The reaction products arising from the high-temperature pyrolysis of mixtures of B2H6 and NH3 in argon have been isolated and characterized in low-temperature matrices. All 11 infrared active fundamentals of aminoborane, H2B=NH2, were identified and l5N and l0B counterparts located for most of the fundamentals. The two lowest frequency fundamentals were observed for the first time. Based on isotopic shifts and a normal-coordinate calculation, reassignment of several of the previously observed gas-phase fundamentals is suggested. Spectra were also obtained of [H2B(NH3)2]+[BH4]" and µ-aminodiborane, H2NB2H5, and band assignments proposed.

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Merged jet copyrolysis of mixtures of Ar/BzH6 and Ar/HzS followed by trapping in a cryogenic matrix has led to the formation, isolation, and characterization of mercaptoborane, H2BSH. Eight of the nine fundamentals of this molecule were observed, including the boron-sulfur stretching mode at 804 cm-l ("B). Local density functional and normal-coordinate calculations were carried out to assist in assigning the observed fundamentals. The position of B-S stretching mode, compared to a range of known compounds, suggests single bond character for the B-S bond. Additional products were observed, including HBS, and suggests further reaction of HzBSH at high temperatures. A mechanism for the reaction is proposed.

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6805in methanol to afford 1-phenyl-1-methoxycyclohexane (2), via the 1 -phenylcyclohexyl carbocation.lOJ1 Extrapolation of the double-reciprocal plot of the quantum yield of formation of 2 vs.[H2S04], after correction for 3Ph2C0 quenching by H2SO4 and for the small (2.2%) inefficiency of conversion of the carbocation to ether,19 affords = 0.36 f 0.01 ( 2 0 )~~ and thus a value of 44.7 f 5 kcal/mol for E,/. The sum of the amplitude factors, & + 4, + q53, equals unity, indicating all the photon energy is recovered as heat within experimental error. Results in cyclohexane are almost identical. However, the inability to measure in aprotic solvent prevents a computation of Et-, in cyclohexane.The values of E T and E, , are the first experimental measurements of the relaxed energy of a short-lived twisted alkene triplet21 and of the heat of geometric isomerization to the most strained cycloalkene known, respectively. The E T value of 56 f 3.4 kcal/mol is but slightly lower than the expected22 spectroscopic triplet energy of 60 kcal/mol. We believe that this corresponds to partial relaxation (Le., <90° rotation) of the tri~1et.I~ Therefore, the triplet surface is nearly flat, similar to the trans to perpendicular region for stilbene triplet.23The energy E,, of 44.7 kcal/mol is remarkably close to the prediction of molecular mechanics (MM) for trans-cyclohexene (E,/ = 42.4 k~a l / m o l ) .~~ Given the AH,-(c-1) = -4.0 f 1.6 kcal/m01~~ and the strain energy E,(c-1) = 1.2 kcal/mo1,26 then AHdt-1) = 41 f 5 kcal/mol and E,(t-1) = 46 f 5 kcal/mol. We note the structural prediction by M M of substantially pyramidalized vinyl carbons in these and related compound^.^,^^ The implications of this for the chemistry of trans-cyclohexenes and for dynamics on the ground-state surface will be discussed in a future report.In conclusion, we have demonstrated the applicability of time-resolved photoacoustic calorimetry to the study of alkene ground and triplet surfaces. Further studies should allow a wealth of thermochemical information on relaxed olefin triplets and strained ground-state isomers. (19) Had we chosen neat alkyl silane as the standard state, PS would have In (Cnat/Cl~) = AS,M -4.0 eu, where (20) (a) Meyers, R. T. Inorg. Chem. 1978, 17,952-958. (b) Martell, A. been even smaller (ASnmt = AS,M C,,, and CIM are concentrations). E.There is considerable uncertainty about the rate constant for a "diffusion-controlled" unimolecular reaction. For a bimolecular

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We report progress in the design of the BepiColombo Mercury Imaging X-ray Spectrometer (MIXS). This instrument consists of two modules; a Wolter I soft X-ray telescope based on radially packed microchannel plate optics (MIXS-T) and a profiled collimator which uses a square pore square packed microchannel plate array to restrict its field of view (MIXS-C). Both instrument modules have identical focal planes (DEPFET macropixel array) providing an energy resolution of better than 200 eV FWHM throughout the mission.The primary science goal of MIXS is to perform X-ray fluorescence spectroscopy of the Hermean surface with unprecedented spatial and energy resolution. This allows discrimination between different regolith types, and by combining with data from other instruments, between competing models of crustal evolution and planetary formation. MIXS will also probe the complex coupling between the planet's surface, exosphere and magnetosphere by observing Particle Induced X-ray Emission (PIXE).