287
Journal of Molecular Structure, 22 (1974) 287-300
(Q Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
MICROW AVE SPECTRUM, CONFORMATION,
INTRAMOLECULAR
HYDROGEN BOND, DIPOLE MOMENT, 14N QUADRUPOLE COUPLING
CONSTANTS AND CENTRIFUGAL DISTORTION OF 2-FLUOROACETAMIDE
K.-M.
MARSTOKK
Department
AND HARALD
of Chemistry,
MØLLENDAL
The University
of Oslo, Blindern,
Oslo 3 (Norway)
(Received 5 October 1973)
ABSTRACT
Mierowave speetra of CHzFCONHz, CHzFCOND(1)H(2), CHzFCONH(l )D(2), and CHzFCONDz are reported. The stable form of the moleeule is shown
to possess a planar FCCONHz skeleton with two out-of-plane hydrogens. The
C-F and C=O bonds are trans to one another and a weak intramoleeular hydrogen
bond is formed between the fluorine atom and the nearest amide group hydrogen
atom stabilizing the identified rotamer. Other conformations are not present in
eoneentrations exeeeding 10 % of the total. Nine vibrationally exeited states were
assigned. Six of these were attributed to the C-C torsional mode and one to the
lowest in-plane bending mode. The first exeited state of -NHz out-of-plane deformation mode was tentatively assigned. Relative intensity measurements yielded
ll4::f::14 cm -1 for C-C torsional mode and 239::f::20 cm -1 for the in-plane bending
mode. The dipole moment was determined as fla = 1.27::f::
0.01 D, flb = 1.67::f::
0.02
D, and fltot = 2.1O::f::0.02D, while the 14N quadrupole eoupling constants were
found to be Xaa = 1.6::f::0.2 MHz, Xbb = 1.6::f::0.2 MHz and Xcc = -3.2::f::0.3
MHz.
~
INTRODUCTION
Several conformations are possible for molecules possessing a CHzF group
attaehed to a earbonyl group. Fig. lillustrates four typieal rotamerie forms of
~roaeetamide
(CHzFCONHz) interehangeable by rotation about the C-C
bond. In conformer I the C-F and C=O bonds are eis and eclipsed. This arrangement is the preferred one in free monofluoroaeetie aeid [1] and in fluoroaeetyl
fluoride [2]. Rotamers analogous to IV where the C-F and C=O bonds are trans
288
.
F
O
H
FO'.
H
.
H
NH2
HNH2
Il. GAUCHE
8=600
I. CIS
8=00
H
O
H
H
O'.
F
H
NH2
Ill. SKEW
8=1200
Fig. 1. Four possible conformations of CH2FCONH2
viewed along the C-C bond. Conformati,
IV differs from the other three in that an intramolecular
hydrogen bond may be forrned betwe
the fluorine atom and the nearest amide group hydrogen atom. This kind of interaction is n
possible in the other three cases.
with respect to the C-C bond have also been found in the two latter compOUl
as their less stable forms. Moreover, the preferred conformation of monofluor
acetone [3] is the trans. An X-ray structure determination of CHzFCONHz 1
Hughes and Small [4] showed that form IV is present in the crystalline sta1
Krueger and Smith [5] utilizing infrared spectroscopy found that a similar situ
tion is present in dilute solutions of carbon tetrachloride. No evidence was fOU
for the stable coexistence of more than the trans rotamer [5]. The stability [5]
this conformation was attributed to the formation of an intramolecular hydrogl
bond formed between the fluorine atom and the nearest amide group hydrog<
atom.
In addition to the conformational behaviour of 2-fluoroacetamide, t]
planarity of the amide group is currently being debated. Microwave spectroscol
is well suited for the study of this problem, because of the high resolution obtai
able. The early microwave investigation of the simplest amide, formamide [E
concluded that this molecule is planar. This was later revised, and the compoUI
was thought to have the two NHz hydrogens slightly out-of-plane [7]. Renewe
t
extensive investigations by Hiro~a et al. [8] have shown that the equili
rium conformation of formamid~s
planar or very nearly so. Very recentl
Steinmetz [9] reported the microwave spectrum of formimide which is also plana
289
The present work was undertaken mainly to study the importanee of intramolecular hydrogen bonding for the conforrnational preferences of 2-fluoroacetamide and, moreover, to investigate the amide group planarity problem. It was
found that the hydrogen-bonded conformation IV mainly, if not exclusively,
exists in the free state. Evidence is presented to show that the amide group of the
molecule is planar or very nearly planar.
