Quartet Spin Multiplicity Research Articles

Infrared Ion Spectroscopic Characterization of the Gaseous [Co(15-crown-5)(H2O)]2+ Complex.

We report fingerprint infrared multiple-photon dissociation spectra of the gaseous monohydrated coordination complex of cobalt(II) and the macrocycle 1,4,7,10,13-pentaoxacyclopentadecane (or 15-crown-5), [Co(15-crown-5)(H2O)]2+. The metal-ligand complexes are generated using electrospray ionization, and their IR action spectra are recorded in a quadrupole ion trap mass spectrometer using the free-electron laser FELIX. The electronic structure and chelation motif are derived from spectral comparison with computed vibrational spectra obtained at the density functional theory level. We focus here on the gas-phase structure, addressing the question of doublet versus quartet spin multiplicity and the chelation geometry. We conclude that the gas-phase complex adopts a quartet spin state, excluding contributions of doublet species, and that the chelation geometry is pseudo-octahedral with the six oxygen centers of 15-crown-5 and H2O coordinated to the metal ion. We also address the possible presence of higher-energy conformers based on the IR spectral evidence and calculated thermodynamics.

Characterization of the electronic states of the biological relevant SSNO molecule.

Using configuration interaction ab initio methods, we investigate the lowest electronic states of doublet and quartet spin multiplicities of SSNO where the one-dimensional cuts of the six-dimensional potential energy surfaces of these electronic states along the stretching and bending coordinates are computed. Mainly, these electronic states are found to be repulsive along the central SN distance. A high density of electronic states is computed even at low excitation energies that may favor their couplings. Therefore, the dynamics of the SSNO electronic states is expected to be very complex. We also characterized the bound electronic states spectroscopically where we derived their equilibrium structures and vibrational frequencies. Our calculations show the importance of taking into account of dynamical correlation, in addition to static correlation, for the accurate description of SSNO electronic excited states and more generally for those of R-NO molecular species. Finally, we highlighted the potential role of SSNO in light-induced NO delivery from SSNO related species in biological media.

Characterization and reactivity of the weakly bound complexes of the [H, N, S](-) anionic system with astrophysical and biological implications.

We investigate the lowest electronic states of doublet and quartet spin multiplicity states of HNS(-) and HSN(-) together with their parent neutral triatomic molecules. Computations were performed using highly accurate ab initio methods with a large basis set. One-dimensional cuts of the full-dimensional potential energy surfaces (PESs) along the interatomic distances and bending angle are presented for each isomer. Results show that the ground anionic states are stable with respect to the electron detachment process and that the long range parts of the PESs correlating to the SH(-) + N, SN(-) + H, SN + H(-), NH + S(-), and NH(-) + S are bound. In addition, we predict the existence of long-lived weakly bound anionic complexes that can be formed after cold collisions between SN(-) and H or SH(-) and N. The implications for the reactivity of these species are discussed; specifically, it is shown that the reactions involving SH(-), SN(-), and NH(-) lead either to the formation of HNS(-) or HSN(-) in their electronic ground states or to autodetachment processes. Thus, providing an explanation for why the anions, SH(-), SN(-), and NH(-), have limiting detectability in astrophysical media despite the observation of their corresponding neutral species. In a biological context, we suggest that HSN(-) and HNS(-) should be incorporated into H2S-assisted heme-catalyzed reduction mechanism of nitrites in vivo.

The electronic structure of vanadium monochloride cation (VCl(+)): tackling the complexities of transition metal species.

