634
J. Phys. Chem. 1991, 95,634-639
in the direction of the dipole moment occurs between the ground and CT or CT excited states. Furthermore, we have observed large s f i t s in t g e emissions in solid crystalline solvents such as acetonitrile at temperatures where a change in the crystalline phase occurs.
Summary and Conclusions Dual-emissive behavior in coordination complexes was first documented with reports of thermally nonequilibrated emissions from triplet mr* excited states localized on bpy and on phen in R h (bpy )2( phen)'+, R h( bpy )(phen) 23+, and related species.**s~' Those studies indicated that weak electronic coupling of the electron distributions on different chelating ligands as well as relatively small energy gaps between excited electron distributions were key elements associated with slow energy transfer between the nonequilibrated states. Quantitative estimates for those complexes indicated that dual emission occurred in cases where excited states were separated by only a few hundred cm-I, but that equilibration resulted when energy gaps became as large as 1600 cm-I. Mixed-chelate/ortho-metalateRh( 111) complexes such as Rh(~py)~(bpy)+ and Rh(bz&(phen)+ display only a single mr* emission in rigid glass and the result is taken as evidence for the strong electronic coupling associated with the highly covalent Rh-C bonds to the ortho-metalating ligands. Similarly, mixed chelate/ortho-metalate complexes of R(I1) and Pd(I1) display only a single emission.47d4 However, rigid glass (48) Maestri, M.;Sandrini, D.; Balzani, V.; Chassot, L.; Jolliet, Ph.; von Zelewsky, A. Chem. Phys. Lett. 1985, 122, 375. (49) Maestri, M.;Sandrini, D.; Balzani, V.; von Zelewsky, A.; Jollict, Ph. Helo. Chim. Acta 1988, 71, 134. (50) Chassot, L.; von Zelewsky, A,; Sandrini, D.; Maestri, M.; Balzani, V. J . Am. Chem. Soc. 1986. 108.6084. (51) Sandrini, D.; M a d , M.; Balzani, V.; Chassot, L.; von Zelewsky, A. J . Am. Chem. Soc. 1987, 109, 7720. (52) Schwarz, R.; Gliemann, G.; Jolliet, Ph.; von Zelewsky, A. Inorg. Chem. 1989, 28, 1053.
samples of the mixed chelate/ortho-metalate complexes of Ir(II1) studied here all display dual emissive behavior. This result cannot be attributed to electronic coupling which should be stronger between the MLCT states in the Ir(II1) complexes than it is for consequently the result the mr* states of the Rh(II1) complexe~;~'J~ is likely to be due to large Franck-Condon barriers originating from a combination of inner-sphere and outer-sphere effects analogous to those encountered in electron-transfer processes. These ortho-metalated Ir(II1) complexes thus appear to afford the opportunity to study the effects of Franck-Condon barriers on energy-transfer rates in situations where strong electronic coupling is assured. Acknowledgment. This work was supported by the Office of Basic Energy Sciences, United States Department of Energy Contract DE-FG03-88ER 13842. (53) Craig, C. A,; Garces, F. 0.; Watts, R. J.; Palmans, R.; Frank, A. J. Photochemistryand Photophysicsof CoordinationCompounds. Cmrd. Chem. Rev. 1990,97, 193. (54) Bar, L.; Gliemann, G.; Chassot, L.; von Zelewsky, A. Chem. Phys. Lett. 1986. 