Pi State Research Articles

Dependence of elastic depolarization cross sections on the potential: OH(X Π2)-Ar and NO(X Π2)-Ar

Elastic tensor and depolarization cross sections are computed for the collision of two exemplary diatomic molecules with (2)Pi electronic ground states-OH and NO-with argon. The interaction of a diatomic molecule in a Pi state with a spherical collision partner must be described by two potential energy surfaces (PESs), corresponding to the two asymptotically degenerate electronic states, of A(') and A(") symmetry. Quantum scattering calculations are most naturally based on the average (V(sum)) and half-difference (V(dif)) of these two PESs. When V(dif) is neglected, the OH(X (2)Pi)-Ar depolarization cross sections are found to be significantly reduced in magnitude, while the NO(X (2)Pi)-Ar cross sections are relatively unaffected. In addition, treating the molecules as closed-shell (1)Sigma(+) species with a corresponding rotational level structure and using (V(sum)) to model the PES, we predict depolarization cross sections which differ significantly from those based on full inclusion of the electronic degeneracy and fine structure of these (2)Pi molecules. This indicates that any single-PES-based simulation of the collisional depolarization of these two molecules would be subject to significant error.

Photodissociation Spectroscopy and Dissociation Dynamics of TiO+(CO2)

TiO(+)(CO(2)) is produced by reaction of laser-ablated titanium atoms with CO(2) and subsequent clustering, supersonically cooled, and its electronic spectroscopy was characterized by photofragment spectroscopy, monitoring loss of CO(2). The photodissociation spectrum consists of a vibrationally resolved band in the visible, with extensive progressions in the covalent Ti-O stretch (952 cm(-1) vibrational frequency and 5 cm(-1) anharmonicity) and in the TiO(+)-CO(2) stretch (186 cm(-1)) and rock (45 cm(-1)). The band origin is at 13 918 cm(-1), assigned using titanium isotope shifts, and the spectrum extends to 17 350 cm(-1). The excited-state lifetime decreases dramatically with increasing internal energy, from 1100 ns for the lowest energy band (v'(TiO) = 0) to <50 ns for v'(TiO) = 3. The long photodissociation lifetime substantially reduces the photodissociation quantum yield at low energy, likely due to competition with fluorescence. The fluorescence rate is calculated to be k(fl) = 7.5 x 10(5) s(-1), based on the measured excited-state lifetimes and relative band intensities. This corresponds to an integrated oscillator strength of f = 0.0056. Electronic structure calculations help to assign the spectrum of TiO(+)(CO(2)) and predict allowed electronic transitions of TiO(+) in the visible, which have not been previously measured. Time-dependent density functional calculations predict that the observed transition is due to B, (2)Pi <-- X, (2)Delta in the TiO(+) chromophore and that binding to CO(2) red shifts the TiO(+) transition by 1508 cm(-1) and lowers the Ti-O stretch frequency by 16 cm(-1). Combining the computational and experimental results, the (2)Pi state of TiO(+) is predicted to lie at T(0) = 15 426 +/- 200 cm(-1), with frequency omega(e) = 968 +/- 5 cm(-1) and anharmonicity omega(e)x(e) = 5 cm(-1). The calculations also predict that there is only one low-lying (2)Sigma state of TiO(+), contrary to conclusions derived from photoelectron spectroscopy of TiO. Prospects for astronomical observation of TiO(+) via the (2)Pi-(2)Delta transition are also discussed.

The electronic spectrum of the fluoroborane free radical. II. Analysis of laser-induced fluorescence and single vibronic level emission spectra

Subsequent to our spectroscopic detection of the HBX (X=F, Cl, Br) free radicals (S.-G. He, F. X. Sunahori, and D. J. Clouthier, J. Am. Chem. Soc. 127, 10814 (2005)), the electronic spectrum of the A (2)A(")Pi-X (2)A(') system of the fluoroborane (HBF) radical in the 600-745 nm region has been studied in detail using the pulsed discharge jet technique. The band system involves a linear-bent transition between the two Renner-Teller components of what would be a (2)Pi state at linearity. Using the results of our theoretical study of the ground and excited state vibrational energy levels and (11)B-(10)B isotope shifts (see the companion paper), the vibrational quantum numbers of the bands in the laser-induced fluorescence (LIF) spectra have been assigned. Rotational and vibrational analyses of the LIF and wavelength resolved emission spectra have been carried out, from which the linear excited state and the bent ground state equilibrium configurations have been confirmed. The ground state molecular geometry of HBF has been determined as r(0)(BH)=1.214(2) A, r(0)(BF)=1.303 4(5) A, and theta=120.7(1) degrees. Based on high-level ab initio calculations and symmetry considerations, predissociation of the excited state into H((2)S)+BF((1)Sigma(+)) on the ground state potential energy surface is identified as the cause of the breaking off of fluorescence in the LIF spectra.

