Triplet Ketene Research Articles

A simplified ab initio treatment of diradicaloid structures produced from stretching and breaking chemical bonds.

The present investigation reports on the prospect of using state specific multireference perturbation theory (SSMRPT) with an improved virtual orbital complete active space configuration interaction (IVO-CASCI) reference function (IVO-SSMRPT) to generate potential energy surfaces (PESs) for molecular systems [such as CH4, C2H6, C2H4, H2O2, LiH, and KN] by stretching and breaking of suitable bonds with modest basis sets. We have also revisited the dissociation energy profile of triplet ketene which exhibits a step-like structure in the observed rate. The application of the method has also been made to the ionization energies of H2O. Although the perturbative corrections are obtained by the diagonalization of the effective Hamiltonian, in IVO-SSMRPT, only one physically relevant solution is achievable. It is parameter free and does not require any threshold to avoid the intruder problem. It is strictly size-extensive and size-consistent provided that local orbitals are used. The PESs obtained with our approach are smooth all along the reaction path. Our estimates are in close agreement with the available reference data indicating that IVO-SSMRPT is a robust paradigm for the accurate computation of ground, excited and ionized states as it captures the mutual inter-play of different flavors of correlation effects in a balanced and accurate way.

Ab Initio Trajectory Study on Triplet Ketene Photodissociation via Statistical Sampling of the Crossing Seam.

Triplet ketene exhibits a steplike structure in the experimentally observed dissociation rate, but its mechanism is still unclear despite many theoretical efforts. A previous surface-hopping simulation at the CASSCF level suggests that nonadiabatic transition from the S0 to T1 states creates the T1 species in a highly nonstatistical manner, which raises the question of whether the use of statistical rate theory is valid in itself for the T1 state. Here, we study this problem by performing ab initio trajectory simulation at the multireference second-order Möller-Plesset perturbation (MRMP) level of theory. Since the MRMP theory is too expensive for such a trajectory calculation, we first construct dual-level potential energy surfaces (PESs) for the S0 and T1 states by calibrating the PESs at the B3LYP level with a limited set of MRMP energies. We then introduce the assumption of vibrational equilibrium on the S0 surface and characterize the S0 → T1 crossing points using the conditional microcanonical distribution on the S0/T1 seam surface. The latter distribution is obtained by running a constrained trajectory on the seam surface by use of an efficient SHAKE-like method. Subsequently, we propagate a number of T1 trajectories from the seam surface to obtain the dissociation rate. The result shows that (i) the S0 → T1 crossing points are localized mainly in the T1 reactant region; (ii) the lifetime on the T1 surface is about 30 ps at the MRMP level, which is 2 orders of magnitude greater than the previous estimate obtained from the surface-hopping simulation at the CASSCF level (∼100 fs); and (iii) the calculated T1 dissociation rate agrees reasonably well with classical transition state theory. These results suggest that the T1 dissociation is rather statistical, given that the T1 trajectories are initiated from the conditional microcanonical distribution on the seam surface.

Multireference coupled-cluster calculation of the dissociation energy profile of triplet ketene

Triplet ketene exhibits a steplike structure in the experimentally observed photodissociation rates, but its mechanism is still unknown despite many theoretical efforts. Here we revisit this problem by calculating the potential energy profile of triplet ketene with the Adamowicz and Mukherjee multireference coupled-cluster (MRCC) theories. At the MRCCSD level, the imaginary frequency of the dissociation barrier is calculated to be about 300icm−1, which is slightly smaller than the previous estimates but is still much greater than the expected maximum value for reproducing the observed steps (100icm−1). This implies that other types of mechanisms (including nonadiabatic ones) may be more plausible for the observed steps.

Reaction of C2H2+ (n·ν2, m·ν5) with NO2: Reaction on the singlet and triplet surfaces