123, 264. (55) Chassot, L.; von Zelewsky, A. Inorg. Chem. 1987, 26, 2814. (56) Bonafede, S.; Ciano, M.; Bdletta, F.; Balzani, V.; Chassot, L.; von Zelewsky, A. J . Phys. Chem. 1986.90, 3836. (57) Sandrini, D.; Maestri, M.; Ciano, M.;Balzani, V.; Lueoend, R.; Deuschel-Cornioley,C.; Chassot, L.; von Zelewsky, A. Gmz.Chim.Ital. 1988, 118, 661. (58) Schwarz, R.; Gliemann, G.; Chassot. L.; Jolliet, P.; von Zelewsky, A. Helu. Chim. Acta 1989, 72, 1. (59) Cornioley-Deuschel, C.; von Zelewsky, A. Inorg. Chem. 1987, 26, 3354. (60) Barigelletti, F.; Sandrini, D.; Maestri, M.; Balzani, V.; von Zelewsky, A.; Chassot, L.; Jolliet, P.; Maeder, U. Inorg. Chem. 1988, 27, 3644. (61) Balzani, V.; Maestri, M.; Melandri, A.; Sandrini, D.; Chassot, L.; Cornioley-Deuschel, C.; Jolliet, P.; Maedcr, U.; von Zelewsky. A. Photo. chemistry and Photophysics of Coordination Compounds;Yersin, H., Vogler, A., Eds.; Springer-Verlag: Berlin. 1987. (62) Wakatsuki, Y.; Yamazaki, H.; Grutsch, P. A.; Santhanam, M.;Kutal, C . J . Am. Chem. Soc. 1985, 107,8153. (63) Craig, C. A.; Watts, R. J. Inorg. Chem. 1989. 28, 309. (64) Schwarz, R.; Gliemann, G.; Jolliet, Ph.; von Zelewsky, A. Inorg. Chem. 1989, 28, 742.
Vibrational Spectroscopic Study of the Temperature- and Pressure-Induced Phase Transition of Norbornyiene N.T.Kawai, I. S.Butler,* and D.F. R. Gilson* Department of Chemistry, McGill University, 801 Sherbrooke St. W.. Montreal, Quebec, H3A 2K6,Canada (Received: July 30, 1990)
The order-disorder phase transition in norbornylene, bicycle[ 2.2.11 hepta-Zene, has been investigated by differential scanning calorimetry, low-temperature infrared and Raman spectroscopy, and high-pressure infrared spectroscopy. When induced by low temperature, a 13 K hysteresis was associated with the phase transition, which occurred at 114 K on cooling. The same transition could be induced by applying 14.9 h 0.3 kbar of pressure, exhibiting a 1.6 f 0.7 kbar pressure hysteresis. The volume change of the transition, AK,was calculated to be 3.1 1.O om3 mol-'. The observed correlation coupling effects suggest that the factor group of the ordered phase is monoclinic or orthorhombic (C, or C,) with two molecules per unit cell, or tetragonal (C, or C,) with Z = 4.
*
Introduction
Many cage hydrocarbon molecules exhibit orientational in the solid state, and have &n termed crysta~.lJ A~~~~ these are the seven-carbon bicyclic hydrocarbons based on nor( 1 ) Sherwood, J. N. The Plastically Crystalline State (OrientarionallyDisordered Crystals); Wilcy-Interscience: New York, 1979. (2) Parsonage, N . G.; Staveley, L. A. K. Disorder in Crystals; Clarcndon, Oxford, U.K., 1978.
0022-3654/9 1 /2095-0634$02.50/0
bornane (bicyclo[2.2.1] heptane), which are well-known to exist as disordered solids over a wide temperature range. We have recently reported an investigation of the solid-phase behavior of the diene analogue, n~rbornadiene.~ The monoalkene, norbornylene (bicyclo[2.2.l]hepta-2-ene), also exhibits a plastically crystalline phase betwe& 129 and 320 K. An adiabatic calori(3) Kawai, N. T.: Gilson. D. F. R.; Butler, I. S.J . Phys. Chem. 1990,91, 5729.
