303
Journal of Molecular Structure, 97 (1983) 303-310
Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
RECENTSTUDlES OF INTERNALHYDROGEN
BONDINGOF ALCOHOLS,AMINESAND THIOLS
HARALDMØllENDAL
Department of Chemistry,
The University
of Oslo, P.O. Box 1033, Blindern,
Oslo 3 (Norway)
ABSTRACT
Same selected results
reviewed and discussed.
of recent
studies
of alcohols,
amines and thiols
are
I NTRODUCTI ON
Free molecules possessing intramolecular
hydrogen bonds have received considerable interest
in recent years. Both electron diffraction
and spectroscopic
methods have been used successfully
in so many cases that a campl ete review of
this dynamic field is not possible within the framework of this short paper.
Rather, a somewhat arbitrary
attention
is given to studies related to work
presently been carried out in Oslo.
There are several reasons why structural
studies of intramolecular
hydrogen
bonding attract
interest
these days. For example, a large number of biologically active molecules con ta i n such bonds which are furthermore crucial to
their biological activity.
Reactivity of many molecules is aften influenced to
a great extent by internal hydrogen banding. The preferred conformation of a
molecule is also aften dependent, if not determined, by this kind of interaction.
In condensed phases as well as in many solutions,
molecules very of ten form
>i"
I
intermolecular
they
free
hydrogen bonds instead
of intramolecular
hydrogen bonds which
hydrogen bonding of
prefer
in the gaseaus state. The study of internal
molecules
is therefore important for a better understanding of this
effect.
Structural
studies
of gaseaus molecules can perhaps best be made by electron
diffraction
or microwave spectroscopy. Both methods are limited to the study of
relatively
small and reasonably volatile campaunds. Electron-diffraction
offers
a rather convenient way to determine heavy-atom band distances and angles,
whereas microwave spectroscopy better can locate hydrogen atoms involved in
hydrogen bonding. The results described in this paper have mainly been obtained
using ane or both of these two methods.
0022-2860/83/$03.00
<91983 Elsevier Science Publishers B.V.
304
RESULTS AND DISCUSSION
Ethanol
derivatives
Ethanol
studied
derivatives
alcohols.
containing
Sa
far
electron-diffraction
molecules
we
have
and
3-buten-l-ol
the
4,5)
double
as
well
that
the
that
at
gas
°c
belongs
work
an
preferred
45
a
were
poorer
quality.
No
computed
to
be
also
for
the
64(3)0
internal
This
is about
seems
atom
to
pull
making
of
in this
This
more
than
double
the
conformation
Accurate
established
up
The
at
accurate
typical
CCCC
dihedral
least
OCCC
angle
dihedral
l-butene
approximately
(ref.
150
7).
closer
(ref.
of
6)
the
present
that
of
angles
dihedral
ethanol
was
to
the
were
of a
hydrogengas
at
were
angle
-ZO
°C.
also
was
found
derivatives
75(3)0
Hydrogen
band
the
the
in
and
from
bonding
hydroxyl
anti.
thus
hydrogen
compact.
H
F
H --+ H/-vH
4--
H'}(
H
(ref.
established
shown
angles
80%
for
angle
had
distances
dihedral
The
atom
solutians
31.4(80)%
is not
band
again
makes
3).
electron-diffraction
2)
More
more
the
using
these
hydrogen
also
while
the
and
6)
It ~as
gas
while
in skew
band
the
dilute
(ref.
work,
is a very
150
3-buten-l-ol
conformation
total.
conformation.
the
viz.
of
most
of
predominate.
hydroxyl
band.
of
Furthermore,
(ref.
bonding.
In each
to
investigation
third
the
identified.
syn.
between
a hydrogen
additional
hydrogen
found
investigations
rotamer.
predominating
from
with
exists
68.6(80)%
predominates
were
been
and
investigated
l).
(CHZOHCHFZ) (ref.
investigation
forms
been
(ref.
simplest
the
up
10%
obtained
conformation
further
an
the
Z,Z-difluoroethanol
has
makes
have
derivatives,
diffraction
stable
that
microwave
bonded
form
exceeding
angles
The
electron
has
ethanol
IR-spectroscopic
less
demonstrated
consentrations
molecules
are
band
conformation
this
to
and
a hydrogen
band.
as
Z)
two
acceptor
spectroscopy
conformation
studied
(CHZOHCHZCH=CHZ) (ref.
15 such
microwave
a hydrogen-bonded
Recently,
In
about
and/or
a proton
I /"F
FF
OH
OH
Il
Fig.
l.
Possible
conformations
of
makes up at least 95% of the gas.
CHFZCHZOH
~ ,
possessing
hydrogen
bonds.
Rotamer
I
305
Two rotamers with intramolecular
hydrogen bonds are possible in the case of
2,2-difluoroethanol.
