A multi-sheeted three-dimensional potential-energy surface for the H-atom photodissociation of phenol

Faraday Discussions
Extending the previous work of Lan et al. [J. Chem. Phys., 122, 224315 (2005)], a multi-state potential model for the H atom photodissociation is presented. All three “disappearing coordinates” of the departing H atom have been considered. Ab initio CASSCF computations have been carried out for the linear COH geometry of C2v symmetry, and for several COH angles with the OH group in the ring plane and also perpendicular to the ring plane. By keeping the C6H5O fragment frozen in a C2v-constrained geometry throughout, we have been able to apply symmetry-based simplifications in the constructions of a diabatic model. This model is able to capture the overall trends of twelve adiabats at both torsional limits for a wide range of COH bend angles.Read more

Theoretical Investigation of the Infrared Spectra of the H5+ and D5+ Cations

The Journal of Physical Chemistry A
Reduced dimensional quantum dynamics calculations of the infrared spectrum of the H5+ and D5+ clusters are reported in both low, 300–2200 cm–1, and high, 2400–4500 cm–1, energy regions. The proposed four-dimensional quantum model describes the motion of the proton between the two vibrating hydrogen molecules. The simulations are performed using time-dependent and time-independent approaches within the multiconfiguration time-dependent Hartree method. Propagation of the wavepackets includes an absorbing scheme to deal with vibrational dissociating states, and to assign the different spectral lines, block improved relaxation computations are performed for both bound and predissociative vibrational states of the systems. The reported computations make use of an analytical ab initio-based potential energy, and “on the fly” DFT dipole moment surfaces. The predominant features in the spectra are assigned to the...  Read more

Microwave Structure for the Propiolic Acid–Formic Acid Complex

The Journal of Physical Chemistry A
New microwave spectra were measured to obtain rotational constants and centrifugal distortion constants for the DCCCOOH···HOOCH and HCCCOOD···DOOCH isotopologues. Rotational transitions were measured in the frequency range of 4.9–15.4 GHz, providing accurate rotational constants, which, combined with previous rotational constants, allowed an improved structural fit for the propiolic acid–formic acid complex. The new structural fit yields reasonably accurate orientations for both the propiolic and formic acid monomers in the complex and more accurate structural parameters describing the hydrogen bonding. The structure is planar, with a positive inertial defect of Δ = 1.33 amu Å2. The experimental structure exhibits a greater asymmetry for the two hydrogen bond lengths than was obtained from the ab initio mp2 calculations. The best-fit hydrogen bond lengths have an r(O1–H1···O4) of 1.64 Å and an r(O3–H2···O2) of 1.87 Å. The average of the...  Read more

Photophysics of fulvene under the non-resonant stark effect. Shaping the conical intersection seam

Faraday Discussions
We introduce a mechanistic strategy to control the excited state lifetime of fulvene based on shaping the topography of an extended seam of intersection with the non-resonant dynamic Stark effect. Fulvene has a very short excited state lifetime due to an energetically accessible seam of intersection which lies along the methylene torsion coordinate, and the initial decay occurs at the seam segment around the planar conical intersection structure. We have followed a three-step approach to simulate the control. First, we have calculated the effect of a non-resonant electric field on the potential energy surface at the ab initio level, including the field in a self-consistent way. The relative energy of the planar segment of the seam is increased by the non-resonant field. In the second step we simulate the control carrying out MCTDH quantum dynamics propagations under a static non-resonant field to derive the main control mechanisms. At moderately intense fields (ε...  Read more

Ultrafast dual photoresponse of isolated biological chromophores: link to the photoinduced mode-specific non-adiabatic dynamics in proteins

Faraday Discussions
The anionic wild-type Green Fluorescent Protein (GFP) chromophore defines an entire class of naturally occurring chromophores, which are based on the oxydized tyrosine side chain. The GFP chromophore exhibits an enriched photoinduced non-adiabatic dynamics in the multiple excited-state decay channels. Deactivation includes vibrational resonant photodetachment and internal conversion. Here, we provide detailed insight into the efficiency of different vibrational modes in promoting a selective photoresponse in the bare GFP chromophore anion. We introduce a general theoretical model that is capable of accounting for the alternative non-equivalent pathways in internal conversion, and we outline the factors, by which the photo-initiated response may be altered in this channel. The topography around the planar minimum in S1 and the two distinct types of the S1/S0 conical intersections obtained through high-level...  Read more