0 1991 American Chemical Society
Phase Transition of Norbornylene metric determination of the entropies of melting and transition gave values of 10.2 and 37.5 J K-' mol-', respectively? A considerable amount of information has been obtained on the plastic phase of norbomylene. Like norbomane and norbornadiene, the structure of this disordered solid is hexagonal close-packed ( a = 6.08 A and c = 9.81 A at 297 K).s This family of compounds is unique in that most organic cage molecules have face-centered cubic disordered phases. Both rotation and lattice diffusion take place in the plastic phase; proton NMR measurements at low temperatures and moderate pressures determined the activation energies for these processes to be 6.3-10.5 and 84.7 kJ mol-', respectively.6.' The range was given for the rotational activation energy since this value was determined to increase with decreasing temperature. Brillouin, Rayleigh, and Raman scattering studies have been performed showing no discontinuities at the melting t r a n ~ i t i o n . ~The , ~ Raman spectroscopy, however, was limited to a study of the 837cm-' peak in the plastic and liquid phases. More recently, by studying a muoniom adduct radical of norbornylene, the rotational regime of the molecules in the plastic phase was determined to be isotropic at room temperature, with anisotropy setting at about 160 K.Io The structure of the low-temperature phase is not yet known. The vibrational spectra of norbornylene have not been as well investigated as those of norbornane and norbornadiene, probably due to the lower molecular symmetry of the monoalkene. However, scaled ab initio STO-3G AND 3-21G harmonic force fields have recently been calculated for all three compounds, and the IR spectra of the vapor and liquid phases of norbornylene, and the Raman spectrum of the liquid phase were reported.!' The skeletal modes of the spectra were assigned according to these force field calculations. No investigation of the solid-state spectra has been reported up to now, and in the present work, the IR and Raman spectra of the disordered and ordered solid phases of norbornylene are presented for the first time. The behavior of both solid phases under pressure was also investigated by observing the changes in the IR spectrum as a function of applied pressure.
Experimental Section Norbornylene (99%) was purchased from Wiley Organics Inc. Gas chromatography showed the sample to contain less than 0.2% impurities, and no further purification was attempted. A Perkin-Elmer DSC-7 differential scanning calorimeter, calibrated by using the phase transition of cyclohexane (Aldrich, Gold Label), was used to measure the transition temperatures and enthalpies of the sample. Typically, the sample was 5-10 mg and was hermetically sealed into aluminum pans. The sample was weighed repeatedly on a Cahn microbalance to ensure that complete sealing of the pan was achieved. A Cryodyne Cryocooler Model 21 cryostat (Cryogenics Technology Inc.) attached to a Cryophysics Model 4025 controller was used to maintain and measure the temperature in the variable-temperature spectroscopic measurements. Infrared and Raman spectra were measured every 10 K both on cooling and heating. Raman spectra were recorded on an I.S.A. spectrometer with a Jobin-Yvon Ramanor U-1000double monochromator, interfaced to an IBM PS/2 Model 60 computer. The 514.532-nm line of an argion ion laser (Spectra Physics, Model 164) was used as the excitation source, with the laser power between 100 and 300 mW at the sample. The sample was sealed in a glass capillary ~~
~~
(4) Westrum, E. F. In Molecular Dynamics and Structure of Solids; Carter. R. S., Rush, J. J., Eds.; National Bureau of S t a n d a h Washington, DC. 1969; p 459. (5) Jackson. R. L.; Strange, J. H. Acta Crysrallogr. 1972, B28, 1645. (6) Folland, R.; RW, S. M.;Strange, J. H. Mol. Phys. 1973, 26, 27. (7) Folland. R.; Jackson, R. L.; Strange, J. H.; Chadwick, A. V. J . Phys. Chem. SolIds 1973, 34, 17 13. (8) Folland, R.; Jackson, D. A.; Rajagopal, S.Mol. Phys. 1975.30,1053. (9) Folland, R.; Jackson, D. A.; Rajagopal, S.Mol. Phys. 1975.30. 1067.
(IO) R i m , M.;Bucci, C.; De Renri, R.; Guidi, G.; Podini, P.; Tedeschi, R.: Scott. C. A. J . Chem. Phvs. 1987. 86. 4198. '(I 1) Shaw, R. A.; Castro,k.; Dutler, R.; Rauk, A.; Wieser. H. J . Chem. Phys. 1988,89,716.
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 635
~
100
110
180
130 140 Temperature
180
180
170
110
(K)
Figure 1. Typical differential scanning calorimetric thermogram of norbornylene scanning at 2.5 K m i d .
