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The infrared spectrum of mono-13C-substituted benzene
Spectrochimica Acta,1965,Vol. 21,~~. 2077to2084.Perganwn PressLtd.Printed
in
Northern Ireland
The infrared spectrum of monoJ3C-substituted benzene SVEND
BRODERSEN
and JBRGEN
CHRISTOFFERSEN*
and BARGE
BAK
and JURGEN
TORMOD
(Received 12 May
NIELSEN?
1965)
Abstract-The infrared spectra were recorded of a mixture of equal parts of r2C,H, and 13C12C,H, in the liquid and in the gas phase. A list is given of the observed frequency shifts due to the 1% substitution together with a preliminary assignment. It is expected that these results will be of value in the determination of the potential function of benzene.
INTRODUCTION
spectra of benzene and of its deuterated derivatives have been the object of numerous papers. The assignment of the fundamental vibrational frequencies is rather certain for most of these compounds. On basis of these assigned frequencies a number of workers have tried to determine the potential function of benzene. The frequencies of even all the deuterated benzenes do not, however, fix the potential, because the information given by the frequencies of the partly deuterated compounds is almost the same as that given by C,H, and C,D, alone. New experimental data are therefore desirable. Recently CALLOMON, DUNN and MILLS [I] have determined one of the zeta constants for both C,H, and The present work is an attempt to determine another kind of experimental C,D,. data, namely the vibrational frequencies of 13CY2C,H,. It is to be expected that an isotopic substitution in benzene in the carbon ring itself will yield informations which are significantly different from the informations obtained on basis of deuterium substitution. The effect on the vibrational frequencies of benzene due to the introduction of a single 13C atom can only be minor THE
VIBRATIONAL
frequency
shifts.
In the limit the relative shift of a certain frequency
is (2/12
-
corresponding to a vibration in which only the carbon atom 2/E)/%% = -4.1% in question moves. In practice the maximum shift should be about -2% or even lower. That the effect is so small has two consequences. On the one hand the experimental errors will play a much more dominating role than in the case of deuterium substitution. On the other hand the effect of anharmonicity on such small frequency shifts may be expected to be negligible. Therefore, in spite of the low precision the results obtained from the present experiments are expected to be theoretically significant. * Department of Chemical Physics, University of Aarhus, Denmark. t Chemical Laboratory V, University of Copenhagen, Denmark. [I] J. H. CALLOMON, T. M. DUNN and I. M. MILLS, to be published. 2077
S. BRODERSEN, J. CHRISTOFFERSEN, B. BAK and J. TORMODNIELSEN
2078
EXPERIMENTAL A sample of 0.55 g of a mixture of equal parts of 12C H, and r3C12C,H, was prepared as described by BAK, CHRISTIANSEN, LIPSCHITZ ‘and NIELSEN [2]. The infrared spectra were recorded partly on a Perkin-Elmer model 221 infrared spectrophotometer equipped with a NaCl-grating unit and a CsBr unit, partly on a Perkin-Elmer model 521. The spectral region covered was 250 to 3800 cm-l, the resolution varying from O-3to 6 cm- l. Spectra were taken at room temperature of the liquid phase using path lengths of 0.025 mm and 0.3 mm and of the gas phase using path lengths of 10 cm and 1 m with vapour pressures up to 80 mm Hg. For comparison corresponding spectra of benzene p.a. were recorded under identical circumstances in all cases. The results are given in Table 1 as well as in Figs. 1, 2 and 3 where essential parts of the spectra are reproduced. In the table only the frequency shift due to Table 1. Frequency
AvW
VW
0 f
673.5 Q 779
Q
1003 Q 1037 Q
1388 M 1482 M
shifts in IR spectrum of WW,H,
Liquid
Gas
VW
0.3
{;;$ "0:; 850
forbidden
969
forbidden
992
>
Av, is smell
-712 -2.2
j, 0.2
-312 (--;:;
1522 PR
= ;I; Oh3
1035
O&2
1147
0*3
1177
Oh2
1248
-2&l
1309 1346
-6+ 0*3 -1.5
*
1
1479
-5.7
f
1
1528
-II
1606 (-__i
1
1393
1586
1623 Q
Remarks
Assignment
AvW
2 : 1643
-2+3
[2] B. BAK, J. J. CHRISTIANSEN, L. LIPSCHITZand J. T. NIELSEN,Acta Chem.&and. 16,2318 (1962).
The infrared spectrum of mono-13C-substituted
2079
benzene
Table 1 (contd.) GM VW 1667 Q
Liquid
-
PC!
