- Email: [email protected]
Cyanogen azide: Ionisation potentials and ab initio SCF Mo calculation
Vo!ume 35, nwnber 2
CHEMCAL PHYSICSLETiERS
CYANOCEN AZIDE: IOIW5ATfON POTENTULS
H.C. Ousted
I?zstilufe, Chemica! Loborufory
1 Scptembei
1975
AWD AB tNlTll0 SCF &fO CALCULATION
Y, ffniversity of Copet~hoger~, Denrnurk
Peter JANSEN Da~zisJxAtomic
Energy Commk-ion,
Rh5. Denn~rk
and tierbert
ST_4FAST *
Receiwd
6 June 1975
The Ne(l) znd He(l) photoelectron spectra of cyanogen azide, NCN3, have Seen recorded at high resolution. Their interprl:tadon is achieved by comparison with the PE spectrum of HN-J and an ab initio LCCO SCF MO cakuhtior?. Dctiations from Koopmans’ theorem of quite different ma_gnitudes are found dependent on the type of ionisation process
I. Introduction Cyanogw azide, NCNJ, is extremely explosive [l]. It is a suitatlIe source for the highly reactive species NCN [2] and for azodicarbonitrile, NCNNCN [2] Microwave [3-S] electron diffraction (61, IR [ 1,79], Raman [9], and UV [2] spectral data are available in the literature, but partly because of the instability of the tide: e-g- upon condensation and thermal or mechanical shock [l], no detailed information about its electronic structure has been reported so far. J?hotoe!ectron(PE) spectroscopy, however, proved to be a cowrenient method of investigation. Ionisation potentials could be measured as well as vibrational frequencies of the cation states. A description of its electronic ground state properties is obtained by both the PE spectrum (within the validity of Koopman’s theorem (101) and by ZRab initio SCFMO. calculation.
2. Method NC& wzs prepared in ~methylph~!ate * To whom cxxespandence
should be addressed.
solution
from BrCN and NaN3, activated by recrystallisation [I ,8], In a typicai run 0.15 g of activated NaN3 were suspended in sufficient dimethylphthalate .with 0.1 g BrCN added. After about 12 hours at room temperature pure NCN, was distilled directly from the solution into the apparatus. No BrCN or Nz (from decomposition of NCN,) w2s detected in the PE spec-
trum (cf. fig. 1). The PE spectra were recorded OR a Perkin-Elmer PS 16 instrument and Calibrated by the xenon and argon peaks (half band width 20-25 meV). The a5 initio LCGO SCF MO calculation [6] was carried out using a 7s, 3p + 4s, 2p basis [I 1J.
3. PE spectra and ab inido SCF MO c&ufntion The Ne(I) and He(r) PE spectra ofcyanogen tide are shown in fig. 1; ionisation potentials and vibrational cation frequencies are summarized in table 1. The Ne(1) spectrum of cyanogen tide displays five separate bands and two more are detected in the high ionisativn energy region of the Kc(r) record. The intensity of the third band decreases slightly, when changing from I%(I) to Kc(E) irradiation, whereas t;Te ‘. 247
Votume 35, m3mber 2
1 September
CHEMICAL PHYSICS LETTERS
1975
NC-N3
Ne 1
He I
IEWI Fig. 1.
-
Ne(I) and He(I) PE spectra
intensities of the fourth and fifth bands increase CORsiderabljr, the first two bmds being regarded unch’an;ed. Most of e&e bzxk show ~br~t~~~~ fute strucbre. These are useful for the band assigilment, if compared wiih: the ndecufar ground state frequencies and the bonG~g properties of the SCF oibitak, rougly estimated from their population analysis’ (tab!e 2). The calcuiated dipole momerit amounts to 3.2? D and
of cyanopn
aide.