9150
3000
2050
1860
I500
1350
1200
1050
000
750
800
450
Wavenumber (em-')
Figure 2. Infrared spectra of norbornylene at (A) 58 K and (B) 145 K.
tube, and the scattered radiation was collected at a 90° angle at a resolution of 1-2 cm-I. Infrared spectra were measured on a Nicolet 6000 FT-IR spectrometer with a liquid nitrogen cooled MCT(B) detector. The sample was sublimed onto a KBr window at 100 K and initially formed a glass. After heating above the phase transition temperature, subsequent cooling resulted in the formation of a polycrystalline solid phase. The sample was then annealed until the spectra of both solid phases remained consistent. The high-pressure IR spectra were measured in a diamond anvil cell (DAC) from High Pressure Diamond Optics (Tucson, AZ), equipped with type IIA diamonds, which are the most IR transparent. The pressure calibrant used was the assymmetric stretching mode of NO3- in a dilute NaBr matrix.I2 A small amount of the calibrant was placed into the 4 W r m hole of a 200 ~.lmthick stainless steel gasket, and the gasket was pressed in the cell to form a thin, uniform layer of calibrant. The DAC was then dismounted, and a large piece of the plastically crystalline sample was placed over the gasket hole. The cell was then quickly reassembled, and enough pressure was applied to keep the volatile sample from escaping the gasket. The DAC was mounted onto an XYZ stage which was aligned on a Spectra-Bench 4X beam condenser (Spectra-Tech). Infrared spectra were obtained upon compression and decompression at a spectral resolution of 4 cm-'.
Results and Discussion Differential scanning calorimetry was performed in both the cooling and heating directions at 5 and 2.5 K min-I. The heating thermograms showed a sluggish phase transition at 127 K with accompanying enthalpy and entropy changes of 4.3 kJ mol-' and 35.9 J K-I mol-'. These results are in agreement with adiabatic calorimetric measurements of Westrum? Like many first-order transitions, hysteresis was associated with this phase transformation, the onset on cooling occurring at 114 K. The different heating and cooling rates showed that the sluggish nature of the phase transition was not caused by too fast a scan rate, but was inherent to the transition. The formation of the metastable phase (12) Klug, D. D.; Whalley, E. Reu. Sei. Insrrum. 1983, 54, 1205.
Kawai et al.
636 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
A
9200
I
1800
3000
,
1200
1400
/I
I I1 I
800
1000
800
kbar
400
Wavenumber (om-')
Figure 3. Raman spectra of norbornylene at (A) 48 K and (B) 135 K.
A
A
3100
3000
2800
2800
Wavenumber (em-')
Figure 5. Infrared spectra of the C-H stretching modes of norbornylene at the indicated pressures upon compression.
B
r
160
.
.
.
v
120
.
.
.
1
'
.
'
BO
u
"
40
Wavenumber (cm-') Figure 4. Raman spectra of the external modes of norbornylene at (A) 48 K and (B) 135 K.
may be caused by the presence of impurities in the sample. Figure 1 shows a typical thermogram of norbornylene in both heating and cooling directions. Variable-temperature IR and Raman spectroscopic studies were used to characterize structural changes occumng during the phase transition Figures 2 and 3 show representative spectra of each phase. No evidence of a mestable phase was detected in the vibrational spectra. Phase I, the disordered or plastic phase of norbornylene, exhibited liquidlike features in both the IR and Raman spectrum. No lattice modes were observed in the lowfrequency Raman spectra, only the broad tail of the Rayleigh line. This confirms that the molecules in this phase undergo rapid tumbling in the hexagonal close-packed lattice, which is expected since, upon melting, no change in correlation times were observed by either NMR6v7or Raman ~pectroscopy.~ All modes are both IR and Raman active, which would be expected for the disordered phase of a molecule with C, symmetry. The low-temperature phase, phase 11, was formed immediately upon cooling through the transition temperature. Many changes in both spectra occurred at this point, including line-narrowing and peak splitting. In addition, nine external modes suddenly appeared in the low-frequency Raman spectrum, indicating that an ordered, crystalline solid had been formed (Figure 4). Table I summaries the vibrational peaks observed for both solid phases of norbornylene. Polarized Raman spectra in CS, solution were in complete agreement with those of ref 1 1. The symmetry assignments of the skeletal modes listed in Table I are those according to STO-3G force-field calculations, and the C H stretching modes have been assigned to previous assignments for norbornane and norbornadiene.lj (13)
Levin, 1. W.;Harris, W.C. Spectrochim.