AvW -8.3
f
0.2
v1+ Vll 1671
-2*2
1716 Q
-8.5
f
1
1713
--s*l
1764 Q
-5.5
f
1
1754
--e+
1814
-0.5
v*+v**
1
va+v17a
1
f
v4 +
Vl7b
v4 +
Vl!a
%x-t
2006 Q
Oh2
1
-6.0
& 0.2
-1*
1959
-6
2004
-3f2
f
1.5
-12&4
1987
2214 PR
1
VI5
%o
+
+Jo
+
v17a
V17b
VlOb +
%?a
%m+
Vl?b
Y+vl?o -cVI + VlPb v5 +
VlB
vs.2+
Vl(lb
t V8b +
v1ea
vso +
Vl80
-4f2
2211
VI6
i Vl3b +
l
1966 M
v9.3+
V18b
V9b +
VlBO
( v9b+ %
2325
-K-&2
2383
-4&2
2483
-9+4
-1163
2672 2594
16f 11 f
(1
1 3
-14f3
2612
2652
-6.6
f
vlOa +
v18b
VlOb +
wla
-14A2
-2&3
2888
17.6 f 1 14 f3
2907 3037
(1
-17.5 -6.5
f f
3 1
3072
-14
f
(3062)
(-4
+ 0.6)
V18b
VI4
{ vsa+
VIP
vso + { v8b +
%?o
v1+ 1 v1+
Vl7b vm
(2;
vSb+
712 v6n +
v1-2
t VI +
V6b +
%a
%a
V180
+
vs.+
V19b
V9b +
v190
V9b +
-9fl
v19a
1 vs+
Vl9b
x8g::::: v1+ { VI+
-7*3
3611 3641 3686 3696
-13f --lo+ --lo+ -11 f
0.5
3 1 3 2
V60 +
v14
%a+
Vl6
G&l { %Bb
(
%a+
v19a
%a+
v18b
Ve.b +
Vl8.l
%a+
Vl8b va,+
Vl9.l
vl+
v6o +
vl8b
VlS
V6b +
Vlllci
Vl +
%a+
VlBb
V?O + i V7b +
11 11
--_ I
11
obs. in Reman
Vq.
2 f
3448
Vl8b
vs+
i
3465 Q
Vl?b %?a
v1+
VI +
3091
+
vaa+
p+v1*
1
2819
visa
v90+
(
2898 M
v15
vlI,
vs+ { vs+
-11*3
2553
v18b
+
vDb+
Oztl (
2386 Q
Remarks
AFlsignment
Av”C
Vl6b VlSO
-
-
-
? v60b +
bob
vsnb +
v8.b +
VI + 2%"b +
-, -, VlOab vllnb
(all possible
2080
S. BRODERSEN, ,]-. CHRISTOFFERSEI~, B. BAX and J . TORMOD ~'[EL
D .3
.2
.1
V I
!
!
I
1280 F i g . 1. S p e c t r a
I
-1
1320 cm
of 0.3 mm
liquid, --mixture,
_ _ . 1 2 C 6 H 6.
D .5
¢, I
I I
I
.4
.3-
/i,,,..,,,, %
#
i I
.2-
v I
I
1580 F i g . 2. S p e c t r a
of 0.3 mm
I
!
I
1620 cm-1 liquid, --mixture,
- - - 1 2 C 6 H 6.
i
2081
The infrared spectrum of mono-‘%-substituted benzene
the 13C-substitution is given, because this magnitude generally is determined to B higher accuracy than is the shifted frequency itself. The inaccuracy indicated is a rough estimate of the probable experimental error in the determination of the frequency shift. The indication of intervals including positive numbers does not * D
.5
I
.4
I I
I
.3
I
I
I
.2
.1
I
I
I
I
_
I
3doo Fig. 3. Spectra of 0.025 mm liquid, -mixture,
31bO cm" ’ - - - 12C,H,.
mean that we accept the possibility of positive frequency shifts, except in the case of hot bands. The columns marked 12C give the frequencies of the correThe Q, PR, or M indicate that both the benzene fresponding bands in W,H,. quency and the frequency shift refer to a Q-branch maximum, a minimum between a P- and a R-branch, or a center of a band without much structure, respectively. Where a Q-branch is observed this will normally yield the best value for the frequency shift. Where a Q-branch is not observed the liquid spectra with the more regular band form often give the best determination. The table include all cases where frequency shifts could be determined with an appreciable accuracy.
