Low-energy Species Research Articles

Abstract 2488: PET imaging of innate immunity activation In Vivo with redox-tuned 4-[18F]fluoro-1-naphthol

Abstract High redox potential reactive oxygen and nitrogen species (ROS/RNS), such as O2 free radicals, superoxide, and hypochlorous acid, generated by activities of the NADPH oxidase-2 (NOX2)/myeloperoxidase (MPO) axis and related enzymes, are key effector molecules of innate immunity in physiological and diseased inflammatory states. Other lower energy species (H2O2, NO) provide adjuvant signaling functions. NOX2- and MPO-derived high energy radicals are known to oxidize naphthol species, wherein the naphthol products bind to proximate proteins and activated myeloid cells. Herein, we present 4-[18F]fluoro-1-naphthol ([18F]4FN), a novel redox-tuned radiopharmaceutical that selectively detects by positron emission tomography (PET) high energy radicals produced by activated innate immunity. The products of human MPO plus H2O2, but not H2O2 alone, rapidly and completely oxidized [18F]4FN. All-trans-retinoic acid-differentiated HL-60 "neutrophil-like" human cells activated with phorbol-12-myristate-13-acetate (PMA) retained [18F]4FN 5-fold over unstimulated cells. 4-ABAH, an MPO-specific inhibitor, or DPI, a broad oxidase inhibitor, blocked cellular retention by >95%. [18F]4FN PET/CT imaging readily discriminated foci of inflammation in vivo in three distinct murine models of acute inflammation: endotoxin-induced whole-body toxic shock, PMA-induced mild contact dermatitis of the ear, and lipopolysaccharide (LPS)-induced ankle arthritis. Mechanistically, in mice in vivo, 4-ABAH reduced inflammation-induced [18F]4FN retention, and Cybb-/- (Nox2-/-) gene-deletion strongly and significantly abrogated PMA-induced [18F]4FN retention. Thus, [18F]4FN shows promise as a robust redox-tuned reporter for imaging activation states of innate immunity by PET/CT, is ready for translation. [18F]4FN PET imaging may find application in a variety of inflammatory states associated with cancer therapy, immunotherapy-related adverse events, as well as other diseases, including arthritis, hepatitis, atherosclerosis, COVID-19, as well as up-staging and monitoring multi-organ inflammation. Citation Format: Seth T. Gammon, Federica Pisaneschi, Vincenzo Paolillo, Sarah Qureshy, David Piwnica-Worms. PET imaging of innate immunity activation In Vivo with redox-tuned 4-[18F]fluoro-1-naphthol [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2488.

(Invited) Water-Splitting Electrocatalysts Made By Pulsed-Laser in Liquids Synthesis

Conversion of solar energy into storable fuels is urgently needed to combat climate change. Efficient, robust materials that are exclusively made of non-precious elements are imperative for tomorrow's global sustainable energy economy and necessitate the development of new nanostructured multi-metallic electrocatalysts that turn our most abundant feedstocks, water or carbon dioxide, into carbon-neutral fuels. The sheer number of possible elemental compositions in multi-metallic nanostructures requires a deep understanding of electrocatalytic mechanisms, including identification of reactive intermediates, to rationally design new materials and guide their optimization.We used pulsed-laser in liquids synthesis to realize series of earth-abundant multi-metallic first-row transition metal oxide and hydroxide nanostructures. The pulsed-laser in liquids technique is a flexible synthetic strategy for surfactant-free nanomaterials with independently controlled compositional, morphological, and structural properties, and defect densities. It permits rapid preparation or modification of tailored, complex nanostructures in sufficiently large quantities to study them in bulk. Importantly, metastable materials become accessible as nanoparticles form from a laser-induced plasma that is confined by the liquid and characterized by very high temperatures and pressures, thus enabling access to extreme regions of the material's phase diagram.Our approach enabled unprecedented atomistic-level structural and mechanistic insights into highly active and robust nickel–iron layered double hydroxide nanocatalysts for water oxidation in base; this understanding allowed rapid optimization. Operando spectroscopy data allowed us to identify a cis-dioxo iron(VI) reactive intermediate as the lowest-energy species before O–O bond formation during water oxidation electrocatalysis. Further, we capitalized on the unique advantages of the laser process for integration of our water-splitting electrocatalysts with photoanodes.

