King Hung Leung

Transient-Resonance Raman and Density Functional Theory Investigation of 4-Biphenylylnitrenium, 2-Fluorenylnitrenium, and Diphenylnitrenium Ions

We present transient-resonance Raman spectra for the 4-biphenylylnitrenium, diphenylnitrenium, and 2-fluorenylnitrenium ions. These spectra display a number of fundamental vibrational bands whose frequencies exhibit good agreement with those computed using BPW91/cc-PVDZ density functional theory calculations for the singlet ground states of the 4-biphenylylnitrenium, diphenylnitrenium, and 2-fluorenylnitrenium ions. Comparison of these arylnitrenium ions with each other and with previous results for structurally similar biphenyl radical cations indicates that the degree of iminocyclohexadienyl character observed in these arylnitrenium ions depends on the relative orientation of the two phenyl rings, the nature of the nitrenium ion moiety, and the ability of the biphenyl-like group to accommodate positive charge through formation of a more planar-like structure with quinoidal-like character.

Resonance Raman intensity analysis investigation of metal-metal bonded transitions: an examination of the1A2u ←1A1g (5dσ* → 6pσ) transition of Pt2(P2O5H2)44−

A preliminary resonance Raman intensity analysis study of the 1A2u 1A1g (5dσ* 6pσ) transition of (n-Bu4N)4[Pt2(P2O5H2)4] in acetonitrile solution at room temperature is reported. The absolute resonance Raman and absorption intensities were simultaneously simulated using wavepacket calculations and a simple model. The best fit parameters indicate that the Pt— Pt bond length changes by about 0.225 A in the initially excited 1A2u state relative to the ground state. This is in good agreement with previous studies on the vibronically structured absorption and emission spectra of low-temperature crystalline (n-Bu4N)4[Pt2(P2O5H2)4] which suggested that the Pt— Pt bond length changes by about 0.21 A in the 1,3A2u states. The resonance Raman intensity analysis demonstrated here can be generally applied to metal– metal bonded electronic transitions for compounds and sample conditions (such as room temperature liquids for many samples) which do not exhibit any vibronic structure. Copyright

Resonance Raman investigation of the short-time dynamics of the ultraviolet photodissociation of bromoform

We report ultraviolet resonance Raman spectra of bromoform (CHBr3) in cyclohexane solution. The resonance Raman spectra show significant intensity in the overtones of the nominal Br-C-Br symmetric bend (v 6), the nominal H-C-Br asymmetric bend (v3), the nominal Br-C-Br symmetric stretch (v 2) and the nominal Br-C-Br asymmetric stretch (v 5) vibrational modes suggesting that the short-time photodissociation dynamics have noticeable multidimensional character. The lack of strong combination bands between several of the Franck-Condon active modes suggests that more than one electronic transition contribute to the resonance Raman spectra. We briefly discuss the ultraviolet short-time photodissociation dynamics of bromoform and the potential implications for the secondary photodissociation reactions of the initially formed CHBr2 radical.

Comparison of the LMCT resonance Raman spectra of PdCl2(PCy3)2 and PdCl2(PPh3)2: effect of the phenyl versus cyclohexyl substituents on the LMCT excited state

Resonance Raman spectra have been obtained at four excitation wavelengths within the A-band ligand to metal charge transfer (LMCT) absorption of trans-dichloro-bis(tricyclo-hexylphosphine) palladium(II) and a resonance Raman intensity analysis was done to elucidate the excited state structure relative to the ground state. Comparison of the present results for trans-dichlorobis(tricyclohexylphosphine) palladium(II) with those previously reported for trans-dichlorobis(triphenylphosphine) palladium(II) shows that the phenyl ligand appears to primarily affect the adjacent P-C vibrational reorganizational energy and does not significantly perturb the Pd-Cl or Pd-P vibrational reorganizational energies.

Time-resolved resonance Raman spectroscopy and density functional study of 2-fluorenylnitrene and its dehydroazepine products

We report time-resolved resonance Raman spectra for 2-fluorenylnitrene and its dehydroazepine products acquired after photolysis of 2-fluorenylnitrene in acetonitrile. The experimental Raman band frequencies exhibit good agreement with the calculated vibrational frequencies from UBPW91/cc-PVDZ density functional calculations for the singlet and triplet states of the 2-fluorenylnitrene as well as BPW91/cc-PVDZ calculations for the two dehydroazepine ring-expansion product species. The decay of the 2-fluorenylnitrene Raman signal and the appearance of the dehydroazepine products suggest the presence of an intermediate species (probably an azirine) that does not absorb very much at the 416 nm probe wavelength used in the time-resolved resonance Raman experiments. Comparison of the singlet 2-fluorenylnitrene species with the singlet 2-fluorenylnitrenium ion species indicates that protonation of the nitrene to give the nitrenium ion leads to a significant enhancement of the cyclohexadienyl character of the phenyl rings without much change of the C-N bond length.

Transient resonance Raman and density functional theory investigation of the ultraviolet photolysis of dibromoacetonitrile in the solution phase

Abstract Ultraviolet photolysis of dibromoacetonitrile in cyclohexane solution was investigated using transient resonance Raman spectroscopy. The experimental Raman vibrational frequencies were compared to those predicted from density functional theory calculations for several probable photoproduct species. Our results indicate that significant amounts of the Br–NCCHBr adduct is produced on the nanosecond time scale. We briefly discuss the probable mechanisms of formation of the Br–NCCHBr species and implications for using substituted dihaloalkanes as carbenoid agents in cyclopropanation reactions.