EXPERIMENTAL
2-Fluoroacetamide purum was purchased from Fluka AG, Buchs, Switzerland and used without further purification. CHzFCOND(1 )H(2), CHzFCONH(1)D(2), (see Fig. 2 for notation) and CHzFCONDz were produced by direct
/°
o"
a
F
r
N~o
O"
H(!)
H(2)
Fig. 2. Projection of stable conformation
exchange
with 99
% DzO
of 2-fluoroacetamide
in the cello Measurements
in the a-b
principal axes plane.
were made at room tempera-
ture employing a conventional Stark speetrometer described briefly in ref. 10.
This apparatus has no facilities for phase-locking the klystrons. Yet, resolution of
better than 0.5 MHz was achieved under favourable conditions. Almost the complete 11.9-29.9 GHz spectral region was searched extensively using recorder
presentation of the spectra.
MICROWAVE
SPECTRUM
AND ASSIGNMENT
OF THE GROUND
VIBRATIONAL
STATE
Based on the X-ray [4] and infrared findings [5], the trans rotamer depicted
in Fig. 2 was expected to be present in the spectrum. Preliminary rotational constants of this form were calculated fr~the
structural parameters of Table l,
290
TABLE 1
PLAUSIBLE
STRUCTURAL
PARAMETERS'
AND OBSERVED AND PREDICTED
LNCO
LNCC
LCCF
LH(I)NC
124.0°
118.7°
110.0°
124.5°
ROTATIONAL
CONSTANTS
2-FLUOROACETAMIDE
C~O
C-F
C-N
C-C
1.254
1.406
1.319
1.533
Å
Å
Å
Å
N-H (l)
N-H (2)
C-H
1.03 Å
1.00 Å
1.093 Å
LH(I)NH(2)
LCCH
LHCH
llC
109
109
Rvtational constants (MHz)
.
Observed
Calculated
Observed
Calculat~
Ao
Bo
Co
CH2FCONH2
9884.358
4059.671
2932.474
9754.775
4019.236
2898.831
CH2FCOND(I)H(2)
9329.76
4055.98
2879.88
9153.58
4016.35
2841.89
Ao
Bo
Co
CH2FCONH(1)D(2)
9293.07
9136.15
3994.87
3957.40
2845.52
2810.60
CH2FCOND2
8804.18
3992.53
2796.93
8614.26
3955.44
2758.22
Not a derived
structure.
See text.
which, with the exception of the CH2F-group hydrogen coordinates, were take
from ref. 4. The dipole moment and its components along the a and b princip
axes were predicted by vectorial addition of bond moments taken from ref.
to be about 0.9 and 1.7 D, respectively.
Search was therefore initially made for the strong low J b-type Q-bran(
transitions. A fairly rich, intense spectrum revealed the existence of many prorr
nent Q-branch transitions. Their characteristic Stark effects and 14N quadrupo
fine structure led to their assignments and the determination of A - C and K. TI
low J, low K-l a-type transitions Were then assigned on the basis of their Stal
effects, positions in the spectrum, intensities, and rigid rotor fit. The rotation
constants obtained in this manner were used to predict additionallow and mediUJ
J lines, which were subsequently measured, corrected for quadrupole fine structUJ
when necessary and fitted to Watson's first order centrifugal distortion formulc
w = Wo - dJJ2(J + 1)2 - dJKJ(J + I )<pi>
-dK<pi>-dwJWoJ(J+
l)-dwKWo
<pi>
(J
utilizing a computer programme [10] based on the perturbation form of eqn. (I
The improved rotational and centrifugal distortion constants thus obtaine
were used to predict further high J transitions. The assignments of these mediUI
to low intensity lines were made oj the basis of their intensities, fit to eqn. (l
and their most useful Stark effectl" For the high J a-type Q-branch transitions
slow Stark effect towards higher frequencies was observed. The high J P- an
291
TABLE 2
SELECTED TRANSITIONS
FOR THE GROUND
Transition
VIBRATIONAL
STATE OF CH2FCONH2
Observed
Obs.-calc.
