Unusual kinetic behavior of the reactions magnesium + oxygen + M and calcium + oxygen + M (M = N2, He) over extended temperature ranges

Abstract

The reaction Mg + 0 2 + Nz was studied by a static-flow technique in which the rate coefficient was determined relative to the diffusion coefficient of Mg('S) atoms in Nz. This yielded k(349K<T<625K) = (2.15 i 0.56) X exp[-( 19.5 f 0.7) kJ mol-'/RTj, where the quoted uncertainty is 2a. This result is in very good accord with the results of a recent flow-tube study which found the rate coefficient to be about 3 times slower when He was the third body. The reaction Ca + 0 2 + M (M = Nz, He) was then studied by the pulsed laser photolysis of a Ca atom precursor, followed by time-resolved laser-induced fluorescence of the resulting Ca(lS) atoms.The reaction with Nz as the third body is between 2 and 3 times faster than the reaction with He, and the rate coefficients for both third bodies increase between 218 and 340 K and then decrease up to 1100 K. A b initio calculations and RRKM theory are then employed to demonstrate that the unusual temperature dependences of both the Mg and Ca reactions are most probably caused by barriers in the entrance channels of these reactions. The recommended rate coefficients between 120 and 2000 K are then given by loglo(k(Mg+Oz+Nz)) = -392.7 + 4 3 9 . 1~ -203.29 + 42.569 -3.4139, loglo(k(Mg+Oz+He)) = -394.5 + 4 4 2 . 4~ -2 0 5 . 0~~ + 4 2 . 9 4~~ -3.4369, loglo(k(Ca+02+Nz)) = -97.75 + 8 4 . 6 6~ -39.859 + 8 . 5 2 2~~ -0.70939, and loglo(k(Ca+Oz+He)) = -93.67 + 7 8 . 0 2~ -35.769 + 7 . 3 6 9~~ -0.58999, where k is in units of cm6 molecule-2 s-I, x = loglo(T'), and the uncertainty in k is estimated to be *30%. Finally, the implications of these results for the chemistry of meteor-ablated Mg and Ca in the upper atmosphere are considered.

Infrared Spectra and Quantum Chemical Calculations of Group 2 MO2, O2MO2, and Related Molecules

“…The case was made for the sharp group 1 band at 515.7 cm -1 in solid argon to be due to a bent OCaO dioxide molecule with valence angle upper limit of 150°. 2 That assignment is reaffirmed by the quantum chemical calculations summarized in Table 4, which predict a strong ν 3 fundamental in the 549-514 cm -1 range and valence angle in the 135-145°range for 3 B 2 OCaO. Furthermore, the 16/18 ratio predicted from CASSCF harmonic frequencies is 1.0344 as compared to the observed 1.0335 ratio.…”

“…The CASSCF approach is in the best agreement with the experimental fundamental. Even the 3 relatively insensitive to level of theory, because both the 3 B 2 and 1 A 1 states have significant Ca 2+ character.…”

Electronic structure of the S1 state in methylcobalamin: Insight from CASSCF/MC‐XQDPT2, EOM‐CCSD, and TD‐DFT calculations

“…The EOM-CCSD approximation provides reasonable results for such states, with errors in the excitation energies on the order of 0.1 eV in calculations for smaller systems [67,68,[84][85][86][87][88] and typically in the 0.3-0.5 eV range for larger polyatomic molecules (see Refs. [89][90][91][92][93] for examples). Let us recall that in the EOM-CCSD method, one represents excited electronic states |W l i by applying the linear excitation operator R ðCCSDÞ l ¼ R l,0 þ R l,1 þ R l,2 (R l,n is the n-body or ntuply excited component of R ðCCSDÞ l ) to the ground state |W 0 i ¼ e T1þT2 |Ui resulting from the standard CCSD [94][95][96][97] calculations.…”

Measurements of the Rates of Reactions of Ground State Atomic Calcium, Ca[4s2(1S0)], with Alkyl Bromides at Elevated Temperature by Time-Resolved Atomic Resonance Absorption Spectroscopy [Ca(41P1 - 41S0); λ = 422.7 nm]

“…The reactions of atomic calcium with N 2 O and O 2 in a fast flow reactor with monitoring by atomic resonance absorption spectroscopy have been reported by Vinckier et al [1,2]. Plane and co-workers have generated Ca( 1 S) by pulsed laser photolysis and monitored the resulting atom by time-resolved laser induced fluorescence in the presence of O 2 , O 3 and N 2 O yielding rate detailed rate measurements [3][4][5][6]. Clay and Husain [7,8] have reported absolute rate data at elevated temperatures for the reactions of Ca( 1 S) with a range of molecules, including CH 3 Cl and C 2 H 5 Cl, by time-resolved atomic resonance absorption spectroscopy following the generation of atomic calcium by broad band pulsed photolysis of a halide precursor.…”

“…These studies have been extended by Adams et al [9] to the reactions of atomic calcium with various higher alkyl chlorides and to that with methyl iodide, and have included diffusional rate measurements of Ca( 1 S 0 ) in various buffer gases yielding diffusion coefficients for this alkaline earth atom. These studies have been influenced both by fundamental considerations and on account of the role of alkaline earth species in flame chemistry [8,10] and to atmospheric chemistry [3][4][5][6]. Rate data for reactions of atomic calcium with molecules such as O 2 , N 2 O and NO 2 have also been reported by Kashireninov et al [11][12][13] using the 'temperature variation of the diffusion flame method' [14], however, the limitations of this method and the quality of the resulting rate data when applied to reactions of alkaline earth atoms have been emphasised in some detail [15].…”