Triplet Methylene Research Articles

Accurate ab initio potential energy surface, rovibrational energy levels and resonance interactions of triplet ( 3 B1 ) methylene.

In this work, we report rovibrational energy levels for four isotopologues of methylene (CH2 , CHD, CD2 , and 13 CH2 ) in their ground triplet electronic state ( 3 B1 ) from variational calculation up to ~10,000 cm-1 and using a new accurate ab initio potential energy surface (PES). Triplet methylene exhibits a large-amplitude bending vibration and can reach a quasilinear configuration due to its low barrier (~2000 cm-1 ). To construct the ab initio PES, the Dunning's augmented correlation-consistent core-valence orbital basis sets were employed up to the sextuple-ζ quality [aug-cc-pCVXZ, X = T, Q, 5, and 6] combined with the single- and double-excitation unrestricted coupled cluster approach with a perturbative treatment of triple excitations [RHF-UCCSD(T)]. We have shown that the accuracy of the ab initio energies is further improved by including the corrections due to the scalar relativistic effects, DBOC and high-order electronic correlations. For the first time, all the available experimental rovibrational transitions were reproduced with errors less than 0.12 cm-1 , without any empirical corrections. Unlike more "traditional" nonlinear triatomic molecules, we have shown that even the energies of the ground vibrational state (000) with rather small rotational quantum numbers are strongly affected by the very pronounced rovibrational resonance interactions. Accordingly, the polyad structure of the vibrational levels of CH2 and CD2 was analyzed and discussed. The comparison between the energy levels obtained from the effective Watson A-reduced Hamiltonian, from the generating-function approach and from a variational calculation was given.

Dissection of the Multichannel Reaction O(3P) + C2H2: Differential Cross-Sections and Product Energy Distributions.

The O(3P) + C2H2 reaction plays an important role in hydrocarbon combustion. It has two primary competing channels: H + HCCO (ketenyl) and CO + CH2 (triplet methylene). To further understand the microscopic dynamic mechanism of this reaction, we report here a detailed quasi-classical trajectory study of the O(3P) + C2H2 reaction on the recently developed full-dimensional potential energy surface (PES). The entrance barrier TS1 is the rate-limiting barrier in the reaction. The translation of reactants can greatly promote reactivity, due to strong coupling with the reaction coordinate at TS1. The O(3P) + C2H2 reaction progress through a complex-forming mechanism, in which the intermediate HCCHO lives at least through the duration of a rotational period. The energy redistribution takes place during the creation of the long-lived high vibrationally (and rotationally) excited HCCHO in the reaction. The product energy partitioning of the two channels and CO vibrational distributions agree with experimental data, and the vibrational state distributions of all modes of products present a Boltzmann-like distribution.

Henry Schaefer wins Stone Award

Henry F. Schaefer III, Graham Perdue Professor of Chemistry and director of the Center for Computational Quantum Chemistry at the University of Georgia, is the winner of the 2018 Charles H. Stone Award, presented every 2 years by the American Chemical Society Carolina-Piedmont Section to an outstanding chemist in the southeastern US. The award recognizes contributions to the field of chemistry through activities in the scientific community, public outreach, education, and research. Schaefer’s research focuses on developing new theoretical and computational methods to solve important problems in chemistry. Among his research contributions is the discovery of the bent structure of triplet methylene.

Mechanism and rate constants of the CH2 + CH2CO reactions in triplet and singlet states: A theoretical study

