Pi State Research Articles

Ni3d–BNπhybridization at theh−BN∕Ni(111)interface observed with core-level spectroscopies

The electronic structure of h-BN films grown on the Ni(111) surface has been studied as a function of film thickness using the synchrotron radiation based spectroscopic techniques: soft x-ray absorption, core-level photoemission and resonant Auger spectroscopy. A manifestation of the strong orbital hybridization between Ni 3d and h-BN pi states has been consistently observed in all spectra, implying a rather strong interfacial interaction between h-BN and the substrate. In the B 1s and N 1s near-edge x-ray absorption fine structure of both bulk h-BN and a single monolayer adsorbed on Ni(111) we observe spectral structures, which can be interpreted as a manifestation of the interlayer conduction-band states of h-BN.

Measurement of Br photofragment orientation and alignment from HBr photodissociation: production of highly spin-polarized hydrogen atoms.

The orientation and alignment of the (2)P(3/2) and (2)P(1/2) Br photofragments from the photodissociation of HBr is measured at 193 nm in terms of a(q) ((k))(p) parameters, using slice imaging. The A (1)Pi state is excited almost exclusively, and the measured a(q) ((k))(p) parameters and the spin-orbit branching ratio show that the dissociation proceeds predominantly via nonadiabatic transitions to the a (3)Pi and 1 (3)Sigma(+) states. Conservation of angular momentum shows that the electrons of the nascent H atom cofragments (recoiling parallel to the photolysis polarization) are highly spin polarized: about 100% for the Br((2)P(1/2)) channel, and 86% for the Br((2)P(3/2)) channel. A similar analysis is demonstrated for the photodissociation of HCl.

Prediction of new phases of nitrogen at high pressure from first-principles simulations.

A rich variety of competing phases is predicted for nitrogen at accessible pressures, including a new metallic chainlike phase very close in energy to the previously predicted cubic gauche phase, and other phases at slightly higher energies, e.g., one with N2 and N6 units. Large energy barriers between structures can account for recent observations of metastability, and we identify a low barrier transition path from the known epsilon phase to the chainlike metallic phase. In analogy to MgB2, the metal is anisotropic with multiple Fermi surfaces formed from pi and sigma states.

H atom transfer along an ammonia chain: tunneling and mode selectivity in 7-hydroxyquinoline.(NH3)3.

Excitation of the 7-hydroxyquinoline(NH(3))(3) [7HQ(NH(3))(3)] cluster to the S(1) (1)pi pi(*) state results in an O-H-->NH(3) hydrogen atom transfer (HAT) reaction. In order to investigate the entrance channel, the vibronic S(1)<-->S(0) spectra of the 7HQ.(NH(3))(3) and the d(2)-7DQ.(ND(3))(3) clusters have been studied by resonant two-photon ionization, UV-UV depletion and fluorescence techniques, and by ab initio calculations for the ground and excited states. For both isotopomers, the low-frequency part of the S(1)<--S(0) spectra is dominated by ammonia-wire deformation and stretching vibrations. Excitation of overtones or combinations of these modes above a threshold of 200-250 cm(-1) for 7HQ.(NH(3))(3) accelerates the HAT reaction by an order of magnitude or more. The d(2)-7DQ.(ND(3))(3) cluster exhibits a more gradual threshold from 300 to 650 cm(-1). For both isotopomers, intermolecular vibrational states above the threshold exhibit faster HAT rates than the intramolecular vibrations. The reactivity, isotope effects, and mode selectivity are interpreted in terms of H atom tunneling through a barrier along the O-H-->NH(3) coordinate. The barrier results from a conical intersection of the optically excited (1)pi pi(*) state with an optically dark (1)pi sigma(*) state. Excitation of the ammonia-wire stretching modes decreases both the quinoline-O-H...NH(3) distance and the energetic separation between the (1)pi pi(*) and (1)pi sigma(*) states, thereby increasing the H atom tunneling rate. The intramolecular vibrations change the H bond distance and modulate the (1)pi pi(*)<-->(1)pi sigma(*) interaction to a much smaller extent.

