Evaluation and assignment of proton and nitrogen hyperfine coupling

Hfcs of l-Picryl-2,2-diphenylhydrazyl

The Journal

of Physical

Chemistry, Vol, 83, No. 26, 1979 3449

Evaluation and Assignment of Proton and Nitrogen Hyperfine Coupling Constants in the Free-Radical l-Picryl-2,2-diphenyIhydrazyl. An NMR, Electron-Nuclear Double Resonance, and Electron-Nuclear-Nuclear Triple Resonance Study Reinhard Biehl," Klaus Mobius, * lbSally

E. O'Connor,lc Robert I. Waiter, *ld and Herbert Zimmermannle

Bruker Analytische Messtechnik Gmbh, D 75 12 Karlsruhe, West Germany; Institut fur Molekulphyslk, Frele Universitat Beriin, D 1000 Berlin 33, West Germany; Chemistry Department, University of Iiiinois at Chicago Circle, Chicago, Illinois 60680; and Max-Planck-Institut fur Medizinische Forschung, D 6900 Heidelberg, West Germany (Received August 13, 1979) Publication costs assisted by Freie Universitat Berlin

Deuterium NMR and proton and nitrogen ENDOR and electron-nuclear-nuclear triple resonance (TRIPLE) measurements have been performed on the stable free-radicalDPPH and ita deuterium or 15N-labeledderivatives in various solvents over the temperature range 160-400 K. The combined data permit the determination of all 12 of the proton hyperfine couplings (hfcs). Nitrogen ENDOR lines were easily detected. Algebraic signs of the hfcs were assigned by TRIPLE and NMR. Four intramolecular reorientation processes were established, and for three of these the activation energies and frequency factors were determined. Unambiguous assignment of several of the hfcs to specific proton positions in the molecule was achieved by studying the temperature dependence of these reorientation processes and also by using specifically deuterated samples. A quantitative comparison of the NMR and the ENDOR/TRIPLE results is given. Within the time scales accessible by NMR and ENDOR a consistent set of hfcs has been obtained. The room temperature ENDOR data, with small modifications, give an acceptable simulation of the experimental ESR spectrum of DPPH.

Introduction The free-radical l-picryl-2,2-diphenylhydrazyl(DPPH; the structure is shown in Figure 1, together with the numbering scheme used here) was first prepared by Goldschmidt and Renn in 1922,2and has been the subject of a variety of chemical and physical studies since that time.3 It was the first free radical for which nuclear hyperfine splitting of an ESR signal was observed; only coupling with the nitrogen 14N nuclei a t positions 1 and 2 (Figure 1) was resolved in these studies by Hutchison and c o - ~ o r k e r s ,A~ variety of methods has subsequently been applied to the precise measurement of these nitrogen coupling constants; probably the best values were determined by Hyde and co-workers6 with the ELDOR technique, Unfortunately, there is evidence that nitrogen hyperfine coupling constants in nitrogen-centered radicals are solvent dependents6 The question has not been fully investigated for solutions of DPPH, but it seems likely that the ratio of the coupling constants aN,/aNais both solvent and temperature dependent in this radical7 Years after the demonstration of nitrogen splitting, thorough solvent purification and deoxygenation enabled Deguchis to resolve a total of some 120 inhomogeneously broadened lines, ascribed to couplings from unspecified ring protons. Complete analysis of this complex hyperfine pattern of DPPH has remained a challenge to magnetic resonance spectroscopists since that time. Neither calculations of spin distribution at various levels of sophisticationg nor various methods of analysis of the ESR spectrum, for example, by comparison of the Fourier transform of the experimental spectrum with that from a simulated ESR spectrum,loa have been adequate to extract the coupling constants. The reasons lie in the complexity of the spectrum: the total number of possible hyperfine lines would be 30 375 if all of the sets of chemically equivalent 14Nand 'H nuclei are also magnetically equivalent. If molecular geometry or internal motions render chemically equivalent sites magnetically nonequivalent, then the predicted number of hyperfine components rises to 995 328 for five 14Nand twelve lH nuclei. Here and subsequently we use "chemically equivalent" to mean sites which cannot be distinguished in a chemical substitution reaction, for ex0022-365417912083-3449$0 1.OO/O

ample, deuteration on the aromatic rings. One can conclude that the observed proton hyperfine splittings in ESR result from subsets of proton couplings with roughly equal values; these subsets in turn must have values which are approximate integral multiples of one another in order to give resolved lines in such a complex system. ENDOR studies of DPPH have been reported by Dalal and coworkers,lla but there remain uncertainties in their interpretation of their spectra. In a very recent communication Dalal et al.llb reported on I4NNMR measurements of the hyperfine couplings of the nitrogen atoms in the nitro groups in the DPPH radical. We report here ENDOR and electron-nuclear-nuclear TRIPLE12 studies of ordinary DPPH and of samples of this radical which are specifically labeled with deuterium at various ring positions. The low-temperature results permit the assignment of twelve coupling constants to the twelve protons in DPPH. Studies at higher temperatures demonstrate the averaging of some of the chemically equivalent but magnetically nonequivalent nuclei as molecular motions increase. Parallel studies of DPPH in which the N1 (picryl) position is labeled with I6N permit the assignment of magnitudes and signs of the hydrazine nitrogen couplings. A preliminary account of the ENDOR and TRIPLE experiments on unlabeled DPPH is included in a review article by Mobius and Biehl.13 Complementary studies at room temperature of the contact-shifted deuterium pulse Fourier transform (PFT) NMR spectra give values for the motionally averaged coupling constants. These studies give more information than the proton NMR contact shifts reported earlier by Sagdeev et ,lalobbecause of the 42-fold line width advantage in the deuterium spectrum. The lower sensitivity of deuterium NMR can be compensated by signal averaging.

