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Rotational barrier of the allylcopper complex. An ab initio molecular-orbital study
Volume
114. number
CHEMICAL
5,6
ROTATiONAL
BARRIER
PHYSKZS
OF THFi ALLYLCOPPER
COMPLEX.
AN AB INITIO MOLECULAR-ORBITAL
STUDY
M. MERCHAN,
I. NEBOT-GIL
R. GONZALEZ-LUQUE,
aad F. TQMAS
Deparrumenro de f+imica Fisica, Cdredra de Qu fmrca General, Faculrad Buqasaf, Volerrcia. Sperm Received
20 December
15 March 1985
LETTERS
de Clencrcrs @imicas,
Vmvers~dad de Vakencru,
1984
Pscudopotent&al nan-cmpirxai calculations have been performed on syn and anti aIiylcopper, and mtermedIate conformatzons It IS found that an unsymmetrtcal structure has the Iowest energy. Using a double-zeta Plus polartzation basis set. the syn md ant1 allylcopper Ire above the mmimum by 3 0 and 7.0 kcai/mol. rcspective[y. These low barrters heights #are related to the covalent character of the complex. and the ongm of the barrier IS attributed to the loss of n delocalization encrpy.
1. introduction The allylcopper complex may be described m terms of an ally1 anion @and and a Cu(I) 3d10 metal atom. The aIIyl aruon 1s the simplest delocalized carbanion with C,, symmetry. When a methylene group is twisted 90”, and pyramidahzatton is allowed, the earbanion lone pan (anti or syn to the double bond) corresponds to a geometry wtth C, symmetry [ 11. Organometalhc compounds and Ion patrs mvolvmg ally1 hgands have been subject of experimental and theoretical work. Thus, alIylaIkali metal compounds have been studted experimentally [3]. Theoretrcai calculations on allylbthtum give a barrier height of 8.4 kcal/mol [3] at STQ-3G level and 15.7 kcal/mol [4] with a 6-3 lG* basis, employing different STO-3G optimized structures. The experrmental barrier is 10.7 kcallrnol [Z] _ On the other hand, the nature of the bonding in brs-(rr-allyl)nrckel was studied with a double-zeta-type wavefunction [S] and the calcuIated value of the barrier to rotatron around the C-C ally1 bond was reported to be PO kcal/mol. Recently, a theoretical study on copper(I)-ally1 anion complexes was carried out f6,7f as a model for a surface complex m the oxidation step of the propene partial oxidation mechanism on solid Cu,O [S]. The a-coordinated syn allylcopper was found to be 516
the most stable among
several CuC31-IS structures studied (71; however, other u-bonded rotarners such as the anti allylcopper were not considered. An ab initio MO study of the nature of the bonding through rotation of the CuCH, group, gotng from the syn to anh allylcopper, is presented here. ln this work three maut geometric confotmattons of the u-coordinated allylcopper complex are exarnined (see fig. 1): syn allylcopper, I; a structure derrved from the syn (or anti) allylcopper by a 90° rotation of the CuCH, group with respect to the C-C-C plane, II; and, anti allylcopper, III. Our arm IS to fmd the most stable conformation for the u-coordinated ally& copper complex and so to determine the rotational barrier height.
2. Method The calculations have been performed within the framework of the pseudopotential technique [9-l 1 J. We have replaced the atomic core electrons of carbon and copper atoms by an effective nonempirical potential function. The present SCF calculations were carried out wtth the PSHONDO algorithm f12], which introduces the pseudopotential proposed by Barthelat and Durand [lo] into the HONDQ program [ 13]_ 0 009-2614/85/S ~o~h-Ho~~d
03.30 Q Elsevier Science Pub&hers Physics oblong Division)
B.V.
Volume 114, number 5,6
15 March 1985
CHEMICAL PHYSICS LETTERS
I
II
III
Frg. 1. Allylcopper conformahons. If the dihedral an$e o IS defmed by the Cu-C(3)-C(2)-C(l) atoms: 0 = O” corresponds. to the syn allylwpper, I; e = 90”, the perpendrcular allylcopper, 11;and, B = 180” represents the ant1 allylcopper, III.
