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Molecular connectivity indexes were tested with regard to their ability to describe the cuticle/water partitioning for a variety of organic chemicals. A model is derived that accounts for 99% of the variation in the published cuticle/water partition coefficients (Kcv¡), which cover the range of 8.5 orders of magnitude. It is shown that the molecular size is directly proportional to the cuticle/water partition coefficients. It is also the most important structural feature that accounts for 89% of the variation in the measured Kcv/ data. Moreover, the molecular connectivity model outperforms in accuracy and speed the traditional empirical models based on 1-octanol/water partition coefficients or water solubilities. Our results and their comparison with previously published models demonstrate that the molecular connectivity model is an accurate predictive tool for the cuticle/water partition coefficients of a wide range of commercial chemicals and that it can be confidently used to assess their potential to penetrate and to accumulate in aboveground vegetation.

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In the bulk gas, the rp(O) value monotonically changes from about 200 to 100 ns with increasing excitation energy (Table I). These values agree fairly well with the fluorescence lifetimes obtained in the supersonic jet.11,12 Accordingly, the slow fluorescence in the bulk gas obviously corresponds to the fluorescence which shows a single exponential decay in the jet. In the bulk gas also, the fast component is very difficult to detect in the decay curve for 338.7-nm excitation (see Figure 6a). However, the molecular level structure even at a very low vibrational level of St cannot be identified as the small molecule limit. As already mentioned, the minute analysis of the experimental results on the quantum yield and decay of the fluorescence at various pressures in the bulk gas indicates that the fluorescence characteristics of acetaldehyde vapor for region-I excitation belong to the intermediate case.The electronic relaxation in St of acetaldehyde vapor after region-I excitation is thus characterized by reversible intersystem crossing to T¡. The Sj -* T, intersystem crossing yield in lower vibrational levels of Si acetaldehyde was reported by Gandini and Hackett5 to be unity at moderately high pressures. Therefore, the lifetime of the fast fluorescence can reasonably be related to the T¡ intersystem crossing rate. The possibility, suggested by Noble and Lee,11•12 that isolated acetaldehyde molecules excited to lower vibrational levels in Sj undergo internal conversion to S0 may be ruled out.Noble and Lee11,12 suggested further that the fast fluorescence, observed in a bulk gas by Atkinson et al.,9 is to be emitted from levels with considerably high rotational quantum numbers. As mentioned previously, however, both the quantum yield and the decay of fluorescence seem to be almost independent of the rotational level excited. It is, therefore, very improbable that the fast component which can be observed in a bulk gas corresponds to fluorescence from high rotational levels.

7 3 + 200 100 0 PPm Figure 4. ,'P NMR spectrum of [Au9(PPh&] [NO,], (CP time 5 ms, pulse interval 5 s, 4710 accumulations, spinning rate 3.5 kHz). ciation reactions.9c The only well-studied compound is [ ( ( M~O ) , P ) , C U ] ' .~~~~~~The Cu atom in (Ph3P)2CuN03 is tetrahedrally coordinated to the two phosphorus atoms and two oxygen atoms of the NO3group.IO The two triphenylphosphine groups are equivalent. The solid-state ''P NMR spectrum of (Ph3P)2CuN03, Figure 2, shows four main peaks at +26.0, +13.1, -7.0, and -31.1 ppm, the outer peaks accompanied by satellite peaks at +28.3 and -33.1 ppm. The 65Cu isotope (abundance 63Cu 69.09%, 30.91%) seems to be responsible for these satellite peaks.Because the crystal structurelo shows that all phosphine groups are equivalent, the splitting of the phosphorus resonance line into four peaks has to be caused by coupling of the phosphorus spin ( I = to the Cu spin ( I = 3/2 for both isotopes). Surprisingly, the splitting between the lines of this quartet increases to higher field. An explanation of this effect may be that due to the presence of a Cu quadrupole interaction not small compared to the Cu Zeeman interaction, the Cu-P dipolar and J tensor interactions are not averaged out by magic angle spinning. Then, in addition to the isotropic J coupling, also the dipolar and J tensor interactions contribute to the splitting. Analogous effects for I3C-l4N couplings have been reported by Lippmaa and co-workers." However, in their case also a broadening of the I3C lines is found, while the ,lP lines of Figure 2 are only 2-ppm wide. In the absence of the Cu quadrupole interaction and without magic angle spinning, the Cu-P dipolar interaction would result in a more than 3-kHz-wide powder pattern, assuming the Cu-P distance to be 2.3 A.12 In this communication no attempt will be made to explain the unequal splittings together with the absence of severe line broadening. Both theoretical and further experimental work is under way to clarify this point.In another copper complex, (Ph3P)3CuCl, the situtation is different. Here the crystal structure of (Ph3P)3CuC1 indicates that there are three molecules per unit cell, two are crystallographically identical, the third molecule being different." The solid-state 31P N M R spectrum of (Ph3P)3CuC1, Figure 3, shows resonances from the two different molecular sites at +9.9, -2.4, -15.3, and -27.9 ppm and at Also satellite peaks appear on the outer signals. Here the splittings within one quartet are not very different, between 900 and 940 Hz. This indicates a smaller Cu quadrupole interaction as compared to (Ph3P)2CuN03, which is reasonable in view of the higher local symmetry (C3J around Cu in (Ph3P)3CuCl.Of the few known gold cluster compounds the spectrum of one of the more interesting clusters, [ A U~( P P~~)~] [NO3]', is given in Figure 4. The spectrum shows two lines, the low-field one asymmetric, at +48.0 and +68.7 ppm.Because the splitting is too large for a dipolar splitting (for a Au-P distance of 2.3 AIz the rigid lattice dipolar...