Circular Dichroism in Mass Spectrometry: Quantum Chemical Investigations for the Differences between (R)-3-Methylcyclopentanone and Its Cation.

In mass spectrometry enantiomers can be distinguished by multiphoton ionization employing circular polarized laser pulses. The circular dichroism (CD) is detected from the normalized difference in the ion yield after excitation with light of opposite handedness. While there are cases in which fragment and parent ions exhibit the same sign of the CD in the ion yield, several experiments show that they might also differ in sign and magnitude. Supported by experimental observations it has been proposed that the parent ion, once it has been formed, is further excited by the laser, which may result in a change of the CD in the ion yield of the formed fragments compared to the parent ion. To gain a deeper insight in possible excitation pathways we calculated and compared the electronic CD absorption spectra of neutral and cationic (R)-3-methylcyclopentanone, applying density functional theory. In addition, electron wavepacket dynamics were used to compare the CD of one- and two-photon transitions. Our results support the proposed subsequent excitation of the parent ion as a possible origin of the difference of the CD in the ion yield between parent ion and fragments.

Laser-driven electron dynamics for circular dichroism in mass spectrometry: from one-photon excitations to multiphoton ionization.

The distinction of enantiomers is a key aspect of chemical analysis. In mass spectrometry the distinction of enantiomers has been achieved by ionizing the sample with circularly polarized laser pulses and comparing the ion yields for light of opposite handedness. While resonant excitation conditions are expected to be most efficient, they are not required for the detection of a circular dichroism (CD) in the ion yield. However, the prediction of the size and sign of the circular dichroism becomes challenging if non-resonant multiphoton excitations are used to ionize the sample. Employing femtosecond laser pulses to drive electron wavepacket dynamics based on ab initio calculations, we attempt to reveal underlying mechanisms that determine the CD under non-resonant excitation conditions. Simulations were done for (R)-1,2-propylene oxide, using time-dependent configuration interaction singles with perturbative doubles (TD-CIS(D)) and the aug-cc-pVTZ basis set. Interactions between the electric field and the electric dipole and quadrupole as well as between the magnetic field and the magnetic dipole were explicitly accounted for. The ion yield was determined by treating states above the ionization potential as either stationary or non-stationary with energy-dependent lifetimes based on an approved heuristic approach. The observed population dynamics do not allow for a simple interpretation, because of highly non-linear interactions. Still, the various transition pathways are governed by resonant enantiospecific n-photon excitation, with preferably high transition dipole moments, which eventually dominate the CD in the ionized population.

Carotenoids as a shortcut for chlorophyll Soret-to-Q band energy flow.

It is proposed that xanthophylls, and carotenoids in general, may assist in energy transfer from the chlorophyll Soret band to the Q band. Ground-state (1Ag ) and excited-state (1Bu ) optimizations of violaxanthin (Vx) and zeaxanthin (Zx) are performed in an environment mimicking the light-harvesting complex II (LHCII), including the closest chlorophyll b molecule (Chl). Time-dependent density functional theory (TD-DFT, CAM-B3LYP functional) is used in combination with a semi-empirical description to obtain the excited-state geometries, supported by additional DFT/multireference configuration interaction calculations, with and without point charges representing LHCII. In the ground state, Vx and Zx show similar properties. At the 1Bu minimum, the energy of the Zx 1Bu state is below the Chl Q band, in contrast to Vx. Both Vx and Zx may act as acceptors of Soret-state energy; transfer to the Q band seems to be favored for Vx. These findings suggest that carotenoids may generally mediate Soret-to-Q energy flow in LHCII.

Resonance Raman and vibronic absorption spectra with Duschinsky rotation from a time-dependent perspective: application to ß-carotene.

The time-dependent approach to electronic spectroscopy, as popularized by Heller and co-workers in the 1980s, is applied here in conjunction with linear-response, time-dependent density functional theory to study vibronic absorption and resonance Raman spectra of ß-carotene, with and without a solvent. Two-state models, the harmonic and the Condon approximations are used in order to do so. A new code has been developed which includes excited state displacements, vibrational frequency shifts, and Duschinsky rotation, i.e., mode mixing, for both non-adiabatic spectroscopies. It is shown that Duschinsky rotation has a pronounced effect on the resonance Raman spectra of ß-carotene. In particular, it can explain a recently found anomalous behaviour of the so-called ν(1) peak in resonance Raman spectra [N. Tschirner, M. Schenderlein, K. Brose, E. Schlodder, M. A. Mroginski, C. Thomsen, and P. Hildebrandt, Phys. Chem. Chem. Phys. 11, 11471 (2009)], which shifts with the change in excitation wavelength.

Non-adiabatic excited state dynamics of riboflavin after photoexcitation.

