J. Phys. Chem. 1981, 85,2007-2091
2087
"'Cd NMR Studies of Solid Cadmium(I1) Complexes P. Gary Mennltt,+Michael P. Shatlock, Victor J. Bartuska, and Gary E. Maclei' Lbpariment Of ChemLstry, Colorado State Universlfy, Fori Colllns, Colorado 80523 (Received: October 22, 1980; In Final Form: April 16, 1981)
W d NMR spectra have been obtained on several complexes, representing a wide range of coordination patterns, by using the cross polarization approach. Powder patterns were obtained on static samples of some of the compounds, from which principal elements of the shift tensors were derived. Using magic-angle spinning, we obtained much narrower resonance lines, giving isotropic l13Cd shifts that can be used as benchmarks for interpreting solution data.
Introduction For some time now we have been concerned with the application of Fourier transform (FT) nuclear magnetic resonance (NMR) techniques to the study of the solution chemistry of metal ions via direct observation of the metal nuclides. The metal-nuclide chemical shift of metal ions has been demonstrated to be a sensitive probe for solvent effects and complexation studies. However, a serious limitation to this approach has been the lack of reliable structure vs. shift correlations to use as the basis of interpreting measured metal-nuclide chemical shifts in terms of chemically useful concepts. Such correlations form the empirical basis for the widespread use of NMR with nuclides such as 'H, '%, 31P,and '9F in determining structural information on solution samples of interest; large tabulations of chemical-shift data are available for these nuclides on compounds for which the solution structures are well known, and can therefore serve as benchmarks for empirical structure shift correlations. This kind of approach is often unwarranted in metal-nuclide work, because the lability and/or uncertainties in metal ion coordination in the systems that one might wish to use as benchmarks preclude the use of such systems for that purpose. Indeed, it is often for elucidating such details of coordination that one would wish to carry out metal-nuclide NMR studies. An approach that we have adopted is to determine the chemical shifts of spin-1/2 metal nuclides (and other pin-'/^ nuclides) in structurally characterized solids as benchmarks for interpreting the chemical shifts of solutions that formally contain these metal ions. As the nature of coordination in these structurally characterized solid species (e.g., species studied by X-ray diffraction) is established, the empirical relationships between chemical shifts of the metal nuclides and coordination details can therefore be established directly. However, until recently it has generally not been possible to obtain high-resolution NMR data on crystalline samples. The recently developed techniques for high-resolution NMR of so1idP now provide the opportunity to study the chemical shift of metal nuclides in well-defined crystalline environments. This paper presents a number of representative W d results which we have obtained on powdered, crystalline samples. The double-resonance experiment of Pines, Gibby, and Waugh3 was used to provide (1) J. Schaefer and E. 0. Stejskal, "Topics in C-13 NMR Spectroscopy", G. C. Levy, Ed., Wiley, New York, 1979,Chapter 4. (2) M. Mehring, "High Resolution NMR Spectroscopy in Solids", Springer-Verlag,New York, 1976, p 107. (3) A. Pines, M. Gibby, and J. S. Waugh, J. Chem. Phys., 66, 1776 (1972). (4)J. Schaefer and E. 0. Stejjskal, J.Am. Chem. Soc., 97,1672(1975).
high-power decoupling of the dipolar interaction with protons and sensitivity enhancement for the less abundant l13Cd nuclide. Information on the chemical-shift anisotropy of the cadmium(I1) ion is retained in the resulting resonance lines, which range in width from tens of Hz to several kHz in a 1.4-T field. A dramatic, further reduction in line width is obtained by utilizing the additional technique of magic-anglespinning.'~~*~ The resulting lines are generally only a few or tens of Hz in width. The resonances obtained in this way are roughly comparableto what would be obtained in solution, if the crystalline environment could be preserved intact in the mobile molecular regime of the liquid state. A major goal of this work is to provide l13Cd chemical-shift benchmarks for a growing body of l13Cd NMR studies in Some of these current studies make use of the l13Cd nuclide as a probe of structure of metalloproteins, with Cd(I1) occurring in the natural protein or as a substitute for some other metal ion, e.g., Zn(II).B**b There have also been some recent reports of solid-state l13Cd NMR data.loa* Experimental Section Measurements. l13Cd NMR measurements were made on the 12.3% abundant l13Cd nuclide at 13.31 MHz on a home-built spectrometer." An external time-shared deuterium field-frequency stabilizer and a JEOL EC-100 data system are incorporated in the system. The nonspinning experiments utilized a crossed-coil probe with a 13-mm insert. The magic-angle spinning (MAS) experiments utilized single-coil electronics and a spinner design of the bullet type." MAS speeds of -2.2 kHz were employed. A 90" proton pulse could be generated in 6 l.ls with either probe, corresponding to a field, H I H of 10 G. The corresponding W d H1of 45 G was adjusted to within 1 dB of the Hartmann-Hahn condition, Y H H ~ H= ycdH1cd.3 The cross-polarization experiments were performed by using a single contact of 1ms. Data acquisition time was limited to 16 ms due to a limitation of the 'H amplifier.
