2850
NOTES 1001
0
C-HEXANE
0
n-HEXANE
2,3 DIMETHYL BUTANE 0 2,2 DIMETHYL BUTANE
v 2,2 DIMETHYL PROPANE
.01'
I
1
0.1
I
0.2
I
0.3
I
0.4
I
0.5
I
0.6
I
0.7
3 DT / p - [ e V ] 2
Figure 5 . Energy-loss rates in hydrocarbon vapors as a function of characteristic electron energy.
obtained. Similarly, because of our use of a characteristic energy there is necessarily some ambiguity in the energy scale of Figure 5. Our conclusion, however, is self-consistent-when the data for the several hydrocarbons are treated identically, near-identical stopping rates are obtained. This conclusion is in agreement with earlier suggestions to this The variations in the radiation-induced liquid conductivities remain unexplained. I n this connection we would point out that the gasphase mobilities of electrons obtained here, when corrected to the density of the liquids, are similar and approximately 7 cm2/sec V in magnitude. Recent measurements of the liquid phase mobilities seem also to be approaching this m a g n i t ~ d e . ~ ~These ' ~ ~ ' ~very large values are of interest, in that they imply that electron-scavenging reactions in the liquid are not diffusion controlled. This clarifies the seemingly paradoxical d i ~ j u n c t i o n 'previously ~ noted between conductivity measurements and susceptibility to geminate ion-pair quenching. But what is most pertinent is that the liquid-phase mobilities no longer cluster around a common value but seem to be rather more sensitive to structural effects than is true in the gaseous state. Again the state of aggregation is implicated as a factor tending to modulate the electrical properties of hydrocarbons. Schmidt and Allen4a have commented on the difficulty in understanding the variations in the radiationThe Journal of Physical Chemistry, Vol. 74, N o . 1.4, 1970
induced conductivities. There is little we can add except to note a semblance of a correlation between these conductivities and the entropies of vaporization of the several hydrocarbons, as obtained from standard vapor pressure data. The significance of any such correlation remains obscure. It seems clear, however, that the conductivity variations are associated with properties of the liquid phase and are not to be understood simply in terms of molecular parameters. (11) M. Inokut,i, R. L. Platzman, and K. Takayanagi, Abstracts, Third lnternational Congress Radiation Research, Cortina D'Ampezzo, Italy, 1966. (12) R. hl. Minday, L. D. Schmidt, and H. T . Davis, J. Chem. Phys., 50, 1473 (1969). (13) E. C. Conrad and J. Silverman, ibid., 51, 450 (1969). (14) J. AM, Warman and S. J. Rzad, {bid., 52, 485 (1970).
Infrared Evidence for the Association of Vanadium Porphyrins
by F. E. Dickson and L. Petrakis Gulf Research and Dezelopment Company Pittsburgh, Penmsylcania 16930 (Received December 18, 1969)
The important role of various porphyrins in biologically significant systems is well known and extensively
2851
NOTES
the results of Tynan and Yena represent a porphyrinrelated association. Nonmetalated porphyrin-porphyrin association has been studied, and energies on the order of -1 kcal/mol have been reported7.8 through the use of nmr techniques. I n addition, the stability of intermolecular complexation in metal porphyrins has been recently observed.@ I n this paper we are providing direct spectroscopic evidence for the existence of two porphyrin forms through the observation of the vanadyl (V=O) stretching mode in the infrared spectrum of vanadyl mesoporphyrin(1X) dimethyl ester (VMP) in a naturally occurring petroleum environment (Figure 1). This model system should closely represent the environment of porphyrin molecules in petroleum asphaltenes.
1050 1000 950 Y
cm"
Figure 1. Infrared spectrum of (A) heavy oil solvent, (B) mesoporphyrin I X dimethyl ester, (C) VMP in CS2 solution, (U) VMP in oil solvent, (E) VMP in KBr pellet; 1050 cm-l t o 950 cm-'.
