93
Journal of Molecular Structure, 23 (1974) 93-101
@ Elsevier Scientific Publishing Company, Amsterdam
- Printed
MICROWAVE SPECTRUM, CONFORMATION,
HYDROGEN BOND, AND DIPOLE MOMENT
BOXALDEHYDE
K.-M.
MARSTOKK
Department
(Received
AND HARALD
of Chemistry,
10 December
in The Netherlands
INTRAMOLECULAR
OF PYRROLE-2-CAR-
MØLLENDAL
The University
of Oslo, Blindern,
Oslo 3 (Norway)
1973)
ABSTRACT
Microwave spectra of C4H3NH-CHO, C4H3ND-CHO, and C4H3NHCH180 are reported. The stable form of the molecule is demonstrated to be planar
with the N-H and C~O bonds in a eis conformation. Other forms of the molecule
are at least l kcal mol-1 less stable. The H(1)' . . O distance is 2.592::tO.006 Å.
Six vibrationally excited states were attributed to the c-c torsional mode, the
symmetrical, and the antisymmetrical aldehyde group deformation vibrations.
Relative intensity measurements yielded 15l::t Il cm -t, for the first frequency,
21O::t17 cm -1 for the second, and 270::t 38 cm -1 for the last mode. The dipole
moment was determined
to be Ila
= 2.47 ::t 0.02
D, Ilb = 0.16::tO.06 D, and Iltot =
2.48::t 0.02 D, respectively.
INTRODUCTlON
In pyrrole-2-carboxaldehyde rotation about the C-CHO bond may produce
rotational isomerism. The plan ar eis and trans rotamers of Fig. l are expected to
be stable forms of the molecule because they are favourable for conjugation,
steric, weak intramolecular hydrogen bonding, and other effects.
To our knowledge, no investigation of the structure of the free molecule has
been carried out, but several researchers have made solution studies. Infrared
[l, 2], ultraviolet [2], and proton magnetic resonance spectroscopy [2-4] have been
applied and showed that the eis form predominates in solution. This conc1usion
has been supported by dielectric relaxation measurements [5].
Weak intramolecular hydrogen bonding between the carbonyl oxygen atom
94
0,'0'/"
N
Q,_o
N
I1
I
H
O
HI
CIS
I
H
TRANS
Fig. 1. Possible plan ar rot ameri c forms of pyrrole-2-carboxaldehyde.
and the hydrogen atom of the N-H bond is pre surnably of importance for the
conforrnational behaviour of the gaseous molecule. Because of aur interest in
this type of interaction, the microwave spectrum was investigated. It was found
that the cis form is preferred, and that the energies of other forms of the molecule
must be at least l kcal mol- 1 higher.
EXPERIMENT
AL
Pyrrole-2-carboxaldehyde was purchased from Schuchardt, Munich, and
used without further purification. C4H3ND-CHO was produced by direct exchange with 99 % D20 in the celloC4H3NH-CH180 was made by dissolving the
compound
in about 20
% enriched
180 water
obtained
from
Prochem,
London.
The water was then removed by distillation under reduced pressure. The molecule
had a vapour pressure of about 60 microns at room temperature at which the
measurements were made. The 11.8-27.9 GHz spectral region was searched utilizing a conventional spectrometer described briefly in ref. 6.
RESULTS
Microwave spectrum and assignment of the ground vibrational state
Based on the findings of the solution studies, the cis rotamer was expected
to be the more stable. Preliminary rotational constants ofthis form were calculated
from the structural parameters of Table l, which, with the exception of the aldehyde group coordinates, were taken from the work of Nygaard et al. [7] on pyrrole.
The ds conformer is depicted in Fig. 2. Survey spectra revealed a relatively simple
a-type R-branch spectrum dose to the predicted one. The rigid rotor fit and Stark
effects of several of the se lines confirmed their assignments. The ground vibrational
state spectrum* is shown in Table 2 and the derived rotational constants are listed
in Table 3. Attempts to fit the Table 2 transitions to Dowling's [8] seven parameter
* The comprehensive list of frequencies measured for the various vibrational states and the two
isotopi c speeies studied is available from the authors up on request or from The Microwave Data
Center, National Bureau of Standards, Washington, D.C., U.S.A.
