Noms
435
effects of the keto form are doubtless small and mainly confined by zero point energy differences of vibrations involving the methylene group. The maximum difference in bond dissociation energies for [Eo(O-H) Eo(C-H)] -- [Eo(O-D) - E0(C-D)]l6 is too small to account for the differences in enthalpy values in Table I. Conceivably, enolization of 3,3-dideuterio-2,4-pentanedione it3 1t:ss favored as a consequence of the mean 0-D bond length being less than that of OH, and therefore the hydrogen bond, -14-0,being longer and weaker. Normal bonds, for example -0-D, involving deuterium always axe shorter than analogous protium bonds due t o anhabrmonic vibrations.” Williams reported18 the following bond distances and angles from an investigation by X-ray crystallography e
2.488
8, c
CcN:
I
wI If this planar, chelated enol structye had involved average” bond lengths (rc-c: = 1.537 A, r c , ~= 1.335, TC--0 = 1.4216, rc-0 = 1.215) and bond angles of either 120” or 105”,the 0-0 separation would be 2.50 A and the hydrogen atom would be >0.30 from the 0-0 line. The acCual O-H distance above is -0.22 greater than Lhe average 0-H distance” in nonhydrogen bonded eoinpounde. Thus, formation of the enol ring apparently is accompanied by bond strains, and the small difference in 10-H and 0-D bond lengths may be unusually important. The data presented here indicate that acidity conketo-enol equilibrium for @-&ketonesin light and heavy water are indeed best explained in terms of protium forming a stronger intramolecular hydrogen bond than deuterium in these systems and that solvent differences between light and heavy water are not dominant. Acknozoledgments. D. W. T. expresses appreciation for a, predoctoral felllowship from the Division of General Medical Sciences, IJnited States Public Health Service. This research was supported in part by the Advanced (16) K. B. Wiberg, Chem. Rev., 55, 713 (1955). (17) 2. E.Sutton, “Tables of Interatomic Distances and Configuran Molecules and Ions” (Special Publication No. 18), The tion i Chemical Sooiety, London, 1965. (18) D. E.Williams, Acta Crystdlogr., 21, 340 (1966).
Research Projects Agency of the Department of Defense, through the Northwestern University Materials Research Center.
Comparative Ligand Field Studies of Manganese(I1) Spectra
by A. Mehra College of Environmental Sciences, University of Wisconsin-Green Bay, Green Bay, Wisconsin 64306 (Received May 18, 1970) Publication costs assisted by the Universitg of Wisconsin--Green Bag
A comparative study of Mn2f spectra in different ligand coordinations is undertaken in this work. The large number of bands in the spectrum of Mn2+allows a free variition of various parameters and provides a good check on ligand field calculations. Spectra in hydrated, fluoride, chloride, and bromide complexes are investigated and regularities have been observed. The energy levels from de configuration of Mn2+and the general features of observed spectra have been reviewed earlier. l-8 The observed band positions in several cases are given in Tables I and 14. Several schemes for fitting the band energies have been sugg e ~ t e d . ~ -One ~ of the best is that of Heidt, Koster, and Johnson4 using free variation of the three crystal field parameters B , C, and Dq. However, it is found that these three parameters are not enough to give good agreement with all the band energies. A similar case is encountered for the free ion where the use of only two electrostatic parameters B and C does not give good agreement for all. the termsae-*’ Consequently, several correction terms for fitting the free ion term values have been suggested. Among these, Trees’ correction term is found to be most Important.loJ1 Following its success for the free ion, we have included this term in our ligand field calculations. The