Integral cross sections and product recoil velocity distributions were measured for reaction of C(2)H(2)(+) with NO(2), in which the C(2)H(2)(+) reactant was prepared in its ground state, and with mode-selective excitation in the cis-bend (2ν(5)) and CC stretch (n · ν(2), n = 1, 2). Because both reactants have one unpaired electron, collisions can occur with either singlet or triplet coupling of these unpaired electrons, and the contributions are separated based on distinct recoil dynamics. For singlet coupling, reaction efficiency is near unity, with significant branching to charge transfer (NO(2)(+)), O(-) transfer (NO(+)), and O transfer (C(2)H(2)O(+)) products. For triplet coupling, reaction efficiency varies between 13% and 19%, depending on collision energy. The only significant triplet channel is NO(+) + triplet ketene, generated predominantly by O(-) transfer, with a possible contribution from dissociative charge transfer at high collision energies. NO(2)(+) formation (charge transfer) can only occur on the singlet surface, and appears to be mediated by a weakly bound complex at low energies. O transfer (C(2)H(2)O(+)) also appears to be dominated by reaction on the singlet surface, but is quite inefficient, suggesting a bottleneck limiting coupling to this product from the singlet reaction coordinate. The dominant channel is O(-) transfer, producing NO(+), with roughly equal contributions from reaction on singlet and triplet surfaces. The effects of C(2)H(2)(+) vibration are modest, but mode specific. For all three product channels (i.e., charge, O(-), and O transfer), excitation of the CC stretch fundamental (ν(2)) has little effect, 2 · ν(2) excitation results in ∼50% reduction in reactivity, and excitation of the cis-bend overtone (2 · ν(5)) results in ∼50% enhancement. The fact that all channels have similar mode dependence suggests that the rate-limiting step, where vibrational excitation has its effect, is early on the reaction coordinate, and branching to the individual product channels occurs later.

Quantum Mechanical Reaction Probability of Triplet Ketene at the Multireference Second-Order Perturbation Level of Theory

Triplet ketene exhibits a steplike structure in the experimentally observed dissociation rates, but its mechanism is still unknown despite many theoretical efforts in the past decades. In this paper we revisit this problem by quantum mechanically calculating the reaction probability with multireference-based electronic structure theory. Specifically, we first construct an analytical potential energy surface of triplet state by fitting it to about 6000 ab initio energies computed at the multireference second-order Mller-Plesset perturbation (MRMP2) level. We then evaluate the cumulative reaction probability by using the transition state wave packet method together with an adiabatically constrained Hamiltonian. The result shows that the imaginary barrier frequency on the triplet surface is 328i cm-1, which is close to the CCSD(T) result (321i cm-1) but is likely too large for reproducing the experimentally observed steps. Indeed, our calculated reaction probability exhibits no signature of steps, reflecting too strong tunneling effect along the reaction coordinate. Nevertheless, it is emphasized that the flatness of the potential profile in the transition-state region (which governs the degree of tunneling) depends strongly on the level of electronic structure calculation, thus leaving some possibility that the use of more accurate theories might lead to the observed steps. We also demonstrate that the triplet potential surface differs significantly between the CASSCF and MRMP2 results, particularly in the transition-state region. This fact seems to require more attention when studying the "nonadiabatic" scenario for the steps, in which the crossing seam between S0 and T1 surfaces is assumed to play a central role.

ChemInform Abstract: Evidence for Stepwise Nitrogen Extrusion and Ring Expansion Upon Photolysis of Phenyl Azide.

The temperature dependence of the products formed on photolysis of two independent precursors of phenylnitrene has been studied. The identity of the product-forming intermediates (triplet phenylnitrene versus ketene imine) produced on photolysis of phenyl azide and benzenesulfoximine in the presence of diethylamine show the same qualitative variation with temperature. An excited state of the nitrene precursor is not responsible for the known temperature dependence of the distribution of stable products. The data are consistent with stepwise formation of singlet phenylnitrene and subsequent expansion to azacycloheptatetraene. The singlet and triplet energies of phenyl azide have been determined by energy transfer

Theoretical Study on the Electronic States of Formylcarbene (HC·CHO) and Triplet Ketene (H2 CCO) and the Ultraviolet Absorption Spectra Attributable to These Molecules

The adiabatic energies, vibrational frequencies, and geometries of the ground and excited electronic states of formylcarbene and the triplet electronic states of ketene are calculated employing the state-of-the art ab initio methods. With the help of these calculations, certain ultraviolet (UV) absorption bands observed in the flash photolysis of oxazole and iso-oxazole are assigned to formylcarbene and triplet ketene.

Analysis of the time-resolved FTIR spectra produced by the photolysis of alkyl phenylglyoxylates

The photochemistry of alkyl phenylglyoxylates (APG) was further investigated using time-resolved infrared spectroscopy. The primary focus was on the analysis of weak transient bands around 1828 and 1730 cm(-1) in the time-resolved FTIR spectra of glyoxylates. The observed transients were assigned to benzoyl and alkyl mandelate ester radicals, respectively. The formation of benzoyl radical was fast and attributed to the Norrish Type I process. In addition, the intensities of the strong FTIR bands around 1680 and 2100 cm(-1) were used to analyze the yields of the triplet state and ketene, respectively. These new and previous data on APG photochemistry are discussed in relation to the acrylate polymerization photoinitiation by alkyl phenylglyoxylates.

