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High resolution studies in the electronic absorption spectra of mono-substituted naphthalenes 1-fluoro- and 1-chloro-naphthalene
JOURNAL OF MOLECULAR SPECTROSCOPY
102, 1- 12 ( 1983)
High Resolution Studies in the Electronic Absorption Spectra of Mono-Substituted Naphthalenes 1-Fluoroand 1-Chloro-naphthalene R. A. SINGHANDS.
N. THAKUR
Lasers and SpectroscopyLaboratory, Department of Physics.Banaras Hindu University, Varanasi-221005, India The vapor phase absorption spectra of I-fluoronaphthalene (l-F%) and I-chloronaphthalene (1-ClN) have been recorded under high resolution. The vibronic bands involving ground and excited state normal vibrations have been assigned on the basis of their rotational contours. Whereas the prominent bands in the spectrum of I-FN have only one type of rotational contour, those of l-ClN show at least three different types of rotational contours. A planar vibration involving the ring carbon atoms appears with slightly more intensity than the 0”,, band in the case of l-ClN and this vibration has been correlated with the in-plane ring deformation vibration which is effective in H-T intensity borrowing mechanism in the electronic spectrum of naphthalene. Fermi-type resonances have been observed in sequence bands of I-RN as well as I-ClN. 1. INTRODUCTION
The vapor phase electronic absorption in naphthalene at 312 nm (~‘Bz~ - jlAg) consists of two types of bands-the electronically allowed bands with the transition moments polarized along the a axis and the vibronically allowed bands with transition moments polarized along the b axis (I, 2). In mono-substituted naphthalenes the k + 2 absorption systems involve ?r* - a electron promotion corresponding closely to that in the k + 2 system of naphthalene. The spectral changes resulting from a substitution of an atom or a group of atoms in naphthalene are expected to depend on both the nature of the substituent and its position on the carbon ring. A rotational band contour analysis of the Ooo bands of mono-derivatives of naphthalene with substituents F, OH, and NH2 has shown that the perturbations of the transition moment orientation in these spectra are not very sensitive to the nature of the substituent (3, 4). It has been found, however, that the electronic transition moment vectors in 2-substituted naphthalene make angles of about -50” with respect to the electronic transition moment vector in naphthalene whereas these angles are about - 15’ in 1-substituted naphthalenes. A recent theoretical investigation by Suzuki and Fujii (5) also shows that the spectral changes caused by introduction of a substituent into naphthalene are mainly dependent on the position of the substituent and almost independent of its electron donating nature. Although this conclusion seems to agree with the results of rotational band contour analyses for the substituents F, OH, and NH2, it is not so for Cl (6). To show that the spectral changes do depend on the nature of the substituent the near ultraviolet absorption spectra of 1-fluoronaphthalene ( 1-FN) and l-chloronaphthalene ( l-ClN) seem to be good examples. The Oooband 1
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2
SINGH AND THAKUR
of the k - 2 system is the most intense of all bands in the case of l-FN, whereas a band at 0 + 450 cm-’ is more intense than the corresponding Oooband in the case of l-ClN. Singh and Singh (7) have made an attempt to explain the anomalous intensity of the 0 + 450 cm-’ band in l-ClN on the assumption that the characteristic features in the k - 2 electronic system of naphthalene are not affected significantly by chlorine substitution. Under this assumption one expects a relatively weak Ooo band with transition moment vector lying along the long in-plane axis of the carbon rings and a more intense vibronic band (0 + 450 cm-‘) with transition moment along the short in-plane axis, as is the case in the naphthalene spectrum. It turns out, however, that although the rotational contours of the 0’0 and the 0 + 450~cm-’ bands in 1-ClN are different, the polarization of the corresponding transition moments bears no similarity to those in the naphthalene spectrum. It has proved impossible to rotationally analyze the Oooband of I-ClN by assuming that the molecule is planar in both the excited and the ground states (6). In the present communication we report the results of a comparative study of the high resolution electronic absorption spectra of l-FN and I-ClN. 2. EXPERIMENTAL
DETAILS
The spectrograph used in the present work was a Jarrell-Ash 3.4-m Ebert spectrograph which has been described in detail elsewhere (8). The absorption spectra were recorded in the eighth order on Kodak spectrum analysis No. 1 plates with a slit width of 10 pm and exposure times of 15 to 25 min. The background continuum
I -10
I
I
-20
I,
I
-30
I
-20
I
I
-50
I.
I
-60
WAVENUMBER
I
I1
-70
-60
I
I
-90
*
I -100
I
I
-110
I
-1
(cm-‘)
FIG. I. Microdensitometer trace of o”,, band and accompanying sequences of I-tluoronaphthalene.
HIGH RESOLUTION
3
SPECTRA OF NAPHTHALENES
was provided by a 500-W xenon arc lamp and the cell temperature could be varied by means of heating tape wound round the 150-cm-long Pyrex tube fitted with quartz windows. The absorption spectra were recorded at various temperatures between 30 and 90°C in the case of 1-fluoronaphthalene and between 70 and 100°C in the case of 1-chloronaphthalene. Plates were measured on a Soviet VI3A comparator and iron arc lines in the sixth and seventh orders were used as standards for calibration. The uncertainty in measurements is *O. 1 cm-’ in the case of sharp rotational peaks and t-O.5 cm-’ in the case of broad rotational features. Rotational band contours were obtained from the photographic plates with a Jarrell-Ash microdensitometer. 3. OBSERVED FEATURES OF THE SPECTRA
I-Fluoronaphthalene The intense bands of this spectra are characterized by a single sharp rotational peak. The rotational contour of the Oooband which is the most intense, and some others that are free from overlapping transitions from neighbouring bands, exhibit a broad and weak peak to the high wavenumber side of the main peak (see Figs. 1, 2).
0
-10
-20 WAVENUMBER
-30
-40
-50
-
(cm-‘)
FIG. 2. Microdensitometer trace of 0 - 459,O + 411, and 0 + 690 bands and accompanying sequences of I-fluoronaphthalene.
