Study of the thermal decomposition of 2-Azidoacetic acid by photoelectron and matrix isolation infrared spectroscopy

Article abstract of DOI:10.1021/ja964354v

2-Azidoacetic acid (N₃CH₂CO₂H) has been synthesized and characterized by a variety of spectroscopic techniques, and the thermal decomposition of this molecule studied by matrix isolation infrared spectroscopy and real- time ultraviolet photoelectron spectroscopy. The results are consistent with the vapor phase thermal decomposition following a pathway involving concerted ejection of molecular N₂ and the simultaneous formation of CO₂ and methanimine (CH₂NH). No evidence was found for the presence of intermediates such as the nitrene NCH₂CO₂H or the imine HNCHCO₂H. At higher temperatures. CH₂NH further decomposes to give HCN and H₂.

Full text of DOI:10.1021/ja964354v

J. Am. Chem. Soc. 1997, 119, 6883-6887
6883
Study of the Thermal Decomposition of 2-Azidoacetic Acid by
Photoelectron and Matrix Isolation Infrared Spectroscopy
J. M. Dyke,† A. P. Groves,† A. Morris,† J. S. Ogden,*,† A. A. Dias,†,§
A. M. S. Oliveira,†,§ M. L. Costa,‡ M. T. Barros,‡ M. H. Cabral,‡ and
A. M. C. Moutinho‡
Contribution from the Department of Chemistry, The UniVersity of Southampton,
Southampton SO17 1BJ, U.K., and CeFITec, Departamento de Fisica de Faculdade de
Ciencas e Technologica, UniVersidade NoVa de Lisboa, 2825, Monte de Caparica,
Lisbon, Portugal
ReceiVed December 18, 1996X
Abstract: 2-Azidoacetic acid (N3CH2CO2H) has been synthesized and characterized by a variety of spectroscopic
techniques, and the thermal decomposition of this molecule studied by matrix isolation infrared spectroscopy and
real-time ultraviolet photoelectron spectroscopy. The results are consistent with the vapor phase thermal decomposition
following a pathway involving concerted ejection of molecular N2 and the simultaneous formation of CO2 and
methanimine (CH2NH). No evidence was found for the presence of intermediates such as the nitrene NCH2CO2H
or the imine HNCHCO2H. At higher temperatures, CH2NH further decomposes to give HCN and H2.
Introduction
In particular, the thermal decomposition of several alkyl
azides (e.g., R2HCN3) has been studied extensively by Bock
and Dammel4-8 using photoelectron spectroscopy. Their work
suggests that for these azides, N2 evolution is accompanied by
a 1,2 hydrogen atom shift to form the imine R2CdNH, which
in some systems can undergo further decomposition to produce
simple hydrocarbons H2 and HCN.
As part of a project designed to characterize and investigate
more complex azides, this paper describes the characterization
and thermal decomposition of azidoacetic acid. By analogy with
the results obtained for decomposition of alkyl azides,7 the
thermal decomposition of N3CH2CO2H might be expected to
yield the imine HNdCHCO2H. Although it is possible to
compute the essential features of its photoelectron (PE) spec-
trum, and thereby check for its formation, it is evident that a
second characterization technique would be highly desirable.
The possibility also exists that, in this more complex azide
system, a quite different decomposition pathway leading to new
products might be favored.
In addition to employing real-time gas phase ultraviolet
photoelectron spectroscopy (PES), our studies on azidoacetic
acid have also incorporated matrix isolation infrared (IR)
spectroscopy. This is a well-established technique for the
stabilization and detection of vapor phase intermediates and
offers not only the possibility of an alternative fingerprint
identification but also the potential for definitive characterization
of any new chemical species Via extensive C/H/N/O isotope
labeling.
This project is also supported by ab initio molecular orbital
calculations of different degrees of sophistication in order to
compute the electronic structures and valence ionization energies
of parent azides and their possible decomposition products.11
Compounds containing the azido group (-N3) have been
recognized for many years as being potentially explosive, and
experimental work on these compounds must therefore be
carried out with care. Nevertheless, the thermal decomposition
of azides is attractive as a synthetic method,1-2 and there is
considerable current interest in the mechanisms of such decom-
positions.