agrees fairly weII with the experimental v&e of a 2199 D [3]: The FE band assignment is sketched out in fig. 2. The frrst two experiment4 ionisation potentials agree ftiriy weti W+I the calcdated ne&tive 3a” and 14a’ orbital energies, respectively. In addition, the shape of the first band with its resolved vibrationlf fine structure confkms, that this ionisation occurs from
Volume 35, number 2 Table
1 Septemberl9?5
CHEMICAL PHYSICS LETTERS
1
Ionisation potentials IP’s (eV), vibrational frequencies (cm-r) of the molcculu azide compared wiih ab initio SCF valence orbital ener~ics eJ(ev) .p
1 2 3 4 5 6 7
Molecular orbital
IP (ev) b)
U+ (cm-’ )
10.96 12.4 13.33 14.16 14.54 16.66 18.0
1840,1200,
g 2rl A 2~ B’ 2.X e 1x D 2n E” *x F *l-l
ground state v and cation states yc of cyanogen
BOO
-1!.16 -12.64 -15.02
?a” 14d
1080 1120,720 1920,480 1960
sound state: a’ fundamentnls v(cm-‘) [9] : ~i =2248 ua = 2198 Y‘q= 921 “5 = 865 u,= 167
13a’ 1 ?a’ 2a“ lla’ la” 103’ 9a’ 8a’ 72’ 6a’
vg = 1246 V6 = 666
-15.90 -17.68 -19.95 -21.01 -21.09 -26.35 -34.45 -37.94 -43.74
__-__...-.-.-.-
a) cf. test.
h) Values refer to the He(I) band ma.ximmum or to the maximum
Table 2 Net atomic and overlap population
analysis of Ihe valence shell SCF orbitals of cyanogen
_,____~__________
Molecular orbital _----3,”
14a’ 13a’
_.-.-
__-._.-
0.00
0.44
0.21
-0.21
0.82 0.50
0.15
0.12
0.34
-0.18
0.13
0.21
0.12
0.29 0.3s 0.10
0.09 0.06
0.09
0.78 0.84 0.87
1.44
la” 1Oa’
0.03
0.00
N3
0.44 0.40
0.35 0.52 0.02 0.02 0.03 _._. .__..
0.22 --
0.27
NJ -_-._-_-..---.--~
whereas the second broad band probably [ 121 contains unresolved deformation modes of NC%. Ln the 1:3-l 5 eV region ionisations certainly arise from the 13a’, 12~1 and ?_a”orbitals (cf. fig. 2, table I). The 14.16 eV and 14.54 eV bands overlap
slightly and mu:ri belong to cation states of different 2Z and ‘IT> because othertise the avoided of two 2Z states would prevent an appearance
of such well resolved vibrational fine structure, ;1s in the higher eners
band. The 13.35 eV
band dkplays zn intense 0 + 0 transition peak and is assigned to the well localized 1% orbital, which can be described as cyano nitrogen “lone p&” (table 2). to the sharpness
-_.--
---
Overlap population
a rr orbital,
With respect
aide - ..--_--
0.91
0.35 a.00
observed
---.-.
0.58 0.62
2a” 1la’
crossing
-
Net atomic population N1 CZ --0.56 0.15
12a’
symmetries
of the highest peak.
of the 14.54 eV band
N5
NiC2 --.-
-0.15
CaNs _..-. _.._____~
0.10
-0.03
NsN‘j
N
0.03 -0.23
-0.02
-0.12.
0.03 0.10
-0.10
0.04
0.22 0.04
-0.07
2.14
0.28 0.00
0.33
0.00
0.03 0.04
0.21 0.16
0.26
0.55 -0.02 __.____-_._~_--
-o.c)3
0.08
-1.03 0.31
0.06
and its 1920 cm-’ progression (vCN = v2 = 2198 should occur from the CN cm-’ [9]) th’c19ronisation bonding 7 orbital 2a”. The broadness of the remaining 14.16 eV band is in agreement with electron ejection from the 13a’ orbital, roughly denotable as “in plane nCN” (table 2). The band at 16.66 eV is straightforwardly assigned to the 1 la’ orbital, the terminal adde nitrogen “lone pair”. No decision about the symmetry of the cation suite at 18 eV is possib!e from the spectram (no resolved vibrational tine structure) or the MO ca.!culation (orbital energy difference only 0.06 eV). Correlation with the assigned PE bands of HNs
[12], however, suggests sn ionisation From the totally rr bonding orbital la”. -
Volume
3.5, number 2
CHEbllCAL
NW,.
