Time-Resolved Kinetic Chirped-Pulse Rotational Spectroscopy in a

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Time-Resolved Kinetic Chirped-Pulse Rotational Spectroscopy in a Room Temperature Flow Reactor Daniel P. Zaleski, Lawrence B Harding, Stephen J Klippenstein, Branko Ruscic, and Kirill Prozument J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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The Journal of Physical Chemistry Letters

Time-Resolved Kinetic Chirped-Pulse Rotational Spectroscopy in a Room Temperature Flow Reactor

Daniel P. Zaleski, Lawrence B. Harding, Stephen J. Klippenstein, Branko Ruscic, and Kirill Prozument*

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, 60439, USA

Corresponding Author *E-mail: [email protected]

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ABSTRACT. Chirped-pulse Fourier transform millimeter-wave spectroscopy is a potentially powerful tool for studying chemical reaction dynamics and kinetics. Branching ratios of multiple reaction products and intermediates can be measured with unprecedented chemical specificity: molecular isomers, conformers and vibrational states have distinct rotational spectra. Here we demonstrate chirped-pulse spectroscopy of vinyl cyanide photoproducts in a flow-tube reactor at ambient temperature of 295 K and pressures of 1–10 µbar. This in situ and time-resolved experiment illustrates the utility of this novel approach to investigating chemical reaction dynamics and kinetics. Following 193 nm photodissociation of CH2CHCN, we observe rotational relaxation of energized HCN, HNC, and HCCCN photoproducts with 10 µs timeresolution, and sample the vibrational population distribution of HCCCN. The experimental branching ratio HCN/HCCCN is compared with a model based on RRKM theory using high level ab initio calculations, which were in turn validated by comparisons to Active Thermochemical Tables enthalpies.

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The nascent distribution of product species and energies is a fundamental property of chemical reaction dynamics and kinetics. These distributions are central to models for the global transformations of complex chemically reactive environments. Theoretical predictions of the incipient product species are complicated by the need to model the interplay between a number of chemical transformations occurring over a complex potential energy surface, including the role of collisions with bath gases in modulating the dynamics. The interpretation of product energy distributions also requires models that proceed beyond statistical theories in order to map the post-transition state dynamics. Quantitative measurements of these distributions provide stringent constraints for theoretical models of chemical reaction dynamics. Unfortunately, it has proven to be extremely challenging to accurately determine these distributions experimentally under well-defined conditions of temperature and pressure. The coupling of flow-tube reactors, which provide well-defined temperature and pressure (T, p) conditions, with photo-ionization mass spectrometry provides an effective means for measuring product species concentrations in chemical kinetics studies; see, for example, Refs. 16. In these studies, it is customary to employ an orifice inserted into the flow on the side of a tubular reactor, through which a small fraction of the molecular sample is expanded, often using a supersonic jet, into a separate vacuum chamber for cooling and analysis. Although, in that configuration, a minimal perturbation to the well-characterized flow is imposed by the probe, the supersonic expansion does alter7-10 the pre-expansion quantum state population distributions of molecular reaction products. These state distributions can encode11-13 important information about the transition states that the system has passed through. In situ and time-resolved spectroscopy, which has been successfully implemented in flow-tube reactors for infrared absorption,14 FTIR emission,15-17 broadband ro-vibrational frequency 3 ACS Paragon Plus Environment


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comb,18 and pure rotational19-22 spectroscopy, bypasses the intermediate step of supersonic cooling. The latter experiments, which provide unsurpassed chemical and quantum state selectivity, were first employed in chemical kinetics studies with traditional millimeter-wave (mm-wave) spectrometers. The invention of the chirped-pulse Fourier transform microwave (CPFTMW) spectroscopy by Pate and coworkers brought about a renaissance of pure rotational spectroscopy.23-31 Broadband spectra with meaningful relative intensities of the rotational transitions separated by tens of GHz can now be acquired in several microseconds. The potential of CP-FTMW spectroscopy and its millimeter-wave region extension, CP-FTmmW spectroscopy,32-35 for chemical dynamics studies of isomerization,25-26,

