Femtochemistry of Norrish Type-I Reactions: II. The Anomalous Predissociation Dynamics of Cyclobutanone on the S1 Surface

Methoxyacetone revisited

Conformational space of methoxyacetone (MA) was studied at the MP2/6-311++G(d,p) and DFT(B3LYP)/6-311++G(d,p) levels of theory. Computations predict MA to adopt four conformations, resulting from internal rotations around the O=C-C-O (Trans, Cis) and C-C-O-C (trans, gauche) dihedral angles. The Tt (Trans-trans) conformer is the most stable. The computed energies of two gauche (Tg and Cg) conformers fall in the 3-8 kJ mol-1 range above Tt and should account for 1/3 of the room-temperature gas-phase equilibrium. The energy of Ct form is 11 kJ mol-1 above Tt, and its expected population is negligible (below 1 %). In our earlier work, MA monomers were isolated in cryogenic argon matrices and characterized by infrared spectroscopy. In the experiment, only the most stable Tt conformer was detected in the sample. Signatures of the other conformers were not detected, either in freshly deposited samples, or in samples subjected to different UV irradiations. We rationalize those observations in terms of computed barriers for intramolecular torsions, indicating occurrence of conformational cooling during deposition. The experimental infrared spectrum of the Tt form is now assigned with the aid of anharmonic DFT computations. Exposure of MA to UV irradiation in the 300-260 nm range led to photolysis, according to the Norrish type II mechanism, resulting in dimer between enol acetone and formaldehyde observed as a cage-confined intermediate photoproduct. The subsequent photolysis resulted in the formation of carbon monoxide as the dominating photoproduct, formed in the Norrish type I photoreaction. Mechanistic interpretation of this photo decarbonylation reaction is presented.

Conformational Space, IR-Induced, and UV-Induced Chemistry of Carvacrol Isolated in a Low-Temperature Argon Matrix

In this work, monomers of carvacrol (5-isopropyl-2-methylphenol), a natural monoterpene exhibiting wide range bioactivity, were trapped in a cryogenic argon matrix and characterized by infrared spectroscopy, while quantum chemical calculations at the B3LYP and MP2 levels were employed to characterize the conformational landscape of the isolated molecule. Four conformers have been localized on the potential energy surface, and the factors accounting for their relative stability were analyzed. The two most stable conformers of carvacrol, differing in the relative orientation of the isopropyl group and both having the OH group pointing away from the vicinal methyl fragment, were identified in the cryomatrix for the first time. The individual spectral signatures of the two conformers were distinguished based on the change in their relative abundance induced by exposing the matrix to broadband infrared light. Matrix-isolated carvacrol was also irradiated with broadband UV light (λ > 200 nm), which resulted in the cleavage of the OH group. Recombination of the released H atom at the ortho- or para-position of the ring resulted in the formation of alkyl-substituted cyclohexadienones. These were found to undergo subsequent valence and open-ring isomerizations, leading, respectively, to the formation of a Dewar isomer and open-chain conjugated ketenes. Decarbonylation of the photoproducts was also observed for longer irradiation times. A mechanistic analysis of the observed photochemical transformations is presented.

Effects of ring-strain on the ultrafast photochemistry of cyclic ketones

Ring-strain in cyclic organic molecules is well-known to influence their chemical reactivity. Here, we examine the consequence of ring-strain for competing photochemical pathways that occur on picosecond timescales. The significance of Norrish Type-I photochemistry is explored for three cyclic ketones in cyclohexane solutions at ultraviolet (UV) excitation wavelengths from 255–312 nm, corresponding to an π* ← n excitation to the lowest excited singlet state (S₁). Ultrafast transient absorption spectroscopy with broadband UV/visible probe laser pulses reveals processes common to cyclobutanone, cyclopentanone and cyclohexanone, occurring on timescales of ≤1 ps, 7–9 ps and >500 ps. These kinetic components are respectively assigned to prompt cleavage of an α C–C bond in the internally excited S₁-state molecules prepared by UV absorption, vibrational cooling of these hot-S₁ molecules to energies below the barrier to C–C bond cleavage on the S₁ state potential energy surface (with commensurate reductions in the energy-dependent α-cleavage rate), and slower loss of thermalized S₁-state population. The thermalized S₁-state molecules may competitively decay by activated reaction over the barrier to α C–C bond fission on the S₁-state potential energy surface, internal conversion to the ground (S₀) electronic state, or intersystem crossing to the lowest lying triplet state (T₁) and subsequent C–C bond breaking. The α C–C bond fission barrier height in the S₁ state is significantly reduced by the ring-strain in cyclobutanone, affecting the relative contributions of the three decay time components which depend systematically on the excitation energy above the S₁-state energy barrier. Transient infra-red absorption spectra obtained after UV excitation identify ring-opened ketene photoproducts of cyclobutanone and their timescales for formation.

