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Dipole moment studies of lactams
321 Journal of Molecular Structure, 23 (1974) 321-327 @ Elsevier Scientific Publishing Company, Amsterdam
- Printed in The Netherlands
DIE’OLE MOMENT STUDIES OF LACTAMS EDMUND R. MALINOWSKI*,
MAGHAR S. MANHAS AND MARK GOLDBERG
Department of Chemistry and Chemical New Jersey 07030 (U.S. A.)
Engineering,
Steaens Institute
of Technology,
Hoboken,
VINCENT FANELLI St. Francis Preparatory
High
(Received
1973)
7 November
School,
Brooklyn,
New
York (US-A.)
ABSTRACT
The electric moments of a series of substituted IV-phenyl-p-lactams were measured in p-xylene solution. This study gives evidence that the dihedral angle between the N-phenyl ring and the fi-lactam ring of the ortho-bromo and orthofluoro derivatives is 52” and 42” with the halogen lying on the side farthest from the keto group. Ultraviolet data appear to substantiate this conclusion. Dipole data indicate that the IV-phenyl-3-azido-4-p-nitrophenyl-B_Iactam compound has a cis configuration with respect to the azido and p-nitrophenyl groups. The moments and conformations of some y-lactams and &lactams are also discussed.
APPARATUS
AND CHEMICALS
The dielectric apparatus used in this study was the same as that described in the earlier work [l], based on the design of Chien [2]. For measuring accurate refractive indices a Brice Phoenix Differential Refractometer, with modifications for temperature variability, was utilized. The dielectric and refractometer cells were thermally controlled to within +0.02 “C of the desired temperature. Zone-refined para-xylene (99.91 % minimum purity), obtained from Litton Chemical Co., was used as the solvent in all dipole studies. This solvent is nonpolar and has a higher boiling point than benzene, most commonly used for dipole studies. Loss due to evaporation during the measurements is minimized and studies at higher temperature are more feasible. The organic compounds under investigation were prepared and purified in this laboratory by methods described previously [3 ]_ * Author
to whom inquiries should be addressed.
322 TREATMENT
OF DATA
Obtaining accurate densities, which are required in dipole measurements, poses a tedious and time-ccnsuming project and is extremely difficult to determine at high or low temperatures_ The simple derivation outlined below shows that dipole
moments
can be measured
accurately
from
dielectric
constant
and re-
fractive index measurements alone. From BSttcher [4] we learn that the polarization of the solute, P. can be expressed in terms of the dielectric constant E, refractive index n, molecular weight M, density d and mole fraction x.
‘_n&!)[(M’d,M’) El
(
p=
-ES(%)J+
Eli-2 +
Subscripts
3M,
* (
d,(q+2)2
2x2
6M,n,
_ )
o
d&:+2j2
I, 2 and 12 refer to the solvent, solute and solution,
dn,, ( ‘ax, ) 0
(1)
respectively. The
subscript zero signifies infinite dilution. For a dilute solution the mole fraction of solute is proportional to the weight fraction of solute w,; i.e. x2 = (MI/M2)~Z_ Therefore P=M,
-Ml
&r--l
[( [( -
E,+2
1
+I
-
&
:
(‘I -‘)
&l--l
n;-1
El+2
n:+2 ) dl
_-_-
1
1
+ d&;“i
2J2 -
d, (:;:2J2]
(2)
where a = (~%,~/i7w~),, /3 = (dd,2/dw2)0 and y = (an12/aw2)o. For nonpolar solvents E is approximately equal to n’. Hence the term in eqn. (2) involving /3is negligible and density measurements of the solutions need not be made. The polarizability expression reduces to P = AI2
(3)
The density and dielectric constant for pure para-xylene have been measured [S] as a function of temperature from 20 to 130 “C. For measurements carried out in para-xylene at 25 “C P = A42 (0.1933 a-O.5808 y)
(4)
Dielectric constants and refractive indices of dilute solutions of organic substances dissolved in zone-refined para-xylene measured as a function of concentration and temperature .are shown in Table I. A least-squares method was
323 TABLE
1
DIPOLE
MOMENT
DATA
02
(I)
x 103
A& x I03
An x 105
I-pheByI-azezidin-2-one 1.562 4.124 5.107 5.989
11.47 31.39 40.63 47.17
13.4 35.1 45.9 53.9I
T a y jl
= 25.0 “C = 7.830 = 0.0888 = 3.24 D
5.613
11.98 14.69 29.87 38.49
15.8 40.7 52.71
T Q y p
= 25.0 “C = 6.980 = 0.0951 = 3.78 D
6.70 15.16 25.69 36.54
9.0 16.3 28.1 39.3I
T u y p.