The first observation of the rhodium monofluoride molecule: jet-cooled laser spectroscopic studies.

Rhodium monofluoride has been observed and spectroscopically characterized. RhF molecules were produced under jet-cooled conditions in a laser vaporization molecular beam source by the reaction of a laser-vaporized rhodium plasma with SF(6) doped in helium, and studied with laser-induced fluorescence spectroscopy under both medium and high resolution. More than 25 bands have been observed in laser-induced fluorescence between 18,500 and 24,500 cm(-1) and five of these have been recorded at 200 MHz resolution. All bands of appreciable intensity have been rotationally analyzed. The ground electronic levels has Omega=2, which is attributed to an inverted (3)Pi state from the 2 delta(4)6 pi(3)12 sigma(1) electron configuration. The ground level rotational constants are B=0.272 45 cm(-1), D=1.035 x 10(-7) cm(-1). Very small ground level Lambda doublings are evident in the spectrum. Excited states having Omega=1, 2, and 3 have been identified. Dispersed fluorescence spectroscopy from 11 excited levels has been used to locate a large number of low-lying vibronic states within the energy range up to 8,000 cm(-1). A ground state vibrational interval of approximately 575 cm(-1) is suggested.

The role of pi sigma* state in intramolecular electron-transfer dynamics of 4-dimethylaminobenzonitrile and related molecules.

Evidence is presented which indicates that the photoinduced intramolecular charge transfer (ICT) in 4-dimethylaminobenzonitrile proceeds by a new mechanism in which pi sigma(C triple bond N) (*) state is the intermediate of a consecutive process that takes the initially excited pi pi(*) state to the fully charge-separated ICT state. The absence of the ICT-state formation in 4-aminobenzonitrile is attributed to the smaller electron-donor strength of the amino group relative to the dimethylamino group, which hinders the pi sigma(*)-->ICT charge-shift reaction.

Does phosphate release limit the ATPases of soleus myofibrils? Evidence that (A)M· ADP·Pi states predominate on the cross-bridge cycle

The ATPases (+/-Ca2+) of myofibrils from rabbit soleus (a slow muscle) and psoas (a fast muscle) have different Ea: -Ca2+, 78 and 60 kJ/mol and +Ca2+, 155 and 71 kJ/mol, respectively. At physiological temperatures, the two types of myofibrillar ATPase are very similar and yet the mechanical properties of the muscles are different (Candau et al. (2003) Biophys J 85: 3132-3141). Muscle contraction relies on specific interactions of the different chemical states on the myosin head ATPase pathway with the thin filament. An explanation for the Ea data is that different states populate the pathways of the two types of myofibril because the rate limiting steps are different. Here, we put this to the test by a comparison of the transient kinetics of the initial steps of the ATPases of the two types of myofibril at 4 degrees C. We used two methods: rapid flow quench ('cold ATP chase': titration of active sites, ATP binding kinetics, k(cat); 'Pi burst': ATP cleavage kinetics) and fluorescence stopped-flow (MDCC-phosphate binding protein for free Pi; myofibrillar tryptophan fluorescence for myosin head-thin filament detachment and ATP cleavage kinetics). We find that, as with psoas myofibrils, the most populated state on the cross-bridge cycle of soleus myofibrils, whether relaxed or activated, is (A)M.ADP.Pi. We propose a reaction pathway that includes several (A)M.ADP.Pi sub-states that are either 'weak' or 'strong', depending on the mechanical condition.

Perturbation analysis for the rotational spectrum of the NiBr radical in the X2Pi3/2 and A2Delta5/2 states.

The millimeter- and submillimeter-wave spectra of the NiBr radical in the X (2)Pi(3/2) and A (2)Delta(5/2) states were observed by a source-modulated microwave spectrometer. The NiBr radical was generated in a dc glow discharge through the mixture of Br(2) vapor and Ar gas by the sputtering reaction with a Ni cathode. Observed transition frequencies were independently analyzed for both electronic states using a standard polynomial expression of a Hund's case (c) approximation. Anomalous behavior of the effective molecular constants in the X (2)Pi(3/2) state was interpreted as the result of the perturbation between the X (2)Pi(3/2) and A (2)Delta(5/2) states. The deperturbed molecular constants were derived using a simplified supermultiplet Hamiltonian including the interaction terms between the two electronic states.

