Inter- and intramolecular O-H···π hydrogen bonding in the methanol-ethene complex and syn-7-norbornenol, probed by IR, 1H NMR and quantum chemistry

Article abstract of DOI:10.1016/S0022-2860(01)00563-4

The geometries and energies of syn-7-norbornenol (1) have been investigated with theoretical and experimental methods, while its epimer anti-7-norbornenol (2) has been investigated by theoretical methods only. It was found that 1 is intramolecularly hydrogen bonded and exists almost exclusively in its hydrogen bonded form, the Anti conformer. Compound 2, for which intramolecular hydrogen bonding is impossible, was found to exist as the Gauche conformer. A theoretical investigation of the model complex between methanol and ethene showed that the interaction energy is -3.1 ± 0.1 kcal mol^-1 (estimated Hartree-Fock basis set limit plus correlation contribution, not including vibrational zero point energy). Electron correlation is essential in the evaluation of the interaction energy and also for the geometry of the complex. It was found that the potential energy surface around the minimum energy structure is flat, with an almost freely rotating methanol part.

Full text of DOI:10.1016/S0022-2860(01)00563-4

Journal of Molecular Structure 567±568 32001) ꢀ19±ꢀꢀ8
www.elsevier.nl/locate/molstruc
Inter- and intramolecular O±H´´´p hydrogen bonding in the
methanol±ethene complex and syn-7-norbornenol, probed by IR,
1
q
H NMR and quantum chemistry
a,
Leif H. Bjerkeseth , Jan M. Bakke , Einar Uggerud
a
b
*
a
Organic Chemistry Laboratories, Norwegian University of Science and Technology, N-7491Trondheim, Norway
b
Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway
Received 6 July 2000; accepted 19 September 2000
Abstract
The geometriesand energiesof syn-7-norbornenol 31) have been investigated with theoretical and experimental methods,
while itsepimer anti-7-norbornenol 32) hasbeen inve st igated by theoretical methodsonly. It wasfound that 1 isintramole-
cularly hydrogen bonded and exists almost exclusively in its hydrogen bonded form, the Anti conformer. Compound 2, for
which intramolecular hydrogen bonding is impossible, was found to exist as the Gauche conformer. A theoretical investigation
2
1
of the model complex between methanol and ethene showed that the interaction energy is 2ꢀ.1 ^ 0.1 kcal mol 3estimated
Hartree±Fock basis set limit plus correlation contribution, not including vibrational zero point energy). Electron correlation is
essential in the evaluation of the interaction energy and also for the geometry of the complex. It was found that the potential
energy surface around the minimum energy structure is ¯at, with an almost freely rotating methanol part. q 2001 Elsevier
Science B.V. All rightsre se rved.
Keywords: Syn-7-Norbornenol; Methanol±ethene complex; Intermolecular hydrogen bonding; Intramolecular hydrogen bonding; Hydrogen
bonding
1
. Introduction
[2], in which the hydrogen bond wascon si dered to be
mainly an electrostatic 3`ionic') interaction. A broader
de®nition waslater given by Pimentel and McClellan
The idea of the classical hydrogen bond was ®rst
0
0
suggested 80 years ago by Latimer and Rodebush [1].
However, the phenomenon did not really reach
common acceptance until Pauling in 19ꢀ9 published
the ®rst edition of `The Nature of the Chemical Bond'
in 1960 [ꢀ], expanding it to A±H´´´A where A and A
not necessarily are electronegative atoms [4].
0
Hydrogen bondswhere A or A are ionic are among
the strongest, while hydrogen bonds of the type X±
H´´´A, where the acceptor A isan electronegative
atom and X ise.g. C, N or O, are termed normal or
weak [5]. At the end of the 1950sand in the 1960 s, p-
systems like a carbon±carbon double or triple bond or
an aromatic ring also became recognised as hydrogen
bond acceptors 3non-classical hydrogen bond) [6,7].
In recent years, the observation of X±H´´´p hydrogen
bondshasbecome rather frequent [5].
q
Dedicated to Professor Marit Trñtteberg on the occasion of her
7
0th birthday.
Corresponding author. Present address: Forsvarets forsknings-
*
institutt, Norwegian Defence Research Establishment, Division for
Protection and Materiel, P.O. Box 25, N-2027 Kjeller, Norway.
Tel.: 147-6ꢀ-80-78-97; fax: 147-6ꢀ-80-75-09.
E-mail address: Leif-Haldor.Bjerkeseth@f®.no
3L.H. Bjerkeseth).
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-2860301)0056ꢀ-4

ꢀ20
L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
Table 1
IR spectroscopic data for syn-7-norbornenol 31) and anti-7-norbornenol 32) in the OH stretch region. All spectra recorded at high dilution in
CCl 3tetrachloroethylene for Ref. [ꢀ0], solvent not given for Ref. [27]). Numbers in parentheses are band half widths
4
2
1
a
Band areas /3%)
Frequencies/cm
Ref.
n
1
n
2
n
ꢀ
n
1
n
2
n
ꢀ
Syn-7-Norbornenol 31)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
572
[27]
624
ꢀ574
[28,29]
[ꢀ0]
[ꢀ1]
575
628 3ꢀ1)
628 325)
628 325)
ꢀ575 322)
b
ꢀ590 318)
18
18
20
82
b
b
ꢀ575 317)
9
7ꢀ
Present study
ꢀ575 317)
80
Present study, corrected
Anti-7-Norbornenol 32)
ꢀ
628
6ꢀ0
[27]
[28,29]
[ꢀ1]
ꢀ
ꢀ
6ꢀ2 327)
a
Integrated absorptions.
b
4
88% pure 1. Recorded at c , 5 mM in CCl . Band 2 wascau se d by an unidenti®ed impurity.
We have previously studied the conformation of
allylic syn-7-norbornenol 31) isideal. In 1, the
hydroxyl group is, from visual inspection of
models, in constant close proximity to the double
bond and has a unique geometrical possibility for
an intramolecular hydrogen bond. ItsOH-epimer
anti-7-norbornenol 32) will be used as a reference
since this cannot form an O±H´´´p bond.
several unsaturated alcohols and other compounds
with a geometrical possibility for an intramolecular
O±H´´´p or O±H´´´F hydrogen bond, and have
discussed the importance of intramolecular hydrogen
bonding [ꢀ±10] for the conformational composition
for such compounds [11±17]. The investigations were
performed by a combination of theoretical calcula-
1
tionsand IR and H NMR spectroscopy. The combi-
To our knowledge, no information about a 1:1
methanol±ethene complex hasbeen publi hs ed until
now. However, several papers describing the analo-
gous1:1 water±ethene complex [16,19±26] have
been published.
nation of experiment and theory is necessary to fully
understand the conformational behaviour in mole-
culeslike the se .
Hydrogen bonded complexesare fundamentally
important for understanding the nature of the
hydrogen bond phenomenon [18]. The geometry/
directionality of the hydrogen bond isl es sw ell
de®ned for the O±H´´´p hydrogen bond than for
stronger hydrogen bonds. We, therefore, considered
it to be of interest to investigate this point from both a
theoretical and experimental point of view:
Compounds 1 and 2 have been the subject of a
1
number of investigations by IR [27±ꢀ2], H NMR
[ꢀ2,ꢀꢀ], MS 3EI ionisation energy) [ꢀ4], photoelectron
spectroscopy [ꢀ0], CNDO/2 [ꢀ5] and ab initio mole-
cular orbital calculations3HF/STO-ꢀG) [ꢀ6]. The
question of intramolecular O±H´´´p hydrogen
bonding [ꢀ±10] has been central to these studies.
Only one band wasreported for 2 in the hydroxyl
stretch region in the IR spectrum [27±29,ꢀ1,ꢀ2]
while both one [27,ꢀ0,ꢀ2] and two [28,29,ꢀ1] bands
3bands1 and 2) have been reported for 1 3Table 1).
However, two bandswere reported for 1 in all the
papers focusing primarily on infrared spectroscopy
[28,29,ꢀ1]. The band for 2 had approximately the
same frequency as band 1 for 1 while band 2 for 1
By studying the intermolecular case, where the
hydroxyl group iscompletely free to move relative
to the double bond. For thispurpoes the 1:1
methanol±ethene complex wascho se n.
By studying the intramolecular case, where the
hydroxyl group hasthe opportunity to interact
with the double bond. For thispurpo se the homo-

