Spin-forbidden Reaction Research Articles

On the Mechanism of the Spin-Nonconserving Chemical Reaction O(3P) + HCCH → CH2(ãA1) + CO(X1Σ+). I. Feasibility

The mechanism of spin-forbidden reaction O(3P) + HCCH(X1 ) → HCCHO(1,23A) → HCCHO(1A) → CH2(a1A1) + CO(X1 ) is discussed. Portions of the 3A‘‘−1A‘, 3A‘−1A‘, and 3A‘‘−3A‘ surfaces of intersection and the spin−orbit coupling matrix elements are determined. An intermediate complex driven model is proposed. Intersystem crossing is expected to occur over the entire region between the intermediate complex, 3HCCHO, and the transition state for the spin-allowed rearrangement, 3HCCHO → 3H2CCO. The barrier for the reaction arises from the formation of the adduct 3HCCHO and is thus the same as that of the spin-allowed process forming CH2(X3B1).

Potential-energy surfaces and their dynamic implications

Accurate density functional and abinitio calculations have been performed to study the potential-energy surfaces (PESs) and their implications for kinetics and dynamics of: (1) the spin-forbidden reaction CH(2Π) +N2→HCN+N(4S); PES characteristics are calculated and used to evaluate the overall rate using non-adiabatic transition-state theory. (2) Gas-phase ion–molecule reactions: C2H2++NH3; PESs are calculated and the mechanism of efficient charge transfer and proton transfer competing with stable complex formation is discussed. C2H2++CH4; the mode-enhancement effect has been elucidated in terms of the new transition state and by direct trajectory calculations.

Electronic structure aspects of the spin-forbidden reaction CH3(X 2A2″)+N(4S)→HCN(X 1Σ+)+H2(X 1Σg+)

Second order configuration interaction wave functions based on molecular orbitals determined from a state-averaged multiconfigurational self-consistent field procedure are used to investigate the intermediate complex driven model for the spin-forbidden reaction CH3(X 2A2″)+N(4S)→HCN(X 1Σ+)+H2(X 1Σ+). The minimum energy crossing point (MECP), the minimum energy point on the surface of intersection connecting the reactant channel, A′′3 potential energy surface, and product channel A′1 potential energy surface, is determined directly, i.e., without a priori characterization of the individual potential energy surfaces. The MECP is found to be 8.2 kcal/mol below of the reactants. The structure at the MECP clearly evinces the incipient formation of a H2 bond. Barrierless paths from the reactants to the intermediate complex-methylnitrene, from the intermediate complex to the MECP, and from the MECP to the products are established. The absence of a barrier on these paths supports the intermediate complex mechanism. Spin–orbit interactions are determined to be ∼30 cm−1 for points on the surface of intersection in the vicinity of the MECP. Spin–orbit interactions and the local potential surface topology at the MECP are used to probe the efficiency of the intersystem crossing using the Landau–Zener model. A reduced dimensionality model is proposed.

The mechanism of the reaction CH(X 2Π) + N 2 (X 1Σ +g) → HCN (X 1Σ +) + N ( 4S). : II. The intermediate complex region

The spin-forbidden reaction CH(X 2Π) + N 2(X 1Σ + g) → HCN (X 1Σ +) + N( 4S) involves a doublet → quartet intersystem crossing. It has been postulated that although the probability for a single pass intersystem crossing is small the existence of a bound intermediate on the lowest doublet potential energy surface increases the overall transition probability by enabling multiple crossings of the doublet—quartet surface intersection. This work considers this proposition, using large scale ((3.2 − 3.8) × 10 6 term), multireference configuration interaction wavefunctions. Two local minima on the lowest doublet potential energy surface are reported, a datively bonded structure N 2 → CH, and a C 2v structure, with HC along the perpendicular bisector of the N 2 bond, which resembles the recently reported geometrical arrangement at the transition state for this reaction, the minimum energy point on doublet—quartet intersection surface. These results enable a clear conceptual picture for the mechanism of this reaction to be presented.

