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Electronic Spectrum of CyclobutanoneRodger F. Whitlock and A. B. F. Duncan Citation: The Journal of Chemical Physics 55, 218 (1971); doi: 10.1063/1.1675511 View online: http://dx.doi.org/10.1063/1.1675511 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/55/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Water-ketones hydrogen bonding: The rotational spectrum of cyclobutanone-water J. Chem. Phys. 123, 164304 (2005); 10.1063/1.2078767 Vacuum Ultraviolet Photochemistry of Cyclobutanone J. Chem. Phys. 57, 2162 (1972); 10.1063/1.1678546 Molecular Zeeman Effect of Cyclobutanone J. Chem. Phys. 53, 3943 (1970); 10.1063/1.1673863 Microwave Spectrum, RingPuckering Potential Function, Ring Structure, and Dipole Moment ofCyclobutanone J. Chem. Phys. 49, 221 (1968); 10.1063/1.1669813 Proton Coupling Constants in Cyclobutanone J. Chem. Phys. 44, 2209 (1966); 10.1063/1.1727010
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THE JOURNAL OF CHEMICAL PHYSICS VOLUME 55, "UMBER 1 1 JULY 19 i 1
Electronic Spectrum of Cyclobutanone*
RODGER F. WHITLOCKt AKD A. B. F. DUNCANt
Department of Chemistry, University of Rochester, Rochester, New York 14627
(Received 10 July 1970)
The electronic spectrum of cyclobutanone is examined to 78 100 em-I. Nine electronic transitions are found, of which five form an ns Rydberg series leading to an ionization potential of 75444 cm-I (9.354
eV). The remaining four transitions are assigned in order of increasing energy as 1I"*<-n, 1I"*<--n', <Tco*<--n, and 11"*<-11". Earlier vibrational analyses of the first two of these are extended and corrected, and the excitation of the carbonyl stretch mode in the 1I"*<--n' transition is demonstrated. Vibrational analyses are also given for the structure observed in the 11"*<-11" and Rydberg transitions. An analysis of existing data on the electronic and photoelectron spectra of acetone is used to support the general assignment of the second electronic transition of ketones as 1I"*<--n'. Second and third ionization potentials of cyclobutanone are predicted to be about 11. 70 and 13.34 eV.
The electronic spectra of compounds containing the carbonyl group (> C=O), where some localization is present in certain states, have been the subject of many investigations1,2; some of these have shown striking similarities. For example, the lowest energy transition involves excitation of an electron from an n orbital, which is almost completely localized on the oxygen atom, to a 71"* orbital localized in the C-O bond. This weak transition is found at about 3000 A in a large number of compounds.3 However, no cycloalkyl ketone has been investigated in complete detail.
In this work, the electronic spectrum of cyclobutanone has been investigated. Prior work on the electronic spectrum of cyclobutanone has shown the existence of three transitions at wavelengths longer than 1650 A,4-6 of which the two at longer wavelengths have vibrational structure. We have discovered several new electronic transitions and analyzed their vibrational structure, as well as correcting and extending published analyses of the known transitions.
The ground state configuration of cyclobutanone in C2v symmetry may be written
••• (ITal)2 (7I"bl )2 (nb2)2, IA I. (1 )
Higher energy configurations may arise from excitation of an electron to 71"* and IT* orbitals. There are also excited configurations involving the ring orbitals and Rydberg configurations. In addition, the possibility exists that the second lone pair of oxygen, designated n', may also be excited at moderate energies.7
EXPERIMENTAL
The transition near 3300 A was investigated with a 1.5-m Bausch & Lomb Littrow spectrograph, which had a wavenumber dispersion of about 50 cm-l/mm and a resolution better than 5 cm-I . The majority of measurements at shorter wavelengths were made with a 2-m vacuum spectrograph with a reciprocal dispersion of 4.15 A/mm and resolution of about 0.2 A. Some measurements of the transition near 2000 A were made on a 1-m vacuum spectrograph with a reciprocal dispersion of 8.35 A/mm and resolution of about 0.5 A.
