A summary of the technical advances that are incorporated in the fourth major release of the Q-Chem quantum chemistry program is provided, covering approximately the last seven years. These include developments in density functional theory methods and algorithms, nuclear magnetic resonance (NMR) property evaluation, coupled cluster and perturbation theories, methods for electronically excited and openshell species, tools for treating extended environments, algorithms for walking on potential surfaces, analysis tools, energy and electron transfer modelling, parallel computing capabilities, and graphical user interfaces. In addition, a selection of example case studies that illustrate these capabilities is given. These include extensive benchmarks of the comparative accuracy of modern density functionals for bonded and non-bonded interactions, tests of attenuated second order Møller-Plesset (MP2) methods for intermolecular interactions, a variety of parallel performance benchmarks, and tests of the accuracy of implicit solvation models. Some specific chemical examples include calculations on the strongly correlated Cr 2 dimer, exploring zeolitecatalysed ethane dehydrogenation, energy decomposition analysis of a charged ter-molecular complex arising from glycerol photoionisation, and natural transition orbitals for a Frenkel exciton state in a nine-unit model of a self-assembling nanotube.Keywords quantum chemistry, software, electronic structure theory, density functional theory, electron correlation, computational modelling, Q-Chem Disciplines Chemistry CommentsThis article is from Molecular Physics: An International Journal at the Interface Between Chemistry and Physics 113 (2015): 184, doi:10.1080/00268976.2014. RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. Authors 185A summary of the technical advances that are incorporated in the fourth major release of the Q-CHEM quantum chemistry program is provided, covering approximately the last seven years. These include developments in density functional theory methods and algorithms, nuclear magnetic resonance (NMR) property evaluation, coupled cluster and perturbation theories, methods for electronically excited and open-shell species, tools for treating extended environments, algorithms for walking on potential surfaces, analysis tools, energy and electron transfer modelling, parallel computing capabilities, and graphical user interfaces. In addition, a selection of example case studies that illustrate these capabilities is given. These include extensive benchmarks of the comparative accuracy of modern density functionals for bonded and non-bonded interactions, tests of attenuated second order Møller-Plesset (MP2) methods for intermolecular interactions, a variety of parallel performance benchmarks, and tests of the accuracy of implicit solvation models. Some specific chemical examples include calculations on the strongly corre...

Knowledge about individual covalent or non-covalent bond strengths is the Holy Grail of many modern molecular sciences. Recent developments of new descriptors for such interaction strengths based on potential constants are summarised in this tutorial review. Several publications for and against the use of compliance matrices (inverse force constants matrix) have appeared in the literature in the last few years. However the mathematical basis for understanding, and therefore interpreting, compliance constants is still not well developed. We therefore summarise the theoretical foundations and point to the advantages and disadvantages of the use of force constants versus compliance constants for the description of both non-covalent and covalent interactions.

Alkene metathesis is an organometallic success story par excellence, and the development of active catalysts for the breaking and making of carbon-carbon double bonds has had a tremendous impact on the development of new methods for the preparation of complex natural products and novel materials. [1,2] The related metathesis of alkynes represents a significantly less-developed synthetic method, [3] although the first homogeneous catalytic systems, for example mixtures of [Mo(CO) 6 ] and phenol additives, and the concept of using alkylidyne complexes in alkyne metathesis were introduced as early as the mid 1970s. [4,5] To date, only a limited number of well-defined alkylidyne complexes are known that fulfill the expectations for an alkyne metathesis catalyst with regard to its activity, substrate compatibility, and required reaction temperature. [1a, 3a, 6] Among these, the neopentylidyne complex [Me 3 CCW-(OCMe 3 ) 3 ] represents the most widely used tungsten-based species for applications such as ring-closing alkyne metathesis (RCAM) and alkyne cross-metathesis (ACM).[7] Furthermore, several catalytically active systems have been established that rely on the activation of molybdenum(III) triamido complexes of the general type [Mo{N(tBu)Ar} 3 ].[8]Herein, we introduce a new design strategy for the development of alkyne metathesis catalysts. This approach draws on the structure of the most active alkene metathesis catalysts, stable molybdenum and tungsten imido alkylidene complexes of type I (Scheme 1). Recently, we reported the preparation of monoanionic imidazolin-2-iminato ligands of type III, which can be described by the two limiting resonance structures IIIA and IIIB, indicating that the ability of the imidazolium ring to stabilize a positive charge leads to highly basic ligands [9] with a strong electron-donating capacity towards early transition metals.[10] Owing to their ability to act as 2s,4p-electron donors, these ligands can be regarded as monodentate analogues of cyclopentadienyl derivatives (C 5 R 5 ) and also as monoanionic imido ligands. Accordingly, substitution of the dinegative arylimido ligand in the alkylidene complex I by a mononegative imidazolin-2-imide allows the concurrent conversion of the metal-carbon double bond into a triple bond, affording alkylidyne complexes of type II with well-preserved structural and electronic integrity and therefore with potentially undiminished catalytic activity.It has been clearly demonstrated for complexes I that alkoxide ligands with electron-withdrawing substituents, for example R' = CMe(CF 3 ) 2 , are beneficial for catalytic performance, because they increase the electrophilicity of the metal. In light of this finding, we set out to synthesize complexes of type II using the readily available starting material [Me 3 CC W{OCMe(CF 3 ) 2 } 3 (dme)], in which the tungsten center is stabilized by dimethoxyethane (dme).[11] Treatment of this complex with the lithium reagent (ImN)Li, obtained from the reaction of 1,3-di-tert-butylimidazolin-2-imine (ImNH) ...

We present an extension to the theory of compliance matrices, which is valid for arbitrary nonstationary points on the potential energy hypersurface. It is shown that compliance matrices computed as the inverse of the covariant Hessian matrix obey the same invariance properties with respect to different internal coordinate systems as they do for stationary points. Furthermore, we demonstrate how the computation of compliance matrices in arbitrary sets of redundant internal coordinates starting from a Cartesian Hessian can be achieved efficiently, and we discuss their potential usefullness in geometry optimization processes

A series of lithium complexes of anionic N-heterocyclic carbenes that contain a weakly coordinating borate moiety (WCA-NHC) was prepared in one step from free N-heterocyclic carbenes by deprotonation with n-butyl lithium followed by borane addition. The reaction of the resulting lithium-carbene adducts with [M(COD)Cl]2 (M = Rh, Ir; COD = 1,5-cyclooctadiene) afforded zwitterionic rhodium(I) and iridium(I) complexes of the type [(WCA-NHC)M(COD)], in which the metal atoms exhibit an intramolecular interaction with the N-aryl groups of the carbene ligands. For M = Rh, the neutral complex [(WCA-NHC)Rh(CO)2] and the ate complex (NEt4)[(WCA-NHC)Rh(CO)2Cl] were prepared, with the latter allowing an assessment of the donor ability of the ligand by IR spectroscopy. The zwitterionic iridium-COD complexes were tested as catalysts for the homogeneous hydrogenation of alkenes, which can be performed in the presence of nonpolar solvents or in the neat alkene substrate. Thereby, the most active complex showed excellent stability and activity in hydrogenation of alkenes at low catalyst loadings (down to 10 ppm).