Journal of Molecular Structure, 49 (1978) 221-237
<9E1sevierScientific Publishing Company, Amsterdam
-
Printed in The Netherlands
MICROWAVE SPECTRUM, CONFORMATIONAL EQUILmRIUM,
INTRAMOLECULAR HYDROGEN BONDING, INVERSION
TUNNELLING, DIPOLE MOMENTS AND CENTRIFUGAL DISTORTION
OF ETHYLENEDIAMINE
K.-M. MARSTOKK and HARALD M(])LLENDAL
Department of Chemistry, The University of Oslo, Blindern, Oslo 3 (Norway)
(Received 14 February 1978)
ABSTRACT
,
The microwave spectrum of ethylenediamine, CHzNHzCHzNHz, has been investigated
in the 12.4-39.5 GHz spectral region. Th~ two N-C-C-N
gauche conformations
denoted I and Il and shown in Fig. l, were assigned. The existence of large fractions of
further conformations is ruled out. Both rotamers I and Il possess an intramolecular
hydrogen
bond. I is favoured
by 0.3 :!:0.2 kcal mol-'
relative
to Il. The
N-C-C-N
angles are 63 :!:20 in both conformers. The average CCN angles are 109 :!:10 in I and
111. 5 :!:10 in Il.
The spectra of both rotamers display splittings caused by double minimum potentials.
In conformation I the a- and c-dipole moment components were of the "inverting" type,
whiIe Il b is "non-inverting". The separation between the (+ )- and the (- )-energylevelsof
the double minimum potential of I is 86.356 :!:0.021 MHz. In conformer Il the a-axis
component of the dipole moment "inverts", while Ilb is "non-inverting". No c-type lines
were observed for this conformation. The energy difference between the (+ )- and the
(-)-states of the double minimum potential of conformation Il is 1.332 :!:0.018 MHz.
The first excited state of the C-C torsional motion was assigned for this conformation
and the energy difference between the (+)- and (-)-states determined as 1.564 :!:0.066 MHz.
The dipole moments were Ila = 1.059:!: 0.007 D, Ilb = 0.787 :!: 0.032 D, Ile =
= 1.179 :!: 0.023 D and Iltot = 1.770 :!: 0.033 D for conformation I; Ila = 1.952 :!: 0.002 D,
Ilb = 0.867 :!: 0.006 D, Ile = 0.538 :!: 0.006 D and Iltot = 2.203 :!: 0.006 D for ri, respectively.
All quartic and two sextie centrifugal distortion constants were determined for I, whiIe
the quartic distortion coefficients were found for Il.
INTRODUCTION
Ethylenediamine is used extensively in both organic and inorganic (coordination) chemistry. The structural properties of the molecule have been
studied by several methods. An electron diffraction study by Yokozeki and
Kuchitsu [1] revealed that the free molecule prefers the gauche conformation
with the N-C-C-Nangle 64:!: 4° from syn. The fraction of any other
rotamer, if present, was estimated to be less than 5% [1]~ However, the
gauche form is not preferred in the crystalline state. In this phase the
molecule takes the N-C-C-N anti conformation [2]. Moreover, the IR
222
spectra of several isotopes have been studied in the gaseous, liquid and
crystalline states as well as in solution [3-5]. Ah initio MO cålculations.
have been made for the molecule by Pople and coworkers [6] who considered several rotameric forms. They found that the two conformations
shown in Fig. 1 were the energetically favoured ones. The conformation
which is designated Il, corresponding to their gGg' [6], was calculated to be
0.26 kcal mol-1 more stable than I (tGg' of Pople et al.). Further rotamers
were calculated to be more than 1 kcal mol-1 less stable than Il.
Both conformations I and Il are stabilized by intramolecular hydrogen
bonds with the two amino groups acting respectively as proton donor and
acceptor within each conformation. In Fig. 1 the amino groups depicted to
the right are acceptors and those to the left are donors. However, the roles
of the amino groups of each conformation may be interchanged so that the
groups to the right become donors and those to the left proton acceptors.
This transformation may be made by appropriate rotation about both C-N
bonds, or perhaps by a combined mot lon involving inversion of one or both
amino groups and rotation about one or both C-N bonds. The conformatiom
reached by such a transformation are completely identical to those of Fig. 1.
The existence of two identical forms of each of the conformations I and Il
is expected to lead to separate double minimum potentials for the appropriatE
transformation motion in each of the two conformations. As will be discussed
in later sections of this paper, the tunnelling frequencies.of the transformatiOl
motion potential of each conformation has been identified. The situation
encountered for ethylenediamine closely resembles that presumed to exist
for ethylene glycol [7]. In the latter molecule, a tunnelling frequency of
about 17.1 GHz and extensive coupling between overall rotation and
tun'nelling was seen [7]. Fortunately, the situation in ethylenediamine is
found to be more easily tractable.
H
1/
/
/00000000000,
<-\
;1-<
</j'--\
H
/
I
H
<--
>
Ho... '.
o o..
\
,0-<
H/-\
H
H
Il
Fig. 1. The most stable conformations
of ethylenediamine.
I is more stable than Il by
0.3 :t 0.2 kcal mol-t. Both rotamers are stabilized by intramolecular
hydrogen bonding.
Interchange
of the hydrogen bonding roles of the two amino groups of each conformation
leads to identical forms with an associated double minimum potential for each conformer.
223
EXPERIMENTAL
Commercial ethylenediamine was purified by ,gas chromatography before
use. Studies were made in the 12.4-39.5GHz spectral region with the cell
cooled to about -30°C in nearly all experiments. Lower temperatures could
'
not be utilized because of insufficient vapour pressure. Vapour pressures of
10-50 microns were employed when the spectra were recotded. A conventional spectrometer described briefly in ref. 8 was ,used.
