A COMPARISON OF SOME ULTRA-VIOLET ABSORPTION SPECT R A OF POLYATOMIC MOLECULES WITH THOSE O F DIATOMIC MOLECULES' W. ALBERT NOYES, J R . Department of Chemistry, Brown University, Providence, Rhode Island Received October 1, 1936 INTRODUCTION
In solving the numerous problems in the field of molecular structure, the methods of spectroscopy must be considered as among the most powerful tools at the disposal of scientists. X-ray and electron diffraction will, at least in the case of simple molecules, furnish information concerning the geometrical distribution of nuclei in molecules in the gas phase, and the added information obtained from the facts of inorganic and organic chemistrywill in most cases permit the assignment of a spatial arrangement to a given molecule. It is possible, in the case of diatomic molecules, to predict the various possible electronic states by using the method of molecular orbitals. Once the various possibilities are known, detailed spectrum analyses permit an assignment of numerical energy values to the various states. An extension of the method of molecular orbitals to polyatomic molecules has been made, so that here again the possible spectroscopic states (at least for the simple molecules) may be predicted with certainty. However, while the principles governing electronic transitions in polyatoinic molecules have been formulated (10, 17), the detailed applications of these ideas have been slow, largely because of the scarcity of adequate experimental data and partly because the enormous superficial complexity of these spectra seems to preclude the assignment of a unique distribution of energy levels. Only in very fern cases have rotational analyses of electronic bands of polyatomic molecules been made, and it is not probable that many more will be accomplished in the near future. The applications of spectroscopy have been confined, therefore, to Raman spectra and to the infra-red and give information only concerning the ground electron state. Attempts to correlate frequencies 1 Presented a t the Symposium on hIolecular Structure, held a t Princeton University, Princeton, New Jersey, December 31, 1936 to January 2, 1937, under the auspices of the Division of Physical and Inorganic Chemistry of the American Chemical Society. 81
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W. ALBERT NOTES, JR.
of diatomic molecules with those observed when groups containing the same atoms are present in a polyatomic molecule ( 7 ) may be of interest in showing that force constants for the bonds are similar, but would seem to give little definite information concerning electronic states. From an experimental standpoint the difficulties in treating polyatomic electronic spectra are obvious. Numerous possibilities of predissociation exist in a molecule with many bonds, so that in many cases even spectroscopic apparatus of a hitherto unattained resolving power might show only diffuse bands. Furthermore, with many modes of vibration the number of allowed vibration transitions may be quite large, even assuming a rigorous application of the selection rules of Heraberg and Teller. It is true, fortunately, that the Franck-Condon principle often restricts the obvious bands to a few members of one type of progression, so that spectra of an unexpected simplicity are sometimes observed. Finally, it now seems certain that very frequently the apparent complexity of these spectra is to be ascribed to a more or less complete intermixing of the bands from several different electron transitions, so that detailed analyses become very difficult. For these and other reasons the obtaining of definite information from these spectra must be accomplished by methods which are not always infallible, and which do not have their counterparts in treating diatomic molecules. We will survey in a rapid fashion the type of information which one can obtain in a few cases. I. THE CARBONYL GROUP I X ALDEHYDES AND K E T O N E S
The electronic structures of aldehydes and ketones have been discussed by Mulliken (18). Recently Henri (7) has contrasted prominent frequency differences observed in the ultra-violet spectra and infra-red and Raman spectra for these substances with values of w e for various states of carbon monoxide. H e has concluded that the electron states of the latter must resemble closely those of the former. The electronic configuration of carbon monoxide in its normal ('Z+) state is (16)
KK(zu)2(yu)2 (w7r) *(xu) This configuration would resemble a Lewis structure with three electronpair bonds between the two atoms (:C:::O:). The w e value for the normal state of carbon monoxide is 2168.9 (31), which does not correspond to the frequency of the carbonyl group. The low excited states of carbon monoxide may be represented probably by adding a vn electron to CO+ (16). The configuration of the latter in its lowest state is probably ~ K ( z u ) 2 ( y u ) 2 ( ( w 7 r ) 4 (2Z+. ~ u ) Thewevalue of COT in its lowest known state is 2211 cm.-I, which differs but little from that of normal CO, although it is higher, indicating that the xu electron exerts perhaps a slight anti-
ABSORPTION SPECTRA OF POLYATOMIC AND DIATOMIC MOLECULES
83
