Predicted predissociation linewidths in the Schumann-Runge bands of O2 compared with recent high resolution measurements

The fine-structure predissociation linewidths for the Schumann-Runge bands were evaluated by the model of Julienne and krauss. The parameters were adjusted and modified to account for the fine-structure data for rotational states of vibrational levels v=0-2, 9-12. The standard deviation of the predicted widths was calculated using the fine-structure data. Green's function method was used to obtain the shifts at the band heads. The analysis showed good agreement between the measured widths and widths obtained using the modified parameters.published_or_final_versio

Theoretical studies of the first-row transition metal phosphides

B3LYP calculations were performed with an extended basis set on the first-row TM phosphides. It was found that the ground states of these phosphides follow those of the isoelectronic sulfides. In addition, the covalent character of these phosphides increase across the series from Sc to Cu, in accordance with the electronegativity difference.published_or_final_versio

Theoretical studies on the photophysical properties of luminescent pincer gold(III) arylacetylide complexes: the role of π-conjugation at the C-deprotonated [C^N^C] ligand

published_or_final_versio

A rapidly-reversible absorptive and emissive vapochromic Pt(II) pincer-based chemical sensor

Selective, robust and cost-effective chemical sensors for detecting small volatile-organic compounds (VOCs) have widespread applications in industry, healthcare and environmental monitoring. Here we design a Pt(II) pincer-type material with selective absorptive and emissive responses to methanol and water. The yellow anhydrous form converts reversibly on a subsecond timescale to a red hydrate in the presence of parts-per-thousand levels of atmospheric water vapour. Exposure to methanol induces a similarly-rapid and reversible colour change to a blue methanol solvate. Stable smart coatings on glass demonstrate robust switching over 104 cycles, and flexible microporous polymer membranes incorporating microcrystals of the complex show identical vapochromic behaviour. The rapid vapochromic response can be rationalised from the crystal structure, and in combination with quantum-chemical modelling, we provide a complete microscopic picture of the switching mechanism. We discuss how this multiscale design approach can be used to obtain new compounds with tailored VOC selectivity and spectral responses

Density functional theory study of alkali metal-noble metal diatomic molecules

Molecular properties, equilibrium bond lengths, dissociation energies, and vibrational frequencies of the ground state, of diatomic molecules formed from alkali metal (Li, Na, K) and noble transition metal (Cu, Ag, Au) have been calculated using density functional theory (DFT) with eight different density functionals. In addition, ab initio wave function based Hartree-Fock (HF) and coupled cluster singles doubles with triple excitations added perturbatively (CCSD(T)) methods are also included for comparisons. The pure density functionals PW91 and BP86 predict well the dissociation energies and harmonic vibrational frequencies but underestimate the bond lengths. CCSD(T) predicts the geometry well but underestimates the dissociation energies and vibrational frequencies. The hybrid HF/DFT B3P86 and B3LYP provide reasonable estimates for all the spectroscopic parameters. From the discrepancy between the computed and experimental bond length of KAg and the vibrational frequency of LiCu, we suggest a reanalysis or reexamination of the experimental spectrum of these two molecules. With only medium sized basis sets and small core relativistic effective core potentials, calculations using DFT functionals often give comparable or even superior results to ab initio wave function based methods in this mixed metal system.link_to_subscribed_fulltex

Emissive or nonemissive? a theoretical analysis of the phosphorescence efficiencies of cyclometalated platinum(II) complexes

