Excited 1A Research Articles

Topology of Conical Intersection Seams and the Geometric Phase.

In this paper, we demonstrate two topological properties of crossing seams, that is, the sets of points in the N-dimensional space of nuclear coordinates where two electronic eigenstates are degenerate. We shall examine the typical case of states of the same spin with accidental degeneracies, whereby the crossing seam is of dimension N - 2. The first property we demonstrate is that a crossing seam has no boundary, therefore, it must either extend asymptotically to infinite values of one or more coordinates or wrap on itself. The second property is that two (or more) crossing seams can intersect each other but in such a way that neither of them ends at the intersection. When N = 3, the crossing seam is a line in a 3D space; this is so in triatomic molecules but also in reduced dimensionality treatments of larger polyatomics. The above-mentioned rules then mean that the crossing seam is a line of infinite length or a closed loop and can split into three branches but not in two. The example of the first two excited 1A' states of H2Cl+ illustrates these rules and shows their usefulness for computational search and characterization of crossing seams.

External Photocatalyst-Free Visible Light-Promoted 1,3-Addition of Perfluoroalkyl Iodides to Vinyldiazoacetates

External Photocatalyst-Free Visible Light-Promoted 1,3-Addition of Perfluoroalkyl Iodides to Vinyldiazoacetates

Non-adiabatic behavior in the homolytic and heterolytic bond dissociation of protonated hydrazine: A guided ion beam and theoretical investigation.

Threshold collision-induced dissociation using a guided ion beam tandem mass spectrometer was performed on protonated hydrazine and its perdeuterated variant. The dominant dissociation pathways observed were endothermic homolytic and heterolytic cleavages of the N-N bond. The data were analyzed using a statistical model after accounting for internal and kinetic energy distributions, multiple collisions, and kinetic shifts to obtain 0 K bond dissociation energies. Comparison with literature thermochemistry demonstrates that both channels behave non-adiabatically. Heterolytic bond cleavage yields NH2+ + NH3 products, but the NH2+ fragment is in the spin-restricted excited 1A1 state and not in the spin-forbidden ground 3B1 state, whereas homolytic bond cleavage leads to dissociation to the NH3+ + NH2 product asymptote with NH2 in its excited 2A1 state rather than the energetically favored 2B1 state. The rationale for the non-adiabatic behavior observed in the homolytic bond cleavage is revealed by detailed theoretical calculations of the relevant potential energy surfaces and the relevant occupied valence molecular orbitals. These calculations suggest that the non-adiabatic behavior results from conservation of the σ and π character of the binding and lone pair electrons on the nitrogen atoms.

An experimental and theoretical investigation of the C(1D) + D2 reaction.

In a previous joint experimental and theoretical study of the barrierless chemical reaction C(1D) + H2 at low temperatures (300-50 K) [K. M. Hickson, J.-C. Loison, H. Guo and Y. V. Suleimanov, J. Phys. Chem. Lett., 2015, 6, 4194], excellent agreement was found between experimental thermal rate constants and theoretical estimates based on ring polymer molecular dynamics (RPMD) over the two lowest singlet potential energy surfaces (PESs). Here, we extend this work to one of its isotopologues, C(1D) + D2, over the same temperature range. Experimental and RPMD results are in good agreement when contributions from both PESs to this chemical reaction are included in the RPMD simulations. The deviation between experiment and the RPMD calculations does not exceed 25% and both results exhibit a slight negative temperature dependence. The first excited 1A'' PES plays a more important role than the ground 1A' PES as the temperature is decreased, similar to our previous studies of the C(1D) + H2 reaction but with a more pronounced effect. The small differences in temperature dependence between the earlier and present experimental studies of C(1D) + H2/D2 reactions are discussed in terms of the use of non-equilibrium populations of ortho/para-H2/D2. We argue that RPMD provides a very convenient and reliable tool to study low-temperature chemical reactions.

Global diabatic potential energy surfaces and quantum dynamical studies for the Li(2p) + H2(X(1)Σ(+)g) → LiH(X(1)Σ(+)) + H reaction.