The diazocarbene (CNN) molecule: characterization of the X 3Sigma- and A 3Pi electronic states.

The ground (X (3)Sigma(-)) and first excited triplet (A (3)Pi) electronic states of diazocarbene (CNN) have been investigated systematically starting from the self-consistent-field theory and proceeding to the coupled cluster with single, double, and full triple excitations (CCSDT) method with a wide range of basis sets. While the linear X (3)Sigma(-) ground state of CNN has a real degenerate bending vibrational frequency, the A (3)Pi state of CNN is subject to the Renner-Teller effect and presents two distinct real vibrational frequencies along the bending coordinate. The bending vibrational frequencies of the A (3)Pi state were evaluated via the equation-of-motion coupled cluster (EOM-CC) techniques. The significant sensitivity to level of theory in predicting the ground-state geometry, harmonic vibrational frequencies, and associated infrared intensities has been attributed to the fact that the reference wave function is strongly perturbed by the excitations of 1pi-->3pi followed by a spin flip. At the highest level of theory with the largest basis set, correlation-consistent polarized valence quadruple zeta (cc-pVQZ) CCSDT, the classical X-A splitting (T(e) value) was predicted to be 68.5 kcal/mol (2.97 eV, 24 000 cm(-1)) and the quantum mechanical splitting (T(0) value) to be 69.7 kcal/mol (3.02 eV, 24 400 cm(-1)), which are in excellent agreement with the experimental T(0) values, 67.5-68.2 kcal/mol (2.93-2.96 eV, 23 600-23 900 cm(-1)). With the EOM-CCSD method the Renner parameter (epsilon) and averaged bending vibrational frequency (omega(2)) for the A (3)Pi state were evaluated to be epsilon=-0.118 and omega(2)=615 cm(-1), respectively. They are in fair agreement with the experimental values of epsilon=-0.07 and nu(2)=525 cm(-1).

Production and state-selective detection of ultracold RbCs molecules.

Using resonance-enhanced two-photon ionization, we detect ultracold, metastable RbCs molecules formed in their lowest triplet state a (3)Sigma(+) via photoassociation in a laser-cooled mixture of 85Rb and 133Cs atoms. We obtain extensive bound-bound excitation spectra of these molecules, which provide detailed information about their vibrational distribution, as well as spectroscopic data on several RbCs molecular states including a (3)Sigma(+), (2) (3)Sigma(+), and (1) (1)Pi. Analysis of this data allows us to predict strong transitions from observed levels to the absolute vibronic ground state of RbCs, potentially allowing the production of stable, ultracold polar molecules at rates in excess of 10(6) s(-1).

The 1 (2)A(1), 1 (2)B(2), and 1 (2)A(2) states of the SO(2) (+) ion studied using multiconfiguration second-order perturbation theory.

The 1 (2)A(1), 1 (2)B(2), and 1 (2)A(2) electronic states of the SO(2) (+) ion have been studied using multiconfiguration second-order perturbation theory (CASPT2) and two contracted atomic natural orbital basis sets, S[6s4p3d1f]/O[5s3p2d1f] (ANO-L) and S[4s3p2d]/O[3s2p1d] (ANO-S), and the three states were considered to correspond to the observed X, B, and A states, respectively, in the previous experimental and theoretical studies. Based on the CASPT2/ANO-L adiabatic excitation energy calculations, the X, A, and B states of SO(2) (+) are assigned to 1 (2)A(1), 1 (2)B(2), and 1 (2)A(2), respectively, and our assignments of the A and B states are contrary to the previous assignments (A to (2)A(2) and B to (2)B(2)). The CASPT2/ANO-L energetic calculations also indicate that the 1 (2)A(1), 1 (2)B(2), and 1 (2)A(2) states are, respectively, the ground, first excited, and second excited states at the ground-state (1 (2)A(1)) geometry of the ion and at the geometry of the ground-state SO(2) molecule. Based on the CASPT2/ANO-L results for the geometries, we realize that the experimental geometries (determined by assuming the bond lengths to be the same as the neutral ground state of SO(2)) were not accurate. The CASPT2/ANO-S calculations for the potential energy curves as functions of the OSO angle confirm that the 1 (2)B(2) and 1 (2)A(2) states are the results of the Renner-Teller effect in the degenerate (2)Pi(g) state at the linear geometry, and it is clearly shown that the 1 (2)B(2) curve, as the lower component of the Renner splitting, lies below the 1 (2)A(2) curve. The UB3LYP/cc-pVTZ adiabatic excitation energy calculations support the assignments (A to (2)B(2) and B to (2)A(2)) based on the CASPT2/ANO-L calculations.