This interest has arisen3-9 because of the potential use of
these materials as high-energy storage sources in a number of
industrial applications and as precursors for the preparation of
single-crystal gallium and silicon nitride epitaxial layers on
semiconductor substrates at relatively low temperatures. The
dipolar character and relative instability of the azido group
enables it to react in a variety of ways depending on the
molecular structure, reagents, and experimental conditions.10
In general, the controlled thermal decomposition of azides
releases molecular nitrogen and, it is believed in the first
instance, a nitrene. This may subsequently undergo rearrange-
ment, hydrogen abstraction, cyclization, dimerization, or addition
to CsH or CdC bonds, and major research efforts have been
made in an attempt to understand how singlet and triplet nitrenes
interconvert and give rise to further products.5-9
* To whom correspondence should be addressed.
† The University of Southampton.
‡ Universidade Nova de Lisboa.
§ Praxis XXI Scholarship holders, Lisbon.
X Abstract published in AdVance ACS Abstracts, June 15, 1997.
(1) (a) Patai, S. The Chemistry of the Azido Group; Interscience: New
York, 1971. (b) Scriven, E. V. F. Azides and Nitrenes; Academic Press:
New York, 1984.
(2) Smith, P. A. S. Open-chain Nitrogen Compounds; Benjamin: Read-
ing, MA, 1961, Vol. 1.
(3) Costa, M. L. S. L.; Almoster Ferreira, M. A. J. Mol. Struct. 1988,
175, 417.
(4) Bock, H.; Dammel, R. Angew. Chem., Int. Ed. Engl. 1987, 26, 504.
(5) Dammel, R. Ph.D. Thesis, University of Frankfurt, Germany, 1985.
(6) Bock, H.; Dammel, R.; Aygen, S. J. Am. Chem. Soc. 1983, 105, 7681.
(7) Bock, H; Dammel, R. J. Am. Chem. Soc. 1988, 110, 5261.
(8) Bock, H.; Dammel, R.; Horner, L. Chem. Ber. 1981, 114, 220.
(9) Boyer, J. H.; Straw, D. J. Am. Chem. Soc. 1953, 75, 1642.
(10) (a) Ishihara, R.; Kanoh, H.; Sugiura, O.; Matsumara, M. Jpn. J.
Appl. Phys. 1992, 31, L74. (b) Bridges, A. S.; Greef, R.; Jonathan, N. B.
H.; Morris, A.; Parker, G. J. Surf. ReV. Lett. 1994, 1, 573. (c) Bu, Y.; Chu,
J. C. S.; Lin, M. C. Surf. Sci. Lett. 1992, 264, L15111.
Experimental Section
Sample Preparation. Azidoacetic acid was prepared from the
reaction between bromoacetic acid (BrCH2CO2H, Aldrich, 95%) and
sodium azide (NaN3, Aldrich 99%). In a typical synthesis, a 1:2 mixture
of BrCH2CO2H and saturated aqueous NaN3 was stirred continuously
in an ice bath for 24 h and subsequently acidified with aqueous HCl
(11) (a) Dyke, J. M.; Warschkow, O. Unpublished work. (b) Costa, M.
L. S. L; Dias, A. A. Unpublished work.
S0002-7863(96)04354-5 CCC: $14.00 © 1997 American Chemical Society

6884 J. Am. Chem. Soc., Vol. 119, No. 29, 1997
Dyke et al.
(1:1) to pH ) 5. The product, N3CH2CO2H, was then extracted with
diethyl ether and dried over anhydrous MgSO4. Final purification to
remove diethyl ether and the last traces of water required prolonged
pumping under vacuum (10-6 mbar) at room temperature. Sample
but above the photon beam. The tantalum insert was heated to ca.
1000 K by radiofrequency induction heating,13,14 and the stainless steel
furnace containing the azidoacetic acid was heated by radiation from
the ceramic insert. Although this system provided a high enough
temperature to obtain full pyrolysis of the parent azide, it was found
that sufficiently controlled pyrolysis could not be achieved.
1
purity was estimated to be >99% on the basis of H NMR and mass
spectrometric data, a summary of which is included in the Results.
Routine precautions were taken to minimize the effect of possible
explosions at all stages in the preparation and handling of azide mate-
rials, but no untoward occurrences were experienced during this work.