A conventional hydrogen discharge lamp was used at wavelengths greater than 1650 A. The xenon and krypton continua were used in the regions 1950-1550 A and 1650-1280 A, respectively. The lamps used for production of these continua were made in this laboratory of fused quartz with LiF windows. It was found that careful degassing of the lamp body and the use of a Ba-AI-Mg getter were essential to the production of continua free from CO emission.
In the case of the 1-m and Littrow spectrographs, cyclobutanone vapor8 was confined in a cell between the light source and spectrograph slit. A quartz cell 1 m long, sealed off with a sample of liquid cyclobutanone in a sidearm, was used with the Littrow spectrograph. The pressure in the cell was varied by holding the liquid at different temperatures. Shorter cells, 15, 17, and 20 cm long, were used with the 1-m vacuum spectrograph; the sample was expanded through a calibrated volume into these cells and pressures were calculated from the volume ratios and equilibrium vapor pressure of the liquid. One of these cells could be electrically heated and was used in hot band studies; its temperature was monitored with a three junction copper-constantan thermocouple and could be kept constant to within 2°C.
The 2-m vacuum spectrograph was used with the sample vapor contained within the spectrograph itself at a low pressure. Vapor was drawn from a reservoir of the liquid and admitted to the spectrograph via a small volume. The pressure was measured with a Veeco thermocouple gauge.
In the investigations carried out with the 1- and 2-m vacuum spectrographs, the sample was routinely degassed before proceeding with an exposure by repeated freeze-thaw cycles and pumping on the liquid itself. This purification procedure was insufficient to remove an impurity which caused sev~ral strong bands to occur on many plates near 1700 A. Measurements of the wavelengths of these bands led to identification of the impurity as biacety1,9 The most prominent bands of biacetyl are reported to be perceptible at a pressure of 2X 10-5 mm Hg in a 4-m path,1O from which it was
218
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ELECTRONIC SPECTRUM OF CYCLOBUTANONE 219
TABLE 1. Vibrational structure of the 3300-1 transition.
n A (n 5 0) B(n 5 2) C(n 5 4)
0 30 979 (106) 31 085 (103) 31 188 (1 281) 32 260 (1 285)
2 33 545 (99) 33 644 (113) 33 757 (1 241) (1 236) (1 208)
3 34 786 (94) 34880 (85) 34 965 (1 226) (1 276)
4 36 012 36 241 (1 237)
5 37 478
(n) G(n 9 0) H(n 9 2) J(n 9 4) 0 31 620 (103) 31 723 (108) 31 831
(1 268) (1 254) (1 281) 32 888 (89) 32 977 (135) 33 112 (1 272) (1 272) (1 260)
2 34 160 (89) 34249 (123) 34372 (1 259) (1 250)
3 35 508 ( 114) 35 622 (1 257) (1 253)
,1 36677 (88) 36 765 (110) 36 875 (1 235)
5 38000
concluded that little biacetyl was present. The biacetyl spectrum in no way obscured the vibrational structure of the cyclobutanone spectrum.
Spectra were photographed on Kodak III -0 and SWR plates and SWR film. Wavelengths in the near uv were measured using lines from an iron arc as standards. Impurity emission lines from the light sources were used as standards at shorter wavelengths. All frequencies reported in this work are in vacuum wavenumbers.
RESULTS
The 3300-A Transition
The near uv, 71'*+-n, transition of cyclobutanone has been previously studied both in solution and in the gas phase.4
-6 The oscillator strength of the transition,
calculated from solution data,6 is about 5X 10-4 • Recent gas phase work by Moule4 was devoted to the region near the origin, particularly the hot band structure, and it was shown that the upper state is slightly nonplanar. However, no consideration was given to the structure of the main body of the transition.
