Article pubs.acs.org/JPCA
Time-Domain Simulations of Transient Species in Experimentally Relevant Environments Tyler W. Ueltschi,† Sean A. Fischer,‡ Edoardo Aprà,‡ Alexander N. Tarnovsky,§ Niranjan Govind,*,‡ Patrick Z. El-Khoury,*,† and Wayne P. Hess*,† †
Physical Sciences Division, Pacific Northwest National Laboratory, P. O. Box 999, Richland, Washington 99352, United States Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States § Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, United States ‡
S Supporting Information *
ABSTRACT: Simulating the spectroscopic properties of short-lived thermal and photochemical reaction intermediates and products is a challenging task, as these species often feature atypical molecular and electronic structures. The complex environments in which such species typically reside in practice add further complexity to the problem. Herein, we tackle this problem in silico using ab initio molecular dynamics (AIMD) simulations, employing iso-CHBr3, namely H(Br)C− Br−Br, as a prototypical system. This species was chosen because it features both a nonconventional C−Br−Br bonding pattern, as well as a strong dependence of its spectral features on the local environment in which it resides, as illustrated in recent experimental reports. We simulate the UV−vis and IR spectra of iso-CHBr3 in the gas phase, as well as in a Ne cluster (64 atoms) and in a methylcyclohexane cage (14 solvent molecules) representative of the previously characterized matrix isolated and solvated iso-CHBr3 species. We exclusively perform fully quantum mechanical static and dynamic simulations. By comparing our condensed phase simulations to their experimental analogues, we stress the importance of (i) conformational sampling, even at cryogenic temperatures, and (ii) using a fully quantum mechanical description of both solute and bath to properly account for the experimental observables.
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INTRODUCTION Thermal and photochemical reaction paths often involve several intermediates, featuring perturbed molecular and electronic structures in far-from-equilibrium situations. Attempting to capture and spectroscopically characterize transient reaction intermediates and/or (photo)products defaults practitioners to two main approaches. The first involves slowing down thermal reaction rates in cryogenic media, whereas the second takes advantage of tools of ultrafast spectroscopy to follow photoinduced spectral evolution in real time. In both cases, operative solute-bath interactions obfuscate spectral assignments, both in frequency- and time-domain measurements. The standard practice of using gas phase simulations to assign condensed phase electronic and vibrational spectra, typically adopted to alleviate the computational burden associated with a full quantum treatment of both solute and solvent, is not formally justified and is often lacking. This is especially the case in solution under ambient conditions, where multiple energetically accessible coupled solute-bath conformations can in principle dictate the overall optical response. Although mixed quantum/classical simulations recover some of the explicit interactions between solute and solvent, this approach does not account for the electronic structure of the © 2016 American Chemical Society
full solute−solvent system. Herein, we illustrate the importance of treating the full solute-bath system using tools of quantum chemistry and stress the importance of conformational sampling to capture the vibrational and electronic properties of transient species in experimentally relevant environments, particularly in solution. For the purpose of this proof-ofprinciple work, we selected iso-CHBr3 (H(Br)C−Br−Br) as a prototypical model system. Because of the environmental relevance of CHBr31,2 and potentially its reactive isomeric form,3 several prior works have been devoted to measuring their spectroscopic properties and describing their electronic structures in different media. Pioneering works of relevance to this report are summarized in the ensuing section. Reid’s group recorded the infrared and UV−vis spectra of iso-CHBr3, formed following 220 nm irradiation of CHBr3 in a Ne cage held at ∼5 K.4 In the same work, the authors explored the CHBr3 → iso-CHBr3 gas phase thermal isomerization reaction path using intrinsic reaction coordinate scans and located a transition state connecting the two isomeric forms. Received: November 30, 2015 Revised: January 10, 2016 Published: January 11, 2016 556
DOI: 10.1021/acs.jpca.5b11710 J. Phys. Chem. A 2016, 120, 556−561
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Figure 1. (A) Total energy as a function of propagation time from a 20 ps constant temperature (5 K) AIMD simulation of an isolated iso-CHBr3 molecule. This calculation employed the PBE functional in conjunction with the def2-SVP basis set and a matching coulomb fitting basis set used for the evaluation of the Coulomb potential. The starting structure comprises iso-CHBr3 minimized at the same level of theory used in AIMD. The vertical lines designate the time steps at which either UV−vis spectra were computed (dashed red) or molecular structures were extracted for subsequent constant energy simulations (solid blue). (B) Conformationally averaged UV−vis spectra computed from 10 different structures randomly selected from the trajectory shown in A. These calculations were performed using TD DFT calculations employing two different density functionals (PBE96 and PBE0) and two different basis sets (def2-SVP and def2-TZVP). Also shown is the experimental UV−vis spectrum of isoCHBr3 in a Ne matrix held at ∼5 K, simulated using the values reported in ref 4. (C) Total energy as a function of propagation time from 5 different 18 ps constant energy AIMD trajectories of the isolated iso-CHBr3 molecule. The same level of theory used in A was used herein, whereas the starting geometries consist of randomly selected structures along the trajectory used in A. The legend designates the time (in ps) at which the structures were taken. (D) Comparison of the infrared spectra from (i) static normal-mode analysis at the optimized iso-CHBr3 minimum, (ii) Fourier transforms of the averaged (over the five trajectories shown in C for a total simulation time of ∼90 ps) dipole autocorrelation functions, and (iii) iso-CHBr3 isolated in a Ne matrix, simulated using the values reported in ref 4.
