5256 Table I.
Proton and Carbon-13 Spectral Data for Nickel(I1) Aminotroponiminates 2a, 2b, and the Corresponding Free Ligands Carbon, ppm
7
Proton isotropic shifte 13Cshift in ligandb I3C shift in complexb l3C isotropic shifts Proton isotropic shifta 13Cshift in ligandb 13Cshift in complexb 1 T isotropic shifts
c-1
ff-C
P-c
r-C
$2.15 -56.1 -83.3 -27.2
-1.17 -79.1 -65.0 $14.1
2a -3.00 -63.7 -86.5 -22.8 2b
$77.2 -54.6 -666.8 -612.2
-36.4 -81.7 $594.6 $676.3
-72.4
7
CHz
C
C
c
C
CHO
-3.95 +20.7 +4.8 -15.9
-100.1 -84.2 $15.9 -99.8
CHI
-146.4 +12.7 -614.5 -627.2
+18.8 -136.8 -533.6 -396.8
-14.3 $39.0 -200.8 -239.8
+
Proton isotropic shifts for 2a and 2b were taken from ref 5b and 6, respectively; = upfield, - = downfield. Carbon-I3 shifts are relative to the carbon resonance of CHIClzused as solvent. The solutions were 0.4 M and, with the proton noise decoupler on the sample temperatures were about 40". The observing frequency was 15.08 MHz. Unobserved.
Table 11. Predicted and Experimental Values for ueonC,/uconHi for the Paramagnetic Forms of the Nickel(I1) Aminotroponiminates
2a 2b 2~
a-C
p-C
-8.4
-18.1
-12.65 -7.93 -14.9
Ligand Position 7-C CHz
Predictee -8.0 Experimental -12.05 -7.60 -18.58 b $4.28 -10.5 b
CHI
+4.03 +16.68
CHO
-
+ k c P z (3) +k H ~ ~ u
yH QxcCPNT
QNCH~PN~
-21.11
a Predicted values are the result of using the same set of p c y ' s for each complex. There are only minor changesS.6 in the experi= mental values between the complexes. The set used is $0.040, pp-cT = -0.020, pr-cT = +0.055, and pc-,* (from ref 5b) = -0.015, Unobserved.
The pattern of the experimental ratios for 2a is the same as that previously observed2 for 2c, and it is important to note that for 2a good agreement is obtained for the y position, which was not observed for 2c, consistent with a rapid attenuation of u delocalization with the intervening bonds. However, the results for 2b are markedly different from those of the other two complexes, in showing close agreement between experimental and predicted values of acon''/ uconH1. The complexes 2b and 2c exhibit proton isotropic shifts within2 ppm for the CY and P positions, but the 13Cisotropic shifts differ by about a factor of two. It appears, therefore, that the mechanisms of the proton and 13Cisotropic shifts are markedly different in these complexes, with the proton ring position being less sensitive to structural change, as witnessed by the substantial constancy of the experimental pci* value^.^,^ Again, it appears that propagation of spin density through u bonds can be important in determining the direction and magnitude of the 13Cisotropic shifts, but is probably of less importance in determining the contiguous proton shifts. In 2b, the situation is probably complicated by the formyl group which may interact strongly with the electrons in the T framework of the cycloheptatriene ring as proposed to explain the apparent anomalous positive isotropic shifts of the formyl proton.6 Further evidence demonstrating the sensitivity of the 13C isotropic shifts to the u spin density is apparent from the shifts of the carbons of the N-alkyl substituents of 2a and 2b. For the CH, group of 2a and can be written as the CH, group of 2b uconCH/uconCH Journal of the American Chemical Society
1
92:17
1 August
where is the pT spin density at the nitrogen, and is p ~ s i t i v e . ~QNcC and Qx;cHH are proportionality constants which are negative and positive, respectively. pcu and pHu are the u spin densities at carbon and proton, respectively, while kC and kH are positive constants. Thus, the T and u contributions to the 13C isotropic shifts are of opposite sign, while they are of the same sign for the proton, consistent with the small experimental ratio. However, the u contribution to the 13C isotropic shift must be dominant, as witnessed by the observed downfield shift. Any T contribution to the shift of the CH, carbon in 2b is expected to be small and u effects appear to be dominant as judged by a between the fourfold increase in the ratio uconCH/uconCH CH2 and CH, groups. It can be seen from these results that the I3C isotropic shifts of the alkyl group carbons are more sensitive to u spin density than are the shifts of proton directly attached to the same carbon. Acknowledgments. We are greatly indebted to Dr. R. E. Benson of the Central Research Department of E. I. du Pont de Nemours and Company for samples of the ligands and complexes. (8) Address correspondence to this author.
David Doddrell, John D. Roberts* Contributior~No. 4072 Gates arid Crellin Laboratories of Chemistry California Institute of Technology, Pasadena, California 91109 Receiced Julie 11, I970
Wavelength Dependence and Orbital Symmetry Controlled Sensitizer Selectivity in the Photochemistry of 2-Benzal-5-methylcyclopentanone
Sir : A number of a,p-unsaturated carbonyl compounds undergo different photochemical reactions in solution with different wavelengths. This behavior implies the existence of two or more reactive excited states which (1) For leading references see (a) E. F. Ullman, Accounts Chem. Res., 1, 353 (1968) and (b) E. F. Ullman and N. Baumann, J . Amer. Chem. SOC.,90,4158 (1968); in press, (c) N. Baumann, M. Sung, and E. F. Ullman, ibid., 90,4157 (1968).
26, 1970
5257 Table I. Photostationary Isomer Ratios and Hydrogen Abstraction Quantum Yields in Isopropyl Alcohol Sensitizer ET, kcalimol
Sensitizer
[l],l o 4 M
84.0 68.0 67.9 65.5 61.8 61.7 60.9 58.9 58.8 58.4 56.6 52.7 48.0
Benzene Triphenylene Fluorene Biphenyl Phenanthrene o-Terpheny 1 Naphthalene 1-Phenylnaphthalene 2-Phenylnaphthalene pTerphenyl Chrysene Fluoranthene Pyrene
1.2 1.2 1.2 1.2 1.2 2.1 1.2 3.2 2.2 1.2 2.6 2.6 1.2
trans
@a
x
102
30
8.4
44
