Structure and hydrogen dynamic behavior in proton sponge cations and organometallic complexes

This thesis focuses on the study of hydrogen dynamic behavior in a series of proton sponge cations as well as in bis(silyl)hydride organometallic complexes. To understand the dynamic behavior, a new tool, i.e. the three-dimensional vibrational Schrödinger equation has been derived and solved. Three different vibrational patterns for hydrogen motion have been revealed. The classification of proton sponges and bis(silyl)hydride complexes into those with localized and delocalized hydrogen behavior has been proposed. For the organometallic complexes the influence of the vibrational motion of the hydrogen on J(Si−H) spin−spin coupling constants has been studied. Also this thesis presents a study of [Cp*Co(SiR3)2(H)2] cobalt complexes and their iridium analogues to detect possible Si···H interactions. Based on molecular geometries, Wiberg bond indices, and J(Si–H) spin-spin coupling constants, at least two residual Si···H interactions have been detected in cobalt complexes while there are only rudimentary Si···H interactions in the case of iridium complexesAquesta tesi es centra en l'estudi del comportament dinàmic d'hidrogen en una sèrie de cations d'esponges de protó, així com en bis(silil)hidrur complexos organometàl•lics. Per entendre el comportament dinàmic, una nova eina com l'equació de Schrödinger vibracional tridimensional s'ha derivat i resolt. S'han trobat tres patrons diferents pel moviment vibracional d'hidrogen. S'ha proposat la classificació d'esponges de protó i bis(silil)hidrur complexos amb el comportament d'hidrogen localitzat i deslocalitzat. Per als complexos organometàl·lics s'ha estudiat la influència del moviment vibracional d'hidrogen en J(Si–H) constants d'acoblament de espín-espín. Una altra part d'aquesta tesi presenta un estudi de [Cp*Co(SiR3)2(H)2] complexos de cobalt i els seus anàlegs d'iridi per detectar possibles interaccions Si···H. Basat en geometries moleculars, en els índexs d'enllaç de Wiberg i constants d'acoblament d'espín-espín J(Si-H), s'han detectat almenys dues interaccions residuals Si···H en els complexos de cobalt, mentre que hi ha interaccions rudimentàries Si···H en el cas dels complexos d'irid

Hydrogen motion in proton sponges

Hydrogen bonds are an essential concept in chemistry and have been studied extensively by both theoretical and experimental methods. An interesting special case is short strong hydrogen bonds present in proton sponges. In principle, three cases are possible. The potential energy surface (PES) of the hydrogen motion indeed has only one symmetric minimum. Alternatively, there can be two minima separated by a small barrier, such that the proton still moves freely between them even at 0 K. Finally, if the barrier is slightly larger, the proton motion can be frozen at low temperature, but still occurs easily at higher temperatur

