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The correlation of vibrational broadening of core lines in x-ray photoelectron spectra with valence bond resonance structures
Volume 33, number
3
CHEMICAL
THE CORRELATION
OF vIB_~TIONAL
FHYSiCS LETTERS
BROADENING
1 Jung
1975
OF
CORZ L3W-S INX-IRAY BHBTOELECTRBN SPECTM b’I?‘B VALENCE BOND RESONANCE STRUCTURES Wl!ixm L. JOLLY and Theodore
F. SCHAAF
G~emisi~ Department, bni?enity of Califomfa, and Itroiggtiic Afaterials Research Division, Lcwznce Bed-eley Labororory, BerkeJey, CaIifomie Pi720, USA Rceived
28 janmy
19i5
V&en core ionization of zn atom in B molecule causes sigr!iflcmt changes il bond orders, the core-hole ion is formed L-Ix strained configuration. Tnis s:rain cases vibrational broadeting of the core lisle. The core-hole ion cm be represented as an ordinary chemicti species by applying the equivalent cores approximation. Then simp!e rules of chssicz! valence bond theochanges in bond orders. Thus I-.. can be used to predict changes in the weighting of resonance structures and corresponding qwlitrtive cixnges iz~ relative linewidths can be predicted.
It has recently been shown that Franck-Cond.on tibrztionai excitation contributes i,mportantly to core photo-llle broadening in mo!ecules [ 1,2]. In general, the photoelectric process yields a molecuIe.ion in which the nuclei are not in their equilibrium positions. The more strained the geometry of the core-hole ion, the broader the photo-line. In the absence of other effects, the core ionization of an atom in a molecule causes a contraction of the valence electron cloud in the immediate vicinity of the atom, corresponding to a reduction of both the atomic _tie and the equilibtium bond distances. This contraction of the equilibrium distances is most pronounced when Ihe atOm is initiaily negatively charged and hence has an easi!y contract :d electron cioud. In such cases the core-hole molecule-ion is formed in a highly strsned configlxation XXI the photo-line is rela!ive!y broad. This generalization accounts for the fact ri),n; the carbon Is line of CH, is broader than that of CF, [I ] , that the sulphur 2p lint! of the terminal sulpilur acon in +AeS,UIJ‘- ion is broader than that of the central sulphur atom in tilat ?.on 13-5 1, and that 5~ nitroger? IS line of _W$ is broader than that of XS;;
[1,5].
important to ret:ognize that the corof atomic charge with linewidth is valid only
however,
r&tier.
it Is
when the formal bond orders of all the bonds in the molecule are unchanged upon ionization of the core. When ionization produces relatively large changes in bond order, the linewidth is broad regardless of the initia! charge on the atom. In cases where ionization of 2 negatively charged atom causes 2 decrease in the order
of the bonds
to that
atom,
a canceliation
of ef-
fects can occur, resulting in an unbroadened line. Sig nificant changes in bond order accompany the core ionizations of some of the molecules listed in table 1, and we shall show that the relative linewidths can be rationalized by the use of simple valence bond resonance concepts and the equivalent cores a?proximation (6,7]. The carbon Is line of t!le outer carbon atoms of C3C2 is broader than that of the middle carbon storn in spite of the fact that the middle carbon atom is more negatively charged [S] The ground state of the molecule can be represented by the valence bond siructure O=C=C=C=O, with sma!l contributions from the following resonance structures
The ion formed by core ionization of the middle cxbon ato;;l c21-1be represented, using the equivalent cores approximation, by the structure G=C=!F=C=O.
