Density Functional Molecular Orbital Calculations Research Articles

Density functional prediction of non cubic isomers of cubane and nitrated cubanes.

Density functional molecular orbital calculations are used to show the existence of two non-cubic isomeric structures of cubane and nitrated cubane. The isomers are shown to have no imaginary frequencies and are thus at a minimum on the potential energy surface. The calculations indicate the isomers are slightly more stable than the cubic structures.

Rotational spectrum and intramolecular hydrogen bonding in 1,2-butanedithiol

The jet-cooled rotational spectrum of 1,2-butanedithiol was observed in the frequency region 2–8 GHz. Two conformers were detected for the molecule, corresponding to anti- and gauche-carbon molecular skeletons, both featuring a gauche arrangement of the two thiol groups. All 13C and 34S monosubstituted isotopologues were additionally assigned in natural abundance for the most stable anti isomer, while only the two 34S species were detected for the weaker gauche conformation. The structural analysis included ground-state effective structures, isotopic substitution coordinates, B3LYP-D3(BJ) density functional molecular orbital calculations and non-covalent interactions mapping with NCIPlot. The structural data confirm that the two thiol groups synchronize their orientation either parallel or antiparallel to support intramolecular S–H⋯S weak hydrogen bonding, reminiscent of the intramolecular hydrogen bond networks observed in adjacent alcohol groups. DFT calculations on 1,2-butanediol and 1,2-ethanedithiol offered structural comparisons with the title compound.

Time Resolved EPR Study on the Photoinduced Long-Range Charge-Separated State in Protein: Electron Tunneling Mediated by Arginine Residue in Human Serum Albumin.

To elucidate how local molecular conformations play a role on electronic couplings for the long-range photoinduced charge-separated (CS) states in protein systems, we have analyzed time-resolved electron paramagnetic resonance (TREPR) spectra by polarized laser irradiations of 9,10-anthraquinone-1-sulfonate (AQ1S(-)) bound to human serum albumin (HSA). Analyses of the magnetophotoselection effects on the EPR spectra and a docking simulation clarified the molecular geometry and the electronic coupling of the long-range CS states of AQ1S(•2-)-tryptophan214 radical cation (W214(•+)) separated by 1.2 nm. The ligand of AQ1S(-) has been demonstrated to be bound to the drug site I in HSA. Molecular conformations of the binding region were estimated by the docking simulations, indicating that an arginine218 (R218(+)) residue bound to AQ1S(•2-) mediates the long-range electron-transfer. The energetics of triad states of AQ1S(•2-)-R218(+)-W214(•+) and AQ1S(-)-R218(•)-W214(•+) have been computed on the basis of the density functional molecular orbital calculations, providing the clear evidence for the long-range electronic couplings of the CS states in terms of the superexchange tunneling model through the arginine residue.

Bisadducts of C70 as Electron Acceptors for Bulk Heterojunction Solar Cells: A Theoretical Study

In the present study, we have used density functional molecular orbital calculations to investigate the energy levels of the frontier orbitals of novel bisadducts of Phenyl-C71-Butyric-Acid-Methyl Ester (bis[70]PCBM) in an attempt to verify if they can be used as electron acceptor in polymer-fullerene solar cells. C70 derivatives are promising materials for these devices because they have a stronger and broader absorption compared to C60 ones, and they can improve the current output of the corresponding polymer-fullerene solar cells. The calculated LUMO levels for the bis[70]PCBM isomers are higher compared to the monoadduct already used as acceptor in such devices. Since the open circuit voltage (Voc) for devices with ohmic contacts is proportional to the difference between the HOMO of the donor polymer and the LUMO of the acceptor, these bisadducts can increase the Voc of polymer solar cells having poly(3-hexylthiophene) as donor.

