Cover Image, Volume 36, Issue 11doi: 10.1002/jcc.23909pmid: N/A
According to high accuracy quantum chemical computations up to the CCSDTQ level, both 1A1 and 3A′2 states of silicon trimer are degenerate. An intersystem crossing between both states appears at a point of α = 68 ± 2° lying only 16 ± 4 kJ/mol above the ground state. On page 805 (DOI: 10.1002/jcc.23856), Nguyen Minh Tam, Tran Dieu Hang, Hung Tan Pham, Huyen Thi Nguyen, My Phuong Pham‐Ho, Pablo A. Denis, and Minh Tho Nguyen probe the chemical bonding of Si3 using Walsh diagrams, electron localization function, and ring current analyses. The singlet three‐membered Si3 exhibits paratropic ring current, which arises exclusively from σ electrons. Attachment of H+ and Li+ cations renders the ring more anti‐aromatic.
Cover Image, Volume 36, Issue 11doi: 10.1002/jcc.23907pmid: N/A
On page 861 (DOI: 10.1002/jcc.23871), Alex Domingo, Celestino Angeli, Coen de Graaf, and Vincent Robert report the intervalence charge transfer (IVCT) between the metal centers of a bi‐iron complex induces a response of the whole electronic structure of the compound. The response effects consist of an electronic reorganization that affects each molecular orbital of the complex differently. The π‐ and n‐type orbitals localized on coordinating atoms undergo the largest polarization, being even larger than that of the Fe 3d‐like orbitals involved in the transfer. The electronic reorganization follows the induced dipole moment by the IVCT, but having a weaker effect on the orbitals close to the Fe center accepting the transferring electron.
Cover Image, Volume 36, Issue 11doi: 10.1002/jcc.23908pmid: N/A
Can allenes and their derivatives perform as efficient leaving groups in catalysis processes? This is the question addressed on page 795 (DOI: 10.1002/jcc.23855) by Nishamol Kuriakose and Kumar Vanka. Specifically, the possibility of these interesting compounds acting as leaving groups in the case of olefin metathesis with the Grubbs catalyst is explored. The investigation is performed with high level quantum chemical calculations using density functional theory (DFT). The results for a range of different allene and substituted allene cases indicate that these compounds would be better by several orders of magnitude in comparison to currently employed leaving groups. This interesting result showcases the potential of this important new class of compounds.
Dividing a complex reaction involving a hypervalent iodine reagent into three limiting mechanisms by ab initio molecular dynamicsSala, Oliver; Lüthi, Hans Peter; Togni, Antonio; Iannuzzi, Marcella; Hutter, Jürg
doi: 10.1002/jcc.23857pmid: 25766580
The electrophilic N‐trifluoromethylation of MeCN with a hypervalent iodine reagent to form a nitrilium ion, that is rapidly trapped by an azole nucleophile, is thought to occur via reductive elimination (RE). A recent study showed that, depending on the solvent representation, the SN2 is favoured to a different extent over the RE. However, there is a discriminative solvent effect present, which calls for a statistical mechanics approach to fully account for the entropic contributions. In this study, we perform metadynamic simulations for two trifluoromethylation reactions (with N‐ and S‐nucleophiles), showing that the RE mechanism is always favoured in MeCN solution. These computations also indicate that a radical mechanism (single electron transfer) may play an important role. The computational protocol based on accelerated molecular dynamics for the exploration of the free energy surface is transferable and will be applied to similar reactions to investigate other electrophiles on the reagent. Based on the activation parameters determined, this approach also gives insight into the mechanistic details of the trifluoromethylation and shows that these commonly known mechanisms mark the limits within which the reaction proceeds. © 2015 Wiley Periodicals, Inc.
Can substituted allenes be highly efficient leaving groups in catalytic processes? A computational investigationKuriakose, Nishamol; Vanka, Kumar
doi: 10.1002/jcc.23855pmid: 25764284
There is considerable interest presently in the chemistry of allenes. The current computational investigation looks into the possibility of using allenes and their derivatives as leaving groups. As it is well known, leaving groups are significant in catalytic processes for generating the active site. A full quantum mechanical study using density functional theory shows that allenes and their derivatives can function as excellent leaving groups. Indeed, the calculations show that they can be several orders of magnitude more effective than existing ligands for this purpose. The modification of second generation Grubbs' catalysts with these ligands suggests that the allene ligand cases that would be most effective are those having electron withdrawing groups, especially those that have the potential for supramolecular interactions between the substituent groups in the free state. © 2015 Wiley Periodicals, Inc.
