The original method of measuring chemical shifts of C 13 was proposed by ... of paramagnetic complexes of Ni 2+ acetylacetonate with organic ligands. ..... I Qcc,C'I ,-~ 14 Oe also follows from data of Fessenden on the EPR spectrum of the.
HYPERFINE
SPLITTING
PARAMAGNETIC
WITH
COMPLEX
C 13
NUCLEI
IN
COMPOUNDS
R. Z . S a g d e e v , u N. M o l i n , ~ . V. D v o r n i k o v , V. A . G r i g o r ' e v ,
UDC
and
T.
541.67 + 538.113
A. Luzina
The measurement of isotope contact shifts for C 13 in a series of paramagnetic complexes of Ni 2+ a c e t y l a c e t o n a t e with organic ligands was carried out. An attachment for double heteronuclear resonance of H ~ - { C '3 } was developed to determine C ta shifts. All measurements were carried out for m e t h y l carbon atoms containing a natural content of the C 13 isotope. Constants of hyperfine splitting a.~ and spin densities PC were found from the values of paramagnetic shifts. Constants a C for Ni + complexes with c~- and B-methylpyridine N-oxides, which are v - e l e c t r o n ligands, were discussed within the framework of the Karplus-Fraenkel theory. It was concluded that in these ligands the hyperconjugation model most adequately describes the e x p e r i m e n t a l distribution of spin density on the methyl group. Values of spin-polarization constants QCC C' - 14 Oe and QCC C ~ + 19 Oe were determined on the basis of e x p e r i m e n t a l data. A linear relation was found between spin densities on the C and H atoms in an i d e n t i c a l position, from which it follows that the laborious procedure o f determining the value of a C in a series of cases can be r e p l a c e d by the determination of the corresponding a H constants.
The study of hyperfine structure from C la nuclei in free radicals by the EPR method made it possible to obtain principle information about mechanisms of hyperfine splittings [1-3]. Analogous data are present in the literature for paramagnetic complex compounds. The NMR method, widely used for the investigation o f h y p e r f i n e splittings in such compounds, encounters definite difficulties upon recording C Is resonance due to the low natural content and r e l a t i v e l y low magnetic moment of the C 13 isotope. The original method of measuring c h e m i c a l shifts of C 13 was proposed by Allen, Becconsall, and Turner [4]. These authors observed satellites in proton resonance spectra due to s p i n - s p i n coupling between H 1 and C ~a upon simultaneous irradiation with a second radiofrequency field having a frequency w2 and amplitude H2. Upon scanning frequency ~o2 at the m o m e n t of resonance w2 = co0C they observed a sharp change in intensity and form of the satellite. Under conditions of a weak radiofrequency field JC13-H >> 7 Ha/2 ~r >> zXu the accuracy of measuring the shift of C la amounts to :E2 Hz. We developed an analogous attachment to a standard JNM-4H-100 high-resolution NMR spectrometer for observance of double heteronuclear resonance of H I - { C 13} and carried out the e x p e r i m e n t a l investigation of p a r a magnetic shifts of C Is in a series of paramagnetic c o m p l e x e s of Ni 2+ a c e t y l a c e t o n a t e with organic ligands. For convenience and speed of mounting the attachment on the operating radiospectrometer, its principal e l e c trical scheme was developed so as not to have an e l e c t r i c a l connection with the spectrometer. The s m a l l changes in construction of the head of the NMR monitor which were necessary for operation of the apparatus are described below upon e x a m i n a t i o n of the block diagram. The block diagram of the attachment together with those units of the radiospectrometer which are used for obtaining double resonance are shown in Fig. 1. The master h i g h - f r e quency quartz generator generates at the output two voltages having frequencies of 5000.8 and 25004.0 kHz. The voltage of frequency 25004.0 kHz acts as a heterodyne on the balance mixer No. 2. The voltage of frequency Institute of C h e m i c a l Kinetics and Combustion, Siberian Branch of the Academy of Sciences of the USSR, Novosibirsk. Translated from Zhurnal Strukturnoi Khimii, Vol. 12, No. 2, pp. 245-251, March-April, 1971. Original article submitted March 18, 1969.