Experimental Section Preparation of Compounds. Methods have been developed for the preparation of picryl-d2 chloride and of various deuterium-labeled di~heny1amines.l~ The latter were converted in the standard way2 to the 1,l-diphenylhydrazines, which were purified and stored as their stable tosylate salts.16 Picrylation was carried out in 0 1979 American Chemlcal Society

3450

Biehi et al.

The Journal of Physical Chemistry, Vol. 83, No. 26, 1979

I1

DPPH

\

1.1-L bi RING-d,,

Flgure 1. Molecular structure of the stable free-radical 1-picryi-2,2diphenyihydrazyl (DPPH). The numbering scheme shown is used throughout the text. Possible intramolecular reorientation processes due to hindered rotation along the axes I-IV are indicated, but the ring orientations around these axes are drawn for clarity, and do not indicate orientations in the actual molecule. A and B identify the phenyl rings.

TABLE I: Identity and Nomenclature for 'H-and 15N-Labeled DPPH Derivatives abbrevn

DPPH-I 5,17-d, PIC-d, DPPH-4,1O-d, PARA-d, DPPH-2,6,8,I2-d, ORTHO-d, DPPH-2,4,6,8,10,12-d6 ORPA-d, DPPH-2,3,4,5,6,8,9,10,11,12-d,, RING-d,, DPPHJ5N, a

ORTHO-d,

d)

PARA-d,

L-I

I

2

labeled compda

c)

no.

1 2 3 4 5 6

Numbering of the labeled positions is shown in Figure

1.

aqueous methanol solution by direct reaction of these tosylate salts with picryl chloride in the presence of sodium carbonate.16a Oxidation of the resulting l-picryl-2,2-diphenylhydrazines to the radicals was carried out with lead dioxidee2 Yields over the five reaction steps from diphenylamine to recrystallized hydrazyl radical fall in the range 50-70%. The deuterium-labeled DPPH radicals prepared in this way, together with abbreviations which will be used for them subsequently, are given in Table I. Deuterium label was incorporated in these radicals at the 94-98% level in all cases except PARA-d2,which was found to contain only ca. 65% label. Label estimates are based upon mass spectra of the diphenylamines, and (more reliably) upon computer simulations of the proton ENDOR or TRIPLE signals of these radicals. DPPH-15N1was prepared from diphenylamine by diazotization with 15N sodium nitrite to give N-[15N]nitrosodiphenylamine, which was reduced to [l5N21-l,ldiphenylhydrazine and isolated as the hydrochloride.16b The free base was treated with picryl chloride to give 1-picryl- [l5N1]-2,2-diphenylhydrazine; subsequent oxidation with lead dioxide gave the desired DPPH-15N1.16c Sample Preparation for N M R Studies. The hydrazyl samples were recrystallized from carbon disulfide and dried in a vacuum desiccator over night prior to use. Solution samples for PFT runs were prepared by stirring excess crystalline hydrazyl for 1 h in spectrograde chloroform to which was added less than 1%chloroform-d as the diamagnetic marker. During the last 10 min of stirring, we added a small amount of solid lead dioxide to the solution to oxidize any remaining hydrazine impurity to the radical. The solutions were then filtered directly into the 5-mm NMR tubes. The concentration of DPPH near saturation in chloroform is ca. 0.6 M. Sample Preparation for ENDORI T R I P L E Studies. The recrystallized hydrazyl samples were pumped on a high vacuum line for several hours. As solvents toluene

1

0

1

2

3

4

5kHz

Figure 2. Deuterium PFT spectra of labeled derivatives of DPPh, taken at 295 K and a frequency of 13.8195 MHz. The reference line for the frequency scale is chloroform-d. Spectra shown are the average of 2048 scans except for PIC-d2, which is the average of 512 scans.

or isopentane, both dried over CaH2 in vacuo, and mineral oils (G 17, G 33 Shell AG, Hamburg), redistilled under vacuum, were used. The solvents were added to the hydrazyl samples under vacuum. For the low temperature measurements a mixture of 50150 toluene-isopentane was used. For the measurements in the temperature range 220-400 K mixtures of toluene and the mineral oils were used; the solvent ratio was optimized for minimum ENDOR line width. Radical concentrations were in the region of 5 x 10-4 M. Instrumentation for Recording NMR Spectra. The deuterium NMR studies were done by the PFT technique on a Bruker B-KR 3225 spectrometer interfaced to a Nicolet 1080 data system. Spectra were taken at 295 f l K at a pulse radio frequency of 13.8195 MHz. Magnetic field drifts and inhomogeneities were minimized by spinning the samples and by locking the field. Sample solvent signals were too broad for an efficient lock, so the protons in tetramethysilane contained in a coaxial tube provided the lock signal. Lines obtained by this method are at least an order of magnitude narrower than spectral lines recorded on these paramagnetic samples without use of sample spin and field lock; for example, deuterium markers then give signal line widths less than 9 Hz. Because of the lower sensitivity of deuterium compared to protons, long-term signal averaging is required. The free induction decay signal was sampled every 50 ps to give a 10-kHz scan range in the frequency domain. Each spectrum was obtained from up to 2048 accumulations. There was no need for further signal-to-noise improvement by data manipulation in the time domain. The real parts of typical Fourier transformed NMR signals are plotted in Figure 2. Proton NMR spectra were also recorded for these samples under conditions which produced overlapping sideb a n d ~ .Samples ~~ were again close to 0.6 M in 10-mm 0.d. NMR tubes. Rather poorly defined NMR spectra were obtained which could not be analyzed by the sideband spectrum deconvolution method. We ascribe this failure to the distinctly lower solubility of the DPPH samples compared to those radical cations to which this method was successfully applied.17 Recording ENDOR f T R I P L E Spectra. ENDOR and TRIPLE experiments were performed both in Berlin and