S. BRODERSEN, J. CHRISTOFFERSEN, B. BAR,
2082
and J. TORMOD NIELSEN
DISCUSSION
W1sC,H, is an asymmetric top of the point group C,,. In Table 2 the distribution of the normal vibrations over the four symmetry species is given, using the numbering of WILSON [3] and LANGSETH and LORD [4]. cr is the operation of reflection in the molecular plane, so that A, and B, contain the in-plane vibrations and A, and B, the out-of-plane vibrations. Table 2. Distribution
C2v
E
4
1
B, A2 B,
1 1 1
c2
a’
1 -1 1 -1
0
1 -1 -1 1
IR forbidden: A,, A, x A,, IR allowed: all others. Raman allowed: all.
of Normal
Frequency
1 1
numbers
1, 2, 6a, 7a, 8a, 9a, 12, 13, 18a, 19a, 20~ 3, 66, 7b, 8b, 9b; 14, 15, 18b, 19b, 20b lOa, 16a, 17a 4, 5, lob, 11, 16b, 17b.
-1 -1 B,
Vibrations
x
B,.
Due to the smallness of the perturbation of the 13C-atom the band forms of Y?sC,H, (K = 0.916) cannot be expected to deviate very much from the band forms of the corresponding bands of 12C,H,. Especially it should be possible to use a measured shift in the position of a Q-branch maximum as a direct measure of the vibrational frequency shift. Formally all transitions of symmetry A,, B, or B, are infrared active. The perturbation of the 13C-atom is, however, so small that the intensities’ cannot be expected to deviate very much from the intensities observed in 12C6H6. Thus primarily those transitions should be observed which are active in 12C,H,. If new transitions do occur they have in a sense borrowed the intensity from the originally active transitions, a process which is in principle not very different from a Fermi resonance. The magnitude of this effect increases with the amplitude of the W-atom in the two states. A large increase in intensity should thus be connected with a large frequency shift. In the spectrum of 13C12C,Hs each of the vibrations corresponding to degenerate vibrations in 12C,H, is split into two components, the cc-component having symmetry A, or A,, and the b-component having symmetry B, or B,. It seems impossible on the basis of this difference in symmetry a priori to tell anything of the relative magnitude of the frequency shifts of the two components. This leaves a specific assignment to be made in close connection with potential function calculations. A preliminary assignment is given in Table 1. It is based on the assignment of the infrared spectrum of 12C,H, by BRODERSEN and LANGSETH [5]. In most cases a number of possible assignments have been given for each line due to the splitting in a- and b-components. For instance the Q-branch observed 2.2 cm-l below the [3] E. BRIGHT WILSON, JR., Phys. Rev. 45, 106 (1934). [4] A. LANGSETH and R. C. LORD, JR., Mat. Fys. Medd. Dan.. Vid. Selsk. 16,no. 6 (1938). [5] S. BRODERSEN and A. LANGSETR, Mat. Fys. Skr. Dan. Vid. Selsk. 1, no. 1 (1956).
The infrared spectrum of mono-13C-substituted
benzene
2083
12CBH, Q-branch at 1037 cm-l could be assigned either to v18a or to v18bor to both, indicating in the first two cases a probably very small shift of the other component. One could have expected, that the observed intensities should make it possible to ascertain whether one or both components was responsible for the new line. Unfortunately this is not so because of the underlying, varying P-branch intensity. One of the most interesting results of the present investigation is the effect of the isotopic substitution on the 1309 cm-l line of liquid 12C,H,. This line has been vibration yr4 by MAIR and HORNIG [6], and the assigned to the Bzu carbon-type assignment was later confirmed by BRODERSEN and LAN~SETH [5]. More recently the correctness of this assignment has been doubted by CALIFANO and CRAWFORD [7] on basis of calculations using a Urey-Bradley potential function. SCHERER and OVEREND [S] have indicated, however, a Urey-Bradley potential function, which is in agreement with the MAIR-HORNIG assignment. As shown in Fig. 1 the isotopic substitution has a double effect on the 1309 cm-’ line, the frequency is lowered by 6 & 1 cm-r and the intensity is increased by some 50%. yr4 is inactive in 12C,H, but is observed in the liquid state due to liquid perturbations. In 13Cr2C,H, y14 becomes formally active and being a typical carbontype vibration it is expected both to gain in intensity and to have the frequency lowered by an appreciable amount, just as observed. Preliminary computations indicate a frequency shift of about -7.7 cm-l. We take this agreement between observation and expectation as an additional proof of the correctness of the MAIR-HORNIG