Journal of Molecular Structure, 48 (1978) 9-23
@Elsevier Scientific Publishing Company, Amsterdam
- Printed
in The Netherlands
MICROWAVE SPECTRUM, CONFORMATIONAL EQUILIBRIUM,
INTRAMOLECULAR HYDROGEN BONDING, THERMODYNAMIC
PARAMETERS, DIPOLE MOMENTS AND CENTRIFUGAL DISTORTION
OF 2-AMINO-1-PROP ANOL (ALANINOL)
BJØRN H. ELLINGSEN, K.-M. MARSTOKK and HARALD MQlLLENDAL
Department of Chemistry, University of Oslo, Blindern, Oslo 3 (Norway)
(Received 21 October 1977)
ABSTRACT
Two conformations
of 2-amino-1-propanol,
CH3CH(NH2)CH2OH,
each with an intramolecular hydrogen bon d are assigned from the microwave spectrum in the 18.6-39.7
GHz
region. Rotamer I has the methyl group approximately
anti to the C-O bond; this group
is gauche to the C-O bond in con former Il. The OCCN angle is 54° :t 2° in form I and
61.5° :t 2° in Il. Hydrogen bond parameters are more favourable in I than in Il and I is
energetically favoured with tJ.G ° = -0.58 :t 0.17 kcal mole-', tJ.HO = -0.50 :t 0.19 kcal
mole-', and tJ.So = 0.3 :t 0.1 e.u. Further conformations
are at least 0.5 kcal mole-Iless
stable than Il. The dipole moment is J.1a= 3.03 :t 0.01 D, J.1b= 0.54 :t 0.03 D, J.1e
= 0.57 :t 0.19 D, and J.1tot = 3.13 :t 0.05 D, respectively, for I, while J.1a= 2.19 :t 0.03 D,
J.1b = 1.46 :t 0.08 D, J.1e= 1.172 :t 0.003 D, and J.1tot = 2.88 :t 0.07 D, respectively,
for Il.
Four vibrationally
excited states belonging to three different normal modes are assigned
for I as are two vibrationally
excited states for Il, mode frequencies being determined
by
relative intensity measurements.
Centrifugal distortion analyses for the ground vibrational
states of the two conformers and the quartic coefficients are reported.
INTRODUCTION
In recent years, several 2-substituted ethanols have been studied in the
free state [1,2]. In cases where the substituents were proton acceptors the
conformations having intramolecular hydrogen bonds were always found
to predominate [1,2]. In the closely related mono-substituted
propanols
where the proton acceptor and the hydroxyl group are placed on neighbouring carbon atoms, two rotamers each with a hydrogen bond are thus
expected to be the favoured forms of the molecule. This situation is demonstrated in Fig. 1 in the case of 2-amino-1-propanol (alaninol), CH3CH(NH2)CH2OH.
An equilibrium exists between conformation I, which has the methyl group
approximately anti to the C-o bond, and rotamer Il where the said group
is roughly gauche to the C-O bond. The hydroxyl gro up is undoubtedly
the proton donor and the amino group the acceptor since this was found in
2-aminoethanol [3]. I and Il are of course interchangeable by appropriate
rotations about the C-C, C-O and C-N bonds.
10
H
CH3
Hr6lH
Hl'!.\VH
"'HO
-.-
~
H
H
C
H3C
.,~H2
OH'"
Il
Fig, 1. The two possible intramolecularly
hydrogen-bonded
conformations
of 2-amino-lpropanol viewed along the CH2OH-CH(NH.)
bond, An equilibrium
exists between these
two rotamers. Dots indicate hydrogen bonds with the hydroxyl group as proton donor
and the amino group as acceptor.
The object of this investigation has been to study the equilibrium of
Fig. 1 and to provide characteristic spectral, structural, and thermodynamic
data. It was also hoped that this information could perhaps give some
indication of the forces responsible for delicate conforrnational equilibria
of the kind shown in Fig. L
Prior to this work, two other similar propanois, viz. CH3CH(OH)CH2NH2
[4], and CH3CH(OH)CH2F [5], have been studied in this laboratory. In the
former molecule, only one conformation similar to rotamer I was assigned.
Other forms were estimated to be at least 1.0 kcal mole-1 less stable. In
CH3CH(OH)CH2F again only one rotamer was found, while other conformers
were estimated to have at least 0.75 kcal mole-1 higher energies. This conformation was, rather surprisingly, similar to I.
EXPERIMENT AL
L-2-amino-1-propanol
(Puriss) and DL-2-amino-1-propanol
(Purum) from
Fluka A.G., Buchs, Switzerland, were used without further purification.
The microwave spectra of the two enantiomers are of course identical. The
compound has a vapour pressure of about 0.2 torr at room temperature.