--
95
TAB LE 1
PLAUSIBLE STRUCTURAL PARAMETERSa,b
PYRROLE-2-CARBOXALDEHYDE
C~O
N-C
C (1)-C (3)
C (2)-C(4)
C(2)-C(5)
N-H
C(1)-H(2)
C(3)-H(3)
C(4)-H(4)
C(5)-H(5)
1.209 Å
1.307 Å
1.382 Å
1.382 Å
1.470 Å
0.996 Å
1.076 Å
1.077 Å
1.077 Å
1.106 Å
AND OBSERVED AND PREDICTED
ROTATIONAL
109.8°
107.7°
121.5°
125.5°
125.5°
121.5°
122.00
115.0°
LCNC
LNCC
LNC(1)H(2)
LC(I)C(3)H(3)
LC(2)C(4)H(4)
LNC(2)C(5)
L C(2)C(5)0
L C(2)C(5)H(5)
Rotational constants (MHz)
Observed
Calculated
Main speeies
Ao
Bo
Co
.
Deuterated speeies
Ao
Bo
Co
7890.35
2085.042
1649.363
7913.32
2078.67
1646.24
7521.55
2084.379
1632.015
7530.49
2078.13
1628.68
7861.22
1986.329
1585.807
7883.25
1980.71
1582.98
180 speeies
Ao
Bo
Co
a
b
Not a derived structure. See text.
See Fig. 2 for notation.
H(4)
b
H(3)
o
Fig. 2. Cis pyrrole-2-carboxaldehyde
Q
projected in the a-b principal axis system.
CONSTANTS
OF
96
T ABLE 2
MICROWAVE
SPECTRUM OF THE GROUND
Transition
Observed
Obs.-calc.
frequency
(MHz)
frequency
(MHz)
11841.61
14021.46
14705.64
14919.26
15151.22
15759.43
17487.62
18219.43
18619.60
18745.10
19072.18
19649.26
20931.61
21646.91
22300.50
21>1 --+ 31>2
31>3 --+ 41>4
30,3 --+ 4o,4
32,2 --+ 42,3
32,1 --+ 42,2
31,2 --+ 41>3
41,4 --+ 51,5
40.4 --+ 50,5
42,3 --+ 52,4
43,2 --+ 53,3
42,2 --+ 52,3
41>3 --+ 51,4
51>5 --+ 61>6
5°'5 --+ 60,6
52,4 --+ 62,5
VIBRATIONAL
0.06
0.11
0.08
0.02
-0.05
0.17
0.03
0.04
0.04
0.01
0.17
0.00
-0.15
-0.06
0.11
STATE OF PYRROLE-2-CARBOXALDEHYDE
Transition
53,3 --+ 63,4
53,2 --+ 63,3
52>3 --+ 62,4
51>4 --+ 61>5
61,6 --+ 71,7
6°,6 --+ 7°,7
62,5 --+ 72,6
65,2 --+ 75>3
65,1 --+ 75,2
63,4 --+ 73,5
63>3 --+ 73,4
62,4 --+ 72,5
61,5 --+ 71,6
71,7 --+ 81>8
Observed
Obs.-calc.
frequency
(MHz)
frequency
(MHz)
22515.10
22559.12
23059.07
23501.54
24353.05
24998.37
25958.15
26224.65
26224.65
26289.98
26388.25
27097.80
27304.25
277 51.87
0.16
-0.10
0.03
-0.02
0.05
-0.03
0.04
-0.09
-0.12
-0.01
-0.14
-0.05
-0.08
0.03
a ::!:0.05 MHz.
TAB LE 3
MOLECULARCONSTANTSFOR C4H3NH-CHO,
lsotopic
C4H3ND-CHO
AND C4H3NH-CH180
C4H3NH-CHO
species
Vibrational state
Number of transitions
a (MHz)
Ground
29
0.086
First ex. C-C torso
25
0.082
Second ex. C-C torso Third ex. C-C,
17
7
0.154
0.082
Av (MHz)
Bv (MHz)
Cv (MHz)
la (UÅ2)
lb (UA2)
le (UA2)
Ie-la-lb
(UA2)
7890.35
::!:0.44
2085.04
::!:0.005
1649.363 ::!:0.005
64.0499 :1:0.0036
242.3817 ::!:0.0006
306.4068 ::!:0.0009
-0.0248::!:0.0037
7805.51
::!:0.44
2085.368 ::!:0.005
1652.165 ::!:0.005
64.7460::!:0.0036
242.3438 ::!:0.0006
305.8871 ::!:0.0009
-1.2027 ::!:0.0037
7718.60 ::!:0.90
2085.556 ::!:0.011
1655.005 ::!:0.01O
65.4~::!:0.0076
242.~20~0.0013
C4H3ND-CHO
C4H3NH-Cl
First ex. sym. de/.