Ultrafast spectroscopy of ring-opening in three cyclic ketones reveals how ring-strain affects Norrish Type-I α-cleavage mechanisms.

Exploring Norrish type I and type II reactions: an ab initio mechanistic study highlighting singlet-state mediated chemistry

Norrish reactions are important photo-induced reactions in mainstream organic chemistry and are implicated in many industrially and biologically relevant processes and in the processing of carbonyl molecules in the atmosphere.

Photochemical Approaches to Complex Chemotypes: Applications in Natural Product Synthesis

The use of photochemical transformations is a powerful strategy that allows for the formation of a high degree of molecular complexity from relatively simple building blocks in a single step. A central feature of all light-promoted transformations is the involvement of electronically excited states, generated upon absorption of photons. This produces transient reactive intermediates and significantly alters the reactivity of a chemical compound. The input of energy provided by light thus offers a means to produce strained and unique target compounds that cannot be assembled using thermal protocols. This review aims at highlighting photochemical transformations as a tool for rapidly accessing structurally and stereochemically diverse scaffolds. Synthetic designs based on photochemical transformations have the potential to afford complex polycyclic carbon skeletons with impressive efficiency, which are of high value in total synthesis.

UV photolysis of α-cyclohexanedione in the gas phase

Ultraviolet absorption spectrum of α-cyclohexanedione (α-CHD) vapor in the wavelength range of 220-320 nm has been recorded in a 1 m long path gas cell at room temperature. With the aid of theoretical calculation, the band has been assigned to the S(2) ← S(0) transition of largely ππ* type. The absorption cross section at the band maximum (∼258 nm) is nearly 3 orders of magnitude larger compared to that for the S(2) ← S(0) transition of a linear α-diketo prototype, 2,3-pentanedione. The photolysis was performed by exciting the sample vapor near this band maximum, using the 253.7 nm line of a mercury vapor lamp, and the products were analyzed by mass spectrometry as well as by infrared spectroscopy. The identified products are cyclopentanone, carbon monoxide, ketene, ethylene, and 4-pentenal. Geometry optimization at the CIS/6-311++G** level predicts that the carbonyl group is pyramidally distorted in the excited S(1) and S(2) states, but the α-CHD ring does not show dissociative character. Potential energy curves with respect to a ring rupture coordinate (C-C bond between two carbonyl groups) for S(0), S(1), S(2), T(1), T(2), and T(3) states have been generated by partially optimizing the ground state geometry at DFT/B3LYP/6-311++G** level and calculating the vertical transition energies to the excited states by TDDFT method. Our analysis reveals that the reactions can take place at higher vibrational levels of S(0) as well as T(1) states.

The photodissociation mechanisms of acrylonitrile: Ab initio calculations on reaction channels and surface intersections

The dissociations of CH(2)CHCN into CH(2)CH+CN and CH(2)C+HCN in the S(0), T(1), and (1)pi(2)pi(C[triple bond]N) ( *) (definitions of pi orbitals can refer to computational details) states, have been explored at the complete active space self-consistent field level of theory employing the Dunning correlation consistent triple-zeta basis set. The lowest energy points of the surface crossing seams have been searched. Two conical intersections, from (1)pi(C[triple bond]N)pi(1) ( *) to (1)pi(2)pi(1) ( *) (CI(1)) and from (1)pi(2)pi(1) ( *) to S(0) (CI(2)), and one intersystem crossing point (T(1)/S(0)) have been located. The energies of all critical points have been recomputed with the multiconfigurational second-order perturbation method. At each conical intersection, derivative coupling and unscaled gradient difference vectors have been analyzed to determine the relaxation channels that the molecule may evolve in after nonradiative decay. Once the molecule is photoexcited to the (1)pi(2)pi(1) ( *) or (1)pi(C[triple bond]N)pi(1) ( *) state, it would relax along the similar pathway: funneling through CI(1) and then CI(2), and finally populate the ground state. Our results show that upon 193 nm photoexcitation, the most probable reaction channel is the ground-state HCN elimination following radiationless decays from excited states through surface crossings, which consists with experimental results J. Chem. Phys. 108, 5784 (1998). The investigated dissociation channels on the (1)pi(2)pi(C[triple bond]N) ( *) surface, which are inaccessible upon 193 nm photoexcitation, may provide information for reactions induced by higher energy excitations.