Precision spectroscopy of pionic 1s states of Sn nuclei and evidence for partial restoration of chiral symmetry in the nuclear medium.

Deeply bound 1s states of pi(-) in (115,119,123)Sn were preferentially observed using the Sn(d,3He) pion-transfer reaction under the recoil-free condition. The 1s binding energies and widths were precisely determined and were used to deduce the isovector parameter of the s-wave pion-nucleus potential to be b1=-(0.115+/-0.007)m(-1)(pi). The observed enhancement of |b(1)| over the free piN value (b(free)1/b1=0.78+/-0.05) indicates a reduction of the chiral order parameter, f*pi(rho)2/f2pi approximately 0.64, at the normal nuclear density, rho=rho(0).

Electronic structure and spectroscopy of the ground and excited states of the HMgO and HMgS radicals

Ab initio RCCSD(T) and MRCI calculations have been carried out for the stable isomer forms of magnesium monohydroxide, HMgO and monohydro sulfide, HMgS. For both molecular systems, it is found that the electronic ground state is a (2)Pi(i) state with linear/linear Renner-Teller components. At the equilibrium geometry of the ground states, the spin-orbit constants A(SO) have been calculated -123 cm(-1) for HMgO and -312 cm(-1) for HMgS. Along the isomerisation path towards the more stable structures MgOH and MgSH, the (2)A' component of the Renner Teller pair correlates with the ground state and the (2)A component with an excited state. Using RCCSD(T) potential energy functions and a variational Renner-Teller approach, rovibronic levels of the X(2)Pi state of both systems have been evaluated for J = 1/2 and 3/2. Strong Fermi resonances are found in HMgO, but not in HMgS. The first electronically excited (2)Sigma(divided by) states are calculated (T-e value at RCCSD(T) level) to lie 8041 cm(-1) above the ground state for HMgS and 4935 cm(-1) for HMgO. (C) 2003 Elsevier B.V. All rights reserved.

Evidence for two types of low-energy charge transfer excitations in Sr2CuO3.

A comparative analysis of electron energy-loss spectroscopy (EELS) spectra for the 1D insulating cuprate Sr2CuO3 with transferred momentum q--> axially and radially to the chain axis allows one to elucidate the structure of the charge transfer gap in in-chain response. It is determined by the superposition of two types of excitations with different magnitudes of dispersion. The low-energy response with q--> radially to the chain direction, but yet within the plane of CuO4 plaquettes, exhibits also a dispersionless peak near 2 eV. The theoretical simulation of the EELS data using exact diagonalizations of an appropriate extended Hubbard Hamiltonian for relevant clusters requires the explicit consideration of low-lying oxygen 2p pi states within the CuO4 plaquette plane beyond the standard pd sigma extended Hubbard model widely used for cuprates with corner-shared CuO4 plaquettes.

Femtochemistry of trans-azomethane: a combined experimental and theoretical study.

