9132
J . Phys. Chem. 1992,96,9132-9139
= l/dl P2 = l/d2 = P I - p2 COS CY = p2 - PI COS a PI
71
T2
Po
= 1/Mo
= l/MCl 1/MHzO Mo is the mass of H 3 0 + minus 2 protons Mcl is the mass of C1- H 2 0 MHl0 is the mass of the H20 molecule PCl
PHzO
+
References and Notes (1) Triolo, R.; Narten, A. H. J . Chem. Phys. 1975, 63, 3624. (2) Gigu€re, P. A. J . Chem. Educ. 1979, 56, 571. (3) Walrafen, G. E.; Chu, Y. C. In Proton Transfer in Hydrogen-Bonded Systems; Bountis, T., Ed.; Plenum: New York, 1992.
(4) Ratcliffe, C. I.; Irish, D. E. In Water Science Reviews; Franks, F., Ed.; Cambridge University Press: Cambridge, 1988. (5) Hams, D. C. Quantitative Chemical Analysis; Freeman: New York, 1982; p 158. (6) Narten, A. H., private conversation, 1992. (7) Cotton, F. A,; Horroch, W. D., Jr. Spectrochim. Acta 1960,16,358. (8) The present C, model involves a 6 X 6 CF matrix. This matrix may be. reduced to a 2 X 2 matrix, whose solution is easy, and to a 4 X 4 matrix, whose solution is difficult bccause it is a nonsymmetric matrix. C and F are each symmetric matrices, but GF is generally a nonsymmetric matrix, unless [C,F]= 0. One cannot use Jacobi's method; see: Maron, M. J. Numerical Analysis; Macmillan: New York, 1982, to obtain the eigenvalues for nonsymmetric matrices. However, we were able to calculate the eigenvalues for our 4 X 4 matrix, readily, using the HQR algorithm for real Hessenberg matrices, see: Vetterling, W. T.; Teukolsky, S.A.; Press, W. H.; Flannery, B. P. Numerical Recipes; Cambridge University Press: New York, 1990. (9) Placzek, G. Rayleigh-Streuung und Raman-Effekt. In Handbuch der Radiologie; Marx, E., Ed.; Akademische Verlag: Leipzig, 1934; Vol. VI, 2, pp 205-374. (10) McQuarrie, D. A. Statistical Mechanics; Harper and Row: New York, 1976. For the BowEinstein correction to the measured Raman intensity, see. p 472, q s 21-17 and 21-18. For a discussion of the isotropic Raman s p c c t " and derivations thereof, see pp 482-489. For G r a d h a r l i e r analysis leading to Gaussian Raman line shapes resulting from dipoltdipole interactions, see p 542. (11) Chu, Y. C. Doctoral dissertation, Physics Department, Howard University, Washington, DC, May, 1991. (12) Walrafen, G. E.; Chu, Y. C.; Hokmabadi, M.S.J . Phys. Chem. 1990, 94, 5658. See eq 1 on p 5660 and discussion thereof. (13) Nakamoto, K. Infrared and Ramon Spectra of Inorganic and Coordination Compounds, 3rd ed.; Wiley-Intersciencc: New York, 1977. (14) Herzbcrg, G. Molecular Spectra and Molecular Structure. II. Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand Reinhold New York, 1945. (15) Walrafen, G. E.; Klein, P. H. J . Chem. Phys. 1987, 86, 6516.
EPR and ENDOR Study of Selected Porphyrin- and Phthalocyanine-Copper Complexes S.P.Creiner,*lt D. L. Rowlands,* and R. W. Kreilick Department of Chemistry, University of Rochester, Rochester, New York 14627 (Received: April 23, 1992; In Final Form: July 30, 1992)
Electron paramagneticresonance spectra and nitrogen ENDOR powder spectra have been obtained from copper complexes of tetraphenylporphyrin, meso-tetrakis(2-methylpyridinium-N-y1)porphyrin(ortho), meso-tetrakis(4-methylpyridinium-Ny1)porphyrin (para), and phthalocyanine at approximately 4 K. All four samples display axial symmetry. The three porphyrins have nearly identical g, and gy values (2.054.2.055) but differ in their g, values (2.186,2.209,2.212). The copper phthalocyanine has axial components equal to 2.060 and a z component of 2.179. CuTPP has the largest axial component of the copper hyperfine tensor, 98.6 MHz compared with values from the other three samples of 80 MHz. The z component of the copper hyperfine tensor is closer in magnitude for CuTPP and copper phthalocyanine (631 and 637 MHz), while for the methylpyridyl species its value is 593 MHz (ortho) and 598 MHz (para). Through analysis of these data, the orientations of molecules contributing to each ENDOR spectrum are readily calculated. The magnitudes of the nitrogen hyperfine and quadrupolar tensors were determined through analysis of the angle-selected ENDOR spectra. The largest component of the nitrogen hyperfine tensor was found to lie along the Cu-N bond axis in each case.
