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Molecular Motion in Frustrated Lewis Pair Chemistry: insights from modelling
Stockholm University, Faculty of Science, Department of Organic Chemistry.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mechanisms of reactions of the frustrated Lewis pairs (FLPs) with carbon dioxide (CO2) and hydrogen (H2) are studied by using quantum chemical modelling. FLPs are relatively novel chemical systems in which steric effects prevent a Lewis base (LB) from donating its electron pair to a Lewis acid (LA). From the main group of the periodic table, a variety of the electron pair donors and acceptors can create an FLP and the scope of the FLP chemistry is rapidly expanding at present. Representative intermolecular FLPs are phosphines and boranes with bulky electron-donating groups on phosphorus and bulky electron-withdrawing groups on boron – e.g., the tBu3P/B(C6F5)3 pair. The intramolecular FLPs feature linked LB and LA centers in one molecule.

Investigations of the FLP reaction mechanisms were carried out using the transition state (TS) and the potential energy surface (PES) calculations plus the Born-Oppenheimer molecular dynamics (BOMD) as an efficient and robust implementation of general ab initio molecular dynamics scheme. In BOMD simulations, quantum and classical mechanics are combined. The electronic structure calculations are fully quantum via the density functional theory (DFT). Molecular motion at finite (non-zero) temperature is explicitly accounted for at non-quantized level via Newton’s equations. Due to recent advancements of computers and algorithms, one can treat fairly large macromolecular systems with BOMD and even include significant portion of the first solvation shell surrounding a large reacting complex in the molecular model.

Main results are as follows. It is shown that dynamics is significant for understanding of FLP chemistry. The multiscale nature of motion – i.e., light molecules such as CO2 or H2 versus a pair of heavy LB and LA molecules – affects the evolution of interactions in the reacting complex. Motion which is perpendicular to the reaction coordinate was found to play a role in the transit of the activated complex through the TS-region. Regarding the heterolytic cleavage of H2 by tBu3P/B(C6F5)3 FLP simulated in gas phase and with explicit solvent, it was found that (i) the reaction path includes shallow quasi-minima “imbedded” in the TS-region, and (ii) tBu3P/B(C6F5)3 are almost stationary while proton- and hydride-like fragments of H2 move toward phosphorous and boron respectively. For binding of CO2 by tBu3P/B(C6F5)3 FLP, it was found that (i) the reacting complex can “wander” along the “potential energy wall” that temporarily blocks the path to the product, and (ii) the mechanism can combine the concerted and two-step reaction paths in solution. The discovered two-step binding of CO2 by tBu3P/B(C6F5)3 FLP involves solvent-stabilized phosphorus-carbon interactions (dative bonding). These and other presented results are corroborated and explained using TS and PES calculations. With computations of observable characteristics of reactions, it is pointed out how it could be possible to attain experimental proof of the results.

Place, publisher, year, edition, pages
Stockholm: Department of Organic Chemistry, Stockholm University , 2015. , 45 p.
Keyword [en]
CO2 capture, hydrogen activation, molecular motion, ab initio molecular dynamics, frustrated Lewis pair, quantum chemical modelling, reaction mechanism
National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-122348ISBN: 978-91-7649-298-7 (print)OAI: oai:DiVA.org:su-122348DiVA: diva2:865717
Public defence
2016-01-26, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 6: Accepted.

 

Available from: 2015-12-29 Created: 2015-10-29 Last updated: 2015-12-30Bibliographically approved
List of papers
1. Ab initio dynamics trajectory study of the heterolytic cleavage of H2 by a Lewis acid [B(C6F5)3] and a Lewis base [P(tBu)3]
Open this publication in new window or tab >>Ab initio dynamics trajectory study of the heterolytic cleavage of H2 by a Lewis acid [B(C6F5)3] and a Lewis base [P(tBu)3]
2013 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 138, no 15, 154305Article in journal (Refereed) Published
Abstract [en]

Activation of H-2 by a frustrated Lewis pair (FLP) composed of B(C6F5)(3) and P(tBu)(3) species has been explored with high level direct ab initio molecular dynamics (AIMD) simulations at finite temperature (T = 300 K) in gas phase. The initial geometrical conditions for the AIMD trajectory calculations, i.e., the near attack conformations of FLP + H-2, were devised using the host-guest model in which suitable FLP conformations were obtained from the dynamics of the B(C6F5)(3)/P(tBu)(3) pair in gas phase. AIMD trajectory calculations yielded microscopic insight into effects which originate from nuclear motion in the reacting complex, e. g., the alternating compression/elongation of the boron-phosphorous distance and the change of the pyramidality of boron in B(C6F5)(3). The ensemble averaged trajectory analysis has been compared with the minimum energy path (MEP) description of the reaction. Similar to MEP, AIMD shows that an attack of the acid/base pair on the H-H bond gives rise to the polarization of the H-2 molecule and as a consequence generates a large dipole moment of the reacting complex. The MEP and AIMD portrayals of the reaction are fundamentally different in terms of the magnitude of the motion of nuclei in B(C6F5)(3) and P(tBu)(3) during the H-2 cleavage. In the AIMD trajectory simulations, geometries of B(C6F5)(3) and P(tBu)(3) appear as nearly frozen on the short time scale of the H-2 cleavage. This is contrary to the MEP picture. Several of the concepts which arise from this work, e. g., separation of time scales of nuclear motion and the time-dependence of the donor-acceptor interactions in the reacting complex, are important for the understanding of chemical reactivity and catalysis.

