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NMR in paramagnetic complexes of radicals with organic ligands
Chemical Physics 15 (1973) 321-330 0 North-liolland Rblishing Company
NMR IN PARAMAGNETIC COMPLEXES OF RADICALS WITH ORGANIC LIGANDS 111.Method of identification of electronic structure of complexes - Experiment N.A. SYSOEVA, A.Yu. KARMILOV and A.L. BUCHACHENKO Imtitutc of Chemicd Physici, Academy of Sciences. Voiob_vemkoe chaussee, 26, Moscow V-334. iJ!XR
Received 22 October
1975
Paramagnetic shifts and broadenings of the NhfR I’mes of different organic molecules which result from their corn&x+ ation with stable nitroxide radical are given and analyzed in terms of the theory reported previously. The main problem that is, what orbit& of the partners arc involved in the for&ion are identified and their electronic structure is established.
of the complex
- is s&cd.
Different
types of complexes
1. Introduction
2. Experimental
In the precedicg papers [1,2] the theory of paramagnetic shifts and line widths of solvent’molecules
The NMR spectra of the iolutions of different organic substances in presence of the stable organic radi-
induced by their complexation with organic radicals was developed, and the theoretical conclusions were summarized, showing what experimental data are required to identify the geometric and the electronic structure of the complexes. In the present paper the experimental results on the paramagnetic shifts and the line widths are discussed in terms of the theoretical treatment given in paper [2] which hereafter is referred to as Part II. The electronic structures of the nitroxide radical complexes with the different ligand molecules are established and their classification is given. The last is based on the taking into account of the atomic or molecular orbital types which the partners of the complex place at common disposal, rather than on a consideration of the interaction types such as hydroging bonding, charge transfer, etc., of which the unambiguous identification.and discrimination is very difficult because of the complex and composite character of the electronic interaction between the partners.
Cal
Me
Me
N--O*
(0
i’;i Me
Me
were recorded at room temperature. The spectrometers used were a high-resolution Varian 4H-100 model and Sov-60 for PMR spectra. The 13C NMR spectra were detected by a IS.1 MHz spectrometer operating in the time sharing mode. The proton paramagnetic shifts were measured with respect to the position of the internal standard (TMS), the 13C paramagnetic shifts were determined in reference toBn external standard. The correction for the volume magnetic susceptibility was determined from the paramagnetic shifts of the cyclohexane protons which weie toting to experience a shift due to magnetic susceptibility only.
:322.‘
.,
I
N.A. Sysoeva et al./NhfI? in paramagizctic complexes ofradicals with orgmic lkar Ids. III
Fig. 2. The molecular model of the hydrogen bonded complex.
Fig. l.The C~R~I/~~~-IS~,C~R~I/~Z~-[N~and([R~]/Avl) [S], ([R~]/AQ)-[N] depcndences for hydroxyl protons of water (W and ethanol (E).
3..Results and discusion
3.1. Competitiveligands -The concentration dependences of the paramagnetic shifts and the line wid!hs in water-ethanol solutions were found to obey the eqs. (7) and (8) of Part II, & shod in fig. I. The slopes of the ([RX ]/Lil)-[S], (&I~A~i)-_ISI and ([R~l1~2)-[N1,([R=jlAvz)_ [N) dependen& for water and ethanol molecules, respectitily, ace of opposite signs - that is, both ligands cornpgte for the same radical drbital and form complexes with ihe hydrogen bond as was shown before [3] . The molecuiar model of the complex is pre-
sented in fig. 2. The equilibrium constant of the complex formation is 0.7 M- 1 for water molecules. Scheme III {see Part II) was found to describe also the proton paramagnetic shifts in the complexes with methanol and chloroform. In accordance with eqs. (7) and (8) of Part II, the slopes of the concentration dependences of the pammagnetic shifts and line widths in methanol-chloroform mixtures are of opposite sign: positive for methanol hydroxyl protons, and negative for chloroform protons. This shows that both Iigands are complexed with the radical in a similar manner, both of them competfng for the lone pair orbital of the radical. The equilibrium constants are 3.0 and 0.6 M- 1 for complexes with methanol and chioroform respectively, the hfc constants are -0.5 and -0.4 G. As will be shown later the chloroform molecules form also complexes through the p-orbitals of the &lorine atoms but their contribution to the hfc constant at the hydrogen atom is much less. The alcohols and aliphatic amines were found to belong also to the competitive ligands showing opposite slopes of the dependences ([RI: ] /S)- [S1. The competitive interaction of water, alcohols, amines, CHCI:, and other similar molecules is not unexpected because all of them form hydrogen bond complexes of n-u type with the lone pair orbitai of nitroxide radical. It is more difficult to predict what kind of complexes can be formed between the radical and the molecules with the lone pair orbitals, double bonds, triple bonds, aromatic systems, etc. The study of the paramagnetic shifts in mixtures of molecules of different types enables to solve the problem what type of radical and ligand orbitak are involved in the formation of complexes.
N.A. Sysoeva et al./NMR in paranta~nerretic complexes of radicals with organic ligands. III
The paramagnetic shifts of 13C NMRlines of cyclohexane and Ccl, molecules were observed. Their complexation with the nitroxide radical (1) was found to be also competitive: the concentration dependences of the 13C paramagnetic shifts in carbon tetrachloridecyclohexane mixture obey the eqs. (7) and (8) of Part II with a positive slope for Ccl,, and a strongly negative slope for the C6H1, molecules. This shows that both ligands form radical complexes with the participation of the same radical orbital. The equilibrium and hfc constants determined from eqs. (7) and (8) providing kC14 gk6H12 Z&e KCCI,,= 0.5 M-‘,
%cWl,)
= 2 G,
‘C&z Q,&H,~)
= 0.05 M-1, = 0.3 G.
