J. Phys. Chem. 1987, 91, 4235-4237 the lifetime: photoi~n-photon~~ and photoelectron-photon34 coincidence measurements show the variation of the fluorescence quantum yields and the pronounced conexponen_tialdecay curves as a result of coupling between the A22+ and B2nistates. The vibronic levels above the intersection point seem to be accurately described in the adiabatic approximation, at least in this case. This might be an explanation for the resolved v , and v 3 vibrational modes above 13.56 eV. According to the calculations of Cederbaum et a1.I8 the inclusion of totally symmetric modes in describing diabatic effects is crucial, as they lower the minimum of the locus of intersection, thus enhancing the mixing of states.
4235
High-resolution PE spectra of the three lowest system of BrCN and ICY have enabled us to reveal spin-orbit coupling net only in the X(211i) but also in the vibrationally complicated B(211i)
systems. In addition, the results have shed more light on some phenomena that cannot be understood if conventional FranckCondon principle were valid. The high-resolution spectra of the 211iand 2Z+ systems indicate the activity of the bending mode ( v 2 ( ~ ) )It. gains intensity through vibronic mixing either within or between states of proper symmetry. The strength and importance of such diabatic effects is dependent on energy separation. If the separation is much greater than the energy of the inducing normal mode, the vibrational pattern can still be analyzed wiLhi_n the adiabatic approximation, as seems to be the case in the A/B systems of BrCN. However, it is expected to lead to complete breakdown of the vibrational progression in that part of the spectrum which corresponcis io the closely spaced electronic systems, as observed in the A/B systems of ICN. According to the Heidelberg group'* the role of the totally symmetric modes in bringing about such nonadiabatic effects is crucial.
(33) Castelluci, E.; Braitbart, L.; Dujardin, G.; Leach, S. Furuduy Discuss. Chem. Soc. 1983. 75. 90. . (34) Maier, J.' PJ'Ochsner, M.; Thommen, F. Faraday Discuss. Chem. SOC.1983, 75, 11.
Acknowledgment. The author thanks Prof. L. Klasinc for drawing her attention to the problem and Prof. T. Cvitai for careful reading of the manuscript. This work was supported by the Republic Council for Science (SIZ-2) of S.R. Croatia.
Conclusion
~
~~~
~
~~
Microwave Studies of eCaprolactone C . Donald Cogley The Pennsylvania State University, The Behrend College, Erie, Pennsylvania 16563 (Received: January 12, 1987)
Frequencies of 60 a-type R-branch rotational transitions were measured for e-caprolactone. Rotational constants derived from the data for the cis-chair conformation were 3201.25 & 0.05, 1920.51 f 0.05, and 1348.73 f 0.05 MHz. Higher energy conformations were not observed, in agreement with predictions of molecular mechanics calculations. Observation of vibrational satellites for a number of lines indicates a small-amplitude, low-frequency vibration.
Introduction
Molecular mechanics calculations have been used with success in recent years to predict conformations of small molecules and microwave spectroscopy has provided experimental verification of these predictions in some cases.1v2 Calculations for ecaprolactone (I) indicated four conformations of the molecule would
uo I
be present in equilibrium in the gas phase with the boat, half-chair, and trans conformations predicted to have energies 2.72, 4.24, and 5.32 kcal/mol relative to the ground-state cis-chair conformatiom2 It was expected, if the relative energy ratios were correct, that only the most stable conformation would give an observable microwave spectrum. We report the results of our microwave investigations which are in agreement with the predictions of the molecular mechanics calculations. Experimental Section
The sample of e-caprolactone was purchased from Aldrich Chemical Co., Milwaukee, WI, and used without purification except for pumping on the sample to remove air and volatile (1) Philip, T.;Cook,R. L.; Malloy, T. B., Jr.; Allinger, N. L.; Chang, S.; Yuh, Y . J. Am. Chem.Soc. 1981, 103, 2151-56. (2) Allinger, N. L. Pure Appl. Chem. 1982, 54, 2515-22.
