Q-Band EPR Spectroscopy of Photogenerated Quartet State Organic

ARTICLE pubs.acs.org/JPCA

Q-Band EPR Spectroscopy of Photogenerated Quartet State Organic Nitreno Radicals Tamara L. Allen and Paul M. Lahti* Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States

bS Supporting Information ABSTRACT: Q-band 34 GHz EPR spectra are reported for quartet state 2-(para-nitrenophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl and 3-(para-nitrenophenyl)-1, 5,6-triphenylverdazyl reactive intermediates generated from the corresponding azido precursors under frozen matrix photochemical conditions, in situ in a Q-band resonator. Comparison of the Q-band spectra to those generated under conventional X-band (9
10 GHz) conditions shows the much superior resolution of transitions in the g > 2 region of the former. Spectral transitions assigned by line shape simulation yield the zero field splittings for the nitreno-radical species.

’ INTRODUCTION The study of interelectronic interactions is extremely important to understand the nature of molecular bonding. The electronic behaviors of molecules with multiple unpaired electrons are particularly fascinating, as well as being rich with experimental and theoretical challenges. Spectral analysis to determine zero field splitting (zfs) is important in such studies, since zfs yields structure
property information regarding interelectronic interactions. However, line shape interpretation can be challenging for organic high-spin molecules having zfs that is significantly greater than the EPR spectral transition energy, due to the nonfirst-order nature of such spectra. This problem can be addressed by obtaining spectra at higher transition energies, i.e., at higher microwave frequencies. Although this is common practice1 for studies of high-spin inorganic systems such as single-molecule magnets, this strategy has seldom been pursued for organic high-spin systems. In this article, we demonstrate the utility of Q-band (34 GHz) EPR spectroscopy for organic high-spin reactive intermediates photogenerated under frozen solution matrix isolation conditions. Quartet state (S = 3/2) nitreno-radical2 systems 2-(paranitrenophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3oxide-1-oxyl and 3-(para-nitrenophenyl)-1,5,6-triphenylverdazyl— NPhNN and NPhVerd, respectively—were photogenerated in frozen glassy solutions in situ in a Q-band resonator with special apparatus. Such use of a higher-field EPR methodology has much promise for study of open shell organic systems with more than two unpaired electrons, in particular because the resolution and separation of transitions in the g > 2 region are markedly superior to the corresponding region in X-band EPR spectra. ’ EXPERIMENTAL METHODS Precursor azides N3PhNN and N3PhVerd were obtained as described in Serwinski et al.2c Dilute solutions of the precursors r 2011 American Chemical Society

in dry 2-methyltetrahydrofuran (MTHF) were placed in 2 mm o. d. quartz tubes with 0.4 mm wall thickness. The tubes were placed into a special-made Bruker E600-1021RL Q-band sample insertion rod having an internal coaxial fiber optic for sample irradiation in situ. The sample was inserted into the ER 5106QT-W Q-band resonator of a Bruker Elexsys-500 EPR system equipped with ER 4118CF-O cryostat that was precooled to 78 K, allowed to freeze for 1 min, and then placed under vacuum and allowed to equilibrate thermally for 10
15 min. The samples were then irradiated while maintaining a low temperature of 78
82 K, using broadband xenon arc light entrained through a quartz lightgathering lens into a fiber optic at the top of the sample stick. Irradiation was continued during the acquisition of multiple Q-band spectra at intervals over about 20 min. The spectra were obtained up to about 17 500 G, and a Hewlett-Packard 5352B frequency counter was used to record the microwave frequencies. Scheme 1 shows the precursor deazetation reactions that give nitreno radicals NPhNN and NPhVerd, which were shown in previous2 work to occur under frozen matrix photochemical conditions similar to these.

’ RESULTS AND DISCUSSION At the present, it is possible to predict ground-state spin multiplicity preferences of high-spin molecules with much confidence,3 to predict the high-spin zfs based on molecular geometries for quintet state dinitrenes and dicarbenes4 and in most cases to simulate5 the expected EPR spectral lineshapes of randomly oriented or powder high-spin samples for comparison to experimental results. A nonexhaustive list of EPR investigations of nitrene-containing organic systems includes S = 3/2 nitreno-radical systems reported by Lahti and co-workers,2 S = 2 Received: February 20, 2011 Revised: April 4, 2011 Published: April 27, 2011 4922

