Solvation and solvent effects on the short-time photodissociation dynamics of CH2I2 from resonance Raman spectroscopy

Resonance Raman spectra of CH2I2 have been obtained at excitation wavelengths of 369, 355, and 342 nm in cyclohexane solution and in methanol solution at excitation wavelengths of 355 and 342 nm. Resonance Raman spectra were also measured for CH2I2 in the vapor phase with an excitation wavelength of 355 nm. The resonance Raman spectra of CH2I2 exhibit most of their intensity in fundamentals, overtones, and combination bands of modes nominally assigned as the I–C–I symmetric stretch, the I–C–I bend, and the I–C–I antisymmetric stretch vibrations. The absorption spectra and resonance Raman intensities of the gas phase and methanol solution phase diiodomethane spectra were simulated using a simple model and time-dependent wave packet calculations. Normal mode coefficients from normal coordinate calculations were used to convert the motion of the wave packet on the excited electronic state surface from dimensionless normal coordinates into internal coordinates of the molecule. The short-time photodissociation dynamics of diiodomethane in the vapor phase shows that the two C–I bonds are lengthening by the same amount, the I–C–I angle becomes smaller, the H–C–I angles become larger, and the H–C–H angle becomes smaller.The two C–I bonds appear essentially equivalent in the Franck–Condon region of the gas phase photodissociation which implies that the molecule chooses which C–I bond is broken after the wave packet has left the Franck–Condon region of the potential energy surface. Comparison of the gas phase resonance Raman spectrum with solution phase spectra obtained in cyclohexane and methanol solvents reveals that the short-time photodissociation dynamics are noticeably changed by solvation with a large solvent-induced symmetry breaking observed. In the Franck–Condon region of the solution phase diiodomethane photodissociation in methanol solvent the two C–I bond become larger by differing amounts, the I–C–I angle becomes smaller, the H–C–H angle becomes smaller, and the H–C–I angles differ from the corresponding gas phase values. During the initial stages of the solution phase photodissociation (at least in methanol and cyclohexane solvents) the two C–I bonds are not the same and this suggests that the molecule chooses which C–I bond will be broken soon after photoexcitation. ©1996 American Institute of Physics.published_or_final_versio

Early-time photodissociation dynamics of chloroiodomethane in the A-band absorption from resonance Raman intensity analysis

We have obtained resonance Raman spectra and absolute Raman cross sections for h 2-chloroiodomethane (fourteen excitation wavelengths between 200 nm and 355 nm) and d 2-chloroiodomethane (for 282.4 nm excitation) in cyclohexane solution. Most of the intensity in the A-band resonance Raman spectra appears in the nominal C-I stretch overtones progression and combination bands of the nominal C-I stretch overtones with the fundamentals of the CH 2 wag, CH 2 scissor, and the Cl-C-I bend or C-Cl stretch fundamentals. The A-band absorption and absolute resonance Raman intensities were simulated using a simple model which included preresonant contributions to the fundamental Raman peaks and time-dependent wave packet calculations. The motion of the wave packet on the excited state surface was converted from dimensionless normal coordinates into internal coordinates using the results of normal coordinate calculations. The A-band short-time photodissociation dynamics of chloroiodomethane shows that the C-I bond lengthens, the I-C-Cl and H-C-I angles become smaller, and the H-C-Cl angles become larger. These internal coordinate motions which are associated with relatively low frequency modes are consistent with a simple impulsive "soft" radical model of the photodissociation and the CH 2Cl group changing to a more planar structure. However, the C-H bond length does not change much and the H-C-H angle (associated with higher frequency modes) becomes slightly smaller which is inconsistent with the "soft" radical model and the CH 2Cl group changing to a more planar structure. This suggests that an impulsive "semirigid" radical model may be more appropriate than the "soft" radical model to qualitatively describe the chloroiodomethane photodissociation. An ambiguity in the assignment of the 724 cm -1 Raman peak and its associated combination bands to combination bands of the nominal C-I stretch overtones with the fundamentals of the Cl-C-I bend or C-Cl stretch fundamentals limits what we are able to determine about the C-Cl bond length changes during the initial stages of the photodissociation. © 1996 American Institute of Physics.published_or_final_versio