The electronic spectrum of the fluoroborane free radical. I. Theoretical calculation of the vibronic energy levels of the ground and first excited electronic states

The fluoroborane (HBF) free radical has a large vibronic interaction which splits the orbitally degenerate (2)Pi state in the linear configuration into two separate electronic states, one strongly bent and one linear. The observed vibrational structure of the electronic transition between the Renner-Teller pair of states is very complex. As an aid to understanding the spectrum, the vibronic energy levels of the ground and first excited states have been calculated from high-level ab initio potential energy surfaces using a variational method. The vibrational frequencies and anharmonicities have been derived from these energy levels and the boron and hydrogen isotope shifts have been predicted. Although the ground state energy levels are for the most part well behaved, the excited state levels show substantial Renner-Teller mixing with nearby ground vibrational states. The calculations in the present work have been successfully used in the companion paper to make vibrational assignments of the laser-induced fluorescence spectra of HBF and DBF.

The Cl + H2 → HCl + H Reaction Induced by IR + UV Irradiation of Cl2 in Solid para-H2: Quantum Model Simulation

Recent experimental investigations by the group of D. T. Anderson (Kettwich, S. C.; Raston, P. L.; Anderson, D. T. J. Phys. Chem. A 2009, 113, DOI 10.1021/jp811206a) show that the reaction Cl + H(2) --> HCl + H in the para-H(2) crystal can be induced by infrared (IR) + ultraviolet (UV) coirradiations causing vibrational pre-excitation of the molecular reactant, H(2)(v=1), and generation of the atomic reactant, Cl((2)P(3/2)), by near-resonant photodissociation of a matrix-isolated Cl(2) molecule in the C (1)Pi(u) state, respectively. The corresponding reaction probability P(v=1) for the reactants Cl + H(2)(v=1) is approximately 0.15; this is approximately 25 times larger than P(v=0) for Cl + H(2)(v=0) (as initiated by pure UV irradiation). We present a simple three-step quantum model which accounts for some important parts of the experimental results and allows predictions for other scenarios, for example, UV photodissociation of the Cl(2) molecule by a laser pulse. The first step, vibrational pre-excitation of H(2), yields the molecular initial state which is described using the Einstein model of the para-H(2) crystal. The second step, photodissociation of Cl(2), generates the Cl((2)P(3/2)) atom approaching H(2)(v=1). In the third step, Cl reacts with H(2)(v=1) much more efficiently than with H(2)(v=0) close to threshold. The ultrashort time domains (approximately 100 fs) of steps 2 plus 3 support one- and then two-dimensional models of photodissociation of Cl(2) by short laser pulses and of the subsequent reaction of the system Cl-H-H embedded in frozen environments. The widths of the corresponding wave function describing the translational motion of the reactants is revealed as a significant parameter which is determined not only by the duration of the laser pulse but, even more importantly, by the width of the Gaussian-type distribution of the center of mass of the H(2) molecule in its Einstein cell. As a consequence, the resulting P(v) are quite robust versus variations of the UV pulse durations, allowing extrapolations to continuous wave irradiation. Quantum dynamics simulations of the reaction reveal that the experimental results are due to energetic and dynamical effects.

