J . Phys. Chem. 1990, 94, 6977-6982
region of the ion. And it may be said that there is no deep minimum in any place on its potential surface. If there were a deep minimum, the intramolecular vibrational redistribution (IVR) should occur from the FC region and reduce the available energy for the complex ion to promote the channel c. As a result, the complex ion would be detected as a metastable species with M = 129 amu. However, the absence of the complex ion in the mass spectra represents that the excess energy after the photoionization destroys promptly the metastable ion before the mass detection. Therefore, it is concluded that the potential surface of the complex ion is rather flat with respect to the reaction coordinate of channel c, having no minimum or a shallow minimum which does not leave the complex ion stable. For the other reaction channel b, there should be a barrier to substitution in spite of the lowest enthalpy of the products. This is similar to their neutral states; the complex in the So and SIstates is quite stable even though the "product" system is thermochemically more stable than the complex. In the neutral ground state, the barrier seems to be quite high because the reaction from C6HsCI + NH3 to C6HSNH2+ HCI in solution requires a basic condition or a high temperature. Very recently we have found
6977
the reaction along channel b in the case of the complex of C&F with NH3 where the energy level of the products due to channel c lies at a much higher region than the complex ion. The results will be reported in a separate paper.
Conclusion The efficient generation of C6HSNH3+is found to occur after the photoionization of the ( ] : I ) complex of C6H5C1-NH3. The bimolecular nucleophilic substitution takes place within the vdW complex ion of C6H5Cl+-NH3 as a precursor of the reaction. The characteristic charge distribution on the aromatic ring upon the photoionization is responsible for the prompt reaction with NH3 as a basic reagent. The ionization initiates the vigorous reaction between moieties in the complex which are nonreactive in its ground state. Acknowledgment. We are grateful to Professor Mitsuo Ito for stimulating discussions. We also thank the referees for discussions about the reaction mechanism. This work was supported by a Grant-in-Aid from the Ministry of Education Japan (No. 01470014).
Electron Spin Echo Envelope Modulation Angle Selection Studies of Axial Pyridine Coordination to Copper( I I ) Benzoylacetonate Jeffrey B. Cornelius,*Ja,t John McCracken,'"*$ R. B. Clarkson,lb R. L. Belford,*.lb and Jack Peisach*'" Department of Molecular Pharmacology, Physiology, and Biophysics, Albert Einstein College of Medicine, 1300 Morris Park Ave., Bronx, New York 10461, and Department of Chemistry and Illinois EPR Research Center, university of Illinois, Urbana, Illinois 61801 (Received: October 30, 1989; In Final Form: April 30, 1990)
Magnetic-field-dependentelectron spin echo envelope modulation (ESEEM) spectroscopy is used to characterize weak ligand hyperfine interactions for 14N-and lsN-labeled pyridine axially coordinated to bis(benzoylacetonato)copper(II) in frozen solution. Because of the g and hyperfine anisotropies for,Cu(II), at any magnetic field within the EPR absorption envelope one obtains overlapping single-crystal-like ESEEM spectra. These spectra reflect the angular dependence of the hyperfine energies of the ligand nuclei. Their analysis yields both magnetic coupling constants and orientations of the principal axes of the pertinent coupling matrices with respect to the Cu(l1) g matrix. Spectra at different microwave frequencies are best simulated within the point-dipole model by locating a nitrogen in the axial position, effectively 2.6 A from the plane defined by the metal and ligand atoms of the molecule. Corrections for defects of the point-dipole model are expected to be small; they would reduce the nitrogen-copper distance to about 2.3 A, in good agreement with known axial pyridine-to-copper(I1) bond lengths. For ISN,the diagonal elements of the A matrix are 1.45, 1.45, and 0.09 MHz; for I4N they are -1.03, -1.03, and -0.06 MHz. The I4N quadrupole parameters are e2qQ = 4.4 MHz and 9 = 0.35, with the principal axis of the electric field gradient oriented coincident with the nitrogen-to-copper bond. The simulations obtained demonstrate the ability to characterize a weakly coupled nitrogen nucleus from the field dependence of the ESEEM spectrum.
