Time-resolved gas-phase kinetic and quantum chemical studies of the reaction of silylene with oxygen

Abstract

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with O(2). The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF(6) bath gas, at five temperatures in the range 297-600 K. The second order rate constants at 10 Torr were fitted to the Arrhenius equation: [see text] The decrease in rate constant values with increasing temperature, although systematic is very small. The rate constants showed slight increases in value with pressure at each temperature, but this was scarcely beyond experimental uncertainty. From estimates of Lennard-Jones collision rates, this reaction is occurring at ca. 1 in 20 collisions, almost independent of pressure and temperature. Ab initio calculations at the G3 level backed further by multi-configurational (MC) SCF calculations, augmented by second order perturbation theory (MRMP2), support a mechanism in which the initial adduct, H(2)SiOO, formed in the triplet state (T), undergoes intersystem crossing to the more stable singlet state (S) prior to further low energy isomerisation processes leading, via a sequence of steps, ultimately to dissociation products of which the lowest energy pair are H2O+SiO. The decomposition of the intermediate cyclo-siladioxirane, via O-O bond fission, plays an important role in the overall process. The bottleneck for the overall process appears to be the T-->S process in H2SiOO. This process has a small spin-orbit coupling matrix element, consistent with an estimate of its rate constant of 1x10(9) s-1 obtained with the aid of RRKM theory. This interpretation preserves the idea that, as in its reactions in general, SiH2 initially reacts at the encounter rate with O2. The low values for the secondary reaction barriers on the potential energy surface account for the lack of an observed pressure dependence. Some comparisons are drawn with the reactions of CH2+O2 and SiCl2+O2.

The reaction between silylene and ammonia: Some gas-phase kinetic and quantum chemical studies

“…The results of these experiments are shown in Figures 1 and 2. Reasonable linear fits were obtained, as expected for second order kinetics, although it should be noted that there is slightly higher scatter than is usually obtained in this kind of study [5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Because the rate constants showed a dependence on laser pulse energy, two plots are shown, one for higher pulse energies (60 mJ) and one for lower values (ca 30 mJ).…”

“…SiH 2 also has a great affinity for electronegative elements and will react with lone pair donor species containing the elements of N, O, F, P, S and Cl amongst others [4]. We, and others, have recently studied the kinetics of its reactions with "O-donor" molecules, H 2 O [5,6], CH 3 OH (CD 3 OD) [5], Me 2 O [7,8], Me 2 CO [9], MeCHO [10,11], CO [12], CO 2 [13] and O 2 [14] itself, and also with "Cldonor" molecules, HCl [15] and CH 3 Cl [16]. We have also previously reported kinetic studies of the reactions of SiH 2 with the N-containing molecules N 2 O [17], NO [18] and even N 2 [19] (although no reaction was found in this case).…”

Defect‐Assisted Covalent Binding of Graphene to an Amorphous Silica Surface: A Theoretical Prediction

“…From our slab (cluster) calculations the overall exothermicity of the O 2 adduction is predicted to be 3.1-3.2 (3.3) eV, consistent with previous theoretical studies that also reported high exothermicities of 2.7-4.1 eV, depending on the silylene substituents (such as :SiX 2 , X= H, CH 3 , Ph). [11,26,27] In addition, the O 2 adduct reaction tends to occur with no significant activation barrier. For instance, the reaction between SiH 2 and O 2 was observed to occur at 297-600 K. [12] For the singlet DOSG (b), the predicted OÀO/SiÀO bond distances and OÀSiÀO bond angle are 1.64 (1.63)/1.64 (1.65) and Table 1.…”

“…[13] An O 2 molecule may readily react with the active divalent Si in SS and form a DOSG (Figure 1 b), that is, O 2 (triplet) + SS (singlet)!DOSG (singlet). Previous studies [12,[26][27][28] suggest that the O 2 adduct reaction would follow three steps. Firstly, one end of the O 2 binds to the divalent Si and forms a silanone O-oxide (> SiCOOC, where C represents a dangling bond) in which the Si becomes pyramidalized (like a silyl radical, SiC) while the Sibound O 2 (SiÀOOC) exhibits superoxo (O 2 À ) character.…”

“…Given the singlet initial (A) and final (E) states, the reaction is assumed to occur on the singlet potential energy surface according to the spin conservation rule. [27] Initially, the graphene was placed 3.3 above the OÀO dimer of DOSG, close to a typical van der Waals distance. In the initial state A, the DOSG-Gr interaction energy is predicted to be 0.05 eV with respect to the fully separated DOSG cluster and graphene sheet (FS), while the OÀO bond is slightly elongated compared to the isolated DOSG case.…”