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Photofragment Imaging, Spectroscopy, and Theory of MnO+ M. David Johnston, Matthew R. Gentry,† and Ricardo B. Metz* Department of Chemistry, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United States
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S Supporting Information *
ABSTRACT: Density functional and ab initio calculations, along with photodissociation spectroscopy and ion imaging of MnO+ from 21,300 to 33,900 cm−1, are used to probe the photodissociation dynamics and bond strength of the manganese oxide cation (MnO+). These studies confirm the theoretical ground state (5Π) and determine the spin−orbit constant (A′ = 14 cm−1) of the dominant optically accessible excited state (5Π) in the region. Photodissociation via this excited 5Π state results in ground state Mn+ (7S) + O (3P) products. At energies above 30,000 cm−1, the Mn+ (5S) + O (3P) channel is energetically accessible and becomes the preferred dissociation pathway. The bond dissociation energy (D0 = 242 ± 5 kJ/mol) of MnO+ is measured from several images of each photofragmentation channel and compared to theory, resolving a disagreement in previous measurements. MRCI+Q calculations are much more successful in predicting the observed spectrum than TD-DFT or EOM-CCSD calculations. 239 ± 10 kJ/mol by measuring the collision energy dependence of the Mn+ + O2 → MnO+ + O reaction using a guided ion beam;14 a subsequent ion beam measurement using the same reaction by Fisher et al. gave a substantially higher value, D0 = 285 ± 13 kJ/mol.15 Although there have been no spectroscopic studies of MnO+, Tono et al. measured photodissociation thresholds for loss of Mn from MnnO+ (n = 3−5)16 and Marks et al. studied photodissociation of (MnO)n+ clusters at 355 nm, observing smaller (MnO)m+ as the most abundant fragments.17 Photofragment imaging18−30 has proven an effective way to determine bond dissociation energies from less spectroscopically informative repulsive states.31 However, imaging measurements tend to be more precise when taken closer to a corresponding product threshold, so that the fragment kinetic energy release is small.32 Fortunately, it is expected that MnO+, like other transition metal species, has a greater density of states near the higher energy thresholds of excited dissociation products. In this study, we use theory at the time-dependent density functional theory (TD-DFT), equations of motion coupled cluster, with single and double excitations (EOMCCSD), and MRCI levels along with photodissociation spectroscopy and ion imaging in order to explore the electronic structure and dynamics of MnO+. We report the photodissociation spectrum from 21,300 to 33,900 cm−1; it is quite structured and contains transitions to several excited electronic states. Photofragment images recorded at multiple wavelengths near the first excited Mn+ (5S) + O (3P) threshold are also discussed. The kinetic energy release (KER) measured from
I. INTRODUCTION Transition metal oxide cations and their roles in catalytic activation have been the subject of much experimental and theoretical research for several decades.1−9 MnO+ is known to activate C−H bonds efficiently,10 and the formation of methanol from its reaction with methane is exothermic. However, unlike some other transition metal oxides, it reacts with methane via hydrogen abstraction to yield MnOH+ and CH3, instead of the more desirable methanol product.2,10,11 Uncertainty remains regarding the precise electronic structure and properties of MnO+, which have proven to be quite challenging to model theoretically.12 The computational difficulties associated with MnO+ arise from the highly multireference nature of the ground state and the presence of numerous low-lying excited states. Density functional theory (DFT) approaches in particular are known to produce highly variable results depending on the method and basis set used. Although all recent high-level calculations agree that the ground state is 5Π, several older calculations, even at levels as sophisticated as CASPT2D (complete active space, with second order perturbation theory), predict that the ground state is 5Σ+.10 Despite the computational burden imposed by the presence of high-spin manganese, Bauschlicher and Gutsev studied the 5Π and 5Σ+ states at the multireference configuration interaction (MRCI) level.12 Recently, very high level electronic structure calculations (at the MRCI level and with relativistic corrections) of several states of MnO+ have been done by Miliordos and Mavridis.13 However, this study, while very thorough, only considers a handful of excited quintet states. Of particular importance to the present study, they do not consider any predissociative quintet states. In addition, experimental studies of MnO+ are sparse. Armentrout et al. measured a bond dissociation energy of D0(Mn+−O) = © 2018 American Chemical Society
Received: August 12, 2018 Revised: September 17, 2018 Published: September 18, 2018 8047
DOI: 10.1021/acs.jpca.8b07849 J. Phys. Chem. A 2018, 122, 8047−8053
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on oxygen (10 electrons in 9 orbitals). The lowest 12 states of each symmetry (plus the ground state) are state averaged, with equal weights. For the MRCI, the active space is expanded to also allow promotions from the 2s orbitals on oxygen, as was done by Miliordos and Mavridis.13 All calculations are carried out in C2v symmetry, for quintet states, and states of each symmetry are calculated separately. In order to obtain the root corresponding to the optically active excited 5Π state, it was necessary to calculate 12 roots in the MRCI, although this state is typically only the eighth state with b1 symmetry. Reported energies include the Davidson correction, so they are at the MRCI+Q level.
these images allows us to determine the bond strength of MnO+.
