Bonding patterns of [Ag2-alanine]0,± hybrid complexes and the implementation of Molecular logic gates.

in International Journal of Quantum Chemistry (2010), 110(12), 2237-2246

Electrochemically Driven Sequential Machines: An Implementation of Copper Rotaxanes

in Chemistry : A European Journal (2009), 15(6), 1310-1313

The post-Born-Oppenheimer regime: dynamics of electronic motion in molecules by attosecond few-cycle spectroscopy

in Physica Scripta (2009), 80(4),

Dynamics of electronic motion when the nuclei are clamped is discussed and shown to be always described as a superposition of adiabatic electronic states. These states are stationary when the nuclei are ... [more ▼]

Dynamics of electronic motion when the nuclei are clamped is discussed and shown to be always described as a superposition of adiabatic electronic states. These states are stationary when the nuclei are clamped but their superposition leads to multiply periodic motion where the natural frequencies are the differences in the energies of the adiabatic electronic states. When one or more of the frequencies are low and the atoms are allowed to move, the electronic rearrangement is commensurate with the motion of the nuclei. This is the usual breakdown of the Born-Oppenheimer approximation. But when the electronic frequencies are higher there is an electronic motion before the nuclei move. The motion can be demonstrated through expectation values such as the multipole moments of the charge distribution. Such superposition states will be excited when the laser pulse width in energy exceeds the spacings of the states. For low-lying valence excited or low Rydberg states this requires a femtosecond or shorter laser pulse. Since the carrier frequency has to be comparable to the excitation energy, the required laser pulses must span only a few cycles. [less ▲]

Combined Mossbauer Spectral and Density Functional Theory Determination of the Magnetic Easy-Axis in Two High-Spin Iron(II) 2-Pyrazinecarboxylate Complexes

in Inorganic Chemistry (2009), 48(17), 8173-8179

A combination of density functional theory (DFT) calculations and Mossbauer spectroscopy has been used to determine that the magnetic easy-axis is coincident with its crystallographic c-axis in [Fe(pca)(2 ... [more ▼]

A combination of density functional theory (DFT) calculations and Mossbauer spectroscopy has been used to determine that the magnetic easy-axis is coincident with its crystallographic c-axis in [Fe(pca)(2)(py)(2)]center dot py, where pac is the 2-pyrazinecarboxylate ligand. This easy-axis bisects the approximately axial O-Fe-O coordination axes of molecules adjacent to each other along the b-axis. In {[Fe(pca)(2)(H2O)]center dot H2O}(n) the easy magnetic axis is not coincident with any of its crystallographic axes nor with the Fe-O(water) coordination axis, but is coincident with one of the Fe center dot center dot center dot Fe axes in the crystal structure. The DFT calculations, which use the B3LYP functional and the 6-311 + +G(d,p) basis set, yield s-electron probability densities and electric field gradient tensors for the iron(II) ion that are in excellent agreement with the observed iron-57 Mossbauer spectral isomer shifts and quadrupole interactions. The gas phase results are very similar for calculations based either on the X-ray structures of the two complexes or on their optimized structures; the optimized structures indicate that the iron to ligand bond distances increase in the absence of any solid-state lattice interactions. The results of a normal coordinate vibrational mode analysis of the two optimized structures are compared with the observed infrared spectra. [less ▲]

Computational, Structural, and Mechanistic Analysis of the Electrochemically Driven Pirouetting Motion of a Copper Rotaxane

in Journal of Physical Chemistry B (2009), 113(18), 6219-6229

A mechanism for the electrochemically driven reorganization of a model copper [2]pseudorotaxane is proposed on the basis of density functional theory computations and validated by comparing to ... [more ▼]

A mechanism for the electrochemically driven reorganization of a model copper [2]pseudorotaxane is proposed on the basis of density functional theory computations and validated by comparing to experimental results. We investigate in detail the ligand reorganization around the Cu ion from a 4 to 5 coordination number that follows the conversion of the oxidation state from +1 to +2. It is found that for both the oxidation and the reduction processes the rearrangement proceeds in a concerted fashion via a single transition state. Energy paths involving stable decoordinated-coordinated intermediates are computed to be higher in energy. The cyclic voltammogram simulated using the computed transition theory state rate constants in solvent medium is in good agreement with the experimental voltammogram. Further, we report on the computed concentration change of stable (Cu-4(+), Cu-5(2+)) and metastable species (Cu-4(2+), Cu-5(+)) during single cyclic voltammetry (CV) cycle as a function of the applied voltage or time (the subscripts 4 and 5 refer to the coordination number of the copper center). [less ▲]

Ultrafast vibrational spectroscopy and relaxation in polyatomic molecules: Potential for molecular parallel computing

in Chemical Physics (2008), 347(1-3), 531-545

The feasibility of controlled ultrafast pumping in the mid IR and the probe of the subsequent intramolecular dynamics is illustrated for vibrational excitation of the two metal carbonyls W(CO)(6) and Mn ... [more ▼]