4
SINGH AND THAKUR
The most prominent sequences have been designated as A, B, and C of which the first two show up to four members and the last one shows up to three members. A puzzling feature of the spectrum is that A’$‘, does not show up in the spectrum but instead there are four rather weak bands in the position of this sequence marked X? in Fig. 1. It is to be noted that both A’ ,C’ 1 and B ’,C’ 1 appear with expected intensities, and A’,B**, LI’,B~~ are perturbed in the same manner as AllB1, but A**B’, , A33B’ , do not show any perturbation. The sequence band marked I;‘, falls precisely in the same position as E’ IB ’ 1 and I;’ ,B ’ , lies at the same position as E’ 1B22; the choice of F’, as a sequence is based, however, on two facts. First, F’ I is more intense than E’ I, and second, there is a one-to-one correspondence in the main sequence intervals in l-FN and l-ClN spectra which supports F' , as a sequence band not related to E’ I. The cold band at 0 + 668 is not accompanied by sequences A, B, and C which is unusual in view of the large intensity of this band, and it may correspond to impurity present in the l-FN sample. 1-Chloronaphthalene The vibronic bands in this molecule can be divided into three groups on the basis of their rotational contours. The Oooband is characterized by a sharp peak accompanied by a rather broad peak on the low wavenumber side (separation = 0.8 cm-‘) and a broader peak on the high wavenumber side (separation = 2.6 cm-‘) (see Figs. 3, 4).
WAVENUMBER
FIG. 3. Microdensitometer l-chloronaphthalene.
( cd)
trace of o”,, 0 - 155 and 0 - 195 bands and accompanying sequence of
HIGH RESOLUTION
-10
SPECTRA OF NAPHTHALENES
-20
-30
-40
WAVENUMBER
FIG. 4. Microdensitometer 1-chloronaphthalene.
-50
5
-’
( cm-‘)
trace of 0 + 372 and 0 + 450 bands and accompanying sequences of
The ratio of the intensities of the main peak and the high wavenumber broad peak is about 2. We represent this contour as type I. The band at 0 + 450 cm-‘, which is comparable in intensity to the Ooobands, has two sharp peaks (separation = 1.0 cm-‘) in addition to the broad peak on the high wavenumber side (separation from nearest sharp peak = 2.6 cm-‘). The intensity of the main peak is about six times as large as that of the broad high wavenumber peak. We call this type of contour as type II. The rotational contours of a few weak bands exhibit only one main peak having a shoulder on the low wavenumber side and a broad peak on the high wavenumber side. The ratio of the intensities of the main peak and the high wavenumber peak is about 4 and we denote these contours as type III. The three most intense sequences in order of decreasing intensity have again been designated by A, B, and C and the first two have been observed up to three members and the third one up to two members. The sequence band A ’rC ’ I is missing, however, and it will be shown later that its explanation is similar to that of the perturbation observed in A’,B’, in the 1-FN spectrum. 4. VIBRATIONAL
ANALYSIS
I-Fluoronaphthalene The Ooo band of the electronic system has been identified at 31 868.2 cm-’ in agreement with the low resolution work (9) and a rotational band contour analysis
6
SINGH AND THAKUR
(4) has shown that this band is type A-type B hybrid. The vibrational structure consisting of about 400 vibronic bands has been analyzed in terms of the ground and excited state fundamentals summarized in Table I and assignments of the prominent bands have been presented in Table II. In general the intensity of bands associated with in-plane vibrations are more than those associated with out-of-plane vibrations. In Table I we have also shown the corresponding naphthalene vibrations both in the ground and the excited states wherever possible. The bands involving out of plane vibrations are very weak with their rotational contours not very well characterized and hence their assignments have been made on the basis of vibrational separations available from the far infrared and Raman spectra. There are some deviations in our descriptions of naphthalene-like vibrations in these molecules from those reported by Michaelian and Ziegler (10). The vibrational frequency of 7 11 cm-’ in I-FN spectrum has been correlated with that of 760.7 cm-’ in the naphthalene spectrum in view of the fact that both of them appear as hot bands in the vapor phase absorption. The vibrational frequency of 459 cm-’ in lFN has been correlated with 506 cm-’ in naphthalene spectrum in view of the fact that the former is associated with the most intense hot band in the 1-m spectrum as is the latter in the naphthalene spectrum. I-Chloronaphthalene The O”, band in this spectrum has been identified at 31 576.1 cm-’ with a rotational contour type I (see Sect. 3). An equally strong vibronic band is observed at 32 026.3
TABLE I Correlation of Ground and Excited State Fundamentals (in cm-‘) of Naphthalene and Substituted Naphthalenes’ mode and symmetry
Normal
1-Fluoronaphthalene Ground st_ate
Excited state
l-Chloronaphthalene Sequence Interval
bCCC,,a”
148.4
105.7
44.1
f!ccc, ,a”
193.9
157.6
35.7
XCX)
239.6
,a’
Y(CX) ,a”
266.9
252.7
c&CCC) ,a’
459.3
410.7
V(CX) ,a’
531.0
499.2
cL(CCC) ,a’
568.0
V(CC) ,a’
710.7
V(K)
,a’
(8771
13.3
Ground state
Excited state
155.2 195.L
167.5
231.3
228.6
Naphthalene Seq"enW Interval
Grwnd state
48.6
ial
171
27.6
195
140
506.1
437.7
14.5
237.6 513.9
450.2
63.8
392.2
372.0
20.4
545.2
536.7
521.2
516
500.7
689.8
657.8
639.5
760.7
702.0
936.2
911.0
31.3
837.1
(827)
788.0
ct(CCC) ,a’
(1010)
946.9
(1003)
913.6
“(CC) ,a’
(1080)
1011.7
(1058)
1023.2
1021
“(CC) ,a’
(1574)
1432.4
(1565)
1400.4
1579
(1624)
1598.0
v(W)
Excited state
>a’ (a) The values shown in parentheses
refer to i.r. spectra
987.4 1434.7 1606.7
(ref.10).
-35.7 -31.3 -26.5 -13.3
752.9
155.2
761.5
763.7
0.0
3.1
-39.7
74’1.6
780.?
-44.1
9.1
-45
736.1
741.5
767.1
-47.2 3
I
-51.b 1
726.3
734.9
-53.1
119.8
-49.3
-57.2
728.4
-61.7
674.3
-67.”
628.6
801.2
-09.3
-66.1
o-459.,
408.9
798.9
601.3
?
O-508.5
-70.2
359.7
-71.3
796.9
798.0
(Pp,
O-568.0
o-531.0
300.?
