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14N Nuclear quadrupole interaction in Cu(II) doped l -alanine
of Molecular Structure, 68 (1980) 203-208 Elsevier Scientific Publishing Company, Amsterdam -
Journal
14N NUCLEAR L-ALANINE
JUAN MURGICH,
QUADRUPOLE
RAFAEL
CALVO
Printed in The Netherlands
INTERACTION
IN Cu(I1) DOPED
and SAUL B. OSEROFF
Centro de QuPmica and Centro de Fisica. Instituto Venezolano Cien tificas, Apartado 1827. Caracas IO1 (Venezuela) (Received
28 September
1979;
de Inuestigaciones
in final form 4 February 1980)
ABSTRACT The ‘“N nuclear quadrupole interaction tensor PN measured by ENDOR in Cu(I1) doped L-alanine is analyzed in terms of the Townes and Dailey theory assuming a tetrahedrally bonded N atom. The resuIts of this analysis are compared with those for the lJN in pure r.,-alanine and it is found that the principal directions of the PN tensor are drastically changed upon metal complexation as a consequence of the higher electron affinity of Cu(II) with respect to C and H. Comparison of the corresponding bond populations in pure and Cu(II) doped L-alanine indicates that the Cu draws 0.11 more electron from the N than the substituted H atom. INTRODUCTION
A large number of 14N nuclear quadrupole resonance (NQR) spectra has been obtained from nitrogen containing compounds, using CW and pulsed RF apparatus or nuclear double resonance methods [ 1, 2]_ However, because of various experimental difficulties, there are few data available for paramagnetic complexes in which a nitrogen containing ligand is coordinated to a metal. Hsieh et al. [3] pointed out that the nuclear double resonance method requires fairly long nitrogen relaxation times. Since in many paramagnetic complexes the unpaired spin density at the N site shortens the relaxation times, it prevents the detection of the NQR spectrum. The Electron Nuclear Double Resonance (ENDOR) technique [4] offers an alternative method for the study of transition metal complexes, and has been extensively used to study magnetically diluted complexes. Valuable information has been obtained about the magnetic hyperfine and nuclear quadrupole interactions in those compounds 143. Several amino acids doped with transition metal ions such as Cu(II) have been studied by EPR and ENDOR [ 5-71. Also, the 14N NQR spectra of several pure amino acids have been reported [ 1, 8]_ It is interesting to compare the values for the nuclear quadrupole interaction of 14N in pure and doped amino acids in order to obtain information about the changes in the orbital population occurring upon metal complexation at the nitrogen atoms. 0022-2860/80/0000-0000/$02.25
o 1980
Elsevier Scientific Publishing Company
204
Having this idea in mind, we analyzed in this work the 14Nnuclear quadrupole interaction in Cu(II) doped L-alanine (CLA) measured by the ENDOR technique [7] and compared these results with those obtained in pure L-alanine by nuclear quadrupole resonance methods [ 91. Our analysis uses the theory of Townes and Dailey and is based on a tetrahedral bonding model for the amino nitrogen. It is found that the substitution of one of the amino protons by the copper ion produces a radical change in the orientation of the electric fieId gradient at the N site. A value of 0.11 electron was obtained for the change in population between the N-Cu orbital and the original N-H orbital. The contribution of the positive charges located at the Cu(II) ion to the electric field gradient (EFG) at the N site is discussed. EXISTING
EXPERIMENTAL
DATA
The crystal structure of L-alanine cNH,CH(CH,)COO-) has been determined and refined by X-rays [ 1lj_ It has an orthorhombic crystal structure (space group P 2,2,2, ) with four molecules per unit cell. The 14N NQR spectrum of L-&nine has been obtained by the nuclear double resonance method at 77 K [9] _ An e*sQ/h = 1.184 MHz value was reported for the quadrupole coupling constant which measures the magnitude of the quadrupole interaction in the z’ principal direction of the electric field gradient, and a value 77= 0.256 for the asymmetry parameter which measures the anisotropy of the EFG around z’. Recently ENDOR experiments [7] have been performed on CLA at 4 K in order to measure the magnetic hyperfine (AN ) and the nuclear quadrupole interaction (PN ) tensors of the nitrogen covalently bonded to the copper ion. The components and the principal directions of PN obtained in these experiments are shown in Table 1.
