Spin‐Orbit Ab Initio Investigation of the Ultraviolet Photolysis of Diiodomethane

Liu, Ya Jun; Vico, Luca De; Lindh, Roland; Fang, Wei

doi:10.1002/cphc.200600737

Cited by 25 publications

(22 citation statements)

References 69 publications

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“…16,17 The electronic structure of CH2I2 has been described qualitatively using an exciton model 8 as well as theoretically using ab initio calculations that include effects of spin-orbit coupling and show that excited states with heavily mixed singlet and triplet character are responsible for the absorption spectrum. 18,19 Photolysis of gas phase CH2I2 in the near-UV leads to energy being partitioned predominantly into internal excitation of the iodomethyl (CH2I) radical rather than translation, with fT reduced to ~0.2. 8,18,[20][21][22][23][24][25] The I* yield is effectively zero following excitation at longer wavelengths, but steadily increases to almost 0.5 at 248 nm.…”

Section: Introductionmentioning

confidence: 99%

UV photodissociation dynamics of CHI₂Cl and its role as a photolytic precursor for a chlorinated Criegee intermediate

Kapnas

Toulson

Foreman

et al. 2017

Phys. Chem. Chem. Phys.

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Photolysis of geminal diiodoalkanes in the presence of molecular oxygen has become an established route to the laboratory production of several Criegee intermediates, and such compounds also have marine sources. Here, we explore the role that the trihaloalkane, chlorodiiodomethane (CHI2Cl), may play as a photolytic precursor for the chlorinated Criegee intermediate ClCHOO. CHI2Cl has been synthesized and its UV absorption spectrum measured; relative to that of CH2I2 the spectrum is shifted to longer wavelength and the photolysis lifetime is calculated to be less than two minutes.The photodissociation dynamics have been investigated using DC slice imaging, probing ground state I and spin-orbit excited I* atoms with 2+1 REMPI and single-photon VUV ionization. Total translational energy distributions are bimodal for I atoms and unimodal for I*, with around 72% of the available energy partitioned in to the internal degrees of freedom of the CHICl radical product, independent of photolysis wavelength. A bond dissociation energy of D0 = 1.73±0.11 eV is inferred from the wavelength dependence of the translational energy release, which is slightly weaker than typical C-I bonds. Analysis of the photofragment angular distributions indicate dissociation is prompt and occurs primarily via transitions to states of A″ symmetry. Complementary high-level MRCI calculations, including spin-orbit coupling, have been performed to characterize the excited states and confirm that states of A″ symmetry with highly mixed singlet and triplet character are predominantly responsible for the absorption spectrum. Transient absorption spectroscopy has been used to measure the absorption spectrum of ClCHOO produced from the reaction of CHICl with O2 over the range 345-440 nm. The absorption spectrum, tentatively assigned to the syn conformer, is at shorter wavelengths relative to that of CH2OO and shows far weaker vibrational structure.3

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Section: Introductionmentioning

confidence: 99%

UV photodissociation dynamics of CHI₂Cl and its role as a photolytic precursor for a chlorinated Criegee intermediate

Kapnas

Toulson

Foreman

et al. 2017

Phys. Chem. Chem. Phys.

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“…These products are predicted to have transitions in the visible or near UV with very large oscillator strength (∼0.5). 17,41,43,44 Femtosecond transient absorption (TA) measurements in acetonitrile 25,45 and picosecond time resolved resonance Raman (TR 3 ) experiments in cyclohexane 40 have shown that A band excitation at 266 nm in solution results in formation of the iso-CH 2 Br-I photoproduct. The visible spectrum of the photoproduct is similar to that observed in low temperature matrix experiments.…”

Section: Introductionmentioning

confidence: 99%

Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH2BrI

Anderson

Spears

Wilson

et al. 2013

The Journal of Chemical Physics

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It is well known that ultraviolet photoexcitation of halomethanes results in halogen-carbon bond cleavage. Each halogen-carbon bond has a dominant ultraviolet (UV) absorption that promotes an electron from a nonbonding halogen orbital (nX) to a carbon-halogen antibonding orbital (σ*C-X). UV absorption into specific transitions in the gas phase results primarily in selective cleavage of the corresponding carbon-halogen bond. In the present work, broadband ultrafast UV-visible transient absorption studies of CH2BrI reveal a more complex photochemistry in solution. Transient absorption spectra are reported spanning the range from 275 nm to 750 nm and 300 fs to 3 ns following excitation of CH2BrI at 266 nm in acetonitrile, 2-butanol, and cyclohexane. Channels involving formation of CH2Br + I radical pairs, iso-CH2Br-I, and iso-CH2I-Br are identified. The solvent environment has a significant influence on the branching ratios, and on the formation and stability of iso-CH2Br-I. Both iso-CH2Br-I and iso-CH2I-Br are observed in cyclohexane with a ratio of ~2.8:1. In acetonitrile this ratio is 7:1 or larger. The observation of formation of iso-CH2I-Br photoproduct as well as iso-CH2Br-I following 266 nm excitation is a novel result that suggests complexity in the dissociation mechanism. We also report a solvent and concentration dependent lifetime of iso-CH2Br-I. At low concentrations the lifetime is >4 ns in acetonitrile, 1.9 ns in 2-butanol and ~1.4 ns in cyclohexane. These lifetimes decrease with higher initial concentrations of CH2BrI. The concentration dependence highlights the role that intermolecular interactions can play in the quenching of unstable isomers of dihalomethanes.