Volume 33, number 2 Tab!e 1 Core !inewidth
data
FG~ ~~~olecules
Core !evcl
hIolscuIe
c Is(outarC) I.11 c !s(in!lerC)0.97
i81
C3%
AI[CH(COCF3)2]3 (CF3)2CQ
x20
[ll)
c 1s (CO) C Is (CF;) c
(111
Is
(CO)
C 1s (CF3) N Is (outer N) N Is (inner N)
[l]
(CH30)2C0
0 Is (C=O)
[ 111
0 Is 0 Is
COF2 [ll] CQClz [II] CO(CH3)*
f\Vhrn (eV) 2)
0 Is
[ill
CFJ [13_] CHsF [ 12)
F Is F Is
1.31 l.Ii 1.01 1.05 0.70 0.59 1.37 1.44 1.15
1.04 1x3 I .36
instrumental conrributions a) Bscause of differtig widths, fwhm v&es from differenr laboratories
to the lize-
cmno:
be
compared.
they have little tenderrcy to contract upon core ionization. We conclude that, because o:‘:l?e CIIZXI~~SIn bond order, the core-ho!e ion an outer carbon atom is formed in a fairly strained configuration, causing a vibrationeIly broadened lir~e. In the carbon 1s spectrum of the trishexefluoroacetylacetonate of aluminum, Al[CK(COCFj)2] 3, the carbonyl carbon atom line is considerably broader than the CF3 carbon atom line. This result is remarkable in view of the fact that the ILnewidths for the carbony1 and C-F3 carbon aroms irk hexaflnoroacetone (table I) and ethyl trifiuoroecetaie cd] arz approxirzate!y the sarre. There seems lo be no structural evi-’ dence that the broadening of the carbonyl carbon iine might be due to the existence of nonequivzlenr cxbcof the nyl groups*, and so we believe rhe broadening carbonyl line is due to vibrational structure. In the ground-state che!ate, the following resonance structures are of equal importance.
of
.
Becnuse the electronegativity lhan
that
of carbon.
of nitrogen is grcaier the contribution of the resonance
structures +O’-C-N=C=O
c--f O=C=N-C=+’
is somewhat greater than the contribution of the analogous s:cuctures in the ground-state CjO, molscule. This change in the weighttig of the resonance structures tends to increase the C-C bond Lengths, in opposition to the rendency for the middle carbon atom to contract and for the C-C bond lengths to s!zorten. The result is essential!y no change in the equilibrium C-C bond lengths znd 2 sii$t shortening of the equi-
librium C-O bond lengths. Consequently
a relatively
is obtained fcr the middle carbon atom. Tlle ion formed by ionizaiion of an outer carbon atom czn be represented by the structure O=C=C=N’=O. However, because of electrostatic interactions, the narrow
Ike
contributions o-f the resonance structures ‘-0X-F W-Oand+OS-(S-=Ni=O are probn,bIy greater, znd that of the resonance structure O=C=C---NiW)i is probably less, than the contributions of the analogous structuies in the ground state. (Note that the favored structures have the 2torr.s with -i1 ~Grnzal charges widely separated, whereas the disfavored structure has t!lesz Etoms adjacent to one another.) Inasmuch as the outer carbon atoms of (1, O2 are positivel_v charged,
CFz-
.
-
0.
b-
d-
II C - CH=
I C - CF3 -
I C=CH
CF,-
.
b II - C -- CF,
However, in the core-ho!e ion, the structure in which thhe oxygen atom beaiin, 0 2 negative formal charge is bonded to the ionized carbon atom is of principa! importxxe. I
.
6i I, CP - C - C\i=iJ7-_CF 3 3 Hence
the ion
is formed
in 3 strained
con:igurztiofl.
