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Raman spectra of gase
423 Journal of Molecular Structure, 17 (1973) 423-425 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
RAMAN
SPECTRA
OF GASES
V.* BROMOCYCLOBUTANE
J. R. DURIG, A. C_ SHING** AND L. A. CARREIRA Department
of Chemistry,
University
of South Carolina,
Columbia,
S.C.
29208 (U.S.A.)
(Received 5 February 1973)
In a recent series of papers’ -4, we have demonstrated that Raman spectroscan be a powerful technique for determining the ring puckering potential functions for small ring compounds. As a part of these studies we have investigated the low frequency Raman spectrum of bromocyclobutane. The Raman spectrum of gaseous bromocyclobutane in the region of the ring puckering vibration is shown in Fig. 1. We find that the broad peak centered at 143.3 cm-’ observed by Blackwell et al. ’ in the far infrared spectrum of bromocyclobutane is actually composed of two lines centered at 149.0 cm-’ and 139.5 cm-‘. The other observed ring puckering transitions at 134.8, 128.5 and 124.0 cm-’ agree reasonably well with the far infrared results.
copy
Fig. 1. Low frequency Raman spectrum of bromocyclobutane of 4 cm-’ at its vapor pressure at room temperature.
recorded with a spectral slitwidth
* For Part lV, see ref. 4. ** Taken in part from the thesis of A. C. S. which was submitted to the Department of Chemistry in partial fulfillment of the Master of Science degree.
424 In order to verify these data we have also recorded the Raman spectrum of bromocyclobutane-d, . This spectrum was found to be somewhat weaker and only
the first three ring puckering transitions were observed at 147, 138 and 131 cm-‘. The bromocyciobutane-d, frequencies confirm our assignment of the 0 ---, 1 and 1 --, 2 transitions in the “light” compound. A least squares fit of the observed frequencies to a calculated potential of the form V(x) = ax4+cx3 --x2 can be seen in Table I. The least square potential TABLE
1
CALCULATED
AND
OBSERVED
RING
PUCKERING
TRANSITIONS
OF BROMOCYCLOBUTANB
AND
BROMO-
CYCLOBLJTANE-d,
Transition
Infrared (cm-‘)
Raman (cm-‘)
Calculated”
143.3
149.0 139.5 134.8 128.5 124.0
146.5 141.4 136.1 129.9 122.8
do O-l l-2 2-3 34 4-5
135.5 129.2 124.0
d, 147 138 131
O-l 1-2 2-3
D For the potential function, see text.
1000-
0% -.40
e
-.20
X-+
0
.2a
Fig. 2. Potential function for the ring puckering wbration of bromocyclobutane calculated using -the potential function V = 3.88 x 105x4+1.10x 105x3-1.35~ 104x2 and a reduced mass of 214.4 amu. Y is in wavenumbers (cm-‘) and x is in A.
425
is determined to be V(X) = 3.88 x 105x4 - 1.35 x 104x2 + 1.10 x 105x3, in which X, the puckering coordinate is in AngstrGms and V(X) is in cm- I_ This potential
function was determined using a reduced mass of 214.4 amu’. The shape of this potential is shown in Fig. 2. It is believed that the transitions observed belong to the equatorial isomer on the basis of the microwave study6*7_However it is not possible from our data to determine the equilibrium angle of the ring. If one assumes planarity at x = 0, an unreasonable angle of puckering is calculated. However, for the asymmetric potential function determined, it is not permissible to assume that V(X) has a m~imum at the pfanar position, and in fact it is a poor approximation to do so. Hence, the value of x (equilibrium) calculated from dV(,)fdx = 0 at x = 0 can be far enough off to give an unrealistic ~(equiiibrium) and puckering angle. The unfortunate fact is that when symmetry does not give dV/& = 0 at the planar position, it is not possible to determine the puckering angle from the far infrared transitions. It was concluded from microwave studies that this molecule is puckered by 29” which seems slightly large relative to the values obtained for the puckering angle in other halocyclobutanes’. The fact that the 0 3 1 and 1 + 2 transitions were not resolved in the far infrared spectrum seems to be due to instrumental problems in this spectral region’, However, the new data lead to a potential consistent with those found for other mono-substituted cyclobutanes2. The authors gratefully acknowledge the financial support given this work by the National Science Foundation by Grant GP-20723. The Raman spectrophotometer was purchased with funds from a National Science Foundation Grant GP-28068.
I 2 3 4 5
6 7 8
J. R. DURIG AND L. A. CARREIRA, J. Cftenz.Phys., 56 (1972) 4996. J. R. DURIG, L. A. CARREIRA AND J. N. WILLIS JR., 3. Cfiem. Nzys., 57 (1972) 275.5. L. A. CARREIRA, R. 0. CARTER AND 3. R. DURIG, J. Chem. Phys., 57 (1972) 3384. J. R. DURIG, A. C. SHIN& L. A. CARFEIRA AND Y. S. Lr, J. Cfzern.P&s., 57 (1972) 4398. C. S. BLACKWELL, L. A. CARREIRA,3. R. DURIG, J. M. KARRIKER AND R. C. LORD, J. Chem. Fhys_ 56 (1972) 1706. W. 6. ROTHSCHILD, J. Chem. Whys., 44 (1966) 2213; 4.5 (1966) 1214. W. G. ROTHSCHILD AND B. P. DAILEY, J. C&em. P&s., 36 (1962) 2931. H. IQM AND W. D. GWEN& J. Ckem. Phys., 44 (1966) 865.