-
-
(5) G. E. Maciel and M. Borzo, J. Chem. Soc., Chem. Commun., 394 (1973). (6)A. Cardin, P.Ellis, J. Odom, and J. Howard, J. Am. Chem. SOC., 97,1672 (1975). (7)R. Kostelnik and A. Bothner-By, J. Magn. Reson., 14,141 (1974). (8) J. J. H. Ackerman, T. V. Orr, V. J. Bartuska, and G. E. Maciel, J . Am. Chem. Soc., 101,341 (1979). (9)(a) J. P. Otvos and I. M. Armitage, Proc. Natl. Acad. Sci. U.S.A., 77,7094 (1980);(b) M. Bain-Ackermanand J. J. H. Ackerman, J.Phys. Chem., 84,3151 (1980). (10)(a) T.T.P. Cheung, L. E. Worthington,P. DUB.Murphy, and B. C. Gerstein, J.Magn. Reson., 41, 158 (1980); (b) E. A. Griffith and E. Amma, J. Chem. Soc., Chem. Commun., 1013 (1979); (c) A. Nolle, Z. Phys. B., 34, 175 (1979). (11)V. J. Bartuska and G. E. Maciel, J. Magn. Reson., 42,312 (1981).
0 1901 American Chemical Society
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The Journal of Physical Chemistty, Vol. 85, No. 14, 1981
This limited the resolution of the instrument to -20 Hz for this work. For the spinning and nonspinning experiments, typically hundreds or thousands of transients were required. Solution l13Cd chemical-shift data were obtained on a Nicolet NT-150 spectrometer, operating a t 33.25 MHz. Solution-state line widths of 1 Hz were typically obtained. All l13Cd spectra, solid state and solution, were referenced to an aqueous 0.1 M Cd(C10J2 solution with ionic strength (I)equal to 4.5, which is arbitrarily assigned a chemical shift of zero. Positive chemical-shift values (6) correspond to lower shielding than the reference. Samples. Reagent-grade commercial samples were used, when they were available. Compounds that could not be purchased were prepared and purified by the methods employed by previous workers who characterized the solid-state structures (see Table 11).
Results and Discussion Powder Spectra. When a cross-polarization (CP) l13Cd spectrum is obtained on a powdered sample without magic-angle spinning, a so-called powder pattern results, from which the principal elements of the chemical-shift tensor (ull, u22, and u33) can be determined.12 For a nonsymmetric system, with ull < u22,< u33,the shielding anisotropy can be defined by ACT=
Menntl et al.
TABLE I : Cadmium-113 Relative Shielding Tensor Elementsa compd
u,,'
6'
395 92 479 372 81 440 410 150 60
-28 0 0
A
+ 022)
+ u22 + 433)
(3) For our present purposes, we define the relative shielding tensor values of each species, with respect to the isotropic average of that species; i.e. u11' = U l l - 8 42; = u22 - 5 u331 = 433 - a (4) '/3(flll
(Positive values of these relative tensor elements correspond to larger shieldings-a convention opposite to the accepted convention for the chemical shift, 6.) From these definitions we see that ACT= 633' Au = ull' - uI'
+
42;)
(5)
for the axial case
(6)
- '/2(~11'
The nature of the line shapes of the powder patterns obtained from NMR experiments on static samples is well understood.12 In favorable cases, the principle elements (ull, 422, u33) can be derived directly from prominent features of the spectra. A somewhat more reliable approach in most cases is to simulate the experimental powder spectra from theoretical line-shape equations,12using the principle elements (and a line width) as fitting parameters. Such an analysis was carried out, by using a computer program, on the spectra of the 12 compounds represented in Table I. Figure 1shows the computer fit (dashed line) obtained in this way for the complex (Me4N)2CdBr4,a case of axial symmetry (i.e., ull = an). For those cases that are not axially symmetric, the u22'value can be obtained di(12)Reference 2, p 21.
