scholarly journals The Role of Vibrational Anharmonicity in the Computational Study of Thermal Spin Crossover

2019 ◽  
Vol 5 (3) ◽  
pp. 49 ◽  
Author(s):  
Jianfang Wu ◽  
Carmen Sousa ◽  
Coen de Graaf
Keyword(s):  
Computational Chemistry ◽  
Spin Crossover ◽  
Computational Study ◽  
Accurate Determination ◽  
Zero Point ◽  
Energy Difference ◽  
Electronic Energy ◽  
Point Energy ◽  
A Minor ◽  
The Impact

Spin crossover in transition metal complexes can be studied in great detail with computational chemistry. Over the years, the understanding has grown that the relative stability of high-spin (HS) versus low-spin (LS) states is a subtle balance of many factors that all need to be taken into account for a reliable description. Among the different contributions, the zero-point energy (ZPE) and the entropy play key roles. These quantities are usually calculated assuming a harmonic oscillator model for the molecular vibrations. We investigated the impact of including anharmonic corrections on the ZPE and the entropy and indirectly on the critical temperature of spin crossover. As test systems, we used a set of ten Fe(II) complexes and one Fe(III) complex, covering different coordination modes (mono-, bi-, and tri-dentate ligands), decreasing coordination number upon spin crossover, coordination by second- and third-row atoms, and changes in the oxidation state. The results show that the anharmonicity has a measurable effect, but it is in general rather small, and tendencies are not easily recognized. As a conclusion, we put forward that for high precision results, one should be aware of the anharmonic effects, but as long as computational chemistry is still struggling with other larger factors like the influence of the environment and the accurate determination of the electronic energy difference between HS and LS, the anharmonicity of the vibrational modes is a minor concern.

scholarly journals THEORETICAL STUDY OF THE [Fe(en)2(NCS)2] COMPLEX WITH REPARAMETERIZED DENSITY FUNCTIONALS

2010 ◽  
Vol 9 (3) ◽  
pp. 432-436
Author(s):  
Yusthinus T. Male ◽  
Djulia Onggo ◽  
Muhamad A. Martoprawiro ◽  
Ismunandar Ismunandar
Keyword(s):  
High Spin ◽  
Hybrid Method ◽  
Density Functional ◽  
Computational Study ◽  
Energy Difference ◽  
Electronic Energy ◽  
Spin States ◽  
High Spin States ◽  
Chemical Studies ◽  
Quantum Chemical Studies

Quantum chemical studies have been carried out on the Fe(en)2(NCS)2 (en = ethylenediamine) complex both in low and high spin states (S = 0 and S = 2) using hybrid exchange-correlation functional (B3LYP) and non-hybrid method (BLYP). Calculations were performed in vacuum and in methanol to study the effect of cis-trans geometry on the structure and energy difference between low-spin (LS) and high-spin (HS) states of iron (II) complexes. Full geometry optimizations of the complexes show that hybrid method consistently gives higher energy difference between LS and HS states than the nonhybrid methods. Calculations with reparameterized density functional theory that showed more reasonable electronic energy splittings in previous research was also carried out. In addition, the computational study of Fe(en)2(NCS)2 in vacuum and methanol with PCM method showed that the complexes tend to adopt cis geometry. This geometry showed much less charge transfer in the substitutions of NCS- ligands compare to trans geometry.   Keywords: Electronic structure, spin states, density functional, frontiers orbitals

Electron transfer in mixed-valence biferrocenium salts: effect of zero-point energy difference and pronounced anion dependence

1991 ◽  
Vol 30 (11) ◽  
pp. 2457-2462 ◽  
Author(s):  
Teng Yuan. Dong ◽  
Chi Chang. Schei ◽  
Tsui Ling. Hsu ◽  
Shyi Long. Lee ◽  
Shiang Jin. Li
Keyword(s):  
Electron Transfer ◽  
Mixed Valence ◽  
Zero Point ◽  
Energy Difference ◽  
Zero Point Energy ◽  
Point Energy

scholarly journals Pinacolone-Alcohol Gas-Phase Solvation Balances as Experimental Dispersion Benchmarks

2020 ◽  
Author(s):  
Charlotte Zimmermann ◽  
Taija L. Fischer ◽  
Martin A. Suhm
Keyword(s):  
Supersonic Jet ◽  
Zero Point ◽  
Energy Difference ◽  
Density Functionals ◽  
Harmonic Frequency ◽  
Point Energy ◽  
Electron Pairs ◽  
London Dispersion ◽  
London Dispersion Forces ◽  
Experimental Dispersion

The influence of distant London dispersion forces on the docking preference of alcohols of different size between the two lone electron pairs of the carbonyl group in pinacolone is explored by infrared spectroscopy of the OH stretching fundamental in supersonic jet expansions of 1:1 solvate complexes. Experimentally, no pronounced tendency of the alcohol to switch from the methyl to the bulkier tert-butyl side with increasing size is found. In all cases, methyl docking dominates by at least a factor of two, whereas DFT-optimized structures suggest a very close balance for the larger alcohols, once corrected by CCSD(T) relative electronic energies. Together with inconsistencies when switching from a C4 to a C5 alcohol, this points at deficiencies of the investigated B3LYP and in particular TPSS functionals even after dispersion correction, which cannot be blamed on zero point energy effects. The search for density functionals which describe the harmonic frequency shift, the structural change and the energy difference between the docking isomers of larger alcohols to unsymmetric ketones in a satisfactory way is open.

