- Email: [email protected]
The 352 nm emission spectrum of difluorodiazirine
JOURNAL
OF biVLECKJLAR
SPECTROSCOPY
%,483-492
(1975)
The 352 nm Emission Spectrum of Difluorodiazirine P. H. HEPBURN,
J. M.
HOLLAS,
AND S. N.
THAKUR~
Chemistry Departmmt, University of Reading, Reading, Berkshire, Englad
Difluorodiazirine fluoresces strongly in the vapor phase showing an extensive band system from about 3.52 to 4.51 nm and with no background continuum. The fluorescence is assigned as AIB1(mr*)-zlA 1and corresponds to the 352 nm absorption system previously studied. The band system is dominated by a progression in Y<‘, the (11N = N stretching vibration, with VP”, the 01 CFa symmetrical deformation vibration, showing a shorter progression. The 00” band and most others show type B rotational contours but type C bands, involving yg”(&, and probably type A bands, involving ~7” (bl) and perhaps vs”(bl) are also observed. The extremely low intensity of bands involving am” is surprising but there seems to be no reason to doubt the assignment from infrared and Raman data. There is a strong vibrational perturbation affecting some quite strong bands in a region within about 275 cm-’ of bands 1,“4#‘, where n = 0 - 3. The cause of the perturbation is not known. There is no evidence for the emission spectrum consisting of more than one band system.
1. INTRODUCTION
When the absorption spectrum of difluorodiazirine (F&N2), or DFD, was recorded, as reported previously (I), a Chance Optical Glass OX7 filter was used to remove, from the continuum, radiation of wavelength below about 250 nm, in order to try to prevent photolysis of the sample. The filter also removed nearly all visible radiation except for some of short wavelength and low intensity. Under these conditions, and in a darkened room, it was observed that the absorption cell glowed brightly with a bluishpurple color. The emitted light was sufficiently intense that it could be observed with a hand spectroscope. It appeared to be banded in the purple and blue regions of the spectrum and, when photographed, it was easily shown to be the fluorescence spectrum of DFD corresponding to the alBl(nr*)-%A1 system studied previously in absorption (I-3). The fluorescence spectrum involves, principally, vibrations in the Xl.41 state. DFD has nine normal vibrations comprised of 4~1 + 1~ -I- 2b1 + 2b2, with the axis labeling convention used previously (1) in which the z-axis is the Cz axis and the x-axis is perpendicular to the CN2 ring. Table 1 in Ref. (I) gives the numbering system which we have adopted for the vibrations and which we use here. All the vibrations in thexlA 1state have been assigned using the vapor phase infrared and liquid phase Raman spectra together with normal coordinate calculations (4) and the infrared spectrum in the solid state in an argon matrix (5). 1Present address:
Physics
Department,
Banaras
Hindu 483
Copyright Q 1975 by Academic Press. Inc. All rights of reproduction in any form reserved.
University,
Varanasi,
India.
484
HEPBURN,
HOLLAS AND THAGUK
2. EXPERIMENTAL
The DFD vapor was contained, at a pressure of 0.3 kNm-*, in the same 40 cm cell as was used for the absorption spectrum (1). The cell had quartz windows, each sealed to a 10 cm long cylindrical piece of quartz joined by a graded seal to the central glass portion of the cell. The diameter of the cell was 3.5 cm. Light from a 500 W ac xenon arc passed through a Chance Optical Glass OXlA filter and then was passed from side to side across the quartz part of the cell three times with two plane mirrors. For some of the spectra photographed there was a region of vapor a few centimeters long, next to the window through which the emitted light passed to the spectrograph, which caused some reabsorption of the fluorescence: for other spectra this region was much shorter resulting in much reduced reabsorption. The spectrum was photographed, in the first order of the grating, with an f/8.4 2 meter Spex spectrograph (6). The grating is 102 X 102 mm, has 1800 grooves per mm and is blazed at 500 nm in the first order. The resolution in this region of the spectrum is about 0.15 cm-l. For the first photograph
taken
an exposure of 3 hr was given with an Ilford
FP4
plate and with a slit width of 100 pm. The second plate was exposed for 4 hr with a slit width of 60 pm: the plate blackening
was about half that on the first plate. The
following plate was exposed for 10 hr with a slit width of only 10 pm in order to obtain maximum resolution. Subsequent plates showed that the amount of DFD in the cell was quickly decreasing due, presumably, to photochemical decomposition and Nz as primary products (5). The spectrum of DFD was calibrated with that of an open iron arc. 3. ROTATIONAL
giving CF2
BAND CONTOURS
A microdensitometer trace of the emission spectrum is shown in Fig. 1. The spectrum is very sharp, in respect of the vibrational structure, and entirely free from any background continuum. Although
the general shapes of rotational
band contours
are clearly distinguished
no
fine structure could be observed even with a 10pm slit width. For example, although the type B ll”, lz”, and ls” bands show three main peaks in each contour, the expected R-branch heads, observed in the 00~ band in absorption [see Fig. 2a in Ref. (I)], and separated by about 0.7-0.9 cm-’ are not observed. contours is not clear.
The reason for the broadening
of
The shapes of rotational contours are, however, sufficiently well defined to be able to distinguish those of typical type B bands, such as members of the l,O (n = O-4) progression, from those of perturbed type B bands, such as 4r” [shown in absorption in Fig. 3a of Ref. (I)], in which the high wavenumber peak is much sharper than in a typical, unperturbed, band. Type C contours, such as those of bands 51” and 110510,are clearly distinguished from type B since they show only two quite sharp peaks. Figure 4a of Ref. (1) shows the type C 51~ band contour in absorption.