Dynamics of OH Formation in the Photodissociation of o-Nitrobenzoic Acid at 295 and 355 nm

Photodissociation dynamics of o-nitrobenzoic acid at 295 and 355 nm is studied by probing the nascent OH photoproduct employing the single-photon laser-induced fluorescence technique. At both of the photolysis wavelengths, the OH fragments are found to be vibrationally cold but have different rotational state distributions. Upon photolysis at 295 nm, the relative population of OH in different rotational states does not follow the Boltzmann equilibrium distribution, whereas upon photolysis at 355 nm, a Boltzmann distribution is observed with a rotational temperature of 1010 +/- 100 K. Between the two spin-orbit states, (2)Pi(3/2) and (2)Pi(1/2), the former is found to be preferentially populated, and the distribution of the Pi(A') state for the Lambda-doublet is dominant at both of the wavelengths studied. Several possible dissociation pathways of o-nitrobenzoic acid leading to formation of the OH fragment are investigated computationally. On the basis of the theoretical and experimental studies, a possible mechanism of OH formation from the photodissociation of o-nitrobenzoic acid at 295 and 355 nm is proposed.

Potential energy curves and interpretation of electronic spectrum of the rhodium monoxide

Potential energy curves of 17 electronic states of rhodium monoxide (RhO) are calculated by multireference configuration interaction with single and double excitations (MRCISD). The ground state of RhO is determined to be a (4)Sigma(-) state with equilibrium bond length of 1.710 A and harmonic vibrational frequency of 825 cm(-1) at the MRCISD level of theory. It dissociates into Rh((4)F)+O((3)P) with a dissociation energy of 3.77/4.26 eV (MRCISD/MRCISD+Q), which is in agreement with the experimental value of 4.19+/-0.43 eV. Two low-lying excited states a (2)Sigma(-) and b(2)Pi are located at 4152 and 7154 cm(-1) above the ground state. The b(2)Pi with the adjacent (2)Delta, (4)Delta, and (2)Pi(II) states can be strongly coupled via spin-orbit interaction leading to a large splitting between b (2)Pi(3/2)-b (2)Pi(1/2) states with the value of 2422 cm(-1), which is comparable with the experimental value of 2400 cm(-1). Two higher doublets, c(2)Pi and d(2)Pi, have the same dominant configuration, 10sigma(2)11sigma(2)12sigma(1)5pi(4)6pi(3)2delta(3), and their transitions to the ground state, i.e., c(2)Pi-->(4)Sigma(-) and d(2)Pi-->(4)Sigma(-), correspond to the two visible bands of RhO.

Axial Asymmetry of the Charge- and Spin-Density Distributions in Π States. Molecular Quadrupole Moments and Hyperfine Coupling Constants of CH, NH, OH, CF, LiO, NO, and FO

The axial asymmetry of the charge- and spin-density distributions in Pi states is studied via second-rank traceless tensors P(ii) (ii = xx, yy, zz), namely, quadrupole moments (Theta(ii)), electric field gradients (q(ii)), and magnetic dipolar (T(ii)) hyperfine coupling constants (hfcc's). In linear molecules, it holds that P(xx) does not = P(yy) does not = P(zz) for Pi, but P(xx) = P(yy) does not = P(zz) for Sigma, Delta, Phi,..., states. Thus, traceless P(ii) in Pi states have two independent parameters, P(parallel) = P(zz) is proportional to [r(m)(3 cos2 theta - 1] and deltaP(perpendicular) = |P(xx) - P(yy)| is proportional to [r(m) sin2 theta], with m = 2(Theta(ii)) or -3(q(ii), T(ii)). All linear states have P(parallel) does not = 0, but only Pi states exhibit deltaP(perpendicular) does not = 0, as shown by hfcc's like c = (3/2)T(zz), and d = |T(xx) - T(yy)|, as well as q0 = (-q(zz)) and |q(2)/2| = |q(xx) - q(yy)|. Little is known about Theta(zz) and deltaTheta(perpendicular) = |Theta(xx) - Theta(yy)| in Pi states since most experimental values (gas-phase) are rotational averages, and several theoretical studies have reported Theta(zz) but assumed deltaTheta(perpendicular) = 0. The diatomics studied here have X2Pi(1/2)(pi1) ground states, like CH and NO, or are of type X2Pi(3/2)(pi3), like OH, CF, LiO, and FO. The A3Pi(sigma pi3) state of NH is also included. Our P(parallel) and deltaP(perpendicular) values--calculated at the experimental R(e)'s with the B3LYP/aug-cc-pVQZ method--reproduce well the available literature data. The properties of the CF and FO radicals are not well-known so that our {c, d} and {q0, q2} values should help future experimental studies of their hyperfine spectra. Excluding OH, the complete quadrupole sets {Theta(zz), deltaTheta(perpendicular)} are new for all species discussed here. For comparison purposes, Theta(zz) of a low-lying Sigma state is also calculated for each X2Pi radical.