frequency'
(MHz)
frequency
(MHz)
-->-->-->-->-->-
20,2
21.1
30,3
31.3
32.2
-->- 43.2
-->-103,7
-->-175,12
-->-288,20
-->-4814.34
13835.99
15111.40
20399.46
19198.27
20976.29
28231.38
12976.53
16458.77
22444.91
13571.94
0.03
-0.03
0.03
-0.01
0.02
0.05
0.04
0.03
-0.04
0.06
-
-->- 11.1
21.2
22,0
22.1
30.3
32.1
-->- 41,4
-->- 41,3
-->- 42,2
-->- 52,3
-->- 53,2
-->- 72,5
-->- 82.6
-->- 93,6
-->-113,8
-->-2211.12
-->-2211.11
-->-2416.9
-->-2416,8
-->-3116,16
-->-3116,15
-->-3019,12
-->-3019,11
-->-4121,21
-->-4121,20
-->-4025,15
-->-4025,16
-->-5126,26
-->-5126,25
-->-5232,21
-->-5232.20
12816.76
18681.73
17622.27
20855.49
15553.57
16607.99
29095.50
13663.93
15923.77
15965.90
28983.37
26358.26
23104.13
24101.10
27921.47
11927.54
11927.54
19039.08
19039.08
12744.02
12744.02
14073.14
14073.14
21227.52
21227.52
19018.54
19018.54
29734.15
29734.15
22255.06
22255.06
-0.06
-0.03
-0.01
-0.03
-0.07
-0.02
-0.01
0.03
0.08
-0.01
-0.05
0.03
0.06
-0.05
0.07
0.08
-0.15
0.09
0.09
-0.05
-0.05
-0.01
-0.01
-0.03
-0.03
0.04
0.04
-0.07
-0.07
0.03
0.01
0.02
0.09
0.13
0.05
0.15
0.12
0.27
0.24
0.39
0.61
0.80
1.71
1.74
3.78
5.89
5.89
- 23.84
- 23.84
19.40
19.40
- 38.29
- 38.29
40.02
40.02
- 89.32
- 89.32
71.33
71.33
-188.24
-188.24
Centrifugal
dist. correction
(MHz)
a-type
10.1
11.0
20.2
21,2
22,1
33.1
103.8
175.13
288.21
4814,31
0.02
0.05
0.05
0.05
0.16
0.46
2.46
- 11.07
- 52.24
-161.65
b-type
00,0
10,1
21,1
21,2
21,2
31,2
30.3
40,4
41,3
51,4
52.3
63,4
81.7
92,7
112,9
2112,9
2112.10
2515,10
2515.11
3017.13
3017.14
3118.13
3118,14
4022,18
4022,19
4124.18
4124,17
5027.23
5027,24
5331,22
5331,23
-->-->-->-->-->-
0.03-"-/
.
:1::0.10 MHz.
Transitions
are corrected
for quadrupole
effects.
-
292
TAB LE 3
MOLECULAR
Vibrational
CONSTANTS FOR CHzFCONHz
state:
AND SEVERAL VIBRATIONALLY
First ex. C-C torso
Ground
133
0.055
Number oftransitions
a(MHz)
Av (MHz)
Bv (MHz)
Cv (MHz)
la (uA Z)
lb (uAZ)
le (uAZ)
dJ (kHz)
dJK (kHz)
dK (kHz)
dWJ X 107
dWK X 106
la+lb-Ie
(uAZ)
IN THE GROUND
9884.358::1::0.004
4059.671 ::1::0.002
2932.474::1::0.001
51.12889 ::1::0.00002
124.48694 ::1::0.00006
172.33776::1::0.00006
-2.06::1::0.05
-31.2::1::1.3
-50.7::1::2.4
8.01 ::1::0.14
8.41 ::1::0.38
3.27807 ::1::0.00009
Second ex. C-C torso Third ex. C-C.