Ab initio and density functional CCSD(T)-F12/cc-pVQZ-f12//B2PLYPD3/6-311G** calculations have been performed to unravel the reaction mechanism of triplet and singlet methylene CH2 with ketene CH2 CO. The computed potential energy diagrams and molecular properties have been then utilized in Rice-Ramsperger-Kassel-Marcus-Master Equation (RRKM-ME) calculations of the reaction rate constants and product branching ratios combined with the use of nonadiabatic transition state theory for spin-forbidden triplet-singlet isomerization. The results indicate that the most important channels of the reaction of ketene with triplet methylene lead to the formation of the HCCO + CH3 and C2 H4 + CO products, where the former channel is preferable at higher temperatures from 1000 K and above. In the C2 H4 + CO product pair, the ethylene molecule can be formed either adiabatically in the triplet electronic state or via triplet-singlet intersystem crossing in the singlet electronic state occurring in the vicinity of the CH2 COCH2 intermediate or along the pathway of CO elimination from the initial CH2 CH2 CO complex. The predominant products of the reaction of ketene with singlet methylene have been shown to be C2 H4 + CO. The formation of these products mostly proceeds via a well-skipping mechanism but at high pressures may to some extent involve collisional stabilization of the CH3 CHCO and cyclic CH2 COCH2 intermediates followed by their thermal unimolecular decomposition. The calculated rate constants at different pressures from 0.01 to 100 atm have been fitted by the modified Arrhenius expressions in the temperature range of 300-3000 K, which are proposed for kinetic modeling of ketene reactions in combustion. © 2018 Wiley Periodicals, Inc.

Theoretical Study on Reaction Pathways of Methyl Radical with Ethylamine

Theoretical Study on Reaction Pathways of Methyl Radical with Ethylamine

Kinetic mechanisms of hydrogen abstraction reactions from methanol by methyl, triplet methylene and formyl radicals

Although H-abstraction reactions from methanol by methyl radical (CH3), triplet methylene radical (3CH2) and formyl radical (HCO) are important for methanol pyrolysis and combustion, the kinetic mechanisms of those reactions are indistinct and need to be better understood. Ab initio calculations were performed to optimize geometries of transition states using Gaussian 09W package with the B3LYP density functional at the 6-311++G (3df, 3pd) basis set. Energy barrier and minimum energy path (MEP) on the reaction potential energy surface were calculated with MP2 method at the same basis set. The canonical variational transition state theory (CVT) with Eckart tunneling correction were employed to compute rate constants over the temperature range 298–2000K. Results show that, for reaction CH3OH+CH3, barrier height for hydroxymethyl channel (R1a) is 12.59kcal/mol and for methoxyl channel (R1b) is 13.82kcal/mol. Rate constants for channels R1a and R1b are k1a=3.635×10−31T5.58exp(−1962/T)cm3molecule−1s−1, k1b=4.342×10−28T4.61exp(−4284/T)cm3molecule−1s−1, respectively. For reaction CH3OH+3CH2, barrier heights for hydroxymethyl channel (R2a) and methoxyl channel (R2b) are 7.68kcal/mol and 9.19kcal/mol, respectively. Rate constant for channel R2a is k2a=5.885×10−28T4.19exp(−1813/T)cm3molecule−1s−1 and for channel R2b is k2b=2.266×10−28T4.26exp(−2833/T)cm3molecule−1s−1. For reaction CH3OH+HCO, barrier heights for hydroxymethyl channel (R3a) and methoxyl channel (R3b) are 21.88kcal/mol and 28.83kcal/mol, respectively. Rate constant for channel R3a is k3a=8.156×10−38T7.37exp(−4900/T)cm3molecule−1s−1 and for channel R3b is k3b=2.007×10−27T4.34exp(−11,605/T)cm3molecule−1s−1. This study is beneficial for improving the kinetic mechanism of methanol pyrolysis and combustion.

Controlling factors in the rates of oxidation of anilines and phenols by triplet methylene blue in aqueous solution.

Anilines and phenols are structurally similar compound classes that both are susceptible to oxidation by excited state triplet sensitizers but undergo oxidation by different mechanisms. To gain an understanding of the factors that control the rate of oxidation of anilines and phenols by triplet excited states, a kinetic study was performed on the oxidation of substituted anilines and phenols by methylene blue. The rate constants of one-electron transfer from anilines to triplet state methylene blue and their dependence on the reaction free energy are well fit to a Sandros-Boltzmann model. The observed rate constants are also well modeled when aniline oxidation potentials derived computationally are used. For phenols, the proton-coupled electron transfer rate constants were found to correlate primarily with O-H bond dissociation free energy and secondarily with phenol pKa. Rate constants for phenols could be modeled using computed bond dissociation free energies. These results provide a basis for predicting aniline and phenol oxidation rates, which could be valuable, for example, in assessing the likely persistence and fate of aniline- and phenol-based aqueous environmental pollutants.