mode
19
0.089
First ex. asym. de/.
mode
15
0.103
Comb. C-C torso and
Ground
Ground
sym. de/. modes
11
0.116
16
0.129
10
0.114
7973.07
::!:0.74
2085.209 ::!:0.01O
1647.960 ::!:0.008
63.3854::!:0.0059
242.3623 ::!:0.0012
306.6676::!:0.0014
0.9199::!:0.0061
7893.57
::!:0.76
2084.869 ::!:0.010
1650.049 ::!:0.007
64.0237 ::!:0.0062
242.4018::!:0.0012
306.2794::!:0.0013
-0.1461
7882.28
::!:1.15
2085.445 ::!:0.014
1650.829 ::!:0.011
64.1155::!:0.0094
242.3348 ::!:0.0015
306.1347 ::!:0.0020
-0.3156::!:0.0097
7521.55
::!:0.84
2084.379 ::!:0.012
1632.015 ::!:0.012
67. 1904::!:0.0075
242.4588::!:0.0014
309.6638 ::!:0.0023
0.0146::!:0.0080
7861.22
::!:l.
1986.329 ::!:O.
1585.807 ::!:O.
64.2873 ::!:O.
254.4271 ::!:O.
318.6870 ::!:O.
-0.0274::!:0.
C4H 3NH-CHO
Conversion
factor
The uncertainties
505376.0 MHz uÅ2.
represent
one standard
deviation.
305.3622 ::!:0.0018
- 2.4349 ::!:0.0079
7621.75
::!:O
2085.512 ::!:O
1657.941 ::!:O
66.3071 ::!:O
242.3271 ::!:O
304.8215::!:0
-3.8127::!:0,
97
centrifugal distortion formula utilizing a computer programme described in [9]
yielded unacceptable centrifugal distortion constants. This is presumably aresult
of the small centrifugal perturbations the observed lines possess.
As indicated in Table 3, the a-type spectrum determines the rotational constants quite accurately. Strong low J b-type transitions were then predicted and
searched for. However, none were identified with certainty. This is in keeping
with the small f-lbdetermined to be less than 0.2 D.
Attempts to resolve the quadrupole fine structure caused by the nitrogen
nucleus proved unsuccessful, probably because of insufficient splittings. Quadrupole coupling constants similar to those determined for pyrrole [7] were assumed
and the splittings predicted to be smaller than about 0.3 MHz for the most intense
components. This is less than the resolution obtainable with our instrument.
The inertial defect is shown in Table 3 to be -0.0248 UA2 which is very
close to its counterparts in ds and trans furan-2-carboxaldehyde [10, 11] and in
trans thiophene-2-carboxaldehyde [10]. The latter molecules were each proved
to be planar.
Vibrationally excited states
The ground vibrational state lines were accompanied by a rich satellite
spectrum. As shown in Table 3, these satellite lines have been assigned to excited
states of the C-C torsional, the symmetrical, and the antisymmetrical deformation
modes of the aldehyde group, respectively. These assignments have been made
because the inerti al defects for the excited states are quite similar to their counterparts in cis furan-2-carboxaldehyde [Il] which was thoroughly analyzed by
Monning et al. [10, 11]. The regular variation of the rotational constants of the
torsional mode upon excitation (Table 3) is typical for a harmonic mode and presents additional evidence for a planar equilibrium conformation of the molecule
[12].
Relative intensity measurements were performed. Most, but not all, of the
precautions of Esbitt and Wilson [13] were observed. The results are shown in
Table 4. The infrared values observed for cis furan-2-carboxaldehyde [11] were
147 cm -1 for the torsional fundamental, 214 cm -1 for the symmetrical deformation mode and 298 cm - 1for the antisymmetrical deformation frequency, respectively. These results are close to those presented in Table 4. This was expected
because the inertial defects are fairly similar in the two molecules for the corresponding excited vibrational states.
The above assignments include all strong lines present in the spectrum.