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dinitrenes by various workers6 since the groundbreaking report7 of 1,3-dinitrenobenzene by Wasserman et al., and various S = 3 aryltrinitrenes.8,9 Chapyshev and co-workers9 have recently reported several EPR studies and reinvestigations of trinitrenes in argon matrices, showing influence of the host matrix with rigorous analyses of the zfs under random orientation conditions. EPR analysis of randomly oriented high-spin (S > 1) organic systems with large zfs can be challenging because a number of their typical X-band spectral transitions can fall in the 500
3400 G region (g > 2). For example, trinitrene zfs is large enough by comparison to the X-band spectrometer frequency of 0.3 cm
1 to give strongly nonfirst-order lineshapes in the lower-field regions.8,9 Since polynitrenes and many other high-spin organic species require photolysis of multiple spin-precursor sites, one frequently must also account for EPR spectral transitions from incompletely photolyzed side products, some at similar resonant fields to transitions coming from the higher spin species. Careful work and good simulation techniques can sort out the various transitions, as shown for example by Koto et al.5c and by Chapyshev and co-workers.9 However, challenges of overlapping peaks for multiple species and the need to analyze nonfirst-order EPR lineshapes keep increasing for systems with higher multiplicity. The use of higher-field spectroscopy to spread out the peaks in high-spin spectra, and increase their first-order nature, mitigates these problems. We chose two quartet state systems to test photochemical generation of organic intermediates in situ in a Q-band EPR resonator. Both NPhNN and NPhVerd exhibit frozen solvent matrix-isolated X-band EPR spectra that our group previously showed2 to arise from quartets that are either ground states or very nearly so. Neither gives significant additional photoproducts, so these seemed ideal as first-case tests for obtaining and analyzing their EPR spectra at frequencies higher than the 9.6
10 GHz X-band used to date. Quartet ground states are expected for both, based on qualitative π-spin-polarization,

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Scheme 2. Qualitative Spin Density Distributions in NPhNN and NPhVerd (for Only the Lower Two Structures, Open Circles = R π-Spin Density Site, Filled Circles = β π-Spin Density Site)

parity based arguments. Scheme 2 shows qualitative spin density distributions of π-orbital bearing atoms on both species, assuming coplanarity of the phenylene and radical units. Polarization of the π-spins in the typical alternating pattern favors a high-spin state in which the radical spin and nitreno nitrogen π-spin have the same sign. Previous computational studies support a highspin preference, and Curie-type studies of X-band EPR signal intensity versus temperature were consistent with either a highspin ground state or near-degeneracy of quartet and doublet states (in the latter, the radical spin is polarized opposite to the nitrene π-spin).2 Qualitatively, a major component of zfs in both quartet systems is expected to arise from the strong interaction between the delocalizable π-spin density on the nitrene nitrogens and the highly localized single electron on the same nitrogen. Even though the nitrene π-spin density is reduced by delocalization, it must have a strong one-center interaction with the localized unpaired electron. Conversely, zfs will be a relatively sensitive function of the delocalization of the nitrene π-spin, with changes in delocalization coming from changing the conjugating radical, as with nitronylnitroxide versus the verdazyl units. Nitreno-radical precursors N3PhNN and N3PhVerd were prepared as described above and photolyzed in degassed, frozen MTHF matrices. Frozen samples photolyzed outside the Q-band resonator at 77 K showed the strong color changes previously reported2 for the transformations in Scheme 1. N3PhNN samples changed from blue to magenta, and N3PhVerd samples changed from green to magenta. The Q-band EPR spectra obtained by in situ photolysis of the precursors in frozen 2-methyltetrahydrofuran (MTHF) matrices are shown in Figure 1. A g ∼ 2 residual monoradical (R) peak is visible in each case from unphotolyzed precursor. In each case, the peaks assigned to quartet NPhNN and NPhVerd (marked “Q”) grew stronger as the photolyses progressed. When photolysis was stopped, the quartet peaks showed no decrease in intensity or line shape changes for the 2
3 h duration of the experiments. However, the quartet peaks disappeared irreversibly if the sample was briefly warmed above 120 K; under the same conditions, the characteristic colors of NPhNN and NPhVerd were also quenched. The reactive nitreno radicals become mobile as the solvent matrix softens at higher temperature, reacting with one another and possibly with the matrix. This is consistent with the behavior previously reported2 for these systems. 4923

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Figure 1. (a)
(c) Experimental Q-band EPR spectrum in various field ranges obtained from photolysis of N3PhNN in MTHF at 78 K at 34.027 GHz, with background subtracted. R = radical peak, Q = quartet state nitreno-radical peak, x = additional, unassigned peak. (d)-(f) Spectra simulated for NPhNN using parameters in Table 1. (g)- (i) Experimental Q-band EPR spectrum in various field ranges obtained from photolysis of N3PhVerd in MTHF at 78 K at 34.055 GHz, with background subtracted. R = radical peak, Q = quartet state nitreno-radical peak. (j)
(l) Spectra simulated for NPhVerd using parameters in Table 1.