Dynamics of the O(3P) + C2H2 reaction from crossed molecular beam experiments with soft electron ionization detection

The reaction between ground state oxygen atoms, O((3)P), and the acetylene molecule, C2H2, has been investigated in crossed molecular beam experiments with mass-spectrometric detection and time-of-flight analysis at three different collision energies, Ec = 34.4, 41.1 and 54.6 kJ mol(-1). From product angular and velocity distribution measurements of the HCCO and CH2 products in the laboratory frame, product angular and translational energy distributions in the center-of-mass frame were determined. Measurements on the CH2 product were made possible by employing for product detection the recently implemented soft electron-ionization (EI) technique with low-energy, tunable electrons, which has permitted suppressing interference coming from the dissociative ionization of reactants, products and background gases. It has been found that the title reaction leads only to two competing channels: H + HCCO (ketenyl) and CO + CH2 (triplet methylene). The branching ratio of cross sections between the two competing channels has been determined to be σ(HCCO)/[σ(HCCO) + σ(CH2)] = 0.79 ± 0.05, independent of collision energy within the experimental uncertainty. This value is in line with that obtained in the most recent and accurate kinetics determination at room temperature as well as with that predicted from recent theoretical calculations based on statistical rate theory and weak-collision master equation analysis and on dynamics surface-hopping quasiclassical trajectory calculations on-the-fly on coupled triplet/singlet ab initio potential energy surfaces. The firm assessment of the branching ratio as a function of translational energy for this important reaction, besides its fundamental significance, is of considerable relevance for the implementation of theoretical models of hydrocarbon combustion.

Temperature Dependent Kinetics (195−798 K) and H Atom Yields (298−498 K) from Reactions of1CH2with Acetylene, Ethene, and Propene

The rate coefficients for the removal of the excited state of methylene, (1)CH(2) (a(1)A(1)), by acetylene, ethene, and propene have been studied over the temperature range 195-798 K by laser flash photolysis, with (1)CH(2) being monitored by laser-induced fluorescence. The rate coefficients of all three reactions exhibit a negative temperature dependence that can be parametrized as k((1)CH(2)+C(2)H(2)) = (3.06 +/- 0.11) x 10(-10) T ((-0.39+/-0.07)) cm(3) molecule(-1) s(-1), k((1)CH(2)+C(2)H(4)) = (2.10 +/- 0.18) x 10(-10) T ((-0.84+/-0.18)) cm(3) molecule(-1) s(-1), k((1)CH(2)+C(3)H(6)) = (3.21 +/- 0.02) x 10(-10) T ((-0.13+/-0.01)) cm(3) molecule(-1) s(-1), where the errors are statistical at the 2sigma level. Removal of (1)CH(2) occurs by chemical reaction and electronic relaxation to ground state triplet methylene. The H atom yields from the reactions of (1)CH(2) with acetylene, ethene, and propene have been determined by laser-induced fluorescence over the temperature range 298-498 K. For the reaction with propene, H atom yields are close to the detection limit, but for acetylene and ethene, the fraction of H atom production is approximately 0.88 and 0.71, respectively, at 298 K, rising to unity by 398 K, with the balance of the reaction with acetylene presumed to be electronic relaxation. Experimental constraints limit studies to a maximum of 1 Torr of bath gas; master equation calculations using an approach that allows treatment of intermediates with deep energy wells have been carried out to explore the role of collisional stabilization for the reaction of (1)CH(2) with acetylene. Stabilization is calculated to be insignificant under the experimental conditions, but does become significant at higher pressures. Between pressures of 100 and 1000 Torr, propyne and allene are formed in similar amounts with a slight preference for propyne. At higher pressures propyne formation becomes about a factor two greater than that of allene, and above 10(5) Torr (300 < T (K) < 600) cyclopropene formation starts to become significant. The implications of temperature-dependent (1)CH(2) relaxation on the roles of (1)CH(2) in chemical mechanisms for soot formation are discussed.