HN3
IL a’ __
10
..- ‘i;
m ._‘..._~_------
_3 a” ..
t
PES
PES
MO _ tl
PHYSICS
1
6
6 ,,___..__.....-. ._-____------
-15
LE’FTEKS
1 September
L975
hrge, because up to now, Roopmms deficiencies for ;r ionisations from cyan0 compounds amounted to less than 2 eV [13]. Evidently theoretical work beyond the firtreeFock Ievel would be necessary to achieve more reliable calculations. But with respect to the limited computer time available, such calculations 2re excluded for most molecules. Comparison between
assigned PE spectra and MO calculations, however, allorv a rou$ estimate of correlation and reorganisation effects connected with the different qpes of ionisation.
_ iP feY1 v
One of the authors (KS.) is grateful to Professor H. Bock for the opportunity of recording the PE
Fig_ Z Correhtion of sE initio valence orbital energies of NCN3 and PE ionisation
Finally
potentids
it can be stated,
of NCX3 and tlN3
that cyano
occur between
spectra.
[ 121.
substitution
of HN3 leads to almost unchanged ionjsatio~ energies of the first two PE b:rnds (AIP N 0.2 eV). Three additional ionisationr which are, roughly
Acknowledgement
13 and 15 eV,
sfea.king, concerned with the cyano nitrogen “lone pair” and the “in plane T,-~” bonds. Considerable energy and “out of plane ids” ~h.ift~ of about I .3 eV relative to HN, are measured for the last two bands of NC??, in the He(I) region.
References fl J F.D. Marsh, 3. Org. Chem. 37 (1972) 2966. [ZI F.D. Marsh and WE. Hermes. J. Am. Chem. SOC. 86 (1964) 4506, X7 (1965) 181% 131 I(, Bolton, R.D. Brown and F.R. Burden, Chem. Phys.
Letters 1.5 (1972) 79. [41 CC. Cost& and H.W. Kroto, Can. iT. Phys. 50 (1972) 1453. [51 G.L. Blackmann, Id B&on, R.D. Brown, F.R. Burden and A. Mishra, J. iMol. Spectry.
[61 A.
47 (1973) 457.
B. Bak, P. Jensen and T.G. Strand, Xcta Chem. &and. 27 (1973) 1531.
4. Discllssion
[71 D.E. Milligm, M.E. Jacox, J.J. Comeford
Comparison between the negative valence orbital energies and the PE ionisation potentials of cyanogen azide reveals, that Koopmans’ theorem is fairly well npplic-sbk deviations
to the first
of about
two
ion&t&.x processes. Urge 3 eV xe connected with the
ejection of electrons cram the cyano and the terminal azirie nitrogn “lone pairs’“.This is why the PE band sequence
is at variance
with the SCF orbital
sequence
(cf. 0 in fig, 2). Sir&r results have already been ob-
tained for other cyano compounds, whenever ionisations occur from well localized canonical nitrogn %t~e pair” orbitals {13,141. On the other hand, deviations of 3 eV are alsc ob=rved for the ion&lions t’rom,the ?i orbit& 2a” uld la”. This is unusually 250
Ah?te~inp~,
and D.E. Mann, J. Chen. Phys. 43 (1965) 756_ 181 HF. ShuiUl znd D.W. Hyslop, J. Chem. Phys. 52 (1975) 881.
(91 B. Bsk, 0. Bang, F. Nicholaisen and 0. Rump, Specaochim. Acta A27’(1971)
1865. 1 (1934) 104; F. Brag%, P-A. Clark, E. Weilbronner and hl. Neuenschwzmder, Angw. Chem. 85 (1973) 414; Angw. Chem. Intern. Ed. 12 (1973) 422, and references
[lot T. Koopmans,?hysica
therein. B. ROOS and P. Siegbah, (1970)
Theoiet.
Chim. Acta 17
199.209.
J.H.D. EJ.and, KLTrans. Roy. Sot. London A268 (1970) 87. H. Stafast, Thesis, Utiverssjty of Frankfurt (I 974). P. Zosmus, H. S&f& and H. Bock, Chem. Phys. Let-
ters 34 (197.5) 275.