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“…The only earlier report of PECs of excited states is by Ya-Jun Liu et al 27 who have investigated UV photolysis pathways using spin-orbit coupled multistate complete active space second order perturbation method and calculated potential energy curves for the first four singlet and triplet excited states. The present work is mainly concerned with explanation of spectral features observed in the UV photoabsorption spectrum.…”

Section: Valence Excited Statesmentioning

confidence: 99%

“…Dipole forbidden states and states with very low oscillator strength are omitted from this table, as also states of mixed nature which are already included in Rydberg assignments. The first six singlet excited states including the forbidden A 2 state have been calculated earlier using the complete active space second order perturbation method (CASPT2) by Liu et al 27 Their results are shown in parentheses in Table VIII for comparison. It may be seen that excitation energies are in overall good agreement except for reordering of some of the states.…”

Section: Valence Excited Statesmentioning

confidence: 99%

“…5 Yet another reason for the interest in CH 2 I 2 is related to the potential for lasing action in excited I 2 produced by photodissociation of CH 2 I 2 . 6 Over the years, a considerable number of experimental 1, 3-22 and theoretical [23][24][25][26][27] investigations on CH 2 I 2 , largely pertaining to photodissociation dynamics in the UV region (<50 000 cm −1 ), have been carried out. Notwithstanding the vast amount of literature available on the UV region, it is rather surprising that there is very little information available on the vacuum ultraviolet (VUV) spectrum of CH 2 I 2 .…”

Section: Introductionmentioning

confidence: 99%

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Electronic state spectroscopy of diiodomethane (CH2I2): Experimental and computational studies in the 30 000–95 000 cm−1 region

Mandal

Singh

Shastri

et al. 2014

The Journal of Chemical Physics

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The electronic absorption spectrum of diiodomethane in the 30,000-95,000 cm(-1) region is investigated using synchrotron radiation; the spectrum in the 50,000-66,500 cm(-1) region is reported for the first time. The absorption bands in the 30,000-50,000 cm(-1) region are attributed to valence transitions, while the vacuum ultraviolet (VUV) spectrum (50,000-95,000 cm(-1)) is dominated by several Rydberg series converging to the first four ionization potentials of CH2I2 at 9.46, 9.76, 10.21, and 10.56 eV corresponding to the removal of an electron from the outermost 3b2, 2b1, 1a2, and 4a1 non-bonding orbitals, respectively. Rydberg series of ns, np, and nd type converging to each of the four ionization potentials are assigned based on a quantum defect analysis. Time dependent density functional theory calculations of excited states support the analysis and help in interpretation of the Rydberg and valence nature of observed transitions. Density functional theory calculations of the neutral and ionic ground state geometries and vibrational frequencies are used to assign the observed vibronic structure. Vibronic features accompanying the Rydberg series are mainly due to excitation of the C-I symmetric stretch (ν3) and CH2 wag (ν8) modes, with smaller contributions from the C-H symmetric stretch (ν1). UV absorption bands are assigned to low lying valence states 1(1)B2, 1(1)B1, 2(1)A1, 3(1)A1, 2(1)B1, and 2(1)B2 and the unusually high underlying intensity in parts of the VUV spectrum is attributed to valence states with high oscillator strength. This is the first report of a comprehensive Rydberg series and vibronic analysis of the VUV absorption spectrum of CH2I2 in the 50,000-85,000 cm(-1) region. The VUV absorption spectrum of CD2I2 which serves to verify and consolidate spectral assignments is also reported here for the first time.

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Spin‐Orbit Ab Initio Investigation of the Ultraviolet Photolysis of Diiodomethane

Cited by 25 publications

References 69 publications

UV photodissociation dynamics of CHI₂Cl and its role as a photolytic precursor for a chlorinated Criegee intermediate

UV photodissociation dynamics of CHI₂Cl and its role as a photolytic precursor for a chlorinated Criegee intermediate

Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH2BrI

Electronic state spectroscopy of diiodomethane (CH2I2): Experimental and computational studies in the 30 000–95 000 cm−1 region

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Spin‐Orbit Ab Initio Investigation of the Ultraviolet Photolysis of Diiodomethane

Cited by 25 publications

References 69 publications

UV photodissociation dynamics of CHI2Cl and its role as a photolytic precursor for a chlorinated Criegee intermediate

UV photodissociation dynamics of CHI2Cl and its role as a photolytic precursor for a chlorinated Criegee intermediate

Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH2BrI

Electronic state spectroscopy of diiodomethane (CH2I2): Experimental and computational studies in the 30 000–95 000 cm−1 region

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UV photodissociation dynamics of CHI₂Cl and its role as a photolytic precursor for a chlorinated Criegee intermediate

UV photodissociation dynamics of CHI₂Cl and its role as a photolytic precursor for a chlorinated Criegee intermediate