Most measurements were made with the cell cooled to about O°C. Lower
temperatures could not be utilized because of the low vapour pressure of the
compound. Most studies were made in the 26.5-39.7 GHz spectral region
while a few lines were measured in the 18.6-26.5 GHz region. Vapour
pressures of 20-50 microns were employed when the spectra were recorded.
A conventional spectrometer described briefly in ref. 6 was used.
RESULTS
Microwaue spectrum and assignment of conformation
I
Preliminary rotational constants were computed for rotamers I and Il by
combining structural parameters taken mainly from 2-aminoethanol [3] and
---
11
propane [7]. Bond moment calculations
[8] yielded
= 2.57 D, J.Lb = 1.00 D,
J.La
and J.Lc= 0.37 D for conformation I, while these parameters were found to
be J.La= 3.68 D, J.Lb= 0.36 D, and J.Lc= 0.24 D by the CNDO/2 method [9].
A major component of the dipole moment along the a-axis was thus expected.
The molecule was found to possess a moderately rich microwave spectrum.
Search was first made in the 26.5-40 GHz region where the a-type R-branch
lines were expected to be the strongest ones in the spectrum. The J = 3 -+ 4,
4 -+ 5, and 5 -+ 6 transitions of this kind were found dose to their predicted
frequencies and a set of unrefined rotational and centrifugal distortion
constants were determined. These parameters could not predict the frequencies
of the much weaker a-type high J Q-branch transitions accurately. These
lines were therefore found by a trial and error procedure. A total of 39
a-type transitions were ultimately assigned as shown in Table 1. The highest
value of J was 64.
The rotational constants appearing in Table 2 are accurately determined
while the centrifugal distortion coefficients are of much poorer quality.
However, they resemble those determined for CH3CH(OH)CH2NH2 [4] as
one would expect since these two molecules are presumed to have similar
geometries and force fields.
N o b-type or c-type lines could be assigned with certainty, although their
hypothetical frequencies could be well predicted. This is in keeping with
the fact that the corresponding dipole moment components are as small as
J.Lb= 0.54:t 0.03 D and J.Lc= 0.57 :t 0.19 D, respectively, producing insufficient intensities for the transitions in question.
Vibrationally
excited states
The ground state lines were accompanied by several satellite transitions
which are presumed to belong to vibrationally excited states. Four of these
were assigned to three different normal modes. *
Sixteen lines of the strongest satellite series were assigned, and the derived
rotational constants are shown in Table 3. The centrifugal distortion constants obtained for this state were too inaccurate to warrant publication.
Relative intensity measurements made large ly as described by Esbitt and
Wilson [10] yielded 126:t 10 cm-1 for this fundamental which is believed
to be the heavy-atom torsional mode. This frequency as well as the changes
in the rotational constants upon excitation are dose to their counterparts
in the dosely related molecule 1-amino-2-propanol
[4].
Attempts were made to see whether the changes in the rotational constants
upon excitation could be reproduced by opening up the OCCN dihedral
angle keeping the other structural parameters fixed. The change in the
B-rotational constant was well reproduced while poor agreement was found
*The microwave spectra of the excited states of the two conformations
are available from
the authors upon request, or from the Microwave Data Center, Molecular Spectroscopy
Section, National Bureau of Standards, Washington D.C. 20234, U.S.A., where the y have
been deposited.
12
TABLE 1
Microwave
spectrum
Transition
20,2
-
Observed
frequencya
(MHz)
30,3
30,3 3,,2 - 4,,3
3.,3 2".
3.,2
40,4
4,,4
32,1 -+ 42,2
40,4
-
50,s
4,,3 -+ 5,,4
4,,4
5.,5
-
42,2
42,3
43"
43,2
50,s
of the ground vibrational
- 52,3
--
52,4
53,2
- 60,6
53,3
- 6,,5
---
18680.56
20419.83
24427.88
27053.14
23547.28
26579.69
29951.36
33506.27
29265.24
33647.29
31664.05
32489.64
32266.67
35371.35
39707.16
34913.06
6.,6
37774.70
52,4
62,s
39314.27
53,2
63,3
38751.84
53,3
63,4
37463.22
122,11
122,,0
27850.20
133,,,
133,10
35473.25
174,,4-174,13
28061.02
195,15 - 195,14
26822.70
226,'6
226,17
34699.09
236,'8
236,'7
30585.96
298,22 -+ 298,2'
39554.67
308,23
308,22
27704.38
329'24
329,23
36699.40
339,25
339,24
36,0,27 - 3610,26 33324.05
3911,29- 39",28 29622.19
39403.41
40",30
40",29
43'2,32
43'2,31 35190.40
30810.66
4613,33
4613,34
49'4,36
49'4,35 26459.16
50'4,37
50 '4,36 36307.02
53" ,38 31380.13
53",39
57 '6,42
57 '6,4' 36778.03
36697.54
64'8,47
64'8,46
5.,4
5,,5
-
-
-
--
-
-
--
Obs. -calc.
