A procedure for computing accurate ab initio quartic force fields: Application to HO2+ and H2O

A procedure for the calculation of molecular quartic force fields (QFFs) is proposed and investigated. The goal is to generate highly accurate ab initio QFFs that include many of the so-called "small" effects that are necessary to achieve high accuracy. The small effects investigated in the present study include correlation of the core electrons (core correlation), extrapolation to the one-particle basis set limit, correction for scalar relativistic contributions, correction for higher-order correlation effects, and inclusion of diffuse functions in the one-particle basis set. The procedure is flexible enough to allow for some effects to be computed directly, while others may be added as corrections. A single grid of points is used and is centered about an initial reference geometry that is designed to be as close as possible to the final ab initio equilibrium structure (with all effects included). It is shown that the least-squares fit of the QFF is not compromised by the added corrections, and the balance between elimination of contamination from higher-order force constants while retaining energy differences large enough to yield meaningful quartic force constants is essentially unchanged from the standard procedures we have used for many years. The initial QFF determined from the least-squares fit is transformed to the exact minimum in order to eliminate gradient terms and allow for the use of second-order perturbation theory for evaluation of spectroscopic constants. It is shown that this step has essentially no effect on the quality of the QFF largely because the initial reference structure is, by design, very close to the final ab initio equilibrium structure. The procedure is used to compute an accurate, purely ab initio QFF for the H(2)O molecule, which is used as a benchmark test case. The procedure is then applied to the ground and first excited electronic states of the HO(2) (+) molecular cation. Fundamental vibrational frequencies and spectroscopic constants from these highly accurate QFFs are compared and contrasted with previous experiment and theory. It is concluded that the spectroscopic constants determined for the X(3)A(") and A(1)A(') states of HO(2) (+) are the most accurately available.     

Resonant Raman spectra and first molecular hyperpolarizabilities of strongly charge-transfer molecules

The effect of vibrational structure on the frequency dependence of the first molecular hyperpolarizability of two thiophene-based charge-transfer chromophores is investigated. A time domain formulation is used to express the polarizability. The new expression includes the solvent-induced inhomogeneous distribution of electronic transition frequencies as well as the effect of the motion of solvent molecules that modulates the vibrational and electronic transition frequencies of the nonlinear optical molecule on which the first molecular hyperpolarizability depends. Resonance Raman scattering and one-photon absorption spectra of the chromophores are measured. By simultaneously fitting the experimental one-photon absorption spectrum and Raman cross sections of vibrational lines derived from resonance Raman scattering to a theoretical model, important parameters needed for the calculation of the first molecular hyperpolarizability are obtained. The first molecular hyperpolarizability is calculated as a function of frequency covering both nonresonance and two-photon resonance regions. The calculated result is compared with the measured hyperpolarizability as a function of frequency of the excitation laser. The resonance Raman-based analysis is shown to account reasonably well for the dispersion of the hyperpolarizability of the two charge transfer chromophores.     

Vibrational dynamics of excited electronic states of molecular iodine

Femtosecond degenerate four-wave-mixing spectroscopy following an initial pump laser pulse was used to observe the wave packet dynamics in excited electronic states of gas phase iodine. The focus of the investigation was on the ion pair states belonging to the first tier dissociating into the two ions I-(1S) + I+(3P2). By a proper choice of the wavelengths of the initial pump and degenerate four-wave-mixing pulses, we were able to observe the vibrational dynamics of the B (3)Pi(u) (+) state of molecular iodine as well as the ion pair states accessible from there by a one-photon transition. The method proves to be a valuable tool for exploring higher lying states that cannot be directly accessed from the ground state due to selection rule exclusion or unfavorable Franck-Condon overlap.     

Theoretical and experimental studies of collision-induced electronic energy transfer from v=0-3 of the E(0g +) ion-pair state of Br2: collisions with He and Ar

Collisions of Br(2), prepared in the E(0(g)+) ion-pair (IP) electronic state, with He or Ar result in electronic energy transfer to the D, D', and beta IP states. These events have been examined in experimental and theoretical investigations. Experimentally, analysis of the wavelength resolved emission spectra reveals the distribution of population in the vibrational levels of the final electronic states and the relative efficiencies of He and Ar collisions in promoting a specific electronic energy transfer channel. Theoretically, semiempirical rare gas-Br(2) potential energy surfaces and diabatic couplings are used in quantum scattering calculations of the state-to-state rate constants for electronic energy transfer and distributions of population in the final electronic state vibrational levels. Agreement between theory and experiment is excellent. Comparison of the results with those obtained for similar processes in the IP excited I(2) molecule points to the general importance of Franck-Condon effects in determining vibrational populations, although this effect is more important for He collisions than for Ar collisions.     