L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
ꢀ21
3. Experimental
3.1. Materials
Syn-7-Norbornenol 31) prepared according to the
procedure above was recrystallised and sublimed to
8% purity 3GC, DB-1). The most abundant by-
product 3unknown) constituted 4% of the remaining
2%. The spectroscopic data were in accordance with
8
Fig. 1. Synthesis of syn-7-norbornenol 31).
21
1
appeared at a 5ꢀ cm lower frequency, indicating
that 1 wasintramolecularly hydrogen bonded.
those previously reported [ꢀ2,ꢀ8±41].
1
.2. H NMR
3
2
. Synthesis
1
The H NMR spectrum of 1 wasrecorded at high
dilution at room temperature in CCl F on a Bruker
ꢀ
Syn-7-Norbornenol 31) wasprepared from norbor-
AM 500 spectrometer, using a sealed capillary tube
with deuterated benzene for locking purposes.
Chemical shifts are reported as ppm relative to
nene 33) in a two-step synthesis as shown in Fig. 1.
Exo-2-chloro-syn-7-acetoxynorbornane 34) was®rts
prepared by heating norbornene 33) with sodium
acetate and copper 3II) chloride in glacial acetic acid
using a catalytic amount of palladium 3II) chloride as
described by Baird [ꢀ7]. Further reaction with potas-
sium tert-butoxide in DMSO asde cs ribed by Baird
TMS. The CCl F solvent was distilled from freshly
ꢀ
Ê
activated ꢀ A molecular sieves and kept over acti-
Ê
vated ꢀ A molecular sieves in a cold room. The high
dilution CCl F solution for NMR 3c , 5 mM) was
ꢀ
prepared in the NMR tube in the cold room and was
Ê
kept over freshly activated ꢀ A molecular sieves for
[
ꢀ7] gave syn-7-norbornenol 31).
some days before measurement.
1
I,J
H NMR 3500 MHz): d 0.86±0.98 32H, m, H ),
H
1
.55 31H, d, J ˆ 12.40 Hz, H ), 1.64±1.76 32H,
C±H
F,G
m, H ), 2.65±2.70 32H, m, H ), ꢀ.6ꢀ 31H, d,
D,E
C
A,B
JC±H ˆ 12.ꢀ5 Hz, H ), 6.00±6.04 32H, m, H ).
ꢀ
J
C±H
denotesthe
J
H and H . For notation, see Fig. 2.
coupling constant between
H±H
C
H
3.3. IR
The IR spectrum of 1 wasrecorded at high
dilution 3c , 5 mM) at room temperature in CCl4
on a single beam Nicolet 20SXC FT-IR spectro-
meter equipped with a Nicolet 620 Data Station
and a TGS detector using IR quartz cells with
1
0 mm path length 3transparent in the hydroxyl
stretch frequency region in IR), 256 scans and
2
1
2
with dry air. The CCl solvent was re¯uxed over
cm
resolution. The instrument was purged
4
and subsequently distilled from P O under N2
5
2
Ê
atmosphere and kept over activated ꢀ A molecular
sieves in a dry box under N2 atmosphere. The
high dilution CCl solution for IR was prepared
4
Fig. 2. Notationsfor syn-7-norbornenol 31) and Newman projec-
tionsof the C±O rotamersof syn-7-norbornenol 31) and anti-7-
1
norbornenol 32). Conformational composition of 1 from H NMR.
and added to the IR cellsin the dry box, and
Ê
waskept over fr eh sly activated ꢀ A
molecular

ꢀ22
L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
Table 2
Calculated conformer populations3%) for syn-7-norbornenol 31) and anti-7-norbornenol 32)
Method 3full geometry
optimisation)
Syn-7-Norbornenol
Anti-7-Norbornenol
ꢀ
a
Gauche
Anti
J
CH±OH/3Hz)
Gauche
Anti
MMPMI
84
ꢀ5
71
52
52
16
65
ꢀ.85
8.90
5.19
7.14
7.14
97
86
ꢀ
b
MMPMI
14
MMX87
MMX87
29
48
b
MNDO
AM1
PMꢀ
48
88
12
98
±
c
100
100
2
±c
±
c
100
d
HF/STO-ꢀG
HF/ꢀ-21G
pp
12
0
88
11.ꢀ
12.5
12.ꢀ
12.ꢀ
12.ꢀ
12.4
12.4
12.4
e
f
100
96
4
5
g
h
HF/6-ꢀ1G
2
98
98
95
95
pp
i
j
HF/6-ꢀ111G
HF/6-ꢀ111G32d,2p)
pp
MP23FU)/6-ꢀ1G
2
5
k
l
2
98
94
6
m
n
1
99
90
10
9
ppo
p
q
MP23FU)/6-ꢀ111G
1
H NMR
1
99
99
91
r
a
a
r
1
a
Calculated from J ˆ 2.2XGauche 1 12.5XAnti and XGauche 1 XAnti ˆ 1, where X isthe molecular fraction; se e Section 4.1.1.
Charge-charge interaction modus.
No Gauche minimum wasfound.
b
c
d
e
f
The electronic energy is 2ꢀ41.49ꢀ9650 hartrees.
The electronic energy is 2ꢀ4ꢀ.8076868 hartrees.
The electronic energy is 2ꢀ4ꢀ.8069ꢀ9ꢀ hartrees.
The electronic energy is 2ꢀ45.7ꢀ56505 hartrees.
The electronic energy is 2ꢀ45.7ꢀ67080 hartrees.
The electronic energy is 2ꢀ45.7442254 hartrees.
The electronic energy is 2ꢀ45.745464ꢀ hartrees.
The electronic energy is 2ꢀ45.7574ꢀꢀ7 hartrees.
The electronic energy is 2ꢀ45.75841ꢀ9 hartrees.
The electronic energy is 2ꢀ46.92ꢀ5650 hartrees.
The electronic energy is 2ꢀ46.9244697 hartrees.
Optimised without second analytical derivatives.
The electronic energy is 2ꢀ46.9467910 hartrees.
The electronic energy is 2ꢀ46.9481449 hartrees.
Experimental data.
g
h
i
j
k
l
m
n
o
p
q
r
sieves for 24 h before measurement. Overlapping
bandswere re so lved by the focas 2.1 program from
Nicolet using a linear baseline approach. The number
of bands and their positions were assessed by use of
Fourier self deconvolution 3FSD). No constraints
were applied to band positions, band widths, band
intensities and the percent Gaussian:Lorentzian band
shape during the curve-®tting process.
Note: Keeping a dilute solution over activated
molecular sieves will reduce the concentration and
make it impossible to measure absolute values of inte-
grated absorptions. However, relative values are
valid.
3.4. Theoretical calculations
Molecular mechanicscalculationswere performed
on a PC with the programs mmpmi 1.0 [42] or mmx
87.200 using the default dipole±dipole electrostatic
mode, or, where noted, charge±charge interaction
mode. mmpmi consists of mm2 3N.L. Allinger, qcpe
Program no. ꢀ95) and mmp1 3N.L. Allinger, qcpe
Program no. ꢀ18) asimplemented by Serena Soft-
ware, Bloomington, Indiana, USA. mmx 87.200
consists of another implementation of mm2 3qcpe
Program no. ꢀ95) and mmp1 3qcpe Program no.
ꢀ18). Input ®leswere made on a PC u si ng PCMODEL