Introduction Electron paramagnetic resonance studies of copper( 11) bis(ketoenolates) such as copper bis(acety1acetonate) (Cu(II)(acac),) and copper bis(benzoy1acetonate) (C~(II)(benzac)~) complexes have focused on information that can be derived from copper hyperfine, quadrupole, and g matrix components obtained by continuous wave (CW) EPR technique^.^^^ By supplementing the central metal information with ligand hyperfine and quadrupole interaction data, a more complete description of the system can be made. Unfortunately, inhomogeneous broadening of the spectral lines makes it impossible to measure weak ligand hyperfine and quadrupole interactions for these complexes by C W EPR 'Present address: Department of Chemistry, Principia College, Elsah, IL 62028. f Present address: Department of Chemistry, Michigan State University, East Lansing, MI 48824.
0022-3654/90/2094-6971.$02.50/0
methods. To overcome this problem, Kirste and van Willigeq4 using both ENDOR and electron-nuclear-nuclear triple resonance (TRIPLE),4 were able to determine the principal components of proton hyperfine coupling matrices of the C H and CH3 protons in frozen-solution and solid-solution samples of Cu(II)(acac),. Kreilick et aL5v6also used ENDOR to study an extramolecular C H proton coupling in a sample of Cu(II)(acac),. Both C ~ ( I l ) ( a c a c and ) ~ Cu(II)(benzac), have been shown to form 1:l adducts with weak Lewis bases such as pyridine.' ( 1 ) (a) Albert Einstein College of Medicine. (b) University of Illinois. (2) Belford, R. L.; Duan, D. C. J . Magn. Reson. 1978, 29, 293-307. (3) Kuska, H. A.; Rogers, M. T. J . Chem. Phys. 1965, 43, 1744-1747. (4) Kirste, B.; van Willigen, H. J . Phys. Chem. 1983, 87, 781-788. (5) Hurst, G. C.; Henderson, T. A.; Kreilick, R. W. J . Am. Chem. SOC. 1985, 107, 7294-7299. ( 6 ) Henderson, T. A.; Hurst, G. C.; Kreilick, R. W. J . Am. Chem. SOC. 1985, 107.1299-7303. (7) Kogane, T.;Yukawai, H.; Hirota, R. Chem. Letf. 1974, 477-478.
0 1990 American Chemical Society
6978 The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 Subsequent to pyridine-d5addition, the hyperfine coupling constant for Cu was shown to decrease, but not so much as to suggest equatorial ligand substitution.* However, the ENDOR spectrum is virtually identical with that seen in the absence of pyridine; I4N resonances are not seen.4 I n the present study, we have used electron spin echo envelope modulation ( ESEEM)*13 spectroscopy to examine weak nitrogen superhyperfine interactions between Cu(II)(benzac)z and axially coordinated pyridine in frozen solution. For the [I5N]pyridine adduct, the number of resolved superhyperfine components and their frequencies proved to be strongly dependent on the effective g value at which the measurements were carried out. A complete analysis of the spectra and their orientation or angle-selected dependence has yielded detailed information concerning the relative principal axis orientations of the hyperfine and nuclear quadrupole coupling matrices with respect to those of the Cu(I1) g matrix as well as the magnetic coupling parameters that describe these interactions. Experimental Section Cu( Il)(benzac)* was precipitated from an aqueous copper sulfate solution to which an ethanolic solution containing two stoichiometric equivalents of benzoylacetone was added. The mixture was neutralized with dilute ammonium hydr0~ide.l~The precipitate was filtered and washed with water and then with ether. Cu(lI)(benza~)~.pyridine complex (2-5 mM Cu) in toluene was prepared either with equimolar concentrations of pyridine and copper or with a large excess of pyridine (30% by volume). CW EPR spectra of each of the samples employed in the ESEEM study were obtained on a Varian E-109 X-band spectrometer and at Q and L bands (35 and 1 GHz, respectively), the latter two to provide additional data needed for computer simulation of the Cu( II)(benzac)z.pyridine EPR spectrum. ESEEM experiments were performed on a spectrometer described in detail elsewhere15using both stripline transmission16and folded stripline reflection type cavities.I5 Electron spin echo envelopes were obtained with both the two-pulse and three-pulse or "stimulated echo" sequences."