scholarly journals Pinacolone-Alcohol Gas-Phase Solvation Balances as Experimental Dispersion Benchmarks

Molecules ◽  
2020 ◽  
Vol 25 (21) ◽  
pp. 5095
Author(s):  
Charlotte Zimmermann ◽  
Taija L. Fischer ◽  
Martin A. Suhm
Keyword(s):  
Supersonic Jet ◽  
Zero Point ◽  
Energy Difference ◽  
Density Functionals ◽  
Harmonic Frequency ◽  
Point Energy ◽  
Electron Pairs ◽  
London Dispersion ◽  
London Dispersion Forces ◽  
Experimental Dispersion

The influence of distant London dispersion forces on the docking preference of alcohols of different size between the two lone electron pairs of the carbonyl group in pinacolone was explored by infrared spectroscopy of the OH stretching fundamental in supersonic jet expansions of 1:1 solvate complexes. Experimentally, no pronounced tendency of the alcohol to switch from the methyl to the bulkier tert-butyl side with increasing size was found. In all cases, methyl docking dominates by at least a factor of two, whereas DFT-optimized structures suggest a very close balance for the larger alcohols, once corrected by CCSD(T) relative electronic energies. Together with inconsistencies when switching from a C4 to a C5 alcohol, this points at deficiencies of the investigated B3LYP and in particular TPSS functionals even after dispersion correction, which cannot be blamed on zero point energy effects. The search for density functionals which describe the harmonic frequency shift, the structural change and the energy difference between the docking isomers of larger alcohols to unsymmetric ketones in a satisfactory way is open.

Proton Acidity and Proton Mobility in ECR-40, a Silicoaluminophosphate That Violates Löwenstein’s Rule

2019 ◽  
Author(s):  
Michael Fischer
Keyword(s):  
Density Functional ◽  
Water Clusters ◽  
Computational Study ◽  
Acid Catalyst ◽  
Ab Initio Molecular Dynamics ◽  
Brønsted Acidity ◽  
Hydrothermal Route ◽  
Proton Mobility ◽  
Brönsted Acidity ◽  
A Minor

<p>The silicoaluminophosphate zeotype ECR-40, which has the MEI topology, contains linkages of AlO<sub>4</sub> tetrahedra via a common oxygen atom, thereby violating the famous “Löwenstein’s rule”. Due to the proven existence of Al-O-Al linkages in this material, it constitutes an ideal model system to study the acidity and mobility of protons associated with such unusual linkages. In addition, their properties can be directly compared to those of protons associated with more common Si-O-Al linkages, which are also present in ECR-40. In this work, static density functional theory (DFT) calculations including a dispersion correction were employed to study the preferred proton sites as well as the Brønsted acidity of the framework protons, followed by DFT-based ab-initio molecular dynamics (AIMD) to investigate the proton mobility in guest-free and hydrated ECR-40. Initially, two different proton arrangements were compared, one containing both H[O6] protons associated with Al-O-Al linkages and H[O10] protons at Si-O-Al linkages, the other one containing only H[O10] protons. The former model was found to be thermodynamically favoured, as a removal of protons from the Al-O-Al linkages causes a local accumulation of negative charge. Calculations of the deprotonation energy showed a moderately higher Brønsted acidity of the H[O10] protons, at variance with previous empirical explanations, which attributed the exceptional performance of ECR-40 as acid catalyst to the presence of Al‑O‑Al linkages. The AIMD simulations (<i>T</i> = 298 K) delivered no appreciable proton mobility for guest-free ECR-40 and for low levels of hydration (one H<sub>2</sub>O per framework proton). Under saturation conditions, framework deprotonation occurred, leading to the formation of protonated water clusters in the pores. Pronounced differences between the two types of framework protons were observed: While the H[O10] protons were always removed from the Si-O-Al linkages, the Al-O-Al linkages remained mostly protonated, but deprotonation did occur to a minor extent. The observation of a degree of framework deprotonation of Al-O-Al linkages differs from the findings reported in a recent computational study of hydrated aluminosilicate zeolites with such linkages (Heard et al., <i>Chem. Sci.</i> <b>2019</b>, <i>10</i>, 5705), pointing to an influence of the overall framework composition. Further inspection of the AIMD results showed that a coordination of water molecules to framework Al atoms occurred in many cases, especially in the vicinity of the Al-O-Al linkages, sometimes resulting in a pronounced modification of the linkages through additional bridging oxygen atoms. Given the changes in the local structure, it can be expected that such modified linkages are especially prone to break upon dehydration. Thus, in addition to elucidating the deprotonation behaviour of protons associated with different types of linkages, the calculations also provide insights into possible reasons for the instability of Al-O-Al linkages, clarifying why Löwenstein’s rule is mostly obeyed in materials that are formed via a hydrothermal route.</p>

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