Journal of Molecular Structure,
@ Elsevier Scientific Publishing
35 (1976)
Company,
57-66
Amsterdam
- Printed
in The Netherlands
l\UCROWAVE SPECTRUM, CONFORMATIONAL PREFERENCE,
BARRIER TO INTERNAL ROTATION, AND CENTRIFUGAL
DISTORTION OF 1-AMINO-2-PROP ANOL
K.-M. MARSTOKK and H. MØLLENDAL
Department of Chemistry, University of Oslo, Blindern, Oslo 3 (Norway)
(Received 13 May 1976)
ABSTRACT
The microwave spectrum of CH,CH(OH)CH2NH2
has been investigated in the 26.539.7 GHz region. One rotamer with an intramolecular
hydrogen bond forme d between
hydroxyl and amino groups was assigned. This conformation
is also characterized
by having
the methyl group anti to the amino group. Other forms, if they exist, must be at least
1 kcal mole-' less stable. Four vibrationally
excited states belonging to three different
normal modes were assigned and the barrier to internal rotation of the methyl group was
found to be 3173:t 100 ca! mole-'.
INTRODUCTION
In 1-amino-2-propanol, CH3CH(OH)CH2NH2, rotation about the three
single bonds C-O, C-N, and CH(OH)-CH2NH2, may give rise to a large
number of conformations. However, Penn and Curl [1] have recently shown
that in the case of the closely related 2-aminoethanol, CH2OHCH2NH2, a
hydrogen-bonded form is preferred. In this rotamer the hydroxyl group acts
as proton donor while the amino group is the proton acceptor. Because of the
dose resemblance between the two named compounds conformers I and n of
Fig. 1 were expected to be the most pro bable low-energy forms of 1-amino2-propanol. These two rotamers each possess an intramolecular hydrogen bond
similar to that of 2-aminoethanol and are the only ones capable of forming
this type of hydrogen bonding. They are interchangable by a rotation of
approximately 120 o about the CH(OH)-CH2NH2 bond followed by
appropriate rotation about the C-N and C-O bonds allowing the intramolecular hydrogen bond to be formed.
Conformation ni of Fig. 1 is induded as just one example of the many
possible rotamers which would not be stabilized by the type of hydrogen
bon ding found in 2-aminoethanol. Such forms are expected to be of high er
energy than I or n because of the lack of this favourable intramolecular
interaction.
Microwave spectroscopy is ideal for studying conformational equilibria
because of the high resolution obtainable, making unambiguous assignments
possible in nearly all cases. As no structural studies appear to have been made
58
"o"
"\J)oc
H2N/
"o'
"6"
"9,WC"'
'NH2
I
~w"
TI
NH2
li
Fig. 1. Three possible conformations
of CH3CH(OH)CH2NH2
viewed along the central
C-C bond. Rotamers I and Il are capable of forming intramolecular
hydrogen bonds with
the hydroxyl group as proton donor and the amino group as proton acceptor. The two
forms differ from each other in that I has the methyl group anti to the amino group while
n has the c-H bond anti. Rotamer nI is just one of the many other conformational
possibilities.
for free l-amino-2-propanol
it was decided to investigate its microwave
spectrum in order to study the roles of hydrogen bonding, steric effects, and
related phenomena in determining its conformational preferences. It was
found that form I is stabilized by at least 1.0 kcal mole-1 relative to other
rotamers.
In paralleI with this work the microwave spectrum of the closely related
2-amino-l-propanol,
CH3CH(NH)2CH2OH, has been assigned [2] and will be
reported in a forthcoming paper.
EXPERIMENT
AL
The l-amino-2-propanol
used was purchased from Fluka A.G., Buchs,
Switzerland and purified by gas chromatography.
The microwave spectrum
was studied on a conventional spectrometer [3] at room temperature and
also with the absorption cells cooled to about -100 C. Measurements were
performed in the spectral region 26.5-39.7 GHz at a vapour pressure of 10
to 50 microns.