-'* Three-pulse echo envelopes were recorded with the time interval (7) between the first two pulses chosen to suppress the proton matrix f r e q ~ e n c y . ~ *Echo , ' ~ envelopes were extrapolated to zero time by means of the dead-time-reconstruction algorithm of Mims.20 Fourier transformation provided the frequency-domain spectra presented in this paper. The computers used for the computations were a DEC 11/780 computer, an FPS array processor, a DEC VAXstation, and IBM AT with floating-point coprocessor. The QPOWA simulation program (see Results and Discussion section) is available on request from R. L. Belford at the Illinois EPR Research Center. Angle Selection. For randomly oriented samples, the continuous wave EPR spectrum consists of contributions from all possible individual single crystal molecular orientations with respect to the laboratory magnetic field direction.z~5~21~z2 For anisotropic systems ( 8 ) Peisach, J.: Blumberg, W . E. Arch. Biochem. Biophys. 1974, 165. 691-708. (9) Mims, W . B.; Peisach, J . J . Chem. Phys. 1978, 69, 4921-4930. (IO) Mims, W . B.; Peisach, J. J . Biol. Chem. 1979, 254, 4321-4323. ( I I ) McCracken, J.; Pember, S . ; Benkovic, S. J.; Villafranca, J.; Miller, R. J.; Peisach. J . J . Am. Chem. SOC.1988, 110, 1069-1074. (12) Kosman, D. J.; Peisach, J.; Mims, W. B. Biochemisrry 1980, 19, 1304-1308. (13) Flanagan, H. L.; Singel, D. J . J . Chem. Phys. 1987,87, 5606-5616. (14) Hon, P.-K.; Pfluger, C. E.; Belford, R. L. Inorg. Chem. 1966, 5 , 5 16-520. (15) McCracken, J.; Peisach, J.; Dooley, D. M. J . Am. Chem. SOC.1987, 109. 4064-4072. (16) Britt, R . D.; Klein, M . P. J. Magn. Reson. 1987, 74, 535-540. (17) Hahn. E. L. Phys. Reo. 1950, 80, 580-594. (18) Mims, W . B. Phys. Reo. B Solid Stare 1972, BS, 2409-2419. (19) Mims. W. B.; Peisach, J . In Biological Magnetic Resonance; Berliner, L. J., Reuben, J., Eds.;Plenum Press: New York, 1981; Vol. 3, pp 213-263. (20) Mims, W . B. J. Magn. Reson. 1984, 59, 291-306. (21) Hoffman. B. M.; Martinsen, J.; Venters, R. A . J. Magn. Reson. 1984, 59, 110-123. (22) Hoffman, 8. M.;Venters, R . A,; Martinsen, J . J . Magn. Reson. 1985, 62, 537-542
Cornelius et ai. only specific orientations contribute to the intensity at a given magnetic field strength.23 Because the ESEEM experiment is performed at a fixed magnetic field and the CW EPR spectrum of Cu(benzac)z.pyridine is dominated by g and metal hyperfine anisotropy, only a select group of orientations is being examined for a particular measurement. Employing the approach of Hurst, Henderson, and K r e i l i ~ k , ~ originally used to interpret "angle-selected ENDOR", the ESEEM simulation calculations begin by determining the molecular orientations that contribute to the EPR spectrum at a given field. It is assumed that the spectra of the Cu(I1) complexes used in this study are dominated by g and metal hyperfine anisotropies. To reduce calculation time, weaker interactions, such as those due to metal quadrupole and ligand hyperfine interactions, are treated by applying a line-shape function to the electron spin echo ( B E ) modulation pattern, rather than including them specifically in the orientation selection process. Also, features resulting from the hyperfine splittings of the 63Cu (69%) and 65Cu (31%) in natural abundance, used in the preparations, are too close together to be distinguished in the ESEEM spectra and only contribute to the effective line widths of the spectral features. Equation 1 gives the spin Hamiltonian used to determine orientations. 7 f e = PJgB + SA1 (1) Here p, is the Bohr magneton, g is the electronic g matrix, A is the hyperfine matrix, B is the external field, and S and I are the electronic and nuclear spin operators. To sufficient accuracy for our purposes, the contributing orientations are solutions that satisfy the approximate (first-order) resonance condition of eq 2. B, = [hv - M , A ( ~ ~ J ) I / P J Z ( ~ , ~ )
(2)