337.2
-74.9
-79.7
793.3
31788.5
O-710.7
O-615.4
252.8
in the Absorption
31157.5
Principal Bands and Their Assignment
TABLE II Spectra of I-Fluoronaphthalene
157.6
946.9
837.1
118.2
689.8
670.9
668.3
662.1
545.2
499.?
410.1
263.3
152.7
211.4
1436.3
1432.4
1380.3
1100.1
1077.8
1011.7
Vapor
8
SINGH AND THAKUR
cm-’ with a type II rotational contour and the excited state frequency of 450 cm-’ associated with this band has been correlated with the ground state frequency of 5 14 cm-’ associated with an intense hot band at 31 062.2 cm-‘. Four prominent hot bands that could not be explained otherwise were taken to be associated with ground state vibrational frequencies of 155, 195, 23 1, and 237 cm-‘. Michaelian and Ziegler (IO) have reported liquid phase infrared vibrational frequencies of 135, 180,220, and 240 cm-’ which involve out of plane vibrations. We have recorded FTIR spectra and have observed vibrational bands at 155, 185,223, and 240 cm-’ in the far infrared. The bands O-l 55 and O-195 cm-’ exhibit a type III rotational contour. It is also observed that the sequence intervals A and B associated with these bands are slightly different from those associated with the Oooband and other prominent vibronic bands. The assignments of prominent bands in I-ClN spectrum are given in Table III using the vibrational frequencies summarized in Table I. The analysis in this case also is based on the measurement of nearly 400 vibrational features in the spectrum. 5. RESULTS AND DISCUSSION
The results of vibrational analyses in the spectra of l-FN and l-ClN have given a number of comparative data which will be discussed in this section. Out of Plane Vibrations Assuming that both the molecules belong to C, point group one can see that at least three out of plane vibrations have been observed in each case. The intensities of the bands associated with the two lowest lying out of plane vibrations in the case of I-ClN are much more than those of the corresponding bands in I-FN spectra but none of these vibrations in the excited electronic states appears as a cold band. The assignments of sequence intervals to these vibrations are based partly on two perturbations observed in the sequence bands that will be discussed later and partly on the observations of what we believe to be bands involving two quanta of the excited state frequencies. Thus a cold band at 32 267.1 cm-’ in the I-FN spectrum involves an excited state interval of 3 15 cm-’ which we have taken as two quanta of an excited state frequency of 157.5 cm-’ and correlated it to the ground state frequency of 194 cm-‘. This accounts for the sequence interval labeled B (=35.6 cm-‘). Similarly a cold band at 3 1 9 11.7 cm-’ in l-ClN spectrum has been identified with two quanta of excited state frequency of 167.8 cm-’ and this explains the sequence interval of 27.5 cm-’ (B) in this case. In-plane Vibration The in-plane vibrations have been identified by comparing the absorption spectra of l-FN and l-ClN with that of naphthalene (1 I, 12). The ground state vibrational intervals of 459 cm-’ in l-FN and 514 cm-’ in l-ClN have been identified with a b3, fundamental of 506 cm-’ in naphthalene. The corresponding excited state fundamentals are found to be 4 11 and 450 cm-’ in l-RN and l-ClN, respectively, and may be compared with the frequency of 438 cm-’ in the naphthalene spectrum. This correlation is based mainly on the identically large activity of this vibrational mode
- 70.7
- 75.8
497.4
500.3
8; c;
11 *1 El 11 % "1 . .
-116.6
459.5
- 80.5
-124.9
451.2
495.6
-126.9
449.2
- 83.4
-150.9
425.2
492.7
-155.2
420.9
- 91.2
-195.1
381.0
484.9
-231.3
- 98.2
-237.6
338.5
344.8
477.9
-308.9
-114.2
-392.2
183.9
267.2
-102.1
-513.9
Ob2.2
461.9
545.0
-536.7
31039.4
474.0
534.3
-604.7
971.4
63.8
66.0
1159.9 1238.1 1400.4 1472.5 1598.0 1878.0
736.0 814.2 976.5 33048.6 174.1 454.1
521.2
450.2
32026.3 097.3
372.0
948.1
335.6
911.7
0.0
3.7
7.0
9.2
228.6
-
-
- 10.3
- 14.5
18.1
- 20.4
- 27.6
Ot521.2
Ot450.2 (N)
cw372.0 G+)
0+2x167.5 (B;)
W228.6
o0 0
1089.2
665.3
- 28.9
- 31.1
1023.2
519.7 599.3
fj Al Bl 11 11 % Dl 2 % 1 El 1 Pl 11 % Dl 1 % 3 % 1 Nl 2 % 1 %
m450.2+639.5+ 788.0
W1598.0
0+450.2+1023.2
lnl400.4
ot450.2+788.0
0+372.0+788.0
O&50.2+639.5
ot1023.2
Dc943.6
822.6
398.7
943.6
w372.@t450.* (PlMl) 00
788.0
364.1
0+639.5 cw788.0
639.5
32215.6
- 41.8
1 El 1 "1 2 B2 1
Spectra of I-Chloronaphthalene
- 43.3
- 48.6
- 55.4
-
-
in the Absorption
III
804.7
576.1
572.4
569.1
566.9
565.8
561.6
558.0
555.7
548.5
547.2
532.8
527.5
520.7
512.3
-657.8
30918.3
31510.1
Principal Bands and Their Assignment
TABLE
W
10
SINGH AND THAKUR
in all the three spectra. In the case of naphthalene the cold band involving 438 cm-’ is the most intense band of the spectrum; in the case of I-ClN the corresponding cold band has an intensity comparable to the Oooband of the spectrum, whereas in l-FN the cold band associated with 411 cm-’ is one of the strong vibronic bands but is less intense than the Oooband. It is also found that both the ground state and the excited state vibrational frequencies combine with other planar vibrations and give rise to strong vibronic bands. We have observed a number of cold bands involving the planar motion of carbon atoms of which those representing stretching in a carbon-carbon bonds are of special interest. In the 3 12-nm system of naphthalene a ground state frequency of 1579 cm-’ involving C-C stretch is changed to 1435 cm-’ in the excited state, whereas those of 102 1 and 76 1 cm-’ in the ground state correspond to 987 and 702 cm-’ in the excited state, respectively. The force field calculations of Neto et al. (23) show that both 1579and 76 1-cm-’ vibrational frequencies involve stretching of the central C-C bond (C,Co) whereas that of 1021 cm-’ does not. If we assume that the normal coordinates do not change significantly in going from ground to the excited electronic state then a relatively large change is expected in the former