Introduction In an earlier paper, it was demonstrated how angle selection from EPR spectra of randomly oriented samples could be used in the analysis of ENDOR spectra to determine the orientation of the ligand nuclei's hyperfine interaction tensor with respect to the central metal atom's g axis system.' The analysis in that paper was confined to the case in which both the g tensor and metal hyperfine interaction were rhombic with coincident axes. In this paper, we consider angle selection for three copper porphyrins, copper tetraphenylporphyrin, copper meso-tetrakis(4-methylpyridinium-N-y1)porphyrin. copper meso-tetrakis(2-methylpyridinium-N-yl)porphyrin,and also copper phthalocyanine. Each 'Current address: Allied-Signal R&T, 50 East Algonquin Rd. Box 5016, Des Plaines, IL 60017. 'Current address: Xerox Corp., 800 Phillips Rd, Bldg 205-99P, Webster, NY 14580.
of the molecules has four nitrogen atoms coordinated in a planar configuration to a central copper atom. Each nitrogen atom is magnetically equivalent to its trans neighbor and has coincident hyperfine axes. Nitrogen atoms cis to one another have hyperfine interactions of equal magnitude but with a different orientation and hence are not equivalent. The EPR spectra of polycrystalline or amorphous samples reflect a "powder" average of all molecular orientations with respect to the magnetic field.2-8 If the g and hyperfine tensors are known, one can associatedistinct sets of molecular orientations with a given resonant field value. This set of molecular orientations is thereby selected for nuclear resonance by the fucd field setting in an ENDOR experiment. The result is an ENDOR spectrum that reflects the angular dependence of the hyperfine energies of ligand nuclei. Once this angular dependence is known, the geometric location of a ligand proton and spin density at that nucleus can be determined?JO For nitrogen ligands, however, the atom's
0022-3654/92/2096-9132$03.00/00 1992 American Chemical Society
Porphyrin- and Phthalocyanindopper Complexes position cannot be determined because the dipolar approximation loses validity when the hyperfine interaction is large. Much of the work done in powder ENDOR has not made full use of the angular selection which is provided by the anisotropy in the EPR spectrum. In cases in which the g anisotropy and metal hyperfine interaction are axial, the EPR spectrum is independent of 4 and one can only relate field positions to 8. If the magnetic field is set to the low-field turning point in the EPR spectrum of an axial system, one may in principle select a single orientation which would result in a "single-crystal" ENDOR spectrum. At fields other than the low-field turning point, one selects orientations at a fned 8 but a range of values of 4. In these cases, a 2-dimensional powder average is observed in the ENDOR spectrum. The ENDOR spectral resolution of axial systems depends on how rapidly 8 changes with the field magnitude. At the turning points of the EPR spectrum, one observes the maximum change in 8 with field, and a distribution of 8 may be selected if the homogeneous EPR line width is relatively large. When this is the case, ENDOR spectra taken at the low-field turning point may be broadened due to imprecise angle selection. A 2-dimensional ENDOR powder pattern will be observed at all fields other than the low-field turning point because a single 8 with a continuous distribution is selected at these field positions. If the anisotropy in g and A leads to opposite contributions to the position of the resonant field (f~ldover~'-'~), one finds unique values for 8 (and 4) which satisfy the resonance condition at the foldover point, and ENDOR spectra at a unique set of angles ("single crystal like") can be observed. The porphyrin molecules presented here exhibit this behavior and have axial g and metal hyperfine tensors with foldover. The nuclear hyperfine interaction tensor is defined as a sum of two terms, the Fermi contact interaction and the dipolar interaction tensor.3*9JoJ4J5 The dipolar interaction has a maximum value when the external field is parallel to the vector connecting the two dipoles. One therefore expects the largest hyperfine interaction when the field is along the copper-nitrogen bond direction. The dipolar shifts observed in the ENDOR spectra therefore depend on the relative orientation of molecules with respect to the external magnetic field. Powder ENDOR usually are taken at fixed magnetic field values where a unique set of angles are selected. One can therefore use anisotropy in the EPR spectra to select a given set of molecular orientations for ENDOR investigations. The ligand hyperfine tensors can have any symmetry or orientation. In the most general case, the tensor is fully asymmetric but remains diagonalizabre, When the g anisotropy is small compared to the average g value, the tensor is only slightly asymmetric and the association of a unique coordinate system with a nucleus becomes a useful concept. For example, under these conditions in organometallic complexes, the maximum hyperfine energy should occur when the field is pointed along the metalnucleus vector (A, axis) and the minimum when the field is near the A,, Ay plane. For porphyrins, the A, axis of the nitrogen ligands may be along either the gx or the gy of the complex. If the ligand hyperfine components are small compared to the g anisotropy and the metal's hyperfine coupling, angle selection is dominated by the g and metal hyperfine anisotropy. If the ligand hyperfine axis system is coincident with the g axis system, one can utilize angles calculated from analysis of EPR spectra in a determination of ENDOR resonance frequencies. If the ligand hyperfine axis system differs from the g axis system, one needs an independent technique to determine the angles between the axis systems in order to utilize the correct angles in calculations of ENDOR frequencies. In some cases, ENDORInduced EPR (EI-EPR) or single-crystal ENDOR can be used to determine these angles between the axis systems prior to analysis of the ENDOR r e s ~ l t s . ' ~ J ~ If the ligand nuclei have a spin greater than the ENDOR spectra may show quadrupole splittings as well as normal hyperfine splitting. Quadrupole couplings in ENDOR spectra have often bem analyzed using an equation from which cross terms have been
The Journal of Physical Chemistry, Vol. 96,No. 23, 1992 9133 omitted. These cross terms contribute significantly to the spectra in some cases, making it necessary to use the complete equation for the quadrupole interaction, to correctly analyze the ENDOR spectra.'J7J8 "ry The equation relating the resonant field position in an EPR spectrum to the principal components of the g tensor (gi), the principal components of the hyperfine tensors (Aji),and the angles (8,4) of the field vector with respect to the g axis system is given by
with the effective A and g values defined as g(894) = (C(gihi)2)1/2
(2)
i
and direction cosines hl = cos 4 sin 8
hz = sin 4 sin 8
h3 = cos 8 (4)