Optical properties of current carrying molecular wires

We consider several fundamental optical phenomena involving single molecules in biased metal-molecule-metal junctions. The molecule is represented by its highest occupied and lowest unoccupied molecular orbitals, and the analysis involves the simultaneous consideration of three coupled fluxes: the electronic current through the molecule, energy flow between the molecule and electron-hole excitations in the leads, and the incident and/or emitted photon flux. Using a unified theoretical approach based on the nonequilibrium Green's function method we derive expressions for the absorption line shape (not an observable but a useful reference for considering yields of other optical processes) and for the current induced molecular emission in such junctions. We also consider conditions under which resonance radiation can induce electronic current in an unbiased junction. We find that current driven molecular emission and resonant light induced electronic currents in single molecule junctions can be of observable magnitude under appropriate realizable conditions. In particular, light induced current should be observed in junctions involving molecular bridges that are characterized by strong charge-transfer optical transitions. For observing current induced molecular emission we find that in addition to the familiar need to control the damping of molecular excitations into the metal substrate the phenomenon is also sensitive to the way in which the potential bias is distributed on the junction.     

Star-shaped molecule of MnII4O6 core with an St=10 high-spin state. A theoretical and experimental study with XPS, XMCD, and other magnetic methods

We report a comprehensive study of the electronic and magnetic properties of a star-shaped molecule comprising a MnII4O6 core. One feature of this compound is weak magnetic coupling constants compared to other similar polyoxo compounds. This leads to complicated low-lying magnetic states in which the ground state is not well separated from the upper-lying states, yielding a high-spin molecule with a giant magnetic moment of up to 20 microB/formula unit. We apply X-ray diffraction and magnetometry as well as other X-ray spectroscopic techniques, namely, X-ray photoelectron spectroscopy, X-ray magnetic circular dichroism, and X-ray emission spectroscopy. We compare our experimental results with ab initio electronic band structure calculations as well as the localized electronic structure around the Mn2+ ions with charge-transfer multiplet calculations.     

Charge localization in stacked radical cation DNA base pairs and the benzene dimer studied by self-interaction corrected density-functional theory

The incomplete cancellation of the electron self-interaction can be a serious shortcoming of density-functional theory especially when treating odd-electron systems. In this work, several popular and potentially viable correction schemes are applied in order to characterize the electronic structure of stacked molecular pairs, consisting of a neutral molecule and adjacent radical cation, as a function of separation distance. The unphysical sharing of the positive charge between adjacent molecules separated by 6-7 A is corrected for by applying a new empirical scheme proposed by VandeVondele and Sprik [Phys. Chem. Chem. Phys. 2005, 7, 1363] with a unique choice of parameters. This method is subsequently applied to characterize the electronic structure of two neighboring guanines excised from a canonical Arnott B-DNA structure and will be used in future investigations of certain model DNA fibers.     

On the possibility of quantum beats in the total fluorescence of large molecules

The temporal evolution of the total emission from excited electronic states of a large molecule which corresponds to the statistical limit for radiationless relaxation is considered within the two-discrete-states model. It is shown that quantum beats in total (spectrally unresolved) emission may be revealed when the excited discrete levels differ in their rate constants for radiationless relaxation even if they have equal rate constants for radiative decay.     

The quantitative description of small molecule binding to abiotic polymers

The variation of equilibrium constants for the binding of small molecules to abiotic polymers as a function of structure is quantitatively described by the intermolecular force (IMF) equation and relationships derived from it. Structural variations in both the small molecules and in the polymers were studied. The data were taken from the literature. The results obtained suggest that the IMF equation may be generally useful for modelling the effect of structural variation on polymer properties which depend on the difference in intermolecular forces between initial and final states. They also provide a model for nonspecific binding to biopolymers.     

C60分子的非线性元激发和电子能谱

1 计算方法在本文中我们利用推广的SSH 模型,采用自洽迭代的数值计算方法,得到C_(60)分子基态、激发态的电子能谱和几何结构.这与分子轨道和密度泛函理论的计算结果是极相符的.在研究导电聚合物,特别是trans-polyacetylene 链的电子态时,苏武沛等提出了著名的SSH 模型,从理论上阐明了非线性元激发(孤子、极化子、双极化子)的物理图象.鉴于C_(60)分子与PA 链有某些相似之处,实验证实了C_(60)分子中的每个C 原子,通过三个σ电     

Rydberg atom mediated polar molecule interactions: a tool for molecular-state conditional quantum gates and individual addressability

We study the possibility to use interaction between a polar molecule in the ground electronic and vibrational state and a Rydberg atom to construct two-qubit gates between molecular qubits and to coherently control molecular states. A polar molecule within the electron orbit in a Rydberg atom can either shift the Rydberg state, or form a Rydberg molecule. Both the atomic shift and the Rydberg molecule states depend on the initial internal state of the polar molecule, resulting in molecular state dependent van der Waals or dipole-dipole interaction between Rydberg atoms. Rydberg atoms mediated interaction between polar molecules can be enhanced up to 10(3) times. We describe how the coupling between a polar molecule and a Rydberg atom can be applied to coherently control molecular states, and specifically, to individually address molecules in an optical lattice, and to non-destructively readout molecular qubits.     