T!le nitrogen !s line of the outer nitrogen atom of N20 is broader than that of the inner nitrogen atom [ I] .This result is consistent with the rough rule that more negatively charged atoms have brosder lines, bur in this case the broadening is dEe to 2 lerqthening, r,ot a shodening, of the equilibrium bond to the outer nitrogen atom. In _NzPO,the equilibrium bond distances ae rN_N = 1 .I29 A aKd ‘N-0 = 1 .i87 A, correspondtig to the resonance smctures
Vo!ume
35, number 2
CiiEhlICAL
1 June 197.5
PHYSICS LETTERS
Core-ioniztion of the outer nitrogen aioin yields an ion Ai& CB~be represented by O=N’=O, which has the equilibrium bond distance rN_O = I. 154 A.. Obviously the ion is formed in a strained configuration. Core ionization of the inner nitrogen atom yields an ion which can be represented by the resonance struttures
In the case of CF3Ne’, one expects considerable rr bonding in the C-F bonc!sz corrcsPonding to signifyani contributions from ‘ho-band” resonance structures of the following type: L7 F-C
ile
as the C-q ion wculd be expected to be a planar triangular species with relatively short C-F bonds, one predicts that the tetrahedral core-hole ion of CF,, is formed in a highly strained state, yielding a broad line. In the case of CH3Nei, no i; bonding can occur and so the core-hole ion of CH3F is not formed in as strained a configuration. Inasmuch
The re!ative weighting of these structures is probably similar to that of the analogous structures of NZO. For ihis reason, and because the inner nitrogen atom is positive!y charged and not subject to much contrac!ion, the core-hole ion is produced in a relatively unstrained state and the line is not strongly vibrational& broadened.
The oxygen ! s lines of the carbon:4 oxygen atoms in (CH, O)zCO and COF, are significantly broader than the corresponding lines c:‘many other carbonyl compounds, such as COClz an4 CO(CH3)2, This broadening is due to changes in the equilibrium distances of the bonds to the carbonyl carbon atom upon core ionization. The carbon:Jl C-O distance increases and the other distances decrease because of the contribution of resonance structures of the foliowirIg type:
i< ‘\
C-F
/ CH30
C-F
/ F
Structures hke these are of less importance in the ions produced by the core ionizaiion of the oxygen atoms in COC!, and CO(CII, j2 because of the reduced rr donor characters of chlotine atoms and methyl groups. The equivalent cores representations of the fluorine Is hole states of CF, and CH,F are, respeciively, i 1 ; - r_ - ;ie7 F
256
H
I and
H - : - ik E
+
This work was supported by the U.S. Atomic Energo Commission and the National Science Foundation (Grant GP4 166 1x).
References [l] U. Gdh, J. Election Spectry. 5 (1974) 985. [I] U. Gelius, S. Svensson, H. Siegbahn, E. Basi!ier, X. FXYZIVand K. Sizgbahn, Chem. Phys. Letters 26 (1974) 1. [3] M. Siepbahn, C. Nordting, A. Pzhlmnn, R. Nordberg, K. Hamrin, J. Hedmmn, G. Johznnrson, T. Ber_gmark, S.-E. Sakson, I. Lindgren and B. Lindberg, ESCA-Atomic, rnolecuhr and solid-s:ate structure studied by means of electron SpectioScoPy (ALmqtiSi and ~~“ikS~~S, UppSda, 1957). [4] K. Siegbahn, J. Electron Spectrj;. 5 (!974) 3. 1.51R.M. Friedman, J. I-iudusand XL. Perlman, Phys. Rev. Letters 29 (1972) 692. [6] !Y.L. Jolly and D.N. Hendrickson, J. Am. Chem. Sac. 92 (1970) 1863. W.L. Jolly, in: Electron spectroscopy, ed. D.A. Shirley (North-Holknd, Amsterdzm, 1972) p?. 629-645. U. Gzkx, C.J. Allan, D.A. ALLison, H. Siegbahn znd I(. Siegbdm, Chem. Phys. Letters 11 (1951) 224. E.A. Shugam ad L.M. Shkol’r&ova: Proc. Acad. Sci. USSR 133 (196Oj 81 I. K. Nzkzmoto and P.J. brcCarthy, Spectroscopy and si,Tctcre of metal che!ate compounds (Wiley, New York, !969). T.F. Sch2af, unpub’tihed dsta. T.D. Thomas, 1. Am. Chem. Sot. 91(1970) 4184.