= 25.0 “C = 6.581 = 0.0715 = 3.91 D
12.79 25.42 40.45 51.65
12.4 25.9 42.0 53.5I
T = 25.0 “C CL = 8.771 y = 0.0906 p = 3.82 D
7.35 14.21 15.01 21.88
17.4 34.9 38.5 50.7I
T = 25.0 “C a = 3.410 y = 0.0822 ,LL= 2.60 D
6.520
4.11 7.77 12.34 15.39
11.1 22.5 31.1 42.7 1
T a y p
f .769 3.500 5.280 6.976
6.76 13.52 20.44 27.95
12.1 22.4 33.6 46.4
(2) I-p-bromophenyl-areridin-2-one
5.890
0
(5)
I-o-bromophenyl-azetidine-2-one 2,087
4.292 4.594
Bl-
A-
/
CH2
G /rN\-/
6.211 )‘=H2
C II
1.638 3.271
ij
4.933
u
(6)
19.5
= 90.0 “C = 2.410 = 0.0653 = 2.55 D
I-o-~t~oTup~lenyl-azeridin-2-one F ‘32
N(
>“2
ii
T = 25.0 “C a =
3.940
y
0.0654 2.42 D
=
p =
0
(7) N-phenyZ-3-azido4-phenyr-8_racram
o\-
/
N’=“kH-N
lc”
II
0
3
1.177 1.926 3.086
8.1 13.6
10.8 17.2
20.9
24.0
3.644 5.559
24.2 37.8
41.8 33.8
T = 25.0 “C a = 6.778 y p =
4.06 D 0.0560
324 TABLE
I (continued) w2 x 103
A~x10~
AnxfO'
2” = 25.0 ‘C 5.215 y = O-0889 p = 4.00 D
a =
N(tH>H-N,
2.03 1 2.226
14.5 17.0
18.2 20.5
T = 25.0 OC
3.375 4.636 5.393
27.6 37.2
43.0
30.4 45.0I 51.0
CL = y = 1’c=
1.335 2,974 4.342 5.876
11.99 26.38 39.33 53.40
12.3 26.5 38.8 53.6
a = y = /.c =
7-950 0.0941 4.74 D
ii
0
(IO) I-phenyI-pyrroiidin-Lone T = 25.0 “C
9.046 o.O!ms 3.66 D
d
(il)
I-p-bronmphenyl-pyrrolidin-Z-one 1so7
12.80
13.2
2.983
24.95 41.33 49.97
26.6 42.0 51-7I
5.668
11.07 22.53 35.03 42.60
15.4 30.1 45.3 56.3I
y = jk =
1.078 2.273 3.340 4.663
12.18 27.06 37.88 49.90
9.8 21.3 28.6 37.2
T = 25.0 ‘C CL = 11.060 y = 0.0837 pr = 4.10 D
4.678 5.668 El-
d0
I so7 2.983 4.678
T = 25.0 OC u = 8.750 y = p =
0.0903 4.39 D
T = 75.0 “C a = 7.505 0.0989 4.57 D
(22) I-phenyI-piperGin-Z-one
325 used to calculate a and y; namely u = C%d& Cof
y _ -p Cc@n ’
(5)
Cd
where de = E~~-E~, An = n12- n, and the sum is taken over each dilution. Values for a and /‘I are shown in Table 1. Polarizabilities were obtained by employing eqn. (3) at the appropriate temperature. Dipole moments, p, listed in Table 1 were calculated by the well-known expression [4] p = 0.012812 (PI-)+
(6)
in which T is the absolute temperature. The uncertainty in the dipole measurements was estimated to be approximately IfrO.03 D.
