8529
J. Phys. Chem. 1991, 95, 8529-8538
Infrared and Raman Study of Monohydrogenated Cyclopentenes-3-h, and - 4 4 , : CH Stretching Vibration and Its Two First Overtones in Gas Phase D. Cavagnat* and S. Banisaeid-Vahedie Laboratoire de Spectroscopie Moleculaire et Cristalline (U.R.A. 124 CNRS), 351 cows de la Liberation, 33405 Talence Cedex, France (Received: March 22, 1991; In Final Form: May 31, 1991) The infrared and Raman spectra of gaseous monohydrogenated cyclopentenes-3-hl (Cy3H) and -4-hl (Cy4H) have been investigated in order to determine the ring-puckering potential in the first three excited states of the CH stretching vibration. All the spectra present a triplet structure accompanied by shoulders and weaker bands, but the relative intensities of the latter and of the central peak of the triplet decrease from d o = 1 to Au = 3. It has been possible to interpret the observed structure and its intensity evolution in terms of transitions from the known ground-state ring-puckering levels to excited-state ring-puckering levels in a potential of increasing asymmetry. The two more intense bands of the triplet are thus assigned to transitions l0,O) lv,O) and l0,l) Iv,~). They correspond to the excitation of the CH oscillator in its axial and equatorial conformation, respectively. The anharmonicity of the axial and equatorial v(CH) vibrations are nearly the same, but are higher in the -3-hl than in the -4-hl derivative. These results confirm nicely the conclusions previously obtained from a similar study of monodeuterated cyclopentenes: in all the isotopic monosubstituted derivatives, the asymmetry of both the kinetic and potential energy terms is definitively evidenced. Furthermore, the observed asymmetry in the potential of the monohydrogenated derivatives is in perfect agreement with the predictions made from the previous study of the monodeuterated ones and with ab initio calculations of the CH bond length difference between axial and equatorial positions.
-
-
Introduction
In a preceding paper, the far-infrared and Raman spectra of the two monohydrogenated cyclopentenes-3hl (Cy3H) and -4hl (Cy4H) were investigated in order to determine the ring-puckering energy function of these molecules in the gas phase.' The asymmetrical form V(x) = V 2 V& + V& previously deduced from the analysis of the spectra of the monodeuterated cyclopentenes-3dl (Cy3D) and -4dl ( C Y ~ Dwas ) ~ found again. This confirms that the isotopic monosubstitution in the allylic positions of cyclopentene induces an asymmetry not only in the kinetic energy function but also in the potential energy function. The two distinct conformers thus produced have an energy difference of 4.3 and 2 cm-' for the 3- and 4-substituted derivatives, respectively. This energy difference is related to the variations of the C H and C D bond lengths (and hence stretching vibrations) of the CHD group in the two conformers. The conformer with a C H bond in the axial position is found to be the most stable one, in good agreement with predictions based on a correlation between ab initio C H bond lengths and spectroscopic vibrational energie~.s-~ This result was corroborated by the study of the CD stretching spectral region of Cy3D. This spectrum exhibits a triplet structure in which the two extreme bands correspond to excitation of the CD oscillator in its equatorial and axial positions. This structure can be explained by transitions between two ring-puckering potentials of reversed asymmetry in the ground and first excited state of u(CD).6 The increase in magnitude and reversal in sign of the asymmetry in the puckering potential in the first excited state of v(CD) was interpreted from the same simple considerations on the vibrational energy of the CHD group. However,as stressed by the authors, the V(CD) stretching region presents many parasitic bands due to overtones and combinations of lower frequency vibrations which can modify the v(CD) pattern, by Fermi resonance phenomena for example. The uncluttered C H stretching spectral region is generally less perturbed, and furthermore the CH oscillator is a more sensitive probe than CD due to its higher frequency.
+
( I ) Cavagnat, D.; Banisacid-Vahedie. S.;Grignon-Dubois, M. J. Phys. Chem. 1991.95, 5073. (2) Rafilipomanana, C.; Cavagnat, D.; Cavagnat, R.; Lassegues, J. C.; Biran, C. J . Mol. Struct. 1985, 127. 283. (3) Sacbo, S.;Cordell, F. R.; Boggs, J. E. J . Mol. Strucr. THEOCHEM 1983, 104, 221. (4) McKean, D. C. J . Mol. Struct. 1984, 113, 251. (5) Aljibury. A. L.; Snyder, R. G.; Strauss, H. L.;Raghavachari, K. J. J. Chem. Phys. 1986.84, 6872. (6) Rafilipomanana. C.; Cavagnat, D.; Lassegues. J. C. J . Mol. Srrucr. 1985, 129, 215.
