Low-Pressure Pyrolysis of tBu2SO: Synthesis and IR Spectroscopic Detection of HSOH

Sulfenic acid (HSOH, 1) has been synthesized in the gas‐phase by low‐pressure high‐temperature (1150 °C) pyrolysis of di‐tert‐butyl sulfoxide (tBu2SO, 2) and characterized by means of matrix isolation and gas‐phase IR spectroscopy. High‐level coupled‐cluster (CC) calculations (CCSD(T)/cc‐pVTZ and CCSD(T)/cc‐pVQZ) support the first identification of the gas‐phase IR spectrum of 1 and enable its spectral characterization. Five of the six vibrational fundamentals of matrix‐isolated 1 have been assigned, and its rotational‐resolved gas‐phase IR spectrum provides additional information on the O–H and S–H stretching fundamentals. Investigations of the pyrolysis reaction by mass spectrometry, matrix isolation, and gas‐phase FT‐IR spectroscopy reveal that, up to 500 °C, 2 decomposes selectively into tert‐butylsulfenic acid, (tBuSOH, 3), and 2‐methylpropene. The formation of the isomeric sulfoxide (tBu(H)SO, 3 a) has been excluded. Transient 3 has been characterized by a comprehensive matrix and gas‐phase vibrational IR study guided by the predicted vibrational spectrum calculated at the density functional theory (DFT) level (B3LYP/6‐311+G(2d,p)). At higher temperatures, the intramolecular decomposition of 3, monitored by matrix IR spectroscopy, yields short‐lived 1 along with 2‐methylpropene, but also H2O, and most probably sulfur atoms. In addition, HSSOH (6), H2, and S2O are found among the final pyrolysis products observed at 1150 °C in the gas phase owing to competing intra‐ and intermolecular decomposition routes of 3. The decomposition routes of the starting compound 2 and of the primary intermediate 3 are discussed on the basis of experimental results and a computational study performed at the B3LYP/6‐311G* and second‐order Møller–Plesset (MP2/6‐311G* and RI‐MP2/QZVPP) levels of theory. The first gas‐phase IR spectroscopic identification of HSOH (1), the lowest member of the sulfur oxo acids, marks a breakthrough in this field after many years of painstaking experimental work. The mechanism of its formation by low‐pressure pyrolysis of tBu2SO has been studied by means of quantum chemical calculations, mass spectrometry, matrix isolation, and gas‐phase FT‐IR spectroscopy.