Spin symmetry and interaction mechanisms in free radical reactions

The principles underlying the spin-allowed and spin-forbidden free radical reactions are sought. Based on symmetry, the interactionmechanism and the corresponding transmission matrix elements for the mixing of reactant/product hyperpotential surfaces ar elucidated. Magnetic multipole, electrostatic, spin-orbit and spin-spin interactions are considered. Four examples are used to illustrate various cases: (1) Xe ( 3P u) + N 2 ( 1Σ g +) → Xe( 1S g) + N 2 ( 3II g), which satisfies Wigner's selection rule for the conservation of total spin angular momentum and for which spin and orbital interactions must be compatible, all of the above interactions are found. (2) The perpendicular mode of photodissociation in O 3 * ( 1B 1) → O( 3P g) + O 2( 3E g −), which is orbitally forbidden and for which only spin-spin interaction is found. (3) The hypothetical reaction CH( 4Σ −) + CH( 2Σ +) → C 2H 2 (Ã 1A u), which is spin forbidden but is made allowed by the C 2h geometry of the reaction coordinate. Only spin interactions are found. (4) The triplet cyclopropylidene to orthogonal singlet allene transformation, which is a Hückel to Möbius transformation, and the reaction coordinate produces orbital angular momentum changes to compensate for the spin change. The detailed interaction matrix elements shown for each channel of reaction give a qualitative indication of the branching ratios for different reaction paths and may be compared with future experimental measurements.

Spin–orbit effects in the decomposition reaction N3H(X̃1A′)→N2(X1Σ+g)+NH(X3σ−, a1Δ)

Using a flexible basis of better than double zeta-polarization quality and configuration interaction (CI) expansions of approximately 200 000 terms the electronic structure aspects of the spin-forbidden decomposition reaction N3H(X̃1A′)→NH(X3Σ−)+N2(1Σ+g) were studied. The spin–orbit interaction (Hso) was treated within the Breit–Pauli approximation including both the microscopic spin–orbit and spin–other–orbit contributions. Matrix elements of Hso between the lowest singlet state Ψ1a′(1A′)≡Ψ[1 1A′(0)] and the components of the lowest triplet state Ψ1a′(3A″) ≡iΨ[1 3A″(0)], Ψ2a′(3A″) ≡i{Ψ[1 3A″(1)] −Ψ[1 3A″(−1)]}/(2)1/2 were determined in the asymptotic region corresponding to N2+NH, at the (experimental) equilibrium geometry of N3H(X̃ 1A′) and in the vicinity of the (approximate) minimum energy singlet–triplet crossing. At the approximate minimum energy crossing we find hsoz ≡〈Ψ1a′(1A′)‖Hso‖ Ψ1a′(3A″)〉 ≊39 cm−1 ≫hsoy ≡〈Ψ1a′(1A′)‖Hso‖ Ψ2a′(3A″)〉 ≊0.45 cm−1. The matrix elements hsoz,hsoy are interpreted in terms of a single configuration model and are compared with analogous quantities in the isolated NH molecule. A qualitative discussion of the decomposition reaction using a Landau–Zener approach is given.

Phosphine photochemistry in the atmosphere of Saturn

A model is presented for the photochemistry of PH 3 in the upper troposphere and lower stratosphere of Saturn that includes the effects of coupling with NH 3 and hydrocarbon photochemistry, specifically the C 2H 2 catalyzed photodissociation of CH 4. PH 3 is rapidly depleted with altitude (scale height ∼35 km) in the upper troposphere when K∼10 4cm 2sec −1; an upper limit for K at the tropopause is estimated at ∼10 5cm 2sec −1. If there is no gas phase P 2H 4 because of sublimation, P 2 and P 4 formation is unlikely unless the rate of the spin-forbidden recombination reaction PH + H 2 + M → PH 3 + M is exceedingly slow. An upper limit P 4 column density of ∼2×10 15cm −2 is estimated in the limit of no recombination. If sublimation does not remove all gas phase P 2H 4, P 2 and P 4 may be produced in potentially larger quantities, although they would be restricted almost entirely to the lowest levels of our model, where T≳100°K. Potentially observable amounts of the organophosphorus compounds CH 3P 2H 2 and HCP are predicted, with column densities of >10 17 cm −2 and production rates of ∼2×10 8cm −2sec −1. The possible importance of electronically excited states of PH x and additional PH 3/hydrocarbon photochemical coupling paths are also considered.