second slower (several picoseconds) mechanism involves photochemical bond breaking followed by solvent cage-assisted recombination of the CHBr2· + Br· radical pair to form isoCHBr3. Very recently, iso-CHBr3 was captured in the gas phase photochemistry of CHBr3 by the same group.6 In the gas phase, iso-CHBr3 is observed 50 fs after 250 nm irradiation of the parent, formed through a process best described as ultrafast S1/ S0 interconversion through a conical intersection accompanied by roaming motion on flat regions of the S1 and S0 potential energy surfaces. Crim and co-workers also captured iso-CHBr3 through its characteristic C−H stretching vibration in a transient infrared absorption scheme, following 267 nm irradiation of CHBr3 in CCl4.7 One lesson learned from the combined aforementioned works is that the electronic structure and thus the spectroscopic properties of iso-CHBr3 are sensitive both to its structure (molecular and electronic) and its surrounding environment. Taking both relevant structural conformations and local environments into account is a challenge from a theoretical perspective, one that we take on in this work. Namely, we report the infrared and UV−vis spectra of gas phase, matrix isolated, and solvated iso-CHBr3 using ab initio molecular dynamics (AIMD) simulations combined with static and time dependent density functional theory (TDDFT) calculations. To gauge the accuracy of our
Notably, natural bond orbital analysis and ensuing natural resonance theory analyses illustrated that the isomerization process involves a transition from a covalent-type resonance structure in the CHBr3 parent well to a predominantly ionic form in the iso-CHBr3 well with a crossover at the transition state. On the basis of these findings, it was suggested that the ion-pair-dominated structures would be preferentially stabilized in condensed phases. Together with Tarnovsky and co-workers’ original observations (vide infra), this analysis stresses that even small geometric displacements of the iso-CHBr3 molecule can affect its underlying electronic structure. Notably, the same effect governs the chemical bonding in isomers of structurally related small halogenated alkanes.3 Tarnovsky and co-workers employed deep-UV through near-IR femtosecond absorption spectroscopy to follow the CHBr3 → iso-CHBr3 photochemical reaction path following 255 nm excitation in solution.5 They found that iso-CHBr3 is a major photochemical product in the liquid phase, formed with a ∼ 35% quantum yield. Two mechanisms for photochemical isomerization taking place on two distinct time scales were identified in the prior work. The first is an ultrafast (tens of femtoseconds) direct photochemical isomerization pathway that connects the photon-accessible CHBr3 Franck−Condon region to the iso-CHBr3 well on the ground electronic state through a conical intersection. The 557
DOI: 10.1021/acs.jpca.5b11710 J. Phys. Chem. A 2016, 120, 556−561
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The Journal of Physical Chemistry A
Figure 2. (A) The experimental UV−vis spectrum of iso-CHBr3 in a Ne matrix held at ∼5 K is simulated using the values reported in ref 4 and compared to (i) vertical transition energies computed at the TD PBE/def2-SVP level from the fully optimized iso-CHBr3Ne64 model, and (ii) conformationally averaged UV−vis spectra computed from 10 different structures randomly selected from a constant energy (∼5 K) AIMD trajectory of the iso-CHBr3Ne64 model of ∼2.5 ps in total simulation time. (B) A comparison of the simulated TD PBE/def2-SVP spectra (i) evaluated at the minimum energy geometry, (ii) conformationally averaged by taking 10 randomly selected snapshots from a constant temperature simulation at 5 K, and (iii) conformationally averaged by taking 10 randomly selected snapshots from a constant temperature simulation at 300 K.
