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UV Photodissociation of η5-C5H5NiNO: An Excited-State Jahn-Teller Distortion Produces a Cartwheeling NO Amber L. Peden, Ryan D. Kieda,‡ Kelsey A. Breck, Joseph R. Basore,† Caleb A. Kent,§ and Jeffrey A. Bartz* Department of Chemistry, Kalamazoo College, 1200 Academy Street, Kalamazoo, Michigan 49006 ReceiVed: June 1, 2010; ReVised Manuscript ReceiVed: August 27, 2010
The 225 nm photodissociation of cyclopentadienylnickel nitrosyl was studied using velocity-mapped ion imaging with 1 + 1′ REMPI detection of the NO (X 2Π1/2,3/2, V′′ ) 0) photofragment. The product recoil energy and angular distributions were measured for selected rotational states of NO. The NO product displays two speeds, a slow product peaked at the center of the ion image and a fast anisotropic product that has an inverted rotational population. In rotational states above J′′ ) 40.5, an even faster anisotropic NO photofragment appears, most likely because the metal-containing dissociation partner emerges in a lower electronic state, increasing the available energy. The µ-v-j vector correlations were measured and are consistent with the orientation µ|v⊥ j. The observed vector correlations arise from an excited-state Jahn-Teller distortion of the parent, a distortion that bends the Ni-NO coordinate prior to dissociation. 1. Introduction Nitric oxide is ubiquitous in biological systems, with roles in neurotransmission, vasodilation, and the immune response. Consequently, the role of metal-NO bonding, in biological systems and nonbiological systems, is an area of intense interest.1 As a ligand, NO is versatile because it can bind to metal centers in a variety of ways. As a cation, NO+, it is isoelectronic with CO, with sp hybridization about the N. In this case NO binds to a metal center in a linear geometry with a 180° bond angle. As an anion, NO-, the N is considered sp2 hybridized and binds to a metal with a bent geometry. Nitric oxide can bind side-on to a metal center, as is seen in the coordination of NO to copper in nitrile reductase.2 The binding mode affects the reactivity of NO, consequently studying systems with dynamic interconversion between coordination geometries is important to understand the factors that drive reactivity. A volatile metal nitrosyl, cyclopentadienylnickel nitrosyl (η5C5H5NiNO or CpNiNO, I), is a model for the interconversion between linear and bent metal-NO bonding. In the ground state, CpNiNO has a linear Ni-NO bond and C5V symmetry.3,4 The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are doubly degenerate in CpNiNO and the related pentamethyl derivative.5-7 In matrix isolation experiments, photoexcitation of CpNiNO results in a lowered wavenumber for ν(NO), indicating a change in Ni-NO bonding.8,9 Because the LUMO in CpNiNO is doubly degenerate, excitation of an electron into one of the two degenerate LUMOs will result in a Jahn-Teller distortion of the excited state. A photoexcited distortion of the Ni-NO bond angle was first * To whom correspondence should be addressed. Tel: (269) 337-7021. Fax: (269) 337-7508. E-mail:
[email protected]. ‡ Current Address: Department of Chemistry, University of Wisconsins Madison, Madison, Wisconsin 53706. † Current Address: Department of Chemistry, Indiana University, Bloomington, Indiana 47401. § Current Address: Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599.