DISCUS!GION
and NMR spectra of N-phenyl-/I-lactams indicate that the n orbitals of the IV-phenyl system and carbonyl system conjugate with the 2p electron pair of the nitrogen forming a planar sp2 nitrogen-bond system. Dipole measurements [7] of the N-phenyl succinimides, where the same structure is present, also provide evidence that the three nitrogen bonds lie in a plane. X-ray studies [SJ have demonstrated that the nitrogen valences are nearly planar and that the N-phenyl ring is coplanar with the /?-lactam ring. From the moments of I-phenyl-azetidin-2-one, its p-bromo and p-chloro derivatives (see Table 1, compounds (1), (2), (3) and (4)), and the moments given in Table 2 we conclude that the vector moment of the parent compound makes an angle of 82” with respect to the N-phenyl bond which is parallel to the carbonhalogen bond. This moment essentially coincides with the carbonyl group since the moment of the IV-phenyl group has been shown to be very small 17). Studies
[6] of the UV
The geometrical orientation between the phenyl ring and the /I?-lactamring poses a challenging problem. If we assume that the two rings are coplanar then TABLE DIPOLE
2 MOhfENTS= USED IN CALCULATIONS
Compound
Dipole moment (0)
Fluorobenzene Bromobenzene Chlorobenzene Ethyl-azide Nitrobenzene
1.49 1.56
1.59 2.12 3.98
a A. L. %fcClellan, Tables of Ekpe~imental Dipoles, W. H. Freeman, San Francisco, 1963.
326 the ortho-substituted derivatives can exist as one of two extreme forms, i.e. cis or trans with respect to the keto group. Our computations show that the cis compound of the fluorine derivative would have a moment of 4.51 D and the trans 1.94 D. Neither of these agree with the observed value, 2.42 D. If the phenyl ring could freely rotate around the N-phenyl bond the moment would be 3.47 D.. This value atso disagrees with the observed moment. If the rotation were restricted due to a potential energy barrier, we could expect the cis and trans forms to exist in equilibrium. From the observed moment, together with the calculated moments for the cis and trans forms, we estimate that the molecule would exist as 87 % trans and 13 % cis. If this were so we might expect the overall measured dipoie moment to vary with temperature. At 90 “C we found the moment of the orthobromo derivative to be 2.55 D, which is identical, within experimental error, to that observed at 25 “C, 2.60 D. Furthermore an increase in temperature should cause an increase in moment and not a decrease as observed. Another possible explanation is apparent. The benzene ring could be twisted out of the plane of the /?-lactam ring occupying a preferential orientation. On this premise, using the component moments listed in Table 2 together with the moments of compounds (11, (5) and (61, we would conclude that the benzene rbg is tilted 52” and 42”, respectively, for the ortho-bromo and ortho-fluoro derivatives, the halogen lying on the side farthest away from the keto group. An examination of the UV spectra of these molecules offers further evidence that the rings are not coplanar. The absorptivity is related to the degree of n-bonding. If the benzene ring of the para-substituted compound is coplanar with the j&la&am ring, then the z-electron ring system would be fully operative over both rings. In the o~ho-substituted compound the twisting of the ring reduces the degree of n-bonding. From simple x-bond molecular-orbital theory we learn that co&I = &,,,(ortho-halogen)/E,,,(parahalogen), where 8 is the angle of twist. .emar was found [6] to be 8 650 and 23 400 for the ortho and para-bromo derivatives, from which we conclude that 0 = 53”. This compares excellently with our estimate from dipole data, 52”. A recent X-ray study [PI of the ortho-bromo derivative indicates a twist of only two degrees. This study was conducted on the compound in the sohd state. The present dipole study gives evidence that the compound does not have the same conformation in the dissolved state as in the solid state. For our dipole moment calculations involving compounds (7), (8) and(g), in which substituents were introduced into the heterocyclic ring, we assumed the following: (a) The rb_C--H and N,--C-H angles are I IO”, and are bisected equally by the #I-la&am plane. (b) The plane of the #+-C-H group is perpendicular to the N-phenyi bond. (c) The plane of the Ns-C-H group is parallel to the N-phenyl bond. (d) The nitrogen valences of the fi-Iactam are planar.