Photocatalytic H2-Evolution by Homogeneous Molybdenum Sulfide Clusters Supported by Dithiocarbamate Ligands.

Irradiation at 460 nm of [Mo3(μ3-S)(μ2-S2)3(S2CNR2)3]I ([2a]I, R = Me; [2b]I, R = Et; [2c]I, R = iBu; [2d]I, R = CH2C6H5) in a mixed aqueous-polar organic medium with [Ru(bipy)3]2+ as photosensitizer and Et3N as electron donor leads to H2 evolution. Maximum activity (300 turnovers, 3 h) is found with R = iBu in 1:9 H2O:MeCN; diminished activity is attributed to deterioration of [Ru(bipy)3]2+. Monitoring of the photolysis mixture by mass spectrometry suggests transformation of [Mo3(μ3-S)(μ2-S2)3(S2CNR2)3]+ to [Mo3(μ3-S)(μ2-S)3(S2CNR2)3]+ via extrusion of sulfur on a time scale of minutes without accumulation of the intermediate [Mo3S6(S2CNR2)3]+ or [Mo3S5(S2CNR2)3]+ species. Deliberate preparation of [Mo3S4(S2CNEt2)3]+ ([3]+) and treatment with Et2NCS21- yields [Mo3S4(S2CNEt2)4] (4), where the fourth dithiocarbamate ligand bridges one edge of the Mo3 triangle. Photolysis of 4 leads to H2 evolution but at ∼25% the level observed for [Mo3S7(S2CNEt2)3]+. Early time monitoring of the photolyses shows that [Mo3S4(S2CNEt2)4] evolves H2 immediately and at constant rate, while [Mo3S7(S2CNEt2)3]+ shows a distinctive incubation prior to a more rapid H2 evolution rate. This observation implies the operation of catalysts of different identity in the two cases. Photolysis solutions of [Mo3S7(S2CNiBu2)3]+ left undisturbed over 24 h deposit the asymmetric Mo6 cluster [(iBu2NCS2)3(μ2-S2)2(μ3-S)Mo3](μ3-S)(μ3-η2,η1-S',η1-S″-S2)[Mo3(μ2-S)3(μ3-S)(S2CNiBu2)2(μ2-S2CNiBu2)] in crystalline form, suggesting that species with this hexametallic composition and core topology are the probable H2-evolving catalysts in photolyses beginning with [Mo3S7(S2CNR2)3]+. When used as solvent, N,N-dimethylformamide (DMF) suppresses H2-evolution but to a greater degree for [Mo3S4(S2CNEt2)4] than for [Mo3S7(S2CNEt2)3]+. Recrystallization of [Mo3S4(S2CNEt2)4] from DMF affords [Mo3S4(S2CNEt2)4(η1,κO-DMF)] (5), implying that inhibition by DMF arises from competition for a Mo coordination site that is requisite for H2 evolution. Computational assessment of [Mo3S4(S2CNMe2)3]+ following addition of 2H+ and 2e- suggests a Mo(H)-μ2(SH) intermediate as the lowest energy species for H2 elimination. An analogous pathway may be available to the Mo6 cluster via dissociation of one end of the μ2-S2CNR2 ligand, a known hemilabile ligand type, in the [Mo3S4]4+ fragment.

Is the Inversion of Phosphorus Trihalides (PF3, PCl3, PBr3, and PI3) a Diradical Process?