RESULTS
Microwave spectrum and assignment of the ground state
The 26.5-39.7 GHz spectral region is characterized by having ""' 100
strong and medium intensity lines as well as a much larger num ber of weak
and very weak transitions. Study of the Stark effects of the strong and
medium intensity lines revealed that many of them showed typical mediumor high-J Q-branch behaviour. In addition, approximately 10 lines with
characteristic R-branch Stark effects were also encountered. Some of the
latter were found to have typical satellites belonging to vibrationally excited
states of the molecule.
Rotational constants were predicted employing the structural data
T ABLE 1
Plausible
structural
parametersa
and observed
and predicted
rotational
Assumed structural parameters
Rotational
Distances (A)
Angles (deg.)
Dihedral
C-O
C-N
C-C
o-H
N-H
C-H
-H2CCO
H,CCO
CCN
CCC
COH
CNHb
CNHc
CCH
HCH
OCCN
NCCC
CCOH
CCNHb
CCNHc
1.396
1.475
1.526
1.139
1.017
1.093
constants
112.1
109.45
108.1
112.4
103.7
110.4
111.3
109.48
109.48
aSee text.
b Amino group hydrogen closest to the oxygen
cThe other amino group hydrogen atom.
angles (deg.)
55.4
0.0
28.3
-101.8
20.5
Ao
Bo
Co
of conformation
constants
I of CH,CH(OH)CH2NH2
(MHz)
Observed
Calculated
Difference
8487.104
3569.380
2767.221
8490.143
3556.460
2762.624
0.04 %
0.4 %
0.2 %
atom.
<:11
<O
60
collected in Table 1. The a-type R-branch transitions were then localized for
rotamer I close to the predicted frequencies and quite accurate rotational
constants were derived; b- and c-type Q-branch lines were predicted and the
former found with ease, while none of the c-type Q-branch lines could be
identified although their frequencies could be very accurately predicted.
This indicates that the component of the dipole moment along the c-axis is
quite small. The a- and b-type lines thus obtained were included in a first
order Watson [4] centrifugal distortion analysis employing SØrensen's
programme [5] and used to predict further transitions. These were
subsequently measured and used to derive improved spectroscopical constants.
In this manner a total of 115 transitions were assigned for the ground
vibrational state. Besides the a- and b-type lines mentioned above, R-branch,
b-type lines were assigned up to J=43, while high-J a-type, Q-branch as well
as b-type, P-branch lines had insufficient intensities to allow definite assignments to be made. 14N is known to possess a small quadrupole moment, but
no lines were found to be split by this effect although a few transitions
appeared to be somewhat broad. Table 2lists 28 selected transitions* and
the derived rotational and first order centrifugal distortion constants are
shown in Table 3.
Vibrationally excited states
The ground state transitions of conformation I of l-amino-2-propanol
were accompanied by a rich but mostly weak satellite spectrum, presumably
belonging to vibrationally excited states of the molecule. As shown in Tables
3 and 5, we succeeded in assigning more than 140 lines belonging to four
excited states of three different normal modes. Low-J a- and b-type R-branch
and medium-J b-type Q-branch transitions were assigned for these excited
states while high-J R-branch lines were too weak to be observed. The latter
transitions supply most information on the centrifugal distortion effect and
their omission is the main reason why the centrifugal distortion coefficients
of the excited states are more poorly determined than in the case of the
ground state.
Crude relative intensity measurements [6] yielded 139::!:: 15 cm-1 for the
lowest of the excited state modes designated TI in Table 3. Because of its
low frequency and the way in which the rotational constants are changed
upon excitation, we were led to assign this mode as the first excited state of
the heavy-atom torsional state. As shown in Table 3 the sec ond excited state
of this mode has also been found. Furthermore, it can be seen that there is a
fairly linear progression in the rotational constants for this mode upon
excitation through the sec ond excited state. This behaviour is typical for
nearly harmonic vibrations.