75
0.071
31
0.149
18
0.138
9.09::1::0.30
9795.62::1::0.04
4052.86::1::0.02
2943.81 ::1::0.02
51.5920::1::0.0002
124.5732::1::0.0006
171.6741 ::1::0.0011
-0.16::1:: 1.25
-63.4::1::26.7
-120.0::1::44.3
2.9::1::3.3
7.73::1::0.66
18.4::1:: 7.7
9839.787
::1::0.007
4056.206
::1::0.003
2938.174::1::0.002
51.36046::1::0.00004
124.59328::1::0.00009
172.00343
::1::0.00011
-2.38::1::0.11
-28.8::1::2.2
-46.3::1::4.2
3.95031
EXCITED STATE
::1::0.00015
4.4911
9752.10::1::0.07
4049.69
::1::0.04
2949.29
:::1:0.04
51.8223
:::1:0.00
124.7937
:::1:0.00
171.3551
:::1:0.00
6.1 :::1:3.4
-305.8:::1:66.1
- 533.5:::1: 111.
-19.7:::1:8.2
88.5:::1:19.2
5.2609
::1::0.0013
:::1:0.00
Conversion factor 505376 MHz uAz.
The uncertainties represent one standard deviation.
a is the standard deviation of the fit.
R-branch lines exhibit a fast Stark effect due to the near degeneracy of the hi)
K- 1 energy levels connected by the a-axis components of the dipole moment.
total of 133 transitions were assigned with a maximum J value of 53. Table 2 lis
41 selected transitions. 'The comprehensive list of frequencies for the groUt
vibrational state as well as for several excited states and the deuterated speeies
available from the authors upon request or from the Microwave Data Cente
National Bureau of Standards, Washington, D.C., D.S.A., where it has bet
deposited.
TABLE 4
CORRELATION
MATRIX FOR THE
ROTATIONAL AND CENTRIFUGAL DISTORTlO1CONSTANTS OF TH
GROUND
VIBRATIONALSTATE
OF CHzFCONHz
A
B
C
dJ
dJK
dK
dWJ
1.000
-0.141
0.182
0.182
0.121
-0.182
1.000
-0.938
-0.937
-0.987
0.937
1.000
1.000
0.868
-1.000
1.000
0.868
-1.000
l. 000
-0.867
dWK
1.000
0.962
0.890
-0.073
-0.019
-0.020
0.117
0.020
l. 000
0.760
-0.016
-0.148
-0.149
0.095
0.149
1.000
293
TABLE 3 (continued)
Fourthex. c-c torso Fifth ex. c-c torso
13
0.129
Sixth ex. c-c torso
First ex. in-p/ane bend First ex. in-p/ane b,
+.first ex. C-C te
7
0.497
8
0.411
5
0.655
15
0.109
9847.3
9666.18
:::!:0.08
9624.44:::!:0.10
9891.93
:::!:0.07
4043.57
:::!:0.05
4040.69
4054.75:::!:0.04
4051.0:::!:0.1
2954.93
:::!:0.06
2928.9:::!:0.1
:::!:0.07
:::!:0.07
:::!:0.3
9709.14:::!:0.11
4046.61
2960.42:::!:0.05
2965.92:::!:0.07
2922.70:::!:0.04
52.0516:::!:0.0006
52.2829
:::!:0.0004
52.5097
51.0897
124.889
:::!:0.002
124.983
:::!:0.002
125.072:::!:0.002
124.6380:::!:0.0012
124.753:::!:0.003
171.028
:::!:0.003
170.711
:::!:0.003
170.394:::!:0.004
172.9141
172.548
:::!:0.0005
-3.1:::!:5.4
51.321
:::!:0.0004
:::!:0.0022
:::!:0.002
:::!:0.006
5.0:::!:2.3
-168.5:::!:35.3
-294.4:::!:55.2
- 59.1 :::!:46.7
20.2:::!:85.3
-9.4:::!: 143.8
4.3:::!: 13.6
- 3.2:::!:25.1
5.913 :::!:0.004
48.5:::!: 10.1
6.555
:::!:0.004
7.188:::!:0.005
3.526:::!:0.007
2.8136:::!:0.0025
Table 3 gives the result obtained from this analysis. Despite the large number
of lines comprising a-type, b-type, P-, Q-, and R-branch transitions used in the
least squares procedure, several correlation matrix elements for the centrifugal
distortioll constants are dose to the absolute value of 1 as shown in Table 4. As
note d before [12], this seems to be an inherent difficulty with this procedure.