assignment. The only previously observed vibrational lines of 13C12CsH, are the y1 and yr2 Raman lines observed by LANGSETH and LORD [4] in natural benzene, which contains 6% of 13C12C,H,. They measured frequency shifts of -8.5 cm-l for y1 and -5 cm-l for yr2. The y1 line is observed in the present infrared spectrum with Av = -8 f 2 in agreement with the Raman value. vr2 is not observed here, probably due to overlapping of the strong vr8 line. Fermi resonance plays a rather important role in the assignment of the present spectrum as indicated in Table 1 by the use of square brackets. The well-known Fermi resonance doublet vs and vr + Ye observed in the Raman spectrum of 12C,H, is also observed in the infrared spectrum of the same liquid due to liquid perturbations as shown in Fig. 2. At a slightly higher frequency the allowed combination vg + v r2 gives rise to a somewhat higher and sharper peak. In the spectrum of liquid r3C1W,H, a series of three lines stands out rather clearly, corresponding to a constant frequency shift of about -10 cm-l. Both the form of the peaks and the intensity seems to indicate a Fermi resonance between the three levels, all three being of the same symmetry, either A, or B,. Another much weaker series of lines seems to indicate the other Fermi-resonance triplet with a constant frequency shift of about -3 cm-l. Fermi resonance between levels involving vs and v1 + vg is observed in three [6] R. D. MAIR and D. F. HORNIG, J. Chem. Phys. 17, 1236 (1949). [7] S. CALIFANO and B. CRAWFORD, JR., Spectrochinz. Acta 16,889 (1960). [8] J. R. SCRERER and J. OVEREND, Spectrochim. Acta 17, 719 (1961).
2084
S. BRODERSEN,
J. CHRISTOFFERSEN, B. BAK and J. TORMOD
NIELSEN
cases of combinations with vl,, vls and v14 respectively, but apparently the resonance with the vs + vls component is lost in all three cases. This may be due to the fact that the transitions to all these levels are allowed in 12CgH6, whereas the transitions to vs and vr + vg are observed in 13Ci2C,H, only because of borrowing intensity from the originally allowed transition to vg + vi2. Thus the vg + vls component has to be the strongest one in the last case and is easily observed, whereas this component may be weak and unobserved in the other three cases. The 3000 to 3100 cm-r region is rather complicated as shown in Fig. 3. This region of the 12C,H, liquid spectrum contains three strong lines assigned as a Fermi resonance triplet consisting of vzO, vs + vls, and y1 + vs + vls. The corresponding spectrum of 13C12C,H, contains at least four lines at 3031, 3058, 3076 and 3084 cm-l. The 3031 cm-l line is obviously one or both components of v2,,, giving a frequency shift of -5.5 cm-l. The 3058 line may be assigned either to one of the four components of vs + v,s giving a much higher frequency shift of - 14 cm-l or to v2, which is formally active, giving a more consistent frequency shift of -4 cm-l. The only possible assignment of the last two lines at 3076 and 3084 cm-l seems to be as two of the four components of vi + vs + vls. It is now worth while to note that in all the other cases of a Fermi resonance considered so far all components apparently are subject to a frequency shift of the same magnitude, consistent with the idea of a mixing of the wave functions. Therefore, one of the many possible explanations would be to interpret the 3058 cm-l line as two blended lines and to accept the presence of two Fermi The one group should then include vZOa,v2, and either v1 + resonance groups. or v1 + vBb + vlsb giving frequency shifts of -5.5, -4, and -7 cm-l v&z+ VlOa respectively. The other group should include either vsa + vlsb or vsb + visa and either v1 + vea + vlsb or v1 + vBb+ vlga giving frequency shifts of -14 and -15 cm-l respectively. Finally it should be remarked that the three lines in the 3600 to 3700 cm-l region obviously again is a Fermi resonance group with a constant frequency shift of -10 or - 11 cm-l. Here the number of combinations is, however, so high that a definite assignment seems almost impossible. Due to the preliminary nature of the assignment given it does not seem appropriate to try assigning a definite frequency shift to each of the 30 normal vibrations. Rather we intend to use the present result for comparison with potential function calculations now being in progress. The results of these calculations will be published separately. Acknowledgemelzt-One of us (S. B.) wants to thank Statens almindelige Videnskabsfond grant towards equipment for this investigation.
