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The single-photon pulsed field ionization photoelectron (PFI-PE) spectrum of trans-2-butene (trans-CH3CH=CHCH3) in the energy range of 73 500–75 850 cm−1 has been measured using vacuum ultraviolet laser sources. The semi-empirical simulation of fine structures resolved in the original PFI-PE band yields a value of 73 624.7±2.0 cm−1 for the ionization energy (IE) of trans-2-butene. The vibrational bands for trans-CH3CH=CHCH3+ resolved in the PFI-PE spectrum are assigned based on ab initio calculations of the vibrational frequencies and Franck-Condon factors (FCFs) for ionization transitions. This assignment has provided reliable vibrational frequencies (ν1+=104 cm−1, ν2+=127 cm−1, ν3+=131 cm−1, ν5+=484 cm−1, ν8+=798 cm−1, ν13+=1164 cm−1, ν14+=1264 cm−1, ν16+=1307 cm−1, ν20+=1407 cm−1, and ν22+=1567 cm−1) for trans-CH3CH=CHCH3+. The PFI-PE spectrum is compared to the recently reported PFI-photoion (PFI-PI) spectrum for trans-2-butene. The major difference observed between the PFI-PE and PFI-PI spectra is th...

The photodissociation of acetophenone (C6H5COCH3) at 193 and 248 nm has been studied using the time-of-flight mass spectrometric technique. For hν=193 nm, two major primary channels, C6H5COCH3+hν→C6H5CO+CH3 [channel (1)] and C6H5+CH3CO [channel (2)], are observed with comparable cross sections. Data analysis shows that ≈30%–50% of primary C6H5CO and CH3CO radicals further decomposes, yielding secondary products C6H5+CO and CH3+CO, respectively. The translational energy release measurements indicate that for both channels (1) and (2) at 193 nm, ≈25%–30% of the available energy is channeled into kinetic energies of the primary photofragments. Measurements at hν=248 nm reveal that the branching ratio of channel (2) to channel (1) is ≈0.01. For channel (1) at hν=248 nm, ≈42% of the available energy is directed as the kinetic energy of the photofragments. The observed maximum kinetic energy release for channel (1) at 248 nm yields a value of 85.0±2.2 kcal/mol for the C6H5CO–CH3 bond dissociation energy at 0 K ...

The kinetic energy release spectra for SH resulting from the 193 nm laser photofragmentation of HSCH2CH2SH have been measured. On the basis of the observed maximum kinetic energy for the formation of HS+CH2CH2SH, a value of 74±2 kcal/mol is derived for the bond dissociation energy of HS–CH2CH2SH at 0 K [D0(HS–CH2CH2SH)]. Angular distribution measurements for SH yield an anisotropic parameter β=−0.4±0.1 for the HS+CH2CH2SH channel, indicating that the C–S bond fission is fast with respect to molecular rotation. The energetics for the formation of HS+CH2CH2SH from HSCH2CH2SH have been investigated using the Gaussian‐2 (G2) and G2(MP3) ab initio quantum chemical procedures. The G2/G2(MP3) calculations give a prediction of 72.5 kcal/mol for D0(HS–CH2CH2SH), in excellent agreement with the experimental value. Ab initio first‐order configuration interaction calculations have also been made to examine the possible excited state of HSCH2CH2SH involved in the photodissociation process and to rationalize the observ...

By combining molecular beam photodissociation and photoionization measurements with ab initio Gaussian‐2 (G2) calculations on the CH3S2 and CH3S+2 systems, we have shown that CH3SS is the dominant isomer formed in the photodissociation process, CH3SSCH3+hν(193 nm) →CH3S2+CH3. The experimental ionization energy for CH3SS (8.97±0.02 eV) and the heat of formation at 0 K for CH3SS+ (217.7±1.2 kcal/mol) are in excellent agreement with the G2 results. The photoionization efficiency spectrum observed for CH3SS is also consistent with the theoretical prediction that the Franck–Condon factor for the photoionization process, CH3SS+hν →CH3SS++e−, is not favorable. Based on the statistical modeling of experimental rates obtained previously for HS loss in the unimolecular decomposition of CH3SSCH+3 and the comparison with G2 ab initio predictions, we conclude that CH2SSH+ is most likely the isomer structure formed near the experimental appearance energy (11.07 eV) observed for the photodissociative ionization process, CH3SSCH3+hν→CH3S+2+CH3+e−.

