Xuming Zheng

Photoisomerization reaction of CH2BrI following A-band and B-band photoexcitation in the solution phase: Transient resonance Raman observation of the iso-CH2I–Br photoproduct

We present nanosecond transient resonance Raman experiments that investigate the photoproduct species formed following A-band and B-band excitation of bromoiodomethane in room temperature cyclohexane solutions. Density functional theory calculations were also performed for several species that have been proposed as photoproducts for photodissociation of bromoiodomethane in the condensed phase. Comparison of the experimental resonance Raman spectra to density functional theory computational results and results for the closely related iso-CH2I–I and iso-CH2Br–Br species demonstrated that the iso-CH2I–Br species is mainly responsible for a transient absorption spectrum that appears after either A-band or B-band photoexcitation of bromoiodomethane in cyclohexane solution. This is in contrast to previous results for low temperature (12 K) solids where mainly the iso-CH2Br–I species was observed following A-band photoexcitation of bromoiodomethane. Further density functional theory computational results indicat...

Transient resonance Raman spectroscopy and density functional theory investigation of iso-polyhalomethanes containing bromine and/or iodine atoms

We report additional transient resonance Raman spectra and density functional theory computations for the products formed following ultraviolet photoexcitation of solution phase polyhalomethanes containing bromine and/or iodine atoms. We show that the iso-polyhalomethane photoproduct is responsible for the intense transient absorption band observed in the 350–470 nm region after ultraviolet excitation of polyhalomethanes in the solution phase. We examine the trends and correlation in the density functional theory optimized geometry and intense electronic absorption transition in the 350–470 nm region for the iso-polyhalomethanes containing bromine and/or iodine atoms. We explore the chemical reactivity of the iso-polyhalomethane species using density functional theory computations for the reaction of iso-CH2Br–Br with ethylene as an example. Our results and comparison with experimental data in the literature indicate that the iso-polyhalomethane species is most likely the methylene transfer agent in the c...

A resonance Raman spectroscopic and CASSCF investigation of the Franck–Condon region structural dynamics and conical intersections of thiophene

Resonance Raman spectra were acquired for thiophene in cyclohexane solution with 239.5 and 266 nm excitation wavelengths that were in resonance with ∼240 nm first intense absorption band. The spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion mostly along the reaction coordinates of six totally symmetry modes and three nontotally symmetry modes. The appearance of the nontotally symmetry modes, the C-S antisymmetry stretch +C-C=C bend mode ν(21)(B(2)) at 754 cm(-1) and the H(7)C(3)-C(4)H(8) twist ν(9)(A(2)) at 906 cm(-1), suggests the existence of two different types of vibronic-couplings or curve-crossings among the excited states in the Franck-Condon region. The electronic transition energies, the excited state structures, and the conical intersection points (1)B(1)/(1)A(1) and (1)B(2)/(1)A(1) between 2 (1)A(1) and 1 (1)B(2) or 1 (1)B(1) potential energy surfaces of thiophene were determined by using complete active space self-consistent field theory computations. These computational results were correlated with the Franck-Condon region structural dynamics of thiophene. The ring opening photodissociation reaction pathway through cleavage of one of the C-S bonds and via the conical intersection point (1)B(1)/(1)A(1) was revealed to be the predominant ultrafast reaction channel for thiophene in the lowest singlet excited state potential energy hypersurface, while the internal conversion pathway via the conical intersection point (1)B(2)/(1)A(1) was found to be the minor decay channel in the lowest singlet excited state potential energy hypersurface.

Solvation effects on the iodoform ultraviolet direct photodissociation reaction: opening the photoisomerization channel

We present transient resonance Raman spectra that demonstrate that the iso-iodoform photoproduct is mainly associated with the ; 400 nm transient absorption band observed following excitation of iodoform in the solution phase. Comparison of our present results with previously reported gas-phase experiments and results for the closely related diiodomethane molecular system suggests that solvation gives rise to noticeable formation of the iso-iodoform photoproduct by some recombination of the initially formed CHI and I photofragments within a solvent cage. q 2000 Elsevier Science B.V. All 2

Resonance Raman intensity analysis of the excited state proton transfer dynamics of 2-nitrophenol in the charge-transfer band absorption

Resonance Raman spectra were obtained for 2-nitrophenol in cyclohexane solution with excitation wavelengths in resonance with the charge-transfer (CT) proton transfer band absorption. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion along more than 15 normal modes: the nominal CCH bend+CC stretch nu(12) (1326 cm(-1)), the nominal CCC bend nu(23) (564 cm(-1)), the nominal CO stretch+NO stretch+CC stretch nu(14) (1250 cm(-1)), the nominal CCH bend+CC stretch+COH bend nu(15) (1190 cm(-1)); the nominal CCH bend+CC stretch nu(17) (1134 cm(-1)), the nominal CCC bend+CC stretch nu(22) (669 cm(-1)), the nominal CCN bend nu(27) (290 cm(-1)), the nominal NO(2) bend+CC stretch nu(21) (820 cm(-1)), the nominal CCO bend+CNO bend nu(25) (428 cm(-1)), the nominal CC stretch nu(7) (1590 cm(-1)), the nominal NO stretch nu(8) (1538 cm(-1)), the nominal CCC bend+NO(2) bend nu(20) (870 cm(-1)), the nominal CC stretch nu(6) (1617 cm(-1)), the nominal COH bend+CC stretch nu(11) (1382 cm(-1)), nominal CCH bend+CC stretch nu(9) (1472 cm(-1)). A preliminary resonance Raman intensity analysis was done and the results for 2-nitrophenol were compared to previously reported results for nitrobenzene, p-nitroaniline, and 2-hydroxyacetophenone. The authors briefly discuss the differences and similarities in the CT-band absorption excitation of 2-nitrophenol relative to those of nitrobenzene, p-nitroaniline, and 2-hydroxyacetophenone.