Note added in proof: Drs. I. M. Mills and J. H. Callomon have bility of a strong Coriolis perturbation between degenerate state. In such a case the observed to the shift in vibrational frequency due to an
for a
called our attention to the possithe two components of a formerly Q-branch shift may not correspond anomalous intensity distribution.
in
Northern Ireland
The infrared spectrum of monoJ3C-substituted benzene SVEND
BRODERSEN
and JBRGEN
CHRISTOFFERSEN*
and BARGE
BAK
and JURGEN
TORMOD
(Received 12 May
NIELSEN?
1965)
Abstract-The infrared spectra were recorded of a mixture of equal parts of r2C,H, and 13C12C,H, in the liquid and in the gas phase. A list is given of the observed frequency shifts due to the 1% substitution together with a preliminary assignment. It is expected that these results will be of value in the determination of the potential function of benzene.
INTRODUCTION
spectra of benzene and of its deuterated derivatives have been the object of numerous papers. The assignment of the fundamental vibrational frequencies is rather certain for most of these compounds. On basis of these assigned frequencies a number of workers have tried to determine the potential function of benzene. The frequencies of even all the deuterated benzenes do not, however, fix the potential, because the information given by the frequencies of the partly deuterated compounds is almost the same as that given by C,H, and C,D, alone. New experimental data are therefore desirable. Recently CALLOMON, DUNN and MILLS [I] have determined one of the zeta constants for both C,H, and The present work is an attempt to determine another kind of experimental C,D,. data, namely the vibrational frequencies of 13CY2C,H,. It is to be expected that an isotopic substitution in benzene in the carbon ring itself will yield informations which are significantly different from the informations obtained on basis of deuterium substitution. The effect on the vibrational frequencies of benzene due to the introduction of a single 13C atom can only be minor THE
VIBRATIONAL
frequency
shifts.
In the limit the relative shift of a certain frequency
is (2/12
-
corresponding to a vibration in which only the carbon atom 2/E)/%% = -4.1% in question moves. In practice the maximum shift should be about -2% or even lower. That the effect is so small has two consequences. On the one hand the experimental errors will play a much more dominating role than in the case of deuterium substitution. On the other hand the effect of anharmonicity on such small frequency shifts may be expected to be negligible. Therefore, in spite of the low precision the results obtained from the present experiments are expected to be theoretically significant. * Department of Chemical Physics, University of Aarhus, Denmark. t Chemical Laboratory V, University of Copenhagen, Denmark. [I] J. H. CALLOMON, T. M. DUNN and I. M. MILLS, to be published. 2077
S. BRODERSEN, J. CHRISTOFFERSEN, B. BAK and J. TORMODNIELSEN
2078
EXPERIMENTAL A sample of 0.55 g of a mixture of equal parts of 12C H, and r3C12C,H, was prepared as described by BAK, CHRISTIANSEN, LIPSCHITZ ‘and NIELSEN [2]. The infrared spectra were recorded partly on a Perkin-Elmer model 221 infrared spectrophotometer equipped with a NaCl-grating unit and a CsBr unit, partly on a Perkin-Elmer model 521. The spectral region covered was 250 to 3800 cm-l, the resolution varying from O-3to 6 cm- l. Spectra were taken at room temperature of the liquid phase using path lengths of 0.025 mm and 0.3 mm and of the gas phase using path lengths of 10 cm and 1 m with vapour pressures up to 80 mm Hg. For comparison corresponding spectra of benzene p.a. were recorded under identical circumstances in all cases. The results are given in Table 1 as well as in Figs. 1, 2 and 3 where essential parts of the spectra are reproduced. In the table only the frequency shift due to Table 1. Frequency
AvW
VW
0 f
673.5 Q 779
Q
1003 Q 1037 Q
1388 M 1482 M
shifts in IR spectrum of WW,H,
Liquid
Gas
VW
0.3
{;;$ "0:; 850
forbidden
969
forbidden
992
>
Av, is smell
-712 -2.2
j, 0.2
-312 (--;:;
1522 PR
= ;I; Oh3
1035
O&2
1147
0*3
1177
Oh2
1248
-2&l
1309 1346
-6+ 0*3 -1.5
*
1
1479
-5.7
f
1
1528
-II
1606 (-__i
1
1393
1586
1623 Q
Remarks
Assignment
AvW
2 : 1643
-2+3
[2] B. BAK, J. J. CHRISTIANSEN, L. LIPSCHITZand J. T. NIELSEN,Acta Chem.&and. 16,2318 (1962).
The infrared spectrum of mono-13C-substituted
2079
benzene
Table 1 (contd.) GM VW 1667 Q
Liquid
-
PC!