Comment on 'Energy transfer and upconversions in cubic Cs2NaYCl6:Er+3 and Cs2NaErCl6'

Well-resolved luminescence from the crystal-field level 2H(2)9/2 aΓ8 at 36236 cm-1 to 34 lower levels has been observed for Cs2NaErCl6 upon pulsed laser excitation at 273.9 nm. The assignments enable the location and identification of the upconversion luminescence state in the study of Hasan et al. [Phys. Rev. B 56, 4518 (1997)] and support a two-photon vibronic excitation mechanism in the latter case. The experimentally determined energy for the 2H(2)9/2 aΓ8 state differs considerably from the calculated value. The temporal behavior of blue upconverted luminescence from the 2G9/2 state is reproduced from an analytical expression, and it is shown that the alternative fitting procedure does not enable the two-ion upconversion rate W to be well determined.published_or_final_versio

Short-time photodissociation dynamics of A-band and B-band bromoiodomethane in solution: An examination of bond selective electronic excitation

We have obtained resonance, Raman spectra and absolute Raman cross section measurements at eight excitation wavelengths in the A-band and B-band absorptions of bromoiodomethane in cyclohexane solution. The resonance Raman intensities and absorption spectra were simulated using a simple model and time-dependent wave packet calculations. Normal mode vibrational descriptions were used with.the results of the calculations to find the short-time photodissociation dynamics in terms of internal coordinates. The A-band short-time photodissociation dynamics indicate that the C-I bond becomes much longer, the C-Br bond becomes smaller, the I-C-Br angle becomes smaller, the H-C-Br angles become larger, the H-C-I angles become smaller, and the H-C-H angle becomes a bit smaller. The B-band short-time photodissociation dynamics indicate the C-Br bond becomes much longer, the C-I bond becomes slightly longer, the I-C-Br angle becomes smaller, the H-C-I angles become larger, the H-C-Br angles become smaller, and the H-C-H angle becomes slightly smaller. Both the A-band and B-band short-time photodissociation dynamics appear to be most consistent with an impulsive "semi-rigid" radical model qualitative description of the photodissociation with the CH 2Br radical changing to a more planar structure in the A-band and the CH 2I radical changing to a more planar structure in the B band. We have carried out a Gaussian deconvolution of the A-band and B-band absorption spectra of bromoiodomethane, as well as iodomethane and bromomethane. The absorption spectra, resonance Raman intensities, and short-time photodissociation dynamics sueeest a moderate amount of coupling of the C-I and C-Br chromophores. © 1996 American Institute of Physics.published_or_final_versio

Direct observation of an isopolyhalomethane O-H insertion reaction with water: Picosecond time-resolved resonance Raman (ps-TR 3) study of the isobromoform reaction with water to produce a CHBr 2OH product

The spectroscopic observation of an isopolyhalomethane O-H insertion reaction with water was obtained using picosecond time-resolved resonance Raman spectroscopy. It was observed that photolysis of low concentrations of bromoform in aqueous solution resulted in noticeable formation of HBr strong acid. It was shown by ab initio calculations that isobromoform can react with water to produce a CHBr 2(OH) O-H insertion reaction product and a HBr leaving group. The implications of the results for the phase dependent behavior of polyhalomethane photochemistry in the gas phase versus water solvated environments were discussed.published_or_final_versio

Comparison of the dehalogenation of polyhalomethanes and production of strong acids in aqueous and salt (NaCl) water environments: Ultraviolet photolysis of CH 2I 2

The ultraviolet photolysis of CH 2I 2 was studied in water and salt water solutions using photochemistry and picosecond time-resolved resonance Raman spectroscopy. Photolysis in both types of environments produces mainly CH 2(OH) 2 and HI products. However, photolysis of CH 2I 2 in salt water leads to the formation of different products/intermediates (CH 2ICl and Cl 2 -) not observed in the absence of salt in aqueous solutions. The amount of CH 2(OH) 2 and HI products appears to decrease after photolysis of CH 2I 2 in salt water compared to pure water. We briefly discuss possible implications of these results for photolysis of CH 2I 2 and other polyhalomethanes in sea water and other salt aqueous environments compared to nonsalt water solvated environments. © 2004 American Institute of Physics.published_or_final_versio