significance of this correctionll and the matrix dements in (1) N. S. Hush and R. J. H. Hobbs, P r o p . Inorg. Chem., 10, 259 (1968). (2) D.S. McClure, Solid State Phys., 9, 399 (1959). (3) C. H. Ballhausen, “Introduction to Ligand Field Theory,” McGraw-Hill, New York, N. Y., 1962. (4) L. J. Heidt, G. F. Koster, and A. M. Johnson, J. Amer. Chem. Soc., 80, 6471 (1958). (5) L. E. Orgel, J. Chem. Phgs., 23, 1004 (1955). (6) J. W.Stout, dbid., 31, 709 (1959). (7) W.Low and G. Rosengarten, J. Mol. Spectrow., 12, 319 (1964). (8) A. Mehra and P. Venkateswarlu, 1. Chem. Phys., 45, 3381 (1966). (9) J. C. Slater, “Quantum Theory of Atomic Structure,” McGrawHill, New York, N. Y., 1960. (10) R. E.Trees, Phys. Rev., 83, 756 (1951); 84, 1089 (1951); and 85, 382 (1952). (11) G. Racah, ibid., 85, 381 (1952). The Journal of P h g h l Chem$stry, Vol. 76,No. 8, 191’1
436
NOTES
Table I: Calculated and Observed Band Energies (cm-1) for the Absorption Spectrum of Mn(ClO&.Aq and MnF2 at Room Temperature r
0bserveda energy
-Mn(C103a*AqCaloulatedb energy (1)
Caloulatedc energy (11)
19,400 22 ,800
18,880 23,013
25 ,200
25,120
28,200 29 ,900 35,000 40,700
28,141 29 ,796 32,747 40,512
18,870 25,275 27,980 29,750 32,960 4.0,810
Observedd energy
19,440 23,500 25,1901 25,5001 28 ,370 30,230 33,060 41,400
-MnFCaloulatede energy (1)
Calculatedf energy (11)
19,464 22,861
19,431 23,505
25,400
25,395
28,409 30,230 35,416 41,080
28,601 30,211 32,817 41,057
I__
CalReference 4. 6 Calculated energies of Heidt, Koster, and Johnson (ref 4) for B = 670, C = 3710, and Dq = 848 ern-'. culated energies in the present case for B = 820, C = 3080, Dq = 780, and CY = 76 cm-1. d Reference 6. Calculated energies Calculated energies after including a, for B = 840, C = 3095, before including (11)for B = 690, C = 3700, and Dq = 860 cm-1. Dq = 3095, and a = 76 Q
'
_I_-_.lls_-
Table 11: Calculated and Observed Band Energies (cm-1) for the Absorption Spectra of MnCla and MnO (the Values Are 'Taken From the Reported Liquid Nitrogen Temperature Spectra) c -
Observeda
Transitions SAi(S)
TI(G)
18,588 21,558
18,424 22,085
15,890 20,210
15,947 19,454
15,905 20,296
23,825
23,700
23,720
23,078
23,078
23 ,720
26,750 28,065 30,500 38,400
26 ,474 28 ,075 32,875 38,180
26,631 28,046 30,415 38,152
25,910 27,912
25,910 27,912
26,093 27 ,906
):;;
4A1(G) %(G) 4T2tD)
WD)
q(P) 44-2@')
-
Calculatedf energy (11)
Observedd energy
18,500
"kztG)
MnOCaloulstede energy (1)
Caloulatedc energy (11)
energy
.-f
M C nCalculated' energy (1)
* Reference a References 17-19. B = 625, C = 3490, and Dq = 760 om''. 0 B = 770, C = 2900, Dp = 670, and a = 76 crn-E. Calculated energy in the present case for B = 22. Calculated energy in ref 22 for B = 601, C = 3540, and Dq = 1010 cm-1. 750, C = 2940, Dq = 940, and (Y = 76 cm-1.
--Table lII : Crysbal Field Parameters and Positions (cm-I) of [ 4A1(G), *E(G)] and 4E(D) Levels of Mnf in Various Coordinations (the Values of Parameters Have Been Obtained in the Present Work as Described in the Text)
915 835 840 835 820 820 810 770 780 760 750 750 a
3235 3080 3095 3070 3080 3000 2980 2900 2910 2955 2860 2940
760 750 745 780 760 710 670 620 590 630 940
26,850 25,278 25,270 25,230 24,960 24 ,740 24 ,590 23 ,560 23,714 23,882 23 ,084 23 ,708
32,340 30,067 30,230 30,060 29 ,750 29 ,454 29 ,100 28 ,065 28,329 28,137 27,505 27,912
13
I4 6 (E
4 15 16 17-19 20 8
17,18 21,22
A. Mehra, unpublished work.
ligand field cam1Z have been discussed earlier. The inclusion of this term1 (called Q term) is also found to improve the accuracy of ligand field calculations. Thus, in. our fitting we consider four parameters, The Journal of Phydcal Chemistry, Vol. 76,No. 3,1071
B, C, Dq, and CY. Due to the lesser percentage contribution of CY term we fix the parameter CY a t the free ion (12) A. Mehrs, J . chm.P ~ Y S .48,4384 , (1968).