were used both as starting structures for TDDFT simulations (panel B) and subsequent constant energy trajectories (panel C). The simulated UV−vis spectra shown in panel B are compared to the experimental spectrum of iso-CHBr3 isolated in a neon matrix at 5 K. The experimental spectrum features a major absorption band centered at 399 nm and a less prominent (∼3% of the total intensity of the blue band) broad absorption feature in the 500−700 nm spectral region, peaking at 561 nm. Two different basis sets and two different density functionals were used to simulate the experimental UV−vis spectrum in Figure 1B, namely the PBE and PBE0 functionals with the def2-SVP and def2-TZVP basis sets. TDDFT vertical transition energies from the minimized energy geometry reveal three excited singlet states in the near UV− visible region, see the Supporting Information. The first two computed electronic transitions feature low oscillator strengths on the order of 10−3 and are separated by 70 and 45 nm at the TD-PBE/def2-SVP and TD-PBE0/def2-TZVP levels of theory, respectively. In the AIMD scheme, we find that both the larger basis set and Hartree−Fock exchange are needed to reproduce the absorption maximum in this region of the spectrum, see Figure 1. For the 399 nm band, however, the situation is reversed. Here, the PBE results converge to the experimental value of ∼399 nm faster than their PBE0 analogues with increasing basis set size. Overall, all of the levels of theory tested herein greatly underestimate the oscillator strengths of the optical transitions in the 500−700 nm region, an observable also alluded to in prior works.4,5 Nonetheless, the nature of all three transitions is invariant to the choice of level of theory. With this in mind, we only consider the PBE/def2-SVP results in the ensuing sections. Figure 1D compares the experimental infrared spectrum of matrix isolated iso-CHBr3 with its simulated static and AIMD analogues. The AIMD spectrum was obtained by averaging the dipole autocorrelation functions from 5 constant energy (5 K) simulations depicted in Figure 1C for a total simulation time of 90 ps. We find that conformational sampling, even at cryogenic temperatures, results in better agreement between the simulated and measured infrared spectra. The effect is observed all throughout the 500−3250 cm−1 spectral region and is particularly evident in the C−H stretch region, see Figure 1D. Furthermore, AIMD infrared spectral simulations at 300 K predict vibrational states that exhibit an even closer
simulations, we directly compare our condensed phase results with their experimental analogues.
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COMPUTATIONAL METHODS All calculations were performed with a local version of NWChem.8 For the time-dependent density functional theory (TDDFT) and normal mode calculations, we used either the PBE exchange-correlation functional9 functional or the PBE010 exchange-correlation functional in conjunction with the def2SVP and def2-TZVP basis sets.11 We ran the molecular dynamics (MD) simulations using the PBE functional and def2SVP basis set with a matching Coulomb fitting basis12 for evaluation of the Coulomb potential. All of the atoms were relaxed throughout the AIMD simulations. For the methylcyclohexane calculations, this involved full AIMD simulations of a system with 2575 basis functions. Additionally, we simulated infrared spectra from the MD simulations using the Fourier transforms of the dipole autocorrelation functions.13,14 We used a time step of 10 atomic units (∼0.2419 fs) for all MD simulations. Using this time step in our AIMD scheme conserved the total energy of the system to within 1 kcal/ mol. For the trajectories used to sample configurations for UV−vis spectral simulations, we ran the MD trajectories in the canonical ensemble, obtained via the stochastic velocity rescaling thermostat of Bussi et al. with a relaxation parameter of 100 atomic units.15 Alternatively, we performed AIMD simulations in the microcanonical ensemble for the trajectories used to generate the infrared spectra and vibrational density of states. TDDFT calculated absorption spectra from the ground state minimum were broadened by Gaussians with a full width at half-maximum (fwhm) of 0.2 eV, while those averaged over snapshots from the MD trajectories were broadened by 0.1 eV; the conformational sampling recovers a portion of the line broadening. We broadened the IR spectra with Lorentzians featuring a fwhm of 10 cm−1. The same broadening parameters were used to simulate the experimental spectra shown in this work and were appropriately referenced in the main text.