The Journal of Physical Chemistry, Vol. 83, No. 26, 1979 3451

Hfcs of l-Picryi-2,2-diphenylhydrazyi

TABLE 11: Coupling Constants from Deuterium PFT Studies of Deuterated Hydrazyl Radicals positions D contact ref chem line D couplin compd

o n ringsa

shift,b ppm

shift,c ppm width? Hz

derived proton coupling,g MHz

constants:>fG

15,17 -76.51 1.48 169 0.152 i 0.001 2.777 3, 5 , 9 , 1 1 -54.42 -0.01 102 0.111 * 0.001 2.023 2, 6, 8, 1 2 113.06 -0.16 500 -0.232 It 0.004 -4.238 475 -0.233 i 0.004 -4.256 2, 6, 8, 1 2 113.97 -0.16 4,lO 114.33 -0.27 590 -0.234 * 0.004 -4.275 4,lO 113.97 -0.27 475 -0.234 f 0.004 -4.275 a See Figure 1 for the numbering scheme. Contact shift is measured relative to the internal marker CDCl,, a t 7.25 ppm, Reference chemical shifts are observed in the diamagnetic hydrazine, relative to the CDCl, reference a t 7.25 vs. Me$. Line widths are determined from the separation of the maxima ppm. Positive shift is downfield from the reference line. and minima in the imaginary part of the Fourier transformed spectra. e Uncertainties in line positions are taken as 5% of the line widths. f Proton coupling constants are computed from the deuterium coupling constants by multiplication by the factor 6.514. g Conversion of hfcs t o MHz is based upon the relationship C L M H=~ 2 . 8 0 2 5 ( g ~ p p ~ / g =~ 2.8044agms, )u~~~~~ since the g factor for DPPH is 2.00366. PIC-d, RING-d ORTHO-d RING-d ,o PAR A-d RING-d,,

in Karlsruhe. In Berlin a Varian E 12 ESR spectrometer equipped with a home-built ENDOR/TRIPLE attachment was used. The experimental setup has been described e1~ewhere.l~ In Karlsruhe the spectra were run on the new high power broad band extension of the Bruker ER 200 ENDOR/TRIPLE attachment to the ER 220 ESR spectrometer (12-in. magnet with ER 036 flux stabilizer, and BV-T-1000 temperature control unit for sample cooling). In this setup samples with up to 5-mm 0.d. could be used. All the ENDOR/TRIPLE spectra shown in Figures 3-6 were obtained by single scans with time constants of typically 3 s. The spectra were recorded with minimum rf power to optimize spectral resolution.

DPPH

I

I

p!

Results and Discussion Deuterium NMR Studies. Under the operating conditions described, the samples fulfill the conditionlafor fast Heisenberg spin exchange: Te[’

Ye

>> an,an

I

10

Here T,, is the characteristic time for Heisenberg electron spin exchange, a, the nuclear hyperfine coupling (hfc) in gauss, and 7, the magnetogyric ratio of the electron. The coupling constant is then given in gauss byla

In this equation, g, and De are the electron g factor and the Bohr magneton, and 7, is the nuclear magnetogyric ratio. B, is the magnetic field at which a nucleus n in a diamagnetic molecule gives its NMR signal, and AB is the shift in that signal when the same nucleus is observed in a paramagnetic molecule. Insertion of appropriate values for the constants gives (for T = 295 K) UD =

-2.0458

X

10-3(AB/BD)

(3)

for deuterium NMR, where (aB/BD) is in parts per million. Proton chemical shifts were calculated from the deuterium data by multiplication by the ratio of the magnetogyric ratios, YH f y~ = 6.514. Representative deuterium NMR spectra of labeled samples of DPPH are given in Figure 2. The downfield signals from the PIC-d2 and RING-dElosamples clearly show that the meta positions in the picryl ring are not equivalent to those in the phenyl rings. Sites in these two sets are averaged on the NMR time scale in these spectra. The broader signals due to ortho deuterons in ORTHO-d4 and to para deuterons in PARA-d2yield less precise data; these positions give the same contact shifts within the precision of our NMR data. Quantitative results are

I

I

12

I

O

li,

I

16

Figure 3. Low-power ENDOR spectra of “all proton DPPH” at 180 and 300 K. Most line positions are strongly temperature dependent: shifts with temperature are marked by arrows.

summarized in Table 11. Each contact shift reported in column three is the average value obtained from a t least five spectra. There was no observable scatter in the line separations, recorded as cursor intervals, for the lines due to meta deuterons. Data for the broader ortho or para deuteron lines scatter within one cursor interval, which is equal to 0.71 ppm under the conditions used for recording these spectra. Thus, scatter is 140% of the estimated uncertainties given in column six for the coupling constants. These uncertainties were estimated on the assumption that the recorded line position has an uncertainty equal to approximately 5 % of the root mean square line width. No effort was made to investigate temperature effects by studying these nearly saturated solutions a t lower temperatures. Indeed, the two-site jump effects observed in the ENDOR/TRIPLE spectra prove that the jump rates for this molecule are beyond the detection range of the NMR experiment. ENDOR f TRIPLE Studies. Assignment Of Hyperfine Couplings. The low-power proton ENDOR spectra of DPPH at 180 and 300 K are given in Figure 3. The spectrum a t 180 K shows immediately ten different pairs of lines centered symmetrically around the free proton frequency, vp. From this it follows that the symmetry group for the DPPH radical is AI, as is indicated in the representation of the molecule in Figure 1. By reducing rf and microwave power levels the doublet around 15.5 MHz in Figure 3 as well as its counterpart on the low-

3452


Biehl et al.