NOTEIS value of ‘76 cm-’, although from a theoretical point of view its value may be slightly lower in complexes, like other parameters. After this, preliminary values of B and C are determined from the two Dq independent transitions to f4&(G), 4E(G)]and “E(D)states and that for Dq isi determined from the position of 4T1(G) band which is most sensitive to the value of Dq. After obtaining this trial set, the parameters are varied over a Elmall range to give a good fit for all bands, particularly the lowenergy ones. I n most of the cases, the trial Bet itself is found to be quite good, which supports our procedure. I n Tables J and I1 results are compared with and without thle inclusion of CY term for different cases, The calculated energies without the a term for Mn(C104)2.Aq arid n/rrtO are taken from references mentioned in the tables.1s-2z For MnFz and MnC12, calculations were also performed without the CY term and reasonable set of parameters, by inspection, were chosein for the calculated energies given in the tables. The agreement is seen to be improved for most of the bands on including a and in particular for the *TI(P) band where the deviation is reduced from about 2000 cm-1 to about 260 cm-l. Regarding the relative improvement among different complexes, it is found that the energy levels in complexes with lesser amount of covalency are better described by this theory in comparison to those with higher covalency. The values of parameters were obtained in this way for all the complexes. These are given in Table I11 with the positions of the Dq-independent [4A1(G), 4E((3;3]and 4E(D) ’lev . A comparison of these and and G shows the validity of value9 of parameters nephelauxe tic series. Similarly, a comparison of Dq values shows the validity of spectrochemical series. It is found for all the MnZ+spectra that the deviations between c a l c ~ l ~ md t ~ dexpbrimental energies are larger for the higket energy bands. The theory used in the present work neglwts the interactions with higher configurations and since the higher lying energy states are closes to excited configurations, the departures from the theory re more pronounced for the bands involving these stateEl. A Icasl-squares fitting is not tried in the present work sinoe, in view of the theory being less accurate tt,t, higher energies, such a fitting makes the (13) 6. Moore, ]Vat. Bur. Stand. (U.S.), Circ., No. 467, 2 (1952). (14) A. Mehra .tnd 1’. Venkateswarlu, J . Chem. Phys., 47, 2334 (1967). (15) A. Mehra a,nd P.Venkateswarlu, ibid., 48, 4381 (1968). (16) 1%. Pappalardo, PhG. Mag., 2, 1397 (1957). (17) R. Pappdardo, 1. Chem. Phys., 31, 1050 (1959). (18) FL. Pappalardo, ibid., 33, 613 (1960). (19) A. Mehra, ibid., 48, 1871 (1968). (20) AL. Mehra, Rhyoica Statu8 Solidi, 29, 847 (1968). (21) 6.W.Pratl and R. Coelho, Phys. Rev., 116,281 (1959). (22) D.a. Muffman, R. L. Wild, and M. Shinmei, J . Chem. Phys., 59, 4092 (1969).
437 set of parameters equally bad for all the levels and is not very useful for interpretation. In our fitting, which gives preference to the lower levels, the errors from configurational mixing are minimized and the parameters may be evaluated for physical meaning in terms of the environment of the ion,
A Gas-Phase Density-Dependent Directly Bonded Coupling Constant by A. Keith Jameson* and John P. lteger Department of Chemistry, Loyola University of Chicago, Chicago, Illinois 60688 (Received September 14, 1970) Publication costs assisted bg Loyola University of Chicago
Solvent shifts of nuclear spin-spin coupling constants have previously been noted.’ Of particular interest are the relatively large shifts of 1J(29Si-’*F)in SiF4in various solvents.2 The shifts are all to larger magnitudes of J relative to the gas and are as large as 5%. One might be tempted to explain solvent shifts in J on the basis of some property of the solution (“reaction field” for instance). If this is so, then there should be no significant dependence of the coupling constant in the gas phase. If such a dependence on density could be found, it would most likely be in cases where large solvent shifts have been observed if the gas-phase shifts are caused by the same mechanism(s) as t,he liquid-phaae shifts. Also J itself should be relatively large in order that changes in J be large enough to be experimentally measurable. On the basis of these considerations, we decided to search for density dependence of coupling constants in SiF4 gas, although Goyle, et ul., found no measurable difference between the coupling constants in SiFe at 30 and 110 atm. The samples were sealed in borosilicate tubing having an inside diameter of 1.2 mm and an outside diameter of 3.9 mm. The volume was usually about 0.1 ml. When the tubes were carefully prepared and properly annealed we could safely obtain pressures as high as 200 atm. A Varian A-56/60 high resolution spectrometer was used. For SiF4,calibrated audio side bands of the 2aSi19Ffluorine singlet were impresse on both the upfield and downfield sides of each member of the 2eSi19F doublet and a linear interpolation was made. Several spectra of each sample were taken. I n all cases the standard deviation in the measured J was about k0.15 Hz. This corresponds t o measurement. of the position (1) For an extensive review see P. Laszlo in ^‘Progress in Nuclear Magnetic Resonance Speotroscopy,” Vol. 3, J. W. Emsley, J Feeney, and L H. Sutcliffe, Ed., Pergamon Press, Oxford, 1967. (2) T. D. Coyle, R . B. Johannesen, F. E. Ebrinckman, and T. C. Farrar, J . Phys. Chern., 70, 1682 (1966).
The Journal of Physical Chemistrgt, Val. 76, No. 41, 1971