frequency
(MHz)
state of conformation
Centrifugal
distortion
(MHz)
0.02
0.13
0.14
0.03
-D.05
-D.14
0.08
-0.13
0.13
-0.02
-0.10
-0.05
-D.18
-0.06
-0.30
-0.10
-0.30
-0.11
-D.23
-D.4 7
0.05
0.11
-0.21
-D.32
-0.65
-0.61
0.05
0.22
0.02
0.00
-0.01
-D.17
-D.02
-D.10
0.10
0.05
-D.08
0.02
-D.07
0.11
0.01
-0.15
0.07
0.00
0.16
-D.02
-D.07
0.18
-0.04
-D.06
-0.19
0.11
-0.18
-D.44
-0.18
-0.45
-0.92
--0.82
-5.14
-6.26
-13.92
-16.62
-23.50
-30.35
-51.50
-65.09
-62.60
-80.25
-95.37
-109.48
-139.29
-157.93
-173.85
-185.84
-241.37
-256.99
-346.96
-474.94
a:t0.10 MHz.
----
--
I of CH3CH(NH2)CH2OH
13
TABLE 2
Spectroscopic
(NHJCH2OH
Ao
AJ
oJ
constantsa
= 7948.403
= 0.51
:t
for the ground
0.077 MHz Bo
0.18 kHz
= 0.163:t 0.024 kHz
aUncertainties
TABLE
AJK
oK
:t
represent
vibrational
= 3640.570
= 3.84
:t
= 3.23:t
one standard
state of conformation
:t
0.011 MHz Co
0.63 kHz
0.56 kHz
AK
I of CH3CH-
= 2747.727:t 0.011 MHz
= 1.5 :t 3.2 kHz
deviation.
3
Rotational
CH20H
Vibrational
constantsa
for vibrationally
state
First ex. heavy-atom torsionb
Second ex. heavy-atom torsionc
First ex. methyl torsiond
First ex. bending mode, or
C-N, or C-O torsion e
aUncertainties
represent
sitions. e4 transitions.
excited
states of conformation
I of CH3CH(NH2)-
Av
Bv
Cv
(MHz)
(MHz)
(MHz)
7904.88
7873.80
:t 1.03
:t 0.42
7930.10
7957.3
:t 0.82
:t 2.2
one standard
deviation.
3636.404 :t 0.086
3632.502:t
0.016
3638.024 :t 0.034
3633.753:t
0.051
b16 transitions.
2745.944 :t 0.087
2743.808 :t 0.014
2745.458 :t 0.040
2743.646:t
0.048
c9 transitions.
d7 tran-
for the A and C rotational constants. This mode is therefore believed to be
more complicated than a simple torsion about the C--c bond.
Nine lines of the second excited state of this mode were assigned. Relative
intensity measurements yielded 181 :t 30 cm-I for this frequency. As seen
in Table 3, the variations upon excitation of this mode are quite linear. It is
therefore concluded that this frequency is nearly harmonic [11].
As indicated in Table 3, the first excited state of the methyl torsion is
believed to be assigned. Since none of the 7 lines measured was split by
the methyl group tunnelling effect this assignment is slightly more uncertain
than in the case of the heavy-atom torsional mode. However, calculations
assuming a hypothetical barrier of about 3 kcal mole-I indicate that the
splitting should be less than the resolution power of our spectrometer.
Relative intensity measurements place this frequency at 199 :t.49 cm-I
close to 221 cm -I derived for 1-amino-2-propanol
[4] for its methyl
torsion. Moreover, the changes in the rotational constants upon excitation
are similar in the two aminopropanols.
The origin of the fourth excited state assigned is uncertain. Our belief is
that this is the lowest bending mode, but it could well be the lowest
torsional mode of the amino- or the hydroxyl group.
14
Dipole moment
The Stark coefficients
of 21,1 -+ 31,2 and
31,2 -+ 41.3 transitions
were
used
to determine the dipole moment. Stark splittings in the 4-11 MHz range
were measured. The experiment was perforrned as previously described [12],
and the inverse squares of the standard deviations of the Stark coefficients
shown in Table 4 were us ed as weights in the least squares procedure. The
results are also presented in Table 4. The total dipole moment is 3.13 :t 0.05 D
which is close to 3.05 :t 0.05 D found for 2-aminoethanol [3]. Bond moment
ca1culations [8] yielded 2.78 D, while 3.71 D was found by the CNDO/2
method [9].
Assignment
of conformation
Il
A preliminary set of rotational constants was computed for conformation
Il based on the same structural parameters as for I. Bond moment calculations
and CNDO /2 computations again indicated that Ila should predominate. The
former method yielded Ila = 2.55 D, Ilb = 1.14 D, and Ile = 0.59 D, respectively, while the CNDO/2 results were Ila = 3.21 D, Ilb = 0.54 D, and
Ile
= 1.56 D, respectively.