The global diabatic potential energy surfaces which are correlated with the ground state 1A′ and the excited state 2A′ of the Li(2p) + H2 reaction are presented in this study. The multi-reference configuration interaction method and large basis sets (aug-cc-pVQZ for H atom and cc-pwCVQZ for Li atom) were employed in the ab initio single-point energy calculations. The diabatic potential energies were generated by the diabatization scheme based on transition dipole moment operators. The neural network method was utilized to fit the matrix elements of the diabatic energy surfaces, and the root mean square errors were extremely small (3.69 meV for , 5.34 meV for and 5.06 meV for ). The topographical features of the diabatic potential energy surfaces were characterized and the surfaces were found to be sufficiently smooth for the dynamical calculation. The crossing seam of the conical intersections between the and surfaces were pinpointed. Based on this new analytical diabatic potential energy surfaces, time-dependent wave packet calculation were conducted to investigate the mechanism of the title reaction. At low collision energies, the product LiH molecule tends to forward scattering, while at high collision energies, the forward and backward scatterings exist simultaneously.

The collisional depolarization of OH(A 2Σ+) and NO(A 2Σ+) with Kr

Quantum beat spectroscopy has been used to measure rate coefficients at 300K for collisional depolarization for NO(A(2)Σ(+)) and OH(A(2)Σ(+)) with krypton. Elastic depolarization rate coefficients have also been determined for OH(A) + Kr, and shown to make a much more significant contribution to the total depolarization rate than for NO(A) + Kr. While the experimental data for NO(A) + Kr are in excellent agreement with single surface quasiclassical trajectory (QCT) calculations carried out on the upper 2A(') potential energy surface, the equivalent QCT and quantum mechanical calculations cannot account for the experimental results for OH(A) + Kr collisions, particularly at low N. This disagreement is due to the presence of competing electronic quenching at low N, which requires a multi-surface, non-adiabatic treatment. Somewhat improved agreement with experiment is obtained by means of trajectory surface hopping calculations that include non-adiabatic coupling between the ground 1A(') and excited 2A(') states of OH(X/A) + Kr, although the theoretical depolarization cross sections still significantly overestimate those obtained experimentally.

Neutral-Fragmentation Paths of Methane Induced by Intense Ultrashort IR Laser Pulses: Ab Initio Molecular Orbital Approach

Instantaneous (laser-field-dependent) potential energy curves leading to neutral fragmentations of methane were calculated at several laser intensities from 1.4 × 10(13) to 1.2 × 10(14) W/cm(2) (from 1.0 × 10(10) to 3.0 × 10(10) V/m) using ab initio molecular orbital (MO) methods to validate the observation of neutral fragmentations induced by intense femtosecond IR pulses (Kong et al. J. Chem. Phys. 2006, 125, 133320). Two fragmentation paths, CH(2) + 2H and CH(2) + H(2), in (1)T(2) superexcited states that are located in the energy range of 12-16 eV were considered as the reaction paths because these states are responsible for Jahn-Teller distortion opening up reaction paths during ultrashort pulses. As field intensity increased, the low-lying excited (1)A(1) states originated from the Jahn-Teller (1)T(2) states were substantially stabilized along the neutral-fragment path CH(4) → CH(2) + 2H and were located below the ionization threshold. On the other hand, the low-lying excited (1)B(1) states, which also originate from the Jahn-Teller (1)T(2) states, were embedded on the ionized state along the dissociation path to CH(2) + H(2). This indicates that ionic fragments, rather than neutral ones, are produced along the CH(2) + H(2) path. The computational results support neutral fragmentations through superexcited states proposed by Kong et al.

