Asian Chemisfry Letter,l'
Vol 7, Nas 2&3 (2003) 61-68
The Role of Intramolecular
Hydrogen Bonding for the Structural and
Conformational Properties oI- 2-Bicyclopropylidenylmethanol
as Studied by
Microwave Spectroscopy Augmented by Quantum Chemical Calculations
Harald Møllendala', Sergej I. Kozhushkovb and Armin de..)Æeijereb
aDeparfmentof Chemistry,Universityof Oslo,P. O,Box
1033Blindern,NO-0315Oslo,Norway
hInstitutfur Organische Chemie der Georg-August-Universitiit, Tammannstrasse 2, DE-37077 Gottingen, Germany
This paper is dedicated to Professor Thorvald Pedersen on the occasion of his retirement
The microwave spectrum of 2-bicyc1opropylidenylmethanol has been studied in the 26-62 GHz spectral range. One
conformer was assigned. This rotamer is stabilized by an intramolecular hydrogen bond. The microwave work has
been assisted by quantum chemical ca\culations at the MP2/6-3Il ++G** level of theory and infTaredspectroscopy.
'-,
<1;>,,~hitaPublicatians. All rights reserved.
"
01 Iiitroduction
:.
A major interest of this laboratory has been the study of intramolecular hydrogen (H) bonding in
isolated molecules.l 2-Bicyclopropylidenylmethanol (BIe) has been chosen for study this time. l'I,ineallstaggered forms are possible for this compound as a result of rotation around the C5-C7 and C7-08
bonds. These rotamers are sketched in Fig. l, and given Roman numerals for reference. Atom numbering
is given on rotamer I.
The forces responsible for the conforrnational make-up of this compound are rather complex.
Intramolecular H bonding is possible because the cyclopropyl group possesses so-called pseudo-n
electrons according to the Walsh bonding model of cyclopropane.2 These electrons which are found on
the outside of the cyclopropyl ring,2 may act as proton acceptors in H bonds.3 Pseudo-n electrons that
may participate in H bonding in BIC are found on the outside of the C4-C5 and C5-C6 bonds. H bonding
is thus possible in conformers I, Vand VIII and may in fact be a decisive factor for the conforrnational
preferences.
,
There are interesting differences between the H bonds in these three rotamers. In I and V H
bonding with the pseudo-n. electrons on the outside of ~~.:ring may take place, In rotamer VIII these
electrons are approached from the inside of the cyclopropyl' ring.'.This is of course less favorable than in
the previous two cases. Moreover, in conformers I",and, VIII :th'epseudo-n. electrons along the C5-C6
edge of ring may get some additional electron density fi:~m:the' n electrons of the C3=C6 double bond,
whereas only the pseudo- n electrons along the C4-C5 side of .the nng are available for H bonding in
conformer V.
BIC thus offers a unique possibility of investigating what conforrnational consequences these
differences in electronic environment of the H bond may result in. This interesting problem was the main
motivation for' carrying out the present research using MW spectroscopy and advanced quantum
chemical calculations as the methods of study.
2 Experimental section
The sample of BIC utilized in this work was synthesized and purified according to the pu'ulished
procedure.4 The MW spectrurn was studied using the Oslo Stark spectrometer which has been described
'Corresponding author, E-mail address:harald.mollendal@kjemi.uio.no
62
Harald Møllendal. Serge) l Kozhushkov and Amlin de Me(jere
briefly in Ref. 5. The 26 - 62 GHz spectral region was investigated. Radio frequency microwave
frequency double resonance (RFMWDR) experiments were made as described by Wordarczyk and
Wilson7 using the same equipment as described in Ref. 8. The spectrum was measured at room
temperature,or withthe microwaveabsorptioncell cooled
.
,>'
HI7
HI!
I 0.0 kl/mol
III
Il
7.43 kl/mol
~
7.71 kl/mol
IV 5.70 kl/mol
~H:
~ K
V 1.74 kl/mol
VII
12.36 kl/mol
VI 5.75 kl/mol
x
IX
VIII 2.58 kl/mol
] 1.28 kl/mol
Fig. I. The nine all-staggered confonnations of Ble. Atom numbering is given for confonner I. The MP2/63 I I++G** energies relative to I are indicated for each rotamer.
to about O °C. Lower temperatures which would have enhanced the intensity of the spectrum
could not be employed owing to insufficient vapor pressure of the compound. The pressure was 4 - 10 Pa
when the spectra were recorded and stored electronically using the computer programs written by Waal.8
The spectrum was very weak. The accuracy of the spectral measurements is therefore no better than
::!:0.12 MHz.
The Ro/e of'/lltral1lo/ecu/ar Hvdrogell BO/ulillgfor the Structural alld COllformatiolla/ .....