Axis-switching in the vibrationless Ã←X̃ transition of the jet-cooled deuterated methyl peroxy radical CD3O2

The jet-cooled high resolution spectrum of the vibrationless A?←X? transition of the deuterated species of the methyl peroxy radical has been recently published in this journal (S. Wu, P. Dupre?, P. Rupper, and T. A. Miller, J. Chem. Phys. 127, 224305 (2007)). The spectrum was analyzed using a rigid-rotor model with quadratic spin-rotation coupling. The analysis was based on the fit of ～350 partially resolved line positions and was quite satisfactory. However, the full simulation of the spectral intensity clearly identifies a lack of ability to reproduce relatively small line clumps ("extra" lines) located between the two main central Q branches. This is indicating of an incomplete initial analysis. In the present paper we reanalyze this electronic transition by considering a reference-frame axis-switching resulting from the nuclear rearrangement associated to the electronic transition (spectra obtained at two different temperatures are considered). The potential energy hypersurfaces of the two electronic states are sufficiently dissimilar to induce changes in the molecule geometry, particularly, the angle COO?, which induces a rotation (～1.7°) of the principal axes of inertia located in the molecule symmetry plane. The present analysis is supported by a global fitting of the spectrum intensity and gives rise to a slightly different set of molecular constants. Attention is paid to the wavefunction symmetry assignment of a non-orthorhombic molecule. Couplings due to the torsion of the methyl group are discussed in the following paper (P. Dupre, J. Chem. Phys. 134, 244309 (2011)).     

Electronic excitation dynamics in multichromophoric systems described via a polaron-representation master equation

We derive a many-site version of the non-Markovian time-convolutionless polaron master equation [Jang et al., J. Chem Phys. 129, 101104 (2008)] to describe electronic excitation dynamics in multichromophoric systems. By treating electronic and vibrational degrees of freedom in a combined frame (polaron frame), this theory is capable of interpolating between weak and strong exciton-phonon coupling and is able to account for initial non-equilibrium bath states and spatially correlated environments. Besides outlining a general expression for the expected value of any electronic system observable in the original frame, we also discuss implications of the Markovian and Secular approximations highlighting that they need not hold in the untransformed frame despite being strictly satisfied in the polaron frame. The key features of the theory are illustrated using as an example a four-site subsystem of the Fenna-Mathews-Olson light-harvesting complex. For a spectral density including a localised mode, we show that oscillations of site populations may only be observed when non-equilibrium bath effects are taken into account. Furthermore, we illustrate how this formalism allows us to identify the electronic and vibrational components of the oscillatory dynamics.     

Molecular structure of proline

The molecular structures of the two lowest-energy conformers of proline, Pro-I and Pro-II, have been characterized by ab initio electronic structure computations. An extensive MP2/6-31G* quartic force field for Pro-I, containing 62,835 unique elements in the internal coordinate space, was computed to account for anharmonic vibrational effects, including total zero-point contributions to isotopomeric rotational constants. New re and improved r0 least-squares structural refinements were performed to determine the heavy-atom framework of Pro-I, based on experimentally measured (A. Lesarri, S. Mata, E. J. Cocinero, S. Blanco, J. C. Lopez, J. L. Alonso, Angew. Chem. 2002, 114, 4867; Angew. Chem. Int. Ed. 2002, 41, 4673) rotational constant sets of nine isotopomers and our ab initio data for structural constraints and zero-point vibrational (ZPV) shifts. Without the ab initio constraints, even the extensive set of empirical rotational constants cannot satisfactorily fix the molecular structure of the most stable conformer of proline, a 17-atom molecule with no symmetry. After imposing the ab initio constraints, excellent agreement between theory and experiment is found for the heavy-atom geometric framework, the root-mean-square (rms) residual of the empirical rotational constant fit being cut in half by adding ZPV corrections. The most significant disparity, about 0.07 A, between the empirical and the best ab initio structures, concerns the r(N...H) distance of the intramolecular hydrogen bond. Some of the experimental quartic centrifugal distortion constants assigned to Pro-II have been corrected based on data obtained from a theoretical force field.     