TABLE 111: Summary of ENDOR/TRIPLE Values for Proton Hyperfine Coupling Constants (ai)and Their Correlation by Intramolecular Jump Processesa intramolecular jump processesa proton positionsb

ai,cad

MHZ

Ie

11, I11

ring A, 4 (para) ring A, 2 or 6 (ortho)

- 5.819

-5.542

ortho phenyl

ring A, 6 or 2 (ortho) picryl ring, 1 5 or 1 7 (meta) ring B, 8 or 1 2 (ortho)

-5.328 +4.180 - 3.334

ortho phenyl

B, 12 or 8 (ortho) B, 10 (para) A or B (meta) B or A (meta)

- 3.234

+2.378 +2.378

ring B or A (meta) picryl ring, 1 7 or 15 (meta) ring A or B (meta)

12.000 t1.773 11.395

ring ring ring ring

IV f

11, IIIg

11, I11

para phenyl

1

meta picryl ortho henyl

P

ortho phenyl

I

I

para phenyl

- 3.059

meta phenyl -

I

I

meta picryl

meta phenyl

t

meta phenyl

meta phenyl

K: a l ’ ~ =, ~ 3 8 . 3 2 0MYz and a I 4 ~=, +22.300 MYz. The numbering scheme and designation of rings are given in Figure 1. Double entries for position designations are used in Arrows cases where the unambiguous assignment to either position is impossible. Observation temperature was 180 K. connect entries for positions which are averaged by each of the jump processes I-IV at higher temperatures. e Process I: E , = 0.224 eV i. 4%; h , = *O.’) s - ‘ . f Process IV: E , is too large to determine at ENDOR frequencies. g Processes ‘03) s- I . 11, 111: E , = 0.325 eV f. 4%; k , = a Two nitrogen hfcs were determined by ENDOR studies at 260

frequency side are resolved into a triplet (see Figure 4). If we assume a Lorentzian line shape in a least-squares fit of this triplet, then hfcs, intensities, and line widths can be evaluated. This procedure gave the 11th hfc out of 1 2 possible proton couplings. The missing hfc can either be hidden by an accidental degeneracy in the hfcs of two protons, or it may be small compared with Tp;l, the transverse electronic relaxation rate of the radical. In the latter case, the ENDOR enhancement of the corresponding transition approaches zero, as was shown by Allendoerfer and Maki.lg The signal-to-noise ratio can be further improved by about a factor of 4 with the aid of high power special TRIPLE,12without any significant change in the appearance of the spectrum. Observation of the ENDOR spectrum over the temperature range to 400 K establishes the existence of several intramolecular jump processes. In Figure 3 some of these changes are indicated by arrows which connect corresponding lines in the low and fast jump limits. Analysis of these changes permits the unambiguous assignment of the position of the missing 12th proton line: the signal at 2.378 MHz (at 180 K) must average with both the 1.395 and the 2.000 MHz signals by jump processes which are fast a t room temperature. Consequently, the 2.378-MHz signal must represent two protons. These are indicated by arrows which connect corresponding lines in the slow and fast jump limit. Assignment of the individual hfcs to specific positions in the DPPH molecule was performed by recording ENDOR/TRIPLE spectra of the specifically deuterated DPPH compounds (Table I). The ENDOR resonance condition for a doublet radical in solution is given by (4) un* = Ivno Iant’2lI where u+ and u- are the high- and low-frequency lines of nucleus n measured relative to the free Larmor frequency uno = (y,/27r)Bo and an is the hfc in frequency units. Because yH/yDE 6.5, deuterium ENDOR lines will normally not overlap the proton ENDOR spectrum. The high-power special TRIPLE spectra of “all proton DPPH” and of compounds 1 , 2 , 3 , and 6 at low temperatures are given in Figure 5. The spectrum of PARA-d2 still exhibits the para phenyl proton signals to quite a large extent, reflecting incomplete labeling in this sample.

*

Figure 4. Part of the proton ENDOR spectrum of “all proton DPPH” obtained under high-resolution conditions (top). A fit to this spectrum by using three Lorentzians gives hfcs, line widths, and intensity ratios (bottom).