They are shown in Fig. l. Further forms are of course
possible, but presumed to be of much higher energy than I or Il, since the
interaction
between the fluorine atom and the hydroxyl group is remarkably
strong as demonstrated in the electron-diffraction
work of the closely related
molecule 2-fluoroethanol
(ref. 8).
Only conformation I was found (ref. 3). This rotamer is more stable than Il
by at least 6 kJjmol. No indication of the presence of further non-hydrogen
bonded conformations was seen in the microwave spectrum.
Two conclusions may be drawn from these findings.
Firstly,
there are hardly
steric reasons for the preference of I as compared to Il as the O...F distance
of the fluorine atom not engaged in hydrogen bonding of the hypothetical
conformation Il is presumably longer than the sum of the van der Waals radii of
fluorine and oxygen atoms. Secondly, there is no attractive
interaction
between
these two electronegative
atoms. Attraction might exist between two strongly
electronegative
repulsion
fluorine atoms as exemplified by CH2FCH2F
(ref. 9,10), while
is apparently
of the hypothetical
present
conformation
between fluorine
and oxygen atoms in the case
Il of CHF2CH20H.
CH3CHYCH2X-type
molecules
Propane derivatives
of the CH3CHYCH2X-type
having one proton accepting and
one proton donating group may have the heavy-atom hydrogen-bonded conformations
of Fig. 2.
CH3.
H-
H
Å/H
H
..-1-..
_H
'-I
-CH3
.
H/'.--/"
Y
X
Y'
X
Il
Fig. 2. Possible
type molecules.
heavy-atom hydrogen-bonded conformations
of CH3CHYCH2X-
306
This kind of equilibrium exists for a large number of molecules. Five such
compounds have recently been studied successfully
by microwave spectroscopy.
Some important results are summarized in Table l.
TABLE l
Results
x
for
y
some CH3CHYCH2X-type molecules.
"'Ho
kJ /mol
OH
NH2
NH2 OH
OH OH
F
OH
OH F
2.0
4
2.4
3
3
{Y-C-C-X
Degrees
54(2) for I; 61.5(2)
54
58.4(10)
59(2)
65(2)
It is seen from this
and that
table
Comment
Ref.
Il not found
11
12
for Il
for I; 53.0(10)
for Il
I I not found
Il not found
that conformation
the heavy-atom dihedral
I predominates
angles are generally
13
14
15
in all cases,
close to 600. The hydroxyl
group is proton donor in both CH3CH(NH2)CH20H
and in CH3CH(OH)CH2NH2.
No signs
of the reverse situation
has been found in any microwave study of molecules
carrying
amino and hydroxyl groups on adjacent
carbon atoms (ref.
11,12,16,17).
The fact that the hypothetical
conformations Il of CH3CHFCH20H
and
CH3CH(OH)CH2F
are at least 3 kJ/mol less stable than I, comes rather surprisingly. Apart from hydrogen-bonding attraction
between the hydroxyl group and
fluorine atom, one might expect additional
stabilisation
between the hydroxyl
group and the methyl group in CH3CHFCH20H,
or between the fluorine atom and the
methyl group in CH3CH(OH)CH2F.
This should lead to a quite stable Il conformation
in both these cases contrary
Both CH3CH(NH2)CH20H
(ref.
energy differences
between I
has I at least 4 kJ/mol more
because weak repulsion might
hydrogen atoms in the latter
to the microwave findings
11) and CH3CH(OH)CH20H
(ref.
(ref.
14,15).
13) have small
and Il as expected. The fact that CH3CH(OH)CH2NH2
stable than the hypothetical
Il form was expected,
exist between the methyl group and the amino group
conformation.
Amines
The amino group may act as proton donor and thereby
form hydrogen bonds.
These interactions
are generally weaker than the corresponding bonds formed by
alcohols. Both hydrogen atoms of the amino group may be used for hydrogen bond
formation. In CH2XCH2NH2-typemolecules the hydrogen-bonded heavy-atom gauche
conformations denoted I and Il and shown in Fig. 3 may exist. In addition,
heavy-atom anti conformations not possessing internal hydrogen bonds are also
307
H
/
Ho...oo. 'o
'0
d'-\
H
V
-
P
"-\
I
rr".,
r
I
H,
,
'.
".
\/l
"l-\
H
H
H
Il
Figo
Possible
30
possibleo
hydrogen-bonded eonformations
The energy differenees
of CH2XCH2NH2-typeo
between heavy-atom gauehe and anti
forms are
generally expeeted to be smaller for amines than for the eorresponding
be~ause amines tend to form weaker hydrogen bondso
Four CH2XCH2NH2-type
eompounds have now been studied.
them are summarized in Table 2.
aleohols
Important findings
for
TABLE2
Results
x
for some CH2XCH2NH2-type
moleeules.
IIHo
LX-C-C-N
Degrees
kJ/mol
NH2
F
OCH3
CN
103(8)
0.4(12)
63(2)
64(2)
for I and Il
for
I;
63(2)
for
-702
61.5(12) for IIb
Oe
e
Il
LN-C-C
Degrees
Ref.