The pseudopotentral parameters and properly optimized basis sets for carbon [14] and copper [ 151 atoms were the same as in reFs. [6,7]. Two different valence basis sets are utrhzed: (a) double-zeta quality except for the polarization p atonuc orbrtals of Cu, where a single-zeta was used, DZ, and (b) which rs the same as (a) but addmg a set of polarization functions for carbon and hydrogen atoms, DZ+P, introducing d orbitals with exponent Cl.8 on the carbons and p orbrtals with exponent 1.1 on the hydrogens [16]. For structures I-III the most sigmficant geometric parameters have been optimized with simple parabolic fits used to determine the minimum for a given parameter, utilizing the DZ basis set.
3. Results and discussion A fist approach to get useful qualitative information about the barrier shape for the allylcopper complex was carried out using the partially optimized geometry of syn aiIyIcopper reported previously [7]. Table I summarizes total and relative energies with vanation of the dihedral angle O(Cu-C(3)-C(2)-C(1)) (see fig. 1). Apparently, the most stable conformation is found for structure II, 0 = 90”. The heights of the potential barriers for the rotation of the CuCH2 group around the C(2)-C(3) bond seem to be relatrvely low. about 2 and 6 kcal/moI going from the perpendicular
Table 1 Total and relative energes of ellylcopper complex versus the &hedral angle. 0 (Cu-C(3)-C(2)-C(l)), at fied geometry a), using the DZ basis set
;)
0 (de@
Et (au)
E, (kcal/mol)
0.0 45.0 90.0 135.0 180.0
-69.247049a) -69.241316 -69.250849 -69.241837 -69.240595
24 22 00 1.9 6.4
Taken from ref. [7].
structure (II) to the syn (I) and anti (111) conforrnatrons, respectively. However, the calculated barner herghts may not be quantttatively correct. Ln what follows, we analyze the influence of geometry optimization and basis set dependence. For a semiquantitatrve evaluation ofbarrier heights, geometnes for structures I, II and III were improved. Conformations I and UI were optnnized in Cs symmetry. The optimized geometrical parameters are bsted m table 2. The most relevant features are a shortening of the C(2)-C(3) bond and a slight lengthening of the C( 1)-C(2) bond for structure II with respect to the C, forms I and III. These trends along with increasing of the Cu-C(3) bond length and the a! angle (see table 2 for deftition), can be attributed to stabikzkg changes In order to get the maximum II delocahzation for structure II. Since it is well known that polarization 517
Volume
114,
number
5.6
CHEMICAL
PHYSICS
Table 2 Geometrical parameters for the DZ optimized allylcopper structures. hstances In A and angles m degree Conformation
Parameter
1
11 1.333
a1 )cta
1.506 2 037 a) 115.3a) 124.2
C(2KC3)
CuC(3) ~C(3)C(2) arb)
lu 1.338
1.333
1.486 2.046 114.3 127.9
1.521 1.992 117.6 125.3
a) Taken from ref. 171. b) mrarmdabutlon angle between the lme bl;ectmg H(4)C(3)H(S) angle and the C(2)C(3) bond.
the
functions are needed to give a quantitative evaluation of barriers [ 17,181, we also Improved our basis set
using the aforementioned DZ+P basis set. Table 3 shows total and relative energes obtained with the two basis sets, DZ and DZ+P, employmg the optimal geometrles. By comparison of tables 1 and 2, it can be inferred that geometry optinuzation increases the barTable 3 Total and relatwe
energes
for the optimal
structures Total
Structure
i(w) Il@erpendlcular) lII(anU)
Table 4 hlulhken
populatron
nnalysls for allylcopper
energes
Relative
(au)
energies
(kcal/mol)
DZ
DZ+P
DZ
DZ+D
-69.247905 -69.253396 -69.241894
-69.301927 -69.306712 -69.295551
3-4 0.0 7.2
3.0 0.0 7.0
structuxes
I-1U
1IJ
ll
DZ
DZ+P
DZ
DZ+P
DZ
DZ+P
-0.229 -0.192 -0.396 0.360
-0.144 -0 147 -0.307 0.343
-0.266 -0.177 -0.407 0.395
-0.178 -0.135 -0.318 o-374
-0.226 -0.201 -0.406 0359
-0.136 -0.156 -0320 0 342
0.677 0.336 0.326
0.720 0.385 0.333
0.671 0313 0.353
0.724 03.55 0.364
charges
total overlap C(l)C(2) C<2xx3) CuC(3)
518