L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
ꢀ2ꢀ
Tight SCF convergence criteria 3asimplemented in
gaussian) was used for all calculations. Whether
the MP2 calculationswere done with frozen core
3FC) or including the core electrons3FU) isevident
from Tables2 and 6. For all the MP2 calculationswith
Pople's basis sets, the core electrons were included.
Geometries were gradient optimised using the Berny
optimiser and default gaussian convergence criteria.
Analytical second derivatives were computed in the
®
nal stage of all geometry optimisations except where
noted in Tables2 and 6. Analytical se cond derivatives
were also calculated for all hydroxyl stretch frequency
calculations. All vibrational frequencies for all calcu-
lated conformersof the 7-norbornenol es rieswere
positive, con®rming that the stationary points were
all local minima. The calculationson the 7-norbor-
nenol series were done without symmetry restrictions.
The geometriesof the methanol±ethene complex 3 Cs)
and itscomponentsmethanol 3 C ) and ethene 3D )
s
2h
were optimised within the indicated point groups.
The geometriesof the componentsof the complex
were allowed to vary freely. Calculation of the basis
set superposition error 3BSSE) [56,57] were done
using the counterpoise method [58] as described by
Frisch et al. [59]. To estimate the BSSE of the proton
acceptor, we added basis functions for methanol to the
optimised geometry of ethene. For the estimation of
the BSSE of the proton donor, we added basis func-
tionsfor ethene to the optim ei sd geometry of
methanol. The location of the additional functions
wasdetermined from the geometry of the
methanol±ethene complex at the same level of theory.
The conformational compositions were calculated
from the relative energies, using the Boltzmann's law
of distribution at 298 K. For the ab initio molecular
orbital calculations, the compositions were calculated
based on total energy 3electron energy) with no
correction for vibrational zero point energy.
Fig. ꢀ. Three-dimensional view of the ab initio 3MP23FU)/6-
pp
ꢀ
111G ) minimum energy structures of the Gauche and Anti
conformersof syn-7-norbornenol 31) and anti-7-norbornenol 32).
version 1.0 or 4.0 from Serena Software. The indivi-
dual point chargesfor mmpmi in charge±charge mode
were calculated with charge2, version 1.0 [4ꢀ±47]
on a PC.
The semiempirical MNDO, AM1 and PMꢀ mole-
cular orbital calculationswere performed with mopac
6
®
.00 [48] on a Digital Vaxstation ꢀ100 Mꢀ8. Input
leswere made by PCMODEL.
Ab initio molecular orbital calculationswere
performed with gaussian 90, Revision I [49], with
gaussian 92, revision C or G.2 [50], or with gaus-
sian 94, revision B.1 or D.4 [51], on the Cray X-MP/
2
16, Cray Y-MP4D/464, Cray J916/8128 and Cray
TꢀE supercomputers in Trondheim. The calculations
were done using either the Hartree±Fock 3HF)
method or Mùller±Plesset perturbation theory to
second order 3MP2 [52]). A large variety of standard
basis sets implemented in gaussian were used, as
indicated in Section 4 below, using standard notation.
We note especially Dunning's correlation consistent
basis sets with and without augmented functions 3cc-
pVxZ and aug-cc-pVxZ, where x ˆ D, T, Q) [5ꢀ±55].
Input ®leswere made by PCMODEL or by hand.
4
. Results and discussion
4.1. Syn-7-Norbornenol ꢀ1)
We have recently studied the conformational
composition of the simplest homoallylic alcohol, ꢀ-
1
buten-1-ol 35), by IR, H NMR and theoretical calcu-
lations[17]. Thismolecule hasal so been st udied by

ꢀ24
L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
2
1
electron diffraction by Trñtteberg et al. [60] and by
microwave spectroscopy by Mùllendal et al. [61].
Only two conformersof 5 have, by visual inspection
of models, a geometrical possibility for intramole-
cular hydrogen bonding. These two conformers were
denoted Conformer 12 and Conformer 13. Our inves-
literature data of 2 showed only one at ꢀ6ꢀ2 cm .
Our data for 1 was close to the result from the most
recent study [ꢀ1] with the exception that we observed
21
a band at ꢀ590 cm , caused by an unidenti®ed
impurity. When corrected for this ps uriousband,
1
380%),
2
1
hastwo bandsat ꢀ628 320%) and ꢀ575 cm
2
1
tigations hs owed that Conformer
13 wasthe only
5ꢀ cm apart. Thisappearsto be the ®r ts time the
relative areasof the hydroxyl st retch bandshave been
reported as the result of a numerical analysis. The low
hydrogen-bonded conformer and that Conformer 12
washardly populated. One of the modelsu es d for
Conformer 12 wasthe homoallylic alcohol ꢀ-cyclo-
penten-1-ol 36) which wasfound to exi ts predomi-
nately with the OH-group in an axial position.
However, no indication of intramolecular hydrogen
bonding in 6 wasfound. For Conformer 13, epicho-
lesterol 37) wasu se d asa model. 7 wasfound to be
intramolecularly hydrogen bonded.
The ꢀ-cyclopenten-1-ol skeleton can be considered
being built into syn-7-norbornenol 31) 3Fig. ꢀ) and
thusforced into a rigid conformation with the OH-
group in an axial position. From visual inspection of
models, the rigid structure in 1 causes a decrease in
the C±C±C bond anglesin the ꢀ-cyclopenten-1-ol
skeleton, thus decreasing the distances between the
proton acceptor and proton donor functions, and
thereby forcing the hydroxyl group and the double
bond to interact.
2
1
frequency band is21 cm lower than that of ꢀ-buten-
21
1-ol 35, ꢀ596 cm [17]) and 14 cm lower than that
21
2
1
of epicholesterol 37, ꢀ589 cm [17]), which both
were found to be intramolecularly hydrogen bonded.
For comparison, the low frequency band of ꢀ-cyclo-
penten-1-ol 36), which wasfound not to be intramo-
2
1
lecularly hydrogen bonded, absorbed at ꢀ601 cm
[17].
Consequently, band 2 380%) of 1 is assigned to the
Anti conformer with a signi®cant O±H´´´p intramo-
lecular hydrogen bond. Band 1 320%) is then assigned
to the Gauche conformer. The population of the
hydrogen bonded conformer is, however, somewhat
uncertain. It hasbeen reported [ꢀ] that hydrogen
bonding increases the hydroxyl stretch molecular
absorption coef®cient 3band 2). However, this rela-
tionship is not always true in case of intramolecular
hydrogen bonding [10]. Furthermore, it hasal so been
reported that bandsof Type II [62±65] 3 Gauche, band
1) might have a larger hydroxyl stretch molecular
absorption coef®cient than bands of Type III 3Anti,
band 2) due to differences in delocalisation possibili-
ties [6ꢀ]. This situation makes population estimates
from IR rather uncertain.
In syn-7-norbornenol 31) and anti-7-norbornenol
2) 3Fig. ꢀ), the only bond that can be rotated isthe
3
C±O bond. Thus 1 and 2 have only two conformers,
which are de®ned by their C±O rotamersonly. The
rotamer where the OH bond in Newman projectionsof
the C±O bond in secondary alcohols is located
between two R-groupsisdenoted
Anti. The other
21
rotamer isdenoted Gauche, in accordance with the
denotationsintroduced in Ref. [62]. Thusthe confor-
mersof 1 and 2 are denoted Anti and Gauche, where
Gauche hasa multiplicity of 2 3Fig. 2). In the Anti
conformation of 1 the O±H bond is situated almost
directly above the CyC double bond and points
towardsit. Thisisthe conformation which from vi su al
inspection of models has a unique geometrical possi-
bility for an intramolecular hydrogen bond.
The high frequency bandsof both 1 3ꢀ628 cm )
2
and 2 3ꢀ6ꢀ2 cm ) absorbed at approximately the
1
same frequencies close to the standard frequency for
2
1
Type II rotamers3ꢀ627 cm [62±65]). Thisindicates
that 2 exists predominately in its Gauche conformer
since 2 shows only one band.
H NMR spectroscopy gives important information
ꢀ
on the conformational composition, especially the J
1
vicinal coupling constant. The observed coupling
constants are the weighted average of the coupling
constants for each of the three rotamers around each
bond. For the H±C±O±H vicinal coupling constant,
1
.1.1. IR and H NMR spectroscopy
4
The IR data for syn-7-norbornenol 31) and anti-7-
norbornenol 32) are given in Table 1. Literature data
J
1808 ˆ 12.5 Hz hasbeen propo se d when the dihedral
21
of 1 showed two bands at ꢀ628 318%) and ꢀ575 cm
82%) in the hydroxyl stretch region in IR, while
angle H±C±O±H ˆ 1808 and J608 ˆ 2.2 Hz when this
3
dihedral angle equals60 8 [12,1ꢀ,66]. Thisgives