The ligand competition for the radical orbitals was shown to exist also between cyclohexane molecules and the aiiphatic groups of the other molecules. The W paramagnetic shifts of dipropylacetylene molecules studied experimentally may be supposed to be due to complexes with different structure: (1) com-
plexes with the triple acetylenic bond, and (2) complexes with aliphatic C-H bonds similar to complexes with CbH,,. If the contribution of complexes of the first type was predominant one should expect a decrease of the paramagnetic shifts along the aliphatic chains starting from the triple bond. However the relative values of these shifts (6,_CH2 = 1.4, 6&CH2 = 1.O and kH3 = 1.2 ppm at [RX] = 0.5 M) indicate that these shifts are induced mostly in complexes with C-H bonds rather than with a triple bond. A similar behavior of the concentration dependences of the t3C paramagnetic shifts was observed in the cyclohexane-tertbutylamine solutions for the carbon atoms of cyciohexane molecules and CH, groups of amine. From the analysis of these dependences in terms of eqs. (7) and (8), Part II, the equilibrium constant for the CH, group of amine was determined to be &.js(amine) = 0.1 M-1, ar-~3 = 0.15 C. This indicates that apart from the standard hydrogen bond complexes formed, as was shown before, via the NHZ protons of amine and the lone pair radical orbital tertbutylamine forms complexes of another unusual type with the participation of CH3 groups. It also follows that CH3 groups of amine and cyclohexane molecules are using the same radical orbit& for the complexation, and that the compIexes of this type produce the main
323
contribution to the paramagnetic 13C shifts of both ligands. It is worthj, of note that the’complexes with the participation of C-H bonds are much less stable than the hydrogen bond complexes considered above, as follows from the comparison of their equilibrium constants. 3.2. Norrcornpctitive
l~ancls
The concentration dependences of the paramagnetic shifts for OH protons in the radical complexes of methanol, ethanol molecules measured in CC14 and C6H,, solutions were found to be the same in both solvents in spite of the fact that Ccl,., and ChH,, molecules, as was shown before, form complexes with the radical with different equilibrium constants. This indicates that the hydrogen bonded ligands on the one hand and carbon tetrachloride and cyclohexane molecules on the other hand use different radical orbitals for their complexation and that their complexation corresponds to Scheme IV, Part II, for noncompetitive ligands. The former were shown before to be formed with the participation of the lone pair orbital of the radical (n-a type complexes), the latter are reasonably assumed to form complexes with the participation of the n-orbital of the radical (n-o type complex in the case of cyclohexane, and rr-n type complex in the case of carbon tetrachloride molecule, where the n-electron orbital of the unpaired electron interacts witi the lone pair orbitals of chlorine atoms). The X-U complexes are of particular interest because they are probably placed on the reaction coordinate and are the precursdrs of the transition state in the atom transfer radical reaction. As was shown in the preceding section all molecules investigated which have aliphatic C-H bonds are competitive ligands since they form complexes with the participation of these bonds, but they do not compete with the hydrogen bonded ligands (water, alcohols, amines). This results in the conclusion that all molecules with aliphatic C-H bonds take part in the formation of n-u complexes. This conciusion was proved by studying of the concentration dependences of the proton and 13C paramagnetic shifts of different molecules’ in solvents with competitive or noncompetitive interactions. The summary of these results is given in tables 1 and 2. The inspection of these tables shows that most of
~~324..‘;:..
-.,..
..-. ^._.
..Tiblcl.
N..k Sys&iet
--,
,...
‘. L,ig&&compcting
al:fNMR
&.para+gwtic
complexes of radicals with Organic ligands.
,:.
with ihesalven<.
-, Functionrtl
..
('3~) jNHl
III _
..
-
Salvcnt
PUP
ti of. complex
K(rK’ f
a.7 0.6
QK2 -0.5 - 0.4
OH protons CH protons
CzHsOH
n-o
CH3OH
n-o
NH2 protons
CH3OH
n-a
_’
CH3 13C
CbHn
r--b
0.1
0.15
n-n 77-n
0.5 0.05
2 0.3
n--o
_
CC&
1%
CqH12
‘3c
HJC-CH2-CH2-Cz =C-CH2-CH2-CH3
CH2, CH3 13C
C6H12 .CC4 Cbhz
Table 2 LjgdndS not competing
with the solvent -
Molecule
Solvent
Functional group
KW’) 3 0.7
- 0.5 - 0.35
0.6
- 0.4
CH3OH C2HsOH
OH protons OH protons
CC&C&t2
n-o
(334,
n-o
(CH&NH2
NH2 CH protons CHJ “C
C6H12 CC4 CHJOH
n-a l-l-lr x--o
CH2 protons
CH30H
K-0
CHCIJ CtjHsOCN3
.c Me Me
CbH12
a(G)
_
N--H
0
-.
Typeof complex
Me
Me
the ligands investigated are able to form railical complexes of different type according to the nature of the chemical group w&h is used for the complexation with the radical. As was pointed out before the concentration dependence of the paramagnetic shifts for NH2 protons of terbutylamine in C!6H11solutions corresponds to Scheme IV (Part If) for noncompetitive ligands. On the contrary the concentntion dependence of the t3C -paramagnetic shifts fcr the CH3 group.in.the same so‘lutions obeys to the equations of Scheme III for the competitive ligands. This shows that the terbutylamine molecules are able-to form complexes-of different structure and stabiIity:.an n-u complex via the ione pairorbital of the radical and the N-H u-bond,
and a rr-u complex via the n-orbital of the unpaired electron and the ahphatic C-H o-bond. The corresponding equilibriurnconstsnts are given in table 2. As seen from table 2 the molecules of CC], and CHCI, belong to noncompetitive ligands since the concentration dependence OFthe paramagnetic shifts of the chloroform proton in CC14solutions obeys eq. (9) of Scheme IV For noncompetitive ligands. This shows that the proton paramagnetkshifts of CHC13molecules are mainly due to n-u complexes with the hydrogen bond rather than to n-q complexes similar to the one formed by CC!, molecules. This conclusion is an accordance with CHC!s-CH30H competitive interaction,as.was pointed out before. The atisole molecules were found to form radical.
..
N.A. Sysoeva er ai. fNMR in pa&magnetic
&plexes
cbmplexes of different types. The NMR lines of the aromatic-protons are high-field shifted, while the NMR !ines of the aromatic carbon atoms experience small paramagnetic shifts to the lower tield (5 = 1 ppm at [RJ = 0.5 M). Therefore the hfc constants at the aromatic carbon atoms are positive while at the aromatic protons they are negative. There is also a linear correlation dependence between the paramagnetic shifts of the carbon atom and the neighbouring proton. For both carbon atoms and protons the pammagnetic shifts were found to be inversely proportional to the electronegativity of both carbon atoms and protons. Such a behavior of the signs and values of the paramagnetic shifts suggests that in this case, too, the hydrogen bond complexes are formed via the C-H ubonds of the aromatic ring. The NMR line of the methoxy carbon atom of anisole was found also to shift strongly to the lower field. To establish the structure of the complexes which are formed between the radical and methoxy group of anisole the 13C pammagnetic shifts of this group were investigated in C,H,, and CH3OH solutions. These shifts appeared to be practically identical in both solvents - that is, the presence of the methanol molecules strongly complexed via the n-radical orbital does not influence the paramagnetic shifts of the methoxy group of anisole. This demonstrates that anisole (with respect to the methoxy group) and methanol molecules are noncompetitive ligands and therefore the methoxy group of anisole forms radical complexes of n-o type via the n-orbital of the unpaired electron and the C-H u-bond of the CH30 group. A similar behavior was observed for sterically hindered amine
c
of ra&cak
wills organic ligands. IIf
Similar complexes were demonstrated to exist brtween the radical and the CHj group of acetone, CH3 and CHZ groups of molecules with fragments
_=Eo& 3
//”
-C-W,
and others. In all these cases, including anisole molecules, paramagnetic shifts to the !ow field were observed for the carbon Xoms of these groups and small shifts to the high field for the neighbouring protons. It demonstrates that there is a positive spin density at the carbon atoms and a small negative spin density at protons in accordance with the results of quantum chemical calculations of K-U complexes of the model system H2NO .a*H’CH3 (fig. 3) in the INDO approximation performed by A. Kabankin. The results are given in table 3. The stabilization energy of the complex at the equilibrium distance r = 2.0 A does not exceed 1 kcal/mole, that is much lower than the bond energy in n-u complexes [4]. The spin density at the 2s A0 of the ligand carbon atom (&) is positive in accordance with experimental finding whereas the spin densityat the Is A0 of the hydrogen atom bonded to the radical is negative. Similar results were obtained by the CNDOjSP method which was described in papers (5,6]. The advantage of this method is that the spin density matrix can be divided into three parts that are due to delocaliiation mechanism (po), spiu polarization (psp) and spin exchange polarization (pep). These contributions are listed in table 4. The inspection of this table shows that the mechanism producing spin density at different atoms in ligand molecule are different. The
Me Me
0
N-H
Me
Me
The paramagnetic shifts of the CH2 protons of this amine in methanol solution are a little higher than in Ccl4 solutions at the same radical and amine concen tration. This indicates that amine molecules form n-u complexes via CH2 groups and the n-orbital of the radical similar.to the competitive CC14 molecules.,
32.5
Fig. 3. The molecular tiodel.of the S-O complex.