I
0 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever [ without permission of the publisher. A copy of this article is available from the publisher for $15.00.
223
Double-resonanceattachment
iNM-4H-100. -I"
radiospectrometer II !
I
Head of NMR ~l monitor
ITransmitter[ 100 MHz J
Amplifier ~ 25 MHz
~1
i
Receiver
t
,
Amplifier
]
I f Graduated I I / Adjustable[ ~ attenuatorU attenuator l- I 1 : 1, 1 : 10, [ ~ Mixer No. 2 J 1 1:1oo i I
I....
5 MHz
I-
H
Iq H I
Frequency
splitter: !0
Frequency splitter: 4
Mixer No. 1
i
t Modulator
detector
4 kHz
I generator I ] 5 MHz; ]
I
25 MHz
Frequency[~ gage
l
]-
T
frequency generator 9-21 kI-Iz
l o Recorder Oscillograph Fig. 1. Block diagram of the attachment for double heteronuclear resonance of H I - {C ~3} . 5000.8kHzaetsthrough the resonance amplifier on the frequency splitter having a splitting coefficient of 10. At the output of this is split a voltage of frequency 500.08 kHz which acts on a frequency splitter having a splitting coefficient of 4. At the output of the latter the voltage of frequency 125.02 kHz acts as a heterodyne on the balance mixer No. 1. To this same mixer is supplied a voltage from a variable audiofrequency generator, the frequency of which is measured with an electronic frequency meter and can be varied in the region of 9-21 kHz. Two side bands of frequency 125.02 - (9-21) and 125.02 + (9-21) kHz are formed at the output of the mixer as a result of conversion. The lower side band is the effective band. The voltage of the output signal from mixer No. 1 is fed to mixer No. 2. Two side bands of frequency 25004 to 125.02 - (9-21) and 25004 + 1 2 5 . 0 2 - (9-21) kHz are formed at the outlet of mixer No. 2 after conversion. The operating frequency band is the upper side band. The voltage at the outlet of mixer No. 2 acts on a oscillatory circuit built into the head of the NMR monitor through the attenuator and high-frequency resonance amplifier. The electric scheme of the oscillatory circuit is presented in Fig. 2. To excite C is transitions a radio-frequency field of the corresponding frequency is formed by the Helmholtz coils LI and L2. They are wound on a glass tube on which hot air is passed to the sample for heating. The arrangement scheme of the coils is shown in Fig. 3. The basic technical characteristics of the attachment are presented below. Excitation frequency C 13 25004 + 125.02 - (9-21) kHz. Interval of frequency change 12 kHz. Accuracy of measuring frequency (maximum) 9 1 Hz. Maximum value of strength of the radio-frequency irradiating field H 2 1 0 e . Voltage of power supply 220 V (50 Hz). Input 180 W. A frequency scan was used to record the proton spectrum. It should be noted that the used method makes it possible to determine c h e m i c a l shifts only for C la nuclei bonded directly with the proton, the signals of which (and
224
c2
T c,
l
::
f')
:i_
,_ ,7gram Ill I / / i l l
2
/
7 #~mm
3
Fig. 2 Fig. 3 Fig. 2. Schematic diagram of the oscillatory circuit: C I = 320, C 2 = (3-20), C a = 43 nF, L1 = L2 = 0.5 MN. Fig. 3. Arrangement scheme of the Helmholtz coils: 1) glass tube for introduction of air; 2) Helmholtz coil; 3) coil of bridge transmitter. correspondingly, the satellites) we observe. Satellites due to further spin-spin coupling between H 1 and C la are normally not observed in the background tails of the intense central signal. The ligands with a natural content of the C ~a isotope investigated by us are enumerated in Table 1. In this work we carried out the measurement of paramagnetic shifts only for methyl C la atoms. This is because the conditions of recording double resonance of H 1 - { C 13} are most favorable for these carbon atoms. To determine absolute paramagnetic shifts in the complex we used the dilution method [5] which