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Infra-red spectra of oxazole and its alkyl derivatives—II
Spectrochimira Acta, 1967,Y’ol.23A,pp. 1336to 1340.PergamonPressLtd. Printedin NorthernIreland
I&a-red spectra of oxazole and its alkyl derivatives-II E. BORELLO,
A.
and ANNA
ZECCHINA
Istituto Chimico dell’Universit8, (Received
APPIANO
Torino, Italy
28 January 1966)
Abstract-Infrared spectra of 4-methyl-oxazole and 2,4-dimethyl-oxazole for the gaseous and liquid states, and of liquid 2,n-hexyl-B-methyl-oxazole have been measured at room temperature between 4000 and 600 cm-l. Comparison with the spectra of oxazole and considerations on the rotational envelopes allow a plausible assignment of the absorptions to be made.
IN this paper we discuss the spectra of 4-methyl-oxazole, 2,4-dimethyl-oxazole,2,nhexyl-&methyl-oxazole; the b.p. were 87°C lOS”C, 203°C respectively at atmospheric pressure [l]. The experimental conditions were indicated in the preceding paper on oxazole [2], as well as the criteria followed in our assignment. The spectra are reported in Figs. 1-5; the observed bands, intensities and proposed assignments are given in Tables l-3.* DISCUSSION
While we can consider the oxazole, with a good approximation, as an oblate, symmetric top (I, N I, < Ia), its methyl derivatives are largely asymmetric tops. Assuming the same bond lengths and bond angles used for oxazole, and the methyl group as a point-mass equal to 15 at the distance of 1.51 A from the nucleus 131, we obtain the following values for the main moments of inertia [a]: 4-methyl: 2,kdimethyl:
I,
= 73.75 x 1040 g-cm2;
I,
= 311.90 x 1040g-cm2;
I,
= 111.50 x lo4
1s = 238.15 x 1O-4o g-cm2;
g-cm2;
I,
= 409.50 x 1040;
Ic = 521.00 x 1040 g-cm2. Thus the rotational contours will be very different in shape from those of the oxazole. Our problem has a remarkable analogy with that examined by WILMSHURST In this case also the tops are quite and BERNSTEIN [3] for toluene and nz-xilene. * We are greatly indebted to Dr. CORNFORTH, National Institute for Medical Research, The Ridgeway, Mill Hill, London, for supplying us with the samples of oxazoles. [I] J. W. CORNFORTH and R. H. CORNFORTH, J. Chem. Sot. 93 (1963). [2] E. BORELLO,A. ZECCEUNA and ANNA APPIANO,Spectrochim. Acta 22,977 (1906). [3] J. K. WILMSHTJRST and H. J. BERNSTEIN,Can. J. Cbm. 35, 911 (1967). [4] R. A. FRILZER, W. J. DUNCANand A. R. COLLAR,Elementary Matrices, CambridgeUniversity Press (1957). 1336
E. BORELLO, A. ZECOHINA and ANNA APPIANO
1336
L
1500
I
1400
1300 cm-l
1200
1100
Fig. 1. I.R. spectrum of 4-methyl-oxazole
&I
l&o
do0 l&o
1200
1100
1000
900
900
700
I
D
900
ma
7w
900
909
700
w
(liq.).
loo0
I
cm-1
Fig. 2. I.R. spectrum of 4-methyl-oxazole
1
(v&p.).
I’
T
SC F
0
52w
woo
I
29co
I
1700
so0
Fig. 3. IX
(500
l4w
uoo
1200
1100
la00
900
cm-1
spectrum of 2,4-dimethyl-oxazole
(liq.).
1337
Infra-red spectra of oxazole and its alkyl derivatives-II
,
1’700
1600
1500
l400
moo
1100
1200
1000
900
900
700
t
900
900
7w
6
cm-’
Fig. 4. I.R.