L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
ꢀ25
Table ꢀ
Ab initio calculated hydroxyl group geometriesfor the conformersof syn-7-norbornenol 31) and anti-7-norbornenol 32). The valuesfor the
corresponding parameters for the Gauche and Anti conformers of the secondary alcohol 2-propanol is given in parenthesis. For notations, see
Fig. 2. For simplicity, the same notation as for 1 isu se d for 2 in thistable
Method
Syn-7-Norbornenol 32-propanol)
Gauche Anti
Anti-7-Norbornenol
D
Gauche
Anti
D
8
H
Ê
O ±H bond lengths3A )
HF/ꢀ-21G
pp
a
a
a
a
a
a
a
a
a
0.9659 30.9666)
0.9668 30.9675)
1 0.0009 310.0009)
0.9662
0.9429
0.9428
0.9ꢀ97
0.9648
0.9661
0.96ꢀ5
2 0.0027
HF/6-ꢀ1G
0.9427 30.94ꢀ0)
0.9428 30.94ꢀ0)
0.9ꢀ96 30.9ꢀ98)
0.964ꢀ 30.9644)
0.9660 30.9661)
0.944ꢀ 30.9440)
0.9445 30.9440)
0.9415 30.9409)
0.9675 30.9655)
0.9694 30.9672)
1 0.0016 310.0010)
1 0.0017 310.0010)
1 0.0019 310.0011)
1 0.00ꢀ2 310.0011)
1 0.00ꢀ4 310.0011)
0.9418
0.9421
0.9ꢀ90
0.9640
0.9660
2 0.0011
2 0.0007
2 0.0007
2 0.0008
2 0.0001
pp
HF/6-ꢀ111G
HF/6-ꢀ111G32d,2p)
pp
MP23FU)/6-ꢀ1G
a
a
a
pp
MP23FU)/6-ꢀ111G
7
C ±O ±H bond angles3 8)
8
H
a
a
a
HF/ꢀ-21G
110.9ꢀ 3110.56)
109.40 3109.77)
2 1.5ꢀ 320.79)
2 0.79 320.ꢀ1)
110.97
109.70
110.ꢀ5
109.97
107.29
108.28
111.9ꢀ
1 0.96
1 1.67
1 1.71
1 1.72
1 0.85
1 0.97
pp
a
a
a
HF/6-ꢀ1G
109.70 3109.81)
108.91 3109.51)
111.ꢀ7
112.06
111.69
108.14
109.25
pp
a
a
a
HF/6-ꢀ111G
HF/6-ꢀ111G32d,2p)
pp
MP23FU)/6-ꢀ1G
110.ꢀ6 3110.55)
109.8ꢀ 3110.ꢀ5)
2 0.5ꢀ 320.20)
2 0.58 320.17)
110.0ꢀ 3110.15)
107.24 3107.29)
109.45 3109.98)
105.ꢀ8 3106.58)
a
a
a
2 1.86 320.71)
2 1.50 320.50)
pp
MP23FU)/6-ꢀ111G
108.27 3108.ꢀ1)
106.77 3107.81)
C
H ±C ±O ±H dihedral angles3 8)
7
8
H
a
a
a
HF/ꢀ-21G
HF/6-ꢀ1G
HF/6-ꢀ111G
7ꢀ.ꢀ 361.1)
180.0
180.0
180.0
180.0
180.0
180.0
58.8
56.8
51.8
5ꢀ.5
57.8
51.5
180.0
180.0
180.0
180.0
180.0
180.0
pp
64.2 359.6)
62.4 360.5)
62.4 361.0)
64.4 361.8)
65.8 365.2)
pp
HF/6-ꢀ111G32d,2p)
pp
MP23FU)/6-ꢀ1G
a
pp
MP23FU)/6-ꢀ111G
a
Ref. [62].
1
JAnti ˆ 12.5 Hz and JGauche ˆ 2.2 Hz for secondary
In conclusion at this point, the IR and the H NMR
investigations indicated that between 80% 3IR) and
99% 3NMR) of compound 1 existed in the Anti
conformation with a hydrogen bond to the p-electrons
of the double bond. Due to the uncertaintiesin the
ꢀ
alcohols. Based on the observed J
312.4 Hz) at
high dilution in non-polar solution, syn-7-norbornenol
CH±OH
31) consists of 99% Anti and 1% Gauche conformers
3Fig. 2).
Table 4
2
1
21
Ab initio calculated hydroxyl stretch frequencies 3cm ) and integrated intensities 3in parenthesis, km mol ) for the conformersof syn-7-
norbornenol 31) and anti-7-norbornenol 32). The frequencies are scaled. The scale factors are given in Table 8. HF/ꢀ-21G scale factor: 0.9516ꢀ
[62]
Method
Syn-7-Norbornenol
Anti-7-Norbornenol
Gauche
Anti
D
Gauche
Anti
D
HF/ꢀ-21G
HF/6-ꢀ1G
ꢀ681.0 314)
ꢀ67ꢀ.7 346)
ꢀ67ꢀ.1 359)
ꢀ67ꢀ.4 361)
ꢀ650.2 324)
ꢀ684.2 317)
ꢀ656.ꢀ 340)
ꢀ652.5 347)
ꢀ650.8 346)
ꢀ606.4 319)
1 ꢀ.2
ꢀ679.8 318)
ꢀ672.5 35ꢀ)
ꢀ674.5 367)
ꢀ67ꢀ.2 367)
ꢀ646.7 3ꢀ0)
ꢀ728.4 310)
ꢀ694.1 3ꢀ0)
ꢀ688.2 3ꢀ6)
ꢀ687.6 3ꢀ6)
ꢀ661.2 314)
1 48.6
pp
2 17.4
2 20.6
2 22.6
2 4ꢀ.8
1 21.6
1 1ꢀ.7
1 14.4
1 14.5
pp
HF/6-ꢀ111G
HF/6-ꢀ111G32d,2p)
pp
MP23FU)/6-ꢀ1G
a
a
21
The corresponding values for the Gauche and Anti conformersof 2-propanol wasꢀ671.9 and ꢀ655.7 cm
3scaled), respectively.

