Effect Of Spin Multiplicity Research Articles

Multistate electronic quenching: Nonadiabatic pathways in NOA2Σ+ + O2X 3Σg - scattering.

The quenching of NOA2Σ+ with O2 as a collisional partner is important for combustion and atmospheric processes. There is still a lack of theoretical understanding of this event, especially concerning the nature of the different quenching pathways. In this work, we provide potential energy surfaces (PESs) of 20 electronic states of this system. We computed the spin-doublet and spin-quartet PESs using SA-CASSCF and XMS-CASPT2. We find two potential quenching pathways. The first one (Q1) is a two-step orientation-specific process. The system first undergoes an electron transfer (NO+X1Σ+ + O2 -X 2Πg) at short distances, before crossing to lower neutral states, such as NOX2Π + O2a 1Δg, O2b 1Σg +, O2X 3Σg -, or even 2 O(3P). The second quenching pathway (Q2) is less orientation-dependent and should be sudden without requiring the proximity conditioning Q1. The Q2 cross section will be enhanced with increasing initial vibrational level in both O2 and NO. It is responsible for the production of NOX2Π with higher O2 excited states, such as O2c 1Σu -, A'3Δu, or A 3Σu +. Overall, this work provides a first detailed theoretical investigation of the quenching of NOA2Σ+ by O2X 3Σg - as well as introduces a weighting scheme generally applicable to multireference, open-shell bimolecular systems. The effect of spin-multiplicity on the different quenching pathways is also discussed.

Electronic structure and reactivity indexes of cobalt clusters, both pure and mixed with NO and [Formula: see text] ([Formula: see text], [Formula: see text] and [Formula: see text]).

Among the most popular motivations for environmental scientists is improving materials that could be useful to fight or avoid pollution. This work shows a study of neutral and cationic cobalt clusters from 4 to 9 atoms ([Formula: see text], q = 0,1 and n = 4-9) to model their separate interaction with contaminant nitric and nitrous oxides. This study is within the framework of the density functional theory in the Kohn-Sham scheme by using BPW91 functional and 6-311G and 6-31G* basis sets to calculate global and local reactivity indexes. The effect of spin multiplicity is also determined. Results on the geometries of pure cobalt clusters agree with previously reported structures. Global minimum energy structures showed a marked preference towards the interaction of nitric and nitrous oxide molecules with cobalt clusters through chemisorptive dissociation, with the dissociation of the corresponding nitrogen oxide. Reactivity indexes reveal an even-odd alternate, which is related to electron counts. Moreover, the chemical potential is lowering after interaction with nitrogen oxides. The Fukui function illustrates the reactive zones with a high probability of chemisorption of more nitrogen oxide molecules.

Spin multiplicity effects in doublet versus singlet emission: the photophysical consequences of a single electron.

Ambient-stable fluorescent radicals have recently emerged as promising luminescent materials; however, tailoring their properties has been difficult due to the limited photophysical understanding of open-shell organic systems. Here we report the experimental and computational analysis of a redox pair of π-conjugated fluorescent molecules that differ by one electron. A π-dication (DC) and π-radical cation (RC) demonstrate different absorption spectra, but similar red emission (λemiss,max = ∼630 nm), excitation maxima (λexc,max = ∼530 nm), fluorescence lifetimes (1–10 ns), and even excited-state (non-emissive) lifetimes when measured by transient absorption spectroscopy. Despite their experimental similarities, time-dependent density functional theory (TDDFT) studies reveal that DC and RC emission mechanisms are distinct and rely on different electronic transitions. Excited-state reorganization occurs by hole relaxation in singlet DC, while doublet RC undergoes a Jahn-Teller distortion by bending its π-backbone in order to facilitate spin-pairing between singly occupied molecular orbitals. This relationship between the excited-state dynamics of RC and its π-backbone geometry illustrates a potential strategy for developing π-conjugated radicals with new emission properties. Additionally, by comparing TDDFT and CIS (configuration interaction singles) excitations, we show that unrestricted TDDFT accurately reproduces experimental absorption spectra and provides an opportunity to examine the relaxed excited-state properties of large open-shell molecules like RC.