Matrix Isolation IR Spectroscopy. Matrix isolation studies on the
thermal decomposition of azidoacetic acid were based around a
conventional closed-cycle cooling unit (APCI “Displex”, model
CSW202) as described elsewhere.12 For these particular studies, the
parent azidoacetic acid vapor was admitted to the system via a PTFE
needle valve at a pressure monitored by a Pirani gauge. The vapor
then passed through a 15 cm length of 5 mm i.d. silica tubing connected
directly to the housing of the Displex. This provided a direct line-of-
sight to a central CsI deposition window, which was maintained at ca.
12 K. Vapors passing through this silica tube could be superheated to
ca. 1000 K, by a concentric cylindrical furnace, and subsequently co-
condensed on the central cooled window12 with a large excess
(>×1000) of matrix gas.
A new pyrolysis system using a resistively heated furnace was there-
fore developed to allow controlled superheating over longer times. In
this inlet system, molybdenum wire was wrapped around an inner glass
tube and connected via tungsten feedthroughs in an outer glass tube to
a 15 V dc power supply with a maximum operating current of 2 A.
Above the heating wire, the glass tube widened and sample vials were
placed on a ledge in this section which was not heated directly. It was
found that the maximum operating temperature reached by this system
was 850 K in the heated section (as measured by a chromel-alumel
thermocouple). The upper region of the furnace containing vials of
azidoacetic acid was heated by conduction, and the temperature reached
was found to be high enough to generate vapor pressures of azidoacetic
acid which gave photoelectron spectra with good signal-to-noise ratios.
Furthermore the operating resolution of the photoelectron spectrometer
(typically 30-35 meV as measured for argon (3p)-1 fwhm ionized with
He IR radiation) was found to be unaffected by this heating system
even at the highest temperature available (ca. 850 K).
Photoelectron spectra were recorded in real time as the furnace
temperature was varied in the temperature range 300-850 K. The onset
of decomposition was marked by the appearance of characteristic PES
bands associated with molecular N2 and CO2. Spectral calibration was
achieved at low ionization energy (9.0-11.0 eV) by CH3I and at higher
energies by reference to the known ionizations of (added) Ar or traces
of H2O present in the system.15,16
In a typical experiment, a sample of azidoacetic acid was pre-cooled
to ca. 281-283 K prior to admission to provide better flow control
and was subsequently co-condensed with nitrogen as the inert matrix
gas. This host was chosen in preference to argon, in order to provide
a more uniform environment for any trapped species, as it was expected
that molecular nitrogen would be generated during thermal decomposi-
tion.
The initial deposition was usually carried out with the superheater
switched off, in order to record the matrix IR spectrum of the parent
azidoacetic acid and to establish a suitable flow rate. Several spectra
were then recorded after periods of deposition with the superheater set
at increasingly higher temperatures, in the anticipation that at some
stage new features arising from decomposition products would be
isolated in the matrix. From time to time, the superheater temperature
was reduced to room temperature to check that the flow of parent
azidoacetic acid remained unchanged. Typical deposition times
between spectra varied from 30 to 90 min.
PE spectra were also obtained for BrCH2CO2H, used in the
preparation of HCO2CH2N3. The lowest energy ionization of this
molecule consists of an intense, closely spaced doublet at 10.61 and
10.95 eV. This feature is assigned to spin-orbit coupling, and the
separation (0.34 eV) is typical of a Br-localized ionization.17 However,
no evidence for BrCH2CO2H was found in photoelectron spectra
recorded for purified samples of azidoacetic acid.
Results
Matrix IR spectra were recorded over the frequency range 4000-
300 cm-1 using either a Perkin-Elmer 983G or a Bio-Rad FTS60
spectrometer equipped with routine spectral subtraction procedures, and
any spectral changes taking place as a result of changes in the inlet
temperature or sample flow rate could readily be detected.
Characterization of N3CH2CO2H. Samples of purified
azidoacetic acid were characterized in the vapor phase by mass
spectrometry and UV photoelectron spectroscopy and in the
1
condensed phase by H NMR, Raman, and IR spectroscopy.
In addition to monitoring changes in IR band intensity during
deposition, it was also possible to investigate the effect of controlled
diffusion on the trapped species. During these studies, the temperature
of the matrix was cycled within the temperature range ca. 12-35 K.