TABLE
1
Principal va!ues of the ‘“N nuclear quadrupole respect to the a, b and c axes of the L-alanine Principal
values
interaction tensor crystal [7 ]
Direction
cosines
(ke) p(l) $) p,b)a =e2 qQ/h
987 97 f* 20 -1084 =--zPN(‘)
+ 20 = + 2170
0.87, 0.60,
-0.14, 0.47,
0.20,
-0.87,0.45
r 40 kHz; 11 = 0.82
0.65 -0.62
+ 0.07.
and direction
Angles (degrees) 39,98,128 53, -62, 78,
-50
51, 64
cosines
with
205 BONDING
AND
14N NUCLEAR
QUADRUPOLE
INTERACTION
DATA
FOR
L-ALANINE
A survey of the NQR data available for the amino nitrogen of several amino acids [ 2, 81 indicates that the values of the quadrupole interaction in L&mine [9] given in the previous section, are typical of compounds that form zwitterions in the solid state. In such structures amino protons form hydrogen bonds with the neighbouring oxygen ions. Edmonds [S] used th? theory of Townes and Dailey [lo] and a tetrahedral bonding scheme for the nitrogen in L-&nine, in order to interpret the NQR data. In the tetrahedral scheme [12] it is assumed that the nitrogen atom is cpvalently bonded through four tetrahedral o orbit&. Finite values of e2 sQ/h are due to differences in the electronegativities of the atoms bonded to the nitrogen
[I31 -
The theory of Townes and Dailey [lo] relates the populations of these four (Torbitals to the values of e2 qQ/h and q. In order to reduce the number of unknowns, it is assumed that two of the N-H (Torbit& are equally populated. If the populations of the G1, C#I~and G3 = G4 orbitals are u NR, oNX and CJNH respectively, we have for a H2 NRX group [lo]
(~NR
- e Nx) + 2(0,x
(~NR
-
t,NX) = P2
-
(I -
0Nr-i) = (4/3)
(e2 qQ/e' 409)(1+ rl)
q/3) (e* qQ/e” QOQ) sin 28
(1) (2)
where cos 20 = (1 + ~)/(3 - r~). The value of e2 q. Q/h is the coupling constant for a single electron in a 2p orbital and is taken equal to -11.88 MHz as we are dealing with a positively charged N atom [ 91. In eqns. (1) and (2) 8 is the angle between the is’ axis of the diagonal EFG tensor and the z axis as shown in Fig. 1. When eNx # 0Na but (T:~:; = (sNH, rl = 0 and the z’ axis is directed along the direction of the N-R orbital. In L-alanine, 77= 0.256 and the direction of the z’ axis is, within a few degrees, parallel to the N-C direction as calculated in ref. 9. The finite value of 17 indicates the existence of inequivalent N-H bonds in the crystal, since X = H’ in this case. BONDING
AND
14N NUCLEAR
QUADRUPOLAR
INTERACTtON
IN CLA
Takeda et al. 1141 proposed a model supported by their EPR data for the Cu(I1) bonding in L-alanine. In such a model, the metal ion is coordinated to three alanine molecules and located at the centre of a nearly planar square formed by two nitrogens and two oxygens. Since the O-O and N-N distances in the unit cell of L-alanine [ll] are more than twice typical Cu-0 and Cu-N distances [ 141, molecular reorienta-Lionsare assumed in order to accommodate the metal ion. Besides the’ r4N ligand to Cu(I1) in CLA, whose quadrupole interaction tensor is shown in Table 1, ENDOR resonances from a second, non equivalent nitrogen ligand have been detected in a lower fre-
206
Fig. 1. The idealized tetrahedral orbitak and their relation to the set of axes xyt and the set of EFG x’y’r’ axes.