The dissociation dynamics of trans-azomethane upon excitation to the S,(n, pi) state with a total energy of 93 kcalmol-1 is investigated using femtosecond-resolved mass spectrometry in a molecular beam. The transient signal shows an opposite pump-probe excitation feature for the UV (307 nm) and the visible (615 nm) pulses at the perpendicular polarization in comparison with the signal obtained at the parallel polarization: The one-photon symmetry-forbidden process excited by the UV pulse is dominant at the perpendicular polarization, whereas the two-photon symmetry-allowed process initiated by the visible pulse prevails at the parallel polarization. At the perpendicular polarization, we found that the two C-N bonds of the molecule break in a stepwise manner, that is, the first C-N bond breaks in approximately 70 fs followed by the second one in approximately 100 fs, with the intermediate characterized. At the parallel polarization, the first C-N bond cleavage was found to occur in 100 fs with the intensity of the symmetry-allowed transition being one order of magnitude greater than the intensity of the symmetry-forbidden transition at the perpendicular polarization. Theoretical calculations using time-dependent density functional theory (TDDFT) and the complete active space self-consistent field (CASSCF) method have been carried out to characterize the potential energy surface for the ground state, the low-lying excited states, and the cationic ground state at various levels of theory. Combining the experimental and theoretical results, we identified the elementary steps in the mechanism. The initial driving force of the ultrafast bond-breaking process of trans-azomethane (at the perpendicular polarization) is due to the CNNC torsional motion initiated by the vibronic coupling through an intensity-borrowing mechanism for the symmetry-forbidden n-pi transition. Following this torsional motion and the associated molecular symmetry breaking, an S0/S1 conical intersection (CI) can be reached at a torsional angle of 93.1 degrees (predicted at the CASSCF(8.7)/cc-pVDZ level of theory). Funneling through the S0/S1, CI could activate the asymmetric C-N stretching motion, which is the key motion for the consecutive C-N bond breakages on the femtosecond time scale.

Variety of the molecular conformation in Peptide nanorings and nanotubes.

Possible molecular conformations in peptide nanorings and nanotubes were theoretically investigated by a mathematical conformation analysis as well as ab initio Hartree-Fock calculations. The mathematical analysis predicts not only the conventional nanorings having an extended-type (E-type) backbone (trans zigzag) but also the novel ones having bound-type (B-type) backbones with a smaller internal diameter. Ab initio calculations for the amino acid substitution reveal that all 20 encoded residues can form both types of the above nanorings as a local minimum. However, the energetically stable type is determined in accordance with the kind of the replaced side chains. Moreover, the present work theoretically reveals that both types of nanorings stack to form nanotubes through inter-ring hydrogen bonds, i.e., larger E-type nanotubes and smaller B-type nanotubes. Electronically, the HOMO and LUMO states of the nanoring and nanotube backbones are formed by the in-plane pi state. The replacement by the appropriate residues is furthermore predicted to intrude additional levels in the energy gap and to form the frontier states localized at the side chains.

Electronic spectra and structures of d2 molybdenum-oxo complexes. Effects of structural distortions on orbital energies, two-electron terms, and the mixing of singlet and triplet states.

Molybdenum-oxo ions of the type [Mo(IV)OL(4)Cl](+) (L = CNBu(t), PMe(3), (1)/(2)Me(2)PCH(2)CH(2)PMe(2)) have been studied by X-ray crystallography, vibrational spectroscopy, and polarized single-crystal electronic absorption spectroscopy (300 and ca. 20 K) in order to investigate the effects of the ancillary ligand geometry on the properties of the MotriplebondO bond. The idealized point symmetries of the [Mo(IV)OL(4)Cl](+) ions were established by X-ray crystallographic studies of the salts [MoO(CNBu(t)())(4)Cl][BPh(4)] (C(4)(v)), [MoO(dmpe)(2)Cl]Cl.5H(2)O (C(2)(v)), and [MoO(PMe(3))(4)Cl][PF(6)] (C(2)(v)()); the lower symmetries of the phosphine derivatives are the result of the steric properties of the phosphine ligands. The Motbd1;O stretching frequencies of these ions (948-959 cm(-)(1)) are essentially insensitive to the nature and geometry of the equatorial ligands. In contrast, the electronic absorption bands arising from the nominal d(xy)() --> d(xz), d(yz) (n --> pi(MoO)) ligand-field transition exhibit a large dependence on the geometry of the equatorial ligands. Specifically, the electronic spectrum of [MoO(CNBu(t)())(4)Cl](+) exhibits a single (1)[n --> pi(xz)(,)(yz)] band, whereas the spectra of both [MoO(dmpe)(2)Cl](+) and [MoO(PMe(3))(4)Cl](+) reveal separate (1)[n --> pi(xz)] and (1)[n --> pi(yz)] bands. A general theoretical model of the n --> pi state energies of structurally distorted d(2) M(triplebondE)L(4)X chromophores is developed in order to interpret the electronic spectra of the phosphine derivatives. Analysis of the n --> pi transition energies using this model indicates that the d(xz) and d(yz) pi(MotriplebondO) orbitals are nondegenerate for the C(2)(v)-symmetry ions and the n --> pi(xz) and n --> pi(yz) excited states are characterized by different two-electron terms. These effects lead to a significant redistribution of intensity between certain spin-allowed and spin-forbidden absorption bands. The applicability of this model to the excited states produced by delta --> pi and pi --> delta symmetry electronic transitions of other chromophores is discussed.