3
CHEMICAL
THE CORRELATION
OF vIB_~TIONAL
FHYSiCS LETTERS
BROADENING
1 Jung
1975
OF
CORZ L3W-S INX-IRAY BHBTOELECTRBN SPECTM b’I?‘B VALENCE BOND RESONANCE STRUCTURES Wl!ixm L. JOLLY and Theodore
F. SCHAAF
G~emisi~ Department, bni?enity of Califomfa, and Itroiggtiic Afaterials Research Division, Lcwznce Bed-eley Labororory, BerkeJey, CaIifomie Pi720, USA Rceived
28 janmy
19i5
V&en core ionization of zn atom in B molecule causes sigr!iflcmt changes il bond orders, the core-hole ion is formed L-Ix strained configuration. Tnis s:rain cases vibrational broadeting of the core lisle. The core-hole ion cm be represented as an ordinary chemicti species by applying the equivalent cores approximation. Then simp!e rules of chssicz! valence bond theochanges in bond orders. Thus I-.. can be used to predict changes in the weighting of resonance structures and corresponding qwlitrtive cixnges iz~ relative linewidths can be predicted.
It has recently been shown that Franck-Cond.on tibrztionai excitation contributes i,mportantly to core photo-llle broadening in mo!ecules [ 1,2]. In general, the photoelectric process yields a molecuIe.ion in which the nuclei are not in their equilibrium positions. The more strained the geometry of the core-hole ion, the broader the photo-line. In the absence of other effects, the core ionization of an atom in a molecule causes a contraction of the valence electron cloud in the immediate vicinity of the atom, corresponding to a reduction of both the atomic _tie and the equilibtium bond distances. This contraction of the equilibrium distances is most pronounced when Ihe atOm is initiaily negatively charged and hence has an easi!y contract :d electron cioud. In such cases the core-hole molecule-ion is formed in a highly strsned configlxation XXI the photo-line is rela!ive!y broad. This generalization accounts for the fact ri),n; the carbon Is line of CH, is broader than that of CF, [I ] , that the sulphur 2p lint! of the terminal sulpilur acon in +AeS,UIJ‘- ion is broader than that of the central sulphur atom in tilat ?.on 13-5 1, and that 5~ nitroger? IS line of _W$ is broader than that of XS;;
[1,5].
important to ret:ognize that the corof atomic charge with linewidth is valid only
however,
r&tier.
it Is
when the formal bond orders of all the bonds in the molecule are unchanged upon ionization of the core. When ionization produces relatively large changes in bond order, the linewidth is broad regardless of the initia! charge on the atom. In cases where ionization of 2 negatively charged atom causes 2 decrease in the order
of the bonds
to that
atom,
a canceliation
of ef-
fects can occur, resulting in an unbroadened line. Sig nificant changes in bond order accompany the core ionizations of some of the molecules listed in table 1, and we shall show that the relative linewidths can be rationalized by the use of simple valence bond resonance concepts and the equivalent cores a?proximation (6,7]. The carbon Is line of t!le outer carbon atoms of C3C2 is broader than that of the middle carbon storn in spite of the fact that the middle carbon atom is more negatively charged [S] The ground state of the molecule can be represented by the valence bond siructure O=C=C=C=O, with sma!l contributions from the following resonance structures
The ion formed by core ionization of the middle cxbon ato;;l c21-1be represented, using the equivalent cores approximation, by the structure G=C=!F=C=O.