To assign zfs parameters to the spectra, we carried out line shape simulations using both the MAKSPC10 program of Sato using eigenfield methods10,11 and the xSophe12 program version 1.1.4.2 using perturbation methods. Both methods gave essentially the same results, assuming both NPhNN and NPhVerd to be S = 3/2 species and using the zfs simulation parameters given in Table 1. Figure 1 shows that the simulations are good fits to the experimental spectra in both cases. The zfs estimated from the Q-band spectra are also close to those estimated from the previous2c X-band studies, confirming the earlier analyses. Two small peaks in the experimental NPhNN spectrum (marked “X”) are not part of the main spectrum, judging by the simulation. These may arise from a twisted conformation of the nitreno radical, given their proximity to two major peaks of the main spectrum, but we have no direct evidence for this speculation. Assignment of the spectral transitions is shown in Figure 2, correlating the experimental Q-band spectrum with a Zeeman energy level diagram for NPhNN, and using canonical spectral axial assignments from xSophe12. The Zeeman diagram was generated using the simulation parameters for NPhNN in Table 1. The spectra for NPhNN and NPhVerd are similar enough that this figure well-represents transitions for both.

Table 1. EPR Simulation Parameters Used to Fit Experimental Q-Band Spectra of NPhNN and NPhVerd NPhNN

NPhVerd

v0 = 34.027 GHz

v0 = 34.055 GHz

S = 3/2

S = 3/2

g = 2.003

g = 2.003

D/hc = 0.263 cm
1 E/hc = 0.000 cm
1

D/hc = 0.280 cm
1 E/hc = 0.000 cm
1

linewidth parameter = 80 G

linewidth parameter = 80 G

Figure 3 correlates the experimental X-band spectrum of NPhNN with a Zeeman splitting diagram generated using zfs parameters from previous2c work. Although most of the peaks at both frequencies can be assigned to canonical transitions, some prominent additional off-axis resonance peaks arise from noncanonical transitions. The presence of strong noncanonical transitions can readily confound analysis of spectra if one considers only canonical axis transition field positions. Actual line shape simulation is therefore quite important for clearer spectral zfs analysis. The line shape simulation, in turn, requires 4924

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Figure 2. (a) Experimental Q-band EPR spectrum from photolysis of N3PhNN at 78 K in MTHF at ν0 = 34.027 GHz. R = radical peak. Inset shows simulated low-field region from Figure 1e with ordinate expansion. Canonical axis assignments for peaks shown as x, y, z, with A = additional transition off the principal axes. (b)
(d) Simulated Zeeman splitting of S = 3/2 substates for (a) showing canonical transitions.

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Figure 3. (a) Experimental X-band EPR spectrum from photolysis of N3PhNN at 78 K in MTHF at ν0 = 9.606 GHz. R = radical peak. Canonical axis assignments for quartet nitreno-radical peaks shown as x, y, and z, with A = additional transition off the principal axes. (b)
(d) Simulated Zeeman splitting of S = 3/2 substates for spectrum in (a), showing canonical transitions, using parameters derived from ref 2c: ν0 = 9.606 GHz, S = 3/2, g = 2.003, D/hc = 0.275 cm
1, E/hc = 0.000 cm
1. 4925

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Figure 4. Simulated Q-band EPR spectrum using NPhNN parameters from Table 1 at ν0 = 34.027 GHz (upper graph), showing angular dependence of resonant transitions as a function of the external field (lower graph). Canonical axis assignments for peaks shown as x,y (angle = 90°) and z (angle = 0°), with A = additional off-axis resonance peak.