Quasiclassical trajectory calculations for Li(22PJ) + H2 → LiH(X1Σ+) + H: Influence by vibrational excitation and translational energy

Ab initio potential energy surfaces and the corresponding analytical energy functions of the ground 1A' and excited 2A' states for the Li(2(2)P) plus H(2) reaction are constructed. Quasiclassical trajectory calculations on the fitted energy functions are performed to characterize the reactions of Li(2(2)P) with H(2)(v = 0, j = 1) and H(2)(v = 1, j = 1) as well as the reaction when the vibrational energy is replaced by collision energy. For simplicity, the transition probability is assumed to be unity when the trajectories go through the crossing seam region and change to the lower surface. The calculated rotational distributions of LiH(v = 0) for both H(2)(v = 0, j = 1) and H(2)(v = 1, j = 1) reactions are single-peaked with the maximum population at j' = 7, consistent with the previous observation. The vibrational excitation of H(2)(v = 1) may enhance the reaction cross section of LiH(v' = 0) by about 200 times, as compared to a result of 93-107 reported in the experimental measurements. In contrast, the enhancement is 3.1, if the same amount of energy is deposited in the translational states. This endothermic reaction can be considered as an analog of late barrier. According to the trajectory analysis, the vibrational excitation enlarges the H-H distance in the entrance channel to facilitate the reaction, but the excess energy may not open up additional reaction configuration.

Photochemical Reactions of Chloranil with Cyclooctene, 1,5‐Cyclooctadiene, and Cyclohexene Revisited

AbstractDepending on the reaction conditions, excited chloranil (3CA) and cis‐cyclooctene (cis‐CO) furnished three types of products, which all contain carbocyclic four‐membered rings. The illumination of a solution in benzene by light bulbs gave rise to the all‐cis 1:1 cycloadduct (11a) in 76 % yield. On its part, this compound reacted with cis‐CO under the action of better sources for UV light with the formation of three isomeric 1:2 cycloadducts. Two of them underwent a photochemical dechlorination leading to a cyclobutene derivative. Albeit rather sluggishly, 3CA and trans‐cyclooctene (trans‐CO) brought about an unsymmetrical 1:1 and a 1:2 cycloadduct. By contrast, excited 11a reacted smoothly with trans‐CO and afforded a 72 % yield of the unsymmetrical 1:2 cycloadduct that resulted from 3CA and cis‐CO in addition to two symmetrical 1:2 cycloadducts. Also, excited 11a and cyclohexene (CH) furnished a bis(cyclobutane) derivative, a significant portion of which was subject to a dechlorination to give a cyclobutene derivative. cis,cis‐1,5‐Cyclooctadiene (COD) and CH were transformed by 3CA to 1:1 and 1:2 cycloadducts, analogous to cis‐ and trans‐CO. In addition to this, 3CA suffered photoreduction in the presence of COD and CH. The [2+2] cycloaddition onto a carbonyl group of 3CA with formation of a spirooxetane occurred to a small extent only with COD, but not at all with cis‐ and trans‐CO and CH.

Photochemical Ring-Opening of Cyclohexadiene: Quantum Wavepacket Dynamics on a Global Ab Initio Potential Energy Surface

We have assembled a global CASSCF potential energy surface for the excited 2A state of the cyclohexadiene -hexatriene system, in two degrees of freedom, with full relaxation in all other degrees of freedom. Quantum wavepacket dynamics on this surface yields simple interpretations of recent experimental data on the ultrafast photochemical ring-opening of cyclohexadiene as well as predictions on preferred product configurations. The feasibility of this system as a model for fulgide molecular switches is discussed.

Dynamics of the C(1D)+H2 reaction: A comparison of crossed molecular beam experiments with quantum mechanical and quasiclassical trajectory calculations on the first two singlet (11A′ and 11A″) potential energy surfaces

The contribution of the first excited 11A″ state potential energy surface (PES) to the reactivity of the C(1D)+H2 system has been examined by comparing the simulations obtained with the theoretical state-to-state differential cross sections (DCSs) with experimental data from cross molecular beam experiments. The calculations were carried out using the quantum scattering and quasiclassical trajectory calculations on the ground and first excited PESs, and the DCSs were convoluted with the various apparatus functions in order to simulate the experimental laboratory angular distributions and time-of-flight distributions. It was expected that the participation of the excited PES could solve the relatively minor discrepancies resulting from the comparison between the experimental data and the simulations using the DCSs obtained on the ground state PES. Nevertheless, the addition of the contribution of the excited 11A″ PES worsens the agreement with the experimental results.