63
The IR spectrum was taken at room temperature at a pressure ofroughly 30 Pa using a Bruker IFS
88 spectrometer. The celliength was 120 m and the resolution was l cm-l. The details of the cell are
described in Ref 9.
3 Ab initio calculations
The Gaussian 98 program packagelOrunning on the IBM RS6000 cluster in Oslo was employed in
the quantum chemical calculations. One reason for carrying out such calculations is that the rotational
constants predicted in these calculations are useful starting points in the analysis of the MW spectrum. It
has beenpointedoue l that accuratestructuresand rotationalconstantsare obtainedusing a large basis
set in Møller-Plesset'2 second order perturbation calculations. The rather large 6-311++G** basis set
provided with the program was used in the present case. Full geometry optimization was carried out for
all nine conformers shown in Fig. l. The structures of all nine rotamers calculated in this mann,=r
appeared to be "normal".
Table l. MP2/6-311++G** structure of conformer I of BIC
Distance/pm
CI-C1
CI-C3
CI-H9
CI-HIO
C2-C3
C2-HII
C2-H 12
C3-C6
C4-CS
C4-C6
C4-H 13
C4-H14
CS-C6
CS-C7
CS-CIS
C7-08
C7-H16
C7-H17
08-H18
IS4.S
147.S
108.7
108.7
147.1
108.7
108.7
131.7
IS4.S
147.3
108.8
108.7
147.3
ISO.9
108.9
142.4
110.0
109.3
96.3
Angle/deg
C2-CI-H9
C2-CI-HIO
C3-C l-H9
C3-CI-HI0
H9-CI-HIO
CI-C2-Hll
CI-C2-H 12
C2-C2-H 11
C3-C2-H 12
H II-C2-H 12
C I-C3-C6
C2-C3-C6
CS-C4-H 13
CS-C4-HI4
C6-C4- H 13
C6-C4-HI4
HI3-C4-HI4
C4-CS-C7
C4-CS-HIS
C6-CS-C7
C6-CS-H1S
C7-CS-HIS
CS-C6-C4
C3-C6-CS
CS-C7-08
CS-C7-HI6
CS-C7-Hl7
OS-C7-HI6
OS-C7-HI7
C7-0S-HI8
116.9
117.3
118.8
118.4
IIS.3
117.2
117.2
118.3
118.S
IlS.3
148.0
148.7
117.2
117.2
118.4
118.S'
IIS.3
119.1
116.S
117.6
117.S
IIS.7
149.0
147.7
111.6
108.S
111.1
110.9
IOS.9
10S.7
Dihedral angle/deg
H9-CI-C2-HII
143.2
-O.S
H9-CI-C2-HI2
-0.1
Hl0-CI-C2-HII
H lO-C] -C2-H 12
-143.7
H9-CI-C3-C6
-76.S
71.7
H lO-C]-C3-C6
-71.7
HI ]-C2-C3-H6
H12-C2-C3-H6
76.0
C I-C3-C6-C4
179.3
S.S
CI.C3-C6-CS
-4.2
C2-C3-C6-C4
-178.0
C2-C3-C6-CS
-2.0
H 13-C4-CS-C7
144.4
H 13-C4-CS-HIS
H14-C4-CS-C7
-14S.S
H14-C4-CS-HIS
0.9
H 13-C4-C6-C3
-70.2
77.7
H 14-C4-C6-C3
67.S
C7-CS-C6-C3
H IS-CS-C6-C3
-77.9
C4-CS-C7-08
-146.0
-23.3
C4-CS-C7-HI6
96.1
C4-CS-C7-HI7
-78.7
C6-CS-C7-08
44.0
C6-CS-C7-HI6
164.4
C6-CS-C7-HI7
67.4
H1S-CS-C7-0S
HIS-CS-C7-HI6
-169.9
-SO.6
H1S CS-C7-HI7
S2.1
CS-C7-08-HI8
-69.4
H 16-C6-08-H IS
173.1
HI 7-C7 -OS- H! S
64
Harald M øliendal. Serge} l Kozhushkov and Armin de Meijere
in these calculations are useful starting points in the analysis of the MW spectrum. It has been
pointed outll that accurate structures and rotational constants are obtained using a large basis set in
Møller-Plesseez second order perturbation calculations. The rather large 6-311++G** basis set provided
with the program was used in the present case. Full geometry optimization was carried out for all nin~
conformers shown in Fig. I. The structures of all nine rotamers calculated in this manner appeared to be
"normal".
The MP2/6"'}11++G** structure of conformer I whose MW spectrum was assigned, is given in
Table l. The conformation-sensitive dihedral angles of all nine rotamers are given in Table 2, while the
predicted rotational constants and components of the dipole moments along the principal inertial axes are
given in Table 3.