II. METHODS The fast ion photofragment imaging mass spectrometer has been described previously in greater detail.31 Manganese oxide cations are created by ablating a rotating manganese disc33 with the second harmonic output of a Nd:YAG laser (Continuum Minilite II) in the presence of a pulsed gas mixture (10% N2O in He). The oxidized manganese ions and other ablation products undergo a supersonic expansion into a vacuum before entering a quadrupole ion guide. Upon exiting the quadrupole, the ions are focused into a refrigerated radiofrequency Paul trap, where they are given time to thermalize with a cold buffer gas. A buffer gas of pure helium is used in the trap, as MnO+ is known to react rapidly with hydrogen.10 The ions are extracted from the trap and accelerated in a typical Wiley−McLaren configuration. Next, the ions are re-referenced to ground potential to avoid floating the entire flight tube at high voltages. The ion beam is guided by several vertical and horizontal deflectors. One of these deflectors is pulsed for a short time, acting as a mass gate by preventing any unwanted m/z from entering the detection region. The instrument has two modes of operation: one for collecting photodissociation spectra and one for photofragment imaging. For photodissociation spectroscopy, the MnO+ ions are photolyzed at the turning point of a reflectron by the frequency doubled output of a pulsed dye laser (Continuum ND6000), which is pumped by the second harmonic of a Nd:YAG (Continuum Powerlite 8020). The reaccelerated fragments are later characterized by their time-of-flight to a microchannel plate (MCP) detector. Photofragment images are collected by directing the ion beam through a small adjustable iris toward a velocity map imaging (VMI) apparatus.21 The ions are dissociated between the first two VMI plates, which are simultaneously pulsed with a high voltage. The resultant electrostatic lens reaccelerates and focuses the newly formed fragments onto the pulsed imaging detector (Photonis), which consists of a microchannel plate stack coupled to a fast phosphor screen. A photomultiplier tube (PMT) and CCD camera are used to record the collisioninduced emission of the phosphor. Images are created by summing the collision events over 20,000−100,000 cycles using NuACQ software.34 The three-dimensional spatial distributions of Mn+ fragments are mathematically reconstructed using the BASEX approach.35 This distribution is then used to determine the kinetic energy release in the photodissociation. The ground and several excited electronic states of MnO+ were calculated using a variety of electronic structure methods. Gaussian 0936 was used for ground state calculations at the density functional (B3LYP) and coupled cluster (CCSD and CCSD(T)) levels, as well as for excited state calculations using time-dependent density functional theory (TD-DFT) and equations-of-motion CCSD (EOM-CCSD). Molpro37,38 software was used for multireference configuration interaction (MRCI) calculations analogous to those of Miliordos and Mavridis13 but seeking to characterize higher-lying states. The MRCI calculations consist of a complete active space selfconsistent field (CASSCF) calculation followed by single and double excitations from the active space. The CASSCF active space consists of the 4s and 3d orbitals on manganese and 2p
III. RESULTS AND DISCUSSION A. Electronic Structure Calculations on MnO+. The density functional and coupled cluster calculations predict that MnO+ has a 5Π ground state, in accord with recent high-level calculations. At the B3LYP/6-311++G(3df,3pd) level, the ground state has re = 1.729 Å and a harmonic vibrational frequency of ωe = 642 cm−1. The A 5Σ+ excited state is calculated to be only 1832 cm−1 higher in energy (Te), with a significantly shorter bond length of 1.586 Å and correspondingly higher vibrational frequency of 926 cm−1. Results at the CCSD(T)/aug-cc-pVTZ level are similar, predicting re = 1.77 Å for the ground state and 1.612 Å for the 5Σ+ excited state, with Te = 2155 cm−1. These results also agree with the calculations of Miliordos and Mavridis.13 Their highest-level results (MRCI, with relativistic correction) predict re = 1.700 Å and ωe = 612 cm−1 for the ground state and re = 1.601 Å with ωe= 940 cm−1 for the 5Σ+ excited state, which lies at 2337 cm−1. They also recommend a dissociation energy of D0 = 230 kJ/mol. The authors note that the ground state of MnO+ is multiconfigurational, so it is somewhat surprising that all three methods produce such similar results. Unfortunately, this agreement does not extend to the higher excited states. Potential energy surfaces for the ground state of MnO+ and all optically accessible quintet excited electronic states below ∼40,000 cm−1 calculated using MRCI+Q/aug-cc-pVQZ are shown in Figure 1. Supplementary