3946 Table 111. Optical States of Bicycle[ 1 .I .O]butane Vibronic State components Frequencyo range, cm-I Oscillator strength Assignment ~ A Bh CC D E ff, P 6 43 600-48 160 48 160-57875 57875-61780 61 780-65000 6 500-74000 5.3 x 10-4 7.4 x 10-2 2.2 x 10-2 2.8 x 1.8 x 10-2 Az BI A2+? ? ? a Frequency range for integration to give oscillator strength. The y bands ride on this broad transition. The 6 bands appear to ride on The 6 bands are assigned as Az, and the underlying continuum presumably a continuum. The oscillator strength for just the 6 bands is 6.6 X corresponds to a different state. 120 lock-in amplifiers which used the 7-Hz signal from the beam modulator driver as the reference. The sample and reference signals (I and lo) were digitized and re- corded using a PDP- 12 computer. In order to improve the signal to noise ratio, the signals were sampled many times (typically 50- 1000) and the Ill0 ratio was derived from the averaged signals. The wave- length marker also was recorded by the computer. After a scan, the sample cell was emptied and the base line was recorded in the same fashion. The final corrected spectrum was obtained as the ratio of the //lo values for the sample run and the base line run. The spectra shown were plotted using the above data (stored on magnetic tape) as input, along with the sample pressure, cell length, and temperature. The optical cross sections are defined by: where the pressure is in millimeters and Lo is the Loschmidt number (2.6868 X IOl9 cm3). This gives the cross section in centimeters*. For convenience, the cross sections in the figures are reported in megabarns (Mb = cm-2). Spectral intensities are often expressed in terms of absorbancy index (A) and this is related to the cross section by A = 257.80 X IO1* uv A comparison of our observed cross sections with those reported for oxygen16 and ethylenel’ indicated an uncertainty of about f20%. The spectra were plotted using a Complot incremental plotter driven by the PDP-12. This allowed any desired section of the spectra to be plotted using any desired plot size. The MO energy levels and wave functions were calculated using GAUSSIAN-7018 and the experimental geometry. The wave functions were plotted using the program written by Jorgensen.I9 References and Notes (1) Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under Grant No. AFOSR-72-2239. (2) Taken in part from the Ph.D. thesis of G.B.E., Yale, 1974. (3) Taken in part from the Ph.D. thesis of K.S.P., Yale, 1975. (4) D. M. Lemal, F. Menger, and G. W. Clark, J. Am. Chem. SOC.. 85, 2529 (1963); E. Vogel, Angew. Chem., 68, 640 (1954). (5) R. Srinivasan. J. Am. Chem. SOC., 85, 4045 (1963). (6) K. B. Wiberg and K. S. Peters, Spectrochim. Acta, in press. (7) F. Birss et al., Can. J. Phys., 48, 1230 (1970). (8) K. W. Cox, M. D. Harmony, G. Nelson, and K. B. Wiberg. J. Chem. Phys., (9) M. Newton and J. Schulman, J. Am. Chem. SOC., 94,767 (1972). 50, 1976 (1969). (IO) G. B. Ellison, Ph.D. Dissertation, Yale University, 1974. (1 1) G. Orlandi and W. Seibrand, J. Chem. Phys., 58, 4513 (1973). (12) P. D. Foo and K. K. Innes, J. Chem. Phys., 80,4582 (1974). (13) D. A. Ramsey in “Determination of Organic Structures by Physical Meth- ods”, Nachod and Phillips, Ed., Vol. 11, Academic Press, New York. N.Y., 1962, pp 317-318. (14) E. F. Pearson and K. K. Innes, J. Mol. Spectrosc.. 30, 232 (1969). (15) H. P. Knauss and S. S. Ballard, Phys. Rev., 48, 790 (1935). (16) A. J. Blake, J. H. Carver, and G. N. Haddad. J. Ouant. Spectrosc. Radiat. Transfer, 8, 451 (1966). (17) M. Zelikoff and K. Watanabe, J. Opt. SOC. Am., 43, 756 (1953). (18) W. J. Hehre, W. A. Lathan, R. Ditchfield, M. D. Newton, and J. A. Pople, Program No. 236, Quantum Chemistry Program Exchange, University of Indiana. (19) W. L. Jorgensen and L. Salem, “The Organic Chemist’s Book of Orbitals”, Academic Press, New York, N.Y., 1973. Electronic States of Organic Molecules. 5. High- Resolution Spectrum of the A State of Bicyclo[ 1.1 .O]butanel Kenneth B. Wiberg,*3a Kevin S. G. Barney Ellison,*b*3a and F. Alberti3b Contribution from the Department of Chemistry, Yale University, New Haven, Connecticut 06520, and the National Research Council, Ottawa, Canada. Received August 9. 1976 Abstract: The A state of bicyclo[ 1.1 .O]butane was examined with a resolution of 0.3 cm-’, permitting the rotational band con- tours for the vibronic components to be observed. The geometry change accompanying this electronic transition was deduced by an analysis of the band contours. The band origin was located (39 477 cm-I) and was found to be at lower energy than the first transition of ethylene (43 769 cm-l). In the preceding paper,4 we presented survey spectra for bicyclobutane and made an initial set of assignments for the observed elect_ronic statetof this saturated hydrocarbon. The lowest state (A) and the C state were characterize! by exten- sive vibronic activity. The system of bands in the C state was labeled the “6” szries while the components with alternating intensity in the A state were referred to as the CY,^' bands. Both of these states were considered to be optically forbidden under one-photon dipole selection rules because they are so weak and were assigned as A2 under the point group C2u. Journal of the American Chemical Society / 99:12 / June 8, I9773947 r 45225 I RESOLUTION 0.3 CM-' BICYCLOBUTANE H6 44861 44681 Figure 1. The a4 and p3 bands of bicyclobutane-do (from left to right). Figure 2. The a5 and 04 bands of bicyclobutane-do. A transition to an optically forbidden state may be observed if intensity is borrowed from a nearby allowed transition. This borrowing of intensity results from mixing of the two states by a vibration. In vibronic coupling, the transition moment has the same symmetry as the state from which the intensity is b~rrowed.~ It this paper we examine the individual vibronic bands of the A state under high resolution. From an analysis of the band contours via calculation of the rotational envelopes, we are able to esjablish the symmetry of the active vibrations that couple the A state and the symmetry of the strong optical state that donates intensity. Experimental Section The vacuum spectrometer used for recording the spectra is a IO m Eagle with a 600 line/" grating yielding a dispersion of 0.75 A/mm in first order. The resolving power is 200 000 which results in a reso- lution of