COMMUNICATIONS TO THE EDITOR
Sept. 5, 1964
357 1
Low frequency absorptions such as those observed could also arise from internal molecular vibrations, lattice modes or other crystal effects, and/or torsional DEPARTMENT OF CHEMISTRY AND ALLANZALKIN LAWRENCE RADIATION LABORATORY J . D. FORRESTER modes (e.g., methyl rotations). For the compounds UNIVERSITY OF CALIFORNIA DAVID H. TEMPLETON listed in Table I, vibrational assignments have been BERKELEY, CALIFORNIA STANLEY M. WILLIAMSON made for benzonitrile,6,' nitrobenzene,s phenylacetylCHARLES W. KOCH ene,g acetonitrile,I0 acetone," and nitromethane.12 RECEIVED JUNE 26, 1964 None of these assignments indicates an internal molecular vibration giving absorption below 130 cm.-'; therefore, the observed bands below 88 cm.-l cannot Far-Infrared Studies of Intermolecular be assigned to intramolecular vibrations. Forces. Dipole-Dipole Complexes Lattice mod& can be eliminated in all the compounds Sir: examined as liquids, and the use of a polyethylene Far-infrared spectra (between 150 and 50 cm.-l) matrix to contain the sample13 minimizes possible of thirteen organic compounds have been obtained in crystallinity effects in the materials examined as this laboratory. A very broad absorption band in the solids. range from 88 to less than 50 cm.-' has been observed Torsion modes can be eliminated from consideration in the spectra of those compounds which have a high in the cases of benzonitrile, where microwave studiesI4 dipole moment. These observations coupled with other have shown a linear CCN structure; nitrobenzene, evidence indicate that these broad, low frequency where Raman data have been used to assign the nitro vibrations are vibrations involving the bond joining torsion a t 130 cm.-l; acetone, for which Fateley and molecules together in dipole-dipole complexes. While Miller's assign a weak band a t 109 cm.-l to the methyl considerable evidence has been reported p r e v i o ~ s l y l - ~ torsion ; and methyl thiocyanate, for which Fateley that highly polar organic molecules can undergo selfand Miller16 assign a broad band a t 131 cm.-] in both association or complexing, this evidence has usually the liquid and solid to the methyl torsion. No methyl been the result of classical, but indirect, studies of torsion would be expected In this region for acetonitrile colligative properties or virial coefficients. The appeardue to the linear CCN structure. ance of these low frequency bands gives a direct means The broad, low frequency band observed in liquid of studying these complexing forces. acetone is not seen in acetone vapor; instead we observe Observed frequencies for the compounds studied are a much weaker, sharp band a t 118 ern.-' (which is problisted in Table I along with the molecular weights and ably the very weak band reported Fateley and MillerI5), TABLE I This disappearance is consistent with dissociation of the dipole-dipole complexes, or clusters of complexes, in the VIBRATIONAL FREQUENCIES OF ORGANIC MOLECULES BETWEEN 50 A N D 150 C M . - ~ vapor state. The broad band observed in liquid aceDipole tonitrile is still evident in the vapor spectrum, but is Molecular moment, Physical Frequencies, much less intense and apparently shifted somewhat to Compound wegbt D.s state cm. - 1 lower frequencies than in the liquid. This would be 103 3.9 Liquid 54vbr Benzonitrile 137 2.56 Liquid 113 p-Chlorobenzonitrile the expected behavior if the complex persisted in 7 2 , 133 128 5 . 9 (calcd.) Solid Phtbalonitrile Persistence the vapor as has been Terephthalonitrile 128 0 Solid 9 5 , 154 of the complex in the vapor for acetonitrile, Nitrobenzene 123 3.98 Liquid