36

discharge,37-40

photolysis,13, 32, 41 and pyrolysis10, 42-43 reactions has been demonstrated in the last decade. However, determination of useful experimental branching ratios10 for kinetic modeling43 hinges on the availability of well-characterized reactors. Implementation of chirped-pulse (CP) spectroscopy within a pulsed uniform flow reactor provided an important advance with the gas flow from a de Laval nozzle maintaining nearly constant temperature and pressure.41,

44

The

setup allows time-resolved investigation of low-temperature chemistry at well-known conditions. However, the technique is limited to ≲100 µs residence time and low temperatures of 20–30 K. Furthermore, the pressures in the flow tend to be on the higher end of what is feasible for effective microwave spectroscopy. In this work, we demonstrate the utility of time-resolved chirped-pulse rotational spectroscopy of photodissociation reaction products in a flow-tube reactor at intermediate (room) temperature. We term it Time-Resolved Kinetic Chirped-Pulse (TReK-CP) spectroscopy. Until now, CP chemical studies have been perceived to be limited to low temperatures. TReK-CP spectroscopy of a room temperature sample and long residence times in the reactor opens the possibility for 4 ACS Paragon Plus Environment

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studying networks of chemical reactions. The present in situ CP-FTmmW detection allows us to study the rotational and vibrational equilibrations of nascent photodissociation products. Here, the utility and quantitative aspects of TReK-CP spectroscopy are illustrated through a study of the photolysis of vinyl cyanide (acrylonitrile, CH2CHCN) at 193 nm. This photolysis has been extensively studied,13,

16, 45-49

but questions still remain13 regarding its dissociation dynamics.

TReK-CP spectroscopy allows us to investigate the HCN, HNC, and HCCCN photoproducts and to deduce the nascent branching ratio for the HCN/HNC pair relative to HCCCN. Such branching ratios are generally difficult to measure with other types of spectroscopy. It is believed that the photolysis induces dissociation of the molecule on its electronic ground state.45, 50 For comparison purposes, we also present statistical theory predictions for the branching ratios arising from the unimolecular decomposition on the ground state. A schematic of the flow tube reactor coupled with the photolysis laser and CP-FTmmW spectrometer is shown in Fig. 1. The reactor is designed to maximize the overlap between the gas sample, photolysis laser beam, and mm-wave beam of a CP-FTmmW spectrometer for better signal. This is achieved with a pair of 3” diameter fused silica mirrors with a high reflectivity dielectric coating for the 193 nm photolysis laser wavelength, labeled “M” in Fig. 1. The mirrors transmit the mm-wave beam sent by the source part of the CP-FTmmW spectrometer and reflect the laser beam thus combining the two inside the reactor as shown with the arrows. The overlapping and co-propagating mm-wave and laser beams interact with the gas sample over the entire 1 meter length of the reactor and are separated by a second dichroic mirror after exiting it. At 1.2 sccm flow, the pressure p = 1 µbar = 0.1 Pa ≈ 0.75 mTorr of neat vinyl cyanide is measured by a capacitance manometer. In other experiments, vinyl cyanide is pre-mixed with



inert gas at 10% by mole and introduced to the reactor. In these runs, the total pressure in the reactor is increased to p = 10 µbar, thus maintaining the partial pressure of CH2CHCN at 1 µbar.

Figure 1. Schematic of the TReK-CP experimental setup. The laser beam is coupled to the flow tube reactor with a pair of laser mirrors “M” with high reflection coating for 193 nm that are also transparent to the mm-wave beam. Fused silica windows “W” transmit both the mm-wave and the laser beams. The precursor vinyl cyanide molecules are introduced at the middle of the reactor and evacuated by a vacuum pump (not shown) through the four ports near the ends of the tube.