327 (e) The moment of the phenyl group attached to the heterocyclic carbon in compounds (7) and (8) is negligible. (f) The moment of the azido group can be taken to be identical to that of ethyl azide_ With this ideal geometry for compounds (7) and (8) we calculated the moments to be 3.69 and 3.71 D, respectively. The experiment&y determined values were 4.06 and 4.00 D. We attribute the discrepancies to the ideafized geometry assumed in the calculations. Slight distortions from the assumed angles and planarity could account for the differences. We cannot deduce whether these molecules exist in the cis or trans configurations relative to the substituents of the fl-lactam ring because the moment of the phenyl group is so small. On the other hand, the nitro group of compound (9) has a very Iarge moment allowing us to make such a distinction. For the cis and trans forms we calculate the moments to be 5.13 and 1.90 D. Comparing these values to the observed moment 4.74 D we conclude that compound (9) has a cis configuration. In comparison to the four-membered heterocychc molecule (1) the fivemembered heterocychc molecuIe (IO) exhibits a sfightly larger dipole moment. This is caused by a change in the ring strain which affects the valence angles of the carbonyi group as well as other things. Using the moment of the para-bromo derivative (11) the moment of the parent molecule (10) is found to lie 72” from the N-phenyl bond. This angle in the &lactams is W-’ indicating that the nitrogen vaIences are different. The exact nature of this difference is not readily apparent from the dipole data. Increasing the temperature to 75 “C causes only a slight increase in the dipole moment of the y-lactam. only one IV-phenyl-b-laetam was investigated, compound (12). Its large moment 4.10 D compared to that of N-phenyl-fi-lactam (3.24 D) again indicates a non-planarity of the nitrogen bonds. The reduced strain in the larger heterocyclic ring allows the molecule to adjust to the steric requirements of the ring.
REFERENCES 1 A. K. Bose, M. S. Manhas and E. R. Malinowski, J. Amer. Gem. Sot., 85 (1963) 2795. 2 J. Y. Chien, J. Chem. Educ., 24 (1947) 494. 3 M. S. Manhas and S. J. Jeng, J. Org. Che/fz., 32 (1967) 1246. 4 C. J. F. Battcher, TIzeory ofHectric Polarisation, Elsevier, New York, 1952. 5 L. M. HeB, Pf2y.s.Rec., 39 (1932) 666. 6 M. S. Manhas, S. Jeng and A. K. Bose, Tetrahedron,
24 (1968) 1237. 7 A. Arcoria, H. Lumbroso and R. Passerini, Bull, Sot. Chim. Fr., 25 (1959) 753. 8 J. L. Luche, H. B. Kagan, R. Parthasarathy, G. Tsoucaris, C. deRango and C. Zelwer,
Tetrahedron, 24 (1968) 1275. 9 J. van der Veen and H. Fujiwara,
private communication.
- Printed in The Netherlands
DIE’OLE MOMENT STUDIES OF LACTAMS EDMUND R. MALINOWSKI*,
MAGHAR S. MANHAS AND MARK GOLDBERG
Department of Chemistry and Chemical New Jersey 07030 (U.S. A.)
Engineering,
Steaens Institute
of Technology,
Hoboken,
VINCENT FANELLI St. Francis Preparatory
High
(Received
1973)
7 November
School,
Brooklyn,
New
York (US-A.)
ABSTRACT
The electric moments of a series of substituted IV-phenyl-p-lactams were measured in p-xylene solution. This study gives evidence that the dihedral angle between the N-phenyl ring and the fi-lactam ring of the ortho-bromo and orthofluoro derivatives is 52” and 42” with the halogen lying on the side farthest from the keto group. Ultraviolet data appear to substantiate this conclusion. Dipole data indicate that the IV-phenyl-3-azido-4-p-nitrophenyl-B_Iactam compound has a cis configuration with respect to the azido and p-nitrophenyl groups. The moments and conformations of some y-lactams and &lactams are also discussed.
APPARATUS
AND CHEMICALS
The dielectric apparatus used in this study was the same as that described in the earlier work [l], based on the design of Chien [2]. For measuring accurate refractive indices a Brice Phoenix Differential Refractometer, with modifications for temperature variability, was utilized. The dielectric and refractometer cells were thermally controlled to within +0.02 “C of the desired temperature. Zone-refined para-xylene (99.91 % minimum purity), obtained from Litton Chemical Co., was used as the solvent in all dipole studies. This solvent is nonpolar and has a higher boiling point than benzene, most commonly used for dipole studies. Loss due to evaporation during the measurements is minimized and studies at higher temperature are more feasible. The organic compounds under investigation were prepared and purified in this laboratory by methods described previously [3 ]_ * Author
to whom inquiries should be addressed.