observed through measurements of changes in nuclear position with EXAFS,10 then by observing structural changes in the pentamethyl analog by low-temperature X-ray diffraction,11 and also by changes in low temperature infrared spectra.12 The photoinduced linkage isomerization results in a side-on η2 coordination in the photoexcited Cp*NiNO (Cp* ) pentamethylcyclopentadiene).13 The Ni-NO bond angle is 92° in the metastable excited state. Density Functional Theory calculations of the ground and excited states indicate that there are two metastable states for CpNiNO, the one observed by low-temperature X-ray diffraction and a second with the NO rotated so that the O is bonded to Ni, forming a CpNiON linkage isomer.14-16 The transition state between the linear Ni-NO ground state and the η2 metastable state has a 137° Ni-NO bond.15 Gas phase photodissociation of CpNiNO was studied near 450 nm, with [2 + 2] resonance-enhanced multiphoton ionization (REMPI) for detecting NO in a mass spectrometer.17 The dissociation populated a broad distribution of NO rotational states. The authors suggest that at 450 nm excitation the NO distribution results from a bent Ni-NO transition state. Photodissociation of CpNiNO near 225 nm, using 1 + 1 REMPI to generate NO+ for detection, yielded an NO rotational distribution fit by a Boltzmann temperature.18 If photoexcited CpNiNO indeed undergoes a Jahn-Teller distortion in the gas phase, then the NO photofragment should show a negative vector correlation between the velocity (v) and the angular momentum (j) vectors. A negative v-j vector correlation would be consistent with the observed Ni-NO bend in condensed phase. The negative v-j vector correlation would also indicate that the observed changes in CpNiNO bonding occur through motions in the excited state rather than through a back reaction of solvent-caged CpNi and NO. Here we report the velocity-mapped ion imaging of NO from the photodissociation of CpNiNO at 225 nm. In a velocitymapping instrument, the initial velocity of the photoproduct is mapped onto an x-y point on a two-dimensional detector. Thus, velocity map imaging gives full speed and angular information about the photofragments. In the present study of CpNiNO
10.1021/jp105026n 2010 American Chemical Society Published on Web 09/24/2010
UV Photodissociation of η5-C5H5NiNO
Figure 1. Schematic representation of the velocity-mapping time-offlight mass spectrometer. Three laser pulses, dissociation, probe, and ionization, intersect the molecular beam at right angles to both the molecular beam and the time-of-flight chamber.
photodissociation, use of linearly polarized dissociation and probe lasers and fits of the resulting ion images determined the vector correlations. The vector correlations reveal the role of an excited-state Jahn-Teller distortion in the photofragmentation process. 2. Experimental Methods Cyclopentadienylnickel nitrosyl was prepared by previously reported procedure and transferred to a Schlenk flask by vacuum methods.19 In an experiment, the compound was treated by three freeze-pump-thaw cycles and then immersed in 40-50 °C water. High-purity He carrier gas (∼600 Torr) was flowed over the sample. The transfer line to the experiment was heated to the same temperature by water circulation. Since CpNiNO has a vapor pressure of 30 Torr at 60 °C,20 the pressure of CpNiNO in the sample was assumed to be less than 20 Torr. The sample was introduced into a velocity-mapping timeof-flight mass spectrometer (Figure 1) with a design similar to that described by Eppink and Parker.21 The source chamber is pumped by a diffusion pump (Varian VHS-6) backed by a mechanical pump (Alcatel 2020A). The sample and carrier gas are introduced to the experiment through a solenoid valve (General Valve Series 9). The resulting molecular beam travels through a skimmer (Beam Dynamics) into the flight tube, which is pumped by a turbomolecular pump (Pfeiffer TPU240). The skimmed beam passes through a 1.5 mm hole in the repeller of the ion optics (Jordan TOF Products), where it interacts with the lasers. In these experiments, the repeller is held at 1250 VDC, extractor at 915 VDC, and the grid at ground. The operating pressure in the source chamber is ∼5 × 10-6 Torr and in the flight tube is 20.5 in Figure 2 is consistent with a prompt, parallel dissociation along the Ni-NO coordinate. Boulet et al. have assigned the 225 nm excitation to the transition from 8a1 to the doubly degenerate 7e1 LUMO (Figure 6).16 Under C5V symmetry, the transition dipole for the E′′ r A′ transition is aligned along the symmetry axis of the molecule, parallel to the Ni-NO bond. The ion images in Figures 2, 3, and 5 are consistent with the parallel assignment.
Figure 5. Ion images from the 225 nm photodissociation of η5C5H5NiNO showing a faster anisotropic NO product appearing at J′′ > 40.5. As the rotational state changes from (a) J′′ ) 39.5, to (b) J′′ ) 43.5, and finally to (c) J′′ ) 46.5, the NO photofragments have more rotational and more translational energy. In these images, the 225 nm dissociation laser is polarized perpendicular to the time-of-flight axis and the probe laser is polarized parallel to the time-of-flight axis. The highest ion intensity is white and the lowest is black.