0 2200
2000
2wo
1700
spectrum of 2,4-dimethyl-oxazole
l500
woo
#)o
,200
1lW
(vap.).
woo
cd
Fig. 5. I.R.
spectrum of 2,n-hexyl-6.methyl-oxazole
(liq.).
asymmetric, with deeply modified contours, compared with those of benzene. Particularly the R- and P-branches of the C-type bands reduce considerably their intensity, often assuming the appearance of shoulders. A comparison between the spectra in vapour phase of 4.methyl-oxazole and toluene, end of 24.dimethyl-oxazole and m-xilene, confirms the analogy between the two cases. On neglecting interaction terms in the potential function between methyl group and ring, and between methyl groups, 4-methyl-oxazole and 2,kdimethyl-oxazole belong to the C, point group. The normal modes are 27 for 4-methyl-oxazole and 36 for 2,4dimethyloxezole. They divide into symmetry species as following: 4-methyl:
18 A’
2,4-dimethyl:
23 A’ (in-plane) + 13 A” (out-of-plane)
(in-plane) + 9 A” (out-of-plane)
Following WILMSHURST and BERNSTEIN, the representation of the internal modes for the methyl groups is: 4methyl:
%‘WW 1W=,)
+ kz(CH,) +
14’VHJ
+ WCH,) + Iq(CH,)
+ WCW + lr,(CH,)
+ + WC&)
Table 1. Infra-red spectrum of 4-methyl-oxazole vapour 3156 3148 VW 3141 3098 3090 VW ( 3082 2992 { 2980 w 2977 m ( 2952 2943 m ( 2936 2896 VW 1620 1614 1606 In i 1600 1532 1525 8 ( 1517 1505 sh 1469 ( 1456 m 1447 w
I- 1397 1389 m ( 1382 1320 sh 1316 1309 m ( 1302 1264 1267 m ( 1249 1230 1224 m ( 1217 1115 1107 8 ( 1103 1077 1069 “8 ( 1062 1045 sh 1015 1011 m 1004 ( 999 962 1 945 w 925 917 “8 ( 910 845 837 s ( 827 755 745 vs 1740 sh 665 666 m ( 645 609 m -
Rotational form
Liquid
Assignment
A
3138 m
W=)
A
3100 sh
vWW
B
2985 w
v~‘WQ
c
2968 w
v.WW
A
2932 m
v,(CHs)
-
2878 w
-
?
1604 m
w
AB
1520 s
Co
-
1500 sh
w
B
1454 m
&WH,)
C
1443 m
&,VW
9
1387 m
WH,)
-
1318 sh
-
A
1309 m
w
-4
1256 m
&CH)
dB
1226 m
v(C-CH,)
-
1190 VW
-
AB
1105 8
WCH,)
A
1063 vs
@W
-
1045 sh
rlWH.J
AB
1006 m
-
1338
954 w
Br
916 m
A
842 m
y(CH)
754 large 754 large
A I/(CH)
656 m
r
611 m
r
Infra-red spectra of oxazole and its alkyl derivatives-II
1339
Table 2. Infra-red spectrum of 2,4-dimethyl-oxazole vapour 3160 VW 3063 VW 2989 i 2978 w 2970 m /2946 2940 m 2934 2896 m 1622 1616 m ( 1610 1698 { 1686 vB 1540 VW 1466 m 1438 w 1396 1389 m ( 1383 1336 sh 1327 ( 1316 a 1269 1264 m ( 1268 1209 1204 m ( 1197 1110 1103 “S 1098 1047 VW 1016 1004 w 974 { 961 w 929 ( 917 8 745 738 vs 729 m 726 678 w 620 w
1
2,4-dimethyl:
Rotational form -
Liquid
Assigmmmt
-
3136 w 3010 VW
vK=)
-
B
2980 w
v,‘W%J
c
2962 w
vaW%)
_4
2932 w
v,(CJI,)
2895 w
-
A
1608 m
w
B
1582 vs
-
1466 m 1438 w
c 4B
1387 s
-
1336 sh
B
1321 VB
AB
1263 m
AB
1203 m
W--U
.4B
1098 “8
~IKCH,)
G
1046 YW
?_L (CH,)
B
1007 w
B
966 w
Br
B
923 8
A
c
740 w
y(cH)
A
730 m
A
C c
678 w 620 w
I:
2y8’(CH,) + 2v,(CH,)
-
+ ~Y,(CH,) + 26,(CH,)
+ 26,(CH,)