Picosecond time-resolved resonance Raman observation of the iso-CH2Cl–I and iso-CH2I–Cl photoproducts from the “photoisomerization” reactions of CH2ICl in the solution phase

We report a preliminary picosecond Stokes time-resolved resonance Raman investigation of the initial formation and subsequent decay of the photoproduct produced following 267 nm excitation of CH2ClI in acetonitrile solution. Density-functional theory computations were done for several probable photoproduct species. Comparison of these computational results and results from a recent femtosecond transient absorption study to our present picosecond resonance Raman spectra indicate that the iso-CH2Cl–I species is mainly produced and associated with the ∼460 nm transient absorption band. The iso-CH2Cl–I species appears to decay and form appreciable amounts of the more stable iso-CH2I–Cl species that is associated with a ∼370 nm transient absorption band after a few hundred ps.

Transient resonance Raman spectroscopy and density functional theory investigation of iso-CHBr2Cl and iso-CCl3Br photoproducts produced following ultraviolet excitation of CHBr2Cl and CCl3Br

We report transient resonance Raman spectra and density functional theory calculations for the photoproducts produced after ultraviolet excitation of CHBr2Cl and CCl3Br in cyclohexane solution. Comparison of the computed vibrational frequencies to the experimental Raman frequencies revealed that the iso-CHBrCl–Br and iso-CHClBr–Br species are mainly responsible for the transient resonance Raman spectrum observed following ultraviolet excitation of CHBr2Cl. Similar comparisons for CCl3Br showed the iso-CCl2Cl–Br species is mainly responsible for the transient resonance Raman spectrum observed following ultraviolet excitation of CCl3Br. Additional density functional theory computations were done to examine the chemical reactions of iso-CH2Br–Cl and iso-CH2Cl–Br with ethylene to give cyclopropane and Br–Cl product. We briefly discuss the possibility for release of reactive halogens into the atmosphere via the photochemical and chemical reactions of iso-polyhalomethane molecules formed after ultraviolet excit...

Effect of geometrical conformation on the short-time photodissociation dynamics of 1-iodopropane in the A-band absorption

We have taken resonance Raman spectra and made absolute Raman cross section measurements at six excitation wavelengths for 1-iodopropane. The resonance Raman spectra have most of their Raman intensity in features that may be assigned as fundamentals, overtones, and combination bands of three Franck–Condon active vibrational modes (the nominal C–I stretch, the nominal CCC bend, and the nominal CCI bend) for the trans and gauche conformations of 1-iodopropane. The resonance Raman and absorption cross sections of the trans and gauche conformations of 1-iodopropane were simulated using a simple model and time-dependent wave packet calculations. The results of the simulations were used in conjunction with the vibrational normal-mode coefficients to find the short-time photodissociation dynamics of trans and gauche conformers of 1-iodopropane in terms of internal coordinate changes. The trans and gauche conformers display significantly different Franck–Condon region photodissociation dynamics, which indicates t...

Role of ribose in the initial excited state structural dynamics of thymidine in water solution: a resonance Raman and density functional theory investigation.

Resonance Raman spectra were obtained for thymidine and thymine with excitation wavelengths in resonance with the approximately 260 nm band absorption spectrum. The spectra indicate that the Franck-Condon (FC) region photodissociation dynamics of thymidine have multidimensional character with motion predominantly along the nominal C5=C6 stretch + C6-H bend nu17 (delta = 0.75, lambda = 468 cm(-1)), the nominal thymine ring stretch + C6-H bend + N1-C1, stretch nu29 (delta = 0.73, lambda = 363 cm(-1)), the nominal thymine ring stretch + C5-CH3/ N1-C1, stretch nu37 (delta = 0.69, lambda = 292 cm(-1)), and accompanied by the moderate and minor changes in the nu40, nu20 and nu23, nu55, nu60, nu61, nu63 modes. A preliminary resonance Raman intensity analysis was done, and these results for thymidine and thymine were compared to each other. The roles of ribose in the FC structure dynamics of thymidine were explored and the results were used to correlate to its lifetime constants tau1 and tau2 for two nonradiative decay channels. Spi/Sn conical intersection versus a distorted structure of Spi,min in the FC region was briefly discussed.

Density functional theory and resonance Raman investigation of the ultraviolet electronic excited states of CF2I2

We report density functional theory calculation results that examine the ultraviolet electronic transitions of CF2I2 and CH2I2. We make preliminary assignments of several transitions to the ultraviolet absorption spectra of CF2I2 and CH2I2. We compare our present results to previous experimental and computational work. We also examine the molecular orbitals involved in the electronic transitions assigned to the absorption spectra.