The feasibility of controlled ultrafast pumping in the mid IR and the probe of the subsequent intramolecular dynamics is illustrated for vibrational excitation of the two metal carbonyls W(CO)(6) and Mn(CO)(5)Br in solution. Pumping and probing is performed by short, 130 fs, pulses centered at about 2000 cm(-1). Frequency resolved measurements of the time delayed probe pulse are performed. Measured two dimensional spectra are fitted by a kinetic scheme that models the vibrational dynamics. Fast relaxation is solvent induced with the solvent acting also as a heat bath. The (several) probe signals in the experiment can be thought of as the response of a finite state logic machine. This suggests that the molecular machine can act as an ultrafast (petaHertz) processor. The number of internal (memory) states of the machine is determined by the number of vibrational states in the kinetic scheme that can fit the observed relaxation. The number of outputs of the machine is the number of the several different available probe signals. It is shown that the machine is massively parallel because in each (sub ps) time step it produces an entire vector as an output and that each component of the output vector is, by itself, a transform over the input. Beyond that, the machine can produce a (finite number of) different output vectors in sequential time steps. (c) 2008 Elsevier BN. All rights reserved. [less ▲]

Implementation of simple logic gates on gold-ammonia bonding patterns in different charge states

in Molecular Physics (2008), 106(2-4), 521-530

An approach is proposed for encoding the gold-ammonia bonding patterns on the conformational manifold of the Au-n(Z)-(NH3)(m), Z=0, +/- 1, clusters in order to implement simple unary and binary logic gate ... [more ▼]

An approach is proposed for encoding the gold-ammonia bonding patterns on the conformational manifold of the Au-n(Z)-(NH3)(m), Z=0, +/- 1, clusters in order to implement simple unary and binary logic gate operations. The gold-ammonia bonding patterns are governed by two different basic ingredients: the anchoring Au-N bond and the nonconventional N-H center dot center dot center dot Au hydrogen bond, and exhibit distinct characteristics as the charge of the cluster varies within Z = 0, +/- 1. The conformers of the small cluster, Au-Z-(NH3)(2), and of the nanometric size one, Au-20(Z), are chosen as illustrative examples. The dependence of the bonding patterns on the overall charge state of the cluster allows to implement the logic gates NOT, FAN-OUT, AND, NAND, OR, NOR, INH, XOR, and XNOR and to identify the outputs using an IR readout protocol. [less ▲]

Pump and probe ultrafast electron dynamics in LiH: a computational study

in New Journal of Physics (2008), 10

A time-dependent multiconfiguration method with a large electronic basis set is used to compute the response of all the electrons of LiH to a few-cycle intense pump field followed by a probe pulse. The ... [more ▼]

A time-dependent multiconfiguration method with a large electronic basis set is used to compute the response of all the electrons of LiH to a few-cycle intense pump field followed by a probe pulse. The ultrashort pump pulse excites a coherent superposition of stationary electronic states and, by changing the pump parameters such as intensity, duration, polarization and phase of carrier frequency, one can steer the motion of the electrons. Particular attention is given to the control provided by the polarization and by the phase. For example, a change in polarization is used to select an electronic wave packet that is rotating in a plane perpendicular to the bond or rotation in a plane containing the bond. The electronic wave packet can be probed by a delayed second pulse. This delayed probe pulse is also included in the Hamiltonian with the result that the frequency dispersed probe spectrum can be computed and displayed as a two-dimensional plot. [less ▲]

Principles of design of a set-reset finite state logic nanomachine

in Journal of Applied Physics (2008), 104(4),

Pulsed electrical set and reset inputs are used to simulate the temporal action of a finite state machine in a three terminal configuration for a variety of arrangements. The gate electrode is necessary ... [more ▼]

Pulsed electrical set and reset inputs are used to simulate the temporal action of a finite state machine in a three terminal configuration for a variety of arrangements. The gate electrode is necessary only if it is of interest to tune the tunneling rate and to compensate for background charges. When the output is the current, a source and drain electrodes are required. If the output is determined by measuring charge occupancy, then a single junction suffices. The electron transfer rates are computed from the free energy change for a single electron transfer to or from a quantum dot of size such that only charge quantization matters. For a small enough dot the device could operate at room temperature. An asymmetric configuration of the source and drain favors a longer term time preservation of the memory of the device. An alternative design that operates with the same energetics and kinetic parameters is to pulse the resistance rather than the voltage. (C) 2008 American Institute of Physics. [less ▲]

On the feasibility of an ultrafast purely electronic reorganization in lithium hydride

in Chemical Physics (2007), 338

Excitation of a coherent electronic wave packet, a linear combination of several electronic stationary states, is discussed with a computational example, the A and B low lying excited states of LiH. Such ... [more ▼]

Excitation of a coherent electronic wave packet, a linear combination of several electronic stationary states, is discussed with a computational example, the A and B low lying excited states of LiH. Such linear combination results in the electron executing a rotation-like motion in the xz plane with a period of 4 fs. The excitation needs to be shorter than this purely electronic reorganization time and yet the mean energy of the photon should be relatively low so that the A and B states are coherently accessed. These two requirements limit the wave form of the few cycle pump pulse. (c) 2007 Elsevier B.V. All rights reserved. [less ▲]