This work explores possible reaction paths for the inversion of a series of trigonal pyramidal phosphorus trihalides, PF3, PCl3, PBr3, and PI3, and it especially addresses the question of whether and when the bonding of the lowest-energy species along the inversion paths should be described as a hyper-open-shell diradical. The various paths for inversion are calculated using a single-reference method within the framework of Kohn-Sham density functional theory and also with multireference wave function methods. Our calculated results using both kinds of methods show that, for all the halogens studied (F, Cl, Br, and I), the lowest-energy singlet path for the inversion occurs by the formation of a C2 v transition structure rather than a D3 h transition structure. This geometrical preference agrees with what has been inferred previously based on closed-shell singlet calculations. But in the present study, we examined not only closed-shell singlet transition states but also open-shell singlet states and triplet states for calculating stationary points and inversion paths, and for some of the phosphorus trihalides, we found that paths involving open-shell configurations are lower in energy than those restricted to closed-shell configurations. We analyzed the changes along the paths in terms of hybridization and orientation of the frontier orbitals and in terms of locally avoided crossings, and the extent of the diradical character was quantified by calculating the effective number of unpaired electrons. Even for the singlet inversion path that goes via a D3 h structure, the barrier for PF3, PCl3, and PBr3 is higher for a closed-shell singlet spin state than for the open-shell singlet configuration. Furthermore, the energy of the triplet D3 h structure is below even that of the open-shell D3 h singlet for PCl3, PBr3, and PI3. This necessitates rethinking the role of open-shell states in nominally closed-shell processes.

Infrared Spectrum of Protonated Corannulene H+C20H10 in Solid para-Hydrogen and its Potential Contribution to Interstellar Unidentified Infrared Bands

Polycyclic aromatic hydrocarbons (PAH) and their derivatives, including protonated and cationic species, are suspected to be carriers of the unidentified infrared (UIR) emission bands observed from the galactic and extragalactic sources. We extended our investigations of infrared (IR) spectra of protonated planar PAH to a nonplanar PAH, corannulene (C20H10), which is regarded as a fragment of a fullerene, C60. The protonated corannulene H+C20H10 was produced on bombarding a mixture of corannulene and para-hydrogen (p-H2) with electrons during deposition at 3.2 K. During maintenance of the electron-bombarded matrix in darkness the intensities of IR lines of protonated corannulene decreased because of neutralization by electrons that were slowly released from the trapped sites. The observed lines were classified into two groups according to their responses to secondary irradiation at 365 nm. Eighteen lines in one group are assigned to the lowest-energy species among five possible isomers, hub-H+C20H10, and ...

Undervalued N3 Coordination Revealed in the Cisplatin Complex with 2'-Deoxyadenosine-5'-monophosphate by a Combined IRMPD and Theoretical Study.

The complex obtained by the reaction of cisplatin and 2'-deoxyadenosine-5'-monophosphate (5'-dAMP) in water has been isolated and detected by electrospray ionization mass spectrometry. The so-formed cis-[PtCl(NH3)2(5'-dAMP)]+ complex has been studied in detail by infrared multiple photon dissociation (IRMPD) spectroscopy in two spectral ranges, namely, 700-1900 and 2800-3800 cm-1, backed by quantum-chemical calculations at the B3LYP/LACV3P/6-311G** level of theory. In agreement with the computational results, the vibrational spectroscopic characterization of cis-[PtCl(NH3)2(5'-dAMP)]+ shows that the sampled ionic population comprises two major isomers, differentiated in the X-H stretching region by their distinct fragmentation patterns. One of these species presents coordination of the platinum moiety at the N3 position of adenine, whereas in the second one, platinum is bound at the N1 position of adenine. IRMPD kinetics have allowed an estimation of their relative proportions. Surprisingly, the most abundant component of cis-[PtCl(NH3)2(5'-dAMP)]+ is the N3 isomer, although it is slightly less stable than the other potential isomers in the gas phase. In contrast, the lowest-energy species, namely, the one showing cisplatin binding to the N7 position of adenine, seems to be the one less represented in the sampled ionic population. These findings suggest that the reaction of cisplatin with 5'-dAMP is governed by the kinetics of the process occurring in solution rather than by the thermodynamic factors.

