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Infrared Frequencies of Amide, Urea, and Urethane Groups 7592(1973). (4) H. Kühne, S. Vaccani, T. K. Ha, A. Bauder, and Hs. H. Günthard, Chem. Phys.
Lett., 38, 449(1976). (5) R. Criegee and G. Weiner, Justus Liebigs Ann. Chem., 546, 9 (1949). (6) P. Groner, I. Stolkin, and Hs. H. Günthard, J. Phys. E, 6, 122 (1973). (7) R. Werder, Thesis ETH, Zurich, No. 3970. (8) A. Murray, III, and D. L Williams, “Organic Syntheses with Isotopes", Part II, pp 1433, 1474. (9) Reference 8, Part III, p 1425. (10) P. Hólemann and K. Clusius, Berichte, 70B, 819 (1937).
(11) E. Br. Wilson, Jr., J. C. Declus, and P. C. Cross, “Molecular Vibrations", McGraw-Hill, London, 1955. (12) K. B. Blick, J. W. DeHaan, and K. Niedensu, Spectrochim. Acta, Part A, 26,
2319(1970).
(13) J. M. Eyster and K. W. Prohovsky, Spectrochim. Acta, Part A, 30, 2041 (1974). (14) H. Hunzlker, J. Mot. Spectrosc., 17, 131 (1965). (15) H. Primas and Hs. H. Günthard, Helv. Chim. Acta, 36, 1659, 1791 (1953). (16) J. A. Lannon, F. D. Verderame, and R. W. Anderson, J. Chem. Phys., 54,
2212(1971).
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Infrared Frequencies of Amide, Urea, and Urethane Groups C, G. Cannon1 Research Department, id Fibres, Harrogate, Yorkshire, United Kingdom (Received February
14, 1975)
Publication costs assisted by id Fibres Limited
A possible interpretation is given with regard to the factors causing frequency shifts of certain characteristic amide bands appearing in related compounds.
The difference between the in-plane OCN group frequencies of the TV-alkyl amide,
0=C-N
O^C-^N
N,N'-dialkyl urea, and ’-alkyl
urethane structures was briefly discussed in a previous paper where the spectra of polyamides, a polyurea, and a polyurethane were described.2 In solid polymer films these structural groups are in comparable states of association and environment. The iV-deuterio polymers have since been prepared and the amide IF frequencies identified. The frequency differences are presented pictorially in Figure 1 where the four structures are arbitrarily positioned along the abscissa axis and the observed frequencies are plotted as ordinates. At first sight the frequency differences are puzzling. For example, in comparison with the amide I and II frequencies of the polyamide (CONH) those of the polyoxalamide (-NHCO-CONH-) are farther apart and those of the polyurea (-NHCONH-) closer together. In the polyurethane (-NHCOO-), however, while amide I is at a much higher frequency amide II is at roughly the same frequency as the polyamide. These shifts are rationalized if we consider that the extent of the - interaction between the C=0 and the nitrogen lone pair electrons increases from left to right in the figure. The two full lines in the figure represent the hypothetical yc=o and yc-N frequencies of the OCN group assuming that no coupling of the type described by Fraser and Price3 exists. These are at approximately 1800 and 1280 cm-1 respectively in the absence of - interaction. If complete conjugation occurred, and a symmetrical distribution of electrons was realized in - —C— + , the two frequencies would be coincident at about 1550 cm-1. The coupling and splitting of these two vibrations give the observed frequencies 1 and IF of the N deuterio groups Figure 1). The further complication of coupling of IF with 5nh to give II and III in the hydrogenated groups is thus responsible for their rather confused pattern of frequencies (- X Figure 1). If this rationalization is accepted then we can conclude that the qualitative picture of increasing - conjugation through this series of structures is correct. This would imply, for example, -
-
decreasing basicity of the N atoms from urethane to oxalamide. The pattern of frequency shifts of I and IF conforms exactly to that expected from vibrational coupling of two modes approaching each other in frequency. Do these frequencies cross over at the point of maximum mixing of vibrations? This usually occurs when the two modes are closest in frequency. The urea group has amide I and IF closest together but this point cannot correspond to the cross over because, were this so, the amide I frequency for the polyamide and polyoxalamide would then have more yc-N character than yc=o· The calculations of Mizushima and his coworkers4 show that amide I has about 80% or more yc=o contribution. We must conclude therefore, that the yc=o and yc-N do not cross over and that the - interaction in the amide groups can never give a structure _eO-C=N+e.5 The limiting extent of interaction occurs in the ~ —C— + structure with a symmetrical distribution of electrons for which yc=o yc-N· Of course this whole analysis, however convincingly presented, rests on the arbitrary position of the four structural —
The Journal
of Physical Chemistry, Voi. 80, No.