The angles 0 and 4 describe the orientation of the magnetic field with respect to the g-axis system. The g matrix and metal hyperfine parameters are determined from simulation of the C W EPR spectra. Hurst, Henderson, and Kreilick5present an equation to calculate the relevant orientations for the simple case in which the A and g matrices are axial and with coincident principal axes. When the metal hyperfine axes are not constrained to be coincident with those of g, 0 and 4 are found iteratively by use of a parabolic search a l g ~ r i t h m . ~For a nearly axial case, the most efficient scheme is for the program to step through 4, searching for the 0 value that satisfies the resonance condition. The orientations determined from this calculation are then used to calculate the modulation function.z4 Nitrogen-15. As aids for determining the 14N quadrupole interaction, ESE data were collected for Cu( II)(benzac)z coordinated to [15N]pyridine. For I5N ( I = the relevant terms in the spin Hamiltonian that describe the superhyperfine splittings are given by eq 3. (3) For our calculations, the hyperfine coupling matrix A was taken to be axial and cast in the form given by the point dipole-point dipole approximation. Following the formalism of Hutchison and M C K ~the~ hyperfine , ~ ~ matrix elements are given by eq 4. AI] = (-PegNPN/hlJ)[g~(3r~T~ -
+ A~so6~~
(4)
rl = cos dN sin ON
rz = sin 4 N sin 8 N r3 = cos 0N
Since the dipolar coupling matrix is neither traceless nor symmetric in this case, where g anisotropy and the dipolar contribution to A,] are large, the hyperfine lines will not necessarily be located symmetrically about the nuclear Zeeman frequency. Simulated modulation functions were obtained through the density matrix formalism developed by Mims18iz4and applied by (23) Riste, G.;Hyde. J . J . Chem. Phys. 1969, 52,4633-4643. (24) Mims, W . B. Phys. Reo. 1972. 86, 3543. (25) Hutchison, C.; McKay, D. B. J . Chem. Phys. 1977,66, 331 1-3330.
Pyridine Coordination to Copper( 11) Benzoylacetonate
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 6979
Reijerse and Keijzers.26 The general expression for a three-pulse echo envelope is given by eq 5.
where
G, = Q,.M.PT.P,.MQ,M.P,P,M
cp = Q,.QT.M.P,.MQ,QTM.P,M Here, Q, P, and M are matrices which are of dimension 21 + 1, with P nd Q being submatrices of the rotation operators that describe the time evolution of the density matrix during the free-precession periods of the superhyperfine frequencies. The modulation depths for these frequencies are given by the elements of M, the transition moment integrals that are obtained from the eigenvectors that diagonalize the Hamiltonian matrix. Although it appears that there are many parameters, only a few are adjustable. Those input parameters which describe the Cu(l1) center, including the g values, the principal values of the copper hyperfine interaction, and the Euler angles relating orientation of the two axis systems, are determined from simulation of the CW EPR spectra. The microwave frequency, the magnetic field, and the timing parameters T and T are taken from the experimental conditions. The number of orientations is chosen to minimize calculation time without introducing distortions. The adjustable parameters include Ai,,, the isotropic part of the hyperfine coupling, reff,the effective dipolar distance, and the angles ON and +N which together with reffspecify the apparent spherical polar coordinates of the superhyperfine nucleus in the principal axis system of the g matrix. The direction of the metal-nitrogen bond coincides with that of the largest hyperfine interaction. Nitrogen-14. Spectral simulation of I4N ESEEM data requires that another term, the nuclear quadrupole interaction, be added to the spin Hamiltonian as shown in eq 6: %N
SANI - gN@NBI ( e 2 q Q / 4 ) [ 3 I Z-2 2
+ q(1;
- I:)]
(6) The nuclear quadrupole interaction is given by five parameters, e2qQ, the quadrupole coupling constant, q, the asymmetry parameter, and three Euler angles which transform the nuclear quadrupole interaction into the principal axis system (PAS) for the g matrix. Since there are six possible states, the resulting matrix to be diagonalized is 6 by 6. As stated above, the eigenvalues give the transition frequencies while the eigenvectors are used in determination of the transition intensities. Assuming that the two electronic manifolds do not interact, one can simplify the problem by solving the two blocks as independent mat rice^.^' The first approach is expected to give more accurate line intensities, but the simulations require 20% more computer time. There is no measurable difference in the transition frequencies. We are exploring cases in which the full diagonalization becomes necessary.