Reactivity of CO on Ni4 cluster- effect of spin multiplicity and H doping-A DFT investigation

We investigated the reactivity of carbon monoxide on tetrahedral Ni4 clusters at different spin multiplicity applying density functional theory calculations considering pure and hybrid functional. The stability of the clusters increases with the increasing spin multiplicity and doping hydrogen in Ni4 cluster. The adsorption or binding energy of CO on Ni4 cluster is thermodynamically feasible process at normal condition whereas dissociation is not feasible. Ab initio molecular orbital analysis shows the orbital overlaps are observed at bridging site, three fold sites, and tetra coordinated centre and formation of δ-bond in the cluster. In NBO analysis, CO binds strongly to the Ni4 cluster not only by two NiC bond (spd hybrid), but also donor-acceptor delocalization interactions, for example, σ type BD(CO) → BD*(NiC) and BD(NiC) → BD*(NiC/CO), two π-type BD(CO) → LP*(Ni) and several diffuse RY*(C) ← LP(Ni) and π*(CO) ← LP(Ni) interactions. Singlet Ni4 cluster shows highest activation energy barrier, 3 eV. H-doped Ni4 cluster decreases dissociation barrier and favors CH bond formation.

Nonlinear optical responses of multiply ionized noble gases: Dispersion and spin multiplicity effects

Nonlinear optical responses of multiply ionized noble gases: Dispersion and spin multiplicity effects

Unusual Manipulative Effects of Spin Multiplicity and Excess Electron Number on the Structure and Nonlinear Optical Response in New Linear and Cyclic Electride Molecules with Multiexcess Electrons

Using the M06-2X density functional theory, unusual manipulative effects of spin multiplicity and excess electron number on the structure and static first hyperpolarizability (β0) are revealed for new electride molecules with multiexcess electrons. (1) For the spin effect on molecular structure, the low spin multiplicity brings bent structures (1L2 and 2L3) for linear isomers. (2) For the dramatic effect of spin multiplicity on β0, the considerable β0 of 22.7 × 104 au for the low spin structure 2L3 is larger by 21 times than that of 1.1 × 104 au for the high spin structure 4L3 due to one excess electron spin reversal. The low spin structure with doubly and singly occupied frontier orbitals is relevant to the low transition energy and complex distribution of electron density, which dramatically enhances β0. (3) For the effect of excess electron number on β0, the β0 value increases rapidly with increasing excess electron number. A long chain-shaped electride molecule (for instance, 2L3) with multiexcess electrons in low spin state may bring a considerable static first hyperpolarizability, which is a novel manipulation strategy of enhancing molecular NLO response.

Effect of Spin Multiplicity on Structure, Second Hyperpolarizability and Vibrational Spectrum of Cyanamide

Effect of Spin Multiplicity on Structure, Second Hyperpolarizability and Vibrational Spectrum of Cyanamide

Interplay among Aromaticity, Magnetism, and Nonlinear Optical Response in All-Metal Aromatic Systems

All-metal aromatic molecules are the latest inclusion in the family of aromatic systems. Two different classes of all-metal aromatic clusters are primarily identified: one is aromatic only in the low spin state, and the other shows aromaticity even in high-spin situations. This observation prompts us to investigate the effect of spin multiplicity on aromaticity, taking Al(4)(2-), Te(2)As(2)(2-), and their copper complexes as reference systems. Among these clusters, it has been found that the molecules that are aromatic only in their singlet state manifest antiaromaticity in their triplet state. The aromaticity in the singlet state is characterized by the diatropic ring current circulated through the bonds, which are cleaved to generate excess spin density on the atoms in the antiaromatic triplet state. Hence, in such systems, an antagonistic relationship between aromaticity and high-spin situations emerges. On the other hand, in the case of triplet aromatic molecules, the magnetic orbitals and the orbitals maintaining aromaticity are different; hence, aromaticity is not depleted in the high-spin state. The nonlinear optical (NLO) behavior of the same set of clusters in different spin states has also been addressed. We correlate the second hyperpolarizability and spin density in order to judge the effect of spin multiplicity on third-order NLO response. This correlation reveals a high degree of NLO behavior in systems with excess spin density. The variance of aromaticity and NLO response with spin multiplicity is found to stem from a single aspect, the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and eventually the interplay among aromaticity, magnetism, and NLO response in such materials is established. Hence, the HOMO-LUMO energy gap becomes the cornerstone for tuning the interplay. This correlation among the said properties is not system-specific and thus can be envisaged even beyond the periphery of all-metal aromatic clusters. Such interplay is of crucial importance in tailoring novel paradigm of multifunctional materials.