Ab initio surfaces of singlet and triplet methylene

Ab initio minimum energy hypersurfaces of singlet and triplet methylene (CH2) are examined using SCSCF. SCSCF + CI, and MCSCF methods with a double-zeta plus polarization basis.

Computational Study of the Reaction of CH2(X3B1) with CH3OH

The reaction of triplet methylene with methanol is a key process in alcohol combustion but surprisingly this reaction has never been studied. The reaction mechanism is investigated by using various high-level ab initio methods, including the complete basis set extrapolation (CBS-QB3 and CBS-APNO), the latest Gaussian-n composite method (G4), and the Weizmann-1 method (W1U). A total of five product channels and six transition states are found. The dominant mechanism is direct hydrogen abstraction, and the major product channel is CH(3) + CH(3)O, involving a weak prereactive complex and a 7.4 kcal/mol barrier. The other hydrogen abstraction channel, CH(3) + CH(2)OH, is less important even though it is more exothermic and involves a similar barrier height. The rate coefficients are predicted in the temperature range 200-3000 K. The tunneling effect and the hindered internal rotational freedoms play a key role in the reaction. Moreover, the reaction shows significant kinetic isotope effect.

Kinetics and Product Branching Ratios of the Reaction of 1CH2 with H2 and D2

The reactions of singlet methylene (a(1)A1 (1)CH2) with hydrogen and deuterium have been studied by experimental and theoretical techniques. The rate coefficients for the removal of singlet methylene with H2 (k1) and D2 (k2) have been measured from 195 to 798 K and are essentially temperature-independent with values of k1 = (10.48 +/- 0.32) x 10(-11) cm(3) molecule(-1) s(-1) and k2 = (5.98 +/- 0.34) x 10(-11) cm(3) molecule(-1) s(-1), where the errors represent 2sigma, giving a ratio of k1/k2 = 1.75 +/- 0.11. In the reaction with H2, singlet methylene can be removed by reaction giving CH3 + H or deactivated to ground-state triplet methylene. Direct measurement of the H atom product showed that the fraction of relaxation decreased from 0.3 at 195 K to essentially zero at 398 K. For the reaction with deuterium, either H or D may be eliminated. Experimentally, the H:D ratio was determined to be 1.8 +/- 0.5 over the range 195-398 K. Theoretically, the reaction kinetics has been predicted with variable reaction coordinate transition state theory and with rigid-body trajectory simulations employing various high-level, ab initio-determined potential energy surfaces. The magnitudes of the calculated rate coefficients are in agreement with experiment, but the calculations show a significant negative temperature dependence that is not observed in the experimental results. The calculated and experimental H to D ratios from the reaction of singlet methylene with D2 are in good agreement, suggesting that the reaction proceeds entirely through the formation of a long-lived methane intermediate with a statistical distribution of energy.

Singlet and Triplet Methylene (CH2) Plus P4: A Computational Study

Ab initio studies of the interaction of singlet and triplet methylene with tetrahedral P4 (white phosphorus) are reported in which singlet methylene “reacts”, giving three 1:1 species: a P−P insertion adduct, a cyclotriphosphirene product, and an apex adduct. Examination of the potential energy surface reveals the connection of the three “products” through two transition states. The structures found computationally are compared with products reported in two experimental studies of reactions of P4 and nucleophilic (stable) carbenes. These computational results suggest that simple carbene reactions with P4 need to be explored experimentally. Triplet methylene generally does not interact with P4 to give any bonded structures, with one noted exception.

A redetermination of the heats of formation of chloro- and dichlorocarbene and the deprotonation of methyl cation, a spin forbidden process?