33 42 0
~ 3 -3 f/z(u11
Under conditions of magic-angle spinning of a solid sample1I2 or under the influence of random tumbling motions of liquids, the isotropic value of the chemical shift (6) is observed, and the relevant shielding is the isotropic average, 8, where =
22 27
-2 0 0 0 0 0 -18 -18 36 54 -116 10 106 1 5 9 -173 24 1 4 8 222 -107 54 54 - 1 6 1 - 3 0 -13 47 68 0 0 0 0 0 -22 -4 28 41 -19 -120 60 6 0 -180 -100 a In pprn with respect t o the isotropic average, E . Nomenclature given in Table 11. Chemical-shift anisotropy defined by eq 1 and 2. Chemical shift o f the isotropic average relative t o a 0.1 M Cd(ClO,), solution with I = 4 . 5 , which is found to have the same chemical shift as solid Cd(C1O4);6H,O.
(1) For an axially symmetric tensor, if uZ2= ull = ul, then ug3= ul1,or if u22= us? = ul, then ull = uII. In either case, the anisotropy is defined to be A U = uil - C T ~ (2)
8
u)~' Aub
u,,'
-11 -11
(Me,N),CdBr, (Me,N),CdI, (Et,N),CdCl, (Et,N),CdBr, (Et,N),CdI, Cd( n-buxan), Cd (exan ) Cd(OH),'Cd(NH,),(SO,);6H2O Cd(ClO,),.GH,O . Cd( acac), Cd(N0,);4H20
i -1 " 'fll0
'
I
"
9 20
'
I
"
\ L '
I
'f 00
"
'
I
"
380
'
l
-.-
360
390
PPH
Figure 1. 113CdNMR powder spectrum of (Me,N),CdBr,. The dashed line shows the computer simulation. The unbroken line is the experimental spectrum (2000 scans).
rectly from the highest-intensity point of the spectrum; in such cases this fact was made a constraint on the curve-fitting procedure, so that only ull and u33 were variable parameters. The ul{, u2;, and 4%' values obtained by the curve-fitting procedure were within 2 or 3 ppm of what had been determined directly from the pertinent features of the spectra. Also, in most cases the quality of the fit with the computer-generated line shape is degraded markedly if any of the tensor elements is varied by more than 3 or 4 ppm. Hence, we believe that the individual tensor elements are reliable to within ca. f3 or 4 ppm. The isotropic shift values represented in Table I cover a range of nearly 580 ppm. The individual chemical-shift anisotropies range up to 222 ppm. Hence, powder patterns of samples with more than one type of cadmium are not likely to yield data on individual tensor elements directly, without more elaborate curve fitting. In general, singlecrystal studies of the samples of this investigation would be necessary to determine the orientation of the principal axes in the crystalline frame. Chemical-ShiftAnisotropies. The range of anisotropies observed for these cadmium compounds does not appear to be unusually large or small when compared with the available data on 13C,I4N,'?F, ?Si, 31P.2J3However, these (13)B.R.Appleman and B. P. Dailey, Adu. Magn. Reson., 7,231-320 (1978). (14)G.E. Maciel, "NMR Spectroscopy of Nuclei other than Protons", T. Axenrod and G. A. Webb, Eds., Wiley-Interscience, New York,1974, p 347.
The Journal of Physical Chemistry, Vol. 85, No. 14, 1981 2089
’’%d NMR Studies of Solid Cadmium(I1) Complexes
300
200
I00
I 1
0
Flgure 2. Spectra of Cd(OH)2 with magic-angle spinning and nonspinning, Nonspinnlng powder pattern represents 600 scans with a line width of 160 ppm, while MAS at 2.2 kHz produces a line width of 10 ppm, in 100 scans. Shift scale is in ppm from 0.1 M Cd(C104)2( I = 4.5).