Study of Acetone Photodissociation over the Wavelength Range 248−330 nm: Evidence of a Mechanism Involving Both the Singlet and Triplet Excited States

Measurements of the acetyl yield from acetone photolysis have been made using laser flash photolysis/laser induced fluorescence. Phi(total)(lambda,p,T) was determined over the ranges: 266 < or = lambda/nm < or = 327.5, 0.3 < or = p/Torr < or = 400 and 218 < or = T/K < or = 295. The acetyl yield was determined relative to that at 248 nm by conversion to OH by reaction with O2. Linear Stern-Volmer plots (1/[OH] vs [M]) describe the data for lambda < 300 nm, but for lambda > 300 nm, nonlinear Stern-Volmer plots were observed. This behavior is interpreted as evidence for dissociation from two excited states of acetone: S1 when the Stern-Volmer plots are linear and both S1 and T1 when Stern-Volmer plots are nonlinear. A model for acetone photolysis is proposed that can adequately describe both the present and literature data. Barriers to dissociation are invoked in order to explain the dependence of pressure quenching of the acetone photolysis yields as a function of wavelength and temperature. This pressure quenching was observed to become more efficient with increasing wavelength, but it was only above approximately 300 nm that a significant T dependence was observed, which became more pronounced at longer wavelengths. This is the first study to observe a T-dependent phi(total)(lambda,p,T). A parametrized expression for phi(total)(lambda,p,T) has been developed and is compared against the recommended literature data by running box model simulations of the atmosphere. These simulations show that acetone photolysis occurs more slowly at the top of the troposphere.

Ultrafast Photodissociation Dynamics of Acetone at 195 nm: I. Initial-state, Intermediate, and Product Temporal Evolutions by Femtosecond Mass-Selected Multiphoton Ionization Spectroscopy

The photodissociation dynamics of the acetone S2 (n, 3s) Rydberg state excited at 195 nm has been studied by using femtosecond pump-probe mass-selected multiphoton ionization spectroscopy. For the first time, the temporal evolutions of the initial state, intermediates, and methyl products were simultaneously measured and analyzed for this reaction to elucidate the complex dynamics. Two mechanisms were considered: (1) the commonly accepted mechanism in which the primary dissociation occurs on the first triplet-state surface, and (2) the recently proposed mechanism in which the primary dissociation takes place on the first singlet-excited-state surface. Our results and analyses supported the validity of the new mechanism. On the other hand, the conventional mechanism was found to be inadequate to describe the observed dynamics. The temporal evolution of methyl products arising from the secondary dissociation of hot acetyl intermediates exhibited a very complex behavior that can be ascribed to the combination of a nonuniform initial vibrational distribution and the competition between dissociation and slow intramolecular vibrational redistribution.

Femtochemistry of trans-azomethane: a combined experimental and theoretical study

The dissociation dynamics of trans-azomethane upon excitation to the S,(n, pi) state with a total energy of 93 kcalmol-1 is investigated using femtosecond-resolved mass spectrometry in a molecular beam. The transient signal shows an opposite pump-probe excitation feature for the UV (307 nm) and the visible (615 nm) pulses at the perpendicular polarization in comparison with the signal obtained at the parallel polarization: The one-photon symmetry-forbidden process excited by the UV pulse is dominant at the perpendicular polarization, whereas the two-photon symmetry-allowed process initiated by the visible pulse prevails at the parallel polarization. At the perpendicular polarization, we found that the two C-N bonds of the molecule break in a stepwise manner, that is, the first C-N bond breaks in approximately 70 fs followed by the second one in approximately 100 fs, with the intermediate characterized. At the parallel polarization, the first C-N bond cleavage was found to occur in 100 fs with the intensity of the symmetry-allowed transition being one order of magnitude greater than the intensity of the symmetry-forbidden transition at the perpendicular polarization. Theoretical calculations using time-dependent density functional theory (TDDFT) and the complete active space self-consistent field (CASSCF) method have been carried out to characterize the potential energy surface for the ground state, the low-lying excited states, and the cationic ground state at various levels of theory. Combining the experimental and theoretical results, we identified the elementary steps in the mechanism. The initial driving force of the ultrafast bond-breaking process of trans-azomethane (at the perpendicular polarization) is due to the CNNC torsional motion initiated by the vibronic coupling through an intensity-borrowing mechanism for the symmetry-forbidden n-pi transition. Following this torsional motion and the associated molecular symmetry breaking, an S0/S1 conical intersection (CI) can be reached at a torsional angle of 93.1 degrees (predicted at the CASSCF(8.7)/cc-pVDZ level of theory). Funneling through the S0/S1, CI could activate the asymmetric C-N stretching motion, which is the key motion for the consecutive C-N bond breakages on the femtosecond time scale.

