He(I) photoelectron spectroscopy of diisocyanogen (CNNC)

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CHEMICAL PHYSICS LETTERS

Volume 155, number 2

He(I) PHOTOELECTRON

SPECTROSCOPY

OF DIISOCYANOGEN

(CNNC)

0. GRABANDT Akzo Research Laboratories, Velperweg76. 6800 SB Arnhem, The Netherlancis

C.A. DE LANGE Loboratoryfor Physical Chemistry, University oftiterdam,

R. MOOYMAN,

Nieuwe Achtergracht127. 1018 WS Amsterdam, The Netherlands

T. VAN DER DOES and F. BICKELHAUPT

Chemical Laboratory Free University, De Boelelaan 1083, 1081 HVAmsterdam, The Netherlands Received 12 November 1988

The He(I) photoelectron spectrum of diisocyanogen ( CNNC ) is presented. The interpretation is mainly based on experimental considerations aided by the results of Hartree-Fock-Slater calculations and strongly supports this novel structure. Indications of large correlation effect; are found.

1. Introduction He( I ) photoelectron spectroscopy (PES ) is a wellestablished method for studying electronic structure of small molecules in the valence region. In the PE spectra of molecules which contain the chemically important cyan0 group (CN ) which is usually bound through its C atom, three ionization phenomena are generally found in the 13- 16 eV region, viz. two of rccN and one of nN type. The ltcN bands Often show vibrational structure arising from the CN stretch, which due to the bonding character of the corresponding orbital has its frequency reduced in going from the neutral molecule to the ion. In contrast, the usually much more localized nN lOtIe pair orbital often gives rise to a rather sharp PE band. The order Of the &-N and nN bands in the PE Spectrum iS not obvious a priori [ 11. The assignment problem associated with CN valence orbitals is aggravated when the molecule under study contains more than one CN group. When two symmetry-related CN groups are present, the delocalized ncN and much more localized nN orbitals give rise to PE bands whose positions are subject to very different correlation contributions. In a study on Se( CN), this has been exemplified by means of relatively simple broken0 009-26 14/89/$ 03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division )

symmetry calculations [ 2 1. In cyanogen (NCCN) the importance of correlation effects on the order of the KCNand nN bands has been established conclusively by means of more sophisticated Green’s function calculations [ 3,4]. In isocyano compounds where the CN group is attached to the molecule through the nitrogen atom, a similar situation prevails. In the PE spectrum, zcN bands associated with delocalized CN orbitals and an nc band corresponding to a localized carbon lone pair orbital are expected. From the point of view of correlation effects it would be interesting to study a compound which contains two symmetry-related isocyano groups. The simplest candidate from a theoretical viewpoint would seem to be the molecule diisocyanogen (CNNC). The various possibilities of bonding two CN groups together have been explored in a theoretical paper in which-neglecting correlation - the geometries, the relative stabilities and the vibrational normal modes of NCCN, CNCN and CNNC were calculated. Cyanogen (NCCN) was found, not surprisingly, to be 66 kcal/mol more stable than diisocyanogen (CNNC) [ 5 1. In a recent experimental paper, diisocyanogen was obtained when the precursor norbomadienone azine was flash-pyrolyzed. A species with mass 52 was genB.V.

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erated which was unstable above zz - 50°C. The 13C and 14N NMR spectra of the pyrolysis product, dissolved in toluene at low temperature, were consistent with the formation of CNNC [ 6 1. In this paper a UV PES study of this interesting unstable species is presented. An analysis of the PE bands in the valence region underlines the importance of correlation and serves as an independent means of unambiguously characterizing the molecule as diisocyanogen ( CNNC 1.