investigated.' I n addition, porphyrins figure importantly in the structure of petroleum residuals, in general, and the asphaltene portion in p a r t i c ~ l a r . ~Dickie ,~ and Yen,3 on the basis of results from several experimental techniques, concluded that asphaltenes were composed basically of unit sheets of pericondensed rings having aliphatic side chains which tend to stack, joined either by actual -C-C- bonds or by some other unspecified form of association, into particles of five or six sheets. The particles associate further into micelles of relatively large size, which account for the high molecular weights observed by colligative measurements, and satisfactorily explain certain anomalous molecular weight mea~urernents.~Altgelt16 on the other hand, observed that molecular weight measurements by dilute solution vapor pressure osmometry did not support the micelle view in that dissociation in the amount required by such a model could not be noted on dilution in various polar and nonpolar solvents. These experiments suggest that if some kind of molecular association does exist in the asphaltene structure then it must be of such strength as not to be affected by dilution with noninteracting solvents, Tynan and Yen6 recently reported the observation of an isotropic anisotropic transition as a function of temperature in the esr spectrum of a natural asphaltene in various solvents. Arrhenius-type calculations based on the assumption that two distinct vanadium(IV) species were represented gave an energy of reaction of 14.3 kcal/mol. Inasmuch as 20-65% of the vanadium(1V) in natural asphaltenes is chelated in porphyrin mo1ecules12it is reasonable to conclude that
+
Experimental Section A 100-mg sample of vanadyl mesoporphyrin IX dimethyl ester was prepared according to the method of Erdman and Ramsey'O using VOSO, and mesoporphyrin IX dimethyl ester. Although this particular porphyrin is not itself found in natural petroleum, it very closely resembles several naturally occurring species, and its solubility in hydrocarbon solvents makes it extremely useful for study. The high-viscosity oil solvent used to duplicate a natural environment was a well-characterized oil fraction from a 22% reduced Kuwait residual having a pour point of 40" and average molecular weight of 760. The oil contained -52% aromatic and -47% saturate material having less than 1 ppm of vanadium and nickel, 0.03% nitrogen, and 2.8% sulfur. Variable temperature infrared spectra were obtained on a Perkin-Elmer Model 421 grating spectrometer using an RIIC variable temperature cell having sodium chloride windows and a path length of 0.2 mm. Spectra in CSZ solution were obtained in a 0.494-mm standard liquid cell. Results and Discussion The frequency of the vanadium-oxygen double bond (vanadyl) stretching mode in vanadium complexes (1) J. E. Folk, "Porphyrins and Metalloporphyrins," Elsevier Publishing Co., New York, N. Y., 1964. (2) E. W.Baker, T . F. Yen, J. P. Diokie, R. E. Rhodes, and L. F. Clark, J . A m e r . Chem. Soc., 89, 3631 (1967). (3) J. P. Dickie and T.F. Yen, A n a l , Chem., 39, 1847 (1967). (4) F. E. Dickson, B. E. Davis, and R. A. Wirkkala, AnaE. Chem., 41, 10, 1335 (1969). (5) K. H. Altgelt, Preprints, Division of Petroleum Chemistry, American Chemical Society, Washington, D. C., Sept 1968. (6) E. C. Tynan and T . F. Yen, FueE, XLVII, 191 (1969). (7) R. J. Abraham, P. A. Burbidge, A. H. Jackson, and D. B. MacDonald, J . Chem. SOC.,B , 620 (1966). (8) D. A. Doughty and C. W. Dwiggins, Jr., J . P h y s . Chem., 73, 423 (1969). (9) H. A. 0. Hill, A. J. MaoFarlane, and R. J. P. Williams, J . Chem. Soc., A , 1704 (1969).
(10) J. E. Erdman, V. G. Ramsey, K. W. Kalenda, and W. E. Hanson, J . Amer. Chem. Soc., 78, 5844 (1956).