quency range [7] _ These results support the model [ 141 for the bonding of Cu(II) to k&nine and also indicate that the square planar model is only approx imate and cannot explain the different orientations found for the principal directions of the tensors g, A,, AN and PN observed in the EPR and ENDOR measurements [ 7, 141. However, this simple configuration of ligands around the copper ion still retains most of the relevant features for CLA and it is possible, for a qualitative analysis, to use the theory of Kivelson and Neiman [ 151 to study the bonding of Cu(I1) in CLA. This theory assumes a &, symmetry around Cu(I1) and allows the bonding parameters to be calculated fiorn the EPR and ENDOR data. Using the experimental results given in ref. 7 values OL*= 0.60 and ~1’~ = 0.54 are obtained for the lB1, > = ((Y/Z)CZ~~-,,~ - ((r’/2) (- cr”,+ DLby + (Y’; - cr$) ground state orbital of Cu(I1) in CLA where d,:-s2 isthe copper atomic orbital and ui = np’ t (1 - n2)*j2 si are the hybridized u orbit& for 0 and N ligands [ 153 . These values indicate a large degree of covalency, similar to that found in the Cu(II) phthalocyanine complex 1151. It is also found, using the experimental data of ref. 7, that the N-Cu bond is closer to an sp 3 hybrid than to an spz hybrid. These results allowed us to apply the tetrahedral bonding model to the amine nitrogen in CLA. Before applying the model of Townes and Dailey to the quadrupolar data for the 14N in CLA in order to obtain the orbital populations, we have to consider the contributions to the electric field gradient of the positive charges located at the Cu(I1) site [ 161. This contribution was calculated assuming a Cu-N distance of 2 a and we obtained +260 kHz and +O.ll to e’qQ/h and to 8 respectively. These
207
quantities should be subtracted from the experimental values in order to obtain the contributions arising from the electrons populating the tetrahedral u bonds. DISCUSSION
The values of e’qQ/h and q in L-alanine and CLA are shown in Table 2. Comparison of these values indicates that the complex formation drastically modifies the EFG at the N site covalently bonded to the Cu ion. Since the ENDOR data provides not only the magnitude of the coupling constant but also determines its sign [4, 71, one may obtain information about the changes in the relative populations without assumptions based only on chemical intuition [ 2, 51 which is frequently the case in the interpretation of conventional NQR data [l] . Considering that e’qQ/h is negative for CLA one obtains from eqn. (2) oNcu - (7Nc< 0 indicating that the Cu ion has a larger electron affinity than C in CLA. A similar conclusion may be obtained regarding the H atom. The values for the differences in u populations obtained with eqns. (1) and (2) and the experimental data are included in Table 2. The substitution of H atoms by the Cu ion induces changes not only in the magnitudes of the components of the EFG but also modifies the direction of its principal axes. The z axis of the EFG tensor which in k&nine is approximately directed along the N-C bond according to the tetrahedral model is in CLA located along a direction 14” away from the N-Cu bond, as seen in Fig. 1. The changes in the axes directions are understood considering the difference of electron affinity between the Cu and the C atoms with respect to the H atom. The Cu ion produces the lowest of the (Jpopulations as seen above and thus
TABLE
2
IiN quadrupole coupling constants, tions for L-alanine and CLA
e2qQ/h q
asymmetry
ONC -
aNH
parameters
“NC
-
ONH’
and relative
ONH’ -
u orbital
ONH
(MHz)
L-alaninea
1.184
0.256
-0.14 ONCu
CLAb CLA=
aData
+ 2.170 + 1.910
0.82 0.71
from ref. 9. These
ENDOR experiment taking into account
-0.12 -
-0.27 -0.24
data correspond
ONH
uNCu
-0.27 -0.12
-0.03 -
ONC
popula-
0
(degrees) 4
uNC-NH
-0.10 -0.13
to the fully deuterated
L-alanine
19 15
since in the
a deuterated sample was used. bData from ref. 7. ‘Data corrected the contributions to the EFG from the charges located at the Cu site.