Search for another form of the molecule proved futile. It is ruled out that additional
forms, which most probably possess sizeable dipole moments, exist in concentrations exceeding 10 per cent of the total. The energy difference between cis
pyrrole-2-carboxaldehyde and other rotamers must thus be more than 1 kcal
98
TABLE 4
RELATIVE INTENSITlES'
AND ENERGY DIFFERENCES OF VIBRATIONALLY
EXCITED
STATES OF PYRROLE-2-
CARBOXALDEHYDE
Transition
Relative intensity
Energy differences
C-C tors.jground state
52'3
0.48
0.49
0.50
0.43
0.50
--+ 52,4
61>5 --+ 71,7
60,6 --+ 70,7
62'5
--+ 72,6
63,3 --+ 73,4
Av:
0.480:1:0.026
Sym.def.jground
60,6 --+ 70,7
62,5 --+ 72,6
73,4
62,4 --+ 72,5
71,7 --+ 81>8
Av:
0.36 :1:0.03
Asym.def.jground
61'6
--+ 72,6
71>7 --+ 81>8
Av:
.
210:1:17 cm-1
state
0.33
0.30
0.23
0.22
--+ 71>7
60,6 --+ 70,7
62'5
state
0.32
0.38
0.33
0.41
0.39
0.34
52,3 --+ 52,4
63,3--+
151:1:11 cm-1
0.27 :1:0.05
270:1:38 cm-1
The uncertainties represent one standard deviation.
T
=
296 oK.
mol-l. This conformational behaviour contrasts that found for furan-2-carboxaldehyde which exists with the trans form 251:!::40 cm -1 more stable than the
cis [11].
Isotopic speeies and mo/ecu/ar structure
The spectra of C4H3ND-CHO and C4H3NH-CHI80 were measured to
obtain additional information about the structural and conformational properties
of the molecule. There is not much difference in the rotational constants of the
cis and trans rotamers. However, the model of Table 1 yields unmistakeably better
agreement between the observed and calculated rotational constants than obtained
99
for the trans form. It is therefore concluded beyond doubt that the ds form has
been identified.
The rotational constants of the three speeies studied furnish insufficient
information for a complete structure determination but the very good agreement
with the model rotational constants (Table 1) presumably indicates that there is
no great difference between the true and the plausible structure.
The non-bonded H (1) . . . O distance can be calculatedfrom the present data
using Kraitchman's method [14]. The result shown in Table 5 was found using
TABLE 5
SUBSTITUTIONCOORDINATES' AND O' . H(1) DISTANCE OF PYRROLE-2-CARBOXALDEHYDE
Atom
Principal
H(1)
O
axes
lal (A)
[bl (A)
0.3090::1:0.0067
2.4755 ::1:0.0005
1.7820::1:0.0011
0.3583 ::1:0.0035
Distance
r(H(1)
. . . O)
2.592
::1:0.006 Å
.
The uncertainties represent one standard deviation calculated from the standard deviations of
the B and C rotational constants.
the B and C values in his equations for a planar rotor. The H(l) . . . O distance of
2.592::1::0.006Å is, as expected, close to 2.651 Å calculated for the Table 1 models.
;/"'"""-
Dipole moment
Stark coefficients of the 30,3 -+ 40,4 transition were used to determine the
dipole moment. This experiment was carried out in the same manner as reported
previously [15] and the statistical treatment of the data of Table 6 was similar to
that described in this paper. As the final result fla
=
2.47::1::0.02 D, flb
=
0.16::1::0.06
D, and fltot = 2.48::1::0.02D are given. The uncertainties quoted represent one
standard deviation. Possible systematic errors have been taken into consideration.
The most recent literature value is 2.18 D for the total dipole moment determined
in benzene solution [5]. Theoretical calculations [16] yielded 1.52 D. The dipole
moment of pyrrole-2-carboxaldehyde of 2.48::1::
0.02 D cannot be predicted by
simple vectorial addition of a "ring" dipole similar to that of pyrrole and an aldehyde group dipole of reasonable magnitude. This is presumably a result of conjugation. The dipole moments of ds and trans furan-2-carboxaldehyde have been
explained in a similar way [10].
100
TABLE 6
STARK COEFFICIENTS
AND DIPOLE MOMENT OF PYRROLE-2-CARBOXALDEHYDE
Transtion
!:.V/E2
(MHz/(V/cm)2)
30.3->-40.4 IMI = O
IMI = l
1MI =2
IMI =3
X 106
obs.
ca/c.
- 3.83 :1:0.03
-2.39:1:0.01
2.20:1:0.01
-3.90
-2.38
2.19
9.80
9.61 :1:0.09
11. = 2.47 :1:0.02 D
I1b = 0.16:1:0.06 D
I1to'= 2.48 :1:0.02 D
The uncertainties represent one standard deviation.