bonding action. The effect of the UT electron will be anti-bonding, and the w e value of CO in its lowest excited (”) state (excepting the K state for which the evidence is somewhat uncertain (14)) is 1739.3. This corresponds quite closely to the carbonyl frequency observed in the infra-red and Raman spectra of aldehydes and ketones (1744 in formaldehyde, 1720 in the aliphatic aldehydes, 1710 in the aliphatic ketones. The latter figures differ by scarcely more than the experimental error among the various homologs (I 1)). When one compares the electronic configuration of the a311 state of carbon monoxide with the electronic configuration of formaldehyde, one notices several important differences (even aside from the difference in symbolism which is necessitated in the consideration of polyatomic molecules). If the structure given for this state of carbon monoxide is correct, there are six bonding electrons ( ( y ~ ~ ) ~ ( w 7 rthree ) ~ > , electrons which are essentially non-bonding ( ( Z U ) ~ ( ~ )and ) one anti-bonding electron (UT), whereas in formaldehyde the structure probably consists (18) of four carbon-oxygen bonding electrons (although these electrons and other electrons play some part in the bond and are partly carbon-hydrogen bonding), four electrons which are primarily carbon-hydrogen bonding (although they are not all localized completely in these bonds), two electrons largely on the oxygen, which are non-bonding, and two more which are non-bonding or slightly anti-bonding (these electrons are again largely but not entirely localized on the oxygen atom). In contrast to the normal state of carbon monoxide which has a very small dipole moment (28), the carbonyl group possesses a considerable moment and the oxygen atom is quite negative (29). Unless one says that merely by accident the effect of the anti-bonding electron so nearly counterbalances the effect of a bonding electron that in both the u311 state of carbon monoxide and in the carbonyl group one has really the equivalent of a typical double bond, there seems to be little reason for believing that the bonds in the two cases are similar. The agreement between the two frequencies may not have much theoretical significance. Some further support for this point of view is found from ionization potentials. The ionization potential of carbon monoxide is 14.1 volts (30), while the electronic term value of the u311 state is 6 volts (31). Therefore the ionization potential of carbon monoxide in this state is about 8 volts, which is considerably lower than that observed for aldehydes and ketones, either as convergence limits of Rydberg series or by electron impact (23, 2, 20). The negative charge on the oxygen in the carbonyl group should tend to lower rather than raise the ionization potential. When one turns his attention to the ultra-violet absorption spectra of aldehydes and ketones, one encounters the difficulty of ascertaining which
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frequency differences are to be ascribed to the carbonyl group. In reality the assignment of such frequencies will have to be done in the same manner as for the Raman effect, Le., by the accumulation of a sufficiently large body of data to render certain the comparison of the frequencies in the various homologs. KO satisfactory vibration analysis of the near ultra-violet bands of formaldehyde has yet been published, although the ideas of Gradstein ( 6 ) concerning the fluorescence may give the clue to the correct solution of the problem. Dieke and Kistiakowsky (1) have carried out a rotational analysis of six of the near ultra-violet bands. These bands are all of a type such that the electric moment is perpendicular to the axis of symmetry, but the authors state that there are bands of another type in the same spectral region, for which the electric moment is parallel to the axis of symmetry. It seems probable, therefore, that more than one upper electron state is involved, a fact that would render the attainment of a correct vibration analysis exceedingly difficult. Although the details of this analysis are still lacking, a series of prominent bands, probably forming a progression, shows a frequency difference between the first and (8, 9) .z The question arises as to whether second members of 1187 the frequency of the carbonyl group in its first excited state is nearly the same in other ketones and aldehydes. While in all probability the carbonyl electron structure is practically the same in all ketones and aldehydes, the symmetry properties of no one of these compounds will be as simple as that of formaldehyde. The rotations of parts of the molecule such as methyl groups must be considered in any complete derivation of selection rules. h’evertheless, the upper electron states of aldehydes and ketones may be expected to have many characteristics in common, and one will expect to find progressions based on carbon-oxygen vibrations in all of their spectra. The complexity of the molecules as a whole will, however, affect the appearance of the spectra and may render the characterization of any definite frequency differences very difficult. It is undoubtedly not sufficient to take differences between absorption maxima as being necessarily characteristic frequencies of vibration in the upper state. I n the near ultra-violet absorption spectra of aldehydes and ketones, relatively few substances give sharp enough bands so that any progressions (even prominent ones) can be designated with certainty. Turning our attention again to carbon monoxide, one finds that the a32 state has a value of w e equal to 1182 (31). The considerable decrease in we from that of the ground state may be taken to indicate that one of 2 Professor KistiakoFsky (private communication) states t h a t this difference is observed also in D,CO.