208
from the N-Cu direction may be explained by the combined effect of the C and Cu atoms on the N u populations. From eqns. (1) and (2) it is seen that whenaNa = %x but 0Na # (TNHthe EFG axes coincide with the xyz system of Fig. 1. The z’ axis in this case bisects the RNX angle. Since the Cu ion has a larger electron affinity than C and a much larger one than H, as found above, it “pulls” the z’ axis toward the N-Cu direction as a consequence of its larger charge deficiency. Additional information about the changes in the orbital populations in CLA is obtained from the fact that the values of cNc - uNH shown in Table 2 are fairly constant in both L-alanine and CLA. Thisconstant value seems to indicate that the main changes in the populations of the e orbitals are localized in the N-Cu bond with small inductive effects on the u bonds. It is possible to estimate the charge transfer from the N ion toward the Cu ion substitution by considering the values of uN~ - (TNHand uNC - UNH. From the values of these quantities shown in Table 2 it can be seen that the Cu ion draws 0.11 more electron from the amino N than the H atom, in CLA. axis
REFERENCES 1 E. A. C. Lucken, Nuclear Quadrupole Coupling Constants, Academic Press, New York, 1969; E. Schempp and P. J. Bray, in D. Henderson (Ed.), Physical Chemistry, Vol. IV, Academic Press, New York, 1970. 2 J. L. Ragle and G. L. Minott III, Advances in Nuclear Quadrupole Resonance, Vol. 3, Heyden, London, 1978, p. 205. 3 Y. N. Hsieh, P. S. Ireland and T. L. Brown, J. Magn. Reson., 21 (1976) 445. 4 L. Kevan and L. D. Kispert, Electron Spin Double Resonance Spectroscopy, J. Wiley, New York, 1976. 5 R. Bottcher, D. Hernhold, S. Wartewig and W. Windsch, J. Mol. Struct., 46 (1978) 363. 6 M. Fujimoto, L. A. McDowell and T. Takui, J. Chem. Phys., 70 (1979) 3694. 7 R. Caivo, S. B. Oseroff and H. C. Abache, J. Chem. Phys., 72 (1980) 760. 8 D. T. Edmonds, Phys. Rep., 29 (1977) 233. 9 M. J. Hunt and A. L. Mackay, J. Magn. Reson., 15 (1974) 402. 10 C. H. Townes and B. P. Dailey, J. Chem. Phys., 17 (1949) 782. 11 H. J. Simpson Jr. and R. E. Marsh, Acta Crystallogr., 20 (1966) 550. 12 D. T. Edmonds and C. P. Summers, J. Magn. Reson., 12 (1973) 134. 13 R. A. Marino and T. Oja, J. Chem. Phys., 56 (1972) 5453. 14 K. Takeda, Y. Arata and S. Fujiwara, J. Chem. Phys., 53 (1970) 854. 15 D. Kivelson and R. Neiman, J. Chem. Phys., 35 (1961) 149. 16 J. Murgich, J. Chem. Phys., 68 (1978) 611.
Journal
14N NUCLEAR L-ALANINE
JUAN MURGICH,
QUADRUPOLE
RAFAEL
CALVO
Printed in The Netherlands
INTERACTION
IN Cu(I1) DOPED
and SAUL B. OSEROFF
Centro de QuPmica and Centro de Fisica. Instituto Venezolano Cien tificas, Apartado 1827. Caracas IO1 (Venezuela) (Received
28 September
1979;
de Inuestigaciones
in final form 4 February 1980)
ABSTRACT The ‘“N nuclear quadrupole interaction tensor PN measured by ENDOR in Cu(I1) doped L-alanine is analyzed in terms of the Townes and Dailey theory assuming a tetrahedrally bonded N atom. The resuIts of this analysis are compared with those for the lJN in pure r.,-alanine and it is found that the principal directions of the PN tensor are drastically changed upon metal complexation as a consequence of the higher electron affinity of Cu(II) with respect to C and H. Comparison of the corresponding bond populations in pure and Cu(II) doped L-alanine indicates that the Cu draws 0.11 more electron from the N than the substituted H atom. INTRODUCTION
A large number of 14N nuclear quadrupole resonance (NQR) spectra has been obtained from nitrogen containing compounds, using CW and pulsed RF apparatus or nuclear double resonance methods [ 1, 2]_ However, because of various experimental difficulties, there are few data available for paramagnetic complexes in which a nitrogen containing ligand is coordinated to a metal. Hsieh et al. [3] pointed out that the nuclear double resonance method requires fairly long nitrogen relaxation times. Since in many paramagnetic complexes the unpaired spin density at the N site shortens the relaxation times, it prevents the detection of the NQR spectrum. The Electron Nuclear Double Resonance (ENDOR) technique [4] offers an alternative method for the study of transition metal complexes, and has been extensively used to study magnetically diluted complexes. Valuable information has been obtained about the magnetic hyperfine and nuclear quadrupole interactions in those compounds 143. Several amino acids doped with transition metal ions such as Cu(II) have been studied by EPR and ENDOR [ 5-71. Also, the 14N NQR spectra of several pure amino acids have been reported [ 1, 8]_ It is interesting to compare the values for the nuclear quadrupole interaction of 14N in pure and doped amino acids in order to obtain information about the changes in the orbital population occurring upon metal complexation at the nitrogen atoms. 0022-2860/80/0000-0000/$02.25
