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Intramolecular Vibrational Energy Redistribution in Aromatic Molecules of Type C6H5X (X ) H, D, F, Cl, CH3, CF3) Rebekka S. von Benten,† Yaxing Liu,† and Bernd Abel*,‡ Institut fu¨r Physikalische Chemie der UniVersita¨t Go¨ttingen, Tammannstrasse 6, D-37077 Go¨ttingen, Germany, and Wilhelm-Ostwald-Institut fu¨r Physikalische und Theoretische Chemie, UniVersita¨t Leipzig, Linne´-Strasse 2, D-04103 Leipzig, Germany ReceiVed: June 13, 2010; ReVised Manuscript ReceiVed: September 11, 2010
Femtosecond IR pump UV probe spectroscopy was employed in the gas phase to study intramolecular vibrational energy redistribution (IVR) in benzene and five monosubstituted derivatives thereof. After selective excitation of the first overtone of the ring CH-stretch vibration, all molecules showed the same two-step redistribution dynamics characteristic for nonstatistical IVR. The nature of the substituent influences mainly the second, slower IVR component. The presence of an internal rotor does not alter the redistribution rate or pathway compared to that of a monatomic substituent of equal mass. Coupling order model calculations reflect the experimental trends well if the polyatomic substituents are regarded as decoupled from the intraring dynamics and modeled as point masses. 1. Introduction Intramolecular vibrational energy redistribution (IVR) is a process that lies at the heart of chemical reactions. In photoexcited reactants, the rate and pathways of IVR govern the rate, pathways and yields of the reaction.1 On the other hand synthetic chemists are well used to adding certain activating/protective groups to molecules or molecular subunits to influence their reactivity. The question that arises naturally is whether this concept of functional groups is transferable to IVR, i.e., whether systematic chemical substitution can help controlling the outcome of photochemical reactions. To answer this question, it is crucial to understand the influence of certain structural features on the time scales and mechanisms of vibrational energy flow. In the past decade IVR in isolated molecules has been investigated experimentally in the time and frequency domains as well as theoretically with some success,2–4 and some structural IVR factors for individual classes of molecules have been identified (for a good overview, see ref 5 and literature cited therein). In this article we report a study on benzene and five monosubstituted derivatives thereof, namely benzene-d1, fluorobenzene, chlorobenzene, toluene, and R,R,R-trifluorotoluene, with time-resolved transient absorption spectroscopy. Of these molecules, benzene and toluene have already been subject to many experimental6–8 and theoretical9,10 benchmark studies on IVR that can well be tied in with the present investigation. Some of these systems have already been part of a study in our group on the impact of chemical substitution on IVR in the solution phase.11 Recent results regarding the solvent influence on IVR indicate, however, that certain substitution effects can be masked in a dense liquid environment.12,13 To elucidate the unperturbed substitution influence, we thus extended the study to the isolated gas phase species. A special focus will lie on the relation between substituent structure (monatomic vs polyatomic), * Corresponding author. E-mail:
[email protected]. † Universita¨t Go¨ttingen. ‡ Universita¨t Leipzig.
substituent mass and vibrational dynamics of the phenyl ring under conditions of nonstatistical IVR. 2. Experimental Methods The time-resolved measurements were conducted with IR pump UV probe spectroscopy as pioneered by Crim14,15 et al. Details of the experimental setup have been published elsewhere.16,11 The principle of the technique is that an IR laser pulse initiates a certain localized, high-frequency, nonstationary vibrational state in the molecule, which in the case of the aromatic molecules described in this paper is the first CH-stretch overtone. This zeroth-order bright state is not Franck-Condon(FC)-active in the electronic UV transition. As IVR proceeds, the increasing population of low-frequency FC-active vibrations causes an increase in the absorption of the time-delayed UV probe pulse, which is tuned to the long wavelength wing of the first electronic transition. The limited number of FC-active vibrations of the molecules can be identified by UV absorption, resonance Raman, and dispersed fluorescence experiments.17–21 In the same way as IVR increases the absorption, subsequent intermolecular vibrational energy transfer (VET) to surrounding molecules decreases the absorption again. As described in previous reports,22 change in optical density can be converted into change in internal energy via calibration against high temperature absorption spectra. The laser system is composed of a Hurricane (SpectraPhysics) Ti:sapphire laser pumping two optical parametric amplifiers, namely a TOPAS (Light Conversion) and a homebuilt two-stage NOPA, with pulse widths of ∼60 fs and a bandwidth of ∼300 cm-1. The IR-pump pulses and time-delayed UV-probe pulses are focused (f ) 80 mm) and overlapped collinearly in the sample cell, a stainless steel cell with sapphire windows (1 mm) and an optical path length of 20 mm. Both collinear overlap and optical path length are necessary to compensate for the intrinsically low sample concentrations in the probed gas phase volume, even though this lessens the actual time resolution (cross correlation of pump and probe pulses) to only 500 ( 200 fs. The temperature of the sample cell was controlled with electrical heating elements and a thermocouple.
