Infrared Spectra of (Z)- and (E)-•C2H3C(CH3)I Radicals Produced upon Photodissociation of (Z)- and (E)-(CH2I)HC═C(CH3)I in Solid para-Hydrogen

Infrared Spectra of (Z)- and (E)-•CH2C(CH3)CHI Radicals Produced upon Photodissociation of (Z)- and (E)-(CH2I)(CH3)C═CHI in Solid para-Hydrogen

Ozonolysis of isoprene is important in atmospheric chemistry because of the abundant emission of isoprene. This process produces the Criegee intermediates CH₂OO, methyl vinyl ketone oxide (MVKO, C₂H₃C(CH₃)OO), and methacrolein oxide (MACRO, CH₂C(CH₃)CHOO). Gaseous MACRO was recently produced and identified in laboratories after photolysis of a mixture of 1,3-diiodo-2-methyl-prop-1-ene [(CH₂I)(CH₃)C═CHI] and O₂, but the conformation-dependent formation mechanism remains unexplored. We report conformation-distinct IR spectra of (E)- and (Z)-(CH₂I)(CH₃)C═CHI isolated in solid p-H₂. Upon irradiation near 300 nm of (E)- and (Z)-(CH₂I)(CH₃)C═CHI in solid p-H₂ at 3.3 K, 3-iodo-2-methyl-prop-1-en-3-yl [•CH₂C(CH₃)CHI] radicals were characterized, with intense infrared absorption lines at 2991.3, 1458.7, 1434.7, 1317.4, 1190.4, 786.3, 677.9, and 467.2 cm^-1 and additional 11 weaker ones assigned to (E)-•CH₂C(CH₃)CHI and intense lines at 3108.5, 3076.2, 3028.5, 2970.0, 1174.2, 796.0, 683.6, and 609.5 cm^-1 and additional 7 weaker ones to (Z)-•CH₂C(CH₃)CHI. The assignments were derived according to the behaviors of secondary photolysis at 495 and 460 nm and a comparison of the vibrational wavenumbers and IR intensities of the observed lines with those calculated with the B2PLYP-D3/aug-cc-pVTZ-pp method. These observations confirm that only the allylic C-I bond, not the vinylic one, was photodissociated at 290 nm, and in solid p-H₂, the excess energy upon photolysis induced no conformational change. When O₂ was present in the matrix, several intense lines at 1147.5, 1025.7, 914.4, and 728.7 cm^-1, and 4 weaker ones were tentatively assigned to the adduct CH₂C(CH₃)CHIOO; the assignments were supported by ¹⁸O₂ isotopic experiments. Unlike in the gaseous phase, the remaining C-I bond of this adduct could not break to form MACRO because of the efficient quenching in a low-temperature matrix.

Second harmonic generation from symmetry breaking stimulated by mixed organic cations in zero-dimensional hybrid metal halides

Mixing cations with different chemical properties to induce the generation of asymmetric structures is a new approach for tuning the optical properties of hybrid organic-inorganic metal halides (HOIMHs). In this study, zero-dimensional (C₉N₃H₁₅)(C₉H₁₃SO)MBr₆ (M = Bi/Sb, [C₉N₃H₁₅]²⁺ = [(C₄N₂H₁₀)(C₅NH₅)]²⁺ and [C₉H₁₄SO]⁺ = [CH₃(C₆H₄)OS(CH₃)₂]⁺) are synthesized. Two different cations cause both compounds to crystallize in the polar space group P2₁2₁2₁, thus resulting in significant phase matchable second harmonic generation under a 1064 nm laser excitation. Thus, (C₉N₃H₁₅)(C₉H₁₃SO)BiBr₆ and (C₉N₃H₁₅)(C₉H₁₃SO)SbBr₆ exhibit intensities that are approximately 1.8 and 1.7 times that of KH₂PO₄, respectively. The results of density functional theory calculations show that both (C₉N₃H₁₅)(C₉H₁₃SO)BiBr₆ and (C₉N₃H₁₅)(C₉H₁₃SO)SbBr₆ exhibit direct bandgaps of 2.95 and 2.81 eV, respectively. Additionally, because of the distortion of the inorganic octahedra, (C₉N₃H₁₅)(C₉H₁₃SO)SbBr₆ exhibited bright yellow emission at room temperature, which is attributed to ns² fluorescence emission. We believe that the symmetry of the HOIMH crystal structure can be broken by introducing spatially differentiated bifunctional organic cations, which consequently enables even-order nonlinear activities.

Infrared characterization of formation and resonance stabilization of the Criegee intermediate methyl vinyl ketone oxide

Methyl vinyl ketone oxide (MVKO) is an important Criegee intermediate in the ozonolysis of isoprene. MVKO is resonance stabilized by its allyl moiety, but no spectral characterization of this stabilization was reported to date. In this study, we photolyzed a mixture of 1,3-diiodo-but-2-ene and O₂ to produce MVKO and characterized the syn-trans-MVKO, and tentatively syn-cis-MVKO, with transient infrared spectra recorded using a step-scan Fourier-transform spectrometer. The O‒O stretching band at 948 cm^-1 of syn-trans-MVKO is much greater than the corresponding bands of syn-CH₃CHOO and (CH₃)₂COO Criegee intermediates at 871 and 887 cm^-1, respectively, confirming a stronger O‒O bond due to resonance stabilization. We observed also iodoalkenyl radical C₂H₃C(CH₃)I upon photolysis of the precursor to confirm the fission of the terminal allylic C‒I bond rather than the central vinylic C‒I bond of the precursor upon photolysis. At high pressure, the adduct C₂H₃C(CH₃)IOO was also observed. The reaction mechanism is characterized.