Polarizable QM/MM Multiconfiguration Self-Consistent Field Approach with State-Specific Corrections: Environment Effects on Cytosine Absorption Spectrum

We present the formulation and implementation of a polarizable quantum mechanics/molecular mechanics (QM/MM) strategy to describe environment effects in multiconfiguration self-consistent field calculations. The strategy is applied to the calculation of the vertical absorption spectrum of cytosine in water. In our approach, mutual polarization of the solute and the solvent is solved self-consistently at the complete-active-space self-consistent-field (CASSCF) level, and the resulting set of charges and dipoles is used to calculate vertical excitation energies using the complete-active-space second-order perturbative (CASPT2) approach and its multistate (MS-CASPT2) variant. In order to treat multiple excited states, we converge the solvent polarization with respect to the state-averaged density of the solute. In order to obtain the final energies, however, we introduce a state-specific correction, where the solvent polarization is recomputed with the density of each state, and demonstrate that this correction brings the excitation energies closer to the values obtained with state-optimized orbitals. Comparison with PCM and nonpolarizable QM/MM calculations shows the importance of specific solute solvent interactions and environment polarization in describing experiments. Overall, the calculated excitations for the π → π* states in water show good agreement with the experimental spectrum, whereas the n → π* appear at energies above 6 eV, approximately 1 eV higher than in the gas phase. Beyond solvents, the new method will allow studying the impact of heterogeneous biological environments in multiple excited states, as well as the treatment of multichromophoric systems where charge transfer and exciton states play important roles

Early events in the photochemistry of 5-diazo Meldrum's acid : Formation of a product manifold in C-N bound and pre-dissociated intersection seam regions

5-Diazo Meldrum's acid (DMA) undergoes a photo-induced Wolff rearrangement (WR). Recent gas-phase experiments have identified three photochemical products formed in a sub-ps scale after irradiation, a carbene formed after nitrogen loss, a ketene formed after WR and a second carbene formed after nitrogen and CO elimination (A. Steinbacher, et al. Phys. Chem. Chem. Phys., 2014, 16, 7290-7298). In this work, ground- and excited-state potential energy surfaces (PESs) have been investigated at the MS-CASPT2// CASSCF level. The key element of the PESs is an extended S0/S1 conical intersection seam along the C-N dissociation coordinate. The C-N predissociated region of the seam is accessed after excitation to the bright S2 state, and decay paths from the seam to the three primary products have been characterized. For the ketene and carbene II products, we show two possible formation pathways, a direct and a stepwise one, which suggests that these products may be formed in a bi-modal fashion. We have also characterized two possible mechanisms for triplet formation, one occurring before C-N dissociation involving a (S1/T2/T1) crossing region, and another one through the carbene. In contrast, excitation to S1 leads to a C-N bound region of the seam from where DMA regeneration or diazirine formation is possible, with a preference for the first case. The results are in good agreement with experimental data. Together with our previous work on diazonaphthoquinone, they show the importance of an extended seam in the photochemistry of a-diazoketones

Research on torque characteristics of a modular arc-linear flux switching permanent-magnet motor

The modular arc-linear flux switching permanent-magnet motor (MAL-FSPM) used for scanning system is researched in this paper by the finite element method (FEM). The MAL-FSPM combines characteristics of the flux switching permanent-magnet motor and linear motor and can realize the direct driving and limited angular movement. Due to the double salient effect, the cogging torque including slot torque and end torque is very high. In order to reduce the cogging torque, a method of magnetizing the permanent magnet in reverse direction combined with rotor segmentation is proposed. By means of the proposed method, peak-peak cogging torque and load torque ripple are reduced effectively. Besides, the smooth operation range and an average load torque of the motor are also optimized. The effectiveness of the method is verified by both 2D and 3D FE models

The excited-state structure, vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-methylcytosine

