An efficient implementation of the orbital-optimized second-order Møller-Plesset perturbation theory (OO-MP2) within the resolution of the identity (RI) approximation is reported. Both conventional MP2 and spin-component scaled (SCS-MP2) variants are considered, and an extensive numerical investigation of the accuracy of these approaches is presented. This work is closely related to earlier work of Lochan, R. C.; Head-Gordon, M. J. Chem. Phys. 2007, 126. Orbital optimization is achieved by making the Hylleraas functional together with the energy of the reference determinant stationary with respect to variations of the double excitation amplitudes and the molecular orbital rotation parameters. A simple iterative scheme is proposed that usually leads to convergence within 5-15 iterations. The applicability of the method to larger molecules (up to ∼1000-2000 basis functions) is demonstrated. The numerical results show that OO-SCS-MP2 is a major improvement in electronically complicated situations, such as represented by radicals or by transition states where spin contamination often greatly deteriorates the quality of the conventional MP2 and SCS-MP2 methods. The OO-(SCS-)MP2 approach reduces the error by a factor of 3-5 relative to the standard (SCS-)MP2. For closed-shell main group elements, no significant improvement in the accuracy relative to the already excellent SCS-MP2 method is observed. In addition, the problems of all MP2 variants with 3d transition-metal complexes are not solved by orbital optimization. The close relationship of the OO-MP2 method to the approximate second-order coupled cluster method (CC2) is pointed out. Both methods have comparable computational requirements. Thus, the OO-MP2 method emerges as a very useful tool for computational quantum chemistry.

A benchmark set of 24 isomerization reactions of large organic molecules (consisting of 24 to 81 atoms) is presented (termed ISOL). The molecules are much larger than what is typically considered in thermochemical tests. To obtain reference isomerization energies, complete basis set (CBS) extrapolations at the (SCS)-MP2 level have been computed that are augmented by perturbative third-order corrections (SCS-MP3 and MP2.5 methods). Based on these carefully examined reference data, a diverse set of common density functionals varying from GGA to double-hybrid functional level with and without dispersion correction (DFT-D) is tested. Double-hybrid and the PBE0 hybrid functionals are found to be the methods of choice for the type of main group thermochemistry examined here. For all isomerizations with an average reaction energy of 22.7 kcal mol(-1) (in a range between 0.5 and 74.5 kcal mol(-1)), PBE0-D, B2PLYP-D and B2GP-PLYP-D yield mean absolute deviations of 2.5, 4.1 and 2.9 kcal mol(-1). Most importantly it is found that the use of a dispersion correction is essential if such large molecules are considered. For all DFT methods the MAD is lowered very significantly by 1.4-5.0 kcal mol(-1) when DFT-D is used. Intramolecular (mainly medium-range) London dispersion interactions account in some cases for more than 50% (41 kcal mol(-1)) of the isomerization energy even though the size of the systems remains unchanged. This study also demonstrates for the first time clearly that typical DFT errors are larger than expected (about 5 kcal mol(-1)) and that chemical accuracy (about 1 kcal mol(-1)) even for these electronically well-behaved molecules is currently not reached by DFT. We propose this new test set as a difficult challenge for electronic structure methods that claim to be routinely applicable to large molecules. We also suggest to use a distance range resolved dispersion energy as a diagnostic for problematic cases in DFT.

The hydrogenation of double bonds is one of the most fundamental transformations [1] in organic chemistry, and has numerous applications in the commodity chemical, agrochemical, pharmaceutical, polymer, and food industries. [2] Despite significant advances in the last 100 years, efforts to improve metal-based technologies for hydrogenation are still the focus of current research. [3] In parallel to these continuing efforts, metal-free strategies for effecting reductions have also been pursued. While organic reagents such as Hantschs esters [4] and silanes [5] have been used as stoichiometric reducing agents, it was not until 2006 [6] that the first metalfree systems, the so-called frustrated Lewis pairs (FLPs), [7] were shown to reversibly activate dihydrogen. This discovery allowed the development of FLP-based catalysts for the reduction of polar unsaturated bonds such as imines, [8] nitriles, [8a,c] aziridines, [8a,c] enamines, [8b] silylenolethers, [9] and aromatic reductions of anilines.[10] Herein, we report the discovery of FLP systems which, while appearing unreactive at room temperature, in fact are capable of dihydrogen activation at temperatures as low as À80 8C. This finding was then exploited for the catalytic hydrogenation of olefins at temperatures between 25 and 70 8C. Experimental and computational data support a plausible mechanism involving protonation of the olefin with subsequent hydride transfer.These FLPs represent the first metal-free hydrogenation catalysts for the reduction of olefins bearing carbocationstabilizing moieties.It is well known that the reactions of olefins with Brønsted acids in the presence of a nucleophilic halide, leads to addition products according to a protonation/addition mechanism. In considering the potential of such a mechanism for FLP hydrogenation of C=C double bonds, it was recognized that while the generated borohydride would act as the nucleophile, this pathway would require the generation of a countercation which was sufficiently acidic to effect protonation of the olefin. While the majority of FLP activations of dihydrogen have been demonstrated for phosphine/borane combinations, [7b] a variety of other donors including amines, [8a, 11] pyridines, [12] carbenes, [13] and phosphinimines [14] have been shown to be effective when paired with boron or aluminum Lewis acids. However, in all of these cases, the generated cations are only weak Brønsted acids and thus are incapable of protonation of olefinic double bonds.Seeking to enhance the Brønsted acidity of the cation generated by the FLP activation of dihydrogen, we initiated investigations employing (C 6 F 5 ) 3 B (1) in combination with the weakly basic phosphine (C 6 F 5 )Ph 2 P (2). An NMR spectroscopic examination of a 1:1 mixture of 1 and 2 at 25 8C resulted in spectra that did not differ from those of the individual components. Exposure of this FLP to hydrogen (5 bar) did not lead to significant changes in the NMR spectra at room temperature. However, the situation altered when the temperatu...

The frustrated Lewis pair tBu 3 P/B(C 6 F 5 ) 3 (1) readily adds SO 2 to yield the zwitterionic adduct tBu 3 P + -S(O)-C 6 H 10 (5)] add SO 2 at À78 C to yield the corresponding six-membered addition products 4a, 4b, 6. The adducts contain a chiral sulfur center. The [B]-O-(O)S-[P] addition products 3, 4b and 6 were characterized by X-ray diffraction. Scheme 1 Reactions of inter-and intramolecular FLPs with SO 2 .

The magnetic, structural, and morphological properties of ultrathin Fe films on Cu 3 Au͑100͒ have been investigated by low-energy electron diffraction including I/V measurements, Auger electron spectroscopy, medium-energy electron diffraction, and magneto-optic Kerr effect. The main aim of these studies was to establish the correlation between film structure and magnetism. For this purpose both films deposited at 300 K ͑RT͒ and at 150 K ͑LT͒ followed by subsequent annealing to 300 K were investigated. Above about 1 monolayer ͑ML͒, the films exhibit a perpendicular magnetization, which switches at 3.2Ϯ0.2 ML for LT and 2.3Ϯ0.2 ML for RT films in-plane. The reduction of the switching thickness from perpendicular to in-plane with growth temperature is caused by an interdiffusion at the Fe film/substrate interface. At somewhat larger thickness a structural transition is observed. This structural transition is not related to the magnetic reorientation. Contrary to other studies no evidence is found for any fcc iron modification. We rather conclude that above 5 ML, the iron film transforms from strained to unstrained bcc͑100͒ Fe.