ABSORPTION SPECTRA O F POLTATOBIIC A S D DIATOMIC MOLECULES
85
the strongly bonding electrons has been excited to an anti-bonding orbital. This state is probably based on that of the ion
K K ( m ) 2 ( yu)Z(w 77)3 (2")'
2rI
The additional electron may be (UT) in carbon monoxide (a"), thus weakening the bond. The decrease in frequency from l i O O to less than 1200 in the carbonyl group upon excitation indicates that a bonding electron is also involved in this transition. Here again, however, there does not seem to be a very close resemblance between the electron state of the carbonyl group and that of carbon monoxide. In the far ultra-violet aldehydes and ketones again show characteristic absorption, but the data in this region are relatively scarce. Detailed data on formaldehyde in the long-wave Schumann region have not been published, although the spectrum has been photographed by Price (23). However, other ketones and aldehydes have been photographed in this region of the spectrum. All or nearly all of them show a frequency difference between prominent bands of approximately 1200 cm.-l, and although the values are not as constant as those obtained by the Raman spectrum for the carbonyl group, it seems probable that this is a frequency characteristic of carbonyl compounds in excited electronic levels. In a few compounds another frequency of about 1050 crn.-' is also noticed. This may possibly be another carbonyl frequency, indicating perhaps that more than one upper electron state is involved in the bands between 1800 and 2000 A.U. It seems probable, therefore, that the carbonyl frequency for the bands in the long-wave Schumann region is little different from that for the upper state of the near ultra-violet bands. The absorption coefficients for these compounds in the region 1800 to 2000 A.U. are very much higher than in the near ultra-violet, although not as high as for bands at still shorter wave lengths. It is not impossible that these are allowed transitions, but involving essentially the same bonding electron as in the near ultra-violet. The bond strength in any case seems to be about the same for the two upper states. At still shorter wave lengths carbonyl compounds show still further electron transitions, some with exceedingly high probabilities, as evidenced by the fact that the absorption coefficients are in general very high. Some frequency differences might possibly be ascribed to the carbonyl group, but detailed analyses of these spectra are lacking. I n at least two cases Rydberg series have been found (23, 2, 20), predicting ionization potentials between 10 and 11 volts, in good agreement with electron-impact values. Ionization results undoubtedly from the removal of a 2p&z non-bonding oxygen electron. It is significant, perhaps, that the bands forming the Rydberg series are, in general, unaccompanied by bands
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W. ALBERT NOYES, JR.
separated by frequencies which could reasonably be ascribed to the carbonyl group. Excitation of the non-bonding electron should affect the bond strengths and interatomic distances relatively little (unless it goes into a bonding or anti-bonding orbital), and hence an application of the TABLE 1 Frequencies chapacteristic of the carbonyl group FREQUEXCY IN Q R O U S D STATE
COMPOCXD
...............
i
(1739.3) (a") H3CO.. . . . . . . . . . . . . ., ' 1744 CHaCHO . . . . . . . . . . . . . 1713 CH3COCH3.. . . . . . . . . . 1706
UPPER STATE FREQUENCY ( N E A R ULTRA-YIOLET)
1
(1182) ( a ' 3 2 )
1
1187 1 GO* 1198 (?)
1
,
1711
'
i
I Cyclopentanone... . ., (1713) Cyclohexanone . . . . . 1714
REFERENCES
(1182) (a'")
,
I
CHsCOC2H5.. . . . . . . . .