The W-band (75–110 GHz) TReK-CP spectrometer used here is a variation of the Argonne Eband CP-FTmmW spectrometer described elsewhere51 in combination with the BrightSpec Wband spectrometer. Pate and coworkers have suggested that fast oscilloscopes with features like FastFrame™ of Tektronix can be used to study molecular spectra with time-resolution of several microseconds.52 We are using the multi-chirp10 approach to target, with narrowband chirps, several known rotational transitions53-55 of CH2CHCN photoproducts HCN, HNC and HCCCN. In this way the limited mm-wave power is concentrated in a few small frequency spans of 30 6 ACS Paragon Plus Environment

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MHz, for improved signal-to-noise ratio (S/N), as opposed to chirping over the up to 23 GHz51 of bandwidth that covers the same transitions and more. In this fine time-resolution mode, unlike in Ref. 10, each free induction decay (FID) is averaged separately, rather than into a common “summary frame”, to record the time-evolution of the spectra following the CH2CHCN photodissociation event at t = 0 as shown in Fig. 2 (a). After 3.5 hours of averaging, we achieve S/N of 230 (HCN), 33 (HNC), and 290 (HCCCN) at the temporal maxima. The FWHM linewidths are 800 kHz with comparable contributions from the Doppler and collisional broadening mechanisms. Reactor details as well as operating principles of the fine timeresolution and medium time-resolution modes of the spectrometer are available in the Supporting Information (SI) section.



Figure 2. The time evolution of the CP-FTmmW signals of HCN, HNC, and HCCCN following photodissociation at t = 0 of CH2CHCN obtained in fine time-resolution mode of the spectrometer. Panel (a) time-dependent CP-FTmmW spectra. Precursor CH2CHCN is flown at p = 1 µbar pressure through the reactor. Excitation chirped pulses are emitted at times t after the laser pulses and with their frequencies centered at 88632, 90664, and 90979 MHz for HCN, HNC, and HCCCN, respectively, with 30 MHz bandwidths. Rotational transitions of HCN, HNC and HCCCN photoproducts in their ground vibrational states53-55 as well as a CH2CHCN 8 ACS Paragon Plus Environment

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transition at 88645.23(10) MHz and a 1ν5 vibrationally excited HCCCN transition at 90987.06(10) MHz are assigned. Panels (b) and (c): the integrated CP-FTmmW line intensities. Panel (a) is shown on a logarithmic time-scale, whereas panel (b) is on a linear time-scale. Integrated lines from Panel (a) are shown with solid circles. Open circles represent the data from flowing CH2CHCN with added argon gas (p =10 µbar). The HNC intensities are magnified 5 times for clarity. Similar plots for tests with helium, xenon and nitrogen as a buffer gas are available in Fig. S2.

The integrated line intensities of the HCN (J = 1–0, the entire hyperfine structure), HNC (J = 1–0), and HCCCN (J = 10–9, vibrational ground state (G.S.) only) are plotted in Figs. 2 (b) and 2 (c) as a function of time t. These are calibrated for variation of the spectrometer chirp amplitude ℰ (), transmission efficiency, and receiver sensitivity across the frequency range. The combination of the latter factors is designated as (). It is important to emphasize that the observed dynamics of the line intensities are not identical to that of the abundances. In order to establish the connection between the two, one needs information about the rotational temperature (Trot) or, more generally, the rotational level population distribution of each molecule at each time-point. That can be achieved if either enough rotational transitions are observed to match their intensities to a rotational Hamiltonian with a Trot, or if there are reasons to believe that the rotational degrees of freedom of all species are thermalized at a common Trot. For linear molecules studied in this work, especially for HCN and HNC, the rotational transitions are too sparse for a reliable experimental determination of Trot.