0022-3654/91/2095-8529$02.50/0
Thus, the present analysis of the v(CH) spectra of Cy3H and Cy4H in the gas phase has been carried out as a logical continuation of this previous study to confirm the results previously obtained from the v(CD) region and to improve our knowledge of the conformational dynamics in the nonrigid molecule. From the vibrational arguments previously developed,'Jf' the asymmetry of the ring-puckering potential in the first excited state of u(CH) is expected to be greatly increased in magnitude but not reversed in sign compared to the ground-state potential. To our knowledge, only one similar comparative analysis has been previously reported for the v(SiH) and v(SiD) spectral regions of l-silacyclobutane-l-dl,7The authors assigned the observed doublets to the axial and equatorial forms, but explained this conformational discrimination only by the asymmetry in the reduced mass caused by the monoisotopic substitution. No account is given for the large splitting of the two first ring-puckering levels in the first excited state of v(SiH) and v(SiD). The analysis of the overtone spectra of the C H stretching vibrations may be another way to investigate the changes of molecular structure and conformation. The dominant transitions in these spectra involve vibrationally excited states whose components have all of the vibrational energy localized in one of the set of equivalent CH oscillators. The energies of these peaks can be fitted to the energy equation of a diatomic Morse oscillation
E = V W , - (V
+
O*)W&~
(1)
where we and WS, are the harmonic frequency and the diagonal local-mode anharmonicity constant, respectively. These parameters are very sensitive to the physical and chemical properties of the CH oscillator such as bond lengthlstrength, hybridization, steric effects, and conformational en~ironment.**~ In particular, the gas-phase overtone spectra provide a very sensitive experimental technique to determine C H bond length changes at the level of accuracy that is provided by ab initio molecular orbital the~ry.~J The overtone spectra of perhydrogenated cyclopentene were previously measured in the liquidlo and in the gas phase" up to the sixth overtone (Av = 7). These spectra are principally composed of two peaks, an intense and well-resolved one corresponding (7) Harthcock, M. A.; Cooke, J. M.; Laane, J. J. Phys. Chem. 1982.86, 4335. (8) Ahmed, M. K.; Henry, B. R. J . Phys. Chem. 1987, 91, 5194. (9) Henry, B. R.; Sowa, M. G. Prog. Anal. Spectrosc. 1989, 12, 349. (IO) Henry, B. R.; Hung, I. F.; MacPhail, R. A,; Strauss, H. L.J. Am. Chem. SOC.1980, 102, 515. ( 1 1) Wong, J. S.; MacPhail, R. A.; Moore, C. B.; Strauss, H. L.J . Phys. Chem. 1982, 86, 1478.
0 1991 American Chemical Society
Cavagnat and Banisaeid-Vahedie
8530 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
the procedure described in the first paper.' As pointed out in that paper, Cy3H is obtained as a pure compound, but Cy4H can only be obtained mixed with Cy3H in almost equal proportion. In both cases, the isotopic purity is higher than 96%. The infrared spectra were recorded under the following conditions. The u(CH) fundamental CH stretching spectra were recorded on a Brucker 1 13V FTIR spectrometer with a resolution of 0.5 cm-l and a path length of 10 cm and the 2u(CH) and 3u(CH) overtone spectra were recorded on a Nicolet 740 FTIR spectrometer with a resolution of 1 cm-' and a path length of 1 m. The gas cells were equipped with CaF, windows. The Raman spectra were recorded on a 224 Dilor triple monochromator equipped with a Hamamatsu R943-02 Peltier effect refrigerated photomultiplier. A Spectra Physics 171 argon ion laser provided a 514.5-nm beam with 4 W of power that was filtered by a Photo-physics laser monochromator and used as the exciting radiation. The spectral resolution was 1.3 cm-I. The gaseous compounds were transferred under vacuum into a cylindrical Pyrex cell equipped with two windows tilted with a Brewster angle for multiple reflections. All the gaseous samples were maintained at room temperature with a pressure of 300 Torr.
d
Results and Discussion
Figure 1. Infrared and Raman spectra of gaseous cyclopentene Cy3H in the CH stretching region. to the ethylenic C H bonds, and a less intense broad one corresponding to the methylenic CH bonds with a complex structure that is badly resolved even in the gas phase at high energy (Au = 6 or 7). In the liquid phase, the authors determine a mean harmonic frequency of 3057.3 cm-I and a mean anharmonicity of 67.3 cm-' for these methylenic u(CH) vibrations.'O Even if the gas-phase spectra are better resolved than in the liquid phase, they are not resolved enough for a decisive assignment of the various methylenic bands. The authors distinguish peaks corresponding to the axial and equatorial C H bonds of the methylenic groups in the 3- and 4-positions by a deconvolution of these broad Au = 6 and hv = I features with four Lorentzians, but do not determine the corresponding local-mode parameters.'' In such molecules with nearly equivalent CH bonds, selective deuteration may be used to identify and assign overtones transitions as belonging to a specific bond.
Experimental Section The two monohydrogenated cyclopentenes-3hl (Cy3H) and -4hl (Cy4H) have been synthesized by organotin route according to
A. Experimental Results. 1. Monohydrogenated Cyclopentene3L (Cy3H). The Raman spectrum of the C H fundamental stretching vibration is isotropic. It exhibits three intense lines at 2885.5, 2909.5, and 2933 cm-' and three weaker ones at 2893,2907, and 2924.5 cm-' (Figure 1). The infrared spectrum shows Q branches at the same frequencies, superimposed on a broad envelope due to the overlap of the P and R wings corresponding to each Q branch (Figure 1). The very weak features at 2869,2873, and 2946 cm-' may be due to isotopic impurities present in small amounts (in particular Cy4H). The frequency difference between the two most intense peaks is 47.5 cm-' in both spectra. The infrared spectra of the first and second overtones present a similar triplet structure, but the central band becomes weaker relative to the others from Au = 1 to Au = 3 (Figure 2). Similarly, a series of shoulders and weak bands are still observed, but their relative intensities also decrease when v increases. One can note that the distance between the two most intense bands is 97 cm-I (Av = 2) and 146 cm-' (Au = 3), i.e. respectively about 2 and 3 times larger than the 47.5-cm-' splitting for Av = 1. In addition, the series of weak broad bands centered at 8099, 8195, and 8505 cm-' in Av = 3 spectrum can be assigned to the third overtone of the u(CD) vibrations (Au = 4). They do not interact with the u(CH) vibrations. Indeed, as shown by Baggott et al. in CHDC12,12the interbond coupling between C H and CD
Av = 3
hv-2 5643
A
5740
.