two frequencies. This is due to the fact that in ‘Bzu state of naphthalene the expansion in C9-Cl0 bond is the largest (2). A perusal of Table I shows that both in l-FN as well as in l-ClN spectra the changes in the two larger C-C stretching frequencies are similar to those in naphthalene but that in the lowest C-C stretching (corresponding to 76 1 cm-’ in naphthalene) is about 3% as against 8% in naphthalene. Sequence Intervals In the 1-ClN spectrum a number of bands involving three of the normal coordinates were identified and were arranged in Deslandres’ schemes. This provided a simple explanation for the occurrence of sequence intervals denoted by B, M, and P. The mean values for the ground and excited state fundamentals responsible for the sequence internal P are 392 and 372 cm-‘, respectively, and the normal vibration involves a large amount of C-Cl stretching. The corresponding values for the ground and excited state fundamentals in l-FN are found to be 531 and 499 cm-‘, respectively, and the normal coordinate involves C-F stretching. Sequence interval M in l-ClN spectrum is attributed to a normal coordinate with ground and excited state frequencies of 5 14 and 450 cm-‘, respectively. The corresponding sequence in l-FN spectrum should involve the ground and excited state frequencies of 46 1 and 4 11 cm-’ but it is missing because of a perturbation described below. The origin of sequence interval B has already been described. It is expected that two of the most intense sequences A and C should originate in the two low lying ground state frequencies. The assignments of these sequence intervals are based on the observations of perturbations in the A’ lB’l band of l-l% and in the A’ ,C’ , band of l-ClN. In l-FN spectrum the bands A’lC’l, A22C’I are observed with sufficient intensities but those corresponding to A’, B1,,A22B’ i are not. A closer examination in the later cases shows that there are four relatively weaker peaks in regions where one expects these bands to appear. We see from Table I that the sum of ground state fundamentals 194 and 267 cm-’ of
HIGH RESOLUTION SPECTRA OF NAPHTHALENES
11
a” symmetry agrees with the ground state ring deformation fundamental of 460 cm-’ of a’ symmetry within 1 cm -‘. Thus, there is a likelihood of Fermi resonance between the combination level and the fundamental level. We believe this to be the reason for the anomalous splittings observed at A ’ J? ’ , . It also turns out that the sum of the sequence intervals A (13.3 cm-‘) and B (35.7 cm-‘) agrees to within 1 cm-’ with the difference between the ground state and excited state fundamentals belonging to the ring deformation vibration. This indicates that the same set of combination and fundamental levels might be in Fermi resonance both in the ground and the excited electronic states. We associate the sequence interval A with the ground state fundamental of 267 cm-’ and indeed there is a weak band around 0 + 253 cm-’ in the 1-FN spectrum. The slight disagreement between the observed and calculated sequence intervals may be attributed to the fact that the observed values refer to separations between the strongest absorption peaks in the bands which may not coincide with the band origins. The third intense sequence interval C is attributed to the remaining low lying fundamental of 148 cm-‘. In I -CIN spectrum, instead of observing a single intense peak in the region of band A’,C’ , we observe a very weak broad structure. This perturbation is again found to be due to an almost exact coincidence between the ground state combination level of (155 + 238 cm-‘) with a fundamental level of 392 cm-‘. The frequencies of the combination level belong to a” symmetry, whereas that of the fundamental level belongs to a’ symmetry, thus fulfilling the conditions for a Fermi resonance. In analogy with l-FN we attribute the sequence intervals A to 23%cm-’ vibration and the sequence intervals C to 155-cm-’ vibration. Unlike the case of I-FN, however, the combination and the fundamental levels in the excited electronic state do not lie close together and hence there is no possibility of a Fermi resonance in the excited state.
6. CONCLUSION
The ring deformation vibration involved in the Herzberg-Teller type of intensity borrowing in the 3 12-nm system of naphthalene also gives rise to strong vibronic bands in I-ClN and l-FN spectra but the activity of this vibration is in the order naphthalene > I-GIN > 1TN. A number of out of plane vibrations have been observed in the spectra of I-ClN and I-FN but more prominently in the former. Fermi-type resonances in sequence bands of I-ClN and I-FN have been found helpful in the assignment of sequence intervals. ACKNOWLEDGMENTS We thank Dr. J. M. Hollas for laboratory facilitiesfor recording the absorption spectra. RAS acknowledges the award of a Senior Research Fellowship of the CSIR (New Delhi).
RECEIVED: September
20, 1979
12
SINGH AND THAKUR REFERENCES
1. K. K. INNES,J. E. PARKIN,D. K. IRVIN, J. M. HOLLAS,AND I. G. Ross, J. Mol. Specfrosc. 16,406414 (1965). 2. J. M. HOLLASAND S. N. THAKUR, Mol. Phys. 22, 203-212 (197 1). 3. J. M. HOLLASAND S. N. THAKUR, Mol. Phys. 25, 1315-1334 (1973). 4. J. M. HOLLASAND S. N. THAKUR, Mol. Phys. 27, 1001-1011 (1974). 5. S. SUZUKIAND T. FIJJII,J. Mol. Spectrosc. 61, 350-359 (1976). 6. R. A. SINGH AND S. N. THAKUR, Mol. Phys. 36, 1053-1059 (1978). 7. S. N. SINGH AND R. D. SINGH, J. Mol. Spectrosc. 65,20-23 (1977). 8. J. CHRISTOFFERSON, J. M. HOLLAS,AND G. H. KIRBY, Pm. R. Sot. A 307,97-l 10 (1968). 9. S. RAMAMURTY,M. J., RAO, AND V. R. RAO, Indian J. Phys. 31, 497-510 (1957). 10. K. H. MICHAEL~ANAND S. M. ZIEGLER,Appl. Spectrosc. 27, 13-21 (1973). 11. D. P. CRAIG, J. M. HOLLAS,M. F. REDIES,AND S. C. WAIT, JR.,Philos. Trans. R. Sot. London Ser. A 253, 543-568 (1961). 12. J. M. HOLLAS,J. Mol. Spectrosc. 9, 138-169 (1962). 13. N. NETO, M. ~CRCECO,AND S. CALIFANO,Spectrochim. Acta 22, 1981-1998 (1966).
