Absorption spectroscopy of (Ind)Ni(PPh3)X (Ind = indenyl, 1-Me-indenyl; X = Cl, Br, Me) and M(Ind)2 (M = Ni, Ru; Ind = indenyl)

1996 ◽  
Vol 74 (11) ◽  
pp. 2194-2200 ◽  
Author(s):  
Malken Bayrakdarian ◽  
Martin J. Davis ◽  
Christian Reber ◽  
Davit Zargarian
Keyword(s):  
Absorption Band ◽  
Absorption Spectroscopy ◽  
Energy Difference ◽  
Molecular Orbital Calculations ◽  
Relaxation Dynamics ◽  
Ligand Interactions ◽  
Key Words Absorption ◽  
Second Category ◽  
Indenyl Complexes ◽  
Homo Lumo

The electronic structures of two types of transition metal indenyl complexes have been studied. The first type, a series of (Ind)Ni(PPh3)X compounds (Ind = indenyl, 1-Me-indenyl; X = Cl, Br, Me) was investigated by absorption spectroscopy and Extended Hückel Molecular Orbital calculations. The energy differences between calculated levels are in good agreement with experimental band positions. For example, the lowest energy singlet–singlet band maximum for (Ind)Ni(PPh3)Cl is at 19 500 cm−1 and the calculated HOMO–LUMO difference is 19 817 cm−1. For X = Me, the calculated energy difference increases to 21 930 cm−1 and the corresponding absorption band is at 22 500 cm−1. The influence of the metal–ligand interactions on the molecular orbitals is discussed. The second category of indenyls, the bis(indenyl) compounds of Ni and Ru, show absorption spectra that are markedly different from those of nickelocene and ruthenocene. For example, in comparison to nickelocene, the first absorption band of Ni(Ind)2 is 5700 cm−1 higher in energy and is more intense by two orders of magnitude; in contrast, the first absorption maximum of Ru(Ind)2 is 6600 cm−1 lower in energy than observed for ruthenocene. The characteristics and relaxation dynamics of the lowest energy excited states are discussed. Key words: absorption spectroscopy, indenyl, nickel, ruthenium, EHMO analysis.

Simulation of UV Absorption Spectra and Relaxation Dynamics of Uracil and Uracil-Water Clusters

2020 ◽  
Author(s):  
Branislav Milovanović ◽  
Jurica Novak ◽  
Mihajlo Etinski ◽  
Wolfgang Domcke ◽  
Nadja Doslic
Keyword(s):  
Aqueous Solution ◽  
Gas Phase ◽  
Absorption Spectra ◽  
Absorption Spectroscopy ◽  
Water Clusters ◽  
Uv Absorption ◽  
Relaxation Dynamics ◽  
Nonradiative Relaxation ◽  
Uv Absorption Spectra ◽  
Uv Absorption Spectroscopy

Despite many studies, the mechanisms of nonradiative relaxation of uracil in the gas phase and in aqueous solution are still not fully resolved. Here we combine theoretical UV absorption spectroscopy...

A comprehensive computational examination of transannular Diels–Alder reactions of unsubstituted C14 trienes — Barriers, template effects, and the Curtin–Hammett principle

2001 ◽  
Vol 79 (8) ◽  
pp. 1284-1292 ◽  
Author(s):  
Saul Wolfe ◽  
Anthony V Buckley ◽  
Noham Weinberg
Keyword(s):  
Free Energy ◽  
Energy Difference ◽  
Diels Alder ◽  
Molecular Orbital Calculations ◽  
Free Energy Difference ◽  
Molecular Mechanics Calculations ◽  
Geometrical Isomers ◽  
Free Energy Of Activation ◽  
Transition Structures ◽  
Cyclic Compounds

A combination of MM3-level molecular mechanics calculations and PM3-level semiempirical molecular orbital calculations has been employed, in conjunction with an algorithm for the comprehensive conformational analysis of cyclic compounds, to obtain 1202 unique 1,3,9-cyclotetradecatriene conformations, distributed over the six possible geometrical isomers, and 70 unique transannular Diels–Alder transition structures leading to the six possible stereoisomeric tricyclic olefins. A kinetic analysis that takes into account all minima of a given geometrical isomer and all transition structures leading to the same tricyclic product leads to a free energy of activation that is almost the same as the free energy difference between the lowest minimum and the lowest transition structure (the Curtin–Hammett principle). A substantial template effect, mainly entropic in origin, is found when the transannular reactions are compared to the Diels–Alder reactions of the cognate 2,4-hexatrienes with the 2-butenes. Although the cyclization of the trans-cis-trans triene favours the cis-anti-cis over the trans-anti-trans product by more than 20 kcal mol–1, the situation is reversed in the acyclic reaction. A cyclic triene that can cyclize directly to a trans-anti-trans tricycle can therefore be proposed.Key words: molecular models, Deslongchamps, Takahashi, trans-anti-trans tricycle, MM3, PM3, transition states.