AvW -8.3
f
0.2
v1+ Vll 1671
-2*2
1716 Q
-8.5
f
1
1713
--s*l
1764 Q
-5.5
f
1
1754
--e+
1814
-0.5
v*+v**
1
va+v17a
1
f
v4 +
Vl7b
v4 +
Vl!a
%x-t
2006 Q
Oh2
1
-6.0
& 0.2
-1*
1959
-6
2004
-3f2
f
1.5
-12&4
1987
2214 PR
1
VI5
%o
+
+Jo
+
v17a
V17b
VlOb +
%?a
%m+
Vl?b
Y+vl?o -cVI + VlPb v5 +
VlB
vs.2+
Vl(lb
t V8b +
v1ea
vso +
Vl80
-4f2
2211
VI6
i Vl3b +
l
1966 M
v9.3+
V18b
V9b +
VlBO
( v9b+ %
2325
-K-&2
2383
-4&2
2483
-9+4
-1163
2672 2594
16f 11 f
(1
1 3
-14f3
2612
2652
-6.6
f
vlOa +
v18b
VlOb +
wla
-14A2
-2&3
2888
17.6 f 1 14 f3
2907 3037
(1
-17.5 -6.5
f f
3 1
3072
-14
f
(3062)
(-4
+ 0.6)
V18b
VI4
{ vsa+
VIP
vso + { v8b +
%?o
v1+ 1 v1+
Vl7b vm
(2;
vSb+
712 v6n +
v1-2
t VI +
V6b +
%a
%a
V180
+
vs.+
V19b
V9b +
v190
V9b +
-9fl
v19a
1 vs+
Vl9b
x8g::::: v1+ { VI+
-7*3
3611 3641 3686 3696
-13f --lo+ --lo+ -11 f
0.5
3 1 3 2
V60 +
v14
%a+
Vl6
G&l { %Bb
(
%a+
v19a
%a+
v18b
Ve.b +
Vl8.l
%a+
Vl8b va,+
Vl9.l
vl+
v6o +
vl8b
VlS
V6b +
Vlllci
Vl +
%a+
VlBb
V?O + i V7b +
11 11
--_ I
11
obs. in Reman
Vq.
2 f
3448
Vl8b
vs+
i
3465 Q
Vl?b %?a
v1+
VI +
3091
+
vaa+
p+v1*
1
2819
visa
v90+
(
2898 M
v15
vlI,
vs+ { vs+
-11*3
2553
v18b
+
vDb+
Oztl (
2386 Q
Remarks
AFlsignment
Av”C
Vl6b VlSO
-
-
-
? v60b +
bob
vsnb +
v8.b +
VI + 2%"b +
-, -, VlOab vllnb
(all possible
2080
S. BRODERSEN, ,]-. CHRISTOFFERSEI~, B. BAX and J . TORMOD ~'[EL
D .3
.2
.1
V I
!
!
I
1280 F i g . 1. S p e c t r a
I
-1
1320 cm
of 0.3 mm
liquid, --mixture,
_ _ . 1 2 C 6 H 6.
D .5
¢, I
I I
I
.4
.3-
/i,,,..,,,, %
#
i I
.2-
v I
I
1580 F i g . 2. S p e c t r a
of 0.3 mm
I
!
I
1620 cm-1 liquid, --mixture,
- - - 1 2 C 6 H 6.
i
2081
The infrared spectrum of mono-‘%-substituted benzene
the 13C-substitution is given, because this magnitude generally is determined to B higher accuracy than is the shifted frequency itself. The inaccuracy indicated is a rough estimate of the probable experimental error in the determination of the frequency shift. The indication of intervals including positive numbers does not * D
.5
I
.4
I I
I
.3
I
I
I
.2
.1
I
I
I
I
_
I
3doo Fig. 3. Spectra of 0.025 mm liquid, -mixture,
31bO cm" ’ - - - 12C,H,.
mean that we accept the possibility of positive frequency shifts, except in the case of hot bands. The columns marked 12C give the frequencies of the correThe Q, PR, or M indicate that both the benzene fresponding bands in W,H,. quency and the frequency shift refer to a Q-branch maximum, a minimum between a P- and a R-branch, or a center of a band without much structure, respectively. Where a Q-branch is observed this will normally yield the best value for the frequency shift. Where a Q-branch is not observed the liquid spectra with the more regular band form often give the best determination. The table include all cases where frequency shifts could be determined with an appreciable accuracy.