Angularly dependent terms appear in both de,#)and A@$). For Cu complexes (in X-band EPR), the g anisotropy is generally the largest interaction, followed by anisotropy in the copper's hyperfine interaction. The EPR spectra of randomly oriented samples are calculated by determining resonant field positions at all values of 8 and 4 with eq 1. In cases in which the g and hyperfine interactions are axial and collinear, one can develop an analytical expression for 8 in terms of the resonant field and the principal components of g and A to solve directly for 8 as a function of field.1° In the more general case, when neither g or A is axial or coincident, there is no analytical expression for the relation between (8,4) and the resonant field, and one must solve eq 1 iteratively to determine the values of 8 and 4 for a given field position.'9,20 When ENDOR spectra are taken at a fixed magnetic field value, one observes nuclear resonance transitions from the group of molecular orientations which are selected by EPR. If one takes ENDOR spectra at a series of EPR fields, one is able to obtain NMR data from molecules at a series of relative orientations with respect to the external field, where the ENDOR frequencies are given by'4J5
with the nuclear Larmor frequency given by VN
= gdNHO/h
(6)
and the quadrupolar term by Pm, =
- uNEITQl(ms/ded))ag- Y~Elff g((ms/g(e,+))'% - vNEITl(ms/g(e&))Ag- V N E l Z
gl(ms/de,+))-%
(7)
with Z = (cos 4 sin 8); + (sin 4 sin 8)j + (cos 8)k'
(8)
The parameters with the bar overhead are tensors, while those with arrows overhead are vectors in these equations. 'E" with the overhead bar is the unit tensor. The superscript 'T" represents
Greiner et al.
9134 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
FIoM (gaurs)
2700
2900
3100
3300
Field (gauss)
Figure 1. EPR simulation and 8 vs field plot for a single isotope of copper with four nitrogen ligands in a planar configuration. The g tensor and metal hyperfine interaction tensor are axial and have the values 2.055, 2.055, and 2.190 and 99, 99, and 631 MHz, while the nitrogen hyperfine values are 53,43, and 43 MHz for one nitrogen pair and 43, 53, and 43 MHz for the other. The microwave frequency was around 9.6 GHz for the simulation. the matrix transpose. The ENDOR frequency is dependent on the relative orientation of the external magnetic field with respect to the g axis system through both the hyperfine and quadrupolar terms. If the molecule of interest has collinear and axial g and A tensors, there is no @ dependence of the EPR signal, and one can only select values of 8 with a continuous distribution of 6’s.The ENDOR spectrum is a 2-dimensional powder pattern which exhibits peaks when du/d8 is zero (turning points). If the vector connecting the nuclear spin to the metal is collinear with the g, axis or if 8 is chosen at Oo, one observes a single-crystal-like spectrum, but in other cases, one observes between four and six ENDOR peaks for each spin nucleus. For spin 1 nuclei, one must include the quadrupolar interaction, which will double the number of ENDOR peaks. Figure 1 show an EPR simulation and corresponding 8 vs field plot for a typical copper porphyrin molecule. For field values near g parallel, molecules whose g, axis line up with the field appear to have four equivalent nitrogen atoms. For four equivalent nuclei of spin 1, there are nine (2N+ 1 = 9) “sets” of orientations contributing to the ENDOR spectra, where a “set” is defined as all the molecular orientations allowed for a single combination of copper and nitrogen nuclear spin quantum numbers. This is depicted in the g-parallel region of Figure 1 by the nine curves in the 0 vs field plots. Angular selection will not result in single-crystal-like ENDOR or even single8 powder ENDOR spectra except at the extreme low-field position because of the large collection of orientations contributingto the EPR spectra at a given field value. Thus, angular selection in these types of EPR spectra results in ENDOR spectra of poorer quality than copper complexes with fewer ligated nitrogen n~clei.’-~’.’~ This effect produces broadened absorption peaks in the g-parallel region of the EPR spectrum (Figure 1). In the g-perpendicular region of the spectrum, the many “finger”-like peaks are indicative of multiple nitrogen hyperfine interactions. One must have near overlap of a series of levels to resolve lines in this region of the spectrum. The curves below the
Figure 2. Expanded view of the g-parallel region of the simulated EPR spectrum of Figure 1 . Field selection at 2800 G selects molecular orientations at nine values of 0 to contribute to the spectral intensity. spectrum serve to show the complexity of the problem. Also, certain qualities can be discussed in reference to this diagram. In the g-parallel region at the mi value for copper of +3/2, we observe 9 lines which separate into 25 curves as we go to larger 8 values. These original nine curves are c a d by the orientation of the field vector in the molecules’ g axis system along an equivalent hyperfine axis for all four nitrogen atoms. At this orientation, the four nitrogen’s energy levels are additive, and one can add the nuclear spin quantum numbers. As the field vector inclines at greater 8 values, each set of nitrogens trans to each other maintains their equivalency, while nitrogens cis to one another become increasingly inequivalent. When the field vector is aligned along either the g, or g, axis (g perpendicular), it is pointing in the direction of maximum hyperfine interaction for one set of nitrogens, while selecting the minimum hyperfine interaction for the two cis nitrogens. The two pairs of nitrogens can be treated as distinct entities, each having a hypothetical I = 2 spin state. This results in each copper manifold being split into five lines from the first I = 2 entity and each of those being split into five additional curves due to the second I = 2 nitrogen. The relative EPR and ENDOR intensity (excluding relaxation effects) at any one field value can be determined by considering the molecular orientation contribution at that field magnitude. Each set of orientations, denoted by its value of 8,and its copper and nitrogen nuclear spin manifold, contributes to the intensity an amount equal to the degeneracy of the set. Thus, for a field value near the g-parallel region of the EPR spectra, the nine sets of contributing orientations (curves of Figure 1) have degeneracies of 1, 4, 10, 16, 19, 16, 10, 4, and 1, respectively. Each set of contributing orientations is also weighted by the corresponding sine of the angle 8. The sine of 8 weighting is necessary, because the population of each orientation varies as the sine of the angle 0 for axial systems. Figure 2 shows an expanded view of the g-parallel region of Figure 1. This diagram illustrates that at low-field values nine sets of orientations each with a different value of 8 contribute to the EPR spectrum. At the field value indicated (2800 G), the set of orientations with the highest value of 8 has a degeneracy equal to 1 and contributes to the EPR spectral intensity the following factor: 1 sin, 0 In this factor, the subscript refers to the curve with degeneracy equal to 1 and the angle 8 is that of curve 1. The set of orien-
The Journal of Physical Chemistry, Vol. 96, No.23, 1992 9135
Porphyrin- and Phthalocyanindopper Complexes
to small changes of the field with angle there. This derivative function plays an important part in the observed intensity and manifests itself in rhombic spectra where the intensity becomes a function of both 8 and 4.
