Excited States Of Methylene Research Articles

Low temperature studies of the rate coefficients and branching ratios of reactive loss vs quenching for the reactions of 1CH2 with C2H6, C2H4, C2H2

The kinetics of the reactions of the first excited state of methylene, 1CH2, with C2H2, C2H4, and C2H6, has been measured over the temperature range 43–298 K by pulsed laser photolysis, monitoring 1CH2 removal by laser induced fluorescence. Low temperatures were obtained using a pulsed Laval expansion (43–134 K), while a slow flow reaction cell was used for temperatures of 160 K and above. The rate coefficients for the reactions with C2H2, C2H4, and C2H6, all showed a strong negative temperature dependence. In combination with other literature data, the coefficients can be parameterized as: kC2H2(43≤T/K≤298)=(3.22±0.15)×10−10×(T/298)(−0.394±0.066)kC2H4(43≤T/K≤298)=(2.16±0.14)×10−10×(T/298)(−0.612±0.089)kC2H6(43≤T/K≤298)=(1.78±0.10)×10−10×(T/298)(−0.545±0.078)Branching ratios for reactive removal of 1CH2 vs quenching to ground state were also determined for all three colliders and for H2 and CH4, at temperatures between 100 and 298 K. The values measured show that the dominant removal process of 1CH2 by H2, C2H2, and C2H4, changes from reactive removal to quenching to ground state 3CH2 as the temperature decreases from 298 K to 100 K, while for CH4 and C2H6, reactive removal drops from around 85% to around 55%. The impacts of the new measurements for Titan's atmosphere are examined using a 1D chemistry and transport model. A significant increase (∼25%) in the mixing ratio of benzene between 500 and 1550 km is calculated, due to the increased production of C3H3 from the reaction of 1CH2 with C2H2.

Low temperature studies of the removal reactions of 1CH2 with particular relevance to the atmosphere of Titan

Methylene, CH2, is one of the major photolysis products of methane by Lyman-α radiation and is involved in the photochemistry of the atmospheres of Titan and the giant planets. The kinetics of the reactions of the first excited state of methylene, 1CH2, with He, N2, O2, H2 and CH4 have been measured over the temperature range 43–160 K by pulsed laser photolysis, monitoring 1CH2 removal by laser induced fluorescence. Low temperatures were obtained with either a pulsed Laval expansion (43–134 K) or a, slow flow reaction cell (160 K). The rate coefficients for the reactions with N2, O2, H2 and CH4 all showed a strong negative temperature dependence. In combination with other literature data, the rate coefficients can be parameterised as:kHe(43 < T/K < 800) = (1.90 ± 0.23) × 10−12 × (T/298)1.74±0.16 × exp((88±23)/T)kN2(43<T/K<800) = (2.29 ± 1.12) × 10−12 × (T/298)−2.15±1.38 × exp((-74±96)/T) + (3.91 ± 0.78) × 10−11 × exp((-469±114)/T)kO2(43<T/K<300) = (6.16 ± 1.09) × 10−11 × (T/298)−0.65±0.14kH2(43<T/K<800) = (1.10 ± 0.04) × 10−10 × (T/298)−0.40±0.06 × exp((11.1±6.9)/T)kCH4(43<T/K<475) = (8.20 ± 0.46) × 10−11 × (T/298)−0.93±0.10 × exp((-20.5±12.8)/T)For the reactions of 1CH2 with H2 and CH4, the branching ratio for quenching to ground state, 3CH2, vs chemical reaction was also determined at 160 and 73 K. The values measured (H2: 0.39 ± 0.10 at 160 K, 0.78 ± 0.15 at 73 K; CH4: 0.49 ± 0.09 at 160 K, 0.64 ± 0.19 at 73 K) confirm trends of an increased proportion of reactive loss with increasing temperature determined at higher temperatures. The impacts of the new measurements for Titan's atmosphere have been ascertained using a 1D chemistry and transport model. A significant decrease (∼40%) in the mixing ratio of ethane between 800 and 1550 km is calculated due to the decrease contribution of methyl production from the reaction of 1CH2 with CH4, with smaller increases in the concentrations of ethene and acetylene. Ethene production is enhanced by more methylene being converted to methylidene, CH, and the subsequent reaction of CH with CH4 to generate ethene. Photolysis of ethene is the major route to acetylene formation.

Isaiah Shavitt: Computational chemistry pioneer

Isaiah (Shi) Shavitt (1925–2012) was a pioneer in using digital computers to study chemical problems. From the days of vacuum tube computers with no floating point arithmetic to the days of massively powerful computers, he showed how we could solve otherwise intractable chemical problems by making use of computers. He started with a statistical mechanical problem and soon switched to quantum mechanical problems. He and his associates showed how the configuration interaction method worked, both in terms of advantages and difficulties. An early problem was the effect of tunneling on kinetics calculations on an H3 potential energy surface. Later problems included the π-electron excited states of benzene and the lowest excited state of methylene. He showed how spin-eigenfunctions could be used efficiently in configuration interaction calculations instead of Slater determinants. His leadership led to the Columbus suite of programs put together with many collaborators.