Results and Discussion CW EPR of the Cu(II))(ben~ac)~.Pyridine Adduct. The CW EPR spectrum of the Cu(II)(benzac),-pyridine adduct was measured and simulated at X band (Figure l ) , at L band, and at Q band with the aid of the program Q P ~ W A . ~ * -From ~ ~ the Q band spectrum, in which field-dependent Hamiltonian terms dominate, the anisotropy in g was determined. No hyperfine structure was resolved in the g , region of the Q-band spectrum, probably as a result of g strain broadening. The X-band and ~~
(26) Reijerse, E. J.; Keijzers, C. P. J . Uagn. Reson. 1987, 71, 83-96. (27) Magliozzo, R. S.;McCracken, J.; Peisach, J. Biochemistry 1987,26, 7923-7931, (28) Nilges, M. J . Ph.D. Thesis, University of Illinois, Urbana, IL, 1979. (29) Belford, R. L.; Nilges, M. J. Presented at the International Electron Paramagnetic Resonance Symposium, Twenty-First Rocky Mountain Conference, Denver, CO, 1979. (30) Maurice, A. M. Ph.D. Thesis, University of Illinois, Urbana, IL,1981.
-80
1
-1001
'
2600
'
'
'
'
2800
'
'
' 3000
'
'
'
'
I ['
3200
'
1 3400
FIELD STRENGTH (GAUSS) Figure 1 . X-band EPR spectrum at 77 K of 5 mM Cu(benzac)2.pyridine adduct in toluene. Simulated (dotted line) using g, = 2.067, gu = 2.062, g, = 2.302, A, = -25 MHz, A, = -25 MHz, A, = -492 MHz, e2qQ = 100 MHz, and 7 = 0.48. Microwave frequency = 9.3743 GHz.
L-band spectra were simulated in order to obtain a unique set of copper hyperfine and quadrupole Hamiltonian parameters that correctly accounted for both spectra. The results are given in the legend to Figure 1. ESEEM Results for the ['"]Pyridine Adduct. ESEEM spectra of C~(II)(benzac)~ with [l5N]pyridine,obtained by Fourier transformation of D E E M data taken at several different magnetic fields (Figure 2), reveal an obvious field dependence. At the low-field edge of the g,, region of the EPR absorption envelope, the two-peak, single-crystal-like ESEEM spectrum shown in Figure 2A is obtained. These two peaks result from the ISN splitting of the M, = 3/2 copper hyperfine energy levels. For data collected at higher fields, additional frequency components are resolved (Figure 2B-D) arising from the inclusion of additional orientations from the other Cu(I1) hyperfine energy levels. In the g, region, all four Cu(I1) hyperfine levels contribute to the ESEEM and a complex spectrum with six clearly resolved peaks was observed (Figure 2D). Data interpretation becomes more clear from a plot of g value versus transition frequency (Figure 3), in which the g value is chosen for the abscissa so that data taken at slightly different microwave frequencies can be included on the same plot. Peaks that were clearly resolved in the ESEEM are represented as crosses, while those frequency components obtained by computer simulations are represented by open squares. The contours are drawn between simulated frequency components that arise from the same Cu(l1) hyperfine state and show the predicted dependence on g value. The simulated transition frequencies were determined from calculations in which isotropic hyperfine coupling and effective radial distance (on a point-dipole model for the electron spin) were varied by means of a simplex algorithm3' to obtain the best fit to the transition frequency at all experimental field values. The results of this analysis show the (x, y , z ) components of the ISN hyperfine coupling matrix, to be (1.45 MHz, 1.45 MHz, 0.09 MHz), respectively, corresponding to a ISN nucleus positioned axially, 2.6 A from the effective position of the electron. This would place the nitrogen atom 2.6 8, from the copper ion were the unpaired electron spin effectively concentrated at the copper nucleus. The effect of a more diffuse (Le., more realistic) electron spin distribution would be to reduce the dipolar coupling somewhat, requiring a shorter copper-to-nitrogen distance to fit the data. An accurate calculation would require a precise knowledge of the spin distribution in Cu(II)(benzac),. Fortunately, however, the effective distance scales as the cube root of any geometrical or spin-density reduction factor. Accordingly, (1) a substantial reduction of effective electron spin density at the Cu nucleus would have only a modest effect on the calculated Cu-N distance and (2) a crude but reasonable estimate of the ( 3 1 ) (a) Deming, S.;Morgan, S. Anal. Chem. 1973, 45, 278A; (b) Mattson, K. J.; Clarkson, R. B.; Belford, R. L. Presented at the Eleventh International EPR Symposium, Denver, CO, 1988; Paper 115.
Cornelius et al.