rier heights about 1 kcal/mol due mainly to the additional stabilization by z de’iocalization for the perpendicular structure, II, as Its optunized parameters show (table 2). In view of these results complete optimization would not be worthwhile since only a few calones could be retrieved from the ad&tional optimization of the C-H distances and angles. On the other hand, by inspectlon of table 3 it is readlly seen that polanzation functions on carbon and hydrogen atoms give a small decrease in rotational barrier heights, contrary to the Isolated ally1 aruon where the influence of polarization functions has proved to be important [1,19]. The geometric trends and expected interactions can be visualized from table 4, where net atomic charges and total overlap population, from Mulliken’s population analyns, are Bven for structures I-III with DZ and DZ+P basis sets. It 1s found that for the mmimal energy structure, II, the posltwe net atomic charge on copper atom is greater than for the C, conformations (implying less charge transfer from copper to ally1 anion): the total overlap population of the Cu-C(3) bond has the smallest value; and an increasmg of the C(2)-C(3) overlap population and a de-
I-HI
I
net atomic C(l) C(2) C(3) cu
15 March 1985
LETTERS
populdt~on 0.693 0324 0343
0.733 0.373 0.349
Vohrrne
114. number
$6
CHEMICAL PHYSICS LBTTERS
creasing of +he C(1)-C(2) overlap population is achieved. These trends are found in both DZ and DZ+P basis sets. In the C, structures I and III the bonding between ally1 anion and Cu+ takes place by CIcharge transfer from the HOMO lone pair of the syn or antr allyi anion, located on C(3), to an s atomic orbital of copper. There are many symmetry-allowed orbital combinations for structure II, due to the low symmetry, but the bonding occurs mainly by charge transfer from the two highest mo~ecuIar orbrtafs of the unsymmetr~cal aUyi amon to an s atomrc orbital of copper, and to pz and dr2 orbitah of copper to a lesser extent. These facts and a gain of a delocahzation are the reasons why the perpendicuJar rotamer, II, 1s the preferred conformatlon. Taking into consideratton these points, we conclude that the ongm of the rr-allylcopper barrier IS, rn large part, the loss of x bonding, as it occurs commonly in a 71system 1171. Our calculated barrier for allylcopper, approximated by the DZ+P energy difference between II and 1(111 is higher n-renergy) IS 3.0 kca.I/mol. Because of the covalent character of the copper-carbon bonding, the rotational bamer height is strongly reduced in relatron to aIlyIlrthium [4] and the isolated ahyl anion f1,20].
The authors are indebted to the Laboratoire de Physique Quantique of Toulouse (France for sending us the PSHONDO program, ps~udopotenti~s, and basis sets, and to the Diputacion Provincial of Valencia (Spain) for computer facditres. MM and RG-L thank the Mmisterio de Education y Crencia of Spain for felfowshrps.
1985
References f 1f P. [2] 131 (41 [S] 161 [7]
f8] [9] IlO] [ 111 [i2] 113 ]
[ 14) (151 [16] [ 171
Acknowledgement
15 Mash
1181 [ 191
[2OJ
Cremaschi, G. hiorosr and hf. Stmonetta, J. h?ol Struct. Tbeochem 85 (1981) 397. T.B. Thompson and W.T. Ford, J. Am. Chem. Sot. 101 (1979) 5459. A. Bongrnr, G. CameRr, G. Qrddlo, P. PaIrmen and A.Uma~-Ronc~,J_Or~nomet_~em. 110 (1976) 1. 1‘. Clark. E.D. Jemmis, P. von R. Schleyer, J.S. Bmkley and J.A. Pople, J. Organomet Chem. 150 (1978) 1. M -M. Rohmer, J. Demuynck and A Vedlard, Tbeoret. Cbim. Acta 36 (I 974) 93 R. Gonz&z-Luque. 1. NebotCd and F. Tom&, Cbem. Phys. Letters 104 (1984) 203. R. Gonz+Iez-Luque, M. Merchlin, I Nebot-Gd, F. Tomis and R. htontaiiand, J. hfol Struct. Theochem, :o be publxhed SW. Gerer, E.V. Rozhkova and Ya.B. Gorokhovatsky. J. Catal. 28 (1973) 341. D. hlcWrUiams and S. Huzmaga, J. Chem. Phys. 63 (1975) 4678. Ph. Durand and J C. Barthelat. nteoret. Glum. Acta 38 (1975) 283. J.C. Barthelat and Ph. Durand, Gazz. Chirn. Ital. 108 (1978) 225. J.P. Daudey, private commumcatron hi. Dupurs, J. Rys and H.F. Kmg, J. Chem. Phys. 65 (1976) ill. J C. Barth&t, private commurucatton. M. Pebsster, J. Chem. Phys 75 (1981) 775. PK. HXI~SZ~R and J-A. PopJe, Theoret. Cbrrn. Acta 28 (1973) 213. P.W Payne and L.C. Allen, m Modern theoretrcal chenustry, Vol. 4, ed. H.F. Schaefer III (Plenum Press. New York, 1977). J A. Pople, m: Modern theoretical chemistry. Vot 4. ed H F. Schaefer III (Pienum Press, New York, 1977). R. Gonz&lez-Luque, I. NebotGd, hl. hlcrchin and F. Tom&, to be pubhhed. J. Chandrasekl’tar, J.G. Andrade and P. yen R. Schleyer. J. Am. CXem.Sot. 103 (1981) 5609.