;,,~~~&‘)(4): _’
.Total energ .(a~)
-40.0836
.:Stabilization iner&kcallmole)
0
:
C
(CHjjaveraged)
:
MCconstants (G)
.-2.10
1.90
1.70
-40.0843
-40.0847
-40.0848
-40.0835
0.46 -1
p2s
.. -.
2.30
o.
PlS.
~1s
‘.
atr_=Z!.lOA.
.m.
-
.,’ H’
..
J
0
0
0
x
0.70
lo-’
;
-5
0.78
x 10-4
1.1o-4
poSithe spin density at th& carbon.atom is induced.me-
dominantly by a spin delocaIization mechanism whereas the main contribution to the negative spin density at hydrogen atom H’ is due to spin polarization. These results are not corisistent with the conclusion made by 1. Moris&ma and coworkers [7,8] that the distribution of the spin dens&r in II-U complexes is due to spin polarization of the ligand u-bond with the unpaired electron of the radial.
-
-0.04
-16 x 1O-4
-0.33 x 10-4
,:
-
-46 x 1O-4
-012699
4 x lo+
14 x 104-
0.0820
0
-1 x 10-4
-0.0178
groups shown in fig. 4 are described by eqs. (16) and (17), Part II. In agreement with the prediction of Scheme VI the ratiosof the interception ,ro the slope for ail aliphatic groups found from these dependencesare pr&ztically identical and equal to 0.07 and 0.08 M-l. Therefore the complexes of the different aliphatic groups of dipropylacetylene molecule are formed with the participation of the same radical orbital, namely the n-orbital of the unpaired electron.
3.3. The iigonds with the different functioml groups The theor&caI expressions for the concentration dependences of the paramagnetic shiits of ligands with different-functional groups were derived in Part II (Schemes VI and VII). Scheme VI was found to comply with the concentration dependences of the paramag&ic shifts experienced by the carbon atoms of the liphatic chains in dipropylacetylene molecules. .I’ was shown bef&e that the shifts are induced in n-o comptexes formed with the aliphatic C-I-I ubonds rather than with the triple bond. The concentration depepdences of the shifts for CH2 and CH3
Fig-
The components ofspin density matrix at l&and atom orbitals in the r-0
Ator;:
A0
1 Is
H’ c-,....
:
2s
-.
p
F0
~0.04x 10-4 ,I&35
x .io-4
-2.16
4. m-
(2) a$
(I
rKH2)
Rx1 /6)-N dependences for CH3 (I), f3-CH, (3) groups of dipropylacetylene.
complex )12NO--:H’CH3,r(O.--H’)
P
X 10-4.
YO.35 x 1o-4
,totaiJ
ep
-0.52
X 10T4
-.
-2.64
. .
._ -,
;.. . .
X 10r4.
1252 x io-4 :
2.51 x 10-4
-. :..
= 1.9 A
;
.. N.A. Sysbevdet al.iNhfRin p4~4~~4gneticcomplt-xes
scheme VII, describing the complex&on of the different functional &and groups with different radical orbitals was found to conform to the concentration dependence o! the paramagnetic shifts fdr NH2 protons and 13C nriclei of CH, groups in tertbutylamine molecules which form complexes of both types, n-u au4 n-u’. This case was considered above in detail. In studying the paramagnetic shifts of n-propylactylene protons both schemes, VI and VII, were found to be valid. The concentration dependence9 of the shifts for the acetylene proton and the a-CH2 protons in C,D,, solutions are identical and the equilibrium constants estimated from these dependences are also almost equal (0.10 and 0.14 M-l respectively). Such a behavior is characteristic for Scheme VI. Ou the other hand the paramagnetic shifts of @-CH2 and CH3 protons do not depend on the concentration of n-propyl acetylene. This shows, in accordance with Scheme VII, ?hat the paramagnetic shifts of acetylene and rr-CH2 protons are induced in complexes of n-u type while the /_LCH, and CH, proton shifts arise in X-U complexes. The equilibrium and hfc constants are K,=O.lM-‘,a,=-0.13 G,andKc~,=O.l4M-‘, aCH2 = -0.01 G, for a-H and a-CH, protons cespectively. We shall show later that this conclusion is not completely correct because the acetylene proton takes also part in the formation of the a-o complex along with n-u complexation but n-u complexes will be shown to be predominant.
of radicalswith&anic ligunds.III
.32-l
‘:
E t
Fig. 5. The ([Rc]I’s)-[S] and ([Rg]/~~)-[S] for a-H protons of dimethyimaleate.
dependences
prediction to be realized the conditions (14) and (15) (Part II) - concerning the relation between the equiiibrium constants of the ligand under consideration and the solvent - should be satisfied. The position of the.
maximum as well as the slopes on both sides of the maximum depend on the difference between these equilibrium constants. To illustrate this approach the concentration dependences of the proton paramagnetic shifts and the line widths of dimethylmaleate molecules
3.4. Two radical orbitals- two ligands competitive interacrion Now we shall consider the more interesting and complicated case where the same functional ligaud group forms complexes with different radical orbitals. This situation arises when some chemical group, for instance actylene proton of phenylacetylene molecule, is able to form complexes of both n-u and II-U type. The theory of this complexation was.given in Part Ii (Scheme V). It was predicted that the concentration dependences of the paramagnetic shifts and the line widths can pass through a maximum pr&ided that the solvent molecules form complexes of one type, n-o or n-u, and interfere with the complexation disturbing the r& lation between the n-u and x-u complexes formed by the chemical group under lnvestlgation.. For this
“\ ON\,/== H&O
/”
/” =\,//” CbC” 3
in CCl, solutions were studied. As seen from figs. 5 and 6 these depeudenc& do pass through-a maximum for both olefinic and CH3 protons. This shows that olefinic and Cl-& groups take part in both n-u, and n-u complexes. From the comparison of the slopes of the concentration dependences it f&lows that the n-u complex formed by the olefinic group is more stable than the n-u complex formed by the CH, group; It is woithy of note-that &specific behavior of .. .. .
:
328
-.
Ar.A. Sysoew et al. fNMR
in par&magneticcomplexes ofradic&
... P
2’.
i
with ot&ic
ligcmd~.III
:-
.
Fig. ?. The ((Rx] {6)-[Sj and ([Rr;l\Av,-(S] dependences for akylene proton of penyIacetyiene in CHsOH solutions.