Locating the Proton in Nicotinamide Protomers via Low-Resolution UV Action Spectroscopy of Electrosprayed Solutions.

Even in relatively simple molecules, the sites of protonation or deprotonation formed upon electrospray ionization can be controversial. This situation means that it is important to develop new approaches for identifying "protomers" and "deprotomers". In this study, we demonstrate that routine, low-resolution UV laser photodissociation spectroscopy can be applied to identify the gaseous protomers of nicotinamide formed upon electrospray. Nicotinamide is an important biological molecule that possesses multiple protonation sites associated with its pyridine and amide groups. We obtain a gas-phase absorption spectrum for protonated nicotinamide that closely resembles the solution-phase spectrum. However, photoexcitation of protonated nicotinamide produces numerous ionic photofragments, and the spectral profiles for production of these photofragments from protonated nicotinamide reveal the existence of two distinctive chromophores, which can be traced to the existence of pyridine and amide protomers. We observe that these protomers are associated with absorption bands centered at 4.96 and 4.73 eV, respectively, with the protomers appearing in an approximate ratio of ∼2:1. The fact that the considerably less favorable amide protomer is observed in substantial quantities in the gas phase is surprising given that the pyridine protomer is the lower-energy species in both solution and gas phase. The high amounts of amide protomers observed here can be explained as arising from asymmetric pyridine protomer-amide bound dimers, present in solution or in the electrosprayed droplets, which lead to enhanced formation of the unexpected amide-protonated isomers.

A radical rebound mechanism for the methane oxidation reaction promoted by the dicopper center of a pMMO enzyme: a computational perspective.

In this article, we investigated the hydroxylation of methane catalyzed by the binuclear copper site of a pMMO enzyme, through a radical rebound mechanism. All intermediates and transition states along the reaction coordinate were located and the energies involved in the mechanism calculated using the B3LYP functional including dispersion effects. Our B3LYP-D2 results show that the singlet state of the (μ-1,2-peroxo)Cu(II)2 complex plays an important role as the lowest energy species prior to C-H bond activation. A crossing between the singlet and triplet PES is suggested to occur before the cleavage of the C-H bond of methane, where the triplet (bis-μ-oxo)Cu(III)2 is very reactive towards activation of the strong C-H bond of methane. The C-H bond activation is the rate-determining step of the reaction, with an activation energy of 18.6 kcal mol(-1) relative to the singlet (μ-1,2-peroxo)Cu(II)2 species. Comparison with previous theoretical results for a non-synchronous concerted mechanism suggests the radical rebound mechanism as a possible alternative pathway.

Bromine-Sensitized Solar Photolysis of CO2

Direct photochemical reduction of CO(2) has generally been accomplished by using transition-metal compounds as electron transfer reagents. Here, we show that elemental bromine can function as an alternative photosensitizer. When sunlight is tightly focused on mixtures of CO(2) and Br(2), in the presence of a polar adsorbent such as silica gel, glass wool, alumina, or titania, a metastable red adduct is formed within seconds and concentrates at the point of illumination. Further illumination causes deposition of a stable black film on the polar adsorbent. Mass spectrometry of the cold-trapped red intermediate shows clusters of peaks corresponding to the expected distribution of isotopomers of C(2)O(4)Br(4)(+), as well as of C(2)O(4)Br(3)(+). DFT computations indicate that the lowest-energy species with the formula C(2)O(4)Br(4) is trans-2,4-dibromo-2,4-dihypobromo-1,3-dioxetane. Formation of this molecule from (2CO(2) + 2Br(2)) would require a minimum of 3 visible photons, two of which would hypothetically be used in formation of as-yet undetected CO(2)Br(2) and the third, in a subsequent photodimerization. By elemental analysis, the final amorphous solid product contains a C/Br atomic ratio >12, suggesting that Br(2) is acting photocatalytically. Even with a poorly optimized optical system, the reaction rate has reached as high as 1.6 mg reduced C with 40 s of solar collection using a 30 cm diameter paraboloid reflector. This rate is consistent with the storage of approximately 1% of incident solar energy.