The theory of surface-enhanced Raman scattering

By considering the molecule and metal to form a conjoined system, we derive an expression for the observed Raman spectrum in surface-enhanced Raman scattering. The metal levels are considered to consist of a continuum with levels filled up to the Fermi level, and empty above, while the molecule has discrete levels filled up to the highest occupied orbital, and empty above that. It is presumed that the Fermi level of the metal lies between the highest filled and the lowest unfilled level of the molecule. The molecule levels are then coupled to the metal continuum both in the filled and unfilled levels, and using the solutions to this problem provided by Fano, we derive an expression for the transition amplitude between the ground stationary state and some excited stationary state of the molecule-metal system. It is shown that three resonances contribute to the overall enhancement; namely, the surface plasmon resonance, the molecular resonances, as well as charge-transfer resonances between the molecule and metal. Furthermore, these resonances are linked by terms in the numerator, which result in SERS selection rules. These linked resonances cannot be separated, accounting for many of the observed SERS phenomena. The molecule-metal coupling is interpreted in terms of a deformation potential which is compared to the Herzberg-Teller vibronic coupling constant. We show that one term in the sum involves coupling between the surface plasmon transition dipole and the molecular transition dipole. They are coupled through the deformation potential connecting to charge-transfer states. Another term is shown to involve coupling between the charge-transfer transition and the molecular transition dipoles. These are coupled by the deformation potential connecting to plasmon resonance states. By applying the selection rules to the cases of dimer and trimer nanoparticles we show that the SERS spectrum can vary considerably with excitation wavelength, depending on which plasmon and/or charge-transfer resonance is excited.     

Ab-initio molecular geometry and normal coordinate analysis of pyrrolidine molecule.

The molecular geometry of pyrrolidine was quantum mechanically calculated using the split valence 6-31G** basis set. Electron correlation energy has been computed employing MP2 method. The molecule showed an envelope form puckered structure with inter-plane angle of 36.4 degrees and has a total energy of -132976.80 kcal mol(-1) of which a -464.86 kcal mol(-1) electron correlation energy. The twist form of the molecule showed a twist angle of 10.2 degrees from planarity and has a total energy of -132976.05 kcal mol(-1) involving -464.097 kcal mol(-1) electron correlation energy. The normal coordinates of the molecule were theoretically analyzed on the basis of the Cs point symmetry of the envelope form. Using initial set of force constants obtained from the ab-initio calculations the fundamental vibrational frequencies were computed. The IR and laser Raman spectra of Pyrrolidine molecules were measured. All the observed vibrational bands including combination bands and overtones were assigned to normal modes with the aid of the potential energy distribution values obtained from normal coordinate calculations. The molecular force field was obtained by refining the initial set of force constants using the least square fit method. The molecular force field was determined by refining the initial set of force constants using the least square fit method instead of using the less accurate scaling factor methods. The determined molecular force field has produced simulated frequencies best match to the observed values. The low frequency molecular out-of-plane deformation modes were observed in both infrared and Raman spectra at 298 and 163 cm(-1). The barrier of ring twisting estimated from the observed ring out-of-plane vibrational mode at 163 cm(-1) was found 3.1 kcal mol(-1).     

Molecular structures of the two most stable conformers of free glycine

The equilibrium molecular structures of the two lowest-energy conformers of glycine, Gly-Ip and Gly-IIn, have been characterized by high-level ab initio electronic structure computations, including all-electron cc-pVTZ CCSD(T) geometry optimizations and 6-31G* MP2 quartic force fields, the latter to account for anharmonic zero-point vibrational effects to isotopologic rotational constants. Based on experimentally measured vibrationally averaged effective rotational constant sets of several isotopologues and our ab initio data for structural constraints and zero-point vibrational shifts, least-squares structural refinements were performed to determine improved Born-Oppenheimer equilibrium (r(e)) structures of Gly-Ip and Gly-IIn. Without the ab initio constraints even the extensive set of empirical rotational constants available for 5 and 10 isotopologues of Gly-Ip and Gly-IIn, respectively, cannot satisfactorily fix their molecular structure. Excellent agreement between theory and experiment is found for the rotational constants of both conformers, the rms residual of the final fits being 7.8 and 51.6 kHz for Gly-Ip and Gly-IIn, respectively. High-level ab initio computations with focal point extrapolations determine the barrier to planarity separating Gly-IIp and Gly-IIn to be 20.5 +/- 5.0 cm(-1). The equilibrium torsion angle tau(NCCO) of Gly-IIn, characterizing the deviation of its heavy-atom framework from planarity, is (11 +/- 2) degrees. Nevertheless, in the ground vibrational state the effective structure of Gly-IIn has a plane of symmetry.