Fig. 6. The ([Rxllsb[Ej and t[Rx]/Avb[S] for CH3 protons of dimethylmaleate.
dependences
the NMR line width for U-I, protons indicates the important role of the complex formation in the line broadening. Perhaps the paramagnetic line broadening induced by the complexation is significant even in the saturated hydrocarbons and its contribution may be comparable with the commonly accepted contribution produced by intermolecular dipolar interaction between the nucleus under consideration and the unpaired electron of the radicaI not invoIved in the complexation. As shown in fig. 7 the concentration dependence of the acetylene-proton paramagnetic shifts of phenyl:acetylene molecules in CHjOH solutions goes through a maximum at [S] = 2.5 M. According to theory. (Part II) this shows that the acetylene proton forms both n-u and n-o complexes. A similar maximum should appear in CC& solutions but as seen from fig. 8 .it was not observed. The reason for this fkt is that in CH30H so!utions the difference of the equilibrium .constaxits Kn(CH30H) and K’(HC%Ph) is large and the condition (15) is fulfilled atlow concentration of pheny&etyIene, while in Ccl4 solutions the difference
.-
of the equilibrium constants K,(CCh) and K,(IIC~ CPh) is small so that the condition (IS) is fulfilled only at the experimentally unreasonable concentration of ihenylacetylene [S] > [So], where [So] is
Fig. 8. The C[Rz]/S)-(Sl and ([Rz]/Avb[S] dependences for acetylene proton of phenylacetylene in CCQ solutions.
the concentration of pure phenylacetylene. In order to estimate the relative contributions of n-u and ‘II-U complexes formed by the acetylene proton of phenylacetylene molecule the concentration dependence of the paramagnetic line broadening was investigated. As seen from fig. 8, in Ccl, solution this dependence is linear with a positive slope while in CH,OH the slope is strongly negative. This indicates that the main contribution to the line width is supplied by one type of complexes, namely the n-u complex. This is a reason for the strongly negative slope of the concentration dependence in CH30H solution. Using equations for the line widths similar to eqs. (9), (10) of Part II, from the dependence ([RX] /A;)- [S] the following parameters for Ccl, solutions in the presence of noncompetitive ligands were found: K, = 0.25M-l, (D + C), = 400 Hz. It should be noted that the change of the solution viscosity was controlled by the change of the intermolecular broadening of the NMR line of TMS. Using these values and analysing the concentration dependences of the pammagnetic shifts in terms of eqs. (1 1) and (12) of Scheme V, Part 11,the equilibrium and hfc constants for n-o and 8-u complexes were determined: u, = -0.08 G,
a, =-0.06
G,
K,=0.25M-I,
K,=O.lOM+
It is clear that, despite of the fact that both complexes are coexisting, the q-u complexes are more stable and that they give the predominant contribution both to the line widths and pararnagnetic shifts for the acetylene proton of phenylacetylene molecule. Similar behavior of the paramagnetic shifts and the line widths was observed for the proton of the CH group of methylformiate molecules. From a comparison of the paramagnetic shifts in CC14and CH,OH solutions the ratio a,K,,/a,K, was found to be equal to 2.5. Therefore these results demonstrate the predominance of the n-u complex contribution to the paramagnetic shift and the line width. A generalization of these results to other molecular systetis may be carried out and the conclusion can be drawn that the hydrogen atom attached to the 5arbon atom with a double or triple bond is able to forin complexes of n-6 and n-o types, the former being predominant since they are more stable and giving the
leading contributiorito the line width.
the paramagnetic shift and
4. Conclusions First of all the experimental results preser.ted in this paper demonstrate that the theory of the paramagnetic shifts and the line widths proposed in Part II provides a powerful and elegant method for the identification
and deciphering
of the different
types
of comp!exes between the nitroxide radicals and the ligand molecules formed in solution. The theory and the method based on it are not limited to the.particular case of the nitroxide radicals considered here.
There is no problem to generalize and expand them to include any paramagnetic species-Iigands interaction. Among them the complexes of molecular oxygen and paramagnetic ioas are of special importance. We have found that this approach is very useful also in the studying of the structure of catalytic complexes both homogeneous and heterogeneous. It is worthy of note that the knowledge of the structure and geometry of the complex is very important for the development of the quantum chemistry of complexes. The experimental results evidence that the radical-l&and interactions responsible for the formation of a complex are strongly localized and that the radical-ligand bonds are definitely oriented and are of a semichemi-
cal nature. Even the concentration dependences of the paramagnetic shifts in complexes of the nitroxide radical with non-polar molecules, such as Ccl4 , CgH12, C4H14,were shown to be in excellent accord with the ones theoretically predicted by Schemes III, IV and VI. This unambiguously indicates that the paramagnetic shifts in complexes of these molecules result from localized and directed electron-electron Interactions rather than from contact pairs with Van der Waals interactions. Indeed the opposite sign of the slopes of the concentration dependences ([R,]/Q-[S] and ([R~]/SN)-[NJ for cyclohexane and carbon tetrachloridc respectively, experimentally observed in cyclohexane-carbon tetrachloride solutions, cannot be explained in terms of contact pairs. This is because the molar volumes of CbH,, and CC14are almost the same and, therefore, the 13C paramagnetic shifts in these molecuIes should not depend perceptibly on a change of .solution composition
-330 :
N.A. Sysoetia et al./NMti in paramagncric complexes Gf radicals with organic ligp)zds.III
if there wei&a maincontributionof contactpairs [9]. Of course this do& not me& that contact pair interaction is absent hi the solutions at all. More than that, the detailed study of the concentration dependences of the paramagnetic shifts and line widths and their analysis in terms of the theory proposed in Part II enables one to iook for any deviations from the theoretical predictions that might be due to contact pair interactions. At last it should be particularly emphasized that the radical complex formation is not a special case of complexation because the general laws concerning the participation, combination and interaction of different orbitals of the complexed molecules are supposed to be identical foi both paramagnetic and diamagnetic complexes, SCthat the structural properties of the paramagnetic complexes can be transferred aImost without modification to the diamagnetic complexes.
References ( 1 J N.A. Sysoeva, A.Yu. Karmilov and A.L. Buchachenko, Chem. Phys. 7 (1975) 123. (21 N.A. Sysocva, A.Yu; Karmilov and A.L. Buchachenko, Chem. F’hys. 15 (1976) 313, Part 11 of the present scrjes. 13) N.A. Sysoevaand A.L. Buchachenko, Zh. Strukt. Khim. I3 (1972) 42. [4] A.S. Kabankin, GM. Zhidomirov and A.L. Buchachenko, 3. Magn.Resonance 9 (1975) 199. [SJ GM. Zhidomirov and N.D. Chuvylkin, Chem. Phys. Lett. 14 (1972) 52. [6J G.M. Zhidomirov and N.D. Chuvylkin, Theoret. Chim. Acta 30 (1923) 197. [7J 1. Morishima,K. Endo and T. Yonezawa, I. Amer. Chem. Sot. 93 (1971) 2048. [SJ 1. Morishima, K. Endo and T. Yonezawa, Chem. Phys. Lett. 9 (1971) 143,203. 191 LE. Orgcl and R.S. Mulliken, J. Amer. Chem. Sot. 79 (1957) 4839.