scholarly journals Diradicalar Character and Ring Stability of Mesoionic Heterocyclic Oxazoles and Thiazoles by Ab Initio Mono and Multi-Reference Methods

Molecules ◽  
2020 ◽  
Vol 25 (19) ◽  
pp. 4524
Author(s):  
Antonio João da Silva Filho ◽  
Lucinêz da Cruz Dantas ◽  
Otávio Luís de Santana
Keyword(s):  
Ab Initio ◽  
Energy Gap ◽  
Experimental Studies ◽  
Energy Difference ◽  
Basis Sets ◽  
Neutral Compounds ◽  
Diradical Character ◽  
Electronic Configurations ◽  
Homo Lumo

Mesoionics are neutral compounds that cannot be represented by a fully covalent or purely ionic structure. Among the possible mesomeric structures of these compounds are the diradical electronic configurations. Theoretical and experimental studies indicate that some mesoionic rings are unstable, which may be related to a significant diradical character, that until then is not quantified. In this work, we investigated the diradical character of four heterocycles: 1,3-oxazol-5-one, 1,3-oxazol-5-thione, 1,3-thiazole-5-one, and 1,3-thiazole-5-thione. The oxazoles are known to be significatively less stable than thiazoles. DFT and ab initio single (B3LYP, MP2, CCSD, and QCISD) and ab initio multi-reference (MR-CISD) methods with three basis sets (6-311+G(d), aug-cc-pVDZ, and aug-cc-pVTZ) were employed to assess the diradical character of the investigated systems, in gas phase and DMSO solvent, from three criteria: (i) HOMO-LUMO energy gap, (ii) determination of energy difference between singlet and triplet wave functions, and (iii) quantification of the most significant diradical character (y0, determined in the unrestricted formalism). All of the results showed that the diradical character of the investigated systems is very small. However, the calculated electronic structures made it possible to identify the possible origin of the oxazoles instability, which can help the design of mesoionic systems with the desired properties.

Syntheses of Chelating Tetrazole-Containing Ligands and Studies of Their Palladium(II) and Ruthenium(II) Complexes

1995 ◽  
Vol 48 (9) ◽  
pp. 1625 ◽  
Author(s):  
AJ Downard ◽  
PJ Steel ◽  
J Steenwijk
Keyword(s):  
Cyclic Voltammetry ◽  
Transition Metal ◽  
Metal Complexes ◽  
Transition Metal Complexes ◽  
Absorption Spectroscopy ◽  
Electronic Absorption ◽  
Ph Control ◽  
Strong Electron ◽  
Ligand Interactions ◽  
Metal Ligand

Eleven chelating tetrazole -containing ligands have been synthesized, and their complexes with palladium(II) and ruthenium(II) prepared. Proton n.m.r. spectroscopy, electronic absorption spectroscopy and cyclic voltammetry have been used to study the nature of the metal-ligand interactions in these complexes. The negatively charged tetrazolate group is shown to be a strong electron donor with very different properties to those of the protonated or alkylated tetrazole group. This leads to pH control of the properties of transition metal complexes containing such ligands.

Saturated N,X-Heterocyclic Carbenes (X=N, O, S, P, Si, C, and B): Stability, Nucleophilicity, and Basicity

2015 ◽  
Vol 68 (9) ◽  
pp. 1438 ◽  
Author(s):  
Zahra Azizi ◽  
Mehdi Ghambarian ◽  
Mohammad A. Rezaei ◽  
Mohammad Ghashghaee
Keyword(s):  
Density Functional ◽  
Energy Gap ◽  
Energy Difference ◽  
Heterocyclic Carbenes ◽  
Structure Stability ◽  
Triplet Energy ◽  
Lumo Energy ◽  
Dft Methods ◽  
Functional Theory ◽  
Homo Lumo

Various saturated five-membered N,X-heterocyclic carbenes (X = N, O, S, P, Si, C, and B) have been studied by ab initio and density functional theory (DFT) methods. The substitutions alter the properties of the reference carbene from the viewpoint of electronic structure, stability, nucleophilicity, and basicity. Our study shows that the oxygen containing carbene (X = O) induces the highest HOMO–LUMO energy gap (ΔEHOMO–LUMO), while carbene with X = N has the widest singlet–triplet energy difference (ΔEs–t). The nucleophilicity of the carbene derivatives increased upon replacement of C, Si, and B, with the effect of the boron substituent being more pronounced. In addition, the basicity of the structure increased for the carbene derivatives with X = C and B with the latter substitution imposing a remarkably higher effect. Moreover, the substitution of boron at the α-position of the carbene increased the nucleophilicity and basicity, while inducing a reduction in the values of ΔEs–t and ΔEHOMO–LUMO.

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