calculations using TD-DFT (B3LYP/6-311++G(3df,3pd)) are in Figure S1. TD-DFT calculations using the newer CAM-B3LYP and M06 density functionals also give very similar results. Potentials calculated at the EOM-CCSD/aug-cc-pVTZ level are in Figure S2. The TD-DFT and EOM-CCSD potentials are quite similar to each other but differ from the MRCI values in ways that are important to the predicted spectra. The numbers below refer to the MRCI+Q results, unless otherwise noted. Two fairly intense transitions are predicted in the visible/near UV. The repulsive 5Σ− state is expected to give a broad feature in the photodissociation spectrum, centered at a vertical energy of ∼26,000 cm−1. This repulsive state is the highest-energy quintet state calculated by Miliordos and Mavridis.13 It is also found at slightly lower energy in the TD-DFT and EOMCCSD calculations. The MRCI calculations predict several excited 5Π states at energies of 24,000−36,000 cm−1 with numerous avoided crossings. Electronic transitions to most of these are predicted to be quite weak. However, transitions to the double-minimum 5Π state shown with a thick line in Figure 1 should be fairly intense, with a predicted integrated oscillator strength of f = 0.053. The electronic transition to this state is predicted to lead to only a short vibrational progression, as the lowest few vibrational levels are localized in the short rMnO well, which has a similar bond length to the ground state. The 8048
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monitoring the resultant formation of Mn+. Several such scans were averaged and divided by laser power and parent signal to produce the spectrum shown in Figure 2. The spectrum is
Figure 3. Photodissociation spectrum of MnO+ from 27,000 to 27,500 cm−1. The simulated spectrum was made in pGopher assuming a (5Π ← X 5Π) transition between states of the same geometry and where A(X 5Π) = 42 cm−1 and A′(5Π) = 14 cm−1. The simulated line width is 7 cm−1 and T = 200 K.
Figure 1. Potential energy curves for quintet states of MnO+ at the MRCI+Q/aug-cc-pVQZ level. Blue lines are 5Π states, red dashed lines are 5Σ− states, red dotted lines are 5Σ+ states, and green dashdotted lines are 5Δ states. The 5Π state with the highest oscillator strength from the ground state is the double-minimum state indicated with a thicker line. The three Φ, two Γ, and one H state in this energy region are not shown, as they are not optically accessible from the ground state.
dominated by a single large feature at 27,240 cm−1 with no clear vibrational progression. Many far less intense peaks, some with partially resolved rotational structure, can also be seen, mostly at higher energies. There is also a very broad absorption of relatively low intensity underlying the spectrum extending from 22,800 to ∼27,800 cm−1. The presence of a single dominant peak suggests that the optically excited state has a very similar geometry to that of the ground state. This feature also contains spin−orbit structure, indicative of a transition to an excited state where Λ > 0. The photofragment images on this peak, which will be discussed in detail in section III.C, imply that ΔΛ= 0 for the corresponding transition. Taken together, this information confirms that the photofragment signal at 27,240 cm−1 is the result of a 5Π ← 5Π transition. An expansion of this portion of the photodissociation spectrum is shown in Figure 3. It is overlaid by a simulated spectrum made in pGopher.39,40 Because ΔΛ = 0 for the transition, the peak spacing only yields the difference in spin−orbit splitting between the two states. Miliordos and
lower-level calculations (Figures S1 and S2) make rather different predictions about the excited 5Π states. First, they predict far fewer states in this region (presumably due to being restricted to one-electron excitation from a single-determinant ground state). Second, they do predict a 5Π state in this region with substantial oscillator strength (f = 0.044, for EOMCCSD), but it has a very flat potential, with a substantially longer bond length (2.0 Å) than the ground state. This should lead to a long vibrational progression in the spectrum, starting at ∼26,000 cm−1, which is not observed. B. Photodissociation Spectrum of MnO+. The photodissociation spectrum of MnO+ from 21,300 to 33,900 cm−1 was recorded by scanning the photolysis laser energy and
Figure 2. Photodissociation spectrum of MnO+ from 21,300 to 33,900 cm−1. 8049
DOI: 10.1021/acs.jpca.8b07849 J. Phys. Chem. A 2018, 122, 8047−8053
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Figure 4. Photofragment image of Mn+ made by dissociating MnO+ at 27,278 cm−1 and the corresponding velocity distribution converted to total kinetic energy release. The image shown has been top/bottom and left/right symmetrized. Laser polarization is vertical.
