B3LYP Level Of Theory Research Articles

Conformational Studies on Rotenoid and Its Biosynthetic Implications

The conformational studies for three rotenone isomers, using a simple rotenoids model has been analyzed by DFT method in B3LYP level of theory and 6-31G∗ basis set to obtain energy. This study shows how a systematic theoretical conformational analysis contributes to understanding molecular level of chemical processes. The gas phase structures are discussed based on thermodynamic values. The results supported that the biological effects of rotenone can be a consequence of conformational restriction and cis preference. The significance of these results is identifying the rotenone toxicity mechanism.

New aqua rhenium oxocomplex; synthesis, characterization, thermal studies, DFT calculations and catalytic oxidations

The aqua rhenium oxocomplex [ReO(OH)(H2O)4]2− (1) has been prepared and characterized by spectroscopy, thermogravimetry, and elemental analysis and its reactivity towards triphenylphosphine has been evaluated. Complex (1) acts as a catalyst precursor in the presence of molecular oxygen for the oxidation of PPh3 to OPPh3. This proceeds through complex intermediates like [Re(PPh3)n]3+ (2), and [ReO(PPh3)n]3+ (3). The newly prepared complex (1) was also employed as catalyst for catalytic oxidation of cyclohexane. The geometry of [ReO(OH)(H2O)4]2− has also been optimized in the singlet state by the DFT method with B3LYP level of theory.

DFT Mechanistic Study on Diene Metathesis Catalyzed by Ru‐Based Grubbs–Hoveyda‐Type Carbenes: The Key Role of π‐Electron Density Delocalization in the Hoveyda Ligand

The catalytic activity and catalyst recovery of two heterogenized ruthenium-based precatalysts (H and NO(2)(4)) in diene ring-closing metathesis have been studied by means of density functional calculations at the B3LYP level of theory. For comparison and rationalization of the key factors that lead to higher activities and higher catalyst recoveries, four other Grubbs-Hoveyda complexes have also been investigated. The full catalytic cycle (catalyst formation, propagation, and precatalyst regeneration) has been considered. DFT calculations suggest that either for the homogeneous and heterogenized systems the activity of the catalysts mainly depends on the ability of the precursor to generate the propagating carbene. This ability does not correlate with the traditionally identified key factor, the Ru...O interaction strength. In contrast, precatalysts with lower alkoxy-dissociation energy barriers and lower stabilities compared with the propagating carbene also present larger C1-C2 bond length (i.e., lower pi character of the C-C bond that exists between the metal-carbene (Ru=C) and the phenyl ring of the Hoveyda ligand). Catalyst recovery, regardless of whether a release-return mechanism occurs or not, is also mainly determined by the pi delocalization. Therefore, future Grubbs-Hoveyda-type catalyst development should be based on fine-tuning the pi-electron density of the phenyl moiety, with the subsequent effect on the metalloaromaticity of the ruthenafurane ring, rather than considering the modification of the Ru...O interaction.

ChemInform Abstract: Ab initio and Density Functional Electronic Structure Study of Molybdenum Oxide Clusters

First principles quantum chemical calculations at various levels of theory (HF, MP2, B3LYP), using the LANL2DZ basis set, provide a satisfactory description of structural, energetic and electronic properties of molybdenum(VI) oxide species of molecular dimensions, formulated as MoO3, MoO42−, MoO4H2, MoO4Na2, Mo2O52+ and Mo2O6. The energetics of all topomers corresponding to global or local minima and saddle points in the potential energy hypersurfaces were computed at the more sophisticated QCISD(T) level as well. The proton affinity of the MoO42− dianion was found to be equal to 1544, 1536 and 1527 kJ mol−1 at the HF, MP2 and B3LYP levels of theory. The formation processes of Mo2O52+(C1) dication from MoO3 and MoO22+(C2v) dication and the Mo2O6 dimer upon dimerization of the MoO3 species are predicted to be exothermic, the energies of formation being equal to 636.0 and 388.7 kJ mol−1 at the B3LYP level, respectively. Finally, the computed spectroscopic properties (harmonic vibrational frequencies and corresponding normal modes, electronic transitions and NMR chemical shifts) of the molecular molybdenum oxides are thoroughly discussed in relation with available experimental data.