After the assignment of rotamer I was completed severallines of medium
intensity remained in the spectrum. Among these lines searches were made for
the relatively strongest a-type R-branch transitions. The 30 3-+ 40 4, 40 4-+ 50 5
and 41 3 -+ 514 transitions
were found
slightly
away from their p~edicted
freque~cies, ~ainly because the OCCN dihedral angle is about 7° larger
than the value found in 2-aminoethanol as will be discussed in the section
on structure determination. The assignments of these three transitions were
confirmed by their Stark effects and a set of unrefined rotational constants
was computed from them. The frequencies of further a-type R-branch lines
TABLE 4
Stark coefficients and dipole moment of conformation
certainties represent one standard deviation)
Transition
2... -->3'.2
3'.2 -->4,,3
= 3.03
= 3.13
-
:t
Cale.
0.01
0.03
0.15
-0.21
-1.96
-7.21
M=O
0.01
0.01
0.03
0.14
-0.20
-0.34
-0.78
-1.51
=l
IMI = 2
1MI = 3
Iltot
(MHz V-'em')'10'
Obs.
M= O -0.21:t
IMI = l -1.99:t
IMI = 2 -7.23:t
IMI
Ila
!:.v/E'
0.01 D
:t 0.05
Ilb
-0.21:t
-o.34:t
-0.77:t
-1.52:t
= 0.54
D.
-
:t
0.03 D
Ile
= 0.57
I of CH3CH(NH,)CH,OH
:t
0.19 D
(Un-
'
15
were then predicted and readily assigned. A total of 10 transitions of this
type were measured. Moreover, 23 b-type and 3 c-type Q-branch lines were
also assigned. No a-type Q-branch lines could be identified owing to their
small intensities. The spectrum is shown in Table 5 and the derived spectroscopic constants appear in Table 6.
TABLE 5
Microwave
CH20H
spectrum
Transition
30 3 --> 40 4
3"2 --> 4"3
3"3 --> 4"4
3;,-->4;2
32:2
--> 42:3
40,4 --> 50"
4'3-->5'4
4"4 -->5,',
42:3 --> 52:4
43" --> 53,2
16,,'2 --> 166,,1
166,10 --> 167,9
176,,2 --> 177,,,
186,13 --> 187,12
19,,15 --> 196,14
197,,2 --> 19.,12
217 15 --> 218 '4
218:13
218,13
238,15
248,16
249,15
258,'7
259,16
279,'8
--> 219:,2
--> 219,13
--> 239,14
--> 249,15
--> 24,0,15
--> 259,16
--> 2510,15
--> 27'0,'7
2810,18
299,20
309,2,
30,0,20
3110,21
33,0,23
3311,22
34",23
35",24
36,2,24
3712,25
--> 2811,'7
--> 2910,19
--> 3010,20
--> 3011,,9
--> 31",20
--> 3311,22
--> 33'2,2'
--> 3412,22
--> 35,2,23
--> 3613,23
--> 3713,24
a:t0.10 MHz.
of the ground
vibrational
Observed
frequencya
(MHz)
Obs. -calc.
frequency
(MHz)
28503.60
31101.68
28013.68
31197.72
29782.49
35105.03
38388.50
34817.89
37009.97
38265.71
32174.90
26769.03
33930.60
35029.30
39601.26
28140.62
39377 .88
36176.20
35028.07
30461.76
28538.55
38110.04
28067.11
36328.41
31320.95
39335.94
32299.93
35615.70
34056.40
33627.74
38723.81
36750.28
36365.89
, 38081.54
39407.49
39080.94
0.04
-0.04
0.03
-0.16
0.14
0.01
-D.01
0.07
0.06
-D.14
0.34
-D.16
-0.25
0.13
-0.27
0.31
0.05
-D.23
-0.12
0.01
-0.02
0.12
0.22
-0.01
0.06
-0.07
-0.17
0.21
0.11
-0.08
0.19
0.03
-0.09
-D.12
0.17
-D.21
state of conformation
Centrifugal
distortion
(MHz)
-D.37
-0.54
-0.38
-D.67
-0.62
-0.70
-1.00
-0.69
-1.05
-1.45
-14.76
-12.48
-17.20
-20.20
-25.00
-19.57
-30.90
-28.81
-28.12
-32.77
-33.58
-41.50
-34.22
-45.14
-46.90
-62.61
-48.01
-51.22
-63.24
-62.47
-66.24
-82.88
-80.83
-80.38
-106.05
-102.27
Il of CH3CH(NH2)-
16
TAB LE 6
Spectroscopic
(NH2)CH2OH
Ao
constantsa
= 6461.620
:!:
for the ground
0.032 MHz Bo
!:..J = 1.65 :!:0.69 kHz
oJ = 0.3420 :!:0.0094 kHz
aUncertainties
represent
Vibrationally
vibrational
= 4144.442
state of conformation
:!:
0.030 MHz Co
!:..JK = 6.35 :!:0.15 kHz
oK
= -0.034
:!:0.117 kHz
one standard
Il of CH3CH-
= 3336.163
!:..K = -7.02:!:
:!:
0.030 MHz
0.40 kHz
deviation.
excited states
Two vibrationally excited states were assigned for conformation Il. On ly
a-type R-branch lines were identified. b-type Q-branch transitions were
searched for extensively but could not be assigned presumably because of
insufficient intensities.