:d
1
I
2800
2800
I 3000
I
I 2700
2800
I 2600
I
I
2800
3000
I 3100
1
J
I 2800
2600
I
'
3200
'11
Experimental Section Both CuTMPyP(4) and CuTMPyP(2) (the para and ortho forms) were obtained as chloride salts from Mid-Century. The samples were weighed out in appropriate amounts and dissolved in water and DMSO or DzO and deuterated DMSO (1:l). These solvent systems form glasses at liquid helium temperatures. Samples of copper phthalocyanine were prepared by conventional techniques.21J2 The copper compounds were mixed with metal-free phthalocyanine in the appropriate concentrations in a sulfuric acid bath. The resulting blue-green powder was precipitated out of solution, rinsed, collected, dried, and then used without further purification. The EPR spectrum of the paramagnetically dilute powder exhibited a free-radical signal at g equal 2. This free radical is believed to be the result of acid precipitation and, as long as ENDOR spectra were not acquired near that region of the EPR spectrum, caused no interference in data collection or analysis. Copper tetraphenylporphyrin (CuTPP) was dissolved in toluene-$ with 5% deuterated chloroform. The concentration of porphyrin was 5 mM, and this sample was found to form a good glass upon freezing. The EPR and ENDOR spectrometer has already been de~cribed.~JOJ~ However, an IBM-PC was used for data acquisition and storage. All samples were placed into 7-mm quartz sample tubes and subsequently cooled to liquid nitrogen temperatures before being placed into the microwave cavity. Experimental ENDOR conditions consisted of frequency modulation at 2 kHz, FM modulation excursion of 60 W z , temperature around 4 K, and rf power over 100 W while saturating the EPR transition. Microwave power typically was between 0.2 and 4 mW. Analysis of the EPR and ENDOR powder spectra was done using spectral simulation techniques previously The nitrogen and copper hyperfine axis systems were considered coincident.
3400
'P
I
I
3300
3500
3200
3400
I
1 3000
3200
I 3400
Flgure 3. EPR powder spectra of copper tetraphenylporphyrin, copper meso-tetrakis(4-methylpyridinium-N-y1)porphyrin (para), copper meso-(2-methylpyridinium-N-yl)porphyrin (ortho), and copper phthalocyanine. The arrows on each of the spectra indicate the positions of the ENDOR spectra were acquired and serve as labels for the spectra on Figures 4-7.
tations with the next highest 8 value has a degeneracy equal to 4 and contributes 4 sin4 8 to the spectral intensity; the next set would contribute 10 times the sine of the angle and so on. Each set of orientations has a different value of 8. Orientations with a higher value of 8 have a larger value of "sin 0" and contribute more to the spectral intensity than do orientations with smaller values of 8 with the same degeneracy. The orientations selected at a given field therefore are biased toward levels with higher 8 values. Spectral intensity is also a function of the derivative of the field with mpct to the angle e? As the field approaches the extremum of its range in 8 (0 or go"), the density of states increases due
Results and Discussion The EPR spectra of the four samples are shown in Figure 3. Each of the EPR spectra contain arrows pointing to locations where the nitrogen ENDOR spectra were acquired. The spectra are similar to one another because the g values and copper hyperfine parameters are nearly equal. The g-parallel values of the porphyrins are almost identical, while the copper phthalocyanine sample exhibits slightly different values. The (methylpyridiniumy1)porphyrins have similar A, and g, values, while the
A
A
I 17
1
16
1
18
I
20
I
22
I
24
I
26
1
28
I
30
I 10
I
I
I
I
I
21
2s
26
27
IO
R # u r r Y (YWO
R . q w r r Y (ultz)
Figure 4. Experimental ENDOR spectra of copper tetraphenylporphyrin for the field values indicated in Figure 3. The experimental spectra are labeled
'EXP", while the best fit ENDOR simulations are found underneath each experimental spectrum and are denoted 'SIM".