The spectrum between 30 585 and 38 000 cm-I appears to be a superposition of about 75 individual peaks on a continuum which has several broad maxima. The broad maxima correspond to the maxima of the solution spectrum, while the superimposed peaks are resolved vibrational transitions. The resolved vi-
D(n 70) E(n 7 2) F(n 7 4)
(118) 31 306 (104) 31 410 (110) 31 520 (100) (1 276) (1 260) (1 264) 32 582 (88) 32 670 (114) 32 784 (104) (1 264) (1 268) (1 279)
(89) 33 846 (92) 33 938 (125) 34063 (97) (1 234) (1 278) (1 267)
(115) 35 080 (136) 35 216 (104) 35 330 (1 257) (1 255) (1 200)
(96) 36 337 (134) 36 471 (59) 36 530 (147) (1 280) (1 256)
(139) 37 617 (110) 37 727
K(n 11 0) L(n 11 2) M(nl14) (111) 31 942 (93) 32 035 (119) 32 164 (96)
(1 277) (1 280) (1 260) (107) 33 219 (96) 33 315 (109) 33 424 (121)
(1 246) (1 257) (1 274) (93) 34 465 (107) 34 572 (126) 34 698 (88)
(1 279) (1 256) (1 274) (122) 35 744 (84) 35 828 (144) 35 972 (40)
(1 231) (100) 36975
brational transitions have about equal peak-to-valley intensities, judging from microphotometer tracings of spectra taken at a sufficiently high pressure. The separation of the vibrational peaks is near 100 cm- I throughout the spectrum.
Vibrational structure first appears at a pressure of 4 mm Hg in a 1-m path and is confined to the neighborhood of the broad maxima. The bands near the origin (30290 cm-I ) do not appear until the pressure is increased to about 42 mm Hg. (This pressure corresponds to the equilibrium vapor pressure of cyclobutanone at room temperature and was the highest pressure used in this work.) At this pressure total absorption extended from about 32300 cm-1 toward higher frequencies.
The broad maxima, which appear at the lowest pressures, are separated by a frequency difference of about 1260 cm-I . This suggests that this frequency is most significant in the analysis of the spectrum. Starting from this point, it is found that all the observed bands can be arranged in 12 progressions in this frequency. In Table I the progressions in 1260 cm-I, which is here called vI', are designated A, B, "', M. The vibrational excitations in the upper state are described by quanta of (vI' V2' V3').
The unsymmetrical vibration which allows the transition is V2', and must be excited in an odd number of quanta. The transitions to the levels V2' = 1 and V2' = 3 are weak, so the band Ao = (30 979 cm-1 ) is designated (050). Excitation of V3' occurs in double quanta to
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220 R. F. WHITLOCK AND A. B. F. DUNCAN
TABLE II. Vibrational structure of the 2050-A transition.
n A B C D E F G
* 48 529 (89) 48 618 (104) 48722 (92) 48 814 (77) 48 891 (104) 48 995 (80) 49 075 (663) (663) (665) (659) (669) (664) (658)
o 49 192 (89) 49 281 (106) 49 387 (86) 49 473 (87) 49560 (98) 49 658 (75) 49 733 (1 107) (1 126) (1 137) (1 133) (1 123) 50 388 (125) 50 513 (97) 50610 (83) 50 693 (88) 50 781 (1 144) (1 130) (1 116) (1 147)
2 51 532 (111) 51 643 51 809 (119) 51 928 (1 086)
3 52 618
H J K L M N o [49 120 (661) ] 49 781 (107) 49 888 (96) 49 984 (80) 50064 (84) 50 148 (76) 50 224
(1 121) (I 116) (I 125) (1 127) (1 161) 50 902 (102) 51 004 (105) 51 109 (82) 51 191 (118) 51 309
2
3
(n) o
P 50 291 (97) (1 116) 51 407 (125)
(1 116) (1 148) 52 018 (134) 52 152 (1 117) 53 135
Q 50 388 (I 144) 51 532
preserve vibronic symmetry. Thus progressions A, D, G, and K are (n m 0); B, E, H, and L are (n m 2); and C, F, J, and Mare (n m 4).
An alternative assignment of the progressions C, F, J, and M is possible. The ring puckering vibration in the lower state is of such low frequency that its first excited levels are appreciably populated at room temperature. We would reasonably expect that absorption might originate from these ground state levels. ~P3
must then be even to preserve vibronic symmetry and the frequency shift of the hot bands must therefore be near nX 100 cm-I. Such bands would tend to overlap members of progressions C, F, J, and M and would have only slightly lessened intensity than the normal bands. In fact, many of the bands probably owe part of their intensity to hot bands, but we have no direct experimental evidence on this point.
