Density functional theory studies comparing the energetics of [4+2] Diels-Alder-like cyclizations of the iminoborane (F3C)(3)C-B equivalent to N-(t-Bu) with substituted cis-2-R-1,3-butadienes (R = CH3, NH2, CF3) predict that some reactions will display regiospecificity derived from the transition state barrier heights. When R is an electronically near-neutral group (CH3), the models predict no preference, and so no regiospecificity. When R is an electron-donating group (NH2), the models predict a strong preference for the 5-R-1-bora-2-azacyclohexa-1,4-diene product; that is, the R group favors proximity to the boron atom. When R is an electronwithdrawing group (CF3) the models predict a preference for the 4-R-1-bora-2-azacyclohexa1,4-diene product, with the R group in proximity to the nitrogen atom. NBO charges provide an explanation for the specificity, in that the atomic charges within the transition state structures match properly for the orientations indicated. Frontier molecular orbital (FMO) coefficients of the reaction participants also support the suggested regiospecificities.

The gas-phase strengths of the SO3-X- bonds (X = Cl, Br, and I) have been determined to be 222 +/- 13, 179 +/- 11, and 161 +/- 9 kJ/mol, respectively, by measuring thresholds for collision-induced dissociation in a flowing afterglow-tandem mass spectrometer. An upper limit of D(SO3-F-) <= 488 +/- 19 kJ/mol was also determined. The periodic trends in the halide affinities of SO3 closely parallel those for SO2 and other Lewis acids. Bond strengths computed using the B3LYP, MP2, QCISD(T), and other models with several basis sets are generally lower than experiment.

Geben und Nehmen: Ein kationisches NHC-Phosphenium-Addukt (siehe Bild; P orange, N lila, C grau, H weiß; NHC=N-heterocyclisches Carben) spaltet sich bei der Reaktion mit [Pt(PPh3)3] in seine Bestandteile und stellt einen NHC-Liganden (starker σ-Donor) sowie einen Phosphenium-Liganden (π-Acceptor) zur Verfügung. Diese ersten Pt-Phosphenium-Komplexe sind durch kurze Pt-P-Bindungen und große Pt-P-Kopplungskonstanten charakterisiert. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2004/z52326_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Quantum mechanical density functional theory (DFT) and coupled DFT/molecular mechanics (QMMM) studies of the compounds (H(3)P)(3)M(eta(1)-SO(2)) and (Me(n)Ph(3-n)P)(3)M(eta(1)-SO(2)) (M = Ni, Pd, Pt; n = 0-3) model the experimental data well, particularly the planar/pyramidal geometries at sulfur. Bond dissociation energy (BDE) calculations confirm that Pd/Pt systems with pyramidal SO(2) ligands exhibit M-S BDEs smaller by 30-50% than Ni systems with planar SO(2). However, scans of the potential energy surfaces show that flexing the planar/pyramidal torsion angle within ranges of 20-30 degrees requires little energy. Bond energy decomposition calculations indicate that the electrostatic Delta E(elstat) term determines the BDE for Pd/Pt molecules where the sulfur is pyramidal, whereas all three terms matter when the sulfur is planar, as for Ni compounds. However, this accounts only for a fraction of the BDE differences; orbital energy matching accounts for the balance.

Computational studies of amine-boranes (X3C)(m)H3-mB-N(CH3)(n)H3-n (X = H, F; m = 0-3; n = 0-3) show that the B3LYP model performs poorly in predicting the structures and B-N bond dissociation energies of such species. A survey of several models shows that the MP2 approach gives the best agreement, but is too computationally intensive for general use. Among several hybrid and pure DFT approaches, the MPW1K model gives the best agreement with experiment and/or with the MP2 model. Scans of the potential surface for rotation around the B-N bond in several molecules and examinations of other amine-boranes suggest that the difficulty with the B3LYP method does not arise from its inability to incorporate nonbonded intramolecular interactions, but from an inherent inability to model the dative bond. The MPW1K approach evidently does this better because it was designed to model "incompletely bound" transition states, which mimic datively bonded systems.

Raman spectra of a number of triply bonded M(2)X(6) (M = Mo, W; X = alkoxide, alkyl) compounds have been obtained. Several exhibit a band assignable to the metal-metal stretching vibration nu(M)(M). This band was not identified in earlier studies of the M(2)(NMe(2))(6) compounds. We have attempted to correlate the Raman vibrational data with structural data from single-crystal X-ray diffraction studies. Diffraction studies of the M(2)(O-1-4-pentyl[2.2.2]bicyclooctyl)(6) species show a crowded environment around the dimetal core, but the M-M-O angles differ substantially from 90 degrees. Thus, this angle does not solely determine the extent to which the metal-metal and ligand-based vibrational modes couple and, in turn, our ability to observe nu(M)(M). Computational studies of model systems confirm the assignment of the band as being nu(M)(M), although the predicted vibrational energies are consistently too high by ca. 7%. The computational results suggest that a nu(M)(M) band may be present in the published spectra of the M(2)(NMe(2))(6) pair.