RESULTS
Microwave spectrum and assignment of conformation
I
Preliminary rotational constants were computed for rotamers I and Il by
combining structural parameters taken Infiinly from the electron diffraction
study [1]. The rotational constants of the two conformations were quite
similar. Bond moment calculations [9] were then performed. The results
were Ila = 0.7 D, Ilb = 0.2 D and Ile = 1.7 D for conformer I: Ila = 1.8 D,
Ilb = 0.1 D and Ile = 0.2 D for Il. The agreementbetween bond moment
calculations and experiment is often reasonably good, and conformation I
was therefore expected to possess a strong c-type spectrum while Il was
predieted to exhibit strong a-type transitions.
,
The microwave spectrum of ethylenediamine was found to be dense and
complicated with absorptions occurring every few megahertz throughout
the entire microwave spectral range. Stark effeet studies of strong transitions
indicated that a majority of them were Q-branch lines. The faet that many
of these were located below 22 GHz was in agreement with predictions for a
c-type spectrum. The search was therefore concentrated on finding the low
J members of the strong ,J1,J-l ~ J2.J-l c-type Q-branch series. The first line
of this series to be identified by its Stark effect was 31.2~ 32,2 at 26612.54 MHz,
while another transition with alm ost identical intensity and very similar
Stark effect was observed at 26441.20 MHz. The splitting of 171.34 MHz
between these two corresponds approximately to two times the tunnelling
frequency which we will denote ~. Another pair of lines with characteristic
Stark effects and identical intensities was located at 24866.12 and
25036.66 MHz and assigned as the 41.3 ~ 42.3 pair. The splitting in this
case was 170.54 MHz which is nearly, although not exactly, that observed
for the previous pair. The mean frequencies of each of these two pairs were
used to obtain a preliminary determination for the average value of"A - C
and Ray's asymmetry parameter". With this information, several more c-type
Q-branch lines were predicted and subsequently measured. All of them were
found to be split. The splittings were about 170 MHz for low value of J and
gradually became smaller as J increased.
b-Type
Q-branch
lines were also well predicted
by the average A
-
C and
" determined from the c-type Q-branch lines. The splittings of the b-type
224
lines were much less than in the case of the c-type transitions. Some of the
former were not resolvably split while others were split by a fairly small
amount, usually less than 3 MHz. The intensities of the individual components of the resolved b-type lines were identical.
In order to make a first determination of all three rotational constants,
low J R-branch b-type lines were searched for as these were expected to be
relatively free from splitting complications. After some searching several of
these were identified by their Stark effect and used to predict c-type R-branc
lines which were readily found. All of these were split by roughly 170 MHz
into two components of identical intensities.
The a-type R-branch lines were next predicted. These transitions were
found to be split by roughly 172 MHz as were the c-type lines. The intensities of the two individual lines of each pair were identical in this case too.
Table 1lists a portion of the spectrum * .
Our interpretation of the spectrum of conformation I of ethylenediamine
is that it is typical of a molecule poss'essing a double minimum potential. ThE
ground state is a symmetric or (+ )-state, while the first vibrational state is an
antisymmetric or (-)-state. The separation, ~, between the (+ )-state and
the (-)-state is about 86 MHz. The selection rules are that of a rigid rotor
plus (+)""* (-) or (-)""* (+) for the a- and c-type transitions, while the b-tYPE
lines obey the ordinary rigid rotor selection rules. The origin of the differenc
of the selection rules are thought to arise from the internal motion of the
amino groups described in the Introduction. An operation which interchangE
the hydrogen bonding roles of the amino groups is presumed to invert the
spatial direction of the a- and c-axes components of the dipole moment. No
inversion takes place for the b-axis component of the dipole moment. Hence
the ordinary rigid rotor selection rules apply in this case.
In order to derive the spectroscopiC; constants characterizing conformatior
I, a computer program written by Nielsen was utilized [10]. A total of
about 180 transitions were assigned for this conformation and 164 of these
were used to derive the spectroscopic constants appearing in Table 2. The
assigned frequencies consisted of all b-type as well as of all c-type Q-branch
transitions of sufficient intensities. The maximum J c-type Q-branch
transitions thus assigned were the 589.49""*5810.49pair. Only low J R-branch
lines of all three dipole moment component types were identified. Many
intermediate and high J b- and c-type R-branch lines are predicted to occur
in the spectrum, but it was not possible to assign them because of their smal]
to moderate intensities making uniqu~ identification in the rich spectrum
virtually impossible. A moderate num ber of high J a-type Q-branch lines
were likewise not assigned for similar reasons.
In the least squares procedure used to derive the parameters of Table 2,
the rotational, the quartic centrifugal, as well as the two sextic constants HJ';"
*The complete microwave spectra of the two conformations are available from the autho
upon request, or Jrom the Microwave Data Center, Molecular Spectroscopy Section, Nati
Bureau of Standards, Washington D.C. 20234, USA, where they have been deposited.
225
TABLE 1
Selected transitions from the microwave spectrum of conformation
Transition
a-type
10.1-. 20,2
11,0 -.21.1
21,2-. 31.3
20,. -. 30.3
3... -. 4.,3
33.1-. 43,.
b-type
00,0 -. 11,1
20.2-. 31,3
60,6 -.61,5
90,9 -. 9.,8
101.9
10.,8
121,11-. 12.,10
14,.,2 -.143,11
c-type
00,0 -. 1.,0
lo,. -.2.,.
5.,4 -. 60,6
31.. -. 3.,.
