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Diisocyanogen or Isocyanogen.

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mentation, which would most suitably be done by a second
laser, enables sequencing of peptides and proteins comparably to Collision-Induced-Dissociation has yet to be investigated.
Received: February 13, 1989 [Z 3172 IE]
German version: Angew. Chem. 10f (1989) 805
100000 150000
Fig. 2. LDI mass spectrum of glucose isomerase. Measured molecular weight:
172420. Sum of 30 single spectra.
as well as a cluster ion
with lower intensity. All
signals can be used to improve mass determination precision.
The fact that glucose isomerase, which consists of four noncovalently bound subunits, is desorbed at least predominantly, if not only as the intact protein is a further proof for the
extreme softness of the matrix-UVLDI technique.
The total sample amount deposited on the target was
500 fmol, spread over an area of several mm2. At least several thousand single spectra can be obtained from such a preparation. An estimate of the sample consumption per single
spectrum gives ca. lo-" mo116] leaving much room for a
further reduction of sample amount necessary.
Fig. 3. LDI mass spectrum of catalase. Measured molecular weight: 236230.
Sum of 50 single spectra.
Figure 3 shows the spectrum of catalase with a measured
molecular weight of 236230, summed over SO single laser
shots. Again, the molecular ion, ( M 4 ) @consisting
of 4 equal
subunits with a molecular weight of 59060 each, is clearly
detected, as well as the doubly charged species (M4)20.Because of the absence of a triply charged molecular ion it can
be concluded that the peak at m/z 59 060 originates from the
singly charged subunit ( M I ) @and not the quadruply charged
molecular ion ( M J 4 @ . Correspondingly the signal at mjz
29 500 represents the doubly charged subunit ( M , ) * @ .
Whether or not the protein mostly decayed into its subunits
upon preparation (and evaporation of the solvent) already.
or the decomposition was predominantly induced by or during the desorption process still needs further investigation.
As in all desorption techniques sample preparation plays
the key role in LDI of high mass molecules. Highly purified
samples and the use of ultrapure water for protein and m a
trix solutions is a prerequisite for a successful mass analysis,
indicating that the salt contamination commonly present can
disturb the homogeneity of the protein-matrix-association
necessary for high mass ion desorption. Under such conditions, all spectra show a very good signal-to-noise ratio. Ths
mass spectra shown are typical for matrix-UVLDI: so far no
fragmentation of covalent bonds has been observed. The
intense signals in the mass range below ca. 1000 Dalton are
caused exclusively by the matrix. Whether a successive fragAngru. Chem. Inl. Ed. Engl. 28 (1989) N o . 6
[l] A. G. Craig, A. Engstrom, H. Bennich, 1. Kamensky, 35th ASMS Conf.
Mass Spectrom. Allied Top., Denver, CO, USA 1987, p. 528.
[2] P. Roepstorff. P. F. Nielsen, K. Klarskov. P. Hojrup, Biomed. Enviranm.
Mass Speclrom. 16 (1988) 9.
[4 R. J. Cotter, Anal. Chem. 60 (1988) 781 A.
[4] K. Tanaka, H. Waki, Y. Ido, S. Akita, Y. Yoshida, T.Yoshida, Rapid Cammun. Mass Speclrom. 8 (1988) 151.
[S] M. Karas, F. Hillenkamp, Anal. Chem. 60 (1988) 2299.
[6] M. Karas, U . Bahr, F. Hillenkamp, h t . J. Mass Spectrum. Ion Processes. in
(71 R. J. Beuhler, J. Appl. P/7ys. 54 (1983) 4118.
Diisocyanogen or Isocyanogen?**
By Lorenz S. Cederbaum,* Francesco Tarantelli,
Hans-Georg Weikert, Markus Scheller, and Horst Koppel
While the stable cyanogen molecule (NCCN) has long
been known, the synthesis of its isomers diisocyanogen (CNNC) and isocyanogen (CNCN) has posed considerable difficulties. Only recently has the experimental formation of CNNC been reported for the first time."] The precursor
norbornadienone azine was flash-pyrolyzed and the ' 3C and
14N NMR spectra of the pyrolysis products were consistent
with the presence of CNNC.['] More recently the He(1) photoelectron (PE) spectrum of CNNC was reported."] This
clean spectrum was interpreted on the basis of experimental
considerations augmented by the results of Hartree-FockSlater calculations. The calculations indicated the presence
of strong correlation effects and made it quite clear that
more sophisticated theoretical methods are necessary to correctly understand the experimental findings.I2]
The PE spectrum of cyanogen has been successfully CdlCUlated in the past using Green's functions theory.[31We have
now computed the PE spectrum of CNNC using both
Green's functions and configuration interaction (CI) methods. The CI result is shown in Figure 1 c. It is seen that the
calculated spectrum does not compare with the experimental
spectrum, which is schematically depicted in Figure 1a. In
particular, the energy difference between the two o-ionization potentials is much smaller than the experimental one.
This difference is very constant with respect to the various
calculations performed. In addition, the calculated nu-ionization potential is, over and beyond the uncertainties of the
theory, too large compared to the experimental value. We
should mention that correlation effects are indeed substantial and an interpretation of the calculated spectrum via orbital energies is rather meaningless.
The geometry of CNNC used for the calculation of the PE
spectrum was optimized at the CI level and was found, due
to the presence of correlation effects, to be somewhat different from the SCF geometry reported in the literatureL4]
[*] Prof. Dr. L. S. Cederbaum, Dr. F. Tarantelli,"' H.-G. Weikert,
M. Scheiier, Dr. H. Koppel
Physikalisch-Chemisches Institut der Universitit
D-6900 Heidelberg (FRG)
Alexander von Humboldt fellow; Permanent adress:
Dipartimenlo di Chimica Universita di Perugia 06100 Perugia (Italy)
We thank C. A . deLange for making the PE spectrum available prior to
publication and for many interesting discussions.