We anticipate that, in our experiments, rotational equilibration of hot photofragments to the ambient room temperature is a plausible explanation for the behavior of the HCN and HCCCN signals in Fig. 2 during the early times, t < 1 ms (the later decay is due to evacuation of the reactor). As the molecules thermalize by collisions with other molecules, the buffer gas,56-58 and the wall5, 59 of the reactor, the rotational level distribution shrinks and the intensity of the J = 1–0 transition increases. At the same time, cyanoacetylene may be forming via a mechanism that is consistent with the observed delay in HCCCN appearance (see below). Two complementary experiments were conducted to investigate other possibilities. From measurements with isotopically labeled vinyl cyanide (see SI), we estimate an upper limit of 6% for the contribution from bimolecular chemistry. In the medium time-resolution mode of the TReK-CP experiment, we use the BrightSpec W-band spectrometer60 in the synchronized High Dynamic Range (syncHDR) regime to obtain broadband CP-FTmmW spectra in the 75–92 GHz frequency range (Fig. S4). By synchronizing to an external trigger, in this mode the BrightSpec spectrometer is set to operate only within a certain time-window following the photolysis event. As shown in SI, these broadband spectra contain multiple transitions of vibrationally excited HCCCN. Using their intensities we obtain the vibrational population distributions (VPDs) averaged over the 0 < t < 400 µs and 1 ms < t < 5 ms time-intervals as shown in Fig. 3. The time-evolution of these VPDs suggest that it is not vibrational relaxation that is responsible for the HCCCN signal ascending in Fig. 2. From lack of the 2ν2 (two quanta of bending at 1411 cm-1)61 J = 1 – 0 line13, 32 in our spectra, we estimate the HCN vibrational temperature to be Tvib ≲ 450 K. The ~1 ms vibrational equilibration time in this experiment is consistent with ≈ 2 µs HNC relaxation half-time observed by Dai and coworkers,16 at 4 Torr in He. It is also worth noting that because HCCCN has too few


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rotational transitions for accurately measuring Trot, the VPDs in Fig. 3 are determined with an assumption that Trot is the same for all vibrational levels. Therefore, the plateaus at t = 0.5–2 ms in Fig. 2 (b) likely indicate completion of rotational equilibration of HCN and HCCCN to the ambient Trot = 295 K. Knowing their rotational temperature, we can derive the [HCN]:[HCCCN] branching ratio from the HCN (1 ms) : HCCCN (600 µs) signal intensity ratio. The absolute abundance of HCN at that time is about 2 × 1011 cm-3 (see SI for details) and its dynamic detection limit is ~ 1 × 109 cm-3 at 8 × 10-6 duty cycle.

Figure 3. Vibrational level population distributions (VPDs) in HCCCN during two time intervals after CH2CHCN photodissociation: 0–400 µs and 1–5 ms. Obtained in the medium timeresolution mode using the sync-HDR regime of BrightSpec spectrometer. Each set of data is fitted with an exponential decay and the resulting vibrational temperatures are indicated on the



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plot. CH2CHCN diluted in argon: the 0–400 µs VPD is Boltzmann at Tvib = 209 K, the 1–5 ms VPD is Boltzmann at Tvib = 238 K. Neat CH2CHCN: the 1–5 ms VPD is Boltzmann at Tvib = 262 K. The HCCCN vibrational modes and the list of rotational transitions used in this plot are available in Tables S1 and S2, respectively.

Branching ratios can be deduced from CP-FTmmW signals as follows. First we consider the CP-FTmmW line intensity ( , ′ ↔ ") of the ′ ↔ " transition of a molecule in the vibrational level , where ′ and " are the total angular momentum labels for the upper and the lower rotational levels involved in the transition, respectively. The two-sided arrow emphasizes the equal role of these levels.62 In the weak-field excitation limit,31-32, 63 in an optically thin sample,64 and in the absence of the chirp edge effects,65 ( , ′ ↔ ") has the following dependence:24, 66-69

(1)

Here k is the Boltzmann constant, and " are the rotational energies of the ′ and " levels, ω is the mm-wave transition frequency,

is the transition strength, µx is the projection of

the electric dipole moment, n is the number density, and L is the length of the reactor. Note that . and are the energy and degeneracy of

the M-degeneracy of J levels is included in

the vibrational level , and Tvib and Trot are the vibrational and rotational temperatures of the ()

molecules. ( ) and ( ) are the rotational partition functions in the ground and in the excited vibrational levels. Because the electric quadrupole splitting due to the 12 ACS Paragon Plus Environment