Microwave spectrum of the OD–OH 2 complex: A strong deuterium isotope effect on angular momentum quenching in the hydroxyl moiety

Microwave spectra of the hydrogen bonded complex 16OD– 16OH 2 have been recorded using pulsed-nozzle Fourier transform microwave spectroscopy. The potential splitting, ρ, which describes the partial quenching of the OD orbital angular momentum upon complexation, is determined to be −142.703173(65) cm −1. Within the spectroscopic model employed, this value implies an energy difference of 202.46 cm −1 between the ground ( 2A′) and first excited ( 2A′′) states of the complex. The observed value of ρ represents a rather large change of 3.85710(11) cm −1 relative to that in the parent complex and implies a 1.30 cm −1 decrease in the 2A′– 2A′′ energy spacing relative to the parent species. Comparison with previous results for the 18OH complex suggests that these changes likely arise from changes in vibrationally averaged geometry upon deuteration. Magnetic hyperfine structure from the deuterium and the water protons is analyzed, as is the nuclear electric quadrupole coupling of the deuterium nucleus. Assuming negligible changes in the axial component of the electric field gradient at the deuterium upon complexation, the deuterium quadrupole coupling constant implies an average angular excursion of the OD bond axis from the vibrationally averaged a-inertial axis of the complex of ∼24°.

On the photodissociation of ozone in the range of 5–9 eV

The photoabsorption spectrum of ozone in the UV range (5–9 eV) is calculated from a short-time wave packet propagation using six potential energy surfaces obtained from ab initio electronic structure calculations. It is shown that the (unnamed) band around 7 eV, which is immediately adjacent to the intense Hartley band, is primarily due to excitation of three electronic states: 5 1A′ (3 1A 1), 6 1A′ (4 1A 1), and 4 1A″ (2 1B 1). Excitation of the state 8 1A′ ( 1B 2) leads to a broad and intense band starting around 8 eV with a maximum near 9.1 eV. In full accord with the recent experimental study of Brouard et al. [M. Brouard, R. Cireasa, A.P. Clark, G.C. Groenenboom, G. Hancock, S.J. Horrocks, F. Quadrini, G.A.D. Ritchie, C. Vallance, J. Chem. Phys. 125 (2006) 133308], the excitation at 193 nm (6.42 eV) involves at least two states (5 1A′ and 4 1A″) different from the state excited in the Hartley band (3 1A′). The dynamics along the dissociation path is discussed in terms of one-dimensional potential curves. Several avoided crossings among the excited 1A′ as well as the 1A″ states point to a complicated fragmentation process. Although a quantitative analysis of branching ratios is not possible on the basis of the present calculations, we surmise, that in addition to O ( 3 P ) + O 2 ( 3 Σ g - ) and O( 1D) + O 2( 1Δ g), the next higher spin-allowed channel, O ( 1 D ) + O 2 ( 1 Σ g + ) , also is likely to be a major product channel, in agreement with experimental observations.

Quantum real wave-packet dynamics of the N(S4)+NO(X̃Π2)→N2(X̃Σg+1)+O(P3) reaction on the ground and first excited triplet potential energy surfaces: Rate constants, cross sections, and product distributions

The reaction N+NO-->N(2)+O was studied by means of the time-dependent real wave-packet (WP) method and the J-shifting approximation. We consider the ground 1 (3)A(") and first excited 1 (3)A(') triplet states, which correlate with both reactants and products, using analytical potential energy surfaces (PESs) recently developed in our group. This work extends our previous quantum dynamics study, and probabilities, cross sections, and rate constants were calculated and interpreted on the basis of the different shapes of the PESs (barrierless 1 (3)A(") and with barrier 1 (3)A(') surfaces, respectively). The WP rate constant (k(1)) shows a weak dependence on T(200-2500 K), as the dominant contribution to reactivity is provided by the barrierless ground PES. There is a good agreement of WP k(1) with the measurements and variational transition state theory (VTST) data, and also between the WP and VTST k(1)(1 (3)A(")) results. Nevertheless, there is a large discrepancy between the WP and VTST k(1)(1 (3)A(')) results. Product state distributions were also calculated for the much more reactive 1 (3)A(") PES. There is an excellent agreement with the experimental average fraction of vibrational energy in N(2)(25+/-3%), the only measured dynamics property of this reaction.