The ionization energy (IE) for CH3SSCH3 has been measured by the pulsed molecular beam photoionization mass spectrometric method. The experimental IE of 8.18±0.03 eV is in excellent agreement with the theoretical prediction of 8.15 eV calculated using the ab initio Gaussian‐2 procedure, indicating that the experimental ionization onset can be assigned as the adiabatic IE for CH3SSCH3. The observation of the adiabatic IE(CH3SSCH3) is attributed to the low potential energy barrier for rotation about the S–S bond, which allows CH3SSCH3 to exist dynamically in a wide range of CSSC dihedral angles.

The photoionization efficiency spectrum of supersonically cooled CH2Br has been measured near its ionization threshold. The adiabatic ionization energy (IE) of CH2Br is determined to be 8.61±0.01 eV, in excellent agreement with the value obtained previously using the He i photoelectron spectroscopic method. We have also performed Gaussian‐2 (G2) calculations on CH2Br+, CH2Br, and CH2Br− which yield values of 8.47 and 0.97 eV for the IE and electron affinity of CH2Br, respectively. The G2 electron affinity is in accord with the literature value of 1.0±0.3 eV calculated from the acidity of CH3Br.

Predictions for the ionization energies (I.E.s) of van der Waals dimers R1R2 and the bond dissociation energies (D0’s) of the dimer ions R1R2+ (R1, R2=He, Ne, Ar, and Kr) have been calculated using the ab initio Gaussian‐2 theoretical procedure. Despite only fair agreements observed between theoretical and experimental geometries, the theoretical I.E.s and D0’s are found to be in excellent agreement with available experimental findings.

Using the Gaussian‐2 (G2) theoretical procedure, we have examined the molecular structures and total energies for CH2SH, CH2S−, CH3S−, CH2SH−, CH3SH−, CH3+, and CH3SH+. Contrary to the relative stabilities of CH3S+(C3v;3A2) and CH2SH+(Cs;1A’), the methylthio radical CH3S(Cs;2A’) and the methylthio anion CH3S−(C3v;1A1) are predicted to be more stable than the mercaptomethyl radical CH2SH(C1;2A) and the mercaptomethyl anion CH2SH−(Cs;1A’) by 9.2 and 38.0 kcal/mol, respectively. The CH2SH−(Cs;1A’) anion may exist in the cis configuration or the less stable trans structure. Combined with the results of previous G2 calculations, this calculation yields predictions for the adiabatic ionization energies (IE) of CH3 (9.79 eV), CH2SH (7.41 eV), and CH3SH (9.55 eV), which are in accord with the experimental IEs of 9.84 eV for CH3, 7.536±0.003 eV for CH2SH, and 9.440 eV for CH3SH. The G2 values for the adiabatic electron affinities (EA) of CH2S, CH2SH to trans‐CH2SH−(Cs;1A’), CH2SH to cis‐CH2SH−(Cs;1A’), and CH3S ar...

The ionization energies (IEs) for the 1-methylallyl, 2-methylallyl, cyclopropylmethyl, and cyclobutyl radicals have been calculated by the wave function based ab initio CCSD(T)/CBS approach, which involves the approximation to the complete basis set (CBS) limit at the coupled cluster level with single and double excitations plus quasiperturbative triple excitation [CCSD(T)]. The zero-point vibrational energy correction, the core-valence electronic correction, and the scalar relativistic effect correction are included in these calculations. The present CCSD(T)/CBS results are then compared with the IEs determined in the photoelectron experiment by Schultz et al. [J. Am. Chem. Soc. 106, 7336 (1984)] The predicted IE value (7.881 eV) of 2-methylallyl radical is found to compare very favorably with the experimental value of 7.90+/-0.02 eV. Two ionization transitions for cis-1-methylallyl and trans-1-methylallyl radicals have been considered here. The comparison between the predicted IE values and the previous measurements shows that the photoelectron peak observed by Schultz et al. likely corresponds to the adiabatic ionization transition for the trans-1-methylallyl radical to form trans-1-methylallyl cation. Although a precise IE value for the cyclopropylmethyl radical has not been directly determined, the experimental value deduced indirectly using other known energetic data is found to be in good accord with the present CCSD(T)/CBS prediction. We expect that the Franck-Condon factor for ionization transition of c-C4H7--bicyclobutonium is much less favorable than that for ionization transition of c-C4H7--planar-C4H7+, and the observed IE in the previous photoelectron experiment is likely due to the ionization transition for c-C4H7--planar-C4H7+. Based on our CCSD(T)/CBS prediction, the ionization transition of c-C4H7--bicyclobutonium with an IE value around 6.92 eV should be taken as the adiabatic ionization transition for the cyclobutyl radical. The present study provides support for the conclusion that the CCSD(T)/CBS approach with high-level energetic corrections can be used to provide reliable IE predictions for C4 hydrocarbon radicals with an uncertainty of +/-22 meV. The CCSD(T)/CBS predictions to the heats of formation for the aforementioned radicals and cations are also presented.