Acra 1973, 29A, 1815.
Analysis of the vibrational spectra showed that several peaks of phase I, regardless of symmetry, split into doublets in phase I1 throughout the spectral region. These splittings can be attributed to solid-state effects occurring in phase I1 which were not present in phase I due to the disordered nature of the latter solid. The spectra of phase I resemble those of a liquid; i.e., the observed IR and Raman peaks are governed by the molecular symmetry, whereas the peaks of phase I1 are influenced by the site and crystal symmetry. No site splitting occurs since all modes are nondegenerate under C, symmetry; therefore, any splitting effects are due to correlation coupling. Since more than half of the observed fundamentals of phase I1 coincide in the Raman and IR spectra, the crystal symmetry is definitely noncentrosymmetric. The possible lattice sites that a molecue of C, symmetry can occupy are C, and CI.Considering the first case, the crystal can have any noncentrosymmetric factor group which has C, as a subset. Of these, only C, and C, will result in doubling of all the modes due to factor group splitting. In the second case, Le., if the molecules occupy general positions, only the C, and C, factor groups can give rise to the observed splitting effects. All four of these possibilities can account for the nine external modes which were observed in the Raman spectrum. The unit cell of phase 11, therefore, is monoclinic or orthorhombic containing two molecules, or tetragonal containing four molecules. Comparing these results with those of our previous analysis of norbornadiene, the ordered phases of these related molecules are not isostructural. A low-temperature phase transition can often be induced by the application of pressure. This was observed to occur in norbornylene by high-pressure IR spectroscopy. Figures 5 and 6 show IR spectral regions at various pressures. At 14.9 f 0.3 kbar, the band profiles changed suddenly, along with a shift of frequencies. Up to 16 kbar, these band shapes and frequencies were altered again, but then remained essentially constant up to 26 kbar except for pressure-induced frequency shifts. Upon decompression, the phase transition occurred more slowly, broadened out between 15 and 13 kbar, with the bulk of the spectral changes occurring at 13.3 f 0.4 kbar. Figure 7 shows a detailed measurement of one mode, the vinyl CH deformation at 1335 cm-I at atmospheric pressure, in both compression and decompression directions. On compression, the band position dropped suddenly at the phase transition, whereas on decompression, the phase transition occurred more slowly about the phase transition point. The 13 K temperature hysteresis is therefore manifested as a 1.6 f 0.7 kbar
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 637
Phase Transition of Norbornylene
TABLE I: Vibntioarl