*The complete list of frequencies for the ground state as well as for the vibrationally
excited states is available from the authors upon request or from the MicrowaveData
Center, Molecular Spectroscopy Section, National Bureau of Standards,
Washington D.C. 20234, D.S.A., where it has been deposited.
61
TABLE
2
Selected
transitions
Transition
a-type
3", -+ 41,3
4.,4 -+ 51,5
4,,3 -+ 5,,4
50,5 -+ 60,6
51,4-+6.,5
b-type
80,8 -+ 8,,7
81,8 -+ 8,,7
12.,11 -+ 12,,10
10,,9 -+ 103,8
63,3 -+ 64"
153,,, -+ 154,11
183,15 -+ 184,14
164,,, -+ 165,11
235,'8 -+ 236,17
40,4 -+ 51,5
50,5 -+ 6.,6
114,7 -+ 123,10
178,,0 -+ 187,11
22",'0 -+ 2311,13
22",11 -+ 2311,,,
2715,,, -+ 2814,15
2715,13 -+ 2814,'4
3217,15 -+ 3316,18
3217,'6
37,0,17
37'0,'8
42'4,'8
42,4,18
a:t0.10
-+ 3316,'7
-+ 3819,'°
-+ 38'9,'9
-+ 43'3,21
-+ 4323,'°
for the ground
Observed
frequency
(MHz)
vibrational
state of CH3CH(OH)CH,NH,
Obs. -calc.
frequency
(MHz)
Centrifugal
distortion
(MHz)
26740.28
29343.05
91481.93
35646.94
39522.22
0.02
-0.12
-0.02
-0.03
-0.04
-
26661.26
31106.22
34552.63
34998.92
36503.72
27416.19
37497.02
34692.59
39110.19
31720.83
36625.83
34772.44
38554.62
26904.43
26904.43
27135.95
27135.95
38585.59
38585.59
38834.39
38834.39
28026.06
28026.06
-0.04
-0.04
-0.06
0.03
0.05
-0.03
-0.13
-0.03
-0.14
-0.07
0.01
-0.01
0.12
0.06
-0.04
-0.05
-0.05
-0.08
-0.08
0.01
0.01
0.01
0.01
a
-
-
-
0.23
0.24
0.45
0.40
0.64
1.38
1.65
4.48
3.55
1.78
7.15
12.99
9.76
23.57
0.27
0.43
1.05
3.29
15.19
15.19
30.65
30.65
45.07
45.07
74.64
74.64
129.94
129.94
MHz.
The vibrational mode called T2 in Table 3 is located at 255
This frequency is tentatively assigned as the first excited state
lowest heavy-atom torsional mode. It is also possible that this
lowest bending vibration. The first excited state of the methyl
torsional mode was also assigned and is discussed below in the
barrier determination.
:t
25 cm -l,
of the secondcould be the
group
section on
In many compounds possessing an amino group, inversion of this group
manifests itself as a prominent feature of the microwave spectrum. This is
not found for l-amino-2-propanol,
presumably because there no two identical
forms to invert between in rotamer L
TABLE
O">
3
""
Spectroscopic
constants
Parameter
Ground state
(115 transitions,
Ay (MHz)
By (MHz)
Cy (MHz)
!:.J (kHz)
!:.JK (kHz)
!:.K (kHz)
oJ (kHz)
o K (kHz)
8487.1037:!:
3569.3804
2767.2212:!:
0.6577
4.237
2.947
0.1311
2.411
for conformation
a = 0.0695)
0.0037
:!:0.0015
0.0012
:!: 0.0022
:!:0.031
:!: 0.015
:!:0.0026
:!: 0.057
I of CH3CH(OH)CH,NH,
TI = la state
(44 transitions,
8439.409
3566.021
2765.596
1.03
4.23
4.19
0.1391
2.30
in the ground
and vibrationally
= 2 state
(22 transitions,
:!:0.024
:!:0.019
:!:0.019
:!:0.29
:!: 0.13
:!:0.97
:!:0.0067
:!:0.15
Uncertainties
represent one standard deviation.
a is the standard deviation of the fit, in MHz.