3.52 nm EMISSION
OF DIFLUORODIAZIRINE
485
-714.5 -896.1 -986.6
0.w
0.13
0.12
0.09
810.9 756.5
8’
B
R’
E!
8’
9’
B
c
8’
8’
A?
c
B
B
B’
B
B
B
B?
MB’
8’
B
** Wavenumbers given are mostly of band origins, judged with the help of information from computed rotational contours. For B’ bands, if the intensity maximum close to the band origin could not be observed, the origin was taken to be 6.5 cm-’ to low wavenumber of the intense high wavenumber peak. For a possible type A band, the wavenumber given is of what is probably the center of the high wavenumber broad maximum.
-1.7
0.08 0.10
938.5
0.02
-761.9
0.18
25980.1
-146.2
0.10
0.02
-728.9
0.04
WI.1
-620.5
0.21
O.Ul
0.07
0.57
0.26
0.16
0.08
0.02
0.26
0.13
0.55
1.08
0.118
0.47
209.0
-6.3
-512.9
0.11
0.21 0.05
050.4
2Q.3
-0.2
-500.7
0.62
8.8
310.0
0.3
-w+.l
0.36
0.02 0.07
0.20
326.1
(15.3)
-265.7
0.22
0.x 0.11
081.6
36”. 8 347.3
-7.7 -0.2
-353.5
“62.8
-2.3
-4118.2
-239.5
0.28
576.7
-7.1
0.1’)
-228.2
0.13
586.1
-0.8
0.06
-119.4
0.0
0.51
1.0 691.8
810.6
0.0
193.lr
0.05
821.3
3.u
162.9
0.03
093.1 26973.9
328.9 280.0
0.01
0.00
0.6
121.1
-1.0
294.1
-1.1 -0.3
515.5 397.7
0.01
327.1
643.5
0.05
27372.3
0.0 -0.u
936.4
0.01
0.00
B
B
0.26
0.61
8’
8’
B
B’
8’
0.10
0.20
0.21
0.08
0.19
* Intensities have been measured from microdensitometer traces, like those in Fig. 1, assuming a linear scale. Errors due to this assumption, and also due to the fact that some bands overlap each other, could be considerable. $ Rotational band contours are either type A, B, C, or B’ where B’ represents the perturbed type B contour typified by that of the 41~ band. t In obtaining the calculated wavenumbers of the bands the fundamental vibration wavenumbers for the _? and A states were all taken to be those given in Table 1 of Ref. (1).
B
0.03
0.09
7
0.03
0.03
30.3 64.5
c
A?
0.06
32.6
0.04
B
B
0.07
0.05
0.05
0.23
B
35.9
B’?
0.10
28.9
B’?
0.05
0.06
-32.8
B
44.7
B/B’
0.19
B
0.18
0.09
0.09
488
HEPRURN,
HOLLAS AXI> THAKUR
4. VIBRATIONAL
ANALYSIS
4.1. al Vibrations
The spectrum is dominated by a long progression in VI”, the vibration which involves principally stretching of the N=N bond. The 00” band, on which the progression is built, shows considerable reabsorption in Fig. 1 but comparison of the intensities of the .5i1 and 1i0511sequence bands shows that 00~should be slightly less intense than 12’. The long progression in ~1” mirrors that in absorption and is consistent with the increase by 3.6 f 0.4 pm (0.036 f 0.004 A) of the N=N bond length in the excited electronic state (1). There is a large negative anharmonicity in ~1” which is illustrated by the wavenumbers of the OOO,li”, lz”, lsO, and lJ” bands in Table 1, which records the wavenumbers, intensities and assignments of all bands. From the four vibrational intervals in vi” values of Xll” and ylri” have been obtained by the method of least squares which gives X11” = -2.9
f
0.2 cm-‘;
ylil”
= -0.42
f
0.03 cm-*.
VZ
The band 2i” (Fig. 1) is fairly weak and shows a perturbed rotational contour. The high wavenumber peak, although sharper than in an unperturbed type B contour, is not as sharp as in 41~. The infrared contour of the VZ’~fundamental is also perturbed (4). The vibration vg” (bJ, which is only 34 cm-’ to low wavenumber, probably causes the perturbation in both the infrared and emission spectra by Coriolis interaction: the two vibrations are connected through a rotation about the y axis. In the region where the 1r02io band is expected there are two bands, both with similar contours to that of 2+, but one of them is 15.2 cm-’ to low wavenumber and the other 74.2 cm+ to high wavenumber of the expected position of 1r02io. The 1121 vibrational level, estimated (excluding anharmonicity) to be at 2845.0 cm-i, is probably involved in Fermi resonance with a nearby level of the same symmetry. If we assume equal and opposite shifts of the two levels affected by Fermi resonance, the nearby level should be at 2904.0 cm-‘. This level could be 3~” + vd” or 4~4” •l- 2vg”, estimated to be at 2915.5 cm-i and 2898.0 cm-‘, respectively: both estimates assume harmonic behavior. The 1202r”band also appears to be involved in a similar Fermi resonance but 1a02io is not perturbed. v3
In our interpretation of the absorption spectrum (1) we accepted the assignment by of 805 cm-’ to ~3” and rejected the suggestion by Bjork et al. (4) o f a wavenumber Simmons et al. (3) and by Lombardi et al. (2) that a strong band at 4i”-274 cm-’ (OOO-775cm-‘) in the absorption spectrum is 31°. The OOO-775cm-’ band is labeled 41°b in Fig. 1. Two main factors led us to this conclusion. First, the band at 804 cm-i in the Raman spectrum is very intense and polarized, and the band at 805 cm-l in the infrared spectrum is an intense type A band : there seems no reason, therefore, to doubt the assignment to v~“. Second, the rotational contour of the 41°b band in the electronic absorption spectrum is almost identical to that of the 4r” band [see Fig. 3 of Ref. (I)].