Quantitative data from these spectra are summarized in Table 111. The large difference in hfcs for the two para protons obviously stems from the possible gauche-anti conformations about the N1-N2 bond of the picryl ring relative to the phenyl rings. The same behavior is found for the four ortho phenyl protons, which are assigned by comparing the spectra of PIC-d2 and ORTHO-& On the basis of their temperature dependence (see below) these ortho protons can be assigned to two groups which belong to the two phenyl rings. Within each group the inequivalence of the ortho protons is rather small (100 and 215 kHz, respectively), while the ratio of the average ortho hfcs of the groups is 1.65. This is comparable to the ratio of 1.9 obtained for the para phenyl hfcs. For theoretical reasons discussed below, the larger ortho and para hfcs can be assigned tentatively to one phenyl ring, while the



o

1

2

3

3,

LMHZ

I

1 is more twisted out of the molecular x-y plane than the other phenyl ring (see Figure 1). If DPPH in solution at low temperature approaches the geometry in the benzeneDPPH crystal reported by then ring A has the smaller and ring B the larger twist angle, as shown in Table 111. This also is suggested by inspection of space-filling molecular models. We refrain from assignment of the meta protons to rings A and B because the MO calculations are less certain at these positions due to the perturbations of the proximate picryl group. ENDORI TRIPLE Studies. Analysis of Hindered Rotation Processes. The clean spectrum of RING-dlo in Figure 5, which exhibits only the two meta picryl protons, shows a classic jump process behavior due to the hindered rotation of the picryl ring when the temperature is varied from 170 to 250 K. The line pattern could be fitted over this temperature range by using the modified Bloch equations.23 By this procedure picryl ring jump rates assigned to process I in Figure 1 were determined. An Arrhenius-type plot of these jump rates is depicted in Figure 7; it shows the expected linear dependence of In k on T-l. From this plot the activation energy, E,, and the preexponential frequency factor, ko, for the hindered rotation of the picryl ring were extracted (Table 111). The same procedure applied to the temperature dependence of the ENDOR spectra of compounds 1 and 4 yielded activation energies and frequency factors for processes I1 and I11 (see Table 111). These spectra confirm the “grouping” of meta and ortho phenyl protons already proposed on the basis of the grouping of coupling constants. Our initial hope that the thermodynamic data would permit unambiguous as-

= +38.320 MHz and

I w-

0

1

2

3

LMHz

Figure 5. High power “special TRIPLE” proton spectra of “all proton DPPH” and of the deuterated compounds at 180 K. Abbreviations used are listed in Table I. The frequency scale gives the difference frequency of the two rf carriers with respect to the free proton frequency, up. Consequently, hfcs can be directly obtained from the line positions by multiplication of the frequency values by two.

smaller ones must belong to the other. The situation is more complicated for the four meta phenyl protons, as can be seen in Table 111. Assignment to sets associated with the two phenyl rings can be carried out by analysis of the temperature dependences of the four hfcs. Association of these two sets of meta with the sets of ortho and para protons which belong to individual rings is a t this stage impossible. The two meta protons on the picryl ring can be assigned in a straightforward manner from the spectrum of RING-d,,. The signs of the hfcs were determined by the general TRIPLE resonance technique.13 By performing these experiments for compounds 1-6 and for the “all proton DPPH”, we obtained relative signs for all the hfcs. The data were self-consistent even for those regions in the spectra where lines strongly overlap. To obtain the absolute signs, TRIPLE experiments were performed in which nitrogen nucleus N1 was taken as the reference nucleus. Because of the magnitude of this nitrogen hfc, on theoretical grounds this value can safely be assumed to be positive. Since ENDOR lines of the 14N nuclei in positions 1 and 2 overlap with the proton ENDOR lines, a specifically 15N-labeled DPPH, 6, was studied. Since ysN/ y14~= 1.40, the 15Nlines then are shifted out of the proton ENDOR spectrum (Figure 6, spectrum at 260 K). The signs of the hfcs obtained by the TRIPLE method are in accordance with the NMR results. The frequencies of these nitrogen ENDOR lines give the nitrogen hfcs: aisNi

3454


Biehl et al.

TABLE IV: Comparison of Proton Hyperfine Coupling Constants from NMR and ENDOR Data

In (khec-')

proton position

from NMR'

av ENDORiTRIPLE datab

meta picryl meta phenyl ortho phenyl para phenyl

+0.988 t 0.725 -1.52 -1.52

+1.06 t 0.727 .-1.55 -1.58

' Data recorded a t 295 K. Fully averaged by motional processes I-IV o n NMR time scale. Arithmetic averages of experimental hfcs recorded a t 180 K. Data for chemically equivalent sites are averaged, -4

TABLE V: Proton Hyperfine Coupling Constants (in Gauss) at Room Temperature; Direct Measurements vs. Averaged Low-Temperature ENDOR Data

a:

-3

- 2 4.0

4.5

5.0

5.5 \

ENDOR data, 300

K' ,

I

1/T x 1O3 [K-'1 Figure 7. Arrhenius plot of the picryl ring jump rate about the C13-N1 axis (process I), from the temperature dependence of the meta picryl proton ENDOR pattern.

signment of phenyl protons to the individual phenyl rings was not sustained because, within experimental error, processes I1 and I11 are characterized by identical values for E, and ko. These values are in the range which is typical of hindered rotation of phenyl substituent^.^^^^^ The jump process denoted by IV in Figure 1 could not be studied, since within the accessible temperature range the positions of the para phenyl ENDOR lines change very little, that is, the system just begins to move from the slow jump limit. One might expect process IV to be lower than processes 1-111, since the N1-N2 bond meets the requirements for a Pauling three-electron bond,26which has a higher rotational barrier than the conventional single bond. The existence of process IV can be deduced by comparison of the NMR and ENDOR hfc data in Tables I1 and 111. In NMR only one time averaged hfc, d , ( N m ) = -4.29 MHz, is observed, while in ENDOR in the temperature range up to 400 K two different para hfcs are obtained at 180 K, with u*,(ENDOR) = -5.819 MHz and uB,(ENDoR) = -3.059 MHz. At 300 K the ENDOR values change to -5.639 and -3.239 MHz, respectively (see Figures 3 and 4). On the basis of a comparison of the NMR and ENDOR data, we estimate that the rate constant for process IV at room temperature is in the range lo4 s-l C k C lo6 s-l, The difference in behavior of processes I-IV in the NMR and ENDOR experiments stems from the fact that in NMR the observed Fermi contact shifts are only some kHz, while in ENDOR the hfcs are directly observed since they are of order of some lo3 kHz. Changes in the spectra due to jump processes can only be detected if 0.1 5 Aolk 5 10 slow jump limit fast jump limit where A w is the difference in the hfcs of the two sites and k is the jump rate. Therefore, when the ENDOR experiment is in the slow jump region, for the same value of the rate constant k the NMR experiment is in the fast jump . region, since AwNm is so much smaller than A W E ~ O RAs a consequence, only the average hfcs are observed by NMR. The average over processes I-IV of the low-temperature ENDOR hfcs should yield values which correspond to the