322 TREATMENT
OF DATA
Obtaining accurate densities, which are required in dipole measurements, poses a tedious and time-ccnsuming project and is extremely difficult to determine at high or low temperatures_ The simple derivation outlined below shows that dipole
moments
can be measured
accurately
from
dielectric
constant
and re-
fractive index measurements alone. From BSttcher [4] we learn that the polarization of the solute, P. can be expressed in terms of the dielectric constant E, refractive index n, molecular weight M, density d and mole fraction x.
‘_n&!)[(M’d,M’) El
(
p=
-ES(%)J+
Eli-2 +
Subscripts
3M,
* (
d,(q+2)2
2x2
6M,n,
_ )
o
d&:+2j2
I, 2 and 12 refer to the solvent, solute and solution,
dn,, ( ‘ax, ) 0
(1)
respectively. The
subscript zero signifies infinite dilution. For a dilute solution the mole fraction of solute is proportional to the weight fraction of solute w,; i.e. x2 = (MI/M2)~Z_ Therefore P=M,
-Ml
&r--l
[( [( -
E,+2
1
+I
-
&
:
(‘I -‘)
&l--l
n;-1
El+2
n:+2 ) dl
_-_-
1
1
+ d&;“i
2J2 -
d, (:;:2J2]
(2)
where a = (~%,~/i7w~),, /3 = (dd,2/dw2)0 and y = (an12/aw2)o. For nonpolar solvents E is approximately equal to n’. Hence the term in eqn. (2) involving /3is negligible and density measurements of the solutions need not be made. The polarizability expression reduces to P = AI2
(3)
The density and dielectric constant for pure para-xylene have been measured [S] as a function of temperature from 20 to 130 “C. For measurements carried out in para-xylene at 25 “C P = A42 (0.1933 a-O.5808 y)
(4)
Dielectric constants and refractive indices of dilute solutions of organic substances dissolved in zone-refined para-xylene measured as a function of concentration and temperature .are shown in Table I. A least-squares method was
323 TABLE
1
DIPOLE
MOMENT
DATA
02
(I)
x 103
A& x I03
An x 105
I-pheByI-azezidin-2-one 1.562 4.124 5.107 5.989
11.47 31.39 40.63 47.17
13.4 35.1 45.9 53.9I
T a y jl
= 25.0 “C = 7.830 = 0.0888 = 3.24 D
5.613
11.98 14.69 29.87 38.49
15.8 40.7 52.71
T Q y p
= 25.0 “C = 6.980 = 0.0951 = 3.78 D
6.70 15.16 25.69 36.54
9.0 16.3 28.1 39.3I
T u y p.