3.52 nm EMISSION OF DIFLUORODIAZIRINE
489
Since the 4r” band is rotationally perturbed it is likely that the 41~6 band also involves the 41 level and therefore cannot be assigned as 31O. Further discussion of the 41°b band is given in Section 4.4. In the emission and absorption spectra the region of 00”805 cm-i, where we expect to observe the 3i” band, is masked by the 41°b band. However, the 274 cm-r interval between bands 4r” and 4i0b decreases regularly to 260,248, and 233 cm-l when associated with the 11O414 1z04ro, and 1304r” bands, respectively, leaving the regions where the 1r03ro, 120310,and L”3ro bands might be expected successively more clear. There are very weak bands in the 1r03io and 1z03roregions but they are not strong enough for the rotational contours to indicate clearly the band type. The bands are recorded in Table 1 and are indicated in Fig. 1. If Q” is active at all it is much more weakly active than any other al vibration whereas, in the A”rBI state, v3’ is slightly more strongly active than ~2’. (However, the assignments of v2’ and vQ’cannot be regarded as conclusive.) v4
There is a short progression of two members in va”, very similar to the progression in ~4’ in the absorption spectrum. There is only small anharmonicity in ~4”. 4.2. a2 and bl Vibrations
The 5r” band involving the a2 vibration vb” is observed in the emission spectrum as well as in absorption (1). The contour is clearly type C. In addition the bands 5&, 5J, 11°510,1r0512,and 120.Sr”are observed in emission. V6
This is a bl vibration which, in odd quanta, could give rise to type A bands. From the infrared spectrum the wavenumber of ve” has been shown to be 1248 cm-’ (1). At about 34 cm-l to high wavenumber of the 2i” band, where the 6r” band would be, there is a weak band whose contour is not sufficiently clear to be certain whether it resembles the computed type A contour shown in Fig. 6b of Ref. (1). The possible 6r” band is marked in Fig. 1.
This vibration is also br and could give rise to type A bands. Bands 701, 5r1701, and 1r0701were tentatively assigned in the absorption spectrum (1) but, in the emission spectrum, we can be more certain where to look for bands involving vr” since its wavenumber is known, from the gas-phase infrared spectrum, to be 481 cm-* (4). In the region of the 4r” band the spectrum is quite crowded due to bands 42’ and 5i” but there is an additional broad peak with its center at OoO-465.7 cm-‘. If this were the broad high wavenumber peak expected [see Fig. 6b in Ref. (I)] for the type A 71~band it should be at about 8 cm-l to high wavenumber of the band origin: this would give ” = 473.7 cm-i. This does not agree very well with the infrared and Raman values W
490
HEPBURN,
HOT,TAS
AXI> THAKC’K
but the discrepancy could be due to the Coriolis interaction between vh”,and ~7” found previously (I) which makes judgment of the position of the band center uncertain in both the emission and gas-phase infrared spectra. The emission spectrum shows other broad peaks, similar to that at 00~465.7 cm-‘, which are probably part of the 1r0710and 1z07robands. These are shown in Fig. 1. 4.3. Sequences Sequences in v5 and ~4, with intervals of about - 120 cm-r and +16 cm-r, respectively, are the most prominent in emission, as in absorption. The sequence band 71’, tentatively assigned in absorption (I), is likely to be partly overlapped by 5r1 but, as can be observed in Fig. 1, there are intensity steps on the low wavenumber edges of bands 51I, lrO.S?, and 1205r1which could be 7r1, 1+7$ and 120711,respectively. The greatest difficulty regarding the assignment of bands in the emission spectrum concerns the pairs of intense bands in the region within about 275 cm-r to low wavenumber of bands 4r”, 11O41O,1~~41~and la0410. The pairs of bands are labeled a, 41°b; Ifa, 11°4rob,etc., where the ‘a’ band is at higher wavenumber than the ‘b’ band, and are shown in Fig. 1. The fact that the ‘b’ bands involve the 41 level is implied by the perturbed contours of the ‘b’ bands which are like that of the 41~ band and characteristic of the excitation of one quantum of v4 in the ground electronic state. Band ‘u’ is 119.8 cm-’ to low wavenumber of 41° and this interval would agree with the assignment of ‘u’ as 4105$, an assignment which has been proposed previously (2). However, the contour of the 41°5r1, band would be expected to be perturbed by Coriolis interaction in a very similar way to that of 41O: but band ‘a’ shows a contour like that of an unperturbed type B band. In addition the intensity of ‘u’ is lower than would be expected for 4r05Q. Suspicions about the assignment of ‘u’ are confirmed by bands llOu, l,Ou, and laoa for which the 119.8 cm-’ interval is reduced to 106.3, 89.5, and 70.0 cm-‘, respectively, and whose intensity increases with respect to 110410,lzo410, and lao4ro so that, for example, 1304r”and ls”u have very similar intensities. The separations of 4r”, 1r04ro, 1$4r”, and 1a0410from their associated ‘b’ bands are 273.8, 259.9, 248.1, and 232.6 cm-l, respectively. Unlike those of the ‘u’ bands, the intensities of the ‘b’ bands remain fairly constant relative to the 4r”, etc., bands with which they are associated. It seems, then, that