-
nuclides have typically been studied in asymmetrical covalent bonding situations. Many of the substances studied here have the cadmium ion located within a nominally or a t least “locally” symmetric environment. It is interesting that apparently small distortions of an octahedral or tetrahedral symmetry are sufficient to produce large chemical-shift anisotropies. For example, the cadmium ions in Cd(C10J2.6H20 and Cd(NH4)2(S04)2-6Hz0 are both octahedrally coordinated by water molecules, and yet their Au values are 0 and 68 ppm, respectively. Small distortions of metal-oxygen bond lengths and angles in the cadmium ammonium sulfate structure have been reported.15 Similar effects can be seen in the measured anisotropies of the tetrahalocadmate(I1) ions: although here there is also an apparent correlation with the size of the counterion. More detailed structural data would be of interest to determine whether these anisotropies are due to a symmetric anion sitting in a distorted environment or whether the tetrahalocadmate(I1) anion, itself, shows distortions from tetrahedral geometry. Wolff, Griffin, and Watson have recently elucidated small distortions of the ammonium ion in ammonium hydrogen oxalate by measuring the electric field gradient tensor at the 14N nucleus.16 Hydrated cadmium chloride offers the opportunity to study the effect of a nonuniform composition of the coordination sphere. This compound has two distinct cadmium ions; one is surrounded by five chlorine atoms and an oxygen atom, while the other type is surrounded by four chlorine atoms and two oxygens. Somewhat surprisingly, the composite line width of the pair of cadmium resonances in cadmium chloride is less than 200 ppm, not much greater than the anisotropy encountered with distorted geometries. The large anisotropy in Cd(OH)2nicely reflects a small deviation from perfect hexagonal close packing.” The shielding tensor is axially symmetric, and symmetry arguments might be advanced to assign the unique axis to be parallel to the axis describing the stacking of successive layers of hydroxide ions. The most shielded direction of the cadmium ion would be in the perpendicular layer of close-packed hydroxide ions. (15)H.Montgomery and F. C. Lingafelter, Acta Crystallogr., 20,728 (1966). (16) E.K.Wolff, R. G. Griffin, and C. Watson, Chem. Phys., 66,5533 (1977). (17)R.G.Wycoff, “Crystal Structures”, Wiley, New York, 1963,Vol. I, p 267.
800
600
400
I
200
0
Flgure 3. Spectrum of Cd(Et,dt~)~spinning at 2.2 kHz about the magic-angle axis. Spectrum obtained in 800 scans. Shift scale is in ppm from 0.1 M Cd(C10,)2 ( I = 4.5).
Magic-Angle Spinning. A dramatic reduction in line width is typically observed when the cross-polarization spectrum of a solid is obtained while spinning the sample at the magic angle. Sample spinning at 2.2 kHz was found to be adequate in most cases to remove the effects of chemical-shift anisotropy in a 1.4-T experiment. Figure 2 shows the CP spectra of Cd(OH)2obtained with and without MAS. In these experiments the line width obtained by using MAS was actually determined by instrumental limitations existing at the time that these measurements were made. Spinning sidebands at f2.2 kHz are noticeable in the CP/MAS spectrum. The obvious improvement in the signal-to-noise ratio with MAS is more impressive when it is noted that the CP/MAS spectrum was obtained in 100 scans, as compared to 600 scans for the nonspinning spectrum. Based on the CP/MAS approach, a survey of Il3Cd isotropic chemical shifts in a variety of compounds was made, and the results are summarized in Table 11. Independent structural data (most of it based on X-ray diffraction) are available for most of the substances, and a description of the coordination sphere of the cadmium ion in each species is given in the table. Comparison of the isotropic shifts measured from powder patterns (Table I) and from MAS spectra (Table 11) reveals a maximum deviation of 8 ppm (for Cd(OH),) and a root-mean-square deviation of 1 ppm. Cd(Et,dtc), (cadmium diethylthiocarbamate) is one of the compounds for which we could not obtain a cross-polarization spectrum without MAS, even with long-term accumulation. The MAS experiment yields the sharp-line spectrum shown in Figure 3 in a matter of hours. The spectrum consists of a series of peaks separated by the spinning frequency of 2.2 kHz. This is a case of “slowspinning”, in which the MAS frequency is considerably less than the shift anisotropy. The intensity pattern of the spinning sidebands in Figure 3 reflects the chemical-shift anisotropy obtained in a nonspinning experiment.1g21 The peak that is independent of the spinning frequency corresponds to the isotropic chemical shift. In several cases, (e.g., HthiCdC14-Hz0,Cd(en)3Clz-Hz0, C O ( N H ~ ) ~ Cresidual ~ C ~ ~ line ) widths of several hundred (18)E.0.Stejskal, J. Schaefer, and R. A. McKay, J . Magn. Reson., 25, 569 (1977). (19)E.T. Lippmaa, W. Alla, and T. Tuhern, Proc. Congr. Ampere, 19th, 113 (1976): (20)R.E.Taylor, R. G. Pembleton, L. M. Ryan, and B. Gerstein, J. Chem. Phys., 71,4541 (1979). (21)M. Maricq and J. S. Waugh, J. Chem. Phys., 70,3300 (1979).