Pt(CN)42- İYONUNUN TİTREŞİM FREKANSLARI VE YAPISAL İNCELEMESİ

The normal mode frequencies and corresponding vibrational assignments of Pt(CN)42- ion have been theoretically examined by means of standard quantum chemical techniques. All normal modes have been assigned to one of six types of motion (CN and Pt-C stretching, Pt-CN in plane and out of plane bending, C-Pt-C in plane and out of plane bending) utilizing the D4h symmetry of Pt(CN)42- ion. Calculations have been performed at HF, BLYP and B3LYP levels of theory using the Lanl2dz effective core basis set. Infrared intensities and Raman activities of vibrational frequencies have also been calculated. Theoretical results have been successfully compared against available experimental data.

Experimental and Computational Thermodynamic Study of Three Monofluoronitrobenzene Isomers

The present work reports the thermodynamic study performed on three monofluorinated nitrobenzene derivatives by a combination of experimental techniques and computational approaches. The standard (p degrees = 0.1 MPa) molar enthalpies of formation in the liquid phase of the three isomers of fluoronitrobenzene were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry. The vapor pressure study of the referred compounds was done by a static method and, from the obtained results, the phase diagrams were elaborated, and the respective triple point coordinates, as well as the standard molar enthalpies of vaporization, sublimation and fusion, at T = 298.15 K, were determined. The combination of some of the referred thermodynamic parameters yielded the standard (p degrees = 0.1 MPa) molar enthalpies of formation in the gaseous phase, at T = 298.15 K, of the studied compounds: Delta(f)H(m)(o) (2-fluoronitrobenzene, g) = -(102.4 +/- 1.5) kJ x mol(-1), Delta(f)H(m)(o) (3-fluoronitrobenzene, g) = -(128.0 +/- 1.7) kJ x mol(-1), and Delta(f)H(m)(o) (4-fluoronitrobenzene, g) = -(133.9 +/- 1.4) kJ x mol(-1). Using the empirical scheme developed by Cox, values of standard molar enthalpies of formation in the gaseous phase were estimated and afterwards compared with the ones obtained experimentally, and both were interpreted in terms of the molecular structure of the compounds. The theoretically estimated gas-phase enthalpies of formation were calculated from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level of theory. The computed values compare very well with the experimental results obtained in this work and show that 4-fluoronitrobenzene is the most stable isomer from the thermodynamic point of view. Furthermore, this composite approach was also used to obtain information about the gas-phase basicities, proton and electron affinities and, finally, adiabatic ionization enthalpies.

DFT versus Møller–Plesset conformational profile and vibrational assignments of non- planar phenoxyacetic acid and 2,3,4,5,6-pentafluorophenoxyacetic acid

The structural stability of phenoxyacetic acid and 2,3,4,5,6-pentafluorphenoxyacetic acid was investigated by the DFT-B3LYP and the ab initio MP2 calculations with the 6-311G ** basis set. For the parent acid the calculations were extended to the MP4(SDQ) level of theory. At the DFT-B3LYP level of calculation the planar Tttp ( transoid O C O H) was predicted to be about 0.5 and 1.3 kcal/mol lower in energy than the non-planar Cgcpp and Tgcpp ( cisoid O C O H) forms, respectively. At the MP2 and the MP4(SDQ) levels the Cgcpp form was predicted to be about 0.8 and 1.4 kcal/mol lower in energy than the Tgcpp and the Tttcp structures, respectively. On the basis of the Møller–Plesset calculations the Cgcpp and the Tgcpp conformations were adopted as the low and high energy structures of phenoxyacetic acid. The observed spectral intensities of phenoxyacetic acid were consistence with the Cgcpp conformation being the predominant form of the acid at room temperature. At the DFT and MP2 levels of theory 2,3,4,5,6-pentafluorophenoxyacetic acid was predicted to exist predominantly in the Cgcpp structure. The vibrational wavenumbers were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of combined theoretical and experimental infrared and Raman data of both molecules.