8600
Figure 2. Infrared spectra of
\ 8460
8320
8180
8040
cm-1
/I
5850
5790
5730
5670
5610
gaseous cyclopentene Cy3H in the first (Au = 2) and second (Au = 3) u(CH) overtone region.
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8531
Monohydrogenated Cyclopentenes
TABLE 1: obsened a d Calculated Frequencies and Intensities of the CH Stretching Transitions in the Raman and Infrared Spectra of Cpseous Cyclopentcln Cy3H md Cy4H'
Cv4HC
Cv3Hb -,--~
transitions (l,n?-(O,n)
(1,O)-(0,O) (l,l)-(O,l) (1,2)-(0,2) (l,3)-(0,3) (1,4)-(0,4) ( 1 S)-(O,5) ( 1,6)-(0,6)
(1,m)-(0,m) ( 1,m ' ) - ( O m 'I (1,3)-(0,2) (I.2)-(0,3) ( 1 ,5)-(0,4) (1,4)-(0,5)
Y
~
Ph
cm-I ,
Y,~,
cm-'
2885.5 2933.0 2893.0 2924.5 2907.0
1.00 0.78 0.33 0.30 0.37
2885.80 2932.30 2893.00 2924.60 2906.60 2910.80
2909.5
0.70 2909.30 2932.90 2884.70 2957.80 2859.60
calculated intensity Raman infrared P ix iy
~ cm-' ,
1.00 0.025 0.98 0.025 0.39 0.010 0.36 0.009 0.32 0.008 0.23 0.006 10.74 0.51 0.013
0.02 0.74 0.03 0.21 0.07 0.07
Iz 1.00 0.25 0.35 0.13 0.21 0.13
0.13
0.29
0.17 0.19
0.13 0.002 0.06
0.05 2964.5 sh 0.16 0.01 0.06
0.05
0.04
0.004 0.004 0.001 0.001
5 X IO-'
v
Ph
val,
cm-'
2933.0 2958.5 2940.0 2951.0 2944.6
1.00 0.9 0.18 0.27 0.30
2933.0 2958.2 2938.9 2952.3 2945.2
2947.5
0.62
2947.5 2947'3 2949.6 2953.0 2962.6 2928.6
calculated intensity Raman infrared infrared
P
1.00 0.99 0.45 0.42 0.37
0'281
0.20 0.85 0.37 0.03 0.12 0.11
ix 0.13 0.66 0.10 0.21 0.12 0.10 0.07 0.12 0.02
I= 1.00 0.45 0.40 0.25 0.27 0.20 0.14 0.24
0.10
0.05 0.001
0.05
0.03
#The levels are labeled lu,n), where u is the vibrational CH stretching quantum number and n the ring-puckeringquantum number; 7 -< m I10 X IO-' -3.0529 X 104x - 8.81 1 X IO-'x2 + 3.2459 X lO-'x' - 1.8688 X IO-IX' - 9.4082X 10-'x5 + 6.5928X IO-Ix6. u = 0: V(x) = -26452~~ + 952~'+ 7540203cm-l. u = 1: V(x) = -26500~~ + 11599~'+ 755000~'cm-I. 'The reduced mass is g(x) = 5.8359 X + 2.9769 X 104x - 4.1121 X IO-'x2 + 4.9844X 10-5~'- 2.1642 X 10-13 - 1.2376 X 10-2x5+ 6.6827 X lo-'#. u = 0: V(x) = -25520~~ 370~'+ 7371242 cm-I. u = 1: V'(x) = -25700~~ + 6400~'+ 751500~'cm-I.
m ' 2 1 I ) . *The reduced mass is Ax) = 5.7251
+
I
2958.5
2958.5
1909.5
2917.5
/ ! A J Raman
cm-1
3000
2950
2900
1
2850
Figure 3. Infrared and Raman spectra of mixed gaseous cyclopentenes Cy3H and Cy4H in the CH stretching region. The bands marked with a star (*) correspond to Cy3H bands (cf. Figure 1).