We herein report a theoretical analysis based on a density functional theory/time-dependent density functional theory (DFT/TDDFT) approach to understand the different phosphorescence efficiencies of a family of cyclometalated platinum(II) complexes: [Pt(NCN)Cl] (1; NCN = l,3-bis(2-pyridyl)phenyl -), [Pt-(CNN)Cl] (2; CNN=6-phenyl-2,2′- bipyridyl -), [Pt(CNC)(CNPh)] (3; CNC = 2,6-diphenylpyridyl 2-), [Pt(RCNN)Cl] (4; R-CNN = 3-(6′-(2″-naphthyl)- 2′-pyridyl)isoquinolinyl -), and [Pt(R-CNC)(CNPh)] (5; R-CNC=2,6bis(2′-naphthyl)pyridyl 2-). By considering both the spin-orbit coupling (SOC) and the electronic structures of these complexes at their respective optimized singlet ground (S 0) and first triplet (T opt 1 ,) excited states, we were able to rationalize the experimental findings that 1 ) 1 is a strong emitter while its isomer 2 is only weakly emissive in CH 2Cl 2 solution at room temperature; 2) although the cyclometalated ligand of 3 has a higher ligand-field strength than that of 1, 3 is nonemissive in CH 2Cl 2 solution at 298 K; and 3) extension of π conjugation at the lateral aryl rings of the cyclometalated ligands of 2 and 3 to give 4 and 5, respectively, leads to increased emission quantum yields under the same conditions. We found that Jahn-Teller and pseudoJahn-Teller effects are operative in complexes 2 and 3, respectively, on going from the optimized S 0 ground state to the optimized T opt 1, excited state, and thus lead to large excitedstate structural distortions and hence fast nonradiative decay. Furthermore, a strong-field ligand may push the two different occupied d orbitais so far apart that the SOC effect is small and the radiative decay rate is slow. This work is an example of electronic-structure-driven tuning of the phosphorescence efficiency, and the DFT/TDDFT approach is demonstrated to be a versatile tool for the design of phosphorescent materials with target characteristics. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.link_to_subscribed_fulltex

Density functional theory studies of [Fe(O)2L]2+: What is the role of the spectator ligand L with different coordination numbers?

Density functional theory (DFT) studies were carried out on [Fe(O) 2(L)]n+ [L = qpy (1), simple amines (2), and tpy (3); qpy = 2,2′:6′,2″:6″,2‴′:6‴′, 2‴′-quinquepyridine and tpy = terpyridine; n = 1 or 2] to study how the coordination number of the spectator ligand L affects the geometries and electronic structures of the complexes. It was found that qpy can act as both a tridentate and pentadentate ligand resulting in [Fe(O)2(qpy)] 2+ (12+) having a trigonal bipyramidal (TBP) geometry in the former case, and a pentagonal bipyramidal (PBP) geometry in the latter case. The difference in coordination geometries has a significant impact on the electronic structures of 12+. With a TBP geometry, 12+ adopts a [FeV(O)2(qpy)+•]2+ formalism where a d3 quartet FeV ion ferromagnetically and antiferromagnetically couples to the qpy cation radical to give close-lying triplet and quintet states (within ca. 0.2 eV). With a PBP geometry, the Fe V ion in 12+ also formally has three unpaired electrons (a d3 quartet) with the fourth unpaired electron localized on a single oxido ligand to give a quintet state. The unoccupied orbital of 12+ in PBP geometry is lower lying in energy and has higher oxido character than when the complex has TBP geometry. Thus, based on the MO energies and oxido character of the unoccupied orbital, 12+ with PBP geometry is proposed to be a more reactive oxidant than 12+ with TBP geometry. On the other hand, 12+ with TBP geometry has a similar electronic structure to heme Cpd I, and it is possible that these two compounds have similar oxygen atom transfer reaction mechanisms. By varying the ligand coordination number using different spectator ligands L, the dioxido-iron complex [Fe(O)2(L)]2+ can change from a high-spin triplet when L = tpy, to a low-spin singlet when L = simple amines, to a quasi-degenerate triplet and quintet state when L = qpy. The electronic structures of [Fe(O)2L]2+ complexes, where L has different coordination numbers, have been studied with DFT. Ligand qpy can act as both a penta- and tridentate ligand. The LUMO of the former has higher oxido character and a lower orbital energy than the latter. The latter coordination geometry also has a similar electronic structure to heme Cpd I, suggesting possibly similar oxygen atom transfer reaction mechanisms for these complexes. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.link_to_subscribed_fulltex

Density functional theory study of trans-dioxo complexes of iron, ruthenium, and osmium with saturated amine ligands, trans-[M(O) 2(NH3)2(NMeH2)2] 2+ (M = Fe, Ru, Os), and detection of [Fe(qpy)(O)2] n+ (n = 1, 2) by high-resolution ESI mass spectrometry