National Category
Atom and Molecular Physics and Optics Physical Chemistry Organic Chemistry
Research subject
Organic Chemistry
Identifiers
urn:nbn:se:su:diva-90787 (URN)10.1063/1.4799932 (DOI)000317814900019 ()
Funder
Knut and Alice Wallenberg Foundation
Note

AuthorCount:2;

Available from: 2013-06-14 Created: 2013-06-11 Last updated: 2017-12-06Bibliographically approved
2. How Frustrated Lewis Acid/Base Systems Pass through Transition-State Regions: H-2 Cleavage by [tBu(3)P/B(C6F5)(3)]
Open this publication in new window or tab >>How Frustrated Lewis Acid/Base Systems Pass through Transition-State Regions: H-2 Cleavage by [tBu(3)P/B(C6F5)(3)]
2014 (English)In: ChemPhysChem, ISSN 1439-4235, E-ISSN 1439-7641, Vol. 15, no 14, 2936-2944 p.Article in journal (Refereed) Published
Abstract [en]

We investigate the transition-state (TS) region of the potential energy surface (PES) of the reaction tBu(3)P+H-2 +B(C6F5)(3)-> tBu(3)P-H(+)+H(-)-B(C6F5)(3) and the dynamics of the TS passage at room temperature. Owing to the conformational inertia of the phosphane center dot center dot center dot borane pocket involving heavy tBu(3)P and B(C6F5)(3) species and features of the PES E(P center dot center dot center dot H, B center dot center dot center dot H vertical bar B center dot center dot center dot P) as a function of P center dot center dot center dot H, B center dot center dot center dot H, and B center dot center dot center dot P distances, a typical reactive scenario for this reaction is a trajectory that is trapped in the TS region for a period of time (about 350 fs on average across all calculated trajectories) in a quasi-bound state (scattering resonance). The relationship between the timescale of the TS passage and the effective conformational inertia of the phosphane center dot center dot center dot borane pocket leads to a prediction that isotopically heavier Lewis base/Lewis acid pairs and normal counterparts could give measurably different reaction rates. Herein, the predicted quasi-bound state could be verified in molecular collision experiments involving femtosecond spectroscopy.

Keyword
ab initio calculations, molecular dynamics, ion pairs, hydrogen, transition states
National Category
Chemical Sciences Physical Sciences
Research subject
Organic Chemistry
Identifiers
urn:nbn:se:su:diva-108988 (URN)10.1002/cphc.201402450 (DOI)000342770500008 ()
Funder
Berzelii Centre EXSELENTKnut and Alice Wallenberg FoundationSwedish National Infrastructure for Computing (SNIC)National Supercomputer Centre (NSC), Sweden
Note

AuthorCount:2;

Available from: 2014-12-04 Created: 2014-11-10 Last updated: 2017-12-05Bibliographically approved
3. Ab Initio Molecular Dynamics Study of Hydrogen Cleavage by a Lewis Base [tBu(3)P] and a Lewis Acid [B(C6F5)(3)] at the Mesoscopic Level-Dynamics in the Solute-Solvent Molecular Clusters
Open this publication in new window or tab >>Ab Initio Molecular Dynamics Study of Hydrogen Cleavage by a Lewis Base [tBu(3)P] and a Lewis Acid [B(C6F5)(3)] at the Mesoscopic Level-Dynamics in the Solute-Solvent Molecular Clusters
2014 (English)In: ChemPhysChem, ISSN 1439-4235, E-ISSN 1439-7641, Vol. 15, no 17, 3714-3719 p.Article in journal (Refereed) Published
Abstract [en]

With the help of state-of-the-art ab initio molecular dynamics methods, we investigated the reaction pathway of the {tBu(3)P + H-2 + B(C6F5)(3)} system at the mesoscopic level. It is shown that: i) the onset of H-2 activation is at much larger boronphosphorus distances than previously thought; ii) the system evolves to the product in a roaming-like fashion because of quasi-periodic nuclear motion along the asymmetric normal mode of PHHB fragment; iii) transient configurations of a certain type are present despite structural interference from the solvent; iv) transient-state configurations with sub-picosecond lifetime have potentially interesting infrared activity in the organic solvent (toluene) as well as in the gas phase. The presented results should be helpful for future experimental and theoretical studies of frustrated Lewis pair (FLP) activity.