2. Experimental The photoelectron spectra were recorded on a homemade spectrometer. The spectrometer was especially designed to minimize the effects of the contamination inherent in working with reactive and unstable species [ 71. It consists of two separately pumped chambers, an ionization chamber and an analyzer chamber. On the He(I) dc discharge lamp in the ionization chamber, a cylindrical brass tube is mounted in which gaseous samples are injected, thus restricting the contamination to a small region and increasing the gas density in the UV photon beam. The spectrometer is under microcomputer control which allows compensation for drift due to contamination by the incorporation of the so-called lock procedure [ 8 1. The precursor molecule used in this work is norbornadienone azine, the synthesis of which is described in refs. [6,9]. The precursor was subjected to flash vacuum thermolysis at 500°C. The stable (benzene) and unstable products were collected in a liquid nitrogen trap [ 6 1. To separate the more volatile products from benzene a distillation was carried out at z - 11 O”C, 10S5 mbar. The purified fraction was then attached to the PE spectrometer and cooled by means of a pentane slush bath. During the experiment the temperature of this bath was slowly raised starting at - 125’ C. At around - 115 oC the gas pressure of the products in this fraction was suitable for sustained PE measurements. The PE spectra obtained in this manner show intense signals due to one component only. The analysis of these spectra leads to the unequivocal identification of the novel, reactive molecule diisocyanogen, CNNC. The noble gases argon, krypton and xenon were 222

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CHEMICAL.PHYSICSLEmERS

added for calibration purposes. Argon was used as a lock signal to increase the long-term stability. A total of 550 scans was obtained at 20 s/scan. The resolution was maintained at 30 meV measured at the Arf 2P3,2 signal.

3. Results and discussion Assuming CNNC to be a symmetric linear molecule, four bands are expected in the He(I) photoionization range. The CN bonding fragment n-orbitals can combine in or out of phase to form n, and n, molecular orbitals. The carbon lone pair combinations can combine in or out of phase to form oB and o, molecular orbit&. The overall photoelectron spectrum of the purifled fraction obtained as described in section 2 is shown in fig. 1. No other PE signals were observed in the He(I) photoionization range (O-21.2 eV). The signals due to argon are indicated. Argon was chosen as lock signal because no overlap with other photoionization phenomena occurred. The spectrum shows three separate photoionization features with intricate vibrational structure located in the regions 12.7-14.2, 14.3-15.0 and 16.1-17.1 eV. In fig. 2 the PE spectrum in the range 12.6-16.0 eV is enlarged. The band structure at the lowest ionization energy is complex. Two spectral contributions other than from the unstable product are apparent. The small amount of He(IP) radiation produced in the discharge lamp leads to small ionization features from Ar at 13.890 and 14.068 eV.

“’

. 160

17.0

160

150

IONIZATION ENERGY

Fig. 1. He (I) photoekctron as a reference.

14 0

130

120

(eV)

spectrum of CNNC with argon added

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CHEMICAL PHYSICS LETTERS

Fig. 2. Expanded He(I) photoelectron spectrum of CNNC in the 12.6-16.0 eV region and preferred assignment.

Based on a careful study of the overall band shape, signals due to the presence of HCN [ 1 ] are indicated. HCN may be generated by two routes. First, the parent compound of norbomadienone azine [ 6 1, quadricyclanone azine, may have remained during the synthesis of norbomadienone azine as an impurity. Upon pyrolysis quadricyclanone azine is known to dissociate into benzene, benzonitrile and hydrogen cyanide [ 91. Alternatively, pyrolysis of norbomadienone azine may lead, by analogy, to asymmetric bond breaking producing minor quantities of hydrogen cyanide. The remaining structure can be explained by the presence of two photoionization phenomena. The first, at 12.873 eV is a sharp band with no associated vibrational structure. The second, with adiabatic ionization energy (IE) and band maximum (which for a broad PE band may deviate from the vertical IE [ lo] ) coinciding at 12.9 11 eV can be fitted with two regular vibrational progressions. Five components with vibrational frequency 2270 cm-’ (w, ) are indicated. From the variation in the spacings, a small anharmonicity w&=55 cm-’ is derived. Starting at each component of wI, indications of a vibrational progression with smaller frequency (wZ) are present. The asymmetryofthepeaksat 13.185, 13.451 and 13.717eV ( wl: v= 1, 2, 3) and the raised background in this band suggest contributions from a vibrational progression with a frequency of approximately 1300 cm-‘. Starting from the ol: v=O, 1 and 2, three, two and one additional small vibrational feature respectively are indicated. On the basis of these two pro-