As indicated in Table 3, la +.lb - le = 3.27807::!::0.00009 UÅ l which is very
dose to its counterparts in trans and cis monofluoroacetic acid [1] where the
values 3.2195 uÅl and 3.2326 uÅl, respectively, have been determined. The latter
conformers were each proved to possess symmetry planes and two out-of-plane
hydrogens.
VIBRATIONAL
SATELLITE SPECTRA
The ground vibrational state lines were accompanied by a rich satellite
spectrum.
The strongest
of the se absorption
lines were about 60
% as
intense as
the corresponding ground state transitions. They are assigned to the first excited
state of the C-C torsional mode because la +.lb - le increases upon excitation [13]
as shown in Table 3. The satellites attributed to its overtone states lie in a series of
steadily decreasing intensity. Six excited states of this fundamental mode were
assigned. As shown in Table 3 and illustrated in Fig. 3, the variation of the rotational constants and la +.lb -le upon excitation is dose to linear. This is expected
for a very nearly harmonic mode [13].
294
MHz
30
A~co
O/°
20
/
/°
10
O/°
,~
2
-10
3
4
5
°"" °""
~o,
-20
6
B.- Bo
'o
°""0
Fig. 3. Variation
of the rotational
v
(A. -Ao) / 10
constants up on excitation
of the C-C torsional
mode.
Another satellite spectrum having about 30 % of the intensity of the grom
state is attributed to a low frequency in-plane bending mode because la+ lbdecreases upon excitation [13] as shown in Table 3. The rotational constants of
combination mode of the above two fundamental frequencies were also dete
mined (Table 3).
Six 'Q-branch
transitions
of about
15 % intens ity of the correspondil
ground state lines were assigned and A-C = 6917.09 MHz and K = -0.6743<
determined. Based on their positions in the spectrum relative to the correspondil
ground state lines, this mode is presumably an out-of-plane fundamental which v
tentative1y assign as the first excited state of the -NHz group out-of-plane defo
mation mode. Crude relative intensity measurements suggest that this mode
about 400cm-1 abovethe ground state.
The spectroscopic constants of the excited states appearing in Table 3 wel
derived by application of Watson's formula (1) in a manner analogous to th:
described for the ground state when a sufficient num ber of transitions were avai
able. In the other cases, a rigid rotor fit was performed making no allowance fe
centrifugal distortion effects. As seen in that table, well determined centrifug:
distortion coefficients could only be obtained for the first excited state of the C-I
torsional mode, whereas their orders of magnitude were found for other vibratior
ally excited states.
A total of more than 300 transitions were definitely assigned for the mai
295
species. This includes all strong lines present in the spectrum. The great majority
of the medium intens ity and weak lines were also accounted for. This extensive
analysis makes two conclusions possible. Firstly, there is largely one rotameric
form of 2-fluoroacetamide, viz. the trans. It is ruled out that further rotamers
exist in concentrations
exceeding 10 % of the total, because bond moment calcula-
tions strongly indicate that additional forms should have sizable dipole moments
making their presence readily detectable if this percentage were exceeded.
The Fig. 2 conformer is thus at least 1.0 kcal mole -1 more stable than any
other form of the molecule. Furthermore, the harmonic behaviour of C-C torsional mode through its sixth excited vibrational state is indicative of a deep
potential well. Any maximum of the rotational barrier for the torsion about the
C-C bond must appear several kcal mole -1 above the minimum.