10.1021/jp105417a 2010 American Chemical Society Published on Web 10/08/2010
Vibrational Energy Redistribution in Aromatic Molecules
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Figure 1. Normalized transient absorption profiles (b) of pure gaseous benzene (a), benzene-d1 (b), fluorobenzene (c), chlorobenzene (d), toluene (e), and R,R,R-trifluorotoluene (f). λpump/λprobe were set to 1678/270, 1670/270, 1658/280, 1663/283, 1678/280, and 1660/275 nm, respectively. Also shown are fits using the model of eq 1 and residuals.
Experiments with benzene/benzene-d1 were conducted at 473 K; for the other measurements the sample temperature was set to 513 K. A pressure gauge directly attached to the sample volume was used to set a constant pressure of 0.5 bar for all substances studied. Transient difference absorptions were measured at 0.5 kHz repetition rate for a particular time-delay until an acceptable signal-to-noise ratio was reached (∼80 000 shots). To identify the optimal pump wavelength, IR spectra of the molecules were recorded with a Cary 5e spectrometer (Varian). Chemicals were purchased in spectroscopy grade quality and used without further purifications. 3. Results and Discussion In the present experiments we excited all six molecules in the two quanta region of the aromatic CH-stretch vibration with a femtosecond laser pulse centered at 1.7 µm. The resulting transient absorption profiles are shown in Figure 1a-f. All signals have the same general form: After excitation (t ) 0), an almost instantaneous rise in absorption is followed by a second, slower rise until a plateau at maximum change in absorption is reached. The only exception is toluene (Figure 1e) where additionally a slow decay of the signal on a 100 ps time scale is observed. The interpretation of increase and decrease in transient absorption has been described in detail elsewhere. Briefly, an increase in absorption is caused by IVR from the initially excited state into nearly isoenergetic dark bath states with contribution of FC-active vibrations. The subsequent decrease of absorption is attributed to energy flow out of these states through vibrational energy transfer in collisions (VET). In this case all aromatic molecules under study exhibit at least two intramolecular redistribution steps, which is a clear indica-
tion of nonstatistical IVR. To compare the individual dynamics quantitatively, the experimental signals were analyzed with a multiexponential model function of the form
(
S(t) ∝ exp -
) [
( )
t t - A · exp - (1) + τVET τIVR
( )]
(1 - A) exp -
t
(2) τIVR
(1)
(1) (2) and τ IVR and relative IVR to extract time constants τ IVR amplitudes A. The time resolution of the pump probe experiment was taken into account by convoluting S(t) with a Gaussian shaped instrument response function of 0.6 ps fwhm. The modeled signals are included in Figure 1a-f as solid lines. Also shown underneath each absorption trace are the residuals of the fits, which leave little uncertainty about the parameter assignment. Due to the rather limited time resolution only an upper (1) is given. In the case of limit for the fast IVR component τ IVR benzene and toluene the collisional energy transfer parameters of Toselli et al.23,24 were used to calculate fixed VET time constants, so that only the IVR parameters remained to be fitted. Such energy transfer data have unfortunately not yet been reported for the other four molecules under study; here τVET was chosen to be at least 3 orders of magnitude larger than (2) to reproduce the plateaus in absorption seen in Figure 1. τ IVR The IVR time constants are not affected by collisions because of the large difference in time scales. Therefore, although the parameter τVET is included in the model function it is omitted in the further discussion. The energy transfer parameters obtained from the fits are summarized in Table 1. The existence
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TABLE 1: Energy Flow Parameters for C6H5X Obtained with Eq 1 and Total Density of States F/states per cm-1 a
X
(1) τ IVR /ps
(2) τ IVR /ps
A
τVET/ps
H D F Cl CH3 CF3