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The Journal of Physical Chemistry, Vol. 85, No. 14, 1987
Mennltt et al.
TABLE 11: Cadmium-11 3 Chemical Shifts of Crystalline Powdered Compounds
compd
attached atoms and approximate first coordination sphere
chemical shift,= ppm
symbol
refb
~~
bis( tetraethylammonium) tetrachlorocadmate thiaminium tetrachlorocadmate monohydrate cadmium n-butyl xanthateC cadmium ethyl xanthateC bis(piperidinium) tetrabromocadmate bis(tetramethy1ammonium) tris(ethylenediamine)cadmium(11) chloride monohydrate cadmium(I1) diethyldithiocarbamate bis(tetraethy1ammonium) tetra bromocadmate cadmium(I1) maleate dihydrate cadmium(11) perchlorate hexahydrate calcium cadmium acetate hexahydrate bis( acetylacetonato)cadmium( 11) cadmium(I1) acetate dihydrate tris(cadmium sulfate) octahydrate cadmium cryptate cadmium nitrate tetrahydrate hexakis(imidazole )cadmium(11) nitrate hexakis(imidazole)cadmium(11) hydroxide nitrate tetrahydrate bis(cadmium(I1) chloride pentahydrate) hexaminecobalt(11) pentachlorocadmate
(Et,N),CdCl,
483
4 C1, tetrahedral
8
HthiCdCl;H,O
460
4 C1, tetrahedral
24
Cd(n-buxan), Cd(exan), (Pd), (Me,N),CdBr, Cd(en ),C1; H,O
44 5 414 396 391 380
4 4 4 4 6
25 26 27 8 28
Cd(Et,dtc), (Et,N),CdBr,
377 374
5 S, tetragonal pyramid 4 Br, tetrahedral
29 8
6 0, octahedral 6 0, octahedral 6 0, octahedral 8 0, dodecahedral
30
Cd(O,CCHCHCO, ). 2H,O Cd(C10, )1.6H,0 CaCd(Ac); 6H,O Cd (acac) Cd(Ac); 2H,O 3CdS0;8H2O Cd[2.2.2.]d Cd(N03),.4H,0 1m6Cd(N03
12
Im6Cd(OH)N0,.4H,0
12 -7 0 -14 -1 8 -46 -58, -45 -62 -100 23 8 272
S, tetrahedral
S, tetrahedral Br, tetrahedral Br, tetrahedral N, octahedral
7 0 6 0, octahedral
80 6 N, octahedral
6 N, octahedral
CdCl; '/,H,O
31 32 33 34 35 36 37 37 38
213 187 134
Cd(2) 1 [Co(",), [CdCl, 1 Cd(OH), Na,Cd( EDTA)
5 C1, 10, octahedral 4 C1, 2 0, octahedral 5 Cl,trigonal bipyramid
39
cadmium(11) hydroxide 158 6 0, octahedral 17 sodium (ethylenediaminetetraaceto). 117,107, 2 N, 4 0, octahedral 41, 42e cadmate 97 bis( tetramethylammonium) 92 4 I, tetrahedral 8 (Me,N),CdI4 tetraiodocadmate bis( tetraethylammonium) 79 4 I, tetrahedral 8 (J3t4N),CdI, tetraiodocadmate cadmium ammonium sulfate 61 6 0, octahedral 15 hexahydrate cadmium(I1) formate dihydrate 26, 21 6 0, octahedral 40 Shifts are referenced to 0.1 M Cd(ClO,), in aqueous solution with I = 4.5. References to structural information. Alkyl xanthate ligand = [S,COR]-. The cryptand ligand of this complex is 4,7,13,16,21,24-hexaoxa1,lO-diazabicyclo[ 8.8.8 Ihexacosane:
FY
,/c3rCo%y
Lov3d Structural information has been inferred from studies on analogous systems incorporating metal nuclides other than cadmium.