aT1 denotes excited states of the lowest heavy-atom torsional mode.
bT2 denotes excited state of the presumed high-frequency
heavy-atom
8400.356
3562.207
2763.095
-0.45
3.82
6.4
0.128
2.54
torsional
mode.
states
T2 = 1b state
TI
a = 0.112)
excited
a = 0.145)
:!:0.051
:!:0.037
:!:0.037
:!:0.56
:!:0.57
:!:3.4
:!:0.022
:!:0.54
(46 transitions,
a = 0.119)
:!:0.028
8487.565
:!: 0.024
3566.995
:!:0.024
2764.017
:!: 0.35
8.55
4.37
:!:0.12
1.48
:!: 0.85
0.1529:!: 0.0064
2.07
:!:0.14
63
TABLE 4
Split Q-branch
and calculated
transitions of the first excited state of the methyl
barriers of CH3CH(OH)CH2NH2
Transition
144 10 -7 14, 9
154:" -715,:10
164,12 -716""
1 7 4,13 -7 17,,12
Observed
frequenciesa
vA (MHz)
vA-vE
39230.62
36803.20
34630.26
33106.79
-1.68
-1.22
-1.15
-0.98
group torsional
Barrier
(cal mole
vibration
-1
)
(MHz)
Average
3085
3200
3203
3203
3173
a:t0.15 MHz.
Barrier to internat rotation of the methyt group
Besides the normal vibrations designated T1 and T2 in Table 3, 33 transitions due to a third excited state were assigned. Four members of the K-l =
4-+5 Q-branch transitions listed in Table 4 were found to be split. Several
other Q-branch lines were also quite broad, but, due to their weakness, resolution of the internal rotation splitting was not achieved.
Calculation of the barrier to internal rotation of the methyl group was
perforrned with the computer program described earlier [7]. The direction
cosines of the methyl group symmetry axis listed in Table 5 were calculated
from the plausible structure discussed below. The moment of inerti a of the
methyl group around this axis was assumed to be 3.20 uA 2. V3 was then
varied to match the splittings exactly with the results indicated in Table 4.
The average barrier is found to be 3173 cal mole-l. The error limit is difficult
to estimate, but i100 cal mole-l seems reasonable when taking in to account
possible systematic errors. From this barrier a torsional frequency of 221 cm-l
is calculated which agrees well with 235 i 25 cm-l obtained by relative
intensity measurements [6].
Ethanol has the CH3CH(OH)- fragment in common with 1-amino-2-prOp2.I101.
It is therefore of interest to compare the methyl group barriers in these two
molecules. The hydroxyl group conformation in rotamer I of 1-amino-2propanol is presumed to resemble that found in anti ethanol much more
closely than that of gauche ethanol because the latter form would have
inferior geometry for hydrogen bonding. In anti ethanol the barrier to internal
rotation of the methyl group is 3329 i 25 cal mole-l [8] which is slightly
higher than in rotamer L
Search for further conformations
The assignments made as described above for rotamer I include all strong
and nearly all medium intensity lines of the spectrum. A careful search was
made among the remaining weak transitions, but we did not succeed in making
assignments to additional forms: nor were we able to resolve the Stark effects
64
TABLE 5
Molecular constants
CH3CH(OH)CH2NH2
Rotational
A
for the first excited
state of the methyl
(MHz) determined
from 33 transitions
constants
= 8475.87
:!:
0.05
B = 3566.03:!: 0.04
Direction cosines
Ab = 0.450
Aa = 0.868
Assumed moment
lex = 3.20 uA2
AC
torsion
of conformation
I of
C = 2764.81 :!:0.04
= 0.214
of inertia of methyl
top about
its symmetry
axis
Reduced barrier
s
= 89.37
Barrier to internal rotation of methyl
V3 = 3173 :!:100 cal mole-1
Uncertainties
represent
one standard
group
deviation
for the rotational
constants.
of any of them. However, due to the spectral positions of some of these
transitions, we do not believe that they all belong to unassigned vibrationally
excited states of conformation I but, perhaps, belong instead to further highenergy rotamers left unassigned. Dipole moment calculations indicate that
most other likely candidates should possess sizeable dipole moments. Very
rough absolute intensity considerations [9] indicate that there can hardly be
large fractions of further forms left unassigned. It thus seems safe to conclude
that rotamer I is at least 1.0 kcal mole-l more stable than any other conformation.