Is Poly(vinylcarbazole) a Good Host for Blue Phosphorescent Dopants in PLEDs? Dimer Formation and Their Effects on the Triplet Energy Level of Poly(N‐vinylcarbazole) and Poly(N‐Ethyl‐2‐Vinylcarbazole)

AbstractThe detailed measurement and analysis of the delayed emission from poly(vinylcarbazole) (PVK) and poly(N‐ethyl‐2‐vinyl‐carbazole) (P2VK) thin films is described. PVK has rapidly become a “polymer of choice” for hosting phosphorescent dopants in PLEDs, especially blue emitters. In this respect it is important to have a full understanding of the triplet properties of this host. It is concluded that in films, the electronic 0–0 peak energy of PVK phosphorescence is found at 2.88 eV (14 K). With an increase of temperature, >44 K, increasing emission from new long lived, lower energy species, previously ascribed to “trap states” in the literature, is observed. Increasing temperature enables thermally assisted triplet exciton hopping to these trap states. Critically it is shown that some of these triplet trap species are ground state triplet dimers in origin for both PVK (2.46 eV) and P2VK (2.1 eV), and not all of them are of excimer nature as previously thought. These species can quench the emission of blue heavy metal complexes doped in PVK and drastically effect performance over lifetime if the dimer formation increases over time and at elevated operating temperature. It is therefore concluded that PVK might not be such an ideal host material for blue phosphorescent emitters.

H·(H2O)n Clusters: Microsolvation of the Hydrogen Atom via Molecular ab Initio Gradient Embedded Genetic Algorithm (GEGA)

A new version of the ab initio gradient embedded genetic algorithm (GEGA) program for finding the global minima on the potential energy surface (PES) of mixed clusters formed by molecules and atoms is reported. The performance of the algorithm is demonstrated on the neutral H·(H(2)O)(n) (n = 1-4) clusters, that is, a radical H atom solvated in 1-4 water molecules. These clusters are of a fundamental interest. The solvated hydrogen atom forms during photochemical events in water, or during scavenging of solvated electrons by acids, and transiently exists in biological systems and possibly in inclusion complexes in the deep ocean and in the ice shield of earth. The processes associated with its existence are intriguingly complex, however, and have been the subject of decades-long debates. Using GEGA, we explicate the apparently extreme structural diversity in the H·(H(2)O)(n) (n = 1-4) clusters. All considered clusters have four basic structural types: type I, where the H radical is weakly coordinated to the oxygen atom of one of the water molecules; type II, where H is weakly coordinated to a H atom of one of the water molecules; type III, consisting of H(2), the OH radical, and n - 1 H(2)O molecules; and type IV, consisting of H(3)O and n - 1 H(2)O. There are myriads of isomers of all four types. The lowest energy species of types I and II are the isoenergetic global minima. H·(H(2)O)(n) clusters appear to be a challenging case for GEGA because they have many shallow minima close in energy some of which are significantly less stable than the global minimum. Additionally, the global minima themselves have high structural degeneracy, they are only weakly bound, and they are prone to dissociation. GEGA performed exceptionally well in finding both the global and the low-energy local minima that were subsequently confirmed at higher levels of theory.

Organic compounds in the C 3H 6O 3 family: Microwave spectrum of cis–cis dimethyl carbonate

Geometry optimization calculations on 13 members of the C 3H 6O 3 family of organic species have been carried out to determine their relative binding energies. Dimethyl carbonate [(CH 3) 2CO 3] is one of the lower energy species in this family, which includes the C 3-sugars 1,3-dihydroxyacetone and glyceraldehyde. The microwave spectrum of dimethyl carbonate has been measured over the frequency range 8.4–25.3 GHz with several pulsed-beam Fourier-transform microwave spectrometers and from 227 GHz to 350 GHz with direct absorption spectrometers. The spectrum of the lowest-energy cis–cis conformer of dimethyl carbonate has been assigned, and ab initio electronic structure calculations of the three possible conformers have been performed. Stark effect measurements were carried out on the cis–cis conformer to provide accurate determinations of the dipole moment components.