Figure 6. Depiction of the doubly degenerate lowest-occupied molecular orbital (LUMO) labeled 7e1 by Boulet et al.16 The LUMO has an antibonding interaction between the Ni metal center and NO ligand. Populating either of these two orbitals will generate a Jahn-Teller instability that will induce a bend in the Ni-NO coordinate.
Promoting an electron to one of the two degenerate 7e1 orbitals in CpNiNO results in a Jahn-Teller instability. As a result, the Ni-NO bond bends. Dissociating from a bent geometry produces the inverted rotational state distribution for the fast NO photoproduct seen here. Similar inverted NO product state distributions have been observed in the photodissociation of CH3ONO and other nitrites, although in nitrites the ground state is bent.29-36 The bent nitrite ground state is mapped onto the excited state where an impulse along the R-NO coordinate generates a product state distribution peaked at high J”. Schinke termed the mapping of the bent ground state geometry into an inverted rotational state distribution, the rotation-reflection principle.37 In CpNiNO, the rotational
TABLE 2: Average Velocities of the Isotropic and Anisotropic Speed Distributions, the Bipolar Moments, and the Molecular Frame Polarization Parameters Found in the Anisotropic Speed Distribution Listed by Rovibronic Transition in NO [A (2Σ, W′ ) 0) r X (2ΠΩ, W′′ ) 0)] Following 225 nm Photodissociation of η5-C5H5NiNO transition
iso
aniso
β20(20)
β20(02)
β00(22)
β20(22)
β20(42)
Re[a(2) 1 (||, ⊥)]
Q11 + P21 (33.5) Q11 + P21 (33.5) Q22 + R12 (35.5) Q11 + P21 (39.5) physical limit: µ|v⊥jb
1000 1000 1100 1200
1400 1400 1300 1300
0.98 1a 0.86 0.99 +1
-0.32 -0.34 -0.21 -0.17 -1/2
-0.22 -0.26 -0.15 -0.12 -1/2
0.21 0.35 0.18 0.20 +1/2
-0.30 -0.35 -0.25 -0.21 -1/2
0.04 0.01 0.09 0.05 0
a
The bipolar moment was fixed at the physical limit. b Adapted from Dixon.24
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Figure 7. Internal motion in the parent η5-C5H5NiNO corresponds to the vectors and vector correlation observed in the laboratory frame. The transition dipole (µ) is aligned with the symmetry axis and the initially linear Ni-NO. As the Ni-NO bond bends, motion around the N-O center-of-mass is asymptotically linked with rotation in NO, shown as the perpendicular vector j (.) in the diagram. Breaking the Ni-NO bond results in the NO velocity (v) aligned preferentially with the transition dipole (µ) and perpendicular to the angular momentum (j). The bipolar moments in the limiting case of µ|v⊥j are indicated, adapted from Dixon.24
product state distribution for the fast NO product is inverted although the ground state is linear in the CpNi-NO coordinate, appearing to violate the rotation-reflection principle. One explanation is that CpNiNO photodissociation is prompt but is not direct. As such, the Jahn-Teller distortion is faster than the dissociation of the Ni-NO bond. The time averaged correlation function of Yang and Bersohn38 was used with the traditional anisotropy parameter, β [) 2β20(20)], and the moment of inertia of CpNiNO39 to estimate an excited-state lifetime of CpNiNO on the order of hundreds of femtoseconds. The Ni-NO bending frequency in the CpNiNO ground state is 490 cm-1,12 corresponding to a vibrational period of 68 fs, a sufficiently short time to permit the Ni-NO bond to bend prior to dissociation. Similar excited-state behavior, starting from a linear ground state and resulting in a bent excited state, has been seen in ICN.40-44 The isotropic peak at the center of the ion image is assumed to be the dissociation of vibrationally hot CpNiNO ground state. Photodissociation experiments with a 450 nm or a 280 nm dissociation beam produced only the slower isotropic product. The lower energy dissociation photons (450 or 280 nm) must have sufficient energy to dissociate CpNiNO in an RRKM fashion, but insufficient energy to access the prompt dissociating state. The combination of bending and breaking the Ni-NO bond results in strong vector correlations between and among velocity, v, angular momentum, j, and transition dipole, µ, as is seen in Table 2. The NO photoproduct emerges with a cartwheel-like motion, cartwheels that are strongly oriented in the laboratory frame. The strong vector correlations begin with internal motions in CpNiNO, depicted in Figure 7, that correlate with the orientations of µ, v, and j. In Figure 7, the values of the five bipolar moments fit here are indicated for the limiting case of a parallel electronic transition, prompt dissociation, and a cartwheeling motion of the NO. Because the CpNiNO undergoes a parallel transition and the molecule dissociates promptly, the NO velocity vector is along the symmetry axis and parallel to the polarization of the dissociation laser. After excitation but before dissociation, the Ni-NO bond bends, an internal motion associated with the angular momentum vector in the free NO.