Proton position assignedd

ENDOR av over I, 11, IIIc

Proton position assignedd

( + + ) 0.706 meta phenyl t 0 . 6 7 3 meta phenyl ( tt ) 0.788 meta phenyl t 0.781 meta phenyl (--) 1.041 ortho phenyl -1.172 ortho phenyl, ring B (-) 1.155 para phenyl (--) 1.893 ortho phenyl (-) 2.011 para phenyl

-1.092 -1.939 -2.076

para phenyl, ring B ortho phenyl, ring A para phenyl, ring A

a Values arranged in order of increasing absolute magnitude. The number of protons per resolved line is indicated by the number of times the algebraic signs are reproDirect assignments not availduced in the parentheses. able. These tentative assignments are based upon the trends observed in the variable temperature studies. Arithmetic averages over processes I, 11, and I11 of the exAssignments based perimental hfcs recorded a t 180 K. upon variable temperature studies of DPPH, together with low temperature studies in the deuterium-labeled samples. e Meta picryl positions not fully averaged a t room temperature.

data obtained by the NMR measurements. The values are given in Table IV; clearly the agreement is good, with differences in the range tip to 7 90. Table V contains our ENDOR data at 300 K, arranged in order of increasing absolute magnitude of the coupling constants. The detailed data on temperature effects and on deuterium-substituted samples which are the basis for our assignments of coupling constants to specific ring sites at 180 K are not available at 300 K, so it is impossible to relate these values to specific rings. However, it is possible to use the trends in hfc values obtained over part of the temperature range from 180 to 300 K to relate the low temperature data to those at room temperature. Tentative assignments to molecular sites on this indirect basis are indicated. For comparison, the averages over jump processes 1-111 of the data recorded a t 180 K are also listed, together with their assignments to ring sites. Clearly the two sets of data correspond closely, within an average error of ca. 5% of their values. Furthermore, the assignments made on a tentative basis for the 300 K data correspond with those made rigorously for the 180 K data. The relatively small deviations between experimental and averaged data in Tables IV and V are substantially larger than the estimated error of f0.005 MHz in the proton line positions. The differences thus appear to be real. They might result from the use of different solvents for the various measurements. They might also result from the use of the theoretical value -yH/-yD = 6.514 to convert the deuterium NMR contact shifts to hfcs. There is good evidence that replacement of hydrogen by deuterium on the aromatic carbon atoms of free radicals can produce



0.73

a

(-0.39)

-1 55 7

073

NO* 1-0.39)

DPPH, fast jump limit Figure 8. Map of hyperfine coupling constants (in gauss) for DPPH in the fast jump limit. All values are taken from ENDOR data in this work except for the picryl I4N hfcs given in brackets, which are from ref l l b . The I4N, hfc was calculated from our measured 15N1hfc by multiplicatlon with y14,/y16, = 0.7129. The proton values listed are obtained as the arithmetic averages over ENDOR data recorded at 180 K, where the 12 protons display 12 unique coupling constants. (Two of these are accidentally identical.) This averaging was carried out with respect to jump processes I-IV to permit direct comparison with the NMR data, since all NMR measurements are made under conditions which fulfill the requirements for the fast jump limit.

isotope effects on the magnitudes of coupling constants as large as several percent.27

Conclusion We believe that this analysis of the DPPH hyperfine structure is a particularly nice example of the power of a combination of the various magnetic resonance techniques. Application of the full armory of NMR, ESR, ENDOR, and TRIPLE resonance gives a consistent picture of the electronic structure and the dynamic processes of DPPH. With the addition of the most recent 14N NMR data for the nitro groups of the picryl ring,llb we collect in Figure 8 a map of the hyperfine coupling constants of DPPH in the fast jump limit. This set of data now awaits rationalization in terms of a consistent MO description. Our results differ from the considerable number of reported values for the coupling constants in DPPH based upon some type of analysis of the ESR, ENDOR, or NMR spectrum of this radical; indeed, these values are not consistent with one another.10J1i28Our data show that averaging over process I is not quite complete on the ESR time scale a t room temperature. Averaging over process IV is not achieved under these conditions. As a result, there are two sets of coupling constants for the two phenyl rings, and the magnetic equivalence of all chemically equivalent sites which is generally assumed in fitting the ESR spectrum is an unwarranted oversimplification. Successful simulation of the experimental ESR spectrum has generally been taken as evidence for correct assignments of coupling constants for a free radical. There are dangers in this assumption, particularly for spectra with broad lines;29generally these will be reproduced equally well by more than one set of hfcs. The experimental ESR spectrum for DPPH has been published on a number of occasions;828J@11a we will not repeat it here. The published spectra are characterized by regularly spaced lines a t an interval variously reported in the range 0.35-0.40 G. There are variations in the apparent intensities of symmetrically placed lines which presumably arise from variations in the line width. Line intensities are maintained in the regions which connect segments of the spectrum which arise from coupling with the various spin states of nitrogen nuclei N1 and N,; this requires that contributions from the overlapping segments must reinforce in the overlap regions.