Density functional theory (DFT) calculations on trans-dioxo metal complexes containing saturated amine ligands, trans-[M(O)2(NH 3)2-(NMeH2)2]2+ (M = Fe, Ru, Os), were performed with different types of density functionals (DFs): 1)pure generalized gradient approximations (pure GGAs): PW91, BP86, and OLYP; 2) meta-GGAs: VSXC and HCTH407; and 3) hybrid DFs: B3LYP and PBE1PBE. With pure GGAs and meta-GGAs, a singlet d2 ground state for trans-[Fe(O) 2(NH3)2(NMeH2)2] 2+ was obtained, but a quintet ground state was predicted by the hybrid DFs B3LYP and PBE1PBE. The lowest transition energies in water were calculated to be at λ ≈ 509 and 515 nm in the respective ground-state geometries from PW91 and B3LYP calculations. The nature of this transition is dependent on the DFs used: a ligand-to-metal charge-transfer (LMCT) transition with PW91, but a π(Fe-O)→π*-(Fe-O) transition with B3LYP, in which it and π* are the bonding and antibonding combinations between the dπ(Fe) and Pπ(O2-) orbitais. The FeVI/V reduction potential of trans-[Fe(O)2-(NH 3)2(NMeH2)2]2+ was estimated to be +1.30 V versus NHE based on PW91 results. The [Fe(qpy)(O) 2]n+ (qpy = 2,2′:6′,2″:6″, 2‴:6‴,2-quinque-pyridine; N = 1 and 2) ions, tentatively assigned to dioxo iron(V) and dioxo iron(VI), respectively, were detected in the gas phase by high-resolution ESI-MS spectroscopy. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.link_to_subscribed_fulltex

Tunneling energy effects on GC oxidation in DNA

Hole-mediated electronic couplings, reorganization energies, and electron transfer (ET) rates are examined theoretically for hole-transfer reactions in DNA. Electron transfer rates are found to depend critically on the energy gap between the donor/acceptor states and the intervening bases-the tunneling energy gap. The calculated distance decay exponent for the square of the electronic coupling, β, for hole transfer between GC base pairs (and pi-electron D/A pairs) ranges from 0.95 to 1.5 Å -1 in the model structures as the tunneling energy gap varies from 0.3 to 0.8 eV (which we argue is the range of energy gaps for GC oxidation probed in recent experiments). We show that the tunneling energy gap depends on the ET reorganization energy, which itself grows rapidly with distance for ET over 1-5 base pairs. Inclusion of the distance dependence of reorganization energies for these hole transfer reactions gives the tunneling rates an apparent decay exponent of ∼1.5-2.5 Å -1. We show that ET rates observed in DNA across one and two base pairs are reasonably well described with single-step ET theories, using our calculated couplings and reorganization energies. However, the computed single-step tunneling (superexchange) ET rates for donor and acceptor species separated by three or more base pairs are much smaller than observed. We conclude that longer-distance ET probably proceeds through thermal population of intermediate hole states of the bridging bases. Switching between mechanisms as distance grows beyond a few base pairs is likely to be a general characteristic of ET in small tunneling energy gap systems.link_to_subscribed_fulltex

Hole size and energetics in double helical DNA: Competition between quantum delocalization and solvation localization

The transition between single step long-range tunneling and multistep hopping transport in DNA electron transfer depends on a myriad of factors including sequence, distance, conformation, solvation and, consequently, hole state energetics. We show that the solvation energetics of hole (radical cation) states in DNA is comparable to the quantum delocalization energetics of the hole. That is, the solvation forces that tend to localize the hole compete with the quantum effects that give rise to hole delocalization. The net result is that the hole states are predicted to be relatively compact (one to three base pairs in length) and that the "trap depth" of these holes is expected to be much shallower than anticipated by gas-phase quantum chemical analysis of base stacks. This analysis predicts guanine oxidation potential dependence on the length of GC runs to be modest (differences <0.1 V for holes from one to three base pairs). The lowering of the trapped hole binding energy has significant implications for the structure and mobility of hole states in DNA.link_to_subscribed_fulltex