Tuning the LUMO level of the acceptor to increase the open-circuit voltage of polymer-fullerene solar cells: A quantum chemical study

In the present study, semiempirical and density functional molecular orbital calculations are performed on fullerene derivatives with varying reduction potentials, successfully used as an electron acceptor in bulk heterojunction solar cells. The geometries of all the compounds were optimized with the semiempirical PM3 method. Density functional theory (DFT) single-point calculations, B3LYP/3-21G*, have been carried out with the aim to investigate the energy levels of the frontier orbitals. We have correlated the theoretical lowest unoccupied molecular orbital (LUMO) levels of different fullerenes with the open-circuit voltage of the photovoltaic device based on the blend of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]–1,4-phenylenevinylene (MDMO–PPV) with the acceptor molecules. We have also investigated the influence of new substituents on the LUMO level of the parent fullerene showing the possibility to further increase the open-circuit voltage of the MDMO–PPV:fullerene device.

Hybridization energies of double strands composed of DNA, RNA, PNA and LNA

The electronic properties of double strands composed of trimeric LNA, PNA, DNA and RNA single strands were investigated by density-functional molecular orbital calculations. The computed hybridization energies for the double strands involving PNA or LNA are larger than those for DNA–DNA and RNA–RNA. The larger stability is attributed to the presence of a larger positive charge of the hydrogen atoms contributing to the hydrogen bonds in the PNA–DNA and LNA–DNA double-strands. These results are comparable to the experimental finding that PNA and LNA single strands display high affinity toward a complementary DNA or RNA single strand.

Density functional molecular orbital calculations on the stability of hydrogen-bonded 4-hydroxyazobenzene dimers

To elucidate the stable structures and electronic properties of hydrogen-bonded 4-hydroxyazobenzene (HOAB) dimers, we performed ab initio molecular orbital calculations based on the density functional theory. The obtained stable structures and total energies for the 15 types of dimers indicate that the trans– trans HOAB dimers are at least 10 kcal/mol more stable than the other cis– cis, cis– trans and trans– cis dimers. In addition, it was found that the dimer of tautomerized trans-HOAB has similar stability to the trans– trans HOAB dimers, suggesting that the tautomerized trans-HOAB dimer contributes to the unique photochemical and spectroscopic behavior of HOAB.

Does type of phosphine affect rotational barrier of vinylidene in the complexes OsHCl(C [dbnd]CH 2)(L) 2 (L = phosphine)?

Density functional molecular orbital calculations at the BP86 level were performed to investigate rotational barrier of vinylidene ligand in the model complexes OsHCl(C CH 2)(L) 2 (L = PMe 3, PH 3, PF 3). Results of calculations indicate that the rotational barrier increases with the increasing π-accepting ability of phosphine. This result has been explained on the basis of the extent of π-interaction between metal and vinylidene ligand in the complexes.

Electronic structures of electron-rich octahedrally condensed transition-metal chalcogenide clusters.

The electronic structures of some electron-rich octahedrally condensed transition-metal chalcogenide clusters are analyzed with the aid of extended Hückel and density functional molecular orbital calculations. A simple orbital approach is developed to analyze the electron counts of these clusters, which do not obey any existing electron-counting rules. Different electron counts are allowed, depending upon the nature of the metal. Optimal counts are discussed. Metal-metal bonding is generally weak in these species. Consequently, their structural arrangements are mainly governed by metal-ligand interactions.

Generation of Supraclusters and Nanoclusters Using Laser Desorption Ionisation Mass Spectrometry

Laser desorption/ionisation of discrete molecular clusters combined with time-of-flight (TOF) or Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry affords spectra in which extensive higher mass clusters are observed. The size of the largest cluster aggregates (or supraclusters) is of the same order of magnitude as nanoclusters. The spectra obtained using TOF mass spectrometry sometimes exhibit post-source decay fragmentation, depending upon the operational conditions employed during data acquisition, which, although providing useful data on the ligand dissociation dynamics, complicate spectral interpretation. Complementary FTICR mass spectra are free of such features. The identities of the supra/nanoclusters generated from the molecular cluster precursors have not been conclusively established but are mostly coordinatively unsaturated. Density functional molecular orbital calculations have identified the possible structures of the comparatively simple electronically unsaturated system, [Ru3(CO)6]−, that provides a clue to the aggregation mechanism.