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RESULTS AND DISCUSSION Figure 1 summarizes the AIMD simulations of gas phase isoCHBr3 at 5 K. Figure 1A shows the total energy as a function of propagation time from an ∼20 ps constant temperature simulation. Randomly selected structures along this trajectory 558
DOI: 10.1021/acs.jpca.5b11710 J. Phys. Chem. A 2016, 120, 556−561
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Figure 3. Left panel compares (i) the experimental transient UV−vis spectrum of iso-CHBr3 in methylcyclohexane, recorded 500 ps after a 255 nm pump pulse excites the parent CHBr3 molecule in the same solvent,5 (ii) vertical transition energies computed at the TD PBE/def2-SVP level from the fully optimized iso-CHBr3(C7H14)14 model, and (iii) conformationally averaged UV−vis spectra computed from 10 different structures randomly selected from a constant energy (∼300 K) AIMD trajectory of the iso-CHBr3(C7H14)14 model of ∼1 ps in total simulation time. The right panel shows the molecular orbitals governing selected excited singlet states in the solvated iso-CHBr3 model (iso-CHBr3(C7H14)14). The predicted transitions are centered at 419 nm (upper) and 630 nm (bottom).
conformation. The inset of Figure 2B shows that conformational sampling at 300 K results in a broader spread in the energies of the electronic transitions, now spanning the 500− 800 nm spectral region. This is reminiscent of the broad absorption features of the recently reported gas-phase isomer spectrum.6 Nonetheless, the oscillator strengths of the calculated transitions are still underestimated. The origin of the orders of magnitude stronger oscillator strengths of the lowlying excited electronic states of iso-CHBr3 in the matrix isolation experiments cannot be reproduced using any of our gas phase/iso-CHBr3Ne64 models. As illustrated below, this is not the case in solution. The experimental and theoretical UV−vis absorption spectra in solution are shown in Figure 3. The experimental spectrum was recorded using transient absorption spectroscopy, following 255 nm excitation of CHBr3 in methylcyclohexane.5 We selected the 500 ps spectrum because no further spectral evolution was observed after this probe time delay, as a result of vibrational cooling.5 Both static and AIMD UV−vis spectral simulations employing the iso-CHBr3(C7H14)14 model capture the experimentally observed spectral features. They both suggest that the predominant spectral feature observed at ∼425 nm in transient absorption, herein assigned to a HOMO2 to LUMO transition, acquires some dynamic (upon photoexcitation) solvent-to-solute charge-transfer character, see Figure 3. Besides a small difference in the predicted absorption maximum within the broad band observed in transient absorption (centered at ∼425 nm), the major difference between the static and conformationally averaged AIMD UV−vis spectra comprises increased absorption strength in the 500−800 nm spectral region. The right panel of the same figure illustrates that the solvent cage plays an intimate role in the low-lying excited electronic states in the solvated iso-CHBr3 system. Namely, dynamically fluctuating solvent to solute charge-transfer states with significant oscillator strength (∼20%) are predicted in this spectral region. Propagating the iso-CHBr3(C7H14)14 system at 300 K, the temperature used in experiment leads to even stronger absorption in the 500−800 nm spectral region. To better understand the intense optical transitions of the solvated iso-CHBr3 species in the 500−800 nm spectral region, we further analyze the predicted transitions that comprise the broadened AIMD UV−vis band centered at
resemblance to the infrared spectrum of iso-CHBr3 in the matrix. This observation is counterintuitive at first blush. For instance, the C−H stretching vibration experimentally observed at 3076 cm−1 in the matrix is predicted to lie at 3116 cm−1 from AIMD infrared spectral simulations at 5 K. At 300 K, the mode is predicted at 3113 cm−1. The improved spectral agreement is attributed to enhanced conformational sampling at the higher temperature, suggesting that multiple conformations of isoCHBr3 are trapped in Ne at 5K. It appears that these relevant conformations are naturally sampled at the higher temperature in the computed AIMD trajectories. That the simulated gas phase spectra of iso-CHBr3 reproduce the electronic and vibrational spectra of iso-CHBr3 in a neon matrix is not fortuitous. Recall that the minimal interaction between solute and rare gas matrices is one of the motivations of matrix isolation spectroscopy. As such, a stringent test of the level of theory used herein to describe iso-CHBr3 in different media is to perform the same spectral simulations in a neon cage, explicitly accounted for in both static and AIMD schemes. The results are shown in Figure 2A, where an iso-CHBr3Ne64 model bears a close resemblance to the gas phase results and reproduces the experimental UV−vis spectra of iso-CHBr3 isolated in a Ne matrix. In fact, using the predominant peak at 399 nm as a reference, we find that the transition predicted at 390 nm using the iso-CHBr3Ne64 model is better-aligned with the experimental value when compared to the gas-phase transition calculated at 383 nm. Inspection of the molecular orbitals associated with the major transition reveals some electron density on the Ne atoms surrounding iso-CHBr3. Though modest, the matrix plays a role in the photophysics of iso-CHBr3. That said, the intensities of the low-energy excited electronic states are still not captured using the iso-CHBr3Ne64 model. As conformational sampling at higher temperature was found to better-account for the vibrational spectra in the matrix, we next attempted to simulate the UV−vis spectra from AIMD simulations at 300 K. Because the overall shape of the UV−vis spectra computed for the gas phase and matrix isolated computational models are qualitatively similar, we employed the former model for computational convenience. The resulting broadened spectrum is shown in Figure 2B, where the 300 K spectrum is compared to its 5 K analogue and to TD-PBE/ def2-SVP vertical transitions from the minimum energy 559
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Figure 4. Oscillator strengths for the solvent to solute charge-transfer transitions, comprising the AIMD UV−vis band centered at 650 nm, as a function of the number of transition vectors (molecular orbital pairs). Also shown are the transition densities for two extreme cases comprising either 1 or 14 transition vectors.
