Reactions of N, N-dichloroalkylamines with solid base as studied by FTIR combined with DFT calculations

Article abstract of DOI:10.1016/S0022-2860(00)00822-X

Products of vacuum gas-solid reactions of N, N-dichloroalkylamines with KOH have been identified by FTIR spectroscopy and DFT calculations. It has been found that the reactions consist of elimination of two Cl atoms accompanied with migration of an H atom, a ring carbon or a methyl group from the α-carbon to the N atom and unstable imines with a C=N double bond are formed.

Full text of DOI:10.1016/S0022-2860(00)00822-X

Journal of Molecular Structure 560 (2001) 337±344
www.elsevier.nl/locate/molstruc
Reactions of N, N-dichloroalkylamines with solid base as studied
by FTIR combined with DFT calculations
*
T. Egawa , M. Ito, S. Konaka
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
Received 2 June 2000; accepted 17 October 2000
Abstract
Products of vacuum gas±solid reactions of N, N-dichloroalkylamines with KOH have been identi®ed by FTIR spectroscopy
and DFT calculations. It has been found that the reactions consist of elimination of two Cl atoms accompanied with migration of
an H atom, a ring carbon or a methyl group from the a-carbon to the N atom and unstable imines with a CyN double bond are
formed. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: Vacuum gas±solid reaction; N, N-dichloroalkylamine; FTIR; Unstable imine; DFT calculations
1. Introduction
the thermal reaction. For the purpose of electron diffrac-
tionstudy,itseemsmoreconvenienttouseN, N-dichlor-
oaminesthanN-chloroaminesbecausetheformercanbe
prepared more easily with high purity [8]. However,
no study has been reported about the products of
VGSR of N, N-dichloroalkylamines with solid base
except for one [9]. In the present study, reaction
products of the following dichloroamines, (CH3)2CH±
NCl2, (CH2)3CH±NCl2, (C2H5)(CH3)CH±NCl2 and
(CH3)3C±NCl2, with KOH have been examined and
identi®ed by FTIR spectroscopy combined with
B3LYP/6-31Gpp DFT calculations, which are known
to reproduce well vibrational frequencies and IR
intensities [10].
We have determined molecular structures of some
unstable species having a CyN double bond by gas
electron diffraction (GED) [1,2]. In these studies,
unstable species were prepared by heating amines up
to about 5008C. On the other hand, syntheses of unstable
imines with a CyN bondbymeans ofvacuum gas±solid
reaction (VGSR) of N-chloroalkylamines have been
reported by Denis et al. [3±5]. These reactions consist
of chlorination of amines by using N-chlorosuccinimide
(NCS) followed by dehydrochlorination of the resulting
monochloroamines by solid base such as tert-BuOK.
They characterized the products by means of NMR
and IR spectroscopy. Bock and Dammel investigated
the products of this reaction system by photoelectron
spectroscopy [6,7]. This reaction seems suitable for
the structural determination of unstable imines by
GED because it requires much milder condition than
2. Experimental
The samples of N, N-dichloroalkylamines were
prepared by chlorination of isopropylamine, cyclobu-
tylamine, sec-butylamine and tert-butylamine
(Nacalai Tesque Inc.). Each amine was condensed
on an excess amount of NCS powder in a glass vessel
* Corresponding author. Fax: 181-11-706-4924.
E-mail address: egawa@sci.hokudai.ac.jp (T. Egawa).
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(00)00822-X

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T. Egawa et al. / Journal of Molecular Structure 560 (2001) 337±344
Fig. 1. Schematic drawing for the FTIR measurement of the reaction products of N, N-dichloroalkylamines and KOH.
in vacuum. The vessel was then kept at room tempera-
ture until the amine was completely dechlorinated for
the amines except cyclobutylamine, for which the
vessel was cooled by an ice bath to control the reac-
tion. Dichlorination was checked by the FTIR spectra
of the products. In the case of isopropylamine,
complete dichlorination was further con®rmed by
GED investigation [8].
The experimental setup is shown in Fig. 1. The N,
N-dichloroamine vapor was passed through a Pyrex-
glass column (15 mm (i.d.) £ 100 mm) containing
KOH powder. The pushing pressure was monitored
by a pirani gauge attached to the end of the column
and was controlled between 250±500 mTorr by using
a needle valve placed between the sample holder of
the dichloroamine and the column. Prior to the experi-
ment, the column was evacuated for hours while
heated up to 908C in order to remove the adsorbed
water from the KOH surface.
propanamine) remained unchanged by passing the
sample through the KOH column at room tempera-
ture. At the column temperature of 908C, the intensi-
ties of the vibrational bands of the parent molecules
diminished to about half the original intensities and
many new bands appeared (see Fig. 2). As a reaction
path, we considered that dehydrochlorination of
(CH3)2CH±NCl2 occurred to form (CH3)2CyNCl (N-
chloro-2-propanimine). Among the new bands,
however, there seemed to be no vibrational band
that can be assigned to the N±Cl stretching. There-
fore, another measurement was carried out by chan-
ging the starting compound to dibromoamine,
(CH3)2CH±NBr2 (N, N-dibromo-2-propanamine) that
was prepared by reaction of isopropylamine and N-
bromosuccinimide to check if the product contains a
halogen atom. As shown in Fig. 2, the wavenumbers
of many vibrational bands of the products of
(CH3)2CH±NCl2 and (CH3)2CH±NBr2 coincided indi-
cating that the product contains no halogen atom. In
comparison with the spectra reported in the literature
[11], it was found that these new bands were assigned
to 2-propanimine, (CH3)2CyNH, that is an unstable
species.
The KOH column was directly connected to an IR
cell with a path length of 10 m. FTIR spectra were
measured by using a BOMEM DA3.16 spectrometer
with a spectral range of 450±4000 cm21 and a resolu-
tion of 0.5 cm21. The cell was continuously evacuated
by a vacuum pump during the measurement.
This product is the same as that of the VGSR of
monochloroamine, (CH3)2CH±NHCl, with solid base
investigated by Guillemin and Denis [5]. They used
tert-BuOK as solid base. They also found that the
reaction of n-C3H7±NDCl with solid base resulted in
the formation of not C2H5±CHyND but C2H5±
CHyNH. Their results show that these reactions
3. Results and discussion
3.1. (CH3)2CH±NCl2
The spectrum of (CH3)2CH±NCl2 (N, N-dichloro-2-