Structural, electrical, electronic and optical properties of melanin films

We present thick, uniform and rather flat melanin films obtained using spray deposition. The morphology of the films was investigated using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Temperature-dependent electrical resistance of melanin thin films evidenced a semiconductor-like character and a hysteretic behavior linked to an irreversible process of water molecule desorption from the melanin film. X-ray Photoelectron Spectroscopy (XPS) was carried out to analyze the role of the functional groups in the primary and secondary structure of the macromolecule, showing that the contribution of the 5,6-dihydroxyindole-2-carboxylic acid (DHICA) subunit to the molecule is about 35%. Comparison of the optical absorption of the thick (800nm) and thin (80nm) films showed a spectral change when the thickness increases. From in vacuum photoconductivity (PC) measured at controlled temperatures, we suggest that the melanin films exhibit a possible charge transport mechanism by means of delocalized pi states along the stacked planar secondary structure.

Interaction of Processive Motor Proteins with ATP analogue Having Syn Conformation with respect to the Adenine-ribose bond

It is known that ATP analogues such as 8-Br-ATP with bulky substitution at the eight position of the adenine ring predominantly assume the syn conformation with respect to the adenine-ribose bond. Previously we have demonstrated that 8-Br-ATP induces intrinsic trp fluorescence enhancement of smooth muscle myosin that reflect the formation of the M∗∗·ADP·Pi state. Moreover, the phosphorylated smooth muscle myosin supported actin translocation using 8-Br-ATP. Contrary, for skeletal muscle myosin, 8-Br-ATP induced neither trp fluorescence enhancement nor actin translocation. Kinesin is also ATP driven motor protein that has strikingly similar structure of motor domain to myosin. In the present study, interaction of kinesin with 8-Br-ATP has been examined. Interestingly, conventional kinesin supported microtubules translocation using 8-Br-ATP. This suggests conventional kinesin adopts the 8-Br-ATP in the normal conformation. However, the sliding velocity was approximately one-fifth of regular ATP. Moreover single molecular measurment using optical tweezers revealed that for kinesin, 8-Br-ATP induced nearly similar force generation with that of ATP. Myosin V is also processive motor protein like kinesin. Interaction of unconventional myosin V with 8-Br-ATP was also analyzed. Myosin V supported actin translocation using 8-Br-ATP. Currently, we are examining single molecular measurment of myosin V in the presence of 8-Br-ATP.

Vibrational relaxation of NO (v = 1–16) with NO, N2O, NO2, He and Ar studied by time-resolved Fourier transform infrared emission

Rates of vibrational quenching of NO (v = 1-16) in collisions with a series of quenching species NO, NO2, N2O, He and Ar have been measured at 295 K. NO (v) was formed both by the O(1D) + N2O reaction and the 193 nm photolysis of NO(2), and time-resolved FTIR emission was used to follow the behaviour of the vibrationally excited species. The trends in quenching rate constants can be explained in terms of V-T transfer, V-V transfer and by the effects of competing processes. He and Ar show trends expected from SSH theory, but with relaxation rates that are considerably higher than those expected from previous studies with closed shell molecules, and the influence of non-adiabatic pathways in the relaxation of the NO 2Pi state is discussed. Relaxation with NO2 shows the influence of resonant energy transfer to the nu3 mode, with rate constants peaking at v = 10. For N2O, relaxation rates show essentially a linear increase with v. A linear increase is expected for the change of the transition moment with v for the harmonic oscillator approximation, and when this is taken into account the "reduced probabilities" (defined as P/v, where P is probability of a gas kinetic collision changing the vibrational level from v to v-1) are approximately independent of the energy lost in the NO molecule. The influence of complex formation far from resonance is invoked in both this and for quenching of low vibrational levels by NO2. Finally, self-quenching shows rates which initially decrease with increasing v, but then show a marked increase, with a minimum value at v = 9. Both V-V and V-T processes are believed to occur. Where previously published data are available, general agreement is observed in this study.

