2853
NOTES
I T
103
Figure 3. Arrhenius-type plot of t h e 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, Japan (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 with ' S different dihedral which involves five J P ~ C H 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).
SOC.Chim. Fr., 1825 (1966). (4) M. Kainosho and A. Nakamura, Tetrahedron, 25, 4071 (1969). (3) C. Benezra and G . Ourisson, Bull.
The Journal of Physical Chemistry, Vol. 74, No. 14, 1970
NOTES This discrepancy led us to examine the nature of J ~ O C in H various phosphates and phosphites more precisely.6 I n this note we report that the pmr spectra’ quinquevalent phosphorus compounds have been examined and suggest that the J ~ v ~ ccould H be explained approximately by the same mechanisms for vicinal JHH.’
. this small with respect to the changes of J P O C ~For reason one can assume that the n electrons play some role in the coupling mechanism. It is well known that there is much less dn-pn conjugation in the P=S or P=Se than in P=O boud10 and then the degree of n contribution in JPOCH may be considerably different for these three systems. As in I and I1 there is an appre-
Table I : P m r Spectral Parameters of Various Acyclic Phosphates JPOCH, JPOCCH, Formula
Series
ab
X
Enc
Hz
3.0 2.8 2.5 3.5 2.5 2.4 3.5 2.5 2.4
11.0 13.7 17.2 8.4 9.8d 11.4 13.7 14.3 14.9 11.0 13.4 14.2 8.4 10.0d 10.1
0 1 2 0 1 2
C1 Br
I 0
S Se Y
0
X=P(OCH&Ha)a
S
P V
Se
His
Hs
a*f
6CHs,O ppm
0.85 1.20 1.45
148.0 150.2 151.7 147.5 150.2 151.5
29.60 30.04 30.34 29.50 30.04 30.30
3.704 3.866 4.025 1.276 1.362 1.466
0.85 0.96 0.80
148.0 148.2 148.5 147.5 147.6 148.5
29.60 29.64 29.70 29.50 29.52 29.70
3 704 3.690 3.688 1.276 1.269 1.322
JlrCH@s
AhsCHn
6CH1,Q ppm
ACHI
4.024 4.232 4.421
+0.208 t0.397
4.024 4.051 4.110
$0.027 tO.086
+ O . 162 f0.321
... f0.086
+0.170
...
I
-0.006
-0.016
...
-0.007 +0.046
...
a Except series I11 (60 MHz; chloroform solution), all compounds were measured a t 100 MHz, neat liquids. T h e number of chlorine atoms substituted on phosphorus. ’ Pauling’s electronegativities. Averaged values of the two slightly different couplings caused by the magnetic nonequivalence of methylene protons. e Determined directly from the expanded spectra, deviation may be less than 0.02 Hz. Calculated from J l a c ~by J 1 3 C H = 500d Hz. Chemical shifts are expressed in p p m from internal TMS.
’
‘
The parameters of various quinquevalent phosphorus compounds are listed in Table I. Quasilinear relationships between the values of JpocH and the s characters of CpH bonds (a2) calculated from JC,H are observed for I and 11. However, an appreciable difference in the slopes was observed for these two series while the s characters of CH, us. the number of chlorines (n) showed almost identical slopes for both. One of the probable reasons for the different slopes in a2-J relations of I and I1 may be that the actual rotamers do not have exactly the staggered form in I1 as has been shown in sodium diethylphosphate by the X-ray study.s The angle deviations from the ideal gauche and trans positions may cause the slope of I1 to differ from that of 1.l Secondly, the stereospecificity of the chlorine substitution effect on J g and J p o c should ~ ~ be c~nsidered.~For 111, JcH values could not be observed with sufficient accuracy, but the electronegativities of the halogens us. J ~ O Clikewise H gave a linear plot. This fact suggests that JpocH increases in absolute value on increasing the a-electron density in the coupling route. Similar relations, the larger s character gives the larger JPOCH, have been observed again for IV and V. I n these cases changes of the bond characters were rather The Journal of Physical Chemistry, Vol. 74, No. 14, 1970
ciable difference between the slopes of a2-Jpoc~relations for I V and V, probably due to similar reasons. The linear relationship between the number of substituted chlorine atoms and the proton chemical shift, the 6Hp, has been reported already by Mavel and Martin6 for I and 11. I n addition to these, quasilinear (5) The expected coupling constant ( J P o c H ~ ~inPtrimethylphosphite ) is expressed as Jexp = 1/8Jt 2/aJQ, where J t and JB represent the P-H coupling constants with dihedral angles of 180 and 60°, respectively. From the 6-.T curve4 we can estimate J t and JQ as 9.3 and 2.5 Hs, respectively. The J P O C H ~value ~ P must be 4.8 Hs,while the observed one is 10.8 Hs. (6) G. Mavel in “Progress in NMR Spectroscopy,” Vol. 1, J. W. Emsley, J. Feeney, and L. H. Sutcliff, Ed., Pergamon Press, Oxford, 1966; and references cited therein. (7) Pmr spectra were recorded on a Varian HA-100 spectrometer operated at 100 MHz. Samples were neat liquids (111 were measured in chloroform solutions) with a small quantity of TMS as the field-frequency controlling signal. The position of resonance line was determined directly with accuracy of 0.1 HI by a HewlettPackard 55124 digital counter; 60-MHz spectra were recorded with a Varian A-60. All samples examined were prepared by the conventional methods unless they were commercially available. (8) Y. Kyogoku and Y . Iitaka, Acta. Crystallgr., 21, 49 (1966). (9) Stereospecificities of the substitution effect on JPOCHhave been examined using a variety of six-membered cyclic phosphates. The results will be published shortly. (10) R. F. Hudson, “Structure and Mechanism in Organophosphorus Chemistry,” Academic Press, London, 1965.