CHEMCAL PHYSICSLETiERS
CYANOCEN AZIDE: IOIW5ATfON POTENTULS
H.C. Ousted
I?zstilufe, Chemica! Loborufory
1 Scptembei
1975
AWD AB tNlTll0 SCF &fO CALCULATION
Y, ffniversity of Copet~hoger~, Denrnurk
Peter JANSEN Da~zisJxAtomic
Energy Commk-ion,
Rh5. Denn~rk
and tierbert
ST_4FAST *
Receiwd
6 June 1975
The Ne(l) znd He(l) photoelectron spectra of cyanogen azide, NCN3, have Seen recorded at high resolution. Their interprl:tadon is achieved by comparison with the PE spectrum of HN-J and an ab initio LCCO SCF MO cakuhtior?. Dctiations from Koopmans’ theorem of quite different ma_gnitudes are found dependent on the type of ionisation process
I. Introduction Cyanogw azide, NCNJ, is extremely explosive [l]. It is a suitatlIe source for the highly reactive species NCN [2] and for azodicarbonitrile, NCNNCN [2] Microwave [3-S] electron diffraction (61, IR [ 1,79], Raman [9], and UV [2] spectral data are available in the literature, but partly because of the instability of the tide: e-g- upon condensation and thermal or mechanical shock [l], no detailed information about its electronic structure has been reported so far. J?hotoe!ectron(PE) spectroscopy, however, proved to be a cowrenient method of investigation. Ionisation potentials could be measured as well as vibrational frequencies of the cation states. A description of its electronic ground state properties is obtained by both the PE spectrum (within the validity of Koopman’s theorem (101) and by ZRab initio SCFMO. calculation.
2. Method NC& wzs prepared in ~methylph~!ate * To whom cxxespandence
should be addressed.
solution
from BrCN and NaN3, activated by recrystallisation [I ,8], In a typicai run 0.15 g of activated NaN3 were suspended in sufficient dimethylphthalate .with 0.1 g BrCN added. After about 12 hours at room temperature pure NCN, was distilled directly from the solution into the apparatus. No BrCN or Nz (from decomposition of NCN,) w2s detected in the PE spec-
trum (cf. fig. 1). The PE spectra were recorded OR a Perkin-Elmer PS 16 instrument and Calibrated by the xenon and argon peaks (half band width 20-25 meV). The a5 initio LCGO SCF MO calculation [6] was carried out using a 7s, 3p + 4s, 2p basis [I 1J.
3. PE spectra and ab inido SCF MO c&ufntion The Ne(I) and He(r) PE spectra ofcyanogen tide are shown in fig. 1; ionisation potentials and vibrational cation frequencies are summarized in table 1. The Ne(1) spectrum of cyanogen tide displays five separate bands and two more are detected in the high ionisativn energy region of the Kc(r) record. The intensity of the third band decreases slightly, when changing from I%(I) to Kc(E) irradiation, whereas t;Te ‘. 247
Votume 35, m3mber 2
1 September
CHEMICAL PHYSICS LETTERS
1975
NC-N3
Ne 1
He I
IEWI Fig. 1.
-
Ne(I) and He(I) PE spectra
intensities of the fourth and fifth bands increase CORsiderabljr, the first two bmds being regarded unch’an;ed. Most of e&e bzxk show ~br~t~~~~ fute strucbre. These are useful for the band assigilment, if compared wiih: the ndecufar ground state frequencies and the bonG~g properties of the SCF oibitak, rougly estimated from their population analysis’ (tab!e 2). The calcuiated dipole momerit amounts to 3.2? D and
of cyanopn
aide.