Six electronic states (X (4)Σ(-), A (4)Π, B (4)Δ, (2)Φ, (2)Δ, (2)Σ(+)) of the vanadium monochloride cation (VCl(+)) are described using large basis set coupled cluster theory. For the two lowest quartet states (X (4)Σ(-) and A (4)Π), a focal point analysis (FPA) approach was used that conjoined a correlation-consistent family of basis sets up to aug-cc-pwCV5Z-DK with high-order coupled cluster theory through pentuple (CCSDTQP) excitations. FPA adiabatic excitation energies (T0) and spectroscopic constants (re, r0, Be, B0, D¯e, He, ωe, v0, αe, ωexe) were extrapolated to the valence complete basis set Douglas-Kroll (DK) aug-cc-pV∞Z-DK CCSDT level of theory, and additional treatments accounted for higher-order valence electron correlation, core correlation, and spin-orbit coupling. Due to the delicate interplay between dynamical and static electronic correlation, single reference coupled cluster theory is able to provide the correct ground electronic state (X (4)Σ(-)), while multireference configuration interaction theory cannot. Perturbations from the first- and second-order spin orbit coupling of low-lying states with quartet spin multiplicity reveal an immensely complex rotational spectrum relative to the isovalent species VO, VS, and TiCl. Computational data on the doublet manifold suggest that the lowest-lying doublet state ((2)Γ) has a Te of ∼11 200 cm(-1). Overall, this study shows that laboratory and theoretical rotational spectroscopists must work more closely in tandem to better understand the bonding and structure of molecules containing transition metals.

Electronic states, conical intersections, and spin-rovibronic spectroscopy of the nitrogen oxide sulfide radical

Highly correlated ab initio methods are used to investigate the lowest electronic states of doublet and quartet spin multiplicities for SNO. One-dimensional cuts of the three-dimensional potential energy surfaces (3D-PESs) of these electronic states along the stretch and bend coordinate are calculated. Several avoided crossings and conical intersections are located for bent and linear configurations. The dynamics on the excited electronic states of SNO are very complex, and suggest that multi-step mechanisms should occur to populate the ground state via radiationless processes or lead to predissociation. In addition, our calculations show that the ground (X̃(2)A(')) and the first excited (Ã(2)A(")(Π)) states of this radical form a linear-bent Renner-Teller system. They correlate to the SNO(1(2)Π) state at linearity. Systematic studies of both components are performed using standard coupled cluster approaches, explicitly correlated coupled cluster technique, and multi-configurational methods in connection with large basis sets. Core-valence and scalar relativistic effects are examined. For both electronic states, the 3D-PESs are mapped in internal coordinates at the RCCSD(T)-F12b∕cc-pVTZ-F12 level. The analytical representations of these potential energy surfaces are incorporated later into perturbative and variational treatments of the nuclear motions. A set of spectroscopic parameters and spin-rovibronic levels calculated variationally are presented. Strong anharmonic resonances are found. These new results allow for the reassignment of earlier experimental IR bands of SNO trapped in cooled argon matrices.

Compound I in horseradish peroxidase enzyme: Magnetic state assessment by quadratric configuration interaction calculations

AbstractQuadratic configuration interaction procedure with single and double electronic excitations (QCISD) has been used, for the first time, to calculate the electronic structure of the Compound I (CpdI), which represents a key intermediate in the catalytic cycle of Horseradish Peroxidase (HRP) enzyme. The QCISD method is applied to lowest quasi‐isoenergetic doublet and quartet spin multiplicity and results compared with density functional theory (DFT/B3LYP) data. This investigation shows that, at present, QCISD is more accurate than DFT‐based approach in discriminating between the two lowest magnetic states of CpdI complex in HRP enzyme. Such a result opens the possibility of theoretically addressing the reaction mechanism leading to CpdI complex in HRP using a correlated wavefunction based approach. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Redox and magnetic switching in 1,3,5-acceptor-substituted benzenes: reversible formation of radical anions, dianions and trianions in doublet, triplet, and quartet spin states

1,3,5-Tris(dicyanovinyl)-substituted benzene 1 can accept one, two, and three electrons stepwise, as shown by (spectro)electrochemical methods. When the corresponding redox stages are attained by K-metal reduction in THF and 2-methyltetrahydrofuran, paramagnetic resonance and optical techniques can identify equilibria between adjacent redox states and different (para)magnetic stages. It can be shown that the dianion can adopt a triplet state whereas the trianion is present in a doublet and quartet spin multiplicity. Similar findings are established for the methoxy-substituted derivative 2. The formation of the different paramagnetic stages is closely connected to the association of the anions with alkali-metal countercations.