Nine transitions were measured for what is believed to be the heavy-atom
torsional mode and the rotational constants appearing in Table 7 were found
by the least squares method assuming a rigid rotor mode!. A frequency of
144:!: 26 cm-1 was determined for this mode by relative intensity measurements. This is not too different from 126 :!:10 cm-1 found for the corresponding fundamental of rotamer I. Attempts to reproduce the observed
changes in the rotational constants up on excitation by opening up the
OCCN dihedral angle were also made in this case. It was found that the
change observed for the C rotational constant agreed closely with the
calculated pattern while discrepancies were seen for the A and B rotational
constants. It is thus concluded that this mode also is more complicated
than a simple torsion about the C-C bonds.
Seven lines of another normal mode were also assigned and the rotational
constants of Table 7 determined in the same manner as for the previous
frequency. The assignment of this mode to a particular fundamental is
uncertain. The assumption that this is the first excited state of the methyl
torsion is favoured, but it could well be the first excited state of the lowest
bend ing mode or the torsional frequency of the amino or hydroxyl group.
A frequency of 225 :!: 26 cm-1 was found for this mode by the relative
intensity method.
TABLE 7
Rotational
CH20H
constantsa
Vibrational
state
First ex. heavy-atom
Uncertainc,d
aUncertainties
--
for vibrationally
excited
Ay
(MHz)
torsionb
represent
By
(MHz)
6471.64:!:
6454.80:!:
one standard
states of conformation
1.13
0.72
deviation.
Cy
(MHz)
4138.216:!:
0.073
4141.01
:!:0.33
b9 transitions.
Il of CH3CH(NH2)
3323.088:!: 0.065
3332.80
:!:0.25
c7 transitions.
dSee text.
~--
17
Amino group inversion is a prominent spectral feature in many compounds.
This is absent for both conformations, presumably because there are no two
identical forms to in vert between in either rotamer.
Dipole moment
Star k coefficients
were measured
for 40,4 --* 50,s and
41,4 --* 51,s transitions
utilizing splittings in the 2-6 MHz range. The statistical treatment of the
data was as for conformation I with the results shown in Table 8.
The dipole moment of this rotamer is seen to be 0.25 D less than for
conformer I. Bond moment calculations predict that these dipole moments
should be almost identical, viz., 2.86 D for n and 2.78 D for I. The CNDO/2
method only partially reproduces the difference observed for the dipole
moment. Computations with this method yielded 3.71 D for rotamer I, and
3.61 D for Il.
Searches for further conformations
After completion of the assignments of the two forms there remained
only a few rather weak transitions in the spectrum. It is not unlikely that
most of these belong to unassigned high J transitions of vibrationally excited
states. That large fractions of further rotamers coexist with the two conformations already accounted for is ruled out. It is felt that further rotamers
would have been noticed if their individual concentration hild exceeded
10-15% of the total. Conformation n, the less stable of the two forms, is
thus conservatively estimated to be at least 0.5 kcal mole-I more stable
than any third rotamer .
Structure
of the two conformations
Only three rotational constants were determined for each of the two
conformations. Consequently, a complete molecular structure cannot be
TABLE 8
Stark coefficients and dipole moment of conformation
certainties represent one standard deviation)
Transition
4.,4 --.5.,5
(Un-
t::.v/E' (MHz V-'cm')'10.
Obs.
40,4 --. 50,5
Il of CH3CH(NH2)CH,OH
0.6
IMI= 1 -11.9:t
IMI= 2 -45.7 :t 0.6
IMI= 3 -102.8 :t 1.0
M =O
IMI= 1
IMI= 2
IMI= 3
-0.58 :t 0.03
12.3 :t 0.6
50.5 :t 1.9
114.6 :t 4.6
J.la = 2.19 :t 0.03 D J.lb = 1.46 :t 0.08 D
J.ltot = 2.88 :t 0.07 D
Calc.
-11.96
-45.90
-102.46
-0.58
12.23
50.63
114.63
J.le = 1.172 :t 0.003 D
---
18
determined. Instead, we had to restrict ourselves to fitting three structural
parameters for each rotamer . The dihedral angles OCCN and OCCC as well
as the CCC angle were selected for fitting because the rotational constants
of the two forms are sensitive to these parameters and because they are
chemically interesting. The interesting dihedral angles HOCC and CCNH
were not chosen because the rotational constants are insensitive even to
rather large variations in these parameters. These angles, as well as the rest
of the structural details of the -CH(NH2)CH2OHfraction of the molecule,
were taken from 2-aminoethanol. Structural data for the methyl gro up and
the CCC angle were transferred from propane [7]. The selected parameters
are shown in Table 9 and the dihedral angles illustrated in Fig. 2.