0022-3654/87/2091-4235$01.50/0
impurities. Microwave spectra were recorded on a HewlettPackard Model 8460 A Stark modulated microwave spectrometer in the R-band (26.5-40.4 GHz) region. Experimental difficulties in obtaining good spectra were encountered due to the low vapor pressure (58.5 Pa at 298 K) of the sample. It was not possible, for example, to increase the intensity of lower J transitions expected in K band (18-26.5 GHz) or X band (8.0-12.4 GHz) by cooling. Since accurate measurements of the M components of low J transitions were precluded by their low intensities, it was not possible to obtain any quantitative dipole moment components for the molecule. A fast scan of the R-band region gives a spectrum dominated by a-type, R-branch transitions, as shown in Figure 1. The five prominent, equally spaced lines led to the initial assignment of the spectrum. Each of these five strong lines is actually due to a pair of overlapping transitions of the type ( J + l)o,J+l Jo,J and ( J + l)l,J+l J 1 , ) The highest frequency line of this type, near 38 794 MHz, arises from J = 14 13 transitions and could not be resolved. When slow scans were run at the lowest possible 90,9and the working pressures (1.33 Pa), the 91,9transition pair at 28 005 MHz were split by 0.6 MHz. Measurement of the splitting between the J = 10 9 transitions and of the separation between the prominent lines in the low-resolution scan gave enough data to calculate line positions for other transitions using a rigid-rotor computer-fitting program. As additional lines were found close to the predicted positions, the data set was expanded and refined until self-consistent results were obtained. Transition assignments, observed line positions, and the difference between observed and calculated line positions
-
0 1987 American Chemical Society
-
-
-
-
-
4236 The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 1
1
1
1
1
27
I
1
1 33
1
1
1
1
1
1
1
39 GHz
I
Cogley TABLE I: Rotational Lines Used in the Determination of Rotational Constants for the Cis-Chair Form of e-Caprolactone" transition obsd obsd - calcd
---
84,s 82,6
%8
74,4 72,s
+-
82,7
91,s 81,7 84.4 74,3 1 0 ~ 1 0 91,9 100,io 90,9 83.5 73.4 +-
I
+
Figure 1. Low-resolution scan of the R-band microwave spectrum of e-caprolactone (pressure = 14 Pa; Stark field = 200 V/cm). Prominent lines are a-type R-branch transitions of the cis-chair conformation.
v=o
93j7
92.7 lO2.9 l01,9 9436
96,3
83.6
82.6 92,8 91,8
+
84,5 86,2
9% 85,4 95.4 +- 85,3 110,Il 100,IO 111,Il 10lJO 94.5 +- 84.4 +
v=I
93,6
83.5 93,7
1°2,8
92,7 l01,9 1°2,9
1.10 112,10
lo,,? 10s,2 108,3
---
94,6
98.1 98.2
+
120.12 12l,12 107.4
+
+
IO6.5
1°6,4 1°5,6 I
I
0
1
IO0
200
Figure 2. The 1 4 0 , ~ ~130,13/141,~4 131,13transition pair with four vibrational satellites. Frequencies in MHz relative to the u = 0 line at 38 793.96 MHz. +
+-
are listed in Table I. Uncertainties in the frequencies listed in Table I were 0.08 M H z or less and represent two standard deviations for 8 or 10 scans through each line. Rotational constants and the asymmetry parameter, K , calculated from the line frequencies are listed in Table I1 along with the corresponding values for conformations based on molecular mechanics calculations. Calculated dipole moment components for each conformation are also included in Table 11. While it was not possible to obtain any dipole moment components from measurement of low J transitions, the movement of the M envelope of the Stark lobes of the strongest transitions was observed qualitatively. A very fast, mirror-image Stark effect was observed for the strong transition pairs. Indeed, the neardegenerate 140,,4 130.13 and 14,,14 131,13transition pair had Stark lobes that were modulated at Stark fields of only about 10 V/cm, one of which moved to lower, and one to higher, frequency. This is indirect evidence for a substantial c component of the dipole moment, since a large Stark effect is to be expected for neardegenerate levels connected by pLc. A number of the strongest transitions were accompanied by a series of closely spaced lines moving off to higher frequency, as shown in Figure 2 for the J = 14 13 transition pair near 38 794 MHz. These series of lines were not predicted by the fitting procedure used to determine the rotational constants and were therefore assumed to be vibrational satellites associated with a low-frequency ring vibration. Values of the assumed vibrational quantum numbers are marked in Figure 2. Transition assignments, observed line positions, and the difference between observed and calculated line positions are listed by assumed vibrational quantum number in Table 111. New values of the rotational constants were calculated by computer program for each vibrational quantum number and these results are listed in Table IV.