analysis of the angular dependencies of spectral resonance fields. This is shown in Figures 4 and 5 for Q-band and X-band simulated spectra of NPhNN. The latter figures show x/y/zcanonical assignments based on transitions that occur at parallel to zfs z-direction (0°) and perpendicular to the z-direction (x/y at 90°). The figures also show how the additional off-axis resonance peaks (“A”) arise where there is not a transition at either 0° or 90° but instead in regions where there is not much change in transition field as a function of angle. Comparison of Figures 2 and 3 illustrates the complementarity of X-band and Q-band spectra for high-spin species. The X-band shows well-resolved spacing for transitions in the higherfield 2 > g > 0.6 region, and the Q-band gives good resolution of transitions in the lower-field g > 2 region. The combination of X-band and Q-band results strengthens the confidence in spectral line assignments by comparison to an analysis done only at lower frequency. The sharpness of the Q-band peaks shows that |E/hc| is essentially zero for both NPhNN and NPhVerd, even less than the limit of 0.002 cm
1 estimated2 from the original X-band study. This is consistent with the axial symmetry expected for NPhNN and NPhVerd. UB3LYP/6-31G* computations with Gaussian13 indicate that the NPhNN and NPhVerd quartet states favor a coplanar geometry between nitrenophenyl and radical units, which will allow conjugative spin overlap from nitrene to radical. We also carried out computations to optimize state geometries and energies using the Becke BL97-D14 hybrid density functional corrected for dispersion interactions by Grimme’s15 method, using the Dunning
Hay double-ζ valence basis set16 with polarization functions (DH*) as implemented in GAMESS.17 Table 2 compares results for the two systems at the two levels of theory. The energy gaps from the planar ground state 4A1 to the excited 2A1 state in NPhNN and NPhVerd at the UB3LYP/ 6-31G* level are 22 and 16 kJ/mol, respectively; at the UB97-D/ DH*, the same gaps decrease somewhat to 12 and 11 kJ/mol. The bisected states lie above the planar states in both systems at both levels of theory. At the UB3LYP/6-31G* level, bisected NPhNN and NPhVerd quartet states lie 48 and 42 kJ/mol above

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Figure 5. Upper graph shows simulated X-band EPR spectrum of NPhNN using parameters derived from ref 2c: ν0 = 9.606 GHz, S = 3/2, g = 2.003, D/hc = 0.275 cm
1, E/hc = 0.000 cm
1. Lower graph shows angular dependence of resonant transitions as a function of the external field (lower graph). Canonical axis assignments for peaks shown as x,y (angle = 90°) and z (angle = 0°), with A = additional off-axis resonance peak.

the planar quartet ground states; at the UB97-D/DH*, the same gaps decrease to 31 and 40 kJ/mol. The bisected states also are computed to have longer transannular C
C bonds (Table 2) than the planar states as a result of deconjugation from twisting. The modest energetic cost of breaking the ring-connecting πbond at all levels is consistent with a large nitreno-radical nature in these systems, as opposed to the dominant contribution of a quinonoidal structure like NPhNN-Q in Scheme 3. The experimental spectra by themselves do not rule out geometries with twisting between the spin unit rings (Scheme 4). It is tempting to assign the small peaks “X” of Figure 1c to a minor conformation of NPhNN that is resolved under Q-band conditions: this would require at least two EPR active isomers to be present. If this is the case, the minor conformation has a significantly larger zfs than the main isomer present. Assuming that the small peaks arise from an S = 3/2 spectrum with transitions analogous to those of the nearest peaks of the main spectrum, and using other simulation parameters from Table 1, |D/hc| ∼ 0.32 cm
1. Such larger zfs seems to fit a loss of cross-ring delocalization of the unpaired nitrene π-electron in a conformer having strong twisting. The computational results of Table 2 show that the total Mulliken spin density on the nitrene nitrogen of the planar 4A1 state of NPhNN is significantly smaller than that of the bisected 4A1 state. This would seem to correlate with a large one-center zfs contribution giving a larger overall zfs in the bisected state. However, the nitrene nitrogen spin density is smaller in NPhVerd compared to NPhNN, even though NPhVerd has the larger experimental zfs. Consideration of the computed spin density on the only nitrene seems not to explain readily the experimental zfs trend between NPhNN and NPhVerd. A detailed zfs estimate includes many spin
spin interactions in the open-shell system, not just the spin density on the nitrene nitrogen. Spin
orbit coupling, while 4926

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generally quite small in systems based on first row elements, can also make a contribution, as pointed out recently by Sugisaki et al.18 Overall, the interplay of spin
spin interactions throughout the molecule is complex enough that the relationship of zfs to twisting or planar geometries is not straightforward. Whatever the weak spectral carrier “X” is, it does seem to be similar to Table 2. Computed Coplanar and Bisected State Energies for NPhNN and NPhVerd relative energy

energy NPhNN

spin density,

ringconnecting

(kJ/mol) nitrene N

(hartrees)

C
C (Å)