Separate matrix isolation studies were also carried out to confirm
the positions of the IR bands of H2O, CO, CO2, HCN, and HCOOH in
N2 matrices and to establish, for the purpose of elimination, the matrix
IR spectra of BrCH2CO2H and the Et2O solvent used in the preparation
of the azidoacetic acid.
The 70 eV electron impact mass spectrum showed a prominent
parent ion peak at 101 amu (55%), together with two intense
fragments at 28 (97% N2+) and 45 (100%, CO2H+) amu. The
other signals observed were all significantly less intense but
included a very weak doublet at 138/140 amu (ca. 1%) arising
from unreacted BrCH2CO2H.
Only two signals of any significance were observed in the
1
300 MHz H NMR spectrum of azidoacetic acid in CDCl3
solution. These were observed at 4.0 and 10.3 ppm relative to
TMS, and their relative positions and integrated intensities (ca.
1.7:1) are consistent with their assignment as methylene and
hydroxyl protons, respectively.
The most intense absorption in the (liquid) IR spectrum was
found at 2117 cm-1, and this was assigned to a vibration of the
N3 group. Other prominent bands were present at 1700-1800
(br, C-O stretch), 1421, 1285, and 1250-1150 (br) cm-1. The
liquid Raman spectrum exhibited counterparts to all the above
bands (2116, 1730 (br), 1422, 1285, and 1225 cm-1) and in
addition showed intense features of 2921 (C-H stretch), 947,
880, 236, and 171 cm-1. The O-H stretching region showed
Photoelectron Spectroscopy. All photoelectron spectra recorded
in this work were obtained using He(I) (21.22 eV) radiation. The
spectrometer used was a single-detector instrument designed for high-
temperature work13,14 with a modified inlet system capable of super-
heating a vapor species immediately prior to photoionization. The vapor
pressure of the parent material, N3CH2CO2H, was insufficient at room
temperature to allow photoelectron spectra to be recorded with
acceptable signal-to-noise. Samples were therefore preheated to ca.
373-423 K in order to generate a greater partial pressure to allow
spectra with greater signal-to-noise ratio to be recorded.
In initial thermal decomposition experiments, a sample of liquid N3-
CH2CO2H was placed in a small stainless steel furnace inside the
spectrometer. A short length of cylindrical ceramic tube with a thin
tantalum inner liner was placed approximately 2 cm beneath the furnace
(15) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. Molecular
Photoelectron Spectroscopy; Wiley Interscience: New York, 1971.
(16) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamaszaki, T.; Iwata, S.
Handbook of HeI Photoelectron Spectra; Japan Scientific Press: Tokyo,
1981.
(17) Eland, J. H. D. Photoelectron Spectroscopy; Butterworths: London,
1974.
(12) See, e.g.: Ogden, J. S.; Wyatt, R. S. J. Chem. Soc., Dalton Trans.
1987, 859.
(13) Morris, A.; Dyke, J. M.; Josland, G. D.; Hastings, M. P.; Francis,
P. D. High Temp. Sci. 1986, 22, 95.
(14) Bulgin, D.; Dyke, J. M.; Goodfellow, F.; Jonathan, N. B. H.; Lee,
E.; Morris, A. J. Electron Spectrosc. Relat. Phenom. 1977, 12, 67.

Thermal Decomposition of 2-Azidoacetic Acid
J. Am. Chem. Soc., Vol. 119, No. 29, 1997 6885
Figure 1. Nitrogen matrix IR spectra obtained during pyrolysis studies
on 2-azidoacetic acid. (a) Spectrum of parent material: bands A-E
listed in Table 1. (b) Spectrum obtained after pyrolysis at ca. 600 K.
(c) Spectrum obtained after pyrolysis at ca. 900 K: new bands F-Q
identified in Table 1.
Figure 2. Vapor phase PE spectra obtained during pyrolysis studies
on 2-azidoacetic acid. (a) Spectrum of parent material: bands I-IX
listed in Table 3. (b) Spectrum obtained after pyrolysis at ca. 680 K.
(c) Spectrum obtained after pyrolysis at ca. 750 K: new bands 1-12
identified in Table 4.