is used in the case of rapid exchange of the ligand between the first coordination sphere of the complex and the solution. The completeness of averaging of the spectra was followed in all cases by studying the temperature dependence of paramagnetic proton shifts. These shifts change linearly with inverse temperature, which indicates complete averaging of the proton spectrum. One can be restricted in the study of the temperature dependence of proton shifts in the investigated complexes, since they exceed in value (in Hz) the paramagnetic shifts for the C la isotope. In this case the completeness of averaging of the proton spectra automatically indicates that the condition of rapid exchange is also fulfilled for C 13. The dependences of the observed paramagnetic shifts on the ratio of mole concentrations of Ni 2+ to ligand are expressed in Fig. 4. Extrapolation of shifts to the ratio [Ni]/[L] = 1 gives a doubled value of shift in a 25p C complex (the investigated complexes evidently have the composition Ni(AA)2 9 2L [5]). We also carried out an analogous procedure for proton shifts. Since the contribution of pseudocontact splitting for nickel complexes, as a rule, is very small [6], it can be considered that in the investigated complexes the paramagnetie shifts are due to isotope contact splitting. To calculate the hyperfine splitting constants a C and a H and correspondingly the spin densities PC and PHin this case the known relations can be used:
6p ~
a. phi--- a n "
2S
v, gf s(s + l)
a~ - -
aa 508.2
V;~
and
3kT
pc
(I)
'
4ac ac2,
ac 2S----1tt2
8,
(1')
where S is the spin of the paramagnetic portion (in this case S = 1); g is its g-factor (in the calculation we assumed g ~. 2); /3 is the Bohr magneton; 7e and YN are the paramagnetic ratios for the electron and nucleus, respectively; aH Is and ac 2s are the hyperfine splitting constants of the proton with the 1s-electron and of the C 13 nucleus with the 2s-electron [7]. The values of 5p(T)and the corresponding values of a i constants in the investigated complexes are presented in Table 1. The investigation of proton paramagnetic shifts [5, 8] showed that in complexes of Ni 2+ with (I) and (II) the unpaired electron is delocalized on the rr-orbits of the ligands. To consider a C constants in these systems we used the theory of isotopic hyperfine splitting with C 13 nuclei in rr-electron radicals which was developed in detail by
225
TABLE 1 Ligand
H
H
I
pcm
~CH 06r' I-I '
acH', Oe
--0,046
--3,3.t0-~
--0,087
+0.082
+20
q-0,026
q-1,86.t0 -4
+0,046
--0,042
--t00
--0,13
--9,4.10-4
--0,3t
+0,i5
+200
+0,26
+18,8.t0 .4
+0,67
+0,04
+60
+0,077
-t-5,52.10-4
-t-0,15
+0,036
Scp Oo30K) *
a ~ H3, O ~
--36
tt
H/~N/\CH,
o H
II
H I
CHa
"(T ~/~:'\H 0 H tt-.,i~/I'I
III
/1(//.\ r1 I H N--It
Am IV
Ctt, H2N--CLCH, \ CH, H
H
I]/CH,
V i_i,/~N:\H
* Shift in weak field is considered positive. I" aHCH is the hyperfine splitting constant for the H atom occupying the same position as the C atom of the CH a group [for example, in the case of (I) for the a-hydrogen atom]. For (IV) the constant OHCH is evahated from aHCHa for (IID calculating the difference in spin densities on the nitrogen atoms in (III) and (IV) [16].
/ Karplns and Fraenkel [1]. For a planar fragment
X1--C
X~ they used the following expression for the a C
\ Xs
constant: 3
$
~
oo t~i
Qx, ~cp, 9
(2)
'i~i
Here ?rr and Oi 7r (i = 1, 2, 3) are the It-electron spin density on the C and Xi atoms, respectively; SC determines the contribution of the 1s-electrons to the a C constant and QC is the contribution of 2s-electrons. CalcuC' lated values of constants for the fragment
\ /
C--H,, where the C' atoms also have sp ~ hybridization, are equal to:
C' SC
=-
12.7 Oe, O~u = -
23,72i Qcca = + 19.47, Gt
G O c c ' - q- 14.44, Qcc, ---- - 13.94oe.