519
114. number
CHEMICAL
5,6
ROTATiONAL
BARRIER
PHYSKZS
OF THFi ALLYLCOPPER
COMPLEX.
AN AB INITIO MOLECULAR-ORBITAL
STUDY
M. MERCHAN,
I. NEBOT-GIL
R. GONZALEZ-LUQUE,
aad F. TQMAS
Deparrumenro de f+imica Fisica, Cdredra de Qu fmrca General, Faculrad Buqasaf, Volerrcia. Sperm Received
20 December
15 March 1985
LETTERS
de Clencrcrs @imicas,
Vmvers~dad de Vakencru,
1984
Pscudopotent&al nan-cmpirxai calculations have been performed on syn and anti aIiylcopper, and mtermedIate conformatzons It IS found that an unsymmetrtcal structure has the Iowest energy. Using a double-zeta Plus polartzation basis set. the syn md ant1 allylcopper Ire above the mmimum by 3 0 and 7.0 kcai/mol. rcspective[y. These low barrters heights #are related to the covalent character of the complex. and the ongm of the barrier IS attributed to the loss of n delocalization encrpy.
1. introduction The allylcopper complex may be described m terms of an ally1 anion @and and a Cu(I) 3d10 metal atom. The aIIyl aruon 1s the simplest delocalized carbanion with C,, symmetry. When a methylene group is twisted 90”, and pyramidahzatton is allowed, the earbanion lone pan (anti or syn to the double bond) corresponds to a geometry wtth C, symmetry [ 11. Organometalhc compounds and Ion patrs mvolvmg ally1 hgands have been subject of experimental and theoretical work. Thus, alIylaIkali metal compounds have been studted experimentally [3]. Theoretrcai calculations on allylbthtum give a barrier height of 8.4 kcal/mol [3] at STQ-3G level and 15.7 kcal/mol [4] with a 6-3 lG* basis, employing different STO-3G optimized structures. The experrmental barrier is 10.7 kcallrnol [Z] _ On the other hand, the nature of the bonding in brs-(rr-allyl)nrckel was studied with a double-zeta-type wavefunction [S] and the calcuIated value of the barrier to rotatron around the C-C ally1 bond was reported to be PO kcal/mol. Recently, a theoretical study on copper(I)-ally1 anion complexes was carried out f6,7f as a model for a surface complex m the oxidation step of the propene partial oxidation mechanism on solid Cu,O [S]. The a-coordinated syn allylcopper was found to be 516
the most stable among
several CuC31-IS structures studied (71; however, other u-bonded rotarners such as the anti allylcopper were not considered. An ab initio MO study of the nature of the bonding through rotation of the CuCH, group, gotng from the syn to anh allylcopper, is presented here. ln this work three maut geometric confotmattons of the u-coordinated allylcopper complex are exarnined (see fig. 1): syn allylcopper, I; a structure derrved from the syn (or anti) allylcopper by a 90° rotation of the CuCH, group with respect to the C-C-C plane, II; and, anti allylcopper, III. Our arm IS to fmd the most stable conformation for the u-coordinated ally& copper complex and so to determine the rotational barrier height.