Figure 5. Photofragment image of Mn+ made by dissociating MnO+ at 30,488 cm−1 and the corresponding velocity distribution converted to total kinetic energy release. Laser polarization is vertical.
C. Images of Mn+ Fragments. MnO+ photofragment images were created by tuning the dye laser to several locations in the photodissociation spectrum and gating the imaging detector on the time-of-flight peak corresponding to Mn+. Photofragmentation channels will be discussed in terms of the state of the Mn+ fragment because the remaining energies after photolysis are not sufficient to electronically excite the neutral O (3P) cofragment. The excitation energy of the dominant transition in the photodissociation spectrum is too low to yield electronically excited Mn+ products so the excess energy is manifested as translational motion. Images taken at photon energies ranging from 27,140 to 27,280 cm−1 (the peak of the spectrum) show fairly anisotropic (β = 0.6) product distributions with radii corresponding to fragments with ∼7000 cm−1 of KER. For example, the velocity distribution shown in Figure 4 is energetically consistent with the formation of Mn+ (7S) photofragments. The higher intensity fragment signal is located at the north and south poles of the image. This vertical
Mavridis have also calculated a spin−orbit constant (A = 42 cm−1) for the 5Π ground state, consistent with our results across many levels of theory. Using this value, we arrive at a spin−orbit constant of A′ = 14 cm−1 for the excited 5Π state based on the spectral simulations. The simulated spectrum has a line width of 7 cm−1 due to the lifetime of the excited state, which precludes measuring rotational constants. The observed spectrum is consistent with the MRCI calculations but not with the TD-DFT or EOM-CCSD predictions. The dominant peak in the spectrum is due to a transition to the 5Π double-minimum state, while the broad, low intensity dissociation from 22,800 to ∼27,800 cm−1 is likely the result of direct excitation to the repulsive 5Σ− state. The smaller peaks in the spectrum are difficult to characterize because the predicted density of electronic states in the region is quite high. They could be due to weak transitions to the many other quintet states predicted to lie in this region or even to spin-forbidden transitions. 8050
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The Journal of Physical Chemistry A anisotropy indicates that the molecules are more effectively photodissociated when they are oriented with the molecular axis parallel to the electric polarization vector of the photolysis laser, consistent with the 5Π ← 5Π transition predicted by the calculations. The observed anisotropy parameter is positive (β = 0.6), confirming a parallel electronic transition (ΔΛ = 0). However, it is below the limiting value (β = 2.0), indicating that the molecular electronic state being accessed is not directly dissociative but is still shorter lived than a rotational period. This time scale is also consistent with the observed 7 cm−1 line width, indicating an excited state lifetime of ∼1 ps. This suggests that the predissociative electronic state being excited in this region likely couples strongly to nearby state(s), eventually yielding ground state photofragments. The MRCI calculations show numerous avoided crossings between 5Π states in this region, as well as a crossing with the repulsive 5Σ− state that leads to ground state products, making such couplings likely. Some images recorded near the lowest energy peak in the spectrum (∼22,740 cm−1) have a slightly perpendicular anisotropy, confirming that dissociation via a Σ or Δ state does contribute to the spectrum in that region, although state assignments are not suggested here. In order to more precisely measure the bond dissociation energy of MnO+, it is advantageous to record images that are energetically close to a product threshold, where photofragments have little kinetic energy.32 The benefit of imaging photofragments with low KER is improved energy resolution; less extrapolation is required to calculate D0. Since no fragmentation is observed at the lowest energies in the photodissociation spectrum, several images were recorded in the higher energy region (>30,000 cm−1), near the onset of Mn+ (5S) production. The image in Figure 5 reveals the production of both ground state Mn+ (7S) + O (3PJ) fragments (outer ring) as well as the higher energy Mn+ (5S) + O (3PJ) channel (inner). We do not resolve the O atom spin−orbit levels, which span a range of 227 cm−1. The images recorded in this region are isotropic for each fragment channel, indicating that the photodissociation process takes longer than the rotational period of the parent ion. The bond dissociation energy of MnO+ is determined by analyzing the KER of images recorded at