Table 1. Principal IR Bands (cm-1) Observed in Matrix Isolation
Studies on the Pyrolysis of Azidoacetic Acida
N2 matrix
previous
matrix studies
(band letter)
assignment
3534 (A)
2118 (B)
1782 (C)
1279 (D)
1138/1131 (E)
3033 (F)
2920 (G)
1637 (H)
1450 (I)
O-H stretching region. In the neat liquid, only broad absorp-
tions are observed, indicative of extensive hydrogen bonding,
but in the matrix-isolated sample, a prominent feature is found
at 3534 cm-1. Under matrix isolation conditions, there is little
possibility of intermolecular hydrogen bonding, and this absorp-
tion is assigned as the O-H stretch in monomeric N3CH2CO2H.
The HeI PES of azidoacetic acid is shown in Figure 2a, and
our assignment of this spectrum was made with reference to
the results of ab initio molecular orbital calculations. An
estimate of the equilibrium geometry was obtained both at the
MP2/6-311G** Hartree-Fock level and at the VWN/TZPP DFT
level using the Gaussian 94 code18 and the Amsterdam Density
Functional code,19,20 respectively. The parameters listed in
Table 2 from the MP2/6-311G** calculations correspond to a
stationary point on the potential energy surface with all real
vibrational frequencies. Other conformers were found to be
higher in energy. The three highest occupied molecular orbitals
in the ground state of this molecule (orbitals 26, 25, and 24)
are essentially nonbonding, whereas orbital 23 is essentially a
π(C-O) bonding orbital. Deeper orbitals are less easily
N3CH2COOH
}
}
3032
2920
1639
1452
1354
1129
1065
3287
748/738
2347
662
CH2NH (ref 24)
1353 (J)
1128 (K)
1065 (L)
3287 (M)
747/737 (N)
2347 (P)
662 (Q)
HCN (refs 23 and 24)
HCN
CO2 (ref 25)
CO2
a Wavenumber accuracy (1 cm-1
.
few distinctive features. In the IR, two very broad absorptions
centered at ca. 3500 and 3050 cm-1 were present but the Raman
showed no discernable features in this region apart from a very
broad, ill-defined band.
Figure 1a shows a typical nitrogen matrix IR spectrum
obtained from a sample of N3CH2CO2H deposited from the
vapor phase without superheating. The absorptions at 3726,
3633, and 1597 cm-1 are due to matrix-isolated H2O arising
from traces of water in the system, but additional intense features
(18) Gaussian 94, Revision C3. Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.;
Keith, T.; Petersen, G. A.; Montgomery, J. A.; Ragavachari, K.; Al-Laham,
M. A.; Zakrzowski, V. G.; Ortiz, J. V.; Foreman, J. B.; Ciolowski, J.;
Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Repogle, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart,
J. P.; Head-Gordon, M.; Pople, J. A. Gaussian Inc., Pittsburgh, PA, 1995.
(19) ADF User’s Guide Version 1.02, Department of Theoretical
Chemistry, Free University, Amsterdam, 1993.
are also present at 3534, 2118, 1782, 1279, and 1138 cm-1
.
The agreement between the positions of these absorptions and
the values listed above for liquid phase azidoacetic acid is very
satisfactory. The results are summarized in Table 1, and it is
found that the only significant difference is to be found in the
(20) Vosko, S. H.; Wilk, L; Nusair, M. Can. J. Phys 1980, 58, 1200

6886 J. Am. Chem. Soc., Vol. 119, No. 29, 1997
Dyke et al.
Table 2. Computed Structural Parameters of Azidoacetic Acid at
the Hartree-Fock MP2/6-311G** Level
Table 4. Assignment of Bands Observed in the He I Photoelectron
Spectrum Recorded for Fully Pyrolyzed Azidoacetic Acida
vertical ionization
band no.