Ultraviolet photodissociation of the SD radical in vibrationally ground and excited states

Ultraviolet (UV) photodissociation dynamics of the SD radical in vibrationally ground and excited states (X (2)Pi(3/2), v'' = 0-5) are investigated in the photolysis wavelength region of 220 to 244 nm using the high-n Rydberg atom time-of-flight (HRTOF) technique. The UV photodissociation dynamics of SD (X (2)Pi(3/2)) from v'' = 0-5 are similar to each other and to that of SH studied previously. The anisotropy parameter of the D-atom product is approximately -1; the spin-orbit branching fractions of the S((3)P(J)) products are essentially constant, with an average S((3)P(2)) : S((3)P(1)) : S((3)P(0)) = 0.51 : 0.37 : 0.12. The UV photolysis of SD is a direct dissociation from the repulsive (2)Sigma(-) state following the perpendicular (2)Sigma(-)-X (2)Pi excitation. The S((3)P(J)) product fine-structure state distributions approach that in the sudden limit dissociation on the single repulsive (2)Sigma(-) curve, but they are also affected by nonadiabatic couplings among the repulsive (4)Sigma(-), (2)Sigma(-), and (4)Pi states. A bond dissociation energy D(0)(S-D) = 29 660 +/- 25 cm(-1) is obtained.

Perturbative calculation of spin-orbit splittings using the equation-of-motion ionization-potential coupled-cluster ansatz

Spin-orbit splittings for (2)Pi states are calculated within coupled-cluster (CC) theory via first-order degenerate perturbation theory. Using the equation-of-motion CC variant for ionization potentials (EOMIP-CC), the two components of the considered (2)Pi state are treated in a balanced way by generating both radical states via annihilation of one electron out of the CC wave function of the corresponding anion. We report on the implementation of the described approach within the CC singles and doubles approximation. To ensure computational efficiency, an atomic mean-field approximation for the spin-orbit integrals is used, resulting in a formulation in terms of one-electron transition-density matrices. Calculations for XH radicals (X=O, S, Se) lead to satisfactory agreement with experiment. For (2)Pi systems that within an EOMIP-CC treatment can only be reached from a triplet reference state (e.g., CF and O(2) (+)) the influence of spin contamination is found to be negligible.

The electronic structure of the two lowest states of CuC

State-of-the-art ab initio quantum mechanical methods and large basis sets are employed for the study of the electronic structure of the first two states of CuC, (4)Sigma(-) and (2)Pi. A one-electron sigma bond state ((4)Sigma(-)) competes with a two-electron sigma-bond state ((2)Pi) for the ground state of the CuC system. The combined effects of core-valence correlation and relativity point to an X-state of (2)Pi symmetry with D(e)=51.9 kcal/mol and r(e)=1.772 A. The (4)Sigma(-) state is predicted to lie 2.1 kcal/mol higher at r(e)=1.787 A.

The Zeeman effect in the (0,0) band of the A Π7−X Σ7+ transition of manganese monohydride, MnH

The Zeeman tuning of the P(1)(0) line (nu=17 568.35 cm(-1)) of the A (7)Pi-X (7)Sigma(+) (0,0) band of manganese monohydride, MnH, has been investigated. The laser induced fluorescence spectrum of a supersonic molecular beam sample was recorded at a resolution of approximately 40 MHz and with field strengths of up to 362.0 mT. The observed spectrum was successfully fitted using a traditional effective Zeeman Hamiltonian to determine an effective magnetic g-factor for the J=2 level of the F(1)-spin component of the A (7)Pi(v=0) state. Spectral predictions of the P(1)(0) line at field strengths used in magnetic trapping experiments are presented.

A coupled-channel model of the Π3u states of N2: Structure and interactions of the 3sσgF3 Π3u and 3pπuG3 Π3u Rydberg states

New and existing spectroscopic data on N(2), obtained using a wide variety of experimental techniques, are interpreted using a coupled-channel Schrodinger-equation (CSE) model of the structure and predissociation dynamics for the interacting Rydberg and valence states of (3)Pi(u) symmetry. As a result, v>0 levels of the 3ppi(u)G(3) (3)Pi(u) Rydberg state are assigned correctly for the first time, leading to the identification of very strong perturbations in the G(3)-state vibrational structure. A four-channel CSE model, which includes the 3ssigma(g)F(3) (3)Pi(u) and 3ppi(u)G(3) (3)Pi(u) Rydberg states and the C(') (3)Pi(u) and C (3)Pi(u) valence states, indicates strong Rydberg-Rydberg coupling between the F(3) and G(3) states, strong Rydberg-valence coupling between the G(3) and C(') states, and weaker coupling between the F(3) and C(') states.