Rotamer I was calculated to have the lowest energy being -910859.61 kl/mol. The calculated
energy differences between this rotamer and the other eight forms are shown on Fig. I. Conformer I is
arbitrarily assigned a relative energy of O kl/mol here. It is expected that ~~se energy differences are
.
correct to within better than :t4 kl/mol.
Interestingly, the three forms I, Vand VIII that are stabilized by internal H bonding, are
calculated to have the lowest energies. I is preferred whereas Vand VIII are predicted to be 1.74 and
2.58 kl/mol, respectively, less stable than I.
Table 2. The conformational sensitive dihedral angles of the nine rotameric forms of BIC
Conformer
I
Il
III
IV
V
VI
VII
VIII
IX
C6-C5-C7-08a
-78.7
-79.7
-83.1
77.9
83.4
81.0
37.5
30.9
36.3
C5-C7-08-B 18a
52.1
-56.8
-174.5
58.5
-52.1
174.4
69.2
-55.2
-177.0
aDihedral angle/deg.
Calculation of vibrational frequencies is rather demanding in the MP2 procedure and was only
carried out for conformer I. The much less expensive density functional theory (DFT) calculations using
the B3LYP procedure\3.14with the 311++G** basis set were carried out for some, but not all, of the nine
conformers. As expected no negative vibrational frequencies were obtained. This is an indication that the
conformers indeed represent energy minima on the conformational energy hypersurface. 15
Moreover, the harmonic force tie1d available for conformer I was used to ca1culate the Watson's Areduction quartic centrifugal distortion constantsl6 employing the ASYM 40 program. 17.18.
The following
values were obtained.for the centrifugal distortion constants: d) = 0.695, d)K = -2.140, dK = 4.340, D)=
0.219 and DK= 1.180 kHz, 'respectively.
4 Microwave spectrum and assignments
Survey spectra reveal at very weak and relatively dense MW spectrum. The peak absorption
coefficients of the strongest lines were approximately 6 x 10-8cm-I. This is not surprising, because the
The Role of lntramolecular
Hydrogen Bonding for the Structural and Conformational
65
.....
rotational constants are qui~e small and the dipole moment is not very large for any one of the nine
rotamers (Table 2). Moreover, each conformer has severallow-ftequency vibrational fundamentais. The
MP2 calculations carried out for rotamer I yielded nine vibrational ftequencies below 500 cm-l. The
partition function of each rotameric form is thus comparatively large under the temperature conditions
employed in the present work rendering a low population in each quantum state. It is also quite possible
that forms other than the one assigned here may be present in considerable concentrations, because the
energy differences between them is predicted to be relatively small (Fig. l). All tbis lead to the
remarkably weak MW spectrum that was observed.
Table 3. Rotational constants and dipole moment components o( the possible confonners BIC as ca\culated
at the MP/6-311++G** level
Confonner A/MHz
I
Il
III
IV
V
VI
VII
VIII
IX
3318.3
3339.1
3376.2
5425.8
5466.5
5492.7
3794.0
3779.5
3863.1
B/MHz
1343.4
1299.6
1301.0
963.8
964.3
967.0
1198.2
1238.5
1206.4
C/MHz
1023.3
1002.6
1003.9
872.2
870.4
873.5
1081.0
1105.5
1093.8
!-liD
1.62
-0.09
-1.07
-1.35
-1.87
-0.70
-0.48
-1.41
1.24
!-It/D
1.51
1.33
0.59
-0.32
I.I2
-1.49
1.46
0.42
-0.91
!-I/D
0.29
-1.20
1.16
1.50
-0.29
-0.78
-0.63
1.40
1.07
It was predicted in the theoretical work above that rotamer I should be the most stable fonn of the molecule.
Searches for this confonner were then made using the MP2 rotational constants as the starting point. The a-type Rbranch spectrum was first searched for since !-lais predicted to be the largest dipole moment component (Table 2). The
high-Kl pair of lines is modulated at just a few V/cm and these transitions were the key to the assignment of this
spectrum. The assigned frequencies were confinned by their fit to a rigid rotor using Watson's A-reduction Trepresentation Hamiltonian,16as well as by RFMWDR experiments in several cases. Both the MP2 and the B3LYP
ca\culations predict a relatively large !-lb'However, no b-type transitions could be assigned with certainty presumably
because they are too weak. Searches for c-type lines were also futile. A total of 85 transitions were used to detennine the
spectroscopic constants shown in Table 4. The full spectrum is available fonn HM upon request.
Table 4. Spectroscopic constantsa of the ground vibrational state ofBIC
A/MHz
B/MHz
C/MHz
LljkHz
LlJK/kHZ
LlK/kHZ
ojkHz
OK/kHZ
No..of transitions:
Root-mean-sguare dev.:
3311.71(18)
1350.364(14)
1027. I 27(1 8)
0.7214(49)
-2.204(13)
4o340b
0.244(12)
1.180b
85
)>.14 MHz
aA-reduction T-representation.16
bQuartic constant preset at the value obtained from the harmonic force field.
