Rudolf Janoschek

IUPAC Critical Evaluation of Thermochemical Properties of Selected Radicals. Part I

This is the first part of a series of articles reporting critically evaluated thermochemical properties of selected free radicals. The present article contains datasheets for 11 radicals: CH, CH2(triplet), CH2(singlet), CH3, CH2OH, CH3O, CH3CO, C2H5O, C6H5CH2, OH, and NH2. The thermochemical properties discussed are the enthalpy of formation, as well as the heat capacity, integrated heat capacity, and entropy of the radicals. One distinguishing feature of the present evaluation is the systematic utilization of available kinetic, spectroscopic and ion thermochemical data as well as high-level theoretical results.

Is the stereomutation of methane possible

Large basis set ab initio calculations at correlated levels, including MP2, single reference, as well as multireference configuration interaction, carried out on the methane potential energy surface, have located and characterized a transition structure for stereomutation (one imaginary frequency). This structure is best described as a pyramidal complex between singlet methylene and a side‐on hydrogen molecule with Cs symmetry. At the single reference CI level, it lies 105 kcal/mol above the methane Td‐ground state but is stable relative to dissociation into CH2(1A1) and H2 by 13 kcal/mol at 0 K (with harmonic zero point energy (ZPE) corrections for all structures). Dissociation of the transition state into triplet methylene and hydrogen also is endothermic (by 4 kcal/mol), but single bond rupture to give CH  3. and H. is 3 kcal/mol exothermic. Thus, it does not appear likely that methane can undergo stereomutation classically beneath the dissociation limit. Confirming earlier conclusions, side‐on insertion of 1A1 CH2 into H2 in a perpendicular geometry occurs without activation energy. Planar (D4h) methane (130.5 kcal/mol) has four imaginary frequencies. Two of these are degenerate and lead to equivalent planar C2v structures with one three‐center, two‐electron bond and two two‐electron bonds and two imaginary frequencies. The remaining imaginary frequencies of the D4h form lead to tetrahedral (Td) and pyramidal (C4v) methane. The latter has three negative eigenvalues in the force‐constant matrix; one of these leads to the Td global minimum and the other to the Cs (parallel) stereomutation transition structure. Multireference CI calculations with a large atomic natural orbitals basis set produce similar results, with the electronic energy of the Cs stereomutation transition state 0.7 ± 0.5 kcal/mol higher than that of CH  3. + H. dissociation products, and a ZPE‐corrected energy which is 5 ± 1 kcal/mol higher. Also considered are photochemical pathways for stereomutation and the possible effects of nuclear spin, inversion tunneling, and the parity‐violating weak nuclear interaction on the possibility of an experimental detection of stereomutation in methane.

Hydrogen/deuterium isotope effects on the 15N NMR chemical shifts and geometries of low-barrier hydrogen bonds in the solid state☆

Abstract In this study, hydrogen/deuterium isotope effects on the geometry and the 15N NMR chemical shifts of strongly hydrogen bonded, solid hydrogen-bisisocyanide salts of the type [MCN⋯L⋯NCM]−X+ 1 (L  H, D) are described both experimentally and theoretically. The theoretical studies include PM 2 6–31+ G(d,p) ab initio calculations of geometries and subsequent calculations of the proton and the deuteron vibrational states in the hydrogen bond of the model compounds [CN⋯L⋯NC]−Li+ 1a (L  H) and 1b (L  D). The Li+, at various fixed CLi distances, represents the external electric field. Furthermore, the 15N NMR chemical shifts were calculated using the Individual Gauge for Localized Orbitals method. The calculated isotope effects depend strongly on the asymmetry of the hydrogen bond caused by the influence of the Li+ cation. These results are compared with those of a solid state 15N cross polarization, magic angle spinning NMR study of the metal-stabilized salts 1c and 1d, where M  Cr(CO)5, X+  AsPh+4 and L  H or D, as well as of those of 1e and 1f, M  Cr(CO)5, and X+  NnPr+4. In the case of 1c and 1d the hydrogen bond is symmetric and isotope effects on the 15N NMR chemical shifts are not observed, as predicted theoretically. By contrast, and in agreement with the calculations, large isotope effects are observed for 1e and 1f, where the hydrogen bond symmetry is lifted by a strong interaction with the counterion. So far, solid state hydrogen/deuterium isotope effects on the NMR chemical shifts of hydrogen bonded nuclei have, to our knowledge, not been observed and may be used as a promising novel tool in hydrogen bond research.