UPPER STATE FREQUENCY ( I B O U T 1900 A . U . )
~
(1160) 1192 1052 (?) 1281 (may be 1187) 1245 1160
* This frequency is obtained as the first difference between peaks on a microphotometer curve. There is no assurance t h a t i t represents the difference between the zero and first vibration levels of the upper state. The same may be said for other aldehydes and ketones in the near ultra-violet, with the exception of formaldehyde and acetone. t This substance shows no discrete structure in the near ultra-violet, although a careful search for such structure was made in this region. However, i t does show a strong fluorescence which would indicate the presence of such a structure. A study of this fluorescence may furnish a clue t o the problem. 1 The band chosen a s the 0,O band for this substance apparently consists of three parts with a maximum separation of about 100 cm.-1 Note: Eastwood and Snow (5) have measured the absorption spectra of several aliphatic aldehydes in the near ultra-violet and, by averaging frequency differences between maxima for each substance, have shown t h a t there is a prominent frequency characteristic of all of these compounds varying between 1021 and J107 em.-' This method of averaging frequency differences in a progression will, of course, give an average lower than the difference between the first two members. Moreover, differences between absorption maxima need not correspond to a definite difference in frequency in either the upper or lower states. Franck-Condon principle would lead to the prediction of the greatest intensities in the 0,O bands. In table 1 one finds a summary of some of the data concerning the frequencies most properly ascribed to the carbonyl group. Carbon monoxide has been included for comparison, although, as pointed out, the electron states of this molecule are not related too closely to those of the carbonyl group.
ABSORPTION SPECTRA O F POLTATOMIC .4SD DIATOJIIC MOLECTLES
8;
From the summary presented in table 1 we find that, whereas the frc>quency of the carbonyl group is not as constant from one compound to another in upper electron states as it is in the ground state, nevertheless there is a frequency jn the neighborhood of 1200 cm.-l which is probably to be ascribed to a carbon-oxygen valence vibration in the upper electronic level, I t is rather surprising that the difference in intensity distribution should be so marked between the two states, since the frequency, and hence the force constants, are approximately the same in both upper states. In the near ultra-violet the intensities are low on the long wave end and increase in a progression, while in the region about 1900 A.U. the bands taken as the 0,O bands are the most intense, and the progressions contain only about four members at most before a region of almost complete transparency is encountered. An application of the Franck-Condon principle would indicate that the tm-o upper configurations have quite different dimensions in the vibrationless state. The products of photochemical reaction (at least in the case of acetone (13)) seem to be identical for the two regions of absorption but, as pointed out by Nulliken (18), this fact seems to have relatively little bearing on the problem a t hand. No precise assignment of electronic structures to the upper states is possible. Both involve the change of a bonding electron, but probably in the near ultra-violet the electron involved has much more influence on the carbonhydrogen or carbon-carbon bonds than does the electron involved at 1900 A.U. In conclusion, it seems to be a coincidence xithout any particular theoretical implications that the frequencies of the carbonyl group agree fairly well with those of certain states of carbon monoxide, although this point of view may be altered as more is known about the states in question. 11. OTHER GRO'C'PS
In the Raman spectra of compounds possessing typical double bonds between atoms of elements in the first period of the periodic table (carbon, nitrogen, and oxygen especially), one always finds a frequency of the order of magnitude of 1600 to 1700 cm.-' Examples are the typical carboncarbon double bond frequency near 1660 or 1670 (12) and the carbonyl group mentioned above. It is not surprising that one of the states of CZ should have a frequency near this figure. In fact the ground state (A311,) has an w e of 1642 (32). The electron configuration of this state bears some resemblance to that of carbon atoms in the double bond, but the correspondence is not 1 to 1. The bond strength would not be expected to be identical with that observed in unsaturated hydrocarbons. Ultraviolet absorption spectra of quite a number of unsattirated hydrocarbons have been investigated, but detailed vibration analyses have not been