Volume 33, number 2 Tab!e 1 Core !inewidth
data
FG~ ~~~olecules
Core !evcl
hIolscuIe
c Is(outarC) I.11 c !s(in!lerC)0.97
i81
C3%
AI[CH(COCF3)2]3 (CF3)2CQ
x20
[ll)
c 1s (CO) C Is (CF;) c
(111
Is
(CO)
C 1s (CF3) N Is (outer N) N Is (inner N)
[l]
(CH30)2C0
0 Is (C=O)
[ 111
0 Is 0 Is
COF2 [ll] CQClz [II] CO(CH3)*
f\Vhrn (eV) 2)
0 Is
[ill
CFJ [13_] CHsF [ 12)
F Is F Is
1.31 l.Ii 1.01 1.05 0.70 0.59 1.37 1.44 1.15
1.04 1x3 I .36
instrumental conrributions a) Bscause of differtig widths, fwhm v&es from differenr laboratories
to the lize-
cmno:
be
compared.
they have little tenderrcy to contract upon core ionization. We conclude that, because o:‘:l?e CIIZXI~~SIn bond order, the core-ho!e ion an outer carbon atom is formed in a fairly strained configuration, causing a vibrationeIly broadened lir~e. In the carbon 1s spectrum of the trishexefluoroacetylacetonate of aluminum, Al[CK(COCFj)2] 3, the carbonyl carbon atom line is considerably broader than the CF3 carbon atom line. This result is remarkable in view of the fact that the ILnewidths for the carbony1 and C-F3 carbon aroms irk hexaflnoroacetone (table I) and ethyl trifiuoroecetaie cd] arz approxirzate!y the sarre. There seems lo be no structural evi-’ dence that the broadening of the carbonyl carbon iine might be due to the existence of nonequivzlenr cxbcof the nyl groups*, and so we believe rhe broadening carbonyl line is due to vibrational structure. In the ground-state che!ate, the following resonance structures are of equal importance.
of
.
Becnuse the electronegativity lhan
that
of carbon.
of nitrogen is grcaier the contribution of the resonance
structures +O’-C-N=C=O
c--f O=C=N-C=+’
is somewhat greater than the contribution of the analogous s:cuctures in the ground-state CjO, molscule. This change in the weighttig of the resonance structures tends to increase the C-C bond Lengths, in opposition to the rendency for the middle carbon atom to contract and for the C-C bond lengths to s!zorten. The result is essential!y no change in the equilibrium C-C bond lengths znd 2 sii$t shortening of the equi-
librium C-O bond lengths. Consequently
a relatively
is obtained fcr the middle carbon atom. Tlle ion formed by ionizaiion of an outer carbon atom czn be represented by the structure O=C=C=N’=O. However, because of electrostatic interactions, the narrow
Ike
contributions o-f the resonance structures ‘-0X-F W-Oand+OS-(S-=Ni=O are probn,bIy greater, znd that of the resonance structure O=C=C---NiW)i is probably less, than the contributions of the analogous structuies in the ground state. (Note that the favored structures have the 2torr.s with -i1 ~Grnzal charges widely separated, whereas the disfavored structure has t!lesz Etoms adjacent to one another.) Inasmuch as the outer carbon atoms of (1, O2 are positivel_v charged,
CFz-
.
-
0.
b-
d-
II C - CH=
I C - CF3 -
I C=CH
CF,-
.
b II - C -- CF,
However, in the core-ho!e ion, the structure in which thhe oxygen atom beaiin, 0 2 negative formal charge is bonded to the ionized carbon atom is of principa! importxxe. I
.
6i I, CP - C - C\i=iJ7-_CF 3 3 Hence
the ion
is formed
in 3 strained
con:igurztiofl.