e
Hz were encountered in the CP/MAS spectra. The possibility that sample impurities could be responsible for these broad lines was ruled out by independent experiments. More likely sources are (1)small deviations from the magic angle, (2) broadening by 14N, as the dipolar interaction with a quadrupolar nucleus need not be averaged out by magic-angle spinning, (3) crystal imperfections, which could give a dispersion of chemical shifts, or (4) polymorphism. Isotropic Chemical Shifts. The ability to obtain isotropic chemical shifts for solids fills a long-standing need in the NMR of metal nuclides. This need is the measurement of chemical shifts of well-defined complexes with labile ligands. The chemical shift measured in solution for such cases is averaged over all possible complexes and is not easily interpreted without ambiguity. In Table I1 examples can be found of cadmium bound to 4,5, and 6 chloro ligands. Recently, we have used these data to assist in the analysis of data on the chemical shift of "3Cd in halide-containing solutions.8 It should be noted
here that a highly erroneous value for the chemical shift of the [CdCl4I2-ion would be obtained by using just the solution data and published equilibrium constants for the pertinent complex ions. For a given ligand atom, there seems to be a definite trend toward increased shielding with increase in coordination number. For sulfur and chlorine there are good illustrations of this in Table 11. Oxygen is interesting in that examples of 6 , 7 , and 8 coordination can be found in the table. However, a corresponding trend is shielding is not so clear with oxygen. Some of the difficulty stems from the variety of oxy ligands involved: OH-, H20, SO,*-, NO:-, and carboxylate (both bridging and chelating). The coordination involving eight 0 in maleate and CaCd(AC)~.~H actually ~ O consists of four close oxygens (-2.3 A) and four at somewhat greater distances (-2.7 A), whereas in Cd(N0 )2.4H20the oxygen atoms are more This may be responsible for the equidistant (-2.5 apparently low shifts observed for maleate and tetraacetate. The range of shifts observed for oxygen ligands
1).
J. Phys. Chem. 1981, 85,2091-2097
in the solid (-260 ppm) is certainly significantly larger than those observed in solution (-100 ppm). Several of the compounds in Table I1 are of interest in relation to other studies. Cadmium n-butylxanthate and cadmium ethylxanthate may represent analogues to the cadmium protein systems (e.g., metallothionines) currently being studied in other laborat~ries.~The tetraalkylammonium halocadmates have been examined by Drakenberg et al. and by Maciel et al. in so1ution.8,22 l13Cd chemical shifts were also obtained on 0.1 M solutions of many of the compounds of Table I1 in H20and/or dimethyl sulfoxide. Almost invariably, and not surprisingly, the shifts obtained in solution are substantially different from the corresponding data obtained in the crystalline state. This simply reflects the complexity of the solution chemistry of these Cd(1I) systems, and in many cases reflects the fact that the solution l13Cd shifts are weighted averages of various species in rapid equilibria.-P The shift values obtained for 0.1 M solutions are (22)T. Drakenberg, N.Bjork, and R. Portanova, J.Am. Chem. Soc., 82,2423 (1978). (23)P. D. Ellis, private communication. (24)M. F. Richardson, K. Franklin, and D. M. Thompson, J. Am. Chem. Soc., 97,3204 (1975). (25)H. M. Rietveld and E. W. Maslen. Acta Crystalloar.. - 18. 429 (1965). (26)Y. Imura and H. Hagihara, Acta Crystallogr., Sect. B, 28, 2271 (1972). (27)G. Marcatrigiano, L. Menaboe, and G . C. Pellacani, J. Mol. Struct., 33, 191 (1976). (28)K. Krishnan and R. A. Plane, Inorg. Chem., 5 , 852 (1966). (29)A. Domenicano, L. Torelli, A. Vaciago, and L. Zambonelli, J. Chem. SOC.A , 1351 (1968). (30)M.L. Post and J. Trotter, J. Chem. SOC.,Dalton Trans., 674 (1972). (31)C. D.West, Z. Kristallogr., A91, 480 (1935). (32)D. A. Langs and C. R. Hare, Chem. Commun., 890 (1967). (33)M. J. Bennett, F. A. Cotton, and R. Eiss, Acta Crystallogr., Sect. B,24, 907 (1968).