Quantum mechanical and quasiclassical trajectory scattering calculations for the C(D1)+H2 reaction on the second excited 1A″1 potential energy surface

Time-independent quantum mechanical (QM) and quasiclassical trajectory (QCT) scattering calculations have been carried out for the C(1D) + H2 --> CH + H reaction at a collision energy of 80 meV on a newly developed ab initio potential energy surface [B. Bussery-Honvault et al., Phys. Chem. Chem. Phys. 7, 1476 (2005)] of 1 1A" symmetry, corresponding to the second singlet state 1 1B1 of CH2. A general good agreement has been found between the QM and QCT rotational distributions and differential cross sections (DCSs). In both cases, DCSs are strongly peaked in the forward direction with a small contribution in the backward direction in contrast with those obtained on the 1 1A' surface, which are nearly symmetric. Rotational distributions obtained on the 1 1A" surface are somewhat colder than those calculated on the 1 1A' surface. The specific dynamics and the contribution of the 1 1A" surface to the overall reactivity of this system are discussed.

A photodissociation study of CH2BrCl in the A-band using the time-sliced ion velocity imaging method

Employing a high-resolution (velocity resolution deltanu/nu<1.5%) time-sliced ion velocity imaging apparatus, we have examined the photodissociation of CH2BrCl in the photon energy range of 448.6-618.5 kJ/mol (193.3-266.6 nm). Precise translational and angular distributions for the dominant Br(2P32) and Br(2P12) channels have been determined from the ion images observed for Br(2P32) and Br(2P12). In confirmation with the previous studies, the kinetic-energy distributions for the Br(2P12) channel are found to fit well with one Gaussian function, whereas the kinetic- energy distributions for the Br(2P32) channel exhibit bimodal structures and can be decomposed into a slow and a fast Gaussian component. The observed kinetic-energy distributions are consistent with the conclusion that the formation of the Br(2P32) and Br(2P12) channels takes place on a repulsive potential-energy surface, resulting in a significant fraction (0.40-0.47) of available energy to appear as translational energy for the photo fragments. On the basis of the detailed kinetic-energy distributions and anisotropy parameters obtained in the present study, together with the specific features and relative absorption cross sections of the excited 2A', 1A", 3A', 4A', and 2A" states estimated in previous studies, we have rationalized the dissociation pathways of CH2BrCl in the A-band, leading to the formation of the Br(2P32) and Br(2P12) channels. The analysis of the ion images observed at 235 nm for Cl(2P(32,12)) provides strong evidence that the formation of Cl mainly arises from the secondary photodissociation process CH2Cl + hnu --> CH2 + Cl.

Theoretical study of isomerization and dissociation of acetylene dication in the ground and excited electronic states