2d,‘(CH,) + 2q(CH,) + 2r,(CHs) + 2t(CH,). As well as for toluene and nz-xilene [3], the ring removes the degeneracy of the internal modes for the free methyl group. This fact is confirmed also by the researches of PERCHARD et al. [5] on the gem-dimethylated molecules. At about 2900 cm-l we have to expect three y(CHs). Following our scheme, Y, only must have a C-type rotational contour; the other two will have an A-, B- or hybrid contour. In the 4-methyl-oxazole the axis of inertia (I,) is practically directed along the C-CH, bond; therefore we must expect for Y, an A-type and for Y’, a B-type contour. This fact is confirmed by the experience. On the contrary [5] J. PERCHARD,M.FOREL
and M.JosIEN,J. Chim. Phye.
61,632 (1964).
1340
E. BORELLO, A. ZECCHINA and ANNA APPIANO Table 3. Infra-red spectrum of 2,n-hexyl-5-methyl-oxazole Liquid 3118 VW
Assignment VP=)
3000 sh 2958 s 2930 vs 2870 sh
-
2868 s 1618 m
UJ
902 m 945 vw-
Br -
1575 8 1460 m
w -
925 VW 885 VW
A
1438 w 1380 w
-
817 m 770 VW
*I’&= -
1340 1310 1253 1217
0 w
740 725 675 625
A r r
VW VW VW m
-
Liquid 1175 w 1128 1088 1043 1010
Assignment -
m m VW m
VW w VW w
WW -
-
1
in the 2,4-dimethyl-oxazole a distinction between the two modes by examining the rotational contour is impossible, because of their hybrid character. The band at 2860-2880 cm-l in vapour phase does not show any recognizable rotational contour; as well as for toluene and m-xilene, it can be considered an In the spectra of the dimethyl-oxides overtone of the 6(CH,) at 1440-1450 cm-l. too an overtone of the 6(CH,) is located in this range [5]. The modes involving the internal bending vibrations of the methyl groups are assignable in the frequency range 1460-1380 cm-l. As expected, in 4-methyloxazole, the modes &(CH,) and J’,(CH,) have an A- and B-type rotational contour respectively. The rocking modes occur at frequencies very near to those found in toluene and m-xilene, and the torsions at frequencies lower than 600 cm-l. For the modes involving nuclear hydrogen atoms, we can deduce, with a good likelihood, that the hydrogen in 4-position is strongly involved in the modes the hydrogen in 2-position in the located, in oxazole, at 3160, 1144, 910 cm-l; modes at 3080, 1085, 840 cm-l and the hydrogen in B-position in the modes at 3138, 1259, 760 cm-r. With regard to the nuclear modes, it is interesting to observe that the breathing mode of alkyl-oxazoles occurs at a lower frequency than oxazole, owing to the coupling with the Y(C-CH,). In efIect we have in alkyl-derivatives a shift towards higher frequencies of the v(C-CH,) and towards lower frequencies of the breathing mode larger than it is possible to foresee. The same effect has been observed in toluene and m-xilene [3]. The modes involving the stretching of the C-CH, bond are indicated in Tables 1 and 2. KATRITZKY and BOULTON [6] observed an analogous unassigned band in It seems probable that the assignthe same frequency range for methyl-isoxazoles. ment is the same as for methyl-oxazoles. In the 2,n-hexyl-5-methyl-oxazole some bands for the various modes of CH, a,ppear, and consequently a certain assignment of the internal modes for methyl groups is not possible. [s] A. R. KATRITZKY and A. J. BOULTON, Spectrochim. Acta 17, 238 (1961).
