J . Phys. Chem. 1989, 93, 3041-3050 law predicts that the product p T l T should be site independent’, if the relaxation is dominated by the interaction with the conduction electrons and should therefore apply for the center and corner carbons. Although the exact value of the Korringa product depends strongly on the dimensionality and on certain microscopic parameters such as the Fermi energy and the ratio of the on-site Coulomb repulsion over the bandwidth,16 we expect the values of K and T , in our system to be in of the same order of magnitude as in the Bechgaard salts. The value of T1 for the corner carbons in the Ni salt ranges from 0.17 to 0.44 s, in agreement with the value of 0.29-0.56 s for (TMTSF)2Re04,as expected since in both systems the corner carbons carry roughly the same spin density. The T1 of 0.44 s for the center carbons is also reasonable, since the magnitude of the expected Knight shift is the same (but opposite in sign) to that of the corner carbons. The methyl group carbons in (TMTSF)2(Ni(tds)2) relax slightly more rapidly than those in the R e o 4 salt. We suggest that this effect is due to the more intimate methyl-methyl contact in the Ni salt resulting from the vertical stacking, as opposed to the alternate stacking in (TMTS F) ,Reo4. Marked changes occur in the corner carbon region of the spectrum of the Ni salt through the transition temperature. Heuer et aL6 have shown by X-ray diffraction that all crystallographically imposed molecular symmetry is lost on passing through the (1 6) Mehring, M. In Low-Dimensional Conductors and Superconductors; Jerome, D., Garon, L. G., Ed.; Plenum: New York, 1987.
3041
transition, via a translation of the cation and anion stacks. The result of the transition is a more intimate inter- and intrastack Se-Se contact and hence enhanced conductivity. They suggest, furthermore, that below T, three distinct local environments can occur for a TMTSF stack, depending on the direction of translation of the two anion stacks on either side of this TMTSF stack. Therefore, we expect the number of resonances to double due to the loss of symmetry and their line width to increase due to the loss of local order. Inspection of Figure 3 shows the doubling of the number of resonances most clearly for the Co carbons. The line width of the Co resonances increases from 125 to 200 Hz and of the Ce carbons from 150 to 300 Hz. A larger range of resonance positions was observed for the corner carbons below T, (Figure 4). Positions both below and above the high-temperature shift were observed. This is consistent with the observed increase in conductivity, and therefore Knight shift, below the transition and with a more inhomogeneous distribution of conduction electrons on the corner sites. The methyl group carbons are relatively unaffected by the transition, which indicates little change in their local spin densities. An effect intermediate between these two cases was observed for the center carbons. A more detailed analysis of the conduction electron densities necessitates the detailed knowledge of the behavior of Tl as a function of temperature. The results of these experiments will be published at a later date. Registry No. (TMTSF),(Ni(tds),), 102784-07-6.
Microwave, Infrared, and Raman Spectra, Structure, and Vibrational Assignment of (Dimethy1amino)difluorophosphine J. R. Durig,* R. J. Harlan,? and P. Groner Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received: August 12, 1988)
The microwave spectrum of five vibrational satellites of (dimethylamino)difluorophosphine, (CH3)2NPF2,has been assigned. Estimated relative intensities predict the lowest fundamental vibrations to be near 35 and 115 cm-’, which are assigned to the NC, out-of-planewagging mode and the PF2 torsion, respectively. The microwave spectra of two isomers of CH3(CD3)NPF2 have been assigned in the vibrational ground state. An improved molecular (ro) structure, obtained by least-squares fitting to the rotational constants of four isotopic species, confirms the semiplanar configuration in which the nitrogen, phosphorus, and carbon atoms lie on the symmetry plane of the molecule. From the Stark effect, a value of 3.1 f 0.1 D was obtained for the pa component of the dipole moment in (CD3),NPF2. The infrared spectra (3200-80 cm-’) of the gaseous and solid phases and the Raman spectra (3200-10 cm-l) of the gaseous, liquid, and solid phases for (CH3),NPF,, CH3(CD3)NPF2, and (CD3)2NPF,have been obtained and a complete vibrational assignment has been proposed. The barrier to internal rotation about the PN bond was determined to be 4356 f 96 cm-’ (12.46 A 0.27 kcal/mol).
Introduction The structure of (dimethy1amino)difluorophosphine has been the subject of considerable interest. From a microwave study’ of the do and d6 isotopic species and an X-ray crystallography study2 it was found that the molecule has C, symmetry with a structure in which phosphorus, nitrogen, and the two carbon atoms lie in the symmetry plane of the molecule (structure I, Figure 1). The data from an electron diffraction s t ~ d yhowever, ,~ were interpreted in terms of an asymmetric gauche structure (11, Figure 1). Nevertheless, it was concluded from all three studies that the molecule contains a very short PN bond which indicates probable T bonding where the electrons from the p orbital of the nitrogen atom delocalize into the empty d orbitals of the phosphorus atom. ‘Taken in part from the thesis of R. J. Harlan which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree.
0022-365418912093-3041$01.50/0
Due to a large number of interfering vibrational satellites, the authors of the microwave study’ were unable to obtain the electric dipole moment of (CHj)ZNPF2 which would determine if the molecule has a symmetry plane. However, no attempt was made to analyze these excited states. From their analysis the authors could not rule out the possibility of an asymmetric structure. In order to determine if the ”semiplanar” structure I (Figure 1) (where C, N, and P atoms are coplanar) was favorable, they carried out a theoretical analysis by performing C N D O calculations and found that the energies are in favor of the “nonplanar” configuration 11. Therefore, they attempted to fit the rotational (1) Forti, P.; Damiani, D.; Favero, P. G. J. Am. Chem. Soc. 1973,95,756. (2) Morris, K. D. Jr.; Nordman, C. E.Inorg. Chem. 1969, 8, 1673. (3) Holywell, G. C.; Rankin, D. W. H.; Beagley, B.; Freeman, J. M. J. Chem. SOC.A 1971, 785.
0 1989 American Chemical Society
3042 The Journal of Physical Chemistry, Vol. 93, No. 8, 1989
Durig et al.
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Figure 3. Low-resolution microwave spectrum of CH3(CD3)NPF2.
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Figure 1. Various configurations of (CH3)*NPF2: (I) semiplanar structure; (11) asymmetric gauche structure; (111) trans structure with
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