oscillators bonded to the same C atom becomes apparent only at high levels of C H bond excitation (around Au = 6) because of the different anharmonicities of the C H and CD vibrations (typically 60 and 30 cm-I for v(CH) and v(CD), respecti~ely.'~.'~ 2. Monohydrogenated Cyclopentew4h, (Cy4H). As already noted, the Cy4H derivative can only be obtained mixed with Cy3H in equal pr0portion.l The infrared and Raman spectra of this mixture are thus the superposition of the spectra of Cy3H and Cy4H. For example, in the v(CH) fundamental region (Figure 3) the lines due to the Cy3H derivative can easily be identified by comparison with Figure 1 and a relatively precise subtraction can be performed. The subtraction coefficient is calculated from the lower frequency band of Cy3H, for example, the band at 2885.5 cm-I in the v(CH) fundamental region. The Raman spectrum thus obtained exhibits three. intense bands at 2933,2947.5, and 2958.5 cm-I, with three weaker ones at 2940, 2944.5, and 2951 cm-' (Figure 4). The frequency difference (12) Baggott. J. E.: Law. D.W.; Mills, I. M. Mol. Phys. 1987, 61, 1309. (13) Duncan,J. L.; McKean, D.C.;Torto, I.; Brown, A.; Ferguson, A. M. J . Chcm. Soc., Faroday Trans. 2 1988.84, 1423.
I R
4
,
I
2980
2960
1
2940
, 2920
CM-1
Figure 4. Infrared and Raman spectra of gaseous cyclopentenc Cy4H in the CH stretching region. These spectra are the result of subtraction of spectra of Figure 1 from spectra of Figure 3. between the two most intense peaks (25.5 cm-') is roughly half that found for Cy3H. The infrared specttum again shows several Q branches emerging from the broad overlapping of the P and R wings. Their frequencies correspond to those of the Raman lines, but, contrary to what is observed in the infrared spectrum of Cy3H, only the Q branch at 2933 cm-'is very intense, the other ones being much weaker (Figure 4). The infrared spectra of the first and second overtones show a similar but more embedded triplet feature, in which the relative intensity of the central band decreases from Au = 1 to Au = 3, but more slowly than in the Cy3H case (Figure 5). A series of shoulders and weak satellite bands are again observed. The distance between the two extreme bands is 51.5 cm-' (Ao = 2) and 73 cm-l (Au = 3), which is again roughly equal to 2 and 3 times the splitting measured for Au = 1 (25.5 cm-I). One can note that the infrared spectrum of the second overtone (Au = 3) of Cy4H is not as simple as the corresponding Au = 3 spectrum of Cy3H.
8532 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 AV
-
Cavagnat and Banisaeid-Vahedie
3
bv = 2
571111.5
57/7q 5771.5
I
J
x10 -.-,a
\
A L L
86b0
890
8480
8420
8360
cm-i
Figure 5. Infrared spectra of gaseous cyclopentene Cy4H in the first (Au = 2) and second (Au = 3) u(CH) overtone region. TABLE II: Observed and Calculated CH Stretching Transitions in the Two First Infrared Overtone Spectra of Gaseous Cyclopenteues Cy3H and Cy4Ha Cv3Hb Cy4HC ual, cm-I vob, cm-’ u ~ cm-l , val, cm-I Ial IaI 1.oo 5643.0 5643.0 5744.5 5744.7 1.oo 5740.0 0.98 0.17 5740.0 5744.0 5654.0 (sh) 5654.4 0.33 5796.0 5796.7 0.98 0.07 0.18 5653.0 5796.9 0.22 5725.1 5731.0 (sh) 0.37 5758.0 5754.3 0.23 5733.4 0.32 5785.0 5786.6 5683.5 0.19 0.15 5683.0 5783.5 0.27 5692.5 5692.5 0.30 5768.9 5753 (sh) 0.14 5755.6 0.15 5744.7 (2,m)-(O,m) 5771.5 5774.8 0.50 0.30 5767.0 0.17 5648.0 5631 .O (sh) 0.17 5633.0 8284.0 8443.0 8443.0 1.oo 0.98 8284.0 8430.0 1 .oo 8429.7 8516.0 8515.8 0.96 0.10 8459.0 8455.3 0.35 843 1.5 0.25 8506.5 8405.3 8502.0 0.26 0.13 8476.4 0.16 8418.2 848 1.O 0.32 8422.0 (sh) 8485.5 0.15 8420.8 0.20 0.1 1 8412.5 8485.3 8491.5 0.17 0.15 8491.0 8426.5 0.20 8449.0 0.20 8512.4 8532.0 (sh) 0.15 0.27 8540.0 8301.0 8448.5 0.18 0.16 8445.0 8325.0 8431.0 (sh) 8299.0 (sh) 0.14 0.30 8428.0 8295.5 0.15 8292.6 8360.0 0.007 8359.6
“The levels are labeled lu,n) where u is the vibrational CH stretching quantum number and n the ring-puckering quantum number. The reduced masses are the same as those used in Table I. b u = 2: V”(x) = - 2 6 5 3 0 ~+ ~ 22735~’ 7558502 cm-I. u = 3: V”’(x) = - 2 6 5 5 0 ~ ~ 33435~’+ 756650~‘cm-l ( m L 5 for Au = 2; m L 7 and m’ 2 4 for Au = 3). c u = 2: V”(x) = - 2 5 8 7 0 ~ ~ 13030~) 766000~‘cm-I. u = 3: V”’(x) = - 2 6 0 5 0 ~ ~ 18330~’+ 7 8 ~ cm-’ ~ (5‘ I m I 7 and m ’ 1 7 for Av = 2 and m ’ L 7, m 2 3 for Au = 3).