24 February 1989

gressions the experimental band shape can be understood in detail. The second band structure at 14.3-15.0 eV shows five distinct peaks with an irregular intensity distribution. The adiabatic ionization energy and band maximum coincide at 14.390 eV. This band structure can also be explained by the interplay of two regular vibrational progressions. The first progression has frequency 2075 cm-’ and three components (14.390, 14.644 and 14.903 eV). The second progression has a frequency of 1160 cm- ‘. Starting from the u= 0 and v= 1 of the larger frequency progression three components of the smaller vibrational progression can be located. This also explains the broadening and asymmetry observed in v= 1 and v=2 of the large progression. The third band in the region 16. l-l 7.1 eV is shown in fig. 3. In this band small signals due to leakage of nitrogen into the vacuum system are present. The vibrational progression due to ionization to the ‘IIu state of N: is shown. The extensive vibrational progression due to the unstable product has an adiabatic ionization energy of 16.106 eV and a band maximum at 16.460 eV. The band structure can be explained on the basis of one vibrational progression with frequency 710 cm-‘. Ten components of this progression can be clearly observed. The specing between the components in this progression is somewhat irregular, hence a reliable anharmonicity constant could not be obtained. The experimental results are compiled in table 1. Summarizing, the measured PE spectrum is intense 0.

I-

Fig. 3. Expanded He(I) photoelectron spectrum of CNNC in the 15.6-17.3 eV region and preferred assignment. 223

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Table 1 Experimental and calculated ionization energies and experimental and without inversion centre for the electronic wavefunctions

‘) b, ‘) ‘)

frequencies.

Exp. IE a1

Calc. IE

Calc. IE

(eV)

D,,

C,,

(o*)-’

12.873

11.74

_C)

(x,)--l

12.911 14.390 16.106 c) 16.460 f)

12.91 12.02 .16.98

12.91 13.00 16.98

The experimental error in the IEs is 0.005 eV. The estimated error in the vibrational frequencies Not converged. d)W&=55+30cm-‘. Adiabatic ionization energy. ‘) Band maximum.

(eV)

(eV)

The calculations

19B9

have been carried out with

b) WI (on-‘)

Ozb’ (cm-‘)

2270 *’ 2075

1300 1160 710

is 40 cm-‘.

and pure. All features in the spectrum can be explained by four ionization phenomena and associated vibrational progressions. Hartree-Fock-Slater (HFS ) LCAO calculations have been performed as an aid in the assignment of the photoionization phenomena. HFS calculations applied to small molecules usually predict absolute IEs to within 1 eV and relative IEs to 0.2-W eV [ 111, The non-local exchange potential in the Hartree-Fock equations is replaced by a local Xa exchange potential [ 12 1. The HFS equations are solved by the method of Baerends et al. [ 13 1. The orbitals are expanded into a Slater-type function basis set. The basis set used was of double-zeta quality and contained an added d-type polarization function (exponent 2.00). The 1s orbitals were treated as frozen core. Two different numerical integration schemes were used, viz. the Diophantine method [ 131 and the te Velde method [ 141 in which better numerical accuracy is achieved while using less integration points. The number of integration points in the Diophantine scheme was 10000, in the te Velde scheme 1716. The ionization energies were calculated using the ASCF approach. The orbit& in the ionic states were allowed to reorganize in the fmed nuclear frame. The geometry used has been calculated by Sana et al. [ 51 in a Hartree-Fock calculation without configuration interaction. A geometry optimization in the CNNC ground state using the semiempirical MNDO method produced a linear, symmetrical structure with parameters within 0.05 au of the result of Sana et al. [ 51. The nuclear geometry used in 224