Direct observation of the transition from the conventional superconducting state to the pi state in a controllable Josephson junction.

We measure the full supercurrent-phase relation of a controllable pi junction around the transition from the conventional 0 state to the pi state. We show that around the transition the Josephson supercurrent-phase relation changes from I(sc) approximately I(c)sin((phi) to I(sc) approximately I(c)sin((2phi). This implies that, around the transition, two minima in the junction free energy exist, one at phi=0 and one at phi=pi, whereas only one minimum exists at the 0 state (at phi=0) and at the pi state (at phi=pi). Theoretical calculations based on the quasiclassical theory are in good agreement with the observed behavior.

Hydrogen atom tunneling in triplet o-methylbenzocycloalkanones: effects of structure on reaction geometry and excited state configuration.

The rates of phosphorescence decay of 4,7-dimethylindanone (2), 6,9-dimethylbenzosuberone (3), and several related compounds have been analyzed between 4 and 100 K to determine the contributions of intramolecular hydrogen atom tunneling from the o-methyl group to the excited state carbonyl oxygen. Changes in the benzocycloalkanone ring size from five to seven not only affect the geometry at the reaction center, but they also affect the electronic configuration of the triplet excited state in a significant manner. While the triplet state of 5,8-dimethyltetralone (1) in nonpolar glasses can be clearly described as having a predominant n,pi configuration, compounds 2 and 3 have a significantly larger contribution of the less reactive pi,pi state. 4,7-Dimethylindanone (2) is stable under cryogenic conditions and in solution at ambient temperature. In contrast, triplet lifetimes and product analysis indicate that 6,9-dimethylbenzosuberone (3) reacts by quantum mechanical tunneling at temperatures as low as 4 K. A surprisingly small isotope effect k(H)/k(D) approximately 1.1 between 4 and 50 K increases steadily up to k(H)/k(D) approximately 5.1 at 100 K. This unusual observation is interpreted in terms of a vibrationally activated quantum mechanical tunneling process with hydrogen atom transfer at the lowest temperatures being mediated by zero-point-energy reaction-promoting skeletal motions. Results presented here indicate that the combined effects of increasing pi,pi character and unfavorable reaction geometry contribute to the diminished reactivity of o-methyl ketones 2 and 3 as compared to those of tetralone 1.

Ultrafast solvent-induced spin-flip and nonadiabatic coupling: ClF in argon solids.

Femtosecond pump-probe spectra show direct evidence for ultrafast solvent-induced spin flip in photodissociation-recombination events of ClF, a light diatomic molecule, for which the spin-orbit coupling is weak. The bound triplet states ((3)Pi) of ClF are probed and the dynamics for excitation to the singlet state ((1)Pi(1)) is compared with excitation to the triplet state B((3)Pi(0)). The population initially excited to the singlet state (1)Pi(1) is transferred to the bound triplet states (3)Pi within tau(f)=0.5 ps. Oscillations in the spectra indicate wave packet dynamics with the triplet state period of 300 to 400 fs in both cases. According to simulations of F(2)/Ar, most of the initially excited singlet state population is converted to repulsive and weakly bound triplet states within approximately 60 fs. In the first ps, 40% of the triplet population accumulates in the weakly bound (3)Pi states, in good accord with the experiment.