The structure of Au6Y+ in the gas phase

The geometric and electronic structure of the Au(6)Y(+) cation is studied by gas phase vibrational spectroscopy combined with density functional theory calculations. The infrared photodissociation spectrum of Au(6)Y(+)·Ne is measured in the 95-225 cm(-1) energy range and exhibits two characteristic absorption bands at 181 cm(-1) and 121 cm(-1). Based on DFT/BP86 quantum chemical calculations, the infrared spectrum is assigned to the lowest energy species found, an eclipsed C(3v) geometry. The 3D structure of Au(6)Y(+) is considerably different from those previously found for both the neutral Au(6)Y (quasi-planar circular geometry) and the anionic Au(6)Y(-) (planar D(6h) symmetry). The different geometries are related to different electronic structures in agreement with 2D and 3D phenomenological shell models for metal clusters.

The Spin Chemistry and Magnetic Resonance of H2@C60. From the Pauli Principle to Trapping a Long Lived Nuclear Excited Spin State inside a Buckyball

One of the early triumphs of quantum mechanics was Heisenberg's prediction, based on the Pauli principle and wave function symmetry arguments, that the simplest molecule, H(2), should exist as two distinct species-allotropes of elemental hydrogen. One allotrope, termed para-H(2) (pH(2)), was predicted to be a lower energy species that could be visualized as rotating like a sphere and possessing antiparallel ( upward arrow downward arrow) nuclear spins; the other allotrope, termed ortho-H(2) (oH(2)), was predicted to be a higher energy state that could be visualized as rotating like a cartwheel and possessing parallel ( upward arrow upward arrow) nuclear spins. This remarkable prediction was confirmed by the early 1930s, and pH(2) and oH(2) were not only separated and characterized but were also found to be stable almost indefinitely in the absence of paramagnetic "spin catalysts", such as molecular oxygen, or traces of paramagnetic impurities, such as metal ions. The two allotropes of elemental hydrogen, pH(2) and oH(2), may be quantitatively incarcerated in C(60) to form endofullerene guest@host complexes, symbolized as pH(2)@C(60) and oH(2)@C(60), respectively. How does the subtle difference in nuclear spin manifest itself when hydrogen allotropes are incarcerated in a buckyball? Can the incarcerated "guests" communicate with the outside world and vice versa? Can a paramagnetic spin catalyst in the outside world cause the interconversion of the allotropes and thereby effect a chemical transformation inside a buckyball? How close are the measurable properties of H(2)@C(60) to those computed for the "quantum particle in a spherical box"? Are there any potential practical applications of this fascinating marriage of the simplest molecule, H(2), with one of the most beautiful of all molecules, C(60)? How can one address such questions theoretically and experimentally? A goal of our studies is to produce an understanding of how the H(2) guest molecules incarcerated in the host C(60) can "communicate" with the chemical world surrounding it. This world includes both the "walls" of the incarcerating host (the carbon atom "bricks" that compose the wall) and the "outside" world beyond the atoms of the host walls, namely, the solvent molecules and selected paramagnetic molecules added to the solvent that will have special spin interactions with the H(2) inside the complex. In this Account, we describe the temperature dependence of the equilibrium of the interconversion of oH(2)@C(60) and pH(2)@C(60) and show how elemental dioxygen, O(2), a ground-state triplet, is an excellent paramagnetic spin catalyst for this interconversion. We then describe an exploration of the spin spectroscopy and spin chemistry of H(2)@C(60). We find that H(2)@C(60) and its isotopic analogs, HD@C(60) and D(2)@C(60), provide a rich and fascinating platform on which to investigate spin spectroscopy and spin chemistry. Finally, we consider the potential extension of spin chemistry to another molecule with spin isomers, H(2)O, and the potential applications of the use of pH(2)@C(60) as a source of latent massive nuclear polarization.

