A photoionization mass spectrometric study of SiH4 at T=150 K reveals the presence of SiH+4 with an adiabatic threshold at 11.00±0.02 eV. The implications for the structure of this Jahn–Teller split state are discussed. The appearance potentials of SiH+2 and SiH+3 are 11.54±0.01 eV and ≤12.086 eV, respectively. The reaction of F atoms with SiH4 generates SiH3 (X 2A1), SiH2 (X 1A1 and a 3B1), and SiH (X 2Π) in sufficient abundance for photoionization studies. The measured adiabatic ionization potentials (eV) are: SiH3, 8.01±0.02; SiH2 (X 1A1), 9.15±0.02 or 9.02±0.02; SiH2 (a 3B1), 8.244±0.025; SiH, 7.91±0.01. The singlet–triplet splitting in SiH2 is either 0.78±0.03 or 0.91±0.03 eV. The dissociation energy of SiH is 2.98±0.03 eV. A Rydberg series is observed, converging to SiH+ (a 3Π) at 10.21±0.01 eV. Heats of formation of the various neutral and ionic species are presented, as are the stepwise bond energies of SiH4.

Photoionization mass spectrometric studies of AsHn (n=1–3)

Gas phase analogues of solvolysis reactions

Through the use of the Active Thermochemical Tables approach, the best currently available enthalpy of formation of HO2 has been obtained as delta(f)H(o)298 (HO2) = 2.94 +/- 0.06 kcal mol(-1) (3.64 +/- 0.06 kcal mol(-1) at 0 K). The related enthalpy of formation of the positive ion, HO2+, within the stationary electron convention is delta(f)H(o)298 (HO2+) = 264.71 +/- 0.14 kcal mol(-1) (265.41 +/- 0.14 kcal mol(-1) at 0 K), while that for the negative ion, HO2- (within the same convention), is delta(f)H(o)298 (HO2-) = -21.86 +/- 0.11 kcal mol(-1) (-21.22 +/- 0.11 kcal mol(-1) at 0 K). The related proton affinity of molecular oxygen is PA298(O2) = 100.98 +/- 0.14 kcal mol(-1) (99.81 +/- 0.14 kcal mol(-1) at 0 K), while the gas-phase acidity of H2O2 is delta(acid)G(o)298 (H2O2) = 369.08 +/- 0.11 kcal mol(-1), with the corresponding enthalpy of deprotonation of H2O2 of delta(acid)H(o)298 (H2O2) = 376.27 +/- 0.11 kcal mol(-1) (375.02 +/- 0.11 kcal mol(-1) at 0 K). In addition, a further improved enthalpy of formation of OH is briefly outlined, delta(f)H(o)298 (OH) = 8.93 +/- 0.03 kcal mol(-1) (8.87 +/- 0.03 kcal mol(-1) at 0 K), together with new and more accurate enthalpies of formation of NO, delta(f)H(o)298 (NO) = 21.76 +/- 0.02 kcal mol(-1) (21.64 +/- 0.02 kcal mol(-1) at 0 K) and NO2, delta(f)H(o)298 (NO2) = 8.12 +/- 0.02 kcal mol(-1) (8.79 +/- 0.02 kcal mol(-1) at 0 K), as well as H(2)O(2) in the gas phase, delta(f)H(o)298 (H2O2) = -32.45 +/- 0.04 kcal mol(-1) (-31.01 +/- 0.04 kcal mol(-1) at 0 K). The new thermochemistry of HO2, together with other arguments given in the present work, suggests that the previous equilibrium constant for NO + HO2 --> OH + NO2 was underestimated by a factor of approximately 2, implicating that the OH + NO2 rate was overestimated by the same factor. This point is experimentally explored in the companion paper of Srinivasan et al. (next paper in this issue).

Active Thermochemical Tables: accurate enthalpy of formation of hydroperoxyl radical, HO2.

A procedure for the calculation of molecular quartic force fields (QFFs) is proposed and investigated. The goal is to generate highly accurate ab initio QFFs that include many of the so-called “small” effects that are necessary to achieve high accuracy. The small effects investigated in the present study include correlation of the core electrons (core correlation), extrapolation to the one-particle basis set limit, correction for scalar relativistic contributions, correction for higher-order correlation effects, and inclusion of diffuse functions in the one-particle basis set. The procedure is flexible enough to allow for some effects to be computed directly, while others may be added as corrections. A single grid of points is used and is centered about an initial reference geometry that is designed to be as close as possible to the final ab initio equilibrium structure (with all effects included). It is shown that the least-squares fit of the QFF is not compromised by the added corrections, and the balan...