Dah (cm-I) for Norbornykae phase I phase I1 Raman IR Raman IR (135 K) (145 K) (48 K) (58 K) 30 w 39 m 53 w 59 vw 66 w 75 w 82 vw 89 vw 95 w 264 w 261 w 380 m 382 m 383 w 469 w 412 w 477 s 477 s 474 w 476 w 663 w 663 vw 664 vw 664 w 665 vw 668vw 668 vw 703 m 110w 712s 708 w 709 s 717 m 764 vw 769 s 769 m 767 vw 768 m 171 s 793 793 m 793 w 807 w 808 809 w 810 w 809 w 815 w 830 vw 832 w 832 m 833 w 832 m 864 vw 867 w 1868vw 871 m 871 m 873 vs 873 m 874vs 876 m 896 w, sh 897 m 899 w, sh 901 m 902m 903 s 903 m 904s 905 m 911 w 923 w 927 w 927 vw 928 m 926vw 931 w 936 vs 936 w 937 w 938 vs 950 w 952 m 952 vw 954m 952vw 964 s 964 w 958vw 965 w IO15 m 1019 w 1019 m lO2Om 1019m 1030m 1033 w 1034 m 1034m 1085 w, sh 1087 w 1093 w, sh 1090 s, br 1090 w, br 1091 s 1099s 1098 w 1116s 1116w,Sh{ 1111 1117sw
I
I
1
{ E; {
1 1 1
{
1
{
1
1 1 1 { ;:-Ui; }
{
1 ;::
{
1125 s a From
llZSm
ll25w 1128s
}
1124m 1127m 1130 m
phase 11
phase I assignment’
Raman (135 K)
IR (145 K) 1144 vw
Raman (48 K)
IR (58 K) 1145 vw
assignment’
1160s 1165 m, br
1164w
1173 vw
lattice modes 1184 vw 1207 w
1207 m 1213 w, sh a”, wIJ a‘. ~ 2
4
a’, Y23 a”, u43
1270 w 1280 m
1271 m
1296 m
1295 m 1329 m, sh 1335 s 1366 w 1386 vw 1418 w
Y~~
1336 vw
a’,
Y~~
1447 s
a’, a”,
Y42 Y~~
U4I
1235 w 1250 m 1266 w, sh
1250 vw 1262 vw
a’,
a“,
1445
1466 w 1544 vw 1566 m, sh 1570 s 1585 w 1590 vw, sh
a’,
VI6
a‘,
VI5
1204w 1210 vw 1247vw 1260 vw 1268 vw 1271 vw 1294w
1570 w
1443 m 1445 m 1450 s
2868 s
2891 vw 2897 vw 2915 s 2934 m, sh
2899 m 2916m 2931 m’ sh
2947 s
2946 s
1238 w 1250m
a”, q3 a”, wj2 a’. wII a’, ul0 a”, wjg
1
1461 w 1468 w
1465 w 1545 vw 1561 vw 1565 w, sh 1570 s 1585 vw 1589vw
A’, 2 X 709 cm-I a‘, u9
a’, us A’, 2 X 769 cm-l
1569 w
a’, u7
1600vw 1611 w, br 1626 vw 1634 vw 1697 w 1708 w 1715 w
1634 vw, br 1699 w 1709 w 1716 w 2852 w 2869 m
a“, wj5 a“, 1 3 ~
1297 w 1329 w, sh 1334s 1368 w 1386 vw 1420 w 1444m 1447 m
1337vw
a”, wl2
1185 vw 1207 w
1265 w, sh 1270m
1611 w, br
a’, wI8
a’, VI7 a”, Y~~
1161 vw, sh
{ 1164w
1466 w
a’, u19
a”, wa
{ ii::;,sh
2866
2869 s 2863
2883 vw 2889 vw 2892 vw 2896 vw 2913s 2928 w 2932 m 2944
2900 m 2916s 2929 s 2932 s
1
1
a‘, 2 X 1447 cm-I
a’ 2-C-bridge CH str a’, I-C-bridge C H str a” a’
a”, uj7
2954 s 2966 s, sh
a”, 2-C-bridge C H str
a’, w l 4 2976 s, br
2976 s 2992 s
a”,u36 at,ul, 3061 s 3135 w
3059m 3135 w
3055 m 3058 s 3136w
1
2971 s, sh 2977 s 2991 m 3048 m 3055 m 3064m 3134w
a’, bridgehead CH str bridgehead C H str vinyl C H str a’, vinyl CH str A‘, 2 X 1570 cm-I
references 1 I and 13.
pressure hysteresis. No pressurevolume data have been reported for norbornylene. However, from our measurement of the temperature and pressure hysteresis of the phase transition, the volume change at the transition can be calculated by using the equation MtATh
= APhAVt
where ASt and AVt are the entropy and volume changes at the transition, respectively, and Aph and ATh are the pressure and temperature hystereses associated with the tran~iti0n.I~The volume change at the transition, AV,, is, therefore, 3.1 f 1 cm3 mol-’, which is comparable to values obtained for other disordered (14) Smith, E. 9. J. Phys. Chem. Solids 1959, 9, 182.