Activation of P4by Singlet Silylene (SiH2): A Computational Study

Ab initio explorations of the interactions of singlet silylene (SiH2) with tetrahedral P4 (white phosphorus) are reported that lead to six stable one-to-one species. Five of these adducts have bonds between silicon and phosphorus: (1) a P−P insertion adduct A, (2) two cyclotriphosphirene rotational isomers Ccis and Ctrans, (3) a structure labeled D, where a hydrogen atom has migrated to phosphorus, and (4) an apical adduct labeled E. Another adduct (B) that has been characterized has no covalent bond between silicon and phosphorus, but SiH2 is bound by ∼5.5 kcal/mol. Characterization of the transition states connecting several of these species leads to a partial understanding of the potential energy surface. The lowest energy species, the C2v insertion adduct A, is substantially more stable than the other adducts. The structures found computationally are compared with the one-to-one insertion adduct of P4 and a nucleophilic (stable) silylene reported by Driess and co-workers. The computational results suggest that non-nucleophilic silylene reactions with P4 might be profitably explored experimentally.

(η5-C4H4S)Cr(CO)3 and (η5-C4H4Se)Cr(CO)3: A DFT Investigation of the Ground-State Singlet and Triplet Surfaces. New Insights into the Mechanism of C−S or C−Se Insertion Reactions

The singlet and triplet surfaces for the interaction of thiophene or selenophene with a chromium tricarbonyl unit were calculated using the B3LYP/LanL2DZ+p model chemistry. The singlet surfaces confirm that the (η5-C4H4E)Cr(CO)3 (E = S or Se) are the lowest energy species. The (η1(C2)-C4H4E)Cr(CO)3 and (κ2(E,C)-C4H4E)Cr(CO)3 species were also located in shallow energy minima. Reaction path modeling on the singlet surfaces provided activation energies for the endothermic insertion process of 172 and 160 kJ mol−1 for E = S or Se, respectively. The activation energy for the insertion process on the triplet surfaces is on the order of 70 kJ mol−1. A novel η3(E,C,C)-η1(C)-coordinated species was located as a transition state between the insertion species and (η5-C4H4E)Cr(CO)3 on the singlet surfaces and an intermediate on the triplet surface for E = S.

Coupled-Cluster Study of Isomers of H2SO2

A theoretical study has been made on six isomers of H2SO2 using coupled-cluster singles and doubles with noniterative triple excitations (CCSD(T)). The isomers studied are sulfoxylic acid (S(OH)2; C2 and Cs conformers), sulfinic acid (HS(=O)OH; 2 C1 conformers), dihydrogen sulfone (H2SO2; C2v), sulfhydryl hydroperoxide (HSOOH; C1), thiadioxirane (Cs), and dihydrogen persulfoxide (H2SOO; Cs). Molecular geometries, harmonic vibrational frequencies, and infrared intensities of all species were obtained using the CCSD(T) method and the 6-311++G(2d,2p) basis set. All aforementioned species were found to be local minima, with the exception of thiadioxirane, which has one imaginary frequency. A prior possible infrared observation of sulfinic acid was reassessed on the basis of the present data. In agreement with previous MP2 results, the present CCSD(T) data provide support for at most 4 of the 8 observed frequencies. The CCSD(T) frequencies and intensities should be of assistance in future identification of H2SO2 isomers by vibrational spectroscopy. Relative energies were calculated using the CCSD(T) method and several larger basis sets. As found previously, the lowest energy species is C2 S(OH)2, followed by Cs S(OH)2, HS(=O)OH, H2SO2, HSOOH, thiadioxirane, and H2SOO. Expanding the basis set significantly reduces the relative energies of HS(=O)OH and H2SO2. The CCSD(T) method was used with extended basis sets (up to aug-cc-pV(Q+d)Z) and basis set extrapolation in two reaction schemes to calculate the DeltaH degrees t (25 degrees C) of C2 S(OH)2. The two reaction schemes gave -285.8 and -282.7 kJ mol-1, which are quite close to a prior theoretical estimate (-290 kJ mol-1).