209 1
as follows (in ppm): cadmium perchlorate, -0.2 in H20and -27.0 in Me2SO;cadmium formate, -10.3 in H20;cadmium sulfate, -1.7 in H20; cadmium maleate, -35.2 in Me2SO; calcium cadmium acetate, -26.5 in H20 and -12.3 in Me2SO;cadmium(I1) acetate, -20.1 in H 2 0 and -12.8 in Me2SO;cadmium ammonium sulfate, -2.7 in H20; cadmium nitrate, -33.0 in H 2 0 and -34.8 in Me2S0. Conclusions l13Cd chemical shifts, as well as shift anisotropies, can be obtained relatively conveniently on solid samples representing specific coordination details of interest. These data can provide unequivocal benchmark shift data that ordinarily cannot be obtained from solution experiments unless extensive studies of pertinent equilibria are also undertaken. This approach should be useful for several spin-1/2 metal nuclides.
Acknowledgment. We gratefully acknowledge support of this research by National Science Foundation Grant No. CHE74-23980 and use of the Colorado State University Regional NMR Center, funded by National Science Foundation Grant No. CHE 78-18581. (34)W. Harrison and J. Trotter, J.Chem. SOC.,Dalton Trans., 956 (1972). (35)J. H. Lipson, h o c . R. SOC.London, Ser. A., 166, 462 (1936). (36)B. Matkovie, B. Ribar, B. Zelenka, and S. W. Peterson, Acta Crystallogr., 21, 719 (1966). (37)A. D. Mighell and A. Santoro, Acta Crystallogr., Sect. E,27,2089 (1971). (38)P. Leligny and J. C. Monier, Acta Crystallogr., Sect. B, 31,728 (1975). (39)E. F. Epstein and I. Bernal, J. Chem. SOC.A , 3628 (1971). (40)K. Osaki, Y.Nakui, and T. Watanabe, J.Phys. SOC.Jpn., 19,717 (1964). (41)Y.Lu, Sci. Sin. (Engl. Ed.), 11, 477 (1962). (42)T. Takeshita and M. Shigeru, Yukagaku, 19,984 (1970).
Photosensitized Geometric Isomerization of Alloocimene. The Triplet Torsional Potential Surface of a Conjugated Triene' Yondani C. C. Butt, Ani1 K. Singh, Bruce H. Baretz, and R. S. H. Liu' Depaftmnt of Chemistry, 2545 The Mall, Universw of Hawaii, Honolulu, Hawaii 96822 (Received November 24, 1980; In Final Form: April 6, 198 1)
The photostationary state compositions and quantum yields of the benzophenone-sensitized geometric isomerization of alloocimene, a model triene, have been studied in detail. The results are discussed in terms of two reaction schemes for four isomer systems (one for steady-state concentrations of isomers and one for partially equilibrated isomeric triplets). A triplet torsional potential surface of the triene is also presented which was constructed to reflect the photochemical properties of the triene. From quenching data, the average lifetime of an alloocimene triplet at room temperature was determined to be 2 x 1O-'s.
-
Introduction Conjugated trienes occupy a unique position in studies of photosensitized isomerization of conjugated polyenes. Previous studies from this laboratory showed that the photochemical properties of trienes are sensitive to structural m ~ i f i c a t i o n s z ~ affect the relative
of several possible excited intermediates (planar, diallyl,
(1)Photochemistry of Polyenes 18. For previous paper in the series, A portion of the work was presented a t the XXIIIrd IUPAC Congress.
(2)Ramamurthy, V.; Liu, R. S. H. J. Am. Chem. SOC.1976, 98, 2935-42. (3)Denny, M.; Liu, R. S. H. Nouo. J. Chim. 1978,2,637-41.
see ref 7.
lmar
+ ill
ill,
M i?tt l e i i e p m t i d i i n i 1
and methylenepentadienyl) in the sensitized reaction. In this paper we discuss results of a study on the sensitized isomerization reaction of alloocimene, a model conjugated
0022-3654/81/2085-2091$01.25/00 1981 American Chemical Society
