An investigation of the germylene addition reaction, GeH₂ + C₂H₂: Time-resolved gas-phase kinetic studies and quantum chemical calculations of the reaction energy surface

Abstract

Time resolved studies of germylene, GeH 2 , generated by laser flash photolysis of 3,4dimethylgermacyclopentene-3, have been carried out to obtain rate constants for its bimolecular reaction with acetylene, C 2 H 2 . The reaction was studied in the gas-phase over the pressure range 1-100 Torr, with SF 6 as bath gas, at 5 temperatures in the range 297-553 K. The reaction showed a very slight pressure dependence at higher temperatures. The high pressure rate constants (obtained by extrapolation at the three higher temperatures) gave the Arrhenius equation:These Arrhenius parameters are consistent with a fast reaction occurring at approximately 30% of the collision rate at 298 K. Quantum chemical calculations (both DFT and ab initio G2//B3LYP and G2//QCISD) of the GeC 2 H 4 potential energy surface (PES), show that GeH 2 þ C 2 H 2 react initially to form germirene which can isomerise to vinylgermylene with a relatively low barrier. RRKM modelling, based on a loose association transition state, but assuming vinylgermylene is the end product (used in combination with a weak collisional deactivation model) predicts a strong pressure dependence using the calculated energies, in conflict with the experimental evidence. The detailed GeC 2 H 4 PES shows considerable complexity with ten other accessible stable minima (B3LYP level), the three most stable of which are all germylenes. Routes through this complex surface were examined in detail. The only product combination which appears capable of satisfying the observed lack of a strong pressure dependence is Ge( 3 P) þ C 2 H 4 . C 2 H 4 was confirmed as a product by GC analysis. Although the formation of these products are shown to be possible by singlet-triplet curve crossing during dissociation of 1-germiranylidene (1-germacyclopropylidene), it seems more likely (on thermochemical grounds) that the triplet biradical, GeCH 2 CH 2 , is the immediate product precursor. Comparisons are made with the reaction of SiH 2 with C 2 H 2 .

Gas phase kinetic and theoretical studies of reactions of germylenes and dimethylstannylene

“…53 Pos sible pressure dependence of the rate constants at elevated temperatures was not checked, because it was assumed that the reaction also approached the high pressure limit at these temperatures, despite the fact that similar addi tion reactions of GeH 2 to C 2 H 4 72 and of SiH 2 to both C 2 H 4 76 and C 2 H 2 79 showed a pressure dependence in the pressure range 1-100 Torr. Later, this reaction was reinvestigated in the wider temperature range (297-553 K) and in the pressure range 1-100 Torr using DMGCP as a precursor 75 and it was found that the reaction does show a slight pressure dependence at higher temperatures. Based on the high pressure limiting rate constants (obtained by eyeball extrapolation), the Arrhenius parameters were determined 75 which were slightly different from those already reported 53 (Table 8).…”

Gas‐Phase Kinetics of Chlorosilylene Reactions II. ClSiH + C2H4: Absolute Rate Measurements and Quantum Chemical and RRKM Calculations for the Prototype π Addition Reaction

“…[13,[27][28][29][30] The pressure dependence of an association reaction corresponds exactly to that of the reverse unimolecular dissociation process, providing there are no other perturbing channels.…”

“…Details of the transitional wavenumbers at other temperatures are given in Table 6. We have assumed, as previously, [13,[27][28][29][30] that geometry changes between reactant and transition state do not lead to significant complications. In modelling the collisional deactivation process, we have used a weak collisional (stepladder) model, [31] because there is overwhelming evidence against the strong collision assumption.…”

“…The fact that the intermediate carbene is not seen is similar to the situation in the isomerisation of silirene [27] and germirene. [28] Table 2 also includes the results of the coupled-cluster calculations. The energy values for the three species investigated, viz.…”

“…[13,[27][28][29][30] The selection of E o , the critical energy, for each possible channel was based upon optimising the fit of the calculations to the experimental pressure dependences. This was carried out mainly at 562 K, because the fitting is most sensitive at the (1) 3016 (1) 3086 (1) 3072(1) 2987 (1) 3072 (1) 3008(1) 2932 (1) 3008 (1) 3006(1) 2917 (1) 3006 (1) 2217(1) 2847 (1) 2217 (1) 1407(1) 1477 (1) 1500 (1) 1394(1) 1472 (1) 1407 (1) 1158(1) 1407 (1) 1394 (1) 1004(1) 1385 (1) 1158 (1) 965(1) 1237 (1) 1004 (1) 945(1) 1200 (1) 965 (1) 891(1) 992 (1) 891 (1) 727(1) 945 (1) 727(1) 700 (1) 891 (1) 700 (1) 693(1) 620 (1) 473 (1) 676(1) 546 (1) 131 (1) 646(1) 461 (1) 125 (1) 484(1) 308 (1) 92 (1) 473(1) 190 (1) 34 (1) 187(1) 158 (1) 32 …”