energy (eV)
assignment
1
2
3
4
5
6
7
8
9
10.62
11.92
12.49
13.60
13.79
14.77
15.58
16.94
17.56
18.08
18.76
19.40
methanimine first band
CO2 first band recorded with HeIâ
methanimine second band
HCN first band (HeIâ, N2)
CO2 first band
methanimine third band
N2 first band
N2 second band
CO2 second band
CO2 third band
N2 third band
bond lengths (Å)
bond angles (deg)
(O1sC1dO2)
(C1sO1sH1)
(O1sC1sC2)
(C1sC2sH2)
(C1sC2sH3)
(C1sC2sN1)
(C2sN1dN2)
(N1dN2dN3)
dihedral (N2dN1sC2sC1)
dihedral (N1sC2sC1sO1)
d(C1dO2)
d(C1sO1)
d(O1sH1)
d(C1sC2)
d(C2sH2)
d(C2sH3)
d(C2sN1)
d(N1dN2)
d(N2dN3)
1.20
124.5
105.7
110.0
109.3
107.7
112.7
115.2
171.5
63.9
1.35
0.97
1.52
1.09
1.10
1.46
1.24
1.15
10
11
12
CO2 fourth band
a The experimental error on the vertical ionization energies is (0.02
eV.
164.3
spectrum of HCN23,24 lie at ca. 3287, 747, and 737 cm-1, and
these correspond within experimental error to three of the new
bands found here (see Table 1). The intense band at 2347 cm-1
is due to CO2. This molecule has been observed many times
during the course of matrix IR studies, and its assignment is
Table 3. Comparison of Experimental and Computed Vertical
Ionization Energies (IEs) of Azidoacetic Acid
experimental IE
band no. (eV)
VWN/TZPP
(eV)
KT × 0.9
KT VIE
(eV)a
MO
(eV)a
26
25
24
23
22
21
20
19
18
I
II
III
IV
V
(9.95)
(11.17)
(11.79)
(12.38)
(14.31)
(14.65)
10.01
10.70
11.85
12.95
13.58
13.71
14.58
15.17
9.63
10.91
11.81
12.31
14.30
14.76
15.83
16.39
17.70
10.47
12.12
13.12
13.68
15.89
16.40
17.59
18.21
19.67
confirmed25 by the accompanying 13C feature at 2283 cm-1
the bending mode at 662 cm-1, and combination bands at 3607
and 3712 cm-1
,
.
All other major features that are present after pyrolysis are
assigned to CH2NH. Table 1 compares the frequencies observed
in these experiments with the nitrogen matrix data previously
obtained by Jacox and Milligan.24 The agreement is very
satisfactory and leaves no room for ambiguity. Four minor
absorptions remain unassigned, and these are identified by
asterisks in Figure 1c. They lie at 3878, 2969, 2887, and 970
cm-1 and are not unique to this azidoacetic acid system. In
particular, they are routinely present in blank experiments, where
no sample is passing through the superheater, and are attributed
to small traces of organic material desorbed from the walls of
the superheater over the prolonged period of deposition.
Photoelectron Studies. Thermal Decomposition and Spec-
tral Assignment. The He I photoelectron spectrum obtained
for azidoacetic acid is shown in Figure 2a, and the PE spectrum
obtained upon partial pyrolysis is shown in Figure 2b. The
temperature of the superheater under these conditions was
approximately 680 K as estimated from chromel-alumel
thermocouple measurements. Figure 2c shows a He I photo-
electron spectrum obtained on complete pyrolysis of azidoacetic
acid, recorded at a furnace temperature of approximately 750
K. Tabulated vertical ionization energies for the photoelectron
bands observed in Figure 2c are shown in Table 4. Calibration
of these spectra was achieved using the known ionization
energies of nitrogen, water, and carbon dioxide.
Signals arising from nitrogen, hydrogen cyanide, and carbon
dioxide are clearly observed in the experimental photoelectron
spectra recorded for the pyrolysis products (Figure 2a,b), being
readily identified by comparison with published spectra.15,16 But
in addition, pyrolysis produces three further features: a broad
band with a vertical ionization energy of 10.62 ( 0.02 eV, a
vibrationally resolved band at 12.49 ( 0.02 eV showing
structure with a mean separation 1280 ( 30 cm-1, and a weak
band at approximately 14.80 eV. The relative intensity of these
three bands remained constant when experimental conditions
were varied, and comparison with literature PE spectra of
methanimine5,8,26 shows that these bands correspond to the three
VI
VII (15.16)
VIII (15.96)
IX
(17.15)
a Obtained from a single-point calculation at the HF/6-311G** level.
described in such simple terms because they are more delocal-
ized. Table 3 compares our experimental vertical ionization
energies (VIEs) with calculations resulting from the application
of Koopmans’ theorem (KT) to the SCF/6-311G** molecular
orbital energies. Values computed at the HF/6-311G** level
were, as expected, higher than the experimental values because
of the use of the Koopmans’ approximation, but scaling by a
factor of 0.921,22 afforded much better agreement with the
experimental values. The VWN/TZPP/DFT VIE values, ob-
tained by the transition-state method also show good agreement
with experiment.