The three angles OCCN, OCCC and CCC, were fitted by minimizing the
sum of the per cent differences between the observed and the calculated
rotational constants. The results are displayed in Table 9. Error limits given
are believed to encompass reasonable structural differences between the
assumed structural parameters and the real ones.
It is seen in Table 9 that the methyl group is almost exactly anti to the
C--D bond in conformation I, and nearly exactly gauche in rotamer IL The
TABLE 9
Plausible structural parametersa and observed
two rotamers of CH3CH(NH2)CH2OH
and ealeulated
rotational
eonstants
Assumed structural parameter common for conformations I and Il
O-H
C-O
C-H
C--C
C-N
N-H
1.139
1.396
1.095
1.526
1.475
1.017
Fitted
A
A
A
A
A
A
structural
LCOHb
LHCOa
LCCO
LCCN
LCNHb
LCNHc
LCCH
Rotational
I
Conformation
54.0:!: 2.00 from syn
180.0:!: 2.00 from syn
112.4 :!: 1.00
28.3
78.2
159.5
Il
61.5 :!:2.00 from syn
60.0 :!:1.00 from syn
112.4 :!: 1.00
constants
Conformation
Obs.
AD (MHz)
Bo (MHz)
Co (MHz)
LCCOHa
LCCNHb
LCCNHc
parameters
Conformation
LOCCN
LOCCC
LCCC
103.70
109.80
112.10
108.10
110.40
111.30
109.80
7948.403
3640.570
2747.727
I
Conformation
Il
Cale.
Diff.
Obs.
Cale.
Diff.
7951.06
3655.65
2753.71
0.03%
0.41 %
0.22%
6461.620
4144.442
3336.163
6465.67
4147.96
3330.25
0.06%
0.08%
0.18%
aSee text and Fig. 2. bSee Fig. 2 for definition
of Ha' Hb and Hc'
--
--
of the
19
CONFORMA TION [
CONFORM ATION Il
H
VIEWED
THE
ALONG
H@H
C-C - BOND
H2N 54'C
HO
H
C
H3C
NH2
60' 61.5'
H@H
OH
HH
VIEWED
THE
ALONG
Hy;r
Ho
O-C - BOND
:
C2H4NH2
, 283'
Ho
H2NH4C2
:'283',
,
.--:..
Hb
VIEWED ALONG
THE -N- C - BOND
159.5'
H,cQ)
H
c
Fig. 2. Definition
CH,oH
H
Hc
N
78.2'
H;cQCH,oH Hb
H
of fitted and assumed dihedral angles of the two conformations.
CCC angle is indistinguishably similar in the two forms and identical to that
of propane within the error limits. However, there is a large, significant
difference in the OCCN dihedral angle of the two conformers. As shown in
Table 9 this angle is approximately 7.5° larger in conformation Il than in I.
The increase in the OCCN angle observed upon going from rotamer I to
Il must lead to a less favourable hydrogen bond situation in the latter conformer. This is bome out by the distances and ang les shown in Table 10. The
hydrogen bond distanee between H and N is seen to increase by about 0.12 Å
from I to Il, and the O-H, , , N angle to decrease from 115.0° to 112.6°.
Both structural differences presumably lead to less hydrogen-bond stabilization in conformation Il than in I.
Energy difference
between the two forms
Relative intensity measurements were used to determine Gibbs' free
energy difference of the two conformations in the following manner.
The peak absorption coefficient o: (cm -1) of any asymmetric-top spectrum
line [13] is given approximately by
V2
2A
l
o: = 3.85,10-14 Tt2f!.(tl~),
T Fu(ABC)2 gl exp(-E~/kT)
(1 )
--
20
TABLE
10
Structural
CH20H
parametersa
of the hydrogen
Structural parameter
Conformation
Ha,..N
Ha...Hb
2.11
2.73
2.82
2.79
115.0°
43.3°
21. 7°
Ha'"
O...N
He
LO-Ha",
N
LHa-o...
N
LHa...N...O
bond in conformations
I
A
A
A
A
Conformation
2.23
2.90
2.89
2.86
112.6°
45.9°
21.5°
I and Il of CH3CH(NH2)-
Il
A
A
A
A
Sum of van der Waals' radiib
H,..H2.4A
O...N
H...N2.7A
2.90 A
aFor definitions of Ha' Hb and He see Fig. 2.
bTaken from ref. 16.
where F v is the fraction of molecules in the particular
state under observation
A, B, and C are the rotational constants, T is the absolute temperature, v is
the absorption frequency, /J.g is the dipole moment component (Debye )
along the g principal inertial axis, A is the line strength of the transition
under study, E; is the rotational energy of the lowest state involved in the
transition, k is Boltzmann's constant, (~v h. is the line breadth at half
intensity (MHz) at T = 300 K and p = 1 torr, a is the symmetry number,
and gl is the reduced nuclear weight.
