Strong, multiple-stranded Cu–N bonds lead to a group of highly emissive 1D-CumIm(L) hybrid yellow phosphors with excellent thermal- and photo-stability.
A novel
allylic 1,6 hydrogen-atom-transfer mechanism is established
through infrared activation of the 2-butenal oxide Criegee intermediate,
resulting in very rapid unimolecular decay to hydroxyl (OH) radical
products. A new precursor, Z/E-1,3-diiodobut-1-ene,
is synthesized and photolyzed in the presence of oxygen to generate
a new four-carbon Criegee intermediate with extended conjugation across
the vinyl and carbonyl oxide groups that facilitates rapid allylic
1,6 H-atom transfer. A low-energy reaction pathway involving isomerization
of 2-butenal oxide from a lower-energy (tZZ) conformer
to a higher-energy (cZZ) conformer followed by 1,6
hydrogen transfer via a seven-membered ring transition
state is predicted theoretically and shown experimentally to yield
OH products. The low-lying (tZZ) conformer of 2-butenal
oxide is identified based on computed anharmonic frequencies and intensities
of its conformers. Experimental IR action spectra recorded in the
fundamental CH stretch region with OH product detection by UV laser-induced
fluorescence reveal a distinctive IR transition of the low-lying (tZZ) conformer at 2996 cm–1 that results
in rapid unimolecular decay to OH products. Statistical RRKM calculations
involving a combination of conformational isomerization and unimolecular
decay via 1,6 H-transfer yield an effective decay
rate k
eff(E) on the order
of 108 s–1 at ca. 3000 cm–1 in good accord with the experiment. Unimolecular decay proceeds
with significant enhancement due to quantum mechanical tunneling.
A rapid thermal decay rate of ca. 106 s–1 is predicted by master-equation modeling of 2-butenal oxide at 298
K, 1 bar. This novel unimolecular decay pathway is expected to increase
the nonphotolytic production of OH radicals upon alkene ozonolysis
in the troposphere.
Reductive condensations of alcohols with aldehydes/ketones to generate ethers are catalyzed by a readily accessible thiourea organocatalyst that operates in combination with HCl. 1,1,3,3-tetramethyldisiloxane serves as a convenient reducing reagent. This strategy is applicable to challenging substrate combinations and exhibits functional group tolerance. Competing reductive homocoupling of the carbonyl component is suppressed.
Atmospheric ozonolysis of biogenic and anthropogenic alkenes generates zwitterionic carbonyl oxide intermediates (R 1 R 2 C�O + O − ), known as Criegee intermediates, with different structural motifs and conformations. This study reports a systematic laboratory study of substituent effects on the electronic spectroscopy of four-carbon Criegee intermediates (CIs) with methyl−ethyl (MECI) and isopropyl (IPCI) groups, which are isomers produced in ozonolysis of asymmetric branched alkenes. The four-carbon CIs are separately generated by an alternative synthetic route, and spectroscopically characterized on the strong π* ← π transition associated with the carbonyl oxide group in a pulsed supersonic expansion with VUV photoionization at 118 nm and UV-induced depletion of the m/z 88 signal. The resultant broad and unstructured UV spectral features for MECI and IPCI are peaked at ca. 320 and 330 nm, respectively, with large absorption cross-sections of ca. 10 −17 cm 2 . Comparisons are made with the four-carbon CIs formed in isoprene ozonolysis, methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide), which have the same backbone connectivity as MECI and IPCI but have extended conjugation across the vinyl and carbonyl groups. A remarkable 50 nm shift of the peak absorption to longer wavelength is observed for MVK-oxide and MACR-oxide compared to MECI and IPCI, respectively. Vertical excitation energies computed theoretically agree well with the experimental findings, confirming that the spectral shifts are caused by the extended π conjugation in the isoprene-derived Criegee intermediates.
The 2-butenal oxide Criegee intermediate [(CH 3 CH�CH)CHOO], an isomer of the four-carbon unsaturated Criegee intermediates derived from isoprene ozonolysis, is characterized on its first π* ← π electronic transition and by the resultant dissociation dynamics to O ( 1 D) + 2-butenal [(CH 3 CH�CH)CHO] products. The electronic spectrum of 2-butenal oxide under jet-cooled conditions is observed to be broad and unstructured with peak absorption at 373 nm, spanning to half maxima at 320 and 420 nm, and in good accord with the computed vertical excitation energies and absorption spectra obtained for its lowest energy conformers. The distribution of total kinetic energy released to products is ascertained through velocity map imaging of the O ( 1 D) products. About half of the available energy, deduced from the theoretically computed asymptotic energy, is accommodated as internal excitation of the 2-butenal fragment. A reduced impulsive model is introduced to interpret the photodissociation dynamics, which accounts for the geometric changes between 2-butenal oxide and the 2-butenal fragment, and vibrational activation of associated modes in the 2-butenal product. Application of the reduced impulsive model to the photodissociation of isomeric methyl vinyl ketone oxide reveals greater internal activation of the methyl vinyl ketone product arising from methyl internal rotation and rock, which is distinctly different from the dissociation dynamics of 2-butenal oxide or methacrolein oxide.
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