+
+
+
All the observed frequencies of these spectra are listed in Tables I and 11. The very good correlation observed between the frequency differences of the principal bands of the fundamental and overtone spectra of the v(CH) vibration shows that, contrary to what is observed in the v(CD) spectral range for Cy3D,6 the v(CH) spectral region is not perturbed by the presence of overtone or combination bands of lower frequency vibrations. Each of the most intense bands of the spectra can be assigned to CH stretching transitions, the lower frequency one to the axial v(CH) vibration and the higher frequency one to the equatorial v(CH) vibration (u = 1-3). We have applied relation 1 to the two principal bands observed in the v(CH) spectra from Av = 1 to Au = 3. The calculated harmonic frequencies and anharmonicities are listed in Table 111. The anharmonicity values are lower than that deduced from the liquid-phase overtone spectra (67.3 cm-l).lo This anharmonicity is roughly the same for the axial and equatorial C H bonds but is slightly higher for the CH in the 3-position than for the C H in the 4-position. Furthermore, the wsxC values calculated from the two successive intervals between the observed
+
+
TABLE HI: Observed Band Maxima of the CH Axial and Equatorial Stretching Overtone Spectra, Harmonic Frequencies (u&,and Anharmonicities (up,) (in cm-I) of Gaseous Cyclopentenes Cy3H and Cy4H“ AE2.0
AEl,O
Au = 1
AE3,0
Au = 2 Au = 3
(W&)1‘2
We
(W&)2‘3
Cy3H
2885.5 eq 2933.0 Cy4H ax 2933.0 eq 2958.5 ax
5643.0 5740.0
8284.0 3013.5 8430.0 3059.0
64.0 63.0
60.2 60.0
5744.5 8443.0 3054.4 5796.0 8516.0 3079.5
60.7 60.5
58.0 59.3
“((u~e)““c is’the
-
I ) u(CH) overtone).
anharmonicity calculated between the u and (u +
-
bands (Au = 1 Au = 2 and Av = 2 Au = 3) are seen to decrease (Table 111). This could indicate the onset of a Fermi resonance, probably with the C H D bending vibration (at 1305 cm-I). This trend must however be confirmed by measurements
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8533
Monohydrogenated Cyclopentenes
P
r"
Figure 6. Principal momenta of inertia of the two conformers (CH axial and CH equatorial) of the monohydrogenated cyclopentenes Cy3H and Cy4H in their bent equilibrium conformation (0 = 22' 2667). -3hl:CH,,: Ic(Z) = 261 X l p g cm2;I B ( Y ) 147 X l p g em2;IA(X) = 144.5 X 10- g cm2. CH,: Ic(Z)= 258 X l o " g cm2;IB(Y)= 148 X l o " g cm2: IA(X) 143.8 X 10- g em2. -4-Ll:CH,: Ic(Z) = 261.6 X 1 o " g em2;IB(X)= 149.6 X 10- g cm2;fA(r)= 142 X 10-g em2. CH,: IC(Z)=258.5X 1@gcm2;Ie(X)= 151.6X 1O4gcm2;IA(Y) = 141 X lO*g em2.
of higher overtones, which are in progress. B. Theoretical Background. The theoretical approach used to analyze these u(CH) profiles is very similar to that already used for the study of the internal rotation of the methyl g r o ~ p ~or~ - I ~ of the u(CD) vibration in C Y ~ D . ~ The basic idea of this theory is to suppose that the fast v(CH) stretching vibration is coupled to the slow puckering motion associated with the nonrigidity of the ring and therefore depends on the puckering coordinate. The total Hamiltonian of the system can thus be written HT = Hv + HI + Hv, (2) where Hv, HI, and Hvl are the Hamiltonians describing respectively the v(CH) mode, the ring-puckering motion, and the coupling between the two motions. If the v(CH) mode is described in the harmonic approximation, HT can be written H-r
(3) where q and x are the coordinates of the u(CH) mode and of the ring-puckering mode, respectively, and 1 / M and g(x) the inverse of their respective reduced masses. The definition of x (Figure 6) and the calculation of g(x) (Table I) are the same as those used in the first paper.' As shown in ref 1, the ring-puckering potential V(x) takes the asymmetrical form V g 2+ V3x3 V&. The linear term Vlx, also allowed by symmetry, would lead to a potential maximum at x' # 0. As x'is likely nearly zero, we have omitted it, as generally done in the previous works (see ref 1 and references therein). In the adiabatic approximation, the total wave function can be written as a product of two wave functions, X(q,x), which describes the fast v(CH) motion and depends slowly on x, and \t(x) which describes the much slower ring-puckering motion.