= 25.0 “C = 6.581 = 0.0715 = 3.91 D
12.79 25.42 40.45 51.65
12.4 25.9 42.0 53.5I
T = 25.0 “C CL = 8.771 y = 0.0906 p = 3.82 D
7.35 14.21 15.01 21.88
17.4 34.9 38.5 50.7I
T = 25.0 “C a = 3.410 y = 0.0822 ,LL= 2.60 D
6.520
4.11 7.77 12.34 15.39
11.1 22.5 31.1 42.7 1
T a y p
f .769 3.500 5.280 6.976
6.76 13.52 20.44 27.95
12.1 22.4 33.6 46.4
(2) I-p-bromophenyl-areridin-2-one
5.890
0
(5)
I-o-bromophenyl-azetidine-2-one 2,087
4.292 4.594
Bl-
A-
/
CH2
G /rN\-/
6.211 )‘=H2
C II
1.638 3.271
ij
4.933
u
(6)
19.5
= 90.0 “C = 2.410 = 0.0653 = 2.55 D
I-o-~t~oTup~lenyl-azeridin-2-one F ‘32
N(
>“2
ii
T = 25.0 “C a =
3.940
y
0.0654 2.42 D
=
p =
0
(7) N-phenyZ-3-azido4-phenyr-8_racram
o\-
/
N’=“kH-N
lc”
II
0
3
1.177 1.926 3.086
8.1 13.6
10.8 17.2
20.9
24.0
3.644 5.559
24.2 37.8
41.8 33.8
T = 25.0 “C a = 6.778 y p =
4.06 D 0.0560
324 TABLE
I (continued) w2 x 103
A~x10~
AnxfO'
2” = 25.0 ‘C 5.215 y = O-0889 p = 4.00 D
a =
N(tH>H-N,
2.03 1 2.226
14.5 17.0
18.2 20.5
T = 25.0 OC
3.375 4.636 5.393
27.6 37.2
43.0
30.4 45.0I 51.0
CL = y = 1’c=
1.335 2,974 4.342 5.876
11.99 26.38 39.33 53.40
12.3 26.5 38.8 53.6
a = y = /.c =
7-950 0.0941 4.74 D
ii
0
(IO) I-phenyI-pyrroiidin-Lone T = 25.0 “C
9.046 o.O!ms 3.66 D
d
(il)
I-p-bronmphenyl-pyrrolidin-Z-one 1so7
12.80
13.2
2.983
24.95 41.33 49.97
26.6 42.0 51-7I
5.668
11.07 22.53 35.03 42.60
15.4 30.1 45.3 56.3I
y = jk =
1.078 2.273 3.340 4.663
12.18 27.06 37.88 49.90
9.8 21.3 28.6 37.2
T = 25.0 ‘C CL = 11.060 y = 0.0837 pr = 4.10 D
4.678 5.668 El-
d0
I so7 2.983 4.678
T = 25.0 OC u = 8.750 y = p =
0.0903 4.39 D
T = 75.0 “C a = 7.505 0.0989 4.57 D
(22) I-phenyI-piperGin-Z-one
325 used to calculate a and y; namely u = C%d& Cof
y _ -p Cc@n ’
(5)
Cd
where de = E~~-E~, An = n12- n, and the sum is taken over each dilution. Values for a and /‘I are shown in Table 1. Polarizabilities were obtained by employing eqn. (3) at the appropriate temperature. Dipole moments, p, listed in Table 1 were calculated by the well-known expression [4] p = 0.012812 (PI-)+
(6)
in which T is the absolute temperature. The uncertainty in the dipole measurements was estimated to be approximately IfrO.03 D.
DISCUS!GION
and NMR spectra of N-phenyl-/I-lactams indicate that the n orbitals of the IV-phenyl system and carbonyl system conjugate with the 2p electron pair of the nitrogen forming a planar sp2 nitrogen-bond system. Dipole measurements [7] of the N-phenyl succinimides, where the same structure is present, also provide evidence that the three nitrogen bonds lie in a plane. X-ray studies [SJ have demonstrated that the nitrogen valences are nearly planar and that the N-phenyl ring is coplanar with the /?-lactam ring. From the moments of I-phenyl-azetidin-2-one, its p-bromo and p-chloro derivatives (see Table 1, compounds (1), (2), (3) and (4)), and the moments given in Table 2 we conclude that the vector moment of the parent compound makes an angle of 82” with respect to the N-phenyl bond which is parallel to the carbonhalogen bond. This moment essentially coincides with the carbonyl group since the moment of the IV-phenyl group has been shown to be very small 17). Studies
[6] of the UV
The geometrical orientation between the phenyl ring and the /I?-lactamring poses a challenging problem. If we assume that the two rings are coplanar then TABLE DIPOLE
2 MOhfENTS= USED IN CALCULATIONS
Compound
Dipole moment (0)
Fluorobenzene Bromobenzene Chlorobenzene Ethyl-azide Nitrobenzene
1.49 1.56
1.59 2.12 3.98
a A. L. %fcClellan, Tables of Ekpe~imental Dipoles, W. H. Freeman, San Francisco, 1963.