アクリジン誘導体の分子内[4+2]および[3+2]環化付加反応ービラジカル中間体の関与ー

アクリジン誘導体の分子内[4+2]および[3+2]環化付加反応ービラジカル中間体の関与ー

Modeling two photon absorption cross sections of open-shell systems

We demonstrate the prospects of computing two photon absorption cross sections of open-shell systems by applying recently developed spin-restricted time-dependent density functional response theory using the pyrrole radical as an example. The spin multiplicity effects on two photon absorption cross sections of this species are investigated for the doublet, quartet, and sextet states. It is found that irrespective of the exchange-correlation functional employed, the two photon cross sections increase with the increase in spin multiplicity. This result indicates that two photon cross sections of paramagnetic compounds can be controlled by manipulating their spin states and this opens new possibilities for design of hybrid magneto-optical materials.

Theoretical Study on the Relationship between Spin Multiplicity Effects and Nonlinear Optical Properties of the Pyrrole Radical (C4H4N·)

The geometrical structure and stability of neutral pi-conjugated C4H4N. with three spin states were investigated by using ab initio and density functional theory (DFT) methods. In addition, the linear and nonlinear optical properties were studied at the same level combined with the finite field approach. The calculated results show that conjugation and stability decreased with increasing spin multiplicity. These reliable UCCSD results show that the polarizability (alpha) values of C4H4N. with the quartet state are maximal, while those of C4H4N. with the doublet state are minimal. The order of betatot values is betasextet > betadoublet > betaquartet. The second hyperpolarizability (gamma) values exhibit positive values. The variation trends of gamma are consistent with alpha.

Ionized State of Hydroperoxy Radical−Water Hydrogen-Bonded Complex: (HO2−H2O)+

Ab initio molecular orbital calculations have been employed to characterize the structure and bonding of the (HO2-H2O)+ radical cation system. Geometry optimization of this system was carried out using unrestricted density functional theory in conjunction with the BHHLYP functional and 6-311++G(2df,2p) as well as 6-311++G(3df,3p) basis sets, the second-order Møller-Plesset perturbation (MP2) method with the 6-311++G(3df,3p) basis set, and the couple cluster (CCSD) method with the aug-cc-pVTZ basis set. The effect of spin multiplicity on the stability of the (HO2-H2O)+ system has been studied and also compared with that of oxygen. The calculated results suggest a proton-transferred hydrogen bond between HO2 and H2O in H3O3+ wherein a proton is partially transferred to H2O producing the O2...H3O+ structure. The basis set superposition error and zero-point energy corrected results indicate that the H3O3+ system is energetically more stable in the triplet state; however, the singlet state of H3O3+ is more stable with respect to its dissociation into H3O+ and singlet O2. Since the resulting proton-transferred hydrogen-bonded complex (O2...H3O+) consists of weakly bound molecular oxygen, it might have important implications in various chemical processes and aquatic life systems.

Spin Multiplicity Effects on the Second Hyperpolarizability of an Open-Shell Neutral π-Conjugated System

The spin multiplicity effects on the second hyperpolarizability (γ) are investigated for a small-size open-shell neutral conjugated model, the C5H7 radical, in the doublet, quartet, and sextet states by using several ab initio molecular orbital and density functional theory methods. The spatial contributions of total, α- and β-electrons to γ are examined to characterize the spin polarization and electron correlation effects on γ. It turns out that the second hyperpolarizability increases with the spin multiplicity, suggesting the interest of designing spin-enhanced nonlinear optical (NLO) systems based on open-shell neutral organic compounds, which also present the possibility of spin control of the NLO properties.

