +/- 8 kJ/mol) relative to Pt(+) + CH(4). The experimental thermochemistry is favorably compared with density functional theory calculations (B3LYP using several basis sets), which also establish the electronic structures of these species and provide insight into the reaction mechanism. Results for the reaction of Pt(+) with methane are compared with those for the analogous palladium system and the differences in reactivity and mechanism 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.     

Metal-benzene and metal-CO bond energies in neutral and ionic C6H6Cr(CO)3 studied by threshold photoelectron-photoion coincidence spectroscopy and density functional theory

photoelectron-photoion coincidence spectroscopy and density functional theory calculations have been used to investigate the dissociation kinetics of the benzene chromium tricarbonyl ion, BzCr(CO)3+ (Bz = C6H6). The dissociation of the BzCr(CO)3+ ion proceeds by the sequential loss of three CO and benzene ligands. The first and third CO and the benzene loss reactions were associated with metastable precursor ions (lifetimes in the microsecond range). By simulating the resulting asymmetric time-of-flight peak shapes and breakdown diagram, the 0 K appearance energies of the four product ions were determined to be 8.33 +/- 0.05, 8.93 +/- 0.05, 9.97 +/- 0.06, and 11.71 +/- 0.06 eV, respectively. Combined with the ionization energy of BzCr(CO)3, 7.30 +/- 0.05 eV, the three successive Cr-CO bond energies in the BzCr(CO)3+ were found to alternate, with values of 1.03 +/- 0.05, 0.60 +/- 0.05, and 1.04 +/- 0.05 eV, respectively, and the Bz-Cr bond energy in BzCr+ is 1.74 +/- 0.05 eV, a trend confirmed by the density functional theory (DFT) calculations. Using the heats of formation of the fully dissociated products, C6H6, Cr+, and CO, the 298 K heats of formation the ionic BzCr(CO)n+ (n = 03) species were determined. By scaling the DFT calculated bond energies for the neutral molecules, the heats of formation of the neutral BzCr(CO)n (n = 03) were also obtained.     

Thermochemistry and dissociative photoionization of Si(CH3)4, BrSi(CH3)3, ISi(CH3)3, and Si2(CH3)6 studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy (TPEPICO) has been used to study the dissociation kinetics and thermochemistry of Me(4)Si, Me(6)Si(2), and Me(3)SiX, (X = Br, I) molecules. Accurate 0 K dissociative photoionization onsets for these species have been measured from the breakdown diagram and the ion time-of-flight distribution, both of them analyzed and simulated in terms of the statistical RRKM theory and DFT calculations. The average enthalpy of formation of trimethylsilyl ion, Delta fH(o)298K(Me(3)Si(+)) = 617.3 +/- 2.3 kJ/mol, has been determined from the measured onsets for methyl loss (10.243 +/- 0.010 eV) from Me(4)Si, and Br and I loss from Me(3)SiBr (10.624 +/- 0.010 eV) and Me(3)SiI (9.773 +/- 0.015 eV), respectively. The methyl loss onsets for the trimethyl halo silanes lead to Delta fH(o)298K(Me(2)SiBr(+)) = 590.3 +/- 4.4 kJ/mol and Delta fH(o)298K(Me(5)Si(2)(+)) = 487.6 +/- 6.2 kJ/mol. The dissociative photoionization of Me(3)SiSiMe(3) proceeds by a very slow Si-Si bond breaking step, whose rate constants were measured as a function of the ion internal energy. Extrapolation of this rate constant to the dissociation limit leads to the 0 K dissociation onset (9.670 +/- 0.030 eV). This onset, along with the previously determined trimethylsilyl ion energy, leads to an enthalpy of formation of the trimethylsilyl radical, Delta fH(o)298K(Me(3)Si(*)) = 14.0 +/- 6.6 kJ/mol. In combination with other experimental values, we propose a more accurate average value for Delta fH(o)298K(Me(3)Si(*)) of 14.8 +/- 2.0 kJ/mol. Finally, the bond dissociation enthalpies (DeltaH(298K)) Si-H, Si-C, Si-X (X=Cl, Br, I) and Si-Si are derived and discussed in this study.     