Peden et al.
Figure 8. Symmeterized experimental, fit, and difference images show the role that β20(42) plays in making an accurate fit of the ion images. Image (a) is the symmeterized version of Figure 3(b), using a different false color scheme to accentuate the dip in intensity along the vertical axis. Image (b) is a synthetic image based on the full parameter fit of the experimental image. Image (c) is a fit of the experimental image excluding the quartic µ-v-j bipolar moment, β20(42). Images (d)-(f) are differences between the images (a)-(c) as indicated. The difference image scales show positive differences in red-orange and negative differences in blue. The similarities in difference images (d) and (f) show that β20(42) has a significant contribution that is evident in both the experimental (a) and fitted (b) images.
As shown in Figure 7, the angular momentum vector in the ejected NO (.) is perpendicular to the plane defined by the bent Ni-NO moiety. The measured bipolar moments appear in Table 2. The vector correlations indicate that the general vector correlations are µ|v⊥j, however the only bipolar moment that reaches the theoretical limit is β02(20), the velocity anisotropy. The velocity anisotropy is large because the parent molecule dissociates promptly, has relatively high moment of inertia, and is cooled in a molecular beam. The bipolar moment β02(42), quartic µ-v-j correlation, is important in fitting the data, even though the value of β02(42) can be determined by other bipolar moments. Costen and Hall have shown how using fewer fit parameters in ICN dissociation accurately determines the vector correlation.43,45 The justification for using fewer parameters in the fit comes from considering the molecular frame polarization parameter, Re[a1(2)(||, ⊥)], 1 2 12 2 1 2 Re[a(2) β (42) 1 (||, ⊥)] ) 2√6 β0(02) - β0(22) 5 7 35 0
[
]
(1)
which is sensitive to the interference of multiple dissociative states.46 In the high-J limit, the contribution of the molecular frame polarization parameter should vanish. Table 2 shows the values of Re[a1(2)(||, ⊥)] calculated from eq 1, which are nearly zero for each transition studied. Because the molecular frame polarization parameter approaches zero, the fits are overdetermined because the values of β02(02) and β02(22) can be used to calculate the expected value of β02(42). Even so, β02(42) is necessary for accurate fits of the images. A parallel set of forward convolution fits using fimage were performed to test whether ion images, such as Figure 8(a), were reasonably fit with fewer parameters. The first set of fits included β02(42) [Figure 8(b)] while the second set [Figure 8(c)] did not.
UV Photodissociation of η5-C5H5NiNO
Figure 9. The possible orientations of the singly occupied NO π* orbital, in the high-J limit, shown relative to the plane of rotation. NO has an unpaired electron that populates one of two π* orbitals. When the unpaired electron occupies the π* orbital in the plane of rotation, it is labeled Π(A′). When the unpaired electron occupies in the π* orbital perpendicular to the plane it is labeled Π(A′′).
