Gaussian-2 and QCI theory with basis sets up to 6-311+G(3df,2p) have been employed to obtain information about the carbenes (methylenes) CH2, CHBr, CHCl, CHF, CHI, CBr2, CBrCl, CBrF, CBrI, CCl2, CClF, CClI, CF2, and CI2. Geometries and vibrational frequencies for the singlet and triplet states, and the singlet−triplet splittings, have been characterized and are discussed in the context of the electronegativities of the substituents. Isodesmic reactions were used to derive a consistent set of enthalpies of formation for the 14 halocarbenes. Where there are measurements, the computed results compare well, and substantial gaps in the current literature are filled.

One of the fastest steps in the initial decomposition of HFCs under combustion conditions is hydrogen atom abstraction by hydroxyl radicals. We have utilized ab initio quantum mechanics and transition-state theory (TST) to calculate the temperature dependence of rate constants for the reactions of OH with CH 4 , CH 3 F, CH 2 F 2 , and CHF 3 . Rate constants calculated using HF/6-31G(d) frequencies and MP2(full)/6-31G(d) structures to evaluate reactant and transition-state partition functions and the Hartree-Fock imaginary frequency, ω i , to compute Eckart tunneling factors, Γ, yielded rate constants that were substantially greater than experiment. Adjustment of the energy barrier to effect agreement between experimental and calculated rate constants at 298 K gave Arrhenius plots that exhibited markedly greater curvature than measured rate constants. When the imaginary frequency and barrier height were calculated by fitting high-level (G2) energies along the reaction path with a semiempirical Eckart function, it was found that the calculated imaginary frequency is a factor of 2.5 lower than the HF/6-31G(d) value, indicating that the energy barrier is considerably broader than predicted by the latter frequency. When the new imaginary frequencies and barrier heights were used to calculate rate constants, it was found that k TST < k expt but that lowering the barrier height (by an average of 4.7 kJ/mol for the four reactions) yields calculated rate constants that are in excellent agreement with experiment at all temperatures.

Bromo-and iodomethanes and the corresponding halogenated methyl radicals have been investigated by ab initio methods. Geometries and vibrational frequencies were derived with quadratic configuration interaction methods at the QCISD/6-311G(d,p) level of theory, and energies via QCISD(T)/6-311+G(3df,2p). Core electrons were represented with relativistic effective potentials. Anharmonicity of the out-of-plane bending modes in the methyl radicals was taken into account by numerical integration of the Schrödinger equation with potentials derived from relaxed scans of these modes. The results are in good accord with experimental data where available. Thermochemistry derived via isodesmic reactions referenced to CH 3 , CH 4 , and monohalomethanes yields excellent accord with new experiments on dihalomethanes and provides recommendations for the more poorly characterized tri-and tetrahalomethanes and halomethyl radicals. For the methanes CH 2 Br 2 , CHBr 3 , CBr 4 , CH 2 I 2 , CHI 3 , CI 4 , CH 2 BrI, CHBr 2 I, and CHBrI 2 we compute ∆ f H°2 98 values of 4.3, 51.6, 110.6, 108.1, 208.5, 321.3, 56.8, 104.8, and 157.1 kJ mol -1 , respectively. For the methyl radicals CH 2 Br, CHBr 2 , CBr 3 , CH 2 I, CHI 2 , CI 3 , CHBrI, CBr 2 I, and CBrI 2 we compute ∆ f H°2 98 values of 166.6, 191.7, 224.0, 217.2, 290.4, 369.1, 241.6, 320.8, and 272.3 kJ mol -1 , respectively. Recommended confidence limits are (3 kJ mol -1 per Br or I atom. Trends in these values and the corresponding C-H bond strengths are discussed and compared with prior experiments, empirical estimation schemes, and ab initio calculations.

A study was designed to investigate the ability of listeners to identify speaker sex from isolated productions of /f/, /θ/, /s/, and /∫/. Nine females and nine males recorded the four fricatives in isolation. The stimuli were randomized and presented via loudspeaker to 10 listeners for sex identifications. The results indicated that the listeners could identify the sex of the speakers from the isolated productions of /s/ and /∫/, but could not from the /f/ and /θ/ productions. Subsequent spectrographic analysis of the /s/ and /∫/ stimuli revealed that the female spectra tended generally to be higher in frequency than the male.