+
As the analytic form of the interaction potential W(q,x) is not known, the functional form of e(x), the energy of the perturbed C H oscillator which acts as an additional effective potential in eq 5, is also unknown. Hence, we have empirically determined the parameters V i , V;, and V,) of the ring-puckering potential V'(x) in the first excited state of v(CH), the linear term being assumed to remain almost zero. This determination is made by fitting the experimental u(CH) transition frequencies with those calculated between the levels of two ring-puckering potentials V'(x) in the first excited u(CH) state and V(x) in the ground state. The parameters V2,V3,and V4 of the latter ground-state potential are already known.' The difference between these two series of parameters contains implicitly the u(CH)/ring-puckering coupling. The values of the V'(x) parameters are further refined by fitting the experimental intensities with those calculated from the following relation 4O,n)+1,$)
P[
--
5 s X I * ( q A **,,Ax)
A ( q A & ( w ) * d x ) d x dq]' (6)
where P is the Boltzman factor exp(
-
v)
l0,n) and I1,n') are respectively the nth and n'th ring-puckering levels in the ground and in the first excited states of u(CH). A(q,x) is the transition operator. It can be expanded to the first order around the equilibrium position q = 0, leading to
The first integral of the relation 6 can be considered as a constant C for the u(CH) vibration:
For the isotropic Raman spectrum, the operator A is the mean polarizability. The corresponding intensity is then in abbreviated notation (9) where ( n l and In) stand respectively for \t*,,, and \th eigenstates. For the infrared spectrum, the situation is slightly more complex. The dipole operator ji yields three Cartesian components along the main molecular axes X,Y,2 (Figure 6). If 7 is assumed to be collinear with the CH bond, these components can be expressed as a function of the ring-puckering coordinate x: for Cy3H: ~ , = y
0.157~
py
= ~ ( 0 . 5 5 5+ 2 . 9 2 8 ~- 3 . 5 7 4 ~ ~ )
pz
= ~ ( 0 . 8 1 6- 1 . 9 9 2 ~- 5.254~')
px
= p(0.577
(10)
for Cy4H:
~~
(14) Cavagnat, D.; Lascombe, J. J. Mol. Specrrosc. 1982, 92, 141. (15) Cavagnat, D.; Lascombe, J. J. Chem. Phys. 1982,76,4336. (16) Cavagnat, D.; Lascombe, J. J . Phys. 1983, 44, 67.
+ 1 . 7 6 8 -~ 1 . 3 5 3 ~ ~ ) ru=O
~~
pz
= p(0.817 - 1 . 2 5 0 ~- 1.916~')
(11)
8534 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
-bo
-20
0
10
-90
-20
0
20
cm-1 -40
un-1
-90
-20
-28
0
0
20
un-'
Figure 7. Calculated infrared vibration-rotation profiles for the CH stretching mode of gaseous cyclopentenes Cy3H and Cy4H when the CH bond is in either an axial or equatorial position. The intensities of the transitions corresponding to these components are given by for Cy3H:
'I
CP($y[0.816(nln) 0
- 1.992(nlxln) -
for Cy4H:
'I
a
CP($)[0.577(nln)
+ 1.768(nlx(n) -
0
IY
'1
a
CP($)1[0.817(nln) 0
Cavagnat and Banisaeid-Vahedie
=0
- 1.25O(nlxln) 1.9 16( nlx2(n)] (13)
0 I
-0.2
I
I
-0,l
0
-
I
I
0,1
0.2
X IA)
Figure 8. Diagram of the u = 0 gastous cyclopentene Cy3H.
u = 1 CH
stretching transitions in
They are characterized by the value of the separation of the P-R wings (APR = 19 cm-I for the A and B type profiles and APR = 27.3 cm-' for the C type profile) and by the intensity of their Q branch (8-10% and 40-4176 for the A and C type profiles, respectively). These values are in good agreement with those previously calculated by another method.20 The profile of the v(CH) band has been calculated for the axial and equatorial position of the C H bond in both Cy3H and Cy4H (Figure 7). This theoretical profile is of BC type, with a greater proportion of B type when the C H bond is in the equatorial position, and a greater proportion of C type when the CH bond is in the axial position, as indicated by the values of PR (APRC= 21.2 and 21 cm-I and APRa = 24.8 and 25.5 cm-' for Cy3H and Cy4H, respectively) and the intensity of the Q branch (Q' = 19.6 and 16.3% and Q = 36.6 and 37.3% for Cy3H and Cy4H, respectively). These theoretical profiles can be used to guide the discussion of the experimental results. C. Interpretation and Calculation of the Spectra. 1. Tbe Fuadp~~ental CH Stretching Vibration (Av = 1). The previous determinations of the ring-puckering potential in an excited state of a vibrational mode (V,(CH~)~' or I ~ , ( C H , )in~ ~perhydrogenated cyclopentene or v(CD) in Cy3D6) show that the parameters Vt2 and V',, which govern the height of the barrier and the splitting of the wells, are very similar to the V2and V4parameters of the potential in the ground state. On the contrary, in the case of an asymmetrical potential,'g6 the V, term is very sensitive to the considered state of vibration. If the ground-state parameter V, is assumed to be positive, two distinct situations are possible for the excited-state parameter V3: V 4 (Table 11). But as the asymmetry increases, the intensity of these last transitions de-
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(24) Dubal,
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H.R.;Crim, F. F.J . Chem. Phys. 1985, 83, 3863.