Comparison of the completely renormalized equation-of-motion coupled-cluster and Quantum Monte Carlo results for the low-lying electronic states of methylene

The left-eigenstate completely renormalized (CR) equation-of-motion (EOM) coupled-cluster (CC) method with singles, doubles, and non-iterative triples, abbreviated as CR-EOMCC(2,3) [M. Włoch et al., Mol. Phys. 104, 2149 (2006); P. Piecuch et al., Int. J. Quantum Chem. 109, 3268 (2009)], and the companion ground-state CR-CC(2,3) methodology [P. Piecuch and M. Włoch, J. Chem. Phys. 123, 224105 (2005); P. Piecuch et al., Chem. Phys. Lett. 418, 467 (2006)] are used to determine the total electronic and adiabatic excitation energies corresponding to the ground and lowest three excited states of methylene. The emphasis is on comparing the CR-CC(2,3)/CR-EOMCC(2,3) results obtained with the large correlation-consistent basis sets of the aug-cc-pCV xZ (x = T, Q, 5) quality and the corresponding complete basis set (CBS) limits with the recently published variational and diffusion Quantum Monte Carlo (QMC) data [P. Zimmerman et al., J. Chem. Phys. 131, 124103 (2009)]. It is demonstrated that the CBS CR-CC(2,3)/CR-EOMCC(2,3) results are in very good agreement with the best QMC, i.e. diffusion MC (DMC) data, with errors in the total and adiabatic excitation energies of all calculated states on the order of a few millihartree and less than 0.1 eV, respectively, even for the challenging, strongly multi-reference C 1 A 1 state for which the basic EOMCC approach with singles and doubles completely fails. The agreement between the CBS CR-CC(2,3)/CR-EOMCC(2,3) and variational MC (VMC) results for the total energies is not as good as in the DMC case, but the excitation energies resulting from the CBS CR-CC(2,3)/CR-EOMCC(2,3) and VMC calculations agree very well.

An experimental and theoretical investigation of the competition between chemical reaction and relaxation for the reactions of 1CH2 with acetylene and ethene: implications for the chemistry of the giant planets

The temperature dependence of the branching ratios for H atom production from the reactions of the first excited state of methylene (a1A1 1CH2) with acetylene and ethene have been measured at approximately 1 Torr total pressure and temperatures of 195, 250 and 298 K by monitoring the production of H atoms using laser induced fluorescence, comparing the signal to that observed from a calibration reaction. For the reaction with acetylene the yield of H increases from 0.28 (195 K) to 0.53 (250 K) to 0.88 at 298 K. The H atom yield from the reaction of 1CH2 with ethene shows similar behaviour, the yields being 0.35 (195 K), 0.51 (250 K) and 0.71 (298 K). The co-products, propargyl (C3H3) and allyl (C3H5) are formed from the dissociation of chemically activated C3H4 and C3H6 intermediates respectively, and are important species in the formation of higher hydrocarbons, including benzene, in the atmospheres of the outer planets and Titan. H atom production is in competition with electronic relaxation to form ground state methylene (X3B1, 3CH2) and collisional stabilization to form C3H4 and C3H6. Master equation calculations have been carried out to demonstrate that for the reaction of 1CH2 with acetylene, collisional stabilization is insignificant under experimental conditions and hence the balance of reaction is due to electronic relaxation. Non-adiabatic transition state theory has been applied to the reaction of 1CH2 with acetylene. The calculations show reasonable agreement with experiment, generally being within the combined errors, and reproduce the negative temperature dependence for electronic relaxation. The implications of the temperature dependence of the absolute rate coefficients for 1CH2 reactions with inert gases, hydrogen, acetylene and ethene and of the branching ratios between chemical reaction and electronic relaxation are discussed.

Electronic states of methylene from ketene photodissociation between 214 and 313 nm

The photolysis of ketene in the presence of cis-2-butene was studied. The effects of pressure and absorbed intensity at 285 and 214 nm were examined, and the effect of small amounts (approximately 5 mole percent) of O/sub 2/ was determined. The hydrocarbon reaction products formed between 250 and 313 nm are consistent with the presence of two reactive electronic states of methylene during photolysis, the ground state, and first excited state. Furthermore, the two states appear to be formed in approximately constant relative proportion over this wavelength region. At 214 nm an increase in the nonstereo-specificity of the methylene addition to the double bond in cis-2-butene can be attributed to the presence of the second excited state. If the second excited state is postulated to be formed from ketene at 214 nm, the results require that it react more rapidly with oxygen than with cis-2-butene, and also indicate that the ground and first excited states of methylene are simultaneously present. (auth)