14

N atom’s

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nuclear spin is either not resolved as in HCCCN and HNC, or the line intensities are summed over the hyperfine structure as in HCN, the effects of the hyperfine interaction are omitted altogether. We also consider here only singlet electronic state molecules with no symmetry operations that permute identical nuclei. ()

In non-floppy molecules ( ) ( ) and the denominator in eq. (1) is the product of the rotational and vibrational partition functions. In that case it is convenient to derive an expression for the CP-FTmmW signal ( ′ ↔ ") summed over all vibrational satellites in the spectrum:

(2)

The vibrational partition function and the vibrational temperature are eliminated, which makes eq. (2) useful when, for example, the VPD is non-thermal. For linear molecules, eq. (2) is straightforward to evaluate. The rotational energy is well approximated by ( 1), where h is Planck’s constant and B is the rotational constant in Hz,

,70 and

(3)



Note that eqs. (1–3) can be used in combination to derive the branching ratios since the proportionality coefficients in those are equal. Take for example HCN for which we do not observe any vibrationally excited levels. Although its lowest vibrational level61 1ν2 lies at 712 cm-1 and may be somewhat populated, our 1 ↔ 0 measurement is blind to that population: the 1ν2 level is associated with ℓ = 1 vibrational angular momentum and does not exist in J = 0. Hence, for nHCN we are employing eq. (1) in order to account for possible ! 1 ( 2) population with higher J. The shortcoming is that we are assuming the vibrational temperature: Tvib = 295 K. For HCCCN, on the other hand, we choose to integrate all the vibrationally excited rotational transitions intensities into ( ′ ↔ ") and use eq. (3) to solve for nHCCCN. The resulting experimental branching ratio is nHCN : nHCCCN = 1.72 ± 0.30. The estimated 1σ uncertainty arises mainly from calibration of the spectrometer, i.e. from determination of () and ℰ (). To gain insight into the experimental results, we conducted a theoretical investigation of vinyl cyanide unimolecular dissociation. The vinyl cyanide potential energy surface was investigated at the CCSD(T)/CBS//B3LYP/6-311G** level of theory (Scheme 1 and Table S3). To assure that all relevant transition states and intermediates were located, the surface was searched using the global reaction route mapping (GRRM) technique,71 which located 44 intermediates and 116 transition states. These include the new transition state recently reported by Vazquez and Martinez-Nunez49 for elimination of HCN via a three-membered ring intermediate (also located using an automated searching method72-73), as well as the key transition states observed in the large body of earlier theoretical work (cf. Ref. 48). The Active Thermochemical Tables (ATcT), a novel approach to deriving accurate and reliable thermochemistry from available thermochemically-relevant determinations,74-77 were used to independently obtain the enthalpies of formation for some of the relevant species as shown in Table S5. Table S6 provides the ATcT 14 ACS Paragon Plus Environment

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0 K enthalpies of reaction, together with the theoretical values, showing that the differences between the two are in the range of 0.1–0.8 kcal/mol. The branching between the different product channels as a function of energy for specific initial rotational states was estimated through the application of RRKM theory implementing the calculated transition state properties. For multistep pathways, preliminary RRKM analyses were used to highlight the dominant bottlenecks, which were then used to predict the branching through that pathway. See SI for further details.

Scheme 1. Schematic energy level diagram displaying the main chemical pathways leading to the HCN, HNC, HCCCN, and CH2CCN products of 193 nm photolysis of CH2CHCN. Energies of the intermediate isomers and the final products are supplemented with the corresponding 15 ACS Paragon Plus Environment


molecular structure pictograms. The structures and energies of the transition states are shown in Table S3. The branchings of the calculated dissociation pathways are given in Figs. S5 and S6, Table S4. The 149.8 kcal/mol energy available for dissociation is comprised of the 193 nm photon energy and CH2CHCN initial thermal energy.