NMR IN PARAMAGNETIC COMPLEXES OF RADICALS WITH ORGANIC LIGANDS 111.Method of identification of electronic structure of complexes - Experiment N.A. SYSOEVA, A.Yu. KARMILOV and A.L. BUCHACHENKO Imtitutc of Chemicd Physici, Academy of Sciences. Voiob_vemkoe chaussee, 26, Moscow V-334. iJ!XR
Received 22 October
1975
Paramagnetic shifts and broadenings of the NhfR I’mes of different organic molecules which result from their corn&x+ ation with stable nitroxide radical are given and analyzed in terms of the theory reported previously. The main problem that is, what orbit& of the partners arc involved in the for&ion are identified and their electronic structure is established.
of the complex
- is s&cd.
Different
types of complexes
1. Introduction
2. Experimental
In the precedicg papers [1,2] the theory of paramagnetic shifts and line widths of solvent’molecules
The NMR spectra of the iolutions of different organic substances in presence of the stable organic radi-
induced by their complexation with organic radicals was developed, and the theoretical conclusions were summarized, showing what experimental data are required to identify the geometric and the electronic structure of the complexes. In the present paper the experimental results on the paramagnetic shifts and the line widths are discussed in terms of the theoretical treatment given in paper [2] which hereafter is referred to as Part II. The electronic structures of the nitroxide radical complexes with the different ligand molecules are established and their classification is given. The last is based on the taking into account of the atomic or molecular orbital types which the partners of the complex place at common disposal, rather than on a consideration of the interaction types such as hydroging bonding, charge transfer, etc., of which the unambiguous identification.and discrimination is very difficult because of the complex and composite character of the electronic interaction between the partners.
Cal
Me
Me
N--O*
(0
i’;i Me
Me
were recorded at room temperature. The spectrometers used were a high-resolution Varian 4H-100 model and Sov-60 for PMR spectra. The 13C NMR spectra were detected by a IS.1 MHz spectrometer operating in the time sharing mode. The proton paramagnetic shifts were measured with respect to the position of the internal standard (TMS), the 13C paramagnetic shifts were determined in reference toBn external standard. The correction for the volume magnetic susceptibility was determined from the paramagnetic shifts of the cyclohexane protons which weie toting to experience a shift due to magnetic susceptibility only.
:322.‘
.,
I
N.A. Sysoeva et al./NhfI? in paramagizctic complexes ofradicals with orgmic lkar Ids. III
Fig. 2. The molecular model of the hydrogen bonded complex.
Fig. l.The C~R~I/~~~-IS~,C~R~I/~Z~-[N~and([R~]/Avl) [S], ([R~]/AQ)-[N] depcndences for hydroxyl protons of water (W and ethanol (E).
3..Results and discusion
3.1. Competitiveligands -The concentration dependences of the paramagnetic shifts and the line wid!hs in water-ethanol solutions were found to obey the eqs. (7) and (8) of Part II, & shod in fig. I. The slopes of the ([RX ]/Lil)-[S], (&I~A~i)-_ISI and ([R~l1~2)-[N1,([R=jlAvz)_ [N) dependen& for water and ethanol molecules, respectitily, ace of opposite signs - that is, both ligands cornpgte for the same radical drbital and form complexes with ihe hydrogen bond as was shown before [3] . The molecuiar model of the complex is pre-
sented in fig. 2. The equilibrium constant of the complex formation is 0.7 M- 1 for water molecules. Scheme III {see Part II) was found to describe also the proton paramagnetic shifts in the complexes with methanol and chloroform. In accordance with eqs. (7) and (8) of Part II, the slopes of the concentration dependences of the pammagnetic shifts and line widths in methanol-chloroform mixtures are of opposite sign: positive for methanol hydroxyl protons, and negative for chloroform protons. This shows that both Iigands are complexed with the radical in a similar manner, both of them competfng for the lone pair orbital of the radical. The equilibrium constants are 3.0 and 0.6 M- 1 for complexes with methanol and chioroform respectively, the hfc constants are -0.5 and -0.4 G. As will be shown later the chloroform molecules form also complexes through the p-orbitals of the &lorine atoms but their contribution to the hfc constant at the hydrogen atom is much less. The alcohols and aliphatic amines were found to belong also to the competitive ligands showing opposite slopes of the dependences ([RI: ] /S)- [S1. The competitive interaction of water, alcohols, amines, CHCI:, and other similar molecules is not unexpected because all of them form hydrogen bond complexes of n-u type with the lone pair orbitai of nitroxide radical. It is more difficult to predict what kind of complexes can be formed between the radical and the molecules with the lone pair orbitals, double bonds, triple bonds, aromatic systems, etc. The study of the paramagnetic shifts in mixtures of molecules of different types enables to solve the problem what type of radical and ligand orbitak are involved in the formation of complexes.
N.A. Sysoeva et al./NMR in paranta~nerretic complexes of radicals with organic ligands. III
The paramagnetic shifts of 13C NMRlines of cyclohexane and Ccl, molecules were observed. Their complexation with the nitroxide radical (1) was found to be also competitive: the concentration dependences of the 13C paramagnetic shifts in carbon tetrachloridecyclohexane mixture obey the eqs. (7) and (8) of Part II with a positive slope for Ccl,, and a strongly negative slope for the C6H1, molecules. This shows that both ligands form radical complexes with the participation of the same radical orbital. The equilibrium and hfc constants determined from eqs. (7) and (8) providing kC14 gk6H12 Z&e KCCI,,= 0.5 M-‘,
%cWl,)
= 2 G,
‘C&z Q,&H,~)
= 0.05 M-1, = 0.3 G.