71.6-.7.,6
51,s -. 5.,3
9..7 -. 93,7
16.,,4 -. 163,14
Observed
frequencya
(MHz)
Obs.-calc.
frequency
(MHz)
(-)-. (+)
(+)-.(-)
f-)(+)
(+)-. (-)
(-)-. (+)
(+)-. (-)
(-)-.(+)
(+)-.(-)
(-)-.(+)
(+)-.(-)
(-)-. (+)
(+)-. (-)
19130.53
19302.91
20006.70
20179.15
27551.03
27722.15
28608.77
28780.42
38412.35
38582.44
38552.14
38724.53
0.01
0.11
-0.18
0.09
-0.02
0.12
0.03
(+)-.
(-) -.
(+)
(-) -.
(+)-.
(-) -.
(+)-.
(-) -.
(+)-.
(-) -.
(+)-.
(-)-.
(+)-.
(-) -.
18877.63
b
36108.56
b
20831.98
20833.16
36516.16
36519.53
28760.85
28762.40
36614.16
36617.40
37595.87
0.15
-0.04
-0.11
0.15
-0.05
0.04
0.00
-0.05
b
19614.92
19787.35
30073.35
30245.57
38876.46
39044.96
26441.20
26612.54
18261.11
18427.77
37006.44
37175.21
39002.71
39167.12
13349.26
13494.56
0.02
-0.06
0.06
0.00
0.14
-0.04
-0.19
-0.11
0.18
0.01
-0.04
0.21
0.08
-0.10
0.04
-0.05
(+)
(-)
(+)
(-)
(+)
(-)
(+)
(-)
(+)
(-)
(+)
(-)
(+)
(-)
(-)-.(+)
(+)-.(-)
(-)-. (+)
(+)
(-)
(-)-. (+)
(+)-.(-)
(-)-.(+)
(+)-. (-)
(-)-.(+)
(+)-. (-)
(-)-.,(+)
(+)->(-)
(-)(+)
(+) -., (-)
(-)-.'(+)
(+)-.(-)
-o.O
0.13
-0.11
-0.03
0.07
I of CH2NH2CH2NH2
Splitting
(MHz)
172.38
172.45
171.12
171.65
170.09
172.39
1.18
3.37
1.55
3.24
172.43
172.22
168.50
171.34
166.66
168.77
164.41
145.30
Centr. dist.
Total
(MHz)
-0.14
--0.14
-0.18
-0.13
-0.17
-0.15
-0.44
-0.43
-0.87
-0.13
-2.01
2.45
-3.69
-3.75
-17.36
-17.64
-10.76
-10.85
-34.33
-34.57
-0.08
-0.05
-0.22
-0.19
-0.93
-0.83
-0.10
0.32
6.53
6.70
-1.96
-1.68
15.25
16.44
72.90
74.36
Sexti
(MR:
r 0.01
0.01
0.0'
0.0'
0.0'
0.0'
0.0'
0.0
0.0'
0.0
0.0
-0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0'
0.0'
0.0'
0.0
0.0
0.1'
0.0
226
TABLE 1 (continued)
Transition
,\
19'.16"'" 194.16
223.19"'" 224.19
84.2
..... 28..24
33..28 -> 336.28
396.33 .....397.33
477.40"'" 478.40
528.44
.....529.44
,
589.49"'"
5810.49
(-)->(+)
(+).....(-)
(-)-> (+)
(+)..... (-)
(-)->(+)
(+)->'(-)
(-).....(+)
(+).....(-)
(-)..... (+)
(+)..... (-)
(-).....(+)
(+)->(-)
(-)4(+)
(+)->;(-)
(-)..... (+)
(+).....(-)
Observed
frequencya
Obs.-calc.
frequency
(MHz)
(MHz)
28945.58
29086.54
15279.93
15406.64
16539.18
16644.68
22252.00
22342.45
22988.60
23058.25
13930.45
13965.48
18595.13
18621.01
18994.47
19005.91
a:!:0.10 MHz. bNot used in least squares
TABLE
procedure
Splitting
(MHz)
Total
S
(MHz)
-0.04
0.11
-0.02
0.04
-0.02
0.01
0.09
-0.02'
-0.09
-0.10
-0.05
-0.04
0.03
0.16
-0.02
187.41
190.05
211.26
216.03
467.40
479.10
949.36
970.01
1631.62
1669.52
1964.81
2043.89
3366.32
3474.69
4767.81
4925.56
140.96
126.7l
105.50
90.45
69.65
35.03
25.88
11.44
-0.12
because
Centr. dist.
of unresolved
4
]
splitting.
2
Spectroscopic constants for the (+)- and the (-)-states
(+ )-state
A (MHz)
B (MHz)
C (MHz)
~r (kHz)
~JK (kHz)
~K (kHz)
oJ (kHz)
o K (kHz)
HJK (Hz)
HK (Hz)
~ (MHz)a
rms (MHz)
I of CH2NH2CH2N
(- )-state
1.4472.133 :!:0.012
5229.2072
:!:0.0050
4405.5612:!:
0.0051
4.85:!: 0.11
-22.52:!:
0.14
80.1 :!:1.6
1.3966 :!:0.0030
11.42 :!:0.21
0.317 :!:0.017
-i1.9:!:
6.2
a~ signifies theseparation
one standard deviation.
of conformation
86.356
0.105
14471.900:!:
0.012
5229.2043
:!:0.0050
4405.4596
:!:0.0051
4.80 :!:0.11
-20.13
:!:0.13
48.2 :!: 1.6
1.3903 :!:0.0030
10.54 :!:0.22
0.383 :!:0.019
-44.0 :!:6.8
:!:0.021
between the (+ )- and the (- )-states. Uncertainties represent
and H K of both the (+ )- and the (- )-states were taken together with the
separation between the (+)- and the (,- )-states, ~, as the variables. N o
coupling terms between inversion and overall rotation were included. This
model yielded a good fit to the observed frequencies as shown in Table 1, as
the root mean square deviation of 0.105 MHz is comparable to the experimental uncertainty of 0.10 MHz.
