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Arguments Concerning the Orbital Sequence in Borazine.

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Arguments Concerning the Orbital Sequence in
BorazineflJ
By Hans Bock and Werner FUSS"~
MO calculations taking account of all valence ele~trons[~-'1
result in an orbital sequence for borazine that is at variance@]
with its photoelectron spectrum[']; in particular, the highest occupied molecular orbital is calculated to be a u type orbital.
The following considerations and experimental data favor a different assignment:
bib
biu
& --+I
1) The completely assigned"] photoelectron spectrum of benzene, a molecule that is isoelectronic with borazine, leads to
the sequence of energy levels characterized in Fig. 1 by orbital
diagrams and irreducible representations of the corresponding
molecuIar orbitals. On changing from Dhh to D,, symmetry,
the irreducible representations eZg and el" coincide in e' and
theirreduciblerepresentations b,, and alg in a;[']. If a perturbation of symmetry D,, is introduced by an alternate raising
and lowering of the nuclear charge of the ring atoms then the
molecular orbitals of the same irreducible representation will
become mixed: The smaller the energy difference between two
levels, the greater their "mutual repulsion". The energy level
shifts resulting from interactions between occupied molecular
orbitals are shown qualitatively in Fig. I by the arrows bib, and
additional changes due to the unoccupied molecular orbitals
by the arrows biu. Whether the orbital sequence is altered within
this benzene-tborazine correlation can be decided experimentally by the assignment of individual bands in the photoelectron
spectrum.
2) Six bands (at least) can be distinguished in the PE spectrum
of borazine (Fig. 2) up to energies of 2 1.2 1 eVL9].The first band
at 10.09 eV is readily assigned to a x ionization on account of
its recognizable vibrational fine structure: The progression of
1360k 100 cm-' can only arise from a ring vibration E' (IR
spectrum of borazine: v I 3= 1465 or vI4 = 1406 ern-'['']) and,
as in the case of benzene, indicates a degenerate level; the progression of 800 t 40 cm-' corresponds to a ring vibration A;
,
-*n
&
I
la2"----
Benzene
Borazine
--la;
Fig. 2. Correlation of the photoelectron spectra of benzene and borazine
191.
(Raman spectrum of borazine: vi = 944 or v4 = 842 cm-' !I1])
----\-and is therefore to be assigned to the "ring bonding" n orbital
le". For the o orbital 4e', which is hardly involved in BIN bond----\-
-
Fig. 1. Qualitative energy level shifts on transition from benzene to borazme.
182
ing, there remains only the structureless second band at 11.40
eV. Moreover, the band at 13.72 eV can be assigned to the
"strongly B/H bonding" orbital 3a; on account of its vibrational
fine structure: This is confirmed primarily by the appearance
Angew. Chern. internat. Edit. / Vol. 10 (1971) / N o . 3
of the B/H stretching vibration 1880 f 100 cm-' of species A;
(Raman spectrum of borazine: v2 = 2442cm-'["]) which, like
the C/H stretching vibration of the lb,, band in the photoelectron spectrum of benzene, is drastically lowered by ionizationl71. On the basis of the qualitative orbital correlations (Fig.
1)and the approximate intensity ratios 2 :2 :0.7 : 0.7 : 5 : (1),the
remaining bands of the borazine photoelectron spectrum are
tentatively assigned in the sequence of degenerate and nondegenerate molecular orbitals e"/e'/a;'/a,'/(e' + a>)/(a'l + e') . . .
(Fig. 2).
3) Further arguments supporting t h e n assignment of the highest
occupied borazine molecular orbital arise from the photoelectron spectra of methyl and halogeno derivatives whose first ionization potentials are shown in Fig. 3.
I
I
X
CH31CH3
F/H
Halogeno substituents X = F,CI should lower the highest occupied u level considerably while in a x system the resonance
effect of X will oppose the inductive effect[121:The first band in
the photoelectron spectrum of B-trifluoroborazine at 10.66 eV
and that of B-trichloroborazine at 10.55 eV therefore clearly
correspond to a x ionization. It also follows from the slight perturbation of the n system that it is the first of the two photoelectron bands of borazine at 10.09 and 11.40 eV (Fig. 2) that
has to be assigned to a n orbital.
Methyl groups should have the effect of raising the highest OCcupied orbital in borazine and its derivatives. According to first
order perturbation theory the change in orbital energy
6eJ = Zcf, 6a,
is given by the perturbation term 6aQ and the square of the
at the perturbed center @. Additivity can
orbital coefficient
be expected in the case of several perturbations: As can be seen
from Fig. 3, the first ionization potential is altered by the same
independently of the B-subamount on N-methylation (-),
stituents, and on B-methylation or B-halogenation (- - -), independently of the N-substituents. Such an additivity is not to
be expected for o orbitals, particularly in the case of B-halogenoborazines.