Theoretical Studies of the Relative Stabilities of Transition Metal Alkylidyne (CH3)2M(⋮CH)(X) and Bis(alkylidene) (CH3)M(CH2)2(X) Complexes

Density functional molecular orbital calculations at the B3LYP level were performed to investigate the relative stabilities of the tautomeric pairs of transition metal alkylidyne (CH3)2M(⋮CH)(X) and bis(alkylidene) (CH3)M(CH2)2(X) complexes (M = W, Mo, Os, and Ru; X = Cl, CH3, CF3, SiH3, and SiF3). Calculation results indicate that the relative stabilities of the bis(alkylidene) tautomers increase with the increasing π-accepting ability of X for the W and Mo complexes. When X is a silyl ligand, it is found that the tautomeric pair for W or Mo have similar stabilities. These results have been explained in terms of π interaction between ligand X and the electron density in the metal−alkylidyne/alkylidene bonds. For the Os and Ru complexes, the bis(alkylidene) tautomers are found more stable no matter what X is. The stabilities of the bis(alkylidene) tautomers for these d2 metal complexes have been related to the bonding characteristic of the orbital that accommodates the two metal d electrons.

Density functional calculations of enthalpies of hydrogenation, isomerization, and formation of small cyclic hydrocarbons

Density functional molecular orbital calculations using a high-level basis set on cyclic hydrocarbons (C≤5) make less severe demands on computer resources than the G2 family of ab initio calculations, and give results that are only slightly less accurate than G2. We present results for B3LYP/6-311+G(3df,2p) calculations at the MP2(full)/6-31G(d) geometry on 30 cyclic and eight acyclic hydrocarbons having two, three, four, or five carbon atoms. The arithmetic mean absolute deviation from 26 experimental results for Δ f H 298 is 2.1 kcal mol −1.

Structural Preferences in η2-Alkenyl Transition-Metal Complexes ML5(η2-alkenyl) and MCpL2(η2-alkenyl)

Density functional molecular orbital calculations have been used to study the preference for coplanarity of the η2-alkenyl and one of the metal−ligand bonds in transition-metal η2-alkenyl complexes. The metal−η2-alkenyl bonding in these complexes involves the Dewar−Chatt−Duncanson type of interactions (the donation of η2-alkenyl π electrons to the metal center and the metal(d)−η2-alkenyl(π*) back-donation) and metal(d)−Cα(p) π bonding. The Dewar−Chatt−Duncanson type of interactions is found to be the dominant factor in determining the structural preference while the metal−Cα π bonding strength remains approximately constant with respect to the η2-alkenyl rotation. When the Cα−Cβ vector of the η2-alkenyl ligand eclipses (is coplanar with) one of the metal−ligand bonds, the metal−η2-alkenyl interactions are the strongest because the mixing of metal d−p orbitals, resulting from the metal−ligand distortion, enhances the metal(d)−η2-alkenyl(π*) back-donation. For η2-alkenyl complexes containing a Cp ligand, the steric effect of the bulky Cp ligand is also operative although the electronic factors are still dominant in determining the structural preference.

Density Functional Study of the Valence-Tautomeric Interconversion Low-Spin [CoIII(SQ)(Cat)(phen)] ⇌ High-Spin [CoII(SQ)2(phen)

The results of self-consistent field (SCF) nonlocal density functional molecular orbital calculations are presented for the various spin states and tautomeric forms of a cobalt complex with two o-quinone-derived ligands. In addition, new variable-temperature solution magnetic susceptibility, EPR, and electronic absorption data are presented to characterize the low-spin [CoIII(3,5-DTBSQ)(3,5-DTBCat)(phen)] to high-spin [CoII(3,5-DTBSQ)2(phen)] valence-tautomeric interconversion, where 3,5-DTBSQ- and 3,5-DTBCat2- are the semiquinonate and catecholate forms of 3,5-di-tert-butyl-o-benzoquinone, respectively, and phen is 1,10-phenanthroline. The solution magnetic susceptibility data were fitted to give ΔH = 2238 cm-1 and ΔS = 118.1 J mol-1 K-1 for the ls-CoIII ⇌ hs-CoII equilibrium. Appreciable changes are seen in the electronic absorption spectrum as the complex changes between the two tautomeric forms. Unrestricted SCF calculations gave J = −594 cm-1 for the parameter characterizing the antiferromagnetic exc...