-
+-
+-
105,s 1 13,9
l10,Il
111,Il 97,3 99j4
96,3
95,5
+
+
95,4 103,8
1°3,7 l2.9 122,ll 121,Il
93,6
1°4,6
94,5
l02.8 112.10
111,IO
1 3 0 ~ 3 120,12 l3l,l3 12l,l* +-
---
+
lI9.2 l9.3 117.5
1°4,7 1°9,1 1°9,2
107.4
1 17.4 +- 107,3
lI5.7 l6,6 l6.5 123.10
-
1°5,6
1°6,5 IO6.4
l3,9 l2,9
122,10
113,8
1°3,7
l32,12 122.11 131,12 12i.ii l5.6 1°5,5 140,14 130,l3 141,14 131.13 l4,7 lO4.6 124,9 l4,8 +
+-
+
'Frequencies in MHz
26 88 1.67 27 295.40 27 347.25 27 437.72 27 725.81 28 005.26 28 005.86 28 464.35 29 141.19 29 932.31 30 063.29 30 101.00 30 175.48 30 225.90 30 359.71 30637.88 30 702.60 30 702.60 31 683.66 31 877.49 32000.35 32468.47 32 782.03 32 767.09 33 360.88 33 374.93 33 374.93 33 399.75 33 399.75 33 507.32 33 694.06 33 763.23 33 806.87 34464.53 34 786.27 34 960.34 35 026.26 35 465.90 35 47 1.41 35 572.41 36 096.85 36096.85 36424.11 36 690.49 36 690.49 36 986.22 37 000.63 37 195.20 37 202.04 37 402.03 37 525.30 37 636.14 37710.32 38 162.68 38 164.48 38 465.86 38 793.95 38 793.95 39222.65 39 369.28
0.18 0.21 0.28 0.26 0.06 0.41 0.03 0.03 0.16 0.18 0.23 0.10 0.10 0.23 0.17 0.07 -0.05 0.28 -0.17 -0.07 0.06 0.08 0.05 0.10 -0.08 0.63 0.74 -0.03 0.08 0.57 0.17 0.06 -0.02 -0.27 -0.02 -0.19 0.02 0.10 -0.10 -0.46 -0.19 -0.15 -0.22 0.84 0.85 0.17 0.01 -0.32 -0.15 -0.30 -0.16 -0.14 -0.22 -0.10 -0.41 -0.76 -0.41 -0.40 -0.72 -0.41