102, 1- 12 ( 1983)
High Resolution Studies in the Electronic Absorption Spectra of Mono-Substituted Naphthalenes 1-Fluoroand 1-Chloro-naphthalene R. A. SINGHANDS.
N. THAKUR
Lasers and SpectroscopyLaboratory, Department of Physics.Banaras Hindu University, Varanasi-221005, India The vapor phase absorption spectra of I-fluoronaphthalene (l-F%) and I-chloronaphthalene (1-ClN) have been recorded under high resolution. The vibronic bands involving ground and excited state normal vibrations have been assigned on the basis of their rotational contours. Whereas the prominent bands in the spectrum of I-FN have only one type of rotational contour, those of l-ClN show at least three different types of rotational contours. A planar vibration involving the ring carbon atoms appears with slightly more intensity than the 0”,, band in the case of l-ClN and this vibration has been correlated with the in-plane ring deformation vibration which is effective in H-T intensity borrowing mechanism in the electronic spectrum of naphthalene. Fermi-type resonances have been observed in sequence bands of I-RN as well as I-ClN. 1. INTRODUCTION
The vapor phase electronic absorption in naphthalene at 312 nm (~‘Bz~ - jlAg) consists of two types of bands-the electronically allowed bands with the transition moments polarized along the a axis and the vibronically allowed bands with transition moments polarized along the b axis (I, 2). In mono-substituted naphthalenes the k + 2 absorption systems involve ?r* - a electron promotion corresponding closely to that in the k + 2 system of naphthalene. The spectral changes resulting from a substitution of an atom or a group of atoms in naphthalene are expected to depend on both the nature of the substituent and its position on the carbon ring. A rotational band contour analysis of the Ooo bands of mono-derivatives of naphthalene with substituents F, OH, and NH2 has shown that the perturbations of the transition moment orientation in these spectra are not very sensitive to the nature of the substituent (3, 4). It has been found, however, that the electronic transition moment vectors in 2-substituted naphthalene make angles of about -50” with respect to the electronic transition moment vector in naphthalene whereas these angles are about - 15’ in 1-substituted naphthalenes. A recent theoretical investigation by Suzuki and Fujii (5) also shows that the spectral changes caused by introduction of a substituent into naphthalene are mainly dependent on the position of the substituent and almost independent of its electron donating nature. Although this conclusion seems to agree with the results of rotational band contour analyses for the substituents F, OH, and NH2, it is not so for Cl (6). To show that the spectral changes do depend on the nature of the substituent the near ultraviolet absorption spectra of 1-fluoronaphthalene ( 1-FN) and l-chloronaphthalene ( l-ClN) seem to be good examples. The Oooband 1
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2
SINGH AND THAKUR
of the k - 2 system is the most intense of all bands in the case of l-FN, whereas a band at 0 + 450 cm-’ is more intense than the corresponding Oooband in the case of l-ClN. Singh and Singh (7) have made an attempt to explain the anomalous intensity of the 0 + 450 cm-’ band in l-ClN on the assumption that the characteristic features in the k - 2 electronic system of naphthalene are not affected significantly by chlorine substitution. Under this assumption one expects a relatively weak Ooo band with transition moment vector lying along the long in-plane axis of the carbon rings and a more intense vibronic band (0 + 450 cm-‘) with transition moment along the short in-plane axis, as is the case in the naphthalene spectrum. It turns out, however, that although the rotational contours of the 0’0 and the 0 + 450~cm-’ bands in 1-ClN are different, the polarization of the corresponding transition moments bears no similarity to those in the naphthalene spectrum. It has proved impossible to rotationally analyze the Oooband of I-ClN by assuming that the molecule is planar in both the excited and the ground states (6). In the present communication we report the results of a comparative study of the high resolution electronic absorption spectra of l-FN and I-ClN. 2. EXPERIMENTAL
DETAILS
The spectrograph used in the present work was a Jarrell-Ash 3.4-m Ebert spectrograph which has been described in detail elsewhere (8). The absorption spectra were recorded in the eighth order on Kodak spectrum analysis No. 1 plates with a slit width of 10 pm and exposure times of 15 to 25 min. The background continuum
I -10
I
I
-20
I,
I
-30
I
-20
I
I
-50
I.
I
-60
WAVENUMBER
I
I1
-70
-60
I
I
-90
*
I -100
I
I
-110
I
-1
(cm-‘)
FIG. I. Microdensitometer trace of o”,, band and accompanying sequences of I-tluoronaphthalene.
HIGH RESOLUTION
3
SPECTRA OF NAPHTHALENES
was provided by a 500-W xenon arc lamp and the cell temperature could be varied by means of heating tape wound round the 150-cm-long Pyrex tube fitted with quartz windows. The absorption spectra were recorded at various temperatures between 30 and 90°C in the case of 1-fluoronaphthalene and between 70 and 100°C in the case of 1-chloronaphthalene. Plates were measured on a Soviet VI3A comparator and iron arc lines in the sixth and seventh orders were used as standards for calibration. The uncertainty in measurements is *O. 1 cm-’ in the case of sharp rotational peaks and t-O.5 cm-’ in the case of broad rotational features. Rotational band contours were obtained from the photographic plates with a Jarrell-Ash microdensitometer. 3. OBSERVED FEATURES OF THE SPECTRA
I-Fluoronaphthalene The intense bands of this spectra are characterized by a single sharp rotational peak. The rotational contour of the Oooband which is the most intense, and some others that are free from overlapping transitions from neighbouring bands, exhibit a broad and weak peak to the high wavenumber side of the main peak (see Figs. 1, 2).
0
-10
-20 WAVENUMBER
-30
-40
-50
-
(cm-‘)
FIG. 2. Microdensitometer trace of 0 - 459,O + 411, and 0 + 690 bands and accompanying sequences of I-fluoronaphthalene.