I&a-red spectra of oxazole and its alkyl derivatives-II E. BORELLO,
A.
and ANNA
ZECCHINA
Istituto Chimico dell’Universit8, (Received
APPIANO
Torino, Italy
28 January 1966)
Abstract-Infrared spectra of 4-methyl-oxazole and 2,4-dimethyl-oxazole for the gaseous and liquid states, and of liquid 2,n-hexyl-B-methyl-oxazole have been measured at room temperature between 4000 and 600 cm-l. Comparison with the spectra of oxazole and considerations on the rotational envelopes allow a plausible assignment of the absorptions to be made.
IN this paper we discuss the spectra of 4-methyl-oxazole, 2,4-dimethyl-oxazole,2,nhexyl-&methyl-oxazole; the b.p. were 87°C lOS”C, 203°C respectively at atmospheric pressure [l]. The experimental conditions were indicated in the preceding paper on oxazole [2], as well as the criteria followed in our assignment. The spectra are reported in Figs. 1-5; the observed bands, intensities and proposed assignments are given in Tables l-3.* DISCUSSION
While we can consider the oxazole, with a good approximation, as an oblate, symmetric top (I, N I, < Ia), its methyl derivatives are largely asymmetric tops. Assuming the same bond lengths and bond angles used for oxazole, and the methyl group as a point-mass equal to 15 at the distance of 1.51 A from the nucleus 131, we obtain the following values for the main moments of inertia [a]: 4-methyl: 2,kdimethyl:
I,
= 73.75 x 1040 g-cm2;
I,
= 311.90 x 1040g-cm2;
I,
= 111.50 x lo4
1s = 238.15 x 1O-4o g-cm2;
g-cm2;
I,
= 409.50 x 1040;
Ic = 521.00 x 1040 g-cm2. Thus the rotational contours will be very different in shape from those of the oxazole. Our problem has a remarkable analogy with that examined by WILMSHURST In this case also the tops are quite and BERNSTEIN [3] for toluene and nz-xilene. * We are greatly indebted to Dr. CORNFORTH, National Institute for Medical Research, The Ridgeway, Mill Hill, London, for supplying us with the samples of oxazoles. [I] J. W. CORNFORTH and R. H. CORNFORTH, J. Chem. Sot. 93 (1963). [2] E. BORELLO,A. ZECCEUNA and ANNA APPIANO,Spectrochim. Acta 22,977 (1906). [3] J. K. WILMSHTJRST and H. J. BERNSTEIN,Can. J. Cbm. 35, 911 (1967). [4] R. A. FRILZER, W. J. DUNCANand A. R. COLLAR,Elementary Matrices, CambridgeUniversity Press (1957). 1336
E. BORELLO, A. ZECOHINA and ANNA APPIANO
1336
L
1500
I
1400
1300 cm-l
1200
1100
Fig. 1. I.R. spectrum of 4-methyl-oxazole
&I
l&o
do0 l&o
1200
1100
1000
900
900
700
I
D
900
ma
7w
900
909
700
w
(liq.).
loo0
I
cm-1
Fig. 2. I.R. spectrum of 4-methyl-oxazole
1
(v&p.).
I’
T
SC F
0
52w
woo
I
29co
I
1700
so0
Fig. 3. IX
(500
l4w
uoo
1200
1100
la00
900
cm-1
spectrum of 2,4-dimethyl-oxazole
(liq.).
1337
Infra-red spectra of oxazole and its alkyl derivatives-II
,
1’700
1600
1500
l400
moo
1100
1200
1000
900
900
700
t
900
900
7w
6
cm-’
Fig. 4. I.R.