326 the ortho-substituted derivatives can exist as one of two extreme forms, i.e. cis or trans with respect to the keto group. Our computations show that the cis compound of the fluorine derivative would have a moment of 4.51 D and the trans 1.94 D. Neither of these agree with the observed value, 2.42 D. If the phenyl ring could freely rotate around the N-phenyl bond the moment would be 3.47 D.. This value atso disagrees with the observed moment. If the rotation were restricted due to a potential energy barrier, we could expect the cis and trans forms to exist in equilibrium. From the observed moment, together with the calculated moments for the cis and trans forms, we estimate that the molecule would exist as 87 % trans and 13 % cis. If this were so we might expect the overall measured dipoie moment to vary with temperature. At 90 “C we found the moment of the orthobromo derivative to be 2.55 D, which is identical, within experimental error, to that observed at 25 “C, 2.60 D. Furthermore an increase in temperature should cause an increase in moment and not a decrease as observed. Another possible explanation is apparent. The benzene ring could be twisted out of the plane of the /?-lactam ring occupying a preferential orientation. On this premise, using the component moments listed in Table 2 together with the moments of compounds (11, (5) and (61, we would conclude that the benzene rbg is tilted 52” and 42”, respectively, for the ortho-bromo and ortho-fluoro derivatives, the halogen lying on the side farthest away from the keto group. An examination of the UV spectra of these molecules offers further evidence that the rings are not coplanar. The absorptivity is related to the degree of n-bonding. If the benzene ring of the para-substituted compound is coplanar with the j&la&am ring, then the z-electron ring system would be fully operative over both rings. In the o~ho-substituted compound the twisting of the ring reduces the degree of n-bonding. From simple x-bond molecular-orbital theory we learn that co&I = &,,,(ortho-halogen)/E,,,(parahalogen), where 8 is the angle of twist. .emar was found [6] to be 8 650 and 23 400 for the ortho and para-bromo derivatives, from which we conclude that 0 = 53”. This compares excellently with our estimate from dipole data, 52”. A recent X-ray study [PI of the ortho-bromo derivative indicates a twist of only two degrees. This study was conducted on the compound in the sohd state. The present dipole study gives evidence that the compound does not have the same conformation in the dissolved state as in the solid state. For our dipole moment calculations involving compounds (7), (8) and(g), in which substituents were introduced into the heterocyclic ring, we assumed the following: (a) The rb_C--H and N,--C-H angles are I IO”, and are bisected equally by the #I-la&am plane. (b) The plane of the #+-C-H group is perpendicular to the N-phenyi bond. (c) The plane of the Ns-C-H group is parallel to the N-phenyl bond. (d) The nitrogen valences of the fi-Iactam are planar.
327 (e) The moment of the phenyl group attached to the heterocyclic carbon in compounds (7) and (8) is negligible. (f) The moment of the azido group can be taken to be identical to that of ethyl azide_ With this ideal geometry for compounds (7) and (8) we calculated the moments to be 3.69 and 3.71 D, respectively. The experiment&y determined values were 4.06 and 4.00 D. We attribute the discrepancies to the ideafized geometry assumed in the calculations. Slight distortions from the assumed angles and planarity could account for the differences. We cannot deduce whether these molecules exist in the cis or trans configurations relative to the substituents of the fl-lactam ring because the moment of the phenyl group is so small. On the other hand, the nitro group of compound (9) has a very Iarge moment allowing us to make such a distinction. For the cis and trans forms we calculate the moments to be 5.13 and 1.90 D. Comparing these values to the observed moment 4.74 D we conclude that compound (9) has a cis configuration. In comparison to the four-membered heterocychc molecule (1) the fivemembered heterocychc molecuIe (IO) exhibits a sfightly larger dipole moment. This is caused by a change in the ring strain which affects the valence angles of the carbonyi group as well as other things. Using the moment of the para-bromo derivative (11) the moment of the parent molecule (10) is found to lie 72” from the N-phenyl bond. This angle in the &lactams is W-’ indicating that the nitrogen vaIences are different. The exact nature of this difference is not readily apparent from the dipole data. Increasing the temperature to 75 “C causes only a slight increase in the dipole moment of the y-lactam. only one IV-phenyl-b-laetam was investigated, compound (12). Its large moment 4.10 D compared to that of N-phenyl-fi-lactam (3.24 D) again indicates a non-planarity of the nitrogen bonds. The reduced strain in the larger heterocyclic ring allows the molecule to adjust to the steric requirements of the ring.
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