We have investigated the S0 → S1 UV vibronic spectrum and time-resolved S1 state dynamics of jet-cooled amino-keto 1-methylcytosine (1MCyt) using two-color resonant two-photon ionization, UV/UV holeburning and depletion spectroscopies, as well as nanosecond and picosecond timeresolved pump/delayed ionization measurements. The experimental study is complemented with spin-component-scaled second-order coupled-cluster and multistate complete active space second order perturbation ab initio calculations. Above the weak electronic origin of 1MCyt at 31 852 cm−1 about 20 intense vibronic bands are observed. These are interpreted as methyl group torsional transitions coupled to out-of-plane ring vibrations, in agreement with the methyl group rotation and out-of-plane distortions upon 1ππ∗ excitation predicted by the calculations. The methyl torsion and ν′1 (butterfly) vibrations are strongly coupled, in the S1 state. The S0 → S1 vibronic spectrum breaks off at a vibrational excess energy Eexc ∼ 500 cm−1, indicating that a barrier in front of the ethylene-type S1 S0 conical intersection is exceeded, which is calculated to lie at Eexc = 366 cm−1. The S1 S0 internal conversion rate constant increases from kIC = 2 · 109 s−1 near the S1(v = 0) level to 1 · 1011 s−1 at Eexc = 516 cm−1. The 1ππ∗ state of 1MCyt also relaxes into the lower-lying triplet T1 (3ππ∗) state by intersystem crossing (ISC); the calculated spin-orbit coupling (SOC) value is 2.4 cm−1. The ISC rate constant is 10–100 times lower than kIC; it increases from kISC = 2 · 108 s−1 near S1(v = 0) to kISC = 2 · 109 s−1 at Eexc = 516 cm−1. The T1 state energy is determined from the onset of the time-delayed photoionization efficiency curve as 25 600 ± 500 cm−1. The T2 (3nπ∗) state lies >1500 cm−1 above S1(v = 0), so S1 T2 ISC cannot occur, despite the large SOC parameter of 10.6 cm−1. An upper limit to the adiabatic ionization energy of 1MCyt is determined as 8.41 ± 0.02 eV. Compared to cytosine, methyl substitution at N1 lowers the adiabatic ionization energy by ≥0.32 eV and leads to a much higher density of vibronic bands in the S0 → S1 spectrum. The effect of methylation on the radiationless decay to S0 and ISC to T1 is small, as shown by the similar break-off of the spectrum and the similar computed mechanismsThis research has been supported by the Schweiz. Nationalfonds (Grant Nos. 121993 and 132540), the Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) from Catalonia (Spain) (Grant No. 2014SGR1202), the Ministerio de Economía y Competividad (MINECO) from Spain (Grant No. CTQ2015-69363-P), and the National Natural Science Foundation of China (Grant No. 21303007

The excited-state structure, vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-methylcytosine

We have investigated the S0 → S1 UV vibronic spectrum and time-resolved S1 state dynamics of jet-cooled amino-keto 1-methylcytosine (1MCyt) using two-color resonant two-photon ionization, UV/UV holeburning and depletion spectroscopies, as well as nanosecond and picosecond timeresolved pump/delayed ionization measurements. The experimental study is complemented with spin-component-scaled second-order coupled-cluster and multistate complete active space second order perturbation ab initio calculations. Above the weak electronic origin of 1MCyt at 31 852 cm−1 about 20 intense vibronic bands are observed. These are interpreted as methyl group torsional transitions coupled to out-of-plane ring vibrations, in agreement with the methyl group rotation and out-of-plane distortions upon 1ππ∗ excitation predicted by the calculations. The methyl torsion and ν′1 (butterfly) vibrations are strongly coupled, in the S1 state. The S0 → S1 vibronic spectrum breaks off at a vibrational excess energy Eexc ∼ 500 cm−1, indicating that a barrier in front of the ethylene-type S1 S0 conical intersection is exceeded, which is calculated to lie at Eexc = 366 cm−1. The S1 S0 internal conversion rate constant increases from kIC = 2 · 109 s−1 near the S1(v = 0) level to 1 · 1011 s−1 at Eexc = 516 cm−1. The 1ππ∗ state of 1MCyt also relaxes into the lower-lying triplet T1 (3ππ∗) state by intersystem crossing (ISC); the calculated spin-orbit coupling (SOC) value is 2.4 cm−1. The ISC rate constant is 10–100 times lower than kIC; it increases from kISC = 2 · 108 s−1 near S1(v = 0) to kISC = 2 · 109 s−1 at Eexc = 516 cm−1. The T1 state energy is determined from the onset of the time-delayed photoionization efficiency curve as 25 600 ± 500 cm−1. The T2 (3nπ∗) state lies >1500 cm−1 above S1(v = 0), so S1 T2 ISC cannot occur, despite the large SOC parameter of 10.6 cm−1. An upper limit to the adiabatic ionization energy of 1MCyt is determined as 8.41 ± 0.02 eV. Compared to cytosine, methyl substitution at N1 lowers the adiabatic ionization energy by ≥0.32 eV and leads to a much higher density of vibronic bands in the S0 → S1 spectrum. The effect of methylation on the radiationless decay to S0 and ISC to T1 is small, as shown by the similar break-off of the spectrum and the similar computed mechanismsThis research has been supported by the Schweiz. Nationalfonds (Grant Nos. 121993 and 132540), the Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) from Catalonia (Spain) (Grant No. 2014SGR1202), the Ministerio de Economía y Competividad (MINECO) from Spain (Grant No. CTQ2015-69363-P), and the National Natural Science Foundation of China (Grant No. 21303007

A global picture of the S-1/S-0 conical intersection seam of benzene

A global picture of the S1/S0intersection seam of benzene is presented. Eleven new conical intersection critical points were located at the CASSCF level, the connectivity was mapped and the energies refined with CASPT 2. There are two seam branches related with pairs of degenerate A1g/B2uand Egstates at D6hsymmetry, respectively, and the two branches are connected by a seam segment of Cssymmetry. The global energy minimum of the seam is the half-boat shaped intersection that leads to a pre-fulvenic intermediate [I.J. Palmer, I.N. Ragazos, F. Bernardi, M. Olivucci, M.A. Robb, J. Am. Chem. Soc. 115 (1993) 673]. Several other intersections that can lead to the same intermediate or vibrationally hot benzene lie in a range of 3.7 eV above the global seam minimum. There is a recurrent connectivity pattern where permutationally isomeric seam segments are connected by intersections of a higher symmetry point groupThis work has been supported by Grant CTQ2008-06696 from the Spanish Ministerio de Ciencia e Innovación (MICINN