The ligand competition for the radical orbitals was shown to exist also between cyclohexane molecules and the aiiphatic groups of the other molecules. The W paramagnetic shifts of dipropylacetylene molecules studied experimentally may be supposed to be due to complexes with different structure: (1) com-
plexes with the triple acetylenic bond, and (2) complexes with aliphatic C-H bonds similar to complexes with CbH,,. If the contribution of complexes of the first type was predominant one should expect a decrease of the paramagnetic shifts along the aliphatic chains starting from the triple bond. However the relative values of these shifts (6,_CH2 = 1.4, 6&CH2 = 1.O and kH3 = 1.2 ppm at [RX] = 0.5 M) indicate that these shifts are induced mostly in complexes with C-H bonds rather than with a triple bond. A similar behavior of the concentration dependences of the t3C paramagnetic shifts was observed in the cyclohexane-tertbutylamine solutions for the carbon atoms of cyciohexane molecules and CH, groups of amine. From the analysis of these dependences in terms of eqs. (7) and (8), Part II, the equilibrium constant for the CH, group of amine was determined to be &.js(amine) = 0.1 M-1, ar-~3 = 0.15 C. This indicates that apart from the standard hydrogen bond complexes formed, as was shown before, via the NHZ protons of amine and the lone pair radical orbital tertbutylamine forms complexes of another unusual type with the participation of CH3 groups. It also follows that CH3 groups of amine and cyclohexane molecules are using the same radical orbit& for the complexation, and that the compIexes of this type produce the main
323
contribution to the paramagnetic 13C shifts of both ligands. It is worthj, of note that the’complexes with the participation of C-H bonds are much less stable than the hydrogen bond complexes considered above, as follows from the comparison of their equilibrium constants. 3.2. Norrcornpctitive
l~ancls
The concentration dependences of the paramagnetic shifts for OH protons in the radical complexes of methanol, ethanol molecules measured in CC14 and C6H,, solutions were found to be the same in both solvents in spite of the fact that Ccl,., and ChH,, molecules, as was shown before, form complexes with the radical with different equilibrium constants. This indicates that the hydrogen bonded ligands on the one hand and carbon tetrachloride and cyclohexane molecules on the other hand use different radical orbitals for their complexation and that their complexation corresponds to Scheme IV, Part II, for noncompetitive ligands. The former were shown before to be formed with the participation of the lone pair orbital of the radical (n-a type complexes), the latter are reasonably assumed to form complexes with the participation of the n-orbital of the radical (n-o type complex in the case of cyclohexane, and rr-n type complex in the case of carbon tetrachloride molecule, where the n-electron orbital of the unpaired electron interacts witi the lone pair orbitals of chlorine atoms). The X-U complexes are of particular interest because they are probably placed on the reaction coordinate and are the precursdrs of the transition state in the atom transfer radical reaction. As was shown in the preceding section all molecules investigated which have aliphatic C-H bonds are competitive ligands since they form complexes with the participation of these bonds, but they do not compete with the hydrogen bonded ligands (water, alcohols, amines). This results in the conclusion that all molecules with aliphatic C-H bonds take part in the formation of n-u complexes. This conciusion was proved by studying of the concentration dependences of the proton and 13C paramagnetic shifts of different molecules’ in solvents with competitive or noncompetitive interactions. The summary of these results is given in tables 1 and 2. The inspection of these tables shows that most of
~~324..‘;:..
-.,..
..-. ^._.
..Tiblcl.
N..k Sys&iet
--,
,...
‘. L,ig&&compcting
al:fNMR
&.para+gwtic
complexes of radicals with Organic ligands.
,:.
with ihesalven<.
-, Functionrtl
..
('3~) jNHl
III _
..
-
Salvcnt
PUP
ti of. complex
K(rK’ f
a.7 0.6
QK2 -0.5 - 0.4
OH protons CH protons
CzHsOH
n-o
CH3OH
n-o
NH2 protons
CH3OH
n-a
_’
CH3 13C
CbHn
r--b
0.1
0.15
n-n 77-n
0.5 0.05
2 0.3
n--o
_
CC&
1%
CqH12
‘3c
HJC-CH2-CH2-Cz =C-CH2-CH2-CH3
CH2, CH3 13C
C6H12 .CC4 Cbhz
Table 2 LjgdndS not competing
with the solvent -
Molecule
Solvent
Functional group
KW’) 3 0.7
- 0.5 - 0.35
0.6
- 0.4
CH3OH C2HsOH
OH protons OH protons
CC&C&t2
n-o
(334,
n-o
(CH&NH2
NH2 CH protons CHJ “C
C6H12 CC4 CHJOH
n-a l-l-lr x--o
CH2 protons
CH30H
K-0
CHCIJ CtjHsOCN3
.c Me Me
CbH12
a(G)
_
N--H
0
-.
Typeof complex
Me
Me
the ligands investigated are able to form railical complexes of different type according to the nature of the chemical group w&h is used for the complexation with the radical. As was pointed out before the concentration dependence of the paramagnetic shifts for NH2 protons of terbutylamine in C!6H11solutions corresponds to Scheme IV (Part If) for noncompetitive ligands. On the contrary the concentntion dependence of the t3C -paramagnetic shifts fcr the CH3 group.in.the same so‘lutions obeys to the equations of Scheme III for the competitive ligands. This shows that the terbutylamine molecules are able-to form complexes-of different structure and stabiIity:.an n-u complex via the ione pairorbital of the radical and the N-H u-bond,
and a rr-u complex via the n-orbital of the unpaired electron and the ahphatic C-H o-bond. The corresponding equilibriurnconstsnts are given in table 2. As seen from table 2 the molecules of CC], and CHCI, belong to noncompetitive ligands since the concentration dependence OFthe paramagnetic shifts of the chloroform proton in CC14solutions obeys eq. (9) of Scheme IV For noncompetitive ligands. This shows that the proton paramagnetkshifts of CHC13molecules are mainly due to n-u complexes with the hydrogen bond rather than to n-q complexes similar to the one formed by CC!, molecules. This conclusion is an accordance with CHC!s-CH30H competitive interaction,as.was pointed out before. The atisole molecules were found to form radical.
..
N.A. Sysoeva er ai. fNMR in pa&magnetic
&plexes
cbmplexes of different types. The NMR lines of the aromatic-protons are high-field shifted, while the NMR !ines of the aromatic carbon atoms experience small paramagnetic shifts to the lower tield (5 = 1 ppm at [RJ = 0.5 M). Therefore the hfc constants at the aromatic carbon atoms are positive while at the aromatic protons they are negative. There is also a linear correlation dependence between the paramagnetic shifts of the carbon atom and the neighbouring proton. For both carbon atoms and protons the pammagnetic shifts were found to be inversely proportional to the electronegativity of both carbon atoms and protons. Such a behavior of the signs and values of the paramagnetic shifts suggests that in this case, too, the hydrogen bond complexes are formed via the C-H ubonds of the aromatic ring. The NMR line of the methoxy carbon atom of anisole was found also to shift strongly to the lower field. To establish the structure of the complexes which are formed between the radical and methoxy group of anisole the 13C pammagnetic shifts of this group were investigated in C,H,, and CH3OH solutions. These shifts appeared to be practically identical in both solvents - that is, the presence of the methanol molecules strongly complexed via the n-radical orbital does not influence the paramagnetic shifts of the methoxy group of anisole. This demonstrates that anisole (with respect to the methoxy group) and methanol molecules are noncompetitive ligands and therefore the methoxy group of anisole forms radical complexes of n-o type via the n-orbital of the unpaired electron and the C-H u-bond of the CH30 group. A similar behavior was observed for sterically hindered amine
c
of ra&cak
wills organic ligands. IIf
Similar complexes were demonstrated to exist brtween the radical and the CHj group of acetone, CH3 and CHZ groups of molecules with fragments
_=Eo& 3
//”
-C-W,
and others. In all these cases, including anisole molecules, paramagnetic shifts to the !ow field were observed for the carbon Xoms of these groups and small shifts to the high field for the neighbouring protons. It demonstrates that there is a positive spin density at the carbon atoms and a small negative spin density at protons in accordance with the results of quantum chemical calculations of K-U complexes of the model system H2NO .a*H’CH3 (fig. 3) in the INDO approximation performed by A. Kabankin. The results are given in table 3. The stabilization energy of the complex at the equilibrium distance r = 2.0 A does not exceed 1 kcal/mole, that is much lower than the bond energy in n-u complexes [4]. The spin density at the 2s A0 of the ligand carbon atom (&) is positive in accordance with experimental finding whereas the spin densityat the Is A0 of the hydrogen atom bonded to the radical is negative. Similar results were obtained by the CNDOjSP method which was described in papers (5,6]. The advantage of this method is that the spin density matrix can be divided into three parts that are due to delocaliiation mechanism (po), spiu polarization (psp) and spin exchange polarization (pep). These contributions are listed in table 4. The inspection of this table shows that the mechanism producing spin density at different atoms in ligand molecule are different. The
Me Me
0
N-H
Me
Me
The paramagnetic shifts of the CH2 protons of this amine in methanol solution are a little higher than in Ccl4 solutions at the same radical and amine concen tration. This indicates that amine molecules form n-u complexes via CH2 groups and the n-orbital of the radical similar.to the competitive CC14 molecules.,
32.5
Fig. 3. The molecular tiodel.of the S-O complex.