+
NOTES
2855
relations between the s character (uz)of the CBH bond and 6HBwere found. Attention should be directed to the 6Hp-uz plots for I and I1 being almost parallel, in contrast to that of J P O C(see H -above). ~ The chemical shifts of methyl protons in I1 are downfield on substituting the ethoxy groups by chlorines. It may be of interest to point out that these shifts also correlate with JPOCCH values. For V, however, such a correlation was not observed, probably due to the same reasons discussed earlier.
when the use of conventional methods3 would be difficult. The chief limitations are that one must stay within the bounds of the Debye theory of dilute solutions and of the linearized theory of microwave cavity p e r t ~ r b a t i o n . ~In particular, aqueous reactions are ruled out because of the abnormally high absorption of microwaves by water. The room temperature (22’) liquid phase bromination of benzene with iodine as catalyst was chosen for the present study. C6He,Br2, and Izhave no dipole moments, but CBHsBrand HBr have dipole moments of 1.52 and 0.78 D , respectively.6
Application of Microwave Cavity Perturbation
Experimental Technique
Techniques to a Study of the Kinetics of
The two parameters of a liquid that can be obtained from microwave cavity measurements are the static dielectric constant e, and the loss factor tan 6. A particular cavity resonance is characterized by the resonant frequency fo and the loaded quality factor Q. A liquid placed in a quartz bottle on the axis of the cylindrical cavity introduces a change in both fo and Q. As shown by Dunsmuir and Powles6 and slate^-,^ the first-order perturbation solution takes the form
Reactions in the Liquid Phase
by A. L. Ravimohan Department of Chemical Engineering, California Institute of Technology, Pasadena, California 91109 (Received March 80,1970)
Polar liquids are known to have broad absorption bands in the microwave region which arise from orientation of the molecules with increasing electric field E , followed by relaxation to thermal equilibrium as E falls to zero. The absorption is expressed as the loss factor tan 6, and for a dilute solution of a polar solute in a nonpolar solvent it is given’ by the Debye theory as
where 6 = loss angle, e, = static dielectric constant, dipole moment of solute molecule, c = concentration of solute in moles per cm3, v = frequency of radiation, v0 = 1/2m, where 7 = relaxation time of solute in solution, N = Avogadro’s number, k g = Boltzmann constant, and T = absolute temperature of the solution. For a mixture of solutes, tan 6 is the sum of the loss factors due to the individuaI solutes. Jackson and Powlesz verified eq 1 for solutions of several polar solutes in benzene by measuring resonant frequency shifts and Q factors of cylindrical microwave cavities partially filled with samples of the solution. The proportionality of tan 6 to the concentration of dipole e, for constant values of other variables in (l),suggests a convenient method of following the kinetics of certain classes of liquid reactions. Consider a reaction in which the reactants have no dipole moments but the products have appreciable ~1’s. The time variation of the product concentration, and hence the extent of reaction, can be followed by measuring tan 6 (and e,) of the reaction mixture as a function of time. The method would be fast, accurate, and convenient, as no samples have to be withdrawn for analysis. It is especially suited to the study of the early stages of the reaction,
€,=I+
W O - fa’) fov
where v = dimensionless form-factor depending on cavity geometry, Ro = radius of microwave cavity, RI = inner radius of quartz bottle, RZ = outer radius of quartz bottle, Eb = dielectric constant of bottle material, E, = dielectric constant of the liquid. When RO>> RI, RI II Rz, and Eb E ,
p =
constant
(Q’L - Q
!),/’E,
(4)
The method used for the measurement of fo and Q was a “transmission” method.’ With the circuit of Figure 1, it is possible to obtain a resonance curve in about 20 sec. A faster sweep may be obtained, if necessary, by using an oscilloscope and high-speed photography. fo (1) FV. Gordy, et aE., “Microwave Spectroscopy,” Dover Publications, New York, N. Y., 1966. (2) W. Jackson and J. G. Powles, Trans. Faraday Soc., 42A, 101 (1946).
(3) K. J. Laidler, “Chemical Kinetics,” McGraw-Hill, New York, N. Y . , 1965. (4) J. C. Slater, “Microwave Electronics,” Van Nostrand, Princeton, N. J., 1950. (5) N. A . Lange, Ed., “Handbook of Chemistry,” 9th ed, McGrawHill, New York, N. Y . , 1956. (6) R . Dunsmuir and J. G. Powles, Phil. Mag., 37, 747 (1946). (7) E. L. Ginzton, “Microwave Measurements,” McGraw-Hill, New York, N. Y., 1957.
The Journal of Physical Chemistry, Vol. 74, No. 14, 1970