S. BRODERSEN, J. CHRISTOFFERSEN, B. BAR,
2082
and J. TORMOD NIELSEN
DISCUSSION
W1sC,H, is an asymmetric top of the point group C,,. In Table 2 the distribution of the normal vibrations over the four symmetry species is given, using the numbering of WILSON [3] and LANGSETH and LORD [4]. cr is the operation of reflection in the molecular plane, so that A, and B, contain the in-plane vibrations and A, and B, the out-of-plane vibrations. Table 2. Distribution
C2v
E
4
1
B, A2 B,
1 1 1
c2
a’
1 -1 1 -1
0
1 -1 -1 1
IR forbidden: A,, A, x A,, IR allowed: all others. Raman allowed: all.
of Normal
Frequency
1 1
numbers
1, 2, 6a, 7a, 8a, 9a, 12, 13, 18a, 19a, 20~ 3, 66, 7b, 8b, 9b; 14, 15, 18b, 19b, 20b lOa, 16a, 17a 4, 5, lob, 11, 16b, 17b.
-1 -1 B,
Vibrations
x
B,.
Due to the smallness of the perturbation of the 13C-atom the band forms of Y?sC,H, (K = 0.916) cannot be expected to deviate very much from the band forms of the corresponding bands of 12C,H,. Especially it should be possible to use a measured shift in the position of a Q-branch maximum as a direct measure of the vibrational frequency shift. Formally all transitions of symmetry A,, B, or B, are infrared active. The perturbation of the 13C-atom is, however, so small that the intensities’ cannot be expected to deviate very much from the intensities observed in 12C6H6. Thus primarily those transitions should be observed which are active in 12C,H,. If new transitions do occur they have in a sense borrowed the intensity from the originally active transitions, a process which is in principle not very different from a Fermi resonance. The magnitude of this effect increases with the amplitude of the W-atom in the two states. A large increase in intensity should thus be connected with a large frequency shift. In the spectrum of 13C12C,Hs each of the vibrations corresponding to degenerate vibrations in 12C,H, is split into two components, the cc-component having symmetry A, or A,, and the b-component having symmetry B, or B,. It seems impossible on the basis of this difference in symmetry a priori to tell anything of the relative magnitude of the frequency shifts of the two components. This leaves a specific assignment to be made in close connection with potential function calculations. A preliminary assignment is given in Table 1. It is based on the assignment of the infrared spectrum of 12C,H, by BRODERSEN and LANGSETH [5]. In most cases a number of possible assignments have been given for each line due to the splitting in a- and b-components. For instance the Q-branch observed 2.2 cm-l below the [3] E. BRIGHT WILSON, JR., Phys. Rev. 45, 106 (1934). [4] A. LANGSETH and R. C. LORD, JR., Mat. Fys. Medd. Dan.. Vid. Selsk. 16,no. 6 (1938). [5] S. BRODERSEN and A. LANGSETR, Mat. Fys. Skr. Dan. Vid. Selsk. 1, no. 1 (1956).
The infrared spectrum of mono-13C-substituted
benzene
2083
12CBH, Q-branch at 1037 cm-l could be assigned either to v18a or to v18bor to both, indicating in the first two cases a probably very small shift of the other component. One could have expected, that the observed intensities should make it possible to ascertain whether one or both components was responsible for the new line. Unfortunately this is not so because of the underlying, varying P-branch intensity. One of the most interesting results of the present investigation is the effect of the isotopic substitution on the 1309 cm-l line of liquid 12C,H,. This line has been vibration yr4 by MAIR and HORNIG [6], and the assigned to the Bzu carbon-type assignment was later confirmed by BRODERSEN and LAN~SETH [5]. More recently the correctness of this assignment has been doubted by CALIFANO and CRAWFORD [7] on basis of calculations using a Urey-Bradley potential function. SCHERER and OVEREND [S] have indicated, however, a Urey-Bradley potential function, which is in agreement with the MAIR-HORNIG assignment. As shown in Fig. 1 the isotopic substitution has a double effect on the 1309 cm-’ line, the frequency is lowered by 6 & 1 cm-r and the intensity is increased by some 50%. yr4 is inactive in 12C,H, but is observed in the liquid state due to liquid perturbations. In 13Cr2C,H, y14 becomes formally active and being a typical carbontype vibration it is expected both to gain in intensity and to have the frequency lowered by an appreciable amount, just as observed. Preliminary computations indicate a frequency shift of about -7.7 cm-l. We take this agreement between observation and expectation as an additional proof of the correctness of the MAIR-HORNIG assignment. The only previously observed vibrational lines of 13C12CsH, are the y1 and yr2 Raman lines observed by LANGSETH and LORD [4] in natural benzene, which contains 