Dynamic Behavior of Hydrogen in Transition Metal Bis(silyl) Hydride Complexes

A series of rhodium complexes CpRh­(SiMe₂X)₂(SiMe₃)­(H) (X = Me, Cl, Br, I), <b>Cp1</b>–<b>Cp4</b>, CpRh­(SiMe₂X)₂(PMe₃)­(H)⁺ (X = Me, Cl, Br, I), <b>Cp5</b>–<b>Cp8</b>, CpRh­(SiMe₃)₂(SiF₃)­(H), <b>Cp9</b>, CpRh­(SiMe₃)₂(SiH₃)­(H), <b>Cp10</b>, TpRh­(SiH₃)₂(SiMe₃)­(H), <b>Tp1</b>, TpRh­(SiH₃)₂(PMe₃)­(H)⁺, <b>Tp2</b>, and TpRh­(SiF₃)₂(PMe₃)­(H)⁺, <b>Tp3</b>, were studied computationally to understand the hydrogen behavior in the Si···H···Si moiety. The hydride ligand interacts with at least one of the silyls, and in many cases with both, but is located asymmetrically with regard to them, giving rise to a double-well potential energy surface (PES) for hydrogen motion. The hydrogen transfer barriers Δ<i>E</i> vary from 0.03 to 3 kcal·mol^–1. For selected complexes <b>Tp1</b>, <b>Tp2</b>, <b>Tp3</b>, and <b>Cp9</b> the three-dimensional PESs were constructed and the vibrational Schrödinger equation was solved. The PES is highly anharmonic in all four cases. The hydrogen is delocalized between two silicons in complexes <b>Tp1</b>, <b>Tp3</b>, and <b>Cp9</b>, but localized around the energy minima in complex <b>Tp2</b>. Complex <b>Tp3</b> is an intermediate case with a substantial tunneling. The delocalized behavior is pertinent to systems with Δ<i>E</i> < 0.25 kcal·mol^–1. For complexes <b>Tp1</b>, <b>Tp2</b>, <b>Tp3</b>, and <b>Cp9</b> the <i>J</i>(Si–H) spin–spin coupling constants were calculated taking into account the vibrational motion of hydride. For <b>Tp1</b>, <b>Tp3</b>, and <b>Cp9</b> both <i>J</i>(Si¹–H) and <i>J</i>(Si²–H) are negative due to simultaneous Si¹···H···Si² interactions, while for <b>Tp2</b> <i>J</i>(Si²–H) is positive, indicating a single Si···H interaction only. Negative <i>J</i>(Si–H) values were obtained even for Si···H distances as large as 2.3 Å (complex <b>Tp3</b>). A possible effect of vibrations on the <i>J</i>(Si–H) values is also discussed

Mixed-Reference Spin-Flip Time-Dependent Density Functional Theory (MRSF-TDDFT) as a Simple yet Accurate Method for Diradicals and Diradicaloids

Due to their multiconfigurational nature featuring strong electron correlation, accurate description of diradicals and diradicaloids is a challenge for quantum chemical methods. The recently developed mixed-reference spin-flip (MRSF)-TDDFT method is capable of describing the multiconfigurational electronic states of these systems while avoiding the spin-contamination pitfalls of SF-TDDFT. Here, we apply MRSF-TDDFT to study the adiabatic singlet–triplet (ST) gaps in a series of well-known diradicals and diradicaloids. On average, MRSF displays a very high prediction accuracy of the adiabatic ST gaps with the mean absolute error (MAE) amounting to 0.14 eV. In addition, MRSF is capable of accurately describing the effect of the Jahn–Teller distortion occurring in the trimethylenemethane diradical, the violation of the Hund rule in a series of the didehydrotoluene diradicals, and the potential energy surfaces of the didehydrobenzene (benzyne) diradicals. A convenient criterion for distinguishing diradicals and diradicaloids is suggested on the basis of the easily obtainable quantities. In all of these cases, which are difficult for the conventional methods of density functional theory (DFT), MRSF shows results consistent with the experiment and the high-level ab initio computations. Hence, the present study documents the reliability and accuracy of MRSF and lays out the guidelines for its application to strongly correlated molecular systems

Synergetic interplay between pressure and surface chemistry for the conversion of sp2-bonded carbon layers into sp3-bonded carbon films

The effects of the interplay between pressure and surface chemistry on the transformation of few-layer graphene into an sp3-bonded carbon film were investigated with first-principles density functional theory calculations including ab initio molecular dynamics. N2H4, H2O, and He were each considered as a candidate pressure medium. Compared with the bulk graphite, the surface chemistry overwhelmingly governed the conversion energetics for nanometer-thick graphene layers. A hydrogen-donating medium reduced the required conversion pressure compared with an inert one; the conversion pressure obtained by using N2H4 was 40% of the corresponding pressure obtained with He. We suggest that pressurizing the cell through hydrogen-donating pressure media has the advantage from the surface chemistry by concentrating hydrogen atoms on carbon surfaces. © 2016 Elsevier Ltd. All rights reserved1441sciescopu