227
Inspection of Table 2 reveals that there is a small but significant difference
between the A and C rotational constants of the (+)- and (-)-states of 0.233
and 0.1016 MHz, respeetively. No difference was found for the B rotational
constants.
Centrifugal distortion is quite large in this molecule, especially for the high
J transitions as indicated in Table 1. The maximum value for centrifugal
distortion was found for the 559.46 -+ 5510.46 (-) -+ (+) transition which was
distorted by + 6762.64 MHz. The centrifugal distortion constants of Table 2
are quite similar in the cases of !::.b!::.JK,Db DK and HJK' Large differences
are seen for DKand HK, It is of course expected that all centrifugal distortion
constants of the two states should be very similar as are the rotational
constants. The reason for the discrepancies found for !::.Kand HK is perhaps
the faet that the model used to fit the data does not take into account
inversion-rotation
coupling effects.
The 14N nudeus is known to possess a small quadrupole moment, but no
splitting of the lines was observed as a result of this effect, although some
low J transitions appeared to be quite broad.
Dipole moment'
The unresolved b-type lines 61.5 -+ 62.4 and 91.8 -+ 92.8 as.well as the
21.1 -+ 31.2 (+) -,t-,(-) a-type and 10.1 -+ 21. I (+) -+ (-) c-type transitions were
used to determine the dipole moment of conformation I of ethylenediamine.
These lines ~ere selected mainly because the complicatiops arising from the
tunnelling frequency, !::.,in the perturbationexpression
of Golden and
Wilson [11] were estimated to be negligible. The experimept w.as performed
as described be fore [12], and the inverse squares of the standard deviations
of the Stark coefficients were used as weights i'n the least squares fit: The
results are givenirfTable 3. The total dipole moment is 1.770:t 0.033 D, in
good agreement with 1.8 D calculated by the bond moment method [9].
Strictly speaking, the a- and c-axes components of the dipole moment
are transition moments rather than permanent dipole moments. However,
no great difference between the two is expeeted in this case since the
tunnelling frequency
is so low (86.356:t 0.021 MHz).
.
.
Assignment of conformation Il
Mter the assignment of the spectrum of conformation I was made,"searches
for the a-type R-branch J = 2 -+ 3 and J = 3 -+ 4 of Il were initiated. These
transitions were predicted to be the strongest ones for this rotamer. These
lines were soon found and their assignments confirmed by their Star k effects,
rigid rotor fit and a characteristic ~splitting of about 2.6 MHz. Although this
splitting is much less than the roughly 172 MHz observed for low J values in
the case of conformation I, its origin is believed to be another independent
double minimum potential analogous to that of rotamer I.
228
TABLE 3
Stark coefficients
and dipole moment
of conformation
Transition
/lv/E2 (MHz V-2 cm2) X lO.
Obs.
Calc.
(+)-+(+)
(-) -+ (-)
(+)-+(+)
(-) -+ (-)
(+)-+ (-)
(+)-+(-)
(+)-+ (-)
6.,5 -+ 62,4
9.,8 -+ 92,7
1o,. -+ 2.,.
2,.. -+ 3..2
Ila
= 1.059
Iltot
I of CH2NH2CH2NH2 a
:t 0.007 D
Ilb
IMI= 6
10.9:t 0.2
11.12
IMI = 9
10.7:t 0.1
10.84
M =O
IMI= 1
IMI= 2
-2.16:t 0.02
22.8 :t 0.2
-9.25 :t 0.09
-2.156
22.32
-9.317
= 0.787
:t
0.032 D
Ile
= 1.179
:t
0.023 D
= 1.770 :t 0.033 D
aUnct:rtainties
represent
one standard
deviation.
The mean frequencies of the a-type R-branch lines were used to predict
b- and c-type Q-branch transitions. Those of the b-type were easily assigned,
while no c-type lines were found. This is in keeping with the small c-axis
dipole moment component of 0.538 :!:0.006 D producing insufficient
intensities for making unique assignments for these weak transitions in
this dense spectrum. None of the b-type lines could be resolved into the
(+ )-state and (-)-state components, presurnably because the rotational
constants are very similar for these two states. This is expected since the
inversion frequency is as small as -1.3 MHz. This conformation thus
possesses an "inverting" a-axis dipole moment component and a "noninverting" Jl.b'Since no c-type lines were definitely identified, the question
as to whether Jl.ebelongs to the "inverting" or "non-inverting" type cannot
be settled. However, model considerations indicate that the c-axis component
of the dipole
moment
is "inverting"
.
A total of 52 transitions were assigned for conformation Il. These inc1ude
alllow J a- and most low'J b-type R-branch lines as well as all the b-type
Q-branch transitions. High J R-branch transitions of b-type and high Ja-type
Q-branch lines were not identified for similar reasons as for the corresponding
lines of conformation L A portion of the spectrum is display ed in Table 4.
In the least squares fitting procedure, the rotational constants of the (+)and the (-)-states were first taken as independent variables with a set of
quartic centrifugal distortion constants assumed to be identical for the two
states. The energy separation, ~, between the two states was also fitted
simultaneously. The rotational constants obtained in this manner were
identical to within their uncertainties for the two states. In the final calculations, the rotational constants of both the (+)- and the (-)-state were
assumed to be identical. The same was assumed for the centrifugal constants.