(a)
The upper index indicates the investigated nucleus, C C ' and CH are the spZ-o-orbitals of the C atoms bonded with C' and H, and C ' C are the o-orbitals of C' atoms bonded with the C atom. As experiments showed [1, 2], the
226
constants presented above describe the experimental data quite well. In the case when the neighboring carbon atom C'has sp a hybridization (as, for example, in the ethyl radical) [2], the following is obtained:
~(c'3), Hz D "
+Sg -
"/
ig
/
+gO+40 -
QcCc, = + 30,0 ce, QcCc, = _ 20,9. oe.
7"
i +2O
'
-~gl
Complexes of Ni(AA)2 with (I) and (II) correspond to the latter case. To describe hyperfine splitting in these complexes it is necessary to choose between two models, a polarization model and a hyperconjugation model. These models [9, 10] were discussed in the literature during the analysis of hyperfine splitting in the ethyl r a d i c a l Both models predict an approximately identical spin density on the methyl protons. On the other hand, these models predict different spin populations of atomic orbitals of the methyl carbon. A critical comparison of both models showed that in the ethyl radical the hyperconjugation model most adequately gives the distribution of spin density [11]. An analogous comparison can also be carried out for complexes of Ni 2+ with (I) and (II). Thus, in the framework of the examined models the following relations between the constants aHCHa and a c C H s [2] can be obtained:
.~
~
I.
t 2
t 4
I 6
J
8
--
~t
~ .f$O
lO
I ,
12
(4)
I
g
Fig. 4. Dependence of observed contact shift on the ratio [Ni]/[L] in complexes of Ni 2+ ace tylacetonate with organic ligands (see Table 1).
polarization model a cCH3 ~ - 2.5 aH CH3,
(5)
hyperconjugation model acCH3 ~ - 0.67 aH CHa,
(6)
The experimentally observed ratio CH3
ac
^
CKa
-~
N~ - - O , a g a ~
(7)
is significantly closer to (6) than to (5). Consequently, in r - e l e c t r o n ligands, as in rr-electron radicals, the hyperconjugation model describes in the best way the experimentaI distribution of spin density. Our experimental results make it possible to determine experimental values of QCC ,C'. Thus it follows from the experimental relation (7) and the similarity of constants aHCH3 and aHCH3 that CH'
acCH' _
aCHH,~a--~-:Q~.
-o.59. =
From which QCC ,C' ~ - 14 Oe. It should be noted that the negative sign of the constant QCC ,C' arising from our experiments is in agreement with theoretical considerations [2]. If possible effects of polarization of = C' - H bonds are considered (theoretical evaluation [3] shows that in the framework of the hyperconjugation model the polarization effect in the ethyl radical can give a contribution to the constant acCH3 of the order of 30%), then the value of QCC ,C' will be smaller. In this way we obtain Ct
[ Q ~ , I ~ < 14 oe:
(s)
It should be noted that I Qcc,C'I ,-~ 14 Oe also follows from data of Fessenden on the EPR spectrum of the ethyl radical (sample containing enrichment of the C 13 isotope to 50%) [12]. Comparison of the experimental value of QCC ,C' with that calculated (4) shows that calculation gives a value of QCC ,C' that is too high. The experimental value of QCC ,C' is close to that calculated for a planar fragment (3). It is not difficult to show that an analogous relation is also observed for QCC ,C' constants. In fact, it follows from the relation O ( O c - c ' ) ~ - P (~ that QCC ,C ~ - 4/3 Q c c ' C ~ + 19 Oe (or less), which approximately corresponds to the value of QCC' for a planar
$
O( 0 C - C ) is the spin density on the o orbitals of the C atom bonded with the C' atom.
227
fragment (3).* In this way a change in hybridization of the second carbon is reflected significantly less on the values of QCC ,C and QCC ,C' than is predicted in [1, 2].