Our calculations show three possible paths for formation of HCCCN. Direct loss of H2 can occur through either TSD2 or TSD5b. Alternatively, since HCCCN + H + H is predicted to lie 4 kcal/mol below the sum of the photon energy and the thermal energy of CH2CHCN, a sequence of two H losses may occur to yield the cyanovinyl radical (CH2CCN) via TSH and then HCCCN. Our calculations predict that the 1,1 elimination of H2 (TSD2), which is followed by :CCHCN → HCCCN isomerization, contributes 87% of the H2 loss flux. Meanwhile, the flux to CH2CCN is predicted to be 4 times that for the H2 loss channels The production of HCCCN from the third pathway depends on the branching between stabilization of CH2CCN and its dissociation to HCCCN + H, which in turn depends on the nascent energy distribution in CH2CCN. The photofragment translational spectroscopy experiments of Lee suggest that a significant fraction of the H atoms (perhaps 50%) arising from the dissociation of vinyl cyanide will have less than 4 kcal/mol translational energy.45 In principle, the corresponding CH2CCN coproducts would have enough internal energy to dissociate. However, the CH2CCN is undergoing collisions with the bath gas that serve to further modulate the energy distributions, which will lead to stabilization of some of the molecules and also further excitation of some of them. Fig. S7 shows the tunneling corrected RRKM predicted dissociation lifetime for CH2CCN as a function of energy above the HCCCN + H asymptote. 16 ACS Paragon Plus Environment

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Note that the reverse barrier for this process is about 3.9 kcal/mol. The quantitative prediction of the nascent energy distribution is challenging, and is left for further work. For now, we simply note that it is reasonable to presume that 1/4 of the CH2CCN molecules dissociate, which would yield a branching to HCCCN that is equivalent to that from the H2 loss channels. Notably, these three paths might be expected to yield quite different internal energy distributions. In particular, the high exothermicities (> 50 kcal/mol, cf. Table S5) from TSD2 and TSD5b suggest that these H2 losses would likely yield HCCCN that is both vibrationally and rotationally hot. However, there has been some suggestion in the literature that a coupling of the decomposition via TSD2 to :CCHCN with the isomerization to HCCCN may lead to an unexpectedly cold HCCCN.45 Nevertheless, it is hard to envision such a coupling transferring the full exothermicity to the H2. Meanwhile, the sequential dissociation process should yield vibrationally cold HCCCN. It is less clear what would be expected for the rotational distribution as the first dissociation step may yield rotationally hot CH2CCN, with a partial conservation of the rotational excitation during the second dissociation. The observed HCCCN VPDs (Fig. 3) are not consistent with very much energy being deposited in the internal motions of HCCCN. In contrast, Lee and coworkers concluded that they observe the HCCCN + H2 elimination channel with about 20 kcal/mol deposited in translational motion. Meanwhile, the Dai group16 observed no hot cyanoacetylene in 193 nm photolysis of CH2CHCN. Notably, contributions from both H2 loss and successive H loss pathways may rationalize these apparently disparate observations. In particular, the present experiments are well described by the sequential H loss channel, which is consistent with our observations of HCCCN thermalizing to room temperature vibrationally from below (Fig. 3). Furthermore, the ~ 1 ms timescale for the rise in the HCCCN population may be related to the timescale for the 17 ACS Paragon Plus Environment