Computational studies comparing the energetics of [4+2] Diels-Alder-like cyclizations of aminoboranes R2B=NR2'(R = H, CF3; R'= H, CH3) with substituted cis-2-R"-1,3-butadienes (R" = CH3, NH2, CF3) show that the regiospecificity observed experimentally derives from the transition state barrier heights rather than the reaction exothermicities. When R" is an electron-donating group such as CH3 or NH2, the model predicts a preference for the 5-R"-1-bora-2-azacyclohex-4-ene product; that is, the R" group favors proximity to the boron atom. When R" is an electron-withdrawing group such as CF3, the model predicts a preference for the 4-R"-1-bora-2-azacyclohex-4-ene product; that is, the R" group favors proximity to the nitrogen atom. The predictions are in accord with experimental results and organic frontier molecular orbital theory, although the aminoboranes exhibit a surprising degree of discrimination between similarly charged carbon atoms.

Density functional theory studies of the series of isomeric d(6) (pentacarbonyl)metal complexes (CO)(5)M(eta(1)-SO(2))(nq), (CO)(5)M(eta(1)-OSO)(nq)(), and (CO)(5)M(eta(2)-SO(2))(nq) (M = Ti-Hf, nq = 2-; M = V-Ta, nq = 1-; M = Cr -W, nq = 0; M = Mn-Re, nq = 1+; M = Fe-Os, nq = 2+) provide accurate structural modeling and quantitative prediction of the relative stabilities of the isomers. The eta(1)-S-bound complexes display planar SO(2) moieties that adopt staggered orientations with respect to the carbonyl ligands, in keeping with experimental observations. The OSO chain in the eta(1)-O-bound complexes generally adopts the u-shape with a staggered orientation. The dianions (CO)(5)(Ti-Hf)(eta(1)-OSO)(2-) differ in that the OSO chain adopts the eclipsed z-shape orientation. The eta(2)-SO(2) complexes exhibit a facial interaction and are stable only for anionic and neutral complexes, supporting the view that this motif involves substantial M --> SO(2) pi-back-bonding. The relative stabilities of the isomers generally follow u-shaped trends both across a row and down a family. This fits with qualitative ideas that the bond dissociation energies (BDEs) for the (CO)(5)M(SO(2))(nq) complexes track competition between relative hardness/softness of the metal fragment and its capacity for pi-back-bonding. Quantitatively, examination of BDEs by bond energy decomposition approaches suggests that electrostatic considerations dominate bonding for the eta(1)-SO(2) complexes and covalent effects dominate for the eta(2)-SO(2) species, while both are important for eta(1)-OSO complexes.

The strengths of the F2ClP−Cl-, POCl3−Cl-, and PSCl3−Cl- bonds have been determined by measuring thresholds for collision-induced dissociation in a flowing afterglow-tandem mass spectrometer. The results are combined with previously determined values for the PF4-, PF3Cl-, POF4-, and PCl4- systems to determine the effect of adjacent ligands on hypervalent bond strengths. Although the addition of electronegative equatorial ligands strengthens bonding to axial halides, the effect is, in some cases, outweighed by the rearrangement energy of the dissociation products. Computational results indicate that the B3LYP/aug-cc-pVTZ method gives particularly good agreement with experiment among the models used here; however, several less resource-intensive methods give acceptable agreement.

The Catalytica process converts methane to methyl bisulfate in good yield at relatively low temperature in fuming sulfuric acid and may help make methane a useful bulk chemical precursor. We have computationally examined the methane C-H activation step of the process. We find that the most likely catalyst for this step is either (bipyrimidine)Pt(OSO3H)(+) or (bipyrimidine)PtCl+. In the former case C-H activation takes place by a-bond metathesis, whereas the latter involves C-H activation by an oxidative addition mechanism. It appears that the need to run the reaction at 180-220 degreesC stems from the fact that methane has to displace either one Cl- ligand or one HSO4- oxygen. Protonation of bipyrimidine by sulfuric acid appears unfavorable, although the resulting highly charged catalysts exhibit lower activation barriers.

Diffuse and polarization functions have been optimized for the LANL2DZ basis set for elements in groups 14-17. The optimized exponents are in most cases similar to those optimized with different effective core potentials, valence basis sets, or computational models. The average of the LANL2DZ results for different models is taken to be the best generalized set of exponents. The extended basis set gives good results (average deviation from experiment 0.11 eV) for atomic electron affinities with the B3LYP model, but is consistently low with the MP2 model. The extended basis set gives similar performance to the all-electron 6-31+G(d) basis set in calculations of vibrational frequencies and bond energies in selected main-group compounds, and is intermediate in speed between the 6-31+G(d) basis set and the unmodified LANL2DZ basis set.

Computational studies of the dimerization of the diboradiazacyclobutadienes (HBNH)2 and (MeBNMe)2 to form the tetraboratetraazacyclooctatetraenes (RBNR)4 suggest that the preferred pathway involves “face-to-face” dimerization of the four-membered ring, followed by asynchronous, one-step scission of the two transannular BN bonds to form the eight-membered monocyclic product. Detection of the proposed intermediates in either system is unlikely, as they lie in shallow potential energy wells. That in certain situations the four- and eight-membered rings establish equilibria is consistent with the comparatively small energy difference between (MeBNMe)2 and (MeBNMe)4.