T!le nitrogen !s line of the outer nitrogen atom of N20 is broader than that of the inner nitrogen atom [ I] .This result is consistent with the rough rule that more negatively charged atoms have brosder lines, bur in this case the broadening is dEe to 2 lerqthening, r,ot a shodening, of the equilibrium bond to the outer nitrogen atom. In _NzPO,the equilibrium bond distances ae rN_N = 1 .I29 A aKd ‘N-0 = 1 .i87 A, correspondtig to the resonance smctures
Vo!ume
35, number 2
CiiEhlICAL
1 June 197.5
PHYSICS LETTERS
Core-ioniztion of the outer nitrogen aioin yields an ion Ai& CB~be represented by O=N’=O, which has the equilibrium bond distance rN_O = I. 154 A.. Obviously the ion is formed in a strained configuration. Core ionization of the inner nitrogen atom yields an ion which can be represented by the resonance struttures
In the case of CF3Ne’, one expects considerable rr bonding in the C-F bonc!sz corrcsPonding to signifyani contributions from ‘ho-band” resonance structures of the following type: L7 F-C
ile
as the C-q ion wculd be expected to be a planar triangular species with relatively short C-F bonds, one predicts that the tetrahedral core-hole ion of CF,, is formed in a highly strained state, yielding a broad line. In the case of CH3Nei, no i; bonding can occur and so the core-hole ion of CH3F is not formed in as strained a configuration. Inasmuch
The re!ative weighting of these structures is probably similar to that of the analogous structures of NZO. For ihis reason, and because the inner nitrogen atom is positive!y charged and not subject to much contrac!ion, the core-hole ion is produced in a relatively unstrained state and the line is not strongly vibrational& broadened.
The oxygen ! s lines of the carbon:4 oxygen atoms in (CH, O)zCO and COF, are significantly broader than the corresponding lines c:‘many other carbonyl compounds, such as COClz an4 CO(CH3)2, This broadening is due to changes in the equilibrium distances of the bonds to the carbonyl carbon atom upon core ionization. The carbon:Jl C-O distance increases and the other distances decrease because of the contribution of resonance structures of the foliowirIg type:
i< ‘\
C-F
/ CH30
C-F
/ F
Structures hke these are of less importance in the ions produced by the core ionizaiion of the oxygen atoms in COC!, and CO(CII, j2 because of the reduced rr donor characters of chlotine atoms and methyl groups. The equivalent cores representations of the fluorine Is hole states of CF, and CH,F are, respeciively, i 1 ; - r_ - ;ie7 F
256
H
I and
H - : - ik E
+
This work was supported by the U.S. Atomic Energo Commission and the National Science Foundation (Grant GP4 166 1x).
References [l] U. Gdh, J. Election Spectry. 5 (1974) 985. [I] U. Gelius, S. Svensson, H. Siegbahn, E. Basi!ier, X. FXYZIVand K. Sizgbahn, Chem. Phys. Letters 26 (1974) 1. [3] M. Siepbahn, C. Nordting, A. Pzhlmnn, R. Nordberg, K. Hamrin, J. Hedmmn, G. Johznnrson, T. Ber_gmark, S.-E. Sakson, I. Lindgren and B. Lindberg, ESCA-Atomic, rnolecuhr and solid-s:ate structure studied by means of electron SpectioScoPy (ALmqtiSi and ~~“ikS~~S, UppSda, 1957). [4] K. Siegbahn, J. Electron Spectrj;. 5 (!974) 3. 1.51R.M. Friedman, J. I-iudusand XL. Perlman, Phys. Rev. Letters 29 (1972) 692. [6] !Y.L. Jolly and D.N. Hendrickson, J. Am. Chem. Sac. 92 (1970) 1863. W.L. Jolly, in: Electron spectroscopy, ed. D.A. Shirley (North-Holknd, Amsterdzm, 1972) p?. 629-645. U. Gzkx, C.J. Allan, D.A. ALLison, H. Siegbahn znd I(. Siegbdm, Chem. Phys. Letters 11 (1951) 224. E.A. Shugam ad L.M. Shkol’r&ova: Proc. Acad. Sci. USSR 133 (196Oj 81 I. K. Nzkzmoto and P.J. brcCarthy, Spectroscopy and si,Tctcre of metal che!ate compounds (Wiley, New York, !969). T.F. Sch2af, unpub’tihed dsta. T.D. Thomas, 1. Am. Chem. Sot. 91(1970) 4184.