ꢀ26
L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
Table 5
Ê
Ab initio calculated distances 3A) between the proton donor 3OH) and proton acceptor functions3C yC) for the Anti conformer of syn-7-
norbornenol 31) and the corresponding distances for the Anti conformer of anti-7-norbornenol 32). For notations, see Fig. 2
Method
Syn-7-Norbornenol
Anti-7-Norbornenol
8
O ´´´C
2
H
H ´´´C
2
O´´´C
3O)H´´´C
HF/ꢀ-21G
HF/6-ꢀ1G
2.94
2.92
2.9ꢀ
2.9ꢀ
2.91
2.9ꢀ
2.58
2.56
2.59
2.58
2.49
2.5ꢀ
2.90
2.90
2.90
2.90
2.89
2.91
2.55
2.54
2.56
2.56
2.49
2.52
pp
pp
HF/6-ꢀ111G
HF/6-ꢀ111G32d,2p)
pp
MP23FU)/6-ꢀ1G
pp
MP23FU)/6-ꢀ111G
estimates of the molecular absorption coef®cients of
the two IR bandsof 1, we suggest that 1 exists almost
exclusively in the Anti form with an intramolecular
hydrogen bond.
The minimal HF/STO-ꢀG calculationsundere ts i-
matesthe Anti conformer of 1 388%). However, all
the other ab initio calculations, even the calculations
with the small basis set ꢀ-21G 3HF), showed that syn-
7
-norbornenol 31) exists as 98±100% Anti as
1
4
.1.2. Theoretical calculations
Syn-7-Norbornenol 31) and anti-7-norbornenol 32)
compared to 99% Anti from H NMR. The energy
differencesbetween the Gauche and the Anti confor-
21
have previously only been subjects to a limited HF/
STO-ꢀG study by Morokuma et al. [ꢀ6]. Only the Anti
conformersand the maxima with O±H bondspointing
in the opposite directions were investigated by these
workers.
merswere calculated to ꢀ.8 kcal mol 3HF/ꢀ-21G),
2
2.6±2.8 kcal mol 3HF calculations with basis sets
1
21
based on 6-ꢀ1G) and ꢀ.0±ꢀ.ꢀ kcal mol 3MP2 calcu-
lations with basis sets based on 6-ꢀ1G).
For anti-7-norbornenol 32), the ab initio calcula-
tionsall sh owed that 90±96% of the moleculesexi st s
as Gauche conformers, in correspondence with the
®nding of only one hydroxyl stretch band in IR at a
frequency compatible with a Type II rotamer. The
energy differencesbetween the Gauche and Anti
Our results are tabulated in Tables 2±5, and the
pp
MP2/6-ꢀ111G minimum energy structures are
given in Fig. ꢀ. In Table 2 we present the calculated
populationsof the two conformers
Anti
multiplicity ˆ 1) and Gauche 3multiplicity ˆ 2) of
3
1
21
and 2. The results depend strongly on the method,
conformerswere calculated to 1.2±1.4 kcal mol
21
demonstrating the need for high-level methods in
describing the weak hydrogen bond interaction prop-
erly.
3HF) and 0.9±1.0 kcal mol 3MP2).
The calculated O±H bond lengths, C±O±H bond
anglesand H±C±O±H dihedral anglesfor the various
ab initio calculationson the Gauche and Anti confor-
mersof 1, 2 and the secondary alcohol 2-propanol
3reference) are tabulated in Table ꢀ. The HF/ꢀ-21G
calculationsare obviou sl y inadequate, and will not be
further commented upon. The O±H bondsof Anti
conformersof se condary alcoholsare typically longer
than in their corresponding Gauche conformers[62±
65]. The calculated O±H bond lengthsfor the Gauche
All molecular mechanicscalculationson 1 severely
underestimates the population of the Anti conformer
1
99% Anti from H NMR). However, for 2, the mole-
3
cular mechanicspopulationsare more in agreement
with the presence of only one hydroxyl stretch band in
IR at a frequency compatible with a Type II rotamer
3Gauche) and with the results from the ab initio calcu-
lations. Using semiempirical MO theory, the Anti
conformer of 1 was severely underestimated using
MNDO. No Gauche minimum were found for 1
using AM1 and PMꢀ and no Gauche minimum was
found for 2 using PMꢀ. Using MNDO, the populations
for 2 are more correctly described, while AM1 under-
estimates the population of the Gauche conformer.
Ê
conformer of 1 are within ^0.000ꢀ A of the calcu-
lated bond lengthsfor the Gauche conformer of 2-
propanol. The difference in O±H bond length between
the Anti and Gauche conformersof 1 wascalculated
Ê
Ê
to 0.0016±0.0019 A 3HF) and 0.00ꢀ2±0.00ꢀ4 A
Ê
3MP2), compared to 0.0010±0.0011 A 3HF and

L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
ꢀ27
MP2) for the difference in O±H bond length between
the Anti and Gauche conformersof 2-propanol. This
shows that the O±H bond length in the Anti conformer
of 1 is slightly, but signi®cantly, prolonged due to
intramolecular hydrogen bonding. The effect was
much more substantial at the correlated level
compared to the uncorrelated level, indicating the
importance of electron correlation in hydrogen
bonding. The ab initio calculations also showed that
the C±O±H bond anglesin 1 are decreased by 0.5±28
in the Anti conformer relative to the Gauche
conformer, somewhat larger than the decrease for
the corresponding conformers of 2-propanol 30.2±
still valid. No high frequency band for the Anti
conformer of 2 was observed in high dilution IR spec-
troscopy 3Table 1). We have previously reported such
discrepancies from the standard frequency pattern
[62±65] for Conformer 14 of ꢀ-buten-1-ol 35) [17]
and 2,2-dimethylpropanol [62].
8
2
H
2
The distances O ´´´C and H ´´´C for 1 and 2 are
tabulated in Table 5. Irrespective of the quality of the
8
2
H
2
wave function, the O ´´´C and H ´´´C distances of 1
are shorter than the sum of the van der Waals radii,
con®rming that 1 isintramolecularly hydrogen
bonded [8,57,67,68]. For comparison, the calculated
hydrogen bond distances are approximately 0.05±
ꢀ
Ê
0.1 A shorter than those to C for Conformer 13 of
0
.78). Thisa los indicatesintramolecular hydrogen
bonding.
For 2, the calculated O±H bond lengthsfor the
pp
ꢀ-buten-1-ol 35) 3HF/6-ꢀ1G ) [17].
Ê
Gauche conformer waswithin ^0.0005 A of the
4.2. Methanol-ethene hydrogen bonded complex
calculated bond lengthsfor the Gauche conformer
of 2-propanol. However, in the Anti conformer, the
O±H bond length was shortened 3instead of prolonged
asfor 2-propanol) and the C±O±H bond angle
increased compared to the Gauche conformer. The
reason for this is most likely steric repulsion between
the hydroxyl group and the upward pointing CH2±
In order to understand how intramolecular O±
H´´´p hydrogen bonding in¯uencesthe t sr uctural
propertiesof syn-7-norbornenol 31) and other homo-
allylic alcohols, a more detailed and accurate quantum
chemical study of a realistic model system would be
useful. The methanol±ethene complex was consid-
ered to be ideal. A key feature of thiscomplex is
that the proton donor and acceptor functionsare
completely free to arrange themselves relative to
each other. The water±ethene complex hasbeen
studied quite extensively, but we consider a system
in which the O±H moiety isa part of an alcohol to be
more relevant for the present purpose, because there is
only one O±H bond to consider.
CH hydrogen atoms3Table ꢀ).
2
We have also calculated the hydroxyl stretch
frequenciesfor 1 and 2. The results are tabulated in
Table 4. All ab initio calculationsin the table 3except
the calculations with the rudimentary basis set ꢀ-21G)
indicate that the Anti conformer of 1 should have a
band at lower frequency than the Gauche band. The
21
MP2 red shift of 44 cm isin good agreement with
the corresponding experimental number 3CCl ),
4
21
which is5ꢀ cm . Thisisin line with our previous
®
4.2.1. General considerations
ndings[17,62].
The fact that the O±H bond length of the Anti
Accurate calculationsof the extremely fragile O±
H´´´p bond requirescertain methodological precau-
tions. This has been discussed various places in the
literature [57,69], but it is necessary to outline the
most important facts here.
The basis set is of utmost importance. A poorly
designed basis set or a too small basis set will lead
to errorsin the variousfactorsthat contribute to an
hydrogen bond. The basis set superposition error
3BSSE) isanother complicating factor. Small ba is s
sets tend to be subject to larger BSSE than larger
basis sets. In addition to an arti®cial higher interaction
energy, BSSE givesan arti®cial hs ortening of the
hydrogen bond length.
conformer of 2 is hs orter than that of the Gauche
conformer also in¯uences the hydroxyl stretch
frequency pattern. In contrast to 1, the Anti conformer
of 2 iscalculated to be higher in frequency 315±
2
1
2
ꢀ
2 cm ) than the Gauche conformer 3ignoring the
-21G calculations). We also note that the absolute
value of the calculated frequency difference decreases
upon increasing the basis set. This is opposite to the
effect seen for 1. For both 1 and 2, the Gauche confor-
mersare calculated to have approximately the sa me
frequency. Consequently, the assignment of the IR
bandsto the conformersfor 1 and 2 from IR only is