Addition Reactions of 1D and 3P Atomic Oxygen with Acetylene. Potential Energy Surfaces and Stability of the Primary Products. Is Oxirene Only a Triplet Molecule? A Theoretical Study

The potential energy surfaces (PES) of the primary addition reactions of atomic oxygen O(1D) and O(3P) on acetylene were done using Density Functional Theory with the standard B3LYP hybrid functional. The crucial areas of these PES were then refined using the UMP2 and CCSD(T) schemes and, in few cases, CASPT2, MCSCF, and MR-AQCC ones. The stability of these primary products was studied. The reactivity of C2H2 and O, especially in low-temperature matrix conditions, is discussed on these bases. The singlet surface shows that the insertion of the oxygen into the C−H bond, resulting in ethynol HC⋮COH, is the main reaction. An addition onto the C⋮C bond, leading to oxirene and/or formylcarbene HC−CHO is nevertheless to be considered. A particular attention was given to the oxirene-formylcarbene region and MR-AQCC calculations predict an extremely flat area in agreement with previous CCSD(T) results. Secondary transposition of formylcarbene into ketene H2CCO could be avoided at very low temperature. From the triplet state of the oxygen atom, the triplet formylcarbene is expected as the major product. Nevertheless, a cyclic nonplanar species, the triplet state of oxirene, is stable on the triplet surface and could be trapped at low temperature, provided an intersystem triplet singlet crossing does not occur, which would lead to singlet oxirene, if stable. If not, this process would lead to singlet formyl carbene and finally to its triplet ground state. Indeed, singlet and triplet surfaces are very close to one another in this part of the PES. Hydrogen transposition giving triplet ketene can be ruled out in matrix conditions. The formation of cyclic three-member carbenes −C−O−CH2−, as well singlet as triplet, is unlikely though both species are rather stable on their respective PES.

Fragmentation surface of triplet ketene

The photofragmentation of ketene to triplet methylene and carbon monoxide is a paradigm for unimolecular dissociation over an exit channel barrier. The geometric structures, quadratic force fields, and harmonicvibrational frequencies of the triplet ketene reactant, the 3B1 CH2+1Σ+ CO products, and both in-plane (CsII) and out-of-plane (CsI) transition states have been determined at the TZ(2d1f,2p) coupled-cluster singles and doubles (CCSD) level of theory. An unusual, shallow minimum at long range [R(C–C)=4.0 Å] has also been discovered and characterized. A rigorous mapping and analytic parametrization has been performed of the TZ(2d1f,2p) CCSD intrinsic reaction paths connecting the CsII transition state to both the reactant and products. Final potential-energy functions along the entire reaction path have been determined with the aid of [(C,O)/H] atomic-orbital basis sets as large as [6s5p4d3f2g1h/5s4p3d2f1g] and electron correlation treatments as extensive as the coupled-cluster method through triple excitations [CCSDT or CCSD(T)]. The final theoretical curve is highlyanharmonic in the transition-state region, displaying a classical barrier of1045 cm-1, a critical C–C distance of 2.257 Å, and a barrier frequency of321i cm-1. Effective barrier frequencies in the 100i cm-1 range which resultfrom RRKM modelling with tunnelling corrections of the observed steplike structure in the triplet ketene dissociation rate constant are thus shown to be physically untenable. Various implications of such abinitio predictions on unravelling the intricacies of the fragmentation dynamics are discussed.

Dynamics of the photodissociation of triplet ketene

Calculations of the microcanonical dissociation rate for vibrationally excited ketene on the first excited triplet surface (T1) are presented. The calculations utilize the quantum reactive scattering methodology of absorbing boundary conditions (ABC) with a discrete variable representation (DVR) to obtain the cumulative reaction probability for dissociation over the barrier. Model 1- and 2-degree of freedom potential energy surfaces for the T1 surface were obtained by fitting to the best available ab initio structures, energies, and frequencies. The dissociation rates in these reduced-dimensionality calculations give good overall agreement with the experimentally measured rates, although the steplike features seen in the experiments are washed out by the tunneling through the narrow barrier predicted in the ab initio calculations. Further model calculations reveal that a barrier frequency of approximately 50–100i cm−1 is required to recover the step structure seen experimentally, which suggests that there is either another transition state region on the T1 surface farther out towards the product channel, or that there is surface-hopping dynamics taking place between the T1 and S0 ketene potential energy surfaces, or that the ab initio barrier frequency is simply too large.