agrees fairly weII with the experimental v&e of a 2199 D [3]: The FE band assignment is sketched out in fig. 2. The frrst two experiment4 ionisation potentials agree ftiriy weti W+I the calcdated ne&tive 3a” and 14a’ orbital energies, respectively. In addition, the shape of the first band with its resolved vibrationlf fine structure confkms, that this ionisation occurs from
Volume 35, number 2 Table
1 Septemberl9?5
CHEMICAL PHYSICS LETTERS
1
Ionisation potentials IP’s (eV), vibrational frequencies (cm-r) of the molcculu azide compared wiih ab initio SCF valence orbital ener~ics eJ(ev) .p
1 2 3 4 5 6 7
Molecular orbital
IP (ev) b)
U+ (cm-’ )
10.96 12.4 13.33 14.16 14.54 16.66 18.0
1840,1200,
g 2rl A 2~ B’ 2.X e 1x D 2n E” *x F *l-l
ground state v and cation states yc of cyanogen
BOO
-1!.16 -12.64 -15.02
?a” 14d
1080 1120,720 1920,480 1960
sound state: a’ fundamentnls v(cm-‘) [9] : ~i =2248 ua = 2198 Y‘q= 921 “5 = 865 u,= 167
13a’ 1 ?a’ 2a“ lla’ la” 103’ 9a’ 8a’ 72’ 6a’
vg = 1246 V6 = 666
-15.90 -17.68 -19.95 -21.01 -21.09 -26.35 -34.45 -37.94 -43.74
__-__...-.-.-.-
a) cf. test.
h) Values refer to the He(I) band ma.ximmum or to the maximum
Table 2 Net atomic and overlap population
analysis of Ihe valence shell SCF orbitals of cyanogen
_,____~__________
Molecular orbital _----3,”
14a’ 13a’
_.-.-
__-._.-
0.00
0.44
0.21
-0.21
0.82 0.50
0.15
0.12
0.34
-0.18
0.13
0.21
0.12
0.29 0.3s 0.10
0.09 0.06
0.09
0.78 0.84 0.87
1.44
la” 1Oa’
0.03
0.00
N3
0.44 0.40
0.35 0.52 0.02 0.02 0.03 _._. .__..
0.22 --
0.27
NJ -_-._-_-..---.--~
whereas the second broad band probably [ 121 contains unresolved deformation modes of NC%. Ln the 1:3-l 5 eV region ionisations certainly arise from the 13a’, 12~1 and ?_a”orbitals (cf. fig. 2, table I). The 14.16 eV and 14.54 eV bands overlap
slightly and mu:ri belong to cation states of different 2Z and ‘IT> because othertise the avoided of two 2Z states would prevent an appearance
of such well resolved vibrational fine structure, ;1s in the higher eners
band. The 13.35 eV
band dkplays zn intense 0 + 0 transition peak and is assigned to the well localized 1% orbital, which can be described as cyano nitrogen “lone p&” (table 2). to the sharpness
-_.--
---
Overlap population
a rr orbital,
With respect
aide - ..--_--
0.91
0.35 a.00
observed
---.-.
0.58 0.62
2a” 1la’
crossing
-
Net atomic population N1 CZ --0.56 0.15
12a’
symmetries
of the highest peak.
of the 14.54 eV band
N5
NiC2 --.-
-0.15
CaNs _..-. _.._____~
0.10
-0.03
NsN‘j
N
0.03 -0.23
-0.02
-0.12.
0.03 0.10
-0.10
0.04
0.22 0.04
-0.07
2.14
0.28 0.00
0.33
0.00
0.03 0.04
0.21 0.16
0.26
0.55 -0.02 __.____-_._~_--
-o.c)3
0.08
-1.03 0.31
0.06
and its 1920 cm-’ progression (vCN = v2 = 2198 should occur from the CN cm-’ [9]) th’c19ronisation bonding 7 orbital 2a”. The broadness of the remaining 14.16 eV band is in agreement with electron ejection from the 13a’ orbital, roughly denotable as “in plane nCN” (table 2). The band at 16.66 eV is straightforwardly assigned to the 1 la’ orbital, the terminal adde nitrogen “lone pair”. No decision about the symmetry of the cation suite at 18 eV is possib!e from the spectram (no resolved vibrational tine structure) or the MO ca.!culation (orbital energy difference only 0.06 eV). Correlation with the assigned PE bands of HNs
[12], however, suggests sn ionisation From the totally rr bonding orbital la”. -
Volume
3.5, number 2
CHEbllCAL
NW,.