Experimental and computational study of the energetics of 5- and 6-aminoindazole

This work aims to study the influence of the amino group in positions 5 and 6 of the benzene ring of indazole. For that purpose, the standard ( p ∘ = 0.1 MPa ) molar enthalpies of formation of 5- and 6-aminoindazole, in the gaseous phase, at T = 298.15 K were determined. These values were calculated from the standard massic energies of combustion, measured by combustion calorimetry, and from the standard molar enthalpies of sublimation, computed from the variation with the temperature of the vapour pressures of each compound, measured by the Knudsen effusion technique. Compound Δ c u ∘ / ( J · g - 1 ) Δ cr g H m ∘ / ( kJ · mol - 1 ) Δ f H m ∘ (g) / ( kJ · mol - 1 ) 5-Aminoindazole (cr) −29299.4 ± 3.7 118.34 ± 0.69 265.1 ± 1.8 6-Aminoindazole (cr) −29255.9 ± 3.9 121.95 ± 0.63 263.0 ± 1.7 The final results for the enthalpies of formation in the gaseous phase are discussed in terms of structural contributions of the amino group. The theoretically estimated gas-phase enthalpies of formation were calculated from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level of theory. The computed values compare very well with the experimental results obtained in this work and show that the 6-aminoindazole is the most stable isomer from the thermodynamic point of view. Furthermore, this composite approach was also used to obtain information about the gas-phase acidities, gas-phase basicities, proton and electron affinities, adiabatic ionization enthalpies and, finally, N–H bond dissociation enthalpies.

Electronic structures and molecular properties of FLBDOB and its derivatives: A combined experimental and theoretical study

Theoretical studies on a new unsymmetrical electrolyte salt, lithium [3-fluoro-1,2-benzenediolato(2-)-o,o′ oxalato]borate (FLBDOB), and its derivatives, lithium bis[3-fluoro-1,2-benzenediolato(2-)-o,o]borate (FLBBB), and lithium bis(oxalate)borate (LBOB) are carried out using density functional theory (DFT) method and B3LYP theory level. Bidentate structures involving two oxygen atoms are preferred. Based on these conformations, a linear correlation is observed between the highest occupied molecular orbital (HOMO) energies and the limiting oxidation potentials measured by linear sweep voltammetry, which supports experimental results that strongly electron-withdrawing substituent anions are more resistant against oxidation. The correlations are also observed between ionic conductivity and binding energy, solubility and theoretical set of parameters of anion, thermal stability and the hardness ( η). Wave function analyses are performed by natural bond orbital (NBO) method to further investigate the cation–anion interactions.

Prediction of the Q-e parameters from radical structures

To construct reliable quantitative structure-property relationship models of Q and e parameters, density functional theory (DFT) calculations were carried out for 56 radicals with structure C1H3–C2HR3• (formed from the monomer + H•) at the B3LYP level of theory with 6-31G(d) basis set. The multiple linear regression technique was used to select the descriptors as inputs for the artificial neural network (ANN). A three-descriptor ANN for the parameter lnQ was developed that produced training set root mean square (rms) error = 0.317 and rms error = 0.313 for an external prediction set. A two-descriptor ANN for the parameter e was built with rms errors of 0.264 for the training set and 0.271 for the prediction set. Comparing to existing models, our ANN models show better or comparable statistical characteristics, which shows that calculating quantum chemical descriptors from radicals to develop ANN models and predict lnQ and e values is feasible.

Molecular structure and vibrational assignments of 2,4-dichlorophenoxyacetic acid herbicide

The structural stability of 2,4-dichlorophenoxyacetic acid was investigated by the DFT-B3LYP and the ab initio MP2 calculations with the 6-311G** basis set. From the calculations at both levels of theory the Cgcpp structure was predicted to be the lowest energy minimum for the acid. The DFT and the MP2 levels disagreed about the nature of the second stable structure of 2,4-dichlorophenoxyacetic acid. At the DFT-B3LYP level of calculation the planar Tttp ( transoid O C O H) and the non-planar Tgcpp ( cisoid O C O H) forms were predicted to be 0.7 and 1.5 kcal/mol, respectively higher in energy than the Cgcpp conformation. At the MP2 level the two high energy Tttp and Tgcpp forms were predicted to be 2.7 and 1.4 kcal/mol, respectively higher in energy than the ground state Cgcpp structure. The Tgcpp form was adopted as the second possible structure of 2,4-dichlorophenoxyacetic acid on the basis of the fact that the Møller–Plesset calculations account better than the DFT ones for the non-bonding O⋯H interactions. The vibrational frequencies of the lowest energy Cgcpp conformer were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of normal coordinate analysis and experimental infrared and Raman data.