cage hydrocarbons, adamantane ( 2 . 3 cm3 mol-’) and 2-methyladamantane (2.8 cm3 Table I1 summarizes the frequency shifts of all the measurable peaks in both phases. There are several effects that are apparent from these data. Zallen observed that, for some simple inorganic solids, d In v/dP decreased with increasing v.I6 This was not observed in the case of phase I of norbornylene; there is no regular trend of d In v/dP values with respect to vibrational frequency. This is due to the different types of bonds within the molecule and the noncubic crystal structure. As would be expected, there (15) Hara, K.;Katou, Y.;Osugi, J. Bull. Chem. Six. Jpn. 1981, 51,687. (16) (a) Zallen, R. Phys. Reo. B 1974, 89, 4485. (b) Zallen. R.;Slade, M.L. Phys. Reo. B 1978, B18. 5175.
. . . . , . . . . ,
1340
. .
.. .
; I 4
Y
A
v
P
5
A
1336
B
A
-
A A A
1334
‘
‘
’
”
’
’
’
“ ’
vjV
i i
A V
8
.
I
. . .
,
A f
h
5
. . .
: Ad
5 1338
.
’
A
~
vi k A A + v A
:
,
I I
: /
:
I
;
I
I
t
_’’
-
vA v
’
‘
’
”
’
’
‘
‘
639
J. Phys. Chem. 1991, 95, 639-656 spectral assignments have been made. As with norbornane and norbomadiene, there exist many possibilities for Fermi resonance between these stretching modes and overtones of the skeletal vibrations. Norbomadiene has been shown to exhibit at least three cases of Fermi resonance.'J3J7 The use of high-pressure vibrational spectroscopy to vary the degree of coupling between the two transitions has been widely documented.'" In our study of norbornadiene, the previously assigned Fermi resonant pairs in the C H stretching region did not conform to the general pressure behavior observed in the cases cited in the literature. It was concluded that the peaks in this region exhibit more complicated
behavior due to the C H bonds being on the outside of the molecule. It was also noted that, in both pairs, the pressure dependence of the lower frequency component was less than half that of the higher frequency component of the pair. Furthermore, no exchange of peak intensities was observed over the 30 kbar range of the measurements. By examining the pressure dependences of the peaks in the C H stretching region of norbornylene, while taking into account the overtone possibilities, there is only one pair that may be considered a Fermi resonance doublet. The overtone of the CH2 scissoring mode at 1466 cm-I may be coupled with a methylene C H stretch to produce the two bands at 2915 and 2976 cm-I.
(17) (a) Butler, 1. S.;Barna, G. J . Raman Spectrarc. 1973,828,1645. (b) Adams, D. M.; Fernando, W. S. Inorg. Chim. Acta 1973, 7 , 277. (18) Sec, for example: (a) Sherman, W. F.; Lewis, S. Spectrochim. Acia 1979,354 613. (b) Wong, P. T. T.; Chagwedera. T. E.; Mantsch, H. H. J . Chem. Phys. 1987, 87, 4487.
Acknowledgment. This research was supported by grants from NSERC and CANMET (Canada) and FCAR (Quebec). N.T.K. also acknowledges the award of a postgraduate scholarship from NSERC.