It may be concluded from the data of Table I that the average values of PI', P2', and p/ are, respectively, 1259,317, and 53 cm-I. To obtain actual values of the normal vibration frequencies, these averages would have to be corrected by inclusion of anharmonic and interaction terms; the data are insufficient for evaluation of these quantities. The upper state vibrations correspond respectively to the following ground state vibrationsll .I2 ; V3" (1816 cm-I); P]9" (394 cm-I); 21'20" (94 cm- I ).
The numbering of the levels of P2v is not the same as that of Moule. Our numbering reflects C2v symmetry, and the levels are numbered consecutively. Moule, on the other hand, employed a labeling 0+, 0-, 1+, 1-, ...
in which the integer refers to the correlative level in C. symmetry and the sign refers to the behavior of the wavefunction on reflection in the molecular plane. He employs such a scheme in accordance with his conclusion that 1'2' has a double minimum potential associated with it. We have employed the notation of C2v
symmetry because our findings give no information on the existence of such a double minimum.
The 2oso-A Transition
The second electronic transition of cyclobutanone begins at about 48530 cm-I and extends to about 54950 cm-I, where there is a minimum of absorption. The vibrational structure of the transition includes about 40 resolved vibrational bands and shows pronounced variations in intensity. An incomplete analysis of the structure was indicated by EI-Sayed5 in earlier work. Vibrational structure could not be recognized beyond 54240 cm-I. The structure of the transition is well developed at a pressure of about 1 mm Hg in a 17-cm path. Increase of the pressure to 42 mm Hg results in total absorption at frequencies higher than about 49000 cm-I, while bringing out a weak group of bands near 48 700 cm-I.
An approximate oscillator strength was calculated from the extinction coefficient given by EI-Sayed. Assuming a parabolic envelope between the limits 2030 and 1820 A, a value f~O.03 was obtained. This is comparable to the strengths of analogous transitions in other ketones and indicates that the transition is allowed. Examination of the temperature dependence
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ELECTRONIC SPECTRUM OF CYCLOBUTANONE 221
of band intensities showed that the bands near 48700 cm-I are hot bands.
The frequency difference between these bands and the corresponding bands near 49 400 cm-I is nearly constant and equal to 665 cm-I. This value may be compared with the value of 670 cm-I assigned to 1'9/1 by Frei and GlinthardY Moreover, the ratio of the pressures at which these two groups appear with equal intensity, 0.047, is close to the computed Boltzmann factor at room temperature, 0.041. Thus the origin of the transition must lie among the bands near 49400 cm-I.
While a superficial inspection of the spectrum suggests that a frequency near 500 cm-I is continued throughout, closer examination shows that alternate groups of bands more nearly resemble one another. This suggests that a frequency near 1100 cm-I is also involved. It is found that the most of the observed bands can be arranged in progressions with a principal frequency difference of 1124 cm-I. The progressions are displayed as vertical series B, C, ... , P in the main part of Table II. The series (B-C-D-E-F-G) are related as a whole to series (H-J-K-L-M-N) and (P-Q) by frequency differences of about 504 em-I in (B-H-P) , (C-J-Q) , (D-K), (F,-L) , (F-M), and (G-N). The differences between the individual series (B-C, C-D, etc.) are about equal to 100 em-I and are shown by horizontal differences. Hot bands from the 1'9" = 1 (665 em-I) level of the ground state accompany the initial members of each series B through H and are shown on the first line of Table II (except that corresponding to H, which is placed to the left.) In addition, there are two bands forming series A, which are 89 cm- I below the initial members of the B series. This series A does not have an observable continuation in the 1124-cm-1 frequencies.
The 1124-cm-1 frequency corresponds to the symmetric carbonyl stretch, 1'/'; the 504-cm-1 frequency to the parallel ring deformation 1'9/1.
The magnitude of the horizontal frequency differences clearly indicates that the ring puckering vibration 1'20 is involved. We interpret series A as a hot band arising from the 21'20/1 level, and B-G as successive double quanta in the upper state.
Two bands not included in the analysis of Table II occur at 49328 and 49826 cm- I as shoulders on Bo and Ho. These may be transitions between odd levels of 1'20/1 and 1'20', but the absence of further bands attributable to this make the assignment doubtful. A continuation of such a progression would be surprising in view of the weakness of the two bands actually seen.