The results of this procedure are shown in Table 5.
229
TABLE 4
Selected
transitions
from the microwave
Transition
a-type
1.,. --+2",
1." --+2."
1". --+2."
2", --+3,,3
2." --+3.,3
2". --+3",
3", --+4",
3", --+4.,3
b-typeb
2." --+3.,3
3,,3
--+ 4.,4
4.,4 --+4.,3
9.,. --+9.,8
3", --+3",
6.,5 --+6,,4
9.,8 --+9,,7
12.,11 --+12"..
3, 3 --+3, ,
6, :.--+ 6,: 5
10,,8 --+103,7
12".. --+123,.
14,,12 --+143,11
spectrum
Observed
frequencya
(MHz)
(-)--+ (+)
(+)--+(-)
(-)--+(+)
(+)--+(-)
(-)--+(+)
(+)--+ (-)
(-)--+(+)
(+)--+ (-)
(-) --+(+)
(+)--+(-)
(-)--+(+)
(+)--+(-)
(-) --+(+)
(+)--+(-)
(-)--+(+)
(+)--+(-)
of conformation
Obs.-calc.
frequency
(MHz)
Il of CH,NH,CH,NH,
Centrifugal
distortion
(MHz)
18192.72
18195.53
18915.86
18918.33
19730,56
19733.57
27261.98
27264.44
28259.88
28262.45
28626.54
28629.25
38339.70
38342.20
39364.45
39367.14
-0.16
-0.02
0.19
0.00
-0.18
0.16
0.11
. -0.10
-0.01
-0.11
-0.11
-0.06
0.03
-0.14
0.09
0.11
0.04
0.04
-0.11
-0.11
-0.12
-0.12
-0.08
-0.08
-0.35
-0.35
-0.01
-0.01
-0.56
-0.56
-1.17
-1.17
35770.75
29965.11
14012.37
34524.85
26796.28
24202.67
25694.32
34617.14
31178.99
37194.93
39504.37
36894.96
36928.20
0.09
0.02
0.04
0.10
-0.05
-0.08
-0.08
-0.04
0.14
-0.11
-0.05
-0.05
0.04
0.08
-1.17
-0.60
-16.95
0.15
1.76
-3.12
-30.60
-0.37
-1.14
14.52
14.72
-1.50
a:l:0.10 MHz. bAll b-type lines include both (+ ) --+ (+) and (-) --+(-) transitions.
Comparison of the rotational and quartic centrifugal distortion constants
of Tables 2 and 5 shows that they are quite similar for both conformations.
This is to be expected since the two rotamers possess fairly similar geometries
and presumably also have similar force fields.
As in the case of conformation I, no splittings due to the two nitrogen
quadrupole nuclei were observed for Il.
230
TABLE 5
Average spectroscopic constants of the (+ )- and (- )-states of conformation
Il of
CH2NH2 CH2NH2 a
Ground
A (MHz)
B (MHz)
C (MHz)
,
tJ.J(kHz)
tJ.JK (kHz)
tJ.K (kHz)
6J (kHz)
6K (kHz)
tJ. (MHz)C
rms (MHz)
aUncertainties
vibrationalatate
torsionalatate"
ånd a (-}1Itate
the separation
stateb
First exciteda C-C
torsional state
14355.536
:!:0.020
5125.3014:!:
0.0090
4356.2924
:!:0.0085
3.85 :!:0.32
-19.73:!:
0.41
51.0 :!:5.1
1.3271 :!:0.0095
9.14 :!:0.45
1.332 :!:0.018
0.091
14451.195
:!:0.093
5090.458 :!:0.036
4338.028 :!:0.035
8.0 :!:1.2
-37.6:!:
2.0
152:!: 25
1.185 :!:0.043
57.1:!: 2.2
1.564 :!:0.066
0.270
represent one standard deviation. bThe "ground state" is the lowest
composed of the (+ )- and the (- )-states. In the "first excited C-C
the heavy atom torsion ia excited. This state too is composed of a (+ )of the transformation motion described in the Introduction. cA signifies
between the (+ )- and the (- ~.tates.
First excited C-C torsional state
One vibrationally excited state of confonnation n was assigned. A total of
. 34 tranaitionS'were measured and uSed to determine the spectroscopic
constants shown in Table 5. These transitions were low J a-type R- as well as
b-type a-branch lines as in the case of the "ground state" of the previous
section..The a-type lines of this excited state were split by about 3.1 MHz
which is only slightly more than that observed for the ground state. No
splitting was seen for the b-type transitions. Since the changes of the
rotational constants upon going from the ground to this state are very dose
to those predicted for opening up thi! NCCN dihedral angle, we assign this
motion as the lowest C-C torsional mode of conformation n. Relative
intensity measurements.made largelyas recommended by Esbitt and, Wilson
[13] yielded 181 :t 20 cm-1 for this fundamental frequency. This is dose
to 186 cm-1 measured in solution by Raman spectroscopy [3].
The faet that the ground state (+)- and (-)-state energy level separation of
1.332:t 0.018 MHz is dose to 1.564;!: 0.066 MHz found for the corresponding separation of t~ fitst excited state of the C-C torsion presumably
means that the transformation motion which interchanges the hydrogen
bonding roles of the amino groups is almost independent of the C-C torsion.
This supports the model discussed in the Introduction where it was assumed
that th~ interchange of the amino groups depends primarily on the C-N
torsional motions and/or inversion of the amino groups.
231
TABLE 6
Stark coefficients and dipole moment of conformation
Transition
21.2'"
31.3b
61.6 ... 62.5
7...'"