2. Method The calculations have been performed within the framework of the pseudopotential technique [9-l 1 J. We have replaced the atomic core electrons of carbon and copper atoms by an effective nonempirical potential function. The present SCF calculations were carried out wtth the PSHONDO algorithm f12], which introduces the pseudopotential proposed by Barthelat and Durand [lo] into the HONDQ program [ 13]_ 0 009-2614/85/S ~o~h-Ho~~d
03.30 Q Elsevier Science Pub&hers Physics oblong Division)
B.V.
Volume 114, number 5,6
15 March 1985
CHEMICAL PHYSICS LETTERS
I
II
III
Frg. 1. Allylcopper conformahons. If the dihedral an$e o IS defmed by the Cu-C(3)-C(2)-C(l) atoms: 0 = O” corresponds. to the syn allylwpper, I; e = 90”, the perpendrcular allylcopper, 11;and, B = 180” represents the ant1 allylcopper, III.
The pseudopotentral parameters and properly optimized basis sets for carbon [14] and copper [ 151 atoms were the same as in reFs. [6,7]. Two different valence basis sets are utrhzed: (a) double-zeta quality except for the polarization p atonuc orbrtals of Cu, where a single-zeta was used, DZ, and (b) which rs the same as (a) but addmg a set of polarization functions for carbon and hydrogen atoms, DZ+P, introducing d orbitals with exponent Cl.8 on the carbons and p orbrtals with exponent 1.1 on the hydrogens [16]. For structures I-III the most sigmficant geometric parameters have been optimized with simple parabolic fits used to determine the minimum for a given parameter, utilizing the DZ basis set.
3. Results and discussion A fist approach to get useful qualitative information about the barrier shape for the allylcopper complex was carried out using the partially optimized geometry of syn aiIyIcopper reported previously [7]. Table I summarizes total and relative energies with vanation of the dihedral angle O(Cu-C(3)-C(2)-C(1)) (see fig. 1). Apparently, the most stable conformation is found for structure II, 0 = 90”. The heights of the potential barriers for the rotation of the CuCH2 group around the C(2)-C(3) bond seem to be relatrvely low. about 2 and 6 kcal/moI going from the perpendicular
Table 1 Total and relative energes of ellylcopper complex versus the &hedral angle. 0 (Cu-C(3)-C(2)-C(l)), at fied geometry a), using the DZ basis set
;)
0 (de@
Et (au)
E, (kcal/mol)
0.0 45.0 90.0 135.0 180.0
-69.247049a) -69.241316 -69.250849 -69.241837 -69.240595
24 22 00 1.9 6.4
Taken from ref. [7].
structure (II) to the syn (I) and anti (111) conforrnatrons, respectively. However, the calculated barner herghts may not be quantttatively correct. Ln what follows, we analyze the influence of geometry optimization and basis set dependence. For a semiquantitatrve evaluation ofbarrier heights, geometnes for structures I, II and III were improved. Conformations I and UI were optnnized in Cs symmetry. The optimized geometrical parameters are bsted m table 2. The most relevant features are a shortening of the C(2)-C(3) bond and a slight lengthening of the C( 1)-C(2) bond for structure II with respect to the C, forms I and III. These trends along with increasing of the Cu-C(3) bond length and the a! angle (see table 2 for deftition), can be attributed to stabikzkg changes In order to get the maximum II delocahzation for structure II. Since it is well known that polarization 517
Volume
114,
number
5.6
CHEMICAL
PHYSICS
Table 2 Geometrical parameters for the DZ optimized allylcopper structures. hstances In A and angles m degree Conformation
Parameter
1
11 1.333
a1 )cta
1.506 2 037 a) 115.3a) 124.2
C(2KC3)
CuC(3) ~C(3)C(2) arb)
lu 1.338
1.333
1.486 2.046 114.3 127.9
1.521 1.992 117.6 125.3
a) Taken from ref. 171. b) mrarmdabutlon angle between the lme bl;ectmg H(4)C(3)H(S) angle and the C(2)C(3) bond.