For each experimental transition in Table 2, either the full fit, with β02(42), or the partial fit, without β02(42), yielded similar values for the other bipolar moments and the product speeds. One slight difference emerged in the χ2 values wherein the χ2 values for the partial fits were at least 10% larger. The full parameter fit, with β20(42), and the partial fit, without 2 β0(42), show differences as the resulting synthetic images are compared. Figure 8(d) is a difference image between the complete fit, including β02(42), and the partial fit. In Figure 8(d) a positive difference is a red-orange color and a negative difference is a blue color. The difference between the experimental image 8(a) and the complete fit 8(b) appears in Figure 8(e) and the difference between the experimental image 8(a) and the partial fit 8(c) appears in Figure 8(f). It is clear in Figure 8(e) that the synthetic image based on the forward convolution fits is not a perfect reproduction of the experimental data, but the differences between the experiment and synthetic images are much more significant in the partial fits, when β02(42) is neglected. In comparing the intensities and patterns of difference images 8(d), 8(e), and 8(f), the two that have the most similarity are 8(d) and 8(f) showing that the difference in contribution of β02(42) is the same in the fit and experimental images. The dip in intensity along the vertical bisector of the symmeterized ion image [Figure 8(a)] gives a visual clue that β02(42) is necessary to fit the data. The intensity dip is a general feature of the ion images with both the dissociation and probe laser polarizations perpendicular to the detector axis for transitions that have a high fraction of Q character. The bipolar moment β20(42) adds a higher order bipolar harmonic that better fits the dip. Although in the high-J limit, the contribution of Re[a1(2)(||, ⊥)] vanishes, and as a result, β20(02) and β20(22) can be used to calculate the expected value of β02(42), but inclusion of β02(42) is necessary to fit the ion images fully. The nascent NO exits the dissociation in a preferred orientation of the unpaired electron. In the high-J limit, the two Λ-doublets of the 2Π ground state of NO correspond to π* orbitals that are in the plane or perpendicular to the plane of rotation, labeled Π(A′) and Π(A′′), respectively, as shown in Figure 9. The strong Q11 + P21 and Q22 + R12 transitions reported in Table 2 arise from the Π(A′′) Λ-doublet. The weaker P11 transitions are poorly fit and originate from the Π(A′) Λ-doublet. The ejected NO has memory of the photodissociation event because the NO shows a preference for Π(A′′) over Π(A′). A series of single-point DFT calculations were performed to understand why the NO shows a preference for having the unpaired electron out of the plane of NO rotation. In the calculations, both the Ni-NO and Cp-Ni-N bonds were bent to keep the center of mass along the Cp-Ni axis. Figure 10
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Figure 10. A plot of the relative energies of the lowest two unoccupied orbitals in η5-C5H5NiNO as the Ni-NO bond bends. At 180° the orbitals are doubly degenerate. As the Ni-NO bond initially bends, the π* orbital in the Ni-NO plane, colored green and yellow for clarity, is lower in energy. As the Ni-NO bond angle goes below 100°, the out-of-plane π* orbital (red and blue) stabilizes as the in-plane π* orbital rises in energy. The in-plane π* orbital is asymptotically connected to the Π(A′) Λ-doublet in NO and the out-of-lane π* is asymptotically connected to the Π(A′′) Λ-doublet. In the photodissociation of η5-C5H5NiNO, the NO photofragment has a higher population in the Π(A′′) Λ-doublet with the unpaired electron out of the plane of rotation. The energies of the in- and out-of-plane orbitals predict that the Ni-NO bond is highly bent during the dissociation process.