72.5
Il of CH2NH2CH2NH28
ÅvlE2 (MHz V-2 cm2) X lO.
Obs.
Calc.
M =0
IMi= 1
IMI=2
IMI=6
IMI= 7
ILa = 1.952 :!:0.002 D
ILtot = 2.203 :!:0.006 D
-2.01 :!: 0.03
9.52 :!: 0.09
43.8 :!: 0.4
-31.9 :!: 0.5
13.3 :!: 0.1
-2.012
-9.489
44.001
31.917
13.281
ILb = 0.867 :!:0.006 D
8Uncertainties
represent. one standard
had identical Stark effects.
deviation.
ILe = 0.538 :!:0.006 D
b ( )... (-)
and (-)...
(+) components
Dipole moment
,
...
,
,
The 21.2-+ 31.3(+) -+ (-)as well as the (-) '-+(+) transition, the 61.5-+ 62.5'
and the 71,.6-+ 72.5transitions' were used ,to determine the dipole moment.
The (+)- and (~)-leveLenergy difference is a very minor complication in this
case and was neglected. The statistical treatment of the data was made in
the same manner as for conformation I with the results shown in Table 6.
The total dipole moment of 2.203 :t 0.006 D is in fair agreement with 1.8 D
calculated by the bond moment method [9J.
..
.
The dipole moment has previously been measured. as 1.96 D in the gas
phase by dielectric measurements [14], 1.92 D has been reported by Tronei
[15] in benzene solution, while Kimura et al. [16] found 1.89 D at 25°C
and 1.84 D1h benzene at 45°C, respeetively.
It will be shown in a later seetion of this paper that the gas phase is.
composed of roughly 2/3 of conformation l and 1/3 of conformation Il.
The average dipole moment of the gas phase is thus 2/3 X 1.71 D + 1/3
X 2.20 D = 1.91 D which is very close to Zahn's old gas phase value of 1.96 D
[14].
, The rotational constants shown in Tables 2 and 5 are quite similar for the
ground states of both conformations, and a discrimination between the two
cannot be made directly from only the rotational constants. To do so, the
components of the dipole moments are very useful. The faet that conformation Il was found to have a predominating /J.a= 1.952 :t 0.002 D and a
quite small /J.e= 0.538 :t 0.006 D, while Ile is the largest component of
rotamer l, was taken as proof for the assignment as this is in quite good
agreement .with the bond moment calculations.
,
232
The remaining unassigned transitions of the spectrum
A total of about 270 lines were assigned for the two conformations. This
includes all the strongest lines of the spectrum as well as a majority of transitions of medium intensity. However, there remain approximately 200 unassigned lines of intermediate intensities and several thousand weak ones. The
Stark effects of some few of the intermediate intensity lines have been well
resolved; for several others partial resolutions have been achieved. A large
fraction of these lines are undoubtedly Q-branch transitions. It is believed
that most of these lines belong to unassigned vibrationally excited states.
Especially the first excited state of the C-C torsion of conformation I
should appear with considerable intensity since this frequency is expected
to be roughly the same as for rotamer Il, namely 180 cm-l. The tunnelling
frequency, .::l,of this mode is presumed to be similar to that of the ground
state as the C-C torsional motion is believed to be almost completely
independent of the transformation potential. Despite considerable efforts,
no assignments could be made.
The two C-N torsional modes of each conformation, however, are
believed to be strongly dependent on the transforrnational barrier. According
to the vibrational analyses [3-5] the lowest C-N torsional fundamental is
238 cm -1, and its first excited state should thus have a fairly intense microwavl
satellite spectrum. The energy separation between the (+)- and the (-)states of this mode is presumed to be much larger than in the ground
vibrational states of each conformation. This WQuid certainly add to the
assignment complications, and no assignments could be made for this mode
for either
rotamer
.
Isotopic species where the amino group has been deuterated are expected
to exhibit less complications from tunnelling and a study of such species will
now be made in this laboratory.
The existence of several other conformers as well as the two assigned in
this work is of course theoretically possible for ethylenediamine. While the
existence of small fractions of further rotamers cannot be completely
precluded by its microwave spectrum, the absolute intensities of the
assigned transitions yield strong evidence that the two identified gauche
rotamers indeed predominate. This is supported by Stark effect studies on
the strongest of the unassigned lines. Scheraga and coworkers [17] have
very recently calculated by the empirical EPEN-method that the gGg
conformation in the notation of the ab initio work [6] is even more stable
than both I and Il by about 0.4 kcal mol-l. The gGg rotamer was calculated
to have a small dipole moment of only 0.3 D, and a direct assignment of
this conformation could have been difficult since the intensities of the
spectrallines depend on the squares of the dipole moment components
along the principal inertial axes. If the results of the EPEN-computations
were correct, about 50% of the gas should consist of the gGg conformation.
This would have led to much lower absolute intensities of the transitions of
233
I and Il than were actually observed. Thus, the microwave spectrum refutes
these calculations. Moreover, the good agreement between the average dipole
moment determined for the gas phase composed of 2/3 of I and 1/3 of Il,
1.91 D, and Zahn's gas phase average dipole moment of 1.96 D [14] is independent evidence against a dominating gGg conformation. Chemically,
it seems improbable that the gGg rotamer should be preferred as it has
no intramolecular hydrogen bond as well as a rather close non-bonded
hydrogen-hydrogen contact between the two amino groups. It is also
interesting to note that this conformation was calculated to be 2.02 kcal mol-1
less stable than Il by Pople et al. [6]. It is thus concluded that free ethylenediamine consists mainly, if not exclusively, of the two N-C-C-N
gauche
conformations shown in Fig. 1.