the expected 1,04105r1 (n = O-3) sequence bands are replaced by pairs of bands 1,Oa and 1,04r0b and that the sum of the intensities of the ‘u’ and ‘b’ bands is very much greater than that of the expected sequence bands which they replace. These facts imply a very strong perturbation of the 1,415r (n = O-3) vibrational levels, perhaps by Fermi resonance, but the source of the perturbation is not known. Since the 1,0410522(n = O-2) and 4~~51~are type B bands, rotationally perturbed in the same way as 41O,in positions quite close to those predicted, it appears that the perturbation does not extend to the 1,,4& or 1,42.51 vibrational levels. The main objections to this explanation of the l,Ou and 1,041°b bands are (i) that it is difficult to see why the total intensity of the lnou and 1,04r0b pairs of bands is so much greater than that of the 1,n4~o.5r1bands which they replace, and (ii) that, in spite
3.52 nm EMISSION
OF DIFLUORODIAZIRINE
491
of an apparently complete ground state vibrational assignment (4), there is no obvious source for a Fermi resonance affecting the 1,4151 levels. An alternative explanation of the Loa and 1,041°b bands which gets over the first of these objections is that the pairs of bands are due to sequences built, not on 1,04r” bands, but on l,O bands. For example bands ‘u’ and 4r”b, separated from 00~ by 620.5 and 774.5 cm”, respectively, might be due to a sequence which shows an interval in a vibration yz, say, of about -700 cm-‘: such a large sequence interval would require v,” to be large. In addition a strong Fermi resonance in the X1 level would have to be invoked in order to account for the fact that bands appear in pairs rather than singly. To account for the perturbed contours of the 1,04r”b bands, the resonance would have to be between X1 and a combination level involving 41. Any such strong Fermi resonance in the X state should be apparent in the infrared, and perhaps the Raman, spectrum and should be identifiable as two bands with the same separation (154.0 cm-l) as ‘u’ and 4r0b. The only two strong bands in the gas phase infrared spectrum (4) to satisfy this requirement are at 1248 and 1091 cm-‘. These have been assigned as vs(bl) and v~(bz), respectively, but the contours of both bands show a strong Q-branch region typical of a type C band, as well as strong rotational perturbation, so that the assignment of I+, might be questioned. However, there is no obvious source of a Fermi resonance between the 81 level and a combination level of bz symmetry involving one quantum of ~4. 5. CONCLUSIONS
The information, regarding ground state vibrations, obtained from the AIB1-LylA I emission spectrum of DFD is in good agreement with that from the infrared and Raman spectra (4, 5). Any misgivings about the assignment of v4”(ur) to a weak Raman line are removed by the observation of many strong type B bands involving v~“, not only in the emission but in the absorption spectrum as well (1-3). The Coriolis interaction between vz” and vg”, giving a distorted rotational contour for VZ”in the infrared spectrum, produces distorted contours in bands involving vz” in the emission spectrum also. The ~4” and VT” Coriolis interaction causes very characteristic distorted rotational contours of all emission bands involving ~4” but could not be detected easily in the infrared spectrum because of the overlapping of contours in a crowded region. The evidence that the bands labeled 1,041°b (n = O-3) involve the 4r level is very strong since they all show contours characteristic of the Coriolis interaction of the 41 level. Therefore the 4r06 band cannot be 31~as was previously suggested (2, 3), not only for this reason but also because the Raman and infrared evidence for vt” seems to be overwhelming (4). The difficulty in interpreting the 1,“a and 1,04r”b pairs of bands might possibly be resolved by a high resolution investigation of the gas phase infrared spectrum in which the overlapping and distortion of band contours, at medium resolution, make unambiguous assignments difficult in some cases. It has been suggested previously (2, 3) that there might be a triplet-singlet system in the region of the A-dy singlet-singlet system. There is no evidence for such a system
492
HEPBURN,
HOLLAS
AND THAKUR
from the emission spectrum. Any possibility that the l,,% or 1,“4i’b bands might belong to a second system is removed by the fact that none of the bands shows reabsorption, which the 00~ band of such a system should. ACKNOWLEDGMENTS One of us (P. H. H.) is grateful to the Science Research Council for a studentship and (S. N. T.) to the Royal Commission for the Exhibition of 18.51 for an Overseas Science Research Studentship.
RECEIVED: July 17, 1974 REFERENCES 1. P. H. HEPBURNANDJ. M. HOLLAS, J. Mol. Spectrosc. 50, 126 (1974). 2. J. R. LOMBARDI,W. KLEMPERER,M. B. ROBIN, H. BASCH, ANDN. A. KUEBLER,J. Chew Phys. 51, 33 (1969). 3. J. D. SIMMONS,I. R. BARTKY, ANDA. M. BASS, J. Mol. Speclrosc. 17, 48 (1965). 4. C. W. BJORK,N. C. CRAIG, R. A. MITSCH, ANDJ. OVEREND,J. Amer. Chem. Sot. 87, 1186 (1965). 5. D. E. MILLIGAN,D. E. MANN, M. E. JACOX,ANDR. A. MITSCH, J. Chem. Phys. 41, 1199 (1964). 6. J. M. HOLLAS AND S. N. THAKUR, Chem. Pkys. 1, 385 (1973).