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8537
Monohydrogenated Cyclopentenes
(AV2/Ao N 0.12% for Cy3H and Cy4H and AV,/Av = -0.7% and 1.9% for Cy3H and Cy4H. respectively) and a strong increase in the cubic term (AV3/Au H 10800 and 6000 cm-* for Cy3H and Cy4H, respectively), which is essentially at the origin of the variation of the barrier height value H (defined as the energy difference between the top and the deeper minimum). H incream linearly with the vibrational quantum number u (Table IV). An 4 increase of the H value was also observed in the first excited v(CD) -0.2 0 0.1 state in Cy3De6 As in the first excited ring-twisting state,' the increase of the potential energy when both CH stretching and ring-puckering motions are simultaneously excited shows that the two motions hinder each other somewhat (Table IV). 2. The Energy Difference AE. The asymmetry of the ringpuckering potential in the excited v(CH) states is greatly increased, v = o but is not reversed in sign compared to the ground-state potential (Table IV). The value of this energy difference LL!? increases linearly with u (A,!?/& = 51.5 and 26 cm-l on average for Cy3H and Cy4H, respectively). As in the ground state,' it can again be supposed that this energy Eo -0.1 0 0.2 difference results primarily from the difference in the vibrational energy of the two conformers corresponding to the C H bond in I I an axial or equatorial position. The major contribution to the -0.2 -0.1 0 0.1 0.2 vibrational partition function difference is again assumed to come x (A, from only the CH and CD stretching modes of the CHD group Figure 11. Diagram of the u = 0 u = 3 CH stretching transitions in and can be written (uCD = 0 and vCH = u): gaseous cyclopentene Cy3H. Mv= h ~ / 2 [ ( 2 ~ l)AB(CH) - AP(CD)] (14) creases to become negligible in the Cy3H case for Au = 3. In ~ ~ As(CD) = r(CD)q with Av(CH) = r(CH)q - D ( C H ) and this last case, the very strong asymmetry of the two potential wells induces a strong localization of the first two ring-puckering wave S(CD)~~. The comparison of the r(CH) spectra of Cy3H and Cy4H and functions in the deeper well and of the third wave function in the of the v(CD) spectra of Cy3D shows that the average frequencies other well and therefore gives rise to a slightly different pattern of these bands yield an isotopic ratio of 1.34, in good agreement of transitions as shown in Figure 1 1. The m a t intense transitions with the isotopic ratio deduced for other different isotopic deare here l0,O) 13.0) and l0,l) )3,2), accompanied by weaker IO,n+l) 13~1)and IO,n+l) (3,n+2) transitions (n 1 I), whose rivatives of cyclopentene.20 Therefore, the energy can be written frequencies are very close to the two corresponding strong ones. M, = hc(u 0.127)Ar(CH) (15) This is in good agreement with the experimentally observed simThe experimentally measured values of Ar(CH) determined plification of the 3 4 C H ) spectrum (Figure 2). D. Discussion of the Results. The discussion will be devoted above are 47.5 and 25.5 cm-' for Cy3H and Cy4H, respectively. to the analysis of the variation of the different parameters The AE, values calculated from relation 15 and these Ar(CH) characterizing the potential energy function associated with the values are listed in Table IV. In consideration of the rough ring-puckering motion in the excited vibrational v(CH) states. approximation involved in (15), these values are in good agreement These results will be also compared to those previously obtained with those determined from the potential calculation. The vibrational origin of the potential asymmetry is thus further confor the first excited v(CD) state in the monodeuterated cyclopentene-3dI ( C Y ~ D ) . ~ firmed. 1. The Potential Barrier Height H. The analysis of the paOn the other hand, the length difference between the axial and rameter values of the ring-puckering potential in the excited v(CH) equatorial C H bond can be evaluated from the measured Ar(CH) states shows a slight increase in the quadratic and quartic terms values and the Pr(CH)/Ar(CH) correlation determined for similar
A
& d
I
I
I
-
+
--
--
+
2164
2152.5 I
2180.5
I
Figure 12. Comparison between experimental (a) and calculated (b) Raman spectra in the u(CD) region of gaseous cyclopentene Cy3D. (u = 0 V(x) = - 2 6 8 5 3 ~+~ 9 1 0 . 7 ~+~781 449x4) cm-l (1) new assignment (this work). ( U C = ~ 1: V ( x ) = - 2 6 0 3 3 ~-~7300x3 + 737 245x4) cm-I (2) previous assignment6 (UCD = 1: V ( x ) = - 2 6 8 7 5 ~-~4 6 2 8 9 + 770971x4) cm-I.