UB3LYP/6-31G* 4

A1 planar


662.2593560

0.0

3.88

2

A1 planar


662.2508591

þ22.3

1.83

4

A1 bisected


662.2412453

þ47.5

3.86

2

A1 bisected


662.2409868

þ48.2

1.85

4

A1 planar


662.0151998

0.0

3.81

2

A1 planar


662.0105168

þ12.3

1.79

4

A1 bisected


662.0034559

þ30.8

3.30

2

A1 bisected


662.0032418

þ31.4

1.80

1.50

1.439

1.59

1.473

1.430 1.473

UB97-D/DH* 1.47

1.445

1.55

1.469

1.460 1.470

NPhVerd UB3LYP/6-31G* 4

A1 planar


583.2401014

0.0

3.84

2

A1 planar


583.2339946

þ16.0

1.81

A1 bisected 2 A1 bisected


583.2241276
583.2236468

þ41.9 þ43.2

3.83 1.83

1.59

1.49

4

1.33

1.462 1.482 1.494 1.495

UB97-D/DH* 4

A1 planar


582.9117445

0.0

3.80

2

A1 planar


582.9076822

þ10.7

1.79

4

A1 bisected


582.8966426

þ39.6

3.80

2

A1 bisected


582.8962078

þ40.8

1.80

1.470 1.482

1.57

1.497 1.498

NPhNN or NPhVerd (based on the similar symmetry of peak positions to the main spectrum). We feel that the tentative zfs estimate of D/hc| ∼ 0.32 cm
1 for a quartet species is therefore justifiable for “X”.

’ CONCLUSIONS Q-band EPR spectra were obtained for two nitreno radicals by in situ photolysis of a matrix isolated sample in the resonator. Photogenerated organic high-spin species in routine frozen solution samples have been virtually unstudied by EPR at higher frequencies as opposed to the typical X-band methods used for studies to date, although Koichi Itoh used K-band EPR spectroscopy for his trailblazing oriented-crystal (not random orientation sample) investigation of quintet 1,3-phenylene-bis(phenylcarbene).19 The random orientation, frozen solution matrix isolated spectra in the present study were successfully simulated and transition assignments made, taking advantage of multiple transitions that are well resolved in the Q-band g > 2 region. Q-band EPR investigation of photogenerated high-spin organic species should be valuable wherever precursor irradiation gives detectable amounts of the desired EPR active species. Studies of numerous inorganic high-spin species have benefitted by comparing X-band EPR spectra to higher frequency spectra.1 Similar benefits will come from analogous studies of organic high-spin species. Extension of the method to where S > 3/2 systems should be particularly valuable, especially in cases where incomplete photolysis gives overlapping spectra from species with different spin quantum numbers (e.g., S = 2 and S = 3 from triazide photolyses), and in cases where multiple conformations or geometries are generated of an intermediate having the same spin quantum number but different zfs. ’ ASSOCIATED CONTENT

bS

Supporting Information. Pictures of the apparatus used to carry out in situ photolysis in the Q-band resonator and archival details of the computations summarized in Table 2. This material is available free of charge via the Internet at http://pubs.acs.org.

Scheme 3. Limiting-Case Nitreno-Radical and Quinonoidal Resonance Contributors to NPhNNa

a

The π-type and σ-type spin orbitals are indicated; half-spins are delocalized between two aminoxyl N
O units.

Scheme 4. Planar and Twisted Conformers of NPhNN, with Loss of Cross-Ring Bonding by Torsiona

a

The π-type and σ-type spin orbitals are indicated; half-spins are delocalized between two aminoxyl N
O units. 4927

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Science Foundation (CHE 0809791, TLA by REU supplement CHE 0834011, UMass Amherst EPR Facility setup by CHE 0443180). ’ REFERENCES (1) (a) Reijerse, E. J. Appl. Magn. Reson. 2010, 37, 795–818. (b) Feng, P. L.; Beedle, C. C.; Koo, C.; Lawrence, J.; Hill, S.; Hendrickson, D. N. Inorg. Chem. Acta 2008, 361, 3465–3480. (c) Lubitz, W.; M€obius, K.; Dinse, K. P. Magn. Reson. Chem. 2005, 43, S2–S3.(d) Grinberg, O. Y.; Berliner, L. J., Eds. Very High Frequency (VHF) ESR/EPR. Biological Magnetic Resonance; Kluwer-Academic-Plenum: New York, NY; 2004; Vol. 22. (e) Reidi, P. R.; Smith, G. In Electron Paramagnetic Resonance, in Specialist Periodical Reports; Royal Society of Chemistry: London, UK; 2000; Vol. 17, pp 164
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