Bonding and singlet–triplet gap of silicon trimer: Effects of protonation and attachment of alkali metal cationsTam, Nguyen Minh; Hang, Tran Dieu; Pham, Hung Tan; Nguyen, Huyen Thi; Pham‐Ho, My Phuong; Denis, Pablo A.; Nguyen, Minh Tho
doi: 10.1002/jcc.23856pmid: 25694392
We revisit the singlet–triplet energy gap (ΔEST) of silicon trimer and evaluate the gaps of its derivatives by attachment of a cation (H+, Li+, Na+, and K+) using the wavefunction‐based methods including the composite G4, coupled‐cluster theory CCSD(T)/CBS, CCSDT and CCSDTQ, and CASSCF/CASPT2 (for Si3) computations. Both 1A1 and 3
A2' states of Si3 are determined to be degenerate. An intersystem crossing between both states appears to be possible at a point having an apex bond angle of around α = 68 ± 2° which is 16 ± 4 kJ/mol above the ground state. The proton, Li+ and Na+ cations tend to favor the low‐spin state, whereas the K+ cation favors the high‐spin state. However, they do not modify significantly the ΔEST. The proton affinity of silicon trimer is determined as PA(Si3) = 830 ± 4 kJ/mol at 298 K. The metal cation affinities are also predicted to be LiCA(Si3) = 108 ± 8 kJ/mol, NaCA(Si3) = 79 ± 8 kJ/mol and KCA(Si3) = 44 ± 8 kJ/mol. The chemical bonding is probed using the electron localization function, and ring current analyses show that the singlet three‐membered ring Si3 is, at most, nonaromatic. Attachment of the proton and Li+ cation renders it anti‐aromatic. © 2015 Wiley Periodicals, Inc.
An ab initio study of nuclear volume effects for isotope fractionations using two‐component relativistic methodsNemoto, Keisuke; Abe, Minori; Seino, Junji; Hada, Masahiko
doi: 10.1002/jcc.23858pmid: 25727195
We investigate the accuracy of two‐component Douglas–Kroll–Hess (DKH) methods in calculations of the nuclear volume term (≡ lnKnv) in the isotope fractionation coefficient. lnKnv is a main term in the chemical equilibrium constant for isotope exchange reactions in heavy element. Previous work based on the four‐component method reasonably reproduced experimental lnKnv values of uranium isotope exchange. In this work, we compared uranium reaction lnKnv values obtained from the two‐component and four‐component methods. We find that both higher‐order relativistic interactions and spin‐orbit interactions are essential for quantitative description of lnKnv. The best alternative is the infinite‐order Douglas–Kroll–Hess method with infinite‐order spin‐orbit interactions for the one‐electron term and atomic‐mean‐field spin‐same‐orbit interaction for the two‐electron term (IODKH‐IOSO‐MFSO). This approach provides almost equivalent results for the four‐component method, while being 30 times faster. The IODKH‐IOSO‐MFSO methodology should pave the way toward computing larger and more general molecules beyond the four‐component method limits. © 2015 Wiley Periodicals, Inc.
Theoretical investigation of the halogen bonded complexes between carbonyl bases and molecular chlorineZierkiewicz, Wiktor; Bieńko, Dariusz C.; Michalska, Danuta; Zeegers‐Huyskens, Thérèse
doi: 10.1002/jcc.23860pmid: 25727322
The halogen bonded complexes between six carbonyl bases and molecular chlorine are investigated theoretically. The interaction energies calculated at the CCSD(T)/aug‐cc‐pVTZ level range between −1.61 and −3.50 kcal mol−1. These energies are related to the ionization potential, proton affinity, and also to the most negative values (Vs,min) on the electrostatic potential surface of the carbonyl bases. A symmetry adapted perturbation theory decomposition of the energies has been performed. The interaction results in an elongation of the ClCl bond and a contraction of the CF and CH bonds accompanied by a blue shift of the ν(CH) vibrations. The properties of the Cl2 molecules are discussed as a function of the σ*(ClCl) occupation, the hybridization, and the occupation of the Rydberg orbitals of the two chlorine atoms. Our calculations predict a large enhancement of the infrared and Raman intensities of the ν(ClCl) vibration on going from isolated to complexed Cl2. © 2015 Wiley Periodicals, Inc.
One‐electron images in real space: Natural adaptive orbitalsMenéndez, Marcos; Álvarez Boto, Roberto; Francisco, Evelio; Martín Pendás, Ángel
doi: 10.1002/jcc.23861pmid: 25691432
We introduce a general procedure to construct a set of one‐electron functions in chemical bonding theory, which remain physically sound both for correlated and noncorrelated electronic structure descriptions. These functions, which we call natural adaptive orbitals, decompose the n‐center bonding indices used in real space theories of the chemical bond into one‐electron contributions. For the n = 1 case, they coincide with the domain natural orbitals used in domain‐averaged Fermi hole analyses. We examine their interpretation in the two‐center case, and show how they behave and evolve in simple cases. Orbital pictures obtained through this technique converge onto the chemist's molecular orbital toolbox if electron correlation may be ignored, and provide new insight if it may not. © 2015 Wiley Periodicals, Inc.