[COS'l'RlIiL'~I'IUS FIiOhl TIIE L)EPART.\lEST OI' CREJIISTRT, TIIE J O H N S
HOPKIXS CNIVERSITY,
~ ~ . 4 l ~ I ' I > f O K18, l i ~ ~ " L R \ I , . ~ S] I I
An Electron Spin Resonance Study of Copper Etioporphyrin 111 BY E. 31.ROBERTS* AND 17. S.Ross1 RECEIVED SEPTEMBER 11, 1959 I'roin the electron sDin resonance (e,s.r.)spectrum of a benzene solution of copper etioporphJ-rin 11,a 60value of 2.0975 was obtained. The spectrum of a caztor oil solution of CuEtio I1 gave g/l = 2.1693 and 6 1 = 2.0616. T h e spectrum of the benzene solution is composed of four absorptions separated by 91.2 gauss. -1hyperfine structure due t o the four-ring nitrogens is observed. The four-nitrogen nuclei cause each of the four copper-hyperfine absorptions to be split into a number of lines. Ideally t h e latter number should be nine. Actually, all nine lines are not seen but this can be explained by the relative intensities of the nitrogen lines being 1: 4 :10: 16: 19: 16: 1 0 : 4 :1 as calculated by a rnultinomial distribution. T h e spacing in the nitrogen hyperfine structure (14.6 gauss) shows that the odd electron can be found in the copper d,z-,z orbital about 74% of the time.
In the abo\-e states the
I/I?~> IJZ~~>
= alwpl>
]hi>
are hybrids give^^ by
-+ + V"
__~.
=
aj '22,
lj721>
=
(2
;/z*4>
= n
Iw21> ~
T24>
- a*ip*2>
(2)
- d17* 1 p.s> -
%'IT21
p*4>
The theory of hybrid orbitals leads to the choice of ( 2 1 a ) i / 2 for a. Overlap has been included in the B1, and hl, orbitals since overlap is the greatest in these two. CY' is given by =
.s =
Cu(3d)
=
d,, d,, d,, d,c
2
-
(3) l/L*.(>
-+
pz:r>i
(4)
Bonding orbitals arc obtained by takitig the positive sign in equations 1 and antibonding orbitals are obtained by taking the negative sign. Figure 2 shows the approximate relative positions of the molecular orbitals. The energy levels are not drawn t o scale. The electronic configuration used in this paper is as follows. The eight sigma electrons from the ring and eight 3d electrons from the copper fill the bonding molecular orbitals shown in Fig. 2 . The odd electron responsible for the paramagnetism is placed in the antibonding IBI,> in the ground state. The B1, state has no orbital degeneracy. Placing the odd electroti in the antibonding ;BL,> (hereafter simply /E1,>) conforms to modern ligand field theory.6 111 fact McGarvey' has shown t h a t placing the odd electron in the 4pz copper orbital predicts results in disagreement with his paramagnetic resonance data on the copper(I1) acetylacetonate. Recent papers by LIeGarvey and 3\Iaki8r9further confirm the theory t h a t the paramagnetic properties of copper (11) acetylacetonate are best explained with the odd electron being placed, in the ground state, partly in the jd,2 - s 2 > copper orbital. i G ) J . S. Gri8ith a n d L E. Orgel, Quavt Xi,o.. 11. 386 (19571 (71 B . R . AIcGarvey, J . P h y s . Chcm., 6 0 , 71 f1951). Ti li 1IcC:arvry ; ~ n d A . 13. l l a h i , .I. (--heriz. I ' I I ? ~ , 29, ::I \I
ELIXTKOP.; SPINICESON~WCE OF COPPEK ETIOPOKPIISRIN I1
Julie 20, l!)W
ide'ol 3 ~ _
_
Sr3 IC15
_
3107
_ 6 , Ei,
.,?3.1
:,i:
?.:.#I..
i
~
'2:
6 2
..
~-
8,i' a,:
A
u,
Although the crystal structure of Cu Etio I1 has been examined in some detail,1° the orientation of the molecule in the crystal lattice is not known. It is thus necessary to study the e.s.r. in solution. The spin Hamiltonian for an electron in a tetragonal electric field is HS
=
BeglH3s3
+ Beg~(H15'1+ IIzS,)+ AS3I3'"
+ B ( S 1 I l C U + SJZ) + + . . . (smaller terms)
1
4?rYhPeBn ( a ' ) 2 P N ( 0 )
9
~ ~~~
Crt
1ClS
Fig. 2.-Energy level scheme for copper etiopurphyriti 11. Here the assumption has been made that p2 = 2p12 - 1 atid that A2 = 18,950 cm.'.
the four copper hyperfine lines. If the strong field approximation is made the spacings in the nitrogen hyperfine structure (h.f.s.) and copper h.f.s. are, respectively
(5)
I2SI"
+ Xt
(6)
where the space fixed Z axis is determined by the homogeneous magnetic field. Xt is defined in reference 9. go and a' are related to giI and gL.
u' =
?
Avcu = a'/'h
+ a'SICU +
go =
E
e:,13r
(cU')2lPX\0)12S,I,"
In equation 5 the direction indicated by the subscript 3 is the symmetry axis of the molecule. The 1 and 2 axes pass through atoms 22 and 21, respectively &, Pn and y , are ~ the Bohr magneton, nuclear magneton and the nuclear gyromagnetic ratio of the nitrogen nucleus. p ~ ( 0 is ) the value of a nitrogen 2s atomic orbital evaluated a t the nitrogen nucleus. The "smaller terms" of equation 3 include the nuclear quadrupole interaction and the interaction of the nuclear moment with the applied field H . By transforming H, of equation 5 into a coordinate system fixed in the laboratory we obtain" 3CS = goO,HS,
P
~-3 a - a 1 7 ~
c-c
'Li
; + (Ell
2EL)
5 ( A + 2B) 1
In solvents of low viscosity we may replace the tiiiic average of Xt by the spatial average which vanishes.12 Herice for solvents of low viscosity we have as the spin Hamiltonian
+
hv HI. = gLPe
The spacings in the spectrum of the high viscosity solution yield the hyperfine structure constants .1 and B . The parameters 811,g l , il and B will depend on the electronic state of the unpaired electron. Abragam and Pryce'j have derived general formulas for the latter parameters. They are = 2 0023 ( 1 -
gi
+
+
+
constant whose value depends on the electronic configuration of the free ion (( = 2 21 for d electrons), the product -PR is the contribution to -1 and B from the well-known Fermi contact interaction. i i l i , m,, and Uii are defined as
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