The reactions of CH 3 +, CH 2Cl + and CHCl 2 + with standard reference bases were examined in a dual cell Fourier transform mass spectrometer. Deprotonation of methyl cation occurs rapidly via a spin forbidden process to afford triplet methylene even when the reaction is nearly thermoneutral. Bracketing results enable us to assign PA(CHCl) = 209.7 ± 2.2 kcal mol −1 and PA(CCl 2) = 205.2 ± 1.9 kcal mol −1, the latter value of which is considerably larger than previous determinations. The resulting heats of formation, however, are in good accord with other measurements and lead to recommended values of Δ H f ° (CHCl) = 74.6 ± 2.4 kcal mol −1 and Δ H f ° (CCl 2) = 53.0 ± 2.6 kcal mol −1. These values correspond to a 2–3 kcal mol −1 lowering of these quantities and are within 1–2 kcal mol −1 of high level G3 and W1 predictions based upon the atomization energies and a series of four isodesmic reactions.

Methyl radical: ab initio global potential surface, vibrational levels and partition function

High-level multi-reference, configuration interaction calculations are performed on the CH3 system. Based on these results, an analytical, global potential energy surface capable of describing the H2 + CH(2Π) → H + CH2( 3B1) reaction is developed. Employing this surface, and a Jacobi coordinates representation of the nuclear motion, the Lanczos method is used to calculate the lowest vibrational states of the methyl radical, CH3( ). The lowest energy levels of the triplet methylene, CH2( 3B1), are also obtained. The CH3 levels associated with excitation of the ν2 out-of-plane umbrella motion display a strong quartic anharmonicity effect, in agreement with earlier experimental and theoretical results. A two-dimensional model based on coupling the umbrella and in-plane symmetric C–H stretch (ν1) modes accounts for the ν2 level patterns. The impact of anharmonicity on the vibrational partition function is assessed.

Progressive Systematic Underestimation of Reaction Energies by the B3LYP Model as the Number of C−C Bonds Increases: Why Organic Chemists Should Use Multiple DFT Models for Calculations Involving Polycarbon Hydrocarbons

[reaction: see text] Computational studies of three different reaction types involving hydrocarbons (homolytic C-C bond breaking of alkanes, progressive insertions of triplet methylene into C-H bonds of ethane, and [2+2] cyclizations of methyl-substituted alkenes to form polymethylcyclobutanes) show that the B3LYP model consistently underestimates the reaction energy, even when extremely large basis sets are employed. The error is systematic and cumulative, such that the reaction energies of reactions involving hydrocarbons with more than 4-6 C-C bonds are predicted quite poorly. Energies are underestimated for slightly and highly methyl-substituted cyclic and acyclic hydrocarbons, so the errors do not arise from structural issues such as steric repulsion or ring strain energy. We trace the error associated with the B3LYP approach to its consistent underestimation of the C-C bond energy. Other DFT models show this problem to lesser extents, while the MP2 method avoids it. As a consequence, we discourage the use of the B3LYP model for reaction energy calculations for organic compounds containing more than four carbon atoms. We advocate use of a collection of pure and hybrid DFT models (and ab initio models where possible) to provide computational "error bars".

Product Study of the Photolysis of Ketene and Ethyl Ethynyl Ether at 193.3 nm

The product distributions of the excimer laser photolysis of ketene (CH2CO) and ethyl ethynyl ether (C2H5OCCH) at lambda = 193.3 nm (ArF) were studied using a time-of-flight mass spectrometer (TOFMS) as an analytical tool. Ketene was photolyzed in bath gases consisting of mixtures of He and H2/D2 at various mixing ratios at constant total pressures of 4 Torr and temperature of about 300 K. Singlet methylene (1CH2) produced in the photolysis of ketene was almost instantaneously converted either to triplet methylene (3CH2) or to methyl radicals in collisions with He and H2 or D2. By extrapolating the methyl and methylene signals to zero time after photolysis, initial concentrations of these radicals were obtained. Analyzing the initial 3CH2 and CH3 concentrations as functions of hydrogen-to-helium ratios as well as simulating the observed traces of reactant and product species resulted in 1CH2 + CO (66 +/- 8)%, as the main product channel of the ketene photolysis with smaller contributions from HCCO + H (17 +/- 7)% and 3CH2 + CO (6 +/- 9)%. Hydrogen atoms, acetylene, ethylene, ethyl, and ketenyl radicals, and small amounts of ketene were observed as primary products of the ethyl ethynyl ether photolysis. Quantification of C2H2, C2H4, C2H5, and CH2CO product leads to a HCCO yield of (91 +/- 14)%.