STRUCTURE
The investigated parent compound yields only three rotational constants.
Consequently, a full structure cannot be determined. In order to reproduce
the observed rotational constants a plausible structure was assumed as shown
in Table 1. The -CH(OH)CH2NH2 moiety was taken mainly from the work
of Penn and Curl [1] on 2-aminoethanol while other structural parameters
were taken largely from propane [10]. The methyl group was assumed to be
exactly anti to the C-N bond in the assigned conformation. As shown in
Table 1, very good agreement is thus found between the observed and
calculated rotational constants, indicating that there is presumably little
difference between the plausible and real geometries. A model of form I is
depicted in Fig. 2.
DISCUSSION
There can be no doubt that the intramolecular hydrogen bond is a major
reason why conformation I is preferred, because there are no indications in
65
H
H'\
\1
c
H
C
c
~
~~/
/
H'
-Fig. 2. Model of the assigned conformation
I
o
--H
I.
the speetrum for the stable eo-existenee of non-hydrogen-bonded
fonns sueh
as, for example, In.
Besides the identified rotamer, the hypothetical confonner n should also
be eapable of forming a hydrogen bond. The reason why conformation I is
more stable than n is perhaps that sterie conditions would be less favourable
in the latter. Computations strongly indicate that there are no non-bonded
eontaets appreciably shorter than the sum of the van der Waals radii of the
involved atoms in eonfonnation I while a short distanee of 2.14 A would
exist in rotamer n between one of the amino group and one of the methyl
group hydrogen atoms. In other words, if the intramoleeular hydrogen bond
was assumed to be identieal in conformations I and n, the above mentioned
hydrogens would be brought into a eontaet whieh would be about 0.26 A
shorter than the sum of their van der Waals radii [11]. This implies that a
small sterie repulsion would be present. In order to alleviate this repulsion,
an intemal rotation eould take plaee within the moleeule. However, this
would lead to a greater distanee between the hydroxyl hydrogen and nitrogen
atoms resulting in a weaker hydrogen bon d than in rotamer L Therefore we
believe that this hypothesis gives a plausible explanation of the greater stability
(at least 1.0 kcal mole-1) of form l eompared to form n.
ACKNOWLEDGEMENTS
Cand.mag. BjØrn H. Ellingsen is thanked
for gas ehromatography
of the sample.
for diseussions and Miss Gerd Teien
66
REFERENCES
1 R. E. Penn and R. F. Curl, Jr., J. Chem. Phys., 55 (1971) 65 l.
2 B. H. Ellingsen, K-M. Marstokk and H. Mfbllendal, to be published.
3 K-M. Marstokk and H. Mfbllendal, J. Mol. Struct., 5 (1970) 205.
4 J. K G. Watson, J. Chem. Phys., 46 (1967)1935.
5 G. O. Sørensen, J. Mol. Spectrosc., 22 (1967) 325.
6 A. S. Esbitt and E. B. Wilson, Jr., Rev. Sci. Instrum., 34 (1963) 90l.
7 K-M. Marstokk and H. Møllendal, J. Mol. Struct., 32 (1976) 19l.
8 J. P. Culot, Fourth Austin Symp., Gas Phase Mol. Struct. 1972, paper T8.
9 K-M. Marstokk and H. Møllendal, J. Mol. Struct., 18 (1973) 247.
10 D. R. Lide, Jr., J. Chem. Phys., 33 (1960) 1514.
11 L. Pauling, The Nature of the Chemical Bond, 3rd edn., Cornell University Press, Ithaca,
N.Y., 1960, p. 260.