o 1980
Elsevier Scientific Publishing Company
204
Having this idea in mind, we analyzed in this work the 14Nnuclear quadrupole interaction in Cu(II) doped L-alanine (CLA) measured by the ENDOR technique [7] and compared these results with those obtained in pure L-alanine by nuclear quadrupole resonance methods [ 91. Our analysis uses the theory of Townes and Dailey and is based on a tetrahedral bonding model for the amino nitrogen. It is found that the substitution of one of the amino protons by the copper ion produces a radical change in the orientation of the electric fieId gradient at the N site. A value of 0.11 electron was obtained for the change in population between the N-Cu orbital and the original N-H orbital. The contribution of the positive charges located at the Cu(II) ion to the electric field gradient (EFG) at the N site is discussed. EXISTING
EXPERIMENTAL
DATA
The crystal structure of L-alanine cNH,CH(CH,)COO-) has been determined and refined by X-rays [ 1lj_ It has an orthorhombic crystal structure (space group P 2,2,2, ) with four molecules per unit cell. The 14N NQR spectrum of L-&nine has been obtained by the nuclear double resonance method at 77 K [9] _ An e*sQ/h = 1.184 MHz value was reported for the quadrupole coupling constant which measures the magnitude of the quadrupole interaction in the z’ principal direction of the electric field gradient, and a value 77= 0.256 for the asymmetry parameter which measures the anisotropy of the EFG around z’. Recently ENDOR experiments [7] have been performed on CLA at 4 K in order to measure the magnetic hyperfine (AN ) and the nuclear quadrupole interaction (PN ) tensors of the nitrogen covalently bonded to the copper ion. The components and the principal directions of PN obtained in these experiments are shown in Table 1.
TABLE
1
Principal va!ues of the ‘“N nuclear quadrupole respect to the a, b and c axes of the L-alanine Principal
values
interaction tensor crystal [7 ]
Direction
cosines
(ke) p(l) $) p,b)a =e2 qQ/h
987 97 f* 20 -1084 =--zPN(‘)
+ 20 = + 2170
0.87, 0.60,
-0.14, 0.47,
0.20,
-0.87,0.45
r 40 kHz; 11 = 0.82
0.65 -0.62
+ 0.07.
and direction
Angles (degrees) 39,98,128 53, -62, 78,
-50
51, 64
cosines
with
205 BONDING
AND
14N NUCLEAR
QUADRUPOLE
INTERACTION
DATA
FOR
L-ALANINE
A survey of the NQR data available for the amino nitrogen of several amino acids [ 2, 81 indicates that the values of the quadrupole interaction in L&mine [9] given in the previous section, are typical of compounds that form zwitterions in the solid state. In such structures amino protons form hydrogen bonds with the neighbouring oxygen ions. Edmonds [S] used th? theory of Townes and Dailey [lo] and a tetrahedral bonding scheme for the nitrogen in L-&nine, in order to interpret the NQR data. In the tetrahedral scheme [12] it is assumed that the nitrogen atom is cpvalently bonded through four tetrahedral o orbit&. Finite values of e2 sQ/h are due to differences in the electronegativities of the atoms bonded to the nitrogen
[I31 -
The theory of Townes and Dailey [lo] relates the populations of these four (Torbitals to the values of e2 qQ/h and q. In order to reduce the number of unknowns, it is assumed that two of the N-H (Torbit& are equally populated. If the populations of the G1, C#I~and G3 = G4 orbitals are u NR, oNX and CJNH respectively, we have for a H2 NRX group [lo]
(~NR
- e Nx) + 2(0,x
(~NR
-
t,NX) = P2
-
(I -
0Nr-i) = (4/3)
(e2 qQ/e' 409)(1+ rl)
q/3) (e* qQ/e” QOQ) sin 28
(1) (2)
where cos 20 = (1 + ~)/(3 - r~). The value of e2 q. Q/h is the coupling constant for a single electron in a 2p orbital and is taken equal to -11.88 MHz as we are dealing with a positively charged N atom [ 91. In eqns. (1) and (2) 8 is the angle between the is’ axis of the diagonal EFG tensor and the z axis as shown in Fig. 1. When eNx # 0Na but (T:~:; = (sNH, rl = 0 and the z’ axis is directed along the direction of the N-R orbital. In L-alanine, 77= 0.256 and the direction of the z’ axis is, within a few degrees, parallel to the N-C direction as calculated in ref. 9. The finite value of 17 indicates the existence of inequivalent N-H bonds in the crystal, since X = H’ in this case. BONDING
AND
14N NUCLEAR
QUADRUPOLAR
INTERACTtON
IN CLA