Theory of spin-forbidden and chelatropic reactions involving atomic oxygen [O(3P)], allene (C3H4), and carbon suboxide (C3O2)

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTheory of spin-forbidden and chelatropic reactions involving atomic oxygen [O(3P)], allene (C3H4), and carbon suboxide (C3O2)Ying Nan Chiu and M. S. F. A. AbidiCite this: J. Phys. Chem. 1982, 86, 17, 3288–3292Publication Date (Print):August 1, 1982Publication History Published online1 May 2002Published inissue 1 August 1982https://doi.org/10.1021/j100214a005RIGHTS & PERMISSIONSArticle Views39Altmetric-Citations9LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (530 KB) Get e-Alerts

Unimolecular reactions of N2O and CO2 at high pressure

An apriori calculation in the framework of the theory of radiationless transitions is presented for the non-adiabatic spin-forbidden reactions of N2O and CO2. The calculation is two-dimensional, incorporating the two stretching degrees of freedom, and the microscopic reaction process is formulated using the relative coordinate system of Rosen as modified by Beswick and Jortner. All microscopic parameters required for the calculation are derived from spectroscopic data; no adjustable parameter is included. For each reaction, a purely theoretical reaction probability function k(E) is synthesised and used to derive a theoretical Arrhenius expression for the infinite-pressure reaction rate. The calculated high-pressure unimolecular dissociation rates for both reactions are in reasonable agreement with experiment. High-pressure recombination rate coefficients are also presented for CO2; they exhibit positive activation energies which increase from 0.5 kcal/mol at room temperature to 8 kcal/mol at 5000 K.

Rates of spin-forbidden organic reactions. 3. Extrusion of nitrogen from methylenepyrazoline

Rates of spin-forbidden organic reactions. 3. Extrusion of nitrogen from methylenepyrazoline

Rates of spin-forbidden organic reactions. Theoretical study of the rate of direct production of triplet methylene by dissociation of singlet diazomethane

Rates of spin-forbidden organic reactions. Theoretical study of the rate of direct production of triplet methylene by dissociation of singlet diazomethane

Rates of spin-forbidden organic reactions. II. Implications for the methylene splitting question

Rates of spin-forbidden organic reactions. II. Implications for the methylene splitting question

Facile spin-forbidden reactions. Barium + sulfur dioxide .far. barium oxide + sulfur monoxide

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTFacile spin-forbidden reactions. Barium + sulfur dioxide .far. barium oxide + sulfur monoxideGregory P. Smith and Richard N. ZareCite this: J. Am. Chem. Soc. 1975, 97, 7, 1985–1986Publication Date (Print):April 1, 1975Publication History Published online1 May 2002Published inissue 1 April 1975https://doi.org/10.1021/ja00840a079RIGHTS & PERMISSIONSArticle Views43Altmetric-Citations28LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (207 KB) Get e-Alerts Get e-Alerts

Collision complex model for spin forbidden reactions: Quenching of O(1D) by N2

A collision complex model based on the RRKM theory of unimolecular decomposition is developed to describe spin-forbidden molecular collision processes. The model is applied to the quenching of O(1D) atoms by molecular nitrogen. The calculated rate constant (300°K) is 7 × 10−11 cm3 mol−1 · sec−1, in excellent agreement with experiment. An average of roughly 30% of the 1.98 eV exothermicity of this process is predicted to go into product vibration. This suggests, in contrast to previous studies, that collision with O(1D) is a very efficient mechanism for vibrational excitation of N2.

Atomic carbon in the atmospheres of Mars and Venus

Photochemical processes involving atomic carbon in the upper atmospheres of Mars and Venus are critically examined. It is shown that carbon is produced mainly by electron- and photon-induced dissociation of CO and CO2, with CO relatively more important for Mars. The important removal mechanism for C is a reaction with O2, although a spin-forbidden reaction with CO2 may also be significant. Detailed calculations are presented for the dayglow emissions from C at 1657 and 1561 A and results are compared with the Mariner 6 and 7 results for Mars. It is indicated that high-quality observations of the C lines can be used to infer valuable information on the abundance of O2.

Variation in yield of the isotopic spin forbidden reaction 12C( 6Li, α 1) 14N

Variation in yield of the isotopic spin forbidden reaction 12C( 6Li, α 1) 14N