J.
1966
The present investigation has eliminated the possibility of the presence of a cis isomer in the solid phnse, and there is no indication of a second isomer in the liquid or vapor phase. Bloom and Sutton encountered some difficulty in purification of their sample and at least two of their runs produced erratic results. They suggested that the work should be rechecked using a sample purified by a different technique but were not able to continue the project because of the war. In light of the spectroscopic evidence and the element of doubt in the dipole moment measurements, the conclusion that biacetyl exists in only the trans, configuration in all phases seems justified.
Conclusions Vibrational assignments have satisfactorily been made for solid biacetyl according to Cth symmetry.
R.DURIG AND J. M. CASPER
Spectra of the liquid :md vapor phascs show no indicntion of L: second isomeric form, and it is concluded that the nonbonded forces betwoen hnlogcns which stabilize the cis isomer of the oxnlyl hdides are not present hctwoen bhe methyls of biacetyl. Thc mulunl exclusion between infrared :md Raman spectra of crystalline biacetyl necessitates :I centrosymmetric structure and the space group is probably C2Aor D2,,. The splitting of tlhe Raman-active librational modes :wid the :rppcar:mce of translational bands in the far-infrared spectrum indicate that there are at least two and possibly four molecules per unit cell. The barrier to methyl rotation has been calculated to be 2.65 ltcal/mol, considerably higher than those found for other acetyl compounds. Acknowledgment. The financial support of the National Aeronautics and Space Administration by Grant NGR-41-002-003is gratefully acknowledged.
Vibrational Spectra and Structure of Organophosphorus Compounds. X.1 Methyl Torsional Frequencies and Barriers to Internal Rotation of Some CH3PXY Cornpounds2~3 by J. R, Durig* and J. M. Casper4 Department of Chemistry, Universitu o/ South Carolina, Columbia, South Carolina ,99208 (Received December 18, 1970) Publication costs assisted by the National Institute of Arthritis and Metabolic Diseases
The far-infrared spectra of CHaPC1,, CHsPOCI,, CHaPOFCl,CHIPOF,, and CHaPSClrin the solid and gaseous states have been recorded from 33 to 350 cm-1. Methyl torsional modes were observed in the aolid state for all compounds except CHaPOFCl. The Raman spectra of CH3PC12,CHaPOF2,and CH31'SCl, in the solid state were recorded and torsional frequencies were observed for CH3PC12and CH3PS('l,. The wave numbers for these internal rotations were 228, 264, 240, and 221 cni-1 for the CH3PC12,CH3POC12,CH8POF2, and CH&'SClt molecules, respectively. The threefold barriers calculated from these assignments arc 3.4, 4.4, 3.6, and 3.2 kcal/mol, respectively. These barrier values are compared with those previously reported for other organophosphorus compounds. TJattice modes were observed in the wave number range 65-1 20 cin-' in the far-infrared spectra for all compounds.
Introduction Barriers to internal rotation are of interest to scientists because of the effect they have on molecular properties. The heat capacity, entropy, and equilibrium constant contain contributions from hindered internal rotation. Many theories have been advanced to explain the nature of the interactions causing a barrier to internal rotation and attempts have been made to preIn dict these barriers from ab initio some cases these attempts have been hindered by inThe Journal of Physical Chemistry, Vol. 76, No. 15, 1071
sufficient and inaccurate experimental results. Theoretical calculations of barrier heights and the various (1) For Part IX, see J. 11. Durig and J. W. Clzirk, J . Cryst. Mol. Struct., 1,43, (1971). ( 2 ) Presented at the Twenty-fourth Symposium on Moleculnr Spectra and Structure, The Ohio State University, 1969, l'wer D7. (3) Taken from the thesis of J. M. C:tsper submitted t o the Deimrtment of ChemistrS., University of South Carolina, in I m r t i d fulfillment of the Ph.D. Degree, June 1971. (4) Predoctoral National Science Foundation Fellow. (5) W. H. Fink and L. C. Allen, J . Phem. Phys., 46, 2261 (1007).
VIBRATIONAL SPEZTIEA ANI) STI~UCTURE OF O~~GANOPHOSPHOI~US COMPOUNDS theories on tho nature of the origin of specific. barriers may be aided by a 1:Lrgc.r number of more accurately detcrmi~iedbarriers to guide the efforts. In addition, vn1uc.s for bm+r heights of molecules with vwied skeletal structures are Iiecdcd to provide more definitive guidclines. I t has bccn proposed by some proponents of neutron inel:istic*scattering tet*hniclues12 that barriers to internal rotation i n the solid phase (minot be obtained from infr:ircd and I