;,,~~~&‘)(4): _’
.Total energ .(a~)
-40.0836
.:Stabilization iner&kcallmole)
0
:
C
(CHjjaveraged)
:
MCconstants (G)
.-2.10
1.90
1.70
-40.0843
-40.0847
-40.0848
-40.0835
0.46 -1
p2s
.. -.
2.30
o.
PlS.
~1s
‘.
atr_=Z!.lOA.
.m.
-
.,’ H’
..
J
0
0
0
x
0.70
lo-’
;
-5
0.78
x 10-4
1.1o-4
poSithe spin density at th& carbon.atom is induced.me-
dominantly by a spin delocaIization mechanism whereas the main contribution to the negative spin density at hydrogen atom H’ is due to spin polarization. These results are not corisistent with the conclusion made by 1. Moris&ma and coworkers [7,8] that the distribution of the spin dens&r in II-U complexes is due to spin polarization of the ligand u-bond with the unpaired electron of the radial.
-
-0.04
-16 x 1O-4
-0.33 x 10-4
,:
-
-46 x 1O-4
-012699
4 x lo+
14 x 104-
0.0820
0
-1 x 10-4
-0.0178
groups shown in fig. 4 are described by eqs. (16) and (17), Part II. In agreement with the prediction of Scheme VI the ratiosof the interception ,ro the slope for ail aliphatic groups found from these dependencesare pr&ztically identical and equal to 0.07 and 0.08 M-l. Therefore the complexes of the different aliphatic groups of dipropylacetylene molecule are formed with the participation of the same radical orbital, namely the n-orbital of the unpaired electron.
3.3. The iigonds with the different functioml groups The theor&caI expressions for the concentration dependences of the paramagnetic shiits of ligands with different-functional groups were derived in Part II (Schemes VI and VII). Scheme VI was found to comply with the concentration dependences of the paramag&ic shifts experienced by the carbon atoms of the liphatic chains in dipropylacetylene molecules. .I’ was shown bef&e that the shifts are induced in n-o comptexes formed with the aliphatic C-I-I ubonds rather than with the triple bond. The concentration depepdences of the shifts for CH2 and CH3
Fig-
The components ofspin density matrix at l&and atom orbitals in the r-0
Ator;:
A0
1 Is
H’ c-,....
:
2s
-.
p
F0
~0.04x 10-4 ,I&35
x .io-4
-2.16
4. m-
(2) a$
(I
rKH2)
Rx1 /6)-N dependences for CH3 (I), f3-CH, (3) groups of dipropylacetylene.
complex )12NO--:H’CH3,r(O.--H’)
P
X 10-4.
YO.35 x 1o-4
,totaiJ
ep
-0.52
X 10T4
-.
-2.64
. .
._ -,
;.. . .
X 10r4.
1252 x io-4 :
2.51 x 10-4
-. :..
= 1.9 A
;
.. N.A. Sysbevdet al.iNhfRin p4~4~~4gneticcomplt-xes
scheme VII, describing the complex&on of the different functional &and groups with different radical orbitals was found to conform to the concentration dependence o! the paramagnetic shifts fdr NH2 protons and 13C nriclei of CH, groups in tertbutylamine molecules which form complexes of both types, n-u au4 n-u’. This case was considered above in detail. In studying the paramagnetic shifts of n-propylactylene protons both schemes, VI and VII, were found to be valid. The concentration dependence9 of the shifts for the acetylene proton and the a-CH2 protons in C,D,, solutions are identical and the equilibrium constants estimated from these dependences are also almost equal (0.10 and 0.14 M-l respectively). Such a behavior is characteristic for Scheme VI. Ou the other hand the paramagnetic shifts of @-CH2 and CH3 protons do not depend on the concentration of n-propyl acetylene. This shows, in accordance with Scheme VII, ?hat the paramagnetic shifts of acetylene and rr-CH2 protons are induced in complexes of n-u type while the /_LCH, and CH, proton shifts arise in X-U complexes. The equilibrium and hfc constants are K,=O.lM-‘,a,=-0.13 G,andKc~,=O.l4M-‘, aCH2 = -0.01 G, for a-H and a-CH, protons cespectively. We shall show later that this conclusion is not completely correct because the acetylene proton takes also part in the formation of the a-o complex along with n-u complexation but n-u complexes will be shown to be predominant.
of radicalswith&anic ligunds.III
.32-l
‘:
E t
Fig. 5. The ([Rc]I’s)-[S] and ([Rg]/~~)-[S] for a-H protons of dimethyimaleate.
dependences
prediction to be realized the conditions (14) and (15) (Part II) - concerning the relation between the equiiibrium constants of the ligand under consideration and the solvent - should be satisfied. The position of the.
maximum as well as the slopes on both sides of the maximum depend on the difference between these equilibrium constants. To illustrate this approach the concentration dependences of the proton paramagnetic shifts and the line widths of dimethylmaleate molecules
3.4. Two radical orbitals- two ligands competitive interacrion Now we shall consider the more interesting and complicated case where the same functional ligaud group forms complexes with different radical orbitals. This situation arises when some chemical group, for instance actylene proton of phenylacetylene molecule, is able to form complexes of both n-u and II-U type. The theory of this complexation was.given in Part Ii (Scheme V). It was predicted that the concentration dependences of the paramagnetic shifts and the line widths can pass through a maximum pr&ided that the solvent molecules form complexes of one type, n-o or n-u, and interfere with the complexation disturbing the r& lation between the n-u and x-u complexes formed by the chemical group under lnvestlgation.. For this
“\ ON\,/== H&O
/”
/” =\,//” CbC” 3
in CCl, solutions were studied. As seen from figs. 5 and 6 these depeudenc& do pass through-a maximum for both olefinic and CH3 protons. This shows that olefinic and Cl-& groups take part in both n-u, and n-u complexes. From the comparison of the slopes of the concentration dependences it f&lows that the n-u complex formed by the olefinic group is more stable than the n-u complex formed by the CH, group; It is woithy of note-that &specific behavior of .. .. .
:
328
-.
Ar.A. Sysoew et al. fNMR
in par&magneticcomplexes ofradic&
... P
2’.
i
with ot&ic
ligcmd~.III
:-
.
Fig. ?. The ((Rx] {6)-[Sj and ([Rr;l\Av,-(S] dependences for akylene proton of penyIacetyiene in CHsOH solutions.