RAMAN
SPECTRA
OF GASES
V.* BROMOCYCLOBUTANE
J. R. DURIG, A. C_ SHING** AND L. A. CARREIRA Department
of Chemistry,
University
of South Carolina,
Columbia,
S.C.
29208 (U.S.A.)
(Received 5 February 1973)
In a recent series of papers’ -4, we have demonstrated that Raman spectroscan be a powerful technique for determining the ring puckering potential functions for small ring compounds. As a part of these studies we have investigated the low frequency Raman spectrum of bromocyclobutane. The Raman spectrum of gaseous bromocyclobutane in the region of the ring puckering vibration is shown in Fig. 1. We find that the broad peak centered at 143.3 cm-’ observed by Blackwell et al. ’ in the far infrared spectrum of bromocyclobutane is actually composed of two lines centered at 149.0 cm-’ and 139.5 cm-‘. The other observed ring puckering transitions at 134.8, 128.5 and 124.0 cm-’ agree reasonably well with the far infrared results.
copy
Fig. 1. Low frequency Raman spectrum of bromocyclobutane of 4 cm-’ at its vapor pressure at room temperature.
recorded with a spectral slitwidth
* For Part lV, see ref. 4. ** Taken in part from the thesis of A. C. S. which was submitted to the Department of Chemistry in partial fulfillment of the Master of Science degree.
424 In order to verify these data we have also recorded the Raman spectrum of bromocyclobutane-d, . This spectrum was found to be somewhat weaker and only
the first three ring puckering transitions were observed at 147, 138 and 131 cm-‘. The bromocyciobutane-d, frequencies confirm our assignment of the 0 ---, 1 and 1 --, 2 transitions in the “light” compound. A least squares fit of the observed frequencies to a calculated potential of the form V(x) = ax4+cx3 --x2 can be seen in Table I. The least square potential TABLE
1
CALCULATED
AND
OBSERVED
RING
PUCKERING
TRANSITIONS
OF BROMOCYCLOBUTANB
AND
BROMO-
CYCLOBLJTANE-d,
Transition
Infrared (cm-‘)
Raman (cm-‘)
Calculated”
143.3
149.0 139.5 134.8 128.5 124.0
146.5 141.4 136.1 129.9 122.8
do O-l l-2 2-3 34 4-5
135.5 129.2 124.0
d, 147 138 131
O-l 1-2 2-3
D For the potential function, see text.
1000-
0% -.40
e
-.20
X-+
0
.2a
Fig. 2. Potential function for the ring puckering wbration of bromocyclobutane calculated using -the potential function V = 3.88 x 105x4+1.10x 105x3-1.35~ 104x2 and a reduced mass of 214.4 amu. Y is in wavenumbers (cm-‘) and x is in A.
425
is determined to be V(X) = 3.88 x 105x4 - 1.35 x 104x2 + 1.10 x 105x3, in which X, the puckering coordinate is in AngstrGms and V(X) is in cm- I_ This potential
function was determined using a reduced mass of 214.4 amu’. The shape of this potential is shown in Fig. 2. It is believed that the transitions observed belong to the equatorial isomer on the basis of the microwave study6*7_However it is not possible from our data to determine the equilibrium angle of the ring. If one assumes planarity at x = 0, an unreasonable angle of puckering is calculated. However, for the asymmetric potential function determined, it is not permissible to assume that V(X) has a m~imum at the pfanar position, and in fact it is a poor approximation to do so. Hence, the value of x (equilibrium) calculated from dV(,)fdx = 0 at x = 0 can be far enough off to give an unrealistic ~(equiiibrium) and puckering angle. The unfortunate fact is that when symmetry does not give dV/& = 0 at the planar position, it is not possible to determine the puckering angle from the far infrared transitions. It was concluded from microwave studies that this molecule is puckered by 29” which seems slightly large relative to the values obtained for the puckering angle in other halocyclobutanes’. The fact that the 0 3 1 and 1 + 2 transitions were not resolved in the far infrared spectrum seems to be due to instrumental problems in this spectral region’, However, the new data lead to a potential consistent with those found for other mono-substituted cyclobutanes2. The authors gratefully acknowledge the financial support given this work by the National Science Foundation by Grant GP-20723. The Raman spectrophotometer was purchased with funds from a National Science Foundation Grant GP-28068.
I 2 3 4 5
6 7 8
J. R. DURIG AND L. A. CARREIRA, J. Cftenz.Phys., 56 (1972) 4996. J. R. DURIG, L. A. CARREIRA AND J. N. WILLIS JR., 3. Cfiem. Nzys., 57 (1972) 275.5. L. A. CARREIRA, R. 0. CARTER AND 3. R. DURIG, J. Chem. Phys., 57 (1972) 3384. J. R. DURIG, A. C. SHIN& L. A. CARFEIRA AND Y. S. Lr, J. Cfzern.P&s., 57 (1972) 4398. C. S. BLACKWELL, L. A. CARREIRA,3. R. DURIG, J. M. KARRIKER AND R. C. LORD, J. Chem. Fhys_ 56 (1972) 1706. W. 6. ROTHSCHILD, J. Chem. Whys., 44 (1966) 2213; 4.5 (1966) 1214. W. G. ROTHSCHILD AND B. P. DAILEY, J. C&em. P&s., 36 (1962) 2931. H. IQM AND W. D. GWEN& J. Ckem. Phys., 44 (1966) 865.