Diapositive 1

1
FTIR Matrix Isolation Spectroscopy and DFT
Studies of Aliphatic Azide-Methyl Sulfide
Photochemistry
M. Algarra1 J. E. Rodriguez-Borges1, M. N. D. S.
Cordeiro2, J. Soto3 J. Mascetti4, M. Lamotte5,
F. Borget6, T. Chiavassa6, J.P. Aycard6 1CIQ and
2REQUIMTE, Department of Chemistry, University of
Porto, 4169-007 Porto, Portugal. 3 Department of
Physical Chemistry, Faculty of Sciences,
University of Malaga, E 29071 Malaga,
Spain. 4LPCM (UMR 5803 CNRS) and 5LPTC (UMR 5472
CNRS) Université Bordeaux 1, 33405 Talence Cedex,
France. 6PIIM (UMR 6633 CNRS), Université de
Provence, Centre St-Jérôme, case 252, 13397
Marseille Cedex 20, France.
Introduction Organic azides are useful reagents
in many fields their ability to form highly
reactive nitrene intermediates makes them
important synthetic reagents, with a number of
potential uses (propellants, semi-conductor
technology, ). Photolysis of methyl azide CH3N3
has been extensively studied and it is clear that
unstable methanimine CH2NH is obtained but the
question wether methylnitrene CH3N is the
intermediate in this reaction is not certain. We
present here photochemical studies of new
aliphatic azide methyl sulfides isolated in
cryogenic matrices.
Experimental details azide vapors were mixed
with Ar, N2, or Xe (1/900) and condensed onto a
Cu plate cooled at 20K. FTIR spectra were
recorded with a Nicolet MAGNA 750 at 10K (0.125
cm-1 resolution). Irradiation was carried out by
means of an Osram 200W high-pressure Hg lamp
(?gt230 nm). Computational details Calculations
of structures and vibrational frequencies of
azides and reaction products were performed by
DFT (B3LYP) and MP2 methods, using Gaussian03.
AEMS
APMS
AMMS
Infrared spectra of AMMS in solid Ar, Xe, and N2
The most intense bands of the IR spectra of
AMMS are na(NNN) and d(CH2). The N3 asymmetric
stretching region exhibit a complex structure,
depending on matrix media, but rather insensitive
to annealing. N3 being a polar moiety, its
stabilization is a function of the polarizability
of the host matrix (N2, Xe). Besides matrix sites
effects, essentially observed in Ar matrix, three
main groups of bands are observed. Actually,
three main conformers of AMMS are calculated in
the range 0.0-3.4 kcal/mol but only two of them
are supposed to be observed in matrices the most
stable one and the one located at 1.6 kcal/mol
above in energy (IR bands surface ratio 10).
As Fermi resonances between N3 asymmetric stretch
and combinations or overtones of the numerous low
frequency modes can be expected, we tentatively
assign these three bands (2133, 2120, 2094 cm-1)
to one or two conformers of AMMS.
? NNN 2158 cm-1 ?CH2 1222 cm-1
?E0
?E3.4 kcal/mol
?E1.6 kcal/mol
Photolysis experiments in solid Ar When AMMS is
irradiated with UV light at 10K, all bands
progressively disappear. After 30 min, new bands
appear in the region 1597-1584 and 1172-1165 cm-1
that we tentatively assign to a nitrene or
thiazirene. After one hour, these new bands
decrease and new ones appear in the ranges
3239-3165, 2948, 2002, 1448-1436, 1352-1330,
1069, 964-962, and 748-671 cm-1, that we
tentatively assign to a thiazetidine or a
thioxime. A different behaviour is observed for
both AEMS and APMS, for which decreasing of
parent bands is followed by the growing of new
bands located in the range 1700-900 cm-1, that do
not change after one to three hours of
irradiation. We tentatively assign them to the
formation of linear imines HNCH-(CH2)1,2-S-CH3.
h?
h?
Conclusions Due to the proximity of sulfur and
nitrogen orbitals, photoirradiation of AMMS leads
to the formation of cyclic species as
intermediate (1-methyl- thiazirene) or as final
product (thiazetidine). NMR studies are in
progress to allow identification of the final
product thiazetidine or thioxime. When there is
more than one CH2 moiety between N and S atoms
(AEMS and APMS), the cyclisation is no more
possible and we observe formation of linear
imines. Intermediate nitrenes have not been
observed for AEMS and APMS.
References M. Frankowski et al., Low Temp.
Phys. 2003, 29 (9-10), 870. J.M. Dyke et al., J.
Phys. Chem. A, 2004, 108, 5299. J.M. Dyke et al.
Chem. Eur. J., 2005, 11, 1665.