T. Egawa et al. / Journal of Molecular Structure 560 (2001) 337±344
339
Fig. 2. (a) FTIR spectrum of N, N-dichloro-2-propanamine measured at room temperature. (b) The spectrum of reaction product of N, N-
dichloro-2-propanamine and KOH at the column temperature of 908C. (c) The spectrum of reaction product of N, N-dibromo-2-propanamine
and KOH at the column temperature of 908C. Arrows in (b) and (c) indicate common peaks for these spectra.
consist of the elimination of the H (D) and Cl atoms
from the N atom and the migration of the H from the
a-carbon (not from solid base) to the N atom. From
the analogy, it is suggested that the reaction revealed
in the present study involves the elimination of the
two Cl atoms and the migration of the H atom from
the a-carbon to the N atom. In order to check if the
reaction is just a thermal decomposition or not, a
control experiment was carried out, where the KOH
powder was replaced with glass wool to obtain a
similar gas ¯ow condition. In this case, nothing
happened even when the column containing the
glass wool was heated over 1008C. Therefore, this
reaction is not just a thermal decomposition but the
existence of the KOH plays an essential role. Hoping
to get some information about the reaction
mechanism, structural optimization of (CH3)2CH±N:
was carried out by using MP2(frozen core) ab initio
method [12] with a 6-31Gpp basis set. Program Gaus-
sian 94 [13] was used. The starting geometry of
(CH3)2CH±N: was formed by removing the two Cl
atoms from the optimized geometry of the C1
conformer of (CH3)2CH±NCl2 [8]. In the course of
the iteration steps of the geometry optimization, the

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T. Egawa et al. / Journal of Molecular Structure 560 (2001) 337±344
Fig. 3. FTIR spectrum of reaction product of N, N-dichloro-cyclobutanamine and KOH measured at the column temperature of 1008C. Almost
all the bands are assigned to 1-pyrroline Ref. [14]. The peaks with asterisks are those of byproducts.
H atom of the a-carbon moved to the N atom and
®nally the structure converged to the imine,
(CH3)2CyNH, with almost monotonous decrease of
the energy. This result indicates that the migration
of the H atom is quite rapid without passing any tran-
sition states and hence KOH does not seem to parti-
cipate in this step. Therefore it is likely that KOH is
involved in the ®rst step of the reaction, that is, the
elimination of the Cl atoms.
Table 1
Observed vibrational wavenumbers of 2-butanimine
Wavenumbers
Intensitya
Vibrational modesb
3263
2981
2971
2938
2894
2747
1652
1455
1385
1286
1091
w
n1 N±H stretch
vs
vs
vs
s
n23 CH3 anti-symmetric stretch
n2 CH3 anti-symmetric stretch
n24 CH3 anti-symmetric stretch
n4 CH3 symmetric stretch
n6 CH2 symmetric stretch
n7 CyN stretch
3.2. (CH2)3CH±NCl2
From the analogy of the above mentioned result, it
was expected that the VGSR of cyclic compound,
(CH2)3CH±NCl2 (N, N-dichloro-cyclobutanamine)
resulted in the formation of (CH2)3CyNH (cyclobuta-
nimine). However, most of the vibrational bands
observed at the column temperature of 1008C were
assigned to 1-pyrroline, whose IR spectrum had
been reported in Ref. [14]. Fig. 3 shows the obtained
spectrum. This result shows that the b-carbon, instead
of the H atom, moved to the N atom to reduce the
strain of the four-membered ring by forming a ®ve-
membered ring.
vw
vs
s
n9 CH3 anti-symmetric deform
n12 CH3 symmetric deform
n14 CH2 wag 1 CNH bend
n16 C±C stretch 1 CH3
rock 1 CNH bend
vs
m
m
1032
1008
967
m
vw
w
s
n30 CH3 rock 1 CyN torsion
n17 C±C stretch 1 CH3 rock
n18 CH3 rock 1 C±C stretch
n31 CyN torsion
878
746
w
w
n32 CH2 rock
n20 skeletal rock
557
a
Abbreviations used: s; strong, m; medium, w; weak, v; very.
Vibrational modes were numbered assuming Cs point group,
b
3.3. (C2H5) (CH3) CH±NCl2
where n1 to n22 belong to the A0 species and n23 to n36 to the A00
species.
In order to examine the priority of the migration,