ꢀ28
L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
Fig. 5. Structural parameters of the ab initio transition structure or
higher order saddle point for the methanol±ethene complex 9 3C ).
s
The methanol H±O±C plane islocated perpendicular to the plane
through the ethene hydrogen atomsand inter se ctsthe carbon atoms
in the ethene CyC bond. r isthe di st ance between the mid-point of
the CyC double bond and the hydroxyl proton. g de®nesthe angle
between the CyC bond and the line between the hydroxyl proton
and the centre of the double bond. The angle u de®nesthe deviation
from a straight line 3u ˆ 1808) between the centre of the double
bond, the hydroxyl proton and the oxygen atom.
Fig. 4. Structural parametersof the methanol±ethene complex
). The methanol H±O±C plane islocated perpendicular to and
bisects the ethene CyC bond, r isthe di st ance between the mid-
8
3C
s
point of the CyC double bond and the hydroxyl proton. g de®nes
the angle between the ethene plane and the line between the
hydroxyl proton and the centre of the double bond. However, the
ethene molecule isnot planar in the complex and the reported value
for g isthe dihedral angle 1-2-ꢀ-4 3the su m of the dihedral angles1-
bond length and a shorter frequency nXH 3nOH
accompanied by a larger intensity IXH 3IOH).
)
2-ꢀ-4 and 6-5-ꢀ-4 wascalculated to 180±181 8 3HF/MP2)). The
angle u de®nesthe deviation from a t sr aight line 3 u ˆ 1808)
between the centre of the double bond, the hydroxyl proton and
the oxygen atom.
In the present work the MP2 method has been used
to estimate electron correlation effects. Scheiner et al.
have reported that the MPꢀ and MP4 contributions
without BSSE correction to the total energy of several
hydrogen bonded complexesare of approximately the
same value, but with opposite signs, making MP2 a
good approximation. With BSSE correction, the MPꢀ
and MP4 contributionsboth approach zero when the
basis set is increased [57]. Mùller±Plesset calcula-
tions are size consistent [57] and among the compu-
tationally fast correlation methods and are thus easy to
extend to larger basis sets [70].
The interaction energieswere corrected for BSSE
[56,57] using the full counterpoise 3CP) method [58].
The CP method hasbeen critic ei ds for both
overestimating and underestimating the true BSSE.
Some researchers have defended the full counterpoise
correction while othershave propoesd their own
A Hartree±Fock wave function may in many
respects be adequate, and provide reasonable esti-
matesof the ba is c propertiesof an intermolecular
hydrogen bond; the interaction energy, the equili-
brium geometry and vibrational frequencies.
Unfortunately, this us cce ss dependson cancella-
tion between variou so surcesof error. It turns
out that electron correlation needsto be included
to give a proper description of electrostatic and
dispersion interactions. The contribution of the
®
rst may be either repulsive or attractive. The
latter strengthens the hydrogen bond, giving an
increased interaction energy, a shorter hydrogen
bond length, a larger X±H 3in thisca es O±H)