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650 nm. No obvious correlations between the structural parameters of the iso-CHBr3 chromophore and the overall magnitude of the transitions were observed (see Supporting Information). Rather, we find that the oscillator strengths of the predicted excited electronic states are correlated with the number of transition vectors (molecular orbital pairs) contributing to the overall electronic transition, see Figure 4. The transition strength increases as a function of the number of contributing transition vectors. A closer inspection of the molecular orbitals involved reveals that all of the aforementioned transition vectors involve the transfer of electron density from solvent molecules to the same solute acceptor state, see the lower right panel of Figure 3. In effect, this analysis suggests that increased electron delocalization across the various solvent molecules in the initial state leads to stronger overall absorption in this spectral region. The concept can be nicely visualized using the transition density plots shown in Figure 4.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b11710. TDDFT simulations performed at different levels of theory and compared to the iso-CHBr3 matrix isolation UV−vis spectra; infrared spectral simulations performed at different levels of theory and compared to the isoCHBr3 matrix isolation infrared spectra; oscillator strengths as a function of select structural parameters of the solute derived from TDDFT snapshots along an AIMD trajectory of the iso-CHBr3(C7H14)14 system. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Phone: 509-371-6160. Fax: 509-371-6145 (N.G.). *E-mail:
[email protected]. Phone: 509-371-6048. Fax: 509-371-6145 (P.Z.E.). *E-mail:
[email protected]. Phone: 509-371-6140. Fax: 509-371-6145 (W.P.H.).
CONCLUSIONS
In summary, we report static and time-domain simulations of a prototypical transient species, iso-CHBr3. Our simulations account for many of the spectral features observed in the UV−vis and vibrational spectra of this species in various media. We find that both conformational sampling, as well as a full quantum treatment of both solute and bath is needed to capture the underlying physics both in cryogenic matrices at low temperatures and in solution under ambient laboratory conditions. Our results suggest that multiple trapped conformations of iso-CHBr3, sampled using our AIMD scheme, contribute to its matrix isolation infrared spectra. Admittedly, the description of charge-transfer excitations is difficult using our functional of choice for this report (PBE). Taken at face value, our results nonetheless also suggest that solvent-to-solute charge-transfer states are responsible for the absorption of solvated iso-CHBr3 in the 500−800 nm spectral region. Overall, the weaker visible absorption of iso-CHBr3 is also reminiscent of the visible absorption bands previously observed for structurally related systems in condensed phases, for example, at ∼545 nm for iso-CH2I2.16,17 Further studies are thus warranted to test the generality of the observations made herein and their applicability to other isomers of poly halogenated alkanes.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS T.W.U. was supported in part by the U.S. Department of Energy, Office of Science, under the Science Undergraduate Laboratory Internship (SULI) program. S.A.F. and N.G. acknowledge support from the U.S. Department of Energy (DOE), Office of Science, Office of Advanced Scientific Computing Research, Scientific Discovery, through the Advanced Computing (SciDAC) program under Award Number KC030106062653. A.N.T. acknowledges support from the national science foundation (CAREER CHE0847707 and CHE-0923360). P.Z.E. acknowledges support from the Laboratory Directed Research and Development Program through a Linus Pauling Fellowship at Pacific Northwest National Laboratory (PNNL) and an allocation of computing time from the National Science Foundation (TGCHE130003). W.P.H. acknowledges support from the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. This work would not have been possible without computing resources provided by PNNL 560
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The Journal of Physical Chemistry A Institutional Computing. This work was performed in EMSL, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle Memorial Institute for the United States Department of Energy under DOE contract number DE-AC05-76RL1830.
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