Takeda et al. 1141 proposed a model supported by their EPR data for the Cu(I1) bonding in L-alanine. In such a model, the metal ion is coordinated to three alanine molecules and located at the centre of a nearly planar square formed by two nitrogens and two oxygens. Since the O-O and N-N distances in the unit cell of L-alanine [ll] are more than twice typical Cu-0 and Cu-N distances [ 141, molecular reorienta-Lionsare assumed in order to accommodate the metal ion. Besides the’ r4N ligand to Cu(I1) in CLA, whose quadrupole interaction tensor is shown in Table 1, ENDOR resonances from a second, non equivalent nitrogen ligand have been detected in a lower fre-
206
Fig. 1. The idealized tetrahedral orbitak and their relation to the set of axes xyt and the set of EFG x’y’r’ axes.
quency range [7] _ These results support the model [ 141 for the bonding of Cu(II) to k&nine and also indicate that the square planar model is only approx imate and cannot explain the different orientations found for the principal directions of the tensors g, A,, AN and PN observed in the EPR and ENDOR measurements [ 7, 141. However, this simple configuration of ligands around the copper ion still retains most of the relevant features for CLA and it is possible, for a qualitative analysis, to use the theory of Kivelson and Neiman [ 151 to study the bonding of Cu(I1) in CLA. This theory assumes a &, symmetry around Cu(I1) and allows the bonding parameters to be calculated fiorn the EPR and ENDOR data. Using the experimental results given in ref. 7 values OL*= 0.60 and ~1’~ = 0.54 are obtained for the lB1, > = ((Y/Z)CZ~~-,,~ - ((r’/2) (- cr”,+ DLby + (Y’; - cr$) ground state orbital of Cu(I1) in CLA where d,:-s2 isthe copper atomic orbital and ui = np’ t (1 - n2)*j2 si are the hybridized u orbit& for 0 and N ligands [ 153 . These values indicate a large degree of covalency, similar to that found in the Cu(II) phthalocyanine complex 1151. It is also found, using the experimental data of ref. 7, that the N-Cu bond is closer to an sp 3 hybrid than to an spz hybrid. These results allowed us to apply the tetrahedral bonding model to the amine nitrogen in CLA. Before applying the model of Townes and Dailey to the quadrupolar data for the 14N in CLA in order to obtain the orbital populations, we have to consider the contributions to the electric field gradient of the positive charges located at the Cu(I1) site [ 161. This contribution was calculated assuming a Cu-N distance of 2 a and we obtained +260 kHz and +O.ll to e’qQ/h and to 8 respectively. These
207
quantities should be subtracted from the experimental values in order to obtain the contributions arising from the electrons populating the tetrahedral u bonds. DISCUSSION
The values of e’qQ/h and q in L-alanine and CLA are shown in Table 2. Comparison of these values indicates that the complex formation drastically modifies the EFG at the N site covalently bonded to the Cu ion. Since the ENDOR data provides not only the magnitude of the coupling constant but also determines its sign [4, 71, one may obtain information about the changes in the relative populations without assumptions based only on chemical intuition [ 2, 51 which is frequently the case in the interpretation of conventional NQR data [l] . Considering that e’qQ/h is negative for CLA one obtains from eqn. (2) oNcu - (7Nc< 0 indicating that the Cu ion has a larger electron affinity than C in CLA. A similar conclusion may be obtained regarding the H atom. The values for the differences in u populations obtained with eqns. (1) and (2) and the experimental data are included in Table 2. The substitution of H atoms by the Cu ion induces changes not only in the magnitudes of the components of the EFG but also modifies the direction of its principal axes. The z axis of the EFG tensor which in k&nine is approximately directed along the N-C bond according to the tetrahedral model is in CLA located along a direction 14” away from the N-Cu bond, as seen in Fig. 1. The changes in the axes directions are understood considering the difference of electron affinity between the Cu and the C atoms with respect to the H atom. The Cu ion produces the lowest of the (Jpopulations as seen above and thus
TABLE
2
IiN quadrupole coupling constants, tions for L-alanine and CLA
e2qQ/h q
asymmetry
ONC -
aNH
parameters
“NC
-
ONH’
and relative
ONH’ -
u orbital
ONH
(MHz)
L-alaninea
1.184
0.256
-0.14 ONCu
CLAb CLA=
aData
+ 2.170 + 1.910
0.82 0.71
from ref. 9. These
ENDOR experiment taking into account
-0.12 -
-0.27 -0.24
data correspond
ONH
uNCu
-0.27 -0.12
-0.03 -
ONC
popula-
0
(degrees) 4
uNC-NH
-0.10 -0.13
to the fully deuterated
L-alanine
19 15
since in the
a deuterated sample was used. bData from ref. 7. ‘Data corrected the contributions to the EFG from the charges located at the Cu site.