Sign reversal of the spin-orbit constant for the C Π3u state of N2

Ab initio calculations are performed at the multireference configuration-interaction level of theory on the diagonal spin-orbit functions for the lowest non-Rydberg states of (3)Pi(u) symmetry in molecular nitrogen. Spin-orbit constants deduced from the ab initio results confirm the recent suggestion, based on new experimental results, that the C (3)Pi(u) state of N(2), long known to be regular in the region of its potential-energy curve minimum, becomes inverted at higher energies. By removing the effects of the crossing C(') (3)Pi(u) state, it is shown that A(v) for the C state changes sign from positive to negative near v=8, corresponding to a change in principal molecular-orbital configuration from (1sigma(g))(2)(1sigma(u))(2)(2sigma(g))(2)(2sigma(u))(3sigma(g))(2)(1pi(u))(4)(1pi(g)) to (1sigma(g))(2)(1sigma(u))(2)(2sigma(g))(2)(2sigma(u))(2)(3sigma(g))(1pi(u))(3)(1pi(g))(2) at an internuclear distance near 1.4 A.

Actomyosin Interaction: Mechanical and Energetic Properties in Different Nucleotide Binding States

The mechanics of the actomyosin interaction is central in muscle contraction and intracellular trafficking. A better understanding of the events occurring in the actomyosin complex requires the examination of all nucleotide-dependent states and of the energetic features associated with the dynamics of the cross-bridge cycle. The aim of the present study is to estimate the interaction strength between myosin in nucleotide-free, ATP, ADP·Pi and ADP states and actin monomer. The molecular models of the complexes were constructed based on cryo-electron microscopy maps and the interaction properties were estimated by means of a molecular dynamics approach, which simulate the unbinding of the complex applying a virtual spring to the core of myosin protein. Our results suggest that during an ATP hydrolysis cycle the affinity of myosin for actin is modulated by the presence and nature of the nucleotide in the active site of the myosin motor domain. When performing unbinding simulations with a pulling rate of 0.001 nm/ps, the maximum pulling force applied to the myosin during the experiment is about 1nN. Under these conditions the interaction force between myosin and actin monomer decreases from 0.83 nN in the nucleotide-free state to 0.27 nN in the ATP state, and increases to 0.60 nN after ATP hydrolysis and Pi release from the complex (ADP state).

Josephson Effect through an Isotropic Magnetic Molecule

We study the Josephson effect through a quantum dot magnet whose spin is isotropic and which is coupled to the dot electron spin via exchange coupling. We calculate the Andreev levels and the supercurrent and examine the intertwined effect of the exchange coupling, Kondo correlation, and superconductivity. The former suppresses Kondo correlations, which triggers phase transitions from the 0 to the pi state, but strong antiferromagnetic coupling restores the 0 state. The asymmetric phase diagram in the exchange coupling suggests that the coupling sign could be determined in experiments.

Do non-centro-symmetric linear X-Y-X molecules exist? The case for the (I)Π2u state of CuCl2

The potential energy surface of the low-lying excited state (I)(2)Pi(u) of CuCl(2) is constructed by using the ionization potential equation-of-motion coupled-cluster method and also the RASPT2 method with a large active space of 21 electrons in 17 orbitals to improve the results. It is found by the multiconfiguration calculation that this state has a barrier of 53 cm(-1) between two equivalent minima in which the linear molecule has a dipole moment. In our computations artifactual symmetry breaking is carefully avoided. Further refinement, including consideration of interaction between the two excited (2)Pi states, yields a somewhat higher barrier between 100 and 500 cm(-1). The mechanism of formation of the double-minimum potential is explained by the pseudo-Jahn-Teller effect theory. Computed spectroscopic constants are in good agreement with experimental ones.