Reaction Mechanisms for the Formation of Mono- And Dipropylene Glycol from the Propylene Oxide Hydrolysis over ZSM‑5 Zeolite

Stepwise and concerted mechanisms for the formation of mono- and dipropylene glycol over ZSM-5 zeolite were investigated. For the calculations, a T128 cluster model of zeolite was used with a QM/QM scheme to investigate the reaction mechanism. The active inner part of zeolite was represented by a T8 model and was treated at the DFT (BP86) level, including D3 Grimme dispersion, and the outer part of the zeolite was treated at the DFTB level. The solvent effects were taken into account by including explicitly water molecules in the cavity of the zeolite. The Gibbs energies were calculated for both mechanisms at 70 °C. In the case of the stepwise mechanism for the monopropylene glycol formation, the rate-limiting step is the opening of the epoxide ring. The activation energy for this process is 35.5 kcal mol^–1, while in the case of the concerted mechanism the rate-limiting step is the simultaneous ring opening of the epoxide and the attack by a water molecule. This process has an activation energy of 27.4 kcal mol^–1. In the case of the stepwise mechanism of the dipropylene glycol formation, the activation energy for the rate-limiting step is the same as for the monopropylene glycol formation, and in the case of the concerted mechanism, the activation energy for the rate-limiting step is 30.8 kcal mol^–1. In both cases (mono- and dipropylene glycol formation), the concerted mechanism should be dominant over the stepwise one. The barrier for monopropylene glycol formation is lower than that for dipropylene glycol formation. Consequently, our results show that the formation of the monopropylene glycol is faster, although the formation of dipropylene glycol as a byproduct cannot be avoided using this zeolite

Probing Evolution of Twist-Angle-Dependent Interlayer Excitons in MoSe2/WSe2 van der Waals Heterostructures

Interlayer excitons were observed at the heterojunctions in van der Waals heterostructures (vdW HSs). However, it is not known how the excitonic phenomena are affected by the stacking order. Here, we report twist-angle-dependent interlayer excitons in MoSe2/WSe2 vdW HSs based on photoluminescence (PL) and vdW-corrected density functional theory calculations. The PL intensity of the interlayer excitons depends primarily on the twist angle: It is enhanced at coherently stacked angles of 0 degrees and 60 degrees (owing to strong interlayer coupling) but disappears at incoherent intermediate angles. The calculations confirm twist-angle-dependent interlayer coupling: The states at the edges of the valence band exhibit a long tail that stretches over the other layer for coherently stacked angles; however, the states are largely confined in the respective layers for intermediate angles. This interlayer hybridization of the band edge states also correlates with the interlayer separation between MoSe2 and WSe2 layers. Furthermore, the interlayer coupling becomes insignificant, irrespective of twist angles, by the incorporation of a hexagonal boron nitride monolayer between MoSe2 and WSe2

Probing Evolution of Twist-Angle-Dependent Interlayer Excitons in MoSe₂/WSe₂ van der Waals Heterostructures

Interlayer excitons were observed at the heterojunctions in van der Waals heterostructures (vdW HSs). However, it is not known how the excitonic phenomena are affected by the stacking order. Here, we report twist-angle-dependent interlayer excitons in MoSe₂/WSe₂ vdW HSs based on photoluminescence (PL) and vdW-corrected density functional theory calculations. The PL intensity of the interlayer excitons depends primarily on the twist angle: It is enhanced at coherently stacked angles of 0° and 60° (owing to strong interlayer coupling) but disappears at incoherent intermediate angles. The calculations confirm twist-angle-dependent interlayer coupling: The states at the edges of the valence band exhibit a long tail that stretches over the other layer for coherently stacked angles; however, the states are largely confined in the respective layers for intermediate angles. This interlayer hybridization of the band edge states also correlates with the interlayer separation between MoSe₂ and WSe₂ layers. Furthermore, the interlayer coupling becomes insignificant, irrespective of twist angles, by the incorporation of a hexagonal boron nitride monolayer between MoSe₂ and WSe₂