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W. ALBERT NOPES, JR
made. In ethylene (24) an upper state frequency of about 1340, which may be a carbon-carbon frequency, has been observed. I n the cis-and trans-dichloroethylenes frequencies between 1400 and 1450, probably characteristic of the double bond, have been found (15). None of the known spectroscopic states of CZhas w e values near these figures, although theoretically additional states are capable of existence, some of which might conceivably have such frequencies. In any case there is no basis at present for a correlation of the spectroscopic states of CZ with those of the carbon-carbon double bond. The upper state frequency of 1000 to 1100 cm.-l observed (4) in organic compounds containing the -N=O group (25) is relatively close to that of nitric oxide in the B211state (33). Triple bonds between atoms of these elements give rise to frequencies in the neighborhood of 2100 and 2200 cm.-l Examples are the frequency of the ground state of carbon monoxide referred to above, the frequency 1975 in acetylene, the frequencies between 2089 and 2201 observed in HCN, ClCN, BrCN, and ICN, respectively, and the frequency 2224.1 in nitrous oxide (34). I n acetylene an upper state difference of about 1760 (24) is observed, which lies in the region of double-bond frequencies and does not differ greatly from the w e of one of the states of CZ(BsII,). I n hydrogen cyanide the upper state frequency of about 900 (23) (which may possibly not be a bond-stretching frequency) does not correspond to that in any of the known states of CX. The same may be said for nitrous oxide, where the upper state difference of about 550 is too low for a bond-stretching frequency (3). This molecule may possibly not be linear in the upper state. I n conclusion, therefore, it seenis quite evident that the correlation between electron states and frequencies of diatomic molecules and the same atoms when they form a group in a polyatomic molecule cannot be carried very far, although some more or less accidental agreements in frequency are observed. HoIvever, characteristic frequencies of groups are obtained for upper electron states just as they are for the ground states as observed in Ranian and infra-red spectra. SUMMARY
1. The electron states of carbon monoxide and of the carbonyl group are contrasted and the dissimilarities pointed out. The agreement between the frequencies probably does not indicate that the types of bond are identical. 2. The frequencies of certain other characteristic bonds in the normal and excited electron states are compared with those of diatomic molecules. Little accurate experimental information is available, but the correlation sariiiot be carried very far.
ABSORPTIOS SPECTRA O F POLYATOMIC .4XD DIATOMIC MOLECULES
89
REFERENCES (I) DIEKEAXD K ~ s ~ ~ a ~ oPhys. a s ~Rev. r : 46, 4 (1934). (2) DCXCAS:J. Chem. Physics 3, 131 (1935). (3) DUSCAN:J . Chem. Physics 4, 638 (1936). (4) DUSCAN,ELLS,AND XOYES:J. Am. Chem. SOC. 68, 1454 (1936). (5) EASTWOOD ASD SNOW:Proc. Roy. SOC. London 149A, 434 (1935). (6) GRADSTEIK: Z. physik. Cheni. 22B, 384 (1933). (7) HENRI:Compt. rend. 199, 849 (1934); 203, 67 (1936). (8) HENRIAND SCHOU:8. Physik 49, 774 (1928). (9) HERZBERG: Trans. Faraday SOC.27, 380 (1931). (10) HERZBERG AND TELLER: Z. physik. Chem. 21B,410 (1933). (11) HIBBEN:Chem. Rev. 18, 36-45 (1936). (12) Reference 11, p. 26 e t seq. (13) HOWEAND NOYES:J. Ani. Chem. SOC.68, 1404 (1936). (14) KAPLAN:Phys. Rev. 36, 1298 (1930). (15) MAHSCKEAXD XOYES:J . Chem. Physics 3, 536 (1935). (16) MULLIKEN: Rev. Modern Phys. 4, 49 (1932). (17) MCLLIKEN:Phys. Rev. 43, 279 (1933). (18) MULLIKEN: J. Chem. Physics 3, 564 (1935). (19) K’EVGI AND JATKAR:J. Indian Inst. Sci. 17A, 175 (1934). (20) NOYES:J. Chem. Physics 3, 430 (1935). (21) NOYES,DUNCAN, AND ~ I A N N I T G J.: Chem. Physics 2, 717 (1934). (22) PIAUX:Compt. rend. 197, 1647 (1933). (23) PRICE:Phys. Rev. 46, 529 (1934). (24) PRICE:Phys. Rev. 47, 444 (1935). (25) PURKIS AND T H o w s o s : Trans. Faraday SOC.32, 1466 (1936). (26) SCHEIBEA N D GRIESEISES:Z. physik. Chem. 26B, 55 (1934) (27) SCHEIBE,PROVESZ, ASD LINsTRdhf: z. physik. Chem. 20B, 295 (1933). (28) ShfyTH: Dielectric Constant and hlolecular Structure, p. 85. The Chemical Catalog Co., Inc., New York (1931). (29) Reference 28, p. 90. (30) S m m r , H. D . : Rev. RIodern Phys. 3, 379 (1931). (31) Cf. SPONER:Molekulspektren, 1-01. I, p. 32. Julius Springer, Berlin (1935). (32) Reference 31, p. 14. (33) Reference 31, p. 36. (34) Reference 31, pp. 76, 77.