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dissociation of the CH2CCN from the first dissociation step (cf. Fig. S7). It is important to recognize though, that we are likely much less sensitive to the H2 loss channels because the signal is distributed over too many rovibrational states. In contrast, the experiments of Lee and coworkers involve slightly lower overall excitation energies due to the effects of supersonic cooling prior to photolysis and thus may not have sufficient energy to induce successive dissociations. However, their mass spectrometry techniques readily observe the molecules arising from the expected broad rovibrational distributions of the H2 loss channels. Most HCN (78%) and HNC (60%) stem from 3-center elimination transition states, with HNC elimination preceded by isomerization (TSI1) to vinyl isocyanide. Both HCN and HNC pathways lead to singlet vinylidene co-products, which has a predicted78 JKaKc = 101–100 transition just below the frequency range of the present spectrometer. Calculated branchings for other CH2CHCN dissociation channels are shown in Figs. S5 and S6. Comparison of the experimental branching ratio [HCN]/[HCCCN]t=1

ms

= 1.72 ± 0.30

determined at t ~1 ms with the theoretical predictions of nascent photoproduct branching may be instructive. For a proper comparison, we must consider the effect on the HCN distributions of the interim kinetics during the first 1 ms. In particular, we note that the HNC signal in Fig. 2 is decaying before thermalization is complete, which may be explained by HNC → HCN isomerization, possibly on the wall of the reactor (compare the HNC and HNC + Ar curves in Fig. 2b). This isomerization can add to the measured HCN signal at t = 1 ms, something that we account for by summing over all computed unimolecular HNC and HCN channels. Observing the evolution of CH2CCN would be informative, but is not feasible in this work (see SI).


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As illustrated in Fig. 4, the theoretical branching ratios obtained using a model with no TSH contribution to HCCCN and with complete conversion of HNC to HCN (dashed line) are in good agreement

with

the

experimental

observations;

([HCN]+[HNC])/[HCCCN]t=0

=

[HCN]/[HCCCN]t=1 ms = 1.72 ± 0.30. However, it seems likely that this agreement is accidental, with the vibrationally cold HCCCN observed here instead primarily arising from the sequential H loss pathway. As noted above, qualitative considerations suggest that the branching through this pathway should be roughly equivalent to that through the H2 loss pathways. It is perhaps worth recalling that Field and coworkers reported CH2CDCN photolysis results that may be pointing to additional HCN channels13 and that there may be additional pathways involving dissociation on the excited state. Further observations of CH2CCN, and/or photodissociation of CH2CDCN aimed at distinguishing chemical pathways to HCCCN and DCCCN could help delineate these possibilities as would calculations of the branching from successive dissociation of CH2CCN.



Figure 4. Comparison of the experimental and theoretical branching ratios. The calculations are performed for three values of the total angular momentum, J, of the CH2CHCN molecules roughly spanning the thermally populated range of J. The branchings are calculated as functions of the energy initially channeled into a CH2CHCN molecule. In this work it is 149.8 kcal/mol— the energy of one photon with 193 nm wavelength added to thermal rovibrational energy. The dashed lines are the branchings that sum over all HCN and HNC calculated channels. Assuming a nearly complete HNC → HCN isomerization at t = 1 ms, these dashed curves are to be compared with the experimental [HCN]:[HCCCN]t=1 ms = 1.72 ± 0.30 branching. The solid lines for nascent HCN and HCCCN predict [HCN]:[HCCCN]t=0 = 1.1.

Concluding, we have demonstrated in situ time-resolved chirped-pulse rotational spectroscopy for chemical dynamics and kinetics studies at room temperature. The unimolecular decomposition of vinyl cyanide was studied under pressures between 1 and 10 µbar. In the fine time-resolution mode we measured rotational equilibration of HCN, HNC, and HCCCN (in two vibrational states) with 10 µs time resolution. Evolution of the vibrational population distribution in HCCCN was observed in the medium time-resolution mode on a 1 ms time-scale. The nonthermal effects, i.e. an incomplete equilibration of energized reaction products prior to the next step in a complex reaction network, can significantly affect the reaction rates.79-80 Extended residence times in the flow reactor at room temperature open a possibility for quantitative measurements of these effects at well-known ambient T and p. We exemplify the quantitative capability of TReK-CP technique by deducing the HCN/HCCCN branching ratio and comparing it with theoretical predictions. This comparison suggests the importance of the hitherto unappreciated successive dissociation channel in CH2CHCN photodissocation. Because the 20 ACS Paragon Plus Environment