Vacuum ultraviolet pulsed field ionization study of ND3: accurate thermochemistry for the ND2-ND2+ and ND3-ND3+ system

The dissociation of energy-selected ND(3) (+) to form ND(2) (+)+D near its threshold has been investigated using the pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method. The breakdown curves for ND(3) (+) and ND(2) (+) give a value of 15.891+/-0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for ND(2) (+) from ND(3). We have also measured the PFI-PE vibrational bands for ND(3) (+)(X;v(2) (+)=0, 1, 2, and 3), revealing partially resolved rotational structures. The simulation of these bands yields precise ionization energies (IEs) for ND(3) (+) X(0,v(2) (+)=0-3,0,0)<--ND(3) X(0,0,0,0). Using the 0 K AE (ND(2) (+)) and IE(ND(3))=10.200+/-0.001 eV determined in the present study, together with the known 0 K bond dissociation energy for ND(3) [D(0)(D-ND(2))=4.7126+/-0.0025 eV], we have determined the D(0)(ND(2) (+)-D), IE(ND(2)), and 0 K heat of formation for ND(2) (+) to be 5.691+/-0.001 eV, 11.1784+/-0.0025 eV, and 1261.82+/-0.4 kJ/mol, respectively. The PFI-PE spectrum is found to exhibit a steplike feature near the AE(ND(2) (+)), indicating that the dissociation of excited ND(3) (+) at energies slightly above the dissociation threshold is prompt, occurring in the time scale 相似文献   

Experimental and computational study on the molecular energetics of indoline and indole

Static bomb calorimetry, Calvet microcalorimetry and the Knudsen effusion technique were used to determine the standard molar enthalpy of formation in the gas phase, at T = 298.15 K, of the indole and indoline heterocyclic compounds. The values obtained were 164.3 +/- 1.3 kJ x mol(-1) and 120.0 +/- 2.9 kJ x mol(-1), respectively. Several different computational approaches and different working reactions were used to estimate the gas-phase enthalpies of formation for indole and indoline. The computational approaches support the experimental results reported. The calculations were further extended to the determination of other properties such as bond dissociation enthalpies, gas-phase acidities, proton and electron affinities and ionization energies. The agreement between theoretical and experimental data for indole is very good supporting the data calculated for indoline.     

Structures and heats of formation of the neutral and ionic PNO, NOP, and NPO systems from electronic structure calculations

High level ab initio electronic structure calculations using the coupled cluster CCSD(T) method with augmented correlation-consistent basis sets extrapolated to the complete basis set limit have been performed on the PNO, NOP, and NPO isomers and their corresponding anions and cations. Geometries for all species were optimized up through the aug-cc-pV(Q+d)Z level and vibrational frequencies were calculated with the aug-cc-pV(T+d)Z basis set. The most stable of the three isomers is NPO and it is predicted to have a heat of formation of 23.3 kcal/mol. PNO is predicted to be only 1.7 kcal/mol higher in energy. The calculated adiabatic ionization potential of NPO is 12.07 eV and the calculated adiabatic electron affinity is 2.34 eV. The calculated adiabatic ionization potential of PNO is 10.27 eV and the calculated adiabatic electron affinity is only 0.24 eV. NOP is predicted to be much higher in energy by 29.9 kcal/mol. The calculated rotational constants for PNO and NPO should allow for these species to be spectroscopically distinguished. The adiabatic bond dissociation energies for the P[Single Bond]N, P[Single Bond]O, and N[Single Bond]O bonds in NPO and PNO are the same within approximately 10 kcal/mol and fall in the range of 72-83 kcal/mol.     

Experimental and quantum-chemical studies on photoionization and dissociative photoionization of CH2Br2

The dissociative photoionization of CH2Br2 in a region approximately 10-24 eV was investigated with photoionization mass spectroscopy using a synchrotron radiation source. An adiabatic ionization energy of 10.25 eV determined for CH2Br2 agrees satisfactorily with predictions of 10.26 and 10.25 eV with G2 and G3 methods, respectively. Observed major fragment ions CH2Br+, CHBr+, and CBr+ show appearance energies at 11.22, 12.59, and 15.42 eV, respectively; minor fragment ions CHBr2+, Br+, and CH2+ appear at 12.64, 15.31, and 16.80 eV, respectively. Energies for formation of observed fragment ions and their neutral counterparts upon ionization of CH2Br2 are computed with G2 and G3 methods. Dissociative photoionization channels associated with six observed fragment ions are proposed based on comparison of determined appearance energies and predicted energies. An upper limit of DeltaH0f,298(CHBr+) < or = 300.7 +/- 1.5 kcal mol(-1) is derived experimentally; the adiabatic ionization energy of CHBr is thus derived to be < or = 9.17 +/- 0.23 eV. Literature values for DeltaH0f,298(CBr+) = 362.5 kcal mol(-1) and ionization energy of 10.43 eV for CBr are revised to be less than 332 kcal mol(-1) and 9.11 eV, respectively. Also based on a new experimental ionization energy, DeltaH0f,298(CH2Br2+) is revised to be 236.4 +/- 1.5 kcal mol(-1).