The photodissociation dynamics of chloroiodomethane (CH2ICl) at 193 nm has been investigated by employing the photofragment time-of-flight (TOF) mass spectrometric method. Using tunable vacuum ultraviolet undulator synchrotron radiation for photoionization sampling of nascent photofragments, we have identified four primary dissociation product channels: CH2Cl + I(2P(1/2))/I(2P(3/2)), CH2I + Cl(2P(1/2))/Cl(2P(3/2)), CHI + HCl, and CH2 + ICl. The state-selective detection of I(2P(3/2)) and I(2P(1/2)) has allowed the estimation of the branching ratio for I(2P(1/2)):I(2P(3/2)) to be 0.73:0.27. Theoretical calculations based on the time-dependent density-functional theory have been also made to investigate excited electronic potential-energy surfaces, plausible intermediates, and transition structures involved in these photodissociation reactions. The translation energy distributions derived from the TOF measurements suggest that at least two dissociation mechanisms are operative for these photodissociation processes. One involves the direct dissociation from the 2 1A' state initially formed by 193 nm excitation, leading to significant kinetic-energy releases. For the I-atom and Cl-atom elimination channels, the fragment kinetic-energy releases observed via this direct dissociation mechanism are consistent with those predicted by the impulsive dissociation models. Other mechanisms are likely predissociative or statistical in nature from the lower 1 1A' and 1 1A'' states and/or the ground X 1A' state populated by internal conversion from the 2 1A' state. On the basis of the maximum kinetic-energy release for the formation of CH2Cl + I(2P(1/2)), we have obtained a value of 53+/-2 kcal/mol for the 0 K bond dissociation energy of I-CH2Cl. The intermediates and transition structures for the CHI + HCl and CH2 + ICl product channels have been also investigated by ab initio quantum calculations at the MP2(full)/6-311G(d) and B3LYP(full)/6-11G(d) levels of theory. The maximum kinetic-energy releases observed for the CHI + HCl and CH2 + ICl channels are consistent with the interpretation that the formation of CHI and CH2 in their ground triplet states is not favored.

Using the same level of ab initio quantum chemical theory as that applied by Becker et al. [J. Chem. Phys. 100, 1759 (1994)], we have reexamined the structures of SF5 and SF+5. Contrary to their report, we find that the SF5/(D3h) and SF+5(C4v) structures are not local minima. The adiabatic ionization energies (IE) for SF4 and SF5, and the heats of formation at 0 K (ΔHf00) for SF4, SF+4, SF5, SF+5, and SF6 have also been calculated using the G2(MP2) procedure [Curtiss et al., J. Chem. Phys. 98, 1293 (1993)]. Excellent agreement is found between the theoretical G2(MP2) and accepted experimental IEs for SF4 and SF5. The theoretical ΔHf00’s for SF4, SF+4 and SF6 are also in excellent accord with the literature values. However, the G2(MP2) calculation indicates that the current recommended experimental ΔHf00(SF5) may be too low by about 10 kcal/mol.

Employing Racah algebra, the general formula for calculating the matrix elements of a weak crystalline field for configuration dn has been derived. Applying this formula the matrix elements for d4 (including spin—orbit coupling interaction) have been calculated and tabulated.

Applying Horie's method, the matrix elements of spin—other‐orbit interaction for configuration d5 have been calculated. The magnetic interaction energy has also been calculated for Mn++ by means of perturbation theory. It is found that spin—other‐orbit interaction plays a very important role for atoms or ions with half‐filled‐shell configuration. Spin—spin interaction is also significant in certain terms.