9136 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
TABLE I: Copper PorpbvrinS X
Y
Greiner et al.
Z
CuTPP g
Cu, MHz “N, MHz
N quad
2.055 98.6 54.2 -0.696
2.055 98.6 42.6 0.826
2.186 631 42.6 -0.130
CuTMPyP(4) g
2.054 80 55.3 -0.770
g
2.055 80 54.8 -0.870
Cu, MHz I4N, MHz N quad
2.054 80 40.5 0.900
2.209 598 40.8 -0.130
CuTMPyP(2) Cu. MHz I4N, MHz N quad
2.055 80 40.1 1.00
2.212 593 40.7 -0.130
Copper Phthalocyanine g
Cu, MHz “N, MHz N quad
2.060 80 57.2 -0.80
2.060 80 42.0 0.950
2.179 637 42.08 -0.150
I
I
I
I
I
I
10
16
20
22
24
20
FnquncY
Single Cryatal-CuTPP g
Cu, MHz ‘*N, MHz
N quad
2.045 -102.7 54.2 -0.6 19
2.045 -102.7 42.8 0.926
A
2.190 -615 44.06 -0,307
g, values of CuTPP and copper phthalocyanine are nearly
equivalent. The two (methylpyridiniumy1)porphyrinsand copper phthalocyanine all have A, and A, values near 80 MHz. The refined EPR and ENDOR parameters as determined by spectral simulation methods are shown in Table I. We estimate this technique has an error limit of about 5 MHz in the measured A, and Ay values, while the error in A, is estimated at 2 MHz. The EPR parameters of capper phthalocyanine were previously determined by Abkowitz et ai.*’ They measured a g parallel of 2.1 57 and g perpendicular at 2.040. These values differ considerably from the values of g we determined (2.179 and 2.060). The differences are probably due to our samples having a mixture of a and fl crystal structures. Abkowitz et al. annealed their samples to form the 6 phase from the a crystal structure. In the absence of annealing,there would be a mixture of both the a and fl phases. The two forms may have different g values, and our results would be an average of these values. In their spectral analysis, Abkowitz et al. assumed equivalence of the four nitrogens for all orientations of the field vector. This type of analysis could produce errors in the analysis of the results in the g-perpendicular region of the spectrum. Experimental and simulated ENDOR spectra are shown in Figures 4, 5,6, and 7 for CuTPP, CuTMPyP(4), CuTMPyP(2), and copper phthalocyanine, respectively. The line widths are very broad in these spectra because of the number of states selected at a given EPR field. The lines are not modulation broadened. Each spectrum was acquired at one of the field locations marked in the corresponding EPR spectrum. The top and third positions are the experimental spectra, while the second and bottom spectra are the best fit ENDOR simulations. In a single-crystal nitrogen ENDOR spectrum, four transitions are observed. In an amorphous sample, a single nitrogen results in four ENDOR transitions also, but each transition is a powder pattern, and each powder pattern has two turning points. The observed experimental ENDOR spectrum of each nitrogen atom is, therefore, a superposition of four powder patterns with eight turning points. Since each of the four nitrogen atoms in any of our samples yields 4 powder patterns and 8 turning points, there are 16 powder patterns with 32 possible turning points in each ENDOR spectrum. The complexity of these pattem can lead to errors in assignments if ENDOR spectra are taken only at a single EPR field. Moreover, the 32 possible turning points are almost never observed due to peak overlap in the powder pattern. The analysis of these
1
I
16
17
1
1 10
10
I
19
I
20
I 23
I
21
I
22
I
24
I
I
25
1 16
27
1
20
I
so
FnsunCY (YH4
Figure 5. Experimental ENDOR spectra of copper meso-tetrakis(4m e t h y l p y r i d i n i u m - N - y l ) ~ ~for h ~the field values indicated in Figure 3. The experimental spectra are labeled ‘EXP”, while the best fit ENDOR simulations are found underneath each experimentalspectrum and are denoted T I M ” .