208
from the N-Cu direction may be explained by the combined effect of the C and Cu atoms on the N u populations. From eqns. (1) and (2) it is seen that whenaNa = %x but 0Na # (TNHthe EFG axes coincide with the xyz system of Fig. 1. The z’ axis in this case bisects the RNX angle. Since the Cu ion has a larger electron affinity than C and a much larger one than H, as found above, it “pulls” the z’ axis toward the N-Cu direction as a consequence of its larger charge deficiency. Additional information about the changes in the orbital populations in CLA is obtained from the fact that the values of cNc - uNH shown in Table 2 are fairly constant in both L-alanine and CLA. Thisconstant value seems to indicate that the main changes in the populations of the e orbitals are localized in the N-Cu bond with small inductive effects on the u bonds. It is possible to estimate the charge transfer from the N ion toward the Cu ion substitution by considering the values of uN~ - (TNHand uNC - UNH. From the values of these quantities shown in Table 2 it can be seen that the Cu ion draws 0.11 more electron from the amino N than the H atom, in CLA. axis
REFERENCES 1 E. A. C. Lucken, Nuclear Quadrupole Coupling Constants, Academic Press, New York, 1969; E. Schempp and P. J. Bray, in D. Henderson (Ed.), Physical Chemistry, Vol. IV, Academic Press, New York, 1970. 2 J. L. Ragle and G. L. Minott III, Advances in Nuclear Quadrupole Resonance, Vol. 3, Heyden, London, 1978, p. 205. 3 Y. N. Hsieh, P. S. Ireland and T. L. Brown, J. Magn. Reson., 21 (1976) 445. 4 L. Kevan and L. D. Kispert, Electron Spin Double Resonance Spectroscopy, J. Wiley, New York, 1976. 5 R. Bottcher, D. Hernhold, S. Wartewig and W. Windsch, J. Mol. Struct., 46 (1978) 363. 6 M. Fujimoto, L. A. McDowell and T. Takui, J. Chem. Phys., 70 (1979) 3694. 7 R. Caivo, S. B. Oseroff and H. C. Abache, J. Chem. Phys., 72 (1980) 760. 8 D. T. Edmonds, Phys. Rep., 29 (1977) 233. 9 M. J. Hunt and A. L. Mackay, J. Magn. Reson., 15 (1974) 402. 10 C. H. Townes and B. P. Dailey, J. Chem. Phys., 17 (1949) 782. 11 H. J. Simpson Jr. and R. E. Marsh, Acta Crystallogr., 20 (1966) 550. 12 D. T. Edmonds and C. P. Summers, J. Magn. Reson., 12 (1973) 134. 13 R. A. Marino and T. Oja, J. Chem. Phys., 56 (1972) 5453. 14 K. Takeda, Y. Arata and S. Fujiwara, J. Chem. Phys., 53 (1970) 854. 15 D. Kivelson and R. Neiman, J. Chem. Phys., 35 (1961) 149. 16 J. Murgich, J. Chem. Phys., 68 (1978) 611.