Barrier to transformation
Very little information is available about the path of the transformation
motion, and no reliable quantitative barrier height can therefore be calculated.
However, the rather small tunnelling frequencies of the two conformations
on the one side, and the light amino groups on the other, lead us into
estimating a rather moderate barrier height in the 4-6 kcal mol-1 range for
each of the conformations. The barrier of rotamer Il is expected to be
somewhat higher than that of I, since ~ is much less in the former case.
Structure of the two conformations
As no isotopic species were studied for the two rotamers, only three
rotational constants are available for each of them. Therefore, a complete
structure cannot be determined. Instead, we restricted ourselves to fitting
two structural parameters for each conformer. The NCCN dihedral angle
and the CCN angle were selected because the rotational constants depend
strongly on these parameters and because they are chemically interesting.
It is of course realized that, strictly speaking, a small difference presumably
exists between the CCN angies when the amino group acts as a proton donor
and when it is an acceptor. The assumed structural parameters were taken
mainly from the electron diffraction study [1]. The amino groups were
assumed to be placed in exactly staggered positions. The NCCN and CCN
angles were then fitted by minimizing the sum of the per cent differences
between the observed and calculated rotational constants. Good agreement
was thereby obtained as shown in Table 7. The error limits given in this
Table are believed to encompass reasonable structural differences between
the assumed structural parameters and the real ones.
The NCCNdihedral angleswhich are 63 :t 20 for both rotamers agree well
with that of the electron diffraction study [1] where the existence of only
one gauche conformation was assumed. The CCN angle was found to be
110.2:t 0.70 by Yokozeki and Kuchitsu [1] which is intermediate between
109:t 10 found for conformation I and 111.5 :t 10 found for Il. Since an
234
TABLE 7
Plausible structural parameters& and observed and calculated rotational constants 'of the
two rotaIIlers of CH,NH,CH,NH,
Assumed structural parameters common for conformations I and Il
C-N
1.469 A
LCCH 109.48°
LHNCC 6000r 1800b
C-C
1.546 A
LCNH 109.48°
C-H
1.093 A
LHCH 109.48°
N-H
1.017 A
LHNH 109.48°
Fitted structural parameters
Conformation I
LNCCN
63 :!:2° from syn
LCCN
109.0:!: 1°
Conformation
Il
63 :!: 2° from syn
111.5:!: 1°
Hydrogen bond parameters (A)
N' . . N
N-H"'N
Rotational
Obs. c
14472.017
5229.206
4405.511
2.893
2.52
2.987
2.64
constants (MHz)
Calc.
14449.45
5223.59
4402.32
aSee text and Fig.!.
rotational constants
Obs.
14355.536
5125.301
4356.292
Diff.
0.16%
0.10%
0.07%
b Amino groups assumed
of (+ )- and (- )-states.
average value was determined byeleetron
agreement between the two methods.
to be in exactly
diffraetion,
Calc.
14362.77
5091.65
4373.18
staggered
positions.
Diff.
0.05%
0.65%
0.39%
cAverag
there is thus good
Energy difference between the two conformations
Relative intensity measurements were made to determine Gibbs' free
energy differenee between the two conformations. Most preeautions of
Esbitt and Wilson [13] were observed. The peak intensities were taken by
slowly seanning over the lines. The earefully seleeted transitions used were
strong and, hopefully, not seriously perturbed by overlapping lines or Stark
eomponents. The pair of lines seleeted for relative intensity measurements
were not widely spaeed in the speetrum thereby minimizing effeets from
wave guide refleetions and deteetor non-linearities. The experimental
material is shown in Table 8. The equilibrium constant K was ealeulated
from [18]
2
L
J
AnBnCn ' AnJlgneXp(-ET. n/k T)
K = [I] =~ vn
(1)
[Il]
Qn
(VI) ( AIBICI )
AIJlgIeXp (-E~.I/kT)
where the symbols have the same meanings as in ref. 18.
Sinee only b-type Q-braneh transitions were employed for conformation
one half of the observed intensities were used for Qn as these unresolved
transitions are eomposed of both the (+) -+ (+) and (-) -+ (-) eomponents.
I]
235
TAB LE 8
Intensity
measurementsa,b,
the ground
vibrational
Conformation
I
Il
I
Il
I
Il
I
Il
I
Il
I
Il
K
Av
-
-
ill [Il]
constants
of conformations
Temp.
(K)
Transition
3,,3 -->4.,.
12',10 -->123,9
5"s -->62,3
6,,6 -->62,s
203,17 -->20.,17
7,,6 -->72,6
132,11 -->133,11
8,,7 -->82,6
3,,3 -->4".
122,10 -->123,9
3,,3 -->4".
142,,2 -->143,11
203,17 -->20.,17
7.,6 -->72,s
203,,7 -->20.,17
707-->716
3,:3 -->4,:,
122,10 -->123,9
203,17 -->20.,17
7.,6 -->72,s
203,17 -->20.,17
70,6 -->7,,6
132,11 -->133,11
81,7--> 82,6
I
Il
I
Il
I
Il
I
Il
I
Il
I
Il
equilibrium
states
(+)-->(-)
243
(+) -->(-)
249
(-)-->(+)
249
(-)-->(+)
(+) -->(-)
249
<+) -->(-)
249
(-)-->(+)
273
(-)-->(+)
273
(+)-->(-)
296
(-.,.-)-->(+)
289
(-)-->(+)
290
(-)-->(+)
290
+o
-. 1 76 -.
38 "'
1
GÅv
"lo..