Irreversible phototautomerization of o-phthalaldehyde through electronic relocation

The potential energy surface for the intramolecular excited state hydrogen transfer (IESHT) in ortho-phthalaldehyde (OPA), which generates an enol ketene, has been studied with ab initio calculations (MS-CASPT2//CASSCF). The goal of our study is to establish the mechanistic factors that make the primary phototautomerization step irreversible. Similar to what we recently described for ortho-nitrobenzaldehyde (NBA) (Migani et al., Chem. Commun., 2011, 47, 6383-6385), the IESHT in OPA is characterized by the relocation of two electrons from the in-plane to the out-of-plane orbital system. Consistent with this, OPA has the same IESHT mechanism as NBA. The first step of ketene formation is the hydrogen transfer, which starts on an (n, π*) state. The reaction coordinate goes through a conical intersection with the ground state and leads to a biradical intermediate with a bent ketene moiety. The second step is the linearization of the ketene moiety, which is associated to a change in the electronic configuration from biradical to ketene. Because of the electron relocation, the reverse transfer is similar to a Woodward-Hoffmann forbidden process with a sizeable barrier. This makes the tautomerization irreversible and allows the ketene to react further to biphthalide and benzaldehyde. Together with our previous NBA study, we establish the electronic relocation mechanism as a new mechanism for IESHT. This mechanism explains the different reactivity of OPA and NBA compared to organic photoprotectors, where the IESHT is reversed on a very short time scale. © the Owner Societies 2012.This work has been supported by Grants CTQ2008-06696 from the Spanish Ministerio de Ciencia e Innovación (MICINN) and SGR0528 from the Catalan Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR). Q. Li acknowledges a Juan de la Cierva fellowship of the MICINN, and A. Migani a Beatriu de Pinós fellowship of the Generalitat de Catalunya (Spain). Financial support from the MICINN (Ministry of Science and Innovation, Spain) and the FEDER fund (European Fund for Regional Development) was provided by grant UNGI08-4E-003.Peer Reviewe

Wave packet dynamics at an extended seam of conical intersection: Mechanism of the light-induced wolff rearrangement

Quantum dynamics calculations on a model surface based on CASPT 2//CASSCF calculations are carried out to probe the traversal of a wave packet through an extended seam of conical intersection during the light-induced Wolff rearrangement of diazonaphtoquinone. The reaction is applied in the fabrication of integrated circuits. It consists of nitrogen elimination and ring rearrangement to yield a ketene. After excitation, the wave packet relaxes and reaches the extended seam. A fraction of the wave packet decays to the ground state at a region of the seam connected to a carbene intermediate, while the remaining part decays at a region leading to the ketene. The passage of the wave packet through the extended seam explains the competition between concerted ketene formation and a stepwise mechanism involving a carbene. The two primary photoproducts are formed in the first 100 fs of the simulation, in agreement with recent ultrafast spectroscopy measurements. © 2012 American Chemical Society.This work has been supported by Grants CTQ2008-06696 and CTQ2011-26573 from the Spanish Ministerio de Ciencia e Innovación (MICINN), SGR0528 from the Catalan Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR), and UNGI08-4E-003 from MICINN and the FEDER fund (European Fund for Regional Development). Q.L. acknowledges a Juan de la Cierva fellowship of the MICINN and A.M. a Beatriu de Pinós fellowship of the Generalitat de Catalunya (Spain).Peer Reviewe

Early events in the photochemistry of 5-diazo Meldrum's acid : Formation of a product manifold in C-N bound and pre-dissociated intersection seam regions

5-Diazo Meldrum's acid (DMA) undergoes a photo-induced Wolff rearrangement (WR). Recent gas-phase experiments have identified three photochemical products formed in a sub-ps scale after irradiation, a carbene formed after nitrogen loss, a ketene formed after WR and a second carbene formed after nitrogen and CO elimination (A. Steinbacher, et al. Phys. Chem. Chem. Phys., 2014, 16, 7290-7298). In this work, ground- and excited-state potential energy surfaces (PESs) have been investigated at the MS-CASPT2// CASSCF level. The key element of the PESs is an extended S0/S1 conical intersection seam along the C-N dissociation coordinate. The C-N predissociated region of the seam is accessed after excitation to the bright S2 state, and decay paths from the seam to the three primary products have been characterized. For the ketene and carbene II products, we show two possible formation pathways, a direct and a stepwise one, which suggests that these products may be formed in a bi-modal fashion. We have also characterized two possible mechanisms for triplet formation, one occurring before C-N dissociation involving a (S1/T2/T1) crossing region, and another one through the carbene. In contrast, excitation to S1 leads to a C-N bound region of the seam from where DMA regeneration or diazirine formation is possible, with a preference for the first case. The results are in good agreement with experimental data. Together with our previous work on diazonaphthoquinone, they show the importance of an extended seam in the photochemistry of a-diazoketones