Flavins are chromophores in light-gated enzymes and therefore central in many photobiological processes. To unravel the optical excitation process as the initial, elementary step towards signal transduction, detailed ultrafast (femtosecond) experiments probing the photo-activation of flavins have been carried out recently [Weigel et al., J. Phys. Chem. B, 2011, 115, 3656-3680.]. The present paper contributes to a further understanding and interpretation of these experiments by studying the post-excitation vibrational dynamics of riboflavin (RF) and microsolvated riboflavin, RF·4H2O, using first principles non-adiabatic molecular dynamics. By analyzing the characteristic atom motions and calculating time-resolved stimulated emission spectra following ππ* excitation, it is found that after optical excitation C-N and C-C vibrations in the isoalloxazine rings of riboflavin set in. The Franck-Condon (vertically excited) state decays within about 10 fs, in agreement with experiment. Anharmonic coupling leads to Intramolecular Vibrational energy Redistribution (IVR) on the timescale of about 80-100 fs, first to (other) C-C stretching modes of the isoalloxazine rings, then by energy spread over the whole molecule, including low-frequency in-plane modes. The IVR is accompanied by a red-shift and broadening of the emission spectrum. When RF is microsolvated with four water molecules, an overall redshift of optical spectra by about 20 nm is observed but the relaxation dynamics is only slightly affected. For several trajectories, a tendency for hydrogen transfer from water to flavin-nitrogen (N5) was found.

Modeling of a violaxanthin-chlorophyll b chromophore pair in its LHCII environment using CAM-B3LYP.

Collecting energy for photosystem II is facilitated by several pigments, xanthophylls and chlorophylls, embedded in the light harvesting complex II (LHCII). One xanthophyll, violaxanthin (Vio), is loosely bound at a site close to a chlorophyll b (Chl). No final answer has yet been found for the role of this specific xanthophyll. We study the electronic structure of Vio in the presence of Chl and under the influence of the LHCII environment, represented by a point charge field (PCF). We compare the capability of the long range corrected density functional theory (DFT) functional CAM-B3LYP to B3LYP for the modeling of the UV/vis spectrum of the Vio+Chl pair. CAM-B3LYP was reported to allow for a very realistic reproduction of bond length alternation of linear polyenes, which has considerable impact on the carotenoid structure and spectrum. To account for the influence of the LHCII environment, the chromophore geometries are optimized using an ONIOM(DFT/6-31G(d):PM6) scheme. Our calculations show that the energies of the locally excited states are almost unaffected by the presence of the partner chromophore or the PCF. There are, however, indications for excitonic coupling of the Chl Soret band and Vio. We propose that Vio may accept energy from blue-light excited Chl.

Chiral distinction by ultrashort laser pulses: electron wavepacket dynamics incorporating magnetic interactions.

The qualitative and quantitative distinction of enantiomers is one of the key issues in chemical analysis. In the last years, circular dichroism (CD) has been combined with laser ionization mass spectrometry (LIMS), applying resonance enhanced multiphoton ionization (REMPI) with ultrashort laser pulses. We present theoretical investigations on the CD in the populations of the first electronic excited state of the REMPI process, caused by the interaction of 3-methyl-cyclopentanone with either left or right circular polarized fs-laser pulses. For this we performed multistate laser driven many electron dynamics based on ab initio electronic structure calculations, namely, TD-CIS(D)/6-311++(2d,2p). For a theoretical description of these experiments, a complete description of the field-dipole correlation is mandatory, including both electric field-electric dipole and magnetic field-magnetic dipole interactions. The effect of various pulse parameters on the CD are analyzed and compared with experimental results to gain further understanding of the key elements for an optimal distinction of enantiomers.

Circular dichroism in ion yields employing femtosecond laser ionization-the role of laser pulse duration.

The circular dichroism (CD) induced by femtosecond laser pulse excitation of 3-methyl-cyclopentanone has been investigated by means of experiment and theory as a function of the laser pulse duration. In the experiment the CD in ion yields is measured by femtosecond laser ionization via a one-photon resonant excited state. In the theoretical part the CD is calculated by solving laser driven quantum electron dynamics for the same resonant excitation based on ab initio electronic structure calculations employing a complete description of the electric field-electric dipole and magnetic field-magnetic dipole interactions. Both the experimentally measured CD in ion yields and the calculated CD in excited state populations exhibit a marked increase of the CD for pulse duration increasing from 50 fs to about 200 fs. Beyond 200 fs pulse duration the CD levels off. The combination of experimental and theoretical evidences indicates that the CD decreases with increasing laser intensity connected to the increased coupling between the excited states.

(TD-)DFT calculation of vibrational and vibronic spectra of riboflavin in solution.

The photophysics and photochemistry of flavin molecules are of great interest due to their role for the biological function of flavoproteins. An important analysis tool toward the understanding of the initial photoexcitation step of flavins is electronic and vibrational spectroscopy, both in frequency and time domains. Here we present quantum chemical [(time-dependent) density functional theory ((TD-)DFT)] calculations for vibrational spectra of riboflavin, the parent molecule of biological blue-light receptor chromophores, in its electronic ground (S(0)) and lowest singlet excited states (S(1)). Further, vibronic absorption spectra for the S(0) --> S(1) transition and vibronic emission spectra for the reverse process are calculated, both including mode mixing. Solvent effects are partially accounted for by using a polarizable continuum model (PCM) or a conductor-like screening model (COSMO). Calculated vibrational and electronic spectra are in good agreement with measured ones and help to assign the experimental signals arising from photoexcitation of flavins. In particular, upon photoexcitation a loss of double bond character in the polar region of the ring system is observed which leads to vibronic fine structure in the electronic spectra. Besides vibronic effects, solvent effects are important for understanding the photophysics of flavins in solution quantitatively.