the
functions are needed to give a quantitative evaluation of barriers [ 17,181, we also Improved our basis set
using the aforementioned DZ+P basis set. Table 3 shows total and relative energes obtained with the two basis sets, DZ and DZ+P, employmg the optimal geometrles. By comparison of tables 1 and 2, it can be inferred that geometry optinuzation increases the barTable 3 Total and relatwe
energes
for the optimal
structures Total
Structure
i(w) Il@erpendlcular) lII(anU)
Table 4 hlulhken
populatron
nnalysls for allylcopper
energes
Relative
(au)
energies
(kcal/mol)
DZ
DZ+P
DZ
DZ+D
-69.247905 -69.253396 -69.241894
-69.301927 -69.306712 -69.295551
3-4 0.0 7.2
3.0 0.0 7.0
structuxes
I-1U
1IJ
ll
DZ
DZ+P
DZ
DZ+P
DZ
DZ+P
-0.229 -0.192 -0.396 0.360
-0.144 -0 147 -0.307 0.343
-0.266 -0.177 -0.407 0.395
-0.178 -0.135 -0.318 o-374
-0.226 -0.201 -0.406 0359
-0.136 -0.156 -0320 0 342
0.677 0.336 0.326
0.720 0.385 0.333
0.671 0313 0.353
0.724 03.55 0.364
charges
total overlap C(l)C(2) C<2xx3) CuC(3)
518
rier heights about 1 kcal/mol due mainly to the additional stabilization by z de’iocalization for the perpendicular structure, II, as Its optunized parameters show (table 2). In view of these results complete optimization would not be worthwhile since only a few calones could be retrieved from the ad&tional optimization of the C-H distances and angles. On the other hand, by inspectlon of table 3 it is readlly seen that polanzation functions on carbon and hydrogen atoms give a small decrease in rotational barrier heights, contrary to the Isolated ally1 aruon where the influence of polarization functions has proved to be important [1,19]. The geometric trends and expected interactions can be visualized from table 4, where net atomic charges and total overlap population, from Mulliken’s population analyns, are Bven for structures I-III with DZ and DZ+P basis sets. It 1s found that for the mmimal energy structure, II, the posltwe net atomic charge on copper atom is greater than for the C, conformations (implying less charge transfer from copper to ally1 anion): the total overlap population of the Cu-C(3) bond has the smallest value; and an increasmg of the C(2)-C(3) overlap population and a de-
I-HI
I
net atomic C(l) C(2) C(3) cu
15 March 1985
LETTERS
populdt~on 0.693 0324 0343
0.733 0.373 0.349
Vohrrne
114. number
$6
CHEMICAL PHYSICS LBTTERS
creasing of +he C(1)-C(2) overlap population is achieved. These trends are found in both DZ and DZ+P basis sets. In the C, structures I and III the bonding between ally1 anion and Cu+ takes place by CIcharge transfer from the HOMO lone pair of the syn or antr allyi anion, located on C(3), to an s atomic orbital of copper. There are many symmetry-allowed orbital combinations for structure II, due to the low symmetry, but the bonding occurs mainly by charge transfer from the two highest mo~ecuIar orbrtafs of the unsymmetr~cal aUyi amon to an s atomrc orbital of copper, and to pz and dr2 orbitah of copper to a lesser extent. These facts and a gain of a delocahzation are the reasons why the perpendicuJar rotamer, II, 1s the preferred conformatlon. Taking into consideratton these points, we conclude that the ongm of the rr-allylcopper barrier IS, rn large part, the loss of x bonding, as it occurs commonly in a 71system 1171. Our calculated barrier for allylcopper, approximated by the DZ+P energy difference between II and 1(111 is higher n-renergy) IS 3.0 kca.I/mol. Because of the covalent character of the copper-carbon bonding, the rotational bamer height is strongly reduced in relatron to aIlyIlrthium [4] and the isolated ahyl anion f1,20].
The authors are indebted to the Laboratoire de Physique Quantique of Toulouse (France for sending us the PSHONDO program, ps~udopotenti~s, and basis sets, and to the Diputacion Provincial of Valencia (Spain) for computer facditres. MM and RG-L thank the Mmisterio de Education y Crencia of Spain for felfowshrps.
1985
References f 1f P. [2] 131 (41 [S] 161 [7]
f8] [9] IlO] [ 111 [i2] 113 ]
[ 14) (151 [16] [ 171
Acknowledgement
15 Mash
1181 [ 191
[2OJ
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