6980 The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 _--
I
..-7---y--
' 4
I
2.201
N
I 2
>-
0
-~
2.25
2.00
2 50
g -va Iue Figure 3. Transition frequency as a function of g value for Cu(ben~ac)~.['~N]pyridine adduct. Experimental (crosses) data obtained with microwave frequency = 9.16 or 9.45 GHz. Simulations (squares) from sin V9 weighting with microwave frequency = 9.45 GHz; A,- = 1.O MHz; ref(= 2.6 A; ON = @N = 0'.
l"Ol 0
O L 0 00
,
,
,
,
,
v
4
,
2.50
9
5 00
E 2700L
FREQUENCY MHZ
1
0
J
2.50
FREQUENCY (MHZ) Figure 4. Fourier transforms of simulated three-pulse echo data as functions of magnetic field. Microwave frequency = 9.45 GHz; T = 145 ns; A, = 1.O MHz; rem= 2.6 A; ON = r # ~=~ 0'. Copper parameters: g, = 2.067; gv = 2.062; g, = 2.302; A, = -25 MHz; A, = -25 MHz; A, = -492 MHz. To interpret the ordinate as a g scale, note that 2700 G and 3250 G correspond to g = 2.5036 and 2.0799, respectively.
L
-
-
i
5 00
2 50
0 00
FREQUENCY MHZ
or 0 00
1
, 2.50
,
,
,
,
I
5 00
FREQUENCY M HZ
Figure 2. Fourier transforms of S E E M data obtained by the three-pulse echo procedure for Cu(ben~ac)~.['~N]pyridineadduct in solution. Microwave frequency = 9.16 GHz. microwave pulse power = 30 W (20 ns fwhm), sample temperature = 4.2 K. ( A ) Field = 2800 G; T = 336 ns. (B) Field = 2950 G; 7 = 318 ns. (C) Field = 3100 G; 7 = 378 ns. (I>) Field = 3240 G; 7 = 360 ns.
reduction factor should yield a satisfactory Cu-N distance. One way to make such a correction is to remove a certain fraction of electron spin density to account for finite size of the Cu d, orbital and the spreading of electron density onto the four oxygen ligand
atoms. Our rough estimate for the spin-density reduction is 30%, making the appropriate distance correction factor 0.89 and thus reducing the Cu-N bond distance to about 2.3 A. In an alternative scheme to estimate the reduction factor, we placed all the electron s in density at about the midpoint of the Cu-0 bonds, Le., at 1 from the Cu nucleus in the x,y plane. This scheme yielded the same result. So little spin density is expected to be spread from the copper d orbitals into nitrogen p orbitals that we ignore this possible source of nitrogen dipolar coupling. The 2.3-A Cu-N distance is quite reasonable when compared with known axial nitrogen adducts of metal complexes. For example, Duckworth, Graddon, Mockler, and Stephenson32report 2.27 A for the axial nitrogen-to-copper bond in the 4-methylpyridine adduct of bis(2-hydroxyacetophenonato)copper(II), where the equatorial copper-to-oxygen bonds lie between 1.91 and 2.00 A. The "slope" of the contours in Figure 3 is determined by the dipolar contribution to the ligand hyperfine coupling, while the minimum spacing between the two frequency components originating from the same Cu(I1) hyperfine state is dependent on the isotropic contact interaction ( 1 .O MHz in this case). The simulations are not very sensitive to the copper hyperfine coupling(fl.5 MHz) and copper g anisotropy. The present analysis ignores the copper quadrupole interaction, which gives rise to mixing of the Cu(I1) hyperfine energy levels, particularly in the g, region of the Cu(I1) EPR spectrum. This would have several consequences, including introducing some error into the transition intensity and the orientation computed for each of the four primary copper hyperfine transitions, and making secondary (doublespin-flip) copper hyperfine transitions somewhat allowed at new
R
(32) Duckworth, V. F.; Graddon, D. P.; Mockler, G. M.; Stephenson, N. C. Inorg. Nucl. Chem. Left. 1967, 3, 471
Pyridine Coordination to Copper( 11) Benzoylacetonate
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 6981
77700.O
t VI
Pz - 14300.O 0.oo
10.00
5.00
FREQUENCY MHZ
0.00
I
I
0
6
FREQUENCY (MHZ)
5.00
10.00
FREQUENCY (MHZ)
6
FREQUENCY MHZ
0.00
5.00
10.00
FREQUENCY MHZ
Figure 6. ( A ) Effect of e2qQ on simulated Fourier transforms of threepulse echo data for Cu(benzac),.[ “Nlpyridine adduct in frozen solution; e29Q ranges from 4.0 to 5.0 MHz. (B) Effect of 1) ranging between 0.20 and 0.50. (C) Effect of r,n ranging between 2.0 and 3.1 A. Other parameters are given in the legend for Figure 5A,B.