OF biVLECKJLAR
SPECTROSCOPY
%,483-492
(1975)
The 352 nm Emission Spectrum of Difluorodiazirine P. H. HEPBURN,
J. M.
HOLLAS,
AND S. N.
THAKUR~
Chemistry Departmmt, University of Reading, Reading, Berkshire, Englad
Difluorodiazirine fluoresces strongly in the vapor phase showing an extensive band system from about 3.52 to 4.51 nm and with no background continuum. The fluorescence is assigned as AIB1(mr*)-zlA 1and corresponds to the 352 nm absorption system previously studied. The band system is dominated by a progression in Y<‘, the (11N = N stretching vibration, with VP”, the 01 CFa symmetrical deformation vibration, showing a shorter progression. The 00” band and most others show type B rotational contours but type C bands, involving yg”(&, and probably type A bands, involving ~7” (bl) and perhaps vs”(bl) are also observed. The extremely low intensity of bands involving am” is surprising but there seems to be no reason to doubt the assignment from infrared and Raman data. There is a strong vibrational perturbation affecting some quite strong bands in a region within about 275 cm-’ of bands 1,“4#‘, where n = 0 - 3. The cause of the perturbation is not known. There is no evidence for the emission spectrum consisting of more than one band system.
1. INTRODUCTION
When the absorption spectrum of difluorodiazirine (F&N2), or DFD, was recorded, as reported previously (I), a Chance Optical Glass OX7 filter was used to remove, from the continuum, radiation of wavelength below about 250 nm, in order to try to prevent photolysis of the sample. The filter also removed nearly all visible radiation except for some of short wavelength and low intensity. Under these conditions, and in a darkened room, it was observed that the absorption cell glowed brightly with a bluishpurple color. The emitted light was sufficiently intense that it could be observed with a hand spectroscope. It appeared to be banded in the purple and blue regions of the spectrum and, when photographed, it was easily shown to be the fluorescence spectrum of DFD corresponding to the alBl(nr*)-%A1 system studied previously in absorption (I-3). The fluorescence spectrum involves, principally, vibrations in the Xl.41 state. DFD has nine normal vibrations comprised of 4~1 + 1~ -I- 2b1 + 2b2, with the axis labeling convention used previously (1) in which the z-axis is the Cz axis and the x-axis is perpendicular to the CN2 ring. Table 1 in Ref. (I) gives the numbering system which we have adopted for the vibrations and which we use here. All the vibrations in thexlA 1state have been assigned using the vapor phase infrared and liquid phase Raman spectra together with normal coordinate calculations (4) and the infrared spectrum in the solid state in an argon matrix (5). 1Present address:
Physics
Department,
Banaras
Hindu 483
Copyright Q 1975 by Academic Press. Inc. All rights of reproduction in any form reserved.
University,
Varanasi,
India.
484
HEPBURN,
HOLLAS AND THAGUK
2. EXPERIMENTAL
The DFD vapor was contained, at a pressure of 0.3 kNm-*, in the same 40 cm cell as was used for the absorption spectrum (1). The cell had quartz windows, each sealed to a 10 cm long cylindrical piece of quartz joined by a graded seal to the central glass portion of the cell. The diameter of the cell was 3.5 cm. Light from a 500 W ac xenon arc passed through a Chance Optical Glass OXlA filter and then was passed from side to side across the quartz part of the cell three times with two plane mirrors. For some of the spectra photographed there was a region of vapor a few centimeters long, next to the window through which the emitted light passed to the spectrograph, which caused some reabsorption of the fluorescence: for other spectra this region was much shorter resulting in much reduced reabsorption. The spectrum was photographed, in the first order of the grating, with an f/8.4 2 meter Spex spectrograph (6). The grating is 102 X 102 mm, has 1800 grooves per mm and is blazed at 500 nm in the first order. The resolution in this region of the spectrum is about 0.15 cm-l. For the first photograph
taken
an exposure of 3 hr was given with an Ilford
FP4
plate and with a slit width of 100 pm. The second plate was exposed for 4 hr with a slit width of 60 pm: the plate blackening
was about half that on the first plate. The
following plate was exposed for 10 hr with a slit width of only 10 pm in order to obtain maximum resolution. Subsequent plates showed that the amount of DFD in the cell was quickly decreasing due, presumably, to photochemical decomposition and Nz as primary products (5). The spectrum of DFD was calibrated with that of an open iron arc. 3. ROTATIONAL
giving CF2
BAND CONTOURS
A microdensitometer trace of the emission spectrum is shown in Fig. 1. The spectrum is very sharp, in respect of the vibrational structure, and entirely free from any background continuum. Although
the general shapes of rotational
band contours
are clearly distinguished
no
fine structure could be observed even with a 10pm slit width. For example, although the type B ll”, lz”, and ls” bands show three main peaks in each contour, the expected R-branch heads, observed in the 00~ band in absorption [see Fig. 2a in Ref. (I)], and separated by about 0.7-0.9 cm-’ are not observed. contours is not clear.
The reason for the broadening
of
The shapes of rotational contours are, however, sufficiently well defined to be able to distinguish those of typical type B bands, such as members of the l,O (n = O-4) progression, from those of perturbed type B bands, such as 4r” [shown in absorption in Fig. 3a of Ref. (I)], in which the high wavenumber peak is much sharper than in a typical, unperturbed, band. Type C contours, such as those of bands 51” and 110510,are clearly distinguished from type B since they show only two quite sharp peaks. Figure 4a of Ref. (1) shows the type C 51~ band contour in absorption.