T. Egawa et al. / Journal of Molecular Structure 560 (2001) 337±344
341
Fig. 4. (a) FTIR spectrum of reaction products of N, N-dichloro-2-butanamine and KOH measured at the column temperature of 1008C. The
band with an asterisk is assigned to the N±Cl stretching mode of the parent molecule. (b) Theoretical IR spectrum of 2-butanimine obtained
from B3LYP/6-31Gpp DFT calculations. (c) Theoretical spectrum of N-propylidene-methanamine. (d) Theoretical spectrum of N-ethylidene-
ethanamine.
the VGSR product of (C2H5)(CH3)CH±NCl2 (N, N-
dichloro-2-butanamine) was investigated. There are
three imines expected as a product of this reaction,
namely, (C2H5)(CH3)CyNH, (C2H5)CHyNCH3 or
(CH3)CHyNC2H5, depending on which group or atom
moves from the a-carbon to the N atom. Nothing had
been reported for the vibrational spectrum of any of
these unstable species. A strong band observed at
878 cm21 (see Fig. 4) is typical of the CyN torsion of
CyNH group and hence this band is indicating the
formation of (C2H5)(CH3)CyNH (2-butanimine). This
was con®rmed by comparing the observed spectrum
with the theoretical ones obtained from B3LYP/6-
31Gpp DFT calculations [15,16]. It has been shown
that not only the vibrational frequencies but also the
intensity pattern of IR spectra predicted by B3LYP
method are quite reliable when the 6-31Gp or higher
basis set is used [10]. As shown in Fig. 4, the theoretical
spectrum of 2-butanimine reproduced the observation
and hence, the main product can clearly be identi®ed
withthisspeciesformedbya migrationofthe H atom. In
addition, gas electron diffraction study of the product of
this reaction has been carried out and the observed data
have been successfully reproduced by assuming the

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T. Egawa et al. / Journal of Molecular Structure 560 (2001) 337±344
Fig. 5. (a) FTIR spectra of reaction products of N, N-dichloro-2-methyl-2-propanamine and KOH measured when 0, 60 and 360 min elapsed
after closing the valves of the cell. Arrows show decaying peaks. (b) Theoretical IR spectrum of N-(1-methylethylidene)-methanamine obtained
from B3LYP/6-31Gpp DFT calculations.
formation of 2-butanimine [17]. Comparison with the
theoretical spectra obtained from MP2(frozen core)/6-
31Gpp ab initio calculations was also carried out. The
results were essentially consistent with the DFT calcu-
lations showing that the product was 2-butanimine but
the reproduction of the observed spectra by the DFT
calculations was slightly better than that of the MP2
calculations. The present result shows that H atom
moves more easily than methyl or ethyl groups. The
observed vibrational bands were assigned by referring
to the DFT calculations as listed in Table 1.
3.4. (CH3)3C±NCl2
Finally, the VGSR product of (CH3)3C±NCl2 (N, N-
dichloro-2-methyl-2-propanamine) was examined in
order to investigate the possibility of a methyl migra-
tion. Theoretical spectrum of the candidate product,
(CH3)2CyNCH3 (N-(1-methylethylidene)-methana-
mine) was prepared from B3LYP/6-31Gpp DFT calcu-
lations. The observed spectrum looked signi®cantly
different from the theoretical prediction because of
the formation of byproducts. In order to pick up the

T. Egawa et al. / Journal of Molecular Structure 560 (2001) 337±344
343
Fig. 6. Reactions revealed in the present study.
vibrational bands of the unstable species, the time
Acknowledgements
evolution of the spectrum was observed after closing
the inlet and outlet valves (see Fig. 1) of the IR cell.
As shown in Fig. 5, it is found that some vibrational
bands decay simultaneously and the positions and
intensities of these bands are in good agreement
with those of the strong bands in the theoretical spec-
trum. In addition, the slow decay of these peaks is
consistent with the fact that the imines of RR0CyNR00
type are relatively more stable than those of
RR0CyNH type [7]. Therefore, it can be concluded
that the migration of the methyl group occurred
although some stable byproducts were also formed.
We thank the Computing Center of the Institute for
Molecular Science, Okazaki National Research Insti-
tutes, for the use of the NEC SX-3 computer and
Library program Gaussian 94.
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The VGSR of N, N-dichloroalkylamines with KOH
has been studied by FTIR combined with B3LYP
calculations. The reactions identi®ed in the present
study are listed in Fig. 6. These reactions consist of
the elimination of the two Cl atoms and the migration
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