The effect of spin multiplicity on the photochemical reaction of a 2-vinylnaphthalene-dichloromaleic anhydride system

The photochemical reaction of a 2-vinylnaphthalene — dichloromaleic anhydride system has been studied in order to elucidate the correlation between the spin multiplicity of an excited-state molecule and reaction pathways in electron donor — acceptor systems. Direct irradiation of the system resulted mostly in the recovery of the starting materials. By contrast, the triplet-sensitized reaction using benzophenone as a sensitizer produced a copolymer and two stereoisomeric cycloadducts of 2-vinylnaphthalene with dichloromaleic anhydride. It is suggested that electron transfer takes place both in the electronically excited singlet and triplet states, although a triplet-state reaction also proceeds simultaneously in the benzophenone-sensitized reaction, and that the reaction pathway of the radical-ion pair generated in the excited singlet state is greatly different from that generated in the excited triplet state.

Chain-Length-Dependent Termination Rate Processes in Free-Radical Polymerizations. 3. Styrene Polymerizations with and without Added Inert Diluent as an Experimental Test of Model

Experiments were performed to test a model for the kinetics of free-radical polymerization systems, including the dependence of the termination rate coefficients on the lengths of both chains involved. The model has few adjustable parameters, the values of which are moreover confined within fairly narrow limits. The data comprised the rate of polymerization in a seeded emulsion polymerization of styrene, with and without benzene as diluent, with initiation by persulfate and by gamma-radiolysis. The latter can be switched off instantly, providing relaxation data which are sensitive to termination kinetics. Data from a single relaxation at a fixed weight-fraction polymer (omega(p)) were fitted to fur the unknown parameters, of which the only significant one is the probability p of reaction between two radicals upon encounter, incorporating the effect of spin multiplicity; this must lie between 0.25 and 1. Modeling using the value so obtained then successfully fitted (a) relaxation data at the same omega(p) but with 15 mol % benzene diluent, (b) relaxation data with and without diluent over the range 0.5 less than or equal to omega(p) less than or equal to 0.8, and (c) chemically initiated data over the same omega(p) range. This provides convincing evidence for the correctness of the termination model, which calculates the termination rate coefficients between two chains from the Smoluchowski equation, incorporating p, with diffusion coefficients (as a function of chain length and of omega(p)) obtained from a ''universal'' scaling law inferred from NMR data, and where the interaction distance for termination is the van der Waals radius of a monomeric unit; contributions from ''reaction diffusion'' (whereby a chain end moves by propagating) are also important at high conversion. The data also support a model for initiator efficiency in emulsion polymerization, this model being based on competition between aqueous-phase propagation (to a sufficient degree of polymerization for surface activity) and termination.

A Striking Effect of Spin Multiplicity on the Photoinduced Electron-Transfer Reaction of a 1-Vinylnaphthalene – Pyromellitic Dianhydride System

Abstract The photochemical reaction of a 1-vinylnaphthalene – pyromellitic dianhydride system is found to show a striking effect of spin multiplicity on the reaction pathway; whereas the radical ion pair generated in the electronically excited singlet state readily undergoes back electron transfer, that produced in the triplet state leads to the formation of a 2 : 1 adduct. The present study presents an example which suggests the control of back electron transfer by spin multiplicity in the photoinduced electron-transfer reaction.

The Photolysis of N,O-Diacyl-N-phenylhydroxylamines

Abstract The photochemical decomposition of the title compounds yielded the rearrangement products derived from 1,3- and 1,5-aroyloxyl migrations in addition to the fragmentation products typical of the aroyloxyl and amido radicals, while only 1,3-aroyloxyl migration was observed on thermolysis. The results of crossover experiments indicate an intramolecular rearrangement, probably involving homolysis of the N–O bond in the excited states. Triplet quenching studies demonstrate that the rearrangement and fragmentation proceed exclusively via the first excited singlet state. No formation of the rearrangement products on triplet sensitization was explained on the basis of the spin multiplicity effects on a radical–radical recombination process within a solvent cage.