The second conclusion possible from the analysis of the vibrationally excited states concems the amide group planarity problem. If the -NH2 group outof-plane deformation mode were governed by a strongly anharmonic double
minimum potential, the spectrum normally observed consists of the ground state
transitions accompanied by a set of excited state lines of eomparable intensities
[14, 15]. If observable, the intensities of the transitions of the next two exeited
states should be greatly redueed eompared to those in the first two states. This
feature was clearly not present in the speetrum. As diseussed above, the only possibility for the first exeited state of the deformation mode appears to be about
400 cm -1 above the ground vibrational state. Thus, the vibrational satellite speetrum strongly tends to refute the existenee of any typieal anharmonie double
minimum potential for the -NH2 group deformation mode.
TABLE 5
RELATIVE INTENSITIES'
AND ENERGY DIFFERENCES
OF VIBRATIONALLY
ACETAMIDE
Relative intensity
Transition
Erergy difference
c-c tors.jground state
42.3 --+
51.4
81.7 --+
82.6
82.6 --+
83.5
92.7
93.6
102.8
--+
0.58
0.54
0.60
0.65
0.54
--+ 103.7
Av. 0.58::1::0.04
In-plane bend.fground
81.7 --+ 82.6
82.6--+ 83.5
92.7 --+ 93.6
114::1::14 cm-1
state
0.36
0.31
0.30
Av. 0.32::1::0.03
239::1::20 cm-l
The uncertainties represent one standard deviation.
. T = 301 K.
EXCITED
STATES OF 2-FLUORO-
296
Relative intensity measurements on the first excited states of the Ctorsional and the in-plane bending modes were performed. Most, but not all
the precautions of ref. 16 were observed. The results are presented in Table
The energy difference between states were derived by assuming that relative
tensity is proportional to the Boltzman factor. The torsional frequency \\
determined to be 114:1:14 cm-l and the in-plane bending mode was 239:1:
cm-l. The torsionalfrequencymaybe approximated [13]fromro(cm-l) = 67.51
where L1= 1/ + lbl - lel - (la0+ lbO- leO). Substituting L1= 0.672 uÅ2 tak
from Table 3, ro = 100 cm-l is calculated which compares well to 114:1:10 cm
obtained by the relative intensity method.
DEUTERATED
SPECIES AND MOLECULAR
STRUCTURE
The spectra of CH2FCOND(1)H(2),
CH2FCONH(1)D(2),
a
CH2FCOND2 were measured in order to obtain additional infOlmation about t
structural and conformational properties of the amide. The spectra were assign
in a straight forward manner and the evaluated spectroscopic constants are list
in Table 6. The values derived for la+ lb - le in the three cases are very dose to t
main speeies counterpart. This is strong evidence for the presenee of a plane
symmetry and two out-of-plane hydrogens which must belong to the CH2F p:
of the molecule.
TABLE 6
MOLECULAR
lsotopic
CONSTANTS FOR THE GROUND
species
Number of transitions
(J (MHz)
Ao (MHz)
Bo (MHz)
Co (MHz)
la (uA 2)
lb (UA2)
le (UA2)
dJ (kHz)
dJK (kHz)
dK (kHz)
dWJ X 107
dWKXl06
la+lb-le
(UA2)
VIBRATIONAL
CH2FCOND(l)H(2)
24
0.088
9329.76::1:0.03
4055.98 ::1:0.03
2879.88 ::1:0.03
54.1682::1:0.0002
124.6002::1:0.0009
175.4851 ::1:0.0017
-2.4::1:1.1
-45.5::1:11.8
- 82.5::1: 17.9
6.9::1:1.9
13.3::1:3.4
3.2833::1:0.0019
Conversion factor 505376 MHz UA2.
The uncertainties represent one standard deviation.
(J is the standard deviation of the tit.