Exploring Two-State Reactivity Pathways in the Cycloaddition Reactions of Triplet Methylene

Spin forbidden 1,2-cycloadditions of triplet methylene to alkenes have been theoretically studied as an example of the two-state reactivity paradigm in organic chemistry. The cycloadditions of triplet methylene to ethylene and the (E)- and (Z)-2-butene isomers show spin inversion after the transition state and therefore with no effect on the reaction rate. A local analysis shows that while triplet methylene addition to alkenes leading to the formation of a biradical intermediate is driven by spin polarization, the ring closure step to yield cyclopropane is a pericyclic process. We have found that at the regions in the potential energy surface where the spin crossover is likely to occur, the spin potential in the direction of increasing spin multiplicity, mu(+)(s), tends to equalize the one in the direction of decreasing spin multiplicity, mu(-)(s). This equalization facilitates the spin transfer process driven by changes in the spin density of the system.

Ab initio post-HF CCSD(T) Calculations for Triplet and Singlet Methylene in Four consecutive Dunning Basis Sets with Extrapolations to Infinite Limits for Various Molecular Properties

Stationary points for four geometrically different states of methylene: bent and linear triplet methylene, bent and linear singlet methylene were investigated using the highly reliable post-HF CCSD(T) method. Extrapolations to the CCSD(T) basis set (CBS) limit from Dunning triple to quintuple correlation consistent polarized basis sets were performed for total energies, for the equilibrium CH distances re(CH), for singlettriplet separation energies, for energy barriers to linearity and for correlation energies. Post-HF calculations with Dunning basis sets of the literature are presented for comparisons.

The effect of temperature on collision induced intersystem crossing in the reaction of 1CH 2 with H 2

Laser flash photolysis of ketene at 308 nm, coupled with H atom vacuum ultraviolet laser induced fluorescence, was used to determine the branching ratio for the CH 3 + H channel (1a) in the reaction of CH 2 1A 1 ( 1CH 2) with H 2, over the temperature range 300–500 K. This reaction channel competes with collision induced intersystem crossing (CIISC) to form triplet methylene, CH 2 3B 1 ( 3CH 2) (channel 1b). The branching ratio for H formation, k 1a/ k 1, was determined by measuring the relative H atom yield in three time resolved measurements of H: (i) in ketene, H 2 mixtures, where H is exclusively formed by reaction 1a, (ii) in ketene, H 2, NO mixtures ([NO] ≪ [H 2]), where H is formed at short times by 1a and at longer times by 3CH 2 + NO, following 1b, and (iii) in ketene, He, NO mixtures ([NO] ≪ [He]), where H is exclusively formed from 3CH 2 + NO, following deactivation of singlet to triplet methylene by He. k 1a/ k 1 was found to increase from 0.85 at 300 K to unity at 500 K, with the yield of CIISC decreasing from 0.15 to zero. This is the first measurement of the temperature dependence of the rate coefficient for CIISC in a reactive system. The rate coefficient for CIISC with an inert gas increases with T. It has been suggested that the fractional yield of CIISC will increase with temperature in reactive systems, thus reducing the rate coefficient for reaction at high temperature, with significant consequences for combustion systems. The present experiments demonstrate that this is not the case for reaction with H 2 and implies a different CIISC mechanism for reactive vs inert collision partners.