Syntheses and Multi-NMR Study offac- andmer-OsO3F2(NCCH3) and the X-ray Crystal Structure (n= 2) and Raman Spectrum (n= 0) offac-OsO3F2(NCCH3)·nCH3CN

Dissolution of the infinite chain polymer, (OsO(3)F(2))(infinity), in CH(3)CN solvent at -40 degrees C followed by solvent removal under vacuum at -40 degrees C yielded fac-OsO(3)F(2)(NCCH(3)).nCH(3)CN (n >/= 2). Continued pumping at -40 degrees C with removal of uncoordinated CH(3)CN yielded fac-OsO(3)F(2)(NCCH(3)). Both fac-OsO(3)F(2)(NCCH(3)).nCH(3)CN and fac-OsO(3)F(2)(NCCH(3)) are yellow-brown solids and were characterized by low-temperature (-150 degrees C) Raman spectroscopy. The crystal structure (-173 degrees C) of fac-OsO(3)F(2)(NCCH(3)).2CH(3)CN consists of two co-crystallized CH(3)CN molecules and a pseudo-octahedral OsO(3)F(2).NCCH(3) molecule in which three oxygen atoms are in a facial arrangement and CH(3)CN is coordinated trans to an oxygen atom in an end-on fashion. The Os---N bond length (2.205(3) A) is among the shortest M---N adduct bonds observed for a d(0) transition metal oxide fluoride. The (19)F NMR spectrum of (OsO(3)F(2))(infinity) in CH(3)CN solvent (-40 degrees C) is a singlet (-99.6 ppm) corresponding to fac-OsO(3)F(2)(NCCH(3)). The (1)H, (15)N, (13)C, and (19)F NMR spectra of (15)N-enriched OsO(3)F(2)(NCCH(3)) were recorded in SO(2)ClF solvent (-84 degrees C). Nitrogen-15 enrichment resulted in splitting of the (19)F resonance of fac-OsO(3)F(2)((15)NCCH(3)) into a doublet ((2)J((15)N-(19)F), 21 Hz). In addition, a doublet of doublets ((2)J((19)F(ax)-(19)F(eq)), 134 Hz; (2)J((15)N-(19)F(eq)), 18 Hz) and a doublet ((2)J((19)F(ax)-(19)F(eq)), 134 Hz) were observed in the (19)F NMR spectrum that have been assigned to mer-OsO(3)F(2)((15)NCCH(3)); however, coupling of (15)N to the axial fluorine-on-osmium environment could not be resolved. The nitrogen atom of CH(3)CN is coordinated trans to a fluorine ligand in the mer-isomer. Quantum-chemical calculations at the SVWN and B3LYP levels of theory were used to calculate the energy-minimized gas-phase geometries, vibrational frequencies of fac- and mer-OsO(3)F(2)(NCCH(3)) and of CH(3)CN. The relative stabilities of the mer- and fac-isomers have been determined and are in accordance with the solution NMR assignments.

Scanning the Potential Energy Surface of Furanosyl Oxocarbenium Ions: Models for Reactive Intermediates in Glycosylation Reactions

A new scanning method with complementary graph to describe the ring potential energy surface of furanoses is introduced. Density functional theory at the B3LYP level of theory with the 6-311G(d,p) basis set is used to calculate the energy of the partially minimized structures. The method is used to determine the correlation between the preferred conformation of oxocarbenium ions that are model intermediates for a glycosylation reaction and recent experimental results. Key disagreements between the predicted geometry and the minima based on the scans described herein indicate that the preferred oxocarbenium ion conformation is not a consistent predictor of preferred stereochemistry of the products.

