Time-resolved Fourier-transform Spectroscopy Research Articles

Reaction dynamics of Cl+CH3SH: rotational and vibrational distributions of HCl probed with time-resolved Fourier-transform spectroscopy.

Rotationally resolved infrared emission spectra of HCl(v=1-3) in the reaction of Cl+CH3SH, initiated with radiation from a laser at 308 nm, are detected with a step-scan Fourier-transform spectrometer. Observed rotational temperature of HCl(v=1-3) decreases with duration of reaction due to collisional quenching; a short extrapolation to time zero based on data in the range 0.25-4.25 micros yields a nascent rotational temperature of 1150+/-80 K. The rotational energy averaged for HCl(v=1-3) is 8.2+/-0.9 kJ mol(-1), yielding a fraction of available energy going into rotation of HCl, fr=0.10+/-0.01, nearly identical to that of the reaction Cl+H(2)S. Observed temporal profiles of the vibrational population of HCl(v=1-3) are fitted with a kinetic model of formation and quenching of HCl(v=1-3) to yield a branching ratio (68+/-5):(25+/-4):(7+/-1) for formation of HCl(v=1):(v=2):(v=3) from the title reaction and its thermal rate coefficient k(2a)=(2.9+/-0.7)x10(-10) cm(3) molecule(-1) s(-1). Considering possible estimates of the vibrational population of HCl(v=0) based on various surprisal analyses, we report an average vibrational energy 36+/-6 kJ mol(-1) for HCl. The fraction of available energy going into vibration of HCl is f(v)=0.45+/-0.08, significantly greater than a value fv=0.33+/-0.06 determined previously for Cl+H2S. Reaction dynamics of Cl+H(2)S and Cl+CH3SH are compared; the adduct CH3S(Cl)H is likely more transitory than the adduct H(2)SCl.

Photodissociation Dynamics of Vinyl Chloride Investigated with a Pulsed Slit-Jet and Time-Resolved Fourier-Transform Spectroscopy

Following photodissociation of vinyl chloride seeded in a He supersonic jet at 193 nm, rotationally resolved infrared emission of HCl (v) are recorded to yield nascent rotational and vibrational distributions. Preliminary results show that the rotational distribution of HCl free from rotational quenching deviates slightly from Boltzmann-type distribution and agrees well with trajectory calculations; a portion of the low-J component observed previously in a flow system is attributed to quenching. The implications for photodissociation dynamics are discussed.

Reaction dynamics of Cl+H2S: Rotational and vibrational distribution of HCl probed with time-resolved Fourier-transform spectroscopy

Following laser irradiation of a flowing mixture of S2Cl2 and H2S at 308 nm to initiate the reaction of Cl+H2S, vibration–rotation resolved emission spectra of HCl(v=1,2) in the spectral region 2436–3310 cm−1 are detected with a step-scan time-resolved Fourier-transform spectrometer. The Boltzmann-type rotational distributions of HCl(v=1) and HCl(v=2) yield rotational temperatures that decrease with reaction time; extrapolation to time zero based on data in the range 0.5–4.0 μs yields nascent rotational temperatures of 1250±70 K and 1270±120 K, respectively; an average rotational energy of 8.3±1.5 kJ mol−1 is determined for HCl(v=1,2), much greater than a previous report. Observed temporal profiles of the vibrational population of HCl(v=1,2) are fitted with a kinetic model that includes formation and quenching of HCl(v=1,2) to yield a branching ratio of 0.14±0.01 for formation of HCl(v=2)/HCl(v=1) and a thermal rate coefficient of k1=(3.7±1.5)×10−11 cm3 molecule−1 s−1. Combining an estimate of the vibrational population of HCl(v=0) based on a surprisal analysis of previous investigations on the reaction Cl+D2S, we report a ratio of vibrational distributions of HCl(v=0):(v=1):(v=2)=0.41:0.52:0.07, which gives an average vibrational energy of 23±4 kJ mol−1 for HCl. Internal energies, especially rotational energy, of HCl derived with this method is more reliable than with previous techniques; the fractions of available energy going into rotation and vibration of HCl are fr=0.12±0.02 and fv=0.33±0.06, respectively.

Three-center versus four-center elimination of haloethene: Internal energies of HCl and HF on photolysis of CF2CHCl at 193 nm determined with time-resolved Fourier-transform spectroscopy

Following photodissociation of 2-chloro-1,1-difluoroethene CF2CHCl) at 193 nm, vibration–rotationally resolved emission spectra of HCl(v⩽3) and HF(v⩽4) in spectral regions 2000–2900 and 3050–4410 cm−1, respectively, are detected with a step-scan time-resolved Fourier-transform spectrometer. All vibrational levels of HCl and HF show Boltzmann-type rotational distributions. HCl has an average rotational energy of 23±4 kJ mol−1 and a vibrational energy of 25±5 kJ mol−1, whereas HF has an average rotational energy of 20±4 kJ mol−1 and a vibrational energy of 48±6 kJ mol−1. The observed internal energy distribution indicates that HCl is produced via the three-center (α,α), but HF via the four-center (α,β) elimination. A modified separate statistical ensemble model predicts an internal energy distribution of HCl slightly greater than experimental observation. A modified impulse model taking into account geometries and displacement vectors of transition states during bond breaking predicts satisfactorily the rotational excitation of HF produced from four-center elimination. Ratios of rate coefficients (0.87:0.13) predicted for three-center or four-center elimination channels based on Rice–Ramsperger–Kassel–Marcus theory are consistent with a branching ratio of 0.88:0.12 determined based on observed populations of HCl and HF, respectively. We also compare these experimental and theoretical results with those of photolysis of vinyl halides (CH2CHX, X=F, Cl, or Br) at 193 nm.