T h e Journal of Physical Chemistry, Vol. 74, No. 14* 1970
2852 has been very well e ~ t a b l i s h e d ~ ~as- ' ~985 em-l i= 50 em-'. The frequency of the absorption has been shown to vary as a function of the type of coordination and/or interaction with neighboring groups. Garvey and Ragsdale,14 for example, investigated a series of 21 oxovanadium (IV) complexes with various substituted pyridine-Y-oxide ligands and observed a 45 em-' decrease in frequency (995 em-'-950 ern-') on transaxial coordination. We have assigned the vanadyl frequency in vanadyl mesoporphyrin IX dimethyl ester to the absorption appearing at 997 cm-I in dilute CSz solution (Figure IC), 986 cm-l in petroleum oil (Figure l D ) , and 994 cm-l in a KBr matrix (Figure 1E). Figure 1 also shows that the assigned absorption does not appear either in nonmetalated mesoporphyrin IX dimethyl ester (Figure 1B) or in the oil solvent (Figure 1A). The 11 cm-I frequency difference observed for the vanadyl mode in CSz and in oil suggests some type of coordination or interaction of the V-0 in and/or with the oil environment. Alternatively, one might suggest that the apparent shift or appearance of the lower frequency is simply the effect of physical environment (egg., liquid-solid). However, the frequency for the vanadyl found in solid KBr matrix, 994 em-', indicated that factors in addition to the physical environment may be causing the appearance of the lower frequency in the oil. The work of Garvey and Ragsdale14 would suggest that the observed lowering might be a result of coordination or interaction with some neighboring ligand in the oil. Previous report^^,^ have shown that nonmetalated porphyrins (copro and meso) tend to dimerize in a configuration with the porphyrin rings "stacked" one over the other. Dickie and Yen3 have proposed that "unit sheets," presumably containing vanadium porphyrins and other heterocycles, in natural asphaltenes also stack with five or six sheets forming a "particle." One might visualize in our system an equilibrium between coordinated and uncoordinated VMP molecules, the coordinated species being bound through the vanadium to a Lewis base site causing the molecules to "stack." The Lewis base sites may be found in various functional groups in the oil environment including other porphyrin molecules, other heterocycles, or possibly the highly aromatic condensed ring systems. If such an equilibrium between coordinated and uncoordinated species exists, then one might expect the intensities of the 986 cm-1 (presumably coordinated vanadyl) and 997 ern-' (uncoordinated as observed in CSz)to vary as a function of temperature. Such experiments in petroleum oil have shown the gradual appearance of a higher frequency absorption a t 1003 em-', in addition to the 986 em-' absorption, as the temperature is increased (Figure 2) and suggest that monitoring the intensity of these two bands as a function of temperature may provide data for a calThe Journal of Physical Chenziatry, Vol. 74, No. 14, 1970
NOTES l
L
I
I
I010
I
I
I
I
I
t
I
970 I010 Y
l
I
I
970 I010
I
I
I
970
cm"
Figure 2. Vanadyl stretching mode of VMP in oil 8s a function of temperature.
culation of the energy of reaction, E , for the equilibrium coordinated ;zt uncoordinated. If the usual assumption is made that the integrated area of the infrared absorption, A , is proportional to the concentration, then the equilibrium constant, K , for dissociation may be expressed in terms of the integrated areas representing the two species, and a plot of In A1003/A986 us. 1/T"K would yield the energy of reaction. The dramatic change of the spectrum of VRlP in oil over the temperature range 33.0-139.3" is shown in Figure 2. Qualitatively, one can observe with increasing temperature the complete disappearance of the 986-cm-l absorption and the appearance and subsequent increase in intensity of the 1003 cm-l absorption. The 986-cm-' absorption reappears on return / A ~ ~ ~ to room temperature, and the ratio A I ~ ~ is~ very close to the starting point. A plot of In (Al003/A986) us. 1/ToK is given in Figure 3 showing a marked temperature dependence. A least-squares analysis of all the points yields an energy of reaction of 11.0 kcal/mol. However, a distinct break in the plot may be observed at -71" indicating possibly two equilibria. Least-squares analysis of the points below 71" gives an energy of 3.3 kcal/mol while analysis of those above 71" yields 17.4 kcal/moI. These results are entirely consistent with those reported by Tynan and Yen6who calculated 14.3 kcal/mol for their suggested equilibrium. The actual mechanism of the coordination is not entirely clear, but the fact that the coordinated vanadyl frequency (986 cm-') is not observed either in CS2 solution nor KBr matrix suggests that the oil medium is definitely involved. An attempt was made to dis(11) N. N. Greenwood, "Spectroscopic Properties of Inorganic and Organometallic Compounds," Vol. I, The Chemical Society, London WIVOBN, 1968. (12) R. J. H. Clark, "The Chemistry of Titanium and Vanadium," Elsevier, Essex, England, 1968. (13) J. Selbin, L. H. Holmes, Jr., and S. P. ILlcGlynn, J . Inorg. Nuc2. Chem., 25, 1359 (1963). (14) R. G. Garvey and R. 0 . Ragsdale, ibid., 29, 745 (1967).