Transitions below 1800 A In the region between 55 500 and 59 500 cm- I , cyclo
butanone has an electronic transition of moderate strength. Work by El-Sayed5 indicated an absence of vibrational structure, which was confirmed by the present work. Such a situation implies that the upper
TABLE III. Vibrational structure of the 59 9()(}-67 600-cm-1 absorption.
A 59 962 (571) 60 533 59 962 (1 100) (1 106) (118) 61 062 (577) 61 639 60 080
B 62 450 (528) 62 978 (1 032) (1 022) 63 482 (518) 64 000 (1 066) (1 152) 64 548 (604) 65 152 (1 177) 65 72S
state of the transition is either dissociated or strongly predissociated. The oscillator strength of the transition was calculated from the data of El-Sayed to be P'-'7X 10-3, from which it may be concluded that the transition is allowed.
Between 59900 and 67600 cm-I a region of very strong absorption occurs. There are about 11 broad bands and a maximum of intensity is reached near 63500 em-I. The strong bands in this region first appear at a pressure of 4 J.L Hg in a 4-m path. As the pressure is increased, the absorption spreads to both higher and lower frequencies. Although extinction coefficients were not measured, the strength of the absorption, as judged by the appearance pressure, is of such magnitude that one or more allowed transitions must be involved.
Most of the bands are symmetrical and have a width (at half-maximum) of about 150 cm-I. The wavenumber accuracy of the reported positions is therefore not great. The bands in the high frequency part of the absorption region are so broad that adjacent bands are not clearly resolved. There is also considerable underlying continuous absorption.
The first band of the absorption includes two relatively sharp peaks with a spacing of 118 cm-t, but no recurrence of this doublet structure is observable. The band at 62 450 cm-I also shows sharp structure in the form of a single small peak surmounting the main band and symmetrically placed; again, no continuation of this structure is observable.
Although the bands of this region appear to form a single extended progression, the absorption region is in fact apparently composed of two electronic transitions, which may be analyzed as shown in Table III. The presence of two transitions is suggested by the asymmetry of the absorption envelope. Moreover, the analyses of the two transitions cannot be extrapolated into each other.
In each transition, two frequency differences are involved, a primary frequency of about 1103 cm-I and a secondary frequency of about 574 cm-I in the first case, and about 1040 and 530 cm-I in the second case.
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222 R. F. WHITLOCK A.ND A. B. F. DUNCAN
TABLE IV. Vibrational structure of the Rydberg transitions.
n=5 68 393 (1105) 69 498 68 393 (537) (573) (55)
68 930 (1141) 70 071 68 448 (568) (506)
69 498 (1079) 70 577
n=6 70 958 70 958 (550) (60)
71 508 (1073) 72 579 71 018
n=7 72 337 (574)
72 900
This interpretation is in harmony with the electronic transitions which have been discussed previously. Since the primary frequency is about twice the secondary frequency in both cases, alternate interpretations in terms of single frequencies in the neighborhood of 500-600 cm- l are possible.
The ground state vibrational mode corresponding to the 1100-cm-1 frequency difference probably is the symmetric carbonyl stretch, va" (1816 cm-l ); that corresponding to the frequency differences 574 and 550 cm- l
is the symmetric ring deformation mode, 7J9" (670 cm- l).
The separation of the doublet members corresponds to 27J20" (94 cm-l
).
Cyclobutanone has a region of relative transparency around 1480 A, beyond which its absorption again rises, to the limit of observation at 1278 A. At a pressure of 10 J1. Hg in a 4-m path, absorption is complete below about 1330 A. As the pressure is increased, the absorption spreads to longer wavelengths until at a pressure of 80 J1. Hg there is nearly total absorption even in the region of transparency.