Assuming a constant perturbation 6ae (CH,), it is also possible
to calculate the ratio of the squares of the B-and N-orbital
coefficients: Insertion of the measured mean orbital energy
changes 6eJ(B-CH3) = 0.55 k 0.05 eV and 6cJ(N-CH3) =
1.1 t 0.1 eV into the perturbation formula yields I c 2 . XcjF;
B 'B.Y
= 1 : 2 for the highest occupied n orbitals of the borazine
derivatives listed in Fig. 3[**1.
Ge
Received: November 13, 1970 [Z 329 IE]
German version: Angew. Chem. 83, 169 (1971)
Fig. 3 . First ionization potentials of borazine and some of its methyl
and halogeno derivatives.
[*] Prof. Dr. H. Bock and DipLChem. W. Fuss
Chemische Institure der Universitar
6 FrankfudMain 70, Ludwig-Rehn-Strasse 1 4 (Germany)
Note added in proof: In Qe meantime, S. D. Peyerimboffand R. J.
Buenker have published an ab initio calcdation [Theor. Chim. Acta 19,
1 (1970)] that fully confirms our assignment.
[l] Photoelectron spectra and molecular properties, Part 2. - Part. 1:
E. Heilbronner, V. Homung, H. Bock, and H. Aft, Angew. Chem. 81,
537 (1969); Angew. Chem. internat. Edit. 8, 524 (1969).
[2] R. Hoffmann, J. Chem. Phys. 40,2474 (1964).
[**I
[3] P. M. Kuznesov and D. F. Shriver, J. Amer. Chem. SOC.90, 1683
(1968).
[4] D. W. Davies, Trans. Faraday SOC.64,2881 (1968).
[ 5 ] D. C. Frost, E G. Herring, C. A. McDowell, and I. A. Stenhouse,
Chem. Phys. Lett. 5, 291 (1970).
[6] a) D. R. Lloyd and N. Lynaugh, Phil. Trans. Roy. SOC.(London) A
268, 97 (1970); b) cf. also D. R. Amstrong and D. T. Clark, Chem.
Commun. 1970, 99; as well as c) the interprelaLion of the ionization
potentials of alkylated borazines given in Ref. (31.
[7] L Asbrink, E. Lindholm, and 0.Edqvisr, Chem. Phys. Lett. 5,609
(1970).
[8J cf.,cg., E. B. Wilson, J. C. Decius, and P. C. Cross: Molecular Vibrations, The Theory of Infrared and Raman Vibrational Spectra.
McGraw-Hill, New York 1955, p. 337.
191 The PE spectra were recorded with a Perkin-Elmer PS 16 instrument
(He I lamp; electrostatic sector field 127"). For higher energies an unavoidable loss of intensity due to measurement conditions has to be taken
into consideration. All the values given refer to vertical ionization potentials; the peak of maximum intensity is given in the case of bands having
a resolved vibrational fine structure. The benzene spectrum was recorded
at low resolution for the sake of better comparison.
[ l o ] K. Niedenzu, W. Sawodny, H. Watanabe, J. W. Dawson, T. Totani,
and W. Weber, Inorg. Chem. 6, 1453 (1967).
(111 B. L. Crawford and J. T Edsall, J. Chem. Phys. 7, 223 (1939).
[12] E. Heilbronner and H. Bock: Das HMO-Model1 und seine Anwendungen. VerIag Chemie, Weinheim/Bergstr. 1968, p. 137.
Angew. Chem. internat. Edit. / Vol. 10 (1971) / N o . 3
Alkaline Earth Metal Halide Hydrates: Use of IR
Spectra for Phase Analysis and Determination of
Possible Structural Types"'
By Heinz D. Lofz, Hans-Jurgen KIiippel, and Georg KhoPl
Investigations of the lattice vibrations of alkaline earth metal
hydroxides have ~ h o w n [ ' - ~that
I IR spectra are much more suitable for the identification and characterization of hydroxides
and hydroxide-hydrates than X-ray patterns, which usually contain many lines and, without measurements on single crystals,
are only difficultly interpretable. It can now be shown, using
alkaline earth metal halide hydrates as example, that lattice
vibration spectra also afford a means of classifying compounds
having the same stoichiometry into particular structural types
and thus considerably simplify the indexing of powder photographs.
Thus it follows from the spectra, and in particular from the number and position of the stretching (3000-3600 cm-') and bending (1600-1700 cm-') modes and the librations (300-800
crn-l) of water of crystallization, that the strontium halide hexahydrates SrC12 6 H 2 0 and SrBr2 . 6 H 2 0 , have, as noted from
X-ray crystallographic dataC41, the same structure and that the
monoclinic barium and strontium halide dihydrates
SrCI, . 2H20L4],BaClz 2H20L4] and BaBr, . 2H20[51, and
BaI, . 2H20[61belong to three different structural types (Fig.
1).
According to their IR spectra (Fig. 2) the monohydrates of the
alkaline earth metal halides, of which we have been able to prepare SrBr2 . H,O, SrI, - H,O, and BaI, . H,O for the first time,
are, with the exception of Srl, . H,O, all isotypic. The crystallographic data subsequently recorded by us for these compounds,
which, according to electron diffraction measurements on
BaCI, H20[71,crystallize in the space group Pnma, are given
in the Table.
183
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