y , z ) components of A(14N), (-1.03 MHz, -1.03 MHz, -0.06
0 0.00
4 ,
,
,
,
,L 5.00
,
,
,
,
10.00
FREQUENCY MHZ Figure 5. Fourier transforms of three-pulse echo data and simulations adduct in frozen solution. ( A and B) for C~(benzac)~-[~~N]pyridine Microwave frequency = 9.38 GHz; magnetic field = 3240 G ; T = 145 ns; microwave pulse power = 30 W; sample temperature = 4.2 K. (C and D) Microwave frequency = 10.80 GHz; magnetic field = 3730 G ; T = 126 ns. ( B and D) Simulations with A i , = 0.7 MHz; re, = 2.6 A; e29Q = 4.4 MHz; 1) = 0.35;ON = & = 0.0’. Copper parameters: g, = 2.302; A , = -25 MHz; A , = -25 MHz; A , = 2.067; gv = 2.062; g, -492 MHz.
orientations that would contribute to nitrogen ESEEM. This could explain why our ESEEM simulations in the g, region do not match the experimental points as well as those done for data collected in the g,,region (Figure 3). Figure 4 shows the magnetic field dependence of the I5N ESEEM spectra simulated with the obtained superhyperfine parameters. [I4N]Pyridine. The spectra of the [14N]pyridineadduct are more complicated to interpret since the I4N nucleus has a higher spin ( I = 1). Thus, six transition frequencies are expected for a given orientation. Moreover, the intensities as well as the frequencies are affected by the nitrogen quadrupole coupling interaction. The [I4N]pyridine hyperfine parameters can be obtained from the results of the [I5N]pyridine analysis given above by scaling all the components of the A matrix by the ratio of the nuclear g values. Then, for [I4N]pyridine one obtains, for the ( x ,
MHz), respectively; Ai, = -0.7 MHz. Unfortunately, the small value of lAi,l for the axially coordinated I4N nucleus causes the observed ESEEM obtained at X-band microwave frequencies to be ~hallow.’~ For C~(II)(benzac)~.pyridine, well-resolved ESEEM spectra could be obtained only in the g, vicinity. Nitrogen-14 ESEEM spectra taken at g = 2.056 at two different microwave frequencies, 9.4 and 10.8 GHz, are shown in Figure 5A and Figure 5C, respectively. As the I4N bond is axial and the coupling is weak, it can be assumed that little of the electron lone pair character of the coordinated nitrogen sp2 hybrid orbital is shared with the Cu(I1). Therefore, a good starting point in a simulation for the nuclear quadrupole coupling parameters is the values obtained from zero field NQR measurements carried out on frozen solutions of pyridine.33 These results provide an upper limit for e2qQ of 4.584 MHz and a starting value for the asymmetry parameter 7 of 0.396. The results of the simulation procedure are shown in Figure 5B and Figure 5D. The I4N quadrupole parameters were determined as e2qQ = 4.4 MHz and 7 = 0.35, with the principal axis of the electric field gradient oriented coincident with the nitrogen copper bond. The quadrupole coupling parameters, interpreted in terms of the Townes-Dailey model,” indicate that the pyridine nitrogen retains much of its lone pair character.35 Because the I4N nuclear quadrupole interaction is stronger than the nuclear Zeeman and superhyperfine splitting interactions, the positions of the peaks in the ESEEM spectra are most sensitive to e2qQ and 7. Figure 6A shows how the higher frequency (>2.0 MHz) peaks in the ESEEM spectrum scale with the value of $qQ, (33) Guibe, L. Ann. Phys., Paris 1961,7, 177. (34) Townes, C. H.; Dailey, B. P. J . Chem. Phys. 1949, 17, 782-796. (35) Hsieh, Y.;Rubenacker, G. V.; Cheng, C. P.; Brown, T. L. J . A m . Chem. SOC.1977, 99, 1384-1389.
6982 The Journal of Physical Chemistry, Vol. 94, No. 18, 1990
while the lower frequency (