Stereoselective isomerization of an ensemble of adsorbed molecules with multiple orientations: stochastic laser pulse optimization for selective switching between achiral and chiral atropisomers.

We present quantum dynamical simulations for the laser driven isomerization of an ensemble of surface mounted stereoisomers with multiple orientations. The model system 1-(2-cis-fluoroethenyl)-2-fluorobenzene supports two chiral and one achiral atropisomers upon torsion around the C-C single bond connecting phenyl ring and ethylene group. An infrared picosecond pulse is used to excite the internal rotation around the chiral axis, thereby controlling the chirality of the molecule. In order to selectively switch the molecules--independent of their orientation on a surface--from their achiral to either their left- or right-handed form, a stochastic pulse optimization algorithm is applied. The stochastic pulse optimization is performed for different sets of defined orientations of adsorbates corresponding to the rotational symmetry of the surface. The obtained nonlinearly polarized laser pulses are highly enantioselective for each orientation.

From stochastic pulse optimization to a stereoselective laser pulse sequence: simulation of a chiroptical molecular switch mounted on adamantane.

Quantum dynamical simulations for the laser-controlled isomerization of 1-(2-cis-fluoroethenyl)-2-fluorobenzene mounted on adamantane are reported based on a one-dimensional electronic ground-state potential and dipole moment calculated by density functional theory. The model system 1-(2-cis-fluoroethenyl)-2-fluorobenzene supports two chiral and one achiral atropisomers upon torsion around the C-C single bond connecting the phenyl ring and ethylene group. The molecule itself is bound to an adamantyl frame which serves as a model for a linker or a surface. Due to the C3 symmetry of the adamantane molecule, the molecular switch can have three equivalent orientations. An infrared picosecond pulse is used to excite the internal rotation around the chiral axis, thereby controlling the chirality of the molecule. In order to selectively switch the molecules--independent of their orientations-- from their achiral to either their left- or right-handed form, a stochastic pulse optimization algorithm is applied. A subsequent detailed analysis of the optimal pulse allows for the design of a stereoselective laser pulse sequence of analytical form. The developed control scheme of elliptically polarized laser pulses is enantioselective and orientation-selective.

Laser-operated chiral molecular switch: quantum simulations for the controlled transformation between achiral and chiral atropisomers.

We report quantum dynamical simulations for the laser controlled isomerization of 1-(2-cis-fluoroethenyl)-2-fluorobenzene based on one-dimensional electronic ground and excited state potentials obtained from (TD)DFT calculations. 1-(2-cis-fluoroethenyl)-2-fluorobenzene supports two chiral and one achiral atropisomers, the latter being the most stable isomer at room temperature. Using a linearly polarized IR laser pulse the molecule is excited to an internal rotation around its chiral axis, i.e. around the C-C single bond between phenyl ring and ethenyl group, changing the molecular chirality. A second linearly polarized laser pulse stops the torsion to prepare the desired enantiomeric form of the molecule. This laser control allows the selective switching between the achiral and either the left- or right-handed form of the molecule. Once the chirality is "switched on" linearly polarized UV laser pulses allow the selective change of the chirality using the electronic excited state as intermediate state.

Photo-induced desorption of NO from NiO(100): calculation of the four-dimensional potential energy surfaces and systematic wave packet studies.

The velocity distributions of the laser-induced desorption of NO molecules from an epitaxially grown film of NiO(100) on Ni(100) have been studied [Mull et al., J. Chem. Phys., 1992, 96, 7108]. A pronounced bimodality of velocity distributions has been found, where the NO molecules desorbing with higher velocities exhibit a coupling to the rotational quantum states J. In this article we present simulations of state resolved velocity distributions on a full ab initio level. As a basis for this quantum mechanical treatment a 4D potential energy surface (PES) was constructed for the electronic ground and a representative excited state, using a NiO5Mg(18+)13 cluster. The PESs of the electronic ground and an excited state were calculated at the CASPT2 and the configuration interaction (CI) level of theory, respectively. Multi-dimensional quantum wave packet simulations on these two surfaces were performed for different sets of degrees of freedom. Our key finding is that at least a 3D wave packet simulation, in which the desorption coordinate Z, polar angle theta and lateral coordinate X are included, is necessary to allow the simulation of experimental velocity distributions. Analysis of the wave packet dynamics demonstrates that essentially the lateral coordinate, which was neglected in previous studies [Klüner et al., Phys. Rev. Lett. 1998, 80, 5208], is responsible for the experimentally observed bimodality. An extensive analysis shows that the bimodality is due to a bifurcation of the wave packet on the excited state PES, where the motion of the molecule parallel to the surface plays a decisive role.