Ab Initio Study of the Potential Energy Surfaces for the Reaction C + CH → C2 + H

The 18 lowest potential energy surfaces of C2H have been investigated with the complete active space multiconfigurational self-consistent-field method. We restricted our study to the doublet and quartet spin multiplicities. Twelve surfaces are issued from the ground-state reactants, while the six others are issued from the first excited state of the reactants. The approach of C toward CH shows no barrier for 6 of the 12 surfaces, obviously making the reaction possible at very low temperatures. The study of the potential energy curves along the reactant and product channels shows that the X, A, a, b, and c states of C2 are expected to be populated by the title reaction, even at very low temperatures. Moreover, six new equilibrium structures corresponding to the excited states of C2H are predicted.

Isomerism of Ni cluster anions and its role on photo-detachment spectroscopy

First principles calculations based on density functional theory and the generalized gradient approximation reveal that a Ni dimer anion can exist in two nearly degenerate forms characterized by different bond lengths and spin multiplicities. Similarly Ni 3 − exhibits two isomeric forms — a linear chain and an equilateral triangle but both having the same quartet spin multiplicity and protected by an energy barrier of about 0.8 eV. The existence of these isomers impacts strongly on the interpretation of the experimental photo-detachment spectra.

Photoinduced electron transfer in linked transition metal donor—acceptor complexes

Several complexes of the general type D −LA have been synthesized in which D −L and AL are well characterized transition metal complexes, and the strength of the donoracceptor electronic coupling has been inferred from measurements of: (a) the oscillators strengths of adjacent metal-to-metal charge transfer (MMCT) absorptions; (b) the shifts in half-wave potentials of Ru(NH 3) 5 3+,2+ couples; and (c) the back electron transfer (BET) behavior observed to result from irradiation of the MMCT absorption band. A special emphasis of this work has been complexes containing degenerate pairs of acceptors (or donors). In a typical complex, a ruthenium(II) center functions as the donor and the acceptor may be of any of several substitution inert metal complexes (Ru 3+, Co 3+, Rh 3+ or Cr 3+). When L1,2-bis(2,2′-bipyridyl-4-yl)ethane, electronic coupling is weak and BET in the photogenerated Ru IIILCo II intermediate is non-adiabatic and nearly identical to that of the outer-sphere Ru(bpy) 3 2+/Co(bpy) 3 3+ (bpy, 2,2′-bipyridine) couple, indicating no special role for the aliphatic linker. When LCN −, the donor—acceptor electronic coupling is very strong and gives rise to intense MMCT absorption bands and to substantial shifts in the D/D − and A/A − electrochemical half-wave potentials. The electronic coupling inferred from the electrochemical shifts is consistently found to be much larger than that inferred from a simple pertubational interpretation of the MMCT absorption bands when the complex contains degenerate acceptors (or donors). Possible origins of this effect are discussed. Picosecond flash photolysis experiments indicate that the electron transfer intermediates (DCNA −) are much longer lived when A is a cobalt(III) complex than when A is a ruthenium(III) complex. The lifetime of the intermediate is further increased when the lowest energy electronic configuration of the cobalt(II) center has quartet spin multiplicity. The kinetic data imply that the electronic coupling matrix element ( H kin DA) appropriate for the BET process in an order of magnitude smaller than the matrix element ( H op DA inferred from MMCT spectroscopy. It is proposed that this is simple symmetry effect: the dπ—dσ electronic transitions are “ x, y-allowed”, while A −-to-D BET is a “ z-allowed” process. Such considerations suggest that orbital symmetries play an important role in this strongly coupled limit.

Helium and neon photoelectron satellites at threshold.

Photoionization of helium and neon to excited satellite states, ${\mathrm{He}}^{+}$ nl and ${\mathrm{Ne}}^{+}$ 1${s}^{2}$2${s}^{2}$2${p}^{4}$nl, was studied with synchrotron radiation and threshold electron analysis. Photoelectron satellites have been directly measured at threshold for the first time to our knowledge. The relative satellite cross sections were determined over the kinetic energy range from 0 to 1 eV. The angular distributions were also evaluated close to threshold. Strong correlation effects were observed in two cases. For He near threshold, the angular-distribution asymmetry parameter \ensuremath{\beta} is near zero for the n=2 satellite and is increasingly negative for the higher-n satellites, in agreement with the theoretical prediction of Greene. In the threshold photoelectron spectrum of Ne, many final states are present, some with quartet spin multiplicity and others with high-L values.