Matrix Isolation Studies. Thermal Decomposition and
Spectral Assignment. Figure 1, parts b and c, shows matrix
IR spectra obtained after passing azidoacetic acid vapor through
the superheater maintained at ca. 600 and 900 K, respectively.
At the lower temperature, the most intense absorptions of the
parent acid are still present, but several new features may be
observed, notably at 2347, 1353, 1065, and 662 cm-1, and the
positions of the new absorptions appearing on superheating are
summarized in Table 1.
With the superheater temperature at ca. 900 K, the parent
absorptions have essentially disappeared (Figure 1c), indicating
that almost complete thermal decomposition of the azide has
taken place.
The assignment of some of these new IR bands is relatively
straightforward. Firstly, on the basis of the earlier work by Bock
et al.,4-8 one might anticipate that at the highest superheater
temperatures HCN would be present as a final decomposition
product. The most intense bands in the nitrogen matrix
(21) (a) Robin, M. B.; Brundle, C. R.; Kuebler, N. A.; Ellison, G. B.;
Wiberg, K. B. J. Chem. Phys. 1972, 57, 1758. (b) Basch, H.; Robin, M. B.;
Kuebler, N. A.; Baker, C.; Turner, D. W. J. Chem. Phys. 1969, 51, 52. (c)
Wiberg, K. B.; Ellison, G. B.; Wendolski, J. J.; Brundle, C. R.; Kuebler,
N. A. J. Am. Chem. Soc. 1976, 98, 7179.
(22) Rabalais, J. W. Principles of UltraViolet Photoelectron Spectroscopy;
John Wiley and Sons: New York, 1977.
(23) King, C. M.; Nixon, E. R. J. Chem. Phys. 1968, 48, 1685.
(24) Jacox, M. E.; Milligan, D. E. J. Mol. Spectrosc. 1975, 56, 333.
(25) Fredin, L.; Nelander, B.; Ribbegard, G. J. Mol. Spectrosc. 1974,
53, 410.

Thermal Decomposition of 2-Azidoacetic Acid
J. Am. Chem. Soc., Vol. 119, No. 29, 1997 6887
lowest energy ionizations of this species. The band centered
at 10.62 eV is the first band of methanimine and is reported5 to
show a vibrational progression with a mean separation of 500
cm-1. This corresponds to excitation of the CNH bending mode.
This structure was not resolved in this work. However, the
second band of methanimine has a vertical ionization energy
of 12.49 eV,5,8,26 and for this feature, the expected structure
was resolved. This band has a vibrational spacing of ca. 1280
cm-1 corresponding to the excitation of the CdN stretching
mode in the cation. This compares with a vibrational frequency
of 1638 cm-1 for the CdN stretching mode in the neutral,27
indicating that ionization has occurred from a bonding CdN
orbital. The third band of methanimine has a vertical ionization
energy of 14.77 eV.5 This band was found previously to exhibit
a complex vibrational pattern, corresponding to excitation of
the CsH and CdN stretching modes in the ion. The experi-
mental spectra in this work are not sufficiently resolved to
identify all the components of this band; however, most
components are seen and the overall band profile is in excellent
agreement with that in the literature.5,8,26 Thus by comparison
of our experimental photoelectron spectra with those recorded
by Peel and Willett26 and Dammel and Bock5,8 for CH2NH,
methanimine is positively identified in the photoelectron
spectrum recorded for the pyrolysis products of azidoacetic acid.
Spectral features indicating the possible formation of an
intermediate nitrene or imine were not observed, and it is
perhaps significant that, in these PES studies, the features
assigned to N2 and CO2 routinely appeared together.