4
SINGH AND THAKUR
The most prominent sequences have been designated as A, B, and C of which the first two show up to four members and the last one shows up to three members. A puzzling feature of the spectrum is that A’$‘, does not show up in the spectrum but instead there are four rather weak bands in the position of this sequence marked X? in Fig. 1. It is to be noted that both A’ ,C’ 1 and B ’,C’ 1 appear with expected intensities, and A’,B**, LI’,B~~ are perturbed in the same manner as AllB1, but A**B’, , A33B’ , do not show any perturbation. The sequence band marked I;‘, falls precisely in the same position as E’ IB ’ 1 and I;’ ,B ’ , lies at the same position as E’ 1B22; the choice of F’, as a sequence is based, however, on two facts. First, F’ I is more intense than E’ I, and second, there is a one-to-one correspondence in the main sequence intervals in l-FN and l-ClN spectra which supports F' , as a sequence band not related to E’ I. The cold band at 0 + 668 is not accompanied by sequences A, B, and C which is unusual in view of the large intensity of this band, and it may correspond to impurity present in the l-FN sample. 1-Chloronaphthalene The vibronic bands in this molecule can be divided into three groups on the basis of their rotational contours. The Oooband is characterized by a sharp peak accompanied by a rather broad peak on the low wavenumber side (separation = 0.8 cm-‘) and a broader peak on the high wavenumber side (separation = 2.6 cm-‘) (see Figs. 3, 4).
WAVENUMBER
FIG. 3. Microdensitometer l-chloronaphthalene.
( cd)
trace of o”,, 0 - 155 and 0 - 195 bands and accompanying sequence of
HIGH RESOLUTION
-10
SPECTRA OF NAPHTHALENES
-20
-30
-40
WAVENUMBER
FIG. 4. Microdensitometer 1-chloronaphthalene.
-50
5
-’
( cm-‘)
trace of 0 + 372 and 0 + 450 bands and accompanying sequences of
The ratio of the intensities of the main peak and the high wavenumber broad peak is about 2. We represent this contour as type I. The band at 0 + 450 cm-‘, which is comparable in intensity to the Ooobands, has two sharp peaks (separation = 1.0 cm-‘) in addition to the broad peak on the high wavenumber side (separation from nearest sharp peak = 2.6 cm-‘). The intensity of the main peak is about six times as large as that of the broad high wavenumber peak. We call this type of contour as type II. The rotational contours of a few weak bands exhibit only one main peak having a shoulder on the low wavenumber side and a broad peak on the high wavenumber side. The ratio of the intensities of the main peak and the high wavenumber peak is about 4 and we denote these contours as type III. The three most intense sequences in order of decreasing intensity have again been designated by A, B, and C and the first two have been observed up to three members and the third one up to two members. The sequence band A ’rC ’ I is missing, however, and it will be shown later that its explanation is similar to that of the perturbation observed in A’,B’, in the 1-FN spectrum. 4. VIBRATIONAL
ANALYSIS
I-Fluoronaphthalene The Ooo band of the electronic system has been identified at 31 868.2 cm-’ in agreement with the low resolution work (9) and a rotational band contour analysis
6
SINGH AND THAKUR
(4) has shown that this band is type A-type B hybrid. The vibrational structure consisting of about 400 vibronic bands has been analyzed in terms of the ground and excited state fundamentals summarized in Table I and assignments of the prominent bands have been presented in Table II. In general the intensity of bands associated with in-plane vibrations are more than those associated with out-of-plane vibrations. In Table I we have also shown the corresponding naphthalene vibrations both in the ground and the excited states wherever possible. The bands involving out of plane vibrations are very weak with their rotational contours not very well characterized and hence their assignments have been made on the basis of vibrational separations available from the far infrared and Raman spectra. There are some deviations in our descriptions of naphthalene-like vibrations in these molecules from those reported by Michaelian and Ziegler (10). The vibrational frequency of 7 11 cm-’ in I-FN spectrum has been correlated with that of 760.7 cm-’ in the naphthalene spectrum in view of the fact that both of them appear as hot bands in the vapor phase absorption. The vibrational frequency of 459 cm-’ in lFN has been correlated with 506 cm-’ in naphthalene spectrum in view of the fact that the former is associated with the most intense hot band in the 1-m spectrum as is the latter in the naphthalene spectrum. I-Chloronaphthalene The O”, band in this spectrum has been identified at 31 576.1 cm-’ with a rotational contour type I (see Sect. 3). An equally strong vibronic band is observed at 32 026.3
TABLE I Correlation of Ground and Excited State Fundamentals (in cm-‘) of Naphthalene and Substituted Naphthalenes’ mode and symmetry
Normal
1-Fluoronaphthalene Ground st_ate
Excited state
l-Chloronaphthalene Sequence Interval
bCCC,,a”
148.4
105.7
44.1
f!ccc, ,a”
193.9
157.6
35.7
XCX)
239.6
,a’
Y(CX) ,a”
266.9
252.7
c&CCC) ,a’
459.3
410.7
V(CX) ,a’
531.0
499.2
cL(CCC) ,a’
568.0
V(CC) ,a’
710.7
V(K)
,a’
(8771
13.3
Ground state
Excited state
155.2 195.L
167.5
231.3
228.6
Naphthalene Seq"enW Interval
Grwnd state
48.6
ial
171
27.6
195
140
506.1
437.7
14.5
237.6 513.9
450.2
63.8
392.2
372.0
20.4
545.2
536.7
521.2
516
500.7
689.8
657.8
639.5
760.7
702.0
936.2
911.0
31.3
837.1
(827)
788.0
ct(CCC) ,a’
(1010)
946.9
(1003)
913.6
“(CC) ,a’
(1080)
1011.7
(1058)
1023.2
1021
“(CC) ,a’
(1574)
1432.4
(1565)
1400.4
1579
(1624)
1598.0
v(W)
Excited state
>a’ (a) The values shown in parentheses
refer to i.r. spectra
987.4 1434.7 1606.7
(ref.10).
-35.7 -31.3 -26.5 -13.3
752.9
155.2
761.5
763.7
0.0
3.1
-39.7
74’1.6
780.?
-44.1
9.1
-45
736.1
741.5
767.1
-47.2 3
I
-51.b 1
726.3
734.9
-53.1
119.8
-49.3
-57.2
728.4
-61.7
674.3
-67.”
628.6
801.2
-09.3
-66.1
o-459.,
408.9
798.9
601.3
?
O-508.5
-70.2
359.7
-71.3
796.9
798.0
(Pp,
O-568.0
o-531.0
300.?