0 2200
2000
2wo
1700
spectrum of 2,4-dimethyl-oxazole
l500
woo
#)o
,200
1lW
(vap.).
woo
cd
Fig. 5. I.R.
spectrum of 2,n-hexyl-6.methyl-oxazole
(liq.).
asymmetric, with deeply modified contours, compared with those of benzene. Particularly the R- and P-branches of the C-type bands reduce considerably their intensity, often assuming the appearance of shoulders. A comparison between the spectra in vapour phase of 4.methyl-oxazole and toluene, end of 24.dimethyl-oxazole and m-xilene, confirms the analogy between the two cases. On neglecting interaction terms in the potential function between methyl group and ring, and between methyl groups, 4-methyl-oxazole and 2,kdimethyl-oxazole belong to the C, point group. The normal modes are 27 for 4-methyl-oxazole and 36 for 2,4dimethyloxezole. They divide into symmetry species as following: 4-methyl:
18 A’
2,4-dimethyl:
23 A’ (in-plane) + 13 A” (out-of-plane)
(in-plane) + 9 A” (out-of-plane)
Following WILMSHURST and BERNSTEIN, the representation of the internal modes for the methyl groups is: 4methyl:
%‘WW 1W=,)
+ kz(CH,) +
14’VHJ
+ WCH,) + Iq(CH,)
+ WCW + lr,(CH,)
+ + WC&)
Table 1. Infra-red spectrum of 4-methyl-oxazole vapour 3156 3148 VW 3141 3098 3090 VW ( 3082 2992 { 2980 w 2977 m ( 2952 2943 m ( 2936 2896 VW 1620 1614 1606 In i 1600 1532 1525 8 ( 1517 1505 sh 1469 ( 1456 m 1447 w
I- 1397 1389 m ( 1382 1320 sh 1316 1309 m ( 1302 1264 1267 m ( 1249 1230 1224 m ( 1217 1115 1107 8 ( 1103 1077 1069 “8 ( 1062 1045 sh 1015 1011 m 1004 ( 999 962 1 945 w 925 917 “8 ( 910 845 837 s ( 827 755 745 vs 1740 sh 665 666 m ( 645 609 m -
Rotational form
Liquid
Assignment
A
3138 m
W=)
A
3100 sh
vWW
B
2985 w
v~‘WQ
c
2968 w
v.WW
A
2932 m
v,(CHs)
-
2878 w
-
?
1604 m
w
AB
1520 s
Co
-
1500 sh
w
B
1454 m
&WH,)
C
1443 m
&,VW
9
1387 m
WH,)
-
1318 sh
-
A
1309 m
w
-4
1256 m
&CH)
dB
1226 m
v(C-CH,)
-
1190 VW
-
AB
1105 8
WCH,)
A
1063 vs
@W
-
1045 sh
rlWH.J
AB
1006 m
-
1338
954 w
Br
916 m
A
842 m
y(CH)
754 large 754 large
A I/(CH)
656 m
r
611 m
r
Infra-red spectra of oxazole and its alkyl derivatives-II
1339
Table 2. Infra-red spectrum of 2,4-dimethyl-oxazole vapour 3160 VW 3063 VW 2989 i 2978 w 2970 m /2946 2940 m 2934 2896 m 1622 1616 m ( 1610 1698 { 1686 vB 1540 VW 1466 m 1438 w 1396 1389 m ( 1383 1336 sh 1327 ( 1316 a 1269 1264 m ( 1268 1209 1204 m ( 1197 1110 1103 “S 1098 1047 VW 1016 1004 w 974 { 961 w 929 ( 917 8 745 738 vs 729 m 726 678 w 620 w
1
2,4-dimethyl:
Rotational form -
Liquid
Assigmmmt
-
3136 w 3010 VW
vK=)
-
B
2980 w
v,‘W%J
c
2962 w
vaW%)
_4
2932 w
v,(CJI,)
2895 w
-
A
1608 m
w
B
1582 vs
-
1466 m 1438 w
c 4B
1387 s
-
1336 sh
B
1321 VB
AB
1263 m
AB
1203 m
W--U
.4B
1098 “8
~IKCH,)
G
1046 YW
?_L (CH,)
B
1007 w
B
966 w
Br
B
923 8
A
c
740 w
y(cH)
A
730 m
A
C c
678 w 620 w
I:
2y8’(CH,) + 2v,(CH,)
-
+ ~Y,(CH,) + 26,(CH,)
+ 26,(CH,)
2d,‘(CH,) + 2q(CH,) + 2r,(CHs) + 2t(CH,). As well as for toluene and nz-xilene [3], the ring removes the degeneracy of the internal modes for the free methyl group. This fact is confirmed also by the researches of PERCHARD et al. [5] on the gem-dimethylated molecules. At about 2900 cm-l we have to expect three y(CHs). Following our scheme, Y, only must have a C-type rotational contour; the other two will have an A-, B- or hybrid contour. In the 4-methyl-oxazole the axis of inertia (I,) is practically directed along the C-CH, bond; therefore we must expect for Y, an A-type and for Y’, a B-type contour. This fact is confirmed by the experience. On the contrary [5] J. PERCHARD,M.FOREL
and M.JosIEN,J. Chim. Phye.