STATE OF DEUTERATED
SPECIES OF 2-FLUOROAC
CH2FCONH(l)D(2)
24
0.1I5
9293.07 ::1:0.04
3994.87 ::1:0.02
2845.52 ::1:0.02
54.3820::1:0.0002
126.5062::1:0.0006
177.6041 ::1:0.0011
-1.0::1: 1.0
-54.4::1:19.1
-96.1 ::1:28.6
7.0::1:2.9
15.7::1:5.6
3.2841 ::1:0.0013
CH2FCC
24
0.11:
8804.18
3992.53
2796.93
57.4018
126.5804
180.6895
-1.2
-57.4
-48.5
5.8
15.7
3.2927
297
T ABLE 7
SUBSTITUTION
Atom
H(1)
H(2)
COORDINATES
Principal
FOR 2-FLUOROACETAMIDE
axes
lal (A)
[bl (A)
[el (A)
0.3268 :1::0.0002
1.3928 :1::0.0001
1.7497 :1::0.0002
1.8335 :1::0.0002
0.0546:1::0.0119
0.0568 :1::0.0100
CH2FCONH2 is the parent molecule.
The uncertainties represent one standard deviation calculated from the standard deviations of
the rotational constants.
Kraitchman's coordinates [17] of the amide group hydrogens may be calculated from the rotational constants of 'Tables 3 and 6. Any of the four isotopic
speeies studied can be employed as the parent molecule. 'Table 7 shows the result
obtained with CH2FCONH2 as the basis molecule. As expected, the c-axis coordinates are very small and not significantly different from zero. This, of course, supports the idea of the presenee of a plane of symmetry.
Assuming appropriate signs for the principal axes coordinates of 'Table 7
and putting c = 0.0, the non-bonded H . . . H distance of the -NH2 group was
calculated to be 1.7216:1::0.0002Å. The corresponding distance in formamide [7] is
1.735 Å, and the X-ray value of 2-fluoroacetamide [4] is 1.668 Å.
'The rotational constants of the main and the three deuterated speeies furnish
insufficient information for a detailed structure determination. However, the
structure of 'Table 1 reproduces the rotational constants and the principal axis
coordinates of 'Table 7 reasonably well. 'This, perhaps, indicates that there is no
great difference between the solid state and the free molecule geometry.
DIPOLE MOMENT
Stark coefflcients of the 61,5 -> 62,4 and 62,4 -> 63,3 transitions were used
to determine the dipole moment. A d.c. voltage was applied between the Stark
septum and the cell with the modulating square wave voltage superimposed. The
d.c. voltage was measured with a digital voltmeter having an accuracy of 0.025 %.
'The electric field was calibrated using the OCS 1 -> 2 transition with /locs =
0.71521 D (ref. 18). A least squares fit using a diagonal weight matrix was performed. 'The weights were chosen as the inverse squares of the experimentally
determined standard deviations of the coefficients appearing in 'Table 8. From the
fit, /la = 1.267:1::0.006D, /lb = 1.67:1::0.01D, and /l!O!= 2.10:1::0.01D were determined. However, taking possible systematic errors into account, the latter
standard deviations are probably toa small, possibly by a factor of 2. As the final
result /la = 1.27:1::0.01 D, /lb = 1.67:1::0.02D, and /l!O!= 2.10:1::0.02D are given.
298
The uncertainties quoted represent one standard deviation. The total dip
moment determined in a benzene solution [19] was 2.64:tO.04 D.
TABLE 8
STARK COEFFICIENTS AND DIPOLE MOMENT OF 2-FLUOROACETAMIDE
Transition
61.5 -+ 62...
62... -+ 63.3
l~vfE21
(MHzf(Vfem)2
[MI =4
IMI =5
IMI =6
IMI=4
IMI =5
IMI=6
!ta =
!tb =
!t,.t =
x 105)
Obs.
Cale.
0.858::1:0.008
1.38 ::1:0.01
2.00 ::1:0.01
2.46 ::1:0.02
4.26 ::I:O.Q3
6.28 ::1:0.06
1.27 ::1:0.01 D
1.67 ::1:0.02 D
2.10 ::1:0.02 D
0.862
1.37
2.00
2.40
4.26
6.53
The uncertainties represent one standard deviation.