2853
NOTES
I T
103
Figure 3. Arrhenius-type plot of the vanadyl stretching modes as a function of temperature.
A
phyrins. Steric repulsion of the bulky propionate side chains were suggested as being responsible for the horizontal displacement. A similar interpretation might be invoked in the case of VRIP. The evidence presented supports the conclusion that two distinctly different vanadyl containing species do exist in equilibrium in petroleum oil with an apparent energy of reaction of 17.4 kcal/mol, and in addition, a lower energy equilibrium of -3.3 kcal/mol suggests a hydrogen bonded or other weakly bonded interaction between porphyrin molecules or porphyrin and solvent molecules. These data lend additional support to current structural theories for asphaltic materials based on associative forces of greater energy than is normally encountered. Also, the energies required in a dissociation process would probably not be observable in solution studies as pursued by Altgelt.5 Acknowledgment. The authors wish to express their appreciation to Dr. Earl Baker of Carnegie-RIellon University for several helpful discussions during the course of this research and to Dr. E. L. McGinnis for the metallation of the porphyrin. I n addition, the assistance of Mr. W. E. Magison in obtaining the infrared data is also appreciated.
B
Figure 4.
Two of several possible modes of association of
VMP: x
= CHa,
y = CHzCHs, z = CH&H&OOCHa.
solve VMP in a nonaromatic oil without success, indicating the aromatic molecules may play a role in the solution of the porphyrin and perhaps the coordination. This observation along with evidence for two equilibria offers several interesting interpretations. The energy of reaction for the equilibrium below 71") 3.3 kcal/ mol, is in the same order of magnitude as hydrogen bonds and other weak, nonspecific interactions while the more energetic equilibrium represented by 17.4 kcal/mol may be associated with coordination of porphyrin molecules through the vanadium in the vertical direction via Lewis base sites. The availability of electrons on oxygen atoms, ring nitrogen atoms, and various r-electron systems offers Lewis base sites for coordination with empty d orbitals of the vanadium. This type of complexing has been observed in other metallo porphyrinse and porphyrin-like systems.l4 Tynan and Yena proposed that a similar coordination be considered as a possible interpretation of their esr data. Several coordinated structures may be visualized including those described in Figure 4. Structure A utilizes the oxygen electrons of another porphyrin ring while B involves coordination with the porphyrin ring nitrogen. Structure B requires a lateral displacement of the rings giving a configuration similar to that proposed by Abraham, et al.,I for dimerization of copropor-
Phosphorus-Proton Spin-Spin Coupling Constants in Acyclic Phosphates
by Masat'sune Kainosho Central Research Laboratories, Ajinomoto Company, Inc., Kawasaki, J a p a n (Received September I,1969)
I n the previous paper' we showed the apparent similar to the well-known angular dependence of JPOCH tendency of vicinal pro ton-pro ton coupling constants for which theoretical calculations by Karplus2 have shown the Fermi contact term dominates. Benezra and Ourisson3 found that an analogous relation to the Karplus curve is operative in case of the vicinal phosphorus-proton coupling constant in P-C-C-H group (JPcc~). Recently a Karplus-like curve has been found for J ~ I I I O C Hthrough a complete analysis of the pmr ]octane spectrum of 2,7,8-trioxa-l-phosphabicyclo[3.2.1 which involves five J P ~ C H with ' S different dihedral angle^.^ However, the expected coupling constant ( J p 0 c H e X * ) for trimethylphosphite based on the $-J curve4 was found to be far different from the observed (1) M. Kainosho, A . Nakamura, and M. Tsuboi, Bull. Chem. SOC, Jap., 42, 1713 (1969). (2) M. Karplus, J . Chem. Phys., 30, 11 (1949); J . Amer. Chem. Soc., 85, 2870 (1963). (3) C. Benezra and G . Ourisson, Bull. SOC. Chim. Fr., 1825 (1966). (4) M. Kainosho and A. Nakamura, Tetrahedron, 25, 4071 (1969).
The Journal of Physical Chemistry, Vol. 74, No. 14, 1970