Transition state vibrational level thresholds for the dissociation of triplet ketene

Rate constants for the unimolecular dissociation of ketene (CH2CO) and deuterated ketene (CD2CO) have been measured at the threshold for the production of CH2 (X̃ 3B1) or CD2 (X̃ 3B1) and CO (X̃ 1Σ+) by photofragmentation in a cold jet. The rate constant increases in a stepwise manner as energy increases. This is in accord with the long-standing premise that the rate of a unimolecular reaction is controlled by flux through quantized transition-state thresholds at each energy level for vibrational motion orthogonal to the reaction coordinate. The first step in rate constant and/or photofragment excitation (PHOFEX) spectrum gives accurate values for the barrier to dissociation above the zero-point energy of the products, 1281±15 cm−1 for CH2CO and 1071±40 cm−1 for CD2CO. The measured rate constants are fit by Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The vibrational frequencies at the transition state obtained from the fits are compared with ab initio results. Vibrational motions at the transition state orthogonal to the reaction coordinate are also revealed in CO product rotational distributions. Calculations using an impulsive model which includes vibrational motions at the transition state reproduce the experimental dependence of the PHOFEX spectra on the CO J state quite well. The small dependence of rate constant on jet temperature (4–30 K) indicates that the Ka quantum number for rotation about its symmetry axis is conserved in the energized ketene molecule.

Evidence for stepwise nitrogen extrusion and ring expansion upon photolysis of phenyl azide

The temperature dependence of the products formed on photolysis of two independent precursors of phenylnitrene has been studied. The identity of the product-forming intermediates (triplet phenylnitrene versus ketene imine) produced on photolysis of phenyl azide and benzenesulfoximine in the presence of diethylamine show the same qualitative variation with temperature. An excited state of the nitrene precursor is not responsible for the known temperature dependence of the distribution of stable products. The data are consistent with stepwise formation of singlet phenylnitrene and subsequent expansion to azacycloheptatetraene. The singlet and triplet energies of phenyl azide have been determined by energy transfer

Photofragmentation of ketene to CH2(~X3B1) + CO. 1. Barrier height and dissociation rate constant

Ketene cooled in a supersonic free jet is photolyzed in the ultraviolet range. The appearance rate of fragment CO from triplet ketene as a function of ultraviolet photon energy is measured by vacuum-ultraviolet laser-induced fluorescence. A tunneling-corrected RRKM theory is used to fit the observed dissociation rate of ketene using the ab initio vibrational frequencies of the transition state. The density of states inferred from the RRKM fit is a factor of 2 greater than the density of states of the ground electronic state (S{sub 0}) and 300 times greater than that of T{sub 1}. Tunneling corrections are necessary to fit the dissociation rate below the top of the exit barrier. The experimental rates show a sharp change in slope at 28,290-cm{sup {minus}1} photolysis frequency. The height of the exit barrier is determined to be 3.79 {plus minus} 0.06 kcal/mol above ground-state fragments. The imaginary frequency that fits the data is (150 {plus minus} 25)i cm{sup {minus}1}, much lower than the ab initio frequency of 523i cm{sup {minus}1}. At higher energies the RRKM theory predicts faster rates than are observed. It is possible that the intersystem crossing rate is relatively independent of energy and becomes the rate-determining step for formationmore » of triplet products at higher energies.« less