A number of bands occur in this region; while most of these are fairly broad, four of them are sharp and narrow. The first two of these are doublets with separations of 55 and 60 cm- l . The two at higher frequencies also appear to be doublets, but their separations could not be measured. The long wavelength members of these doublets form a Rydberg series, the frequencies of the members of which are accurately expressed (to within 2 cm- l ) by the formula
Pn = 75 444- R/ (n-1.0555)2, n=5-8,
where R is the Rydberg constant. The other bands in this region form vibrational pro
gressions associated with the Rydberg transitions. As with other transitions of cyclobutanone, it is possible to analyze the observed transitions in terms of two frequencies, one about twice as large as the other. This analysis is shown in Table IV. The vibrational bands rapidly become diffuse and weak with increasing vibrational quantum number, so the accuracy of measurement is not too great for many of them.
The Rydberg series formula was found by extrapolation from the known ionization potentialsl3 of cyclohexanone (9.14 eV) and cyclopentanone (9.26 eV) to obtain an approximate ionization potential for cyclobutanone of 9.37 eV. The series formula constants were then refined to improve the fit, giving a final value for the ionization potential of cyclobutanone of 9.354 eV.
The observed electronic transitions of cyclobutanone are summarized in Table V. They are listed in order of increasing energy, following a summary of certain properties of the ground state. The following data are included: assignments, state symmetries, origins of the transitions, frequencies of the three most prominently involved vibrations, and, in some cases, the oscillator strengths.
DISCUSSION
The lowest energy transition may be assigned without doubt as 1['*<c-n (1A2<c-1AI). Its intensity, location, and structure are in harmony with the results for other ketones and aldehydes. The prominence of the carbonyl stretching mode suggests that the bond lengthens on excitation of an electron from an essentially nonbonding (n) orbital to an an tibonding (1['*) orbital.
Four transitions (13, C, D, and J~) are observed in the cyclobutanone spectrum between the 1['*<c-n transition and the Rydberg series member at 68393 cm-I .
We will consider the assignments of these primarily in terms of intravalence transitions, noting that this does not preclude interpretation of some of them as Rydberg transitions also.
The transition 13 has analogs in other ketone spectra, and has been assigned by different authors7
.14 as
crco*<c-n or 1['*<c-n'. The polarizations corresponding to these two assignments are b2 (y) and bl (x), respectively, so direct studies of the polarization of 13 would decide
X A B C E
D F G Ii ]
TABLE V. Summary of data on observed electronic states of cyclobutanone.
Oscillator Assignment Origin Pa Pg 2P20 strength
'A, 1816' 670' 94b
7r*+--n 'A2 30 291 0 1260 980 3X1O-' 7r*+--n' 'B, 49 281 1124 504 104 3XlO-2
(jco*~n 'B2 (55 500--59 500) ( Continuous) 7XlO-a
7r*+--7r 'AI 62 450 (1040) 528 (2XlO- l )
ns Rydberg states n=4 59 962 (1100) 574 118 n=5 68 393 (1108) 546 (55) n=6 70 953 (1073) 550 (60) n=7 72 337 574 n=8 73 170
11 Reference 11. b Reference 12. c Reference 4.
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ELECTRONIC SPECTRUM OF CYCLOBUTANONE 223
the issue. However, we have no data on the polarization of the cyclobutanone spectrum, and such studies would present formidable experimental difficulties.
An argument may be made in favor of the assignment 7r*<c-n' on the basis of the higher ionization potentials of acetone, observed in its photoelectron spectrum. Photoelectron spectroscopy can in principle determine the ionization potentials of all orbitals in a molecule, and Koopmans' theoreml5 equates the observed ionization potentials with the corresponding orbital energies. The second ionization potential of acetone is found to be 12.16 eV.16
If the transition of acetone at 51 181 cm-I, analogous to B of cyclobutanone, is assigned as the 7r*<c-n' transition, then an approximate second ionization potential can be calculated as follows. The energy difference of the 7r*<c-n and 7r*<c-n' transitions, ~jI=2.511 eV, should be close to the difference in energies of the nand n' orbitals. While there is certainly some shifting of the other orbital energy levels upon excitation of an electron to the 7r* orbital, we could reasonably expect that such energy shifts would be about equal in the two cases and would cancel when the term difference is taken. Therefore, we would expect that the ionization potential of the n' orbital of acetone would be nearly equal to the first ionization potential (9.71 eV) plus the term difference (2.51 eV), or 12.22 eV. The agreement with the observed value is excellent.