Ab initio calculations employing the configuration interaction method including Davidson's corrections for quadruple excitations have been carried out to unravel the dissociation mechanism of acetylene dication in various electronic states and to elucidate ultrafast acetylene-vinylidene isomerization recently observed experimentally. Both in the ground triplet and the lowest singlet electronic states of C2H2(2+) the proton migration barrier is shown to remain high, in the range of 50 kcal/mol. On the other hand, the barrier in the excited 2 3A" and 1 3A' states decreases to about 15 and 34 kcal/mol, respectively, indicating that the ultrafast proton migration is possible in these states, especially, in 2 3A", even at relatively low available vibrational energies. Rice-Ramsperger-Kassel-Marcus calculations of individual reaction-rate constants and product branching ratios indicate that if C2H(2)2+ dissociates from the ground triplet state, the major reaction products should be CCH+(3Sigma-)+H+ followed by CH+(3Pi)+CH+(1Sigma+) and with a minor contribution (approximately 1%) of C2H+(2A1)+C+(2P). In the lowest singlet state, C2H+(2A1)+C+(2P) are the major dissociation products at low available energies when the other channels are closed, whereas at Eint>5 eV, the CCH+(1A')+H+ products have the largest branching ratio, up to 70% and higher, that of CH+(1Sigma+)+CH+(1Sigma+) is in the range of 25%-27%, and the yield of C2H++C+ is only 2%-3%. The calculated product branching ratios at Eint approximately 17 eV are in qualitative agreement with the available experimental data. The appearance thresholds calculated for the CCH++H+, CH++CH+, and C2H++C+ products are 34.25, 35.12, and 34.55 eV. The results of calculations in the presence of strong electric field show that the field can make the vinylidene isomer unstable and the proton elimination spontaneous, but is unlikely to significantly reduce the barrier for the acetylene-vinylidene isomerization and to render the acetylene configuration unstable or metastable with respect to proton migration.

Dynamics of the O(1D) D2 reaction: A comparison between crossed molecular beam experiments and quasiclassical trajectory calculations on the lowest three potential energy surfaces

This paper describes crossed beam experiments and quasiclassical trajectory (QCT) calculations for the 'insertion' reaction O(1D)+D2 at a collision energy (Ec=25.9 kJ mol-1) much higher than the calculated barrier (∼8.4 kJ mol-1) for the competitive 'abstraction' mechanism, which takes place along the excited state potential energy surfaces (PES). Adiabatic QCT calculations were carried out on the ground 11A' and first excited 11A'' PESs developed by Dobbyn and Knowles. Non-adiabatic contributions from the excited 21A' PES to the reaction were considered by means of a trajectory surface hopping method. QCT calculations were also performed at Ec=22.2 kJ mol-1 to compare with previous experimental results. Excellent agreement was found between experiment and QCT predictions at Ec=22.2 kJ mol-1, while at the higher Ec of 25.9 kJ mol-1 only a qualitative agreement was noted. In all cases, the comparison was significantly improved with respect to QCT calculations on a previous version of the ground state PES.

Ultrafast non-adiabatic laser-induced photodissociation dynamics of CpMn(CO)3. An ab initio quantum chemical and dynamical study

The ultrafast laser driven dynamics of CpMn(CO)3, so-called cymantrene, have been investigated by wavepacket propagations on adiabatic coupled CASSCF/MR-CCI potential energy curves calculated for the electronic ground and low-lying metal-centered (MC) excited 1A′ and 1A″ states as a function of one Mn–CO bond elongation. The main features of the absorption spectrum as well as the electronic states populations are investigated by the time-dependent approach including numerical non-adiabatic couplings. The photo-induced simulations use selected laser pulse parameters close to the experimental ones (Daniel et al., Chem. Phys., 2001, 267, 247), where the frequencies correspond to the four main resonant Franck-Condon transitions which can prepare either the parent neutral molecule [CpMn(CO)3]* or the first photofragment [CpMn(CO)2]*.

CHEMICAL REACTIONS IN THE O(1D) + HCl SYSTEM II.

Using the accurate global potential energy surfaces for the 11A′, 11A′′, and 21A′ states reported in the previous sister Paper I, detailed quantum dynamics calculations are performed for these three adiabatic surfaces separately for J = 0 (J: total angular momentum quantum number). Overall reaction probabilities for O + HCl → OH + Cl and H + ClO, the branching ratio between the two reactions, effects of the initial rovibrational excitation, and product rovibrational distributions are evaluated in the total energy region E tot ≤ 0.9 eV. Significant contributions to the overall reaction dynamics are found from the two excited 11A′′ and 21A′ potential energy surfaces, clearly indicating the insufficiency of the dynamics only on the ground 11A′ surface. The detailed dynamics on the excited surfaces are reported in the third paper of this series.