109(1) for I; 111.5(10) for Ila
110(1) for I; 114.5(10) for Il
112.2(12) for Il
e
18
19
20
21
a
bAverage values.
I not found
~Preliminary result.
Not yet determined.
o
As shown in Table 2, both I and Il has been found for CH2NH2CH2NH2'
CH2FCH2NH2'
and CH2CNCH2NH2
using mierowavespeetroseopy, and they have approximately the same energy. In eontrast,
eonformation I of CH2NH2CH20CH3
was not
identified
and estimated to be 7.2 kJ/mol less stable than Ilo It is diffieult
to explain this unexpeeted behaviour of this maleeule.
308
Heavy-atom anti
conformations
of Table 2 and not found.
were searched for
A high-temperature
CH2NH2CH2NH2
revealed no anti conformation
gauche conformations
are thus very stable.
indicate
stable
that
weak hydrogen bonds exist
gauche conformations
Some additional
Il.
effect
This additional
force
of the nitrogen nucleii
also prefer very stable
It
groups
is also
in both forms
be explained
as both CH2FCH2F(ref.
gauche conformations.
to note that
18).
The very
by hydrogen bonding alone.
hydrogen bonds of I and
with
9,10)
the electronegativity
and CH20HCH2F(ref.
by 3-50 (ref.
rotameric
Four hydrogen-bonded
situation
8)
donating
18,19).
exists
conformations
than in the
shown in Fig.
4 are
H
'H
H
/
H-N
molecules
study of
the N-C-C angle of the proton
typically
In CHF2CH2NH2a more complicated
the four
(ref.
augments the internal
in I than in Il
CH2XCH2NH2molecules.
all
(ref.
22). The hydrogen-bonded
The geometri es of these two rotamers
has perhaps some connection
interesting
is smaller
can hardly
presumably
for
electron-diffraction
\
!/
..l\
HH
1/
F
N
li F
\
V
,/-\
F
,
Il
H
il
/
rF
N
\c-c Y
1 \
HH
")-(
H
ill
Fig.
4. Possible
IV
hydrogen-bonded
conformations
of CHF2CH2NH2'
possible for CHF2CH2NH2'In addition,
a rotameric form without
may exist.
Conformations I, H, and III were assigned for this
microwave spectroscopy
(ref.
23).
Conformation
IV as well
as the rotamer with-
out a hydrogen bond were not seen and presumed to be at least
stable
than any one of I,
Conformation
three.
less
III
than III
about 600 in I and Il,
stable
by 1.5(7)
2 kJ/mol
less
or Ill.
which contains
Rotamer I is less
stable
Il,
a hydrogen bond
molecule using
two hydrogen bonds is
than III
kJ/mol
by 1.0(7)
(ref.
23).
the most stable
kJ/mol,
and rotamer
The N-C-C-F dihedral
and the N-C-C angle is 50 larger
in Il
and III
of the
Il
is
angles are
than in I.
309
Thiols
The mercapto group is generally considered to be a very weak proton donor.
Only few gas phase studies have been made for thiols capable of forming hydrogen
bonds. Recently, CH20HCH2SH
was investigated
(ref. 24). In this case, one
conformation was assigned and the mercapto group found to be proton acceptor
with the hydroxyl group as proton donor. In CH2SHCH2Cl(ref. 25) only a heavyatom anti form without a hydrogen bond, was seen. Two heavy-atom gauche forms
~,
were found for CH2SHCH2NH2;
the high-energy form of these has an internal
hydrogen bond with the mercapto group as proton donor.
In CH2SHCH2CN
the existence of both the hydrogen-bonded gauche conformation
of Fig. 5 as well as the anti forms were expected. Anti Il and the hydrogen-
"
H
H
S-H
H
I
~,
/
N
/
~..
IC-C""
C
IC -
C
I
\-'"
N
ANTI l
H
/
\"'H
H
ANTI II
N
cl
\
!/H
/IC-\,
H
H
GAUCHE
Fig. 5. Rotameric forms of CH2SHCH2CN.
bonded gauche conformation
stable
were assigned
(ref.
by 2(2) kJ/mol. The heavy-atom dihedral
26). The latter
is the more
angle has a normal value of
65(3)0. Anti I was not found and estimated to be at least 3 kJ/mol less stable
than the hydrogen-bonded gauche conformation. Thiols seem to prefer the gauche
form for the H-S-C-C dihedral angle (ref. 27,28), and the finding that Anti Il
is more stable than Anti I by at least l kJ/mol is in keeping with this trend.
310
ACKNOWLEDGEMENT
I am most grateful
constructional
Research
to my co-worker
Cand.Real.
work and pleasent cooperation
Council
K.-M. Marstokk for his skillful
through many years. The Norwegian
for Science and the Humanities
is thanked for financial
support.
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