spectra is simplified by considering certain symmetries of the copper porphyrins and copper phthalocyanine. Before beginning computer simulationsof the ENDOR spectra, the spectra were analyzed to derive initial estimates of the hyperfine splittings through analysis of the EPR data and ENDOR resonances. Beginning with the g-perpendicular spectra, the orientational distribution of each powder pattem can be thought of as consisting of a continuum of directions between the maximum (parallel to the Cu-N bond) and minimum (perpendicular to the Cu-N bond) hyperfine axes. Each pair of trans nitrogen atoms has an equivalent hyperfiie and quadrupole interaction tensor with coincident axes. Even though nitrogen atoms cis to one another also have coincident hyperfine axes, the magnitudes of their minimum and maximum hyperfiie interactions are not equivalent along the same direction. The magnitudes of every nitrogen’s minimum and maximum hyperfine axes are equivalent, even if they are not in the same direction. The same turning point frequencies occur for all nitrogens because the field selects all orientations between the maximum and minimum hyperfine axes for each nitrogen. Turning points from different nitrogens occur coincidentally and overlap. In a first approximation, these spectra
The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 9137
Porphyrin- and PhthalocyanineCopper Complexes
b
ddd 1 17
k, “
17
10
21
FnquncY (W)
23
25
10
20
18
22
1 10
1 21
F ” m Y
1
sa
1 15
(mkl
a4
R w ” c Y (WI
Figure 6. Experimental ENDOR spectra of copper meso-tetrakis(2-methylpyridinium-N-y1)porphyrinfor the field values indicated in Figure 3. The experimental spectra are labeled “EXP”,while the best fit ENDOR simulations are found underneath each experimental spectrum and are denoted
‘SIM”. can be analyzed as if they resulted from only one nitrogen, and the molecular orientations which contribute to the spectrum result from the field vector selecting only those orientations on a small distribution of 8 values above and below the gx-gyplane. Thus, a calculation of transition frequencies resulting from the powder pattern orientation distribution was performed, iteratively adjusting the magnitude of the hyperfine and quadrupolar interactions to bring the eight turning point resonances to agree with the peaks observed in the spectra. We can describe this more clearly by examining the spectra acquired around the g-perpendicular region of the EPR spectrum in Figures 4 c z , 5d, 6 n , and 7d. In Figure 4d, five peaks are observed. In this region of the EPR spectrum,the angular selection consists of orientations near the gx-gy plane where both the maximum and minimum hyperfine axes for our “single” nitrogen are selected. This results in an eight-peak ENDOR spectrum, four peaks for each of the high- and low-frequency turning points. Only five peaks are observed because of the overlap of the highand low-frequency turning points with concomitant intensity varying throughout the spectrum. We can assign the transitions which occur at the extremum of the spectrum to the maximum and minimum theoretical frequencies calculated for the eight transitions by eq 5. The second highest peak in the experimental spectrum can be assigned to the second highest calculated frequency because initial calculations show this transition stands apart from the others. Only five calculated frequencies remain to be fitted to three experimental peaks. Two of the five must be near enough to two other resonances to be hidden in the line width, leaving the last peak easily assigned to the last remaining resonance. The ENDOR spectra of Figures 4-7 a q W at low-field values near the g-parallel region of the EPR spectrum exhibit only two broad peaks. Theoretically, there should be four peaks. Two pairs of transitions must be close enough to be hidden within the line width. Since the nitrogen resonances are centered by A/2, the initial values of the hyperfine splitting can be approximated from these spectra by averaging the two resonance frequencies and then multiplying by two. Using these numbers as starting values, an iterative process was begun by calculating the transition frequencies for the ENDOR resonances and adjusting them until the four resonances fall within the experimental line width. The magnitude of the quadrupolar tensor is iteratively adjusted in the calculation to bring the two pairs of resonances to agree with the two peaks observed in the spectrum. After obtaining starting values of the hyperfiie and quadrupolar couplings as described above, full computc~simulations commence to calculate the ENDOR frequencies as a function of angle and field, resulting in a synthesized spectra. Thus, refined hyperfine and quadrupolar couplings are obtained. While optimizing the
ENDOR parameters, however, one must consider the g-parallel transitions simultaneously with the g-perpendicular transitions because the trace of the quadrupolar tensor must always be set to zero. In Figures 4-7, the simulated spectra (second from the top and bottom) reproduce major features of the experimental spectra. Not only are the peak positions at the correct frequencies but the relative intensities match the experimental spectra. The EPR and optimized ENDOR parameters from the simulation techniques for CuTPP, CuTMPyP(4), CuTMPyP(2), and copper phthalocyanine are listed in Table I. The hyperfine and quadrupolar values obtained from a single-crystal CuTPP ENDOR studyz4are also given for comparison. We could not determine the signs of any components of the hyperfine or quadrupolar tensors from our data, so previously reported values were as~umed.*“~~ Figures 4-7 show a dramatic dependence of the ENDOR spectrum on the EPR field position (orientation). Two ENDOR lines are observed at low EPR fields, while five lines are observed at higher fields. These spectra illustrate the necessity of obtaining data at a series of field positions before attempting analysis of the ENDOR spectra. The nitrogen hyperfine and quadrupole coupling constants used to simulate spectra must reproduce experimental results at all fields in the EPR spectra. If data are taken at only a single EPR field, large errors may result from the analysis of the ENDOR spectrum. The nitrogen hyperfine values determined by this study are similar for all four porphyrins. The components of the quadrupolar tensors vary slightly between samples, and all but copper phthalocyanine have a Q, component around -0.13 MHz. The error associated with the components of the quadrupolar interaction are fairly large. We estimate the error to be about 20 kHz based on ENDOR simulations. Table I allows a comparison between our data for an amorphous sample of CuTpP in a glass and single-crystal data. The g tensors for these two types of samples are nearly identical, but z components of the hyperfie and quadrupole couplings are somewhat different. The z components of the copper and nitrogen hyperfine coupling constants differ by only 2-3%, while the z components of the quadrupole coupling constants differ by more than 50%. Figure 8 shows a plot of experimental and theoretical CuTPP I4N ENDOR spectra. The spectrum on top of the diagram is a simulation of the experimental spectrum in Figure 4a using the singlecrystal data. The spectrum in the middle is a reproduction of Figure 4a, and the simulation using our data is shown on the bottom. The centers of the two simulations differ because of the difference in AzN. The z component of the quadrupolar tensor produces the largest ENDOR shifts around this center frequency. This fgure clearly illustrates the difference in the ENDOR spectra between a sample having the single-crystal hyperfine and quad-
Greiner et al.
9138 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
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Figure 8. Nitrogen ENDOR simulations using the singlecrystal data of CuTPP2' (top) and the data from this work (bottom). The spectrum in the middle of the diagram is the experimental spectrum from Figure 4a.