Keyword
ab initio calculations, frustrated Lewis pairs, hydrogen cleavage, molecular dynamics, transient configurations
National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
urn:nbn:se:su:diva-111380 (URN)10.1002/cphc.201402519 (DOI)000345513200003 ()
Funder
Berzelii Centre EXSELENTKnut and Alice Wallenberg FoundationSwedish National Infrastructure for Computing (SNIC)National Supercomputer Centre (NSC), Sweden
Note

AuthorCount:2;

Available from: 2015-01-07 Created: 2015-01-02 Last updated: 2017-12-05Bibliographically approved
4. Multiple-Pathways of Carbon Dioxide Binding by a Lewis Acid [B(C6F5)(3)] and a Lewis Base [P(tBu)(3)]: The Energy Landscape Perspective
Open this publication in new window or tab >>Multiple-Pathways of Carbon Dioxide Binding by a Lewis Acid [B(C6F5)(3)] and a Lewis Base [P(tBu)(3)]: The Energy Landscape Perspective
2014 (English)In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 114, no 4, 289-294 p.Article in journal (Refereed) Published
Abstract [en]

Using the reaction-relevant two-dimensional potential energy surface, an accurate reaction-pathway mapping and ab inito molecular dynamics, it is shown that CO2 capture by P(tBu)(3) and B(C6F5)(3) species has many nearly degenerate reaction-routes to take. The explanation of that is in the topography of the transition state (saddle) area. An ensemble of asynchronous reaction-routes of CO2 binding is described in fine detail.

Keyword
carbon dioxide, frustrated Lewis pairs, phosphines, boranes, van der Waals complexes
National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
urn:nbn:se:su:diva-100640 (URN)10.1002/qua.24558 (DOI)000329509400001 ()
Funder
Knut and Alice Wallenberg Foundation
Note

AuthorCount:2;

Available from: 2014-02-14 Created: 2014-02-10 Last updated: 2017-12-06Bibliographically approved
5. Uncovering the Role of Intra- and Intermolecular Motion in Frustrated Lewis Acid/Base Chemistry: Ab Initio Molecular Dynamics Study of CO2 Binding by Phosphorus/Boron Frustrated Lewis Pair [tBu(3)P/B(C6F5)(3)]
Open this publication in new window or tab >>Uncovering the Role of Intra- and Intermolecular Motion in Frustrated Lewis Acid/Base Chemistry: Ab Initio Molecular Dynamics Study of CO2 Binding by Phosphorus/Boron Frustrated Lewis Pair [tBu(3)P/B(C6F5)(3)]
2014 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 53, no 9, 4598-4609 p.Article in journal (Refereed) Published
Abstract [en]

The role of the intra- and intermolecular motion, i.e., molecular vibrations and the relative motion of reactants, remains largely unexplored in the frustrated Lewis acid/base chemistry. Here, we address the issue with the ab initio molecular dynamics (AIMD) study of CO2 binding by a Lewis acid (LA) and a Lewis base (LB), i.e., tBu(3)P + CO2 + B(C6F5)(3) -> tBu(3)P-C(O)O-B(C6F5)(3) ([1]). Reasonably large ensemble of AIMD trajectories propagated at 300 K from structures in the saddle region as well as trajectories propagated directly from the reactants region revealed an effect arising from significant recrossing of the saddle area. The effect is that transient complexes composed of weakly interacting reactants nearly cease to progress along the segment of the minimum energy pathway (MEP) at the saddle region for a (subpicosecond) period of time during which the dominant factor is the light-to-heavy type of relative motion of the vibrating reactants, i.e., the bouncing-like movement of CO2 with respect to much heavier phosphine and borane as main contributor to the mode that is perpendicular to the MEP-direction. In terms of how P...C and B...O distances change with time, the roaming-like patterns of typical AIMD trajectories, reactive and nonreactive alike, extend far beyond the saddle region. In addition to the dynamical portrayal of [1], we provide the energy-landscape perspective that takes into account the hierarchy of time scales. The verifiable implication of the effect found here is that the isotopically substituted (heavier) LB/LA pair should be less reactive that the normal and thus lighter counterpart.

National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
urn:nbn:se:su:diva-104394 (URN)10.1021/ic500284q (DOI)000335547400040 ()
Funder
Berzelii Centre EXSELENTKnut and Alice Wallenberg Foundation
Note

AuthorCount:2;

Available from: 2014-06-12 Created: 2014-06-10 Last updated: 2017-12-05Bibliographically approved
6. Ab initio molecular dynamics with explicit solvent discovers a two-step pathway in the Frustrated Lewis Pair reaction
Open this publication in new window or tab >>Ab initio molecular dynamics with explicit solvent discovers a two-step pathway in the Frustrated Lewis Pair reaction
(English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765Article in journal (Refereed) Accepted
National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
urn:nbn:se:su:diva-121802 (URN)
Available from: 2015-10-16 Created: 2015-10-16 Last updated: 2017-12-01

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