Verlagsgesellrchajl m b H , 0-6940 Weinheim,1989
0570-0833/89/0606-0761 $02.50/0
peated the calculation of the PE spectrum of CNCN using
the same procedures and basis sets as for the newly computed CI geometry (CNC’N’: RCN= 1.180 RNc,= 1.320 A,
R,.,. = 1.157 A). The technical details of the Green’s function and CI calculations of both CNCN and CNNC will be
given elsewhere.16] The resulting PE spectrum of CNCN is
shown in Figure I b. It compares very convincingly with the
experimental spectrum (Fig. 1 a). It is now obvious that the
measurements in Ref. 121 were performed on CNCN.[’]
Received: February 17, 1989 [Z 3178 IE]
German version: Angew. Chem. 101 (1989) 770
CAS Registry numbers:
diisocyanogen, 78800-21-2; cyanoisocyanogen, 83951-85-3
Fig. 1. a) The experimental [2] (schematic drawing) and b), c) calculated PE
(RcN= 1.181 A,R,, = 1.291 A). This difference in geometry is, however, by far too small to lead to substantial
changes of the ionization potentials. In particular, the
0,- 0“separation remains unchanged. To remedy the situation we searched for a linear but non-symmetric CNNC (a
non-linear CNNC would exhibit six ionization potentials
instead of the four found in the PE spectrum). Although we
have invested considerably in this search and the potential
energy surface has been found to be very flat along the asymmetric distortion, no additional minima could be detected
and we conclude that CNNC is indeed symmetric. Moreover, we must conclude that the molecule whose PE spectrum has been measured is not diisocyanogen.
Since the molecule that has been synthesized contains two
C atoms and two N atoms, we must assume that either a
different isomer has been obtained, or that CNNC has isomerized to the more stable CNCN before the PE spectrum
was recorded. The spectrum, however, does not exhibit signs
of an additional isomer and, therefore, we tend to assume
that it is not CNNC which has been synthesized. B is one
possible explanation for the formation of CNCN from norbornadienone azine, whereas A would correspond to the
formation of CNNC as suggested in Ref. [I].
Since the NMR measurements are carried out in the condensed phase we cannot rule out the formation of ordered
The ionization potentials of CNCN have been calculated
before”’ and agree well with the experimental values.[*]To
be consistent with the calculation on CNNC, we have re762
Erlagsgesellschaft mhH. 0-6940 Wemheim. 1989
[I] T. van der Does, F. Bickelhaupt, Angew. Chem. 100 (1988) 998; Angew.
Chem. Int. Ed. Engl. 27 (1988) 936.
[2] 0. Grabant, C. A. delange, R. Mooyman, T. van der Does, F. Bickelhaupt,
Chem. Phys. L r t f . 155 (1989) 221.
131 L. S. Cederbaum, W. Domcke, W. von Niessen, Chem. Phys. I0 (1975) 459.
[4] M. Sana, G. Leroy, J. Mot. Strucl. 76 (1981) 259.
[5] W. von Niessen, L. S. Cederbaum, J. Schirmer. G. H. F. Diercksen, W. P.
Kraemer, J. Electr. Spectr. 28 (1982) 45.
[6] F. Tarantelli, L. S . Cederbaum et al., unpublished.
[7] Authors’ note: After submission of our manuscript a relevant paper has
appeared (F. Stroh and W. Winnewisser, Chem. Ph.ys. Let/. 155 (1989) 21)
in which the experimental infrared and microwave spectra of the molecule
of [I] are presented and analyzed and likewise attributed to CNCN.
Surprising Reactions of Decamethylsilicocene
with n-Systems of the Type X = C = Y
B y Peter Jutzi* and Andreas Mohrke
We recently reported the synthesis of decamethylsilicocene
1.[’]This metallocene-like x-complex is so far the only divalent silicon compound that is stable under normal conditions. In the framework of our investigations of the chemistry of 1 we have, inter aha,[*] carried out reactions with
simple organic compounds of the type X = C =Y. In the following we describe some, in part, surprising observations we
made in the reaction of 1 with carbon disulfide, carbon dioxide, and phenyl isothiocyanate.
Already at room temperature 1 reacts with carbon disulfide in benzene solution to give the cyclic thioester 3 in almost quantitative yield. This suggests that the reaction involves an initial [2 I]-cycloaddition leading to the unstable
thiasilirane 2, which immediately dimerizes to give 3
[Eq. (all.
The reaction of 1 with carbon dioxide proceeds quite differently: passage of CO, into a solution of 1 in toluene at
room temperature surprisingly leads to almost quantitative
formation of the spiro compound 7. The reaction probably
proceeds as follows: the initial [2+ I]-cycloaddition product
4 formed analogously to Equation (a) loses carbon monoxide to give the silanone 5 , which reacts with carbon dioxide
in a [2 + 2]-cycloaddition to give the cyclic silanediyl carboxylate 6; this then reacts with further silanone 5 to give the
final product 7 [Eq. (b)]. The carbon monoxide formed in
this reaction sequence was detected in the form of a carbonyl
complex.[3’If the reaction according to Equation (b) is carried out in the presence of acetone the hydroxysilylenol ether
Prof. Dr. P. Jutzi, Dipl.-Chem. A. Mohrke
Fakultat fur Chemie der Universitat
Postfach 8640, D-4800 Bielefeld (FRG)
0570-OR33I89j0606-0762 $02.50/0
Angew. Chem In!. Ed. Engl 28 (1989) N o . 6
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isocyanogen, diisocyanogen
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