As a test of the validity of this argument, we may consider the 7r*<C-7r transition of acetone at 65 213 cm-l. The difference [(7r*<c-n)- (7r*<C-7r)] is 34919 cm-l or 4.251 eV, so the third ionization potential of acetone should be 9.71+4.25=13.96 eV. The observed valuel6
is 13.94 eV. Again, the agreement is excellent. It may be inferred that the assignment of band B of cyclobutanone is therefore properly 7r*<c-n'.
The third transition of cyclobutanone, C, is completely different from the other transitions. Clearly the upper state is, as pointed out in Results, either naturally unstable or strongly predissociated through perturbations by a nearby state. It is probable that there is strong predissociation, in view of the continuous absorption which underlies the entire vacuum ultraviolet spectrum. A definite assignment of this transition cannot be made on the basis of the present results alone. Simpson7 has suggested that the corresponding transitions in other ketones may in fact be the IJCo *<c-n transitions, which would be consistent with the assignment of band B as 7r*<c-n'. Certainly the transition IJco*<c-n should occur. McMurryl4 c~lculated an approximate oscillator strength for it as 2X 10-2 • EISayed,5 using more recent results for the orbitals involved, calculated the same quantity and obtained a value of 3X 10-2
• While both of these estimates are higher than is observed, the IJCo *<c-n cannot be excluded on intensity grounds. EI-Sayed proceeds to argue that the changing carbonyl hydridization should result in
changing intensity of this band in the cyclic ketones, but reference to the work of Holdsworth and Duncanl7
on noncyclic ketones suggests that other factors also cause an intensity variation for this transition. All in all, it appears that the assignment IJco*<c-n is consistent with the observed transition C, but a more conclusive argument for the assignment cannot be made.
The interpretations of transitions D and Ii are more clearly discussed together. We believe D to be the n=4 member of the Rydberg series described in Results, and Ii to be the 7r*<C-7r transition. While the calculated position of this Rydberg series member is 62 787 cm-1, slightly higher in energy than the observed origin of Ii, the intensity of Ii is more nearly that expected for the 7r*<C-7r transition. D is only slightly stronger than the Rydberg transition F. The observed patterns of intensity in D and E also support these assignments. D, like F, has no clear maximum of intensity; Ii displays a pronounced maximum, which would be expected to occur in conjunction with a transition in which the molecule undergoes a geometry change, such as the 7r*<C-7r transition.
The Rydberg series observed in cyclobutanone may be interpreted as an ns series on the basis of the magnitude of the Rydberg defect 0= 1.0555. This is in agreement with the findings for other ketones.
The presence of some vibrational structure associated with the Rydberg series members of cyclobutanone implies that the n electrons are not completely nonbonding. Some mixing of the n orbital with the b2
ring orbitals may be inferred from this. The ionization potential found for cyclobutanone
corresponds to a wavelength of 1325.5 X, beyond which the photoionization continuum should occur. This is probably the cause of the strong absorption setting in near 1330 X.
From arguments of the same form as used above in the discussion of acetone, we may predict that the second ionization potential of cyclobutanone, corresponding to the n' orbital, will be about 11.70 eV, while the third ionization potential, corresponding to the 7r orbital, will be about 13.34 eV. Examination of the photoelectron spectrum of cyclobutanone would therefore be of great interest as a test of this analysis in particular, and would be of significance to our general understanding of carbonyl spectra.
* Supported in part by the U.S. Office of Kaval Research. t Work based on a thesis submitted in partial fulfillment of the
requirements for the degree of Doctor of Philosophy at the University of Rochester.
t To whom correspondence should be addressed. 1 S. R. LaPaglia, J. Mol. Spectry. 10, 240 (1963). 2 J. W. Sidman, Chern. Rev. 58, 689 (1958). 3 R. S. Mulliken, J. Chern. Phys. 3, 564 (1935). 'D. C. Maule, Can. J. Phys.47, 1235 (1969). 5 A. Udvarhazi and M. A. EI-Sayed, J. Chern. Phys. 42, 3335
(1965) . 6 S. W. Benson and G. B. Kistiakowsky, J. Am. Chern. Soc.
64,80 (1942).
