I-Chia Chen

Photofragments CH3(X̃2A2″)+HCO(X̃2A′) from acetaldehyde: Distributions of rotational states and preferential population of K doublets of HCO

The relative rotational populations of HCO dissociated from acetaldehyde excited to the S1 state at photolysis energies 31 412, 31 771, 32 165, 32 728, and 33 239 cm−1 are obtained. Fluorescent spectra B 2A′−X 2A′(0,0) of HCO were detected; the intensity is corrected for both the fluorescent quantum yield and the effect of axis switching to obtain the populations of rotational states of HCO. All measured populations of rotational states of HCO, displaying a Gaussian‐type pattern, cannot be described according to a Boltzmann distribution. The average energy partitioned to rotation of HCO along the b and c axes is [(270–465)±25] cm−1, about 11% of the available energy, but the average energy partitioned along the a axis remains the same (32±2) cm−1 for all photolysis energies. The rotational energy of HCO is less than that predicted according to an impulsive model calculated with the ab initio transition state structure. Preferential population of Ka=1 doublet states Nu1 and Nl1 is observed. The ratio of ...

I. Three-center versus four-center HCl-elimination in photolysis of vinyl chloride at 193 nm: Bimodal rotational distribution of HCl (v<=7) detected with time-resolved Fourier-transform spectroscopy

Following photodissociation of vinyl chloride at 193 nm, fully resolved vibration-rotational emission spectra of HCl in the spectral region 2000–3310 cm−1 are temporally resolved with a step-scan Fourier-transform spectrometer. Under improved resolution and sensitivity, emission from HCl up to v=7 is observed, with J>32 (limited by overlap at the band head) for v=1–3. All vibrational levels show bimodal rotational distribution with one component corresponding to ∼500 K and another corresponding to ∼9500 K for v⩽4. Vibrational distributions of HCl for both components are determined; the low-J component exhibits inverted vibrational population of HCl. Statistical models are suitable for three-center (α, α) elimination of HCl because of the loose transition state and a small exit barrier for this channel; predicted internal energy distributions of HCl are consistent but slightly less than those observed for the high-J component. Impulse models considering geometries and displacement vectors of transition sta...

Three-center versus four-center elimination in photolysis of vinyl fluoride and vinyl bromide at 193 nm: Bimodal rotational distribution of HF and HBr (v⩽5) detected with time-resolved Fourier transform spectroscopy

Following photodissociation of vinyl fluoride (CH2CHF) and vinyl bromide (CH2CHBr) at 193 nm, fully resolved vibration–rotational emission spectra of HF and HBr in spectral regions 3050–4900 and 2000–2900 cm−1, respectively, are temporally resolved with a step–scan Fourier transform spectrometer. With a data acquisition window 0–5 μs suitable for spectra with satisfactory ratio of signal-to-noise, emission from HX (with X = F or Br) up to v=6 is observed. All vibrational levels show bimodal rotational distributions. For CH2CHF, these two components of HF have average rotational energies ∼2 and 23 kJ mol−1 and vibrational energies ∼83 and 78 kJ mol−1, respectively; the values are corrected for small quenching effects. For CH2CHBr, these two components of HBr correspond to average rotational energies ∼4 and 40 kJ mol−1, respectively, and similar vibrational energies ∼68 kJ mol−1. The separate statistical ensemble (SSE) model is suitable for three-center (α, α) elimination of HX because of the loose transiti...

Vibrational levels of the transition state and rate of dissociation of triplet acetaldehyde

Fluorescence decay of the S1 state of d4-acetaldehyde is measured. Below the dissociation threshold for formation of fragments of formyl and methyl radicals, single exponential decays of fluorescence are observed. Biexponential decay is observed when the excitation energies are near and above the dissociation threshold. Hence, in this region the mechanism for intramolecular relaxation of energy reaches the “intermediate case.” Strong coupling between S1 and T1 states or reversible reaction for S1↔T1 results in this biexponential behavior in the fluorescence decay of S1. Rates of appearance of DCO from dissociation of acetaldehyde are measured. The stepwise increases in plots of both rate of appearance of formyl radicals and rate of decay of excited acetaldehyde versus excitation energy for both isotopic variants of acetaldehyde are observed. According to fits to Rice–Ramsperger–Kassel–Marcus (RRKM) theory, these sharp increases correspond to vibrational levels of transition state in the triplet surface. F...

A rapid SNAP-tag fluorogenic probe based on an environment-sensitive fluorophore for no-wash live cell imaging.

One major limitation of labeling proteins with synthetic fluorophores is the high fluorescence background, which necessitates extensive washing steps to remove unreacted fluorophores. In this paper, we describe a novel fluorogenic probe based on an environment-sensitive fluorophore for labeling with SNAP-tag proteins. The probe exhibits dramatic fluorescence turn-on of 280-fold upon being labeled to SNAP-tag. The major advantages of our fluorogenic probe are the dramatic fluorescence turn-on, ease of synthesis, high selectivity, and rapid labeling with SNAP-tag. No-wash labeling of both intracellular and cell surface proteins was successfully achieved in living cells, and the localization of these proteins was specifically visualized.

Predissociation mechanism and spin‐rotation constant of the HCO B̃ 2A′ state

Formyl radicals produced from photolysis of acetaldehyde at 310 nm were supersonically cooled and detected via the B–X transition using the laser‐induced fluorescence (LIF) technique. Spectra at 0.16 cm−1 resolution and fluorescence lifetimes of HCO B(0,0,0), (0,0,1), and (0,1,0) levels were measured. The observed lifetimes decrease rapidly with variation of the rotational quantum number Ka from 0 to 2 but slowly with the rotational quantum number N from 0 to 8. Experimental data indicate that the B state is coupled to a predissociating state via an a‐type Coriolis interaction to account for the rotationally dependent lifetime for the low vibrational levels of the HCO B state. Correction of the fluorescence quantum yield for individual rotational states is necessary to obtain accurate ground state populations by LIF when using the B–X transitions. The intensity distribution of the two spin states observed implies a negative value of the spin‐rotation parameter for the B state, in contrast with the...