Probing the Bonding and Electronic Structure of Single Atom Dopants in Graphene with Electron Energy Loss Spectroscopy

Nano Letters
A combination of scanning transmission electron microscopy, electron energy loss spectroscopy, and ab initio calculations reveal striking electronic structure differences between two distinct single substitutional Si defect geometries in graphene. Optimised acquisition conditions allow for exceptional signal-to-noise levels in the spectroscopic data. The near-edge fine structure can be compared with great accuracy to simulations and reveal either an sp3-like configuration for a trivalent Si or a more complicated hybridized structure for a tetravalent Si impurity.Read more

Excited state dynamics of Photoactive Yellow Protein chromophores elucidated by high-resolution spectroscopy and ab initio calculations

Faraday Discussions
We report on experimental high-resolution spectroscopic studies in combination with advanced theoretical calculations that focus on the excited-state dynamics of various forms of the chromophore of the Photoactive Yellow Protein (PYP), and the dependence of these dynamics on conformational and isosteric structure, as well as the biological environment. Three-colour nanosecond multiphoton ionization pump–probe studies confirm and extend previous conclusions that the dominant decay channel of the lowest excited ππ* state (the so-called V′ state) of methyl-4-hydroxycinnamate is picosecond internal conversion to the adiabatically lower nπ* state, and enable us to resolve apparent contradictions with picosecond pump–probe studies. Comparison of multiphoton ionization and laser induced fluorescence excitation spectra leads to the assignment of the hitherto elusive excitation spectrum of the V(ππ*) state. Complexation of methyl-4-hydroxycinnamate with water radically changes the...  Read more

CO2 Vibrational State Distributions From Quasi-Classical Trajectory Studies of the HO + CO → H + CO2 Reaction and H + CO2 Inelastic Collision

The Journal of Physical Chemistry A
Quasi-classical trajectory studies have been carried out for the HO + CO → H + CO2 reaction and H + CO2 inelastic collision on a recently developed global potential energy surface based on a large number of high-level ab initio points. The CO2 vibrational state distributions for these processes have been determined using an original normal-mode analysis method. It was found that the CO2 product of the reaction is highly excited in both the Fermi-linked bending and symmetric stretching modes, but little population was found in the antisymmetric stretching mode. The substantial excitation of the CO2 vibration, while consistent with the geometry of the transition state in the exit channel, is in disagreement with available experimental data. For the inelastic collision, the CO2 is much less excited despite much higher total energies. In addition, excitations in all vibrational modes were found, in good agreement with...  Read more

Ab Initio Potential Energy and Dipole Moment Surfaces for CS2: Determination of Molecular Vibrational Energies

The Journal of Physical Chemistry A
The ground state potential energy and dipole moment surfaces for CS2 have been determined at the CASPT2/C:cc-pVTZ,S:aug-cc-pV(T+d)Z level of theory. The potential energy surface has been fit to a sum-of-products form using the neural network method with exponential neurons. A generic interface between neural network potential energy surface fitting and the Heidelberg MCTDH software package is demonstrated. The potential energy surface has also been fit using the potfit procedure in MCTDH. For fits to the low-energy regions of the potential, the neural network method requires fewer parameters than potfit to achieve high accuracy; global fits are comparable between the two methods. Using these potential energy surfaces, the vibrational energies have been computed for the four most abundant CS2 isotopomers. These results are compared to experimental and previous theoretical data. The current potential energy surfaces are shown to accurately reproduce...  Read more

Variational Calculations of Vibrational Energies and IR Spectra of trans- and cis-HOCO Using New ab Initio Potential Energy and Dipole Moment Surfaces

The Journal of Physical Chemistry A
We report ab initio potential energy and dipole moment surfaces that span the regions describing the minima of trans- and cis-HOCO and the barrier separating them. We use the new potential in three types of variational calculations of the vibrational eigenstates, for zero total angular momentum. Two use the code MULTIMODE (MM) in the so-called single-reference and reaction path versions. The third uses the exact Hamiltonian in diatom–diatom Jacobi coordinates. The single-reference version of MM is limited to a description of states that are localized at each minimum separately, whereas the reaction-path version and the Jacobi approach describe localized and delocalized states. The vibrational IR spectrum for zero total angular momentum is also reported for the trans and cis fundamentals and selected overtone and combination states with significant oscillator strength.Read more