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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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Articles you may be interested inIntense, hyperthermal source of organic radicals for matrix-isolation spectroscopy Rev. Sci. Instrum. 74, 3077 (2003); 10.1063/1.1574397Luminescence spectroscopy of matrix-isolated atomic cadmium J. Chem. Phys. 110, 3903 (1999); 10.1063/1.478244 ESR and optical spectroscopy of matrixisolated ethylnitreneThe mechanism for trapping new chemical species by condensing the products of a microwave discharge with inert matrices has been investigated. Variation of geometrical, electrical, and chemical parameters of the Ar, HCI, CI 2 system indicated that the major product species-HCl2 radical or anion-was formed under conditions where neither ions nor atomic species produced in the discharge were condensed in the matrix. The mechanism for forming the product species is vacuum ultraviolet photolysis of the sample during deposition with radiation from the microwave discharge, since a coaxial orifice discharge tube provided photolysis and produced the product species, while studies with an off-axis orifice discharge tube, which could not serve as a photolysis source, did not produce the product. The H atom-Cl 2 reaction gave HCI using both discharge tubes, but the HCl l species was produced only with the coaxial tube. Hence, this species requires vacuum ultraviolet light in addition to Hand CI atoms for its production, and its identification as HCI, is strongly supported. Similar observations of ArnD+ and 0 3 with the coaxial tube, but not with the off-axis tube, confirm their identification as charged species.

The matrix isolation technique has been employed to isolate several intermediates, in sequence, in the oxidation of CH3OH by CrCl2O2. Consistent with previous theoretical calculations, a hydrogen-bonded complex formed initially after twin jet deposition and was enhanced by matrix annealing to 33 K. This complex was photodestroyed by Hg arc irradiation with λ < 500 nm, and led to the production of HCl and ClCrO2OCH3. These species were also produced by room temperature reaction of the two precursors in a flow system followed by rapid matrix trapping. Heating the flow reaction zone above 150 °C led to destruction of ClCrO2OCH3, and production of CH2O and HCl. Above 250 °C bands due to CH2O were destroyed and bands due to CO and CO2 grew in. High-level density functional calculations (B3LYP/6-311G*) were carried out to identify potential intermediates in this system and to provide theoretical vibrational spectra for comparison to experiment.

The photoreactivity of (3-methyl-2H-azirin-2-yl)-phenylmethanone, 1, is wavelength-dependent (Singh et al. J. Am. Chem. Soc. 1972, 94, 1199-1206). Irradiation at short wavelengths yields 2P, whereas longer wavelengths produce 3P. Laser flash photolysis of 1 in acetonitrile using a 355 nm laser forms its triplet ketone (T(1K), broad absorption with λ(max) ~ 390-410 nm, τ ~ 90 ns), which cleaves and yields triplet vinylnitrene 3 (broad absorption with λ(max) ~ 380-400 nm, τ = 2 μs). Calculations (B3LYP/6-31+G(d)) reveal that T(1K) of 1 is located 67 kcal/mol above its ground state (S(0)) and has a long C-N bond (1.58 Å), and the calculated transition state to form 3 is only 1 kcal/mol higher in energy than T(1K) of 1. The calculations show that 3 has significant 1,3-carbon iminyl biradical character, which explains why 3 reacts efficiently with oxygen and decays by intersystem crossing to the singlet surface. Photolysis of 1 in argon matrixes at 14 K produced ketene imine 7, which presumably is formed from 3 intersystem crossing to 7. In comparison, photolysis of 1 in methanol with a 266 nm laser produces mainly ylide 2 (λ(max) ~ 380 nm, τ ~ 6 μs, acetonitrile), which decays to form 2P. Ylide 2 is formed via singlet reactivity of 1, and calculations show that the first singlet excited state of the azirine chromophore (S(1A)) is located 113 kcal/mol above its S(0) and that the singlet excited state of the ketone (S(1K)) is 85 kcal/mol. Furthermore, the transition state for cleaving the C-C bond in 1 to form 2 is located 49 kcal/mol above the S(0) of 1. Thus, we theorize that internal conversion of S(1A) to a vibrationally hot S(0) of 1 forms 2, whereas intersystem crossing from S(1K) to T(1K) results in 3.

Publication costs assisted by the U S . Office of Naval ResearchThe infrared spectra have been obtained for a number of 1:l hydrogen bonded complexes isolated in a nitrogen matrix at 15'K. The complexes studied and their absorptions (successively, us and Yb, given in cm-l) are as follows: (CH3)2O -HCl, 2280,525; (CH&N -HC1, 1595, 1370; (CH3)sN -HBr, 1870,1600; H3N HBr, 1390, 1211; (CH3)zCO -HBr, 1700; HzO HBr, 2310, 410. For each acid, the value of us in the complex depends systematically, though not monotonically, upon base strength, as measured by proton affinity. We define the normalized proton affinity difference, A = [PA(B) -PA(X-)]/[PA(B) + PA(X-)I, and find that plots of both Au,/uSo and Yb against A correlate all of the systems studied (including H3N a HCl and HzO -HC1). We call such a plot a vibrational correlation diagram and we associate the trends with an increasing extent of proton transfer as A and base strength increase. According to this interpretation, the proton is "completely shared" if A is near -0.23. Partial proton transfer occurs if A is less negative than -0.23, increasing progressively a8 base strength increases.

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