Fig. 5. Dependence between spin densities on H(PH) and C(Pc) atoms in an identical position.
It is expedient to dwell slightly at length on the results which can be associated with this conclusion. In [1, 2] in the examination of hyperfine splitting the values of spin-polarization constants Q are assumed to be dependent both on one-center and two-center double integrals, while on the strength of the significant dependence of the latter on interatomic distance and hybridization the obtained values of the QCC, C' constants for sp~-sp z and spZ-spz of C - C ' bonds are different (-13.94 and - 2 0 . 9 0 e [1, 2]). On the other hand, it is interesting to note that a calculation scheme has recently been proposed in the literature which, only considers one-center exchange integrals [13-15]. In particular, the INDO scheme [15] used in calculating hyperfine splitting constants of a whole series of free radicais led to good agreement of theoretical values with experimental values. The conclusions obtained by us on the constancy of values of spin-polarization constants can evidentIy indicate the validity of introducing only one-center exchange integrals into calculations schemes.
In complexes of Ni 2+ with N-methylaniline, tert-butylamine, and B-picoline the unpaired electron is delocalized along o-bonds. Hyperfine splitting with C 13 nuclei in o-electron systems has not been discussed in the literature, as a result of which it is difficult at this time to make definite conclusions on mechanisms of such interactions. However, attention should be paid to the fact that in all of the studied complexes (also including zr- electron ligands) the spin density on C 13 and the H 1 atom in the same position (for example, on the methyl carbon and the NH proton in N-methylaniline) are approximately identical (Fig. 5). This conclusion is very significant, since from it follows that the laborious procedure of determining a C values can be replaced in a series of cases by the determination of the corresponding a H constants. Thus, it is possible at the present time to predict values and signs of aC constants in many cases. Further accumulation of experimental data and the development of theoretical concepts in this area wflI make it possible to analyze in detail the regularities of hyperfine splitting with C 13 nuclei in o-systems. The authors express their gratitude to P. V. Schasmev and N. D. Chuvylkin for helpful discussion, to V. S. Kuzhutov for mounting the apparatus, and to 7u. M. Berus for participation in the experiment. LITERATURE CITED 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
M. Karplus and G. K. Praenkel, I. Chem. Phys., 3___5_5,1312 (1961). H . L . Strauss and G. K. Fraenkel, J. Chem. P h i . , 3_~5, 1738 (1961). G . M . Zhidomirov and P. V. Schastnev, Teor. Eksp. Khim., 1_, 649 (1965). L H. Alien, L K. Becconsall, and D. W. Turner, J. Sci. Instrum., 41, 673 (1964). E.E. Zaev and Yu. N. Molin, Zh. Strukt. Khim., 7, G80 (1966). D.R. Eaton and W. D. Phyllips, in: Advances in Magnetic R~sonance, Vol. 1, Academic Press, New York (1965). J. R. Morton, Chem. P~v., 4_ 453 (1964). R. W. Kluiber and W. Dew Horrocks, I. Amer. Chem. Soc., 87_, 5350 (1965). A.D. McLachlan, Mol. Phys., 1_, 233 (1958). D.B. Chesnut, I. Chem. Phys., 29.__,43 (1958). G.M. Zhidomirov and P. V. Schastnev, Zh. Strukt. Khim., 8, 974 (1967). R.W. Fessenden, I. Phys. Chem., 71___.74 (1967). R.W. Dixon, Mol. Phys., 12_._,83 (1967). H. Kato, T. Yonezawa, and H. Koneshi, Bull. Chem. Soc.Japan, 40, 1017 (1967).
*The experimental value of the constant t a c 1 = 39.07 Oe for the methylene carbon in the ethyl radical also agrees with this value of QCC,C' [12].
228
15. 16.
J.A. Pople, D. L. Beveridge, and P. A. Dobosh, J. Chem. Phys., 47._, 2026 (1967). Yu. N. Molin, Doctoral Dissertation [in Russian], Institute of Chemical Kinetics and Combustion, Novosibirsk (1970).
229