6% of 13C12C,H,. They measured frequency shifts of -8.5 cm-l for y1 and -5 cm-l for yr2. The y1 line is observed in the present infrared spectrum with Av = -8 f 2 in agreement with the Raman value. vr2 is not observed here, probably due to overlapping of the strong vr8 line. Fermi resonance plays a rather important role in the assignment of the present spectrum as indicated in Table 1 by the use of square brackets. The well-known Fermi resonance doublet vs and vr + Ye observed in the Raman spectrum of 12C,H, is also observed in the infrared spectrum of the same liquid due to liquid perturbations as shown in Fig. 2. At a slightly higher frequency the allowed combination vg + v r2 gives rise to a somewhat higher and sharper peak. In the spectrum of liquid r3C1W,H, a series of three lines stands out rather clearly, corresponding to a constant frequency shift of about -10 cm-l. Both the form of the peaks and the intensity seems to indicate a Fermi resonance between the three levels, all three being of the same symmetry, either A, or B,. Another much weaker series of lines seems to indicate the other Fermi-resonance triplet with a constant frequency shift of about -3 cm-l. Fermi resonance between levels involving vs and v1 + vg is observed in three [6] R. D. MAIR and D. F. HORNIG, J. Chem. Phys. 17, 1236 (1949). [7] S. CALIFANO and B. CRAWFORD, JR., Spectrochinz. Acta 16,889 (1960). [8] J. R. SCRERER and J. OVEREND, Spectrochim. Acta 17, 719 (1961).
2084
S. BRODERSEN,
J. CHRISTOFFERSEN, B. BAK and J. TORMOD
NIELSEN
cases of combinations with vl,, vls and v14 respectively, but apparently the resonance with the vs + vls component is lost in all three cases. This may be due to the fact that the transitions to all these levels are allowed in 12CgH6, whereas the transitions to vs and vr + vg are observed in 13Ci2C,H, only because of borrowing intensity from the originally allowed transition to vg + vi2. Thus the vg + vls component has to be the strongest one in the last case and is easily observed, whereas this component may be weak and unobserved in the other three cases. The 3000 to 3100 cm-r region is rather complicated as shown in Fig. 3. This region of the 12C,H, liquid spectrum contains three strong lines assigned as a Fermi resonance triplet consisting of vzO, vs + vls, and y1 + vs + vls. The corresponding spectrum of 13C12C,H, contains at least four lines at 3031, 3058, 3076 and 3084 cm-l. The 3031 cm-l line is obviously one or both components of v2,,, giving a frequency shift of -5.5 cm-l. The 3058 line may be assigned either to one of the four components of vs + v,s giving a much higher frequency shift of - 14 cm-l or to v2, which is formally active, giving a more consistent frequency shift of -4 cm-l. The only possible assignment of the last two lines at 3076 and 3084 cm-l seems to be as two of the four components of vi + vs + vls. It is now worth while to note that in all the other cases of a Fermi resonance considered so far all components apparently are subject to a frequency shift of the same magnitude, consistent with the idea of a mixing of the wave functions. Therefore, one of the many possible explanations would be to interpret the 3058 cm-l line as two blended lines and to accept the presence of two Fermi The one group should then include vZOa,v2, and either v1 + resonance groups. or v1 + vBb + vlsb giving frequency shifts of -5.5, -4, and -7 cm-l v&z+ VlOa respectively. The other group should include either vsa + vlsb or vsb + visa and either v1 + vea + vlsb or v1 + vBb+ vlga giving frequency shifts of -14 and -15 cm-l respectively. Finally it should be remarked that the three lines in the 3600 to 3700 cm-l region obviously again is a Fermi resonance group with a constant frequency shift of -10 or - 11 cm-l. Here the number of combinations is, however, so high that a definite assignment seems almost impossible. Due to the preliminary nature of the assignment given it does not seem appropriate to try assigning a definite frequency shift to each of the 30 normal vibrations. Rather we intend to use the present result for comparison with potential function calculations now being in progress. The results of these calculations will be published separately. Acknowledgemelzt-One of us (S. B.) wants to thank Statens almindelige Videnskabsfond grant towards equipment for this investigation.
Note added in proof: Drs. I. M. Mills and J. H. Callomon have bility of a strong Coriolis perturbation between degenerate state. In such a case the observed to the shift in vibrational frequency due to an
for a
called our attention to the possithe two components of a formerly Q-branch shift may not correspond anomalous intensity distribution.