Fig. 6. The ([Rxllsb[Ej and t[Rx]/Avb[S] for CH3 protons of dimethylmaleate.
dependences
the NMR line width for U-I, protons indicates the important role of the complex formation in the line broadening. Perhaps the paramagnetic line broadening induced by the complexation is significant even in the saturated hydrocarbons and its contribution may be comparable with the commonly accepted contribution produced by intermolecular dipolar interaction between the nucleus under consideration and the unpaired electron of the radicaI not invoIved in the complexation. As shown in fig. 7 the concentration dependence of the acetylene-proton paramagnetic shifts of phenyl:acetylene molecules in CHjOH solutions goes through a maximum at [S] = 2.5 M. According to theory. (Part II) this shows that the acetylene proton forms both n-u and n-o complexes. A similar maximum should appear in CC& solutions but as seen from fig. 8 .it was not observed. The reason for this fkt is that in CH30H so!utions the difference of the equilibrium .constaxits Kn(CH30H) and K’(HC%Ph) is large and the condition (15) is fulfilled atlow concentration of pheny&etyIene, while in Ccl4 solutions the difference
.-
of the equilibrium constants K,(CCh) and K,(IIC~ CPh) is small so that the condition (IS) is fulfilled only at the experimentally unreasonable concentration of ihenylacetylene [S] > [So], where [So] is
Fig. 8. The C[Rz]/S)-(Sl and ([Rz]/Avb[S] dependences for acetylene proton of phenylacetylene in CCQ solutions.
the concentration of pure phenylacetylene. In order to estimate the relative contributions of n-u and ‘II-U complexes formed by the acetylene proton of phenylacetylene molecule the concentration dependence of the paramagnetic line broadening was investigated. As seen from fig. 8, in Ccl, solution this dependence is linear with a positive slope while in CH,OH the slope is strongly negative. This indicates that the main contribution to the line width is supplied by one type of complexes, namely the n-u complex. This is a reason for the strongly negative slope of the concentration dependence in CH30H solution. Using equations for the line widths similar to eqs. (9), (10) of Part II, from the dependence ([RX] /A;)- [S] the following parameters for Ccl, solutions in the presence of noncompetitive ligands were found: K, = 0.25M-l, (D + C), = 400 Hz. It should be noted that the change of the solution viscosity was controlled by the change of the intermolecular broadening of the NMR line of TMS. Using these values and analysing the concentration dependences of the pammagnetic shifts in terms of eqs. (1 1) and (12) of Scheme V, Part 11,the equilibrium and hfc constants for n-o and 8-u complexes were determined: u, = -0.08 G,
a, =-0.06
G,
K,=0.25M-I,
K,=O.lOM+
It is clear that, despite of the fact that both complexes are coexisting, the q-u complexes are more stable and that they give the predominant contribution both to the line widths and pararnagnetic shifts for the acetylene proton of phenylacetylene molecule. Similar behavior of the paramagnetic shifts and the line widths was observed for the proton of the CH group of methylformiate molecules. From a comparison of the paramagnetic shifts in CC14and CH,OH solutions the ratio a,K,,/a,K, was found to be equal to 2.5. Therefore these results demonstrate the predominance of the n-u complex contribution to the paramagnetic shift and the line width. A generalization of these results to other molecular systetis may be carried out and the conclusion can be drawn that the hydrogen atom attached to the 5arbon atom with a double or triple bond is able to forin complexes of n-6 and n-o types, the former being predominant since they are more stable and giving the
leading contributiorito the line width.
the paramagnetic shift and
4. Conclusions First of all the experimental results preser.ted in this paper demonstrate that the theory of the paramagnetic shifts and the line widths proposed in Part II provides a powerful and elegant method for the identification
and deciphering
of the different
types
of comp!exes between the nitroxide radicals and the ligand molecules formed in solution. The theory and the method based on it are not limited to the.particular case of the nitroxide radicals considered here.
There is no problem to generalize and expand them to include any paramagnetic species-Iigands interaction. Among them the complexes of molecular oxygen and paramagnetic ioas are of special importance. We have found that this approach is very useful also in the studying of the structure of catalytic complexes both homogeneous and heterogeneous. It is worthy of note that the knowledge of the structure and geometry of the complex is very important for the development of the quantum chemistry of complexes. The experimental results evidence that the radical-l&and interactions responsible for the formation of a complex are strongly localized and that the radical-ligand bonds are definitely oriented and are of a semichemi-
cal nature. Even the concentration dependences of the paramagnetic shifts in complexes of the nitroxide radical with non-polar molecules, such as Ccl4 , CgH12, C4H14,were shown to be in excellent accord with the ones theoretically predicted by Schemes III, IV and VI. This unambiguously indicates that the paramagnetic shifts in complexes of these molecules result from localized and directed electron-electron Interactions rather than from contact pairs with Van der Waals interactions. Indeed the opposite sign of the slopes of the concentration dependences ([R,]/Q-[S] and ([R~]/SN)-[NJ for cyclohexane and carbon tetrachloridc respectively, experimentally observed in cyclohexane-carbon tetrachloride solutions, cannot be explained in terms of contact pairs. This is because the molar volumes of CbH,, and CC14are almost the same and, therefore, the 13C paramagnetic shifts in these molecuIes should not depend perceptibly on a change of .solution composition
-330 :
N.A. Sysoetia et al./NMti in paramagncric complexes Gf radicals with organic ligp)zds.III
if there wei&a maincontributionof contactpairs [9]. Of course this do& not me& that contact pair interaction is absent hi the solutions at all. More than that, the detailed study of the concentration dependences of the paramagnetic shifts and line widths and their analysis in terms of the theory proposed in Part II enables one to iook for any deviations from the theoretical predictions that might be due to contact pair interactions. At last it should be particularly emphasized that the radical complex formation is not a special case of complexation because the general laws concerning the participation, combination and interaction of different orbitals of the complexed molecules are supposed to be identical foi both paramagnetic and diamagnetic complexes, SCthat the structural properties of the paramagnetic complexes can be transferred aImost without modification to the diamagnetic complexes.
References ( 1 J N.A. Sysoeva, A.Yu. Karmilov and A.L. Buchachenko, Chem. Phys. 7 (1975) 123. (21 N.A. Sysocva, A.Yu; Karmilov and A.L. Buchachenko, Chem. F’hys. 15 (1976) 313, Part 11 of the present scrjes. 13) N.A. Sysoevaand A.L. Buchachenko, Zh. Strukt. Khim. I3 (1972) 42. [4] A.S. Kabankin, GM. Zhidomirov and A.L. Buchachenko, 3. Magn.Resonance 9 (1975) 199. [SJ GM. Zhidomirov and N.D. Chuvylkin, Chem. Phys. Lett. 14 (1972) 52. [6J G.M. Zhidomirov and N.D. Chuvylkin, Theoret. Chim. Acta 30 (1923) 197. [7J 1. Morishima,K. Endo and T. Yonezawa, I. Amer. Chem. Sot. 93 (1971) 2048. [SJ 1. Morishima, K. Endo and T. Yonezawa, Chem. Phys. Lett. 9 (1971) 143,203. 191 LE. Orgcl and R.S. Mulliken, J. Amer. Chem. Sot. 79 (1957) 4839.