Femtochemistry of Norrish type-I reactions: IV. Highly excited ketones--experimental

Femtosecond dynamics of Norrish type-I reactions of cyclic and acyclic ketones have been investigated in real time for a series of 13 compounds using femtosecond-resolved time-of-flight mass spectrometry. A general physical description of the ultrafast processes of ketones excited into a high-lying Rydberg state is presented. It accounts not only for the results that are presented herein but also for the results of previously reported studies. For highly excited ketones, we show that the Norrish type-I reaction is nonconcerted, and that the first bond breakage occurs along the effectively repulsive S2 surface involving the C-C bond in a manner which is similar to that of ketones in the S1 state (E. W.-G. Diau et al. ChemPhysChem 2001, 2, 273-293). The experimental results show that the wave packet motion out of the initial Franck-Condon region and down to the S2 state can be resolved. This femtosecond (fs) internal conversion from the highly excited Rydberg state to the S2 state proceeds through conical intersections (Rydberg-valence) that are accessed through the C=O stretching motion. In one of these conical intersections, the internal energy is guided into an asymmetric stretching mode. This explains the previously reported pronounced nonstatistical nature of the reaction. The second bond breakage involves an excited-state acyl radical and occurs on a time scale that is up to one order of magnitude longer than the first. We discuss the details regarding the ion chemistry, which determines the appearance of the mass spectra that arise from ionization on the fs time scale. The experimental results presented here, aided by the theoretical work reported in paper III, provide a unified picture of Norrish reactions on excited states and on the ground-state potential energy surfaces.

Femtochemistry of Norrish type-I reactions: III. Highly excited ketones--theoretical

Time-dependant density functional theory (TDDFT) and ab initio methods (CASSCF and CASMP2) are applied here for the investigation of the excited-state potential energy surfaces of ketones studied experimentally in the accompanying paper, number IV in the series. The aim is to provide a general and detailed physical picture of the Norrish type-I reaction from S0 and S1 potentials (papers I and II) and from higher-energy potentials (papers III and IV). Particular focus here is on reactions following excitation to the 3s, 3p, and 3d Rydberg state and to the (nz-->pi*) and (pi-->pi*) valence states. It is shown that the active orbitals in the CASSCF calculations can be chosen so that accurate results are obtained with a small active space. Dynamic corrections of the state-specific CASSCF energies at the multireference MP2 level do not improve the results for the Rydberg states but are significant for the valence states. The geometries of the Rydberg states are similar to the ground state; the S1 and other valence states are not. A common property of the valence states is the elongated CO bond and the pyramidalization of the carbonyl carbon atom. As a consequence, these valence states cross all Rydberg states along the CO stretching coordinate and provide an efficient pathway down to the 3s Rydberg states (S2) through a series of conical intersections (CIs). The nonadiabatic coupling vector of the CI between the (pi-->pi*) and the 3s Rydberg states guides energy channeling into the asymmetric CC-stretching mode. The energy demand for the CC bond breakage (Norrish type-I) on the S2 surface is lower than that of the CI leading to the S1 state. This CC bond breakage leads to a linear excited state acetyl radical (3s Rydberg). Crossing a small barrier the 3s acyl radical can access a CI leading either to a second CC bond breakage or to a hot ground-state acetyl radical. The barriers for the Norrish type-I reaction on the various excited-state surfaces can be rationalized within the framework of valence-bond theory. The dynamic picture of the Norrish type-I reactions is now clear: The excitation to high-energy states leads to the nonconcerted breakage of the alpha-CC bonds by an "effective downhill" potential in space involving the active excitation center CO, CC stretching, and CCO bending nuclear motions, but not, as usually thought, a direct repulsive potential along the CC bond. In our accompanying paper (part IV), it is shown that the results from the experimental investigations of Norrish type-I reactions on the femtosecond timescale are consistent with these theoretical results.