Electronic Spectroscopy of the Cyanogen Halides W.S. Felps, K.Rupnik, and S. P.McClynn* Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803 (Received: June 25, 1990; In Final Form: August 6, 1990)
The electronic absorption spectra of the cyanogen halides, XCN where X = CI, Br, and I, have been investigated in the range 3100-1050 A. The spectra are analyzed in terms of vibronic structure, oscillator strengths, and effective quantum numbers. The spectra of the cyanogen halides exhibit no regular Rydberg structure. The absence of regularity is shown to be a direct consequence of the presence of intravalence excitations and Rydberglvalence interactions. In confirmation of the above, the intravalence transitions arising from the 2a (5a, 37r, and 6a) configurational excitations are observed and assigned. In addition, we presume to extract "term values" for the antibonding 5u, 3r, and 64 MOs and then use these to predict the energies of the remaining nine low-energy intravalence excitations of the cyanogen halides, {4u; 1r; 3u) {Sa;3a; 64. There is a danger in this, in that we seem, too blithely perhaps, to make use of a simple one-electron MO model in situations where it is known that many-electron effects may dominate. We believe that the use of one-electron considerations is moderated by the extensive use of a vast amount of empirical, correlative experimental data. Finally, all excited states that arise from the 12 low-energy intravalence excitations are correlated with the states of the separated halogen atom and CN radical such that hotoprocesses in the cyanogen halides may be rationalized. The A-band and a-band continua are assigned as 2a 5a,'3 ll and 2a 3r;',3A,'*32+,'93Z-configurational excitations, respectively. The 4a 5a;'2+ transition is associated with the discrete structure atop the a continuum; the intense, discrete band systems that lie to the blue of the B and C Rydberg band systems are associated with the 2a 6a;'*'ll intravalence transitions. The states that arise from the remaining eight configurational excitations are shown to be mostly dissociative in nature. The correlation scheme predicts (i) CN (X22+)to be the primary product of photolysis within the A continua; (ii) CN a211 toi) be the primary product of photolysis from the onset of the a continua up to photon energies of 80000 cm-' for each of the cyanogen halides, and (iii) CN m22+) to be a primary product for photon energies greater than 105000,94000, and 80000 cm-'in ClCN, BrCN, and ICN, respectively.
-
-
P
-
-
-
-
-
Introduction The advent of UV and VUV lasers has generated considerable interest in the high-energy, high-resolution photochemistry of the cyanogen halides. However, most of the new photochemical investigations are predicted on absorption spectra obtained in 1966 using a 1-m Cary Model 14 spectrophotometer for X > 1340 AIo and a 21-ft vacuum spectrograph for 1250 A < X < 1840 A.Ib It is known that some of these spectra are inaccurate or incomplete and that the assignments need reconsideration. We demonstrate these problems by citing a few seemingly trivial, but important examples: (i) The more recent UV2 and VUVZaspectra of ICN suggest that the two lowest energy continuous absorption systems (Le., ( I ) (a) King, G. W.; Richardson, A. W. J . Mol. Specirosc. 1966,21,339. (b) King, 0 . W.; Richardson, A. W. J . Mol. Specrrosc. 1966. 21, 353. (2) (a) Myer, J. A.; Samson, J. A. R. J . Chem. Phys. 1970,52,266. (b) Holdy, K. E.; Klotz, L. C.; Wilson. K. R. 1.Chem. Phys. 1970 52,4588. (c) Ling, J. H.; Wilson, K. R. J . Chem. Phys. 1975. 63, 101.
0022.365419 112095-0639$02.50/0
-
the so called A and a bands) are actually centered 100 A to the red and blue, respectively, of the maxima reported in 1966.'* (ii) Some shorter wavelength studies, Xmin 1050 A, dealt',' with photodissociation and predissociation of the a band and with the discrete Rydberg band systems at shorter wavelengths. These studies indicate that the maxima of the a band lie at considerably shorter wavelengths in ClCN, BrCN, and ICN than was reported in 1966.'* (iii) It has recently been shownSathat the A and a bands of ClCN are actually two separate and distinct features and that
-
(3) West, G. A.; Berry, M. J. Chem. Phys. L r t . 1978. 56, 423. (4) (a) Macpherson, M. T.; Simons, J. P. J . Chem. Soc., Faraday Trans. I1 1979, 75, 1572. (b) Ashfold, M. N. R.; Simons, J. P. J . Chem. Soc., Faraday Trans. I1 1978. 74, 280. (c) Ashfold, M. N. R.; Macpherson, M. T.; Simons, J. P. Top. Curr. Chem. 1979,86, 1 . (5) (a) Felps, W. S.; McGlynn, S. P.; Findley, G. L. J . Mol. Spectrosc. 1981,86, 71. (b) Rabalais, J. W.; McDonald, J. M.; Scherr, V.; McGlynn. S.P. Chem. Rev. 1971, 71, 73.
0 1991 American Chemical Society