3.52 nm EMISSION
OF DIFLUORODIAZIRINE
485
-714.5 -896.1 -986.6
0.w
0.13
0.12
0.09
810.9 756.5
8’
B
R’
E!
8’
9’
B
c
8’
8’
A?
c
B
B
B’
B
B
B
B?
MB’
8’
B
** Wavenumbers given are mostly of band origins, judged with the help of information from computed rotational contours. For B’ bands, if the intensity maximum close to the band origin could not be observed, the origin was taken to be 6.5 cm-’ to low wavenumber of the intense high wavenumber peak. For a possible type A band, the wavenumber given is of what is probably the center of the high wavenumber broad maximum.
-1.7
0.08 0.10
938.5
0.02
-761.9
0.18
25980.1
-146.2
0.10
0.02
-728.9
0.04
WI.1
-620.5
0.21
O.Ul
0.07
0.57
0.26
0.16
0.08
0.02
0.26
0.13
0.55
1.08
0.118
0.47
209.0
-6.3
-512.9
0.11
0.21 0.05
050.4
2Q.3
-0.2
-500.7
0.62
8.8
310.0
0.3
-w+.l
0.36
0.02 0.07
0.20
326.1
(15.3)
-265.7
0.22
0.x 0.11
081.6
36”. 8 347.3
-7.7 -0.2
-353.5
“62.8
-2.3
-4118.2
-239.5
0.28
576.7
-7.1
0.1’)
-228.2
0.13
586.1
-0.8
0.06
-119.4
0.0
0.51
1.0 691.8
810.6
0.0
193.lr
0.05
821.3
3.u
162.9
0.03
093.1 26973.9
328.9 280.0
0.01
0.00
0.6
121.1
-1.0
294.1
-1.1 -0.3
515.5 397.7
0.01
327.1
643.5
0.05
27372.3
0.0 -0.u
936.4
0.01
0.00
B
B
0.26
0.61
8’
8’
B
B’
8’
0.10
0.20
0.21
0.08
0.19
* Intensities have been measured from microdensitometer traces, like those in Fig. 1, assuming a linear scale. Errors due to this assumption, and also due to the fact that some bands overlap each other, could be considerable. $ Rotational band contours are either type A, B, C, or B’ where B’ represents the perturbed type B contour typified by that of the 41~ band. t In obtaining the calculated wavenumbers of the bands the fundamental vibration wavenumbers for the _? and A states were all taken to be those given in Table 1 of Ref. (1).
B
0.03
0.09
7
0.03
0.03
30.3 64.5
c
A?
0.06
32.6
0.04
B
B
0.07
0.05
0.05
0.23
B
35.9
B’?
0.10
28.9
B’?
0.05
0.06
-32.8
B
44.7
B/B’
0.19
B
0.18
0.09
0.09
488
HEPRURN,
HOLLAS AXI> THAKUR
4. VIBRATIONAL
ANALYSIS
4.1. al Vibrations
The spectrum is dominated by a long progression in VI”, the vibration which involves principally stretching of the N=N bond. The 00” band, on which the progression is built, shows considerable reabsorption in Fig. 1 but comparison of the intensities of the .5i1 and 1i0511sequence bands shows that 00~should be slightly less intense than 12’. The long progression in ~1” mirrors that in absorption and is consistent with the increase by 3.6 f 0.4 pm (0.036 f 0.004 A) of the N=N bond length in the excited electronic state (1). There is a large negative anharmonicity in ~1” which is illustrated by the wavenumbers of the OOO,li”, lz”, lsO, and lJ” bands in Table 1, which records the wavenumbers, intensities and assignments of all bands. From the four vibrational intervals in vi” values of Xll” and ylri” have been obtained by the method of least squares which gives X11” = -2.9
f
0.2 cm-‘;
ylil”
= -0.42
f
0.03 cm-*.
VZ
The band 2i” (Fig. 1) is fairly weak and shows a perturbed rotational contour. The high wavenumber peak, although sharper than in an unperturbed type B contour, is not as sharp as in 41~. The infrared contour of the VZ’~fundamental is also perturbed (4). The vibration vg” (bJ, which is only 34 cm-’ to low wavenumber, probably causes the perturbation in both the infrared and emission spectra by Coriolis interaction: the two vibrations are connected through a rotation about the y axis. In the region where the 1r02io band is expected there are two bands, both with similar contours to that of 2+, but one of them is 15.2 cm-’ to low wavenumber and the other 74.2 cm+ to high wavenumber of the expected position of 1r02io. The 1121 vibrational level, estimated (excluding anharmonicity) to be at 2845.0 cm-i, is probably involved in Fermi resonance with a nearby level of the same symmetry. If we assume equal and opposite shifts of the two levels affected by Fermi resonance, the nearby level should be at 2904.0 cm-‘. This level could be 3~” + vd” or 4~4” •l- 2vg”, estimated to be at 2915.5 cm-i and 2898.0 cm-‘, respectively: both estimates assume harmonic behavior. The 1202r”band also appears to be involved in a similar Fermi resonance but 1a02io is not perturbed. v3
In our interpretation of the absorption spectrum (1) we accepted the assignment by of 805 cm-’ to ~3” and rejected the suggestion by Bjork et al. (4) o f a wavenumber Simmons et al. (3) and by Lombardi et al. (2) that a strong band at 4i”-274 cm-’ (OOO-775cm-‘) in the absorption spectrum is 31°. The OOO-775cm-’ band is labeled 41°b in Fig. 1. Two main factors led us to this conclusion. First, the band at 804 cm-i in the Raman spectrum is very intense and polarized, and the band at 805 cm-l in the infrared spectrum is an intense type A band : there seems no reason, therefore, to doubt the assignment to v~“. Second, the rotational contour of the 41°b band in the electronic absorption spectrum is almost identical to that of the 4r” band [see Fig. 3 of Ref. (I)].