TAB LE 9
l"N
NUCLEAR QUADRUPOLE
SPLITTINGS
MINE THE QUADRUPOLE
COUPLING
Transition
F-+F'
OF 2-FLUOROACETAMIDE
MICROWAVE
LINES USED TO DETI
CONSTANTS
Frequency
(MHz)
Obs.
Cale.
Centre
12817.29
16.73
13662.92
50.5 -+ 51...
35 -+
-+ 35}
5-+5
18110.61
60.6 -+ 61.5
64-+4
-+ 6}
6-+6
23589.66
70.7 -+ 71.6
75-+5
-+ 7)
7-+7
12817.16
16.68
13662.97
64.28
64.59
18110.58
11.97
12.25
23589.65
91.04
91.27
29760.52
61.85
62.04
20854.29
55.83
22632.64
33.88
25034.35
35.47
28056.69
7.76
7.97
12816.76
40... -+ 41.3
1 -+ 1
1-+2
4-+4
00.0 -+ 11.1
86-+6
-+ 8)
21.2 -+ 22.1
31.3 -+ 32.2
41... -+42.3
51... -+ 52.3
2-+2
3-+3
3-+3
4-+4
4-+4
5-+5
5-+5
4-+4
6 -+ 6)
64.58
12.14
91.42
29760.52
62.20
20854.41
55.82
22632.60
33.88
25034.22
35.50
28056.75
58.05
12663.93
18111.58
23590.64
29761.46
20855.49
22633.57
25035.17
28057.46
299
14N
QUADRUPOLE
COUPLlNG
CONSTANTS
Several of the observed lines were split due to quadrupole coupling of the
14N nudeus with the molecular rotation. Nine transitions of which splittings were
resolved are listed in Table 9. The diagonal elements of the quadrupole coupling
tensor were determined from these transitions as Xaa= 1.6:t 0.2 MHz, Xbb=
1.6:t0.2 MHz, and Xcc= -3.2:t0.3 MHz. Computer programme [20] MB09
was used to evaluate these parameters. The above values were also used to calculate the centre frequencies of the split excited state lines.
Xccis dose to -3.6 MHz determined for forrnamide [6] and -3.41:t0.07
MHz obtained for formimide [9].
DISCUSSION
2-fluoroacetamide exists as the trans form in the crystalline state [4], in
solution [5], and in the gas phase. The remarkable stability of this conformer in
the latter state can be mainly ascribed to two different intramolecular forces, viz.
the five-membered intramolecular hydrogen bond and the electrostatic interaction
of the bond dipoles.
The hydrogen bond geometry is probably unfavourable for the formation
of astrong intramolecular bond. Utilizing the plausible structure of Table l, the
non-bonded H(l) . . . F distance is computed to be about 2.31 Å which is approximately 0.25 Å shorter than the sum of the pertinent Van der Waals' radii [21].
The N . . . F distance is roughly 2.65 Å compared to 2.85 Å which is the sum of
the N and F Van der Waals' radii [21]. The N-H (l ) . . . F angle is far from linear.
The plausible structure yields 98°. Moreover, the LH(I)'"
F-C is about 89°
and the N-H (l ) and C-F bonds are approximately 7° from being paralleI. These
values are hopefully correct to within a few hundredths of an Å and a few degrees.
It is difficult to estimate quantitatively how large the hydrogen bond stabilisation
energy is, but the 1-3 kcal mole -1 range appears reasonable.
The identified rotamer has a geometry which is very favourable for electrostatic bond dipole stabilisation. The c=o and C-F bonds are trans and there is
thus a minimum of bond dipole repulsion. There should also be considerable attraction between the C-F and the two N-H bond dipoles. Moreover, the N-H(l)
and the C-F bonds are dose to being parallel which is the most favourable arrangement. The electrostatic stabilisation energy is perhaps as important as hydrogen bonding in this case. It is difficult to estimate quantitatively its contribution,
but an interaction energy in the 1-3 kcal mole -1 range is expected.
The two effects discussed above both tend to stabilize the Fig. 2 form,
possibly by several kcal mole -1, and largely explain the infrared and microwave
evidence for the non-existence of additional rotamers.
300
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1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
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