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Figure 7. Experimental ENDOR spectra of copper phthalocyanine for the field values indicated in Figure 3. The experimental spectra are labeled 'EXP", while the best fit ENDOR simulations are found underneath each experimental spectrum and are denoted "SIM".
rupole coupling constants vs an amorphous sample with the coupling constants reported in this paper. The match between the experimental spectrum and the simulation with our data is superior to the simulation using the single-crystal data. The z components of the hyperfine and quadrupole coupling constants appear to measure the local environment in the two types of samples, with Q, being particularly sensitive to changes in local environment. Lau and LmZ7list references to other EPR studies of metalloporphyrins and their derivatives, which show differences in the g and hyperfine, ACY,tensors reported from different laboratories. These discrepancies are probably due to differing local environments of the central metal ion. The hyperfine components of the proton ENDOR for the singlecrystal and the frozen glass samples of CuTPP exhibited dissimilarities as well (not shown). The dif€crenca in the magnetic parameters of the single-crystal and frozen solution samples of the CuTPP imply the matrix cage surrounding the paramagnetic ion induces differing local environments. This is not unusual as optical spectroscopy in organic vs S h p o l ~ k i i ~solvents * * ~ ~ clearly demonstrates disparate sites
amongst solute mole~ules.~9~* In an ENDOR spectrum from an amorphous sample, one can only observe an ensemble average of the magnetic parameters from differing molecular sites with the same relative orientation. Thus, analysis of single-crystal and amorphous EPR and ENDOR spectra will not always yield identical magnetic parameters. The difference in the nitrogen hyperfine and quadrupolar tensors for crystal and amorphous samples reported here is confirmation of lattice site disparities between single-crystal and amorphous samples.
conclusion Our results demonstrate how ENDOR spectra taken at a series of EPR field positions (angles) a n be used to determine hyperfiine and quadrupolar tensors. ENDOR spectra collected from the g-perpendicular region of the EPR spectrum have often been ignored because of the large number of contributingorientations but are necessary for a complete analysis of ENDOR data. Molecular symmetry allows one to make some simple approximations to elucidate the components of the hyperfine and quadrupolar interactions which previously could only be determined from singlecrystal experiment% ENDOR data have been collected and analyzed for the copper complexes of TPP, TMPyP(2), TMPyP(4), and phthalocyanine. These data allowed us to determine the g values, copper and ligand nitrogen hyperfine couplings, and the nitrogen quadrupolar couplings. Acknowledgment. This work was supported by National Institute of H d t h Grant GM-22793. We express our appreciation to Drs.Thomas Henderson and Boklye Kim for many helpful discussions. References and Notes (1) Greiner, S.P.; Kreilick, R. W.; Kraft, K. A. J . Am. Chem. Soc. 1992, 114, 391. (2) Sands, R. H. Phys. Rev. 1955,99, 1222.
( 3 ) Wertz, J, E.; Bolton, J. R. Electron Spin Resonance: Elemental Theory and Practical Applications; McGraw-HiU New Yo&, 1972. (4) Poole, C. P.;Farach, H. A. Bull. Magn. Reson. 1980, 1, 162. (5) Hoffman, B. M.; Martinsen, T.; Venters, R. A. J. Mu@. Reson. 1984, 59, 110. (6) Malstrom, T.; Vanngard, T. J. Mol. Biol. 1960. 2, 118. (7) Roberts, E. W.; Kasky, W. S.J. Am. Chem. Soc. 1960. 82, 3006. (8) Nieman, R.;Kivelson, D. J . Chem. Phys. 1961, 35, 156. (9) Henderson, T. A.; Hurst, G. C.; Kreilick, R. W. J. Am. Chem. Soc. 1985. -107.1299. (10) HUrsi: G. C.; Henderson, T. A.; Krelick, R. W. J. Am. Chem. Soc. 1985. -. - -,-107. , 1294. -. (11) Rist, G.; Hyde, J. S.J . Chem. Phys. 1970, 52, 4633. (12) Orchinnkov, I. V.; Konstantinov, V. N. J. M a p . Reson. 1978, 32, 179.
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J. Phys. Chem. 1992,96,9139-9144 (13) Chen, I.; Ablanvitz, M.; Sharp,J. H. J. Chem. Phys. 1%9,50,2237. (14) Weil, J.; Anderson, J. J. Chem. Phys. 1961, 35, 1410. (15) Iwaaaki, M. J. Mugn.Reson. 1974, 16,417. (16) Schweiger, A. ENDOR of TrunsitionMetal Complexes with eganjc Lfgunds;Springer-Verlag: Berlin, 1982. (17) Greiner, S. P.; Kreilick, R. W. J. Mugn.Reson., in press. (18) Greiner, S. P. Thesis, Department of Chemistry, University of Rochester, 1987. (19) Greiner, S. P.; Baumgarten, M. J. J . Mugn.Reson. 1989, 83, 630. (20) Kneubuhl, F. K. J . Chem. Phys. 1960, 33, 1074. (21) Abkowitz, M.; Chen, I.; Sharp, J. H. J. Chem. Phys. 1968,48,4561. (22) Beltran-Lopez, V.; Caatro-Tello, J. J. Magn. Reson. 1982, 47, 19.
