Theoretical predictions of enforced structural changes in molecules

A core idea in the context of mechanochemistry is that applying an external tensile force along a reaction coordinate should enhance the chemical reaction of interest. Here, we analyze perturbed generic molecular structures: schematic models of triatomics, ABC, and tetraatomics, AABB. They are used to demonstrate that pulling does usually not use the “reaction coordinate,” but opens new reaction pathways. Within development of these models we use the concept of Newton trajectories for a theory of mechanochemistry. However, we find cases where the theory of Newton trajectories is not applicable. For all cases, we define the curve of force‐displaced stationary points, and we discuss the importance of barrier breakdown points and valley‐ridge inflection points. The examples use Morse potentials for bonds and simple angle functions and are demonstrated by assumed real values for the potential parameters. On the basis of the systematic study of some generic models we explain a set of already observed experimental mechanochemical phenomena in specific molecular systems and we apply the results to the strength of chemical bonds.

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“…We use a model for the effective PES, V f ( x ) for a fixed force, f , via the approximation in x

V_{f} ((), x) = V ((), x) - f^{T} \cdot boldx

where the superscript T denotes transpose, and x is the vector of the nonredundant, internal coordinates of the molecule.…”

Section: Introductionmentioning

confidence: 99%

“…[35][36][37] We use a model for the effective PES, V f (x) for a fixed force, f, via the approximation in x. [5,10,26,29,[38][39][40][41][42][43][44][45][46][47][48]…”

Section: Introductionmentioning

confidence: 99%

Toward a theory of mechanochemistry: Simple models from the very beginnings

Quapp

Bofill

Ribas‐Ariño

2018

Int J of Quantum Chemistry

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“…[8] The smallest building block of spin crossover systems is often an octahedrally coordinated transition metal center with its 3d orbitals split by the ligand field environment. [13][14][15] These techniques quantify the change in the molecular PES under external stresses and the resulting changes in observables. [9] Using effective nuclear forces scaled by their distances from the molecular centroid we here model spin crossover in octahedral metal-ligand complexes as a function of hydrostatic pressure.…”

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confidence: 99%

“…[12] Optimal molecular geometries under external forces can be computed with an assortment of techniques that yield a force-modified potential energy surface (FMPES). [13][14][15] These techniques quantify the change in the molecular PES under external stresses and the resulting changes in observables. Examples include shifts of signals in infrared [16][17][18] and optical spectra, [18][19][20] as well as changes in reaction kinetics.…”

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confidence: 99%

Quantum Chemical Modeling of Pressure‐Induced Spin Crossover in Octahedral Metal‐Ligand Complexes

2019

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Spin state switching on external stimuli is a phenomenon with wide applicability, ranging from molecular electronics to gas activation in nanoporous frameworks. Here, we model the spin crossover as a function of the hydrostatic pressure in octahedrally coordinated transition metal centers by applying a field of effective nuclear forces that compress the molecule towards its centroid. For spin crossover in first-row transition metals coordinated by hydrogen, nitrogen, and carbon monoxide, we find the pressure required for spin transition to be a function of the ligand position in the spectrochemical sequence. While pressures on the order of 1 GPa are required to flip spins in homogeneously ligated octahedral sites, we demonstrate a fivefold decrease in spin transition pressure for the archetypal strong field ligand carbon monoxide in octahedrally coordinated Fe 2 + in [Fe(II)(NH 3 ) 5 CO] 2 + .Pressure-induced spin-flips of transition metal sites involve changes in Coulomb energy, closed shell repulsions, covalent bonding energy and crystal field energy. [1,2] Using the computational Extreme-Pressure PCM (XP-PCM) protocol, [3][4][5] it has been shown that high pressures cause drastic electronic rearrangements, causing first row transition metals with electronic configurations d n (4 � n � 8) to favor low spin configurations at high pressures. [6] In a metal-organic framework (MOF) with exposed Fe(II) sites, a distinct step as a function of atmospheric pressure has been observed in the adsorption isotherm of carbon monoxide, for which a cooperative spin-transition mechanism involving interacting iron centers in the MOF on introduction of the strong-field ligand carbon monoxide has been proposed. [7] In general, the use of MOFs in the separation of industrially relevant gases like hydrogen, nitrogen and carbon monoxide holds a lot of promise, due to the large surface areas, the thermal stability and the adjustability of various parameters of the MOFs. [8] The smallest building block of spin crossover systems is often an octahedrally coordinated transition metal center with its 3d orbitals split by the ligand field environment. Spin-flip at high pressures can be attributed to the increase in splitting of the 3d levels at the metal site such that the potential energy required to maintain a high spin configuration surpasses the spin pairing energy. [9] Using effective nuclear forces scaled by their distances from the molecular centroid we here model spin crossover in octahedral metal-ligand complexes as a function of hydrostatic pressure. We find the pressure required for spin transition to be a function of ligand position in the spectrochemical sequence [10] and demonstrate that the spin transition pressure can be tuned by an adequate choice of the ligand field. Finer quantum chemical effects at play in the process involve a change in the covalent binding energy, Pauli repulsion, and charge transfer as a function of pressure, as demonstrated with an Energy Decomposition Analysis (EDA) [11] scheme.Any external force on...

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“…Using the quantum chemical External Force is Explicitly Included( EFEI) [20][21][22] method to apply forces to molecules, it is shownt hat the transannular carbon-carbonb ond in Dewar benzenei sm echanicallyl abile. At the B3LYP [23][24][25] /cc-pVDZ [26] level of theory, this bond length is 1.60 in the dihydroxy-dinitro-derivative shown in Figure 1A.T he B3LYP model was chosenb ecause it was provent ob er obust in reproducing the correct rupture forcesa nd bond lengths in various mechanochemicalp ulling scenarios [2,27,28] as wella si nabenchmark of spectroscopic properties relevant for this study (see Supporting Information).…”

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confidence: 99%

Mechanical Switching of Aromaticity and Homoaromaticity in Molecular Optical Force Sensors for Polymers

Stauch

2018

Chemistry A European J

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The sensing of mechanical stress in polymers is indispensable for investigating the origin and propagation of cracks that lead to material failure and for designing mechanically responsive polymers. Here the unique approaches of using the force-induced switching of aromaticity and homoaromaticity in molecular optical force sensors for the real-time measurement of mechanical forces acting in stretched polymers are suggested. The mechanical switching of aromaticity in Dewar benzene is an irreversible event, whereas the degree of π-orbital overlap in homoaromatic compounds like homotropylium can be adjusted progressively over a wide range of forces. Using computational methods, it is demonstrated that both approaches lead to significant changes in the visible part of the UV/Vis spectra of the force sensors upon application of weak forces (pN-nN). Polymers that incorporate such molecular force sensors therefore change their color well before material failure occurs.

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Theoretical predictions of enforced structural changes in molecules

Cited by 94 publications

References 22 publications

Toward a theory of mechanochemistry: Simple models from the very beginnings

Toward a theory of mechanochemistry: Simple models from the very beginnings

Quantum Chemical Modeling of Pressure‐Induced Spin Crossover in Octahedral Metal‐Ligand Complexes

Mechanical Switching of Aromaticity and Homoaromaticity in Molecular Optical Force Sensors for Polymers

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