N2, and CH2NH and the subsequent formation of HCN only at
the highest temperatures. The observation that N2 and CO2
appeared to be produced simultaneously in the PES studies
effectively precludes stepwise reaction schemes involving the
nitrene or imine, as might be envisaged in the possible reactions
3-8 below:
N3CH2CO2H f NCH2CO2H + N2
(3)
nitrene
parent azide
NCH CO H HNdCHCO H
f
(4)
(5)
2
2
2
nitrene
imine
nitrene or imine f HNdCH2 + CO2
f HCN + HCOOH
f HCN + CO2 + H2
(6)
(7)
f CO + H2O + HCN
(8)
In the above equations, only those molecules which were
observed are underlined.
Conclusions
Azidoacetic acid has been synthesized and characterized by
a range of techniques, including IR and UV PE spectroscopy.
In particular, the photoelectron spectrum of azidoacetic acid
recorded with He I radiation consists of nine bands in the 9.0-
21.0 eV ionization energy region. Ab initio calculations have
been performed for azidoacetic acid, and application of Koop-
mans’ theorem to the computed orbital energies yields vertical
ionization energies which are in satisfactory agreement with
experimental values. The characters of the four uppermost
occupied molecular orbitals of azidoacetic acid are essentially
N3 nonbonding, N3 nonbonding, and O 2p (lone pair), O 2p
(lone pair), and π(CO) in nature as obtained from examination
of the molecular orbitals calculated at the HF/6-311G** level
of theory.
Mechanism of Gas-Phase Thermal Decomposition of
Azidoacetic Acid. On the basis of the experimental evidence
provided by the PES and matrix IR experiments, we propose
that the decomposition of 2-azidoacetic acid occurs via a
concerted process involving the formation of a five-membered
ring transition state which decomposes as shown below to form
molecular nitrogen, carbon dioxide, and methanimine.
Thermal decomposition studies of azidoacetic acid reveal
nitrogen, carbon dioxide, and methanimine as the only observ-
able initial decomposition products, and a mechanism has been
proposed which is consistent with these findings. In the flow
system used in these experiments, significant decomposition was
observed above 650 K which was essentially complete by 750
K. At temperatures above 900 K, there was evidence of
methanimine decomposition, resulting in the formation of
hydrogen cyanide and (probably) molecular hydrogen.
Previous studies have shown that, when heated alone under
conditions similar to those used in this study, methanimine
begins to decompose above 1200 K.5 This difference in
decomposition temperature is almost certainly due to CH2NH
being formed here with a high internal energy as a result of the
initial pyrolysis of the azidoacetic acid.
Further heating then results in the decomposition of metha-
nimine to form hydrogen cyanide: the other product of this
reaction step being almost certainly molecular H2. The detection
of this product by matrix isolation IR is precluded by selection
rules, and hydrogen was not seen in the PES work, probably
due to its low photoionization cross section and/or overlap with
other bands. Indeed it has been reported5 that the presence of
molecular H2 in PES studies on alkyl azide pyrolyses could only
be observed after spectral subtraction. However, its formation
in the matrix studies could be inferred from the otherwise
inexplicable rise in pressure in the vacuum system at the highest
pyrolysis temperatures employed.
Acknowledgment. The authors thank the EU for supporting
this research with a grant under the EC Science-Twinning
scheme and acknowledge the contribution made by Mr. O.
Warschkow for assistance with the electronic structure calcu-
lations.
Our results therefore indicate that the initial decomposition
of azidoacetic acid can be written as
N3CH2CO2H f HNdCH2 + CO2 + N2
(1)
JA964354V
and that at higher temperatures (>900 K)
HNdCH2 f HCN + H2
(26) Peel, J. B.; Willett, G. D. J. Chem. Soc., Faraday Trans. 2 1975,
71, 1799.
(27) (a) Hamada, Y.; Hashigushi, K.; Tsuboi, M.; Koga, Y.; Kondo, S.
J. Mol. Spectrosc. 1984, 105, 70. (b) Duxbury, G.; LeLerre, M. L. J. Mol.
Spectrosc. 1982, 92, 326. (c) Duxbury, G.; Kato, H.;. LeLerre, M. L. J.
Chem. Soc., Faraday Discuss. 1981, 71, 97. (d) Allegrini, M.; Johns, J. W.
C.; McKellar, A. R. W. J. Chem. Phys. 1979, 70, 2829.
(2)
The scheme summarized in the above equations not only
accounts for all the species observed in the pyrolysis but also
is consistent with the apparently simultaneous generation of CO2,