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(23) Hurst, G.; Kraft, K.; Schultz, R.;Kreilick, R. W. J . Mugn.Reson. 1982, 49, 159. (24) Brown, T. G.; Hoffman, B. M. Mol. Phys. 1980, 39, 1073. (25) Manoharan, P. T.; Rogers, J. In Electron Spin Resonunce of Metul Complexes; Yen, T. F., Ed.;Plenum Press: New York, 1969. (26) Maki, A. H.; McGarvey, B. R.J. Chem. Phys. 1958, 29, 31. (27) Lau, P. W.; Lin, W. C. J. Inorg. Nucl. Chem. 1975, 37, 2389. (28) Shpolskii, E. V.; Klimova, L. A. Opt. Spectrosc. 1971, 22, 19. (29) Mobius, K.; Frohling, W.; Lendzian, F.; Lubitz, W.; Piato, M.; Winscom, C. J. J. Phys. Chem. 1982,86,4491. (30) Dinse, K. P.; Winscom, C. J. J. Lumin. 1979, 18-19, 500. (31) Dinse, K. P.; Winscom, C. J. J. Chem. Phys. 1978, 68, 1337.
Intracluster Polymerlzatlon Reactions within Acetylene and Methylacetylene Cluster Ions M. Todd Coolbaugb, Stephanie G. Wbitney, Gopalakrishaan Vaidyanathan, and James F. Garvey*l+ Department of Chemistry, Acheson Hall, State University of New York at Buffalo, Buffalo, New York 14214 (Received: May 5, 1992; In Final Form: July 16, 1992)
We report the observation of large (n up to -25) acetylene and methylacetylene cluster ions-the largest yet reported from ionization of neutral (C2H2J,or (CH3CCHJ,clusters. The cluster ion intensity distributions of both systems display prominent magic numbers at n = 3. This finding is indicative of intracluster ion-molecule reactions giving rise to what are most likely benzene and trimethylbenzene ions from (C2H2Jn+ and (CH3CCH),,+ cluster ions, respectively. The acetylene cluster ion intensity distributionsobserved under efficient clustering conditions are further characterized by unexpected features, most notably a sharp break at n = 7 and a strong magic number at n = 14. The mass spectra of methylacetylene display a less dramatic break at n = 7 and a weak magic number around n = 10. These surprising structures may arise as a result of the formation of particularly stable covalently bonded molecular ions formed via intracluster polymerization reactions.
I. ~troducti06 Gas-phase clusters present the experimentalist excellent op portunities to study processes of chemical importance in greatly simplified environments. One such process which has begun to attract rcccnt interest is ionic polymerization reactions. Clusters allow one to study ionic reactions in a solvated environment without complications due to the need to maintain electroneutrality, i.e. there is no need to consider effects due to neutralization reactions or counterions. It is also possible to investigate the chemistry of single ions. Although the use of a molecular beam cluster source results in a distribution of cluster sizes, essentially all of the reaction products observed in the cluster mass spectrum (CMS) arise from reactions of a single ion within the cluster. The study of polymerizationreactionsin clusters pennits one to gain insight into the importance of several factors in determining the course of reaction. Studying the reactivity patterns as a functionof cluster size has shown the crucial role solvent molecules play in the intracluster polymerization reactions by stabilizing the highly excited intermediates formed by the first few addition reactions. We have studied fhe positive ion chemistry of several olefin van der Waals clusters (ethene, 1, l-difluoroethene, and The small clusters: show ample evidence of the effects of solvent-induced changes in the ion chemistry, particularly the stabilization of highly energetic intermediates. We have also presented results indicating that sequential ion-molecule addition reactions (intracluster cationic %olymcrization~)takeplace within the cluster ions.'-' The findings of these studies are entirely consistent with previous bulk phase studies and suggest that clusters can provide insight into the initial stages of ionic polymerization. Several other groups are now also investigating intracluster polymerization reactions, both cationid and anionic! We have recently begun an investigation of the cluster ion chemistry of the alkynes acetylene (ethyne, ACE) and methyl'Alfred P. Sloan Foundation Fellow 1991-1993.
acetylene (propyne, MACE). Ionic polymerization reactions involving these molecules have been implicated in the formation of polyaromatic hydrocarbons (PAHs) in flames' and during pyrolysis of alkynes.* We will be reporting here the largest acetylene "clusterwions observed to date. The CMS of the alkyne clusters shows evidence of ionic intracluster polymerization reactions for both ACE and MACE. The CMS of both acetylene and methylacetylenedisplay magic numbers at n = 3, which may be indicative of the formation of benzene and trimethylbenzene ions, respectively. The CMS of acetylene also display several features at higher sizcs, the most intriguing of which is a magic number at n = 14. This feature may indicate the formation of a particularly stable, covalently bound ion.
II.
Experimental Section The experimental setup has been described in detail elsewhereg and is shown schematically in Figure 1. In brief, it consists of a continuous molecular beam cluster source of the Campargue design coupled to a chamber housing a mass spectrometer. The cluster beams were generated by expansion of gas through a 2%" supersonic nozzle. The nozzle assembly is equipped with a shroud through which fluid from a circulating chiller could be passed. The pertinent expansion conditions-pressure (Po) and temperature (To)-are reported in the figure captions. The acetylene (Scott, 99.696,dissolved in acetone) was expanded neat after passing through an activated charcoal fdter (Matheson 454) to remove as much acetone vapor as possible. This purification step is absolutely necessary in order to observe neat acetylene clusters since the acetone impurity in the beam can lead to the nearly exclusive observation of mixed acetylene/acetone cluster ions.'O Due to safety considerations, expansion pressures Po < 2 atm were utilized for all experiments. Overall clustering efficiencycould be increased by reducing the nozzle tempcraturq however, as Towas decreased, the relative amounts of acety-
0022-365419212096-9139$03.00/00 1992 American Chemical Society