3.52 nm EMISSION OF DIFLUORODIAZIRINE
489
Since the 4r” band is rotationally perturbed it is likely that the 41~6 band also involves the 41 level and therefore cannot be assigned as 31O. Further discussion of the 41°b band is given in Section 4.4. In the emission and absorption spectra the region of 00”805 cm-i, where we expect to observe the 3i” band, is masked by the 41°b band. However, the 274 cm-r interval between bands 4r” and 4i0b decreases regularly to 260,248, and 233 cm-l when associated with the 11O414 1z04ro, and 1304r” bands, respectively, leaving the regions where the 1r03ro, 120310,and L”3ro bands might be expected successively more clear. There are very weak bands in the 1r03io and 1z03roregions but they are not strong enough for the rotational contours to indicate clearly the band type. The bands are recorded in Table 1 and are indicated in Fig. 1. If Q” is active at all it is much more weakly active than any other al vibration whereas, in the A”rBI state, v3’ is slightly more strongly active than ~2’. (However, the assignments of v2’ and vQ’cannot be regarded as conclusive.) v4
There is a short progression of two members in va”, very similar to the progression in ~4’ in the absorption spectrum. There is only small anharmonicity in ~4”. 4.2. a2 and bl Vibrations
The 5r” band involving the a2 vibration vb” is observed in the emission spectrum as well as in absorption (1). The contour is clearly type C. In addition the bands 5&, 5J, 11°510,1r0512,and 120.Sr”are observed in emission. V6
This is a bl vibration which, in odd quanta, could give rise to type A bands. From the infrared spectrum the wavenumber of ve” has been shown to be 1248 cm-’ (1). At about 34 cm-l to high wavenumber of the 2i” band, where the 6r” band would be, there is a weak band whose contour is not sufficiently clear to be certain whether it resembles the computed type A contour shown in Fig. 6b of Ref. (1). The possible 6r” band is marked in Fig. 1.
This vibration is also br and could give rise to type A bands. Bands 701, 5r1701, and 1r0701were tentatively assigned in the absorption spectrum (1) but, in the emission spectrum, we can be more certain where to look for bands involving vr” since its wavenumber is known, from the gas-phase infrared spectrum, to be 481 cm-* (4). In the region of the 4r” band the spectrum is quite crowded due to bands 42’ and 5i” but there is an additional broad peak with its center at OoO-465.7 cm-‘. If this were the broad high wavenumber peak expected [see Fig. 6b in Ref. (I)] for the type A 71~band it should be at about 8 cm-l to high wavenumber of the band origin: this would give ” = 473.7 cm-i. This does not agree very well with the infrared and Raman values W
490
HEPBURN,
HOT,TAS
AXI> THAKC’K
but the discrepancy could be due to the Coriolis interaction between vh”,and ~7” found previously (I) which makes judgment of the position of the band center uncertain in both the emission and gas-phase infrared spectra. The emission spectrum shows other broad peaks, similar to that at 00~465.7 cm-‘, which are probably part of the 1r0710and 1z07robands. These are shown in Fig. 1. 4.3. Sequences Sequences in v5 and ~4, with intervals of about - 120 cm-r and +16 cm-r, respectively, are the most prominent in emission, as in absorption. The sequence band 71’, tentatively assigned in absorption (I), is likely to be partly overlapped by 5r1 but, as can be observed in Fig. 1, there are intensity steps on the low wavenumber edges of bands 51I, lrO.S?, and 1205r1which could be 7r1, 1+7$ and 120711,respectively. The greatest difficulty regarding the assignment of bands in the emission spectrum concerns the pairs of intense bands in the region within about 275 cm-r to low wavenumber of bands 4r”, 11O41O,1~~41~and la0410. The pairs of bands are labeled a, 41°b; Ifa, 11°4rob,etc., where the ‘a’ band is at higher wavenumber than the ‘b’ band, and are shown in Fig. 1. The fact that the ‘b’ bands involve the 41 level is implied by the perturbed contours of the ‘b’ bands which are like that of the 41~ band and characteristic of the excitation of one quantum of v4 in the ground electronic state. Band ‘u’ is 119.8 cm-’ to low wavenumber of 41° and this interval would agree with the assignment of ‘u’ as 4105$, an assignment which has been proposed previously (2). However, the contour of the 41°5r1, band would be expected to be perturbed by Coriolis interaction in a very similar way to that of 41O: but band ‘a’ shows a contour like that of an unperturbed type B band. In addition the intensity of ‘u’ is lower than would be expected for 4r05Q. Suspicions about the assignment of ‘u’ are confirmed by bands llOu, l,Ou, and laoa for which the 119.8 cm-’ interval is reduced to 106.3, 89.5, and 70.0 cm-‘, respectively, and whose intensity increases with respect to 110410,lzo410, and lao4ro so that, for example, 1304r”and ls”u have very similar intensities. The separations of 4r”, 1r04ro, 1$4r”, and 1a0410from their associated ‘b’ bands are 273.8, 259.9, 248.1, and 232.6 cm-l, respectively. Unlike those of the ‘u’ bands, the intensities of the ‘b’ bands remain fairly constant relative to the 4r”, etc., bands with which they are associated. It seems, then, that