Dehydrocyclooctatetraene. Photoelectron Spectroscopy of the C8H6 Anion

The photoelectron spectrum of the dehydrocyclooctatetraene negative ion, C8H6-, is reported. The spectrum strongly resembles that previously reported for the cyclooctatetraene anion, indicating that the structure of C8H6- is very similar to that of C8H8-. Two electronic states of dehydrocyclooctatetraene are observed in the photoelectron spectrum. The lowest energy feature is assigned to singlet 1,3,5-cyclooctatrien-7-yne, while the higher energy band corresponds to a triplet state of dehydrocyclooctatetraene. The electron affinity of C8H6 is found to be 1.044 ± 0.008 eV, and the energy difference between the singlet and triplet states is 0.708 ± 0.006 eV. Vibrational activity is observed in the photoelectron spectrum and assigned using a simple potential energy surface. Stretching of the triple bond in cyclooctatrienyne is found to have a frequency of 2185 cm-1, essentially what is expected for a triple bond within an eight-membered ring. Ab initio and density functional molecular orbital calculations on...

Resonance Raman and quantum chemical studies of short polyene radical cations

The results of our investigations of the geometric and vibrational structures of some short conjugated polyene radical cations are reported. The radical cations of 1,3 — butadiene and three of its deuterated isotopomers, trans — and cis — 1,3 — pentadiene, 2 — methyl — 1,3 — butadiene, and E — and Z — 1,3,5 — hexatriene have been studied. The radical cations were generated radiolytically in a glassy Freon matrix and investigated by optical absorption and resonance Raman spectroscopy. Ab initio and density functional molecular-orbital calculations have been carried out to predict equilibrium structures and to assist assignment of the resonance Raman spectra. A new and improved scaled quantum mechanical force field for the butadiene radical cation was also determined. The presence of more than one rotamer was observed in all the polyene radical cations we investigated.

Theoretical and experimental studies of F 3SiCO + and F 3SiOC +

Density-functional molecular orbital calculations and multi-collision-induced dissociation (CID) experiments in the gas phase have been performed on isomers associated formally with the addition of CO to SiF 3 +. Calculations at the BLYP/6-31G(d,p) level of theory show that the isomer F 3SiCO + has an energy lower than that of F 3SiOC + by 17.8 kcal mol −1, and thus establish that the potential energy surface for these ions is similar to those for the analogous H 3SiCO + and (CH 3) 3SiCO + ions in this respect. The transition structure for the interconversion of F 3SiCO + and F 3SiOC + is 7.5 kcal mol −1 in energy below that of dissociated SiF 3 + + CO, a situation which is similar qualitatively to that for H 3SiCO +, but differs qualitatively from that for (CH 3) 3SiCO +. The calculations also indicate that the isomerization of F 3SiCO + to F 3SiOC + requires 35.0 kcal mol −1. Multi-collision CID experiments performed with a selected-ion flow tube (SIFT) apparatus suggest that both F 3SiCO + and F 3SiOC + are produced by the reaction of SiF 3 + with CO in an abundance ratio of approximately 0.85: 0.15, and are stable in the gas phase.

Comparison of the reactivity of [2.2]paracyclophane and p-xylene

The relative abilities of [2.2]paracyclophane (C16H16) and p-xylene (C6H4Me2-1,4) to form arene tricarbonyl complexes from chromium hexacarbonyl has been studied in dioxane using the Strohmeier reflux method, and the rate constants contrasted. The reactions are found to proceed more quickly with [2.2]paracyclophane by ca. 25%. Density functional molecular-orbital calculations have rationalised this observation, and indicate that the enhanced reactivity of the [2.2]paracyclophane system relative to p-xylene is a consequence of repulsive interactions between the two arene decks in the former, which are relieved to some extent by co-ordination of the electron-withdrawing Cr(CO)3 fragment.