shows how the initially degenerate LUMO changes as a function of Ni-NO angle. As the Ni-NO bond bends, the π* orbital that is asymptotically connected to the Π(A′) Λ-doublet is in the Ni-NO plane. Conversely, the π* orbital that is asymptotically connected to the Π(A′′) Λ-doublet is perpendicular to the Ni-NO plane. As the Ni-NO angle changes from linearity, the in-plane π* orbital lowers in energy, reaching a minimum at 120°. The out-of-plane π* orbital energy lowers slightly, as the Ni-NO bond bends. At an Ni-NO angle of 100°, the energies of both the in-plane and out-of-plane π* orbitals cross. At angles below 100°, the energy of the in-plane π* orbital increases sharply as out-of-plane orbital, the one that asymptotically leads to the Π(A′′) Λ-doublet in NO, decreases in energy. Slight bends in the Ni-NO bond will not result in a NO photofragment in the Π(A′′) Λ-doublet, however extreme bending motions will. The most surprising result is the appearance of NO photoproducts with higher translational energy and higher rotational energy above J′′ ) 40.5. Fits to the ion image from the Q11 + P21 (J′′ ) 46.5) transition in Figure 5(c) give similar bipolar moments to those of the lower NO rotational states. As a result, it is assumed that the dissociation pathway to produce the faster NO products at higher J′′ states is similar to that of the other anisotropic NO products. Since one of the photoproducts, CpNi, contains a metal, there are low-lying CpNi electronic states that affect the energy available to NO. A TDDFT calculation on CpNi finds the lowest excited state is 310 cm-1 above the ground state. The difference in translational energy release between the fastest products in Figure 5, parts (a) and (c), is around 300 cm-1, accounting for the gain in translational energy. The Ni-NO bond bending that results in higher NO rotation must facilitate a curve crossing in the CpNiNO to another dissociating surface. On the second dissociating surface, the CpNi fragment has lower electronic energy, which increases the energy available to partition into photofragment recoil. 5. Conclusions Photodissociation of the organometallic compound, CpNiNO, at 225 nm generates an NO photofragment from bent excited
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state arising from a Jahn-Teller distortion. Forward convolution fits to the photoejected NO show strong vector correlations between the transition dipole, velocity, and angular momentum. The initially linear Ni-NO bond bends significantly prior to dissociation, consistent with changes in bonding observed in the condensed phase. Although DFT and TDDFT calculations give some information on why the excited state bends and shows a preference for the Π(A′′) Λ-doublet a more exhaustive computational study of the excited state behavior of CpNiNO would give a more detailed explanation for the observations shown here and a better understanding of metal-NO bonding in general. Acknowledgment. This work was supported by the National Science Foundation (Awards CHE-0420928 and CHE-0314745). A.P. acknowledges summer support from the Hutchcroft Fund. R.K. received summer support from the Heyl Foundation. K.B. acknowledges support from the Howard Hughes Medical Institute through an Undergraduate Science Education Program grant to Kalamazoo College (Award 52005128). The authors thank Professor Joseph Cline for providing the fimage program. References and Notes (1) Richter-Addo, G. B.; Legzdins, P.; Burstyn, J. Chem. ReV. 2002, 102, 857–1270, Thematic Issue on Nitric Oxide Chemistry. (2) Tocheva, E. I.; Rosell, F. I.; Mauk, A. G.; Murphy, M. E. P. Science 2004, 304, 867–870. (3) Cox, A. P.; Thomas, L. F.; Sheridan, J. Nature 1958, 181, 1157– 1158. (4) Cox, A.; Brittain, A. Trans. Faraday Soc. 1970, 66, 557–562. (5) Field, C.; Green, J.; Mayer, M.; Nasluzov, V.; Rosch, N.; Siggel, M. Inorg. Chem. 1996, 35, 2504–2514. (6) Li, X.; Tse, J.; Bancroft, G.; Puddephatt, R.; Tans, K. Inorg. Chem. 1996, 35, 2515–2523. (7) Green, J.; Underwood, C. J. Organomet. Chem. 1997, 528, 91–94. (8) Crichton, O.; Rest, A. J. Chem. Soc., Chem. Commun. 1973, 1973, 407–407. (9) Crichton, O.; Rest, A. J. J. Chem. Soc., Dalton Trans. 1977, 986– 993. (10) Chen, L. X.; Bowman, M. K.; Wang, Z.; Montano, P. A.; Norris, J. R. J. Phys. Chem. 1994, 98, 9457–9464. (11) Fomitchev, D. V.; Furlani, T. R.; Coppens, P. Inorg. Chem. 1998, 37, 1519–1526. (12) Schaiquevich, P. S.; Güida, J. A.; Aymonino, P. J. Inorg. Chim. Acta 2000, 303, 277–281. (13) Fomitchev, D. V.; Furlani, T. R.; Coppens, P. Inorg. Chem. 1998, 37, 1519–1526. (14) Boulet, P.; Buchs, M.; Chermette, H.; Daul, C.; Furet, E.; Gilardoni, F.; Rogemond, F.; Schlaepfer, C. W.; Weber, J. J. Phys. Chem. A 2001, 105, 8999–9003. (15) Boulet, P.; Buchs, M.; Chermette, H.; Daul, C.; Gilardoni, F.; Rogenmond, F.; Schlaepfer, C. W.; Weber, J. J. Phys. Chem. A 2001, 105, 8991–8998.
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