= 0.39 Flgure 9. Downfield ESR half-spectra simulated with = 0.48 G,’Ib and various values for the hfcs for protons G and aMnko) and the hydrazine nitrogens. (Curve A) Single segment simulated with proton hfcs for the fast exchange limit. (Curve B) Single segment with 180 K ENDOR proton couplings. (Curve C) Proton hfcs from 180-K ENDOR, and ENDOR values for aN1= 9.74 G and aN2= 7.95 G. (Curve D) Same as curve C, except aN changed to 9.94 G. (Curve E) Simulation with proton hfcs from 30b-K ENDOR, and hydrazine nlrogen couplings as in curve C. (Curve F) Proton hfcs as In curve E, with aN, = 9.54 G and = 7.95 G.

We first attempted to simulate the spectrum with proton coupling constant values averaged over processes I-IV (Table IV). The results, shown for one segment of the spectrum in Figure 9A, are unsatisfactory because line spacings and intensities do not display the regular overall pattern observed for DPPH. When the 180-K ENDOR hfcs averaged arithmetically over processes 1-111 (Table V, column 3) are used, the results are much better (see Figure 9B). Inclusion of the coupling constants for the hydrazine nitrogens in the simulation gives the familiar five-segment pattern of DPPH. Its appearance is sensitive to the values for these large coupling constants. Thus, simulation with the ENDOR values for uN1 and aNz gives poor results due to destructive interference in the regions which join the center section of the spectrum to the outer sections (see Figure 9C). Adjustment of either of these values by 0.1 or 0.2 G improves the spectrum. We give in Figure 9D the simulation with aN1 = 9.94 G and UN, = 7.95 G. Note that this is an adjustment in the value of uN1 of only ca. 3 % . When the 300-K ENDOR values (Table V, column 1) are used for the simulation, the spectrum in Figure 9E is obtained. This shows some destructive interference a t the spectrum center. Again, adjustment of either uN1or uN2 improves the simulation; spectrum 9F is obtained with uN1

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= 9.54 G and uN2= 7.95 G. The simulation is essentially identical if the values are 9.64 and 8.05 G. The simulations in parts D and F of Figure 9 are qualitatively equally good, but quantitative examination shows that Figure 9F is about 1G narrower. Consequently, the interval between the first and last maxima corresponds better to the experimental spectrum. The spacing of the closest lines varies in the reported spectra, but averages about 0.02 G less than in these simulations. This spacing is only weakly dependent upon the values used for uo,p(nitro).Thus, reduction of ao(nitro) from 0.39 to 0.35 decreases this line spacing by less than 0.02 G. Reduction in up(nitro)also affects the spacing only weakly, and introduces some bizarre changes in the shape of the spectrum envelope. It is clear that, while the appearance of the spectrum is quite sensitive to small changes in at least some of the hfcs, it cannot be used to establish the superiority of a particular set of values. In all of our simulations, the line width used is 0.14 G, the minimum value we found necessary to remove structure on the individual lines of the spectrum. Note that the spectrum produced by sets of 2 , 2 , 1 , 2 , 2 , 1 , 2hydrogen and 2, 1, 1, 1 nitrogen atoms contains 131 220 lines. The observed spectrum contains about 150 lines, and is so underdetermined that we consider it rash to claim superiority for a particular set of parameters on the basis of a qualitative fit to the ESR spectrum. Instead, we consider the consistency of the data reported here for DPPH at 180 K, at room temperature, and in the fast jump limit to be the best evidence for their validity. The simulations do support our claim that averaging over processes 1-111, but not IV, occurs at room temperature. This has been suggested previously by Balaban and c o - w ~ r k e ron s ~ the ~ basis of their analysis of the ESR spectra of certain DPPH analogues.

Acknowledgment. Support for early stages of the work at UICC was provided by Grant GP 33518X from the National Science Foundation. Summer support for S. E.O'C. was provided by grants from the UICC Research Board. Computer services were provided by the UICC Computer Center. The work at Berlin was supported by the Deutsche Forschungagemeinschaft (Sfb 161). Many helpful discussions with Dr. M. Plato are gratefully acknowledged.

References and Notes (1) (a) Bruker Analytische Messtechnik GmbH and Freie UniversttitBerlin. (b) Frele Universitat Berlin. (c) University of Illinois; work taken from the Ph.D. thesis of S.E.O'C.; (d) University of Illinois. (e) MaxPlanck-Institut. (2) S. Goldschmldt and K. Renn, Berichfe, 55, 628-643 (1922).