Small carbanions and their parent neutral systems in the multiconfigurational SCF + CI approach

Abstract The carbanions C−, CH−, CH−2, and CH−3 and their parent neutral systems are investigated in order to obtain molecular structures and electron affinities. At the most sophisticated level of theory, MC SCF + CI, the calculated electron affinities deviate at most by 0.15 eV from the experimental values. Althouh the stability of CH−3 is not achieved the results query its hitherto predicted non-planarity.

Has the benzene molecule an extra stability

Abstract Different possibilities for the definition of the stabilization energy of the benzene molecule are described and critically discussed by means of quantum-chemical calculations. The results query the hitherto assumed stabilizing effect of π-electron delocalization.

A comparison of silylidene-amines, -phosphanes and -arsanes: syntheses and quantum chemical calculations

Abstract A review of recent results of the syntheses, spectroscopic properties—including some cyclovoltametric measurements — and aspects of the molecular structures of silylideneamines (silanimines, R 22 SidNR), silylidenephosphanes (phosphasilenes, R 2 SiPR) and silylidenearsanes (arsasilenes, R 2 SiAsR) is presented. Owing to the distinctly different SiX bond polarities (XN, P, As), the molecular structure and the reactivity of silanimines are quite different from those of the phosphorus and arsenic analogs. The electronic structure of phosphosilenes is distinguishable from that of arsasilenes by means of cyclovoltametric measurements. Whereas the SiP system undergoes a reversible one-electron reduction, the arsenic analogs show exclusively an irreversible reduction process. The tremendous differences between the SiN, SiP and SiAs bonds are also explored by theoretical investigations of the parent systems H 2 SiNH, H 2 SiNSiH 3 , H 2 SiPH, H 2 SiPSiH 3 and H 2 SiAsH.

Novel carbon suboxides and subsulpffldes (C5O2, C5S2, C4O2 and C2S2: assignment of UV and IR spectra by quantum chemical calculations

Abstract A variety of recently synthesized unexpected carbon suboxides and subsulphides is reviewed. Quantum-chemical calculations are shown to be indispensable for the assignment of IR and UV spectra. Hunds rule is revisited and critically discussed by means of different levels of ab initio CASSCF, configuration interaction and MPn.

Critical points of the conformational potential energy surface of carbonic acid: H2CO3

Abstract The conformational potential energy surface E  E ( T 1 , T 2 ) of H 2 CO 3 , where T 1 is the torsional angle for HO 1 CO 2 and T 2 is the torsional angle for O 1 CO 2 H, revealed that the anti—anti conformation is the global minimum. Additional local minima were also found. The next higher energy conformation was the syn—anti conformation, and a degenerate pair of right handed and left handed helical conformations were the highest on the energy scale. The syn—syn conformation turned out to be a transition structure sandwiched between the two helical conformation.

Ab initio quantum chemical study of the SN2 reaction, CH3F+H− → CH4+F−, in the gas phase

Abstract The reaction profile of the title S N 2 reaction has been studied by the ab initio method at various levels. The geometries of the van der Waals complexes of reactants and products and of the transition state have been optimized at the MP2/6-311G** level. It has been found that the reaction path loses C 3v symmetry and the reaction path is bifurcated in the product region. All the stationary point energies have been evaluated at the MP2 and MCSCF CI levels with various basis sets. The vibrational frequencies of the stationary points have been calculated using the SCF and, for some modes, MP2 methods.

Bonding properties of chlorine oxides ClOx (x=1–4) in the density functional theory

Abstract The geometries, energies, heats of formation and bond dissociation energies of the chlorine oxides ClO x ( x =1–4) were computed at the restricted open shell BECKE3LYP/cc-pV5Z level; vibrational wavenumbers and intensities were calculated employing the cc-pVQZ basis set. The mean absolute deviation between the calculated and newest experimental enthalpies of formation was 5.3 kJ mol −1 . A tetrahedral-like C 2v minimum geometry of the recently described ClO 4 radical was calculated, whereas the hitherto assumed C 3v structure was characterized as an energetically higher-lying second-order saddlepoint.