Detection of ClCO with time-resolved Fourier-transform infrared absorption spectroscopy

Abstract ClCO was produced as a reaction intermediate after irradiation of a flowing mixture of Cl 2 /CO/Ar at 355 nm. A step-scan time-resolved Fourier-transform infrared spectrometer operated in absorption mode was employed to detect ClCO. A transient spectrum at a resolution of 0.13 cm −1 partially reveals rotational structure with the Q-branch peaked at 1884.59 cm −1 ; the transition is associated with the C–O stretching ( ν 1 ) mode of ClCO. Calculations with density-functional theory (B3LYP/aug-cc-pVTZ) predict the geometry, vibrational and rotational parameters of 35 ClCO and 37 ClCO. A simulated absorption spectrum based on these parameters agrees satisfactorily with experimental observation.

Axis switching in the B̃ 2A′–X̃ 2A′ transition of HCO and fluorescence lifetimes of the B̃ 2A′(0,0,0) rotational states

Fluorescence spectra of the B 2A′–X 2A′(0,0) band of thermalized HCO at 298 K were recorded. The lifetimes of nascent HCO B 2A′(0,0,0) states produced from acetaldehyde photolyzed at a wavelength of 310.9 nm in a supersonic jet were obtained for rotational states to N=26, 24, and 18 for Ka=0, 1, and 2, respectively. The lifetimes of these states decrease as the Ka and N quantum numbers increase; they vary in the ranges 24–89 ns for Ka=0 states, 12–52 ns for Ka=1 states and 14–18 ns for Ka=2 states. An a‐type Coriolis interaction between the B 2A′ and A 2A″ states is proposed to account for predissociation of the low vibrational levels of B 2A′. The spectral intensity shows anomalous behavior in that the intensity ratio of transitions QR0/QP0 is nearly 2; similar phenomena are observed for other branches. The mechanism of axis switching in the B 2A′–X 2A′ transition is because the geometry is altered during the excitation; consequently the principal inertial axes are rotated to account for the inte...

PHOTODISSOCIATION OF HNO3 AT 193 NM : NEAR-INFRARED EMISSION OF NO DETECTED BY TIME-RESOLVED FOURIER TRANSFORM SPECTROSCOPY

Rotationally resolved emission of NO, produced from photolysis of HNO3 at 193 nm, in the near infrared region (8900–9300 cm−1) was recorded with a step‐scan Fourier‐transform interferometer at a resolution of 0.1 cm−1. The emission is assigned as NO D 2Σ+−A 2Σ+ (v′,v″)=(0,0) band with rotational states N′=17–42. Emission from selective rotational states of NO D 2Σ+ was observed when HNO3 was photolyzed with an ArF excimer laser having a narrow bandwidth ≊0.01 nm. The experimental results indicate that the D 2Σ+ state of NO is formed via absorption of another 193 nm photon by NO (v″=1) in the ground electronic state. The measured distribution of intensity implies that NO is produced highly rotationally excited; the most likely mechanism for formation of NO is from the unstable NO2 fragment undergoing secondary dissociation.

Metal-metal bonding and structures of metal string complexes Cr3(dpa)4Cl2, Cr3(dpa)4(NCS)2, and [Cr3(dpa)4Cl2](PF6) from IR, Raman, and surface-enhanced Raman spectra.

We recorded infrared, Raman, and surface-enhanced Raman scattering (SERS) spectra of metal-string complexes Cr(3)(dpa)(4)X(2) (dpa = di(2-pyridyl)amido, X = Cl, NCS) and [Cr(3)(dpa)(4)Cl(2)](PF(6)) and dipyridylamine (Hdpa) to determine their vibrational frequencies and to study their structures. For the SERS measurements these complexes were adsorbed on silver nanoparticles in aqueous solution to eliminate the constraints of a crystal lattice. From the results of analysis of the vibrational normal modes we assign the infrared band at 346 cm(-1) to the Cr(3) asymmetric stretching vibration of the symmetric form and the Raman line at 570 cm(-1) to the Cr-Cr stretching mode for the unsymmetric form of Cr(3)(dpa)(4)Cl(2). Complex Cr(3)(dpa)(4)Cl(2) exhibits both symmetric (s-) and unsymmetric (u-) forms in solution but Cr(3)(dpa)(4)(NCS)(2) only the s-form. The structures for both complexes in their ground states have the s-form. The oxidized complex [Cr(3)(dpa)(4)Cl(2)](PF(6)) has only a u-form for which the Cr-Cr stretching mode is assigned to the band at 570 cm(-1). From the variation with temperature from 23 to 60 degrees C of the intensity of this line, we obtained the proportion of the u-form Cr(3)(dpa)(4)Cl(2); the enthalpy change is thus obtained to be DeltaH = 46.2 +/- 3.3 kJ mol(-1) and the entropy change is DeltaS = 138 +/- 10.3 J K(-1) mol(-1) for the reaction u-Cr(3)(dpa)(4)Cl(2) <--> s-Cr(3)(dpa)(4)Cl(2). From the spectral intensities and band frequencies in SERS spectra, Hdpa is expected to adsorb on a silver nanoparticle with the amido nitrogen and pyridyl rings tilted from the silver surface, whereas the trichromium complex with the chromium ion line is orthogonal to the silver surface normal in aqueous silver solution.