DISCUSSION
The stable form of pyrrole-2-carboxaldehyde is the ds both in solution
[1-5] and in the gas phase. The stabil ity ofthis rotamer is pro babl y mainly caused
by three different effects, viz. conjugation, interaction between the pyrrole ring
and the carbonyl group dipoles, and a weak intramolecular hydrogen bond.
T~
rotamer has a geometry which is very favourable for the two former
effects. The planar cis and trans forms allow a maximum overIap between the
n-orbitals of the pyrrole ring and the Sp2 hybridized orbitals of the carbon and
oxygen atoms ofthe carbonyl group. Conjugation may thus explain the preference
of planar forrns, but does not account for the stability of the ds form compared
to the trans. The dipole interaction effect is thought to be a main facto r stabilizing
the ds conformer. Theoretical caIculations [16, 17] indicate that the pyrrole ring
dipole moment is directed along the bisector of the CNC angle with the positive
end directed towards the nitrogen atom. The plausible structure of Table l indicates that the bisector of the CNC angle and C=O bond are about 9° from being
paralleI which is the most favourable direction for dipole-dipole interaction. In
the trans rotamer, on the other hand, these dipoles would tend to repe! each other
and thus destabilize this form. In addition, intramolecular hydrogen bonding is
possible in the ds but not the trans rotamer. This effect is expected to be small,
because of rather unfavourable geometry. The O . . . H distance is for example
2.592 A which is almost exactly the sum of the hydrogen and oxygen atoms van
der Waals' radii [18] (2.60 A). Furthermore, from the Table l structure, the
O . . . N distance is computed
to be roughly 2.83
A compared to 2.90 A being the
sum of the van der Waals' radii of the two atoms [18]. The N-H'
---
. . O angle is
101
calculated to be about 90° and is thus very far from linear. The H . . . O=C angle
is approximately 81°.
It is very difficult to assess the quantitative importance of the two latter stabilization effects which are thought to be the main reasons for the preference of the
cis form to the trans. The hydrogen bond is weak and not expected to stabilize
the cis rotamer as compared to the trans by more than 2 kcal mol-l at most. The
dipole-dipole interaction is perhaps more important although conjugation makes
it rather difficult to estimate its quantitative contribution.
REFERENCES
1
2
3
4
5
6
7
8
9
lO
11
12
13
14
15
16
17
18
R. Alan Jones and A. G. Moritz, Spectrochim. Acta, 21 (1965) 295.
R. Alan Jones and P. H. Wright, Tetrahedron Lett., 53 (1968) 5495.
S. Gronowitz, A.-B. Hornfelt, B. Gestblom and R. A. Hoffman, Ark. Kemi, 18 (1961) 133.
S. Shimokawa, H. Fukni and J. Sohma, Mol. Phys., 19 (1970) 695.
C. W. N. Cumper and J. W. M. Wood, J. Chem. Soc., B, (1971) 1811.
K.-M. Marstokk and H. Møllendal, J. Mol. Struct., 5 (1970) 205.
L. Nygaard, J. T. Nielsen, J. Kirchheiner, G. MaItesen, J. Rastrup-Andersen and G. O.
Sørensen, J. Mol. Struct., 3 (1969) 491.
J. M. Dowling, J. Mol. Speetrase., 6 (1961) 550.
K.-M. Marstokk and H. Møllendal, J. Mol. Struct., 15 (1973) 137.
F. Monning, H. Dreizler and H. D. Rudolph, Z. Naturforsch., 20a (1965) 1323.
F. Monning, H. Dreizler and H. D. Rudolph, Z. Naturforsch., 21a (1966) 1633.
D. R. Herschbach and V. W. Laurie, J. Chem. Phys., 40 (1964) 3142.
A. S. Esbitt and E. B. Wilson, Jr., Rev. Sei. Instrum., 34 (1963) 901.
J. Kraitchman,plmer. J. Phys., 21 (1953) 17.
K.-M. Marsto\k and H. Møllendal, J. Mol. Struct., 16 (1973) 259.
R. D. Brown and B. A. W. Coller, Theor. Chim. Acta, 7 (1967) 259.
D. W. Davies and W. C. Mackrodt, Chem. Commun., (1967) 1226.
L. Pauling, The Nature of the Chemical Band, Cornell University Press, Ithaca, N.Y., 1960,
p.260.