11, 1976
1248
D. H. Finseth, C. Sourisseau, and F. A. Miller
groups along the abscissa axis. However, be found which gave a
no
other order could
rational pattern fitting all the condi-
tions.
Acknowledgment. Thanks are due to Dr. B. C. Stace for preparing the deuterated polymers and recording their spectra.
References and Notes Postgraduate Research Associate with Richard C. Lord, 1953. C. G. Cannon, Spectrochim. Acta, 16, 302 (1960). R. D. B. Fraser and W. C. Price, Nature (London), 170, 490 (1952). S. Mizushima et al„ J. Chem. Phys., 24, 408 (1956); 29, 611 (1958). (5) Unless O protonation occurs in a proton donor medium when the 1669-cm-1 band is indeed yc=Nl Stewart and L. J. Muenster, Can. J. Chem., 39, 401 (1) (2) (3) (4)
(1961).
Vibrational Spectra and Force Field of Tricarbonyl(trimethylenemethane)iron-Ai6 and -d61a,b Dennis H. Finseth, Claude Sourisseau, and Foil A. Miller*10 Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received January 26, 1976)
Complete vibrational spectra are presented for tricarbonyl(trimethylenemethane)iron, [(H2C)3C]Fe(CO)s, and for its perdeutero derivative. The latter has been made for the first time. The data include Raman spectra for the gas, liquid, and solid and infrared spectra for the gas and solid. A vibrational assignment has been made for 49 of the 50 active modes (25 in each molecule). All but 8 or 10 of these are certain. From the assignments a force field has been deduced which (a) reproduces the observed frequencies of both isotopic molecules with an average error of less than 1%, and (b) provides descriptions of the normal modes. From selected frequencies and force constants the following conclusions are drawn about the bonding. (1) All the C-H stretches are above 3000 cm-1, indicating that the methylene carbons are unsaturated. (2) The C-C average stretching frequency, force constant, and bond distance all indicate that these bonds are unsaturated. (3) The iron-ligand stretching and tilting force constants are relatively large and indicate strong metal-ligand bonding. It appears that a model with bonds between iron and the three C-C orbitals is better than one with a bond along the iron-central carbon line. These data, assignments, and force constants differ somewhat from earlier work on the hydrogenic compound. Some comments are made on the reliability and significance of force constants.
Introduction Trimethylenemethane, C(CH2)3, is of considerable interest because of its unusual valence. In the free state it is a reactive diradical,2a but it can be stabilized by formation of its tricar-
bonyliron complex to give tricarbonyl(trimethylenemethane)iron, [(H2C)3C]Fe(CO)3. For brevity, the hydrogen version of the complex will hereafter be called he and the deuterium version de. Tricarbonyl(trimethylenemethane) iron was first prepared by Emerson et al. in 1966,2b and was found to be fairly stable. The deuterated molecule has not been reported heretofore. The structure of the complex, obtained by electron diffraction,3 is shown in Figure 1. Note that the CC3 portion is nonplanar with the outer carbons displaced toward the iron atom. The angle Fe-C-C is 76.4°. The central carbon is 0.34 Á out of the plane of the three methylene carbons in the direction away from the iron atom. The Fe-central C distance is 1.938 Á whereas the Fe-outer C distance is 2.123 Á. In addition, the plane of each CH2 group is tilted about 14.4° relative to the extension of the corresponding C-C line, the tilt being away from the iron atom. There has also been an x-ray diffraction study of a related compound in which one hydrogen atom has been replaced by a phenyl group, with very similar results.4 Proton and 13C NMR spectra indicate that The Journal of Physical Chemistry, Vol. 80, No. 11, 1976
the six methylene protons are equivalent and the three methylene carbons are equivalent, or else are exchanging rapidly.2b An early, very low resolution photoelectron spectrum was interpreted as giving ionization potentials for the orbitals in the hydrocarbon portion of the complex.5 It seemed to us that the vibrational spectrum would be useful and interesting, so a study of the infrared and Raman spectra of he was undertaken.6 We hoped to make a complete vibrational assignment, and through that to answer some questions concerning the bonding. For example, do the C-C bonds behave spectroscopically more like single or double bonds? Also, one can imagine two extreme models for the bonding between iron and trimethylenemethane. In one case the bonding is directed from the iron to the central carbon atom; in the other case it is directed from the iron to the three methylene carbons or C-C bonds. The actual situation is probably intermediate, but just where is it between these two extremes?
During the course of the work a paper on the same subject by Andrews and Davidson appeared.7 This was followed by another by Andrews, Davidson, and Duce dealing with the force constants.8 These papers will be referred to hereafter as AD and ADD, respectively. Our work is considerably more extensive. It differs from that of AD and ADD in the following ways. (1) We have made the first preparation of dg, and report