van der Waals Work up to 1966 or 1967 is reviewed in A. R. Forrester, J. M. Hay, and R. H. Thomson, "Organic Chemistry of Stable Free Radicals", Academic Press, New York, 1968, Chapter 4. C. A. Hutchison, R. C. Pastor, and A. G. Kowalsky, J . Chem. Phys., 20, 534-535 (1952). J. S.Hyde, R. C. Sneed, Jr., and G. H. Rist, J . Chem. Phys., 51, 1404-1416 (1969). (a) For nitroxyl radicals: Y. Y. Lim and R. S.Drago, J. Am. Chem. SOC.,93, 891-894 (1971). (b) For para-substltutednitrobenzene anion radicals: P. L. Kolker and W. A. Waters, J . Chem. SOC., 1136-1141 (1964). (a) N. S.Garifyanov, A. V. Ilyasov, and Yu. V. Yablokov, Lbk/. Akad. Nauk SSSR, 149, 876-879 (1963); (b) Yu. M. Ryzhmanov and A. A. Egorova, ibid., 191, 148-150 (1970). Y. Deguchi, J. Chem. Phys., 32, 1584-1585 (1960). (a) R. I.Walter, J . Am. Chem. Soc., 88, 1930-1936 (1966); (b) V. A. Gubanov, V. I.Koryakov, A. K. Chlrkov, and R. 0. Matevosyan, Zh. StrUkt. Khim., 11, 941-946 (1970); 12, 538-541 (1971). (a) V. A. Gubanov, V. I.Koryakov, and A. K. Chirkov, J. Magn. Reson., 11, 326-334 (1973); (b) R. E. Sagdeev, Yu. N. Molin, V. I. Koryakov, A. K. Chirkov, and R. 0. Matevosyan, Org. M g n . Reson., 4,365-368 (1972). (a) N. S. Dalai, D. E. Kennedy, and C. A. McDowell, Chem. Phys. Lett., 30, 186-189 (1975), and precedlng papers; (b) N. S. Daiai, J. A. Ripmeester, and A. H. Reddoch, J . Magn. Reson., 31, 471-477 (1976). (a) K. P. Dlnse, R. Biehl, and K. Moblus, J . Chem. Phys., 61, 4335-4341 (1974); (b) R. Blehl, M. Plato, and K. Moblus, ibid., 63, 3515-3522 (1975). K. Moblus and R. Biehl In "Multiple Electron Resonance Spectroscopy", M. M. Dorio and J. H. Freed, Ed., Plenum Press, New York, 1979. Unpublished work of W. A. Doak and G. Putz with R. I.Waiter. S.E. O'Connor and R. I.Walter, J . Org. Chem., 42, 577 (1977). (a) M. M. Chen, A. F. D'Adamo, Jr., and R. I.Walter, J. Org. Chem., 26, 2721-2727 (1961); (b) K. Clusius and M. Vecchi, Helv. Chlm. Acta, 38, 933-937 (1953); (c) R. H. Polrler, E. J. Kahler, and F. Benlngton, J. Org. Chem., 17, 1437-1445 (1952). G. A. Pearson and R. I.Walter, J. Am. Chem. Soc., 99, 5262-5266 (1977). For general reviews, see (a) R. W. Krelllck In "NMR of Paramagnetic Molecules", 0. N. LaMar, W. Dew. Horrocks, Jr., and K. H. Holm, Ed., Academlc Press, New York, 1973; (b) E. DeBoer and H. van Wllligen in "Progress in NMR Spectroscopy", Vol. 2, J. W. Emsley, J. Feeney, and L. H. Sutcllffe, Ed., Pergamon Press, Oxford, 1967. R. D. Allendoerfer and A. H. Maki, J . Magn. Reson., 3, 396-410 (1970). R. Blehl, K. Hinrichs, H. Kurreck, W. Lubltz, U. Mennenga, and K. Roth, J . Am. Chem. SOC.,99, 4278-4286 (1977). R. Biehl, Ph.D. Thesls, Free University of Berlin, 1974. D. E. Williams, J . Am. Chem. Soc., 88, 5665-5666 (1966). H. S. Gutowsky, D. M. McCall, and C. P. Slichter, J . Chem. Phys., 21, 279-292 (1953). M. Plato, R. Biehi, K. Mobius, and K. P. Dinse, 2.Naturforsch. A , 31, 169-176 (1976). C. v. Borczyskowski and K. Moblus, Chem. Phys., 12, 281-290 (1976). L. Pauling, "The Nature of the Chemical Bond", 3rd ed, Cornell University Press, Ithaca, N.Y., 1960, pp 341-343. R. G. Lawlor and 0. K. Fraenkel, J . Chem. Phys., 49, 1126-1139 (1966); R. G. Lawlor, J. R. Bolton, M. Karplus, and 0. K. Fraenkel, ibld., 47, 2149-2165 (1967). 2. Panlotis and H. H. Gunthard, MI".Chlm. Acta, 51, 561-564 (1968). 0. A. Pearson, M. Rocek, and R. I.Walter, J. Phys. Chem., 82, 1165-1192 (1976). A. T. Balaban, M. T. Capriou, N. Nagolta, and R. Baican, Tetrahedron, 33, 2249-2253 (1977), and references quoted there.

Research on Quantum Beats in Phosphorescence at Leiden J. H. van der Waals Center for the Study of Exclted States of Molecules, Huygens Laboratory, Lelden, The Netherlands (Recelved September 18, 1070) Pubilcatlon costs assisted by Ruksuniversitelt Lelden

G. N. Lewis and his school1 identified the optical pumping cycle underlying the phosphorescence of organic molecules. On excitation into a higher singlet state, intersystem crossing (ISC) occurs to the metastable triplet state T, from which the molecules decay back to the ground state So with the emission of light. Weissman, in a n ingenious experiment, showed t h a t this 0022-3654/79/2083-3456$0 1.OO/O

"phosphorescence" corresponds to electric dipole radiationS2 We specifically consider azaaromatic molecules with an n r * state S1 and r r * state T. One then expects the spin-orbit coupling (SOC) to occur locally near the N nucleus and such that in the ISC process the spin S = 1 is "created" with S in the " n r plane" determined by the 0 1979 American Chemlcal Society

Evaluation and assignment of proton and nitrogen hyperfine coupling

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