Both conformations have Cs symmetry and therefore one assumes that
a = gl = 1. It is further assumed that (~v)1..is 8 times the dipole moment [14].
This leads to the following relation between the fraction of molecules in the
ground vibrational state of conformation I (F~=o)relative to the corresponding
fraction of rotamer Il (Fv~o)
2
I
F~;o
Fv=o
.
=~
an
~
2.1..
J
AnBnCn
( VI ) ( AIBICI )
2
An /J.gn exp(-E/,nlkT)
AI /J.gIexp(-E
T,IlkT)
(2)
Only a-type transitions were utilized to calculate FJ=o/FJ!o. Substituting
the values for the rotational constants and the components of the dipole
moments of the two conformations along the a-axes, eqn. (2) reduces to
F~~o
Fv=o
= 0.766 al An
an AI
(.!:lL
VI )
2
eXP(-E~.nlkT)
(3)
exp(-ET,IlkT)
This equation is related to Gibbs' free energy difference of the ground
vibrational states, ~G~=o, by
FJ=o
o ~Gv=o - -RT ln
lI
(4)
F v=o
~
21
In order to minimize systematic errors arising from reflections in the wave
guide or different frequency-dependent
properties of the detector system,
transitions which were reasonably near each other in frequency were selected
for relative intensity measurements. Care was taken to en sure that these lines
were completely modulated and not contaminated by Stark components.
Moreover, it was hoped that the selected lines were not overlapped by other
weaker unassigned transitions. The results are presented in Table 11. A value
of ilG~;o -o.58:t 0.17 kcal mole-1 was derived from six selected measurements.
Since the variation of the free energy with temperature was not studied,
it is not possible to derive the entropy difference of the two rotamers by the
familiar eqn. (8). An alternative method was therefore used to evaluate this
quantity.
The partition function is given approximately by [15]
.
Q = Q rot. Q.v,b.
3/2
=~
( 8n3 l A l B l e )t (kT ) TT. ( 1
a . h3
- e-hw ilkT)-l
where a, k, and T have the same meanings as was discussed for
lA, IB, and le are the principal moments of inertia, h is Planck's
and Wi the normal mod e frequency.
Jf it is assumed that the products of all normal modes of the
are identical with the exception of the aCCN torsional modes,
ratio of the partition functions is found
I
Q v;o - A Il B Il C Il t 1 - e-1.44wIlIT
A IB I CI '
1 - e-1.44wIIT
Q I~
v-o -
(
(5)
l
eqn. (1),
constant,
two rotamers
the following
)
(6)
where WI and WIIare the torsional modes of the two farms determined
126:t 10 cm-1 and 144:t 26cm-l, respectively.
The entropy difference is given by [15]
to be
TABLE 11
Relative intensities and Gibbs' free energy difference
conformations
I and Il
Conformation
Conformation I
Transition
Freq uency
Transition
(MHz)
3,,2-->4,,3
133,11 -->133,,0
195,,5 -->195".
195,15 -->195".
4,,4 -->5,,5
40,4 --> 50,5
27053.14
27850.20
28061.02
28061.02
29265.24
29951.36
of the ground
Il
Frequency
vibrational
states of
T
l:I.Go
"'n
(K)
(kcal mole-')
3.82
1.12
1.65
1.14
2.10
5.03
276
268
268
276
275
273
-0.59
-0.74
-0.77
-0.60
-0.28
-0.47
Av.
-o.58:t
(MHz)
3,,3--> 4".
3,,3 -->4".
3,,3 -->4,,4
30,3 -->4o,.
30,3 --> 4o,.
32,2 --> 42,3
28013.58
28013.58
28013.58
28503.60
28503.60
29782.49
0.17
22
flSO- v-o
= RIn
Q~=o+
RT~
II
dT
Q v=O
(In Q~=o
)
Q
II
v=O
(7)
At 273 K the entropy difference is calculated to be 0.3 :!:0.1 cal mole-l K.
The enthalpy difference flH~=o at 273 K is then calculated from
flG~=o
= flH~=o -
TflS~=o
(8)
and found to be -0.50:t
0.19 kcal mole-l. Thus, conformation I is favoured
energetically while there is very Httle entropy difference between the two
rotamers.
This result contrasts with that obtained by the CNDO /2 method which predicted conformation n to be the more stable by about 1 kcal mole-l.
It would be desirable to construet a potential surface for the conformational behaviour of 2-amino-1-propanol.
Unfortunately, this is not possible
because of insufficient data. Even a reasonable path between conformations
I and n is impossible to construet because it is probably quite complicated
as the C---C, C-N, and C-O torsional mod es would have to be considered.
Inversions of the amino- and hydroxyl groups could also be of importanee
for the transformation of I into n and vice versa.