the expected 1,04105r1 (n = O-3) sequence bands are replaced by pairs of bands 1,Oa and 1,04r0b and that the sum of the intensities of the ‘u’ and ‘b’ bands is very much greater than that of the expected sequence bands which they replace. These facts imply a very strong perturbation of the 1,415r (n = O-3) vibrational levels, perhaps by Fermi resonance, but the source of the perturbation is not known. Since the 1,0410522(n = O-2) and 4~~51~are type B bands, rotationally perturbed in the same way as 41O,in positions quite close to those predicted, it appears that the perturbation does not extend to the 1,,4& or 1,42.51 vibrational levels. The main objections to this explanation of the l,Ou and 1,041°b bands are (i) that it is difficult to see why the total intensity of the lnou and 1,04r0b pairs of bands is so much greater than that of the 1,n4~o.5r1bands which they replace, and (ii) that, in spite
3.52 nm EMISSION
OF DIFLUORODIAZIRINE
491
of an apparently complete ground state vibrational assignment (4), there is no obvious source for a Fermi resonance affecting the 1,4151 levels. An alternative explanation of the Loa and 1,041°b bands which gets over the first of these objections is that the pairs of bands are due to sequences built, not on 1,04r” bands, but on l,O bands. For example bands ‘u’ and 4r”b, separated from 00~ by 620.5 and 774.5 cm”, respectively, might be due to a sequence which shows an interval in a vibration yz, say, of about -700 cm-‘: such a large sequence interval would require v,” to be large. In addition a strong Fermi resonance in the X1 level would have to be invoked in order to account for the fact that bands appear in pairs rather than singly. To account for the perturbed contours of the 1,04r”b bands, the resonance would have to be between X1 and a combination level involving 41. Any such strong Fermi resonance in the X state should be apparent in the infrared, and perhaps the Raman, spectrum and should be identifiable as two bands with the same separation (154.0 cm-l) as ‘u’ and 4r0b. The only two strong bands in the gas phase infrared spectrum (4) to satisfy this requirement are at 1248 and 1091 cm-‘. These have been assigned as vs(bl) and v~(bz), respectively, but the contours of both bands show a strong Q-branch region typical of a type C band, as well as strong rotational perturbation, so that the assignment of I+, might be questioned. However, there is no obvious source of a Fermi resonance between the 81 level and a combination level of bz symmetry involving one quantum of ~4. 5. CONCLUSIONS
The information, regarding ground state vibrations, obtained from the AIB1-LylA I emission spectrum of DFD is in good agreement with that from the infrared and Raman spectra (4, 5). Any misgivings about the assignment of v4”(ur) to a weak Raman line are removed by the observation of many strong type B bands involving v~“, not only in the emission but in the absorption spectrum as well (1-3). The Coriolis interaction between vz” and vg”, giving a distorted rotational contour for VZ”in the infrared spectrum, produces distorted contours in bands involving vz” in the emission spectrum also. The ~4” and VT” Coriolis interaction causes very characteristic distorted rotational contours of all emission bands involving ~4” but could not be detected easily in the infrared spectrum because of the overlapping of contours in a crowded region. The evidence that the bands labeled 1,041°b (n = O-3) involve the 4r level is very strong since they all show contours characteristic of the Coriolis interaction of the 41 level. Therefore the 4r06 band cannot be 31~as was previously suggested (2, 3), not only for this reason but also because the Raman and infrared evidence for vt” seems to be overwhelming (4). The difficulty in interpreting the 1,“a and 1,04r”b pairs of bands might possibly be resolved by a high resolution investigation of the gas phase infrared spectrum in which the overlapping and distortion of band contours, at medium resolution, make unambiguous assignments difficult in some cases. It has been suggested previously (2, 3) that there might be a triplet-singlet system in the region of the A-dy singlet-singlet system. There is no evidence for such a system
492
HEPBURN,
HOLLAS
AND THAKUR
from the emission spectrum. Any possibility that the l,,% or 1,“4i’b bands might belong to a second system is removed by the fact that none of the bands shows reabsorption, which the 00~ band of such a system should. ACKNOWLEDGMENTS One of us (P. H. H.) is grateful to the Science Research Council for a studentship and (S. N. T.) to the Royal Commission for the Exhibition of 18.51 for an Overseas Science Research Studentship.
RECEIVED: July 17, 1974 REFERENCES 1. P. H. HEPBURNANDJ. M. HOLLAS, J. Mol. Spectrosc. 50, 126 (1974). 2. J. R. LOMBARDI,W. KLEMPERER,M. B. ROBIN, H. BASCH, ANDN. A. KUEBLER,J. Chew Phys. 51, 33 (1969). 3. J. D. SIMMONS,I. R. BARTKY, ANDA. M. BASS, J. Mol. Speclrosc. 17, 48 (1965). 4. C. W. BJORK,N. C. CRAIG, R. A. MITSCH, ANDJ. OVEREND,J. Amer. Chem. Sot. 87, 1186 (1965). 5. D. E. MILLIGAN,D. E. MANN, M. E. JACOX,ANDR. A. MITSCH, J. Chem. Phys. 41, 1199 (1964). 6. J. M. HOLLAS AND S. N. THAKUR, Chem. Pkys. 1, 385 (1973).