L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
ꢀ29

ꢀꢀ0
L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338

L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
ꢀꢀ1
Table 7
Ab initio calculated geometry parametersfor the methanol±ethene complex 8 3C
s
) ascompared with methanol 3 C
s
)s. For notations, see Fig. 4
Ê
Ê
Ê
Ê
r 3A) O´´´C 3A) 3O)H´´´C 3A) O±H 3A)
Method
g 38)
u 38)
C±O±H 38)
MeOH
8
D
MeOH
8
D
pp
HF/6-ꢀ1G
HF/6-ꢀ111G
96.66 182.06 2.68 ꢀ.68
101.92 175.15 2.77 ꢀ.77
101.71 174.0ꢀ 2.77 ꢀ.76
100.21 177.82 2.81 ꢀ.81
100.5ꢀ 175.61 2.78 ꢀ.78
97.64 18ꢀ.69 2.72 ꢀ.72
99.46 178.60 2.75 ꢀ.75
99.50 178.48 2.78 ꢀ.78
99.18 176.84 2.77 ꢀ.77
101.ꢀ6 17ꢀ.98 2.78 ꢀ.77
101.40 17ꢀ.99 2.80 ꢀ.79
94.56 189.08 2.ꢀ9 ꢀ.41
11ꢀ.ꢀ7 165.7ꢀ 2.ꢀ8 ꢀ.ꢀ9
85.40 195.71 2.4ꢀ ꢀ.4ꢀ
92.46 185.16 2.ꢀ2 ꢀ.ꢀ5
90.54 195.55 2.4ꢀ ꢀ.44
92.70 189.57 2.ꢀ6 ꢀ.ꢀ8
89.22 194.64 2.ꢀ6 ꢀ.ꢀ7
91.72 190.6ꢀ 2.ꢀ2 ꢀ.ꢀꢀ
91.57 190.87 2.ꢀ4 ꢀ.ꢀ7
91.16 191.07 2.ꢀ0 ꢀ.ꢀ2
89.22 194.70 2.ꢀ7 ꢀ.ꢀ8
91.74 190.56 2.ꢀ4 ꢀ.ꢀ5
91.ꢀ9 191.16 2.ꢀ6 ꢀ.ꢀ8
2.76
2.85
2.84
2.88
2.86
2.80
2.8ꢀ
2.86
2.84
2.86
2.87
2.48
2.47
2.52
2.41
2.5ꢀ
2.45
2.46
2.41
2.44
2.40
2.46
2.4ꢀ
2.46
0.942ꢀ 0.94ꢀ7 0.0014 109.64 109.50 2 0.14
0.942ꢀ 0.94ꢀ6 0.001ꢀ 110.55 110.41 2 0.14
0.9ꢀ91 0.9407 0.0016 110.0ꢀ 109.95 2 0.08
0.9ꢀ97 0.9410 0.001ꢀ 110.01 109.87 2 0.14
0.9ꢀ81 0.9ꢀ95 0.0014 110.10 110.00 2 0.10
0.9449 0.9460 0.0011 109.09 108.94 2 0.16
0.9ꢀ90 0.940ꢀ 0.001ꢀ 109.92 109.84 2 0.08
0.9ꢀ80 0.9ꢀ92 0.0012 110.2ꢀ 110.10 2 0.1ꢀ
0.9420 0.94ꢀ4 0.0014 109.98 109.86 2 0.12
0.9ꢀ9ꢀ 0.9406 0.001ꢀ 110.25 110.18 2 0.08
0.9ꢀ81 0.9ꢀ94 0.001ꢀ 110.22 110.21 2 0.01
0.9622 0.9640 0.0018 107.ꢀꢀ 106.7ꢀ 2 0.60
0.96ꢀ7 0.9657 0.0020 108.57 108.ꢀ6 2 0.21
0.96ꢀ7 0.9661 0.0024 108.57 107.79 2 0.78
0.9592 0.9629 0.00ꢀ7 108.01 107.ꢀ5 2 0.66
0.9589 0.9612 0.002ꢀ 107.40 106.58 2 0.82
0.9570 0.9604 0.00ꢀ4 108.1ꢀ 107.ꢀ6 2 0.77
0.9652 0.9665 0.001ꢀ 106.29 105.71 2 0.58
0.9581 0.9610 0.0029 107.56 106.88 2 0.68
0.9657 0.9685 0.0028 107.92 106.91 2 1.00
0.9590 0.9615 0.0025 108.14 107.12 2 1.01
0.9655 0.9670 0.0015 106.ꢀꢀ 105.69 2 0.65
0.9594 0.9626 0.00ꢀ2 107.4ꢀ 106.79 2 0.64
0.9657 0.9690 0.00ꢀꢀ 107.94 106.92 2 1.02
pp
HF/6-ꢀ111G32d,2p)
pp
HF/6-ꢀ1111G
HF/6-ꢀ1111G32d,2p)
HF/cc-pVDZ
HF/cc-pVTZ
HF/cc-pVQZ
HF/aug-cc-pVDZ
HF/aug-cc-pVTZ
HF/aug-cc-pVQZ
pp
MP23FU)/6-ꢀ1G
MP23FU)/6-ꢀ111G 3#1)
pp
MP23FU)/6-ꢀ111G 3#2)
pp
MP23FU)/6-ꢀ111G32d,2p)
pp
MP23FU)/6-ꢀ1111G
MP23FU)/6-ꢀ1111G32d,2p)
MP23FU)/cc-pVDZ
MP23FU)/cc-pVTZ
MP23FU)/aug-cc-pVDZ
MP23FU)/aug-cc-pVTZ
MP23FC)/cc-pVDZ
MP23FC)/cc-pVTZ
MP23FC)/aug-cc-pVDZ
variant [57,71±77]. Nevertheless, there seems to be a
consensus that the CP correction gives a correct order-
of-magnitude estimate of BSSE [78].
calculationson 9 were done with various basis sets
up to 6-ꢀ1111G32d,2p), using both HF and MP2.
The calculationsrevealed that the potential energy
surface 3PES) is very ¯at with respect to variations
of u and g 3Fig. 6; see Fig. 4 for de®nitions). In most
cases 8 wasfound to be a minimum 3with zero
imaginary frequencies), while 9 wasfound to be a
transition structure or a higher order saddle point.
When high accuracy isaimed for, it isin su f®cient to
carry out single calculations. A more systematic
approach isneeded, like e.g. the Dunning'scorrela-
tion consistent basis sets which offer a systematic
improvement to the correlation energy with each
increment in the cardinal number [5ꢀ±55,78]. It has
been found that the correlation energiesobtained with
these basis sets converge smoothly to apparent
complete basis set limits [78,79] as demonstrated by
Feller [70,80]. Many other propertiesal so converge to
well-de®ned limits[79].
21
The energy differenceswere le ss than 0.ꢀ kcal mol
Furthermore, the absolute value of the imaginary
.
frequency in the two cases where 8 wasfound to be
21
a transition structure was below 20 cm , which prob-
ably isof the order of the numerical preci si on. The
substantial decrease in the distance r in going from HF
to MP2 3Table 7 and Fig. 7) results from not including
the attractive dispersion forces in the former wave
function.
4
.2.2. Results
We performed ab initio molecular orbital calcula-
tionson the two methanol±ethene complexeswith Cs
symmetry, 8 and 9, asde®ned in Fig s. 4 and 5. The
results from the calculations on 8 are shown in Tables
The calculated HF and MP2 interaction energiesfor
8 are summarised in Table 6. With HF, all the calcula-
tionsgave uncorrected interaction energiesin the
21
6
±8. The geometry optimisations and frequency
range 21.2±1.5 kcal mol
with the exception of

ꢀꢀ2
L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
n
n
n
n

L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
ꢀꢀꢀ
pp
s
Fig. 6. Relative interaction energy for the methanol±ethene complex 8 3C ) obtained using single point calculations 3MP23FU)/6-ꢀ1G ) as a
function of both u and g, starting with the fully optimised minimum energy structure at the same level of theory and keeping all other
parameters constant. No correction for BSSE. For notations, see Fig. 4.
the two smallest basis sets, which gave somewhat
higher absolute values due to BSSE. We observed
that the counterpoise 3CP) correction with HF
was , 0.ꢀ kcal mol with the exception of the two
smallest basis sets and that the CP corrected interac-
givesa HF interaction energy of approximately
2
1
21.2 kcal mol , vibrational zero point energy not
included.
2
1
The BSSE wasgreatly reduced with HF u si ng fairly
large basis sets. However, the BSSE is much less
susceptible to reduction at correlated levels and is
often non-negligible [57]. It is necessary to use a
large ¯exible basis set with multiple polarisation and
diffuse functions to accurately evaluate the interac-
tions[18,70,75,78,80].
Our MP2 calculationswere ®r st done with the core
electronsincluded 3FU). However, Feller et al. [80]
have recently pointed out that the Dunning'scorrela-
tion consistent basis sets cc-pVxZ and aug-cc-pVxZ
only were designed to recover valence correlation.
tion energiesall were in the range
21.2±
pp
21
1
.ꢀ kcal mol with the exception of the 6-ꢀ1G
calculations. This shows that, when using 6-
pp
111G or cc-pVTZ and larger basis sets, the HF
ꢀ
interaction energies are not largely basis set depen-
dent and that these basis sets are almost free of BSSE.
The very large basis set aug-cc-pVQZ gave the smal-
21
lest HF value for BSSE, 0.02 kcal mol . The conver-
gence of the interaction energy is hs own in Fig. 8,
using the method of Feller [70,80]. Extrapolation