vibrational

24 February

the HFS calculations was a linear configuration with interatomic distances C-N 2.205 au and N-N 2.400 au. The HFS calculations were performed allowing two symmetries for the electronic wavefunctions: D,, and C,,. The latter lacks the inversion centre and the mirror plane perpendicular to the molecular axis, thereby allowing charge localization on one side of the molecule upon ionization. The results of the calculations are given in table 1. In the Dolh case, four ionizations in the He ( I ) range are predicted: two orbitals of x-type which are each twofold degenerate and two of a-type. The delocalized n-type ionizations converge to IE values close to the experimentally observed ones. However, the much more localized o-type ionizations converge to IE values completely different from experimentally observed ionization phenomena. The broken symmetry calculations in C,, again gives rise to four ionizations. The n-type ionizations are hardly influenced compared to the IEs calculated in the full symmetry of the molecule. One of the a-type ionizations shifts to higher IE, the other shows serious convergence problems. The situation is very similar to that previously found in Se(CN)2 [ 2 1. The obvious conclusion is that electron correlation plays a crucial role in describing the ionic states obtained from o-type ionizations. In table 2, Mulliken population analyses of the Dmh orbitals are shown both for the ground state neutral and the ionic states. Neither in DWh nor in C,, does significant d-orbital participation occur. In D,, the IC, orbital in particular shows strongly delocalised

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Table 2 Population analysis in D,, c 2s

c Zp,“’

C 2P.v

N 2~,,

og neutral o8 ion

0.64 0.64

0.34 0.35

7t,neutral b, ns ion

0.43 0.31

0.52 0.56

u. neutral a, ion

0.69 0.14

0.28 0.24

?r, neutral n, ion

0.14 0.11

0.84 0.86

a>The z-axis is tbe molecular axis. b, In the R orbitals some d orbital participation occurs.

character. Both o-type molecular orbitals are combinations of the carbon 2s and 2p, atomic orbitals. The reorganization effects calculated upon ionization are small. In C,, symmetry no significant changes occur in the population analysis of the neutral ground state of the molecule. In the ionic states only the composition of the ionized D orbitals shows radically different behaviour. Despite convergence problems both Q orbitals appear to be localised completely on one carbon atom with 70% 2s and 30% 2p, character. The preferred electronic assignment of the ionization phenomena based on experimental considerations and supported by our calculations, starting from low IE and assuming Dooh symmetry, is ( c~)-‘, (IQ-‘, (o,)-’ and (zJ-‘. The assignment of the degenerate x-type ionizations is underlined by considering the total intensity of these two band structures compared to the o-type ionizations. Also, from the population analysis ionization of the x-type orbitals is expectred to give rise to appreciable vibrational progressions due to the delocalized character of these orbitals. Finally, the calculated IEs are in fair agreement with the experimentally observed ones. The other two ionization phenomena are of G type where the preferred order of assignment is taken to be that predicted by the Dmh calculations. This is also in agreement with the preferred assignment in cyanogen [l]. In a linear, four-atomic molecule there are seven vibrational normal modes: two twofold degenerate bending vibrations (symmetric and asymmetric) and three stretch vibrations (two symmetric and one asymmetric). In the case of CNNC, the two symmetric stretch vibrations can be characterized as mainly CN stretch ( w , ) and mainly NN stretch ( w, ) . From a preliminary analysis of recent infrared and

Raman experiments performed by Stufkens et al. [ 151 a CN stretch frequency of 2290 cm-’ and an NN stretch of 976 cm-’ were obtained. In interpreting the vibrational structure in the PE bands of CNNC the molecule is assumed linear in both neutral and ionic states, and only totally symmetric normal modes are considered. From symmetry the ng orbital of CNNC possesses a nodal plane intersecting the molecular axis between the nitrogen atoms, whereas in A, no such no&l plane is present. Therefore, while both orbitals show CN bonding character, np is expected to be NN antibonding and A, to be NN bonding. Upon ionization of IC, the antibonding NN character should decrease and an increase in the frequency of the normal mode dominated by the NN stretch is predicted. The corresponding decrease in CN bonding character should lead to a lowering of the frequency of the normal mode dominated by the CN stretch. Similarly, upon ionization of IC, a lowering of both the NN and CN stretch frequencies is predicted. Comparing these predictions with experiment, in n, the CN stretch (w, ) appears to be marginally lowered upon ionization. The NN stretch ( w2) shows a dramatic increase of approximately 330 cm- ‘. This behaviour can be understood from the results of the population analysis. Although n[8is strongly delocalized over the entire molecule, the electron density is large on the nitrogen atoms. Upon ionization of x, the PE band only shows the NN stretch with appreciable decrease in frequency. The population analysis of this orbital shows an approximately 852 localization on the nitrogen atoms. The observed dominance of the NN stretch vibration and the lowering of its frequency on ionization are in line with this computational result. The HFS calculations on o, and o, suggest a 225