61,632 (1964).
1340
E. BORELLO, A. ZECCHINA and ANNA APPIANO Table 3. Infra-red spectrum of 2,n-hexyl-5-methyl-oxazole Liquid 3118 VW
Assignment VP=)
3000 sh 2958 s 2930 vs 2870 sh
-
2868 s 1618 m
UJ
902 m 945 vw-
Br -
1575 8 1460 m
w -
925 VW 885 VW
A
1438 w 1380 w
-
817 m 770 VW
*I’&= -
1340 1310 1253 1217
0 w
740 725 675 625
A r r
VW VW VW m
-
Liquid 1175 w 1128 1088 1043 1010
Assignment -
m m VW m
VW w VW w
WW -
-
1
in the 2,4-dimethyl-oxazole a distinction between the two modes by examining the rotational contour is impossible, because of their hybrid character. The band at 2860-2880 cm-l in vapour phase does not show any recognizable rotational contour; as well as for toluene and m-xilene, it can be considered an In the spectra of the dimethyl-oxides overtone of the 6(CH,) at 1440-1450 cm-l. too an overtone of the 6(CH,) is located in this range [5]. The modes involving the internal bending vibrations of the methyl groups are assignable in the frequency range 1460-1380 cm-l. As expected, in 4-methyloxazole, the modes &(CH,) and J’,(CH,) have an A- and B-type rotational contour respectively. The rocking modes occur at frequencies very near to those found in toluene and m-xilene, and the torsions at frequencies lower than 600 cm-l. For the modes involving nuclear hydrogen atoms, we can deduce, with a good likelihood, that the hydrogen in 4-position is strongly involved in the modes the hydrogen in 2-position in the located, in oxazole, at 3160, 1144, 910 cm-l; modes at 3080, 1085, 840 cm-l and the hydrogen in B-position in the modes at 3138, 1259, 760 cm-r. With regard to the nuclear modes, it is interesting to observe that the breathing mode of alkyl-oxazoles occurs at a lower frequency than oxazole, owing to the coupling with the Y(C-CH,). In efIect we have in alkyl-derivatives a shift towards higher frequencies of the v(C-CH,) and towards lower frequencies of the breathing mode larger than it is possible to foresee. The same effect has been observed in toluene and m-xilene [3]. The modes involving the stretching of the C-CH, bond are indicated in Tables 1 and 2. KATRITZKY and BOULTON [6] observed an analogous unassigned band in It seems probable that the assignthe same frequency range for methyl-isoxazoles. ment is the same as for methyl-oxazoles. In the 2,n-hexyl-5-methyl-oxazole some bands for the various modes of CH, a,ppear, and consequently a certain assignment of the internal modes for methyl groups is not possible. [s] A. R. KATRITZKY and A. J. BOULTON, Spectrochim. Acta 17, 238 (1961).