3.75
9.34
1.39
2.61
11.9
6.88
7.69
6.86
3.76
.9.34
3.76
11.6
11.9
6.88
11.9
4.11
3.76
9.34
11.9
5.88
11.9
4.11
7.69
6.86
261_
=-0.28
aSee text. blntensities
of lines belonging
in this Table. See text.
and
I and
Gibbs'
free energy
differences
of
Il of CH2NH2CH2NH2
0/ (arbitrary
unit)
exp (-E/kT)
4.4
3.5
4.7
2.66
9.6
2.66
7.8
2.6
2.7
3.4
2.7
3.4
6.8
2.1
6.8
1.26
1.5
1.86
7.0
1.95
7.0
1.9
4.6
2.7
0.992
0.854
0.972
0.962
0.668
0.947
0.834
0.940
0.988
0.867
0.988
0.816
0.683
0.961
0.683
0.966
0.989
0.878
0.697
0.954
0.697
0.958
0.865
0.942
K
Go
kcal moC'
2.2
-0.38
2.5
-0.45
1.8
-0.29
2.2
-0.39
1.4
-0.17
1.6
-0.23
1.4
-0.18
1.6
-0.26
1.4
-0.20
1.8
-0.34
1.3
-0.16
1.9
-0.37
:!:0.10 kcal moC'
to con format ion Il were twice the values shown
Measurements were made at about -25, oDe and room temperature: It was
hoped that !:lJIOand !:lSocould be determined from eqn. (2)
>
!:lGo = !:lHo -T!:lSo
(2)
If the data of Table 8 are fitted to this equation !:lHo and !:lSo are obtained
with large standard deviations and correlations. !:lSo is found to be insignificantly different from zero from this fit. This is expected since the two
gauche conformations are so similar, The standard deviation obtained for
!:lHo from this fit, is felt to be bit worse than is actually the case. Since
!:lSOis probably very dose to zero, !:lJIOis expected to be very dose to the
average value of !:lGo = ~O,28:t 0.10 kcal morI. Thus, !:lHo= ~O.3:t 0.2
kcal morI is preferred for the energy difference with conformation I as
the energetically favoured rotamer. The gas phase is then roughly composed
of 2/3 of conformation I and 1/3 of Il.
a
236
It is interesting to note that conformer n was calculated by Pople et al. [6]
as being 0.26 kcal mori more stable than 1. Although this in opposition to
the experimental findings, the fact that rotamers I and n were calculated to
be very dose in energy is substantiated by this work.
DlSCUSSION
lntramolecular hydrogen bonding is undoubtedly an important factor in
the preference for conformations l and nsinee conditions for forming this
type of bond are optimal in these two rotamers. The hydrogen bonds are
probably of rather moderate strength as can be conduded from the nonbonded distances shown in Table 7. The N. . . N distanee is 2.89 Å in conformation l, and is roughly 0.1 Å longer in Il. These values are very dose to
3.0 Å wttich is twice the sum of the van der Waals' radius of the nitrogen
atom [19]. The N-H" . N distances of 2.52 and 2.64 Å for l and n,
respectively, are more uncertainthan
the corresponding N... N distanees,
since the experimental data are relatively insensitive to the exact orientation
of the amino groups. However, these two non-bonded distances are about
0.2 and 0.1 Å, respectively, shorter than the sum of the van der Waals' radii
of a hydrogen and a nitrogen atom [19].
It is also interesting to note that the non-bonded distances of the hydrogen
bond are more favourable in conformation l than in n. It is thus possible
that the hydrogen bond in l is the stronger one, and that this may largely
explain why this conformation is energetically favoured.
Aremarkable finding of the electron diffraction work [1] was that even
at 118°C at least 95% of ethylenediamine prefers the hydrogen-bonded
N-C-C-..N gauche conformations. This means that both gauche rotamers
are rather unexpectedly stable. It is difficult to believe that the rather
moderate strength 'intramolecular hydrogen bonds present in the conformations alone can explain the high stabilities of the two gauche conformations. Some other additional stabilisation effect seems to be necessary
to account for the electron diffraction observation.
Recent studies have revealed that several other 1,2-disubstituted
ethane
derivatives, XCH2CH2 Y, prefer very stable X-C--C-Y
gauche conformations
provided that both X and Y contain electronegative elements, and provided
that the molecules are isoelectronic to ethylenediamine. Thus, CH2OHCH2OH
[20,21, 7]..CH2OHCH2F [22], CH2OHCH2NH2 [23] and CH2FCH2F [24]
which all meet these requirements, are found to prefer very stable gauche
conformations. lntramolecular hydrogen bonding is present in the first three
of these compounds, but in CH2FCH2F this effect is not possible. Yet,
gauche CH2FCH2F is considerably more stable than the anti conformation
[24]. No universally accepted explanation exists to account for this
observation.
Since all these isoelectronic compounds prefer rather unusually stable
gauche conformations, we believe that the kind of effect which stabilizes
237
CH2FCH2Falso exists in the other members of this series in addition to the
intramolecular hydrogen bond thus augmenting the stabilities of the gauche
rotamers.
ACKNOWLEDGEMENT
Cand. Scient. Claus J. Nielsen is thanked for making his program ASMIXX
available, and for discussions.
REFERENCES
1
2
3
4
5
6
A Yokozeki and K. Kuchitsu, Bull. Chem. Soc. Jpn., 44 (1971) 2926.
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15 P. Trunei, Compt. Rend., 203 (1936) 563.
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Ithaca, N.Y., 1960, p. 260.
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21 L. Fernholt, personal communication.
22 K Hagen and K. Hedberg, J. Am. Chem. Soc., 95 (1973) 8263.
23 R. E. Penn and R. F. Curl Jr., J. Chem. Phys., 55 (1971) 651.
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