8538
J. Phys. Chem. 1991, 95, 8538-8541
compound^.^ It is found to be (2.8 f 0.2) X A for Cy3H and (1.5 f 0.1) X A for Cy4H. These values are in good agreement with the a b initio calculated ones (2 X 10” A and 1 x 10-3 Q.3 3. Comparison with the u(CD) Results. As already noted, the previous study of the v(CD) spectra of the monodeuterated cyclopentene-3dl (Cy3D)* was faced with the delicate problem of the presence in this spectral region of harmonic or combination bands. As seen above, the v(CH) spectral region is free from these parasitic vibrations and is able to guide and to improve the analysis of the v(CD) spectral region of Cy3D. The axial and equatorial v(CD) frequencies calculated from the axial and equatorial u(CH) frequencies with the isotopic ratio 1.34 are 2152.5 and 2188 cm-l, respectively, for Cy3D, and 2188 and 2207.5 cm-’, respectively, for Cy4D. These frequencies effectively correspond to the frequencies of bands present in the v(CD) spectra of C Y ~ DHowever, .~ the authors have assigned the two intense bands at 2152.5 and 2176 cm-I to axial and equatorial v(CD) vibrations, respectively, and the band at 2188.5 cm-l, in spite of its relatively high intensity, to a combination or harmonic vibration because of its temperature behavior (Figure 12). But this latter is very similar to that of the equatorial v(CH) band in the monohydrogenated cyclopentene~.~~ A calculation based on the assignment of the band at 2152.5 cm-I to the axial v(CD) and of the band at 2188.5 cm-I to the equatorial v(CD) leads to a new ring-puckering potential which still presents a reversed asymmetry compared to the ground-state potential. The Raman spectrum calculated with this new potential reproduces rather well the experimental one (Figure 12). The band at 2176 cm-’, whose frequency and intensity are not very well calculated, could, however, correspond to the transition )0,3) 11,3) calculated a t 2173 cm-l, perturbed by a Fermi resonance with a combination or harmonic band at the same frequency. This latter feature would be at the origin of the shoulder observed at 2170 cm-l in the experimental spectrum which was not calculated. A study of the v(CD) overtone spectra could confirm this assignment. However, it is more difficult to perform this experiment than it was for u(CH) because of the weaker anharmonicity of the v(CD) vibration.
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(25) Cavagnat. D.; Cavagnat, R.; Comut, J. C.; Banisaeid-Vahedie, S. To be published.
Conclusion In this study, the analysis of the infrared and Raman spectra of the C H stretching vibration of gaseous monohydrogenated cyclopentenes-3hl (Cy3H) and -4hl (Cy4H) in the fundamental and in the two first overtone spectral regions shows that the fundamental u(CH) spectral region is not perturbed by harmonic or combination bands of lower frequency vibrations, contrary to what happens in the v(CD) spectral region for monodeuterated cyclopentenes.6 Each of the observed bands of these spectra can thus be assigned to different v(CH) transitions between two ring-puckering potentials in the ground and in the first excited v(CH) states. The asymmetry of the ring-puckering potential in these excited states increases in magnitude but is not reversed in sign compared to the ground-state potential. This constitutes a further argument to c o n f m the hypothesis of the vibrational origin of this asymmetry. In the fundamental v(CH) region, the most intense lower frequency band, due to the transition l0,O) ll,O), is assigned to the axial v(CH), and the most intense higher frequency band, due to the transition l0,l) Il,l), is assigned to the equatorial v(CH). The more or less intense intermediate frequency lines are due to the transitions l0,l) 11,n’) with n = n’ > 1. This assignment is corroborated by the comparison between experimental and theoretical calculated infrared v(CH) profiles. The v(CH) first overtones are similarly analyzed. A linear increase of the asymmetry in the puckering potential is observed with increasing stretching vibrational quantum number v and leads to a grouping of the transition frequencies around the axial and equatorial frequencies, in good agreement with the experimental observation. The anharmonicity value is found to be slightly higher for the C H bond in the 3-position than for the CH bond in the 4-position but almost equal for the axial and equatorial conformation. A coherent interpretation of the v(CH) and v(CD) spectra of the monosubstituted isotopic derivatives of cyclopentene in the gas phase is thus obtained.
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Acknowledgment. We thank very much R. M. Cavagnat, J. Derouault, and T. Buffeteau for their help during the spectra recording. We are also indebted to Y. Guissani and J. C. Leicknam for the calculation of the infrared band profiles and to L. Lespade for helpful discussions about overtone spectra. We are grateful to J. C. LassEgues for a critical reading of this paper. Registry No. Cy3H, 133931-44-9; Cy4H, 133931-45-0.
Zero-Field Splitting of the LiganbLIgand Charge-Transfer Emitting Manifold of Zinc( I I ) Complexes Shigeru Ikeda, Seiichi Yamamoto, Kaoru Nozaki, Takeshi Ikeyama; Tohru Ammi,* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan
J. A. Burt, and C. A. Crosby* Department of Chemistry, Washington State University, Pullman, Washington 991 64-4630 (Received: March 22, 1991; In Final Form: May 20, 1991) Zero-field splittings of the emitting ligand-ligand charge-transfer (LLCT) states were observed for Zn(4-CH30-PhS)2(phen) and Zn(4-CI-PhS)2(phen). The spin-axis-independent zero-field-splitting parameters D* are 1.63 GHz for Zn(4-CH30PhS)2(phen) and 2.25 GHz for Zn(4-CI-PhS)2(phen). The measurements confirm the triplet nature of the emitting state at low temperature. The calculated splittings due to spin dipolespin dipole interaction are much smaller than the experimental values. Mixing with the phenanthroline-localized %T* state is suggested to account for the large measured values of D*.
Introduction Mixed-ligand complexes of the type Zn(X-phS)&hen) (where X-phS is a substituted benzenethiol and phen is ],lo‘Department of Chemistry, Miyagi University of Education, Sendai 980, Japan.
0022-3654/91/2095-8538$02.50/0
phenanthroline) exhibit a broad emission band in the visible region of the spectrum. The emitting excited state has been assigned by Crosby et al.l-’ to a ligand-ligand charge-transfer (LLCT) (1) Truesdell,
K.A.; Crosby,G. A. J . Am. Chem. SOC.1985,107, 1788.
0 1991 American Chemical Society