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224 R. F. WHITLOCK AND A. B. F. DUNCAN
7 E. E. Barnes and W. T. Simpson, J. Chem. Phys. 39, 670 (1963) .
8 "Puriss" grade from Columbia Organic Chemical was used. 9 V. R. Ells, J. Am. Chem. Soc. 60, 1864 (1938). 10 A. B. F. Duncan, J. Chem. Phys. 8, 444 (1940). 11 K. Frei and H. H. Giinthard, J. Mol. Spectry. 5, 218 (1960). 12 L. H. Scharpen and V. W. Laurie, J. Chem. Phys. 49, 221
(1968) .
THE JOURNAL OF CIIEMICAL PJIYSICS
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14 H. L. McMurry, J. Chem. Phys. 9, 231 (1941). 15T. Koopmans, Physica 1,104 (1934). 16 M. J. S. Dewar and S. D. \Vorley, J. Chem. Phys. 50, 654
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VOLUME 55, NUMBER 1 1 JULY 1971
Moment Analysis of Magnetic Circular Dichroism: Diamagnetic Molecular Solutions
P. J. STEPHE:-lS*
Department of Chemistry, University of Southern California, Los Angeles, California 90007
AND
R. L. MOWERY AND P. N. SCHATZ
Department of Chemistry, Unii'ersityof Virginia, Charlottes~>ille, Virginia 22901
(Received 24 July 1970)
The method of moments enables more rigorous analysis of magnetic circular dichroism data than rigid shift models. Moment analysis is here applied to magnetic circular dichroism measurements on room temperature solutions of Mn04-, CrO;-, PdCI 42-, SbCl6-, ~i (c:t';-) 42- , Fe(CK) 64-, triphenylene, and coronene. Qualitative conclusions previously reached on the basis of rigid shift models are shown to be very little affected. More accurate values for the magnetic moments of degenerate excited state manifolds are obtained. The new data for Fe(CK)64- support the assignment of the 47000 cm- 1 absorption band to an in tramolecu lar, 1,.-->1, u metal-to-ligand transition.
INTRODUCTION
Recent measurements of magnetic circular dichroism (MCD) have demonstrated the value of this technique in solving spectroscopic problems. At present, the most powerful and rigorous approach to the analysis of the MCD of broad absorption bands is the method of moments. Moment analysis was introduced by Henry, Schnatterly, and Slichter1 in studying color centers and has already been much employed in that field. 2 In a recent paper3 (henceforth designated as I) an attempt was made to generalize the formalism of Henry, Schnatterly, and Slichter to facilitate its application to a wider variety of absorbing systems. Particular emphasis was placed on molecular solutions, a category of primary concern to chemists. In this paper we make specific application to solutions of diamagnetic molecular species.4 MCD data for a wide range of such species have been published over the last few years.;; In particular, measurements for a variety of simple, symmetrical molecules have been used to evaluate magnetic moments and to make assignments of degenerate excited states by means of rigid shift theories. Our principal goal is the re-evaluation of these data using moment analysis. 'Ve also take this opportunity to present and discuss new :VIeD measurements on Fe(CN)64-.
THEORY
In I, expressions were derived for the zeroth and first moments of absorption bands and their accompanying MCD in molecular solutions. For the A----+J band of a diamagnetic solution, the results reduce to
(I, (A ----+J)O)o = (h2a 2.iY/3hn) ~o (A ----+J),
(k (A----+J) °)1 = (27r2O'.W/3h2Jl)
X {~l(A----+J) -hwO~o(A----+J) I, (11k (A----+J) )0= - (4rr2O'.2lY/3hn) ffio(A----+J) II,
(l1k(A~J) )1= - (4rr2O'.2LY/3n2n)
X !al(A----+J)+ffil(A----+J) -hwOffio(A----+J) Ill. (1)
Ie is the absorption coefficient (1,0 in zero fIeld) and 11k = kL - kH= 1,-- k+ is the circular dichroism in magnetic field II. ~foments are defined about the circular frequency WO by
(k(A----+J)O)n= 1 k(A----+J)O(w-wO)"dw, band
(11k (A----+J) )n= 1 /1k(A----+J) (w-wO)"dw. (2) band
0'.2/11 is a solvent effective field correction (n being here
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