Reaction paths for the dissociation ã 3A″ CH2CO→X̃ 3B1 CH2 + X̃ 1Σ+ CO

An ab initio investigation of the (CIIs) in-plane bent 3A″ CH2CO→X̃ 3B1 CH2+X̃ 1∑+CO and the (CIs) out-of-plane bent 3A′ CH2CO→X̃ 3B1 CH2+X̃ 1∑+CO dissociation paths has been performed. Geometrical structures, vibrational frequencies, and quadratic force constants have been determined at the DZP SCF and DZP CISD levels of theory for the X̃ 1A1, 3A″, and 3A′ states of ketene and for the 3A″ and 3A′ transition states for dissociation. The DZP CISD structure for à 1A″ ketene is also reported. Final energetic predictions for triplet ketene dissociation have been obtained from large-basis (QZ2P and QZ2P+f) UMP4(SDTQ) calculations at the DZP CISD geometries. The CIIs stationary point for 3A″ ketene dissociation is a true transition state with r(C–C)=2.071 Å at the DZP CISD level of theory. The corresponding CIs stationary point for 3A′ ketene is actually a super transition state for the interconversion of two equivalent 3A″CIIs transition states for dissociation. Final theoretical predictions of Te=19 400 cm−1 and T0=19 150 cm−1 are made for the adiabatic excitation energy of the ã 3A″ state of ketene, and a value of 22.3 kcal/mol is proposed for the 3A″ dissociation energy.

Rotational state distribution of CO photofragments from triplet ketene

The nascent rotational state distribution of CO(v″=0,J″) following excimer laser photolysis of ketene at 351 nm has been determined under collisionless conditions in a flow cell. At this low excitation energy dissociation can only take place on the triplet potential surface leading to CH2(X̃ 3B1) and CO(X̃ 1Σ+). The available energy permits only the vibrational ground state of CO to be populated. The observed rotational distribution of CO(v″=0,J″) deviates drastically from a phase space theory statistical distribution as well as from a thermal one. A Boltzmann plot of this distribution exhibits a population inversion for J″<13. The nonstatistical behavior is attributed to a barrier along the dissociation path. The fragments are repelled too rapidly for energy to be randomized between them. Thus the photofragmentation dynamics of triplet ketene contrasts markedly with dissociation on the singlet surface which has no barrier and gives a statistical CO rotational state distribution. An impulsive model calculation for the ab initio transition state geometry is in surprisingly good agreement with the experimental energy partitioning among the fragment degrees of freedom. This suggests that the CCO bond angle is strongly bent at the top of the barrier and that the barrier height is a substantial fraction of available energy.

ChemInform Abstract: High‐Temperature Reactions of Triplet Methylene and Ketene with Radicals.

Chemischer InformationsdienstVolume 17, Issue 38 Preparative Organic Chemistry ChemInform Abstract: High-Temperature Reactions of Triplet Methylene and Ketene with Radicals. P. FRANK, P. FRANKSearch for more papers by this authorK. A. BHASKARAN, K. A. BHASKARANSearch for more papers by this authorTH. JUST, TH. JUSTSearch for more papers by this author P. FRANK, P. FRANKSearch for more papers by this authorK. A. BHASKARAN, K. A. BHASKARANSearch for more papers by this authorTH. JUST, TH. JUSTSearch for more papers by this author First published: September 23, 1986 https://doi.org/10.1002/chin.198638110Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume17, Issue38September 23, 1986 RelatedInformation

High-temperature reactions of triplet methylene and ketene with radicals

The reactions of CH/sub 2/ and ketene with CH/sub 2/ and H have been investigated at elevated temperatures in the postshock region behind reflected shocks. Atomic (ARAS) and molecular resonance absorption spectrometry was used to record simultaneously H-atom and CO-molecule concentration profiles. The thermal decomposition of very low initial concentrations of ketene (2-50 ppm CH/sub 2/CO/Ar) served as a source for methylene. The experiments were conducted in the temperature range of 1650-2800 K at total densities of 5 x 10/sup -6/ to 1.3 x 10/sup -5/ mol cm/sup -3/. For temperatures above 2000 K the following rate coefficients (k in cm/sup 3/ mol/sup -1/ s/sup -1/) were deduced: CH/sub 2/ + CH/sub 2/ = C/sub 2/H/sub 2/ + 2H, k = (10 +/- 2) x 10/sup 13/; CH/sub 2/ + H = CH + H/sub 2/, k = (8 +/- 2) x 10/sup 12/. For temperatures between 1650 and 1850 K a value of (1.8 +/- 0.6) x 10/sup 13/ is deduced for the reaction of ketene with H atoms. 30 references, 9 figures, 1 table.

On the absence of triplet ketene in ketene excited in the region 2800–3660 Å

The sensitized biacetyl emission technique was used in an attempt to detect triplet ketene molecules, which might be formed when ketene is excited by light ranging in wavelength from 2800 to 3600 Å. No sensitized emission was observed in this exciting wavelength region thus confirming the recent results of Voisey (1) which also show that triplet ketene is not produced when ketene is excited by light between 2800 and 3200 Å, and that the triplet CH2 found in such systems must arise from some source other than triplet ketene.