Synthesis, spectroscopic and structural studies of 2,2,2-trifluoroethyl phosphorodiamidate complexes with tin(IV) chloride

The two octahedral complexes SnCl 4 · 2(O)P(NR 2) 2OCH 2CF 3 (R = Me ( 1) or Et ( 2)) have been prepared from SnCl 4 and the ligands (R 2N) 2P(O)OCH 2CF 3 in chloroform solution. Both adducts have been characterised by ( 31P and 119Sn) NMR, IR spectroscopy and elemental analysis. The NMR data show that the complexes exist as mixtures of cis and trans isomers in solution with the latter isomer being the predominant species. The structure of 1 has been determined by X-ray crystallography. Accordingly, the structure is centrosymmetric and the two ligands are bound trans to each other in the octahedral tin complex. DFT/B3LYP calculations show that trans configuration does indeed lead to the lowest energy species. Comparison of the structural, NMR and theoretical data of both complexes with those related to SnCl 4 · 2L (L = (Me 2N) 3P(O) and (Me 2N) 2P(O)F) further supports the important effects of the nature of the substituents in the ligand on the stereochemistry of the complex formed.

On the existence of MHn species with M=Al, Ga and n=4, 5, 6. Computational study of structures, stabilities and bonding

Based on second-order perturbation theory (MP2) predictions with large 6-311 + + G(3df, 3pd) basis set we have reviewed the possible structures and stabilities of a series of neutral MH(n)(M = Al, Ga; n = 4, 5, 6) species. For AlH4 and AlH5, our results confirm the previous theoretical findings, which indicate the dihydrogen C(s) complexes (2A') AlH2(H2) and (1A') AlH3(H2), respectively, as the lowest energy isomers. We found, similarly, C(s) (2A') GaH2(H2) and (1A') GaH3(H2) van der Waals complexes as the most stable species of the gallium analogues GaH4 and GaH5. The calculated H2 dissociation energies (D(e)) for AlH2(H2) and AlH3(H2) are of the order 1.8-2.5 kcalmol(-1), whereas this range of values for GaH2(H2) and GaH3(H2) is 1.4-1.8 kcalmol(-1) . Symmetry-adapted perturbation theory (SAPT) was used to analyze the interaction energies of these dihydrogen complexes (for n = 5) to determine why the Ga species show a smaller binding energy than the Al species. The SAPT partitioning of the interaction energy showed significant differences between AlH3(H2) and GaH3(H2), resulting from the much stronger "hydride" character of the aluminum species. The experimental observation of AlH2(H2) and AlH3(H2), and likely GaH3(H2), via low-temperature matrix isolation has been reported recently by Pullumbi et al. and Andrews et al., supporting the theoretical predictions. For n = 6, we found the degenerate C2(2A) and C(s)(2A') MH2(H2)2 "double H2" type van der Waals complexes as the lowest energy species for both M = Al and Ga.

Radicals Derived from Guanine: Structures and Energetics

The electron affinities (EA) of five radicals derived from guanine by removing a hydrogen atom are predicted by using four carefully calibrated (Chem. Rev. 2002, 102, 231) density functional methods. The most stable guanine radical arises from the removal of the H10b atom (Fig. 1) from the NH2 group. The theoretical adiabatic electron affinities (EAad) for the five possible guanine radicals are substantial, in the range of 2.18-2.99 eV, which is much higher than that for the closed-shell guanine molecule (-0.17 eV). The N9 dehydrogenated radical has the highest EAad (2.99 eV), and its related anion is the lowest energy species among those studied in the present research. The energy difference between the two N10-sited (amino group) radicals is 4.8 kcal/mol, indicating that these two structures are not conventional internal-rotation conformers.