HN3
IL a’ __
10
..- ‘i;
m ._‘..._~_------
_3 a” ..
t
PES
PES
MO _ tl
PHYSICS
1
6
6 ,,___..__.....-. ._-____------
-15
LE’FTEKS
1 September
L975
hrge, because up to now, Roopmms deficiencies for ;r ionisations from cyan0 compounds amounted to less than 2 eV [13]. Evidently theoretical work beyond the firtreeFock Ievel would be necessary to achieve more reliable calculations. But with respect to the limited computer time available, such calculations 2re excluded for most molecules. Comparison between
assigned PE spectra and MO calculations, however, allorv a rou$ estimate of correlation and reorganisation effects connected with the different qpes of ionisation.
_ iP feY1 v
One of the authors (KS.) is grateful to Professor H. Bock for the opportunity of recording the PE
Fig_ Z Correhtion of sE initio valence orbital energies of NCN3 and PE ionisation
Finally
potentids
it can be stated,
of NCX3 and tlN3
that cyano
occur between
spectra.
[ 121.
substitution
of HN3 leads to almost unchanged ionjsatio~ energies of the first two PE b:rnds (AIP N 0.2 eV). Three additional ionisationr which are, roughly
Acknowledgement
13 and 15 eV,
sfea.king, concerned with the cyano nitrogen “lone pair” and the “in plane T,-~” bonds. Considerable energy and “out of plane ids” ~h.ift~ of about I .3 eV relative to HN, are measured for the last two bands of NC??, in the He(I) region.
References fl J F.D. Marsh, 3. Org. Chem. 37 (1972) 2966. [ZI F.D. Marsh and WE. Hermes. J. Am. Chem. SOC. 86 (1964) 4506, X7 (1965) 181% 131 I(, Bolton, R.D. Brown and F.R. Burden, Chem. Phys.
Letters 1.5 (1972) 79. [41 CC. Cost& and H.W. Kroto, Can. iT. Phys. 50 (1972) 1453. [51 G.L. Blackmann, Id B&on, R.D. Brown, F.R. Burden and A. Mishra, J. iMol. Spectry.
[61 A.
47 (1973) 457.
B. Bak, P. Jensen and T.G. Strand, Xcta Chem. &and. 27 (1973) 1531.
4. Discllssion
[71 D.E. Milligm, M.E. Jacox, J.J. Comeford
Comparison between the negative valence orbital energies and the PE ionisation potentials of cyanogen azide reveals, that Koopmans’ theorem is fairly well npplic-sbk deviations
to the first
of about
two
ion&t&.x processes. Urge 3 eV xe connected with the
ejection of electrons cram the cyano and the terminal azirie nitrogn “lone pairs’“.This is why the PE band sequence
is at variance
with the SCF orbital
sequence
(cf. 0 in fig, 2). Sir&r results have already been ob-
tained for other cyano compounds, whenever ionisations occur from well localized canonical nitrogn %t~e pair” orbitals {13,141. On the other hand, deviations of 3 eV are alsc ob=rved for the ion&lions t’rom,the ?i orbit& 2a” uld la”. This is unusually 250
Ah?te~inp~,
and D.E. Mann, J. Chen. Phys. 43 (1965) 756_ 181 HF. ShuiUl znd D.W. Hyslop, J. Chem. Phys. 52 (1975) 881.
(91 B. Bsk, 0. Bang, F. Nicholaisen and 0. Rump, Specaochim. Acta A27’(1971)
1865. 1 (1934) 104; F. Brag%, P-A. Clark, E. Weilbronner and hl. Neuenschwzmder, Angw. Chem. 85 (1973) 414; Angw. Chem. Intern. Ed. 12 (1973) 422, and references
[lot T. Koopmans,?hysica
therein. B. ROOS and P. Siegbah, (1970)
Theoiet.
Chim. Acta 17
199.209.
J.H.D. EJ.and, KLTrans. Roy. Sot. London A268 (1970) 87. H. Stafast, Thesis, Utiverssjty of Frankfurt (I 974). P. Zosmus, H. S&f& and H. Bock, Chem. Phys. Let-
ters 34 (197.5) 275.