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marked lone pair character for both orbitals, which would predict rather sharp photoionization phenomena in their PE spectrum. At the same time the results of the broken symmetry calculations underline the importance of configuration interaction (CI) in both a, and 0”. Inclusion of CI in the case of 0, would lead to an electronic configuration containing additional contributions from other orbitals which by symmetry would all contain a nodal plane between the nitrogen atoms. The qualitative result of CI will be an orbital which is CN bonding and NN antibonding. This would create a situation completely analogous to that described above for xK Therefore, upon ionization a &induced lowering of the CN stretch and an increase in the NN stretch frequencies would be expected. This interpretation is indeed supported by the experimental results. In the case of the op orbital the photoionization features between 12.7 and 14.2 eV can be understood by assuming a structureless PE band. Obviously correlation effects play an important role in the PE spectroscopy of CNNC. In order to place the considerations advanced above for the otype orbitals on a firmer basis, calculations which take account of configuration interaction in a proper way are called for. Such calculations are presently in progress [ 161. Preliminary theoretical results show that because of correlation effects the molecule CNNC is not symmetric. Deviations from a symmetric structure can be important in a more detailed understanding of the molecule.

4. Conclusion It is shown by UV photoelectron spectroscopy that flash pyrolysis of norbomadienone azine produces a novel reactive species which can be unambiguously characterized as diisocyanogen (CNNC), thus confirming the preliminary assignment based on physical properties, mass and NMR spectra [ 6 I. The PE spectrum obtained is consistent with that of a linear molecule. The results of ab initio Hartree-FockSlater calculations in which both D,,, and C,, symmetries are allowed for the electronic wavefunctions,

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strongly indicate the importance of correlation effects, in particular for o-type orbitals. A more detailed study of such correlation effects will be very useful.

Acknowledgement The authors are grateful to the Netherlands organisation for scientific research (NWO) for equipment grants and financial support (OG) and to F.M. Bickelhaupt for helpful discussions.

References [ 1 ] D.W. Turner, C. Baker, A.D. Baker and C.R. Btundle, Molecular photoelectron spectroxopy ( Wiley-Interscience, NewYork, 1970). [Z ] G. Jonkers, C.A. de Lange, L. Noodleman and E.J. Baerends, Mol. Phys. 46 (1982) 609. [3] L.S. Cederbaum, W. Domcke and W. von Niessen, Chem. Pllys. 10 (1975) 459. [4] L.S. Cederbaum, W. Domcke, J.Schitmer and W. von Niessen, Physica Scripta 21 ( 1980) 48 1. [5] M. SanaandG. Leroy, J. Mol. Struct. 76 (1981) 259. [6] T. van der Does and F. Bickelhaupt, Angew. Chem. 100 ( 1988) 998. [7] D.M. de Leeuw, Ph. D. Thesis, Vrije Universiteit, Amsterdam ( 1979). [8] O.Grabandt, H.G. Muller and CA. de Lange, Computer Enhanced Spectry. 2 (1984) 33. [9] A. Riemann and R.W. Hoffmann, Chem. Ber. 118 (1985) 2544. [lo] L.S. Cederbaum and W. Domcke, i;Advances in chemical physics, Vol. 36, eds. I. Prigogine and S.A. Rice (Wiley, New York, 1977) p. 205. [ 111 E.J. Baerends, J.G.Snijders, CA. de Lange and G. Jonkers, in: Local density approximations in quantum chemistry and solid state physics, eds. J.P. Dahl and J. Avery (Plenum Press, New York, 1984). [12] J.C. Slater,Phys.Rev. 81 (1951) 385; 91 (1953) 528. [ 131 E.J. Baerends and P. Ros. Intern. J. Quantum Chem. Symp. 12 (1978) 169. [ 141 P.M. Boerrigter, G. te Velde and E.J. Baerends, Intern. J. Quantum Chem. 33 (1988) 87. [ 151 D.Stutkens, private communication. [ 161 F. Tarantelli and L.S. Cederbaum, private communication.