Ultraviolet f→f emission and crystal field analysis for Er3+ in Cs2NaErCl6

Luminescence is reported from the 2I11/2 level of Er3+, in the cubic elpasolite lattices Cs2NaErCl6 and Cs2NaYCl6. Altogether, with the use of ultraviolet laser excitation, 11 transitions from 2I11/2 Γ7 (at 40 668 cm-1) to lower term multiplets have been observed and assigned. Transitions are also reported from the 2K13/2 Γ6 level at 32 613 cm-1. The absence of emission from 2P3/2 (at 31 367 cm-1) under the experimental conditions is rationalized. Up-conversion to 2H(2)9/2, which is not due to two-photon absorption, is reported for Cs2NaErCl6 under blue pulsed laser excitation. Trap emission from 2G9/2 defect sites has been observed under ultraviolet excitation. A preliminary investigation has been made of the electronic absorption spectra of Cs2NaErCl6 and 58 Kramers quartet and doublet levels have been assigned, with a further 18 levels uncertain. The energy-level fit to 58 levels with total degeneracy 180 has been performed with a mean deviation of 20.4 cm-1, which is improved to 16.8 cm-1 if an empirical correction to the diagonal reduced matrix element of U4 for the 2H(2) term is included. The average error is similar for the 18 uncertain levels (total degeneracy 52). A comparison is included with the energy-level parametrizations of other Er3+ systems.published_or_final_versio

Observation of a HI leaving group following ultraviolet photolysis of CH 2I 2 in water and an ab initio investigation of the O-H insertion/HI elimination reactions of the CH 2I-I isopolyhalomethane species with H 2O and 2H 2O

Ultraviolet/visible absorption and pH measurements that indicate significant production of H + and I - product following ultraviolet photolysis of CH 2I 2 in liquid water are presented. As such, the chemical reactivity of isodiiodomethane (CH 2I-I) with H 2O and 2H 2O is explored using ab initio calculations and compared to previous results. The CH 2I-I isopolyhalomethane species is found to react with water by an O-H insertion/HI elimination reaction mechanism that forms a HI leaving group.published_or_final_versio

4f-5d transitions of Pr3+ in elpasolite lattices

The 4f5d→4f2 emission spectra of Cs2MPrCl6 (M = Na,Li) and CS2NaYCl6:Pr3+ have been recorded at temperatures down to 10 K. The spectra of Pr3+ in the cubic host Cs2NaYCl6 are the most clearly resolved, and 15 transitions to terminal crystal field levels of symmetry representations Γ5g and Γ4g have been observed and assigned, thereby inferring that the symmetry representation of the lowest 4f5d crystal field level is Γ3u. Each transition is characterized by strong progressions in two totally symmetric vibrational modes. The relative displacement of the potential energy curves for the 4f2 and 4f5d crystal field levels, along the α1g internal mode coordinate, is small, being only about 5 pm. The 10-K ultraviolet absorption spectra of CS2NaYCl6:Pr3+ are assigned to transitions from the [3H4] Γ1g electronic ground state to terminal Γ4u crystal field levels of 4f5d. Nontotally symmetric gerade vibrational modes only provide minor intensity contributions. The large energy gap between the d-f emission and f-d absorption spectra of Pr3+ in the cubic elpasolite host is rationalized. The 8-K excitation spectra of Cs2NaPrCl6 and Cs2NaYCl6:Pr3+, excited by synchrotron radiation, show that the transitions to 4f5d fall into two groups. The energy levels and wave vectors of the (independent) 4f2 and 4f5d configurations of Pr3+ have been calculated using a model which includes spin-orbit coupling and crystal field and Coulomb interactions, as well as the configuration interaction of 4f2 with 4f6p. Using the eigenvector of the predominantly high-spin, lowest excited crystal field level of 4f5d, the emission intensities are reasonably well simulated. However, the refinement of the 4f2→4f5d absorption intensities requires a more detailed knowledge of the crystal field energy level scheme of 4f5d. The configuration interaction of 4f5d with 4f6s and 4f5g is discussed.published_or_final_versio

Luminescent zinc(ii) and copper( i) complexes for high-performance solution-processed monochromic and white organic light-emitting devices

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