DlSCUSSION
Intramolecular hydrogen bonding is undmlbtedly an important factor in
determining why 2-amino-1-propanol prefers the two conformations of
Fig. 1 to other forms which necessarily would lack this kind of favourable
interaction.
It is also interesting to note that conformation I is more stable by about
0.5 kcal mole-l. The hydrogen-bonding conditions are probably better in
this rotamer than in Il as indicated in Table 10 and this may be the reason
why I is preferred to n. The worsened hydrogen-bonding situation in n
relative to I was caused by the 7.50 OCCN angle opening. It is possible that
this increase of the OCCN angle results from steric repulsion. Calculations
indicate that there are two critical non-bonded contacts between hydrogen
atoms in conformation n. The first of the se exists between the methyl
group and the hydroxyl group while the other is found between the methyl
and amino groups. Both these hydrogen-hydrogen
distances were found to
be slightly less than the sum of their van der Waals' radii which is 2.4 Å [16].
Furthermore, it was found that an opening of the OCCN angle resulted in
increased distances between both the se hydrogen-hydrogen
distances. The
observed conformation thus seems to represent a compromise between
steric repulsion and hydrogen bond attraetion.
Finally, we wish to discuss why the title compound exists with two
intramolecularly hydrogen-bonded
conformations while only one rotamer
corresponding to I was found for the closely related 1-amino-2-propanol
[4].
The hypothetical conformation n of the latter molecule is sketched on the
23
H
H
Hr<:xH
H3C~.~H2
O-H'"
H
$
H
C.
H3C
H2N.' .
..~
CONFORMATION Il OF
CONFORMATlON Il
2-AMINO -1- PROPANOL
1-AMINO - 2 -PROPANOL
OF
Fig. 3. Rotamer
Il of 2-amino-1-propanol
(left) and of 1-amino-2-propanol
(right) viewed
along the C-C bond. Note the crowded
situation
between the methyl and amino groups
of the latter conformer
and the possibility
of interaction
between the methyl
group and
the oxygen lon e pairs of the former rotamer.
right in Fig. 3. This rotamer must, if it exists, be at least 1.0 kcal mole-1 less
stable than the identified form which was similar to I [4].
A prominent non-bonded steric interaction resulting from the faet that
the amino group is proton acceptor and the hydroxyl gro up proton donor in
the two aminopropanols is perhaps the reason for the difference in their
conforrnational behaviour. Computations indicate that the methyl group is
rather close to the hydroxyl group in rotamer Il of 2-amino-l-propanol
and
dose to the amin o group in rotamer Il of l-amino-2-propanol.
In the title
compound there could be some stabilization between the methyl gro up and
hydroxyl gro up lone pair. An opposite situation presumably exists in conformer Il of ref. 4. It is likely that there is a repulsive interaction between
the amino and methyl groups in this conformation. The presumed opposing
non-bonded interactions present in the two rotamers are thus perhaps the
cause for their different conforrnational equilibria.
REFERENCES
10. Bastiansen, H. M. Seip and J. E. Boggs, in J. D. Dunitz and J. A. Ibers (Eds.),
Perspectives in Structural Chemistry, Vol. 4, Wiley, New York, 1971.
2 E. B. Wilson, Jr., Chem. Soc. Rev., (1972) 293.
3 R. E. Penn and R. F. Curl, Jr., J. Chem. Phys., 55 (1971) 651.
4 K-M. Marstokk and H. Møllendal, J. Mol. Struct., 35 (1976) 57.
5 K-M. Marstokk and H. M<$llendal,J. Mol. Struct., 40 (1977) 1.
6 K-M. Marstokk and H. M<$llendal,J. Mol. Struct., 5 (1970) 205.
7 D. R. Lide, Jr., J. Chem. Phys., 33 (1960) 1514.
8 C. P. Smyth, Dielectric Behavior and Structure, McGraw-Hill, New York, 1955, p. 244.
9 J. A. Pople and D. L. Beveridge, Approximate Molecular Orbital Theory, McGraw-Hill,
New York, 1970.
10 A. S. Esbitt and E. B. Wilson,Jr., Rev. Sci. Instrum., 34 (1963) 901.
11 D. R. Herschbach and V. M. Laurie, J. Chem. Phys., 40 (1964) 3142.
12 K-M. Marstokk and H. Møllendal, J. Mol. Struct., 16 (1973) 259.
13 W. Gordy and R. L. Cook, Microwave Molecular Spectra, Interscience, New York,
1970, p. 195.
14 W. Gordy, W. V. Smith and R. F. Trambarulo, Microwave Spectroscopy, Wiley, New
York, 1953, p. 96.
15 S. Glasstone, Theoretical Chemistry, Van Nostrand, Princeton, New Jersey, 1944,
Chapter VIII.
16 L. Pauling, The Nature of the Chemical Bond, 3rd edn., Cornell University Press,
Ithaca, N.Y., 1960, p. 260.
--