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Boltzmann population difference between rotational levels at T ≥ 300 K peaks for transitions at higher mm-wave frequencies or in the terahertz region, as can be shown with eqs. (1–3), employment of higher frequency spectrometers is desired for increased TReK-CP sensitivity. With such an advancement, studies in a heated flow-tube reactor may be within reach. In future studies, higher operating pressures will be explored while paying close attention to diffusion of reactive photoproducts to the reactor’s walls and the possibility of wall-mediated chemistry. Whereas wall reactions may add to the gas-phase chemistry under investigation, wall reactions themselves can be a focus of TReK-CP studies. For example, TReK-CP spectroscopy of catalytic reaction81-82 products desorbed into the gas phase from a prepared wall of the flow-tube reactor is an intriguing possibility.



Acknowledgments We thank Stephen Pratt for making valuable comments and suggestions on the manuscript. We gratefully acknowledge discussions with Brooks Pate, Laurent Wiesenfeld, Barratt Park, Robert Tranter, Albert Wagner, and Michael Heaven. We express gratitude to Alexander Heifetz and Sasan Bakhtiari for lending the Agilent analog signal generator. This material is based on work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under contract No. DEAC02-06CH11357.

Supporting Information The Supporting Information (SI) section contains detailed description of the experimental setups and theoretical methodologies, and additional experimental, theoretical and ATcT data.

References

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To vacuum pump CP-FTmmW Spectrometer The Receiver

8 cm M

Photolysis laser beam

Mass flow controller Capacitance manometer Precursor molecules

To vacuum pump W

W λ = 193 nm

Millimeter-wave beam (chirped pulses + FID)

M

λ = 3 mm 6 cm

66 cm

To vacuum pump

CP-FTmmW Spectrometer

Flow Tube Reactor L=1m

ACS Paragon Plus Environment

Millimeter-wave beam (chirped pulses) To vacuum pump

The Source


hν = 6.4 eV

(a) JKa Kc = 335 27−324 30

t=0

J = 1−0

{

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Vib. G.S. 1ν5

(b)

(c)


J = 1−0

J = 10−9

Page 32 of 36

Page 33 of 36


2 0 0

H C C C N J = 9 - 8 r o t a t i o n a l l i n e i n t e n s i t y , µV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

N e a t , l a t e t i m e : t ∈ 1 −5 m s A r , e a r l y t i m e : t ∈ 0 −4 0 0 µs A r , l a t e t i m e : t ∈ 1 −5 m s

1 5 0

T

= 2 6 2 K

v ib

T

1 0 0

v ib

= 2 3 8 K

T

5 0

v ib

= 2 0 9 K

0 0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0


V ib r a tio n a l e n e r g y , c m

6 0 0 -1

7 0 0


B r a n c h in g R a tio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

H C N (H C N H C N (H C N H C N (H C N E x p e

2 .5

/H C + H /H C + H /H C + H r im

C C N C C C N C C C N C e n t

N , )/H N , )/H N , )/H

J = 6 C C J = 3 C C J = 6 C C

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C N , J = 6 0 C N , J = 3 0 6 C N , J = 6 6

2 .0

1 .5

1 .0

0 .5

0 .0

1 2 0

1 2 5

1 ACS 3 0 Paragon 1 3 5 Plus1 Environment 4 0 1 4 5

E n e r g y ( k c a l/m o l)

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1 5 5

1 6 0

Page 35 of 36

193 nm photon energy + thermal rovibrational energy

+ TSI6

TSD2

+

TSD1

TSD7

TSD8

TSD5b

TSD3

TSH + H

TSD12 TSI2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27


TSI1

+

+

+ +


e Journal of PhysicalPage Chemistry 36 of 36 Lett 1 2 3 4 5 hν ACS 6 Paragon Plus Environment 7 8 T, p

Time-Resolved Kinetic Chirped-Pulse Rotational Spectroscopy in a

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