Scaling of Computed Cyano-Stretching Frequencies and IR Intensities of Nitriles, Their Anions, and Radicals

Specialized scale factors and scaling equations have been determined for both cyano stretching vibrational frequencies and IR intensities of nitrile molecules, anions and radicals (269 data points in all). Various double and triple-zeta basis sets, viz. 3-21G, 6-31G, 6-311G, 6-31G*, 6-311G*, 6-31+G, 6-311+G, 6-31G**, 6-31+G*, 6-311+G*, 6-31++G**, 6-311++G**, and aug-cc-pVDZ at the B3LYP and HF theory levels have been applied. Besides the theory and basis set, the mean deviations between scaled computed and experimental spectroscopic features have been found to depend also on the series studied: frequencies or intensities, and molecules, anions, or radicals. Use of scaling equations instead of scale factors has given slightly worse results for frequencies, but essentially better ones for infrared intensities.

Azo-containing pyridine amide ligand. A six-coordinate nickel(II) complex and its one-electron oxidized species: Structure and properties

A new potentially tridentate ligand HL 11 consisting of 2-pyridinecarboxamide unit and azo functionality has been used, in its deprotonated form, to prepare a nickel(II) complex which has been structurally characterized. The ligand L 11(−) affords a bis-complex [Ni II(L 11) 2] ( 1). In 1, the two L 11(−) ligands bind to the Ni II center in a mer configuration. The relative orientations within the pairs of pyridyl-N, deprotonated amido-N, and azo-N atoms are cis, trans, and cis, respectively. The Ni IIN 2(pyridyl)N′ 2(amide)N″ 2(azo) coordination environment is severely distorted from ideal octahedral geometry. The Ni–N am (am = amide) bond lengths are the shortest and the Ni–N azo bond lengths are the longest. Complex 1 exhibits a quasireversible Ni III/Ni II redox process. Moreover, the complex displays two ligand-centered (azo group) quasireversible redox processes. Spectroscopic (absorption and EPR) properties have been studied on coulometrically-generated nickel(III) species. To understand the nature of metal-ligand bonding interactions Density Functional Theory (DFT) calculations have been performed on 1 at the B3LYP level of theory. Calculations have also been done for closely related nickel(II) complexes of deprotonated pyridine amide ligands and comparative discussion has been made using observed results.

Molecular structure and conformations of 1-methoxy-2,3,3,4,4-pentafluorocyclobut-1-ene

The structure of 1-monomethoxy-2,3,3,4,4-pentafluorocyclobut-1-ene (F5MCB) has been studied as a part of a series of investigations of the structures of substituted cyclobutenes. The experimental method was gas-phase electron diffraction (GED) augmented by molecular orbital calculations at the HF and B3LYP levels of theory with basis sets 6-31G(d), 6-311+G(3df,2p), and cc-pvtZ. The matter of most interest in these compounds is the length of the C3–C4 bond, which has been found to be significantly longer than the corresponding bond in cyclobutene itself when both ligands to the double bond are fluorine or chlorine atoms, but about the same as in cyclobutene when both substituents are methoxy groups. This result has been verified by the additional data provided by this study of F5MCB: the C3–C4 bond is about the same as in the dimethoxy compound and in cyclobutene. In the gas-phase at 25 °C, F5MCB exists as a mixture of two conformers, one with the methyl group cis to the double bond (about 70%) and one with it trans (about 30%), in good agreement with theory that predicts 63–70% cis. Some values (with estimated 2 σ uncertainties) of the more important parameters ( r g/Å; ∠ α/°) are for the cis/ trans forms r(〈C–H〉) = 1.101 (17)/1.121, r(C C) = 1.329 (36)/1.328, r(C1–C4) = 1.498 (19)/1.504, r(C2–C3) = 1.483 (19)/1.485, r(C3–C4) = 1.581 (21)/1.576, r(C2–F6)=1.346 (12)/1.339, r(C3–F7) = 1.347 (10)/1.349, r(C4–F9) = 1.344 (10)/1.350, r(C1–O5) = 1.337 (26)/1.343, r(O5–C11) = 1.469 (37)/1.493, ∠(C4C1C2) = 93.3 (8)/93.1, ∠(C1C2C3) = 96.4 (8)/96.6, ∠(C2C3C4) = 84.4 (8)/84.4, ∠(C3C4C1) = 85.0 (9)/85.9, ∠(C2C1O5) = 141.0 (14)/135.2, ∠(C1C2F6) = 135.4 (14)/135.1, ∠(C1O5C11) = 117.3 (33)/117.8, ∠(F9C4F10) = 108.3 (13)/107.7, ∠(F7C3F8) = 107.7 (13)/107.6, ∠(XC4C3) = 134.4 (15)/134.4, and ∠(X′C3C4) = 134.2 (15)/133.9, where X and X′ are, respectively, the bisectors of the F9C4F10 and F7C3F8 angles. The structure is discussed with reference to the structures of similar molecules.