Molecular arrangements and reorientation behavior in a dibenzopyrene-derivative ferroelectric columnar liquid crystal as studied by time-resolved Fourier-transform ir spectroscopy.

Polarized, time-resolved Fourier-transform infrared spectroscopy was employed to study the orientational order and the reorientation dynamics of a diskotic ferroelectric liquid crystal. In the shear oriented cell the dibenzopyrene derivative forms two different field-dependent columnar phases that show a tripling in the spontaneous polarization. These field-dependent phases are analyzed with respect to the dependence of the infrared absorbance from the polarization plane. In this way it was confirmed that the high-field phase is characterized by a homogeneous orientation of the tilt-plane formed by the core normals n and the column axis N. In contrast, in the low-field phase the columns exhibit several different tilt-planes. The orientational order parameter of the columns is determined. It was also detected that the average orientation of the alkyl tails of the molecules is not lying in the plane of the disklike core. By monitoring the evolution of the infrared bands in the course of the electric-field-induced reorientation, we found that the reorientation process is divided into three steps: A fast initial response followed by a slowing down of the reorientation is observed, which then is followed by an acceleration of the reorientation. In the high-field phase the fast initial electrical induced process can be assigned to a rotation of the molecules around the column axis by a few degrees. During the subsequent ferroelectriclike response the molecules rotate around the column axis by approximately 180 degrees. Other models for this switching mechanisms could be excluded.

Time-resolved spectroscopy of stable molecules

Specific solutions to record with the appropriate dynamic range, time-resolved vibrational spectra of unstable molecules with Fourier-transform spectroscopy (FTS) are shortly commented. It is also shown that time-resolved FTS (TRFTS) brings surprising advantages to the study of stable molecules when coupled to intracavity laser absorption system.

Time-Resolved FT-IR Spectroscopic Investigation of the pH-Dependent Proton Transfer Reactions in the E194Q Mutant of Bacteriorhodopsin

The photoreaction of the E194Q mutant of bacteriorhodopsin has been investigated at various pH values by time-resolved step-scan Fourier-transform infrared difference spectroscopy employing the attenuated total reflection technique. The difference spectrum at pH 8.4 is comparable to the N-BR difference spectra of the wild type with the remarkable exception that D85 is deprotonated. Since the retinal configuration is not perturbed by the E194Q mutation, it is concluded that there is no interaction of D85 with retinal during the lifetime of the N state. At pH 6, a consecutive state to the O intermediate is detected in which D212 is transiently protonated. The comparison with wild-type bacteriorhodopsin reveals that protonation of D212 represents an intermediate step during proton transfer from D85 to the proton release group in the final stage of the reaction cycle. The described effects are more pronounced in the E194Q mutant than in the E204Q mutant demonstrating different roles of these two glutamates/glutamic acids at least in the final stages of the catalytic cycle of bacteriorhodopsin.

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 states during bond breaking predict substantial rotational excitation for three-center elimination of HCl but little rotational excitation for four-center (α, β) elimination; observed internal energy of the low-J component is consistent with that predicted for the four-center elimination channel. Rate coefficients 33.8 and 4.9×1011 s−1 for unimolecular decomposition predicted for three-center and four-center elimination channels, respectively, based on Rice-Ramsberger-Kassel-Marcus theory are consistent with the branching ratio of 0.81:0.19 determined by counting vibrational distribution of HCl to v⩽6 for high-J and low-J components. Hence we conclude that observed high-J and low-J components correspond to HCl (v, J) produced from three-center and four-center elimination channels, respectively.

Fast time-resolved Fourier-transform spectroscopy for the study of transient chemical reactions

We describe the most recent implementation of the data acquisition system which we have developed for fast time-resolved (FTR) Fourier transform spectroscopy (FTS) and report spectra that were obtained by using this instrument. This FTRPTS data system operates in conjunction with any continuous-scan Michelson interferometer, giving it the capability to record many time-delayed spectra of a transient event, with a minimum time resolution of 1 µs. The sensitivity and the spectral resolution of the complete system are the same as those that would be obtained if the interferometer were used in conventional steady-state spectroscopy. To illustrate the performance of the FTRPTS system, we recorded emission spectra from the products of transient chemical reactions of H atoms with CF(3)Cl, CF(2)Cl(2), CFCL(3), and NO(2). These are laser-initiated reactions involving atoms with energies that correspond to a temperature of approximately 27,000 K and lifetimes of a few microseconds, but the FTRPTS system records the time evolution of their products with high signal-to-noise ratio.

Time-resolved Fourier transform spectroscopy with 100-ps acquisition time

As a step toward the realization of high-information time-resolved Fourier-transform spectroscopy, in a first experiment we have achieved time-resolved spectra with our modified high-information Connes-type interferometer as we explored its time-resolution capabilities. To this purpose, a well-defined test source made of a He–Ne laser intensity modulated by an acousto-optic crystal was built. Recorded spectra demonstrate that time resolutions ranging from 40 ns to 100 ps are possible. The spectra showed a 2-order-of-magnitude improvement over the best previous experimental results.

Molecular reaction mechanisms of proteins as monitored by time-resolved FTIR spectroscopy

Although infrared spectroscopy is a classical method for analytical and structural studies, it can also provide valuable insights into the mechanism of chemical reactions. Time-resolved Fourier-Transform infrared difference spectroscopy has recently successfully proved itself as a powerful new method for studies of molecular reaction mechanisms with the generation of nanosecond time resolution for proteins up to 120 kDa in size.