multiple wavelengths. The total KER from each of the energetically accessible dissociation pathways is plotted against the photon energy at which the corresponding image was recorded (Figure S3). The bond strength (D0 = 242 ± 5 kJ/mol) is determined by subtracting the total internal energy of the relevant fragments41 from the y-intercepts of the linear trendlines. This value is compared with other published experimental and theoretical values in Table 1. The D0 reported here is in good agreement with the original ion beam measurement of Armentrout et al.14 but not with the later measurement of Fisher et al.15 As for theory, the highest level ab initio result, at the MRCI level with Davidson and relativistic corrections13 slightly underestimates the bond strength; DFT methods tend to overestimate it.12,42 This level of agreement is very encouraging for such a challenging molecule. Smoes and Drowart measured the dissociation energy of neutral MnO to be 3.83 ± 0.08 eV using a high-temperature Knudsen cell.43 Combining this value with our measurement of D0(Mn+−O) and the ionization energy of Mn atom (7.434 eV)41 yields an ionization energy of 8.76 ± 0.09 eV for MnO. This is in excellent accord with the MRCI value, 8.6 eV.13
Table 1. Experimental and Theoretical Ground State Bond Dissociation Energies (D0) of MnO+ (5Π) D0 of MnO+ (kJ/mol)
experiment
239 ± 10 285 ± 13 242 ± 5
ion beam14 ion beam15 VMI (present study) theory
230a 233 225 255 251 273b
C-MRCI+DKH2+Q13 MRMP42 IC-MRCI+Q12 B3LYP12 B3LYP42 CASPT2D10
a
The De obtained was converted to D0 by subtracting a zero-point energy of 3.6 kJ/mol. bThe D0 obtained assumes a 5Σ+ ground state and not the 5Π ground state reported here.
IV. CONCLUSIONS Photofragment spectroscopy and ion imaging from 21,300 to 33,900 cm−1 have been used to probe the bond dissociation energy and dynamics of MnO+. The experimental results are compared to electronic structure calculations performed at the TD-DFT, EOM-CCSD, and MRCI levels and to theoretical results available in the literature. The photodissociation spectrum displays a dominant feature centered at 27,200 cm−1. The lack of a vibrational progression indicates that the excited state has a similar bond length to the ground state. The presence of spin−orbit structure within this peak along with the vertical anisotropy of the corresponding photofragments indicates that the fragments result from a predissociative 5Π ← 5 Π transition. This finding experimentally confirms the expected identity of the ground state (5Π) of MnO+ and, using the previously calculated theoretical spin−orbit constant for the ground state,13 provides a spin−orbit constant (A′ = 14 cm−1) for the 5Π excited state. Other features in the photodissociation spectrum are much less intense, irregularly spaced, and difficult to assign to specific excited states due to the high density of electronic states in the region. The MRCI calculations are much more successful at predicting the observed electronic spectrum than the TD-DFT or EOMCCSD calculations. Images at energies above 30,000 cm−1 indicate the opening of a new photodissociation channel corresponding to Mn+ (5S) + O (3P) fragments, which quickly dominates the Mn+ (7S) ground state pathway. By photolyzing MnO+ at several energies and measuring the corresponding kinetic energy releases from the associated photofragment images, we obtain the bond dissociation energy of MnO+ (D0 = 242 ± 5 kJ/mol).
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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.8b07849. Complete refs 36 and 37; potential energy curves for quintet states of MnO+ calculated using TD-DFT at the B3LYP/6-311++G(3df,3pd) and EOM-CCSD/aug-ccpVTZ levels; and plot of excitation energy versus total kinetic energy release for several images of Mn+ products resulting from photodissociation of cold MnO+ used to obtain the bond dissociation energy of MnO+ (PDF) 8051
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ricardo B. Metz: 0000-0003-1933-058X Present Address †
Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO 80309. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Science Foundation under Award No. CHE-1566407 is gratefully acknowledged. MRG acknowledges the financial support of the University of Massachusetts Amherst undergraduate research assistant fellowship. The MOLPRO calculations were carried out at the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Professor Arthur Suits for providing the NuAcq data acquisition package and Professor Hanna Reisler for the BASEX analysis software.
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DOI: 10.1021/acs.jpca.8b07849 J. Phys. Chem. A 2018, 122, 8047−8053
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DOI: 10.1021/acs.jpca.8b07849 J. Phys. Chem. A 2018, 122, 8047−8053