A theoretical study of salicylate oxidation for ADME prediction

The salicylic acid oxidation has been explained by a mechanism involving single electron transfer oxidation and single hydrogen transfer using quantum chemistry calculations at the B3LYP theory level, together with the 6-311G** basis set. These methods were employed to obtain energy (E), ionization potential (IP), bond dissociation energies (BDE), and spin density distribution of the salicylic acid. The results show no discrepant behaviors between electron and hydrogen transfer in the regioselective hydroxylation of salicylic acid by cytochrome P-450. The unpaired electron remains localized on the O7 phenolic oxygen (0.26 and 0.38), C1 carbon atoms at the carbonyl group (0.12 and 0.28), C2 carbon atom at the hydroxyl group (0.15 and 0.00), C3 carbon atom at the hydroxyl group (0.22 and 0.30), and C5 carbon atom (0.40 and 0.41) for cation free radical and semiquinone form, respectively. Calculations of spin densities showed that chemistry reactivity is more favored in the positions C5 > C3 > C1 to form salicylate derivatives. These results supported the salicylic acid as scavenger derivatives in the lipid peroxidation. Furthermore, we suggest a conventional proton and secondary electron abstraction, and semiquinone form by [1,5] hydrogen shift between phenol and carbonyl groups.

Diphenoxido‐Bridged CoII and ZnII Complexes of Tripodal N2O2 Ligands: Stabilisation of MII‐Coordinated Phenoxyl Radical Species

AbstractThree new tripodal ligands with an N2O2 donor set, namely2‐tert‐butyl‐6‐({(2‐hydroxybenzyl)[2‐(2‐pyridyl)ethyl]amino}methyl)‐4‐methylphenol (H2L1), 2‐tert‐butyl‐6‐({(2‐hydroxybenzyl)[2‐(2‐pyridyl)ethyl]amino}methyl)‐4‐methoxyphenol (H2L2) and 2‐tert‐butyl‐6‐({[2‐(dimethylamino)ethyl](2‐hydroxybenzyl)amino}methyl)‐4‐methoxyphenol (H2L3) have been synthesised. Treatment of the ligands with Co(CH3CO2)2·4H2O or [Zn(H2O)6][ClO4]2 in the presence of Et3N provides the corresponding CoII and ZnII complexes of composition [MII2(L1)2] [M = Co (1) (single‐crystals are a solvate with the composition [CoII2(L1)2]·2CHCl3, i.e. 1·2CHCl3); M = Zn (2)], [MII2(L2)2] [M = Co (3); Zn (4)] and [CoII2(L3)2] (5). Crystallographic analyses reveal that the complexes have closely similar diphenoxido‐bridged structures. Each metal centre assumes MIIN2O3 coordination. The geometry around each metal ion in 1·2CHCl3 (τ = 0.76), 2 (τ = 0.77), 3 (τ = 0.74), 4 (τ = 0.76) and 5 [one CoII (τ = 0.49) and the other CoII (τ = 0.63)] is intermediate between ideal square‐pyramidal (τ = 0) and trigonal‐bipyramidal (τ = 1). Temperature‐dependent magnetic studies reveal weak intramolecular antiferromagnetic exchange couplings for all the three CoII complexes(–J = 1.84, 1.32 and 5.70 cm–1 for 1, 3 and 5, respectively). Spectroscopic properties of the complexes have also been investigated. Cyclic voltammetric (CV) measurements of 1 and 2 show an irreversible oxidative response at Epa (anodic peak potential) in the range 0.60–0.75 V relative to the SCE (saturated calomel electrode), whereas two successive quasi‐reversible oxidative responses can be observed for 3–5 at E1/2 values in the range 0.40–0.53 V relative to the SCE. Oxidative responses are due to the formation of MII‐coordinated phenoxyl radical species. The metal‐coordinated phenoxyl radical species, generated by two‐electron coulometric oxidation of 3–5, were characterised by CV and by adsorption and EPR spectroscopy. The stability of such species was determined by measuring the decay constant (absorption spectroscopy), which reveals that the phenoxyl radical species of 5 is more stable than that of 3 and 4. EPR spectroscopic studies (120 K) of coulometrically generated two‐electron oxidised species of 4 in CH2Cl2 (containing 0.1 M TBAP) at 298 K reveal a combination of an isotropic S = ${1 \over 2}$ signal at g = 2 [(phenolato)(phenoxyl radical)‐coordinated dizinc(II) complex] and a spin‐triplet resonance (S = 1) that gives rise to the symmetric split‐line pattern [bis(phenoxyl radical)‐cordinated dizinc(II) complex]. To pinpoint the site of oxidation (metal‐ or ligand‐centred) in each case, DFT calculations were performed at the B3LYP level of theory.