337.2
-74.9
-79.7
793.3
31788.5
O-710.7
O-615.4
252.8
in the Absorption
31157.5
Principal Bands and Their Assignment
TABLE II Spectra of I-Fluoronaphthalene
157.6
946.9
837.1
118.2
689.8
670.9
668.3
662.1
545.2
499.?
410.1
263.3
152.7
211.4
1436.3
1432.4
1380.3
1100.1
1077.8
1011.7
Vapor
8
SINGH AND THAKUR
cm-’ with a type II rotational contour and the excited state frequency of 450 cm-’ associated with this band has been correlated with the ground state frequency of 5 14 cm-’ associated with an intense hot band at 31 062.2 cm-‘. Four prominent hot bands that could not be explained otherwise were taken to be associated with ground state vibrational frequencies of 155, 195, 23 1, and 237 cm-‘. Michaelian and Ziegler (IO) have reported liquid phase infrared vibrational frequencies of 135, 180,220, and 240 cm-’ which involve out of plane vibrations. We have recorded FTIR spectra and have observed vibrational bands at 155, 185,223, and 240 cm-’ in the far infrared. The bands O-l 55 and O-195 cm-’ exhibit a type III rotational contour. It is also observed that the sequence intervals A and B associated with these bands are slightly different from those associated with the Oooband and other prominent vibronic bands. The assignments of prominent bands in I-ClN spectrum are given in Table III using the vibrational frequencies summarized in Table I. The analysis in this case also is based on the measurement of nearly 400 vibrational features in the spectrum. 5. RESULTS AND DISCUSSION
The results of vibrational analyses in the spectra of l-FN and l-ClN have given a number of comparative data which will be discussed in this section. Out of Plane Vibrations Assuming that both the molecules belong to C, point group one can see that at least three out of plane vibrations have been observed in each case. The intensities of the bands associated with the two lowest lying out of plane vibrations in the case of I-ClN are much more than those of the corresponding bands in I-FN spectra but none of these vibrations in the excited electronic states appears as a cold band. The assignments of sequence intervals to these vibrations are based partly on two perturbations observed in the sequence bands that will be discussed later and partly on the observations of what we believe to be bands involving two quanta of the excited state frequencies. Thus a cold band at 32 267.1 cm-’ in the I-FN spectrum involves an excited state interval of 3 15 cm-’ which we have taken as two quanta of an excited state frequency of 157.5 cm-’ and correlated it to the ground state frequency of 194 cm-‘. This accounts for the sequence interval labeled B (=35.6 cm-‘). Similarly a cold band at 3 1 9 11.7 cm-’ in l-ClN spectrum has been identified with two quanta of excited state frequency of 167.8 cm-’ and this explains the sequence interval of 27.5 cm-’ (B) in this case. In-plane Vibration The in-plane vibrations have been identified by comparing the absorption spectra of l-FN and l-ClN with that of naphthalene (1 I, 12). The ground state vibrational intervals of 459 cm-’ in l-FN and 514 cm-’ in l-ClN have been identified with a b3, fundamental of 506 cm-’ in naphthalene. The corresponding excited state fundamentals are found to be 4 11 and 450 cm-’ in l-RN and l-ClN, respectively, and may be compared with the frequency of 438 cm-’ in the naphthalene spectrum. This correlation is based mainly on the identically large activity of this vibrational mode
- 70.7
- 75.8
497.4
500.3
8; c;
11 *1 El 11 % "1 . .
-116.6
459.5
- 80.5
-124.9
451.2
495.6
-126.9
449.2
- 83.4
-150.9
425.2
492.7
-155.2
420.9
- 91.2
-195.1
381.0
484.9
-231.3
- 98.2
-237.6
338.5
344.8
477.9
-308.9
-114.2
-392.2
183.9
267.2
-102.1
-513.9
Ob2.2
461.9
545.0
-536.7
31039.4
474.0
534.3
-604.7
971.4
63.8
66.0
1159.9 1238.1 1400.4 1472.5 1598.0 1878.0
736.0 814.2 976.5 33048.6 174.1 454.1
521.2
450.2
32026.3 097.3
372.0
948.1
335.6
911.7
0.0
3.7
7.0
9.2
228.6
-
-
- 10.3
- 14.5
18.1
- 20.4
- 27.6
Ot521.2
Ot450.2 (N)
cw372.0 G+)
0+2x167.5 (B;)
W228.6
o0 0
1089.2
665.3
- 28.9
- 31.1
1023.2
519.7 599.3
fj Al Bl 11 11 % Dl 2 % 1 El 1 Pl 11 % Dl 1 % 3 % 1 Nl 2 % 1 %
m450.2+639.5+ 788.0
W1598.0
0+450.2+1023.2
lnl400.4
ot450.2+788.0
0+372.0+788.0
O&50.2+639.5
ot1023.2
Dc943.6
822.6
398.7
943.6
w372.@t450.* (PlMl) 00
788.0
364.1
0+639.5 cw788.0
639.5
32215.6
- 41.8
1 El 1 "1 2 B2 1
Spectra of I-Chloronaphthalene
- 43.3
- 48.6
- 55.4
-
-
in the Absorption
III
804.7
576.1
572.4
569.1
566.9
565.8
561.6
558.0
555.7
548.5
547.2
532.8
527.5
520.7
512.3
-657.8
30918.3
31510.1
Principal Bands and Their Assignment
TABLE
W
10
SINGH AND THAKUR
in all the three spectra. In the case of naphthalene the cold band involving 438 cm-’ is the most intense band of the spectrum; in the case of I-ClN the corresponding cold band has an intensity comparable to the Oooband of the spectrum, whereas in l-FN the cold band associated with 411 cm-’ is one of the strong vibronic bands but is less intense than the Oooband. It is also found that both the ground state and the excited state vibrational frequencies combine with other planar vibrations and give rise to strong vibronic bands. We have observed a number of cold bands involving the planar motion of carbon atoms of which those representing stretching in a carbon-carbon bonds are of special interest. In the 3 12-nm system of naphthalene a ground state frequency of 1579 cm-’ involving C-C stretch is changed to 1435 cm-’ in the excited state, whereas those of 102 1 and 76 1 cm-’ in the ground state correspond to 987 and 702 cm-’ in the excited state, respectively. The force field calculations of Neto et al. (23) show that both 1579and 76 1-cm-’ vibrational frequencies involve stretching of the central C-C bond (C,Co) whereas that of 1021 cm-’ does not. If we assume that the normal coordinates do not change significantly in going from ground to the excited electronic state then a relatively large change is expected in the former