A procedure for computing accurate ab initio quartic force fields: Application to HO2+ and H2O.

This paper reports a comprehensive theoretical examination of the re and r0 geometrical structures, harmonic and anharmonic vibrational frequencies, and infrared intensities ofX2Π SiH, X 1A1, a 3B1, and A1B1 SiH2, X2A1 SiH3, and X1A1 SiH4. Both self-consistent-field (SCF) and configuration interaction singles and doubles (CISD) methods have been employed with two basis sets (B1 and B2) of double-zeta plus polarization (DZ +P) and triple-zeta plus double polarization (TZ+2P) quality. For SiH the B2 CISD parameters re = 1.5214 A, r0 = 1.5326 A, ωe = 2062 cm−1, and v0 = 1985 cm−1 are in excellent agreement with the experimental values re = 1.5201 A, r0 = 1.5313 A, ωe = 2042 cm−1, and v0 = 1971 cm−1. The B2 CISD αi rovibrational constants for SiH2D2, SiH4, SiD4, and SiHD3 when utilized to correct experimental A0, B0, and C0 rotational constants give re = 1.4735 A for SiH4. With the same procedure, re(SiH) = 1.5141 A and θe (HSiH) = 92.0° are found for X 1A1 SiH2, and re(SiH) = 1.4871 A and θe(HSiH) = 121.8° are found for A1B1 SiH2. All of these experimentally derived re parameters are in excellent agreement with the B2 CISD re structures for X 1A1 SiH4, X1A1 SiH2, and A1B1 SiH2. For a3B1 SiH2, re = 1.4768 A, θe = 118.3°, and Te = 19.6 kcal/mol are predicted. The B2 CISD harmonic vibrational frequencies for the SiH stretches of SiH4 are only 1–2% too large, and the corresponding theoretical anharmonicities are in error by less than 7 cm−1. Using the SiH4 data to separately scale the B2 CISD ω3 and x3 values for X1A1 SiH2, ν3 = 1981 cm−1 is obtained, which confirms the recent experimental frequencies of Fredin, Hauge, Kafafi, and Margrave. In a similar fashion ν1 = 2117 cm−1, ν2 = 892 cm−1, and ν3 = 2180 cm−1 are predicted for a3B1 SiH2 and ν1 = 2016 cm−1 and ν3 = 2115 cm−1 for A1B1 SiH2. The B2 CISD geometrical parameters for X2A1 SiH3 are re(SiH) = 1.4768 A and θe(HSiH) = 111.1°, and the final (scaled) B2 CISD fundamental frequencies are ν1 = 2150 cm−1, ν2 = 773 cm−1, ν3 = 2180 cm−1, and ν4 = 933 cm−1. These frequencies definitely show that the 1999, 1955, and 996 cm−1 IR absorptions of Milligan and Jacox are not attributable to SiH3. Finally, at the highest level of theory used here, a classical inversion barrier of 5.8 kcal/mol is obtained for SiH3.

Geometrical structures, force constants, and vibrational spectra of SiH, SiH2, SiH3, and SiH4

Vacuum ultraviolet photoelectron spectroscopy of transient species. XVII. The SiH3(X 2A1) radical

A partial HeI photoelectron spectrum of the formyl radical has been obtained. In the band associated with the first ionization potential a long vibrational series was observed for both HCO and DCO and analysis of the vibrational structure, which is attributed to excitation of the deformation vibration in the ion, gave values of and 620 ± 30 cm-1 in the HCO and DCO cases respectively. For HCO the first vertical ionization potential has been measured as 9·31 ± 0·01 eV. Evidence is presented to show that the formyl radical provides an example of a rare case in photoelectron spectroscopy where the adiabatic ionization potential is not directly observed. However, with the aid of Franck-Condon calculations, the first adiabatic ionization potential of HCO is estimated as 8·27 ± 0·01 eV. Use of this value allows the heat of formation of HCO, ΔH ⦵ f(298) (HCO) and the C-H bond dissociation energy in formaldehyde to be calculated as 28·0 ± 5·4 kJ mol-1 and 354·0 ± 6·3 kJ mol-1 respectively.

M.J. Winter

Papers

Vacuum ultraviolet photoelectron spectroscopy of transient species. XVII. The SiH3(X 2A1) radical

The first ionization potential of the formyl radical, HCO(X2A'), studied using photoelectron spectroscopy

Vacuum ultraviolet photoelectron spectroscopy of transient species: Part 13. Observation of theX3A″state of HO2+