Vibrational and chiroptical spectroscopic characterization of γ‐turn model cyclic tetrapeptides containing two β‐Ala residues

The optical spectroscopic characterization of gamma-turns in solution is uncertain and their distinction from beta-turns is often difficult. This work reports systematic ECD and vibrational circular dichroism (VCD) spectroscopic studies on gamma-turn model cyclic tetrapeptides cyclo(Ala-beta-Ala-Pro-beta-Ala) (1), cyclo(Pro-beta-Ala-Pro-beta-Ala) (2) and cyclo(Ala-beta-Ala-Ala-beta-Ala) (3). Conformational analysis performed at the 6-31G(d)/B3LYP level of theory using an adequate PCM solvent model predicted one predominant conformer for 1-3, featuring two inverse gamma-turns. The ECD spectra in ACN of 1 and 2 are characterized by a negative n-->pi* band near 230 nm and a positive pi-->pi* band below 200 nm with a long wavelength shoulder. The ECD spectra in TFE of 1-3 show similar spectra with blue-shifted bands. The VCD spectra in ACN-d(3) of 1 and 2 show a +/-/+/- amide I sign pattern resulting from four uncoupled vibrations in the case of 1 and a sequence of two positive couplets in the case of 2. A -/+/+/- amide I VCD pattern was measured for 3 in TFE-d(2). All three peptides give a positive couplet or couplet-like feature (+/-) in the amide II region. VCD spectroscopy, in agreement with theoretical calculations revealed that low frequency amide I vibrations (at approximately 1630 cm(-1) or below) are indicative of a C(7) H-bonded inverse gamma-turns with Pro in position 2, while gamma-turns encompassing Ala absorb at higher frequency (above 1645 cm(-1)).

Density functional theory study on LBDOB and its derivatives: Electronic structures, energies, and molecular properties

Theoretical studies on unsymmetrical electrolyte salts, lithium [1,2-benzenediolato(2-)-O,O′ oxalato]borate (LBDOB), and its derivatives, lithium bis[1,2-benzenediolato(2-)-O,O′]borate (LBBB), and lithium bis(oxalate)borate (LBOB) are carried out using density functional theory (DFT) method and B3LYP theory level. Bidentate structures involving two oxygen atoms are preferred. Based on these conformations, a linear correlation was observed between the highest occupied molecular orbital (HOMO) energies and the limiting oxidation potentials measured by linear sweep voltammetry, which supports experimental results that strongly electron-withdrawing substituent anions are more resistant against oxidation. The correlations were also observed between ionic conductivity and binding energy solubility and theoretical set of parameters of anion, thermal stability and the hardness ( η). Wave function analyses have been performed by natural bond orbital (NBO) method to further investigate the cation–anion interactions.