Pyrolysis of the Simplest Carbohydrate, Glycolaldehyde (CHO-CH₂OH), and Glyoxal in a Heated Microreactor

Both glycolaldehyde and glyoxal were pyrolyzed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from CHO-CH₂OH and HCO-CHO were detected and identified by vacuum ultraviolet (VUV) photoionization mass spectrometry. Complementary product identification was provided by argon matrix infrared absorption spectroscopy. Pyrolysis pressures in the microreactor were about 100 Torr, and contact times with the microreactors were roughly 100 μs. At 1200 K, the products of glycolaldehyde pyrolysis are H atoms, CO, CH₂î-?O, CH₂î-?Cî-?O, and HCO-CHO. Thermal decomposition of HCO-CHO was studied with pulsed 118.2 nm photoionization mass spectrometry and matrix infrared absorption. Under these conditions, glyoxal undergoes pyrolysis to H atoms and CO. Tunable VUV photoionization mass spectrometry provides a lower bound for the ionization energy (IE)(CHO-CH₂OH) ≥ 9.95 ± 0.05 eV. The gas-phase heat of formation of glycolaldehyde was established by a sequence of calorimetric experiments. The experimental result is Δ_fH₂₉₈(CHO-CH₂OH) = -75.8 ± 1.3 kcal mol^-1. Fully ab initio, coupled cluster calculations predict Δ_fH₀(CHO-CH₂OH) of -73.1 ± 0.5 kcal mol^-1 and Δ_fH₂₉₈(CHO-CH₂OH) of -76.1 ± 0.5 kcal mol^-1. The coupled-cluster singles doubles and noniterative triples correction calculations also lead to a revision of the geometry of CHO-CH₂OH. We find that the O-H bond length differs substantially from earlier experimental estimates, due to unusual zero-point contributions to the moments of inertia.