66
Harald Møllendal, Serge) l Kozhushkov and Armin de Mei)ere
It was found in the fitting proeedure that significant values could only be detennined for three of
the quartic centrifugal distortion constants, namely !>..J,
!>..JK
and 5J. The two remaining constants !>..K
and
5Kwere kept constant at the values obtained from the harmonic force field in the final tit. Interestingly,
the experimental values of !>..J,
!>..JK
;md 5J obtained in this manner (Table 4) are in quite good agreement
with the ones obtained from the harmonic force field obtaind in the ab initio calculations (see above).
Attempts to assign vibrationally excited states of this rotamer failed owing to its weak spectrum.
It is seen in Table 3 that conformations I, Il and III have rather similar rotational constants that
are quite different from the rotational constants of the six other rotamers. The rotational constants in
Table 4 deviate from the predicted MP2 values of I (Tabl~ 3) by just a few MHz. The deviations are
slightly larger in the cases of Il ahd Ill, but not so large that these two altematives can be excluded on
this basis alone. Fortunately, other criteria are available. The faet that the MP2 calculations above
indicate that I is more stable than Il by as much as 7.43 kl/mol (Fig. l), and more stable than III (by
7.71 kl/mol) is one indication that I has indeed been assigned and not confused with any other rotamer.
In addition, conformation Il is predicted to have a small Ila and henee not the type of spectrurn that was
observed, whereas III should have a predominating c-type spectrurn in contrast to the a-type spectrum
that was observed.
A third indieation comes from the infrared spectrurn in the O-H stretching region shown in Fig. 2.
This spectrum is weak owing to the low vapor pressure (roughly 30 Pa at room tempereature) of BIC.
There is also some interference from water vapor in this speetrurn presurnably because the sample
contained a small amount of water. The appearance of the O-H stretching band is typieal of compounds
possessing weak internal H bonds. The major peak ofthis eornplex band falls at about 3638 cm-I. There
are other maxima around 3650 and 3615 cm-l. These peaks are red-shifted eompared to methanol whose
gas-phase O-H stretching frequency is loeated at 3682 em-I.19
Extensive searehes including RFMWDR were next carried out in an atternpt to fmd VandIor
VIII, but no assignments could be made. It is thus assumed that rotamer I is the most stable rotameric
form ofBIC in agreement with the MP2 calculations.
1.0
Q)
(.)
c:
ro
:I:'
'E
'"
c:
~
f-
0.8
3680
3640
3600
Wavenumber
lem"
3560
Fig. 2. Gas-phase infrared spectrum ofBIC in the O-H stretching region
The Role of Intramolecular Hydrogen Bonding jCJrthe Structural and Conformational
.....
67
The very good agreement between the observed (Table 4) and calculated (Table 3) rotational
constants is one indication that the MP2 structure in Table l is indeed close to the "true" structure, in
accordance with the finding of Helgaker et al. 11
5 Discussion
It is not surprising that BIC prefers a conformation that is stabilized by an internal H bond
because this has beenfoundfor severalotherrelatedcyclopropylderivatives.1,3
In rotamerI, the distancesbetween
HI8 andC5 and between H18 and C6 as calculated from the structure in Table l, are 250 pm and 280 pm,
respectively. This is less than the sum of the van der Waals radii of H (120 pm) and aromatic carbon
(170 pm) thus being 290 pm.20
The orientation of H18 relative to the carbon atoms of the cyclopropyl ring is obviously of
importance. This atom is so close (280 pm) to C6 in I that the 1telectrons of the C3=C6 double bond may
contribute electron density to the H bond in addition to the pseudo-1telectrons along the edge of the ring
on its outside. The electronic environment of the H bond in rotamer V is sirnilar, but there are no
supporting 1telectrons from the double bond. This may explain why I is more stable than V by about 1.7
kl/mol.
Conditions for H bond interaction with the 1t electrons of the double bond are best in VIII,
because the distance berNeen H18 and C6 is 250 pm in this conformer, as calculated form the MP2
structure (not given in Table l). However, there is little interaction with the pseudo-1t electrons here
because they are concentrated on the outside of the ring, making this rotamer less stable than I by about
2.6 kl/mol.
A rough value for the strength of the H bond in I can be inferred form the energy difference
between this rotamer and conformers Il and III that are without this interaction. Il and III are both about
7.5 klimolless stable than I and this is taken to be an approximate value for the H bond strength in I.
Acknowledgement
Anne Horn and Karl-Magnus Marstokk are thanked for their expert help and assistance. Professor
Claus J. Nielsen is thanked for obtaining the infrared spectrum. This work has received support from the
Research Council of Norway (Prograrnme for Supercomputing) through a grant of computer time.
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[Reeeived 6.2.2003 ]
.:.