ꢀꢀ4
L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
pp
) obtained using single point calculations 3HF/6-ꢀ1G and MP23FU)/
Fig. 7. Relative interaction energy for the methanol±ethene complex 8 3C
s
pp
-ꢀ1G ) as a function of r, starting with the fully optimised minimum energy structures at the same levels of theory and keeping all other
6
parameters constant. No correction for BSSE. For notations, see Fig. 4.
Thus we also performed MP2 calculations using
frozen core 3FC). The geometrieswith and without
core electronsin the MP2 calculationswere found
to be almost identical 3Table 7). However, from
Table 6 it isevident that the u es of frozen core
decreased the uncorrected absolute interaction energy
and the BSSE, especially with aug-cc-pVxZ, while
the CP corrected interaction energiesare almo st iden-
tical in both cases.
Due to the very ¯at PES around the minimum
energy structure, some of the frozen core MP2 energy
calculations were done using the corresponding
MP23FU) geometries. For the same reasons, some of
the energy calculationswere done on optim ie sd
geometries with smaller basis sets 3Table 6). The
effectsof ignoring or including geometry relaxation
is small as demonstrated by the MP23FU)/aug-cc-
pVTZ energy calculationswith both relaxed and
corrected by FC calculations. The smallest basis sets
21
gave a BSSE of 1.9 kcal mol . However, the very
large basis set aug-cc-pVQZ reduced BSSE to
2
1
0.27 kcal mol . We note that asthe BSSE wasgradu-
ally reduced by increasing the size of the basis set, the
CP corrected absolute interaction energy was gradu-
ally increased while the uncorrected absolute interac-
tion energy wasgradually reduced [70,80] 3Table 6,
Fig. 9). When using the counterpoise procedure it is
only possible to compensate partly for the missing
part of the correlation energy for basis sets smaller
than aug-cc-pVTZ. In this case it is necessary to use
very large basis sets up to aug-cc-pVQZ to estimate
the interaction energy of 8. Extrapolation using the
method of Feller [70,80] in Fig. 9 givesan interaction
21
energy of approximately 2ꢀ.1 ^ 0.1 kcal mol ,
vibrational zero point energy not included.
The large and almost systematic increase in the
absolute difference between the CP corrected interac-
tion energiesfor the uncorrelated 3HF) and correlated
3MP2) calculations with the same basis set upon
increasing the number of basis functions shows that
electron correlation isa very isg ni®cant and non-
negligible contribution to the interaction energy
3Table 6, Figs. 8±9).
®
xed geometry in Table 6.
Asexpected, the MP2 BSSE wasquite large. It was
reduced by approximately a factor of two when the
cardinal number wasincrea se d by 1 [78] 3Table 6).
Thiswasnot the ca se for the MP23FU)/aug-cc-pVxZ
series where BSSE was hardly reduced by increasing
the cardinal number. Asexpected [80], thiswas

L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
ꢀꢀ5
Fig. 8. Convergence of the HF interaction energiesof 8 as a function of the size of the basis set using Dunning's correlation consistent basis sets
cc-pVxZ and aug-cc-pVxZ 3x ˆ D through Q), with and without the counterpoise correction.
The HF calculationsgive a minimum energy st ruc-
ture with g ˆ 99 2 1028, u ˆ 174 2 1798, r ˆ
basis sets used, less than the sum of the van der
Waalsradii, con®rming the hydrogen bonded nature
of the interaction 3see Table 7). The MP2 calculations
ꢀ
:75 2 2:81 A while the MP2 calculationsgive a
2
Ê
with the largest basis set gave a distance of 2.40 A,
which is signi®cantly less than the sum of the van der
Waals radii 3see also Section 4.1.2).
The differencesin O±H bond length between
methanol and 8 are also given in Table 7. The MP2
value of the O±H bond length increases by approxi-
Ê
mately 0.00ꢀ A, and the MP2 C±O±H bond angle
decreases by approximately 1.08.
We also calculated the hydroxyl stretch frequencies
minimum energy structure with g ˆ 91 2 928,
ꢀ
u ˆ 191 2 1968, r ˆ 2:ꢀ0 2 2:4ꢀ A 3Table 7, the
smallest basis sets and the calculations with imaginary
frequenciesand double minima were excluded). This
shows that even if a very large basis set with MP2 is
necessary to evaluate the interaction energy, the
geometry of the complex isnot os dependent upon
the basis set used. The calculations with the largest
ꢀ
basis sets gave g ˆ 1018, u ˆ 1748, r ˆ 2:80 A 3HF)
ꢀ
and g ˆ 918, u ˆ 1918; r ˆ 2:ꢀ0 A 3MP2). With MP2
for methanol and the methanol±ethene C complex
s
using Dunning's correlation consistent basis sets we
observed that r decreased upon increasing the cardinal
number. However, we may not have reached the
asymptotic limit for r with MP23FU)/aug-cc-pVTZ.
With MP2, the 3O)H´´´C distances were, for all
38). The result is shown in Table 8. With the largest
basis set and MP2, the red shift for the complex is
21
52 cm . The corresponding calculated integrated
intensities for the complex is seven times that of
methanol alone. The larger red shift and larger

ꢀꢀ6
L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
Fig. 9. Convergence of the MP23FC) interaction energiesof 8 as a function of the size of the basis set using Dunning's correlation consistent
basis sets cc-pVxZ and aug-cc-pVxZ 3x ˆ D through Q), with and without the counterpoise correction.
intensity with MP2 compared to HF shows again the
importance of electron correlation for intermolecular
hydrogen bonding.
tively) in 8 3MP2/aug-cc-pVTZ) were for the Anti
conformer of 1 < 70 and ꢀ04.28, respectively
3MP2/6-ꢀ111G ).
pp
The reason for this large difference in geometry is
that in 1 the proton donor and acceptor functionsare
forced together by the strained skeleton while the
calculationsof 8 givesthe minimum energy st ructure
when the proton donor and acceptor functionscan
approach each other freely. However, an arrangement
of the proton donor and acceptor functionsasfor 1 is
not possible for the complex 8 due to steric con¯icts
between some of the ethene and methanol protons.
Thismakesthe water±ethene complex without su ch
con¯icts an attractive probe for further investigations.
This has, however, recently been investigated [24]. It
wasfound to have a geometry 3MP2/6-ꢀ1 11G32d,2p)
4.3. Comparisons between intra- and intermolecular
hydrogen bonding
The calculationson both 1 and 8 showed prolonga-
tion of the O±H bond, a smaller C±O±H bond angle
and a red frequency shift upon interaction. The effects
are more substantial at the electron correlated level
than with HF. The O´´´C and 3O)H´´´C distances for
1
are all shorter than the sum of the van der Waals
radii. The same is not the case for the complex 8
where only the MP2 3O)H´´´C distances are less
then the sum of the van der Waals radii.
ꢀ
From Fig. ꢀ and Table 7 it isevident that the
geometriesof 1 and 8 are very different. The corre-
sponding values to g and u 391 and 1918, respec-
very similar to 8 with r ˆ 2:ꢀ6 A; g ˆ 71:468 and u ˆ
189:708: The interaction energy was et si mated to
22.5 kcal mol 3MP2/NHFL3ꢀd,2p)) in contrast to
21

L.H. Bjerkeseth et al. / Journal of Molecular Structure 567±568 ꢀ2001) 319±338
ꢀꢀ7
2
1
2ꢀ.1 ^ 0.1 kcal mol for 8. For comparison, the inter-
action in the water dimer hasbeen e st imated to 25.0 ^
Acknowledgements
21
0.1 kcal mol [70,80] while the interaction energy for
the methanol±water complex hasbeen e tsi mated to
4.9 kcal mol [18]. These values are not corrected
for vibrational zero point energy.
Financial support for this research from The Norwe-
gian Research Council for Science and the Humanities
3NAVF) isgratefully acknowledged. Thiswork hasalso
received support from The Research Council ofNorway
21
2
3Programme for Supercomputing) through a grant of
computing time 3Cray X-MP/216, Cray Y-MP4D/464,
Cray J916/8128 and Cray TꢀE in Trondheim).
5
. Conclusions
1
From IR, H NMR and ab initio calculations,
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Gauche conformer 3Fig. ꢀ). For 1, many effects
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due to lack of dispersion. The MP2 interaction
energiesgreatly depend on the bais es tswhile
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