Uncertainties in measured frequencies 5
0.08 MHz.
Discussion Comparison of experimentally determined and calculated values of the rotational constants in Table I1 confirms the prediction that the cis-chair conformation of t-caprolactone is the most stable. The structure of this conformation is shown in Figure 3. Most of the measurable lines in the R-band spectrum are accounted for by calculations based on the rotational constants of the cis-chair form and no evidence was found for any lines that would correspond to the other possible conformations. The large c-component of the dipole moment calculated for the cis-chair form also agrees with our qualitative observation of a fast, mirror-image Stark effect
The Journal of Physical Chemistry, Vol. 91, No. 16. I987
Microwave Studies of eCaprolactone
4231
TABLE II: Values of Rotational Constants and Dipole Moment Components for r-Caprolactone Conformations from Experimental Work and Molecular Mechanics Calculations" A B C K Pa Pb PC Plat exvt cis-chair boat half-chair trans
3201.25 3144 2994 2624 3324
* 0.05
1920.51 f 0.05 1942 2015 2414 1860
1348.73 f 0.05 1363 1485 1629 1357
-0.383 -0.349 -0.298 0.578 -0.488
3.52 3.28 2.06 3.81
I .44
0.04 0.33 0.04 0.15
1.90 1.46 0.1 1
3.80 3.80 2.52 3.82
" Rotational constants in MHz, dipole moments in Debye. TABLE IV: Values of Rotational Constants, in MHz, by Vibrational State
TABLE III: Rotational Lines Used in the Determination of Rotational Constants by Vibrational State" vibrational auantum number
transition
3
obsd
obsd - calcd
28041.06 28 041.78 30 097.70 30 132.61 30742.15 30 742.15 32 805.07 32817.36 33 442.94 33 442.94 35 507.35 35 512.85 36 143.72 36 143.72 37 565.79 37 675.00 37741.79 38 207.84 38 209.77 38 844.50 38 844.50 30 778.66 30 778.66 33 482.77 33 482.77 36 186.98 36 186.98 38251.39 38 249.55 38891.16 38891.16 37 603.13 35 545.68 35 551.06 37705.33 37 770.64 33 519.96 33 519.96 36 227.34 36 227.34 38 288.41 38 290.25 38 934.66 38 934.66 37 637.84 37 738.56 37 797.37
0.49 0.25 0.44 -1.74 0.46 0.13 0.38 -1.98 0.24 0.13 0.26 0.19 0.03 -0.01 0.19 0.32 0.40 0.13 0.01 -0.20 -0.21 0.23 0.54 0.17 0.27 0.07 0.1 1 0.07 0.19 -0.1 1 -0.10 -0.14 0.28 0.3 1 -3.65 2.00 0.26 0.36 0.18 0.22 0.31 0.23 -0.01 -0.00 0.07 -3.36 1.82
Frequencies in MHz. Uncertainties in measured frequencies 5 0.08 MHz.
-
for the ( J + l)o,J+l Jo,Jand ( J + l)lJ+l JIJtransition pairs. The boat and the half-chair forms are predicted to have substantial c-dipole components but their predicted rotational constants do not agree well with the values derived from our spectra.
vibrational auantum number
A 3201.2 3198.3 3193.8 3191.8
0 1
2 3
'
0
B 1920.5 1922.9 1925.6 1927.5
C 1348.7 1350.6 1352.2 1353.8
Lr
Figure 3. Structure of the cis-chair conformation of c-caprolactone. Small circles represent H atoms; intermediate circles C atoms; and large circles 0 atoms.
The rotational constants for the trans-chair form agree with our values fairly well but the trans form is predicted to have a small c-dipole component. If this conformation were to be observed, transitions would be modulated only at much larger Stark fields than were actually used in the experiment. We conclude that the observed microwave spectrum of t-caprolactone is due solely to the chair conformation. Finally, we note, from the values listed in Table IV, that the rotational constants vary smoothly with assumed vibrational quantum number. This behavior is an indication of a small-amplitude, low-frequency vibration which has been observed in other small-ring molecule^.^ Acknowledgment. We thank the Langley Research Center, Hampton, VA, for an equipment loan which has aided us greatly in our studies. We also thank Professor Robert L. Cook, Department of Physics, Mississippi State University and Professor Norman L. Allinger, Department of Chemistry, University of Georgia for helpful discussions and correspondence during this project. Registry No. I, 502-44-3.
+-
(3) Malloy, T. B., Jr.; Bauman, L. E.; Carreira, L. A. In Topics in Sfereochemisrry, Vol. 11, Allinger and Elid, Eds.; Wiley: New York, 1979.