two frequencies. This is due to the fact that in ‘Bzu state of naphthalene the expansion in C9-Cl0 bond is the largest (2). A perusal of Table I shows that both in l-FN as well as in l-ClN spectra the changes in the two larger C-C stretching frequencies are similar to those in naphthalene but that in the lowest C-C stretching (corresponding to 76 1 cm-’ in naphthalene) is about 3% as against 8% in naphthalene. Sequence Intervals In the 1-ClN spectrum a number of bands involving three of the normal coordinates were identified and were arranged in Deslandres’ schemes. This provided a simple explanation for the occurrence of sequence intervals denoted by B, M, and P. The mean values for the ground and excited state fundamentals responsible for the sequence internal P are 392 and 372 cm-‘, respectively, and the normal vibration involves a large amount of C-Cl stretching. The corresponding values for the ground and excited state fundamentals in l-FN are found to be 531 and 499 cm-‘, respectively, and the normal coordinate involves C-F stretching. Sequence interval M in l-ClN spectrum is attributed to a normal coordinate with ground and excited state frequencies of 5 14 and 450 cm-‘, respectively. The corresponding sequence in l-FN spectrum should involve the ground and excited state frequencies of 46 1 and 4 11 cm-’ but it is missing because of a perturbation described below. The origin of sequence interval B has already been described. It is expected that two of the most intense sequences A and C should originate in the two low lying ground state frequencies. The assignments of these sequence intervals are based on the observations of perturbations in the A’ lB’l band of l-l% and in the A’ ,C’ , band of l-ClN. In l-FN spectrum the bands A’lC’l, A22C’I are observed with sufficient intensities but those corresponding to A’, B1,,A22B’ i are not. A closer examination in the later cases shows that there are four relatively weaker peaks in regions where one expects these bands to appear. We see from Table I that the sum of ground state fundamentals 194 and 267 cm-’ of
HIGH RESOLUTION SPECTRA OF NAPHTHALENES
11
a” symmetry agrees with the ground state ring deformation fundamental of 460 cm-’ of a’ symmetry within 1 cm -‘. Thus, there is a likelihood of Fermi resonance between the combination level and the fundamental level. We believe this to be the reason for the anomalous splittings observed at A ’ J? ’ , . It also turns out that the sum of the sequence intervals A (13.3 cm-‘) and B (35.7 cm-‘) agrees to within 1 cm-’ with the difference between the ground state and excited state fundamentals belonging to the ring deformation vibration. This indicates that the same set of combination and fundamental levels might be in Fermi resonance both in the ground and the excited electronic states. We associate the sequence interval A with the ground state fundamental of 267 cm-’ and indeed there is a weak band around 0 + 253 cm-’ in the 1-FN spectrum. The slight disagreement between the observed and calculated sequence intervals may be attributed to the fact that the observed values refer to separations between the strongest absorption peaks in the bands which may not coincide with the band origins. The third intense sequence interval C is attributed to the remaining low lying fundamental of 148 cm-‘. In I -CIN spectrum, instead of observing a single intense peak in the region of band A’,C’ , we observe a very weak broad structure. This perturbation is again found to be due to an almost exact coincidence between the ground state combination level of (155 + 238 cm-‘) with a fundamental level of 392 cm-‘. The frequencies of the combination level belong to a” symmetry, whereas that of the fundamental level belongs to a’ symmetry, thus fulfilling the conditions for a Fermi resonance. In analogy with l-FN we attribute the sequence intervals A to 23%cm-’ vibration and the sequence intervals C to 155-cm-’ vibration. Unlike the case of I-FN, however, the combination and the fundamental levels in the excited electronic state do not lie close together and hence there is no possibility of a Fermi resonance in the excited state.
6. CONCLUSION
The ring deformation vibration involved in the Herzberg-Teller type of intensity borrowing in the 3 12-nm system of naphthalene also gives rise to strong vibronic bands in I-ClN and l-FN spectra but the activity of this vibration is in the order naphthalene > I-GIN > 1TN. A number of out of plane vibrations have been observed in the spectra of I-ClN and I-FN but more prominently in the former. Fermi-type resonances in sequence bands of I-ClN and I-FN have been found helpful in the assignment of sequence intervals. ACKNOWLEDGMENTS We thank Dr. J. M. Hollas for laboratory facilitiesfor recording the absorption spectra. RAS acknowledges the award of a Senior Research Fellowship of the CSIR (New Delhi).
RECEIVED: September
20, 1979
12
SINGH AND THAKUR REFERENCES
1. K. K. INNES,J. E. PARKIN,D. K. IRVIN, J. M. HOLLAS,AND I. G. Ross, J. Mol. Specfrosc. 16,406414 (1965). 2. J. M. HOLLASAND S. N. THAKUR, Mol. Phys. 22, 203-212 (197 1). 3. J. M. HOLLASAND S. N. THAKUR, Mol. Phys. 25, 1315-1334 (1973). 4. J. M. HOLLASAND S. N. THAKUR, Mol. Phys. 27, 1001-1011 (1974). 5. S. SUZUKIAND T. FIJJII,J. Mol. Spectrosc. 61, 350-359 (1976). 6. R. A. SINGH AND S. N. THAKUR, Mol. Phys. 36, 1053-1059 (1978). 7. S. N. SINGH AND R. D. SINGH, J. Mol. Spectrosc. 65,20-23 (1977). 8. J. CHRISTOFFERSON, J. M. HOLLAS,AND G. H. KIRBY, Pm. R. Sot. A 307,97-l 10 (1968). 9. S. RAMAMURTY,M. J., RAO, AND V. R. RAO, Indian J. Phys. 31, 497-510 (1957). 10. K. H. MICHAEL~ANAND S. M. ZIEGLER,Appl. Spectrosc. 27, 13-21 (1973). 11. D. P. CRAIG, J. M. HOLLAS,M. F. REDIES,AND S. C. WAIT, JR.,Philos. Trans. R. Sot. London Ser. A 253, 543-568 (1961). 12. J. M. HOLLAS,J. Mol. Spectrosc. 9, 138-169 (1962). 13. N. NETO, M. ~CRCECO,AND S. CALIFANO,Spectrochim. Acta 22, 1981-1998 (1966).